Substituent‐Influenced Spin Cross­over in FeIII Quinolylsalicylaldimin­ates

Abstract

AbstractThe 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 FeIII centres with one of the qsal‐4‐OMe ligands significantly distorted from planarity. In contrast, in 5 the FeIII 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.