Journal of the American Chemical Society, Vol.140, No.26, 8287-8302, 2018
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 boolean AND C)AuCl(PR3) and LiHBEt3 afford the first examples of gold(III) phosphino hydrides (C boolean AND C)AuH(PR3) (R = Me, Ph, p-tolyl; C boolean AND C = 4,4'-di-tert-butylbiphenyl-2,2'-diyl). The X-ray structure of (C boolean AND C)AuH(PMe3) was determined. LiHBEt3 reacts with (C boolean AND C)AuCl(py) to give [(C<^>C)Au(H)Cl](-), whereas (C<^>C)AuH(PR3) undergoes phosphine displacement, generating the dihydride [(C boolean AND C)AuH2](-). Monohydrido complexes hydroaurate dimethylacetylene dicarboxylate to give Z-vinyls. (C boolean AND N boolean AND 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 delta -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(d(pi))-based orbitals and "deshielding" sigma(Au-H)-type MOs, which are mixed to some extent upon inclusion of spin-orbit (SO) coupling. The resulting H-1 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 H-1 hydride shifts, including the sign change, upon mutual cis and trans ligand alternation.