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Deciphering the Influence of Meridional versus Facial Isomers in Spin Crossover Complexes
Timothée Lathion1, Laure Guénée2, Céline Besnard2
1Department of Inorganic and Analytical Chemistry, University of Geneva, 30 quai E. Ansermet, 1211, Geneva 4, Switzerland.
Abstract:
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 FeII low-spin ↔FeII high-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.
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