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

We report syntheses and H activation involving model complexes of mono-iron hydrogenase (Hmd) derived from acyl-containing pincer ligand precursors bearing thioether () or phosphine () donor sets. Both complexes feature pseudo-octahedral iron(II) dicarbonyl units. While the pincer adopts the expected -CNS (pincer) geometry, the ligand unexpectedly adopts the -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 also results in deligation of the thioether sulfur donor, generating an open coordination site from the acyl unit. In contrast, maintains a CO ligand from the acyl site both in the parent and dearomatized complexes (the -PPh donor is to acyl). The dearomatized - was competent for H activation (5 atm D plus phenolate as base), which is attributed to both the basic site on the ligand framework and the open coordination site to the acyl donor. In contrast, the dearomatized - was for H activation, which is ascribed to the blocked coordination site from acyl (occupied by CO ligand). These results highlight the importance of both (i) the open coordination site to the organometallic acyl donor and (ii) a pendant base in the enzyme active site.