Cationic complexes of iridium: Diiodobenzene chelation, electrophilic behavior with olefins, and fluxionality of an Ir(I) ethylene complex

The synthesis of a series of dicationic Ir(Ill) complexes is described. Reaction of Ir(CO)(dppe)l (dppe = 1,2-bis(diphenylphosphino)ethane)) with RI (R = CH3 and CF3) results in formation of the lr(111) precursors IrR(CO)(dppe)(1)2 (R = CH3 (1a) and CF3 (1b)). Subsequent treatment with AgOTf (OTf = triflate) generates the bis(triflate) analogues IrR(CO)(dppe)(OTf)(2) (R = CH3 (2a) and CF3 (2b)), which undergo clean metathesis with NaBARF (BARF = B(3,5-(CF3)(2)C6H3)(4)(-)) in the presence of 1,2-diiodobenzene (DIB) forming the dicationic halocarbon adducts [lrR(CO)(dppe)(DIB)][BARF](2) (R = CH3 (3a) and CF3 (3b)). Complexes 3a and 3b demonstrate facile exchange chemistry with acetonitrile and carbon monoxide forming complexes 4 and 5, respectively. NMR investigation of the mechanism reveals that the process proceeds through an eta(1)-diiodobenzene adduct, where labilization at the coordination site trans to the alky( group occurs first. Complex 3a reacts with ethylene forming the cationic iridium(l) product [lr(C2H4)(2)(CO)(dppe)][BARF] (6), which demonstrates fluxional behavior. Variable-temperature NMR studies indicate that the five-coordinate complex 6 undergoes three dynamic processes corresponding to ethylene rotation, Berry pseudorotation, and intermolecular ethylene exchange in order of increasing temperature based on NMR line shape analyses used to determine the thermodynamic parameters for the processes. The DIB adducts 3a and 3b were also found to promote olefin isomerization of I-pentene, and polymerization/oligomerization of styrene, a.-methylstyrene, norbomene, beta-pinene, and isobutylene via cationic initiation.