Spin-States of Diastereomeric Iron(II) Complexes of 2,6-Bis(thiazolin-2-yl)pyridine (ThioPyBox) Ligands and a Comparison with the Corresponding PyBox Derivatives

This report investigates homoleptic iron(II) complexes of thiazolinyl analogues chiral PyBox tridentate ligands: 2,6-bis(4-phenyl-4,5-dihydrothiazol-2-yl)pyridine (L1Ph), 2,6-bis(4-isopropyl-4,5-dihydrothiazol-2-yl)pyridine (L1iPr), and 2,6-bis(4-tert-butyl-4,5-dihydrothiazol-2-yl)pyridine (L1t-Bu). Crystallographic data imply the larger more flexible rings reduce steric clashes between R substituents in homochiral [Fe((R)-L1R)2]2+ or [Fe((S)-L1R)2]2+ (R = Ph, iPr, t-Bu), compared to their (L2R) analogues. Conversely, heterocyclic S atoms are close contact with heterochiral [Fe((R)-L1Ph)((S)-L1Ph)]2+, giving it a sterically hindered ligand environment than that [Fe((R)-L2Ph)((S)-L2Ph)]2+ (L2Ph 2,6-bis(4-phenyl-4,5-dihydrooxazol-2-yl)pyridine). Preformed [Fe((R)-L1Ph)((S)-L1Ph)]2+ [Fe((R)-L1iPr)((S)-L1iPr)]2+ do not racemize by redistribution CD3CN solution, but [Fe(L1iPr)2]2+ [Fe(L1t-Bu)2]2+ both undergo partial displacement solvent. Homochiral [Fe(L1Ph)2]2+ exhibit spin-crossover equilibria CD3CN, centered at 344 ± 6 K 277 1 respectively, while congeners essentially low-spin within liquid range These diastereomers show greater difference spin-state behaviors was previous found for [Fe(L2Ph)2]2+. Gas-phase DFT calculations (B86PW91/def2-SVP) [Fe(L1R)2]2+ [Fe(L2R)2]2+ reproduce most observed trends, they overstabilize high-spin state SCO-active ca. 1.5 kcal mol–1. might reflect influence intramolecular dispersion interactions on spin states these compounds. Attempts model this dispersion-corrected functionals B97-D2 PBE-D3 were less successful our original protocol, confirming molecules challenging computational problem.