A Stable Planar-Chiral <i>N</i>‑Heterocyclic
Carbene with a 1,1′-Ferrocenediyl Backbone
Alex R. Petrov
Anatoli Derheim
Jan Oetzel
Michael Leibold
Clemens Bruhn
Stefan Scheerer
Steffen Oßwald
Rainer F. Winter
Ulrich Siemeling
10.1021/acs.inorgchem.5b01064.s003
https://acs.figshare.com/articles/dataset/A_Stable_Planar_Chiral_i_N_i_Heterocyclic_Carbene_with_a_1_1_Ferrocenediyl_Backbone/2152801
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.
2015-07-06 00:00:00
hydrolysis products
carbon monoxide
THF
IR data
electrochemical data
parameter value
square wave voltammetry
addition product
HOMO energies
chemical properties
ambiphilic NHCs
XRD
betainic enolate
electrochemical investigation
carbene precursor
racemic compound
ambiphilic nature