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.