Diversities of Coordination Geometry Around the Cu2+Center in Bis(maleonitriledithiolato)metalate Complex Anions: Geometry Controlled by Varying the Alkyl Chain Length of Imidazolium Cations

Abstract

Six new ion-pair metal-bis(dithiolene) complexes with the formulas [C9H14N4][Cu(mnt)2] (1a), [C10H16N4][Cu(mnt)2] (1b), [C11H18N4][Cu(mnt)2] (1c), [C12H20N4][Cu(mnt)2] (1d), [C13H22N4][Cu(mnt)2] (1e), and [C14H24N4][Cu(mnt)2] (1f) have been synthesized starting from Cu(II) salt, Na2mnt (disodium maleonitriledithiolate), and bromide salts of alkyl-bis(imidazolium) cations [C8H12(CH2)nN4Br2] (n = 1–6, a–f). In this series of ion-pair compounds 1a–1f, a common [Cu(mnt)2]2– complex anion is associated with alkyl imidazolium cations of varied alkyl chain lengths. We have described a systematic study of deviation from square planar geometries (in terms of distortion) around the metal ion in customary square planar metal-dithiolene complexes. The distortion in the geometry around the metal ion can be explained on the basis of center of symmetry along C–H···Cu supramolecular interaction and unbalanced supramolecular interactions, such as S···H, N···H, and M···S type weak contacts. Dianionic copper(II) complexes 1a–1f show an electronic absorption in the near-infrared (NIR) region, which has been attributed to the charge transfer transition from the highest occupied molecular orbital level of copper dithiolate anion [Cu(mnt)2]2– to the lowest unoccupied molecular orbital level of alkyl imidazolium cation [C8H12(CH2)nN4]2+. All these compounds are unambiguously characterized by single crystal X-ray crystallography and further characterized by IR, 1H NMR, electron spin resonance, LC/MS spectroscopic techniques, and electrochemical studies.