Spin Crossover Behavior in the Iron(II)-2-pyridyl[1,2,3]triazolo[1,5-a]pyridine System: X-ray Structure, Calorimetric, Magnetic, and Photomagnetic Studies

Compounds [Fe(tzpy)3](BF4)2 (1), [Fe(tzpy)2(NCS)2]•S (S = 2CHCl3 (2), H2O (3)), and [Fe(tzpy)2(NCSe)2] (4) (tzpy is 3-(2-pyridyl)[1,2,3]triazolo[1,5-a]pyridine) have been synthesized and characterized. 1 crystallizes in the monoclinic noncentrosymmetric system, Cc space group, Z = 4, with a = 11.4680(6) (angstrom), b = 27.449(2) (angstrom), c = 12.4510(8) (angstrom), (beta)= 108.860(5) °, V = 3709.0(4) (angstrom)^3, and T = 293(2) K. The structure consists of mononuclear [Fe(tzpy)3]2+ diamagnetic species, which stack via -interactions. Disordered BF4- anions fill the voids generated by complex cations. 2 crystallizes in the triclinic system, P space group, Z = 1, with a = 8.3340(4) (angstrom), b = 8.6520(4) (angstrom), c = 11.6890(6) (angstrom), (alpha)= 89.113(2) °, (beta)= 81.612(2) °, (gamma)= 77.803(2) °, V = 814.90(7) (angstrom)3, and T = 293(2) K. The structure consists of mononuclear [Fe(tzpy)2(NCS)2] neutral species, which interact each other via -staking defining layers separated by two-dimensional arrays of CHCl3. The average Fe-N bond distance, 2.176(3) (angstrom), corresponds to what is expected for an iron(II) ion in the high-spin state. Compounds 2-4 undergo thermal-driven spin conversion. The regular solution model was applied to account for the corresponding to thermodynamic parameters. The intermolecular interaction parameter, the characteristic temperature, and the enthalpy and entropy changes associated with the spin conversion were estimated as = 0.86 (2), 0.89 (3), and 3.79 (4) kJ mol^-1, T1/2 = 75 (2), 118 (3), and 251 K (4), (delta)H = 3.67 (2) and 4.08 (3) kJ mol^-1, and (delta)S = 34 (2) and 34.5 (3) J K-1 mol-1. (delta)H = 8.75 kJ mol^-1 and (delta)S = 34.8 J K^-1 mol^-1 were estimated from calorimetric measurements and used as fixed parameters for 4. A quantitative light-induced excited spin state trapping (LIESST) effect was observed for 3, and the high-spin to low-spin relaxation was studied in the temperature region 20-63 K.