Synthetic Models for the Active Site of the [FeFe]-Hydrogenase: Catalytic Proton Reduction and the Structure of the Doubly Protonated Intermediate

Abstract

This report compares biomimetic HER catalysts with and without the amine cofactor (adtNH): Fe2(adtNH)(CO)2(dppv)2 (1NH) and Fe2(pdt)(CO)2(dppv)2 (2; (adtNH)2− = (HN(CH2S)22−, pdt2− = 1,3-(CH2)3S22−). These compounds are spectroscopically, structurally, and stereodynamically very similar but exhibit very different catalytic properties. Protonation of 1NH and 2 each give three isomeric hydrides beginning with the kinetically favored terminal hydride, which converts sequentially to sym and unsym isomers of the bridging hydrides. In the case of the amine, the corresponding ammonium-hydrides are also observed. In the case of the terminal amine hydride [t-H1NH]BF4, the ammonium/amine-hydride equilibrium is sensitive to counteranions and solvent. The species [t-H1NH2](BF4)2 represents the first example of a crystallographically characterized terminal hydride produced by protonation. The NH--HFe distance of 1.88(7) Å indicates dihydrogen bonding. The bridging hydrides [µ-H1NH]+ and [µ-H2]+ reduce near −1.8 V, about 150 mV more negative than the reductions of the terminal hydride [t-H1NH]+ and [t-H2]+ at −1.65 V. Reductions of the amine hydrides [t-H1NH]+ and [t-H1NH2]2+ are irreversible. For the pdt analog, the [t-H2]+/0 couple is unaffected by weak acids (pKaMeCN 15.3) but exhibits catalysis with HBF4•Et2O, albeit with a TOF around 4 s−1 and an overpotential greater than 1 V. The voltammetry of [t-H1NH]+ is strongly affected by relatively weak acids and proceeds at 5000 s−1 with an overpotential of 0.7 V. The ammonium-hydride [t-H1NH2]2+ is a faster catalyst with an estimated TOF of 58,000 s−1 and an overpotential of 0.5 V.

A Reversible Proton Relay Process Mediated by Hydrogen‐Bonding Interactions in [FeFe]Hydrogenase Modeling

“…[12] The in-situ FTIR measurements ( Figure 1a and S1 in the Supporting Information) reveal the decay of 1 mH,w hich t distance of 3.130(4) i ss horter than the sum of van der Waals radii( 3.35 ). [16] Rauchfuss [9] and Ta larmin et al [7] have reportedt hat model complexes possessing N-protonation are able to form hydrogen-bond interactions with conjugate bases of acids, whereas this work provides ad ifferent perspecScheme2.The interconversionb etween 1SHNMe 2 Ph (left) and 1 mH (right). tive on how acid interacts with the secondary coordination site.…”

“…[6] It has been suggested from intensive investigations on complex modeling that the azadithiolate unit could serve as as econdary coordinations ite for protona ccepting [7, 8] andr elay [9] to the catalytic Fe center.L ess knowni st he reverser oute, which is proton migration of the Fe-hydride to the aza nitrogen bridgehead. Suggestionso n the nitrogen atom's potentialr ole for facilitation of the transfer have been made.…”

Synthesis and Characterization of Bio‐Inspired Diiron Complexes and Their Catalytic Activity for Direct Hydroxylation of Aromatic Compounds

“…[28][29][30][31][32][33] However, the well-known oxygen sensitivity of the hydrogenases and their active-site biomimetics poses a problem for their development as alternatives to platinum in fuel cell applications. As reported, the oxygenation of the diiron (Fe II ) organometallics that model the diiron subsite in the active site of [FeFe]-hydrogenase enzymes would occur at the dithiolato-sulfur site or at the Fe-Fe bond.…”

Crystallographic Characterization of a Fully Rotated, Basic Diiron Dithiolate: Model for the Hred State?

“…[11] In both complexes, the major isomer crystallized with dppv spanning apical and basal sites. 4 A C H T U N G T R E N N U N G (diphos)] compounds without bulky dithiolates. [11] The CO ap groups become slightly bent with Fe- Crystallographic analysis of 1 Et revealed two very different molecules in the asymmetric unit, in both of which the dppv is dibasal.…”