Substituent‐Influenced Spin Crossover in Fe^III Quinolylsalicylaldiminates

Abstract

Abstract The new ligand Hqsal‐4‐OMe was synthesized, and four iron(III) complexes, [Fe(qsal‐4‐OMe) 2 ]Y · CH 2 Cl 2 [Hqsal‐4‐OMe = 4‐methoxy‐ N ‐(8‐quinolyl)salicylaldimine; Y = ClO 4 ( 1 ), NO 3 ( 2 )] and [Fe(qsal‐4‐OMe) 2 ]Y (Y = PF 6 ( 3 ), OTf ( 4 )] as well as the isomer [Fe(qsal‐5‐OMe) 2 ]PF 6 · CH 2 Cl 2 ( 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 · CH 2 Cl 2 (Y = ClO 4 , NO 3 ) and [Fe(qsal‐4‐OMe) 2 ]PF 6 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.