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

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• 作者：Alex R. Petrov ; Anatoli Derheim ; Jan Oetzel ; Michael Leibold ; Clemens Bruhn ; Stefan Scheerer ; Steffen O脽wald ; Rainer F. Winter ; Ulrich Siemeling
• 刊名：Inorganic Chemistry
• 出版年：2015
• 出版时间：July 6, 2015
• 年：2015
• 卷：54
• 期：13
• 页码：6657-6670
• 全文大小：578K
• ISSN：1520-510X

文摘

This paper focuses on the stable, ferrocene-based N-heterocyclic carbene (NHC) rac-[Fe{(畏⁵-t-BuC₅H₃)NpN}₂C:] (A鈥?Np, Np = neopentyl), which is planar-chiral due to the two tert-butyl substituents in 3,3鈥?positions. A鈥?Np was synthesized in nine steps starting from 1,1鈥?di-tert-butylferrocene (1), the first step being its 3,3鈥?dilithiation to afford rac-[Fe(畏⁵-t-BuC₅H₃Li)₂] (rac-fc鈥睱i₂, 2). The structures of rac-fc鈥?SiMe₃)₂ (3), rac-fc鈥睟r₂ (4), rac-fc鈥?N₃)₂ (5), and the immediate carbene precursor [A鈥?NpH]BF₄ were determined by single-crystal X-ray diffraction (XRD). The chemical properties of A鈥?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. 鈭?.0 eV) and low singlet鈥搕riplet gaps (ca. 35 kcal/mol). A Tolman electronic parameter value of 2050 cm^鈥? was derived from IR data of cis-[RhCl(A鈥?Np)(CO)₂], indicating the high donicity of A鈥?Np as a ligand. Consistent with its ambiphilic nature, A鈥?Np was found to react readily with carbon monoxide, affording the betainic enolate (A鈥?Np)₂CO as four stereoisomers, viz. (R_pR_p-A鈥?Np)鈺怌(O^{鈥?/sup>)(R_pR_p-A鈥?Np+), (S_pS_p-A鈥?Np)鈺怌(O鈥?/sup>)(S_pS_p-A鈥?Np+), (R_pR_p-A鈥?Np)鈺怌(O鈥?/sup>)(S_pS_p-A鈥?Np+), and (S_pS_p-A鈥?Np)鈺怌(O鈥?/sup>)(R_pR_p-A鈥?Np+). The former two isomers were structurally characterized as a racemic compound by single-crystal XRD. A鈥?Np was found to react swiftly with dichloromethane, affording the addition product A鈥?NpH鈥揅HCl₂ in a reaction that is unprecedented for diaminocarbenes. A-NpH鈥揅HCl₂ was obtained analogously. Both compounds were structurally characterized by single-crystal XRD. An electrochemical investigation of A鈥?Np by cyclic and square wave voltammetry revealed a reversible oxidation of the carbene at a half-wave potential of 鈭?.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鈥?Np were found to be reversibly oxidized at half-wave potentials of 鈭?.418 and 鈭?.437 V, respectively.}