Effect of positive-charges in diphosphino-imidazolium salts on the structures of Ir-complexes and catalysis for hydroformylation

The effect of positive charges in the diphosphino-imidazolium salts of L2 was investigated in terms of coordinating character, structures of corresponding Ir-complexes and catalysis for hydroformylation. It was found that the involved positive charges exhibited strong electron-withdrawing effect on the neighbored phosphine fragment, rendering L2 more π-acceptor ability than the corresponding neutral counterpart of L1. Consequently, the changed coordinating ability of L1 and L2 led to the variety in the structures and components for the Ir-complexes (Ir-L1a, Ir-L1b, Ir-L2a, and Ir-L2b). The complexation of L2 with Ir(acac)(CO)₂ led to a novel five-coordinate Ir(II)-complex of Ir-L2a chelated by a PCC (phosphine-carboanion-carbene) pincer in tripodal mode, whereas the complexation of L1 with Ir(acac)(CO)₂ led to a four-coordinate square-planar Ir(I)-complex of Ir-L1a chelated by a PCP (phosphine-carboanion-phosphine) pincer. In addition, the different catalytic performances of these Ir-complexes ligated by L1 and L2 for hydroformylation of olefins were investigated.