Flipping hosts in hyperfine fields of paramagnetic guests

Nuclear magnetic resonance (NMR) spectroscopy of paramagnetic systems is an advancing field that contributes to characterization of the structure and magnetism of molecules, molecular assemblies, biomolecules, and materials. In this account, we utilize the hyperfine field intrinsic to ruthenium(III) coordination compounds to uncover details of their supramolecular host-guest binding with cyclodextrins. We interpret perturbations of the 1H NMR shifts of the host to suggest the binding mode of the interaction. We demonstrate how the size of the cyclodextrin cavity and the structural modification of the guest affect whether the binding occurs through a head or tail portal of the cyclodextrin and attempt to explain the observed differences in terms of molecular shapes and the charge-distribution complementarity between the host and guest. Paramagnetic NMR spectroscopy of host-guest systems is shown to be a useful tool for supramolecular and structural chemists investigating modes of molecular encapsulation. [Display omitted] •Paramagnetic NMR spectroscopy is an excellent tool for determining host-guest binding•Hyperfine field of guest is calculated via a g-tensor and relativistic DFT•Geometry and binding constants of weakly bound supramolecular complexes are determined•Head versus tail binding is governed by the host-guest complementarity Novotný et al. demonstrate the power of paramagnetic nuclear magnetic resonance spectroscopy in determining the binding constants and geometry of supramolecular host-guest complexes. By exemplifying with ruthenium metallodrugs and cyclodextrin carriers, they show that a slight structural modification of the guest or host modulates their complementarity and impacts the mode of binding.