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 (H(2)L(3OMe)), 4-methoxysalicylaldehyde (H(2)L(4OMe)), and salicylaldehyde (H(2)L(H)) to act as chelating and/or bridging ligands in Ni(II) complexes was investigated. Three clusters of different nuclearities, [Ni(3)(L(3OMe))(2)(OAc)(2)(MeOH)(2)]∙2MeOH∙MeCN (1∙2MeOH∙MeCN), [Ni(2)(HL(4OMe))(L(4OMe))(OAc)(MeOH)(2)]∙4.7MeOH (2∙4.7MeOH), and [Ni(4)(HL(H))(2)(L(H))(2)(OAc)(2)]∙4MeOH·0.63H(2)O·0.5MeCN·HOAc (3∙4MeOH·0.63H(2)O·0.5MeCN·HOAc), were prepared from Ni(OAc)(2)∙4H(2)O and the corresponding ligand in the presence of Et(3)N. 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(3OMe))(py)] (4), [Ni(L(4Ome))(py)] (5) and [Ni(L(H))(py)] (6). 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(2)(L)(EtOH)] complexes as precursors and applying the metal-exchange procedure, the mononuclear complexes [Ni(HL(3OMe))(2)]∙MeOH (7∙MeOH) and [Ni(HL(H))]∙2MeOH (9∙2MeOH) and hybrid organic–inorganic compound [Ni(2)(HL(4OMe))(2)(CH(3)OH)(4)][Mo(4)O(10)(OCH(3))(6)] (10) were achieved. The octahedral complexes [Ni(HL)(2)] (7–9) can also be obtained by the direct synthesis from Ni(Oac)(2)∙4H(2)O and the appropriate ligand under specific reaction conditions. Crystal and molecular structures of 1∙2MeOH∙MeCN, 2∙4.7MeOH, 3∙4MeOH∙0.63H(2)O∙0.5MeCN∙HOAc, 4, 5, 9∙2MeOH, and 10 were determined by the single-crystal X-ray diffraction method.