# Unhindered Biplane-Shaped N-Heterocyclic Carbene Ligands as a Remedy for Low Reactivity of Crowded Substrates in Ruthenium Catalyzed Olefin Metathesis

Pawel Krzesiński, Chiara Dinoi, Iker Del rosal, Laure Vendier, Stéphanie Bastin, Vincent César, Anna Kajetanowicz, Karol Grela

► To cite this version:

Pawel Krzesiński, Chiara Dinoi, Iker Del rosal, Laure Vendier, Stéphanie Bastin, et al. Unhindered Biplane-Shaped N-Heterocyclic Carbene Ligands as a Remedy for Low Reactivity of Crowded Substrates in Ruthenium Catalyzed Olefin Metathesis. 2024. hal-04542322

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# Biplane-like small N-heterocyclic carbenes as effective ligands in challenging Ru-catalyzed metathesis of sterically crowded olefins

Pawel Krzesiński, Chiara Dinoi, Iker Del rosal, Laure Vendier, Pavel Kumandin, Adrian Sytniczuk, Stéphanie Bastin, Vincent César, Anna Kajetanowicz, Karol Grela

► To cite this version:

Pawel Krzesiński, Chiara Dinoi, Iker Del rosal, Laure Vendier, Pavel Kumandin, et al.. Biplane-like small N-heterocyclic carbenes as effective ligands in challenging Ru-catalyzed metathesis of sterically crowded olefins. Chem Catalysis, 2025, 5 (5), pp.101292. 10.1016/j.checat.2025.101292. hal-04542322v2

HAL Id: hal-04542322

https://hal.science/hal-04542322v2

Submitted on 18 Jun 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Unhindered Biplane-Shaped N-Heterocyclic Carbene Ligands as a Remedy for Low Reactivity of Crowded Substrates in Ruthenium Catalyzed Olefin Metathesis

Paweł Krzesiński,<sup>a,b</sup> Chiara Dinoi,<sup>c</sup> Iker del Rosal,<sup>c</sup> Laure Vendier,<sup>b</sup> Stéphanie Bastin,<sup>b,*</sup> Vincent César,<sup>b,*</sup> Anna Kajetanowicz,<sup>a,*</sup> Karol Grela<sup>a,*</sup>

**Keywords:** Olefin Metathesis, Ruthenium Catalysts, N-Heterocyclic Carbene, Catalyst Design, Tetrasubstituted Double Bond Formation

**Abstract:** Despite notable progress, olefin metathesis methods for preparing sterically crowded C-C double bonds remain scarce. They are commonly based on specialized ruthenium catalysts with sterically reduced N-heterocyclic carbene (NHC) ligands, able to accommodate more crowded olefinic substrates during the catalytic steps. Yet, although being highly active, these complexes are rather unstable, mainly due to intramolecular C-H activation at the *ortho* position of an *N*-aryl group of the NHC ligand, leading to catalyst deactivation. Considering that the deleterious C-H activation process requires the rotation of the *N*-aryl arm of the NHC ligand, we introduced a second decker of aromatic groups in benzimidazolylidene-based *N*-phenyl NHC ligands, which leads to robust and highly efficient ruthenium metathesis catalysts in challenging metathesis reactions of tri- and tetra-substituted olefins. The beneficial effect of these upper aromatic “wings” on the stability and activity of the Ru-complexes is rationalized through the experimental determination of the stereoelectronic properties of the NHC ligands, complemented by DFT calculations on the nature of the through-space interactions between the aromatics and on the decomposition pathway of the second-generation Hoveyda precursors.

<figure><img src="image_2.png" /></figure>

<footnote><sup>a</sup> Biological and Chemical Research Centre, Faculty of Chemistry, University of Warsaw, Żwirki i Wigury 101, 02-089 Warsaw, Poland</footnote>

<footnote><sup>b</sup> LCC-CNRS, Université de Toulouse, CNRS, 205 route de Narbonne, 31077 Toulouse cedex 4, France</footnote>

<footnote><sup>c</sup> LPCNO, Université de Toulouse, INSA Toulouse, CNRS, UPS, 135 Avenue de Rangueil, 31077 Toulouse, France.</footnote>
# Unhindered Biplane-Shaped N-Heterocyclic Carbenes as Powerful Ligands in Challenging Ru-Catalysed Olefin Metathesis of Sterically-Crowded Substrates

Paweł Krzesiński,<sup>a,b</sup> Chiara Dinoi,<sup>c</sup> Iker del Rosal,<sup>c</sup> Laure Vendier,<sup>b</sup> Pavel Kumandin,<sup>a</sup> Adrian Sytniczuk,<sup>a</sup> Stéphanie Bastin,<sup>b,*</sup> Vincent César,<sup>b,*</sup> Anna Kajetanowicz,<sup>a,*</sup> Karol Grela<sup>a,*</sup>

**Keywords:** Olefin Metathesis, Ruthenium Catalysts, N-Heterocyclic Carbene, Catalyst Design, Tetrasubstituted and Crowded Double Bond Formation

**Abstract:** Despite notable progress, olefin metathesis methods for preparing sterically crowded C-C double bonds remain scarce. They are commonly based on specialised ruthenium catalysts with sterically reduced N-heterocyclic carbene (NHC) ligands, able to accommodate more crowded olefinic substrates during the catalytic steps. Yet, although being highly active, these complexes are rather unstable. In the case of Ru complexes containing NHC ligands with *N*-aryl groups, the catalyst deactivation is mainly due to intramolecular C-H activation at the *ortho* position of the *N*-aryl group of the NHC ligand. Considering that the deleterious C-H activation process requires the rotation of the *N*-aryl arm of the NHC ligand, we introduced a second decker of aromatic groups in benzimidazolylidene-based *N*-phenyl NHC ligands, which led to robust and highly efficient ruthenium metathesis catalysts in challenging metathesis reactions of tri- and tetra-substituted olefins. The beneficial effect of these upper aromatic “wings” on the stability and activity of the Ru-complexes is rationalised through the experimental determination of the stereoelectronic properties of the NHC ligands, complemented by DFT calculations on the nature of the through-space interactions between the aromatics and on the decomposition pathway of these second-generation Hoveyda precursors.

## INTRODUCTION

Olefin metathesis has become a device of prominent importance for C–C double bond formation thus earning an outstanding position in the organic synthesis toolbox.<sup>1–4</sup> Especially well-defined ruthenium-based alkylidene complexes are widely used as catalysts due to their stability towards air and moisture. The discovery of N-heterocyclic carbenes (NHCs)<sup>5–10</sup> has made a great impact in the field, giving birth to even more stable and efficient second generation catalysts (e.g. **Ru1** and **Ru2**, Figure 1).<sup>11, 12</sup>

Despite the incredible progress made in the discipline, there are still many hurdles to overcome, one

<footnote><sup>a</sup>Biological and Chemical Research Centre, Faculty of Chemistry, University of Warsaw, Żwirki i Wigury 101, 02-089 Warsaw, Poland</footnote>

<footnote><sup>b</sup>LCC-CNRS, Université de Toulouse, CNRS, 205 route de Narbonne, 31077 Toulouse cedex 4, France</footnote>

<footnote><sup>c</sup>LPCNO, Université de Toulouse, INSA Toulouse, CNRS, UPS, 135 Avenue de Rangueil, 31077 Toulouse, France.</footnote>
INTRODUCTION

Olefin metathesis has become a device of prominent importance for C–C double bond formation thus earning an outstanding position in the organic synthesis toolbox.¹⁻⁴ Especially well-defined ruthenium-based alkylidene complexes are widely used as catalysts due to their stability towards air and moisture. The discovery of N-heterocyclic carbenes (NHCs)⁵⁻¹⁰ has made a great impact in the field giving birth to even more stable and efficient second generation catalysts (e.g. **Ru1** and **Ru2**, Figure 1).¹¹,¹²

Despite the incredible progress made in the discipline, there are still many hurdles to overcome, one of which is the ineffectiveness of these popular, general-purpose ruthenium catalysts in the formation of tetrasubstituted or crowded C–C double bonds (Figure 1).¹³,¹⁴,¹⁵

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Selected general-purpose catalysts (**Ru1**, **Ru2**) versus sterically reduced ones (**Ru3**–**Ru8**) designed for the formation of tetrasubstituted C–C double bonds, and associated decomposition products (**Ru9**, **Ru10**). In the inset is shown an example of a challenging metathesis reaction.¹⁶</figcaption></figure>

In 2007, Grubbs and Schrodi reported a practical solution to this essential problem by developing a series of ruthenium catalysts bearing NHC ligands with sterically less hindered, mono-*ortho* substituted N-aryl groups (**Ru3** and **Ru4**, Figure 1).¹⁶,¹⁷ The proposed explanation for their higher activity is fairly straightforward and widely accepted—the smaller demand of the “chopped” NHC
of which is the ineffectiveness of these popular, general-purpose ruthenium catalysts in the formation of tetrasubstituted or crowded C–C double bonds (Figure 1).¹³⁻¹⁵

<figure><img src="image_2.png" /><figcaption>**Figure 1.** A. Selected general-purpose catalysts (Ru1, Ru2) versus sterically reduced ones (Ru3-Ru8) designed for the formation of crowded C–C double bonds and their mode of deactivation leading to decomposition products (Ru9, Ru10). In the inset is shown an example of a challenging ring-closing metathesis reaction.¹⁶ B. Challenging cross-metathesis reaction during the synthesis of *Largazole* illustrating the above problem.</figcaption></figure>

In 2007, Grubbs and Schrodi reported a practical solution to this essential problem by developing a series of ruthenium catalysts bearing NHC ligands with sterically less hindered, mono-*ortho* substituted *N*-aryl groups (Ru3 and Ru4, Figure 1).¹⁶,¹⁷ The proposed explanation for their higher activity
ligand allows for more facile coordination of sterically crowded olefins to the ruthenium metal center.¹⁸ About the same time, other structures were developed (e.g. **Ru5** — **Ru7**, Figure 1) based on the same concept.¹⁷,¹⁹,²⁰,²¹,²²

Although these new catalysts are more efficient in the formation of crowded and tetrasubstituted olefins than popular general purpose complexes, such as **Ru1** and **Ru2**, it was quickly observed that they strongly suffer from decreased stability and increased sensitivity to air and other poisons.¹⁹,²³ Blechert *et al*. investigated the mechanism of deactivation of **Ru6**, which consisted of sequential pericyclic cyclization, oxidation, and elimination steps yielding the metathesis-inactive product **Ru9** (Figure 1).¹⁹ Particularly, when both arms of the NHC ligand are sterically reduced, the resulting catalyst (e.g. **Ru7**) is extremely prone to deactivation, following pathways including C-H activation in *ortho N*-phenyl position, which after insertion in alkylidene gives the permanently deactivated complex **Ru10** (Figure 1).²³⁻²⁵

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Selected approaches to stabilize small-NHC ruthenium catalysts, A) by using stabilizing ligands, B, C) by restricting aryl ring rotation.²⁶⁻²⁸,²⁹</figcaption></figure>

The instability of ruthenium complexes induced by the use of low sterically hindered NHC ligands can be mitigated to some extent by using indenylidene²⁶,²⁷ or thioether benzylidene ligands that are known to increase the thermodynamic stability of Ru catalysts (Figure 2A).²⁸ It is clear that the initial step in catalyst decomposition is the rotation of the *N*-aryl until which is coplanar with the imidazolidinylidene ring,³⁰ therefore some effort has been put into the synthesis of NHC-based Ru complexes with restricted *N*-aryl rotation while maintaining the low steric demand of the NHC
is fairly straightforward and widely accepted—the smaller demand of the “chopped” NHC ligand allows for more facile coordination of sterically crowded olefins to the ruthenium metal centre.¹⁸ At about the same time, other structures were developed (e.g. **Ru5** – **Ru7**, Figure 1) based on the same concept.¹⁷,¹⁹ (A conceptually different idea for forcing tetrasubstituted C=C bonds formation in olefin metathesis is to use specially reinforced SIMes-bearing catalysts at higher temperatures (up to 140 °C), e.g. Refs.: 20, 21 and some other examples of Ru-based catalysts specially-designed for sterically-demanding substrates can be found in Refs.: 22-28)

Although these new catalysts are more efficient in the formation of crowded and tetrasubstituted ol-
efins than popular general purpose complexes, such as **Ru1** and **Ru2**, it was quickly observed that
they strongly suffer from decreased stability and increased sensitivity to air and other poisons.¹⁹, ²⁹
Blechert *et al*. investigated the mechanism of deactivation of **Ru6**, which consisted of sequential
pericyclic cyclisation, oxidation, and elimination steps yielding the metathesis-inactive product
**Ru9** (Figure 1).¹⁹ Particularly, when both arms of the NHC ligand are sterically reduced, the result-
ing catalyst (e.g. **Ru7**) is extremely prone to deactivation, following pathways including C-H activa-
tion in the *ortho N*-phenyl position, which after insertion in alkylidene gives the permanently deac-
tivated complex **Ru10** (Figure 1).²⁹⁻³¹
ligand.30-32 Our group has recently published an example how such an approach can deliver enhanced catalyst stability and productivity (Figure 2B). 29

Perhaps due to their limited applications, benzimidazolylidene ruthenium catalysts did not gain much interest. Delaude and co-workers reported, in their scholarly work, the synthesis of ruthenium catalysts having benzimidazolylidene NHC ligand with *ortho*-tolyl (Ru7, Figure 1) and mesityl N-substituents.33,34 We reasoned that such a ligand architecture would be ideal for the installation of a second decker of aromatic rings, which would hinder the *N*-aryl rings' rotation. According to this working hypothesis, catalysts **Ru11** and **Ru12** (Figure 2C), featuring the smallest possible set of aryl *N*-substituents at the NHC ligand, *i.e.* phenyl groups, may display higher stability than their parent complex **Ru5**, while their activity in the challenging metathesis of sterically crowded bonds would be maintained and, hopefully, even enhanced. Electronically-different phenyl (in **Ru11**) and pentafluorophenyl (in **Ru12**) groups were chosen as second-decker aryl groups to investigate the possible peculiarities of these hitherto unknown architectures, such as a potential presence of π-π interactions and modulated electronic properties.

## RESULTS

**Synthesis of NHC precursors.** Commercially available 4,7-dibromo-2,1,3-benzothiadiazole (1) was chosen as a precursor for both phenyl and pentafluorophenyl substituted benzimidazolium salts 5 and 8 (Scheme 1). Suzuki coupling allowed for the installation of phenyl groups in the backbone motif, while subsequent reductive sulfur extrusion gave access to *o*-phenylenediamine derivative 2 in 72% yield over two steps. Next, the bicyclic benzimidazole skeleton was obtained using a standard cyclization protocol with triethyl *ortho*-formate, affording 3 in quantitative yield. Products 2 and 3 were obtained on a gram scale without purification by column chromatography. In the final step, the desired benzimidazolium salt 5 was obtained using copper-catalyzed arylation with diphenyliodonium tetrafluoroborate.35 Worth highlighting is that such direct arylation of NH imidazole to imidazolium salt has never been reported in the literature.35-37 This protocol gave a mixture of the desired *N*,*N'*-diphenylbenzimidazolium salt 5 (36% isolated yield) and neutral *N*-phenylbenzimidazole 4 (58% isolated yield) which were separated using column chromatography. The succeeding arylation of isolated 4 gave salt 5 in 77% yield, therefore the two subsequent arylation reactions lead to compound 5 in an 81% overall yield.

Synthesis of pentafluorophenyl substituted derivative **8** turned out to be a more synthetic challenge (see Supporting Information for experimental details on pre-ligands synthesis). A Kumada-Corriu coupling was successfully employed to graft pentafluorophenyl groups into the benzothiadiazole motif. The subsequent reductive sulfur extrusion step gave *o*-phenylenediamine derivative **6** in 55% yield over the two steps. Unlike the synthesis of salt 5, the *N*-phenyl groups were first installed
A) Using stabilizing ligands (known)

<figure><img src="image_3.png" /><figcaption>Ru11</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Ru12</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Ru13</figcaption></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

Euonyminol octaacetate

B) Restricting aryl wings rotation by bonding them (known)

<figure><img src="image_9.png" /><figcaption>Ru14a</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Ru14b</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Ru15</figcaption></figure>

C) Restricting aryl wings rotation by stacking them (this work)

<figure><img src="image_12.png" /><figcaption>Ru16</figcaption></figure>

<figure><img src="image_13.png" /></figure>

Restricted rotation?

<figure><img src="image_14.png" /></figure>

π-π Stacking?

<figure><img src="image_15.png" /></figure>

Better stability (vs Ru5)?

