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

Abstract

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-(cy)Prpz)4]2.(CH3OH) ((cy)Pr = 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-(cy)Prpz)4]2 and Fe[HB(3-(cy)Prpz)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.

Syntheses, Structures and Electronic Properties of Zwitterionic Iron(II) and Cobalt(II) Complexes Featuring Ambidentate Tris(pyrazolyl)methanide Ligands

“…Comparable values have been reported for both HS and LS complexes of iron(II). [35] As with the corresponding cobalt complex, the coordination sphere around Fe II at both temperatures and, hence, both spin states, is trigonally distorted, although the distortion is much more pronounced for HS-3 (average N À Fe À N angles: 88.098 and 91.918 (LS); 84.608 and 95.408 (HS)). As a consequence of the shorter iron-nitrogen distances and much less trigonally distorted octahedral geometry, the C1···Fe1 distances are smaller for the low-spin com-…”

The Effect of Ligand Design on Metal Ion Spin State—Lessons from Spin Crossover Complexes

“…While not all were crystallographically characterized, examples that might be expected to adopt more twisted L ligands are high-spin, or exhibit significantly reduced SCO T ½ values, in the solid state [63,64]. Comparably distorted ligand conformations, imposed by intermolecular steric contacts rather than by ligand design, may also be responsible for stabilization of the high-spin state in some other crystalline metal complexes [65][66][67][68][69][70].…”

Syntheses and Electronic Properties of Rhodium(III) Complexes Bearing a Redox-Active Ligand

“…Such changes in the ligand framework lead to subtle, but difficult-to-rationalize, differences in the (RhN-NC) torsion angles, the so-called "pyrazolyl ring-twisting" that describes the "fit" of the ligand to the metal center. 21 …”