Molecular structure and nitrite-bonded study on copper(II) complexes of N,N-dialkyl,N′-benzyl-ethylenediamine; synthesis, spectroscopic characterization, X-ray structure, steric effect and density functional theory calculations

Abstract

Presented here is an investigation on the geometric (molecular) structures, spectroscopic properties and electronic structures of copper(II)-nitrite complexes as a function of steric effects, utilizing a set of closely related co-ligands. The prepared complexes, with the general formula [CuLR(η2-ONO)2] where LR is N,N-dialkyl,N′-benzyl-ethylenediamine and R is methyl, ethyl or isopropyl moieties, were structurally characterized by physico-chemical and spectroscopic methods. An X-ray diffraction study on CuLMe(η2-ONO)2] and CuLEt(η2-ONO)2] reveals that the copper(II) center in both compounds is located in a distorted octahedral environment through coordination of two amine nitrogen atoms and four oxygen atoms of the nitrite ligands. Depending on the steric crowding of the co-ligand, the coordination mode is either symmetric or asymmetric bidentate, as is evident from X-ray crystallography and IR spectroscopy. The relative stability of linkage isomers of these compounds, along with a tert-butyl derivative complex, was investigated using density functional theory (DFT) calculations. The calculated results demonstrated that in all cases the bidentate η2-ONO isomer is more stable than the four-coordinated nitro isomer (η1-NO2). However, the differences in the relative stability decrease as the steric hindrance in the co-ligand increases. The vibrational spectra of the complexes were assigned using DFT calculations. In support with the X-ray structure, the results reveal that νas(N–O) splits into two bands, with the separation increasing from [CuLMe(η2-ONO)2] through to [CuLtert-buty(η2-ONO)2]. That is at least partially dictated by steric factors within the molecules, imposed by the alkyl groups of the co-ligand. The UV–Vis absorption spectra are presented and analyzed with the help of time-dependent density functional theory (TD-DFT) calculations.