<figure><img src="image_16.png" /></figure>

Better activity (vs Ru5)?

<caption>Ru17</caption>

**Figure 2.** Selected approaches to stabilise small-NHC ruthenium catalysts, A) by using stabilising ligands (in an inset is shown the application of **Ru12** in the total synthesis of *Euonyminol octaacetate* is shown<sup>15</sup>) B, C) by restricting aryl ring rotation. See, Refs.: 32-35.

Because the observed instability of new catalysts bearing chopped NHC ligands limits their applicability in target oriented syntheses,<sup>36</sup> a number of effort was invested in its improvement. It was, *inter alia*, found that the stability of the Ru complex can be improved in some extent by using indenylidene<sup>32, 33</sup> or thioether benzylidene ligands<sup>34</sup> that are known to increase the thermodynamic stability of Ru catalysts **Ru11–Ru13** (Figure 2A). Indeed, **Ru12** was found to be very productive in a challenging RCM reaction en route to the natural products *Euonymine* and *Euonyminol Octaacetate* (Figure 3A).<sup>15</sup> As it is clear that the initial step in catalyst decomposition is the rotation of the *N*-aryl until it is coplanar with the imidazolidinylidene ring,<sup>37</sup> some effort has been put into the synthesis of NHC-based Ru complexes with restricted *N*-aryl rotation while maintaining the low steric demand of the NHC ligand.<sup>37-39</sup> Our group has recently published an example showing how such an
through a Buchwald-Hartwig amination, giving compound **7** in 21% isolated yield. In a final step, the benzimidazolium salt **8** was obtained with quantitative yield using a modified cyclization protocol using triethyl orthoformate in the presence of HBF₄×●Et₂O.

<figure><img src="image_5.png" /></figure>

**Scheme 1.** Synthesis of benzimidazolium salts **5** and **8**. Reaction conditions: a) 2.1 equiv. PhB(OH)₂, 4 equiv. K₂CO₃, 0.8 mol% Pd(PPh₃)₄, toluene/water 2.5 : 1 v/v, reflux, overnight; b) 9 equiv. NaBH₄, 6 mol% CoCl₂×6H₂O, EtOH/THF 3:1 v/v, reflux, 3 h; c) (EtO)₃CH (excess), 6 equiv. HClaq, reflux, overnight; d) 2 × 2 equiv. Ph₂IBF₄, 2 × 5 mol% Cu(OAc)₂×H₂O, DMF, 100 °C, 24 h, then 48 h; e) 2 equiv. Ph₂IBF₄, 7.5 mol% Cu(OAc)₂×H₂O, DMF, 100 °C, overnight; f) 3 equiv. BrMgC₆F₅, 10 mol% Pd(PPh₃)₃, THF, 70 °C, overnight; g) 3 equiv. NaBH₄, 7 mol% CoCl₂×6H₂O, EtOH/THF 3:1 v/v, 50 °C, 5 min.; h) PhBr (excess), 2.5 mol% Pd₂(dba)₃, 7.5 mol% P'Bu₃, 1.5 equiv. KHMDS, 160 °C, 30 min.; i) (EtO)₃CH (excess), 1 equiv. HBF₄×Et₂O, 60 °C, 30 min.

**Synthesis of ruthenium catalysts.** The corresponding Hoveyda-Grubbs complexes were obtained by displacement of the tricyclohexylphosphine ligand in ruthenium precursor **Ru13** by the free NHCs generated using KOC(CF₃)CH₃ as a base (Scheme 2A). Complex **Ru12** was isolated using column chromatography, whereas complex **Ru11** was purified by simple crystallization from a tetrahydrofuran-pentane mixture. Both complexes (brown-orange solids) are stable for months when stored in a protective atmosphere. It is noteworthy that upon the synthesis of known **Ru5** (see Supporting Information for details),¹⁷ we noticed its fragility when column chromatography was performed with regular solvents. Even with carefully degassed dry solvents, complex **Ru5** was obtained in only 19% yield, which is considerably lower than in the case of **Ru11** and **Ru12**, 72% and 43% respectively. The molecular structures of complexes **Ru11** and **Ru12** were confirmed by X-ray diffraction experiments on single crystals grown by layering dichloromethane complex solutions of **Ru11** and **Ru12** with pentane (respectively, Scheme 2B and C, Figure 3, Table 2, and Supporting Information for details).
approach can deliver enhanced catalyst stability and productivity (Figure 2B).³⁵

Perhaps due to their limited applications, benzimidazolylidene ruthenium catalysts did not gain much interest. Delaude and co-workers reported, in their scholarly work, the synthesis of ruthenium catalysts having benzimidazolylidene NHC ligand with *ortho*-tolyl (**Ru7**, Figure 1) and mesityl *N*-substituents.⁴⁰, ⁴¹ We reasoned that such a ligand architecture would be ideal for the installation of a second decker of aromatic rings, which would hinder the *N*-aryl rings' rotation. According to this working hypothesis, catalysts **Ru16** and **Ru17** (Figure 2C), featuring the smallest possible set of aryl *N*-substituents at the NHC ligand, *i.e.* phenyl groups, may display higher stability than their parent complex **Ru5**, while their activity in the challenging metathesis of sterically crowded bonds would be maintained and, hopefully, even enhanced. Electronically-different phenyl (in **Ru16**) and pentafluorophenyl (in **Ru17**) groups were chosen as second-decker aryl groups to investigate the possible peculiarities of these hitherto unknown architectures, such as a potential presence of $\pi-\pi$ interactions and modulated electronic properties.

## RESULTS

**Synthesis of NHC precursors.** Commercially available 4,7-dibromo-2,1,3-benzothiadiazole (1) was chosen as a precursor for both phenyl and pentafluorophenyl substituted benzimidazolium salts 5 and 8 (Scheme 1). Suzuki coupling allowed for the installation of phenyl groups in the backbone motif, while subsequent reductive sulphur extrusion gave access to *o*-phenylenediamine derivative 2 in 72% yield over two steps. Next, the bicyclic benzimidazole skeleton was obtained using a standard cyclisation protocol with triethyl orthoformate, affording 3 in quantitative yield.⁴² Products 2 and 3 were obtained on a gram scale without purification by column chromatography. In the final step, the desired benzimidazolium salt 5 was obtained using copper-catalysed arylation with diphenyliodonium tetrafluoroborate.⁴³ Worth highlighting is that such direct arylation of NH imidazole to imidazolium salt has never been reported in the literature, however, examples of N-R substituted imidazole arylations to corresponding imidazolium salts,⁴³; as well as arylations from NH imidazoles to corresponding N-aryl imidazoles are known.⁴⁴ This protocol gave a mixture of the desired *N*,*N'*-diphenylbenzimidazolium salt 5 (36% isolated yield) and neutral *N*-phenylbenzimidazole 4 (58% isolated yield) which were separated using column chromatography. The succeeding arylation of isolated 4 gave salt 5 in 77% yield, therefore the two subsequent arylation reactions lead to compound 5 in an 81% overall yield.

Synthesis of pentafluorophenyl substituted derivative 8 turned out to be a more synthetic challenge (see Supporting Information for experimental details on pre-ligands synthesis). A Kumada-Corriu coupling was successfully employed to graft pentafluorophenyl groups into the benzothiadiazole motif. The subsequent reductive sulphur extrusion step gave *o*-phenylenediamine derivative 6 in
<figure><img src="image_6.png" /><figcaption>**Scheme 2.** A) Synthesis of **Ru11** and **Ru12**; conditions A: i) 1.75 equiv. of **5**, 1.75 equiv. KOC(CF₃)CH₃, benzene, rt, 30 min., ii) 1 equiv. **Ru13**, 60 °C, 1 h; conditions B: i) 1.75 equiv. of **8**, 2 equiv. KOC(CF₃)CH₃, benzene, rt, 15 min., ii) 1 equiv. **Ru13**, 60 °C, 30 min. B) XRD structure of **Ru11**, ellipsoids are drawn at 50% probability. C) XRD structure of **Ru12**, ellipsoids are drawn at 50% probability.</figcaption></figure>

**Evaluation of electronic and steric properties of new NHC ligands and catalysts.** An in-depth characterization of the electronic properties of the three ligands was then carried out (Scheme 3 and Table 1). The overall electron-donating ability of the NHC ligands was first quantified by recording the average stretching frequency, *v*COav, of the carbonyl ligands in complexes **Rh1-Rh3**, which is correlated to the Tolman Electronic Parameter (TEP) value of the ligand by a well-established linear correlation.¹⁰ Additionally, the σ-donating and the π-accepting abilities of the NHCs can be independently assessed by measuring the $^1J_{CH}$ coupling constant between the carbon and the hydrogen atoms on the pre-carbenic position in the azolium precursors and by recording the chemical shift of $^{77}$Se nuclei in the seleno adducts **Se1-Se3** respectively.³⁸ The experimental data first confirmed that the three benzimidazole-based NHCs are less donating than their imidazol(in)e-based congeners **IMes** and **SIMes**. Within the benzimidazolylidene series, the presence of the two
55% yield over the two steps. Unlike the synthesis of salt **5**, the *N*-phenyl groups were first installed through a Buchwald-Hartwig amination, giving compound **7** in 21% isolated yield. In a final step, the benzimidazolium salt **8** was obtained in quantitative yield using a modified cyclisation protocol using triethyl orthoformate in the presence of HBF₄×Et₂O.

<figure><img src="image_17.png" /></figure>

**Scheme 1.** Synthesis of benzimidazolium salts **5** and **8**. Reaction conditions: a) 2.1 equiv. PhB(OH)₂, 4 equiv. K₂CO₃, 0.8 mol% Pd(PPh₃)₄, toluene/water 2.5 : 1 v/v, reflux, overnight; b) 9 equiv. NaBH₄, 6 mol% CoCl₂×6H₂O, EtOH/THF 3:1 v/v, reflux, 3 h; c) (EtO)₃CH (excess), 6 equiv. HClaq, reflux, overnight; d) 2 × 2 equiv. Ph₂IBF₄, 2 × 5 mol% Cu(OAc)₂×H₂O, DMF, 100 °C, 24 h, then 48 h; e) 2 equiv. Ph₂IBF₄, 7.5 mol% Cu(OAc)₂×H₂O, DMF, 100 °C, overnight; f) 3 equiv. BrMgC₆F₅, 10 mol% Pd(PPh₃)₃, THF, 70 °C, overnight; g) 3 equiv. NaBH₄, 7 mol% CoCl₂×6H₂O, EtOH/THF 3:1 v/v, 50 °C, 5 min.; h) PhBr (excess), 2.5 mol% Pd₂(dba)₃, 7.5 mol% P'Bu₃, 1.5 equiv. KHMDS, 160 °C, 30 min.; i) (EtO)₃CH (excess), 1 equiv. HBF₄×Et₂O, 60 °C, 30 min.

**Synthesis of ruthenium catalysts.** The corresponding Hoveyda-Grubbs complexes were obtained by displacement of the tricyclohexylphosphine ligand in ruthenium precursor **Ru18** by the free NHCs generated using KOC(CF₃)CH₃ as a base (Scheme 2A). Complex **Ru17** was isolated using column chromatography, whereas complex **Ru16** was purified by simple crystallisation from a tetrahydrofuran-pentane mixture. Both complexes (brown-orange solids) are stable for months when stored in a protective atmosphere. It is noteworthy that upon the synthesis of known **Ru5** (see Supporting Information for details),¹⁷ we noticed its fragility when column chromatography was performed with regular solvents. Even with carefully degassed dry solvents, complex **Ru5** was obtained in only 19% yield, which is considerably lower than in the case of **Ru16** and **Ru17**, 72% and 43% respectively. The molecular structures of complexes **Ru16** and **Ru17** were confirmed by X-ray diffraction experiments on single crystals grown by layering dichloromethane complex solutions of **Ru16** and **Ru17** with pentane (respectively, Scheme 2B and C, Figure 3, Table 2, and Supporting Information for details).
distal phenyl groups in **BI*Ph**³⁹ slightly increased its σ-donation but also its π-acidity relative to the unsubstituted **BIPh**, leading to a slightly stronger overall electronic donation with a TEP value of 2052.2 cm⁻¹. In contrast, perfluorination of the backbone-aryl groups induced a noticeable decrease in electronic donation with a TEP value ranging from 2052.2 cm⁻¹ for **BI*Ph** to 2056.2 cm⁻¹ for **BIF*Ph**, as could be anticipated in view of the strong electron-withdrawing effect of fluorine atoms. Overall, starting from the unsubstituted **BIPh**, the installation of the second aryl layer at the rear of the benzimidazolyl core generates the more electron-donating **BI*Ph** ligand and the less electron-donating **BIF*Ph** ligands.

<figure><img src="image_7.png" /></figure>

**Scheme 3.** A) Structures and abbreviations of NHC ligands studied herewith. B) Synthesis of rhodium carbonyl complexes and selenium adducts. Reaction conditions: a) i) 2.2 equiv. of benzimidazolium precursor, 2.2 equiv. KOC(CF₃)₂CH₃, THF, rt, 30 min., ii) 1 equiv. [RhCl(COD)]₂, overnight; b) CO (excess), DCM, rt, 30 min.; c) i) 1 equiv. of benzimidazolium precursor, 1 equiv. KOC(CF₃)₂CH₃, THF, rt, 30 min., ii) 2 equiv. Se, overnight.

<table><caption>**Table 1.** Measured stereoelectronic parameters of SIMes, IMes, BIPh, BI*Ph and BIF*Ph</caption><thead><tr><th>Ligand</th><th>v<sub>CO</sub><sup>av</sup>[a]<br>[cm<sup>-1</sup>]</th><th>TEP value[b]<br>[cm<sup>-1</sup>]</th><th><sup>1</sup>J<sub>CH</sub> <sup>40</sup></th><th>δ(<sup>77</sup>Se)[c] [ppm]</th><th>%V<sub>bur</sub> [%][d]</th></tr></thead><tbody><tr><td>SIMes</td><td>lit. 2037.5[e]</td><td>2050.2</td><td>lit. 206[f]</td><td>lit. 110[g]</td><td>33.7[h]</td></tr><tr><td>IMes</td><td>2037.5<br>(lit. 2037.6)[i]</td><td>2050.2<br>(2050.3)</td><td>225<br>(lit. 225)[f]</td><td>27<br>(lit. 27)[g]</td><td>n.a.[j]</td></tr><tr><td>BIPh</td><td>2043.0</td><td>2054.6</td><td>224</td><td>127</td><td>32.9[k]</td></tr><tr><td>BI*Ph</td><td>2040.0</td><td>2052.2</td><td>222</td><td>160</td><td>31.6</td></tr><tr><td>BIF*Ph</td><td>2045.0</td><td>2056.2</td><td>226</td><td>198</td><td>31.6</td></tr></tbody></table>

Selected bond lengths and angles of complexes **Ru11** and **Ru12** are presented in Table 2, along with

<footnote>[a] Measured in CH₂Cl₂. [b] TEP = (0.8001 × vCOav) + 420.0 cm⁻¹. [c] Measured in CDCl₃. [d] Calculated from XRD structures of the corresponding Grubbs-Hoveyda complexes. [e] From ref 41. [f] From ref 38. [g] From ref 42. [h] XRD structure retrieved from ref 43,44 [i] From ref 45. [j] n.a. = not available; no single crystal XRD structure of IMes-derived Grubbs-Hoveyda complex has been reported. [k] XRD structure retrieved from ref 17,46</footnote>
<figure><img src="image_18.png" /></figure>

5 (X = H)

8 (X = F)

Conditions A or B

Ru16 (X = H), Cond. A, 72%

Ru17 (X = F), Cond. B, 43%

<figure><img src="image_19.png" /></figure>

Ru16

<figure><img src="image_20.png" /></figure>

Ru17

**Scheme 2.** A) Synthesis of **Ru16** and **Ru17**; conditions A: i) 1.75 equiv. of **5**, 1.75 equiv. KOC(CF₃)CH₃, benzene, rt, 30 min., ii) 1 equiv. **Ru18**, 60 °C, 1 h; conditions B: i) 1.75 equiv. of **8**, 2 equiv. KOC(CF₃)CH₃, benzene, rt, 15 min., ii) 1 equiv. **Ru18**, 60 °C, 30 min. B) XRD structure of **Ru16**, ellipsoids are drawn at 50% probability. C) XRD structure of **Ru17**, ellipsoids are drawn at 50% probability. CCDC-2341751 (**Ru16**) and CCDC-2341750 (**Ru17**) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service via www.ccdc.cam.ac.uk/structures.

