Protonation of Nickel–Iron Hydrogenase Models Proceeds after Isomerization at Nickel

Abstract

Theory and experiment indicate that the protonation of reduced NiFe dithiolates proceeds via a previously undetected isomer with enhanced basicity. In particular, it is proposed that protonation of (OC)3Fe(pdt)Ni(dppe) (1; pdt2– = –S(CH2)3S–; dppe = Ph2P(CH2)2PPh2) occurs at the Fe site of the two-electron mixed-valence Fe(0)Ni(II) species, not the Fe(I)-Ni(I) bond for the homovalence isomer of 1. The new pathway, which may have implications for protonation of other complexes and clusters, was uncovered through studies on the homologous series L(OC)2Fe(pdt)M(dppe), where M = Ni, Pd (2), and Pt (3) and L = CO, PCy3. Similar to 1, complexes 2 and 3 undergo both protonation and 1e– oxidation to afford well-characterized hydrides ([2H]+ and [3H]+) and mixed-valence derivatives ([2]+ and [3]+), respectively. Whereas the Pd site is tetrahedral in 2, the Pt site is square-planar in 3, indicating that this complex is best described as Fe(0)Pt(II). In view of the results on 2 and 3, the potential energy surface of 1 was reinvestigated with density functional theory. These calculations revealed the existence of an energetically accessible and more basic Fe(0)Ni(II) isomer with a square-planar Ni site.

Nickel‐Molybdenum and Nickel‐Tungsten Dithiolates: Hybrid Models for Hydrogenases and Hydrodesulfurization

“…The propensity of iron tricarbonyls 4, 5 and 6 to form hydrides [10] prompted investigation into whether protonation of the present group 6 tetracarbonyls would also proceed. Stable examples of heptacoordinate carbonyl hydrides of Mo II and W II are known, [14,15] and complexes 7 and 9 were examined here.…”

“…[10] Complexes like 4 are among the few functional synthetic models for nickel-iron hydrogenase, a key enzyme in anaerobic H 2 metabolism and an aspirational target for synthetic catalysts. [11] Described now are NiS 2 M (M = Mo, W) derivatives that blend hydrodesulfurization and hydrogenase themes into a promising catalytic motif.…”

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.…”

“…21 This work addresses this weakness. Redox at the nickel centers typically induces changes in the coordination geometry, such that Ni I species (cf.…”

Biomimetic Complexes for Production of Dihydrogen and Reduction of CO2

“…However, catalysis by the complex is likely mechanistically quite distinct from the enzyme. Recent computational work in combination with the experimental evaluation of Ni-Pd and Ni-Pt analogues suggests that protonation occurs at the Fe site of the reduced, mixed valence Fe(0)-Ni(II) complex, whereas interaction with substrate in the enzyme takes place at the nickel [53]. This iron protonation is thought to be induced by a large geometry change at the nickel site from tetrahedral to square planar.…”