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

We report the synthesis and reactivity of a model of [Fe]-hydrogenase derived from an anthracene-based scaffold that includes the endogenous, organometallic acyl(methylene) donor. In comparison to other non-scaffolded acyl-containing complexes, the complex described herein retains molecularly well-defined chemistry upon addition of multiple equivalents of exogenous base. Clean deprotonation of the acyl(methylene) C-H bond with a phenolate base results in the formation of a dimeric motif that contains a new Fe-C(methine) bond resulting from coordination of the deprotonated methylene unit to an adjacent iron center. This effective second carbanion in the ligand framework was demonstrated to drive heterolytic H activation across the Fe(ii) center. However, this process results in reductive elimination and liberation of the ligand to extrude a lower-valent Fe-carbonyl complex. Through a series of isotopic labelling experiments, structural characterization (XRD, XAS), and spectroscopic characterization (IR, NMR, EXAFS), a mechanistic pathway is presented for H/hydride-induced loss of the organometallic acyl unit ( pyCH-C[double bond, length as m-dash]O → pyCH+C[triple bond, length as m-dash]O). The known reduced hydride species [HFe(CO)] and [HFe(CO)] have been observed as products by H/H NMR and IR spectroscopies, as well as independent syntheses of PNP[HFe(CO)]. The former species ( [HFe(CO)]) is deduced to be the actual hydride transfer agent in the hydride transfer reaction (nominally catalyzed by the title compound) to a biomimetic substrate (Im = fluorinated imidazolium as hydride acceptor). This work provides mechanistic insight into the reasons for lack of functional biomimetic behavior (hydride transfer) in acyl(methylene)pyridine based mimics of [Fe]-hydrogenase.