**Evaluation of electronic and steric properties of new NHC ligands and catalysts.** An in-depth characterisation of the electronic properties of the three ligands was then carried out (Scheme 3 and Table 1). The overall electron-donating ability of the NHC ligands was first quantified by recording the average stretching frequency, $v_{CO}^{av}$, of the carbonyl ligands in complexes **Rh1-Rh3**, which is correlated to the Tolman Electronic Parameter (TEP) value of the involved ligand by a well-established linear correlation.¹⁰ Additionally, the σ-donating and the π-accepting abilities of the
the corresponding ones for the previously reported complex **Ru5**,¹⁷,⁴⁶ which may serve as the reference compound. The geometrical parameters describing the structures of complexes **Ru11** and **Ru12** correlate well with the one of reference **Ru5**, as illustrated by the superimposed structures (Figure 3B). The lengths of the Ru-C<sub>NHC</sub> and the Ru=C<sub>alkylidene</sub> in the three complexes do not differ of more than three estimated standard deviations, as well as the geometry index (τ<sub>5</sub>),⁴⁷ which characterizes the distortion of the square-pyramidal geometry [**Ru5**: 0.35; **Ru11**: 0.38; **Ru12**: 0.40]. However, Ru-O bond distances significantly increase in order **Ru12**, **Ru5**, **Ru11**, respectively, which perfectly correlates with the NHCs electronic donation abilities (overall and σ-donation) rising in this order **BI*FPh** < **BIPh** < **BI*Ph**, reflecting structural *trans* effect from **BI*FPh** to **BIPh** and to **BI*Ph**.⁴⁸ Interestingly, such observation differs from phosphine derived second generation OM catalysts (e.g. **Ru1**) where the increased π-acidity of NHC ligand accounts for stronger *trans* effect as the resulting metal-phosphine π-backdonation weakens.⁴⁹ Obviously isopropoxy ligand is a purer σ-donor hence stronger σ-donating NHC will have a stronger *trans* effect. Such a trend can also be observed in other examples in the literature although, to the best of our knowledge, no such assumption has been made so far.⁵⁰

The relative steric constraints of the parent unsubstituted **BIPh** and the two new double-decker **BI*Ph** and **BIF*Ph** ligands were quantified by establishing their topographical steric maps in complexes **Ru5**, **Ru11**, and **Ru12** respectively, using the SambVca2 web application (Figure 3A),⁵¹ and were compared to the steric hindrance brought by the **SIMes** ligand in the general-purpose catalyst **Ru2**.⁴³,⁴⁴ Interestingly, it appeared that the substitution of the benzimidazolyl backbone led to a slight decrease of the %V<sub>bur</sub> from 32.9% for **BIPh** in **Ru5** to 31.6% for **BI*Ph** and **BIF*Ph** in **Ru11** and **Ru12**, which arises from a higher orthogonality of the *N*-phenyl groups relative to the benzimidazolyl heterocycle, characterized by the increase of the dihedral angles C7-N2-C8-C9 and C2-N1-C10-C11 from 48.21° and 62.91° in **Ru5** to 69.23° and 77.17° in **Ru11** and 77.30° and 66.24° in **Ru12**. This may be ascribed to the substitution of the backbone of the benzimidazolyl ring by phenyl and pentafluorophenyl rings in **Ru11** and **Ru12** respectively, which hinders the *N*-phenyl rotation as hypothesized. This through-space effect of the backbone substitution is the opposite of the buttressing effect observed in amino-decorated NHC catalysts, in which the bulky amino substituents on the backbone forced the *N*-aryl groups to be twisted.⁴⁵,⁵² As expected, all three benzimidazolylidene ligands provide a lower steric pressure than the standard **SIMes** ligand [%V<sub>bur</sub>(**SIMes**) = 33.7%], due to the presence of small *N*-phenyl groups.
NHCs can be independently assessed by measuring the ¹J<sub>CH</sub> coupling constant between the carbon and the hydrogen atoms on the pre-carbenic position in the azolium precursors and by recording the chemical shift of ⁷⁷Se nuclei in the seleno adducts **Se1-Se3** respectively.⁴⁵ The experimental data first confirmed that the three benzimidazole-based NHCs are less donating than their imidazol(in)e-based congeners **IMes** and **SIMes**. Within the benzimidazolylidene series, the presence of the two distal phenyl groups in **BI*Ph** (Names for substituted BIPh NHC ligands were adopted in analogy with well-known Cp and Cp* naming scheme) slightly increased its σ-donation but also its π-acidity relative to the unsubstituted **BIPh**, leading to a slightly stronger overall electronic donation with a TEP value of 2052.2 cm⁻¹. In contrast, perfluorination of the backbone-aryl groups induced a noticeable decrease in electronic donation with a TEP value ranging from 2052.2 cm⁻¹ for **BI*Ph** to 2056.2 cm⁻¹ for **BIF*Ph**, as could be anticipated in view of the strong electron-withdrawing effect of fluorine atoms. Overall, starting from the unsubstituted **BIPh**, the installation of the second aryl layer at the rear of the benzimidazolyl core generates the more electron-donating **BI*Ph** ligand and the less electron-donating **BIF*Ph** ligands.

<figure><img src="image_21.png" /><figcaption>**Scheme 3.** A) Structures and abbreviations of NHC ligands studied herewith. B) Synthesis of rhodium carbonyl complexes and selenium adducts. Reaction conditions: a) i) 2.2 equiv. of benzimidazolium precursor, 2.2 equiv. KOC(CF₃)₂CH₃, THF, rt, 30 min., ii) 1 equiv. [RhCl(COD)]₂, overnight; b) CO (excess), DCM, rt, 30 min.; c) i) 1 equiv. of benzimidazolium precursor, 1 equiv. KOC(CF₃)₂CH₃, THF, rt, 30 min., ii) 2 equiv. Se, overnight.</figcaption></figure>

<table><caption>**Table 1.** Measured stereoelectronic parameters of **SIMes**, **IMes**, **BIPh**, **BI*Ph** and **BIF*Ph**</caption><thead><tr><th>Ligand</th><th><i>v</i><sub>CO</sub><sup>av[a]</sup><br>[cm<sup>-1</sup>]</th><th>TEP value<sup>[b]</sup><br>[cm<sup>-1</sup>]</th><th><sup>1</sup>J<sub>CH</sub></th><th>δ(<sup>77</sup>Se)<sup>[c]</sup> [ppm]</th><th>%V<sub>bur</sub> [%]<sup>[d]</sup></th></tr></thead><tbody><tr><td><b>SIMes</b></td><td>lit. 2037.5<sup>[e]</sup></td><td>2050.2</td><td>lit. 206<sup>[f]</sup></td><td>lit. 110<sup>[g]</sup></td><td>33.7<sup>[h]</sup></td></tr><tr><td><b>IMes</b></td><td>2037.5</td><td>2050.2</td><td>225</td><td>27</td><td>n.a.<sup>[i]</sup></td></tr></tbody></table>
<figure><img src="image_8.png" /><figcaption>**Figure 3.** A) Steric maps of NHC ligands in complexes: left—**Ru5**, center— **Ru11**, right— **Ru12**. Values in the four corners of the maps are the %V<sub>bur</sub> of the NHC ligand in the corresponding quadrant. B) Superimposed structures of **Ru5** (gray), **Ru11** (red) and **Ru12** (blue).</figcaption></figure>

Furthermore, when **Ru11** and **Ru12** are compared, although dihedral angles between the *N*-phenyl groups and the benzimidazolyl ring do not clearly stand out one from another, dihedral angles between NHC core and phenyl or pentafluorophenyl upper deck substituents (angles C2-N1-C10-C11 and C5-C6-C14-C15, respectively) explicitly increase from 62.50°, 63.79° to 70.32°, 79.38°, respectively. This observation could be explained by stronger π-π interactions between the two aromatic planes in the structure of **Ru12**.

<table><caption>**Table 2.** Selected, corresponding bond lengths (Å) and angles (deg) in X-ray crystal structures of complexes **Ru5**, **Ru11**, and **Ru12**. Atom labeling corresponds to Figure 3B and not to CIF files.</caption><thead><tr><th></th><th>Ru5<sup>46</sup></th><th>Ru11</th><th>Ru12</th></tr></thead><tbody><tr><td>Ru1-C1</td><td>1.9618(10)</td><td>1.968(3)</td><td>1.9585(17)</td></tr><tr><td>Ru1-C16</td><td>1.8365(11)</td><td>1.830(3)</td><td>1.8326(17)</td></tr><tr><td>Ru1-O1</td><td>2.2692(7)</td><td>2.2807(19)</td><td>2.2539(12)</td></tr><tr><td>C1-Ru1-O1</td><td>176.19(4)</td><td>175.61(9)</td><td>177.09(5)</td></tr></tbody></table>
<table><thead><tr><th></th><th>(lit. 2037.6)<sup>[i]</sup></th><th>(2050.3)</th><th>(lit. 225)<sup>[f]</sup></th><th>(lit. 27)<sup>[g]</sup></th><th></th></tr></thead><tbody><tr><td><strong>BIPh</strong></td><td>2043.0</td><td>2054.6</td><td>224</td><td>127</td><td>32.9<sup>[k]</sup></td></tr><tr><td><strong>BI*Ph</strong></td><td>2040.0</td><td>2052.2</td><td>222</td><td>160</td><td>31.6</td></tr><tr><td><strong>BIF*Ph</strong></td><td>2045.0</td><td>2056.2</td><td>226</td><td>198</td><td>31.6</td></tr></tbody></table>

[a] Measured in CH₂Cl₂. [b] TEP = (0.8001 × vCO₃) + 420.0 cm⁻¹. [c] Measured in CDCl₃. [d] Calculated from XRD structures of the corresponding Grubbs-Hoveyda complexes. [e] From ref. 46. [f] From ref. 45. [g] From ref. 47.

[h] XRD structure retrieved from ref. 48 and the Cambridge Crystallographic Data Centre (CCDC) under accession number 1100116. [i] From ref. 49. [j] n.a. = not available; no single crystal XRD structure of IMes-derived Grubbs-Hoveyda complex has been reported. [k] XRD structure retrieved from ref. 17 and the Cambridge Crystallographic Data Centre (CCDC) under accession number 605939.

Selected bond lengths and angles of complexes **Ru16** and **Ru17** are presented in Table 2, along with the corresponding ones for the previously reported complex **Ru5**,¹⁷ which may serve as the reference compound. The geometrical parameters describing the structures of complexes **Ru16** and **Ru17** correlate well with the one of reference **Ru5**, as illustrated by the superimposed structures (Figure 3B). The lengths of the Ru-C<sub>NHC</sub> and the Ru=C<sub>alkylidene</sub> in the three complexes do not differ by more than three estimated standard deviations, as well as the geometry index (τ₅),⁵⁰ which characterises the distortion of the square-pyramidal geometry [Ru5: 0.35; Ru16: 0.38; Ru17: 0.40]. However, Ru-O bond distances significantly increase in the order **Ru17**, **Ru5**, **Ru16**, respectively, which perfectly correlates with the NHCs electronic donation abilities (overall and σ-donation rising in the order BI*FPh < BIPh < BI*Ph), and which reflects the structural *trans* effect from BI*FPh to BIPh and to BI*Ph.⁵¹ Interestingly, such observation differs from phosphine derived second generation olefin metathesis catalysts (e.g. **Ru1**) where the increased π-acidity of NHC ligand accounts for a stronger *trans* effect as the resulting metal-phosphine π-backdonation weakens.⁵² Obviously isopropoxy ligand is a purer σ-donor hence stronger σ-donating NHC will have a stronger *trans* effect. Such a trend can also be observed in other examples in the literature although, to the best of our knowledge, no such assumption has been made so far.⁵³

The relative steric constraints of the parent unsubstituted **BIPh** and the two new double-decker **BI*Ph** and **BIF*Ph** ligands were quantified by establishing their topographical steric maps in complexes **Ru5**, **Ru16**, and **Ru17** respectively, using the SambVca2 web application (Figure 3A),⁵⁴ and were compared to the steric hindrance brought by the **SIMes** ligand in the general-purpose catalyst **Ru2**.⁴⁸,⁵⁵ Interestingly, it appeared that the substitution of the benzimidazolyl backbone led to a slight decrease of the %V<sub>bur</sub> from 32.9% for **BIPh** in **Ru5** to 31.6% for **BI*Ph** and **BIF*Ph** in **Ru16** and **Ru17**, which arises from a higher orthogonality of the *N*-phenyl groups relative to the benzimidazolyl heterocycle. The latter is characterised by the increase of the dihedral angles C7-N2-C8-C9 and C2-N1-C10-C11 from 48.21° and 62.91° in **Ru5** to 69.23° and 77.17° in **Ru16** and 77.30° and 66.24°
<table><tr><td>Cl1-Ru1-Cl2</td><td>155.303(10)</td><td>153.04(3)</td><td>153.163(14)</td></tr><tr><td>C7-N2-C8-C9</td><td>48.21</td><td>69.23</td><td>77.30</td></tr><tr><td>C2-N1-C10-C11</td><td>62.91</td><td>77.17</td><td>66.24</td></tr><tr><td>C5-C6-C14-C15</td><td>-</td><td>62.50</td><td>79.38</td></tr><tr><td>C4-C3-C12-C13</td><td>-</td><td>63.79</td><td>70.32</td></tr></table>

In order to rationalize the effect of the insertion of a phenyl or pentafluorophenyl ring in the backbone of the benzimidazolyl group of **Ru11** and **Ru12** respectively, we carried out a DFT study at the B3PW91-D3 level of theory. The optimized structure of **Ru11** and **Ru12** (11opt and 12opt, respectively), are shown in Figure S11. Selected bond distances and angles are listed in Table S11 and compared with those experimentally observed. The computed bond distances and angles accurately reproduce the experimental values with variations of less than 0.04 Å. Taking into account the free rotation of both the *N*-phenyl and the benzimidazolyl-phenyl or pentafluorophenyl rings, the disparities observed in the torsion angles between the experimental and computed structures are likely to be attributed to packing interactions within the crystal lattices.

To better characterize the interactions between the *N*-phenyl and the benzimidazolyl-phenyl or pentafluorophenyl rings, the natural charges of complexes **11opt** and **12opt** have been computed by NBO analysis (Figure S12). In complex **11opt**, the carbon atoms of both the *N*-phenyl and benzimidazolyl-phenyl rings display negative charges (in the range -0.235 to -0.249 and -0.227 to -0.250 for the *N*-phenyl and benzimidazolyl-phenyl groups respectively). Conversely, in complex **12opt**, while the carbon atoms of the *N*-phenyl display negative charges (in the range -0.230 to -0.254), those of the pentafluorophenyl rings display positive charges (in the range 0.300 to 0.393). According to the model proposed by Hunter and Sanders,⁵³ the electron-withdrawing F substituents would reduce the negative quadrupole moment of the second-decker aromatic rings, thereby favoring parallel displaced sandwich conformations in complex **12opt**. In order to check for the possible presence of π-π orbital interactions between the *N*-phenyl and the backbone-phenyl or -pentafluorophenyl rings, we also carried out molecular orbital analysis of compounds **11opt** and **12opt**. As previously suggested,⁵⁴ indeed, when two aromatic rings are near enough to each other, their π orbital overlapping may provide an additional binding force to their stacking interaction (Figures S13 and S14). While for complex **11opt**, no π orbital overlapping between the *N*-phenyl and the backbone-phenyl rings is observed, in complex **12opt**, on the other hand, the HOMO-15 orbital displays a small π orbital overlapping between the *N*-phenyl and the backbone-pentafluorophenyl
in **Ru17**, which is translated in the topographical steric maps by a decrease of the %V<sub>bur</sub> values in the east quadrants and by values in NW and SW quadrants closer together in **Ru16** and **Ru17** relative to **Ru5**. This may be ascribed to the substitution of the backbone of the benzimidazolyl ring by phenyl and pentafluorophenyl rings in **Ru16** and **Ru17** respectively, which hinders the *N*-phenyl rotation as hypothesised. This through-space effect of the backbone substitution is the opposite of the buttressing effect observed in amino-decorated NHC catalysts, in which the bulky amino substituents on the backbone forced the *N*-aryl groups to be twisted.<sup>49, 56</sup> As expected, all three benzimidazolylidene ligands provide a lower steric pressure than the standard **SIMes** ligand [%V<sub>bur</sub>(SIMes) = 33.7%], due to the presence of small *N*-phenyl groups.

