# Transition metal (Rh and Fe) complexes and main-group (Se and B) adducts with N,N'-diphosphanyl NHC ligands: a study of stereoelectronic properties

Pengfei Ai, Andreas A Danopoulos, Pierre Braunstein

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Pengfei Ai, Andreas A Danopoulos, Pierre Braunstein. Transition metal (Rh and Fe) complexes and main-group (Se and B) adducts with N,N'-diphosphanyl NHC ligands: a study of stereoelectronic properties. Dalton Transactions, 2016, 45 (11), pp.4771 - 4779. 10.1039/c6dt00318d. hal-05020253

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4771

# Transition metal (Rh and Fe) complexes and main-group (Se and B) adducts with N,N'-diphosphanyl NHC ligands: a study of stereoelectronic properties†

Pengfei Ai,ª Andreas A. Danopoulos*ª,ᵇ and Pierre Braunstein*ª

Attempts to evaluate experimentally the donor characteristics of the N,N'-bis(di-tert-butylphosphanyl)-imidazole-2-ylidene (PCNHC P) hybrid ligand are described. Thus, reactions of PCNHC P with [Rh(µ-Cl)(COD)]₂ and [Rh(µ-Cl)(CO)₂]₂ led to the formation of the mononuclear and dinuclear complexes, [Rh(PCNHC P,κP,κCₙH₄)₂Cl](PCNHC P-RhCl) and [Rh(CO)(PCNHC,κP,κCₙH₄,κN)]₂ (PCNHC-RhCO), respectively, the latter resulting after *in situ* cleavage of one (t-Bu)₂P-Nimid bond of PCNHC P. With ligands acting as a P,C-chelate, a straightforward evaluation of the Tolman electronic parameter (TEP) of the CNHC donor is problematical; the viability of dangling P- and bound CNHC-donors (i.e. κCₙH₄) has been observed in the trinuclear Fe(ii) chain complex [Fe₃Cl₂(µ-Cl)₄(THF)₂(PCNHC P,κCₙH₄)₂] (PCNHC P-Fe), obtained by the reaction of PCNHC P with [Fe₄Cl₈(THF)₆] and, recently, established on Cr¹¹, Co¹¹ and Au¹ centers. Evaluation of the π-accepting properties of the PCNHC P (and the related Dipp-PCNHC) was based on the ⁷⁷Se NMR chemical shifts of the corresponding NHC-Se adducts, PCNHC P-Se (and Dipp-PCNHC-Se), which were prepared from the free PCNHC P (and Dipp-PCNHC) and Se. The π-acidity of PCNHC P is found to be higher than that of Dipp-PCNHC but lower than that of SIPr. The donor ability of the CNHC in PCNHC P was explored by its reaction with the Lewis acids tris(pentafluorophenyl)borane (B(C₆F₅)₃) and tris (pentafluorophenyl)boroxine ([C₆F₅)BO]₃), which resulted in stable donor-acceptor adducts with no FLP reactivity. The steric properties of PCNHC P and Dipp-PCNHC are conformation dependent, with the percent buried volume (%Vbur) of PCNHC P in the structurally characterised conformer calculated at 67.6, the largest value currently reported for NHC ligands.

Received 22nd January 2016,
Accepted 2nd February 2016

DOI: 10.1039/c6dt00318d

www.rsc.org/dalton

## Introduction

N-heterocyclic carbene (NHC) ligands have become ubiquitous in molecular chemistry and find extensive applications in e.g. catalytic transformations, medicinal chemistry and material sciences.¹ Both experimental and theoretical studies on their electronic properties have established synergism resulting from their strong σ donor and weaker π acceptor properties. The most commonly used method for the experimental evaluation of the donor properties of NHCs is the Tolman electronic parameter (TEP), originally developed for phosphines by Tolman in 1970,² which is based on the value of the A₁ν(CO) infrared-stretching frequencies in [Ni(CO)₃L], or the average

ν(CO) frequencies in cis-[Ir(CO)₂CIL] and cis-[Rh(CO)₂CIL] (Chart 1), the latter two complexes being more widely used nowadays owing to the toxicity of [Ni(CO)₄].²,³ However, the TEP quantifies the overall NHC electronic properties and does not separate the σ donation and π back-donation components.

Recently, Bertrand and coll.⁴ and Ganter and coll.⁵ separately reported the use of phosphinidene (NHC)PPh and selenium adducts (NHC)Se (Chart 1) to probe the NHC π back-bonding ability, on the basis of the δₚ chemical shift in the ³¹P NMR and the δₚSe chemical shift in the ⁷⁷Se NMR, respectively. These adducts can be represented by two limiting canonical structures: the polarized form A with a NHC-E (E = PPh or Se) single bond indicating little π accepting ability and the resonance form B with a NHC-E (E = PPh or Se) double bond consistent with a high π accepting ability of NHCs. In contrast to the narrow range of TEP values, the chemical shifts of the phosphinidene/selenium adducts are very sensitive to subtle changes of the electronic structures of NHCs and offer a finer grading of the NHCs.

Various groups have recently reported N-phosphanyl-func- tionalized NHC ligands, which feature the two adjacent, strong σ-donors groups linked by a direct P-N bond.⁶ Because of the

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

<footnote>ªLaboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France.
E-mail: danopoulos@unistra.fr, braunstein@unistra.fr</footnote>

<footnote>ᵇInstitute for Advanced Study, USIAS, Université de Strasbourg, France</footnote>

