# New Pyridine Dicarbene Pincer Ligands with Ring Expanded NHCs and their Nickel and Chromium Complexes

Evangelos a A Papangelis, Katrin Pelzer, Christophe Gourlaouen, Dominique

Armspach, Pierre Braunstein, Andreas A Danopoulos, Corinne Bailly,

Nikolaos Tsoureas, Dimitrios Triantafyllos Gerokonstantis

► To cite this version:

Evangelos a A Papangelis, Katrin Pelzer, Christophe Gourlaouen, Dominique Armspach, Pierre Braunstein, et al.. New Pyridine Dicarbene Pincer Ligands with Ring Expanded NHCs and their Nickel and Chromium Complexes. Chemistry - An Asian Journal, 2024, 19, 10.1002/asia.202400169. hal-05266431v2

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# New Pyridine Dicarbene Pincer Ligands with Ring Expanded NHCs and their Nickel and Chromium Complexes

Evangelos Papangelis,[a] Katrin Pelzer,[b] Christophe Gourlaouen,[c] Dominique Armspach,[b] Pierre Braunstein,[b] Andreas A. Danopoulos,[a] Corinne Bailly,[d] Nikolaos Tsoureas,[a] and Dimitrios Triantafyllos Gerokonstantis[e]

This paper is dedicated to Prof. Andy Hor, Prof. Xianjun Loh and Dr. Yun Zong from Agency for Science, Technology and Research (A\*STAR), Singapore, Prof. Fuwei Li from University of Chinese Academy of Sciences, China, Prof. Guoqin Xu from National University of Singapore, Singapore and Prof. Jian-Ping Lang from Soochow University, Suzhou, China, for their outstanding contributions to chemistry and long-lasting friendship.

The pincer complexes $[Ni^{II}Br(CNC)]Br (4)$, $[Cr^{III}Br_{3}(CNC)] (5a)$ and $[Cr^{III}Br_{2.3}Cl_{0.7}(CNC)] (5b)$, where $CNC = 3,3'-(pyridine-2,6-diyl)bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)$, were obtained from the novel ligand $CNC$, generated in situ from the precursor $(CHNCH)Br_{2}$ and $[Ni^{II}Br_{2}(PPh_{3})_{2}]$ or from $[Cr^{II}(N(SiMe_{3})_{2})_{2}(THF)_{2}]$ and $(CHNCH)Br_{2}$ by aminolysis, respectively. The tetrahedally distorted square planar ($\tau_{4}\approx0.30$) geometry and the singlet ground state of Ni in 4 were attributed to steric constraints of the $CNC$ backbone. Computational methods highlighted the dependence of the coordination geometry and the singlet-triplet energy difference on the size of the N-

substituent of the tetrahydropyrimidine wingtips and contrasted it to the situation in 5-membered imidazolin-2-ylidene pincer analogues. The octahedral $Cr^{III}$ metal center in 5a and 5b is presumably formed after one electron oxidation from $CH_{2}Cl_{2}$. 4/MAO and 5a/MAO were catalysts of moderate activity for the oligomerization and polymerization of ethylene, respectively. The analogous $(CH^{N}N^{CH})Br_{2}$ precursor, where $(CH^{N}N^{CH}) = 3,3'-(pyridine-2,6-diylbis(methylene))bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-1-ium)$, was also prepared, however its coordination chemistry was not studied due to the inherent instability of the resulting free $C^{N}N^{C}$ ligand.

## Introduction

The study of chemical (structure, stoichiometric and catalytic reactivity), physical (photophysical, magnetic), biological and medicinal properties of metal complexes comprising one or more N-heterocyclic carbene (NHC) donors is a topical area of research, in particular with pincer-type ligand designs.[1,2] In addition to the precise spatial tuning of the pincer architecture, the electronic and steric modulation of the NHC group(s) constitute attractive features to design, develop and tune functional complexes.[1,3] Accumulating evidence, also originat-

ing from diverse computational methodologies, points to the versatility of the NHC functionality in modulating the σ-donor and π-donor/acceptor interactions with the metal center. Towards this end, manipulation of the energies of the NHC HOMO and LUMO (and the respective energy difference between them) are instrumental.[1,3a,b,4] The nature of the heterocycle, where the $C^{NHC}$ is integrated in, is crucial for this purpose; thus, the σ-donation combined with the minimal π-acceptance of the imidazol- and imidazolin-based NHC functionalities are compared and contrasted to the pronounced σ-donation of the mesoionic NHCs and the σ-donation and π-

[a] E. Papangelis, Prof. A. A. Danopoulos, Prof. N. Tsoureas
Inorganic Chemistry Laboratory, Department of Chemistry
National and Kapodistrian University of Athens
Panepistimiopolis Zografou, 15771 Athens, Greece
E-mail: adanop@chem.uoa.gr

[b] Dr. K. Pelzer, Prof. D. Armspach, Prof. P. Braunstein
Equipe Confinement Moléculaire et Catalyse, Institut de Chimie de
Strasbourg, UMR 7177 CNRS,
Université de Strasbourg,
4, rue Blaise Pascal, CS-90032, 67081 Strasbourg Cedex, France
E-mail: braunstein@unistra.fr

[c] Dr. C. Gourlaouen
Laboratoire de Chimie Quantique, Institut de Chimie de Strasbourg, UMR
7177 CNRS,
Université de Strasbourg,
4, rue Blaise Pascal CS-90032, 67081 Strasbourg Cedex, France
E-mail: gourlaouen@unistra.fr

[d] Dr. C. Bailly
Fédération de Chimie "Le Bel" - UAR2042
BP 296R8
1, rue Blaise Pascal, 67008 Strasbourg Cedex, France

[e] D. T. Gerokonstantis
Laboratory of Analytical Chemistry, Department of Chemistry,
National and Kapodistrian University of Athens,
Panepistimiopolis Zografou, 15771 Athens, Greece

Supporting information for this article is available on the WWW under
https://doi.org/10.1002/asia.202400169

