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-Me(2)pz)(3)](2)}(BF4)(2) (1) {Fe[HC(pz)(3)](2)}(BF 4)(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 l igand with Fe(BF4). 6H(2)O. Complex 1 is high-spin in thr solid state and i n solution at 298 K. In the solid phase, it undergoes a decrease in magneti c 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 sub sequent cooling to 5 K. Crystallographically, then is only one iron (II) si te in the ambient-temperature solid-state structure, a structure that clear ly shows the complex is high-spin. Mossbauer spectral studies show conclusi vely that the magnetic moment change observed at lower temperatures arises from the complex changing fi om a high-spin state at higher temperatures to a 50:50 mixture of high-spin and low-spin states at lower temperatures. Co mplexes 2 and 3 are low-spin in the solid phase at room temperature. Comple x 2 in the solid phase gradually changes over to the high-spin state upon h eating above 295 K and is completely high-spin at ca. 470 K. In solution, v ariable-temperature H-1 NMR spectra of 2 show both high-spin and low spin f orms are present, with the percentage of the paramagnetic form increasing a s the temperature increases. Complex 3 is low-spin at all temperatures stud ied in both the solid phase and solution.,An X-ray absorption spectral stud y has been undertaken to investigate the electronic spin states of (Fe[HC(3 ,5 Me(2)pz)(3)](2)}(BF4)(2) and {Fe[HC(pz)(3)](2)}(BF4)(2). Crystallographi c information: 2 is monoclinic, P2(1)/n, a = 10.1891(2) Angstrom, b = 7.622 3(2) Angstrom, c = 17.2411(4) Angstrom, beta = 100.7733(12)degrees, Z = 2; 3 is triclinic, P (1) over bar, a = 12.4769(2) Angstrom, b 12.7449(2) K, c = 13.0215(2) Angstrom, alpha = 83.0105(8)degrees, beta = 84.5554(7)degrees, gamma = 62.5797(2)degrees, Z = 2.