Abstract
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 H2 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 H2/hydride-induced loss of the organometallic acyl unit (i.e. pyCH2–C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O → pyCH3+C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O). The known reduced hydride species [HFe(CO)4]− and [HFe3(CO)11]− have been observed as products by 1H/2H 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 ([TolIm](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.
Highlights
The search for earth abundant substitutes for precious metal catalysts in energy-related chemical transformations has led researchers to investigate biological precedents that utilize rstrow transition metals.[1,2,3,4,5,6] Of these enzymes, the [FeFe] and [NiFe] H2ases have been studied in detail for their redox active sites for the generation and metabolism of dihydrogen (H2).[7,8,9] Less studied is the ‘third hydrogenase’ — namely the redox inactive [Fe]-hydrogenase (Hmd)
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
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
Summary
The search for earth abundant substitutes for precious metal catalysts in energy-related chemical transformations has led researchers to investigate biological precedents that utilize rstrow transition metals.[1,2,3,4,5,6] Of these enzymes, the [FeFe] and [NiFe] H2ases have been studied in detail for their redox active sites for the generation and metabolism of dihydrogen (H2).[7,8,9] Less studied is the ‘third hydrogenase’ — namely the redox inactive [Fe]-hydrogenase (Hmd). The 1H NMR spectrum of 1 in d8-THF solution (Fig. S2†) exhibits diamagnetic proton resonances with the characteristic methylene proton resonances observed as diastereotopic doublets at 3.97 and 4.52 ppm consistent with the ligation of the anionic acyl (–CH2C]O) group to the iron center. The 2H NMR spectrum (Fig. 5A) of the reaction was monitored, revealing new resonances at 2.59 ppm and À14.90 ppm, corresponding to deuteration of the 2-methylpyridine moiety of the Anth$CH3NSMe ligand and an Fe–D species, respectively.
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