Proton induced coupling reactions in dinuclear σ-alkynyl-μ-methylene-rhodium complexes

Addition of two equivalents of HBF4 to suspensions of the di-alkynyl-di-μ-methylene-dirhodium complexes [(C5Me5)2Rh2(μ-CH2)2(C2R)2] (2, R=Ph; 3, R=p-C6H4Me) (prepared from the chloro-complex [(C5Me5)2Rh2(μ-CH2)2Cl2] (1), and RC2MgCl), gave the unexpected products syn- and anti-[(C5Me5)2Rh2(μ-η5,η5′-{XC6H4CH(CH2)CC(CH2)CHC6H4X}][BF4]2 (4, X=H; and 5, X=p-Me). The solid-state structure of syn-5, determined by single crystal X-ray diffraction, was shown to contain a hydrocarbon skeleton of two linked η5-allylbenzenes. Complexes 4 and 5 underwent a dynamic process in solution; this was examined by VT-NMR spectroscopy for 5 and was shown to involve η5-/η3-/η5-migration of the allylbenzene fragment, with a free energy of activation of 62 kJ mol−1. The complexes 2 and 3 reacted with HCl to give 1 and RC2H.