The Structural Characterisation of new Pyridinyl Alcohol Cu(II) Complexes and their Application as Catalysts in the Oxidation of n–octane

In this study a series of pyridinyl alcohol ligands (2-pyridinyl-1-cyclohexanol (1), 2-pyridinyl-1,1-diphenyl methanol (2) and 2-pyridinyl-1-phenyl-1-isopropylmethanol (3) have been coordinated to Cu (Cu 1–3). These complexes have been characterised by single-crystal X-ray diffraction. The solid-state structures of the Cu complexes obtained were dependent on the temperature and method used to grow crystals. In the selective oxidation of n-octane the copper catalysts gave yields of up to 25 % towards C8 oxygenates with predominant selectivity to C2-C4 octanones. The exception was a chlorido-bridged dimer, which showed high selectivity to 1-octanol. Products from activation of the C-1 position predominately form in higher yield in the catalytic reactions of copper(II) complexes that exhibit narrower N−Cu−O bite angles. The activity of the Cu catalysts can be attributed to metal-ligand cooperative catalysis that involves the formation of Cu(II) peroxo intermediates. Characterisation of the recovered catalyst, in combination with the addition of a radical inhibitor during the partial oxidation reactions, indicated that the catalytic reaction progresses via a combination of an oxygen rebound mechanism and a radical chain reaction, with the latter mechanism usually dominating.