Group 4 Metallocene Complexes Supported by a Redox-Active O,C-Chelating Ligand

Abstract

Group 4 metallocene complexes Cp2M[OC] (Cp = η5-C5H5; M = Ti (1) or Zr (2); [OC] = κ2-O,C-OC6H2-2-CPh2-4,6-tBu2) supported with a redox-active bidentate O,C-ligand were successfully synthesized through salt metathesis. X-ray crystallographic results showed that both complexes maintained similar bent metallocene coordination geometry. A close inspection of the structural parameters revealed a remarkably long Ti–C bond in 1, indicating concomitant intramolecular charge transfer upon metathesis and coordination. The electronic structure of 1 was investigated by electron paramagnetic resonance (EPR) measurements, revealing the presence of a Ti(III) center and one [OC]•– radical anion. In contrast, the diamagnetic complex 2 was found to comprise a Zr(IV) center and an alkyl/aryloxo dianion. Divergent catalytic reactivity was observed for the two group 4 derivatives in the reaction between alkenes and HBpin. Specifically, dehydrogenative boration products (i.e., vinyl boronate esters) were obtained exclusively in the presence of the Zr complex (18 examples, up to 90% yield), while the Ti compound selectively promoted the formation of alkyl boronate esters via hydroboration (19 examples, up to 99% yield).