Coordination modes of the 1,3-dithiosquarate (1,3-dtsq) ligand. Syntheses, crystal structures, and magnetic properties of [Ni(tren)(1,3-dtsq)(H2O)] and [Ni-2(tren)(2)(1,3-dtsq)](ClO4)(2) [tren equals tris(2-aminoethyl)amine]

The first X-ray crystallographic stucture determinations of metal complexes of the 1,3-dithiosquarate (1,3-dtsq, dianion of 3-hydroxy-4-mercapto-2-thioxo-3-cyclobuten-1-one) are reported herein. Compounds [Ni(tren)(1,3-dtsq)(H2O)] (C10H20N4NiO3S) (1) and [Ni-2(tren)(2)(1,3-dtsq)](ClO4)(2) (C16H36Cl2N8Ni2O10S2) (2) (where tren is tris(2-aminoethyl)amine) both crystallize in the monoclinic system, space group P2(1)/c, with a = 8.519(1) Angstrom, b = 13.931(1) Angstrom, c = 13.587(3) Angstrom, beta = 106.705(8)degrees, and Z = 4 for 1 and a = 8.663(1) Angstrom, b = 14.496(2) Angstrom, c = 11.617(2) Angstrom, beta = 98.135(9)degrees, and Z = 2 for 2. The structure of 1 consists of mononuclear, neutral [Ni(tren)(1,3-dtsq)(H2O)] complex units, whereas that of 2 is made up of dinuclear dithiosquarate-bridged cations, [Ni-2(tren)(2)(1,3-dtsq)](2+), and uncoordinated perchlorate anions. The nickel environment is distorted octahedral in both cases, the tren group acting as a tetradentate ligand through its four nitrogen atoms and the dithiosquarate being monodentately coordinated through one of the sulfur atoms in 1 and bridging bis-bidentate in 2. A water molecule which is hydrogen bonded to the neighboring carbonyl-oxygen atom of the dithiosquarate completes the octahedron around the metal atom in 1. The intradimer metal-metal separation in 2 is 7.096(1) Angstrom. Variable-temperature magnetic susceptibility measurements of complex 2 reveal the occurrence of a weak intramolecular antiferromagnetic coupling. The ability of the 1,3-dithiosquarate ligand to transmit electronic effects between metal ions has been analyzed through extended Huckel calculations. The theoretical influence of the position (1,2-dithiosquarate versus 1,3-dithiosquarate) and the number of sulfur (1,2,3,4-tetrathiosquarate) atoms on the exchange coupling through bis-bidentate thiosquarates is also considered.