Fastener Effect on Uniaxial Chemical Pressure for One-Dimensional Spin-Crossover System, [FeII(NH2-trz)3](CnH2n+1SO3)2.xH2O

• Published in: Bulletin of the Chemical Society of Japan Vol. 88; no. 3; p. 419
• Main Authors: Kamebuchi, Hajime, Nakamoto, Akio, Yokoyama, Toshihiko, Kojima, Norimichi
• Format: Journal Article
• Language: English
• Published: 01.01.2015
• Subjects: Covalence; Fasteners; Hysteresis; Ligands; Orbitals; Spectra; Three dimensional; Van der Waals forces
• ISSN: 0009-2673; 1348-0634

One-dimensional spin-crossover (SCO) polymers of formula [FeII(NH2-trz)3](CnH2n+1SO3)2.xH2O (trz: 4-substituted-1,2,4-triazole; n = 1-9) have been synthesized. They displayed hysteretic SCO transition around 280-350 K. The spin transition temperatures (T1/2) in the heating and cooling processes increased, and the hysteresis width became narrower with increasing alkyl chain length (n). From the analysis of EXAFS, the nearest-neighbor Fe-Fe distance decreased with increasing n, and it had a close relationship with T1/2. This is highly indicative of the increase of intrachain interaction of [FeII(NH2-trz)3] with n being attributed to the uniaxial chemical pressure effect induced by van der Waals force between alkyl chains, so-called "fastener effect." To elucidate the physicochemical effect on T1/2, the ligand field and Racah parameter (B) were estimated by analysing the pre-edge region in Fe-K edge XANES spectra based on the ligand-field theory. B decreased with increasing n, which was closely consistent with the behavior of T1/2 and 57Fe Mossbauer isomer shift (IS). Judging from these results, a covalency between FeII 3d and ligand pi * orbital is enhanced by the fastener effect, leading to the expansion of the 3d orbital, i.e. the remarkable decrease of B and IS, which is the dominating factor on T1/2 in our system.