# Synthesis and Coordination of a Bisphosphine-[NHC-borane] Compound: A Ligand Framework for Bimetallic Structure Featuring a Boron-Bridging Moiety

Aurèle Camy, Laure Vendier, Christian Bijani, Israel Fernández, Sébastien Bontemps

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Aurèle Camy, Laure Vendier, Christian Bijani, Israel Fernández, Sébastien Bontemps. Synthesis and Coordination of a Bisphosphine-[NHC-borane] Compound: A Ligand Framework for Bimetallic Structure Featuring a Boron-Bridging Moiety. Inorganic Chemistry, 2023, 62 (23), pp.9035-9043. 10.1021/acs.inorgchem.3c00785. hal-04169429

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# Synthesis and coordination of a bisphosphine-[NHC-borane] compound: a ligand framework for bimetallic structure featuring a boron-bridging moiety

Aurèle Camy,ª Laure Vendier,ª Christian Bijani,ª Israel Fernández*ᵇ and Sébastien Bontemps*ᵃ

<sup>a</sup>LCC-CNRS, Université de Toulouse, CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 04, France

<sup>b</sup> Departamento de Química Orgánica I and Centro de Innovación en Química Avanzada (ORFEO—CINQA), Facultad de Ciencias Químicas, Universidad Complutense de Madrid 28040 Madrid (Spain)

**ABSTRACT.** We report herein the synthesis of a bisphosphine-[NHC-BH₃] compound and its coordination to gold. The ligand is shown to support a bimetallic structure bisphosphine-[NHC-BH₃](AuCl)₂. The abstraction of one chloride from the gold metal center triggers the activation of the BH₃ fragment leading to the reductive elimination of H₂ and the formation of a dicationic Au₄²⁺ complex featuring Au centers at the +0.5 oxidation state, via a (µ-H)Au₂ intermediate, characterized *in situ* at 183 K. The reactivity of the Au₄ with thiophenol led to the re-oxidation of the gold metal centers to a (µ-S(Ph))Au₂ complex. In the different complexes, the borane moiety was shown to bridge Au₂ core via weak interaction with [BH], [BCl] and [BH₂] moieties.

## INTRODUCTION
In the field of ligand design, elegant strategies were developed for the preparation of bimetallic structures.<sup>1-8</sup> Defined molecular multimetallic systems are indeed of high interest to understand and eventually emulate the activity of polymetallic enzymatic or heterogeneous catalysts. In some cases, the ligand framework supporting bimetallic structures incorporates one moiety bridging the two metal centres.<sup>9-13</sup> This feature serves both as a structuring site and also as an additional reactive center. O, S, N and C atoms are usual bridging elements, found either in metalloenzymes<sup>14-18</sup> and/or in heterogeneous catalysts (Chart 1a).<sup>19-20</sup>

Although boron is not known to be a bridging element in metalloenzymes, it constitutes a specific and powerful doping agent in various energy materials, notably because of its p-dopant properties.<sup>21-23</sup> We thus believe that developing ligand frameworks supporting bimetallic systems with boron-containing bridging fragments at the molecular level is important (Chart 1a). Moreover, the very versatile behavior of this element in the vicinity of transition metals<sup>24</sup> – as borate (coordinated or not),<sup>25-27</sup> Z-type borane,<sup>28-32</sup> boryl,<sup>33-34</sup> and terminal or bridging borylene<sup>35-36</sup> – should offer a very broad scope of coordination modes in bimetallic systems. To the best of our knowledge, however, only bisphosphine-boryl ligands – initially developed by Yamashita, Nozaki *et al.* as pincer ligands for mononuclear complexes<sup>37-39</sup> – were shown to support bimetallic structures of Ni and Co<sup>12</sup> and recently of Cu.<sup>9</sup> Interestingly, the bimetallic Ni and Co complexes were shown to reversibly react with H<sub>2</sub> via bifunctional activation between the metal centers and the boron-containing moiety, which alternated between boryl and hydroborane forms.

