Nitrosyl Derivatives of Diiron(I) Dithiolates Mimic the Structure and Lewis Acidity of the [FeFe]-Hydrogenase Active Site

This study probes the impact of electronic asymmetry of diiron(I) dithiolato carbonyls. Treatment of Fe2(S2CnH2n)(CO)6−x(PMe3)x compounds (n = 2, 3; x = 1, 2, 3) with NOBF4 gave the derivatives [Fe2(S2CnH2n)(CO)5−x(PMe3)x(NO)]BF4, which are electronically unsymmetrical because of the presence of a single NO+ ligand. Whereas the monophosphine derivative is largely undistorted, the bis(PMe3) derivatives are distorted such that the CO ligand on the Fe(CO)(PMe3)(NO)+ subunit is semibridging. Two isomers of [Fe2(S2C3H6)(CO)3(PMe3)2(NO)]BF4 were characterized spectroscopically and crystallographically. Each isomer features electron-rich Fe(CO)2PMe3 and electrophilic Fe(CO)(PMe3)(NO)+ subunits. These species are in equilibrium with an unobserved isomer that reversibly binds CO (ΔH = −35 kJ/mol, ΔS = −139 J mol−1 K−1) to give the symmetrical adduct [Fe2(S2C3H6)(μ-NO)(CO)4(PMe3)2]BF4. In contrast to Fe2(S2C3H6)(CO)4(PMe3)2, the bis(PMe3) nitrosyl complexes readily undergo CO substitution to give the (PMe3)3 derivatives. The nitrosyl complexes reduce at potentials that are ∼1 V milder than their carbonyl counterparts. Results of density functional theory calculations, specifically natural bond orbital analysis, reinforce the electronic resemblance of the nitrosyl complexes to the corresponding mixed-valence diiron complexes. Unlike other diiron dithiolato carbonyls, these species undergo reversible reductions at mild potentials. The results show that the novel structural and chemical features associated with mixed-valence diiron dithiolates (the so-called Hox models) can be replicated in the absence of mixed-valency by the introduction of electronic asymmetry.