© 2024 The Authors. Chemistry - An Asian Journal published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
acceptance of the Ring Expanded (RE) and cyclic Alkyl Amino Carbene (cAAC) NHCs.[5] Therefore, a good scope for the NHC selection could be in place to differentially (de)stabilize metal oxidation states, M–C**NHC** and M-coligand interactions, metal nucleophilicity, promote/suppress lability etc. As part of a pincer ligand design, NHC functionalities can be installed at the wingtip(s) or bridgehead and subjected to the constraints of the rigid ligand structure, further modulating π-interactions via variation of torsion angles and steric interactions of the constituent functionalities.[6] In this respect, the magnitude of the N–C**NHC**–E (E = heteroatom, usually N or C) angle can covertly accentuate the steric demands of the NHC functionality(ies) through enforcing distal bulky substituents to proximal positions. Despite the above mentioned options offered by the NHC functionality as a ligand building block, the majority of the studies with pincer ligand systems are based on the usage of 5-membered imidazol- and, to a lesser extent, imidazolin-based NHCs as side arms (type I in Figure 1).[1a,c,j] In a limited number of studies, mesoionic triazol-based wingtips (type II in Figure 1) have been employed;[7] in contrast, wingtips based on the RE-NHCs (type III in Figure 1) are unknown.[8] Interestingly, the picture regarding the nature of the NHC as bridgehead functionality shows more abundant usage of RE-6 NHCs, that have led to remarkable structural and reactivity differences compared to analogous 5-membered moieties (in particular, with donor atoms OC**NHC**O of aryloxide,[9] NC**NHC**N of pyridine or imine,[10] PC**NHC**p[11] of dialkyl- or diaryl-phosphane). Herein, we report the synthesis of pyridine dicarbene pincer (pro)-ligands featuring RE-6 NHC wingtips (type III, E = N) and their usage in accessing Ni and Cr complexes of catalytic

relevance. In the present study, the distinctive steric profile of the RE-6 NHC wingtip was manifested as (tunable) geometrical distortion at the metal with ground state electronic structure implications.

## Results and Discussion

### Proligand and Ligand Synthesis

The synthetic strategy to access the precursors (CHNCH)Br₂ and (CH^N^CH)Br₂ is depicted in Scheme 1. Although it should be general enough to accommodate any aromatic substituent at the six membered tetrahydropyrimidine, the choice of the mesityl group was dictated by the increased stericities anticipated in the resultant CNC complexes due to the obtuse N–C**NHC**–N angle.

All steps in Scheme 1 proceed with good to excellent isolated yields at multigram scale. The proligands were isolated as colourless powders and characterized spectroscopically (see Experimental). Deprotonation of (CHNCH)Br₂ with two equivalents of NaN(SiMe₃)₂ led to the pincer ligand CNC which could be isolated as an oily substance; it was stable at room temperature in C₆D₆ for at least 24 h, and was characterized by ¹H and ¹³C{¹H} NMR spectroscopies. Although the NMR spectra were in agreement with the symmetric structure proposed for CNC, it was impossible to observe in the ¹³C{¹H}-NMR spectrum a signal assignable to the carbene carbon C**NHC**, despite longer acquisition times and pulse delays to complete relaxation. In contrast, attempts to isolate at room temperature and characterize C^N^C were not successful and its future use for complexation reactions presumably should not be based on its *in situ* generation and trapping with metal precursors, but on 'coordinated base' methodologies or other C–H metalation protocols. Lastly, despite CNC being stable (*vide supra*), its long-term storage was avoided and for using it in metal complex syntheses, *in situ* generation in the presence of metal precursors was favored.

### Computational Studies on the CNC Ligand

Since the molecular structure determination of CNC by X-ray diffraction methods was impossible due to the lack of crystallinity (*vide supra*), gaining insight into its metrical data, preferred conformation(s), stability and electronic structure were attempted by computational methods. According to DFT calculations, the computed CNC(DFT) in the gas phase can adopt three energetically unequal conformations (A, B, C), that differ in the relative orientation of the heterocyclic rings, where the two C**NHC** and the one N**Py** lone pair bearing atoms are located (Scheme 2). In the most stable conformer C repulsions between neighbouring lone pairs are minimized. Cartesian coordinates and selected metrical data of the three optimized structures are given in the Supporting Information (Tables S3, S5 and S6).

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Common (type I, E = N, C etc.) and known (type II, E = N) symmetrical pincer ligand designs with NHC wingtips; type III, (E = N, n = 0, 1, m = 1) is the subject of this paper.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Scheme 1.** The synthetic sequence leading to the pro-ligands (CHNCH)Br₂ and (CH^N^CH)Br₂ and the ligand CNC.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Scheme 2.** The three conformers of CNC(DFT): A: all syn lone pairs; B: syn, anti lone pairs; C: all anti lone pairs. The all syn conformer D was employed in computations for comparison purposes (see text for details). * The numbers denote the Gibbs free energies; the energy of conformer C was set to 0.0.</figcaption></figure>

The singlet-triplet energy difference Δ*E*<sub>S-T</sub> and the energy of the HOMO of the conformer **A** (resembling best the metal-bound conformation of the ligand) was computed at *ca.* -2.11 eV and -6.91 eV, respectively. In comparison, the Δ*E*<sub>S-T</sub> and the energy of the HOMO of the relevant pincer ligand **D** with 5-membered NHCs were calculated at *ca.* -3.55 eV and -7.48 eV respectively, confirming the superior σ-donor characteristics of **A** when bound to a metal center; the energies of the LUMO in **A** and **D** are comparable (see Supporting Information, Table S9).

Interestingly, in addition to the difference of the magnitude of the ΔE<sub>S-T</sub>, there are disparities in the *nature* of the computed HOMOs, LUMOs and SOMOs of **A** and **D** in their singlet and triplet states: (i) in the singlet states, although in both **A** and **D** the nature of the HOMOs has clearly C<sup>NHC</sup> and N<sup>pyr</sup> lone pair character, in the HOMO of **D** there is significant contribution of the π system of the pyridine, which is almost absent in **A**; conversely, the LUMOs of both **A** and **D** have C<sup>NHC</sup> and pyridine π character; (ii) moreover, in the triplet state in **A**, the energetically highest SOMO that displays C<sup>NHC</sup> character, featuring two degenerate minima, each being localized on one of the RE-NHCs, results in the pyramidalization of the corresponding RE-NHC group (Figure 2). Conversely, π pyridine character, and ‘dearomatization’ of the pyridine are two features found in the triplet state in **D** (see also Supporting Information, Table S10). Recently, substantial localization of spin density in the π system

<figure><img src="image_6.png" /><figcaption>**Figure 2.** MOs of **A** (from left to right): HOMO, LUMO of the singlet state and SOMO of the triplet state.</figcaption></figure>

of the pyridine bridgehead in radical anions of imidazol-2-ylidene analogues of **D** was experimentally observed and computationally demonstrated.[12]

Ni and Cr Complexes with the CNC Ligand

In view of our interest in the catalytic oligomerization of ethylene,[13] we employed the ligand **CNC** to access Cr and Ni complexes that are of relevance to this reaction (*vide infra*). The synthetic methods employed to access the well-defined complexes **4, 5a** and **5b** are given in Scheme 3.

