Reaction Chemistry of the Carbenoid Butadienyl Complex Ion [CpCo(σ,η⁴‐C₄HMe₄)]⁺ Formed by Protonation of the Cyclobutadiene Complex [CpCo(C₄Me₄)]

Abstract

The protonation of [CpCo(C 4 Me 4 )] (1) with excess acid, which produces the carbenoid ion 3 + , is shown to be reversible under conditions of low proton activity. Treatment of 3 + with one equivalent of NEt 3 mainly produces 1 (60 %, isolated yield), and in DMSO the solvent acts as base transforming [3]BF 4 into 1 (67 %, isolated yield). Deprotonation of [3]BF 4 with excess NEt 3 affords (cyclopentadienyl)[(2-5-η)-(3Z,4Z)-3,4-dimethylhexa-1,2,4-triene]cobalt (5) in high yield (83 %). The structure of 5 has been determined by X-ray work. In the reaction of 3 + with pyridine four reaction channels could be unveiled by low-temperature NMR spectroscopy: i) Reversible deprotonation to form 5 in a kinetic shunt; ii) reversible nucleophilic addition at the carbenoid center to form a pyridinium ion complex 6 + as a low-temperature species with a terminal anti-Me group; iii) irreversible isomerization of 6 + to the more stable stereoisomer 7 + with a terminal syn-Me group; iv) very slow formation of 1. Using very high concentrations of pyridine (mixing at -60°C, slowly warming to 20°C) allowed the preparation of [7]BF 4 in near quantitative yield (97 %). The pyridine rings in 6 + and 7 + display hindered rotation (NMR), and the structure of [7]CF 3 SO 3 shows considerable steric crowding. On treatment with a large excess of CF 3 CO 2 H ( Ն 7 equiv.) the hexatriene complex 5 quantitatively reverts to the carbenoid 3 + . Reaction of 5 with (C 5 H 5 NH)CF 3 SO 3 (-80°C, 3 weeks) produced

N-Cobaltocenium Amide as Reactive Nucleophilic Reagent for Donor–Acceptor Bimetallocenes

“…Electron-poor metallocenes such as Cp 2 Ti­(CCR) 2 or [Cp 2 Co] + (Cc + ) , were successfully incorporated as electron acceptors in CT systems. Compared to the ubiquitous Fc/Fc + couple, the Cc/Cc + couple has been less often employed in CT and PET schemes. ,, This underdeveloped attention is certainly due to the more difficult and less established substitution chemistry of Cc + . Only in the last years have the groups of Bildstein and Heck put forward reliable routes toward important Cc + key building blocks, such as [Cc-Br] + , [Cc-NO 2 ] + , [Cc-N 3 ] + , [Cc-COOH] + , or [Cc-NH 2 ] + . , …”

Metallocenium Chemistry and Its Emerging Impact on Synthetic Macromolecular Chemistry

“…28 Substi-tuted derivative 32 can be prepared from half-sandwich complex CpCo(C 9 H 16 ) through the addition of tert-butyl isocyanide. 29 Planar-chiral cobaltoceniums 33-35 were prepared using achiral trisubstituted Cp's as starting materials in the same way. 30 A more-complicated helical cobaltocenium was reported by Katz, Geiger, Rheingold, and co-workers (Figure 5).…”

Reactions of Metal-Complexed Carbocyclic 4π Systems

“…93 The σ:η 4 -butadienyl complex 161 was deprotonated with NEt 3 , affording the hexatriene complex 164 as an isomer of the CpCo-capped cyclobutadiene 159 (Scheme 42). 94 Careful investigations of this protonation/deprotonation equilibrium have shown that the cyclobutadiene complex 159 is thermodynamically more stable than the hexatriene complex 164. Complex 161 also shows a fascinating ring formation (and in terms of starting from the cyclobutadiene in total a ring enlargement) when it is reacted with tert-butyl isocyanide.…”

“…Complex 161 also shows a fascinating ring formation (and in terms of starting from the cyclobutadiene in total a ring enlargement) when it is reacted with tert-butyl isocyanide. 94 This very fast reaction is carried out at -35 °C, and the resulting cobaltocenium ion 165 was obtained in about 90% yield.…”

“…The σ:η 4 -butadienyl complex 161 was deprotonated with NEt 3 , affording the hexatriene complex 164 as an isomer of the CpCo-capped cyclobutadiene 159 (Scheme ) . Careful investigations of this protonation/deprotonation equilibrium have shown that the cyclobutadiene complex 159 is thermodynamically more stable than the hexatriene complex 164 .…”