Structure−Function Correlations in Iron(II) Tris(pyrazolyl)borate Spin-State Crossover Complexes

Iron(II) poly(pyrazolyl)borate complexes have been investigated to determine the impact of substituent effects,
intramolecular ligand distortions, and intermolecular supramolecular structures on the spin-state crossover (SCO)
behavior. The molecular structure of Fe[HB(3,4,5-Me3pz)3]2 (pz = pyrazolyl ring), a complex known to remain high
spin when the temperature is lowered, reveals that this complex has an intramolecular ring-twist distortion that is
not observed in analogous complexes that do exhibit a SCO at low temperatures, thus indicating that this distortion
greatly influences the properties of these complexes. The structure of Fe[B(3-cyPrpz)4]2·(CH3OH) (cyPr = cyclopropyl
ring) at 294 K has two independent molecules in the unit cell, both of which are high spin; only one of these
high-spin iron(II) sites, the site with the lesser ring-twist distortion, is observed to be low-spin iron(II) in the 90 K
structure. A careful evaluation of the supramolecular structures of these complexes and several similar complexes
reported previously revealed no strong correlation between the supramolecular packing forces and their SCO behavior.
Magnetic and Mössbauer spectral measurements on Fe[B(3-cyPrpz)4]2 and Fe[HB(3-cyPrpz)3]2 indicate that both
complexes exhibit a partial SCO from fully high-spin iron(II) at higher temperatures, respectively, to a 50:50 high-spin/low-spin mixture of iron(II) below 100 K. These results may be understood, in the former case, by the differences
in ring-twisting and, in the latter case, by a phase transition; in all complexes in which a phase transition is observed,
this change dominates the SCO behavior. A comparison of the Mössbauer spectral properties of these two complexes
and of Fe[HB(3-Mepz)3]2 with that of other complexes reveals correlations between the Mössbauer-effect isomer
shift and the average Fe−N bond distance and between the quadrupole splitting and the average FeN−NB intraligand
dihedral torsion angles and the distortion of the average N−Fe−N intraligand bond angles.