Redox-inactive metal ions promoted the catalytic reactivity of non-heme manganese complexes towards oxygen atom transferElectronic supplementary information (ESI) available: Experimental details of catalytic epoxidation, GC-MS graphs, and EPR spectra. See DOI: 10.1039/c4dt03993a

• Main Authors: Choe, Cholho, Yang, Ling, Lv, Zhanao, Mo, Wanling, Chen, Zhuqi, Li, Guangxin, Yin, Guochuan
• Format: Journal Article
• Language: English
• Published: 07.05.2015
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c4dt03993a

Redox-inactive metal ions can modulate the reactivity of redox-active metal ions in a variety of biological and chemical oxidations. Many synthetic models have been developed to help address the elusive roles of these redox-inactive metal ions. Using a non-heme manganese( ii ) complex as the model, the influence of redox-inactive metal ions as a Lewis acid on its catalytic efficiency in oxygen atom transfer was investigated. In the absence of redox-inactive metal ions, the manganese( ii ) catalyst is very sluggish, for example, in cyclooctene epoxidation, providing only 9.9% conversion with 4.1% yield of epoxide. However, addition of 2 equiv. of Al 3+ to the manganese( ii ) catalyst sharply improves the epoxidation, providing up to 97.8% conversion with 91.4% yield of epoxide. EPR studies of the manganese( ii ) catalyst in the presence of an oxidant reveal a 16-line hyperfine structure centered at g = 2.0, clearly indicating the formation of a mixed valent di-μ-oxo-bridged diamond core, Mn III -(μ-O) 2 -Mn IV . The presence of a Lewis acid like Al 3+ causes the dissociation of this diamond Mn III -(μ-O) 2 -Mn IV core to form monomeric manganese( iv ) species which is responsible for improved epoxidation efficiency. This promotional effect has also been observed in other manganese complexes bearing various non-heme ligands. The findings presented here have provided a promising strategy to explore the catalytic reactivity of some di-μ-oxo-bridged complexes by adding non-redox metal ions to in situ dissociate those dimeric cores and may also provide clues to understand the mechanism of methane monooxygenase which has a similar diiron diamond core as the intermediate. The oxygenation efficiency of a manganese catalyst can be sharply improved by a Lewis acid which causes the dissociation of a diamond Mn 2 ( iii , iv ) core.