In this context, we aimed at designing a ligand framework that would support a bimetallic structure and feature a central bridging boron-containing moiety (Chart 1b). We selected NHC-borane as the central moiety of the ligand – because of the known ability of carbene to stabilize various boron-containing species<sup>40-41</sup> – with two adjacent phosphine buttresses. We expected to stabilize
the coordination of the central NHC-borane moiety with adjacent phosphine buttresses to expand
the coordination chemistry of carbene-borane compounds. Compared to the wealth of reports of
organic and main group chemistry of carbene-borane,⁴⁰⁻⁴¹ the coordination chemistry of
unsupported carbene-borane is indeed limited to the following examples: i) NHC-BH₃ were shown
to weakly bind Mn, Cr, Mo and W metal centres,⁴² ii) a B-H bond was activated at Fe and W alkyl
precursors,⁴³⁻⁴⁴ iii) NHC-haloborane led to a very original NHC-borido Co complex⁴⁵ and finally
iii) the coordination of carbene-cyanoboryl generated three carbene-boryl Au complexes.⁴⁶⁻⁴⁸

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

<figure><img src="image_3.png" /><figcaption>**Chart 1.** a) Classical bridging moieties in bimetallic complexes, b) Highlights of this work</figcaption></figure>

Herein we present the synthesis of the first compound of this type and its coordination toward
gold. As it will be described, the synthesized bisphosphine-[carbene-borane] is able to support
bimetallic structures. Interestingly, upon chlorine abstraction from Au(I), the BH₃ fragment is
involved in reductive elimination of H₂ at gold leading to the isolation of a Au₄ cluster. The
formation of the cluster, its bonding description and its reactivity toward thiol were studied by
experimental and theoretical means. Particular attention was paid to the versatile coordination
behavior of the borane fragment as a central bridging moiety.

**RESULTS AND DISCUSSION**
The bisphosphine-benzimidazolium compound **1** (Scheme 1) was synthesized according to the literature procedure in five steps with an overall yield of 40%.⁴⁹⁻⁵⁰ In toluene, the addition of KHMDS at room temperature followed by $Me_3N-BH_3$ at 90 °C to compound **1**, led to the generation of compound **2**. In these optimized conditions, compound **2** was isolated in 83% yield on a 3-gram scale (Scheme 1). It is characterized in $tol-d_8$ by a sharp singlet at 16.6 ppm in $^{31}$P{$^1H$} NMR and a quartet at -34.1 ppm ($^{1}$J$_{B-H}$ = 89 Hz) in $^{11}$B NMR analyses. The hydride signal, integrating for three hydrogen atoms, appears as a singlet at 2.13 ppm in $^1H}${$^{11}$B} NMR analysis. The carbenic carbon linked to $BH_3$ is characterized by a broad signal at 183.3 ppm in $^{13}$C{$^1H$} analysis, at the upper limit of the large range (153.3-186.0 ppm) established by Curran *et al*. from an exhaustive literature survey of carbene-borane compounds.⁴⁰

<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Synthesis of compound **2**.</figcaption></figure>

Single crystals suitable for X-Ray diffraction (XRD) analysis were obtained from a concentrated pentane solution (Figure 1a). The compound crystallized in the triclinic space group P-1 with six molecules exhibiting very similar features in the asymmetric unit (Figure S97).⁵¹ As shown in Figure 1a, two dihydrogen bonds⁵² are detected with [C-H···H-B] distances of 2.08 and 2.15 Å, shorter than the sum of Van der Waals radii of 2.65 Å.⁵³ While these types of weak hydrogen interactions have been mostly detected with metal hydride as the H-bond acceptor,⁵⁴ dihydrogen interaction [C-H···H-B] were reported intramolecularly with azolylborane adducts⁵³ and intermolecularly with NHC-BH₃ compounds.⁵⁵ In both cases, the reported hydridic-protic distances are marginally longer than that found in **2** (2.22 and 2.24 Å for the shortest, respectively),
while the reported intermolecular [N–H···H–B] distances of 2.02 Å are significantly shorter in
$NH_3-BH_3$.⁵⁶

