Analysis of a Pentacoordinate Iron Dicarbonyl as Synthetic Analogue of the Hmd or Mono-Iron Hydrogenase Active Site

Abstract

Pentacoordinate iron dicarbonyls, (NS)Fe(CO)₂P (NS=2-amidothiophenylate, P=PCy₃ (4), PPh₃, (5), and P(OEt)₃ (6)) were prepared as potential biomimetics of the active site of the mono-iron hydrogenase, [Fe]-H₂ase. Full characterization including X-ray diffraction, density functional theory (DFT) computations, and Mössbauer studies for complexes 5 and 6 find that, despite similar infrared v(CO) pattern and absorption frequencies as the active site of the [Fe]-H₂ase, the geometrical distortions towards trigonal bipyramidal, the negative isomer shift parameters, and the differences in CO-uptake reactivity are due to the “non-innocence” of the NS ligand. Ligand-based protonation with a strong acid, HBF₄⋅Et₂O, interrupted the extensive π-delocalization over Fe and NS ligand of complex 4 and switched on CO uptake (1 bar) to form a CO adduct, mer-[(H-NS)Fe(CO)₃(PCy₃)]⁺ or 4(CO)-H⁺. The extrinsic CO is reversibly removed on deprotonation with Et₃N to regenerate complex 4. In a ¹³CO atmosphere, concomitant CO uptake by 4-H⁺ and exchange with intrinsic CO groups provide a facile route to ¹³C-labeled 4(CO)-H⁺ and, upon deprotonation, ¹³C-labeled complex 4. DFT calculations show substantial Fe character in the LUMO of 4-H⁺ typical of the d⁶ Fe^II in a regular square-pyramidal geometry. Thus, the Lewis acidity of 4-H⁺ makes it amenable for CO binding, whereas the dianionic NS ligand renders the iron center of 4 insufficiently electrophilic and largely of Fe^I character.