Two-step spin transition in a 1D Fe(II) 1,2,4-triazole chain compound.

A thermochromic 1D spin crossover coordination (SCO) polymer [Fe(ßAlatrz)3](BF4)2⋅2 H2O (1⋅2 H2O), whose precursor ßAlatrz, (1,2,4-triazol-4-yl-propionate) has been tailored from a ß-amino acid ester is investigated in detail by a set of superconducting quantum interference device (SQUID), (57)Fe Mössbauer, differential scanning calorimetry, infrared, and Raman measurements. An hysteretic abrupt two-step spin crossover (T1/2(↓) = 230 K and T1/2(↑) = 235 K, and T1/2(↓) = 172 K and T1/2(↑) = 188 K, respectively) is registered for the first time for a 1,2,4-triazole-based Fe(II) 1D coordination polymer. The two-step SCO configuration is observed in a 1:2 ratio of low-spin/high-spin in the intermediate phase for a 1D chain. The origin of the stepwise transition was attributed to a distribution of chains of different lengths in 1⋅2 H2O after First Order Reversal Curves (FORC) analyses. A detailed DFT analysis allowed us to propose the normal mode assignment of the Raman peaks in the low-spin and high-spin states of 1⋅2 H2O. Vibrational spectra of 1⋅2 H2O reveal that the BF4(-) anions and water molecules play no significant role on the vibrational properties of the [Fe(ßAlatrz)3](2+) polymeric chains, although non-coordinated water molecules have a dramatic influence on the emergence of a step in the spin transition curve. The dehydrated material [Fe(ßAlatrz)3](BF4)2 (1) reveals indeed a significantly different magnetic behavior with a one-step SCO which was also investigated.

Ligand substitution effects on the charge transport properties of the spin crossover complex [Fe(Htrz)1+y-x (trz)2-y (NH2trz) x ](BF4)y·nH2O.

The complex dielectric permittivity of a series of spin crossover complexes, with variable ligand stoichiometry [Fe(Htrz)1+y-x (trz)2-y (NH2trz) x ](BF4) y ·nH2O, has been investigated as a function of temperature in a wide frequency range. In each compound, a substantial drop of the conductivity and permittivity is evidenced when going from the low spin to the high spin state, albeit with decreasing amplitude for increasing ligand substitution (i.e. for increasing x). The deconvolution of the dielectric spectra using the Havriliak-Negami equation allowed to extract the dipole and conductivity relaxation times, their distributions as well as the dielectric strengths in both spin states. Remarkably, no clear correlation appears between the conductivity changes and the lattice properties (Debye temperature) in the dilution series. We rationalize these results by considering the dimensionality of the system (1D), wherein the charge transport occurs most likely by hopping along the [Fe(Rtrz)3] n n+ chains.

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

We report on the effect of hydrostatic pressure on the electrical conductivity and dielectric permittivity of the [Fe(Htrz)2(trz)](BF4) (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.