<figure><img src="image_22.png" /><figcaption>**Figure 3.** A) Steric maps of NHC ligands in complexes: left—**Ru5**, centre— **Ru16**, right— **Ru17**. Values in the four corners of the maps are the %V<sub>bur</sub> of the NHC ligand in the corresponding quadrant. B) Superimposed structures of **Ru5** (gray), **Ru16** (red) and **Ru17** (blue)</figcaption></figure>

Furthermore, when **Ru16** and **Ru17** are compared, although dihedral angles between the *N*-phenyl groups and the benzimidazolyl ring do not clearly stand out one from another, dihedral angles between NHC core and phenyl or pentafluorophenyl upper deck substituents (angles C2-N1-C10-
rings. The analysis of the interaction between the *N*-phenyl and the second decker-phenyl or pentafluorophenyl rings by comparing their different *i*)* centroid's distances (3.607 and 3.427 Å for **11opt** and **12opt** respectively), *ii*)* quadrupole moments and *iii*)* π orbital overlapping suggests therefore that the *N*-phenyl ring interacts more strongly with the backbone-pentafluorophenyl ring in **12opt** than with the backbone-phenyl ring in **11opt**.

**Catalysts stability in solution.** The stability of the two newly obtained biplane-type NHC ruthenium complexes **Ru11** and **Ru12** was then quantitatively monitored and compared with the benchmark **Ru5**. Since NHC ligand remains coordinated to the metal center throughout the entire catalytic cycle, the differences in complexes' stability should directly translate to the stability of actual propagating species, thus conveying their effectiveness in catalysis. Gratifyingly, **Ru11** and **Ru12** remained perfectly unchanged for approximately three weeks in solution at ambient temperature, while **Ru5** slowly degraded to around 50% of the initial amount (Figure 4A, left). This fact confirmed our previous observation that upon isolation the top-covered complexes **Ru11** and **Ru12** are considerably more stable than **Ru5**. Furthermore, the differences in stability were even more pronounced at higher temperature (Figure 4A, right), where **Ru5** disappeared completely after about 27 hours, while **Ru11** and **Ru12** were still detectable even after more than 100 hours in solution at 80 °C. Furthermore, the elevated temperature allowed to distinguish **Ru11** from **Ru12** in terms of stability and **Ru12** proved to be the more stable catalyst, which we assign to the beneficial impact of the π-π interactions between *N*-phenyl groups and phenyl or pentafluorophenyl backbone motifs. To unambiguously confirm the nature of the catalyst deactivation pathway, we attempted to isolate the degradation product of **Ru5**.<sup>23,55</sup> After stirring **Ru5** in dichloromethane at 40 °C for one week, the expected, tethered η<sup>6</sup>-arene-NHC Ru(II) complex **Ru14** was isolated (Figure 4B), whose formation should proceed through the activation of an *ortho* C-H bond of one *N*-phenyl group followed by alkylidene insertion. The molecular structures of complex **Ru14** was confirmed by X-ray diffraction experiments on single crystals grown by layering dichloromethane complex solution with pentane (Figure 4C).
C11 and C5-C6-C14-C15, respectively) explicitly increase from 62.50°, 63.79° to 70.32°, 79.38°, respectively.

<table><caption>**Table 2.** Selected, corresponding bond lengths (Å) and angles (deg) in X-ray crystal structures of complexes **Ru5**, **Ru16**, and **Ru17**. Atom labelling corresponds to Figure 3B and not to CIF files.</caption><thead><tr><th></th><th>Ru5<sup>[a]</sup></th><th>Ru16</th><th>Ru17</th></tr></thead><tbody><tr><td>Ru1-C1</td><td>1.9618(10)</td><td>1.968(3)</td><td>1.9585(17)</td></tr><tr><td>Ru1-C16</td><td>1.8365(11)</td><td>1.830(3)</td><td>1.8326(17)</td></tr><tr><td>Ru1-O1</td><td>2.2692(7)</td><td>2.2807(19)</td><td>2.2539(12)</td></tr><tr><td>C1-Ru1-O1</td><td>176.19(4)</td><td>175.61(9)</td><td>177.09(5)</td></tr><tr><td>Cl1-Ru1-Cl2</td><td>155.303(10)</td><td>153.04(3)</td><td>153.163(14)</td></tr><tr><td>C7-N2-C8-C9</td><td>48.21</td><td>69.23</td><td>77.30</td></tr><tr><td>C2-N1-C10-C11</td><td>62.91</td><td>77.17</td><td>66.24</td></tr><tr><td>C5-C6-C14-C15</td><td>-</td><td>62.50</td><td>79.38</td></tr><tr><td>C4-C3-C12-C13</td><td>-</td><td>63.79</td><td>70.32</td></tr></tbody></table>

In order to rationalize the effect of the insertion of a phenyl or pentafluorophenyl ring in the backbone of the benzimidazolyl group of **Ru16** and **Ru17** respectively, we carried out a DFT study at the B3PW91-D3 level of theory. The optimised structure of **Ru16** and **Ru17** (Ru16-opt and Ru17-opt, respectively), are shown in Figure S9. Selected bond distances, angles, and torsion angles are listed in Table S15 and compared with those experimentally observed. The computed bond distances and angles accurately reproduce the experimental values with variations of less than 0.04 Å and 1.0°. Taking into account the free rotation of both the *N*-phenyl and the benzimidazolyl-phenyl or penta-fluorophenyl rings, the disparities observed in the Cl1–Ru–Cl2 angles and in some of the torsion angles between the experimental and computed structures (Table S15) are likely to be attributed to packing interactions within the crystal lattices.

To better characterize the interactions between the *N*-phenyl and the benzimidazolyl-phenyl or benzimidazolyl-pentafluorophenyl rings, the natural charges of complexes **Ru16-opt** and **Ru17-opt** have been computed by natural bond orbital (NBO) analysis (Figure S10). In complex **Ru16-opt**, the carbon atoms of both the *N*-phenyl and benzimidazolyl-phenyl rings display negative charges (in the range -0.217 to -0.260 and -0.227 to -0.250 for the *N*-phenyl and benzimidazolyl-phenyl groups respectively). Conversely, in complex **Ru17-opt**, while the carbon atoms of the *N*-phenyl

<footnote>[a] Structure retrieved from the Cambridge Crystallographic Data Centre (CCDC) under accession number 605939.</footnote>
<figure><img src="image_9.png" /><figcaption>**Figure 4.** A) Stability test in solution at ambient and elevated temperature measured by ¹H NMR (1,3,5-trimethoxybenzene added as reference). Lines are visual aid only. Conditions: (left) CD₂Cl₂ at 22 °C, under argon; (right) C₆D₆ at 80 °C, under argon. B) Study of the decomposition of the complex **Ru5** into the complex **Ru14**. C) XRD structure of complex **Ru14** (ellipsoids are drawn at 50% probability).</figcaption></figure>

**Computational study.** In order to explain the slower decomposition kinetics of complexes **Ru11** and **Ru12** compared to that of **Ru5**, we hypothesized that the insertion of an aromatic plane parallel to the *N*-aryl rings could prevent the rotation of the *N*-phenyl group in consequence constraining its aromatic CH activation and therefore its decomposition. Therefore, starting from complexes **5opt**, **11opt** and **12opt** we computed the TS Gibbs free energy barriers associated with the *N*-phenyl rotation (Figure 5), by DFT calculations at the B3PW91-D3 level of theory. Complexes **11opt** and **12opt** display higher *N*-phenyl rotation TS energies (19.3 and 22.7 kcal·mol⁻¹) than compound **5opt** (17.4 kcal·mol⁻¹). This may be ascribed to the increased steric hindrance of compounds **11opt** and **12opt** compared to that of their simpler analog **5opt**. In addition, in agreement with the higher π-π stacking interaction expected between the *N*-phenyl and the second decker-pentafluorophenyl rings, the rotation of the *N*-phenyl ring is 3.4 kcal·mol⁻¹ higher for **12opt** than for **11opt**. Thus, this TS energy trend well reflects the influence of both i) the increased steric bulk of **11opt** and **12opt** compared to **5opt** and ii) the F substitution on the strength of the phenyl-pentafluorophenyl π-π stacking interaction. However, the different catalyst deactivation kinetics measured for **5opt**, **11opt** and **12opt** cannot be ascribed to this *N*-phenyl rotation process, as the corresponding TS Gibbs free energy barriers can be easily overcome at room temperature, unlike experimentally observed.
rings display negative charges (in the range -0.228 to -0.254), those of the benzimidazolyl-penta-
fluorophenyl rings display positive charges (in the range 0.300 to 0.393). According to the Hunter
and Sanders model applied to complex **Ru16**,⁵⁷ the π electron density on both the *N*-phenyl and ben-
zimidazolyl-phenyl rings creates a quadrupole moment with a partial negative charge above and be-
low both aromatic faces and a partial positive charge around the periphery of each ring. The electro-
static repulsion exerted by the quadrupole moments of these two electron-rich aromatics results in
a respective distancing of the *N*-phenyl and benzimidazolyl-phenyl rings, leading to ring-centroid
distances of 3.607 and 3.612 Å in **Ru16-opt** and 3.734 and 3.901 Å in **Ru16**, Figure 4). By replacing
the phenyl with the pentafluorophenyl groups, on the other hand, the electron-withdrawing F sub-
stituents polarize the π electron density away from the aromatic core of the pentafluorophenyl rings
in **Ru17**, providing a reversal in the direction of the overall quadrupole moment. The central area of
relative electron deficiency in the pentafluorophenyl rings exerts now an electrostatic attraction to-
wards the central area of relative electron richness in the *N*-phenyl rings, the shorter ring centroid
distances of 3.427 and 3.450 Å in **Ru17-opt** and 3.597 and 3.704 Å in **Ru17** (Figure 4) reflecting the
approaching of the *N*-phenyl and benzimidazolyl-pentafluorophenyl rings closer to each other. As
represented in Figure 4, interestingly, the analysis of the electrostatic potential surfaces of com-
plexes **Ru16-opt** and **Ru17-opt** confirms this model, showing the presence of two electron-rich
aromatics and one electron-rich and one electron-deficient aromatic, respectively. According to the
polar/π Hunter and Sanders model, therefore, complex **Ru17** displays a stronger electrostatic inter-
action between the stacked aromatic rings than its analogue **Ru16**, leading to shorter ring centroid
distances between the pentafluorophenyl and the *N*-phenyl rings compared to those between the
phenyl and the *N*-phenyl rings.

<figure><img src="image_23.png" /><figcaption>**Figure 4.** Electrostatic potential surfaces for complexes **Ru16-opt** and **Ru17-opt**, fixing the positive and negative charge densities to the blue and red colour, respectively.</figcaption></figure>

A molecular orbital (MO) analysis of compounds **Ru16-opt** and **Ru17-opt** was also carried out in
order to probe a potential intramolecular orbital interaction between the *N*-phenyl and the back-
bone-phenyl or -pentafluorophenyl rings. Indeed, when two aromatic rings are near enough to each
Therefore, we wondered whether the slower decomposition of **Ru11** and **Ru12** at ambient temperature and the need for higher temperatures to observe their degradation could be instead ascribed to a particular step in the decomposition mechanism.

<figure><img src="image_10.png" /><figcaption>**Figure 5.** Calculated Gibbs free energy barrier associated with the rotation of *N*-phenyl substituent in complexes **5opt**, **11opt** and **12opt**, computed at the B3PW91-D3 level of theory.</figcaption></figure>

Inspired by the work reported some years ago by the group of Cavallo for complex **Ru7** (Figure 1),²⁵,³⁰ we thus performed the calculation of the whole decomposition pathway for compounds **Ru5**, **Ru11** and **Ru12**. For the sake of clarity in the comparison, their corresponding Gibbs free energy profiles are overlapped in Figures 6 and 7. Starting from compounds **5opt**, **11opt** and **12opt**, the first step involves a Berry pseudo rotation of the alkylidene *O*¹Pr arm bringing the *O*¹Pr group in *trans* position to one of the chloride ligands. The reaction is endergonic by 6.1, 10.1 and 8.8 kcal·mol⁻¹, with an associated barrier of 28.8, 33.8 and 33.4 kcal·mol⁻¹, for **TS-1(Ru5)**, **TS-1(Ru11)** and **TS-1(Ru12)** respectively. Through a second Berry pseudo rotation process, one of the *N*-phenyl rings may then move in *trans* position to the alkylidene *O*¹Pr arm, engaging one of its *ortho*-C–H bonds into an agostic interaction with the Ru metal center. The reaction is endergonic by 24.2 and 28.1 kcal·mol⁻¹ for **I-2(Ru5)** and both **I-2(Ru11)** and **I-2(Ru12)**, respectively, the corresponding barriers measuring 27.6, 29.3 and 30.3 kcal·mol⁻¹ for **TS-1(Ru5)**, **TS-1(Ru11)** and **TS-1(Ru12)**, respectively.

To prepare the H transfer from the *N*-phenyl cycle to the benzylidene group, the *N*-phenyl ring
other, their π orbital overlapping may provide an additional binding force to their stacking interac-
tion (see Figures S11 and S12 for the most representative MOs).⁵⁸ For both the **Ru16-opt** and **Ru17-opt** complexes, the HOMO-15 orbital is the only molecular orbital involving a small overlapping between the π orbitals of the *ipso* and *ortho* carbons of both the *N*-phenyl and the backbone-phenyl or -pentafluorophenyl rings, respectively. This small overlapping is slightly stronger for **Ru17-opt** than for **Ru16-opt**, as confirmed by an NBO analysis showing a slightly stronger second order stabilizing interaction between the *N*-phenyl and the second decker pentafluorophenyl rings in **Ru17-opt** (5.90 kcal·mol⁻¹), compared to that between the *N*-phenyl and the second decker-phenyl rings in **Ru16-opt** (1.49 kcal·mol⁻¹).

To conclude, therefore, the interaction between the stacked aromatic groups in **Ru16-opt** and **Ru17-opt** has mostly an electrostatic nature, resulting from the interactions between the quadrupole moments of the stacked aromatic rings. The charge alternation in the resulting quadrupolar moments leads to the *N*-phenyl ring interacting more strongly with the pentafluorophenyl ring in **Ru17-opt** than with the phenyl ring in **Ru16-opt** and, as a consequence, to shorter centroid's distances in **Ru17-opt** (3.607 and 3.612 Å) than in **Ru16-opt** (3.427 and 3.450 Å). In view of the little NBO energy interaction between the π orbitals of the stacked rings in both **Ru16-opt** and **Ru17-opt**, the small π-π orbital overlapping is rather a consequence of the spatial proximity of the stacked aromatic rings, with the slightly stronger π-π orbital overlapping in complex **Ru17-opt** coming from the closer proximity of the *N*-phenyl ring with the pentafluorophenyl ring in **Ru17-opt** than with the phenyl ring in **Ru16-opt**.

**Stability of the complexes in solution.** The stability of the two newly obtained biplane-type NHC ruthenium complexes **Ru16** and **Ru17** was then quantitatively monitored and compared with the benchmark **Ru5**. Since the NHC ligand remains coordinated to the metal centre throughout the entire catalytic cycle, the differences in complexes' stability should directly translate to the stability of actual propagating species, thus conveying their effectiveness in catalysis. Gratifyingly, **Ru16** and **Ru17** remained perfectly unchanged for approximately three weeks in solution at ambient temperature, while **Ru5** slowly degraded to around 50% of the initial amount (Figure 5A, left). This result confirmed our previous observation that upon isolation the top-covered complexes **Ru16** and **Ru17** are considerably more stable than **Ru5**. Furthermore, the differences in stability were even more pronounced at higher temperature (Figure 5A, right), where **Ru5** disappeared completely after about 27 hours, while **Ru16** and **Ru17** were still detectable even after more than 100 hours in solution at 80 °C. Furthermore, the elevated temperature allowed to distinguish **Ru16** from **Ru17** in terms of stability, with the **Ru17** resulting the most stable catalyst.

To unambiguously confirm the nature of the catalyst deactivation pathway, we attempted to isolate
must undergo a rotation, allowing the re-formation of an *ortho* C-H agostic interaction with the metal on the same side as the Ru-alkylidene bond. This rotation process is an equilibrium reaction, with the **I-3** intermediate lying at +1.0, -0.5 and -0.1 kcal·mol⁻¹ relative to the **I-2** intermediate, for complexes **Ru5**, **Ru11** and **Ru12** respectively. In the next step, the transfer of the agostic activated proton to the nearby α-C atom of the benzylidene group may then occur, affording a Ru-benzyl complex containing a new σ-bond between the Ru center and the *ortho*-C atom of the NHC *N*-phenyl ring. Interestingly, while the barrier for the formation of the Ru···C-H agostic interaction through **TS-3** is almost identical between compound **Ru5** and compounds **Ru11** and **Ru12** (31.4, 33.7 and 32.6 kcal·mol⁻¹, respectively), the barrier for the transfer of the agostic activated proton to the benzylidene group through **TS-4** is considerably higher for complexes **Ru11** and **Ru12** (35.1 and 36.1 kcal·mol⁻¹) than for **Ru5** (28.1 kcal·mol⁻¹). This difference of 7.0 and 8.0 kcal·mol⁻¹ between the **TS-4** energy of **Ru11** and **Ru12** with that of **Ru5** is probably due to the presence of the two parallelly stacked aryl rings on the benzimidazolylidene backbone which rigidify the NHC skeleton, thus reducing the flexibility of the *N*-phenyl ring during the proton transfer process in **TS-4**. In the corresponding products, lying at 21.3, 28.5 and 29.2 kcal·mol⁻¹ for complexes **Ru5**, **Ru11**, and **Ru12** respectively, the benzylidene group is transformed into a benzyl group and a new Ru-C bond with an *ortho* position of the *N*-phenyl ring is created.