The proximity between the hydrogen atoms of methylene and borane moieties was also observed
in solution by NMR spectroscopy. Nuclear Overhauser Effects (NOE) were indeed detected
between these nuclei by ¹H/¹H NOESY experiment at 298 K in tol-d₈ (Figure S11-12) and the non-
equivalence of the methylenic proton was hypothesized based on the broadening of the associated
doublet at 4.81 (d, ²Jₚ-H = 2.6 Hz) at 173 K (Figure S13).

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

**Figure 1.** a) Structure of compound 2. Thermal ellipsoids drawn at 30% probability. Hydrogen atoms are omitted for clarity except in $BH_3$ and $CH_2$ moieties which were located in the difference Fourier map and refined freely without any constraint. Selected bond lengths [Å] and angles [°]: C108-B4 1.604(5), B4-H41 1.121(4), B4-H42 1.107(4), B4-H43 1.105(4), H41···H10L 2.085, H42···H10K 2.150, B4-H41-H10L 109.2, C107-H10K-H42 129.8, B4-H42-H10K 106.1, C109-H10L-H41 131.5; b) Contour map of Laplacian function ($\nabla^2\rho$) of compound 2 with relevant bond paths (black dashed line) and Bond Critical Points (BCPs, green spheres).

These interactions were characterized by Density Functional Theory (DFT) calculations by having
an electrostatic ([B]H and H[C] natural charges of -0.10 and +0.28, respectively) and also a
covalent contribution. The latter is observed in the Contour map of Laplacian function by the
occurrence of a Bond Critical Point (BCP) and a Bond Path (BP) running between the two
hydrogen atoms (Figure 1b) and described in the NBO analysis as the donation from the doubly-
occupied σ(B–H) molecular orbital (MO) to the empty σ*(C–H) MO, with a stabilization energy
value of -2.4 kcal/mol.

With compound **2** in hands, we were eager to probe its ability to support bimetallic structure and to observe the role that the central NHC-BH₃ fragment would play. Compound **2** was thus reacted with two equivalents of AuCl(SMe₂) in THF, producing the bimetallic gold(I) complex **3**, isolated in 83% yield (Scheme 2). This compound is characterized by one singlet at 63.6 ppm in ³¹P{¹H} NMR analysis, indicative of the coordination of both phosphine moieties. Satisfyingly, the BH₃ fragment is retained upon coordination, as indicated by a well-defined quartet at -33.7 ppm in ¹¹B NMR analysis. The chemical shift of the carbenic carbon at 184.2 ppm is very close to the signal of the starting compound **2**.

X-Ray diffraction analysis showed that the complex crystallized in orthorhombic space group Pna2₁ with two molecules in the asymmetric unit of identical geometry. This solid-state analysis confirms the formation of a di-gold complex. The phosphines are linearly coordinated to the Au-Cl fragments (P1-Au1-Cl1 176.82(15)°, P2-Au2-Cl2 173.52(12)°) and these P-Au-Cl segments are both pointing to the same direction, while the central NHC-BH₃ fragment is oriented at the opposite (Figure 2). As in ligand **2**, short hydrogen contacts between the methylene and the borane moieties are observed in the solid-state (2.2, 2.2 and 2.6 Å) as well as in solution, as evidenced by cross-peaks in ¹H/¹H NOESY experiment at 298 K in CD₂Cl₂ (Figures S26).
<figure><img src="image_6.png" /><figcaption>**Scheme 2.** Synthesis of compounds 3 and 4.</figcaption></figure>

