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

Abstract

Compounds [Fe(tzpy)(3)](BF(4))(2) (1), [Fe(tzpy)(2)(NCS)(2)].S (S = 2CHCl(3) (2), H(2)O (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) A, b = 27.449(2) A, c = 12.4510(8) A, beta = 108.860(5) degrees, V = 3709.0(4) A(3), and T = 293(2) K. The structure consists of mononuclear [Fe(tzpy)(3)](2+) diamagnetic species, which stack via pi-interactions. Disordered BF(4)(-) anions fill the voids generated by complex cations. 2 crystallizes in the triclinic system, P one macro space group, Z = 1, with a = 8.3340(4) A, b = 8.6520(4) A, c = 11.6890(6) A, alpha = 89.113(2) degrees, beta = 81.612(2) degrees, gamma = 77.803(2) degrees, V = 814.90(7) A(3), and T = 293(2) K. The structure consists of mononuclear [Fe(tzpy)(2)(NCS)(2)] neutral species, which interact each other via pi-staking defining layers separated by two-dimensional arrays of CHCl(3). The average Fe-N bond distance, 2.176(3) A, 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 Gamma = 0.86 (2), 0.89 (3), and 3.79 (4) kJ mol(-1), T(1/2) = 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.

Structure and Electronic Configuration of an Iron(II) Complex in a LIESST State: A Pump and Probe Method

“…[22] For 1 B, the value of c m T remains almost constant as the temperature is lowered until 102 K, where it drops abruptly to nearly zero. However, 1 A exhibits somewhat gradual transition between 180 to 120 K. The small residual signal at 120-140 K is due to the presence of small amount of 1 B.…”

“…Apparently, these complexes display strongly solvent-dependent magnetic behavior; [22] (2) and 2.193(1) for metal to pyridine, 2.181(2) and 2.174(1) for metal to triazole, and 2.097(3) and 2.107(2) for metal to thiocyanate, which are very similar to those of the CHCl 3 -solavated structure (2.204(2), 2.211(2), and 2.113(3) ). [22] The difference in Fe À N py bond length between A and B appears to be significant; however, the two crystal structures were measured at different temperature (300, 150 K), and the difference may be smaller when corrected for thermal libration. The molecules in the crystal form a sheet structure along the ac and ab plane for 1 A and 1 B, respectively, through p-p interactions between the adjacent parallel tzpy planes, as shown in Figure 3 a.…”

“…-a]pyridine) (tzpy) was first used for iron(II) SCO compounds by Niel et al [22] The solvated complexes [Fe…”

[(Pyridylcarbonyl)pyridyl]triazolopyridines, Useful Ligands for the Construction of Polynuclear Coordination Compounds – Synthesis, Crystal Structure and Magnetic Properties of a Novel Tetranuclear Copper(II) Cubane

“…On one hand, a part of the structure is analogue to that of the 3-(pyridin-2-yl)-triazolopyridines, which coordinates through the N2 and N11 atoms, i. e. to Cu II , and Fe II ions. [15,16] In particular, the Fe II derivatives with NCS -and NCSe -coordinating anions undergo spin crossover behaviour. On the other hand, the 2,2Ј-dipyridyl ketone moiety is also present.…”

Cu(II)-assisted oxidation of quinoline-2-carbaldehyde hydrazone to give [1,2,3]triazolo[1,5-α]quinoline: The first example of Cu(I) complex containing [1,2,3]triazolo-ligand

“…While the ligand behavior of azines was widely investigated towards Zn II , Fe II , Co II , Ni II and Cu II ions, the only reaction leading to a triazolo-containing ligand complex was in the present of copper(II) ions, indicating that the Cu II ion is necessary for promoting the cyclisation [3]. The subsequent experiments carried out under mild conditions without any other strong oxidants confirmed that O 2 in air acted as the oxidant for the ring-closure [4]. Thus, Amadei.…”