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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 –CO → pyCH 3 +CO). The known reduced hydride species [HFe(CO) 4 ] − and [HFe 3 (CO) 11 ] − have been observed as products by 1 H/ 2 H 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](BAr F ) = 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.
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Diazomethane umpolung atop anthracene: an electrophilic methylene transfer reagent
Formal addition of diazomethane's terminal nitrogen atom to the 9,10-positions of anthracene yields H 2 CN 2 A ( 1 , A = C 14 H 10 or anthracene). The synthesis of this hydrazone is reported from Carpino's hydrazine H 2 N 2 A through treatment with paraformaldehyde. Compound 1 has been found to be an easy-to-handle solid that does not exhibit dangerous heat or shock sensitivity. Effective umpolung of the diazomethane unit imbues 1 with electrophilicity at the methylene carbon center. Its reactivity with nucleophiles such as H 2 CPPh 3 and N-heterocyclic carbenes is exploited for CC bond formation with elimination of dinitrogen and anthracene. Similarly, 1 is demonstrated to deliver methylene to a nucleophilic singlet d 2 transition metal center, W(ODipp) 4 ( 2 ), to generate the robust methylidene complex [ 2 CH 2 ]. This behavior is contrasted with that of the Wittig reagent H 2 CPPh 3 , a more traditional and Brønsted basic methylene source that upon exposure to 2 contrastingly forms the methylidyne salt [MePPh 3 ][ 2 CH].
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- Award ID(s):
- 1664799
- PAR ID:
- 10119260
- Date Published:
- Journal Name:
- Chemical Science
- Volume:
- 9
- Issue:
- 6
- ISSN:
- 2041-6520
- Page Range / eLocation ID:
- 1540 to 1543
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c7sc04506a