Complex 3 was reacted with different chloride abstractors.⁵⁷ Optimized reaction conditions were found with the addition of 1 equivalent of $GaCl_3$ at low temperature leading to the very clean formation of complex 4, isolated in 58% yield. Complex 4 is characterized at 243 K in $CD_3CN$ by two triplets in $ ^{31}P \{ ^{1}H \} $ NMR spectrum at 122.9 and 119.0 ppm ($ ^{3}J_{P-P} = 23.1$ Hz, $ ^{3}J_{P-P} = 24.6$ Hz), suggesting a AA'BB' system. The formation of a cluster of gold atoms was hypothesized to account for i) the presence of four phosphorus nuclei coupling through metal centers and ii) the large phosphorus deshielding compared to the signal of complex 3 ($\delta P = 63.6$) suggesting a change at the metal center. Of note is the characterization of the carbenic carbon at 173.8 ppm in $ ^{13}C \{ ^{1}H \} $ NMR spectrum, almost 10 ppm high-field shifted compared to complex 3.
<figure><img src="image_7.png" /><figcaption>**Figure 2.** Structure of compound 3. Thermal ellipsoids drawn at 30% probability. Co-crystallized CH₂Cl₂ and hydrogen atoms are omitted for clarity except in the BH₃ and CH₂ moieties which were located in the difference Fourier map and refined freely without any constraint. Selected bond lengths [Å] and angles [°]: B1-C1 1.607(16), B1-H1A 1.09(3), B1-H1B 1.09(12), B1-H1C 1.13(12), H1A···H17B 2.6, H1B···H17B 2.2, H1C···H8A 2.2, Au1-P1 2.230(3), Au2-P2 2.231(3), Au1-Cl1 2.288(3), Au2-Cl2 2.284(3), P1-Au1-Cl1 176.82(15), P2-Au2-Cl2 173.52(12), C17-H17B-H1A 107, B1-H1B-H17B 102, C17-H17B-H1B 132, B1-H1C-H8A 106, C8-H8A-H1C 135.</figcaption></figure>

XRD analysis showed that complex 4 is a dicationic Au₄ cluster stabilized by two ligands, which crystallized in a triclinic space group P-1 with one molecule in the asymmetric unit (Figure 3). Two bisphosphine-NHC-borane ligands bridge the Au atoms with a single phosphine moiety per Au and four similar P–Au bond lengths (2.287(3) to 2.290(3) Å). The four Au atoms adopt a nearly tetrahedral geometry (Au-Au-Au angles from 59.408(16) to 60.484(16)°). Although Au₄ clusters are rare in Au cluster chemistry,⁵⁸⁻⁶⁴ the T_d geometry is the usual arrangement adopted by these dicationic species. The Au–Au distances are measured in a narrow range of 2.6979(6) to 2.7274(7)
Å and are rather similar to those in the only two other examples [R₃PAu]₄²⁺ reported to date with P'Bu₃ and PMes₃.⁶²⁻⁶³ While these 50 valence electrons complexes were not synthesized from the P-Au-X precursors, related [R₃PAu]₄X₂²⁺ complexes (R = aryl, X = halogen) were synthesized from the addition of half an equivalent of a halogen abstractor as in the synthesis of complex 4.⁵⁹

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

**Figure 3.** Full (a) and partial (b) structure of compound 4. Thermal ellipsoids drawn at 30% probability. The two GaCl₄ anions and hydrogen atoms are omitted for clarity, except hydrides on boron which were located in the difference Fourier map and refined with some constraints on distances and angles. Because of these constraints, no esd are provided for bond and angles associated with H atoms. Selected bond lengths [Å] and angles [°]: C7-B2 1.613(15), C25-B1 1.643(18), B2-H22 0.969, B2-H21 0.962, B2-Cl2 1.939(12), B1-H12 0.945, B1-H11 0.949, B1-Cl1 1.898(16), H11-Au4 2.549, H11-Au1 3.058, H22-Au3 2.583, H22-Au2 2.979.

