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(S₂C _nH_2n)(CO)_6-x(PMe₃)_x compounds (n = 2, 3; x = 1, 2, 3) with NOBF₄ gave the derivatives [Fe₂(S₂C_nH_2n)(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)(PMe ₃)(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 [Fe ₂(S₂C₃H₆)(μ-NO)(CO) ₄(PMe₃)₂]BF₄. In contrast to Fe ₂(S₂C₃H₆)(CO)₄(PMe ₃)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 H_ox models) can be replicated in the absence of mixed-valency by the introduction of electronic asymmetry.