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[ClO4](2)center dot 6H(2)O by 1 equiv. 2,6-bis((4S)-4-phenyl-4,5-dihydrooxazol-2-yl)pyridine ((S)-(LPh)-Ph-1) and 2,6-bis((4R)-4-phenyl-4,5-dihydrothiazol-2-yl)pyridine ((R)-(LPh)-Ph-2) cleanly affords [Fe((S)-(LPh)-Ph-1)((R)-(LPh)-Ph-2)][ClO4](2); [Fe((R)-L(1)iPr)((S)-L(2)iPr)][ClO4](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)-(LR)-R-1)((S)-(LR)-R-1)][ClO4](2) and [Fe((R)-(LR)-R-2)((S)-(LR)-R-2)][ClO4](2) (R = Ph or iPr) species. The spin states of [Fe(LR)(bimpy)][ClO4](2) and [Fe(LR)(bpp)][ClO4](2) (LR = (LR)-R-1 or (LR)-R-2; 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 H-1 NMR, their solution T-1/2 values appear unaffected by this and correlate well with their spin state energies from gas phase DFT calculations. The high-spin state of [Fe((LR)-R-2)(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 sigma-basicities and pi-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.