An unusual example of allyl-to-alkynyl migration in a phenylacetylide-bridged heterobinuclear complex of rhodium and iridium

Abstract

The reactivity of the alkynyl-bridged complex [RhIr(CO)2(μ2-η1:η2-C2Ph)(dppm)2][X] (X = BF4 (1a), SO3CF3 (1b); dppm = Ph2PCH2PPh2) with electrophiles has been demonstrated. Protic acids HX (X = BF4, SO3CF3) first yield the oxidative-addition products [RhIr(X)(CO)2(μ-H)(μ-C2Ph)(dppm)2][X], which under carbon monoxide result in displacement of the weakly coordinating BF4- or SO3CF3- anions and subsequent conversion to the vinylidene-bridged [RhIr(CO)4(μ-CC(H)Ph)(dppm)2][X]2. Reaction of 1 with allyl halides yields the allyl vinylidene-bridged compounds [RhIr(Y)(CO)(μ-CC(Ph)CH2CHCH2)(μ-CO)(dppm)2][X] (Y = Br (5), Cl (6)), by coupling of the alkynyl and allyl groups at the β-position of the alkynyl moiety. NMR studies at low temperatures show coordination of allyl halide at Ir at −80 °C, followed by allyl halide loss and subsequent oxidative addition at −50 °C. The oxidative-addition intermediates, [RhIr(η1-CH2CHCH2)(CO)2(μ-Y)(μ-C2Ph)(dppm)2][X] (Y = Br (9), Cl (10)), rearrange to the allylvinylidene products (5 and 6) at ambient temperature. Although halide removal from compounds 5 and 6, using AgBF4, does not result in destabilization of the allylvinylidene fragment, resulting instead in replacement of halide by fluoborate ion, the reaction of 1 with allyl halide in the presence of a silver salt does not lead to coupling of the allyl and alkynyl moieties, but gives [RhIr(η3-C3H5)(CO)(μ-C2Ph)(μ-CO)(dppm)2][X]2 (13). Addition of halide ion to this η3-allyl complex at ambient temperature again leads to formation of 5 or 6. On the basis of these results a mechanism is proposed for the allyl/alkynyl coupling reaction.