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

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 45; no. 28; pp. 11369 - 11383
• Main Authors: Lv, Zhanao, Zheng, Wenrui, Chen, Zhuqi, Tang, Zhiming, Mo, Wanling, Yin, Guochuan
• Format: Journal Article
• Language: English
• Published: England 01.01.2016
• Subjects: Catalysis; Catalysts; Crystallography, X-Ray; Electron Spin Resonance Spectroscopy; Lewis acid; Ligands; Metal ions; Metals - chemistry; Molecular Structure; Olefins; Oxidation; Oxidation-Reduction; Oxygen - chemistry; Oxygen atoms; Ruthenium; Ruthenium Compounds - chemistry; Spectrophotometry, Ultraviolet
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c6dt01077f

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)[double bond, length as m-dash]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)[double bond, length as m-dash]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)[double bond, length as m-dash]O with Al(iii), Ru(iv)[double bond, length as m-dash]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)[double bond, length as m-dash]O/Al(iii) to olefin and the oxidation of Ru(iii)-O-Ru(iii) back to the active Ru(iv)[double bond, length as m-dash]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.