A Synthetic, Structural, Magnetic, and Spectral Study of Several Fe[tris(pyrazolyl)methane]₂(BF₄)₂ Complexes: Observation of an Unusual Spin-state Crossover
The complexes Fe[HC(3,5-Me2pz)3]2 (BF4)2 (1), Fe[HC(pz)3]2(BF4)2 (2), and Fe[PhC(pz)2(py)]2(BF4)2 (3) (pz = 1-pyrazolyl ring, py = pyridyl ring) have been synthesized by the reaction of the appropriate ligand with Fe(BF4)2·6H2O. Complex 1 is high-spin in the solid state and in solution at 298 K. In the solid phase, it undergoes a decrease in magnetic moment at lower temperatures, changing at ca. 206 K to a mixture of high-spin and low-spin forms, a spin-state mixture that does not change upon subsequent cooling to 5 K. Crystallographically, there is only one iron(II) site in the ambient-temperature solid-state structure, a structure that clearly shows the complex is high-spin. Mössbauer spectral studies show conclusively that the magnetic moment change observed at lower temperatures arises from the complex changing from a high-spin state at higher temperatures to a 50:50 mixture of high-spin and low-spin states at lower temperatures. Complexes 2 and 3 are low-spin in the solid phase at room temperature. Complex 2 in the solid phase gradually changes over to the high-spin state upon heating above 295 K and is completely high-spin at ca. 470 K. In solution, variable-temperature 1H NMR spectra of 2 show both high-spin and low-spin forms are present, with the percentage of the paramagnetic form increasing as the temperature increases. Complex 3 is low-spin at all temperatures studied in both the solid phase and solution. An X-ray absorption spectral study has been undertaken to investigate the electronic spin states of Fe[HC(3,5-Me2pz)3]2 (BF4)2 and Fe[HC(pz)3]2(BF4)2. Crystallographic information: 2 is monoclinic, P21/n, a = 10.1891(2) Å, b = 7.6223(2) Å, c = 17.2411(4) Å, β = 100.7733(12)°, Z = 2; 3 is triclinic, P1̄, a = 12.4769(2) Å, b = 12.7449(2) Å, c = 13.0215(2) Å, α = 83.0105(8)°, β = 84.5554(7)°, γ = 62.5797(2)°, Z = 2.
D. L. Reger and C. A. Little and A. L. Rheingold and M. Lam and L. M. Liable-Sands and B. Rhagitan and T. L. Concolino and A. Mohan and G. J. Long and V. Briois and F. Grandjean, "A Synthetic, Structural, Magnetic, and Spectral Study of Several Fe[tris(pyrazolyl)methane]₂(BF₄)₂ Complexes: Observation of an Unusual Spin-state Crossover," Inorganic Chemistry, American Chemical Society (ACS), Jan 2001.
The definitive version is available at http://dx.doi.org/10.1021/ic001102t
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© 2001 American Chemical Society (ACS), All rights reserved.