M^IV/Co^–I (M = Zr, Hf) Bis(phosphinoamide) Complexes with η⁶- and η⁴-Arenes

Abstract

Despite their relevance to catalysis, low-/subvalent cobalt complexes are difficult to synthesize and isolate. Consequently, very few “cobaltate” complexes are known and there is a lack of architectural diversity in this field. Lewis acidic d0 group IV metals have been demonstrated to stabilize Co–I centers via metal–metal bonds. Herein, we report the synthesis of bis(phosphinoamide) heterobimetallic MIV/Co–I arene complexes (M = Zr, Hf). The driving force to maintain the aromaticity of the arenes dictates the hapticity of the Co-bound arene ligands and influences the binding arrangement of the phosphinoamide ligands. Benzene and toluene were observed to bind η6 to the Co–I center, forcing dissociation of one of the phosphinoamide ligands, whereas η4-coordination of anthracene allows both phosphinoamide ligands to remain bound to the Co center. The identity of both the arene and the group IV metal ion starkly influence the lability of the arene. For instance, the Co-bound benzene ligand in the ZrIV/Co–I benzene complex rapidly exchanges with C6D6 in solution, whereas toluene/C6D6 exchange is much slower and appreciable C6H6/C6D6 exchange is not observed for the Hf analogue. The ZrIV/Co–I benzene complex loses benzene upon repeated exposure to vacuum to form an arene-free tetrametallic dimer.