Bridging cyanides from cyanoiron metalloligands to redox-active dinitrosyl iron units.

Cyanide, as an ambidentate ligand, plays a pivotal role in providing a simple diatomic building-block motif for controlled metal aggregation (M-CN-M'). Specifically, the inherent hard-soft nature of the cyanide ligand, i.e., hard-nitrogen and soft-carbon centers, is due to electronic handles for binding Lewis acids following the hard-soft acid-base principle. Studies by Holm and Karlin showed structural and electronic requirements for cyanide-bridged (por)FeIII-CN-CuII/I (por = porphyrin) molecular assemblies as biomimetics for cyanide-inhibited terminal quinol oxidases and cytochrome-C oxidase. The dinitrosyliron unit (DNIU) that exists in two redox states, {Fe(NO)2}9 and {Fe(NO)2}10, draws attention as an electronic analogy of CuII and CuI, d9 and d10, respectively. In similar controlled aggregations, L-type [(η5-C5R5)Fe(dppe)(CN)] (dppe = diphenyl phosphinoethane; R = H and Me) have been used as N-donor, μ-cyanoiron metalloligands to stabilize the DNIU in two redox states. Two bimetallic [(η5-C5R5)(dppe)FeII-CN-{Fe(NO)2}9(sIMes)][BF4] complexes, Fe-1 (R = H) and Fe*-1 (R = CH3), showed dissimilar FeIIC[triple bond, length as m-dash]N-{Fe(NO)2}9 angular bends due to the electronic donor properties of the [(η5-C5R5)Fe(dppe)(CN)] μ-cyanoiron metalloligand. A trimetallic [(η5-C5Me5)(dppe)FeII-CN]2-{Fe(NO)2}10 complex, Fe*-2, engaged two bridging μ-cyanoiron metalloligands to stabilize the {Fe(NO)2}10 unit. The lability of the FeII-CN-{Fe(NO)2}9/10 bond was probed by suitable X-type (Na+SPh-) and L-type (PMe3) ligands. Treatment of Fe-1 and Fe*-1 with PMe3 accounted for a reduction-induced substitution at the DNIU, releasing [(η5-C5R5)Fe(dppe)(CN)] and N-heterocyclic carbene, and generating (PMe3)2Fe(NO)2 as the reduced {Fe(NO)2}10 product.