Synthesis and Oxidation Catalysis of [Tris(oxazolinyl)borato]cobalt(II) Scorpionates

The reaction of CoCl₂·THF and thallium tris(4,4-dimethyl-2-oxazolinyl)phenylborate (TlTo^M) in tetrahydrofuran (THF) provides To^MCoCl (1) in 95 % yield; however, appropriate solvents and starting materials are required to favor 1 over two other readily formed side-products, (To^M)₂Co (2) and {HTo^M}CoCl₂(3). ESR, NMR, FTIR, and UV/Vis spectroscopies were used to distinguish these cobalt(II) products and probe their electronic and structural properties. Even after the structures indicated by these methods were confirmed by X-ray crystallography, the spectroscopic identification of trace contaminants in the material was challenging. The recognition of possible contaminants in the synthesis of To^MCoCl in combination with the paramagnetic nature of these complexes provided impetus for the utilization of X-ray powder diffraction to measure the purity of the To^MCoCl bulk sample. The X-ray powder diffraction results provide support for the bulk-phase purity of To^MCoCl in preparations that avoid 2 and 3. Thus, 1 is a precursor for new [tris(oxazolinyl)borato]cobalt chemistry, as exemplified by its reactions with KOtBu and NaOAc to give To^MCoOtBu (4) and To^MCoOAc (5), respectively. Compound 5 is a catalyst for the oxidation of cyclohexane with meta-chloroperoxybenzoic acid (mCPBA), and the rate constants and selectivity for cyclohexanol versus cyclohexanone and ε-caprolactone were assessed.