Of note, the borane moiety was modified during the reaction, since the borane is not a BH₃ anymore but a BH₂Cl unit. The central borane participates in the coordination because for each ligand, a B–H bond points toward a Au–Au bond. Although short Au–H distances within the sum of van der Waals radii (2.86 Å) are observed (Figure 3b),⁶⁵ the residual density around the cluster
could not be completely resolved (see ESI for details). We thus turned to theoretical calculations
to further discuss the nature – if any – of the [BH] interaction in complex 4. B–H···Au interactions
have indeed been observed in few systems with either tris(mercaptoimidazolyl)borate or
carbaborane ligands, but not investigated further.⁶⁶⁻⁷⁰ The geometry of the cluster 4 was well
reproduced by DFT. Any attempt to incorporate two hydrogen atoms into the Au₄ cluster did not
lead to a minimum. This confirms the loss of one hydrogen atom per ligand in agreement with the
detection of dihydrogen by ¹H NMR analysis accounting for the reduction of the Au centers
(Figure S91). Our calculations show that in complex 4, the four metal centers behave very similarly
with nearly identical bond orders (QTAIM Au-Au delocalization index of ca. 0.37) and electronic
charges (ranging from +0.11 to +0.22), in line with a +0.5 oxidation state for each Au metal center
as calculated by Pyykkö et al. on the parent [AuPH₃]₄²⁺ complex.⁷¹ To gain quantitative insight
into the bonding between the cluster and the ligand, the Energy Decomposition Analysis (EDA)–
Natural Orbitals for Chemical Valence (NOCV) method was employed. Using [P₂(NHC-
BH₂Cl)Au₄]²⁺ and P₂(NHC-BH₂Cl) ligand as fragments, the computed total interaction energy
(ΔE int = -187.2 kcal/mol) indicates a rather strong bonding, which is mainly dominated by the
electrostatic attractions (ΔE elstat = -340.8 kcal/mol) and, to a lesser extent, also by orbital
interactions (ΔE orb = -185.5 kcal/mol). The contribution from dispersion forces (ΔE disp = -55.6
kcal/mol) is comparatively weaker, yet not negligible.

The interaction of the central BH₂Cl moieties with the Au cluster was then evaluated. According
to the NCIPlot method, there exists a stabilizing non-covalent interaction between the BH
fragments and the Au₄ cluster (Figure 4a). In addition, QTAIM analysis confirms the occurrence
of BCPs and associated BPs between the bridging hydride and the two nearest gold atoms (Figure
S99). As shown by the NOCV method, this interaction corresponds to the donation from the
occupied σ(B-H) MO to the σ*(Au-Au) MO with a weak yet noticeable stabilizing energy ΔE<sub>orb</sub>
= -8.6 kcal/mol (Figure 4b). For a more complete description of the main orbital interactions in 4,
see the supporting information (Figure S100).

<figure><img src="image_9.png" /><figcaption>**Figure 4.** (a) Contour plots of the reduced density gradient isosurfaces (density cutoff of 0.04 a.u.) for compound 4. (b) Plot of the deformation density of the pairwise orbital interactions involving the σ(B-H)→σ*(Au-Au) interaction. The color code of the charge flow is red → blue.</figcaption></figure>

