Thiodiacetate and Oxydiacetate Cobalt Complexes: Synthesis, Structure and Stereochemical Features

A series of cobalt complexes containing the dianionic ligands X(CH,COO)(2) 2- (X = S, tda; or X = O, oda) is reported. The complexes [Co(tda)(bipy)(H2O)center dot-4H(2)O (2), which was characterised by X-ray crystallography, [Co(tda)(phen)(H2O)] (3), [Co(tda){(MeO)(2)bipy}(H2O)]-2H(2)O (4) and [Co(tda)(dpp)(H2O)]center dot 2H(2)O (5) are obtained by reaction of the precursor complex [Co(tda)(H2O)], (1a) with the bidentate N-donor ligands 2,2'-bipyridine (bipy), o-phenanthroline (phen), 4,4'dimethoxy-2,2'-bipyridine [{(MeO)2bipy] and 2,3-bis(2-pyridyl)pyrazine (dpp), respectively. The reaction of la with di(2-pyridyl) ketone unexpectedly displaces tda from the coordination sphere and leads to the Co-III species [Co(OC(OH)(2-pyridyl)2-puridil)(2)}(2)](+)[Htda]center dot 3H(2)O (6), as confirmed by an X-ray analysis. Three new oxydiacetate complexes of cobalt are also synthesised by treating the known complex [(Co(oda)(H2O)2)-H2O], with 4,4'-di-tert-butyl-2,2'-bipyridine (tBu(2)bipy), 2,3-bis(2-pyridyl)pyrazine (dpp) and 4,4'-bipyridine (4,4'-bipy). The complexes [Co(oda)(tBu(2)bipy)(H2O)]center dot l.5H(2)O (7), [Co(oda)(dpp)(H2O)j-2H(2)O (8) and [Co(oda)(H2O)2)2(P4,4'-bipy)]-2H(2)O (9) are structurally characterised (8 as a methanol solvate). Complex 9 is binuclear, in contrast to 7 and 8 (mononuclear), and magnetic susceptibility measurements down to 2 K for this complex show high-spin noncorrelated Con ions with a typical Curie-Weiss behaviour. The Co(oda) fragment in 7-9 adopts a planar (mer) rather than a folded (fac) conformation, the latter of which is common to all the M(tda) complexes but is only occasionally observed in the M(oda) ones. In the final section of the paper, the key geometric parameters that control the stereochemistry of the two ligands are determined from a statistical analysis based on the structurally characterised Co(oda) and Co(tda) complexes.