The orange, diamagnetic Ni complex **4** was obtained in good yields (*ca.* 60%) by the reaction of the *in situ* generated **CNC** with [NiBr<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>]. In contrast, the pink-purple, para-magnetic complex **5a** was obtained in moderate yields (*ca.* 57%) after metalation of the **CNC** by aminolysis of [Cr<sup>II</sup>{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>(THF)<sub>2</sub>] with the precursor (**CHNCH**)Br<sub>2</sub> in THF, extraction into CH<sub>2</sub>Cl<sub>2</sub> and isolation by solvent evaporation; the mechanism of the one electron oxidation of the metal and the origin of the third bromide co-ligand in **5a** is vague. Attempts to crystallize **5a** by pentane diffusion into a CH<sub>2</sub>Cl<sub>2</sub> solution over one week led to the isolation of **5b** in the form of pink-purple crystals. The molecular structures of complexes **4** and **5b** determined crystallographically are shown in Figures 3 and 4 with important metrical data given in the caption of the Figures.[14] Noticeably, diamagnetic **4** was amenable to study by ¹H-NMR spectroscopy, while **5a** was silent in both ¹H-NMR and X-band EPR spectroscopy, in agreement with the presence of a high spin Cr<sup>III</sup> center.[15]

The coordination geometry at Ni in **4** deviates substantially from the expected square planar (τ<sub>4</sub>≈0.30),[16] whereas the bromide co-ligand is situated *ca.* 1.30 Å above the plane defined by the three other donors (C<sup>NHC</sup> and N<sup>pyr</sup>); the Ni center is only 0.16 Å above this plane. Both tetrahydropyrimidine six-membered rings adopt a half-chair conformation, thus maintaining the approximate C<sub>s</sub> symmetry of the complex (relative to the plane defined by the N<sup>pyr</sup>-Ni-Br), also in order to alleviate requirements arising from the planarity of the N-C<sup>NHC</sup>-N substructure. The structural features of **4** are to be contrasted

<figure><img src="image_7.png" /><figcaption>**Scheme 3.** Synthesis of Ni<sup>II</sup> and Cr<sup>III</sup> complexes with the CNC ligand.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 3.** Two views of one of the two cations in the asymmetric unit of 4; the second cation, two bromide anions, one toluene molecule and hydrogen atoms are omitted for clarity. Important metrical data (Å and °) are: Ni–N<sup>Pyr</sup> = 1.810(8), Ni–C<sup>NHC</sup> = 1.942(10) and 1.958(10), Ni–Br = 2.2816(15), Br–Ni–N<sup>Pyr</sup> = 155.5(3), C<sup>NHC</sup>–Ni–C<sup>NHC</sup> = 162.0(4), C<sup>NHC</sup>–Ni–N<sup>Pyr</sup> = 81.7(4) and 82.2(4), C<sup>NHC</sup>–Ni–Br = 98.6(3) and 99.4(3), N–C<sup>NHC</sup>–N = 119.4(9); the second cation in the asymmetric unit shows very similar metrical data (see also Supporting Information, Table S1).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 4.** View of the molecular structure of 5b; one CH<sub>2</sub>Cl<sub>2</sub> molecule and hydrogen atoms are omitted for clarity. Important metrical data (Å and °) are: Cr–N<sup>Pyr</sup> = 1.998(2), Cr–C<sup>NHC</sup> = 2.172(2) and 2.170(2), Cr–Br = 2.413(8), 2.4818(5) and 2.5016(5), Br–Cr–N<sup>Pyr</sup> = 177.4(2), C<sup>NHC</sup>–Cr–C<sup>NHC</sup> = 153.81(7), N–C<sup>NHC</sup>–N = 116.66(19) (See also supporting information). The halogen atom site *trans* to the N<sup>Pyr</sup> is partially occupied by Br (30%) and Cl (70%); only the former is depicted for clarity (see also Supporting Information, Table S1).</figcaption></figure>

with those of the analogous complex with the five-membered (benz)imidazol-2-ylidene pincer already reported (imidazol, wingtip substituents: Me,<sup>[17]</sup> DiPP,<sup>[18]</sup> Mes,<sup>[19]</sup> benzimidazol: <sup>a</sup>Bu<sup>[20]</sup>) where virtually planar environments around the Ni<sup>II</sup> centers are observed. The Ni–C<sup>NHC</sup> bond distances in 4 are marginally longer than those reported for Ni<sup>II</sup>-imidazol-2-ylidene complexes, possibly due to steric reasons (see Supporting Information, Table S2). In complex 5b the metal adopts an octahedral coordination geometry. Here too, the two tetrahydropyrimidine six-membered rings of the pincer adopt half-

chair conformation and an approximate C<sub>s</sub> symmetry of the complex (relative to the plane defined by the N<sup>Pyr</sup>–Cr–Br<sub>2</sub>) is obtained. The Cr–C<sup>NHC</sup> bond distances in 5b are in the range reported for the other pyridine dicarbene Cr complexes, i.e. (2.087(6), and 2.120(6) for Cr<sup>III</sup><sup>[21]</sup>), (2.125(12) and 2.122(10) for Cr<sup>II</sup><sup>[22]</sup>).

## Computational Studies on the Geometric and Electronic Structure of the [Ni(CNC)X]<sup>+</sup> Analogues

In order to get insight into the factors that may be responsible for the unusual geometry around the Ni center in 4, DFT calculations were undertaken. Thus, the computed structure of the cation in the singlet state 4BrMes<sup>s</sup> is in good agreement with the experimental and replicates the unusual features observed. In order to probe whether steric interactions could be held responsible for the structure of 4BrMes<sup>s</sup>, the structures of additional cations were computed: (i) with different NHC substituents (i.e. H, Me, adamantyl (Ad)), and (ii) with X coligands differing in size (Cl, Br, I). From the optimized structures were extracted: (i) the geometric parameter τ<sub>4</sub> and (ii) the concomitant energy difference E<sub>T</sub>–E<sub>S</sub> = ΔE<sub>TS</sub> and used to quantify the departure from the square planar coordination geometry and the nature of the ground state electronic structure, respectively; the results are combined in Table 1.

