Synergistic oxygen atom transfer by ruthenium complexes with non-redox metal ions

Abstract

Non-redox metal ions can affect the reactivity of active redox metal ions in versatile biological and heterogeneous oxidation processes; however, the intrinsic roles of these non-redox ions still remain elusive. This work demonstrates the first example of the use of non-redox metal ions as Lewis acids to sharply improve the catalytic oxygen atom transfer efficiency of a ruthenium complex bearing the classic 2,2′-bipyridine ligand. In the absence of Lewis acid, the oxidation of ruthenium(II) complex by PhI(OAc)₂ generates the Ru(IV)O species, which is very sluggish for olefin epoxidation. When Ru(bpy)₂Cl₂ was tested as a catalyst alone, only 21.2% of cyclooctene was converted, and the yield of 1,2-epoxycyclooctane was only 6.7%. As evidenced by electronic absorption spectra and EPR studies, both the oxidation of Ru(II) by PhI(OAc)₂ and the reduction of Ru(IV)O by olefin are kinetically slow. However, adding non-redox metal ions such as Al(III) can sharply improve the oxygen transfer efficiency of the catalyst to 100% conversion with 89.9% yield of epoxide under identical conditions. Through various spectroscopic characterizations, an adduct of Ru(IV)O with Al(III), Ru(IV)O/Al(III), was proposed to serve as the active species for epoxidation, which in turn generated a Ru(III)–O–Ru(III) dimer as the reduced form. In particular, both the oxygen transfer from Ru(IV)O/Al(III) to olefin and the oxidation of Ru(III)–O–Ru(III) back to the active Ru(IV)O/Al(III) species in the catalytic cycle can be remarkably accelerated by adding a non-redox metal, such as Al(III). These results have important implications for the role played by non-redox metal ions in catalytic oxidation at redox metal centers as well as for the understanding of the redox mechanism of ruthenium catalysts in the oxygen atom transfer reaction.