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