Thus, when the substituents R at the wingtips are either Me, Mes or Ad, the Ni in the corresponding complex cations is in a distorted square planar environment with increasing τ<sub>4</sub> values as the steric bulk of the R group becomes larger. Conversely,

<table><caption>**Table 1.** Computed structural and electronic parameters of the cation in 4 as a function of the wingtip substituent R and the halide coligand.<sup>[a]</sup></caption><thead><tr><th>Complex<sup>[b]</sup></th><th>Spin Multiplicity</th><th>τ<sub>4</sub><sup>[16]</sup></th><th>ΔE<sub>TS</sub><sup>[c]</sup></th></tr></thead><tbody><tr><td>4ClMes<sup>s</sup></td><td>Singlet</td><td>0.30</td><td>7.7</td></tr><tr><td>4ClMes<sup>T</sup></td><td>Triplet</td><td>0.71</td><td></td></tr><tr><td>4BrMes<sup>s</sup></td><td>Singlet</td><td>0.33</td><td>7.0</td></tr><tr><td>4BrMes<sup>T</sup></td><td>Triplet</td><td>0.70</td><td></td></tr><tr><td>4Imes<sup>s</sup></td><td>Singlet</td><td>0.40</td><td>4.6</td></tr><tr><td>4Imes<sup>T</sup></td><td>Triplet</td><td>0.72</td><td></td></tr><tr><td>4BrH<sup>s</sup></td><td>Singlet</td><td>0.12</td><td>27.9</td></tr><tr><td>4BrH<sup>T</sup></td><td>Triplet</td><td>0.35</td><td></td></tr><tr><td>4BrMe<sup>s</sup></td><td>Singlet</td><td>0.32</td><td>11.6</td></tr><tr><td>4BrMe<sup>T</sup></td><td>Triplet</td><td>0.56</td><td></td></tr><tr><td>4BrAd<sup>s</sup></td><td>Singlet</td><td>0.45</td><td>-6.7</td></tr><tr><td>4BrAd<sup>T</sup></td><td>Triplet</td><td>0.68</td><td></td></tr><tr><td>ImidCIMes<sup>S[d]</sup></td><td>Singlet</td><td>0.12</td><td>14.5</td></tr><tr><td>ImidCIMes<sup>T[d]</sup></td><td>Triplet</td><td>0.70</td><td></td></tr></tbody></table>

[a] For the computation methodology see the Supporting Information; [b] The notations 4XR<sup>s</sup> and 4XR<sup>T</sup> refer to computed structures with R wingtip substituent, X coligand in the singlet or triplet states, respectively; [c] in kcal/mol; [d] ImidCIMes<sup>s</sup> and ImidCIMes<sup>T</sup> refer to computed structures of pyridine bis(imidazolin-2-ylidenes) with Mes wingtip substituent, Cl coligand in the singlet or triplet state, respectively.
when R=H, (4BrH⁵), minimal distortion is observed ($τ_4$≈0.12). Moreover, increasing the size of the halide co-ligand also results in increased distortion away from the square planar.

This picture is further clarified by perusal of the steric maps derived after calculations of the % buried volume and its spatial distribution by employing the *SambVca* web toolkit[3c] and cartesian coordinates obtained experimentally (4) or by DFT methodology (as in Table 1); full details and representative results are found in the Supporting Information (Figure S18) and Figure 5, respectively.

Finally, the calculations indicated that an increased distortion from the square planar geometry results in a smaller triplet-singlet energy difference $ΔE_{TS}$ in the calculated complexes with the bulkiest R (i. e. in 4BrAd), the triplet is stabilized vs. the singlet state (by -6.7 kcal/mol). For the sake of comparison, DFT calculations were implemented on the analogous imidazolin-2-ylidene pincer complex (R=Mes), showcasing that it neither exhibits the aforementioned distortion ($τ_4$≈0.12) nor a narrow triplet-singlet energy difference ($ΔE_{TS}$≈14.5 kcal/

mol). The disparate features were attributed to the wider N–C$^{NHC}$–N angles of the RE-NHCs that force the NHC substituents (R) to close proximity with each other and the metal center (the calculated average ≈N–C$^{NHC}$–N in cations with RE-6 and the corresponding imidazol-2-ylidenes were ca. 117° and 104° respectively).

## Catalytic Studies

Ligand design is paramount for tuning the activity and selectivity of Ni- and Cr-based ethylene oligomerization/ polymerization catalysts. In particular, selectivities such as formation of oligomers vs. polymers, α-olefins vs. internal and branched alkenes are of high industrial interest and as a result many studies have been dedicated to the understanding of the reaction mechanisms at work.[23] So far, pyridine bis(imidazol-2-ylidene) Ni complexes activated with organoaluminium compounds have not been studied for their ability to catalyze alkene oligomerization/polymerization reactions; moreover, pyridine bis-imidazol-2-ylidene chromium complexes have been claimed to behave as efficient catalysts for ethylene oligomerization.[21,24] To extend the scope of CNC-type pincer ligands, the complexes **4** and **5a** were tested for these reactions. A summary of the results obtained is given in Table 2.

All catalytic tests were conducted in chlorobenzene because the complexes are poorly soluble in toluene. It is evident that both **4** and **5a** exhibit moderate activity[25] in terms of ethylene conversion, leading respectively to a mixture of oligomers and polymers with traces of oligomers, respectively. Although **5a** is predominantly a polymerization catalyst, the small amounts of oligomers detected turned out to be exclusively α-olefins. The polyethylene obtained was insoluble at room temperature in solvents commonly used for the elucidation of polymer microstructures and therefore it was not further studied.

In attempts to gain insight into the behavior of **4** in catalysis, its stoichiometric reactivity with various methylating agents (MeLi, MeMgCl) was pursued but led to intractable reaction mixtures. Additionally, after assuming that methylated species of type 6DFT (Figure 6) could be a reasonable product from the reaction of **4** with MAO and a catalysis relevant precursor to the active species, its stability and plausible

<figure><img src="image_10.png" /><figcaption>**Figure 5.** Steric maps of computed structures of the cations (from top left clockwise): 4BrMes⁵, 4BrH⁵, ImidCIMes⁵ and 4BrAd⁵; the origin of the cartesian coordinates is at the Ni, the z-axis and the xz-plane coincide with the Ni–N$^{pyr}$ vector and the C$^{NHC}$–N$^{pyr}$–C$^{NHC}$ plane, respectively.</figcaption></figure>

<table><caption>**Table 2.** Summary of the catalytic oligo-(poly)-merization studies using **4** and **5a**.</caption><thead><tr><th>Entry</th><th>Cat</th><th>TOF<sup>a</sup></th><th>%C<sub>4</sub><sup>b</sup></th><th>%C<sub>6</sub><sup>b</sup></th><th>%C<sub>8</sub><sup>b</sup></th><th>%C<sub>10</sub><sup>b</sup></th><th>%PE</th></tr></thead><tbody><tr><td>I</td><td>4</td><td>20504</td><td>70(48)</td><td>21(20)</td><td>6(21)</td><td>2(0)</td><td>—</td></tr><tr><td>II<sup>c</sup></td><td>4</td><td>—</td><td>73(5)</td><td>24(2)</td><td>3(22)</td><td>—</td><td>—</td></tr><tr><td>III<sup>d</sup></td><td>4</td><td>7022</td><td>60(45)</td><td>29(11)</td><td>9(3)</td><td>2(0)</td><td>—</td></tr><tr><td>IV</td><td>5a</td><td>8744<sup>e</sup></td><td>3(100)</td><td>5(100)</td><td>2(100)</td><td>—</td><td>90</td></tr><tr><td>V</td><td>5a</td><td>728<sup>e</sup></td><td>1(100)</td><td>—</td><td>—</td><td>—</td><td>100</td></tr><tr><td>VI<sup>f</sup></td><td>5a</td><td>18206<sup>e</sup></td><td>2(100)</td><td>1.5(100)</td><td>1(100)</td><td>0.5(95)</td><td>95</td></tr></tbody></table>

