Chemistry of transition-metal complexes containing functionalized phosphines. Part 2. Cationic rhodium(I) complexes stabilized by hemilabile diallylphosphines

Cationic rhodium(I) complexes of the type [Rh(COD){κ3(P,C,C)-RP(CH2CHCH2)2}][BF4] (R = tBu 3a, Ph 3b, iPr2N 3c) have been synthesized starting from the chloro-bridged dimer [Rh2(μ-Cl)2(COD)2] and fully characterized by 31P{1H}, 1H and 13C{1H} NMR. X-ray diffraction analysis of 3a and 3c showed a square planar geometry around of the rhodium atom, with allyl-phosphine ligands in a chelating fashion due to coordination of only one of the allylic double bonds while the other allyl moiety is located away from the metal center. Crystals of 3c showed the presence of the two enantiomers in the asymmetric unit. In solution, a dynamic equilibrium of exchange between the two allylic double bonds was detected by low temperature NMR analysis. Hemilabile properties of diallylphosphine ligands have been demonstrated by ligand exchange reactions and the reversible displacement of coordinated allylic double bond by acetonitrile have been observed. DFT calculations have been performed to explain the order of reactivity on removing the acetonitrile ligand under high vacuum.