Diversities of coordination geometry around the Cu < sup > 2+ < /sup > 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 [C 9H 14N 4][Cu(mnt) 2] (1a), [C 10H 16N 4][Cu(mnt) 2] (1b), [C 11H 18N 4][Cu(mnt) 2] (1c), [C 12H 20N 4][Cu(mnt) 2] (1d), [C 13H 22N 4][Cu(mnt) 2] (1e), and [C 14H 24N 4][Cu(mnt) 2] (1f) have been synthesized starting from Cu(II) salt, Na 2mnt (disodium maleonitriledithiolate), and bromide salts of alkyl-bis(imidazolium) cations [C 8H 12(CH 2) nN 4Br 2] (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 [C 8H 12(CH 2) nN 4] 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. © 2012 American Chemical Society.