Structural, magnetic, and Mössbauer spectral study of the electronic spin-state transition in [Fe{HC(3-Mepz)2(5-Mepz)}2](BF4)2.

The complex [Fe{HC(3-Mepz)(2)(5-Mepz)}(2)](BF(4))(2) (pz = pyrazolyl ring) has been prepared by the reaction of HC(3-Mepz)(2)(5-Mepz) with Fe(BF(4))(2) x 6 H(2)O. The solid state structures obtained at 294 and 150 K show a distorted iron(II) octahedral N(6) coordination environment with the largest deviations arising from the restrictions imposed by the chelate rings. At 294 K the complex is predominately high-spin with Fe-N bond distances averaging 2.14 A, distances that are somewhat shorter than expected for a purely high-spin iron(II) complex because of the presence of an admixture of about 80% high-spin and 20% low-spin iron(II). At 294 K the twisting of the pyrazolyl rings from the ideal C(3v) symmetry averages only 2.2 degrees, a much smaller twist than has been observed previously in similar complexes. At 150 K the Fe-N bond distances average 1.99 A, indicative of an almost fully low-spin iron(II) complex; the twist angle is only 1.3 degrees, as expected for a complex with these Fe-N bond distances. The magnetic properties show that the complex undergoes a gradual change from low-spin iron(II) below 85 K to high-spin iron(II) at 400 K. The 4.2 to 60 K Mössbauer spectra correspond to a fully low-spin iron(II) complex but, upon further warming above 85 K, the iron(II) begins to undergo spin-state relaxation between the low- and high-spin forms on the Mössbauer time scale. At 155 and 315 K the complex exhibits spin-state relaxation rates of 0.36 and 7.38 MHz, respectively, and an Arrhenius plot of the logarithm of the relaxation rate yields an activation energy of 670 +/- 40 cm(-1) for the spin-state relaxation.

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-(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.

Directional control of pi-stacked building blocks for crystal engineering: the 1,8-naphthalimide synthon.

Incorporating the 1,8-naphthalimide group into bis(pyrazolyl)methane ligands triggers the association of their rhenium(i) complexes into directionally ordered dimers in both solution and solid state, as demonstrated by ES+/MS, PGSE-NMR and X-ray diffraction studies.