<figure><img src="image_11.png" /><figcaption>**Figure 6.** The first part of the Gibbs Free energy profile for the deactivation reaction of catalysts **Ru5** (black), **Ru11** (red) and **Ru12** (blue), computed at the B3PW91-D3 level of theory.</figcaption></figure>
the degradation product of **Ru5** (Previous attempts toward isolation and characterisation of its decomposition products focus only on its PCy₃ derivative²⁹). After stirring **Ru5** in dichloromethane at 40 °C for one week, the expected, tethered η⁶-arene-NHC Ru(II) complex **Ru19** was isolated (Figure 5B), whose formation should proceed through the activation of an *ortho* C-H bond of one *N*-phenyl group followed by alkylidene insertion. The molecular structures of complex **Ru19** was confirmed by X-ray diffraction experiments on single crystals grown by layering dichloromethane complex solution with pentane (Figure 5C).

<figure><img src="image_24.png" /><figcaption>**Figure 5.** A) Stability tests in solution at room and high temperature, measured by ¹H NMR (1,3,5-trimethoxybenzene added as reference). Lines are visual aid only. Conditions: (left) CD₂Cl₂ at 22 °C, under argon; (right) C₆D₆ at 80 °C, under argon. B) Study of the decomposition of the complex **Ru5** into the complex **Ru19**. C) XRD structure of complex **Ru19** (ellipsoids are drawn at 50% probability).</figcaption></figure>

**Computational study.** In order to explain the slower decomposition kinetics of complexes **Ru16** and **Ru17** compared to that of **Ru5**, we hypothesised that the insertion of an aromatic plane parallel to the *N*-aryl rings could prevent the rotation of the *N*-phenyl group, thereby constraining its aromatic C–H activation and thus its decomposition. Starting from complexes **Ru5-opt**, **Ru16-opt** and **Ru17-opt**, we therefore computed, at the DFT-B3PW91-D3 level of theory, the Gibbs free energy barriers associated to the *N*-phenyl rotation, keeping the alkylidene isopropoxy O<sup>i</sup>Pr ligand in axial position (Figure 6). The transition states corresponding to the *N*-phenyl rotation display higher energies for the **Ru16-opt** and **Ru17-opt** species (19.3 and 22.7 kcal·mol<sup>-1</sup>) than for compound **Ru5-opt** (17.4 kcal·mol<sup>-1</sup>). This is likely due to the increased steric constrain between the *N*-phenyl and
From intermediate **I-4**, a Berry pseudo rotation brings the benzyl *O*Pr arm in *trans* position to the NHC ring, affording intermediate **I-4'** by an equilibrium process (Figure 7). The following step involves, in a single step, both *i*) the transfer of an α hydrogen from the benzyl group to the Ru atom and *ii*) the insertion of the newly formed benzylidene into the σ-bond of the Ru-C<sub>ortho</sub> atom of the *N*-phenyl ring. For complexes **Ru5**, **Ru11**, and **Ru12** respectively, this elementary step is exergonic by 6.9, 10.4 and 13.8 kcal·mol<sup>-1</sup>, with the corresponding transition state measuring 29.7, 34.7 and 33.7 kcal·mol<sup>-1</sup>.

The resulting intermediate **I-5** may then undergo a reductive elimination step which breaks the
Ru–hydride bond, by transferring the hydrogen to the formerly benzylic C center. This leads to
intermediate **I-6** with associated barriers of 20.2, 22.4 and 24.4 kcal·mol<sup>-1</sup> for complexes **Ru5**, **Ru11**
and **Ru12** respectively. Intermediate **I-6** is only a kinetic intermediate to the decomposition product
**I-8**, which can be reached from **I-6** through a series of haptotropic shifts involving the Ru atom and
the Ph group of the formerly benzylidene moiety, as confirmed by the X-Ray structure of complex
**Ru14** (Figure 4). The formation of **I-8** is slightly exergonic by only –3.2, –0.2, and –2.3 kcal·mol<sup>-1</sup>
for compounds **Ru5**, **Ru11**, and **Ru12**, respectively, in contrast to the stability of about 20
kcal·mol<sup>-1</sup> obtained for the analog of complex **Ru5**, which contains the same benzylidene ligand
without the *O*Pr substituent (**Ru10**, Figure 1).<sup>25</sup> This difference is likely to be ascribed to the deco-
ordination of the *O*Pr ligand during the formation of the final η<sup>6</sup> product **I-8**, which destabilizes
the system with respect to the starting compound.
C-aryl groups, upon rotation around the N–C<sub>Ph</sub> bond, in compounds **Ru16-opt** and **Ru17-opt**, with respect to their simpler analogue **Ru5-opt**. As shown in Figure 6, this steric constrain is geometrically traduced by an out-of-plane distortion within the bicyclic benzimidazolylidene ligand of the **TSrot-Oax** species, with the N1–C2–C7–N2 and C3–C2–C7–C6 dihedral angles deviating from the expected 0° on going from 4.6 and 7.1° for **Ru5** to 9.5° and 15.0° for **Ru16** and 9.2° and 15.4° for **Ru17**. Another consequence of the steric constrain resulting from the addition of the benzimidazolyl-phenyl or benzimidazolyl-pentafluorophenyl rings is the distortion from the expected -180° value of the Ru–C1–N1–C2 dihedral angle, which measures -154.4° in **TSrot-Oax** (*Ru5*), -147.8° in **TSrot-Oax** (*Ru16*) and -149.2° in **TSrot-Oax** (*Ru17*) (Figure S13).

Although the crossing of this transition state requires more energy for complexes **Ru16-opt** and **Ru17-opt** than for **Ru5-opt**, the different catalyst deactivation kinetics measured for **Ru5**, **Ru16**, and **Ru17** in Figure 5 cannot be ascribed to this *N*-phenyl rotation process, as the corresponding Gibbs free energy barriers can be easily overcome at room temperature, unlike experimentally observed. Therefore, we wondered whether the slower decomposition of **Ru16** and **Ru17** at room temperature and the need for higher temperatures to observe their degradation could be instead ascribed to a particular step in the decomposition mechanism.

<figure><img src="image_25.png" /><figcaption>**Figure 6.** Calculated Gibbs free energy barriers associated with the rotation of the *N*-phenyl ring in complexes **Ru5-opt**, **Ru16-opt** and **Ru17-opt**, with the alkylidene isopropoxy *O*Pr ligand in axial position, along with some geometrical parameters for the **TSrot-Oax** structures computed at the B3PW91-D3 level of</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 7.** The second part of the Gibbs Free energy profile for the deactivation reaction of catalysts **Ru5** (black), **Ru11** (red), and **Ru12** (blue), computed at the B3PW91-D3 level of theory.</figcaption></figure>

Considering the full mechanistic profile, the path between intermediates **I-3** and **I-5** involves transition states with the Gibbs free energy values of **Ru11** and **Ru12** considerably higher than those of **Ru5**. This indicates that the influence of the insertion of an aromatic plane parallel to the *N*-phenyl rings seems to have a particularly significant impact on the **TS-4** and **TS-5** transition states. Therefore, we carried out a geometrical analysis of structures of those key transition states for compounds **Ru5**, **Ru11** and **Ru12** displaying in Figure 8 the most significant geometrical parameters.

In all the three TS-4 transition states (Figure 8A), the aromatic *N*-phenyl ring is heavily rotated, so that the activated *C–H* bond can properly interact with the Ru center and thus be transferred to the alkylidene moiety, the C9-H and C49-H bond distances measuring 2.023, 2.092, 2.056 Å and 1.645, 1.599, 1.616 Å for complexes **Ru5**, **Ru11** and **Ru12** respectively. As a consequence, the *N*-phenyl ring arranges in a quasi-coplanar fashion with respect to the benzimidazolylidene heterocycle, and any substitution on the benzimidazolylidene moiety results in a steric clash that disfavors the *N*-phenyl rotation, increasing the **TS-4** energy. This increased steric repulsion is geometrically reduced by an out-of-plane distortion of the two benzimidazolylidene rings. Indeed, while in **TS-
theory.

Inspired by the work reported some years ago by the groups of Cavallo and Suresh for complex **Ru7** (Figure 1),³⁰, ³¹, ³⁷ we decided to compute the *N*-phenyl C-H activation step involved in the H transfer to the benzylidene group for the three compounds **Ru5**, **Ru16** and **Ru17**, keeping the alkylidene isopropoxy *O*^Pr group in axial position. As shown in Figure S14, however, the computed energy barriers (36.3, 41.9 and 42.7 kcal·mol⁻¹ for **TS-CHact-Oax** of **Ru5**, **Ru16**, and **Ru17**, respectively) are too high to explain the experimentally observed decomposition kinetics of the three compounds. This is particularly true for the unsubstituted precursor (**Ru5**) for which a barrier of 36.3 kcal·mol⁻¹ cannot match with the decomposition process observed at room temperature (Figure 5), even if slow. We thus wondered whether the isomerisation of the *O*^Pr group from the axial to the equatorial position could decrease the C-H activation barrier. We therefore computed the whole decomposition pathway for the **Ru5**, **Ru16** and **Ru17** pre-catalysts, considering as the first step the isomerisation process moving the *O*^Pr arm from the axial to the equatorial position. For the sake of clarity in the comparison, their corresponding Gibbs free energy profiles are overlapped in Figures 7 and 8. Starting from compounds **Ru5-opt**, **Ru16-opt** and **Ru17-opt**, the first step involves a Berry pseudo rotation of the alkylidene *O*^Pr arm, bringing the *O*^Pr group in *trans* position to one of the chloride ligands. The reaction is endergonic by 6.1, 10.1 and 8.8 kcal·mol⁻¹, with an associated barrier of 28.8, 33.8 and 33.4 kcal·mol⁻¹, for **TS-1(Ru5)**, **TS-1(Ru16)** and **TS-1(Ru17)** respectively. Through a second Berry pseudo rotation process, one of the *N*-phenyl rings may then move in *trans* position to the alkylidene *O*^Pr arm, engaging one of its *ortho*-C-H bonds into an agostic interaction with the Ru metal centre. The reaction is endergonic by 24.2 and 28.1 kcal·mol⁻¹ for **I-2-Oeq(Ru5)** and both **I-2-Oeq(Ru16)** and **I-2-Oeq(Ru17)**, respectively, the corresponding barriers measuring 27.6, 29.3 and 30.3 kcal·mol⁻¹ for **TS-1(Ru5)**, **TS-1(Ru16)** and **TS-1(Ru17)**, respectively.

To prepare the H transfer from the *N*-phenyl cycle to the benzylidene group, the *N*-phenyl ring must undergo a rotation around the *N*--*C*<sub>Ph</sub> bond, allowing the re-formation of an *ortho* C-H agostic interaction with the metal on the same side as the *Ru*--*C*<sub>alkylidene</sub> bond. This rotation process is an equilibrium reaction, with the **I-3-Oeq** intermediate lying at +1.0, -0.5 and -0.1 kcal·mol⁻¹ relative to the **I-2-Oeq** intermediate, for complexes **Ru5**, **Ru16** and **Ru17** respectively. In the next step, the transfer of the agostic activated proton to the nearby *α*-C atom of the benzylidene group may then occur, affording a ruthenium-benzyl complex containing a new *σ*-bond between the ruthenium center and the *ortho*-C atom of the NHC *N*-phenyl ring. Relative to the most stable preceding intermediates (**Ru5-opt**, **Ru16-opt** and **Ru17-opt**), interestingly, while the barrier for the formation of the *Ru*···C-H agostic interaction through **TS-3** is almost identical between compound **Ru5** and compounds **Ru16** and **Ru17** (31.4, 33.7 and 32.6 kcal·mol⁻¹, respectively), the barrier for the transfer of
**4(Ru5)** the two rings of the bicyclic benzimidazolylidene are almost perfectly planar (dihedral angles N2-C7-C2-N1 = 0.5°, C6-C7-C2-C3 = 0.9°), they clearly deviate from planarity in **TS-4(Ru11)** and **TS-4(Ru12)** (N2-C7-C2-N1 = 5.7° and 5.5°, C6-C7-C2-C3 = 9.5° and 10.3° respectively), due to the steric clash between the *N*-phenyl and *C*-aryl groups upon rotation around the *N*-C<sub>Ph</sub> bond. The crossing of this transition state, therefore, requires more energy for complexes **Ru11** and **Ru12** than for **Ru5**, reflecting their slower experimental decomposition shown in Figure 4.

Figure 8B shows the geometry of the transition states **TS-5** of compounds **Ru5**, **Ru11** and **Ru12**. While for **TS-5(Ru5)** the *C*-*C* bond formation occurs once the benzylic *H* has been already transferred to the Ru center, in the case of **Ru11** and **Ru12** these two processes occur simultaneously. As previously seen for **TS-4**, the kinetic barrier associated with **TS-5** is considerably higher for complexes **Ru11** and **Ru12** (34.7 and 33.7 kcal·mol<sup>-1</sup>) than for **Ru5** (29.7 kcal·mol<sup>-1</sup>), and arises from the same distortion of the benzimidazolylidene core due to steric clash between the stacked exocyclic aryl groups.

<figure><img src="image_13.png" /><figcaption>**Figure 8.** Geometrical parameters of the **TS-4** and **TS-5** structure of complexes **Ru5**, **Ru11** and **Ru12**.</figcaption></figure>

In conclusion, therefore, the computed decomposition profile indicates that the steric repulsion between the *N*-phenyl and the second decker phenyl- and pentafluorophenyl groups strongly affects the rate-determining step of the reaction which involves *i*) the transfer of the *N*-phenyl agostic proton to the nearby *α*-C atom of the benzylidene group (**TS-4**) and *ii*) the transfer of the benzylic
the agostic activated proton to the benzylidene group through **TS-4** is considerably higher for complexes **Ru16** and **Ru17** (35.1 and 36.1 kcal·mol⁻¹) than for **Ru5** (28.1 kcal·mol⁻¹). This difference of 7.0 and 8.0 kcal·mol⁻¹ between the **TS-4** energy of **Ru16** and **Ru17** with that of **Ru5** is likely due to the presence of the two parallelly stacked aryl rings on the benzimidazolylidene backbone which rigidify the NHC skeleton, thus reducing the flexibility of the *N*-phenyl ring during the proton transfer process in **TS-4** (see below). Interestingly, the C-H activation barriers obtained here for **TS-4** with the O<sup>i</sup>Pr group in equatorial position are considerably lower than those for **TS-CHact-Oax**, presenting the O<sup>i</sup>Pr group in axial position (Figure S14), (28. 1, 35.1 and 36.1 kcal·mol<sup>-1</sup> vs. 36.3, 41.9 and 42.7 kcal·mol<sup>-1</sup> for **Ru5**, **Ru16** and **Ru17** respectively), underlining the relevance of the pseudo-Berry rotation as the first step in the decomposition process. In the corresponding products **I-4-Oeq**, lying at 21.3, 28.5 and 29.2 kcal·mol<sup>-1</sup> for complexes **Ru5**, **Ru16**, and **Ru17** respectively, the benzylidene group is transformed into a benzyl group and a new Ru–C bond with the *ortho* C of the *N*-phenyl ring is created.

<figure><img src="image_26.png" /><figcaption>**Figure 7.** The first part of the Gibbs Free energy profile for the deactivation reaction of pre-catalysts **Ru5** (black), **Ru16** (red) and **Ru17** (blue), computed at the B3PW91-D3 level of theory.</figcaption></figure>

From intermediate **I-4-Oeq**, a Berry pseudo rotation brings the benzyl O<sup>i</sup>Pr arm in *trans* position to the NHC ring, affording intermediate **I-4'-Oax** by an equilibrium process (Figure 8). The following reaction involves, in a single step, both *i*) the transfer of an α hydrogen from the benzyl group to the Ru atom and *ii*) the insertion of the newly formed benzylidene species into the σ-bond of the Ru–C<sub>ortho</sub> atom of the *N*-phenyl ring. For complexes **Ru5**, **Ru16**, and **Ru17**, respectively, this elementary
H to Ru coupled with the C-C bond formation (TS-5). The corresponding kinetic barrier is thus considerably higher for compounds **Ru11** and **Ru12** than for **Ru5**, in accordance with the decomposition kinetics reported in Figure 4.