<footnote>†Electronic supplementary information (ESI) available: Crystal data for PCP-RhCl·2CH₂Cl₂, PCNHC-RhCO, PCNHC P-Fe, PCNHC P-Se, PCNHC P-B1·THF and PCNHC P-B2-toluene. CCDC 1449234–1449239. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt00318d</footnote>
<figure><img src="image_4.png" /><figcaption>**Chart 1** Three experimental methods used to evaluate the electronic properties of NHC ligands.</figcaption></figure>

rigidity and ease of tuning of their steric and electronic properties, they have not only been used as bridging ligands for $d^{10}$ coinage metals but also as small bite angle (ca. 68.2°) chelating ligands for platinum group or late transition metals.⁶⁻⁹ We have recently reported the synthesis of the new tridentate ligand $N,N'$-bis(di-tert-butylphosphanyl)-imidazole-2-ylidene ($PC_{NHC}P$) (Scheme 1) and its behaviour as bridging ligand with coinage metals or palladium, resulting in polynuclear complexes,⁶,⁷,⁸ and as $PC_{NHC}$ chelating ligand with chromium or palladium.⁸,⁹ However, a more precise experimental estimate of the stereoelectronic properties of $C_{NHC}$ donor in the $PC_{NHC}P$ ligand is not straightforward, due to the high nucleophilicity of both donors and the propensity of formation of small ring chelates with the metals used as probes for the TEP. Herein, we describe Rh(i) carbonyl complexes with $PC_{NHC}P$ which display $κP$, $κC_{NHC}$ coordination, thus rendering the evaluation of the TEP cumbersome; in contrast, the selective access of NHC-Se adducts permits the evaluation of the $π$ accepting ability of the $C_{NHC}$ in $PC_{NHC}P$; furthermore we give an estimate of the steric bulk of $PC_{NHC}P$, and describe adducts with $B(C_6F_5)_3$ and $FeCl_2$ in which $C_{NHC}$ coordination predominates.

## Results and discussion

### Mono- and dinuclear rhodium complexes and trinuclear iron complex

Since no rhodium complex with the $PC_{NHC}P$ has yet been prepared, with the objective to prepare cis-[RhCl(COD)$_2$($PC_{NHC}P$)] we firstly reacted $PC_{NHC}P$ with $[Rh(μ-Cl)(COD)]_2$ in THF in a 1:1 ligand/Rh ratio (Scheme 1). However, instead of the expected adduct $[RhCl(COD)(PC_{NHC}P,κC_{NHC})]$, the mononuclear complex $[Rh(PC_{NHC}P,κP,κC_{NHC})_2]Cl$ ($PC_{NHC}P-RhCl$) was isolated, the two $PC_{NHC}P$ ligands acting as $P_2C$-chelates. This was confirmed by single-crystal X-ray diffraction analysis (Fig. 1).

The unit cell of $PC_{NHC}P-RhCl$ contains two similar complex cations with slightly different metrical data and isolated chloride anions (Rh-Cl distance 7.418/7.359 Å). In each cation, the tetracoordinated Rh atom occupies a centre of symmetry and is chelated by two $PC_{NHC}P$ ligands through their $C_{NHC}$ and one phosphorus donor, with a bite angle of 67.26(11)/67.31(13)°. The solution $^{31}$P{$^1$H} NMR spectrum of $PC_{NHC}P-RhCl$ contains a doublet at $δ$ 113.1 ($^{1}$J(P-Rh) = 146.9 Hz) and a singlet at $δ$ 83.8,

<figure><img src="image_5.png" /><figcaption>**Scheme 1** Reactions of $PC_{NHC}P$ with $[Rh(μ-Cl)(COD)]_2$, $[Rh(μ-Cl)(CO)_2]$ and $[Fe_4Cl_8(THF)_6]$.</figcaption></figure>
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 1** Thermal ellipsoid representation (30% probability level) of the structure of one cation in $PC_{NHC}P-RhCl$. H atoms, the *t*-Bu methyl groups, the second complex cation and the chloride anions are omitted for clarity. Selected bond lengths (Å) and angles [°] (the two values are for the two independent cations in the unit cell): Rh1–C1 2.077(4)/2.082(4); Rh1–P1 2.300(1)/2.304(1), C1–Rh1–P1 67.26(11)/67.31(13).</figcaption></figure>

corresponding to the coordinated and the uncoordinated phosphines, respectively. The latter value is slightly different from that of the free $PC_{NHC}P$ (δ 98.7),⁶⁻⁹ indicating electronic communication between the P donors on coordination.

In view of the lability of the COD ligand that prevented the preparation of [RhCl(COD)(PC$_{NHC}P$)], the complex [Rh(µ-Cl)(CO)$_2$]$_2$ was used as precursor and was directly reacted with PC$_{NHC}P$ in THF. However, instead of the expected cis-[RhCl(CO)$_2$(PC$_{NHC}P$,κC$_{NHC}$)], the neutral, dinuclear complex [Rh(CO)(PC$_{NHC}$,κP,κC$_{NHC}$,κN)]$_2$ (PC$_{NHC}$-RhCO) containing two monovalent PC$_{NHC}$ ligands was obtained after cleavage of one (t-Bu)$_2$P-N$_{imid}$ bond of PC$_{NHC}P$ (Scheme 1). The formation of (t-Bu)$_2$PCl was evidenced in the reaction mixture as the only other P-containing product (δ 145 in $ ^{31}$P{$^{1}$H} NMR). A similar P-N bond cleavage reaction has also been observed when PC$_{NHC}P$ was reacted with [AuCl(tht)] and rationalised by the reactivity of the P-N bond toward the nucleophilic chloride ligand.⁶⁻⁹ The $ ^{31}$P{$^{1}$H} NMR spectrum of PC$_{NHC}$-RhCO contains a doublet at δ 128.4 ($ ^{1}$J(P-Rh) = 142.8 Hz). The structure of this complex exhibits an approximate C$_2$ symmetry and the square-planar environment around each Rh atom contains one chelating PC$_{NHC}$ ligand, bonded through the P and C$_{NHC}$ 2 electron donors which form an angle of 67.3(3)/67.2(3)°, a negatively charged N atom from the other PC$_{NHC}$ ligand and one carbonyl ligand trans to C$_{NHC}$ (Fig. 2). Related dinuclear rhodium complexes with bridging arylimidazolide-N$^3$,C$^2$ ligands containing a central six-membered ring have been recently reported.¹⁰

