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

Abstract

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 (L 1 Ph), 2,6-bis(4-isopropyl-4,5-dihydrothiazol-2-yl)pyridine (L 1 iPr), and 2,6-bis(4tert-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)-L 1 R) 2 ] 2+ or [Fe((S)-L 1 R) 2 ] 2+ (R = Ph, iPr, or t-Bu), compared to their PyBox (L 2 R) analogues. Conversely, the larger heterocyclic S atoms are in close contact with the R substituents in heterochiraland [Fe(L 1 iPr) 2 ] 2+ exhibit spin-crossover equilibria in CD 3 CN, 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(L 1 Ph) 2 ] 2+ and [Fe(L 1 iPr) 2 ] 2+ show a greater difference in their spin-state behaviors than was previous found for [Fe(L 2 Ph) 2 ] 2+ . Gas-phase DFT calculations (B86PW91/def2-SVP) of the [Fe(L 1 R) 2 ] 2+ and [Fe(L 2 R) 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.

Heteroleptic iron(ii) complexes of chiral 2,6-bis(oxazolin-2-yl)-pyridine (PyBox) and 2,6-bis(thiazolin-2-yl)pyridine ligands – the interplay of two different ligands on the metal ion spin sate

“…Both 1 and 2 are low-spin at the temperature of measurement, which is consistent with the magnetic properties described below (Table S2§). Their ligand conformations resemble those in low-spin, heterochiral [Fe(L 1 Ph) 2 ][ClO 4 ] 2 , 28,29 [Fe(L 2 Ph) 2 ][ClO 4 ] 2 and [Fe(L 2 iPr) 2 ][ClO 4 ] 2 30 (Fig. 1).…”

“…That is unsurprising, since its homoleptic congeners [Fe(( R )-L 1 Ph)(( S )-L 1 Ph)][ClO 4 ] 2 and [Fe(( R )-L 2 Ph)(( S )-L 2 Ph)][ClO 4 ] 2 are also low-spin. 28–30 More informative is 2 which exhibits an SCO equilibrium on cooling. This is centred near room temperature but is very gradual and incomplete, reflecting the poor crystallinity of the sample (Fig.…”

“…We, and others, have used chiral 2,6-bis(4,5-dihydrooxazol-2-yl)pyridine (PyBox; L 1 R, Scheme 1) 27 and 2,6-bis(4,5-dihydrothiazol-2-yl)pyridine (ThioPyBox; L 2 R) ligands to probe the spin states of different diastereomers of [Fe(L 1 R) 2 ] 2+ and [Fe(L 2 R) 2 ] 2+ derivatives. 28–30 The comparison was possible because some heterochiral [Fe(( R )-LR)(( S )-LR)] 2+ (LR = L 1 R or L 2 R) complexes are stable to racemisation in solution. That should reflect destabilisation of homochiral [Fe(( R )-LR) 2 ] 2+ and [Fe(( S )-LR) 2 ] 2+ by steric clashes between their ‘R’ substituents, which are not present in the heterochiral isomer.…”

“…20 DFT calculations confirmed [Fe(( R )-LR)(( S )-LR)] 2+ have lower energy than the corresponding [Fe(( R )-LR) 2 ] 2+ species when R = Ph or iPr. 30…”

“…31 The chirality of such compounds is another attractive feature, since chiral-at-molecule SCO complexes are still quite rare. 28–30,32 Such compounds have potential for switchable ferroelectric or chiroptical properties in the solid state. 33,34…”