Diversity of Coordination Modes in a Flexible Ditopic Ligand Containing 2-Pyridyl, Carbonyl and Hydrazone Functionalities: Mononuclear and Dinuclear Cobalt(III) Complexes, and Tetranuclear Copper(II) and Nickel(II) Clusters

Syntheses, crystal structures and characterization are reported for four new complexes [Cu4Br2(L)(4)]Br-2 (1), [Ni-4(NO3)(2)(L)4(H2O)1(NO3)(2) (2), [Co-2(L)(3)](ClO4)(3) (3) and [Co(L)(2)](ClO4) (4), where L- is the monoanion of the ditopic ligand N'-(1-(pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide (LH) built on a picolinoyl hydrazone core fragment, and possessing a bidentate and a tridentate coordination pocket. The tetranuclear cation of 1.0.8H(2)O center dot MeOH is a strictly planar, rectangular [2 x 2] grid. Two 2.21011 L- ligands bridge adjacent Cu-II atoms on the short sides of the rectangle through their alkoxide oxygen atoms, and two 2.11111 ligands bridge adjacent Cu-II atoms on the long sides of the rectangle through their diazine groups; two metal ions are 5-coordinate and two are 6-coordinate. The tetranuclear cation of 2.0.2H(2)O center dot 3EtOH is a square [2 x 2] grid. The two 6-coordinate NiII atoms of each side of the square are bridged by the alkoxide 0 atom of one 2.21011 L- ligand. The dinuclear cation of 3.0.8H(2)O center dot 1.3MeOH contains two low-spin octahedral CoIII ions bridged by three 2.01111 L- ligands forming a pseudo triple helicate. In the mononuclear cation [Co(L)(2)](+) of complex 4, the low-spin octahedral Co-III center is coordinated by two tridentate chelating, meridional 1.10011 ligands. The crystal structures of the complexes are stabilized by a variety of pi-pi Tr stacking and/or H-bonding interactions. Compounds 2, 3 and 4 are the first structurally characterized nickel and cobalt complexes of any form (neutral or anionic) of LH. The 2.01111 and 1.10011 coordination modes of L-, observed in the structures of complexes 3 and 4, have been crystallographically established for the first time in coordination complexes containing this anionic ligand. Variable-temperature, solid-state dc magnetic susceptibility and variable-field magnetization studies at 1.8 K were carried out on complexes 1 and 2. Antiferromagnetic metal ion...metal ion exchange interactions are present in both complexes. The study reveals that the cation of 1 can be considered as a practically isolated pair of strongly antiferromagnetically coupled (through the diazine group of L-) dinulear units. The susceptibility data for 2 were fit to a single-I model for an S = 1 cyclic tetramer. The values of the J parameters have been rationalized in terms of known magnetostructural correlations. Spectral data (infrared (IR), ultraviolet/visible (UV/VIS), H-1 nuclear magnetic resonance (NMR) for the diamagnetic complexes) are also discussed in the light of the structural features of 1-4 and the coordination modes of the organic and inorganic ligands that are present in the complexes. The combined work demonstrates the ligating flexibility of L-, and its usefulness in the synthesis of complexes with interesting structures and properties.