Since PC$_{NHC}$ chelation was observed here with PC$_{NHC}$P-RhCl and previously with Pd(II) and Cr(III) centres,⁸,⁹ but not with Cr(II) or Co(II),⁹,¹¹ we wondered whether the κC$_{NHC}$ coordination mode is more common with 3d metals. Thus, reaction of PC$_{NHC}P$ with [Fe$_4$Cl$_8$(THF)$_6$] in THF inside a glovebox

<figure><img src="image_8.png" /><figcaption>**Fig. 2** Thermal ellipsoid representation (30% probability level) of the structure of PC$_{NHC}$-RhCO. H atoms and the *t*-Bu methyl groups are omitted for clarity. Selected bond lengths (Å) and angles [°]: Rh1–C1 2.071(10), Rh2–C12 2.080(9), Rh1–C23 1.833(10), Rh2–C24 1.842(11), Rh1–N4 2.080(8), Rh2–N2 2.065(8), Rh1–P1 2.249(3), Rh2–P2 2.243(3), C23–O1 1.166(12), C24–O2 1.152(12); C1–Rh1–P1 67.3(3), C12–Rh2–P2 67.2(3).</figcaption></figure>

(Scheme 1) afforded colourless, paramagnetic, air-sensitive crystals after crystallisation by addition of pentane. Single-crystal X-ray diffraction established the formation of a centrosymmetric trinuclear Fe(II) chain complex [Fe$_3$Cl$_2$(µ-Cl)$_4$(THF)$_2$(PC$_{NHC}P$,κC$_{NHC}$)$_2$] (PC$_{NHC}P$-Fe) in which each outer Fe atom is coordinated to the C$_{NHC}$ donor of one PC$_{NHC}P$ ligand, with both P donors remaining uncoordinated (Fig. 3). The linear metal chain is assembled by two µ-chlorido ligands that connect the outer Fe atoms to the central Fe2. The outer Fe atoms adopt a tetrahedral coordination geometry consisting of three chloride ligands and one C$_{NHC}$ donor while the central iron is in an octahedral environment with four chloride ligands occupying equatorial positions and two THF molecules the axial positions. The higher occurrence of this coordination mode with 3d metals may be rationalised by the reduced M-P bond energies and the reduced size and strength of ligand fields of 3d rendering structures with dangling P donors energetically more accessible compared to 4d and 5d metals.

### NHC adducts with main group elements (Se, B)

Based on the results we shifted our attention to alternative adducts that may disclose information on the donor characteristics of the PC$_{NHC}P$. The first attempt focused on the synthesis of phosphinidene adducts of NHCs, which carry information on the π-accepting strength of the NHC precursor.⁴ Two known synthetic routes were explored for the synthesis of the adducts (Scheme 2): (i) the direct reaction of PC$_{NHC}P$ with pentaphenyl-cyclopentaphosphane ((PPh)$_5$); (ii) the reaction of PC$_{NHC}P$ with PhPCl$_2$ followed by reduction with Mg. However, both failed: the former gave no reaction even after heating to 70 °C, and the latter led to the cleavage of a P-N bond of PC$_{NHC}P$ by PhPCl$_2$ with formation of (t-Bu)$_2$PCl.
<figure><img src="image_9.png" /><figcaption>**Fig. 3** Thermal ellipsoid representation (30% probability level) of the structure of PC<sub>NHC</sub>P-Fe. H atoms and the t-Bu methyl groups are omitted for clarity. Selected bond lengths (Å) and angles [°]: Fe1–C1 2.103(6), Fe1–Cl1 2.2587(19), Fe1–Cl2 2.3807(17), Fe1–Cl3 2.4118(18), Fe2–Cl2 2.4781(14), Fe2–Cl3 2.4755(14), Fe2–O1 2.158(4); Fe1–Cl2–Fe2 91.98(5), Fe1–Cl3–Fe2 91.30(5), C1–Fe1–Cl1 112.98(16), C1–Fe1–Cl2 115.81(15), C1–Fe1–Cl3 115.31(16), Cl1–Fe1–Cl2 111.90(7), Cl1–Fe1–Cl3 109.59(6), Cl2–Fe1–Cl3 88.99(6), O1–Fe2–Cl3' 88.33(11).</figcaption></figure>

The second attempt focused on the synthesis of selenium adducts; herein, the π-accepting properties of NHCs precursors can also be correlated with the ⁷⁷Se NMR chemical shift observed.⁵ Thus, reaction of elemental selenium with PC<sub>NHC</sub>P in a 1:1 ratio in THF afforded selectively the desired adduct PC<sub>NHC</sub>P-Se, with intact phosphine groups (Scheme 2), as indicated by a ³¹P NMR shift (s, δ 91.0) (cf. PC<sub>NHC</sub>P δ s, 98.7). The structure of the product was further confirmed by an X-ray diffraction analysis (Fig. 4). The Se–C<sub>NHC</sub> bond distance 1.839(8) Å is intermediate between that of a Se–C single bond (aver. 1.94 Å) and of a typical Se=C double bond (aver. 1.74 Å).¹² Its value is similar to that found in other selenium–NHC adducts.⁵b,¹²a–c Compared to the free carbene PC<sub>NHC</sub>P, the

<caption>**Fig. 4** Thermal ellipsoid representation (30% probability level) of the structure of PC<sub>NHC</sub>P-Se. H atoms and the t-Bu methyl groups are omitted for clarity. Selected bond lengths (Å) and angles [°]: C1–Se1 1.839(8), C1–N1 1.36(1), C1–N2 1.370(9), N1–P1 1.760(7), N2–P2 1.775(7); N1–C1–N2 108.2(6).</caption>

N–C<sub>NHC</sub> bond lengths (1.36(1)/1.370(9) Å, cf. 1.376(1)/1.378(1) Å in PC<sub>NHC</sub>P) are shortened and the N1–C1–N2 angle (108.2(6)°, cf. 102.5(1)° in PC<sub>NHC</sub>P) enlarged.

