Magnetic Behavior of Spin-Crossover Iron(III) Complexes Adsorbed on the Surface of Silicon Dioxide and Metal Dilution Effects on Magnetism

Abstract

The effects of metal dilution and adsorption on the surface of silicon dioxide on the spin-crossover behavior of [Fe(acpa)2]PF6, [Fe(acpa)2]BPh4, and [Fe(bzpa)2]PF6, which undergo fast spin-state interexchange on the 57Fe Mössbauer timescale have been studied by using 57Fe Mössbauer and EPR spectroscopies, powder X-ray diffraction and magnetic susceptibility, where Hacpa is N-(1-acetyl-2-propylidene)-2-pyridylmethylamine and Hbzpa N-(1-benzoyl-2-propylidene)-2-pyridylmethylamine. Solid solution compounds, [FexCo1−x(acpa)2]BPh4(x=0.035 and 0.074) and [Fe0.05Co0.95(bzpa)2]PF6 exhibit spin-crossover phenomena. No difference in the spin-state interexchange rate is observed between the diluted compound and the corresponding undiluted one as far as the rates are evaluated by Mössbauer spectroscopy. Mössbauer spectra for [Fe0.074Co0.926(acpa)2]PF6 exhibit only a pair of quadrupole doublets assigned to the low-spin state in the temperature range from 78 to 298 K. The spin-state interexchange rates for [Fe(acpa)2]BPh4 and [Fe(bzpa)2]PF6 adsorbed on the surface of silicon dioxide become slow compared to those for the corresponding pure complexes and the narrowing of the spin-state transition temperature range is observed. On the contrary no detectable differences in the spin-state interexchange rates are observed between the Mössbauer spectra for [Fe(acpa)2]PF6 adsorbed on the surface of silicon dioxide and that for the pure complexes. These observations are interpreted in terms of a spin-state transition behavior in these complexes which is affected by the packing state in the solid state, which is dependent on the characteristic structure of the respective complex.

Iron(III) Spin Crossover Compounds

“…The 57 Fe Mössbauer spectra for [Fe x Co 1-x (acpa) 2 ]BPh 4 (x=0.035 and 0.074) show that the transition has been displaced to a higher temperature than that for the pure Fe(III) compound [156]. This may again be related to the difference in ionic radii for low spin Co(III) compared to high spin Fe(III) (see below).…”

“…This observation suggests that the geometry about the Fe(III) ion is more distorted in the diluted complex than in the neat compound. A signal observed at g=4 is characteristic for a high spin ferric complex in a rhombically distorted environment [156].…”