Coinage metal complexes with bridging hybrid phosphine–NHC ligands: synthesis of di- and tetra-nuclear complexes

Abstract

A series of P–NHC-type hybrid ligands containing both PR₂ and N-heterocyclic carbene (NHC) donors on meta-bis-substituted phenylene backbones, L^Cy, L^tBu and L^Ph (R = Cy, tBu, Ph, respectively), was accessed through a modular synthesis from a common precursor, and their coordination chemistry with coinage metals was explored and compared. Metallation of L^Ph·n(HBr) (n = 1, 2) with Ag₂O gave the pseudo-cubane [Ag₄Br₄(L^Ph)₂], isostructural to [Ag₄Br₄(L^R)₂] (R = Cy, tBu) (T. Simler, P. Braunstein and A. A. Danopoulos, Angew. Chem., Int. Ed., 2015, 54, 13691), whereas metallation of L^R·HBF₄ (R = Ph, tBu) led to the dinuclear complexes [Ag₂(L^R)₂](BF₄)₂ which, in the solid state, feature heteroleptic Ag centres and a ‘head-to-tail’ (HT) arrangement of the bridging ligands. In solution, interconversion with the homoleptic ‘head-to-head’ (HH) isomers is facilitated by ligand fluxionality. ‘Head-to-tail’ [Cu₂Br₂(L^R)₂] (R = Cy, tBu) dinuclear complexes were obtained from L^R·HBr and [Cu₅(Mes)₅], Mes = 2,4,6-trimethylphenyl, which also feature bridging ligands and heteroleptic Cu centres. Although the various ligands L^R led to structurally analogous complexes for R = Cy, tBu and Ph, the rates of dynamic processes occurring in solution are dependent on R, with faster rates for R = Ph. Transmetallation of both NHC and P donor groups from [Ag₄Br₄(L^tBu)₂] to Au^I by reaction with [AuCl(THT)] (THT = tetrahydrothiophene) led to L^tBu transfer and to the dinuclear complex [Au₂Cl₂L^tBu] with one L^tBu ligand bridging the two Au centres. Except for the silver pseudo-cubanes, all other complexes do not exhibit metallophilic interactions.