A Stable Planar-ChiralN-Heterocyclic Carbene with a 1,1′-Ferrocenediyl Backbone

Abstract

This paper focuses on the stable, ferrocene-based N-heterocyclic carbene (NHC) rac-[Fe{(η(5)-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(η(5)-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.

Redox‐Active N‐Heterocyclic Germylenes and Stannylenes with a Ferrocene‐1,1′‐diyl Backbone

“…DFT calculations indicate an appreciable delocalisation of the charge and spin density onto the carbene centre for the oxidised form . Nevertheless, the large anodic shift of the E 1/2 value by 0.410 V in 1′Np C relative to the corresponding 3,3′‐di‐ tert ‐butyl‐1,1′‐bis(neopentylamino)ferrocene precursor ( E 1/2 =−0.720 V) indicates sizable electron donation of the amino substituents to the carbene carbon atom, thus rendering oxidation less facile . With the availability of the present new compounds, it was of great interest to study the effect of the nature of the divalent tetrel atom on the electrochemical properties.…”

“…Previous electrochemical studies have revealed that the ferrocenylene backbone endows ferrocene‐derived NHCs with reversible electrochemistry, as exemplified for the 3,3′‐di‐ tert ‐butyl‐substituted compound [Fe{(η 5 ‐ t Bu‐C 5 H 3 )NNp} 2 C:] ( 1′Np C). The rather low half‐wave potential ( E 1/2 ) of −0.310 V (vs. ferrocene/ferrocenium) for the 0/+ couple of 1′Np C shows these unusual ferrocene compounds to be rather electron rich . DFT calculations indicate an appreciable delocalisation of the charge and spin density onto the carbene centre for the oxidised form .…”

“…With the availability of the present new compounds, it was of great interest to study the effect of the nature of the divalent tetrel atom on the electrochemical properties. There has been documented high reactivity of carbenes 1 Np C and 1′ Np C toward chlorinated solvents, including dichloromethane, that give rise to an addition product derived from the formal insertion of the carbene carbon atom into a C−H bond, and even trace amounts of water, which leads to the corresponding ring‐opened 1,1′‐disubstituted amino(formamido)ferrocene . Therefore, all these studies were conducted inside a nitrogen‐filled glovebox with oven‐dried glassware and [ n Bu 4 N][PF 6 ]; furthermore, the THF/[ n Bu 4 N][PF 6 ] electrolyte was dried and stored over a potassium mirror (see Table for the results).…”

“…Ligands with a redox tunability superior to that of the NHC system 1 R C were the aim of Breher's study . Our work was stimulated more by the exceptional alkyl(amino)carbene‐like reactivity of such ferrocene‐based NHCs, which we reported recently, and focussed on 1 Np C and its 3,3′‐di‐ tert ‐butyl‐substituted homologue [Fe{(η 5 ‐ t Bu‐C 5 H 3 )NNp} 2 C:] ( 1′ Np C) . Due to the ambiphilicity of these stable NHCs, these compounds are suitable for the activation of fundamentally important small molecules such as ammonia and carbon monoxide .…”

“…In our work on ferrocene‐based NHCs, the neopentyl substituent was particularly advantageous in terms of the stability of the carbene and the solubility and crystallinity of the carbene and its precursors and derivatives –. The neopentyl substituent has also proved to be useful in the context of stable monomeric N‐heterocyclic germylenes and stannylenes .…”