The mechanism of the formation of **4** was also investigated to assess the role of the new ligand
platform and in particular of the central NHC-BH₃ moiety. More generally, such investigation
could improve the understanding of Au nanoparticle nucleation processes, since Au nanoparticles
can be generated from borohydride and even NHC-BH₃ reductants.⁷²⁻⁷³ The formation of **4** was
first followed by NMR analyses at room temperature in CD₂Cl₂. In these conditions, ³¹P{¹H} NMR
spectrum recorded after 5 min shows the complete disappearance of complex **3** and the presence
of the two major triplets characterizing complex **4**. In addition, three other sets of two triplets are
observed in the same area (122 to 111 ppm) which disappeared within 3 h, to the benefit of the
signals of complex **4**, as the only signature observed besides a small singlet at 79.9 ppm,
unidentified so far (Figure S90). The sets of triplets likely correspond to other Au₄ clusters, given the similarities of these signals with complex 4, both in terms of chemical shifts in ³¹P and ¹H NMR spectra and in terms of multiplicity of the phosphorus nuclei. We assumed that the BH₂Cl fragment may adopt different conformations toward the Au₄ core, leading to different conformers of 4. In agreement with this assumption, DFT calculations located another conformer lying only 0.8 kcal/mol higher in energy than 4. In this new conformer 4-iso (Figure S98), the B-Cl bond points toward Au-Au bonds for each ligand. In line with this assumption, ROESY experiment showed that the two inequivalent methylene-phosphine buttresses of complex 4 are in dynamic exchange with each other. This suggests an easy rotation around the B–C bond, facilitated by the weak interaction between the borane fragment and the Au₄ core (Figures S39-41).

The initial elementary step of the reaction is presumed to be the chloride abstraction by GaCl₃.

After this step, our calculations pointed to the energetically accessible cationic dinuclear (μ-Cl)Au₂⁺ and also to the parent (μ-H)Au₂⁺ (Scheme 3). The reaction was then investigated at a lower temperature, in order to identify possible intermediates occurring before the formation of 4.

Solutions of both reagents were cooled and mixed in an NMR tube at 193 K. Four singlet signals were observed in ³¹P{¹H} NMR spectrum at 193 K in the range of 82.0 to 72.0 ppm along with a minor signal corresponding to complex 3. While the mixture did not evolve at this temperature or below, raising the temperature to 223 K led to signal broadening. After 50 minutes at 223 K, the temperature was lowered back to 183 K. The signals sharpened and only two remained at 74.9 and 78.7 ppm. The former signal at 74.9 ppm is related to a triplet in ¹H NMR spectrum at 6.62 ppm (²J<sub>P-H</sub> = 92.4 Hz), which resolves into a singlet upon selective ³¹P decoupling (Figure S95). These features are indicative of a proton coupling with two phosphorus signals that we attributed to the (μ-H)Au₂⁺ compound with a calculated δ<sub>Pcalc</sub> of 76.4 ppm close to the experimental value. The
methylene signals associated with this compound are observed along with the signal for the BH₂Cl fragment at 3.9 ppm (δH(calc) = 4.4 ppm) in ¹H NMR analysis. Very few [Ln(µ-H)Au]₂ systems have been reported in the literature and bridging hydrides were not always directly observed.⁷⁴ While bridging hydrides were characterized between 0.42 and -1.95 ppm in [NHC(µ-H)Au]₂ complexes,⁷⁵⁻⁷⁶ a broader range at lower field between 2.83 (²Jₚ-H = 90.9 Hz) and 8.29 (²Jₚ-H = 49 Hz) was observed in case of [(PR₃)ₙ(µ-H)Au]₂ complexes (n = 1 or 2).⁷⁷⁻⁷⁹

The ³¹P chemical shift of (μ-Cl)Au₂⁺ species was calculated to be 78.9 ppm (Scheme 3). By comparison, the signal detected at 78.7 ppm could thus be the signature of this species, although the signals observed initially at 72.1 and 82.0 ppm could also correspond to the bridged (μ-Cl)Au₂⁺ complex.

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

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

<table><thead><tr><th></th><th>(μ-Cl)Au<sub>2</sub><sup>+</sup></th><th>(μ-H)Au<sub>2</sub><sup>+</sup></th></tr></thead><tbody><tr><td>E<sub>rel</sub></td><td>0.0 kcal/mol</td><td>+6.2 kcal/mol</td></tr><tr><td>δ<sub>P</sub>(calc)</td><td>av. 78.9 (81.3, 76.6)</td><td>av. 76.4 (77.7, 75.1)</td></tr><tr><td>δ<sub>P</sub>(exp)</td><td>78.7</td><td>74.9</td></tr><tr><td>δ<sub>H</sub>(exp)</td><td>-</td><td>6.61 (t, ²J<sub>P-H</sub> = 92.4 Hz, μ-H), 3.9 (br, BH<sub>2</sub>Cl)</td></tr></tbody></table>

