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Article Dans Une Revue Chemistry - A European Journal Année : 2018

Deciphering the influence of meridional versus facial isomers in spin crossover complexes

Résumé

Chelate coordination of non-symmetrical didentate pyrazine-benzimidazole (L1) or pyridine-benzimidazole (L2) N-donor ligands around divalent iron in acetonitrile produces stable homoleptic triple-helical spin crossover [Fe(Lk)3]2+ complexes existing as mixtures of meridional (C1-symmetry) and facial (C3-symmetry) isomers in slow exchange on the NMR timescale. The speciation deviates from the expected statistical ratio mer/fac=3:1, a trend assigned to the thermodynamic trans-influence, combined with solvation effects. Consequently, the observed spin state FeIIlow-spin↔FeIIhigh-spin equilibria occurring in [Fe(Lk)3]2+ refer to mixtures of complexes in solution, an issue usually not considered in this field, but which limits rational structure-properties correlations. Taking advantage of the selective and quantitative formation of isostructural facial isomers in non-constrained related spin crossover d-f helicates (HHH)-[LnFe(Lk)3]5+ (Ln is a trivalent lanthanide, Lk=L5, L6), we propose a novel strategy for assigning pertinent thermodynamic driving forces to each spin crossover triple-helical isomer. The different enthalpic contributions to the spin state equilibrium found in mer-[Fe(Lk)3]2+ and fac-[Fe(Lk)3]2+ reflect the Fe−N bond strengths dictated by the trans-influence, whereas a concomitant solvent-based entropic contribution reinforces the latter effect and results in systematic shifts of the spin crossover transitions toward higher temperature in the facial isomers.

Dates et versions

hal-01961784 , version 1 (20-12-2018)

Identifiants

Citer

T. Lathion, L. Guénée, C. Besnard, Azzedine Bousseksou, C. Piguet. Deciphering the influence of meridional versus facial isomers in spin crossover complexes. Chemistry - A European Journal, 2018, 24 (63), pp.16873-16888. ⟨10.1002/chem.201804161⟩. ⟨hal-01961784⟩

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