Preparation of Cobalt‐Containing Ligands with NHC‐ and/or P‐Coordinating Sites and Their Application in Heck Reactions: The Formation of an Unexpected Cobalt‐Containing Zwitterionic Complex

An unexpected cobalt‐containing zwitterionic complex, [{(μ‐PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 }{μ,η‐(R1)PC≡CCH 2 (R2)}] ( 8 ; R1 = Ph 2 Bz; R2 = 3‐CoBr 3 ‐1‐benzimidazole), has been obtained during an attempt to modify the imidazole moiety in the alkyne‐bridging dicobalt complex [{(μ‐PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 }{μ,η‐Ph 2 PC≡CCH 2 (IM)}] ( 5 ; IM = benzimidazole) to a carbene‐type substituent by reaction with benzyl bromide. As revealed in the crystal structure of 8 , it is clear that the benzyl bromide was nucleophilically attacked by the phosphane moiety of 5 with the result the addition of the benzyl moiety. This phosphane site in 8 is in a tetrahedral environment and presumably cationic in nature. In addition, an unusual anionic species CoBr 3 – was found to have attached itself to the free nitrogen site of the imidazole moiety. Thus 8 can be regarded as an unusual metal‐containing zwitterionic complex. When the phosphane site of 5 was replaced by a non‐reactive substituent, for example, [{(μ‐PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 }{μ,η‐RC≡CCH 2 (IM)}] ( 5′ : R = H; 5 _O : R = Ph 2 (O)P), nucleophilic attack took place exclusively on the imidazole moiety and led to the formation of [{(μ‐PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 }{μ,η‐RC≡CCH 2 (IM′)} + ]Br – [ 9′ : R = H; 9_O : R = Ph 2 (O)P; IM′ = 3‐benzyl‐1 H ‐benzimidazole]. Complex 9′ could act as a carbene precursor, which was demonstrated by the formation of the bis‐ 9′ ‐coordinated palladium complex [ trans ‐({(μ‐PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 }{μ,η‐HC≡CCH 2 (IM′)}) 2 PdBr 2 ] ( 11 ) on treatment with [Pd(cod)Cl 2 ]. Fair‐to‐good efficiencies were observed in Heck reactions employing either isolated 11 or in situ 9′ /PdCl 2 (2:1) as the catalytic precursor.