Crucial Role of the Heteroatom and Bite Angle Influencing Spin States and Dinuclear Fe(II) Helicate Formation in 1,3,4-Thia/Oxadiazole Ligand

Helical complexes can serve as sensors in various applications, detecting the presence of specific guest molecules by binding to them or responding to their presence. We report the synthesis of two novel iron(II) helicates together with a mononuclear iron(II) compound. The new ligand LTDA-Hel formed the triple-stranded helicates [Fe2(LTDA-Hel)3](ClO4)4·3 H2O (C1) and [Fe2(LTDA-Hel)3](BF4)4·5H2O (C2), as anticipated, whereas the new ligand LODA-Hel formed a mononuclear Fe(II) complex [Fe(H2O)2(LODA-Hel)2](ClO4)2 (C3) with coordinated aqua coligands. All three complexes have been studied by single crystal X-ray crystallography to investigate the effect of the size of the O/S heteroatom in the helix formation. Complexes C1 and C2 are diamagnetic in both solid state and solution phase, with solution NMR confirming their stability. Due to the small helical cavity, guest sensing with Li+ ions was tested using VT7Li NMR spectra.