Re-evaluating selectivity as a determining factor in peroxidative methane oxidation by multimetallic copper complexes†
A series of multimetallic copper(II) complexes have been re-investigated for methane oxidation with H2O2. The preparation and properties of trinuclear copper(II) complexes of the form [Cu3(triazole)n(OH2)12−n] (n = 8, 10) are reported. While these complexes are trimeric in the solid-state, 1H NMR studies suggest that facile ligand dissociation occurs in solution. The oxidation of cyclohexane with H2O2 catalyzed by [Cu3(triazole)n(OH2)12−n] (n = 8, 10) is compared against a literature known oxo-centered tetrameric cluster (Angew. Chem., Int. Ed., 2005, 44, 4345) and these catalysts display moderate activities. The series have also been investigated in methane oxidation at 30 bar and 40 °C. Analytical techniques including a solvent suppression 1H NMR method have been applied to quantify the liquid- and gas-phase products. The multi-metallic copper(II) complexes and copper(II) nitrate control samples produce only methanol and CO2. While TONs for methanol production range from 1.4–4.6 in all cases approximately 50 times the amount of CO2 is produced relative to methanol. We conclude that selectivity is a determining factor in methane oxidation under these conditions and should be considered in future studies.