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.

Complexation of Fe[ClO 4 ] 2 ·6H 2 O by 1 equiv. 2,6-bis((4 S )-4-phenyl-4,5-dihydrooxazol-2-yl)pyridine (( S )-L 1 Ph) and 2,6-bis((4 R )-4-phenyl-4,5-dihydrothiazol-2-yl)pyridine (( R )-L 2 Ph) cleanly affords [Fe(( S )-L 1 Ph)(( R )-L 2 Ph)][ClO 4 ] 2 ; [Fe(( R )-L 1 iPr)(( S )-L 2 iPr)][ClO 4 ] 2 (L 1 iPr = 2,6-bis(4-isopropyl-4,5-dihydrooxazol-2-yl)pyridine; L 2 iPr = 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 )-L 1 R)(( S )-L 1 R)][ClO 4 ] 2 and [Fe(( R )-L 2 R)(( S )-L 2 R)][ClO 4 ] 2 (R = Ph or iPr) species. The spin states of [Fe(LR)(bimpy)][ClO 4 ] 2 and [Fe(LR)(bpp)][ClO 4 ] 2 (LR = L 1 R or L 2 R; bimpy = 2,6-bis(1 H -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 1 H 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(L 2 R)(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.