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

Abstract

AbstractThe protonation of [CpCo(C4Me4)] (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 NEt3 mainly produces 1 (60 %, isolated yield), and in DMSO the solvent acts as base transforming [3]BF4 into 1 (67 %, isolated yield). Deprotonation of [3]BF4 with excess NEt3 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]BF4 in near quantitative yield (97 %). The pyridine rings in 6+ and 7+ display hindered rotation (NMR), and the structure of [7]CF3SO3 shows considerable steric crowding. On treatment with a large excess of CF3CO2H ( ≥ 7 equiv.) the hexatriene complex 5 quantitatively reverts to the carbenoid 3+. Reaction of 5 with (C5H5NH)CF3SO3 (–80 °C, 3 weeks) produced [6]CF3SO3 with retention of the stereochemistry at C‐4 (NMR), while the same reaction at ambient temperature and in presence of a large excess of pyridine is accompanied by stereoisomerization and afforded [7]CF3SO3 (94 %). Nucleophilic addition of excess PPh3 to 3+ gave a phosphonium salt [8]BF4, again with stereoisomerization, in near quantitative yield (98 %). Addition of CNtBu with concomitant rearrangement produced a (tert‐butylamino)cobaltocenium salt [9]CF3SO3 (90 %). Nucleophilic substitution of the pyridine moiety of 7+ with 4‐picoline gave the 4‐picoline additionproduct [10]CF3SO3 (96 %). With CN– a hexa‐2,4‐dienenitrile complex [CpCo(η4‐C4HMe4CN)] (11) was obtained (85 %), which on protonation gave an aminocobaltocenium ion [CpCo{C5Me4(NH2)}]+ (12+), isolated as [12]BF4 (94 %). Acid removes the pyridine moiety from 7+ to produce a carbenoid ion 13+ with a 4‐Me group in syn‐position (a stereoisomer of 3+) which was characterized by NMR spectroscopy. Deprotonation of 13+ affords a hexatriene complex 14 (a stereoisomer of 5) in good yield (57 %). Thermolyses of 5 and 14 show that 14 is the lower‐energy isomer and produce a dinuclear complex [(μ‐C5H3Me3)(CoCp)2(Co–Co)] (15) (91 %) with a bridging penta‐2,4‐dienylidene ligand. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)