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Deciphering the Influence of Meridional versus Facial Isomers in Spin Crossover Complexes
Author(s) -
Lathion Timothée,
Guénée Laure,
Besnard Céline,
Bousseksou Azzedine,
Piguet Claude
Publication year - 2018
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201804161
Subject(s) - spin crossover , chemistry , isostructural , spin states , acetonitrile , homoleptic , geminal , pyridine , crystallography , computational chemistry , stereochemistry , inorganic chemistry , crystal structure , organic chemistry , metal
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( L k ) 3 ] 2+ complexes existing as mixtures of meridional ( C 1 ‐symmetry) and facial ( C 3 ‐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 Fe II low‐spin ↔Fe II high‐spin equilibria occurring in [Fe( L k ) 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( L k ) 3 ] 5+ (Ln is a trivalent lanthanide, L k = 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( L k ) 3 ] 2+ and fac ‐[Fe( L k ) 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.