X-RAY-ABSORPTION SPECTROSCOPIC STUDIES ON THE IRIDIUM(III) COMPLEXES

Iridium(III) complexes, [Ir(H)(-C=CPh)(PhCN)(CO)(PPh3)(2)]ClO4(1), [Ir -CH=CH=CH=CH(CO)(PhCN)(PPh3)(2)]ClO4 (2), [CpIr(CH3CN)(eta(3)-CH2CHCH Ph)]CF3SO3 (3), have been synthesized and characterized by X-ray absor ption spectroscopic analysis in order to correlate their molecular and electronic structures around iridium ion with their catalytic propert ies. According to the Ir L-III-edge X-ray absorption spectra for those complexes, the white lines for all the complexes represent a single p eak, which suggests that the iridium(III) ions in complexes are in low spin state. From the least square fittings to XANES (X-ray Absorption Near Edge Structure) spectra, it has been found that the white line p osition for the complex (3) is shifted to a lower energy side and its area is smaller compared to the others, indicating that the ligand to metal charge transfer for the former becomes more significant than the latter. Such a finding can be easily understood from the difference i n ligand and local symmetry between complexes (1,2) and (3). On the ot her hand, We have also performed the EXAFS (Extend X-ray Absorption Fi ne Structure) analyses to obtain a detailed information on crystal str ucture parameters like bond distances, Debye-Waller factor, etc., on t he basis of model structures deduced from the similar compounds. The s hapes of EXAFS spectra clearly show the structural difference between the complexes (1,2) and (3), which is well consistent with XANES resul ts. The bond distances, d(lr-C) and d(Ir-N), determined from EXAFS ana lyses are reciprocally proportional to the IR frequencies, v(C=O) and v(N=C), of the corresponding ligands, indicating that the bonds, Ir-C and Ir-N, are competing with the bonds, CIO and N=C.