[a] Turnover frequency in: (mol C₂H₄)·(mol complex·h)⁻¹. [b] Percentage of olefin-product fractions with chain number of C atoms as indicated by the index (numbers in parentheses indicate the percentage of linear α-olefins); [c] 1 bar of C₂H₄ and 30 min run; [d] Initial temperature = 60 °C; [e] Turnover frequency was calculated only for the polymerization; [f] 20 bar C₂H₄.
<figure><img src="image_11.png" /><figcaption>**Figure 6.** One plausible deactivation step of a catalysis relevant precursor 6DFT via methyl migration into the Ni–C<sup>NHC</sup> bond computed by DFT. *The numbers denote the Gibbs free energies (for the sake of comparison the reactant's energy was set to 0.0).</figcaption></figure>

deactivation reactivity was investigated by computational means. In particular, the decomposition by migration of a methyl group from Ni to the C<sup>NHC</sup> to 6DFTmig was explored. This elementary step has been previously observed experimentally and/or implicated in the rigid pyridine dicarbene pincer ligands of the imidazol-2-ylidene type on a range of metals and with pincer ligands with bridgehead NHC donor.[18,26]

The results show that **6DFTmig** is very stable compared to **6DFT**; furthermore, it is accessible **via** a low energy barrier directly by concerted migration of one CH₃ from the Ni to the C<sup>NHC</sup>.

## Conclusions

The synthesis of the novel pyridine dicarbene pincer ligands, where the carbene is a RE-NHC, that are described herein, unraveled subtle electronic structure and steric profile differentiation from the established and widely studied analogues based on the imidazol-2-ylidenes. The donor-acceptor electronic characteristics of the new pincers stem from the nature of the constituent functional groups, while the steric profile is arising from conformational preferences of the ligand backbone on metal coordination. The diverse tuning handles in the ligand design promise to be useful for future applications in functional metal complexes. The computational studies have shown a gradual RE-NHC substituent and metal coligand dependent transition to pyramidalized four-coordinate Ni<sup>II</sup> geometries, accompanied by triplet-singlet energy difference modulation. That the complexes **4** and **5a** show unremarkable catalytic activity in the oligo(poly)merization of ethylene may be attributed to catalyst deactivation due to migration elementary steps. The extension of the use of the novel pyridine RE-NHC pincer ligands to other metals and oxidation states, in particular 3d metals of relevance to catalysis, and the manipulation of

singlet-triplet energy difference of pincer NHC complexes with potential applications in photocatalysis are directions that are currently under exploration.

## Experimental Section

General experimental guidelines are given in the Supporting Information. Compound 2 was obtained by following a method of the literature (see Supporting Information).

### Compound 3·HBr

To a round-bottom flask equipped with side-arm was added compound 2 (18.000 g, 50.828 mmol), HC(OEt)₃ (37.660 g, 254.140 mmol) and 2–3 drops conc. HCOOH(aq). With the side arm open, the mixture was stirred for 16 h at 100 °C, whereupon a clear orange-red solution was obtained. After cooling to rt, diethyl ether was added and a sticky, off-white-to-orange solid precipitated, that was isolated by filtration and washed with diethyl ether affording a free-flowing solid. If the final solid product remains sticky (an indication of residual HC(OEt)₃ present), it is dried at 80–90 °C for ca. 16 h under vacuum. Yield: 7.197 g (50%). ¹H-NMR (400 MHz, CD₃OD, 25 °C): δ = 8.17 (s, 1H, NCHN), 7.05 (s, 2H, HAr), 3.69 (t, 2H, CH₂), 3.57 (t, 2H, CH₂), 2.31 (s, 3H, p-CH₃), 2.28 (m, 2H, CH₂), 2.26 (s, 6H, o-CH₃); ¹³C[¹H]-NMR (100 MHz, CD₃OD, 25 °C): δ = 155.39 (NCHN), 141.32 (CAR), 138.39 (CAR), 136.11 (CAR), 130.93 (CAR), 47.70, 38.64, 21.00, 19.88, 17.49; HRMS (ESI): m/z calcd for C₁₃H₂₀N₂Br: 203.1543 [M]⁺; found: 203.1556.

### Compound 3

In two separate Schlenk flasks were added compound **3·HBr** (6.459 g, 22.806 mmol) in diethyl ether (ca. 50 mL) and ¹BuOK (2.815 g, 25.087 mmol) in diethyl ether (ca. 50 mL). After cooling the resulting mixtures to –10 °C, the base was added to the suspension of the salt and the mixture was stirred for 3–4 h at rt during which time the appearance of the precipitate changed. Removal of the precipitate by filtration through a pad of Celite and evaporation of the colourless filtrate under reduced pressure to dryness afforded the product as an off-white solid. Yield: 3.690 g (80%). m.p. 100–103 °C; ¹H-NMR (400 MHz, CDCl₃, 25 °C): δ = 6.90 (s, 3H, HAr & NCHN), 3.45 (t, 2H, CH₂), 3.33 (t, 2H, CH₂), 2.27 (s, 3H, p-CH₃), 2.22 (s, 6H, o-CH₃), 2.01 (m, 2H, CH₂); ¹³C[¹H]-NMR (100 MHz, CDCl₃, 25 °C): δ = 149.41 (NCHN), 140.00 (CAR), 137.46 (CAR), 136.73 (CAR), 129.42 (CAR), 45.62, 43.28, 21.53, 21.02, 17.87; HRMS (ESI): m/z calcd for C₁₃H₁₉N₂: 203.1543 [M + H]⁺; found: 203.1573.