For comparison with PC<sub>NHC</sub>P-Se, the selenium adduct Dipp-PC<sub>NHC</sub>-Se of a ligand containing only one phosphine group was prepared under similar reaction conditions (Scheme 2). The ⁷⁷Se NMR spectra feature a triplet at δ 166.9 (²J<sub>SEP</sub> = 46.5 Hz) for PC<sub>NHC</sub>P-Se and a doublet at δ 131.1 (²J<sub>SEP</sub> = 40.0 Hz) for Dipp-PC<sub>NHC</sub>-Se. Accordingly, and by comparison with literature data, it can be concluded that the π-acidity of PC<sub>NHC</sub>P is stronger than that of Dipp-PC<sub>NHC</sub> and that both ligands are stronger π-acids than SIMes (δ 110 ⁷⁷Se NMR), but weaker π-acids than SIPr (δ 190 ⁷⁷Se NMR) (Scheme 3).¹³

To further explore the donor abilities of the C<sub>NHC</sub> and P donors in PC<sub>NHC</sub>P, the latter was reacted with tris(pentafluorophenyl)borane B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> in toluene (Scheme 4). The ¹¹B NMR spectrum of the resulting product (PC<sub>NHC</sub>P-B1) shows a single resonance at δ −17.5, which is indicative of a four-coordinate boron centre and suggests the formation of a NHC–borane Lewis adduct with a stable B–C<sub>NHC</sub> bond.

Broad NMR resonances were not only observed for the three pentafluorophenyl groups in the ¹⁹F NMR and ¹³C NMR spectra but also for the phosphine groups (one broad singlet at δ 110.5) and C<sub>NHC</sub> (broad multiplet at δ 177.1). This, together with the singlet at δ 7.69 for the two protons on the imidazolylidene ring, indicates fast free rotation of the B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> moiety around the B–C<sub>NHC</sub> bond. A single-crystal

<figure><img src="image_10.png" /><figcaption>**Scheme 2** Reactions of PC<sub>NHC</sub>P and DIPP-PC<sub>NHC</sub> with PhPC<sub>2</sub>, (PPh)<sub>5</sub> and elemental selenium.</figcaption></figure>

<figure><img src="image_11.png" /></figure>
<figure><img src="image_12.png" /><figcaption>**Scheme 3** Comparative π-acidity of NHC ligands based on the ⁷⁷Se NMR data (in CDCl₃).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Scheme 4** Synthesis of the adducts PC<sub>NHC</sub>P-B-1 and PC<sub>NHC</sub>P-B-2.</figcaption></figure>

X-ray diffraction analysis showed that the three C₆F₅ groups adopt a typical C₃ propeller-type orientation (Fig. 5). For steric reasons, the lone pair on the P atoms is oriented toward the borane moiety.

Unlike other NHC-borane adducts,¹⁴ PC<sub>NHC</sub>P-B1 exhibits no frustrated-Lewis-pair (FLP) reactivity toward H₂ or THF, which is consistent with a rather strong donor-acceptor C<sub>NHC</sub> → B interaction, despite a moderate elongation of this bond (1.688(3) Å) when compared to that in the adduct IPr/B(C₆F₅)₃

(1.663(5) Å)¹⁴ᵃ and in 1,3,4,5-tetramethyl-1,3-imidazole-2-ylidene/B(C₆F₅)₃ (1.640(2) Å).¹⁵

Another donor-acceptor adduct was obtained by reaction of PC<sub>NHC</sub>P with a suspension of tris(pentafluorophenyl)boroxine [[C₆F₅)BO]₃ in toluene (Scheme 4). An immediate reaction occurred with formation of a clear solution. A crystallographic analysis established again the formation of one C<sub>NHC</sub> → B bond in the boroxine adduct PC<sub>NHC</sub>P-B2, leaving the other two B atoms intact (Fig. 6). The B-C<sub>NHC</sub> bond distance 1.670(4) Å

<figure><img src="image_14.png" /><figcaption>**Fig. 5** Thermal ellipsoid representation (30% probability level) of the structure of PCP-B1. H atoms, the t-Bu methyl groups, all the F atoms and one molecule of THF are omitted for clarity. Selected bond lengths (Å) and angles [°]: B1–C1 1.688(3), B1–C20 1.659(3), B1–C26 1.660(3), B1–C32 1.670(3); N1–C1–N2 104.98(15), C20–B1–C26 111.11(16), C20–B1–C32 112.03(16), C26–B1–C32 101.70(15), C20–B1–C1 106.09(15), C26–B1–C1 110.95(16), C32–B1–C1 115.06(15).</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 6** Thermal ellipsoid representation (30% probability level) of the structure of PC<sub>NHC</sub>P-B-2. H atoms, the t-Bu methyl groups, all the F atoms and one molecule of toluene are omitted for clarity. Selected bond lengths (Å) and angles [°]: C1–B1 1.670(4), C20–B2 1.590(4), C26–B3 1.586(4), C32–B1 1.643(4), N1–C1–N2 105.6(2), O1–B1–C1 108.6(2), O3–B1–C1 105.0(2), C32–B1–C1 113.7(2).</figcaption></figure>
is slightly shorter than that in **PC<sub>NHC</sub>P-B1**. Obvious differences were observed between the three B–C<sub>6</sub>F<sub>5</sub> moieties: (i) the B1–C32 bond (1.643(4) Å) is elongated compared to the other two (1.590(4) Å and 1.586(4) Å); (ii) the B2–C20 and B3–C26 bonds are coplanar with the boroxine ring while the B1–C32 bond bends out of the boroxine plane, away from the **PC<sub>NHC</sub>P** ligand with a distance of 1.393 Å between the C23 atom and the boroxine plane. This results from the coordination of B1 to C<sub>NHC</sub> forming a tetrahedral environment around the B1 atom.

The <sup>31</sup>P NMR spectrum shows one singlet at δ 97.6 for the phosphine groups and a doublet is observed in the <sup>1</sup>H NMR spectrum at δ 1.15 (<sup>3</sup>J<sub>HP</sub> = 12.9 Hz) for the *t*-Bu groups, which indicates free rotation of the boroxine moiety around the B–C<sub>NHC</sub> bond. The <sup>11</sup>B NMR spectrum contains a broad signal at δ 26.4 and a singlet at δ −0.6 ppm corresponding to the three- and four-coordinate B atoms, respectively. Similarly to **PC<sub>NHC</sub>P-B1**, it exhibited no frustrated-Lewis-pair (FLP) reactivity toward H<sub>2</sub> or THF.

