Low-temperature doublet luminescence and absorption spectra of multidentate hexaam(m)ine Cr(III) complexes. Effects of ligand induced distortion from Oh symmetry

Abstract

Luminescence and absorption spectra of the complexes [Cr([18]aneN6)]Br 3, [Cr(sen)]Br 3, [Cr(taptacn)]Br 3, trans-[Cr(tn) 2(NH 3) 2](ClO 4) 3, cis-[Cr(tn) 2)NH 3) 2](ClO 4) 3 and [Cr([20]aneN6)]Br 3 have been obtained at low temperatures. ([18]aneN6 = 1,4,7,10,13,16-hexaazacyclooctadecane, [20]aneN6 = 1,4,7,11,14,17-hexaazacycloeicosane, sen = 4,4′,4″-ethylidynetris-(3-azabutan-1-amine), taptacn = 1,4,7-tris(aminopropyl)-1,4,7-triazacyclononane, tn = 1,3-diaminopropane). The observed temperature dependence of luminescence band intensities, together with the comparison of the luminescence and absorption spectra, has allowed the assignment of the 2E g electronic origins, generally found to be split into two orbital components. The energies of the lowest energy doublet component {the origin splitting} were found to be 14 646{29}, 14 729{16}, 14 818{5?}, 15 108{44}, 15 14 680{72} cm −1 for the above sequence of complexes, respectively. In addition to the differences in 2E g splitting, considerable variance was found in the proportion in which the luminescence intensity appeared in the electronic origins as opposed to the vibronic origins. None of the spectra gave evidence for progressions. The data reveal the effect of symmetry lowering at the metal centre on both band intensities and doublet splittings. The observed splittings are compared with those calculated from the angular overlap model based on X-ray diffraction determinations of the coordination geometries around the metal centre. Trans-[Cr(tn) 2(NH 3) 2][(ClO 4) 3 is interesting because it shows the usual doublet splitting plus an additional solid state splitting of 5 cm −1 observed in the electronic origin and repeated with varying magnitude in many of the lines in the spectrum. This is attributed to the presence of two crystallographically non-equivalent complex sites in the crystal, which we were unfortunately not able to verify by X-ray diffraction owing to disorder in the perchlorate anions.