Lability of Ta–NHC adducts as a synthetic route towards heterobimetallic Ta/Rh complexes

Abstract

We report the synthesis and characterization of a series of original tantalum/rhodium heterobimetallic species assembled by a bifunctional alkoxy-N-heterocyclic carbene (NHC) ligand platform (noted L). The heterotrimetallic [Ta(CH₂^tBu)(CH^tBu)(μ-L)Rh₂(COD)₂Cl₂]_n, 2, and heterobimetallic [Ta(μ-L)(CH^tBu)(CH₂^tBu)₂Rh(COD)Cl], 4, complexes are obtained upon treatment of [Ta(L)(CH^tBu)(CH₂^tBu)₂], 1, with [Rh(COD)Cl]₂. To avoid parasistic reactivity arising from the neopentylidene fragment in 1, the peralkyl compound {Ta(L)[OSi(O^tBu)₃](CH₂^tBu)₃}, 5, resulting from the 1,2-addition of tris(tertbutoxysilanol) across the TaC alkylidene motif, is prepared. An unanticipated silanol–NHC adduct, {HOSiO^tBu₃}{Ta(L)[OSi(O^tBu)₃](CH₂^tBu)₃}, 6, is formed when 1 is treated with two equivalents of HOSi(O^tBu)₃. Finally, treatment of 5 with [Rh(COD)Cl]₂ provides the heterobimetallic complex {Ta(μ-L)[OSi(O^tBu)₃](CH₂^tBu)₃Rh(COD)(Cl)}, 7, in high yield. This work highlights the reactivity of Ta–NHC adducts and the aptitude of the NHC motif to transfer from Ta to Rh which is used with profit as an efficient synthetic route to access early/late heterobimetallic complexes.