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A Stable Planar-Chiral N‑Heterocyclic Carbene with a 1,1′-Ferrocenediyl Backbone

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posted on 06.07.2015, 00:00 by Alex R. Petrov, Anatoli Derheim, Jan Oetzel, Michael Leibold, Clemens Bruhn, Stefan Scheerer, Steffen Oßwald, Rainer F. Winter, Ulrich Siemeling
This paper focuses on the stable, ferrocene-based N-heterocyclic carbene (NHC) rac-[Fe­{(η5-t-BuC5H3)­NpN}2C:] (A′-Np, Np = neopentyl), which is planar-chiral due to the two tert-butyl substituents in 3,3′-positions. A′-Np was synthesized in nine steps starting from 1,1′-di-tert-butylferrocene (1), the first step being its 3,3′-dilithiation to afford rac-[Fe­(η5-t-BuC5H3Li)2] (rac-fc′Li2, 2). The structures of rac-fc′(SiMe3)2 (3), rac-fc′Br2 (4), rac-fc′(N3)2 (5), and the immediate carbene precursor [A′-NpH]­BF4 were determined by single-crystal X-ray diffraction (XRD). The chemical properties of A′-Np were found to be very similar to those of its tert-butyl-free congener A-Np, both being ambiphilic NHCs with rather high calculated HOMO energies (ca. −4.0 eV) and low singlet–triplet gaps (ca. 35 kcal/mol). A Tolman electronic parameter value of 2050 cm–1 was derived from IR data of cis-[RhCl­(A′-Np)­(CO)2], indicating the high donicity of A′-Np as a ligand. Consistent with its ambiphilic nature, A′-Np was found to react readily with carbon monoxide, affording the betainic enolate (A′-Np)2CO as four stereoisomers, viz. (RpRp-A′-Np)C­(O)­(RpRp-A′-Np+), (SpSp-A′-Np)C­(O)­(SpSp-A′-Np+), (RpRp-A′-Np)C­(O)­(SpSp-A′-Np+), and (SpSp-A′-Np)C­(O)­(RpRp-A′-Np+). The former two isomers were structurally characterized as a racemic compound by single-crystal XRD. A′-Np was found to react swiftly with dichloromethane, affording the addition product A′-NpH–CHCl2 in a reaction that is unprecedented for diaminocarbenes. A-NpH–CHCl2 was obtained analogously. Both compounds were structurally characterized by single-crystal XRD. An electrochemical investigation of A′-Np by cyclic and square wave voltammetry revealed a reversible oxidation of the carbene at a half-wave potential of −0.310 vs ferrocene/ferrocenium (THF/NBu4PF6). The electrochemical data previously published for A-Np were identified to be incorrect, since unnoticed hydrolysis of the NHC had taken place, affording A-Np(H2O). The hydrolysis products of A-Np and A′-Np were found to be reversibly oxidized at half-wave potentials of −0.418 and −0.437 V, respectively.