Study of the Charge Transfer Band Observed in Dialkoxo-Bridged Binuclear Copper(II) Complexes

Abstract

Abstract The near-ultraviolet absorption band characteristic of dialkoxo-bridged binuclear copper(II) complexes was reinvestigated by using a series of binuclear copper(II) complexes with N,N-dialkyldiaminoalcohols, Cu2{R2N(CH2)mNH(CH2)nO}2X′2 (abbreviated as Cu(R-m-n)X′, X′ = ClO4, BF4). In this study eighteen complexes including ten new compounds were prepared and classified into two types, i.e., type-A and B according to Kida’s classification, based on their temperature dependencies of magnetic susceptibilities (80–300 K) and reflectance spectra. The crystal structures of Cu(n-Bu-3-2)ClO4 (10), Cu(H-3-3)BF4·EtOH (11b), and Cu(Me-3-3)ClO4 (12a) were determined by the single-crystal X-ray diffraction method. The detailed analysis of the structures and reflectance spectra led to the same conclusion as Kida’s one in assigning the near-ultraviolet absorption band. That is, the absorption was ascribed to the charge transfer (CT) from the bridging oxygen’s nonbonding orbital to vacant copper d-orbital. On the other hand, in assigning the origin of the appearance of the CT band in the near-ultraviolet region, the conclusion different from Kida’s one was presented. The origin was interpreted in terms of the red-shift of the band owing to the electrostatic repulsion between the bridging oxygen atoms. Moreover, the unusually low frequency and low intensity of the CT band of 12a were also reasonably explained.