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.