Substituent-influenced spin crossover in Fe^III quinolylsalicylaldiminates

The new ligand Hqsal-4-OMe was synthesized, and four iron(III) complexes, [Fe(qsal-4-OMe)2]Y·CH2Cl2 [Hqsal-4-OMe = 4-methoxy-N-(8-quinolyl)salicylaldimine; Y = ClO4 (1), NO3 (2)] and [Fe(qsal-4-OMe)2]Y (Y = PF6 (3), OTf (4)] as well as the isomer [Fe(qsal-5-OMe)2]PF6·CH2Cl2 (5), were prepared. UV/Vis spectroscopic studies indicated that the complexes are high-spin in solution and exhibit a ligand-to-metal charge-transfer band around 400 nm. X-ray crystallographic studies on [Fe(qsal-4-OMe)2]Y·CH2Cl2 (Y = ClO4, NO3) and [Fe(qsal-4-OMe)2]PF6 at 97 K revealed high-spin Fe^III centres with one of the qsal-4-OMe ligands significantly distorted from planarity. In contrast, in 5 the Fe^III centre is low-spin at low temperature, and the room-temperature structure of 5 shows about 25 % spin crossover (SCO). The structure of [Fe(qsal-4-OMe)2]Y·sol shows weak π-π interactions between neighbouring cations, while 5 exhibits stronger π-π interactions that link it into a 1D chain. SQUID magnetometric studies revealed that 1-4 are essentially high-spin, whereas 5 undergoes gradual and almost complete SCO up to 350 K. A combination of distortions to the qsal-4-OMe ligand and intermolecular C-H···O interactions involving both the coordinated phenolate oxygen atom and the OMe group seem to be responsible for the loss of SCO.