Rhodium and Iridium Complexes of Bulky Tertiary Phosphine Ligands. Searching for Isolable Cationic MIII Alkylidenes

Cyclometalated chloride complexes of rhodium and iridium based on (η5-C5Me5)­MIII fragments that result from the metalation of the xylyl substituent of a coordinated PR2(Xyl) phosphine (Xyl = 2,6-Me2C6H3) have been prepared by reaction of the appropriate metal precursor with the corresponding phosphine. For iridium, the four complexes 1a–d, derived from the phosphines PiPr2(Xyl), PCy2(Xyl), PMe2(Xyl), and PPh2(Xyl), respectively, have been prepared, whereas for rhodium only the complexes 2a,d, derived from PiPr2(Xyl) and PMe2(Xyl), respectively, have been studied. Chloride abstraction from compounds 1 and 2 by NaBArF (BArF = B­(3,5-C6H3(CF3)2)4) leads to either cationic dichloromethane adducts or to cationic hydride alkylidene structures resulting from α-H elimination. The rhodium complexes investigated yield only dichloromethane adducts. However, in the iridium system the less sterically demanding phosphines PMe2(Xyl) and PPh2(Xyl) also provide dichloromethane adducts as the only observable products, whereas for the bulkier PiPr2(Xyl) and PCy2(Xyl) ligands the hydride alkylidene formulation prevails. Nonetheless, variable-temperature NMR studies reveal that in solution each of these two structures exists in equilibrium with undetectable concentrations of the other by means of facile reversible α-H elimination and migratory insertion reactions. Reactivity studies on the cationic hydride alkylidene complexes of iridium are reported as well.