Deciphering the Influence of Meridional versus Facial Isomers in Spin Crossover Complexes

Chemistry : a European Journal
Timothée LathionClaude Piguet

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 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...Continue Reading

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Citations

Jan 7, 2021·Dalton Transactions : an International Journal of Inorganic Chemistry·Neel DeorukhkarClaude Piguet
Nov 13, 2018·Inorganic Chemistry·Naheed BibiAndré Luiz Barboza Formiga
Oct 12, 2021·Chemical Communications : Chem Comm·Lauren L K TaylorImogen A Riddell

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