X-ray absorption spectroscopic studies on the iridium(III) complexes

Iridium(III) complexes, [Ir(H)(-C≡CPh)(PhCN)(CO)(PPh₃)₂]ClO₄(I), [Ir-CH=CH-CH=CH(CO)(PhCN)(PPh₃)₂]ClO₄ (2), [Cp*Ir(CH₃CN)(η³-CH₂CHCHPh)]CF ₃SO₃ (3), have been synthesized and characterized by X-ray absorption spectroscopic analysis in order to correlate their molecular and electronic structures around iridium ion with their catalytic properties. According to the Ir L_III-edge X-ray absorption spectra for those complexes, the white lines for all the complexes represent a single peak, 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 position 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 in ligand and local symmetry between complexes (1,2) and (3). On the other hand, We have also performed the EXAFS (Extend X-ray Absorption Fine Structure) analyses to obtain a detailed information on crystal structure parameters like bond distances, Debye-Waller factor, etc., on the basis of model structures deduced from the similar compounds. The shapes of EXAFS spectra clearly show the structural difference between the complexes (1,2) and (3), which is well consistent with XANES results. The bond distances, d(Ir-C) and d(Ir-N), determined from EXAFS analyses 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, C≡O and N≡C.