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

Abstract

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)Fe^III–CN–Cu^II/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 Cu^II and Cu^I, 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)Fe^II–CN–{Fe(NO)₂}⁹(sIMes)][BF₄] complexes, Fe-1 (R = H) and Fe*-1 (R = CH₃), showed dissimilar Fe^IICN–{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)Fe^II–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 Fe^II–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.