EFFECTS OF CYCLOPENTADIENYL AND PHOSPHINE-LIGANDS ON THE BASICITIES AND NUCLEOPHILICITIES OF CP'IR(CO)(PR(3)) COMPLEXES

Basicities of the series of complexes CpIr(CO)(PR(3)) [PR(3) = P(p-C6H 4CF3)(3), P(p-C6H4F)(3), P(p-C6H4Cl)(3), PPh(3), P(p-C6H4CH3)(3), P(p- C6H4OCH3)(3), PPh(2)Me, PPhMe(2), PMe(2), PEt(3), PCy(3)] have been me asured by the heat evolved (Delta H-HM) when the complex is protonated by CF3SO3H in 1,2-dichloroethane (DCE) at 25.0 degrees C. The -Delta H-HM values range from 28.0 kcal/mol for CpIr(CO)[P(p-C6H4CF3)(3)] to 33.2 kcal/mol for CpIr(CO)(PMe(3)) and are directly related to the bas icities of the PR(3) ligands in the complexes. For the more basic pent amethylcyclopentadienyl analogs, the -Delta H-HM values range from 33. 8 kcal/mol for the weakest base CpIr(CO)[P(p-C6H4CF3)(3)] to 38.0 kca l/mol for the strongest CpIr(CO)(PMe(3)). The nucleophilicities of th e Cp ' Ir(CO)(PR(3)) complexes were established from second-order rate constants (k) for their reactions with CH3I to give [Cp ' Ir(CO)(PR(3 ))(CH3)]I-+(-) CD2Cl2 at 25.0 degrees C. There is an excellent linear correlation between the basicities (Delta H-HM) and nucleophilicities (log k) of the CpIr(CO)(PR(3)) complexes. Only the complex CpIr(CO)(PC y(3)) with the bulky tricyclohexylphosphine ligand deviates dramatical ly from the trend. In general, the pentamethylcyclopentadienyl complex es react 40 times faster than the cyclopentadienyl analogs. However, t hey do not react as fast as predicted from electronic properties of th e complexes, which suggests that the steric size of the Cp ligand red uces the nucleophilicities of the CpIr(CO)(PR(3)) complexes. In addit ion, heats of protonation (Delta H-HP) of tris(2-methoxyphenyl)phosphi ne, tris(2,6-dimethoxyphenyl)phosphine, and tris(2,4,6-trimethylphenyl )phosphine were measured and used to estimate pK(a) values for these h ighly basic phosphines.