Synthesis and Structural Characterization of [Ir4(m-CO)(CO)7{m4-h3-Ph2PC(H)C(Ph)PCBut}(m-PPh2)]: Alkyne-Phosphaalkyne Coupling and Formation of a Novel 2-phosphabutadienylphosphine Ligand

Reaction of [Ir4(mu-H)(CO)9(Ph2PCºCPh)(mu-PPh2)] 1 with PºCBu t in CH2Cl2, at 35 °C, for 4 h yields the novel compound [Ir4(mu-CO)(CO)7{mu4-eta³-Ph2PC(H)C(Ph)PCBu t}(mu-PPh2)] 2, which contains the 2-phosphabutadienylphosphine chain. Compound 2 is also formed upon thermolysis of [Ir4(CO)10(Ph2PCºCPh)(PPh2H)] 3 in the presence of PºCBu t in thf, at 40 °C, for 48 h. Small amounts of [Ir4(mu-CO)(CO)7(mu3-eta²-HCCPh)(mu-PPh2)2] 4 are always obtained from both reactions, because of the competing rates of the transformations of 1 and 3 into 4 and of their reactions with PºCBu t. Compound 2 was characterized by analytical and spectroscopic studies such as FAB ms, ¹H, 31P,13C, 2D31P-¹H HETCOR, nOe difference and DEPT NMR experiments, which led to its formulation and established the coupling between the coordinated Ph2PCºCPh and PºCBu t and the migration of the hydride to the Calpha of the Ph2PCºCPh ligand. However, it was impossible to establish unambiguously if cleavage of the P-Csp bond of the Ph2PCºCPh ligand had occurred and the mode of interaction of the organophosphorus chain. An X-ray diffraction study of compound 2 established a butterfly arrangement of iridium atoms with the new ligand interacting with the metal framework via four sigma bonds and the PPh2 phosphorus lone pair.