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

摘要

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) with NOBF4 gave the derivatives [Fe2(S2CnH2n)(CO)5-x(PMe3)x(NO)]BF4 (x = 1, 2, 3) which are electronically unsymmetrical due to the presence of a single NO⁺ ligand. Whereas the mono phosphine 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•K) 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) nitrosyls 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. DFT calculations, specifically NBO values, reinforce the electronic resemblance of the nitrosyl complexes with the corresponding mixed-valence diiron complexes. Unlike other diiron dithiolato carbonyls, these species undergo reversible reductions at mild conditions. 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 introducing electronic asymmetry.