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, ß = 100.7733(12)°, Z = 2; 3 is triclinic, P1-, a = 12.4769(2) Å, b = 12.7449(2) Å, c = 13.0215(2) Å, α = 83.0105(8)°, ß = 84.5554(7)°, γ = 62.5797(2)°, Z = 2