Hydrogenase Models

Abstract

This review will highlight major advances in the synthesis and study of active site analogs for the [NiFe]‐, [FeFe]‐, and [Fe]‐hydrogenases (H 2 ases). In their biological role, the [NiFe]‐ and [FeFe]‐ H 2 ases utilize a combination of redox and acid–base reactions for the activation and formation of H 2 . Both reactions have been demonstrated in models. Relevant to the [NiFe]‐H 2 ases, models have been developed that form hydrides via the biologically relevant pathways of protonation or activation of H 2 . These hydrides serve as catalysts for the reduction of protons or oxidation of H 2 via mixed‐valence intermediates. These studies suggest that the hydride bridges the two metals unsymmetrically, and the Fe– H … Ni center engages in dihydrogen bonding to protic reagents as an intermediate in the formation and activation of H 2 . For the [FeFe]‐H 2 ases, the most informative models feature both an azadithiolate cofactor, which serves as a proton relay, and a terminal hydride ligand. Diiron species undergo oxidation to mixed‐valence Fe(I)Fe( II ) cations, several of which are capable of oxidizing H 2 . The Fe(I)Fe(I) species catalyze hydrogen evolution via the formation of iron hydrides. Catalysis is faster when the hydride is a terminal ligand, but also proceeds for isomeric species with a bridging hydride ligand. Models for [Fe]‐H 2 ases feature biomimetic coordination spheres consisting of acyl, thiolate, and 2‐substituted pyridine ligands bound to a redox‐inactive ferrous center.

Models of the Ni-L and Ni-SIa States of the [NiFe]-Hydrogenase Active Site

“…7,10 [FeFe]-hydrogenase models are fairly advanced and have strong resemblance and relevance to the enzymatic system. 3,4,11 In contrast, models of the oxygentolerant [NiFe]-hydrogenases are less mature, and comparisons to the enzymatic system are sometimes less applicative. 3,4,12,13 …”

“…3,4,11 In contrast, models of the oxygentolerant [NiFe]-hydrogenases are less mature, and comparisons to the enzymatic system are sometimes less applicative. 3,4,12,13 …”

Generation of New Artificial Metalloproteins by Cofactor Modification of Native Hemoproteins

“…[32][33][34] To understand the reaction mechanism and the catalytic reactivity of the H-cluster, a series of synthetic model complexes containing dithiolatebridged Fe 2 (CO) n clusters were designed and studied. [35][36][37] Generation of a protein with catalytic activity similar to that of the wild type enzyme was reported when the apo form of [FeFe]-hydrogenase from Chlamydomonas reinhardtii was treated with a synthetic mimic of the [2Fe] cluster. [38,39] This finding encouraged us to construct welldesigned artificial hydrogenase.…”

Preparation and Protonation of Fe2(pdt)(CNR)6, Electron-Rich Analogues of Fe2(pdt)(CO)6

“…600 MHz 1 H NMR spectrum (CD 2 Cl 2 , −40 °C) of [ 3 H] + generated in situ from 3 and [HNEt 3 ]BAr F 4 .…”

“…Synthetic analogues of this site most often include disubstituted species, e.g., Fe 2 (SR) 2 (CO) 4 L 2 , but tri- and tetrasubstituted derivatives have also been prepared. 1 The tetrasubstituted species are often subject to significant steric congestion as is evident in the unusual acid–base properties of Fe 2 (pdt)(CO) 2 (dppv) 2 and Fe 2 (pdt)(CO) 2 (PMe 3 ) 4 (pdt 2− = CH 2 (CH 2 S − ) 2 , dppv = cis -C 2 H 2 (PPh 2 ) 2 , Figure 1). 6,7 In other work, bulky dithiolates have been shown to stabilize unusual, but biomimetic coordination geometry at one of the Fe centers.…”