Critical biological electron-transfer processes involving high-valent oxometal chemistry occur widely, for example in haem proteins [oxoiron(IV); FeIV (O)] and in photosystem II. Photosystem II involves Ca 2+ as well as high-valent oxomanganese cluster species. However, there is no example of an interaction between metal ions and oxoiron(IV) complexes. Here, we report new findings concerning the binding of the redox-inactive metal ions Ca2+ and Sc3+ to a non-haem oxoiron(IV) complex, [(TMC)FeIV (O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). As determined by X-ray diffraction analysis, an oxo-Sc3+ interaction leads to a structural distortion of the oxoiron(IV) moiety. More importantly, this interaction facilitates a two-electron reduction by ferrocene, whereas only a one-electron reduction process occurs without the metal ions. This control of redox behaviour provides valuable mechanistic insights into oxometal redox chemistry, and suggests a possible key role that an auxiliary Lewis acid metal ion could play in nature, as in photosystem II.
Bibliographical noteFunding Information:
This work was supported by a Grant-in-Aid (no. 20108010 to S.F.) and a Global COE program, ‘the Global Education and Research Center for Bio-Environmental Chemistry’ from the Ministry of Education, Culture, Sports, Science and Technology, Japan (to S.F.), and NRF/MEST through a WCU project (R31-2008-000-10010-0) (to S.F. and W.N.) and the Creative Research Initiatives Program (to W.N.). Crystallographic data for [(TMC)FeIV(O)–Sc(OTf)4(OH)]havebeendepositedwiththeCambridgeCrystallographic Data Center under reference numbers CCDC-742067 (X-ray).