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

Abstract

The reaction of Fe(C≡CC≡N)(dppe)Cp (1) with one-half equivalent of [trans-Fe(N≡CMe)2(dppx)2][BF4]2 (dppx = dppe ([2][BF4]2) or dppm ([3][BF4]2)) affords trimetallic [trans-Fe{N≡CC≡CFe(dppe)Cp}2(dppx)2][BF4]2 (dppx = dppe [4][BF4]2; dppx = dppm [5][BF4]2). Both [4][BF4]2 and [5][BF4]2 undergo three, one-electron oxidation processes, arising from sequential oxidation of the two terminal Fe(C≡CR)(dppe)Cp moieties and finally the central Fe(N≡CR)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(C≡CC≡N) 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(N≡CR)2(dppx)2}(2+) to terminal {Fe(C≡CR)(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 -C≡C-C≡N{Fe(dppx)2}N≡C-C≡C- bridge. The tetracationic complexes [4](4+) and [5](4+) generated by a further stepwise oxidation exhibit a single {Fe(N≡CR)2(dppx)2}(2+)→{Fe(C≡CR)(dppe)Cp}(+) IVCT transition.