Between Ni(mnt)2 and Ni(tfd)2 dithiolene complexes:: the unsymmetrical 2-(trifluoromethyl)acrylonitrile-1,2-dithiolate and its nickel complexes

A novel 1,2-dithiolate ligand, that is, the 2-(trifluoromethyl)acrylonitrile-1,2-dithiolate, abbreviated here as ffadt, is prepared from the corresponding cyclic dithiocarbonate. This ligand, substituted with both a CN and a CF3 group, is compared with the well-known maleonitrile- and bis(trifluoromethyl)ethane-1,2-dithiolates. The preparation, electrochemical properties, and X-ray crystal structures of the square-planar nickel complexes, in both their dianionic diamagnetic (Ni(tfadt)(2)](2-) and their monoanionic paramagnetic [Ni(tfadt)(2)](.)- forms, are reported, as n-Bu4N+, PPh4+, and (18-crown-6)Na+ salts, respectively. In the [(18-crown-6)Na](2)[Ni(tfadt)(2)] salt, each CN moiety of the [Ni(tfadt)(2)](2-) dianion is coordinated to a (18-crown-6)Na+ cation through a CN . . . Na interaction [N . . . Na = 2.481(3) A], affording an axle with wheels model where two MeOH molecules act as axle caps. On the other hand, in [(18-crown-6)Na][Ni(tfadt)21, each (18-crown-6)Na+ cation is coordinated on both sides by the CN groups of two monoanionic [Ni(tfadt)(2)](.)- complexes with N . . . Na+ distances at 2.434(5) and 2.485(4) angstrom, giving rise to heterobimetallic chains with alternating (18-crown-6)Na+ and [Ni(tfadt)(2)](2-) ions. These two examples demonstrate the attractive ability of the CN moieties in the [Ni(tfadt)(2)](2-,.)- complexes to coordinate metallic cationic centers. The paramagnetic salts of the anionic [Ni(tfadt)(2)](.-) complex follow Curie-type law in the 2-300 K temperature range, indicating the absence of intermolecular magnetic interactions in the solid state. The complexes are found in their trans form in all crystal structures, while density functional theory calculations establish that both forms have essentially the same energy. A cis-trans interconversion process is observed by variable-temperature NMR on the dianionic [Ni(ftfadt)(2)](2-) complex with a coalescence temperature T-c of 260 K and a free energy of activation of 51-53 kJ mol(-1).