#### (CHNCH)Br₂

In a small Schlenk flask were mixed as solids **3** (2.390 g, 11.814 mmol) and 2,6-dibromopyridine (1.399 g, 5.907 mmol). The mixture was placed under N₂, the vessel closed, fully immersed into an oil-bath and was left at 140 °C for 16 h during which time it gradually solidified to an off-white solid. After removal of the Schlenk from the oil bath, allowing it to cool to room temperature, breaking up the solid with a spatula and washing it with diethyl ether (3 x 20 mL), the product was obtained as a colorless solid. Yield: 1.200 g (93%). ¹H-NMR (400 MHz, CD₃CN, 25 °C): δ = 9.11 (s, 2H, NCHN), 8.18 (t, ³J<sub>H,H</sub> = 8.13 Hz, 1H, p-CH₃pyr), 7.46–7.44 (d, ³J<sub>H,H</sub> = 8.22 Hz, 2H, m-CH₃pyr), 7.04 (s, 4H, HAr), 4.12 (t, 4H, CH₂), 3.77 (t, 4H, CH₂), 2.43 (m, 4H, CH₂), 2.31 (s, 6H, p-CH₃), 2.22 (s, 12H, o-CH₃); ¹³C[¹H]-NMR (100 MHz, CD₃CN, 25 °C): δ = 153.85 (NCHN), 150.82 (CAR), 144.45 (CAR), 141.36 (CAR), 137.96 (CAR), 135.47 (CAR), 130.70 (CAR),
112.43 (CAR), 48.25, 43.97, 20.97, 19.63, 17.80; MS (50 eV): m/z (%): 240 (26) [M]$^{2+}$, 560 (16) [M + $^{79}$Br]$^{+}$, 562 (16) [M + $^{81}$Br]$^{+}$, 498 (100) [M + H$_{2}$O]$^{+}$; HRMS (ESI): m/z calcd for C$_{31}$H$_{39}$N$_{5}$Br$_{2}$: 240.6597 [M]$^{2+}$; found: 240.6598.

(CH^N^N^CH)Br₂

In a round-bottom flask were added 0.392 g (1.483 mmol) of 2,6-bis(bromomethyl)pyridine and 0.600 g (2.966 mmol) of 3. After addition of dry CH$_{3}$CN (20 mL) the mixture was stirred at rt for 16 h during which time a suspension had formed. Volatiles were removed under reduced pressure, the colorless solid residue was washed with diethyl ether and dried under vacuum. Yield: 0.694 g, (70%). ¹H-NMR (400 MHz, CDCl$_{3}$, 25 °C): δ = 9.24 (s, 2H, NCHN), 7.72–7.69 (t, $^{3}$J$_{H,H}$ = 7.69 Hz, 1H, p-CHpyr), 7.56-7.55 (d, $^{3}$J$_{H,H}$ = 7.64 Hz, 2H, m-CHpyr), 6.89 (s, 4H, HAr), 5.33 (s, 4H, CH$_{2}$), 3.66 (t, 4H, CH$_{2}$), 3.61 (t, 4H, CH$_{2}$), 2.32-2.29 (br, 4H, CH$_{2}$), 2.26 (m, 18H, o-CH$_{3}$ & p-CH$_{3}$); ¹³C (¹H)-NMR (100 MHz, CDCl$_{3}$, 25 °C): δ = 155.19 (NCHN), 154.08 (CAR), 139.99 (CAR), 138.89 (CAR), 136.44 (CAR), 134.50 (CAR), 130.19 (CAR), 123.27 (CAR), 59.38, 46.24, 44.48, 21.12, 19.49, 18.25; MS (25 eV): m/z (%): 508 (14) [M]$^{+}$, 254 (74) [M]$^{2+}$, 588 (100) [M + $^{79}$Br]$^{+}$, 590 (100) [M + $^{81}$Br]$^{+}$; HRMS (ESI): m/z calcd for C$_{33}$H$_{43}$N$_{5}$Br$_{2}$: 254.6754 [M]$^{2+}$; found: 254.6758.

In Situ Generation and Characterization of the CNC

In a Young NMR-tube were placed (CHNCH)Br₂ (0.025 g, 0.039 mmol) and NaHMDS (0.016 g, 0.085 mmol). The tube was cooled at -30°C, C₆D₆ (0.6 mL) was added and the mixture was left to reach rt while manually agitating for 30 min. Finally, the ¹H- and ¹³C(¹H)-NMR spectra of the reaction mixture were recorded; the spectra demonstrated the formation of a single product corresponding to the CNC, despite the fact that an anticipated downfield signal in the ¹³C(¹H)-NMR spectrum assignable to CNHc could not be observed. The spectrum remained unchanged for at least 24 h. ¹H-NMR (400 MHz, C₆D₆, 25 °C): δ = 7.08 (br, 3H, p-CHpyr and m-CHpyr), 6.70 (s, 4H, HAr), 3.37 (br, 4H, CH₂), 2.69 (br, 4H, CH₂), 2.14-2.12 (m, 18H, o-CH₃ and p-CH₃), 1.65 (br, 4H, CH₂); ¹³C(¹H)-NMR (100 MHz, C₆D₆, 25 °C): δ = 157.84 (CAR), 144.54 (CAR), 139 (CAR), 136.67 (CAR), 134.10 (CAR), 129.70 (CAR), 105.84 (CAR), 44.27, 39.30, 21.58, 21.07, 18.01, 6.56.

Complex 4

In a Schlenk flask were added NaHMDS (0.075 g, 0.411 mmol) and (CHNCH)Br₂ (0.120 g, 0.187 mmol). After cooling the mixture of the solids to -78 °C, THF (ca. 20 mL) was added and the suspension was stirred for 2 h reaching rt, when it assumed a yellow coloration; subsequently, it was added dropwise to a dark green solution of [NiBr₂(PPh₃)₂] (0.139 g, 0.187 mmol) in THF (10 mL), affording an orange suspension that was stirred for 16 h at rt. Evaporation of the volatiles under reduced pressure, washing of the solid residue with toluene (3x15 mL), extraction into CH₂Cl₂ (ca. 20 mL) and filtration through a pad of Celite afforded a clear orange filtrate from which 4 was obtained as an orange solid after removing the volatiles under reduced pressure. Yield: 0.078 g (60%). Orange crystals of 4 suitable for SC-XRD were obtained by slow diffusion of toluene to a CH₂Cl₂ solution at room temperature. ¹H-NMR (400 MHz, CDCl₃, 25 °C): δ = 8.08 (t, 1H, p-CHpyr), 7.03-7.01 (d, 2H, m-CHpyr), 6.72 (s, 4H, HAr), 3.92 (t, 4H, CH₂), 3.47 (t, 4H, CH₂), 2.39 (s, 12H, o-CH₃), 2.37 (m, 4H, CH₂), 2.23 (s, 6H, p-CH₃).

