Crystal structure of tris(2-dicyclohexylphosphino-2′,6′-dimethoxy-1,1′-biphenyl-κP)-μ-oxoethenylidene-triangulo-trigold(I) bis(trifluoromethanesulfonyl)imide

The title compound contains a ketenylidene bridge that caps a tri-gold cluster. This is the first reported tri-gold ketenylidene with atomic distances indicative of bonding interaction between the gold atoms.


Chemical context
Metal clusters containing ketenylidenes are of interest for their wide range of applications. For instance, ketenylidenes are useful for facilitating C-C bond formation and cleavage (Went et al., 1987), metal cluster building (Sailor & Shriver, 1985), and as potential intermediates for carbon monoxide chemistry . One of the first transitionmetal ketenylidene complexes described was a tricobalt cluster reported by Seyferth et al. in 1974(Seyferth et al., 1974. Since then, the scope of ketenylidene clusters has been expanded to include metals such as osmium (Went et al., 1987), ruthenium (Sailor & Shriver, 1985), molybdenum (Ramalakshmi et al., 2015), and manganese (Crespi & Shriver, 1986) to name a few.
However, relatively few ketenylidenes involving gold have been reported. Work by Green and co-workers uncovered a surface-bound gold ketenylidene [Au 2 CCO], which serves as a reactive intermediate in the aerobic oxidation of acetic acid on Au/TiO 2 surfaces (Green et al., 2012). More recently, Daugherty and co-workers reported the first instance of a tri-gold ketenylidene (Daugherty et al., 2017). In that case, the AuÁ Á ÁAu distances suggest that there is no bonding interaction between the metal atoms.
Herein, we describe the first crystal structure analysis of a tri-gold ketenylidene in which the atomic distances suggest a bonding interaction between the gold atoms

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. Four molecules are present in the unit cell (Z = 4) and there is one component in the asymmetric unit. The title ISSN 2056-9890 compound consists of three molecules of (2-dicyclohexylphosphino-2 0 ,6 0 -dimethoxy-1,1 0 -biphenyl)gold(I) bis(trifluoromethanesulfonyl)imide capped by a ketenylidene unit (C C O) to form a tri-gold cluster. The tri-gold cluster has an overall charge of +1, with trifluoromethanesulfonylimide serving as the counter-ion. As shown in Fig. 1, the ketenylidene atoms (C C O) form an angle of 88.1 (5) with the mean Au1-Au2-Au3 gold plane.

Supramolecular features
In the crystal structure of the title compound, the discrete complexes are arranged into columns along the b axis (Fig. 2). Within these columns, the ketenylidene atoms alternate between the +a and Àa-axis directions. The only other similar cluster with Au was reported by Daugherty and co-workers (Daugherty et al., 2017), and it does not show this alternating arrangement. However, in this earlier case, the Au atoms are all bonded to the carbon of N-heterocyclic carbene ligands, rather than to phosphines. As such, the title compound is the first trimetallic ketenylidene cluster of any metal that involves the metal bound to only phosphine and the ketenylidiene bridge, rather than the more common C O ligand found in most trimetallic metal complexes in the CSD.

Database survey
The Cambridge Structural Database (CSD, Version 5.42, February 2021;Groom et al., 2016)  The molecular structure of the title compound with select atom labeling. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity reasons.
The only other known all gold(I) cluster (LEBFOQ, Daugherty et al., 2017) differs from both the reported title compound as well as the other structures in the CSD, as it bears N-heterocyclic carbene ligands attached to gold, rather than either phosphines or carbon monoxide ligands. Additionally, the gold(I) atoms in this previously reported cluster were too far apart from each other to have any metal-metal bonding interaction.
Thus, the reported ketenylidene cluster differs from similar compounds in the CSD in both the title cluster's unique phosphine ligands and the short Au-Au bonding interactions.

Synthesis and crystallization
The title compound was observed during scope studies related to the gold(I)-catalyzed synthesis of trisubstituted indolizine 2 from 2-propargyloxypyridine 1 (Rossler et al., 2019). While initial studies had shown that treatment of pyridine 1 with methyl ketones in the presence of alcohols and (2-dicyclohexylphosphino-2 0 ,6 0 -dimethoxy-1,1 0 -biphenyl)gold(I) bis(trifluoromethane-sulfonyl)imide could provide trisubstituted indolizines 2 in moderate to good yields, when the methyl ketone was replaced with acetic anhydride, an unknown organic product and the title ketenylidene cluster 3 were observed (Fig. 3). In an attempt to determine the organic product of the reaction, crystals were grown by the slow evaporation of a concentrated ethanol solution over several weeks at room temperature. Using this method, a few tiny yellow needle-shaped crystals, suitable for X-ray diffraction, were obtained and analyzed. However, rather than revealing the structure of the organic product as expected, the X-ray structure revealed the title ketenylidene-bridged tri-gold cluster 3. Subsequent studies aimed at the independent synthesis of cluster 3 and related species stoichiometrically were unsuccessful.

Refinement
Crystal data, data collection, and refinement details are collected in Table 1. All non-hydrogen atoms were refined anisotropically. Hydrogen-atom positions were calculated geometrically (C-H = 0.95-1.00 Å ) and refined using a riding model with U iso (H) = 1.2U eq (C) or 1.5U eq (C-methyl). Reaction scheme. The title ketenylidene 3 was discovered as an unexpected by-product of a reaction exploring the gold(I)-catalyzed rearrangement of 2-propargyloxypyridine 1.

Tris(2-dicyclohexylphosphino-2′,6′-dimethoxy-1,1′-biphenyl-κP)-µ-oxoethenylidene-triangulo-trigold(I) bis(trifluoromethanesulfonyl)imide
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.77 e Å −3 Δρ min = −0.95 e Å −3 Special details Experimental. Data was collected using a BRUKER CCD (charge coupled device) based diffractometer equipped with an Oxford low-temperature apparatus operating at 173 K. A suitable crystal was chosen and mounted on a nylon loop using Paratone oil. Data were measured using omega scans of 0.5° per frame for 30 s. The total number of images were based on results from the program COSMO where redundancy was expected to be 4 and completeness to 0.83Å to 100%. Cell parameters were retrieved using APEX II software and refined using SAINT on all observed reflections.Data reduction was performed using the SAINT software which corrects for Lp. Scaling and absorption corrections were applied using SADABS6 multi-scan technique, supplied by George Sheldrick. The structure was solved by the direct method using the SHELXT program and refined by least squares method on F2, SHELXL, incorporated in OLEX2. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. The structure was refined by Least Squares using version 2014/6 of XL (Sheldrick, 2008) incorporated in Olex2 (Dolomanov et al., 2009). All non-hydrogen atoms were refined anisotropically. Hydrogen atom positions were calculated geometrically and refined using the riding model.′