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

This paper focuses on the stable, ferrocene-based <i>N</i>-heterocyclic carbene (NHC) <i>rac</i>-[Fe­{(η<sup>5</sup>-<i>t-</i>BuC<sub>5</sub>H<sub>3</sub>)­NpN}<sub>2</sub>C:] (<b>A′-Np</b>, Np = neopentyl), which is planar-chiral due to the two <i>tert</i>-butyl substituents in 3,3′-positions. <b>A′-Np</b> was synthesized in nine steps starting from 1,1′-di-<i>tert</i>-butylferrocene (<b>1</b>), the first step being its 3,3′-dilithiation to afford <i>rac</i>-[Fe­(η<sup>5</sup>-<i>t</i>-BuC<sub>5</sub>H<sub>3</sub>Li)<sub>2</sub>] (<i>rac</i>-fc′Li<sub>2</sub>, <b>2</b>). The structures of <i>rac</i>-fc′(SiMe<sub>3</sub>)<sub>2</sub> (<b>3</b>), <i>rac</i>-fc′Br<sub>2</sub> (<b>4</b>), <i>rac</i>-fc′(N<sub>3</sub>)<sub>2</sub> (<b>5</b>), and the immediate carbene precursor [<b>A′-Np</b>H]­BF<sub>4</sub> were determined by single-crystal X-ray diffraction (XRD). The chemical properties of <b>A′-Np</b> were found to be very similar to those of its <i>tert</i>-butyl-free congener <b>A-Np</b>, 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<sup>–1</sup> was derived from IR data of <i>cis</i>-[RhCl­(<b>A′-Np</b>)­(CO)<sub>2</sub>], indicating the high donicity of <b>A′-Np</b> as a ligand. Consistent with its ambiphilic nature, <b>A′-Np</b> was found to react readily with carbon monoxide, affording the betainic enolate (<b>A′-Np</b>)<sub>2</sub>CO as four stereoisomers, viz. (<i>R</i><sub>p</sub><i>R</i><sub>p</sub>-<b>A′-Np</b>)C­(O<sup>–</sup>)­(<i>R</i><sub>p</sub><i>R</i><sub>p</sub>-<b>A′-Np</b><sup>+</sup>), (<i>S</i><sub>p</sub><i>S</i><sub>p</sub>-<b>A′-Np</b>)C­(O<sup>–</sup>)­(<i>S</i><sub>p</sub><i>S</i><sub>p</sub>-<b>A′-Np</b><sup>+</sup>), (<i>R</i><sub>p</sub><i>R</i><sub>p</sub>-<b>A′-Np</b>)C­(O<sup>–</sup>)­(<i>S</i><sub>p</sub><i>S</i><sub>p</sub>-<b>A′-Np</b><sup>+</sup>), and (<i>S</i><sub>p</sub><i>S</i><sub>p</sub>-<b>A′-Np</b>)C­(O<sup>–</sup>)­(<i>R</i><sub>p</sub><i>R</i><sub>p</sub>-<b>A′-Np</b><sup>+</sup>). The former two isomers were structurally characterized as a racemic compound by single-crystal XRD. <b>A′-Np</b> was found to react swiftly with dichloromethane, affording the addition product <b>A′-Np</b>H–CHCl<sub>2</sub> in a reaction that is unprecedented for diaminocarbenes. <b>A-Np</b>H–CHCl<sub>2</sub> was obtained analogously. Both compounds were structurally characterized by single-crystal XRD. An electrochemical investigation of <b>A′-Np</b> 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/NBu<sub>4</sub>PF<sub>6</sub>). The electrochemical data previously published for <b>A-Np</b> were identified to be incorrect, since unnoticed hydrolysis of the NHC had taken place, affording <b>A-Np</b>(H<sub>2</sub>O). The hydrolysis products of <b>A-Np</b> and <b>A′-Np</b> were found to be reversibly oxidized at half-wave potentials of −0.418 and −0.437 V, respectively.