Complexes 5a and 5b

Inside a N₂ filled glove box in a Schlenk flask were placed (CHNCH)Br₂ (0.185 g, 0.289 mmol) and THF (10 mL) giving an off-white suspension. To this was added [Cr(N(SiMe₃)₂)₂](THF)₂ (0.178 g, 0.346 mmol) and the mixture was stirred for 16 h at rt, during which time a grey suspension and HN(SiMe₃)₂ were formed. The volatiles were removed under reduced pressure, the solid residue was removed from the glove box and washed with diethyl ether. Although the nature of the metal species at this stage was unknown, the residue was extracted into CH₂Cl₂ giving a light-brown solution. It is plausible that CH₂Cl₂ may act as one electron oxidant which could account for the formation of 5a. Evaporation of all volatiles under reduced pressure afforded a light-brown solid which upon washing with THF turned pink purple. The evaporation step after the extraction with CH₂Cl₂ must be performed rapidly in order to suppress formation of 5b by halogen exchange with the solvent. After drying the residue under vacuum, analytically pure 5a was obtained. Yield: 0.127 g (57%). Pink purple crystals of 5b suitable for SC-XRD were grown by slow diffusion of pentane to a CH₂Cl₂ solution at room temperature over one week. Elemental analysis, calcd (%) for C₃₁H₃₇Br₃CrN₅: C 48.27, H 4.83, N 9.08; found: C 48.14, H 5.60, N 8.77.

General Procedure for Ethylene Oligomerization and
Polymerization Catalysis

The catalytic reactions were performed in a magnetically stirred (1200 rpm) 145 mL stainless steel autoclave. A 125 mL glass container was used to avoid corrosion of the autoclave walls. The precatalyst solution was prepared by dissolving 1×10⁻⁵ mol of the complex in chlorobenzene. This solution was injected into the reactor under an ethylene flux, followed by the cocatalyst solution (400 equiv. of MAO in toluene). After injection of the catalyst and cocatalyst solutions under a constant low flow of ethylene, which is considered as the t₀ time, the reactor was immediately pressurized to 10 bar of ethylene. The 10 bar working pressure was maintained through a continuous feed of ethylene from a bottle placed on a balance to allow monitoring of the ethylene uptake. The reaction mixture was stirred for the given reaction time. At the end of each test, a dry ice bath was used to rapidly cool the reactor. When the inner temperature reached 0 °C, the ice bath was removed, allowing the temperature to slowly rise to 18 °C. The gaseous phase was then transferred into a 10 L polyethylene tank filled with water. An aliquot of this gaseous phase was transferred into a Schlenk flask, previously evacuated, for GC analysis. The amount of ethylene consumed was thus determined by differential weighting of the bottle (accuracy of the scale: 0.001 g). To this amount of ethylene, the remaining ethylene (calculated using the GC analysis) in the gaseous phase was subtracted. The reaction mixture in the reactor was quenched in situ by the addition of ethanol (10 mL), transferred into a Schlenk flask, and separated from the metal complexes by trap-to-trap evaporation into a second Schlenk flask previously immersed in liquid N₂ to avoid loss of product for GC analysis. The liquid phase was analyzed by GC, whereas the solid phase (when polymers are formed) was washed several times with a 1:1 mixture of ethanol/HCl before being weighted. Analysis of the solid polyethylene phase by GPC or NMR was not possible due to the poor solubility of the former in commonly utilized solvents. Each catalytic test was performed at least twice to ensure the reproducibility of the results.
**Supporting Information**

General experimental guidelines, methodology and spectroscopic data, crystallographic data and computational details are given in the Supporting Information. The authors have cited additional references within the Supporting Information.

**Acknowledgements**

The Special Accounts for Research Grants (S.A.R.G.) of the National and Kapodistrian University of Athens (NKUA) for the provision of partial support, access to the single crystal X-ray Diffractometer of the Core Facility at NKUA and the HPC of the University of Strasbourg for computational time are gratefully acknowledged. We thank the CNRS and the MESRI (Paris) for support (P.B., D.A., K. P.). We also thank Dr. E. Sakki for carrying out mass spectroscopic measurements and Fani Kournavou for the design of the Table of Contents/Abstract graphic.

**Conflict of Interests**

The authors declare no conflict of interest.

**Data Availability Statement**

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords:** Pincer • Nickel • Chromium • Oligomerization • N-heterocyclic Carbene

[1] a) D. Pugh, A. A. Danopoulos, *Coord. Chem. Rev.* **2007**, *251*, 610-641; b) D. Zhang, G. Zi, *Chem. Soc. Rev.* **2015**, *44*, 1898-1921; c) R. E. Andrew, L. González-Sebastián, A. B. Chaplin, *Dalton Trans.* **2016**, *45*, 1299-1305; d) Y. Liu, P. Persson, V. Sundström, K. Wärnmark, *Acc. Chem. Res.* **2016**, *49*, 1477-1485; e) S. Hameury, P. de Frémont, P. Braunstein, *Chem. Soc. Rev.* **2017**, *46*, 632-733; f) V. Charra, P. de Frémont, P. Braunstein, *Coord. Chem. Rev.* **2017**, *341*, 53-176; g) A. A. Danopoulos, T. Simler, P. Braunstein, *Chem. Rev.* **2019**, *119*, 3730-3961; h) R. Arevalo, P. J. Chirik, *J. Am. Chem. Soc.* **2019**, *141*, 9106-9123; i) Q. Liang, D. Song, *Chem. Soc. Rev.* **2020**, *49*, 1209-1232; j) V. Arora, H. Narjinari, P. G. Nandi, A. Kumar, *Dalton Trans.* **2021**, *50*, 3394-3428; k) Y. Wang, B. Zhang, S. Guo, *Eur. J. Inorg. Chem.* **2021**, *2021*, 188-204; l) A. A. Danopoulos, T. Simler, P. Braunstein, in "Comprehensive Organometallic Chemistry IV" (COMC-IV), K. Meyer, D. O'Hare, G. Parkin (Editors-in-Chief); T. H. Warren (Section Editor), Elsevier, **2022**, Vol. 7.11, pp 632-758; m) I. Ligielli, A. A. Danopoulos, P. Braunstein, T. Simler, in "Comprehensive Organometallic Chemistry IV" (COMC-IV), K. Meyer, D. O'Hare, G. Parkin (Editors-in-Chief); T. H. Warren (Section Editor), Elsevier, **2022**, Vol. 8.07, pp 427-574; n) F. He, K. P. Zois, D. Tzeli, A. A. Danopoulos, P. Braunstein, *Coord. Chem. Rev.* **2024**, *514*, 215757.

[2] a) A. Melaiye, R. S. Simons, A. Milsted, F. Pingitore, C. Wesdemiotis, C. A. Tessier, W. J. Youngs, *J. Med. Chem.* **2004**, *47*, 973-977; b) D. van der Westhuizen, C. A. Slabber, M. A. Fernandes, D. F. Joubert, G. Kleinhans, C. J. van der Westhuizen, A. Stander, O. Q. Munro, D. I. Bezuidenhout, *Chem. Eur. J.* **2021**, *27*, 8295-8307.

