Ligand-Modulated Nuclearity and Geometry in Nickel(II) Hydrazone Complexes: From Mononuclear Complexes to Acetato- and/or Phenoxido-Bridged Clusters.

The propensity of 4-hydroxybenzhydrazone-related ligands derived from 3-methoxysalicylaldehyde (HL), 4-methoxysalicylaldehyde (HL), and salicylaldehyde (HL) to act as chelating and/or bridging ligands in Ni(II) complexes was investigated. Three clusters of different nuclearities, [Ni(L)(OAc)(MeOH)]∙2MeOH∙MeCN (∙2MeOH∙MeCN), [Ni(HL)(L)(OAc)(MeOH)]∙4.7MeOH (∙4.7MeOH), and [Ni(HL)(L)(OAc)]∙4MeOH·0.63HO·0.5MeCN·HOAc (∙4MeOH·0.63HO·0.5MeCN·HOAc), were prepared from Ni(OAc)∙4HO and the corresponding ligand in the presence of EtN. The hydrazones in these acetato- and phenoxido-bridged clusters acted as singly or doubly deprotonated ligands. When pyridine was used, mononuclear complexes with the square-planar geometry seemed to be favoured, as found for complexes [Ni(L)(py)] (), [Ni(L)(py)] () and [Ni(L)(py)] (). Ligand substituent effects and the stability of square-planar complexes were investigated and quantified by extensive quantum chemical analysis. Obtained results showed that standard Gibbs energies of binding were lower for square-planar than for octahedral complexes. Starting from [MoO(L)(EtOH)] complexes as precursors and applying the metal-exchange procedure, the mononuclear complexes [Ni(HL)]∙MeOH (∙MeOH) and [Ni(HL)]∙2MeOH (∙2MeOH) and hybrid organic-inorganic compound [Ni(HL)(CHOH)][MoO(OCH)] () were achieved. The octahedral complexes [Ni(HL)] (-) can also be obtained by the direct synthesis from Ni(Oac)∙4HO and the appropriate ligand under specific reaction conditions. Crystal and molecular structures of ∙2MeOH∙MeCN, ∙4.7MeOH, ∙4MeOH∙0.63HO∙0.5MeCN∙HOAc, , , ∙2MeOH, and were determined by the single-crystal X-ray diffraction method.