GROUP-5 TETRATHIOMETALATES - SIMPLIFIED SYNTHESES AND STRUCTURES

The tetrathiometalates [NbS4]3- and [TaS4]3-, previously known only in the form of intractable solids prepared at high temperatures, have been synthesized in the reaction system M(OEt)5/(Me3Si)2S/LiOMe (1:4:3) in acetonitrile solution at ambient temperature The species [VS4]3- Was prepared similarly from VO(OMe)3. The initial product in these systems is Li3[MS4].4MeCN (M = V, Nb, Ta), as established by the structure of the niobium compound. Li3[NbS4].4MeCN crystallizes in orthorhombic space group Fddd with a = 6.0772 (6) angstrom, b = 24.682 (4) angstrom, c = 25.683 (4) angstrom, and Z = 8. Pure and stable products were obtained by recrystallization of the TMEDA adducts to afford Li3[MS4].2TMEDA (M = V (dark red), Nb (light yellow), Ta (colorless) (TMEDA = N,N,N',N'-tetramethylethylenediamine)). These compounds are isomorphous and crystallize in tetragonal space group P4n2BAR with a = 13.902 (3)/13.983 (6)/13.994 (2) angstrom, c = 6.0474 (9)/6.152 (3)/6.1633 (5) angstrom, and Z = 2 for M = V/Nb/Ta. The crystal structures reveal discrete, essentially tetrahedral [ MS4] 3- species with imposed D2d symmetry and four S...S edges bridged by Li+ in distorted tetrahedral coordination. Opposite edges are bridged by Li+ to form a linear chain of [MS4]3- units; two other edges are bridged by Li+ coordinated to two MeCN ligands or one TMEDA ligand. The differences in colors of the three chromophores arise from a progressive high-energy shift of the first LMCT band (1t1 --> 2e) in the series M = V (501, 561 nm), Nb (340 nm), Ta (300 nm). [NbS4]3- and [TaS4]3- are the simplest of the few discrete soluble chalcogenoniobates and -tantalates that have thus far been prepared; structures of these species are illustrated. The role of Li+ in stabilizing and solubilizing the product compounds is considered, and unknown tetrathiometalates that might be obtained by the cation-stabilizing aspect of the methods used here are pointed out.