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

Abstract

Complexation of Fe[ClO₄]₂·6H₂O by 1 equiv. 2,6-bis((4S)-4-phenyl-4,5-dihydrooxazol-2-yl)pyridine ((S)-L¹Ph) and 2,6-bis((4R)-4-phenyl-4,5-dihydrothiazol-2-yl)pyridine ((R)-L²Ph) cleanly affords [Fe((S)-L¹Ph)((R)-L²Ph)][ClO₄]₂; [Fe((R)-L¹iPr)((S)-L²iPr)][ClO₄]₂ (L¹iPr = 2,6-bis(4-isopropyl-4,5-dihydrooxazol-2-yl)pyridine; L²iPr = 2,6-bis(4-isopropyl-4,5-dihydrothiazol-2-yl)pyridine) was prepared by a similar route. The compounds exhibit thermal spin-crossover in solution, at temperatures midway between the corresponding [Fe((R)-L¹R)((S)-L¹R)][ClO₄]₂ and [Fe((R)-L²R)((S)-L²R)][ClO₄]₂ (R = Ph or iPr) species. The spin states of [Fe(LR)(bimpy)][ClO₄]₂ and [Fe(LR)(bpp)][ClO₄]₂ (LR = L¹R or L²R; bimpy = 2,6-bis(1H-benzimidazol-2-yl)pyridine; bpp = 2,6-di(pyrazol-1-yl)pyridine) are also reported, with most examples exhibiting gradual spin-crossover in solution and the solid state. Although some products undergo partial ligand exchange in solution by ¹H NMR, their solution T_½ values appear unaffected by this and correlate well with their spin state energies from gas phase DFT calculations. The high-spin state of [Fe(L²R)(bpp)]²⁺ is more stabilised than expected, compared to the other [Fe(LR)L]²⁺ complexes studied (L = bimpy, bpp or terpy). That is explained by an interplay between the relative σ-basicities and π-acidities of the two ligands in each molecule. The steric influence of their phenyl or isopropyl ‘R’ substituents stabilises the heteroleptic complexes by up to 5 kcal mol⁻¹, compared to analogues lacking these groups.