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 Fe-2(S2Cn,H-2n)(CO)(6-x)(PMe3)(x). compounds (n = 2, 3; x = 1, 2, 3) with NOBF4 gave the derivatives [Fe-2(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 [Fe-2(S2C3H6)(CO)(3)(PMe3)(2)(NO)]BF4 were characterized spectroscopically and crystallographically. Each isomer features electron-rich Fe(CO)(2)PMe3 and electrophilic Fe(CO)(PMe3)(NO)(+) subunits. These species are in equilibrium with an unobserved isomer that reversibly binds CO (Delta H = -35 kJ/mol, Delta S = -139 J mol(-1) K-1) to give the symmetrical adduct [Fe-2(S2C3H6)(mu-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 similar to 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 H-ox models) can be replicated in the absence of mixed-valency by the introduction of electronic asymmetry.