Hydrogen Activation by Biomimetic [NiFe]-Hydrogenase Model Containing Protected Cyanide Cofactors

Abstract

Described are experiments that allow incorporation of cyanide cofactors and hydride substrate into active site models [NiFe]-hydrogenases (H2ases). Complexes of the type (CO)2(CN)2Fe(pdt)Ni(dxpe), (dxpe = dppe, 1; dxpe = dcpe, 2) bind the Lewis acid B(C6F5)3 (BArF3) to give the adducts (CO)2(CNBArF3)2Fe(pdt)Ni(dxpe), (1(BArF3)2, 2(BArF3)2). Upon decarbonylation using amine oxides, these adducts react with H2 to give hydrido derivatives Et4N[(CO)(CNBArF3)2Fe(H)(pdt)Ni(dxpe)], (dxpe = dppe, Et4N[H3(BArF3)2]; dxpe = dcpe, Et4N[H4(BArF3)2]). Crystallographic analysis shows that Et4N[H3(BArF3)2] generally resembles the active site of the enzyme in the reduced, hydride-containing states (Ni-C/R). The Fe-H…Ni center is unsymmetrical with rFe-H = 1.51(3) and rNi-H = 1.71(3) Å. Both crystallographic and 19F NMR analysis show that the CNBArF3− ligands occupy basal and apical sites. Unlike cationic Ni-Fe hydrides, [H3(BArF3)2]− and [H4(BArF3)2]− oxidize at mild potentials, near the Fc+/0 couple. Electrochemical measurements indicate that in the presence of base, [H3(BArF3)2]− catalyzes the oxidation of H2. NMR evidence indicates dihydrogen bonding between these anionic hydrides and ammonium salts, which is relevant to the mechanism of hydrogenogenesis. In the case of Et4N[H3(BArF3)2], strong acids such as HCl induce H2 release to give the chloride Et4N[(CO)(CNBArF3)2Fe(pdt)(Cl)Ni(dppe)].

Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

“…The bis(triarylborane) adduct ([ 47 (CO)]) undergoes decarbonylation with Me 3 NO to presumably afford a coordinatively unsaturated electrophile that, despite its lack of a positive charge, extracts H − from H 2 in the presence of the Me 3 N decarbonylation coproduct (Figure 47). 368 …”

“…X-ray crystal structure of anion [ 47 ( μ -H)] − (bottom; F and non-hydridic H atoms omitted for clarity). 368 …”

H2 and O2 activation by [NiFe]hydrogenases – Insights from model complexes

“…In that structure, as was hypothesized by past enzyme and model studies [1,7,[10][11][12][13], where Ni-SIa is defined as '0' [7]. For example, the hydride species, Ni-R, is placed at the 14.…”

“…The literature includes reports of numerous [NiFe]H 2 ase model complexes, but heterolytic cleavage of H 2 molecules using the NiFe complexes was only accomplished in 2013 [11,12]. Since the acetonitrile (CH 3 CN) ligand in the Ni-SIa model shown in Fig.…”

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

“…1,2 The possibility of connecting synthetic complexes to enzyme states was established with [(dppe)Ni(pdt)( μ -H)Fe(CO) 3 ] + 14,21 . A related nickel–iron hydride [(amine) 2 Ni(SR) 2 ( μ -H)FeL 3 ] + catalyzes the reduction of redox dyes by H 2 , a characteristic assay for the hydrogenases.…”