Crystal Structure, Magnetic Properties, and 57Fe Mössbauer Spectroscopy of the Two-Dimensional Coordination Polymers [M(1,2-bis(1,2,4-triazol-4-yl)ethane)2(NCS)2] (MII = Fe, Co)

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Abstract
New coordination polymers of the formula [M(btre)2(NCS)2] (btre = 1,2-bis(1,2,4-triazol-4-yl)ethane; MII = Fe, Co) have been synthesized, and their crystal structures have been determined at 293 K by X-ray analysis. The FeII compound (C7H8FeN7S2) crystallizes in the monoclinic space group P21/n, a = 12.439(5) Å, b = 8.941(2) Å, c = 9.321(3) Å, β = 90.88(2)°, V = 1036.6(6) Å3, Z = 2, 3791 reflections [I > 3σ(I)], RF = 0.036, wR2 = 0.123. The CoII compound is isostructural to the FeII compound. The crystal structure consists of a 2D sheet in which the metal ions are linked by bis monodentate (N1, N1‘) 1,2,4-triazole ligands. The structure is stabilized by π-bond interactions between two adjacent sheets and by S···S interactions. Temperature-dependent SQUID, 57Fe Mössbauer, and X-ray diffraction measurements indicate that [Fe(btre)2(NCS)2] retains a HS ground state upon cooling from 293 K down to 8 K. The surprising absence of spin-crossover behavior for this Fe(II)−1,2,4-triazole polymeric coordination compound that has been confirmed by pressure experiments up to ∼12 kbar and by light irradiation experiments at 10 K is discussed on the basis of its structural features. Insight into the origin of the cooperative effects of the spin transition in [Fe(btr)2(NCS)2]·H2O (btr = 4,4‘-bis-1,2,4-triazole) is also given thanks to a re-evaluation of its distortion parameters in the high- and low-spin states.

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