Spin crossover in six-coordinate [Fe(L) 2(NCX) 2] compounds with L = DPQ = 2,3-bis-(2′-pyridyl)-quinoxaline, ABPT = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole and X = S, Se: synthesis, magnetic properties and single crystal studies

Abstract

The iron(II) compounds of formulae [Fe(DPQ) 2(NCS) 2]·CO(CH) 3) 2(DPQ = 2,3-bis-(2′-pyridyl)-quinoxaline) ( 1) and [Fe(ABPT) 2-(NCX) 2] (ABPT = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) X = S ( 2) and Se ( 3) were synthesized and the crystal structure of 1 determined by X-ray diffraction methods. It crystallizes in the monoclinic system P2 1/n, a = 12.057(2), b = 22.474(5), c = 15.004(7) A ̊ , β = 109.76(3)°, V = 3826 A ̊ 3, Z = 4 and T = 293 K . The structure is made up of discrete [Fe(DPQ) 2(NCS) 2] units. Each metal atom is in a distorted FeN 6 octahedral environment, the FeN bonds ranging from 2.013(8) Å to 2.425(8) Å. Variable-temperature magnetic susceptibility data in the temperature range 290–4.2 K revealed that 1 is high spin, in contrast to 2 and 3 which show a moderately cooperative high spin ↔ low spin (HS ↔ LS) transition, centred at T 1 2 = 186 K and 224 K for 2 and 3 respectively. The thermodynamic model of Slichter and Drickamer was applied to account for the magnetic data. The intermolecular interaction parameter, the enthalpy and entropy changes associated with the spin transition of 2 were estimated as Γ = 2.3 kJ mol −1, ΔH = 10.7 kJ mol −1 and ΔS = 58 J mol −1K −1 respectively.