Scaffold-based [Fe]-hydrogenase model: H2 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-2 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-2/hydride-induced loss of the organometallic acyl unit (i.e. pyCH(2)-C=O -> pyCH(3)+C O). The known reduced hydride species [HFe(CO)(4)](-) and [HFe3(CO)(11)](-) have been observed as products by H-1/H-2 NMR and IR spectroscopies, as well as independent syntheses of PNP[HFe(CO)(4)]. The former species (i.e. [HFe(CO)(4)](-)) is deduced to be the actual hydride transfer agent in the hydride transfer reaction (nominally catalyzed by the title compound) to a biomimetic substrate ([(Tol)Im](BArF) = 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.

Automated summary

The research focuses on the synthesis and reactivity of a model of [Fe]-hydrogenase derived from an anthracene-based scaffold. It demonstrates the maintenance of well-defined chemistry upon addition of exogenous base, and the formation of a dimeric motif containing a new Fe-C(methine) bond through deprotonation of the acyl(methylene) C-H bond with a phenolate base.This study provides mechanistic insight into the lack of functional biomimetic behavior in acyl(methylene)pyridine based mimics of [Fe]-hydrogenase, shedding light on the reasons behind this phenomenon.