Shape‑, Size‑, and Functional Group-Selective Binding of Small Organic Guests in a Paramagnetic Coordination Cage

The host–guest chemistry of the octanuclear cubic coordination cage [Co₈L₁₂]¹⁶⁺ (where L is a bridging ligand containing two chelating pyrazolyl-pyridine units connected to a central naphthalene-1,5-diyl spacer via methylene “hinges”) has been investigated in detail by ¹H NMR spectroscopy. The cage encloses a cavity of volume of ca. 400 ų, which is accessible through 4 Å diameter portals in the centers of the cube faces. The paramagnetism of the cage eliminates overlap of NMR signals by dispersing them over a range of ca. 200 ppm, making changes of specific signals easy to observe, and also results in large complexation-induced shifts of bound guests. The cage, in CD₃CN solution, acts as a remarkably size- and shape-selective host for small organic guests such as coumarin (<i>K</i> = 78 M^–1) and other bicyclic molecules of comparable size and shape such as isoquinoline-<i>N</i>-oxide (<i>K</i> = 2100 M^–1). Binding arises from two independent recognition elements, which have been separately quantified. These are (i) a polar component arising from interaction of the H-bond accepting O atom of the guest with a convergent group of CH protons inside the cavity that lie close to a <i>fac</i> tris-chelate metal center and are therefore in a region of high electrostatic potential; and (ii) an additional component arising from the second aromatic ring (aromatic/van der Waals interactions with the interior surface of the cage and/or solvophobic interactions). The strength of the first component varies linearly with the H-bond-accepting ability of the guest; the second component is fixed at approximately 10 kJ mol^–1. We have also used ¹H–¹H exchange spectroscopy (EXSY) experiments to analyze semiquantitatively two distinct dynamic processes, viz. movement of the guest into and out of the cavity and tumbling of the guest inside the host cavity. Depending on the size of the guest and the position of substituents, the rates of these processes can vary substantially, and the rates of processes that afford observable cross-peaks in EXSY spectra (e.g., between free and bound guest in some cases; between different conformers of a specific host·guest complex in others) can be narrowed down to a specific time window. Overall, the paramagnetism of the host cage has allowed an exceptionally detailed analysis of the kinetics and thermodynamics of its host–guest behavior