<table><caption>**Table 1** Percent buried volume (%V<sub>bur</sub>)</caption><thead><tr><th rowspan="2">Compounds</th><th colspan="2">%V<sub>bur</sub> for M-NHC length at</th></tr><tr><th>2.00 Å</th><th>2.28 Å</th></tr></thead><tbody><tr><td><img src="image_0_0.png" alt="A structure with a t-butyl group attached to a phenyl ring, which is attached to a p-tolyl group. The p-tolyl group has a t-butyl group attached to it."></td><td>67.6</td><td>63.1</td></tr><tr><td><img src="image_0_1.png" alt="A structure with a t-butyl group attached to a phenyl ring, which is attached to a p-tolyl group. The p-tolyl group has a t-butyl group attached to it."><br>PC<sub>NHC</sub>P</td><td>32.5</td><td>28.3</td></tr><tr><td><img src="image_0_2.png" alt="A structure with a t-butyl group attached to a phenyl ring, which is attached to a p-tolyl group. The p-tolyl group has a t-butyl group attached to it. The molecule in the center is a selenocysteine residue."></td><td>36.4</td><td>31.7</td></tr><tr><td><img src="image_0_3.png" alt="A structure with a t-butyl group attached to a phenyl ring, which is attached to a p-tolyl group. The p-tolyl group has a t-butyl group attached to it."><br>[CrCl<sub>2</sub>(PC<sub>NHC</sub>P-κC<sub>NHC</sub>)<sub>2</sub>]<sup>9</sup></td><td>39.4</td><td>36.2</td></tr><tr><td><img src="image_0_4.png" alt="A structure with a t-butyl group attached to a phenyl ring, which is attached to a p-tolyl group. The p-tolyl group has a t-butyl group attached to it."><br>Dipp-PC<sub>NHC</sub></td><td>35.8</td><td>31.0</td></tr><tr><td><img src="image_0_5.png" alt="A structure with a t-butyl group attached to a phenyl ring, which is attached to a p-tolyl group. The p-tolyl group has a t-butyl group attached to it."><br>[CrCl<sub>2</sub>(Dipp-PC<sub>NHC</sub>-κC<sub>NHC</sub>)<sub>2</sub>]<sup>9</sup></td><td></td><td></td></tr></tbody></table>

## Percent buried volume of the NHC ligands

Lastly, the steric properties of **PC<sub>NHC</sub>P** and **Dipp-PC<sub>NHC</sub>** were evaluated from the percent buried volume (%V<sub>bur</sub>) using the SambVca software,<sup>16</sup> and metrical data from the crystal structures. The results are compiled in Table 1 assuming a 3.50 Å for the sphere radius, omitted hydrogen atoms and scaled bond radii by 1.17, as recommended. Rotation of the phosphine group around the P–N bond has dramatic effect on the steric environment at the carbene centre, especially for **PC<sub>NHC</sub>P**. The %V<sub>bur</sub> of the free carbene **PC<sub>NHC</sub>P** reaches a value up to 67.6 (M–NHC 2.00 Å), for the conformation observed crystallographically, featuring *anti* lone pair arrangement; this value is the largest value among all reported NHC ligands (Table 1).<sup>17</sup> In the conformation with syn arrangement of the lone pairs which is commonly found on κC<sub>NHC</sub> coordination or adduct formation the %V<sub>bur</sub> decreases to 32.5 for **PC<sub>NHC</sub>P-Se** and 36.4 for [CrCl<sub>2</sub>(**PC<sub>NHC</sub>P**-κC<sub>NHC</sub>)<sub>2</sub>]<sup>9</sup> (M–NHC 2.00 Å). Compared to the %V<sub>bur</sub> values found in **PC<sub>NHC</sub>P**, that for **Dipp-PC<sub>NHC</sub>** is smaller (39.4, M–NHC 2.00 Å) but similar (35.8, M–NHC 2.00 Å) to that in the Cr(II) complexes.

## Conclusions

Some stereoelectronic properties of the hybrid ligands **PC<sub>NHC</sub>P** and **Dipp-PC<sub>NHC</sub>** have been evaluated. Initial attempts to prepare cis-[RhCl(CO)<sub>2</sub>(**PC<sub>NHC</sub>P**-κC<sub>NHC</sub>)] by the reaction of **PC<sub>NHC</sub>P** with [Rh(μ-Cl)(COD)]<sub>2</sub> or [Rh(μ-Cl)(CO)<sub>2</sub>]<sub>2</sub> led instead to the formation of rhodium complexes with a chelating **PC<sub>NHC</sub>P**,κP,κC<sub>NHC</sub> or **PC<sub>NHC</sub>P**,κP,κC<sub>NHC</sub>,κN ligand, and the additional coordination of the phosphine group hampered the evaluation of the TEP of the sole C<sub>NHC</sub> donor. However, the π-accepting properties of **PC<sub>NHC</sub>P** and **Dipp-PC<sub>NHC</sub>** were determined from the <sup>77</sup>Se NMR chemical shift of the corresponding selenium adducts. The σ-donor ability of the carbene donor in **PC<sub>NHC</sub>P** was confirmed by the isolation of two stable donor-acceptor adducts with borane or boroxine. The free carbene donor of **PC<sub>NHC</sub>P** shows the largest value reported for percent buried volume in NHC ligands in the conformation of the free **PC<sub>NHC</sub>P** established crystallographically. Currently, it appears that a coordination mode through the C<sub>NHC</sub> donor (κC<sub>NHC</sub>) is more common with 3d metals.

