EXAFS and EPR Studies of the Alkene Oxidation Catalyst Species trans-[CrIII(bpb)(L)2]n+ and CrV Oxidation Products (bpb=N,N'-Bis(2-pyridinecarboxamido)-1,2-benzene)

Abstract

The structures of trans-[CrIII(bpb)(OH2)2]+ and trans-[CrIII(bpb)(OH2)Cl] (bpb = N,N′-bis(2-pyridinecarboxamido)-1,2-benzene) have been determined by multiple-scattering analysis of their extended X-ray absorption fine structure (EXAFS) spectra. This is the first reported structural characterizations of these complexes, which have been used as catalysts in the oxidation of alkenes and the industrially important coupling reaction of epoxides with CO2. The formation of CrV species, which are likely catalytic intermediates, was observed when trans-[CrIII(bpb)(OH2)Cl] was treated with oxidants: PbO2, iodosylbenzene, or tert-butylhydroperoxide. The intermediates in these reactions were studied using X-band and Q-band electron paramagnetic resonance (EPR) spectroscopy to probe the ability of the bpb ligand to stabilize CrV–oxido complexes. Several CrV species were generated in such oxidation reactions that may be the reason for the lack of selectivity when the CrIII species are used as oxidation catalysts in the presence of oxidants.