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)(2)P (NS=2-amidothiophenylate, P=PCy(3) (4), PPh(3), (5), and P(OEt)(3) (6)) were prepared as potential biomimetics of the active site of the mono-iron hydrogenase, [Fe]-H(2)ase. 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]-H(2)ase, 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, HBF(4).Et(2)O, interrupted the extensive pi-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)(PCy(3))](+) or 4(CO)-H(+). The extrinsic CO is reversibly removed on deprotonation with Et(3)N to regenerate complex 4. In a (13)CO atmosphere, concomitant CO uptake by 4-H(+) and exchange with intrinsic CO groups provide a facile route to (13)C-labeled 4(CO)-H(+) and, upon deprotonation, (13)C-labeled complex 4. DFT calculations show substantial Fe character in the LUMO of 4-H(+) typical of the d(6) 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.

Hydrogen-activating models of hydrogenases

“…[4,[27][28]71]. Some selected examples are shown in Figure 2, and several of them have very similar structure with the [Fe]-hydrogenase [72][73][74][75][76][77][78][79]. However, until now, none of the [Fe]-hydrogenase models can activate H 2 , even those structurally very similar to the enzyme.…”

Structure and Function of [Fe]‐Hydrogenase and its Iron–Guanylylpyridinol (FeGP) Cofactor

“…[22,[28][29][30][31] For example, the geometry of the iron site of [Ph 2 PC 6 H 4 C(O)]Fe(SPh)(CO) 3 -is similar to that found in the proposed structure of CO-and cyanide-inhibited [Fe]-hydrogenase, respectively. [31] Recently, two model iron compounds of the FeGP cofactor containing a five-membered metallacycle with an acyl ligand were reported.…”

The Third Hydrogenase: More Natural Organometallics

“…27 These complexes are five-coordinate, leaving a vacant site which can be viewed as similar to the "solvent" site in the [Fe]-hydrogenase. Only 27a shows any tendency to bind carbon monoxide under ambient conditions.…”