Structural, Magnetic and Mössbauer Spectral Studies of Salts of Bis[2,6-bis(pyrazol-3-yl)pyridine]iron(II)—a Spin Crossover System

Abstract

The magnetic and Mossbauer spectral properties of hydrated and anhydrous complex salts [Fe( bpp )2] X2, where bpp = [2,6-bis(pyrazol-3-yl)pyridine] and X = PF6, BF4, Br, I, NO3, are indicative of a temperature-induced singlet (1A1) ↔ quintet (5T2) transition in the complex cation. The nature of the transition depends on the anion and the degree of hydration. The transition is displaced to lower temperatures in the anhydrous species and for the hydrated is only partial above room temperature. The transition in the hydrated hexafluorophosphate salt occurs in two steps, and that in the anhydrous hexafluorophosphate is incomplete, a high residual fraction of quintet state species being retained below the transition temperature. For the anhydrous complexes the transition is discontinuous and that in the complex iodide is associated with a narrow thermal hysteresis with transition temperatures Tc ↓ = 203 K and Tc ↑ = 205 K. The structures of [Fe( bpp )2]I2.4H2O and [Fe( bpp )2][BF4]2.3H2O have been determined at 298 K. Both complex salts are essentially low spin at this temperature and have an average Fe-N distance of 1.95 Ǻ. The structures reveal in both instances an extended hydrogen-bonded network involving the uncoordinated >NH groups of the ligands, the anions and lattice water. It is suggested that it is this bonding which is responsible for the favouring of the singlet state in the hydrated salts. Bis [2,6-bis(pyrazol-3-yl)pyridine]iron(II) diiodide tetrahydrate : triclinic, space group Pī , a 8.276(8), b 8.305(7), c 21.941(27) Ǻ, α 98.11(5), β 90.08(6), γ 90.82(6)°, Z 2. Bis [2,6-bis(pyrazol-3-yl)pyridine]iron(II) bis ( tetrafluoroborate ) trihydrate : monoclinic, space group C2/c, a 31.222(15), b 14.456(3), c 12.907(5) Ǻ, β 99.40(2)°, Z 8.

Coordination Dependence of Magnetic Properties within a Family of Related [FeII2] Complexes of a Triazine‐Based Ligand

“…[24][25][26] The system shown here is peculiar in that it is dinuclear, unlike most previously reported examples. Also, it has the potential of acting as a building block in the formation of extended structures, a very attractive prospect, as high-dimensional SC systems are still rare.…”

A Guideline to the Design of Molecular‐Based Materials with Long‐Lived Photomagnetic Lifetimes

“…The selected family was the [Fe(bpp) 2 ]X 2 ·n H 2 O series (bpp = 2,6-bis(pyrazol-3-yl)pyridine), well-known for exhibiting various thermal spin-crossover behavior depending on the nature of the anion and the degree of hydration; the iron(ii) metal center is surrounded by six donor nitrogen atoms from the two ligand molecules coordinated in an octahedral meridional plane. [22][23][24][25] Moreover, in some cases unusual long-lived lifetimes have been found for the metastable HS state generated both by thermal trapping and by the LIESST effect. [6,7,26] Consequently, we systematically determined the thermal spin transition and the limit temperature of the LIESST phenomenon of all the series of hydrated and dehydrated [Fe(bpp) 2 ]X 2 ·n H 2 O complexes, with X = PF 6 , NCS, NCSe, BF 4 , Br, and I.…”

Anion-dependent spin crossover in solution for an iron(ii) complex of a 1H-pyrazolyl ligand

“…in 9:1 v/v (CD3)2CO:D2O (circles) and pure (CD3)2CO (squares) §. The black data points are from this work, while the white circles are the I -and PF6 -salts measured by Goodwin et al 12 The solvent dependence of these data is discussed in ref. 20.…”

“…9 The low-spin state of these complexes is favoured in the presence of strongly associating halide anions, which interact with the chelating N-H groups at the periphery of the H2bip ligands. Earlier investigations of aniondependent SCO in other compounds had shown negative results, 11,12 possibly because those studies were performed in aqueous solution or water-containing solvent mixtures. Water tends to disrupt host:guest interactions to anions, all other things being equal, because of its polarity and strong hydrogen-bonding character.…”

“…2). 12 The enthalpy and entropy of SCO for four of the salts in , which is a potential side-reaction in the high-spin form of the complex, would have the opposite effect of raising H and S. 4 For comparison, the three 1X2 salts that are soluble in pure (CD3)2CO were also measured in that solvent (Table 1 The invariance of these spectra with the anion present is inconsistent with the Evans method data, since max of this MLCT band should increase with T½ which raises the low-spin fraction of the complex at room temperature.…”