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

Cyclometalated chloride complexes of rhodium and iridium based on (eta(5)-C5Me5)M-III fragments that result from the metalation of the xylyl substituent of a coordinated ER2(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 PiPr(2)(Xyl), PCy2(Xyl), PMe2(Xyl), and PEh(2)(Xyl), respectively, have been prepared, whereas for rhodium. only the complexes 2a,d, derived from (PPr2)-Pr-i(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 alpha-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 (PPr2)-Pr-i(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 alpha-H elimination and Migratory insertion reactions: Reactivity studies on the cationic hydride alkylidene complexes of iridium are reported as well.