Diversities of Coordination Geometry at Cu2+ Center in the Bis(maleonitriledithiolato)cuprate Complexes:  Syntheses, Magnetic Properties, X-ray Crystal Structural Analyses, and DFT Calculations

Four new ion-pair complexes, consisting of bis(maleonitriledithiolato)cuprate dianion with the derivatives of benzylpyridinium (1−3) or monoprotonated 1,4-diazabicyclo-[2.2.2]octane (4), have been prepared and characterized structurally and magnetically, and a diversity of the coordination geometry at Cu2+ center of [Cu(mnt)2]2- was observed. Highly distorted square-planar geometry at the tetracoordination Cu2+ center of [Cu(mnt)2]2- was found in 1 and 4. Two chelate rings make a dihedral angle of 27.2° in 1 and 20.5, 34.3, and 19.7° for three crystallographically independent [Cu(mnt)2]2- molecules in 4. Interestingly, the distortion is not ascribed to steric repulsion between ligands or weakly coordinating interaction from the nearest anions, countercations, or solvent molecules but to the weak supramolecular interactions between anion and cations [such as S···H, N···H, or S···I]. In 2, the Cu2+ center in [Cu(mnt)2]2- possesses approximate square-planar coordination geometry, and the dihedral angle between two chelate rings is 5.8°. In 3, the Cu2+ ion of [Cu(mnt)2]2- lies on a symmetric center (2/m), and the anion has C 2h symmetry. The coordination S atoms and central Cu2+ ion of [Cu(mnt)2]2- are strictly coplanar with perfect square-planar coordination geometry because of the symmetric constraint. However, the anion is nonplanar because the ligand fragment is bent away from the CuS4 plane with a dihedral angle of 13.8°. X-band EPR spectra at 293 and 110 K failed to give any available information about the electronic structural difference between the anions of square-planar and distorted square-planar geometries because no hyperfine splitting was detected. Density functional theory (DFT) has been implemented to calculate the single-point energy for each [Cu(mnt)2]2- in 1−4 and optimize the molecular geometry of [Cu(mnt)2]2-. On the basis of these theoretic analyses, the reasons for the structural fluctuations between square-planar and distorted square-planar coordination geometry were explored.