Long range charge transfer in trimetallic mixed-valence iron complexes mediated by redox non-innocent cyanoacetylide ligands

The reaction of Fe(CCCN)(dppe)Cp (1) with one-half equivalent of [trans-Fe(NCMe)2(dppx)2][BF4]2 (dppx = dppe ([2][BF4]2) or dppm ([3][BF4] 2)) affords trimetallic [trans-Fe{NCCCFe(dppe)Cp}2(dppx) 2][BF4]2 (dppx = dppe [4][BF4] 2; dppx = dppm [5][BF4]2). Both [4][BF 4]2 and [5][BF4]2 undergo three, one-electron oxidation processes, arising from sequential oxidation of the two terminal Fe(CCR)(dppe)Cp moieties and finally the central Fe(NCR) 2(dppx)2 fragment. The redox products [4]n+ and [5]n+ (n = 3, 4) have been characterised by UV-vis-NIR and IR spectroelectrochemistry. The shifts in the characteristic ν(CCCN) bands upon oxidation demonstrate not only the localised electronic structure of the trications, but also the redox non-innocence of the cyanoacetylide ligands. The trimetallic [formally Fe(ii/ii/iii) mixed-valence] complexes [4]3+ and [5]3+ feature two distinct IVCT transitions, one associated with charge transfer from the central 18-electron {Fe(NCR)2(dppx) 2}2+ to terminal {Fe(CCR)(dppe)Cp}+ moiety, and a lower energy transition involving charge transfer between the terminal Fe fragments which are separated by the redox active 9-atom, 10-bond -CC-CN{Fe(dppx)2}NC-CC- bridge. The tetracationic complexes [4] 4+ and [5]4+ generated by a further stepwise oxidation exhibit a single {Fe(NCR)2(dppx)2}2+→ {Fe(CCR)(dppe)Cp}+ IVCT transition. This journal is © the Partner Organisations 2014.