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

Abstract

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.

Scaffold-based [Fe]-hydrogenase model: H2 activation initiates Fe(0)-hydride extrusion and non-biomimetic hydride transfer

“…The structure of 2 unequivocally confirms deprotonation of the methylene proton as proposed (but not unambiguously proven) in another acyl-containing model compound (a mer -CNS dicarbonyl) recently published by our group, 26 suggesting that this mechanism is broadly applicable. Furthermore, deprotonation of the methylene-acyl moiety has been observed in another model compound by Song and coworkers through a suggested keto–enol tautomerization and acylation mechanism, although the analogous intermediate was not identified in that case.…”

Biomimetic Hydrogenation Catalyzed by a Manganese Model of [Fe]‐Hydrogenase

“…Many Fe complexes have been made to model the active site of [Fe]‐hydrogenase . Among them, only few complexes could activate H 2 –. Complex 1 (Figure c), the best structural model, is inactive towards H 2 presumably because the 2‐OH group first needs to be deprotonated.…”

Diversifying Metal–Ligand Cooperative Catalysis in Semi‐Synthetic [Mn]‐Hydrogenases

“…[11] Note that deprotonation of methylene(acyl) Angewandte group by ab ase was also reported for Fe models of [Fe]hydrogenase,b ut the resulting enolate anion did not have ac atalytic role. [9] In the case of 1d (Supporting Information, Figure S13), no deprotonation is needed to create abasic site. Thus,t he reaction occurs by replacement of aC Oby H 2 followed by heterolytic H 2 cleavage via aMn-O cooperation.…”