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

Abstract

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.

Supramolecular Coronation of Platinum(II) Complexes by Macrocycles: Structure, Relativistic DFT Calculations, and Biological Effects

“…As expected, experimentally determined binding constants indicate a significantly higher affinity of compound 2a for the CB7 cavitand than for β-CD. Our results correspond with the binding constants previously obtained for the interaction of organic salts 46 or ruthenium(III) compounds 47,48 bearing an adamantyl anchor with either CB7 or β-CD macrocycles.…”

Nature of NMR Shifts in Paramagnetic Octahedral Ru(III) Complexes with Axial Pyridine-Based Ligands

“…Although we did not arrive at a full understanding of the solvent effects on the electronic structure and hyperfine NMR shifts, this represents an extremely interesting field for further investigation. The results we obtained systematically expand our previous pNMR studies of Ru(III) compounds, 8,9,30,31,40 add new information about the distribution of spin density in openshell molecules, and enhance our knowledge of hyperfine effects in transition-metal complexes in general. The synthesis (I, II) of symmetric ruthenium and rhodium coordination compounds (1, 2) corresponds to the preparation of KP-type Ru(III) compounds by previously reported synthetic procedures.…”

“…Solvent nucleophiles (H 2 O or OH – ) can, in principle, replace either chloride (equatorial ligand) or pyridine (axial ligand), see structures in Figure . When H 2 O is substituted for Cl – in compound 1a , the charge of the complex is changed from −1 to 0, but the axial organic ligand (4-Me-Py) is affected only indirectly by a modulation of the paramagnetic NMR effects propagated from the paramagnetic center toward the pyridine moiety. ,,, In the case of pyridine, the axial 4-Me-Py ligand is dissociated from the ruthenium center as manifested by the appearance of NMR resonances belonging to a diamagnetic form of the 4-Me-Py ligand.…”

Modes of Micromolar Host–Guest Binding of β-Cyclodextrin Complexes Revealed by NMR Spectroscopy in Salt Water

“…Values of association constants can then, theoretically, range from 10 8 to 10 15 M –1 . These values were reported for adamantane derivatives with a long noncharged chain in the position one or even for anionic complexes of Ru­(III) and singly positively charged adamantane derivatives with a cationic part in the optimal position, respectively.…”

“…Another feature of inclusion complexes between CB7 and adamantane derivatives is their tight fit. As a result, the adamantane cage cannot move significantly from the optimal position inside the cavity contrary to derivatives of linear aliphatic hydrocarbons as has been indicated by molecular dynamic simulations . Because adamantane-based ligands are tightly anchored within the complex and ion–dipole interactions between the cationic part of the guest and the CB7 portal cannot surpass the hydrophobic forces, the contribution of the ion–dipole interaction should be easily adjusted by changing the length of a rigid linker between adamantane cage and the cationic part.…”