CNS and CNP Iron(II) Mono-Iron Hydrogenase (Hmd) Mimics: Role of Deprotonated Methylene(acyl) and the trans-Acyl Site in H2 Heterolysis.

We report syntheses and H2 activation involving model complexes of mono-iron hydrogenase (Hmd) derived from acyl-containing pincer ligand precursors bearing thioether (CNS Pre ) or phosphine (CNP Pre ) donor sets. Both complexes feature pseudo-octahedral iron(II) dicarbonyl units. While the CNS pincer adopts the expected mer-CNS (pincer) geometry, the CNP ligand unexpectedly adopts the fac-CNP coordination geometry. Both complexes exhibit surprisingly acidic methylene C-H bond (reversibly de/protonated by a bulky phenolate), which affords a putative dearomatized pyridinate-bound intermediate. Such base treatment of Fe-CNS also results in deligation of the thioether sulfur donor, generating an open coordination site trans from the acyl unit. In contrast, Fe-CNP maintains a CO ligand trans from the acyl site both in the parent and dearomatized complexes (the -PPh2 donor is cis to acyl). The dearomatized mer-Fe-CNS was competent for H2 activation (5 atm D2(g) plus phenolate as base), which is attributed to both the basic site on the ligand framework and the open coordination site trans to the acyl donor. In contrast, the dearomatized fac-Fe-CNP was not competent for H2 activation, which is ascribed to the blocked coordination site trans from acyl (occupied by CO ligand). These results highlight the importance of both (i) the open coordination site trans to the organometallic acyl donor and (ii) a pendant base in the enzyme active site.