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

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.