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, L_m, where pz is a pyrazolyl ring, yields the monofluoride bridged, binuclear [M₂(μ-F)(μ-L_m)₂](BF₄)₃ complexes. In contrast, a similar reaction of L_m with Ni(BF₄)₂·6H₂O yields dibridged [Ni₂(μ-F)₂(μ-L_m)₂](BF₄)₂. The solid state structures of seven [M₂(μ-F)(μ-L_m)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-L_m)₂](BF₄)₃ retains its dimeric structure in solution. The [Ni₂(μ-F)₂(μ-L_m)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-L_m)₂](BF₄)₃ and [Co₂(μ-F)(μ-L_m)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-L_m)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-L_m)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).