SPIN TRANSITION IN FE(PY)2L(NCS)2 COMPLEXES, WHERE PY = PYRIDINE AND L = 2,2'-BIPYRIMIDINE (BPYM) AND 1,10-PHENANTHROLINE (PHEN) - MAGNETIC, CALORIMETRIC, AND MOSSBAUER-EFFECT INVESTIGATION - CRYSTAL-STRUCTURE OF [FE(PY)2BPYM(NCS)2].0.25PY

The abrupt high-spin (S=2)↔ low-spin (S = 0) transitions in solid [Fe(py)2bpym(NCS)2] 0.25py (1) and [Fe(py)2phen- (NCS)2] 0.5py (2) complexes (with py = pyridine, bpym = 2,2′ -bipyrimidine, and phen = 1,10-phenanthroline) have been investigated by variable-temperature magnetic susceptibility, 57Fe Mössbauer-effect spectrometry, and differential scanning calorimetry (DSC) measurements. The structure of the high-spin form of 1 was determined. Crystal data at 293 K: space group tetragonal l4|1/a, a = 18.107 (4) Â c = 29.40 (2) Â Z = 16; the noncoordinated pyridine molecules present a high thermal motion and/or a positional disorder. In the complex molecules, the pyridine rings are in trans positions and the NCS-groups in eis positions. The transition obtained for 1 is one of the sharpest reported so far; it is centered at Tc↓ ≃ 113.5 K and TCₓ ↓ 116.5 K. The transition presented by 2 is also abrupt, though slightly more gradual; it is centered at Tc↓ₓ Tc↑ₓ 106 K. Mösbauer and magnetic susceptibility data lead to similar thermal dependence of the high-spin molecular fraction. The Debye temperatures, evaluated from the variation of the Mössbauer total absorption as a function of temperature, provide evidence for the rearrangement of the crystal lattice upon the spin transition. The temperature dependence of ΔEq HSshows that, as expected, the ligand field is less distorted from octahedral symmetry in 1 than in 2. The enthalpy and entropy changes associated with the spin transition are evaluated from the DSC measurements; ΔH = 6.5 ± 0.5 kJ mol−1(1), 3.7 ± 0.5 kJ mol −1(2); ΔS = 56 ± 4 J mol −1K−1(1), 37 ± 5 J mol−1K−1(2). The lattice vibrational contribution of the entropy variation is shown to be very weak. © 1990, American Chemical Society. All rights reserved.