Spin transition in iron Fe(py)2L(NCS)2 complexes where py = pyridine and L = 2,2'-bipyrimidine (bpym) and 1,10-phenanthroline (phen): magnetic, calorimetric, and Moessbauer-effect investigation. Crystal structure of [Fe(py)2bpym(NCS)2].0.25py

Abstract

The abrupt high-spin ( S = 2) -low-spin ( S = 0) transitions in solid [Fe(py)2bpym(NCS),].0.25py (1) and [Fe(py),phen-(NCS),].O.Spy (2) complexes (with py = pyridine, bpym = 2,2'-bipyrimidine, and phen = 1 ,IO-phenanthroline) have been investigated by variable-temperature magnetic susceptibility, 57Fe Mossbauer-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 14,/a, a = 18.107 (4) A, c = 29.40 (2) A, 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 cis positions.The transition obtained for 1 is one of the sharpest reported so far; it is centered at Tcl = 113.5 K and Tct = 116.5 K. The transition presented by 2 is also abrupt, though slightly more gradual; it is centered at Tcl = Tct = 106 K. Mossbauer and magnetic susceptibility data lead to similar thermal dependence of the high-spin molecular fraction. The Debye temperatures, evaluated from the variation of the Mossbauer total absorption as a function of temperature, provide evidence for the rearrangement of the crystal lattice upon the spin transition. The temperature dependence of AEqHS shows 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: AH = 6.5 f 0.5 kJ mol-' ( I ) , 3.7 f 0.5 kJ mol-' (2): AS = 56 f 4 J mol" K-' (l), 37 f 5 J mol" K-' (2). The lattice vibrational contribution of the entropy variation is shown to be very weak.

Extrinsic and intrinsic magnetic properties of Co1−x Fex Sb3

“…Indeed, a discontinuous transition from low to high spin transition has also been observed in different iron compounds, whether inorganic [32,33] or organic [34]. These compounds, however, have in common that the iron ions are in the divalent state, as the ground state of octahedral d 6 complexes have a low spin ground state (S = 0, t 6 2g eg 0 ) close to the high spin excited state (S = 2, t 4 2g e 2 g ) [35][36][37][38]. In principle, such a transition can also occur for Fe 3+ since the ground state is in the low spin state, but we do not have any example.…”

Theoretical Study of Spin Crossover in 30 Iron Complexes

“…[Fe(py)2phen(NCS)2], 49 17: [Fe(py)2bpym(NCS)2], 49 18: [Fe((NH2)2sar)] 2+ , 50 19: [Fe(HC(pz)3)2] 2+ , 51 20: [Fe(HC(3,5-Me2pz)3)2] 2+ , 52 21: [Fe(tp [10]aneN3)] 2+ , 53 22: [Fe(lpp [9]aneN3)] 2+ , 54 23: [Fe(btpa)] 2+ , 55 and 24: [Fe(tptMetame)] 2+ , 56 as well as for the iron(III) complexes 25: [Fe(acac)2trien] + , 57…”

Pressure-induced spin-state crossovers at room temperature in iron(II) complexes: comparative analysis; a XANES investigation of some new transitionsDedicated to the memory of Professor Olivier Kahn.

“…In Fig. 5, the (P 1/2 , T 1/2 ) set related to compound 1 (0.32 GPa, 270 K) can be compared with the corresponding data reported for other mononuclear metal complexes of the type FeL n L 0 m (NCS) 2 (L ¼ bidentate ligand, n ¼ 1 or 2): a (0.80 GPa, 8 176.2 K 31,32 ), b (0.65 GPa, 8 178.8 K 31,32 ) and c (1.00 GPa, 8 115 K 32,33 ). These compounds are good candidates for comparison purposes: they neither include solvent molecules nor exhibit intermolecular hydrogen bonds.…”

“…Regarding compound c, only the DS HL value related to the solvate cÁ0.25py was reported, 56 ± 4 J K À1 mol À1 . 33 As this value presumably includes the additional contribution of an order-disorder transition of non-coordinated pyridine molecules (these being anticipated to show a positional disorder in the HS phase on the basis of the large thermal coefficient of their atoms), 33 the DS HL and hence DV HL values of c are likely not to significantly deviate from those of compounds a and b.…”