Unlocking structural diversity in gold(III) hydrides: Unexpected interplay of cis/trans-influence on stability, insertion chemistry, and NMR chemical shifts

The synthesis of new families of stable or at least spectroscopically observable gold(III) hydride complexes is reported, including anionic cis-hydrido chloride, hydrido aryl and cis-dihydride complexes. Reactions between (C^C)AuCl(PR3) and LiHBEt3 afford the first examples of gold(III) phosphino hydrides (C^C)AuH(PR3) (R = Me, Ph, p-tolyl; C^C = 4,4′-di-tert-butylbiphenyl-2,2′-diyl). The X-ray structure of (C^C)AuH(PMe3) was determined. LiHBEt3 reacts with (C^C)AuCl(py) to give [(C^C)Au(H)Cl]–, whereas (C^C)AuH(PR3) undergoes phosphine displacement, generating the dihydride [(C^C)AuH2]-. Monohydrido complexes hydroaurate dimethylacetylene dicarboxylate to give Z-vinyls. (C^N^C)Au pincer complexes give the first examples of gold(III) bridging hydrides. Stability, reactivity and bonding characteristics of Au(III)-H complexes crucially depend on the interplay between cis and trans-influence. Remarkably, these new gold(III) hydrides extend the range of observed NMR hydride shifts from δ 8.5 to +7 ppm. Relativistic DFT calculations show that the origin of this wide chemical shift variability as a function of the ligands depends on the different ordering and energy gap between “shielding” Au(dπ)-based orbitals and “deshielding” σ(Au-H)-type MOs, which are mixed to some extent upon inclusion of spin-orbit (SO) coupling. The resulting 1H hydride shifts correlate linearly with the DFT optimized Au-H distances and Au-H bond covalency. The effect of cis ligands follows a nearly inverse ordering to that of trans ligands. This study appears to be the first systematic delineation of cis ligand influence on M-H NMR shifts and provides the experimental evidence for the dramatic change of the 1H hydride shifts, including the sign change, upon mutual cis and trans ligand alternation.