Probing the dinucleating behaviour of a bis-bidentate ligand:: Synthesis and characterisation of some di- and mononuclear cobalt(II), nickel(II), copper(II) and zinc(II) complexes of 3,5-di(2-pyridyl)-4-(1H-pyrrol-1-yl)-4H-1,2,4-triazole

As a probe of the dinucleating ability of the known but little studied bis-bidentate ligand 3,5-di-(2-pyridyl)-4-(1H-pyrrol-yl)-4H-1,2,4-triazole (pldpt) its reactivity towards MX2.6H(2)O (M = Co-II, Ni-II and Zn-II; X = ClO4- and BF4-) as well as Cu(ClO4)(2).6H(2)O, in a 1:1 metal-to-ligand molar ratio in MeCN, has been investigated. In the case of Co-II, Ni-II and Zn-II, these reactions gave dinuclear complexes M-2(II)(pldpt)(2)-X-4(MeCN)(m)(H2O)(n), whereas for Cu-II initially the mononuclear complex [Cu-II(pldPt)(2)(CIO4)2] was isolated, followed by the dinuclear complex [CU2II(pldpt)(2)(MeCN)(2)(H2O)(2)](ClO4)(4). The use of the strongly polar aprotic co-solvent DMF resulted in the partial breakdown of the initial dinuclear entities in the case of Co-II and Ni-II but not in the case of Zn-II. In all five of the structurally characterised dinuclear complexes the (N',N-1,N-2,N")(2) double-bridging coordination mode is realised with distorted octahedral N4Y2-coordinated metal centres (Y = DMF, H2O or MeCN). The two mononuclear complexes feature the common trans-(N',N-1)(2) coordination mode with axial DMF or ClO4- co-ligands. The near-perpendicular orientation [82.4(3)-88.8(1)degrees] of the pi-electron-rich 4-(1H-pyrrol-1-yl) substituent with respect to the triazole ring of pldpt, observed in all of these structures, means that no pi-interactions are expected between these rings so any electronic interaction is likely to be small. Whether a di- or mononuclear complex of pldpt forms is therefore primarily determined by a number of other factors, including the reaction stoichiometry, the nature of the counterions and the solvent, as well as the relative solubility of the various possible products. Clearly the nature of the N-4 substituent can have a major impact on the last of these factors. Magnetic studies carried out on the dinuclear complexes revealed that the triazole bridges mediate relatively weak antiferromagnetic coupling between the two metal centres.