**Scheme 3.** Intermediates (μ-Cl)Au₂⁺ and (μ-H)Au₂⁺. Selected computed bond lengths [Å]: (μ-Cl)Au₂⁺ Au-Au 3.467, Au-Cl 2.429, Au-HB 4.938; (μ-H)Au₂⁺ Au-Au 2.758, Au-H 1.772, Au-HB 4.670.

When the temperature of the solution was raised to room temperature, these signals disappeared to the benefit of the cluster **4** as the major compound, suggesting that (μ-H)Au₂⁺ complex is an intermediate in the reaction. Since no other intermediate could be observed during the temperature
rise, the last steps involved in the formation of **4** remain to be deciphered. While further
investigations will be necessary to disclose these steps, the reductive elimination of H₂ from a
putative Au₄(H)₂ cluster obtained from dimerization of (μ-H)Au₂⁺ is ruled out i) by our
calculations showing that the addition of H₂ to the final Au₄ complex **4** does not lead to any
minimum and ii) by the observation of H₂ early on at low temperature along with (μ-Cl)Au₂⁺ and
(μ-H)Au₂⁺ complexes and prior to the observation of any Au₄ cluster.

Although complex **4** proved to be sensitive toward air and light, it was stable enough for reactivity
studies. The addition of 2 equivalents of GaCl₃ to the isolated tetragold complex **4** led to the clear
formation of complex **5** (Scheme 4), which could not be isolated, but was *in situ* characterized. In
the ³¹P{¹H} NMR spectrum two triplets were observed at 125.2 and 122.4 ppm (³Jₚ-ₚ = 24.5 Hz),
indicating that the Au₄ structure was conserved, and in the ¹¹B{¹H} NMR spectrum, by a singlet
at -29.4 ppm. These key signals, very similar to those in **4**, suggest minimal structural changes. A
tentative structure of complex **5** is depicted in Scheme 2. We propose that the additional GaCl₃
interacts with the chlorine atom of each BH₂Cl moiety forming a donor-acceptor BCl₃→GaCl₃
complex. In agreement with this hypothesis, this structure was located as a minimum on the
potential energy surface by DFT calculations (Figure S98), which indicate that its formation is
strongly exothermic (4 + 2 GaCl₃ → 5; ΔE = -60.5 kcal/mol).
<figure><img src="image_12.png" /><figcaption>**Scheme 4.** Synthesis of compounds 5 and 6.</figcaption></figure>

Complex 4 was also reacted with thiophenol given the importance of thiol in gold surface chemistry.<sup>80-82</sup> A clean reaction is observed *in situ* leading to the isolation of complex 6 in 86% yield upon H<sub>2</sub> loss. The complex was characterized in solution by a singlet at 77.1 ppm in <sup>31</sup>P({<sup>1</sup>H}) analysis and at -30.2 ppm in <sup>11</sup>B({<sup>1</sup>H}) NMR analysis. Single crystals suitable for X-Ray diffraction analysis were obtained from a mixture of CH<sub>2</sub>Cl<sub>2</sub> and Et<sub>2</sub>O at low temperature. The solid state analysis showed that complex 6 is a bimetallic complex featuring a (μ-S(Ph))Au<sub>2</sub> core stabilized by one bisphosphine-carbene borane ligand (Figure 5a). The formation of this species is also computed to be strongly exothermic according to our DFT calculations (4 + 2 PhSH → 2 6 + H<sub>2</sub>; ΔE = -44.4 kcal/mol). The gold metal centers of complex 4 have thus been re-oxidized formally by 0.5 e- to Au(I) in complex 6 (computed Au-charge of +0.24). This oxidation of a molecular gold cluster by a thiol upon loss of dihydrogen was observed once<sup>83</sup> and is related to the reaction of thiol with Au surfaces,<sup>80-81, 84</sup> while other (μ-S)Au<sub>2</sub> complexes were obtained by other routes.<sup>74</sup>
Interestingly, the BH₂Cl fragment is interacting with the Au centers through two hydrogen-gold interactions as shown by Au1···H1A 2.80(5) Å and Au2···H1B 2.70(5) Å distances at the upper limit of the sum of van der Waals radii (2.86 Å).⁶⁵ BCP (and BP) were indeed found between the B-H and gold atoms and the interaction is described by the NBO method as the donation from the doubly-occupied σ(B-H) MO to the empty σ*(Au-P) MO, with a stabilization energy value of -2.1 kcal/mol.

