Electron-Rich Vaska-Type Complexes trans-[Ir(CO)Cl(2-Ph2PC6H4COOMe)(2)] and trans-[Ir(CO)Cl(2-Ph2PC6H4OMe)(2)]: Synthesis, Characterisation and Reactivity

The in-situ-generated dimeric precursor [Ir(CO)(2)Cl](2) reacts with four molar equivalents of the ligands 2-Ph2PC6H4-COOMe (a) and 2-Ph2PC6H4OMe (b) to afford tetracoordinated complexes of the type trans-[Ir(CO)ClL2] (1a, 1b), where L = a and b. The IR spectra of 1a and 1b in CHCl3 solution show the terminal nu(CO) bands at around 1957 and 1959 cm (1), respectively, which are significantly lower in frequency compared to Vaska's complex, trans-[Ir(CO) Cl-(PPh3)(2)] (1965 cm(-1)) and substantiate the enhanced electron density at the metal centre. The single-crystal X-ray structure of 1a indicates iridium-oxygen (ester group) distances [Ir center dot center dot center dot O(2) 3.24 angstrom Ir center dot center dot center dot O(5) 3.29 angstrom] and angle [O(5)center dot center dot center dot Ir center dot center dot center dot O(2) 157.25 degrees] suggesting a long-range intramolecular "secondary" Ir center dot center dot center dot O interaction resulting in a pseudo-hexacoordinated complex. Complex 1b reacts with O-2 to generate [Ir(O-2)(CO)-Cl(2-Ph2PC6H4OMe)(2)] (2b), while 1a remains unreactive. Complex 2b shows a distorted octahedral structure with peroxo O-O linkage (O2-O3 1.47 angstrom). The kinetic study of the reaction of 1b and Vaska's complex towards dioxygen addition reveals that the rate of dioxygen addition to 1b is about three times faster than Vaska's complex. Complexes 1a and 1b undergo oxidative addition with small molecules like CH3I and I-2 to produce Ir-III carbonyl species of the type [Ir(CO)Cl(CH3)IL2] (3a, 3b) and [Ir(CO)ClI2L2] (4a, 4b), where L = a, b.