**Catalytic activity studies.** The catalytic performance of the new, “stacked” complexes was compared with known benchmark **Ru5** using a set of model reactions. First, we checked their activity in ring closing metathesis (RCM) with tetrasubstituted C-C double bond formation. ¹³,¹⁴ For this purpose, diethyl 2,2-di(2-methylallyl)malonate (**10a**) was chosen as the model substrate and the time-conversion plots of its RCM reaction were constructed with 0.2 mol% of each catalyst, in toluene, at 80 °C (Figure 9). All complexes exhibited high activity, affording considerable conversions within the first 30 minutes. **Ru5** is one of the most active catalysts designed for this reaction,¹⁷ and the new catalysts bear the same small *N*-substituents at the NHC ligands. Thus said, the reaction catalyzed by **Ru5** reached its limits at a significantly lower conversion than that recorded with **Ru11** and **Ru12**. This can be attributed to a more stable 14-e⁻ catalytic species formed from the latter catalysts versus those generated from **Ru5**. Interestingly, although **Ru11** gave higher conversions than **Ru12** within the first hour of the reaction, after this point the trend reversed, leading to 96% conversion with **Ru12** versus 91% with **Ru11** after 5 hours (and 72% for the benchmark **Ru5**).

<figure><img src="image_14.png" /><figcaption>**Figure 9.** Time-conversion curves in RCM of diene **10a**. Conditions: catalyst 0.2 mol%, PhMe, 80 °C under argon. Measured by GC with 1,3,5-trimethoxybenzene added as internal standard. Lines are visual aid only.</figcaption></figure>

To further examine the differences in catalyst productivity, the previously described reaction of **10a** was reattempted at *two times lower catalyst loading* of 0.1 mol% (Scheme 4). The trend observed earlier was maintained and **Ru5** gave a rather poor conversion for such reaction (50%), whereas **Ru11** and **Ru12** gave much higher conversions of 81% and 92%, respectively. The three **BIPh**-type
step is exergonic by 6.9, 10.4 and 13.8 kcal·mol⁻¹, with the corresponding transition state measuring 29.7, 34.7 and 33.7 kcal·mol⁻¹ with respect to the most stable preceding intermediates (**Ru5-opt**, **Ru16-opt** and **Ru17-opt**).

The resulting intermediate **I-5-Oax** may then undergo a reductive elimination step which breaks the Ru–hydride bond, by transferring the hydrogen to the formerly benzylic C centre. This leads to intermediate **I-6-Oax** with associated barriers of 20.2, 22.4 and 24.4 kcal·mol⁻¹ for complexes **Ru5**, **Ru16** and **Ru17**, respectively. Intermediate **I-6-Oax** is only a kinetic intermediate to the decomposition product **I-8**, which can be reached from **I-6-Oax** through a series of haptotropic shifts involving the ruthenium atom and the Ph group of the formerly benzylidene moiety, as confirmed by the X-ray structure of complex **Ru19** (Figure 5). The formation of **I-8** is slightly exergonic by only –3.2, –0.2, and –2.3 kcal·mol⁻¹ for compounds **Ru5**, **Ru16**, and **Ru17**, respectively, in contrast to the stability of about 20 kcal·mol⁻¹ obtained for the analogue of complex **Ru5**, which contains the same benzylidene ligand without the O<sup>i</sup>Pr substituent (**Ru10**, Figure 1).³¹ This difference between compounds **Ru5**, **Ru16**, **Ru17** and compound **Ru10** is likely due to the decoordination of the O<sup>i</sup>Pr ligand and during the formation of the final η<sup>6</sup> product **I-8**, which destabilizes the system with respect to the starting compound.

<figure><img src="image_27.png" /><figcaption>**Figure 8.** The second part of the Gibbs Free energy profile for the deactivation reaction of pre-catalysts **Ru5** (black), **Ru16** (red), and **Ru17** (blue), computed at the B3PW91-D3 level of theory.</figcaption></figure>
catalysts were further tested in five more challenging RCM reactions featuring tetrasubstituted double C-C bond formation. Similar was the case of the formation of the six-membered malonate **11b** and five-membered tosylamide **13b** where **Ru11** and **Ru12** gave up to 32 percentage points (%p) higher substrate conversions compared to **Ru5**. The fluorinated complex **Ru12** slightly but noticeably outperformed **Ru11** in the two latter reactions. The superiority of the new catalysts **Ru11** and **Ru12** was even more pronounced in the more challenging formation of a less thermodynamically favored seven-membered ring (compounds **12b** and **14b**). In these latter cases, a much higher catalyst loading of 2 mol% with malonate **12a** and 1 mol% with tosylamide **14a** was required to reach suitable conversions⁵⁶,¹³,¹⁴ and it is worth noting that **Ru11** and **Ru12** gave conversions 4-8 times higher than **Ru5**. Again, **Ru12** was slightly better than **Ru11** which overall correlates with the observed trend in stability tests.

<figure><img src="image_15.png" /><figcaption>**Scheme 4.** Scope and limitations of catalysts **Ru5**, **Ru11**, and **Ru12** in RCM of challenging substrates. %p – percentage point difference from **Ru5** in a substrate conversion, determined by gas chromatography; a by-product **15a'** was formed (see Figure 10), reaction was conducted for 8 h; b improvement in formation of main product **15b** from replacing **Ru5** with **Ru11** or **Ru12**.</figcaption></figure>

For all substrates discussed so far, the desired RCM products were obtained as the sole product. However, the RCM reaction of ether **15a** led to the formation of a rearranged byproduct **15a'** resulting from a double bond migration along the alkyl chain (Scheme 4 and Figure 10A). This can be caused by catalyst decomposition products (probably ruthenium hydride species).⁵⁷⁻⁵⁹ The observed formation of **15a'** stands in agreement with the previous report of Grubbs and co-workers, although
At that point, the rate-determining step (rds) for the three complexes could be identified, by taking
the most stable preceding intermediates as references, i.e. the starting complexes **Ru5-opt**, **Ru16-opt**, and **Ru17-opt**.⁵⁹ Taking into account that the systematic error for the DFT method on activation barriers is estimated at around 4 kcal·mol⁻¹,⁶⁰,⁶¹ the rate determining step in the **Ru5** system can be associated with any of the transition states between **TS-1** and **TS-5**. In the **Ru16** and **Ru17** systems, on the other hand, the rds can be associated with **TS-1** and with the transition states between **TS-3** and **TS-5**. Despite the presence in the first part of the profile of a rather flat energy surface for each of the three compounds, the Gibbs free energy profiles of **Ru16** and **Ru17** are always higher than that of their **Ru5** analogue, traducing the slower decomposition reaction observed for **Ru16** and **Ru17** compared to **Ru5**. Taking the starting complexes **Ru5-opt**, **Ru16-opt** and **Ru17-opt** as reference, indeed, the Gibbs free energy associated to the transition states from **TS-1** to **TS-5** are in the range 27.6 to 31.4 kcal·mol⁻¹ for complex **Ru5**, whereas in the range 29.3 to 35.1 kcal·mol⁻¹ and 30.3 to 36.1 kcal·mol⁻¹ for complexes **Ru16** and **Ru17**, respectively. Among all the transition states, those that show the most marked difference between the **Ru5** complex and the **Ru16** and **Ru17** counterparts are **TS1** (28.8 vs 33.8 and 33.4 respectively), **TS4** (28.1 vs 35.1 and 36.1 respectively) and **TS5** (29.7 vs 33.7 and 34.7 respectively). To explain this difference, we carried out a molecular orbital (MO), NPA (natural population analysis) charge and geometrical analysis of these key transition states for the three compounds.

In **TS-1**, the increase of the energy from 28.8 to 33.4 and 33.8 kcal·mol⁻¹, ongoing from **Ru5** to **Ru17** to **Ru16** respectively, is mainly due to the insertion of additional phenyl or pentafluorophenyl planes parallel to the *N*-aryl rings, which constrains the Berry rotation process. This increased steric constrain is geometrically traduced by an out-of-plane distortion of the bicyclic benzimidazolylidene ligand, with the N1–C2–C3–C4 and N2–C7–C6–C5 dihedral angles deviating more and more from the expected 180° on going from **Ru5** (−179.3 and 179.6°) to **Ru17** (177.7 and −177.1°) and to **Ru16** (175.1 and −174.7°) (Figure 9A). On the other hand, as displayed in Figures S15 and S16, the analysis of the frontier MOs and NPA charges does not show any significant difference between the three species.

With regard to **TS-4**, in all three compounds (Figure 9B), the aromatic *N*-phenyl ring is heavily rotated, so that the activated C–H bond can properly interact with the Ru centre and thus be transferred to the alkylidene moiety, the C11–H and C16–H bond distances measuring 2.023, 2.092, 2.056 Å and 1.645, 1.599, 1.616 Å for complexes **Ru5**, **Ru16** and **Ru17**, respectively. As a consequence, the *N*-phenyl ring arranges in a quasi-coplanar fashion with respect to the benzimidazolylidene heterocycle and any substitution on the benzimidazolylidene moiety results in a steric constrain that disfavours the *N*-phenyl rotation, increasing the **TS-4** energy. This increased steric repulsion is geometrically traduced by an out-of-plane distortion of the two benzimidazolylidene rings. Indeed, while in
this isomerization product was not characterized there.¹⁷ Catalyst **Ru5** offered a very poor conversion of **15a**, forming **15b** and **15a'** in comparable amounts (**15a**:**15b**:**15a'**ratio of 96:1.4:2.7). Interestingly, in this example **Ru11** outperformed **Ru12**, both in terms of conversion and selectivity toward **15b** (**15a**:**15b**:**15a'** ratio for **Ru11** reached 24:75:0.9 versus 51:44:5.4 for **Ru12** after 8 hours at 80 °C). This observation could be explained by the subtle difference in activity of **Ru11** and **Ru12** that was observed in RCM of **10a** (Figure 9).

<figure><img src="image_16.png" /><figcaption>**Figure 10.** A) Reaction profile of substrate **15a** RCM. B) *o*-isopropoxystyrene (**16**) release during RCM of diene **15a**.</figcaption></figure>

To verify the latter hypothesis, the reaction profile for RCM of **15a** was established, comparing **Ru11** performance with **Ru12** (Figure 10A). The difference in catalyst activity is more pronounced in this challenging RCM example, and complex **Ru11** is considerably more active than **Ru12**.
**TS-4(Ru5)** the two rings of the bicyclic benzimidazolylidene are almost perfectly planar (dihedral angles N2-C7-C2-N1 = 0.5°, C6-C7-C2-C3 = 0.9°), they clearly deviate from planarity in **TS-4(Ru16)** and **TS-4(Ru17)** (N2-C7-C2-N1 = 5.7° and 5.5°, C6-C7-C2-C3 = 9.5° and 10.3° respectively), due to the steric constrain between the *N*-phenyl and *C*-aryl groups, upon rotation around the *N*–*C*<sub>Ph</sub> bond. The crossing of this transition state, therefore, requires more energy for complexes **Ru16** and **Ru17** than for **Ru5**, reflecting their slower experimental decomposition shown in Figure 5. On the other hand, the analysis of the frontier MOs and NPA charges does not show any significant difference between the three species (Figures S17 and S18).

<figure><img src="image_28.png" /><figcaption>**Figure 9:** Geometrical parameters of A) the TS-1(Ru5), TS-1(Ru16) and TS-1(Ru17) complexes; B) the TS-4(Ru5), TS-4(Ru16) and TS-4(Ru17) complexes, and C) the TS-5(Ru5), TS-5(Ru16) and TS-5(Ru17) complexes.</figcaption></figure>
(15a:15b:15a' ratio after 1 h reached 28:71:0.3 with **Ru11** versus 63:36:0.8 with **Ru12**). After the first hour of the reaction, the formation of **15b** slows significantly, indicating that most of the catalystically active species has been spent. After that point, the formed decomposition products start to isomerize **15a**, which is available in a higher quantity in the case of the reaction with **Ru12**. Those factors account for a higher amount of **15a'** observed with **Ru12** after 8 h.

To further probe the origin of **Ru11** higher activity, the release of *o*-isopropoxystyrene (16) (relative to the last point measured at 8 h) versus time was plotted (Figure 10B). The formation of 16 can serve as an indirect measure of how catalyst initiation progresses throughout the reaction. For both complexes, 16 release finishes within the first 4 h, although for **Ru11** the slope is considerably steeper than for **Ru12**. This is in line with the hypothesis that **Ru11** is a more active catalyst than **Ru12** probably due to a faster initiation rate resulting from a higher *trans* effect of more electron donating **BI*Ph** NHC ligand.

<figure><img src="image_17.png" /><figcaption>**Scheme 5.** Scope and limitations of catalysts **Ru5**, **Ru11**, and **Ru12** in CM and cycloisomerization of challenging substrates. <sup>a</sup> Reaction conducted in perfluorotoluene. %p – percentage point difference from **Ru5**.</figcaption></figure>

To further evaluate the scope and limitations of catalysts **Ru11** and **Ru12**, several challenging cross metathesis (CM) reactions were investigated (Scheme 5). First, we studied the reaction between ethyl chrysanthemate (17) and Z-1,4-diacetoxy-2-butene (18). The challenge of this transformation lies in the low reactivity of the trisubstituted double C-C bond in 17.<sup>27</sup> Catalysts **Ru11** and **Ru12** gave similar conversions of 17, 69% and 73% respectively, giving product 19 selectively, whereas
complexes.

Analogously to **TS-4**, the barrier associated with **TS-5** is considerably higher for complexes **Ru16** and **Ru17** (34.7 and 33.7 kcal·mol⁻¹) than for **Ru5** (29.7 kcal·mol⁻¹). This difference may be explained by the geometrical distortion of the benzimidazolylidene core in **Ru16** and **Ru17**, the presence of stacked exocyclic aryl groups increasing the steric constrain around the metal centre (Figure 9C). In **TS-5(Ru5)** the two rings of the bicyclic benzimidazolylidene moiety are almost planar (dihedral angles N2–C7–C2–N1 = 0.6°, C6–C7–C2–C3 = 1.1°) whereas in **TS-5(Ru16)** and **TS-5(Ru17)** they clearly deviate from planarity (N2–C7–C2–N1 = 4.2 and 4.6°, C6–C7–C2–C3 = 8.1° and 9.3° respectively). In addition, the **TS-5(Ru16)** and **TS-5(Ru17)** species display a longer Ru–C16 bond distance (1.930 and 1.940 Å) with respect to **TS-5(Ru5)** (1.908 Å), the distortion of the bicyclic benzimidazolylidene in **TS-5(Ru16)** and **TS-5(Ru17)** cumbering the metal coordination environment (Figure 9C). As a consequence, while for **TS-5(Ru5)** the C–C bond formation occurs once the benzylic H has been already transferred to the ruthenium centre (C16–H = 1.750 Å and C16–C11 = 2.105 Å), in the case of **TS-5(Ru16)** and **TS-5(Ru17)** these two processes occur simultaneously (C16–H = 1.357 and 1.344 Å and C16–C11 = 1.913 and 1.894 Å). Finally, the analysis of the frontier MOs and NPA charges does not show any significant difference between the three **TS-5** species, except the more negative NPA charges on the C16 centre of the **TS-5(Ru16)** and **TS-5(Ru17)** species, as a consequence of their larger C16–Ru bond distances (Figures S19 and S20).

In conclusion, we have computed by DFT calculations the full decomposition profile of the precatalytic species **Ru5**, **Ru16** and **Ru17**. Interestingly, compounds **Ru16** and **Ru17** display higher kinetic barriers than their analogue **Ru5**, indicating a slower decomposition process of **Ru16** and **Ru17**, in accordance with the experimental data reported in Figure 5. This behaviour is likely due to the steric hindrance of the additional second decker phenyl- and pentafluorophenyl groups, which induces a distortion of the bicyclic benzimidazolylidene scaffold and the consequent increase of the energies of the corresponding rate-determining steps (**TS-1**, **TS-4** and **TS-5**). The similar energetic profiles obtained for the **Ru16** and **Ru17** species underlines the little effect of the aromatic-aromatic interaction, observed in **Ru17**, in the stabilisation of its potential energy surface with respect to **Ru16**.

