Piezoresistive Effect in the [Fe(Htrz)₂(trz)](BF₄) Spin Crossover Complex

Abstract

We report on the effect of hydrostatic pressure on the electrical conductivity and dielectric permittivity of the [Fe(Htrz)(trz)](BF) (Htrz = 1H-1,2,4,-triazole) spin crossover complex. Variable-temperature and -pressure broad-band impedance spectrometry revealed a piezoresistive effect of more than 1 order of magnitude for pressures as low as 500 bar, associated with a large pressure-induced hysteresis of 1700 bar. The origin of the piezoresistive effect has been attributed to the pressure-induced spin state switching in the complex, and the associated P,T phase diagram was determined.

Halogenated Alkyltetrazoles for the Rational Design of FeII Spin‐Crossover Materials: Fine‐Tuning of the Ligand Size

“…For 9,t he spin transition of the first iron site occurs between 265 and 180 K, whereas the two remaining iron sites undergo spin transition within 25 K, achieving the complete LS state at 155 K. During the determination of the magnetics usceptibility in heating mode, the second step of the spin transition wasf ound to show ab road 10 Kh ysteresis between 165 and 190 K. Above this temperature, the heating and cooling curves are againi dentical, as observed over the whole temperature regime for compounds 8, 10,a nd 11. This exceptional behavior of the chlorine-substitutedd erivative 9 within thiss eries was also observed in the halogenated1ethyl-1H-tetrazoles;[ Fe(2-Cl-2Tz) 6 2Tz) 6 ](ClO 4 ) 2 at wo-step transition with a1 7K broad hysteresis in the second step. [21a] To investigate the impact of the anion, [Fe(3-Cl-3Tz) 6 ](SbF 6 ) 2 (12)w as prepared.…”

“…Although all the halogen-containing materials exhibit an early complete spin transition, the hydroxy substituent in complex 7 causes av ery gradual and incomplete transition between 50 and 200 K. Although the hydroxy groups in the complex were initially expected to act as hydrogen-bonding partners, thusb uffering the cooperativity and enhancing the abruptness of the spin transition, the magnetic data show their presence to be unfavorable. This exceptional behavior of the chlorine-substitutedd erivative 9 within thiss eries was also observed in the halogenated1ethyl-1H-tetrazoles;[ Fe(2-Cl-2Tz) 6 ](BF 4 ) 2 (2Tz = 1-(2-chloroethyl)-1H-tetrazole) shows at wo-step transition and [Fe(2-Cl-Scheme1.Synthesis of the halogen-substituted 1-propyl-1H-tetrazoles. The high-spin fraction remaining after the spin transition increases inversely with halogen atomic number.T he fluorine compound (8) shows ac ompletely low-spinstate whereas the iodine complex (11)s hows 21 %r esidual high-spin configuration.…”

“…In contrast to the halogenated ethyl derivatives, [21b] no correlation between the spin-transition behavior of complexes 8-11 and the increasing size of its halogen is observed. [21a] To investigate the impact of the anion, [Fe(3-Cl-3Tz) 6 ](SbF 6 ) 2 (12)w as prepared. The T 1/2 valuesf or the series are 175 K( Br, 10), 212/160K (Cl, 9), 194 K (F, 8), and2 00 K( I, 11).…”

“…[Fe(3-I-3Tz) 6 ](BF 4 ) 2 (11): Fe(BF 4 ) 2 ·6H 2 O( 64.6 mg, 0.19 mmol, 1equiv) and tetrazole 6 (278 mg, 1.2 mmol, 6.1 equiv) were dissolved in MeCN (10 mL) and stirred at 50 8Cf or 18 h. After evaporation of the solvent the residual solid was triturated in CH 2 Cl 2 (10 mL) to give complex 11 as aw hite solid (135 mg, 42.5 %). IR: n CH(Tz) = 3139 cm À1 ;e lemental analysis calcd (%) for C 24 Figure S30.…”

“…Building on the work of Gütlich and co-workerso n [Fe(3Tz) 6 ](BF 4 ) 2 (3Tz = 1-propyl-1H-tetrazole), [16b, 20] which features an abrupt,c omplete spin transition at 130 Kw ith as mall hysteresis, as eries of terminala nd bridging alkyltetrazoles with various alkyl chain lengths were prepared. Longer tails on the tetrazoles resulted in ak ind of shock-absorber effect, reducing the internal cooperativity of the system and resulting in only partials pin transitionsw ithouta ny significant increasei nt he spin-transitiont emperature T 1/2 .F inally,h alogenation of 1ethyl-1H-tetrazole [21] was found to be au seful modification of the ligand,r aising the value of T 1/2 of its Fe II SCO complex by about7 0K as compared with the alkyl-substituted equiva-lent.…”