Coordination chemistry of [HFe(CN)(2)(CO)(3)](-) and its derivatives: toward a model for the iron subsite of the [NiFe]-hydrogenases

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 pK(a) approximately 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)(2)L](-) (L = P(OPh)(3) and PPh(3)). 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 degrees C) with DeltaH(double dagger) = 51.6 kJ mol(-1) and DeltaS(double dagger) = -83.0 J K(-1) mol(-1). These ligands are displaced under an atmosphere of CO. With cis-Ph(2)PCH=CHPPh(2) (dppv), we obtained the monocarbonyl, [HFe(CN)(2)(CO)(dppv)](-), a highly basic hydride (pK(a) > 23.3) that rearranges in solution to a single isomer. Treatment of [K(18-crown-6)][HFe(CN)(2)(CO)(3)] with Et(4)NCN 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)(PPh(3))](-) 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 H(2) 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 degrees 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, CH(3)). 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.

Figure 1

IR spectra (ν_CO region only) for the reaction of [K(18-crown-6)][1H(CO)₂] with 10 equiv of P(OPh)₃ in MeCN solution at 20 °C.

Figure 2

Molecular structure of the anion [1H(dppv)]⁻ with thermal ellipsoids set at the 35 % probability level. Cation, solvates, and non-hydride hydrogen atoms were omitted for clarity.

Figure 3

Molecular structure of the anion [1H(CN)(CO)]^2– with thermal ellipsoids set at the 35 % probability level. [K(18-crown-6)]⁺ cations and acetonitrile solvate was omitted for clarity.

Figure 4

Solution (MeCN) IR spectra of [K(18-crown-6)][1H(CO)₂] (bottom) and [K(18-crown-6)][1Me(CO)₂] (top).

Figure 5

Fe site of the Ni-R and Ni-C states in the [NiFe]-hydrogenases (A) and [1H(dppv)]⁻ (B)

Scheme 1

Scheme 2