Zwitterionic Ru(III) Complexes: Stability of Metal–Ligand Bond and Host–Guest Binding with Cucurbit[7]uril

A wide range of ruthenium-based coordination compounds have been reported to possess potential as metallodrugs with anticancer or antimetastatic activity. In this work, we synthesized a set of new zwitterionic Ru­(III) compounds bearing ligands derived from N-alkyl (R) systems based on pyridine, 4,4′-bipyridine, or 1,4-diazabicyclo[2.2.2]­octane (DABCO). The effects of the ligand(s) and their environment on the coordination stability have been investigated. Whereas the [DABCO-R]⁺ ligand is shown to be easily split out of a negative [RuCl₄]⁻ core, positively charged R-pyridine and R-bipyridine ligands form somewhat more stable Ru­(III) complexes and can be used as supramolecular anchors for binding with macrocycles. Therefore, supramolecular host–guest assemblies between the stable zwitterionic Ru­(III) guests and the cucurbit[7]­uril host were investigated and characterized in detail by using NMR spectroscopy and single-crystal X-ray diffraction. Paramagnetic ¹H NMR experiments supplemented by relativistic DFT calculations of the structure and hyperfine NMR shifts were performed to determine the host–guest binding modes in solution. In contrast to the intramolecular hyperfine shifts, dominated by the through-bond Fermi-contact mechanism, supramolecular hyperfine shifts were shown to depend on the “through-space” spin–dipole contributions with structural trends being satisfactorily reproduced by a simple point-dipole approximation.