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

The iridium(III) complexes, 1-3, which are the key intermediates for a variety of well established reaction pathways for the metal-catalyzed reactions of alkynes such as oligomerization or polymerization, have been characterized by X-ray absorption spectroscopic analysis. 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 the complexes are in a 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 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. According to EXAFS (extended X-ray absorption fine structure) analyses, the spectral shapes clearly show the structural difference in the complexes between 1 or 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, ν(C ≡ O) and ν(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.