Re-evaluating selectivity as a determining factor in peroxidative methane oxidation by multimetallic copper complexes

Abstract

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.