## Experimental section

### Synthesis and characterisation

**General methods.** All manipulations involving organometallics were performed under argon using standard Schlenk techniques. Solvents were dried using standard methods and distilled under nitrogen prior to use or passed through columns of activated alumina and subsequently purged with nitrogen or argon. <sup>1</sup>H, <sup>13</sup>C{<sup>1</sup>H}, and <sup>31</sup>P{<sup>1</sup>H} NMR spectra were recorded at 298 K, unless otherwise specified, on a Bruker Avance 400, 500 or 600 spectrometer and referenced to the residual solvent resonance (<sup>1</sup>H and <sup>13</sup>C) or external 85% H<sub>3</sub>PO<sub>4</sub>

<figure><img src="image_16.png" /></figure>
<figure><img src="image_17.png" /></figure>

in D₂O (³¹P). The ⁷⁷Se NMR spectra were referenced to external KSeCN in D₂O at a specific concentration with a chemical shift of -329.0 ppm (0.25 mol L⁻¹). Abbreviation used are s = singlet, d = doublet, t = triplet, q = quartet, sept = septet, m = multiplet, dd = doublet of doublets, ddt = doublet of doublet of triplets, dm = doublet of multiplets, br = broad, br d = broad doublet, br m = broad multiplet. Elemental analyses were performed by the “Service de microanalyses”, Université de Strasbourg. The preparations of **PCNHC**P and **Dipp-PCNHC** have been reported before.⁶,⁷,⁹ The precursors [Rh(µ-Cl)(COD)]₂,¹⁸ [Rh₂(µ-Cl)Cl(CO)₄],¹⁹ [(C₆F₅)BO]₃²⁰ and [Fe₄Cl₈(THF)₆]²¹ were prepared according to the literature.

**Synthesis of PCNHC**P-*RhCl*. To a solution of [RhCl(COD)]₂ (0.030 g, 0.061 mmol) in THF (5 ml) was added a solution of **PCNHC**P (0.100 g, 0.281 mmol) in THF (5 ml) at room temperature and the reaction mixture was stirred for 2 h. After evaporation of the volatiles, the residue was washed with pentane (20 ml) and dried under vacuum to give a bright yellow powder (0.100 g, 96%). X-ray quality crystals were obtained by slow diffusion of pentane into its CH₂Cl₂ solution. Analysis: Found (Calcd for C₃₈H₇₆ClN₄P₄Rh) (%): C, 53.57 (53.61), H, 8.89 (9.00), N, 6.75 (6.58). ¹H NMR (400 MHz, CD₂Cl₂): δ 7.15 (d, 2H, ²Jₚₘ = 2.1 Hz, im-*H*), 7.13 (s, 2H, im-*H*), 1.52 (t, 36H, ³Jₚₘ = 7.4 Hz, C(CH₃)₃), 1.21 (d, 36H, ³Jₚₘ = 12.5 Hz, C(CH₃)₃). ¹³C{¹H} NMR (125 MHz, CD₂Cl₂): δ 186.1 (ddt, ²Jₚ₄ = 53.5 Hz, ¹Jₖ-Rh = 40.4 Hz, ²Jₚ₄ = 11.2 Hz, NCN), 124.2 (d, ²Jₚ₄ = 6.5 Hz, im-*C*), 122.2 (im-*C*), 36.8 (q, ¹Jₚ₄ = 2.0 Hz, C(CH₃)₃), 35.2 (d, ¹Jₚ₄ = 29.6 Hz, C(CH₃)₃), 29.5 (q, ²Jₚ₄ = 4.5 Hz, C(CH₃)₃), 29.1 (d, ²Jₚ₄ = 15.7 Hz, C(CH₃)₃). ³¹P{¹H} NMR (161 MHz, CD₂Cl₂): δ 113.1 (d, ¹Jₚ-Rh = 146.9 Hz), 83.8 (s).

**Synthesis of PCNHC**P-*RhCO*. To a solution of [Rh₂(µ-Cl)₂(CO)₄] (0.088 g, 0.226 mmol) in THF (5 ml) was added a solution of **PCNHC**P (0.160 g, 0.449 mmol) in THF (5 ml) at room temperature and the reaction mixture was stirred for 2 h. It was then was concentrated to *ca*. 2 ml and 20 ml pentane were added to precipitate the product. The supernatant was removed by filtration and the solid was dried under vacuum to give a brown powder (0.066 g, 44%). X-ray quality crystals were obtained by slow diffusion of pentane into its THF solution. Analysis: Found (Calcd for C₂₄H₄₀O₂N₄P₂Rh₂) (%): C, 42.01 (42.12), H, 5.79 (5.89), N, 8.43 (8.19). ¹H NMR (400 MHz, CD₂Cl₂): δ 7.04 (t, 2H, ³Jₚₘ = 1.8 Hz, im-*H*), 6.95 (s, 2H, im-*H*), 1.40 (d, 36H, ³Jₚₘ = 15.0 Hz, C(CH₃)₃). ¹³C{¹H} NMR (125 MHz, CD₂Cl₂): δ 194.09 (dd, ¹Jₖ-Rh = 61.9 Hz, ²Jₚ₄ = 12.9 Hz, CO), 177.3 (dd, ¹Jₖ-Rh = 35.8 Hz, ²Jₚ₄ = 24.2 Hz, NCN), 132.3 (d, ³Jₚ₄ = 5.3 Hz, im-*C*), 119.4 (d, ²Jₚ₄ = 7.0 Hz, im-*C*), 37.1 (d, ¹Jₚ₄ = 10.3 Hz, C(CH₃)₃), 28.4 (d, ²Jₚ₄ = 6.6 Hz, C(CH₃)₃). ³¹P{¹H} NMR (161 MHz, CD₂Cl₂): δ 128.4 (d, ¹Jₚ-Rh = 142.8 Hz).

**Synthesis of PCNHC**P-*Fe*. In the glovebox, 2 ml THF was added at room temperature to a solid mixture of **PCNHC**P (0.020 g, 0.056 mmol) and [Fe₄Cl₈(THF)₆] (0.012 g, 0.013 mmol). After it was stirred for 0.5 h, the solution was layered with 10 ml pentane and crystallization for four days yielded colourless and extremely air-sensitive crystals (0.014 g, 67%), which were suitable for X-ray diffraction. Due to the air sensitivity of this complex, satisfactory elemental analyses could not be obtained.

