Between Ni(mnt)_2 and Ni(tfd)_2 Dithiolene Complexes:the Unsymmetrical 2-(Trifluoromethyl)acrylonitrile-1,2-dithiolate and Its Nickel Complexes

摘要

A novel 1,2-dithiolate ligand,that is,the 2-(trifluoromethyl)acrylonitrile-1,2-dithiolate,abbreviated here as tfadt,is prepared from the corresponding cyclic dithiocarbonate.This ligand,substituted with both a CN and a CF_3 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]~(centre dot -) forms,are reported,as n-Bu_4N~+,PPh_4~+,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)_2],each (18-crown-6)Na~+ cation is coordinated on both sides by the CN groups of two monoanionic [Ni(tfadt)_2]~(centre dot -) complexes with N...Na~+ distances at 2.434(5) and 2.485(4) A,giving rise to heterobimetallic chains with alternating (18-crown-6)Na~+ and [Ni(tfadt)_2]~(centre dot -) ions.These two examples demonstrate the attractive ability of the CN moieties in the [Ni(tfadt)_2]~(2-,centre dot -) complexes to coordinate metallic cationic centers.The paramagnetic salts of the anionic [Ni(tfadt)_2]~(centre dot -) 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(tfadt)_2]~(2-) complex with a coalescence temperature T_c of 260 K and a free energy of activation of 51-53 kJ mol~(-1).