[3] a) H. V. Huynh, *Chem. Rev.* **2018**, *118*, 9457-9492; b) D. Munz, *Organometallics* **2018**, *37*, 275-289; c) L. Falivene, Z. Cao, A. Petta, L. Serra, A. Poater, R. Oliva, V. Scarano, L. Cavallo, *Nat. Chem.* **2019**, *11*, 872-879.

[4] D. M. Andrada, N. Holzmann, T. Hamadi, G. Frenking, *Beilstein J. Org. Chem.* **2015**, *11*, 2727-2736.

[5] a) M. Melaimi, R. Jazzar, M. Soleilhavoup, G. Bertrand, *Angew. Chem. Int. Ed.* **2017**, *56*, 10046-10068; b) A. Vivancos, C. Segarra, M. Albrecht, *Chem. Rev.* **2018**, *118*, 9493-9586.

[6] a) A. Eizawa, K. Arashiba, H. Tanaka, S. Kuriyama, Y. Matsuo, K. Nakajima, K. Yoshizawa, Y. Nishibayashi, *Nat. Commun.* **2017**, *8*, 14874; b) A. Egi, H. Tanaka, A. Konomi, Y. Nishibayashi, K. Yoshizawa, *Eur. J. Inorg. Chem.* **2020**, *2020*, 1490-1498; c) S. Takaoka, A. Eizawa, S. Kusumoto, K. Nakajima, Y. Nishibayashi, K. Nozaki, *Organometallics* **2018**, *37*, 3001-3009.

[7] R. Maity, B. Sarkar, *JACS Au* **2022**, *2*, 22-57.

[8] T.-a. Koizumi, T. Tomon, K. Tanaka, *Organometallics* **2003**, *22*, 970-975.

[9] J. Long, D. M. Lyubov, G. A. Gurina, Y. V. Nelyubina, F. Salles, Y. Guari, J. Larionova, A. A. Trifonov, *Inorg. Chem.* **2022**, *61*, 1264-1269.

[10] a) V. Friese, S. Nag, J. Wang, M.-P. Santoni, A. Rodrigue-Witchel, G. S. Hanan, F. Schaper, *Eur. J. Inorg. Chem.* **2011**, *2011*, 39-44; b) J. A. Thagfi, G. G. Lavoie, *Organometallics* **2012**, *31*, 7351-7358; c) H. Z. Kaplan, B. Li, J. A. Byers, *Organometallics* **2012**, *31*, 7343-7350; d) R. M. Brown, J. Borau Garcia, J. Valjus, C. J. Roberts, H. M. Tuononen, M. Parvez, R. Roesler, *Angew. Chem. Int. Ed.* **2015**, *54*, 6274-6277; e) Y. Jiang, C. Gendy, R. Roesler, *Organometallics* **2018**, *37*, 1123-1132.

[11] F. He, C. Gourlaouen, H. Pang, P. Braunstein, *Chem. Eur. J.* **2022**, *28*, e202104234.

[12] R. P. Yu, J. M. Darmon, C. Milsmann, G. W. Margulieux, S. C. E. Stieber, S. DeBeer, P. J. Chirik, *J. Am. Chem. Soc.* **2013**, *135*, 13168-13184.

[13] a) T. Simler, P. Braunstein, A. A. Danopoulos, *Organometallics* **2016**, *35*, 4044-4049; b) X. Ren, C. Gourlaouen, M. Wesolek, P. Braunstein, *Angew. Chem. Int. Ed.* **2017**, *56*, 12557-12560; c) Y. Li, K. Pelzer, D. Sechet, G. Creste, D. Matt, P. Braunstein, D. Armspach, *Dalton Trans.* **2022**, *51*, 11226-11230; d) F. He, C. Gourlaouen, H. Pang, P. Braunstein, *Chem. Eur. J.* **2022**, *28*, e202104234.

[14] Deposition Numbers 2328279 (for 4), 2328280 (for 5b) 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.

[15] G. Leniec, *ACS Appl. Opt. Mater.* **2023**, *1*, 1114-1121.

[16] L. Yang, D. R. Powell, R. P. Houser, *Dalton Trans.* **2007**, 955-964.

[17] K. Inamoto, J.-i. Kuroda, K. Hiroya, Y. Noda, M. Watanabe, T. Sakamoto, *Organometallics* **2006**, *25*, 3095-3098.

[18] D. Pugh, A. Boyle, A. A. Danopoulos, *Dalton Trans.* **2008**, 1087-1094.

[19] K. Inamoto, J.-i. Kuroda, E. Kwon, K. Hiroya, T. Doi, *J. Organomet. Chem.* **2009**, *694*, 389-396.

[20] D. H. Brown, B. W. Skelton, *Dalton Trans.* **2011**, *40*, 8849-8858.

[21] D. S. McGuinness, V. C. Gibson, D. F. Wass, J. W. Steed, *J. Am. Chem. Soc.* **2003**, *125*, 12716-12717.

[22] D. Pugh, J. A. Wright, S. Freeman, A. A. Danopoulos, *Dalton Trans.* **2006**, 775-782.

[23] a) T. Agapie, *Coord. Chem. Rev.* **2011**, *255*, 861-880; b) H. Olivier-Bourbigou, P. A. R. Breuil, L. Magna, T. Michel, M. F. Espada Pastor, D. Delcroix, *Chem. Rev.* **2020**, *120*, 7919-7983; c) N. Patel, V. Valodkar, G. Tembe, *Polym. Chem.* **2023**, *14*, 2542-2571.

[24] D. S. McGuinness, V. C. Gibson, J. W. Steed, *Organometallics* **2004**, *23*, 6288-6292.

[25] V. C. Gibson, S. K. Spitzmesser, *Chem. Rev.* **2003**, *103*, 283-316.

[26] a) A. A. Danopoulos, N. Tsoureas, J. C. Green, M. B. Hursthouse, *Chem. Commun.* **2003**, 756-757; b) C. Romain, D. Specklin, K. Miqueu, J.-M. Sotiropoulos, C. Fliedel, S. Bellemin-Laponnaz, S. Dagorne, *Organometallics* **2015**, *34*, 4854-4863; c) E. Despagnet-Ayoub, M. K. Takase, J. A. Labinger, J. E. Bercaw, *J. Am. Chem. Soc.* **2015**, *137*, 10500-10503.

Manuscript received: February 16, 2024

Revised manuscript received: April 14, 2024

Accepted manuscript online: April 15, 2024

Version of record online: May 22, 2024