Exploring Ligand Effects on Structure, Bonding, and Photolytic Hydride Transfer of Cationic Gold(I) Bridging Hydride Complexes of Molybdocene and Tungstenocene.

Abstract

A diverse family of heterobimetallic bridging hydride adducts of the type [LAu(μ-H)₂MCp₂][X] (L = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene, IPr; 1,3-bis(1-adamantyl)imidazole-2-ylidene, IAd; 1,3-bis(2,6-di-iso-propylphenyl)-5,5-dimethyl-4,6-diketopyrimidinyl-2-ylidene, ^DippDAC; triphenylphosphine, PPh₃; 2-di-tert-butylphosphino-2',4',6'-triisopropylbiphenyl, ^tBuXPhos; X = SbF₆^-, BF₄^- or TfO^-) was synthesized by reacting group VI metallocene dihydrides Cp₂MH₂ (Cp = cyclopentadienyl anion; M = Mo, W) with cationic gold(I) complexes [LAu(NCMe)][X]. Trimetallic [L'Au₂(μ-H)₂WCp₂][X]₂ and tetrametallic [L'Au₂{(μ-H)₂WCp₂}₂] [X]₂ complexes (L' = rac-2,2'-bis(diphenylphosphino)-1,1'-binaphthalene or bis(diphenylphosphinomethane)) were obtained by reacting digold [L'{Au(NCMe)}₂][X]₂ with Cp₂WH₂ in a 1:1 and a 1:2 stoichiometry. Accessing such a broad structural diversity allowed us to pinpoint roles played by the ancillary ligands and group VI metals on the bonding properties of this family of bridging hydrides. In particular, a clear effect of the ligand on the interaction energy and electronic structure was observed, with important implications on photolytic reactivity. UV or visible light irradiation, indeed, leads to the selective cleavage of the heterobimetallic Au(μ-H)₂M arrangement and formation of molecular gold hydrides. The photolysis was found to be chromoselective (wavelength-dependent), which can be ascribed to different charge redistributions upon excitation to the first (Kasha's reactivity) and higher (anti-Kasha's reactivity) excited states.