Coordination Chemistry of [HFe(CN)2(CO)3]− and Its Derivatives: Toward a Model for the Iron Subsite of the [NiFe]-Hydrogenases

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Abstract
The photoreaction of Fe(CO)5 and cyanide salts in MeCN solution affords the dianion [Fe(CN)2(CO)3]2−, conveniently isolated as [K(18-crown-6)]2[Fe(CN)2(CO)3]. Solutions of [Fe(CN)2(CO)3]2− oxidize irreversibly at −600 mV (vs Ag/AgCl) to give primarily [Fe(CN)3(CO)3]−. Protonation of the dianion affords the hydride [K(18-crown-6)][HFe(CN)2(CO)3] with a pKa ≈ 17 (MeCN). The ferrous hydride exhibits enhanced electrophilicity vs its dianionic precursor, which resists substitution. Treatment of [K(18-crown-6)][Fe(CN)2(CO)3] with tertiary phosphines and phosphites gives isomeric mixtures of [HFe(CN)2(CO)2L]− (L = P(OPh)3 and PPh3). Carbonyl substitution on [1H(CO)2]− by P(OPh)3 is first-order in both the phosphite and iron (k = 0.18 M−1 s−1 at 22 °C) with ΔH‡ = 51.6 kJ mol−1 and ΔS‡ = −83.0 J K−1 mol−1. These ligands are displaced under an atmosphere of CO. With cis-Ph2PCH=CHPPh2 (dppv), we obtained the monocarbonyl, [HFe(CN)2(CO)(dppv)]−, a highly basic hydride (pKa > 23.3) that rearranges in solution to a single isomer. Treatment of [K(18-crown-6)][HFe(CN)2(CO)3] with Et4NCN resulted in rapid deprotonation to give [Fe(CN)2(CO)3]2− and HCN. The tricyano hydride [HFe(CN)3(CO)2]2− is prepared by the reaction of [HFe(CN)2(CO)2(PPh3)]− and [K(18-crown-6)]CN. Similar to the phosphine and phosphite derivatives, [HFe(CN)3(CO)2]2− exists as a mixture of all three possible isomers. Protonation of the hydrides [HFe(CN)2(CO)(dppv)]− and [HFe(CN)3(CO)2]− in acetonitrile solutions releases H2 and gives the corresponding acetonitrile complexes [K(18-crown-6)][Fe(CN)3(NCMe)(CO)2] and Fe(CN)2(NCMe)(CO)(dppv). Alkylation of [K(18-crown-6)]2[Fe(CN)2(CO)3] with MeOTf gives the thermally unstable [MeFe(CN)2(CO)3]−, which was characterized spectroscopically at −40 °C. Reaction of dppv with [MeFe(CN)2(CO)3]− gives the acetyl complex, [Fe(CN)2(COMe)(CO)(dppv)]−. Whereas [Fe(CN)2(CO)3]2− undergoes protonation and methylation at Fe, acid chlorides give the iron(0) N-acylisocyanides [Fe(CN)(CO)3(CNCOR)]− (R = Ph, CH3). The solid state structures of [K(18-crown-6)][HFe(CN)2(CO)(dppv)], Fe(CN)2(NCMe)(CO)(dppv), and [K(18-crown-6)]2[HFe(CN)3(CO)2] were confirmed crystallographically. In all three cases, the cyanide ligands are cis to the hydride or acetonitrile ligands.

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