**Synthesis of PCNHC**P-*Se*. To a solution of **PCNHC**P (0.050 g, 0.140 mmol) in THF (5 ml) was added a suspension of selenium (0.011 g, 0.139 mmol) in THF (8 ml) at room temperature and the reaction mixture was stirred for 1 h. After evaporation of the solvent to dryness, the residue was washed with pentane (10 ml) and dried under vacuum to give a white powder (0.057 g, 93%). X-ray quality crystals were obtained by slow diffusion of pentane into its THF solution at -30 °C. Analysis: Found (Calcd for C₁₉H₃₈N₂P₂Se) (%): C, 52.30 (52.41), H, 8.78 (8.80), N, 6.73 (6.43). ¹H NMR (600 MHz, CDCl₃): δ 7.19 (s, im-*H*), 1.25 (d, 36H, ³Jₚₘ = 12.4 Hz, C(CH₃)₃). ¹H NMR (400 MHz, CD₂Cl₂): δ 7.18 (s, im-*H*), 1.22 (d, 36H, ³Jₚₘ = 12.3 Hz, C(CH₃)₃). ¹³C{¹H} NMR (125 MHz, CD₂Cl₂): δ 176.6 (t, ²Jₚ₄ = 34.3 Hz, NCN), 122.6 (t, ²Jₚ₄ = 4.3 Hz, im-*C*), 35.9 (d, ¹Jₚ₄ = 33.7 Hz, C(CH₃)₃), 29.3 (d, ²Jₚ₄ = 17.7 Hz, C(CH₃)₃). ³¹P{¹H} NMR (243 MHz, CDCl₃): δ 91.0 (s). ⁷⁷Se NMR (114 MHz, CDCl₃): δ 166.9 (t, ²Jₚₐₙ = 46.5 Hz).

**Synthesis of Dipp-PCNHC**P-*Se*. A synthetic procedure similar to that for **PCNHC**P-*Se* was applied using **Dipp-PCNHC** (0.050 g, 0.134 mmol) and selenium (0.010 g, 0.127 mmol). Compound **Dipp-PCNHC**P-*Se* was obtained as a white powder (0.055 g, 92%). Analysis: Found (Calcd for C₂₃H₃₇N₂P₂Se) (%): C, 61.01 (61.19), H, 8.20 (8.26), N, 6.41 (6.20). ¹H NMR (600 MHz, CDCl₃): δ 7.44 (t, 1H, ³Jₚₘ = 7.7 Hz, Ar-*H*), 7.31 (dd, 1H, ³Jₚₘ = 2.1 Hz, ⁴Jₚₘ = 0.3 Hz, im-*H*), 7.26 (d, 2H, ³Jₚₘ = 7.7 Hz, Ar-*H*), 6.87 (dd, 1H, ³Jₚₘ = 2.1 Hz, ³Jₚₘ = 1.3 Hz, im-*H*), 2.50 (sept, 2H, ³Jₚₘ = 6.9 Hz, iPr-*CH*), 1.32 (d, 18H, ³Jₚₘ = 12.5 Hz, C(CH₃)₃), 1.28 (d, 6H, ³Jₚₘ = 6.9 Hz, iPr-*CH*), 1.10 (d, 6H, ³Jₚₘ = 6.9 Hz, iPr-*CH*). ¹³C{¹H} NMR (125 MHz, CDCl₃): δ 169.1 (d, ²Jₚ₄ = 36.1 Hz, NCN), 146.1 (Ar-*C*), 134.9 (Ar-*C*), 130.2 (Ar-*CH*), 124.4 (Ar-*CH*), 122.0 (d, ²Jₚ₄ = 7.6 Hz, im-*C*), 121.7 (d, ³Jₚ₄ = 1.3 Hz, im-*C*), 35.9 (d, ¹Jₚ₄ = 33.1 Hz, C(CH₃)₃), 29.3 (d, ²Jₚ₄ = 17.5 Hz, C(CH₃)₃), 29.0 (iPr-*CH*), 24.6 (iPr-*CH*), 23.5 (iPr-*CH*). ³¹P{¹H} NMR (243 MHz, CDCl₃): δ 90.5 (s). ⁷⁷Se NMR (114 MHz, CDCl₃): δ 131.1 (d, ²Jₚₐₙ = 40.0 Hz).

**Synthesis of PCNHC**P-B1. To a solid mixture of free carbene **PCNHC**P (0.035 g, 0.098 mmol) and B(C₆F₅)₃ (0.051 g, 0.100 mmol) was added 3 ml toluene at room temperature and the reaction mixture was stirred for 2 h. After removal of the supernatant by filtration, the residue was washed with pentane (10 ml) and dried under reduced pressure to give a white powder (0.075 g, 87%). X-ray quality crystals were obtained by slow diffusion of pentane into its THF solution. Due to the sensitivity to air, satisfactory elemental analyses could not be obtained. ¹H NMR (400 MHz, d⁸*-THF): δ 7.69 (s, 2H, im-*H*), 1.26 (d, 18H, ³Jₚₘ = 13.1 Hz, C(CH₃)₃), 1.01 (d, 18H, ³Jₚₘ = 12.9 Hz, C(CH₃)₃). ¹³C{¹H} NMR (125 MHz, d⁸*-THF): δ 177.1 (br m, NCN), 149.9 (br d, ¹Jₚ₄ = 235.2 Hz, o-ArCF), 140.4 (br d, Jₚ₄ = 244.0 Hz, p-ArCF), 137.5 (br d, ¹Jₚ₄ = 238.0 Hz, m-ArCF), 128.1 (im-*C*), 123.6 (br, ArCB), 37.0 (d, ¹Jₚ₄ = 30.1 Hz, C(CH₃)₃), 36.7 (d, ¹Jₚ₄ = 31.1 Hz, C(CH₃)₃), 29.4 (d, ²Jₚ₄ = 18.3 Hz, C(CH₃)₃), 29.0 (d, ²Jₚ₄ = 18.5 Hz, C(CH₃)₃). ³¹P{¹H} NMR (161 MHz, d⁸*-THF): δ 110.5 (br). ¹¹B NMR (128 MHz, d⁸*-THF): δ -17.5 (s). ¹⁹F NMR (564 MHz, d⁸*-THF): δ -122.0 (br, 2F, o-ArCF), -125.5 (br, 2F, o-ArCF), -132.1 (br, 2F, o-ArCF), -162.6 (br, 4F, p-ArCF), -168.2 (br, 2F, m-ArCF), -168.7 (br, 3F, p-ArCF), -169.1 (br, 2F, m-ArCF).
<figure><img src="image_18.png" /></figure>

