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

作者：Petrov Alex R; Derheim Anatoli; Oetzel Jan; Leibold Michael; Bruhn Clemens; Scheerer Stefan; Osswald Steffen; Winter Rainer F; Siemeling Ulrich^*

来源：Inorganic Chemistry, 2015, 54(13): 6657-6670.

DOI：10.1021/acs.inorgchem.5b01064

This paper focuses On the stable, ferrocene-based N-heterocyclic carbene (NHC) rac-[Fe{(eta(5)-t-BuC5H3)NpN}(2)C:] (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(eta(5)-t-BuC5H3Li)(2)] (rac-fc'Li-2, 2). The structures of rac-fc'(SiMe3)(2) (3), rac-fc'Br-2 (4), rac-fc'(N-3)(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)(2)CO as four stereoisomers, viz. (R(p)R(p-)A'-Np)= C(O-) (RpRp-A'-Np+), (SpSp-A'-Np)= C(O-)(SpSp-A'-Np+), (RpRp-A'-Np)=(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.

• 出版日期2015-7-6