**Catalytic activity studies.** The catalytic performance of the new, “stacked” complexes was compared with known benchmark **Ru5** using a set of model reactions. First, we checked their activity in ring closing metathesis (RCM) with tetrasubstituted C-C double bond formation.¹³, ¹⁴ For this purpose, diethyl 2,2-di(2-methylallyl)malonate (**10a**) was chosen as the model substrate and the time-conversion plots of its RCM reaction were constructed with 0.2 mol% of each catalyst, in toluene, at 80 °C (Figure 10). All complexes exhibited high activity, affording considerable conversions within
known **Ru5** provided only 26% of conversion. Next, the functionalization of the geminal C-C double bond of 4-methylene-1-tosylpiperidine (20) with either **18** or amylene (22) was probed. In the case of reaction with **18**, the use of **Ru11** and **Ru12** allowed for conversions up to 18 percent-points (%p) higher compared to benchmark **Ru5**, leading to 76% conversion for **Ru12**.

Noteworthy, in the example of CM of 20 with amylene,⁶⁰,⁶¹ the double-decker NHC catalysts **Ru11** and **Ru12** gave almost quantitative conversions of 20, 93% and 91% respectively, which was 5 times higher than in the case of **Ru5** reaching only 18%. Last but not least, the highly challenging 1,5-enyne metathesis of substrate 24 to obtain crowded and strained cyclobutene derivative 25 was attempted.⁶² An additional difficulty arising in this reaction is that its selectivity can be compromised by a competing self-CM side reaction and by degradation of the highly strained resulting product. We were pleased to find that catalysts **Ru11** and **Ru12** gave good yields of 59% and 53% respectively, whereas with **Ru5** only traces of product were observed which amounted to 6% yield. Interestingly, similarly to the case of the RCM reaction of 15a, in this example, catalyst **Ru11** performed visibly better than its fluorinated counterpart **Ru12**, which we attribute to **Ru11** higher activity. Remarkably, the use of the new catalysts allowed for a two-fold diminution in catalyst loading compared to the general-purpose SIMes complex **Ru2** used at 20 mol% loading.⁶²

## CONCLUSION

In summary, we developed the two new biplane-type NHC ligands **BI*Ph** and **BIF*Ph** based on a benzimidazolylidene core, whose lower wings are set as *N*-phenyl rings to reduce their steric hindrance on the first coordination sphere and whose upper wings are phenyl or pentafluorophenyl rings, respectively. The corresponding second-generation Grubbs-Hoveyda complexes **Ru11** and **Ru12** were generated and the effect of the introduction of the second decker of aromatic rings on their stereoelectronic properties, stability, and catalytic efficiency was evaluated, taking the mono-plane-type, unsubstituted **BIPh** ligand as a reference. According to the nature of the aromatic rings, **BI*Ph** was shown to be more electron-donating than **BIPh**, while **BIF*Ph** is less donating. Conversely, the second aromatic decker led in both cases to slightly less hindered NHC ligands compared to **BIPh** by forcing the *N*-phenyl rings to stay in a more orthogonal position relative to the benzimidazolylidene core.

More importantly, both complexes **Ru11** and **Ru12** were shown to be significantly more stable than the known **BIPh**-supported complex **Ru5**. This stability gain was rationalized through DFT calculations of the decomposition pathways. Although the aromatic rings of the second decker cannot stop the rotation of the *N*-phenyl groups in **Ru11** and **Ru12**, they strongly contribute to the increase in energy of the rate-determining step, by forcing a strong distortion of the carbenic heterocycle through steric constraints. The rate-determining step involves proton transfer of one of the
the first 30 minutes. Noteworthy, **Ru5** is one of the most active catalysts designed for this reaction,¹⁷ and the new catalysts bear the same small *N*-substituents at the NHC ligands. However, the reaction catalysed by **Ru5** reached its limits at a significantly lower conversion than that recorded with **Ru16** and **Ru17**. This can be attributed to a more stable 14-*e*⁻ catalytic species formed from the latter catalysts versus those generated from **Ru5**. Interestingly, although **Ru16** gave higher conversions than **Ru17** within the first hour of the reaction, after this point the trend reversed, leading to 96% conversion with **Ru17** versus 91% with **Ru16** after 5 hours (and 72% for the benchmark **Ru5**).

<figure><img src="image_29.png" /><figcaption>**Figure 10.** Time-conversion curves in RCM of diene **10a**. Conditions: catalyst 0.2 mol%, PhMe, 80 °C under argon. Measured by GC with 1,3,5-trimethoxybenzene added as internal standard. Lines are visual aid only.</figcaption></figure>

To further examine the differences in catalyst productivity, the previously described reaction of **10a** was reattempted at *two times lower catalyst loading* of 0.1 mol% (Scheme 4). The trend observed earlier was maintained and **Ru5** gave a rather poor conversion for such reaction (50%), whereas **Ru16** and **Ru17** gave much higher conversions of 81% and 92%, respectively. The three **BIPh**-type catalysts were further tested in five more challenging RCM reactions featuring tetrasubstituted double C-C bond formation. Similar was the case of the formation of the six-membered malonate **11b** and five-membered tosylamide **13b** where **Ru16** and **Ru17** gave up to 32 percentage points (%p) higher substrate conversions compared to **Ru5**. The fluorinated complex **Ru17** slightly but noticeably outperformed **Ru16** in the two latter reactions. The superiority of the new catalysts **Ru16** and **Ru17** was even more pronounced in the more challenging formation of a less thermodynamically favoured seven-membered ring (compounds **12b** and **14b**). In these latter cases, a much higher— however, still considered low in the context of the challenging formation of C–C tetrasubstituted
ortho positions of the N-phenyl groups to the nearby carbon atom of the benzylidene moiety fol-
lowed by migration of the benzylic proton to ruthenium with the simultaneous carbon-carbon bond
formation. Moreover, the observed increased stability of the ruthenium precursors **Ru11** and **Ru12**
was also translated to the active species and enabled their implementation as highly efficient, stable
pre-catalysts in challenging olefin metathesis reactions for the formation of tri- and tetra-substi-
tuted C-C double bonds. Complex **Ru11** bearing the more electron-donating and *trans* effect in-
ducting **BI*Ph** ligand revealed also more active than its **Ru12** counterpart bearing the less electron
donating **BIF*Ph** ligand.

Overall, while it is well known that ruthenium catalysts bearing NHC ligands with low steric de-
mand are more fragile, which impairs their productivity, we showed here that the adjunction of a
second decker of aromatic groups in the NHC scaffold is a viable and promising strategy to access
very stable and efficient olefin metathesis catalysts. Because the formation of sterically hindered
olefins has always been the Achilles' heel of the ruthenium-catalyzed metathesis, this special trait
exhibited by the newly obtained complexes appears of paramount importance. We believe that the
present report will open up new possibilities for further developments of olefin metathesis catalysts
and will significantly extend applications of this transformation in natural product synthesis and
medicinal chemistry.

ACKNOWLEDGEMENTS

This work has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 860322.

C. D. and I. d. R. are indebted to CALMIP for HPC resources (Grant 2017-[p17010]).

NOTES

We are grateful to Ekaterina Gulyaeva for the creation of the graphical abstract.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

SUPPORTING INFORMATION

Detailed experimental procedures, copies of NMR spectra, computational methods, Cartesian coor-
bonds¹³, ¹⁴—catalyst loading of 2 mol% with malonate **12a** and 1 mol% with tosylamide **14a** was required to reach suitable conversions and it is worth noting that **Ru16** and **Ru17** gave conversions 4-8 times higher than **Ru5**. Again, **Ru17** was slightly better than **Ru16** which overall correlates with the observed trend in stability tests.

<figure><img src="image_30.png" /><figcaption>**Scheme 4.** Scope and limitations of catalysts **Ru5**, **Ru16**, and **Ru17** in RCM of challenging substrates. %p percentage point difference from **Ru5** in a substrate conversion, determined by gas chromatography. a The reaction was conducted for 8 h and by-product **15a'** was formed (see Figure 10). b Improvement observed in the formation of the main product **15b** by replacing **Ru5** with **Ru16** or **Ru17**.</figcaption></figure>

For all substrates discussed so far, the desired RCM products were obtained as the sole product. However, the RCM reaction of allylic ether **15a** led to the formation of a rearranged by-product **15a'** resulting from a double bond migration along the alkyl chain (Scheme 4 and Figure 11A). Such an olefin isomerisation was already shown to result from the degradation of the metathesis catalysts, generating ruthenium-hydride species.⁶²⁻⁶⁵ The observed formation of **15a'** stands in agreement with the previous report of Grubbs and co-workers, although this isomerisation product was not characterised there.¹⁷ Catalyst **Ru5** offered a very poor conversion of **15a**, forming **15b** and **15a'** in comparable amounts (**15a**:**15b**:**15a'**ratio of 96:1.4:2.7). Interestingly, in this example **Ru16** outperformed
dinates of DFT-optimized structures, and crystallographic details. Crystal structures have been de-
posited with the Cambridge Crystallographic Data Centre CCDC). The data can be obtained free of
charge via www.ccdc.cam.ac.uk/structures.

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12 Vougioukalakis, G. C. & Grubbs, R. H. Ruthenium-Based Heterocyclic Carbene-Coordinated Olefin Metathesis Catalysts. *Chem. Rev.* **2009**, *110*, 1746-1787.

13 Mukherjee, N., Planer, S. & Grela, K. Formation of tetrasubstituted C–C double bonds via olefin metathesis: challenges, catalysts, and applications in natural product synthesis. *Org. Chem. Front.* **2018**, *5*, 494-516.

14 Lecourt, C. et al. Natural products and ring-closing metathesis: synthesis of sterically congested olefins. *Nat. Prod. Rep.* **2018**, *35*, 105-124.

15 This limitation is particularly unfavorable in the context of total synthesis of natural and biologically active compounds, see: Wang, Y., Nagai, T., Watanabe, I., Hagiwara, K. & Inoue, M. Total Synthesis of Euonymine and Euonyminol Octaacetate. *J. Am. Chem. Soc.* **2021**, *143*, 21037-21047.

16 Stewart, I. C. et al. Highly Efficient Ruthenium Catalysts for the Formation of Tetrasubstituted Olefins via Ring-Closing Metathesis. *Org. Lett.* **2007**, *9*, 1589-1592.

17 Berlin, J. M. et al. Ruthenium-Catalyzed Ring-Closing Metathesis to Form Tetrasubstituted Olefins. *Org. Lett.* **2007**, *9*, 1339-1342.

18 Ragone, F., Poater, A. & Cavallo, L. Flexibility of N-Heterocyclic Carbene Ligands in Ruthenium Complexes Relevant to Olefin Metathesis and Their Impact in the First Coordination Sphere of the Metal. *J. Am. Chem. Soc.* **2010**, *132*, 4249-4258.

19 Vehlow, K., Gessler, S. & Blechert, S. Deactivation of Ruthenium Olefin Metathesis Catalysts through Intramolecular Carbene–Arene Bond Formation. *Angew. Chem. Int. Ed.* **2007**, *46*, 8082-8085.
**Ru17**, both in terms of conversion and selectivity toward **15b** (15a:15b:15a' ratio for **Ru16** reached 24:75:0.9 versus 51:44:5.4 for **Ru17** after 8 hours at 80 °C). This observation could be explained by the subtle difference in activity of **Ru16** and **Ru17** that was observed in RCM of **10a** (Figure 10).

To verify the latter hypothesis, the reaction profile for RCM of **15a** was established, comparing **Ru16** performance with **Ru17** (Figure 11A). The difference in catalyst activity is more pronounced in this challenging RCM example, and complex **Ru16** is considerably more active than **Ru17** (15a:15b:15a' ratio after 1 h reached 28:71:0.3 with **Ru16** versus 63:36:0.8 with **Ru17**). After the first hour of the reaction, the formation of **15b** slows significantly, indicating that most of the catalytically active species has been spent. After that point, the formed decomposition products start to isomerise **15a**, which is available in a higher quantity in the case of the reaction with **Ru17**. Those factors account for a higher amount of **15a'** observed with **Ru17** after 8 h.

To further probe the origin of **Ru16** higher activity, the release of *o*-isopropoxystyrene (16) (relative to the last point measured at 8 h) versus time was plotted (Figure 11B). The formation of 16 can serve as an indirect measure of how catalyst initiation progresses throughout the reaction. For both complexes, 16 release finishes within the first 4 h, although for **Ru16** the slope is considerably steeper than for **Ru17**. This is in line with the hypothesis that **Ru16** is a more active catalyst than **Ru17** probably due to a faster initiation rate resulting from a higher *trans* effect of more electron donating **BI*Ph** NHC ligand.
20

A conceptually different idea for forcing tetrasubstituted C=C bonds formation in olefin metathesis is to use specially reinforced SIMes-bearing catalysts at higher temperatures (up to 140 °C), eg. Refs. 21-22.

21

Bantreil, X., Schmid, T. E., Randall, R. A. M., Slawin, A. M. Z. & Cazin, C. S. J. Mixed N-heterocyclic carbene/phosphite ruthenium complexes: towards a new generation of olefin metathesis catalysts. *Chem. Commun.* **2010**, *46*, 7115-7117.

22

Vorfalt, T., Leuthäußer, S. & Plenio, H. An [(NHC)(NHC<sub>EWG</sub>)RuCl<sub>2</sub>(CHPh)] Complex for the Efficient Formation of Sterically Hindered Olefins by Ring-Closing Metathesis. *Angew. Chem. Int. Ed.* **2009**, *48*, 5191-5194.

23

Hong, S. H., Chlenov, A., Day, M. W. & Grubbs, R. H. Double C-H Activation of an N-Heterocyclic Carbene Ligand in a Ruthenium Olefin Metathesis Catalyst. *Angew. Chem. Int. Ed.* **2007**, *46*, 5148-5151.

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Chung, C. K. & Grubbs, R. H. Olefin Metathesis Catalyst: Stabilization Effect of Backbone Substitutions of N-Heterocyclic Carbene. *Org. Lett.* **2008**, *10*, 2693-2696.

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Kuhn, K. M., Bourg, J.-B., Chung, C. K., Virgil, S. C. & Grubbs, R. H. Effects of NHC-Backbone Substitution on Efficiency in Ruthenium-Based Olefin Metathesis. *J. Am. Chem. Soc.* **2009**, *131*, 5313-5320.

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39

Names for substituted BIPh NHC ligands were adopted in analogy with well-known Cp and Cp\* naming scheme.
<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /><figcaption>**Figure 11.** A) RCM reaction profile of substrate **15a**. B) Relative *o*-isopropoxystyrene (16) release during RCM of diene **15a**.</figcaption></figure>

To further evaluate the scope and limitations of catalysts **Ru16** and **Ru17**, several challenging cross metathesis (CM) reactions were investigated (Scheme 5). First, we studied the reaction between ethyl chrysanthemate (17) and Z-1,4-diacetoxy-2-butene (18). The challenge of this transformation lies in the low reactivity of the trisubstituted double C-C bond in 17.<sup>33</sup> Catalysts **Ru16** and **Ru17**
40

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Deposited at the Cambridge Crystallographic Data Centre (CCDC) under accession number 1100116.

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Deposited at the Cambridge Crystallographic Data Centre (CCDC) under accession number 605939.

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50

Ru-O bond length decreases when less electron donating ligand are employed, e.g. Vougiukalakis, G. C. & Grubbs, R. H. Ruthenium-Based Olefin Metathesis Catalysts Coordinated with Unsymmetrical N-Heterocyclic Carbene Ligands: Synthesis, Structure, and Catalytic Activity. *Chem. Eur. J.* **2008**, *14*, 7545-7556.

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César, V. et al. Ruthenium Catalysts Supported by Amino-Substituted N-Heterocyclic Carbene Ligands for Olefin Metathesis of Challenging Substrates. *Chem. Eur. J.* **2017**, *23*, 1950-1955.

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55

Previous report featuring isolation and characterization of the decomposition product from BIPh ruthenium olefin metathesis catalyst focuses only on its PCy₃ derivative, ref. 23.

56

However, such loading is still considered low in the context of the challenging formation of C–C tetrasubstituted bonds, cf. Refs. 13-14.

57

Higman, C. S., Plais, L. & Fogg, D. E. Isomerization During Olefin Metathesis: An Assessment of Potential Catalyst Culprits. *ChemCatChem* **2013**, *5*, 3548-3551.

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Krompiec, S. et al. The role of the functional group in double bond migration in allylic systems catalysed by ruthenium hydride complexes. *J. Mol. Catal. A Chem.* **2006**, *253*, 132-146.

