ic5b01064_si_001.pdf (8.38 MB)
A Stable Planar-Chiral N‑Heterocyclic Carbene with a 1,1′-Ferrocenediyl Backbone
journal contribution
posted on 2015-07-06, 00:00 authored by Alex R. Petrov, Anatoli Derheim, Jan Oetzel, Michael Leibold, Clemens Bruhn, Stefan Scheerer, Steffen Oßwald, Rainer F. Winter, Ulrich SiemelingThis
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.