Synthetic and mechanistic studies on tripledecker complexes with bridging 1 H-borole ligands

Abstract

A mechanistic model for the nucleophilic degradation of tripledecker complexes like μ-(η 5-C 4H 4BPh)[Rh(η-C 4H 4BPh)] 2 is proposed. The rate-determining step is a lateral nucleophilic attack at one of the metal centres which is followed by fast addition of two further ligand molecules. The nucleophilic degradation of μ-(η 5-C 4H 4BPh)[Rh(η 5-C 4H 4BPh)] 2 by e.g. PMe 3, P(OMe) 3, CNBu t and NH 3 is reversible. Stacking reactions, mainly by means of electrophilic addition to 1-pheynylborataferrocene ion, provide a strategy for the synthesis of new heterobimetallic tripledecker complexes including e.g. the group 5 compounds μ-(η 5-C 4H 4BPh)[M(CO) 4](FeCp) (M = V, Nb, Ta) and the group 6 anions [μ-(η 5-C 4H 4BPH)[M(CO) 3](FeCp)] − (M = Cr, Mo, W). In this context the electron count of tripledecker complexes is discussed. In the presence of strong σ-donor/gp-aceptor ligands as outer ligands these complexes obey a strict 30e rule. The combination of an electrophilic stacking reaction and a nucleophilic degradation reaction may result in a net ligand transfer. Examples demonstrate the transfer of borole ligands from Fe to Re, from Co to Ru and Ir via tripledecker intermediates.