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

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

**Figure 5.** a) Structure of compound 6. Thermal ellipsoids drawn at 30% probability. GaCl₄ anion and hydrogen atoms are omitted for clarity, except hydrides on boron which were located in the difference Fourier map and refined freely without constraints. Selected bond lengths [Å] and angles [°]: B1-C11 1.610(4), B1-H1A 1.10(5), B1-H1B 1.12(5), B1-C11 1.929(4), Au1-P1 2.2689(7), Au2-P2 2.2658(7), Au1-Au2 3.1263(2), Au1-S1 2.3275(7), Au2-S1 2.3291(7), Au1···H1A 2.80(5), Au2···H1B 2.70(5), Au1-S1-Au2 84.34(2), P1-Au1-S1 176.74(2), P2-Au2-S1 178.79(3); b) Contour map of Laplacian function (∇²ρ) of compound 2 with relevant bond paths (black dashed line) and Bond Critical Points (BCPs, green spheres).

## CONCLUSION

In conclusion, we have developed a new ligand platform that supports bimetallic structures and features a central NHC-borane moiety able to bridge gold centers with different coordination
modes. This versatile coordination behavior likely results from the weak nature of the borane-digold interaction and the easy rotation around the NHC-B bond. Moreover, the borane moiety was shown to participate in the reductive elimination of H₂ to form a Au₄ cluster that was re-oxidized upon thiol reaction. These results further expand the coordination chemistry and the reactivity of NHC-borane compounds.<sup>40-41, 43-44</sup> On-going investigations are directed at exploiting the rich chemistry associated with NHC-borane compounds and at expanding the coordination study to other transition metals.

## AUTHOR INFORMATION

### Corresponding Author

**Israel Fernández** - Departamento de Química Orgánica I and Centro de Innovación en Química Avanzada (ORFEO—CINQA), Facultad de Ciencias Químicas, Universidad Complutense de Madrid 28040 Madrid (Spain); Email: israel@quim.ucm.es

**Sébastien Bontemps** - LCC-CNRS, Université de Toulouse, CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 04, France; Email: sebastien.bontemps@lcc-toulouse.fr

### Author

**Aurèle Camy** - LCC-CNRS, Université de Toulouse, CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 04, France

**Laure Vendier** - LCC-CNRS, Université de Toulouse, CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 04, France

**Christian Bijani** - LCC-CNRS, Université de Toulouse, CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 04, France
## Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

## ASSOCIATED CONTENT

### Supporting Information.

The supporting information are available free of charge. It includes experimental details, compounds synthesis and characterization and DFT method.

## ACKNOWLEDGMENT

AC and SB thank the CNRS and the French Ministry for financial support. IF thanks financial support from the Spanish MCIN/AEI/10.13039/501100011033 (Grants PID2019-106184GB-I00 and RED2018-102387-T). For the purpose of Open Access, a CC-BY public copyright license has been applied by the authors to the present document and will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this submission.

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## Graphic for Table of Contents

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

A unique bisphosphine-(NHC-borane) compound was shown to support bimetallic structure leading to the isolation of a rare Au₄ cluster with a B-H fragment bridging two Au centers.