Monofluoride Bridged, Binuclear Metallacycles of First Row Transition Metals Supported by Third Generation Bis(1-pyrazolyl)methane Ligands: Unusual Magnetic Properties

The reaction of M(BF4)2·xH2O, where M is Fe, Co, Cu, and Zn, and the ditopic, bis(pyrazolyl)methane ligand m-[CH(pz)2]2C6H4, Lm, where pz is a pyrazolyl ring, yields the monofluoride bridged, binuclear [M2(μ-F)(μ-Lm)2](BF4)3 complexes. In contrast, a similar reaction of Lm with Ni(BF4)2·6H2O yields dibridged [Ni2(μ-F)2(μ-Lm)2](BF4)2. The solid state structures of seven [M2(μ-F)(μ-Lm)2](BF4)3 complexes show that the divalent metal ion is in a five-coordinate, trigonal bipyramidal, coordination environment with either a linear or nearly linear M−F−M bridging arrangement. NMR results indicate that [Zn2(μ-F)(μ-Lm)2](BF4)3 retains its dimeric structure in solution. The [Ni2(μ-F)2(μ-Lm)2](BF4)2 complex has a dibridging fluoride structure that has a six-coordination environment about each nickel(II) ion. In the solid state, the [Fe2(μ-F)(μ-Lm)2](BF4)3 and [Co2(μ-F)(μ-Lm)2](BF4)3 complexes show weak intramolecular antiferromagnetic exchange coupling between the two metal(II) ions with J values of −10.4 and −0.67 cm−1, respectively; there is no observed long-range magnetic order. Three different solvates of [Cu2(μ-F)(μ-Lm)2](BF4)3 are diamagnetic between 5 and 400 K, thus showing strong antiferromagnetic exchange interactions of −600 cm−1 or more negative. Mössbauer spectra indicate that [Fe2(μ-F)(μ-Lm)2](BF4)3 exhibits no long-range magnetic order between 4.2 and 295 K and isomer shifts that are consistent with the presence of five-coordinate, high-spin iron(II).