Computational, electrochemical, and spectroscopic studies of two mononuclear cobaloximes: the influence of an axial pyridine and solvent on the redox behaviour and evidence for pyridine coordination to cobalt(i) and cobalt(ii) metal centres

Abstract

[Co(dmgBF₂)₂(H₂O)₂] 1 (where dmgBF₂ = difluoroboryldimethylglyoximato) was used to synthesize [Co(dmgBF₂)₂(H₂O)(py)]·0.5(CH₃)₂CO 2 (where py = pyridine) in acetone. The formulation of complex 2 was confirmed by elemental analysis, high resolution MS, and various spectroscopic techniques. The complex [Co(dmgBF₂)₂(solv)(py)] (where solv = solvent) was readily formed in situ upon the addition of pyridine to complex 1. A spectrophotometric titration involving complex 1 and pyridine proved the formation of such a species, with formation constants, log K = 5.5, 5.1, 5.0, 4.4, and 3.1 in 2-butanone, dichloromethane, acetone, 1,2-difluorobenzene/acetone (4 : 1, v/v), and acetonitrile, respectively, at 20 °C. In strongly coordinating solvents, such as acetonitrile, the lower magnitude of K along with cyclic voltammetry, NMR, and UV-visible spectroscopic measurements indicated extensive dissociation of the axial pyridine. In strongly coordinating solvents, [Co(dmgBF₂)₂(solv)(py)] can only be distinguished from [Co(dmgBF₂)₂(solv)₂] upon addition of an excess of pyridine, however, in weakly coordinating solvents the distinctions were apparent without the need for excess pyridine. The coordination of pyridine to the cobalt(II) centre diminished the peak current at the E_pc value of the Co^I/0 redox couple, which was indicative of the relative position of the reaction equilibrium. Herein we report the first experimental and theoretical ⁵⁹Co NMR spectroscopic data for the formation of Co(I) species of reduced cobaloximes in the presence and absence of py (and its derivatives) in CD₃CN. From spectroelectrochemical studies, it was found that pyridine coordination to a cobalt(I) metal centre is more favourable than coordination to a cobalt(II) metal centre as evident by the larger formation constant, log K = 4.6 versus 3.1, respectively, in acetonitrile at 20 °C. The electrosynthesis of hydrogen by complexes 1 and 2 in various solvents demonstrated the dramatic effects of the axial ligand and the solvent on the turnover number of the respective catalyst.