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

摘要

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((CH2Bu)-Bu-t)((CHBu)-Bu-t)(mu-L)Rh-2(COD)(2)Cl-2](n), 2, and heterobimetallic [Ta(mu-L)((CHBu)-Bu-t)((CH2Bu)-Bu-t)(2)Rh(COD)Cl], 4, complexes are obtained upon treatment of [Ta(L)((CHBu)-Bu-t)((CH2Bu)-Bu-t)(2)], 1, with [Rh(COD)Cl](2). To avoid parasistic reactivity arising from the neopentylidene fragment in 1, the peralkyl compound {Ta(L)[OSi((OBu)-Bu-t)(3)]((CH2Bu)-Bu-t)(3)}, 5, resulting from the 1,2-addition of tris(tertbutoxysilanol) across the Ta=C alkylidene motif, is prepared. An unanticipated silanol-NHC adduct, {(HOSiOBu3)-Bu-t}{Ta(L)[OSi((OBu)-Bu-t)(3)]((CH2Bu)-Bu-t)(3)}, 6, is formed when 1 is treated with two equivalents of HOSi((OBu)-Bu-t)(3). Finally, treatment of 5 with [Rh(COD)Cl](2) provides the heterobimetallic complex {Ta(mu-L)[OSi((OBu)-Bu-t)(3)]((CH2Bu)-Bu-t)(3)Rh(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.