End-on and side-on π-acid ligand adducts of gold(I): Carbonyl, cyanide, isocyanide, and cyclooctyne gold(I) complexes supported by N-heterocyclic carbenes and phosphines

摘要

N-Heterocyclic carbene ligand SIDipp (SIDipp = 1,3-bis(2,6- diisopropylphenyl)imidazolin-2-ylidene) and trimesitylphosphine ligand have been used in the synthesis of gold(I) cyanide, t-butylisocyanide, and cyclooctyne complexes (SIDipp)Au(CN) (3), (Mes_3P)Au(CN) (4), [(Mes _3P)_2Au][Au(CN)_2] (5), [(SIDipp)Au(CN ~tBu)][SbF_6] ([6][SbF_6]), [(SIDipp) Au(cyclooctyne)][SbF_6] ([8][SbF_6]), and [(Mes 3P)Au(cyclooctyne)][SbF_6] ([9][SbF_6]). A detailed computational study has been carried out on these and the related gold(I) carbonyl adducts [(SIDipp)Au(CO)][SbF_6] ([1][SbF _6]), [(Mes_3P)Au(CO)][SbF_6] ([2][SbF _6]), and [(Mes_3P)Au(CN~tBu)]~+ ([7]~+). X-ray crystal structures of 3, 5, [6][SbF_6], [8][SbF_6], and [9][SbF_6] revealed that they feature linear gold sites. Experimental and computational data show that the changes in π-acid ligand on (SIDipp)Au~+ from CO, CN~-, CN ~tBu, cyclooctyne as in [1]~+, 3, [6]~+, and [8]~+ did not lead to large changes in the Au-C_(carbene) bond distances. A similar phenomenon was also observed in Au-P distance in complexes [2]~+, 4, [7]~+, and [9]~+ bearing trimesitylphosphine. Computational data show that the Au-L bonds of "naked" [Au-L]~+ or SIDipp and Mes3P supported [Au-L]~+ (L = CO, CN~-, CN~tBu to cyclooctyne) have higher electrostatic character than covalent character. The Au←L σ-donation and Au→L π-back-donation contribute to the orbital term with the former being the dominant component, but the latter is not negligible. In the Au-CO adducts [1]~+and [2]~+, the cationic gold center causes the polarization of the C-O σ and π orbitals toward the carbon end making the coefficients at the two atoms more equal which is mainly responsible for the large blue shift in the CO stretching frequency. The SIDipp and Mes_3P supported gold(I) complexes of cyanide and isocyanide also exhibit a significant blue shift in υì..._(CN) compared to that of the free ligands. Calculated results for Au(CO)Cl and Au(CF _3)CO suggest that the experimentally observed blue shift in νì..._(CO) of these compounds may at least partly be caused by intermolecular forces.