Cooperative activating effects of metal ion and Brønsted acid on a metal oxo species

Metal oxo (M[double bond, length as m-dash]O) complexes are common oxidants in chemical and biological systems. The use of Lewis acids to activate metal oxo species has attracted great interest in recent years, especially after the discovery of the CaMn(4)O(5) cluster in the oxygen-evolving centre of photosystem II. Strong Lewis acids such as Sc(3+) and BF(3), as well as strong Brønsted acids such as H(2)SO(4) and CF(3)SO(3)H, are commonly used to activate metal oxo species. In this work, we demonstrate that relatively weak Lewis acids such as Ca(2+) and other group 2 metal ions, as well as weak Brønsted acids such as CH(3)CO(2)H, can readily activate the stable RuO(4)(−) complex towards the oxidation of alkanes. Notably, the use of Ca(2+) and CH(3)CO(2)H together produces a remarkable cooperative effect on RuO(4)(−), resulting in a much more efficient oxidant. DFT calculations show that Ca(2+) and CH(3)CO(2)H can bind to two oxo ligands to form a chelate ring. This results in substantial lowering of the barrier for hydrogen atom abstraction from cyclohexane.