**Synthesis of PCNHC-P-B2.** To a suspension of [(C₆F₅)BO]₃ (0.116 g, 0.199 mmol) in toluene (2 ml) was added a solution of PCNHC-P (0.072 g, 0.202 mmol) in toluene (2 ml) at room temperature and stirring was maintained for 1 h. The reaction mixture was concentrated to 2 ml and 20 ml pentane was added to precipitate the product. After filtration, the residue was dried under reduced pressure to give a white powder (0.120 g, 64%). X-ray quality crystals were obtained by cooling down its toluene solution to -30 °C for two days. Analysis: Found (Calcd for C₃₇H₃₈B₃F₁₅N₂O₃P₂) (%): C, 47.72 (47.37), H, 4.17 (4.08), N, 2.97 (2.99). ¹H NMR (400 MHz, *d*⁸-THF): *δ* 7.66 (s, 2H, *im*-H), 1.15 (d, 36H, ³J<sub>HP</sub> = 12.9 Hz, C(CH₃)₃). ¹³C{¹H} NMR (125 MHz, *d*⁸-THF): *δ* 175.9 (br, NCN), 149.9 (dm, ¹J<sub>CF</sub> = 249.7 Hz, *o*-ArCF), 148.7 (dm, ¹J<sub>CF</sub> = 241.6 Hz, *o*-ArCF-BCNHC), 142.8 (dm, *J*<sub>CF</sub> = 255.3 Hz, *p*-ArCF), 140.3 (dm, ¹J<sub>CF</sub> = 241.4 Hz, *p*-ArCF-BCNHC), 137.8 (dm, ¹J<sub>CF</sub> = 250.7 Hz, overlapping *m*-ArCF and *m*-ArCF-BCNHC), 126.0 (im-*C*), 124.9 (br, ArCB), 109.7 (br, ArCB-CNHC), 35.9 (d, ¹J<sub>CP</sub> = 35.2 Hz, C(CH₃)₃), 29.0 (d, ²J<sub>CP</sub> = 17.7 Hz, C(CH₃)₃). ³¹P{¹H} NMR (161 MHz, *d*⁸-THF): *δ* 97.6 (s). ¹¹B NMR (128 MHz, *d*⁸-THF): *δ* 26.4 (br, ArB), -0.6 (s, ArB-CNHC). ¹⁹F NMR (282 MHz, *d*⁸-THF): *δ* -132.4 (dm, 4F, ³J<sub>FF</sub> = 22.4 Hz, *o*-ArCF), -135.3 (m, 2F, *o*-ArCF-BCNHC), -156.0 (tt, 2F, ³J<sub>FF</sub> = 20.0 Hz, ⁴J<sub>FF</sub> = 3.1 Hz, *p*-ArCF), -164.0 (t, 1F, ³J<sub>FF</sub> = 20.3 Hz, *p*-ArCF-BCNHC), -166.1 (m, 4F, *m*-ArCF), -168.5 (m, 2F, *m*-ArCF-BCNHC).

X-ray crystallography

Summary of the crystal data, data collection and refinement for the structures of PCP-RhCl·2CH₂Cl₂, PCNHC-RhCO, PCNHC-P-Fe, PCNHC-P-Se, PCNHC-P-B-1·THF and PCNHC-P-B-2·toluene are given in Table S1.†

For PCNHC-P-RhCl·2CH₂Cl₂, PCNHC-P-Fe, PCNHC-P-Se, PCNHC-P-B-1·THF and PCNHC-P-B-2·toluene, X-ray diffraction data collection was carried out on a Bruker APEX II DUO Kappa-CCD diffractometer equipped with an Oxford Cryosystem liquid N₂ device, using Mo-Kα radiation (λ = 0.71073 Å). The crystal-detector distance was 38 mm. The cell parameters were determined (APEX2 software)²² from reflections taken from three sets of 12 frames, each at 10s exposure. The structure was solved by direct methods using the program SHELXS-97.²³ The refinement and all further calculations were carried out using SHELXL-97.²⁴ The H-atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on *F*². A semi-empirical absorption correction was applied using SADABS in APEX2.²²

For PCNHC-RhCO, X-ray diffraction data collection was carried out on a Nonius Kappa-CCD diffractometer equipped with an Oxford Cryosystem liquid N₂ device, using Mo-Kα radiation (λ = 0.71073 Å). The crystal-detector distance was 36 mm. The cell parameters were determined (Denzo software)²⁵ from reflections taken from one set of 10 frames (1.0° steps in phi angle), each at 20 s exposure. The structures were solved by direct methods using the program SHELXS-97.²³ The refinement and all further calculations were carried out using

SHELXL-97.²⁴ The H-atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on *F*². A semi-empirical absorption correction was applied using MULscanABS in PLATON.²⁶

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

The USIAS, CNRS, Région Alsace and Communauté Urbaine de Strasbourg are acknowledged for the award of fellowships and a Gutenberg Excellence Chair (2010–11) to AAD. We thank the CNRS and the MESR (Paris) for funding and Dr L. Karmazin and Miss C. Bailly (Service de Radiocristallographie, UdS) for the determination of the crystal structures. We are grateful to the China Scholarship Council for a PhD grant to P.A.

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