A Synthetic, Structural, Magnetic, and Spectral Study of Several {Fe[tris(pyrazolyl)methane]₂}(BF₄)₂Complexes:  Observation of an Unusual Spin-State Crossover

Abstract

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, P2(1)/n, a = 10.1891(2) A, b = 7.6223(2) A, c = 17.2411(4) A, beta = 100.7733(12) degrees, Z = 2; 3 is triclinic, P1, a = 12.4769(2) A, b = 12.7449(2) A, c = 13.0215(2) A, alpha = 83.0105(8) degrees, beta = 84.5554(7) degrees, gamma = 62.5797(2) degrees, Z = 2.

A Study of the Electronic Spin‐State Crossover in {Fe[HC(3,4,5‐Me3pz)3]2}(BF4)2

“…The hyperfine parameters of the single quadrupole doublet observed at 160 K and above clearly indicate that 1 is high spin above this temperature. [2,7,11] Further, the hyperfine parameters of the single quadrupole doublet observed between 4.2 and 100 K suggest that 1 is low spin at these temperatures. The linear temperature dependence of the high-spin isomer shift observed between 110 and 300 K is expected and, within the high-temperature limit, leads to an effective vibrating mass [19] of 64 g mol Ϫ1 .…”

“…[8,11] The solution phase 1 H NMR spectrum is broad with chemical shifts ranging from 41 to Ϫ43 ppm, a range that is indicative of a paramagnetic high-spin iron() complex.…”

“…In general, this torsion angle decreases as the size of the metal ion in- (22) 111.38 (19) creases, because the bite size of these ligands is largely fixed, and increasing the tilting is an important way to accommodate larger metal ions. [10] In the fully high-spin structure [11] of {Fe[HC (3, [10,13] Unfortunately, we have been unable to obtain the low-temperature crystal structure of 1 in order to follow how these angles change upon spin-state crossover, because the crystals shatter upon cooling to low temperature.…”

“…Interestingly, the low-spin complex, {Fe[HC(pz) 3 ] 2 }(BF 4 ) 2 , undergoes a gradual spin-state transition [11] to the high-spin state upon heating above 295 K, a transition which is very similar to that observed [7] in the analogous borate complex, Fe[HB(pz) 3 3 ] 2 is known [7] to remain high-spin even upon cooling to 1.7 K and to show a spin-state crossover to the lowspin state at high pressure, [7] and because {Fe[HC(3,5-Me 2 pz) 3 ] 2 }(BF 4 ) 2 has a unique spin-state crossover behavior. Herein we report the synthesis, the single-crystal Xray structure and magnetic analysis, and Mössbauer spectroscopic studies of this new complex, a complex that does show a complete spin-state crossover from the high-spin to the low-spin state upon cooling.…”

Structure:function relationships in molecular spin-crossover complexes

“…49 The average Fe-N2-N1-C5 torsion in the high-spin phase of that crystal is 168.0(3)1, while in the low-temperature phase it is 161.7(3)1 for the high-spin iron site and 179.0(3)1 for the low-spin molecule. 49 In this case, reduction in the pyrazole twist angle for half of the molecules in the crystal enforces an increased ligand twist in the other half, in compensation. Hence, it was proposed that solid high-spin complexes of the [Fe(pz 3 CH) 2 ] 2+ type, whose structure deviates too strongly from the regular symmetry preferred by the low-spin form, are unable to undergo spin-crossover.…”