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Synthesis, Structure, and Unusual Reactivity of β-Halovinyl Cobalt Porphyrin Complexes

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journal contribution
posted on 06.03.2006 by Joseph M. Fritsch, Noah D. Retka, Kristopher McNeill
The preparation, structures, and reactivity of tetraphenylporphyrin (TPP) cobalt halovinyl complexes are reported. β-Halovinyl complexes of (TPP)Co(E-CHCHX) (X = Br and I) were prepared from the insertion of acetylene into the cobalt halide bonds of the corresponding halide complexes. The reactivity of these compounds and of the previously reported (TPP)Co(E-CHCHCl), was studied in depth, and it was found that complex reactivity increased with the leaving group ability of the halide. A trans-dichlorovinyl cobalt porphyrin complex, (TPP)Co(Z-CClCHCl), was also prepared through the reaction of (TPP)CoNa and TCE. The structures of (TPP)Co(E-CHCHBr), (TPP)Co(Z-CClCHCl), and (TPP)Co(C2H) are reported. The C−C bond length of the vinyl group was found to vary for the β-halovinyl complexes (TPP)Co(E-CHCHX) from 1.211 Å for X = Br to 1.234 Å for X = Cl and 1.320 Å for (TPP)Co(Z-CClCHCl). A comparison of these structures to many chlorovinyl cobalt complexes shows that trans-2-halo substitution results in a dramatically decreased vinyl C−C bond length. The mechanism of halide substitution for the β-halovinyl complexes was investigated with kinetic experiments that indicated a dissociative mechanism and supported the intermediacy of a cobalt acetylene complex.