Tetranuclear Nickel complexes composed of pairs of dinuclear LNi2 fragments linked by 4,4′-bipyrazolyl, 1,4-bis(4′-pyrazolyl)benzene, and 4,4′-bipyridazine:: Synthesis, structures, and magnetic properties

The ability of the ligands 4,4'-bipyrazolyl (H(2)bpz), 1,4-bis(4'pyrazolyl)benzene (H(2)bpzb), and 4,4'-bipyridazine (bpdz) to link two dioctahedral LNi2 units has been examined. The following complexes were prepared: [(LNi2II)-Ni-1(Hbpz)][BPh4] (6[BPh4], [(LNi2II)-Ni-1(bpdz)][ClO4](2) (7[ClO4](2)), [((LNi2II)-Ni-1)(2)(bpzb)]-[BPh4]2 (8[BPh4]2), and [(L-2 Ni-2(II))(2)(bpz)][BPh4](2) (9[BPh4](2)), where (L-1)(2-) and (L-2)(2-) represent macrocyclic hexaaza-dithiophenolate ligands. All complexes have been characterised by UV/Vis spectroscopy, IR spectroscopy, and X-ray crystallography. Whereas (Hbpz)(-) and bpdz in 6[BPh4](2) and 7[ClO4](2) act as bidentate ligands coordinating to only one [LNi2](2+) unit, in 8[BPh4](2) and 9[BPh4](2) the (bpzb)(2-) and (bpz)(2-) units are tetradentate linkers. This is qualitatively explained in terms of the absence or presence of steric repulsions between the tBu groups of the supporting ligands and the length of the coligands. The structures of the tetranuclear complexes differ mainly in the distance between the center of the Ni center dot center dot center dot Ni axes of the isostructural [LNi2] units {14.040(l) angstrom in B[BPh4](2), 9.184(l) angstrom in 9[BPh4](2)). The two Ni(2-)pyrazolato planes in 9[BPh4](2) are coplanar. An analysis of the temperature-dependent magnetic susceptibility data for 9[BPh4](2) reveals the presence of weak ferromagnetic exchange interactions between the Ni-II ions in the binuclear[(LNi2)-Ni-2] subunits with values for the magnetic exchange constant J(1) of 23.97 cm(-1) (H = -2JS(1)S(2)). The exchange coupling across the dipyrazolato bridge is less than 0.1 cm(-1), suggesting that no significant interdimer exchange coupling occurs in 9[BPh4](2).