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 ((LPh)-Ph-1), 2,6-bis(4-isopropyl-4,5-dihydrothiazol-2-yl)pyridine (L(1)iPr), and 2,6-bis(4-tert-butyl-4,5-dihydrothiazol-2-yl)pyridine (L(1)t-Bu). Crystallographic data imply the larger and more flexible thiazolinyl rings reduce steric clashes between the R substituents in homochiral [Fe((R)-(LR)-R-1)(2)](2+) or [Fe((S)-(LR)-R-1)(2)](2+) (R = Ph, iPr, or t-Bu), compared to their PyBox ((LR)-R-2) analogues. Conversely, the larger heterocyclic S atoms are in close contact with the R substituents in heterochiral [Fe((R)-(LPh)-Ph-1)((S)-(LPh)-Ph-1)](2+), giving it a more sterically hindered ligand environment than that in [Fe((R)-(LPh)-Ph-2)((S)-(LPh)-Ph-2)](2+) ((LPh)-Ph-2 = 2,6-bis(4phenyl-4,5-dihydrooxazol-2-yl)pyridine). Preformed [Fe((R)-(LPh)-Ph-1)((S)-(LPh)-Ph-1)](2+) and [Fe((R)-L(1)iPr)((S)-L(1)iPr)](2+) do not racemize by ligand redistribution in CD3CN solution, but homochiral [Fe(L(1)iPr)(2)](2+) and [Fe(L(1)t-Bu)(2)](2+) both undergo partial ligand displacement in that solvent. Homochiral [Fe((LPh)-Ph-1)(2)](2+) and [Fe(L(1)iPr)(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((LPh)-Ph-1)(2)](2+) and [Fe(L(1)iPr)(2)](2+) show a greater difference in their spin-state behaviors than was previous found for [Fe((LPh)-Ph-2)(2)](2+). Gas-phase DFT calculations (B86PW91/def2-SVP) of the [Fe((LR)-R-1)(2)](2+) and [Fe((LR)-R-2)(2)](2+) complexes reproduce most of the observed trends, but they overstabilize the high-spin state of SCO-active [Fe(L(1)iPr)(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.