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)3. 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](BF 4)3 retains its dimeric structure in solution. The [Ni2(μ-F)2(μ-Lm)2](BF 4)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(ll) 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)are diamagnetic between 5 and 400 K, thus showing strong antiferromagnetic exchange interactions of -600 cm-1 or more negative. Mössbauer spectra indicate that [Fe 2(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, highspin iron(II). © 2009 American Chemical Society.



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