59

Schmidt, B. Catalysis at the Interface of Ruthenium Carbene and Ruthenium Hydride Chemistry: Organometallic Aspects and Applications to Organic Synthesis. *Eur. J. Org. Chem.* **2004**, *2004*, 1865-1880.

60

Chatterjee, A. K., Sanders, D. P. & Grubbs, R. H. Synthesis of Symmetrical Trisubstituted Olefins by Cross Metathesis. *Org. Lett.* **2002**, *4*, 1939-1942.
gave similar conversions of **17**, 69% and 73% respectively, giving product **19** selectively, whereas known **Ru5** provided only 26% of conversion. Next, the functionalisation of the geminal C-C double bond of 4-methylene-1-tosylpiperidine (**20**) with either Z-1,4-diacetoxy-2-butene (**18**) or amylene (**22**) was probed. In the case of reaction with olefin **18**, the use of **Ru16** and **Ru17** allowed for conversions up to 18 percent-points (%p) higher compared to benchmark **Ru5**.

<figure><img src="image_33.png" /><figcaption>**Scheme 5.** Scope and limitations of catalysts **Ru5**, **Ru16**, and **Ru17** in CM and cycloisomerisation of challenging substrates. <sup>a</sup>Reaction conducted in perfluorotoluene. %p percentage point difference from **Ru5**.</figcaption></figure>

Noteworthy, in the example of CM of **20** with amylene (**22**),<sup>66, 67</sup> the biplane-type NHC catalysts **Ru16** and **Ru17** gave almost quantitative conversions of **20**, 93% and 91% respectively, which was 5 times higher than in the case of **Ru5**, reaching only 18%. Last but not least, the highly challenging 1,5-enyne metathesis of substrate **24** to obtain crowded and strained cyclobutene derivative **25** was attempted.<sup>68</sup> An additional difficulty arising in this reaction is that its selectivity can be compromised by a competing homo-dimerisation side reaction and by degradation of the highly strained resulting product. We were pleased to find that catalysts **Ru16** and **Ru17** gave good yields of 59% and 53% respectively, whereas with **Ru5** only traces of product were observed, which amounted to 6% yield. Interestingly, similarly to the case of the RCM reaction of **15a**, in this example, catalyst **Ru16**
61 Ablialimov, O., Kędziorek, M., Malińska, M., Woźniak, K. & Grela, K. Synthesis, Structure, and Catalytic Activity of New Ruthenium(II) Indenylidene Complexes Bearing Unsymmetrical N-Heterocyclic Carbenes. *Organometallics* **2014**, *33*, 2160-2171.

62 Debleds, O. & Campagne, J.-M. 1,5-Enyne Metathesis. *J. Am. Chem. Soc.* **2008**, *130*, 1562-1563.
performed visibly better than its fluorinated counterpart **Ru17**, which we attribute to **Ru16** higher activity. Remarkably, the use of the new catalysts allowed for a two-fold diminution in catalyst loading compared to the general-purpose **SIMes** complex **Ru2** used at 20 mol% loading.⁶⁸,⁶⁹ It should be noted that some differences in solubility were noticed at such high catalyst loadings. In this respect, complexes **Ru5** and **Ru16** performed better in toluene, which was able to dissolve them at the reaction temperature, while perfluorotoluene could not solubilise them entirely at this temperature. On contrast, complex **Ru17** dissolves better in perfluorotoluene, therefore this solvent was used in a test, in which **Ru16** gave also a similarly satisfactory yield of product 25.

Previously (Figure 5, Figure 10 and Schemes 4 and 5) we compared the newly designed catalysts with their closest analogue **Ru5**, in order to see the effect of additional aryl upper wings installation on the catalysts' stability and activity as we believe that comparing them with other catalyst designs will be like comparing apples with oranges.

In the next stage of this study, we opted to search for other traits that the new catalysts can exhibit, owing to their small-NHC ligands and overall enhanced stability. To achieve this, we have selected diastereoselective ring-rearrangement metathesis (DRRM)—a transformation that has been applied in the synthesis of a number of fused carbocycles as well as N- and O-heterocycles and various natural products.⁷⁰ According to our previous work,⁷¹,⁷² we used substrate **26** to check if sterically-reduced NHC ligand can influence the *cis/trans* selectivity of this transformation (Scheme 6A). We found that within the tested catalysts, **Ru16** and **Ru17** gave the highest stereoselection towards *trans*-27 (up to 2.8:1). Catalysts **Ru3** and **Ru2**, bearing gradually enlarged symmetrical NHC ligands gave greater proportions of *cis*-isomer, and finally the SIPr-substituted catalyst **Ru20** lead to a 1:1 mixture of *cis* and *trans*-27. A similar tendency was also observed by Blechert in his work⁷⁰.

Furthermore, a model CM reaction of allyl benzene (28) and 1,4-diacetoxy-2-butene (29) was chosen to assess the *E/Z* selectivity profile of different-size NHC catalysts (Scheme 6B).⁷³ The plot shown in Scheme 6B illustrates the *E/Z* ratio of product 30 as a function of conversion. Interestingly, the *E/Z* ratio profiles found for **Ru16** and **Ru17** show significant difference compared to the previously reported SIMes second- and third-generation catalysts **Ru1**, **Ru2** and **Ru21**.⁷³ All SIMes-based catalysts led to a product with lower *E/Z* ratios (~3:1) at low conversion, then the (*E*)-product amount increased slowly to 10:1. Interestingly, the small-NHC **Ru16** and **Ru17** offered low *E/Z* ratios *at much higher conversion* as compared to SIMes catalysts, which at the same conversion (70-75%) gave much higher amount of (*E*)-30. Presumably, the difference between these two catalyst classes can be rationalized on the basis of the smaller steric foot-print of **Ru16** and **Ru17** which translates to better inherent diastereoselectivity of these catalysts. At conversion higher than 80% the new catalysts promoted enough secondary metathesis events isomerising the product to the thermodynamically favoured *E*-isomer, similarly as their more bulky SIMes analogues did (Scheme 9B).
<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /><figcaption>**Scheme 6.** Investigating other features of the new catalysts in selected applications. A) Model Diastereo-selective Ring Rearrangement Metathesis (DRRM). B) *E/Z*-Stereoselectivity profile in the model CM reaction.

<sup>a</sup>Data taken from Ref. 74. TBS = tert-butyl(dimethyl)silyl.</figcaption></figure>

Finally, we decided to compare the newly designed catalysts with other Ru catalysts designed for challenging olefin metathesis substrates. While some of the model substrates presented in Figure 5, Figure 10 and Schemes 4 and 5 were also tested with other catalysts, the various conditions and loadings applied in the literature make the direct comparison of these systems impossible (although, see SI, Tables S12, S13, and S14 for a compilation of literature results).
<figure><img src="image_36.png" /><figcaption>**Scheme 7.** Exploring applications with challenging polyfunctional alkenes. A) Comparative study of selected small-NHC catalysts in RCM of isopulegol derivative **32**. Conversion calculated by GC. In parenthesis, conversion after 2 h of reaction. B) RCM of biomass-derived diene **34** leading to musk-smelling macrocycle. Yields of isolated pure product. In parenthesis, yield obtained with 1 mol% of catalyst and 2 mol% of TFQ. The *E/Z* ratio was determined by GC analysis. C) CM of sclareol. Yields of isolated pure product. The *E/Z* ratio was determined by NMR analysis. D) CM reaction leading to simplified analogue of statins. Catalysts was added in 5 equal portions over 4 h. Yields of isolated pure product. The *E/Z* ratio was determined by NMR analysis. <sup>a</sup> In brackets conversion of reaction conducted in perfluorobenzene. <sup>b</sup> Data taken from Ref. 35. <sup>c</sup> Data taken from Ref. 75 TFQ = tetrafluoroquinone.</figcaption></figure>

To provide the reader (and the potential user) with a bigger picture, we selected a number of additional experiments, utilising more complex substrates, including natural products or API (Active Pharmaceutical Ingredient) analogues. First a RCM reaction of (-)-L-isopulegol (a monoterpenic alcohol that is widely employed in the flavour and perfume industry) derivative **32** was attempted (Scheme 7A). This RCM reaction—leading to formation of a tetrasubstituted C–C
double bond—is known to be problematic for a second-generation catalyst used in 1 mol% loading under standard conditions (conversion ~30%).<sup>76</sup> In our tests we conducted RCM of **32** under previously reported conditions (benzene-*d*<sub>6</sub>, 60 °C, 2 h, 0.25 mol% of catalyst),<sup>35</sup> comparing **Ru16** and **Ru17** with selected known “small-NHC” catalysts. We found that (with the only example of macrocyclic-NHC derived **Ru14b**)<sup>35</sup> new complex, **Ru16** was giving the highest conversion (89%) of diene **29** into bicyclic product **30** after 2 h. Prolongation of the reaction time to 20 h increased conversion to 91% (Scheme 6C). Interestingly, the fluorinated analogue **Ru17** led to lower conversion (78%) in benzene, probably due to imperfect solubility in this solvent (cf. discussion about reaction **24**→**25**; Scheme 5), while in perfluorobenzene at the same temperature the conversion was improved to 85%. The non-stabilised parent catalyst **Ru5** gave lower conversion (53%) under the same conditions. Interestingly, the specialised SITol catalysts that are commercially available (**Ru3** and **Ru4**) gave 76 and 79% conversion in this reaction, while their close cousin **Ru11** offered only 61% of conversion. The SIXyl version of the latter, **Ru12**, led to 64% of conversion to product **30** (66% after 20 h), which is interesting in the light of very good activity observed for the same catalyst in RCM *en route* to *Euonymine* and *Euonyminol octaacetate* (cf. Figure 2A).<sup>15</sup> This again shows<sup>77-79</sup> that no single catalyst can outperform all others in all cases, and thus there is still a demand for new specialised tools in the context of challenging metathesis used for crowded and polyfunctional substrates.

Because in Scheme 4 we limited our model RCM trial to small-ring products only, next we decided to check the applicability of new catalysts in metathesis leading to larger macrocyclic rings, such as macrocyclic musk compounds.<sup>80</sup> Despite the waste majority of substrates in musk synthesis using olefin metathesis in a diluted solution (For an alternative high-temperature reactive distillation method, see Ref 81) constitutes simple terminal dienes that can undergo RCM with standard first and second-generation Ru catalysts (For one of key early examples, see Ref 82, and for a review, see Ref 83) some of biomass-based substrates are more difficult to be cyclised by RCM. To this class of dienes belong esters of oleic acid with alcohols bearing a geminally trisubstituted C-C double bonds, such as citronellol, geraniol, or nerol.<sup>75</sup> We noted that with these dienes general purpose catalysts, such as **Ru22** (Scheme 7B) give only moderate yields of 40-48%.<sup>75</sup> However, we were pleased to note that with the new BI\*Ph-catalysts family the same RCM reaction proceeds well, providing access to musk-smelling macrolactone **37** in a good isolated yield of 79-84% under identical conditions (Scheme 7B).

Next, we opted to provide an example in which a crowded allylic alcohol derivative is used as a substrate.<sup>84</sup> Because this motif is present in many natural and biologically active substances and drugs,<sup>85</sup> we decided to test our new catalysts with these substrates. As a first target we selected (-)-sclareol—a bicyclic diterpene alcohol found in clary sage (*Salvia sclarea*)—as it contains an
interesting tertiary allylic alcohol fragment. We were pleased to find that with help of **Ru16** this transformation can be conducted in satisfactory isolated yield (78%), while the fluorinated analogue **Ru17** gives slightly worse result (71%), but still visibly higher than the yield noted with stabilisation-deprived **Ru5** (Scheme 7D).

Finally, we attempted another challenging CM, that originated from a closed industrial project on Rosuvastatin valorisation.⁸⁶ As we checked previously,⁸⁷ a protected sterically crowded allyl alcohol derivative **38** is rather unreactive toward styrene-type substrates in presence of general-purpose catalysts, thus making CM-based synthesis of statin analogues⁸⁸ very difficult. To check if better results can be obtained with our new small-NHC catalysts **Ru16** and **Ru17**, we attempted CM of **38** with 3,4-dimethoxystyrene (39) and observed an important increase in productivity (59-58% of isolated yield) compared to results obtained with the commercial SIMes-based catalyst **Ru22** (38%, Scheme 7D).

## CONCLUSION

In summary, we developed the two new biplane-type NHC ligands **BI*Ph** and **BIF*Ph** based on a benzimidazolylidene core, whose lower wings are set as *N*-phenyl rings to reduce their steric hindrance on the first coordination sphere and whose upper wings are phenyl or pentafluorophenyl rings, respectively. The corresponding second-generation Grubbs-Hoveyda complexes **Ru16** and **Ru17** were generated and the effect of the introduction of the second decker of aromatic rings on their stereoelectronic properties, stability, and catalytic efficiency was evaluated, taking the mono-plane-type, unsubstituted **BIPh** ligand as a reference. According to the nature of the aromatic rings, **BI*Ph** was shown to be more electron-donating than **BIPh**, while **BIF*Ph** is less donating. By computing the electrostatic potential surfaces for complexes **Ru16-opt** and **Ru17-opt**, we have shown that complex **Ru16-opt** contains two electron-rich aromatics whereas complex **Ru17-opt** alternates electron-rich and electron-deficient aromatics. According to the polar/π Hunter and Sanders model, therefore, complex **Ru17-opt** displays a stronger electrostatic interaction between the stacked aromatic rings than its analogue **Ru16-opt**, leading to shorter ring centroid distances between the pentafluorophenyl and the *N*-phenyl rings (3.427 and 3.450 Å) in **Ru17-opt** compared to those between the phenyl and the *N*-phenyl rings (3.607 and 3.612 Å) in **Ru16-opt**.

More importantly, both complexes **Ru16** and **Ru17** were shown to be significantly less susceptible towards decomposition than the known **BIPh**-supported complex **Ru5**. This enhanced stability gain was rationalised through a DFT study of the involved decomposition pathways. Although the aromatic rings of the second decker cannot prevent the rotation of the *N*-phenyl groups in **Ru16** and **Ru17**, they strongly contribute to increase the energy of the rate-determining step, by imposing a strong distortion of the carbenic heterocycle through steric constraints. Among the transition states
of the decomposition profile, those that show the most marked Gibbs free energy difference between the **Ru5** complex and the **Ru16** and **Ru17** counterparts are **TS-1** (28.8 vs 33.8 and 33.4 respectively), **TS-4** (28.1 vs 35.1 and 36.1 respectively) and **TS-5** (29.7 vs 33.7 and 34.7 respectively), involving, respectively, a Berry pseudo-rotation (**TS-1**), the proton transfer from one of the *ortho* positions of the *N*-phenyl groups to the nearby carbon atom of the benzylidene moiety (**TS-4**) and the migration of the benzylic proton to ruthenium with the simultaneous carbon-carbon bond formation (**TS-5**). The observed increased stability of the ruthenium precursors **Ru16** and **Ru17** was also translated to the active species and enabled their implementation as highly efficient, stable pre-catalysts in challenging olefin metathesis reactions for the formation of tri- and tetra-substituted C-C double bonds. Complex **Ru16**, bearing the more electron-donating and *trans* effect inducting **BI*Ph ligand, revealed more active than its **Ru17** counterpart bearing the less electron donating **BIF*Ph ligand.

Overall, while it is well known that ruthenium catalysts bearing NHC ligands with low steric demand are more fragile, which impairs their productivity, we showed here that the adjunction of a second decker of aromatic groups in the NHC scaffold is a viable and promising strategy to access very stable and efficient olefin metathesis catalysts. Because the formation of sterically hindered olefins has always been the Achilles' heel of the ruthenium-catalysed metathesis, this special trait exhibited by the newly obtained complexes appears of paramount importance. We believe that the present report will open up new possibilities for further developments of olefin metathesis catalysts and will significantly extend applications of this transformation in natural product synthesis and medicinal chemistry.

## SUPPLEMENTAL INFORMATION

Supplemental information can be found on-line.

Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under accession numbers CCDC 1100116 and 605939. These data can be obtained free of charge from the CCDC via https://www.ccdc.cam.ac.uk/structures/.

## NOTES

We are grateful to Ekaterina Gulyaeva for the creation of the graphical abstract.

## CONFLICT OF INTEREST

The authors declare no competing interests.
## ACKNOWLEDGEMENTS

This work has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 860322. C.D. and I.d.R. are indebted to CALMIP for HPC resources (Grant 2017-[p17010]).

P.K., A.S, A.K, and K.G. are grateful to MAESTRO project financed from the National Science Centre, Poland on the basis of a decision DEC-2019/34/A/ST4/00372. For the purpose of Open Access, the author has applied a CC-BY public copyright licence to any Author Accepted Manuscript (AAM) version arising from this submission".

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