Probing coordination geometry and electronic nature of Cu center in mixed ligand N,N,N′,N'-tetramethylethylenediamine copper complexes using XAFS

Abstract

X-ray absorption fine structure (XAFS) has been investigated at the Cu K-edge in the copper complexes: [Cu tmen ox]·4H2O(1), [Cu tmen bipy](ClO4)2 (2), [Cu tmen en](SO4)·4H2O (3), [Cu tmen gly](ClO4) (4), [Cu tmen phen](ClO4)2 (5), where tmen = N,N,N′,N'-tetramethyl-ethylenediamine, ox = oxalate ion, en = ethylenediamine, gly = glycinate ion, bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline, to probe the coordination geometry and electronic nature of the Cu metal center. Crystal structure of 1 is known to be distorted octahedral and the same result has been obtained from EXAFS analysis. Structures of 2–5 are not available. From analysis of XANES and its derivative spectra, the structures of 2 and 3 have been estimated to be square pyramidal and of 4 and 5 to be square planar. From EXAFS analysis, the same structures have been arrived at. Four O/N atoms form the square plane in 2–5 with Cu atom at center of the square. In 2 and 3, one O/N atom is at apical position forming the pyramid. Ab-initio XANES calculations have been performed to obtain simulated XANES spectra as well as p-DOS and d-DOS for the absorbing Cu metal center and these have been compared and correlated with the experimental spectra. The splitting of Cu K-edge into two edges K1 and K2 in both the experimental and simulated spectra have been correlated with the theoretical p-DOS. The trends in the relative intensities of the two peaks in the derivative XANES spectra and in the p-DOS have been observed to be similar in the three geometries. The occurrence of pre-edge has been correlated with the d-DOS.