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 [Co(8)L(12)](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 (1)H NMR spectroscopy. The cage encloses a cavity of volume of ca. 400 Å(3), 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 CD(3)CN solution, acts as a remarkably size- and shape-selective host for small organic guests such as coumarin (K = 78 M(-1)) and other bicyclic molecules of comparable size and shape such as isoquinoline-N-oxide (K = 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 fac 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 (1)H-(1)H 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.

Selective guest recognition by a self-assembled paramagnetic cage complex.

A cubic cage complex assembled from twelve bis-bidentate ligands and eight Co(II) ions provides a cavity that selectively recognises and binds coumarin in MeCN solution. The cage portals are large enough to allow guest exchange, but small enough to provide a kinetic trap; the cage paramagnetism facilitates detailed NMR analysis.

Structures, host-guest chemistry and mechanism of stepwise self-assembly of M4L6 tetrahedral cage complexes.

The ligand L(bip), containing two bidentate pyrazolyl-pyridine termini separated by a 3,3'-biphenyl spacer, has been used to prepare tetrahedral cage complexes of the form [M(4)(L(bip))(6)]X(8), in which a bridging ligand spans each of the six edges of the M(4) tetrahedron. Several new examples have been structurally characterized with a variety of metal cation and different anions in order to examine interactions between the cationic cage and various anions. Small anions such as BF(4)(-) and NO(3)(-) can occupy the central cavity where they are anchored by an array of CH···F or CH···O hydrogen-bonding interactions with the interior surface of the cage, but larger anions such as naphthyl-1-sulfonate or tetraphenylborate lie outside the cavity and interact with the external surface of the cage via CH···π interactions or CH···O hydrogen bonds. The cages with M = Co and M = Cd have been examined in detail by NMR spectroscopy. For [Co(4)(L(bip))(6)](BF(4))(8) the (1)H NMR spectrum is paramagnetically shifted over the range -85 to +110 ppm, but the spectrum has been completely assigned by correlation of measured T(1) relaxation times of each peak with Co···H distances. (19)F DOSY measurements on the anions show that at low temperature a [BF(4)](-) anion diffuses at a similar rate to the cage superstructure surrounding it, indicating that it is trapped inside the central cage cavity. Furthermore, the equilibrium step-by-step self-assembly of the cage superstructure has been elucidated by detailed modeling of spectroscopic titrations at multiple temperatures of an acetonitrile solution of L(bip) into an acetonitrile solution of Co(BF(4))(2). Six species have been identified: [Co(2)L(bip)](4+), [Co(2)(L(bip))(2)](4+), [Co(4)(L(bip))(6)](8+), [Co(4)(L(bip))(8)](8+), [Co(2)(L(bip))(5)](4+), and [Co(L(bip))(3)](2+). Overall the assembly of the cage is entropy, and not enthalpy, driven. Once assembled, the cages show remarkable kinetic inertness due to their mechanically entangled nature: scrambling of metal cations between the sites of pure Co(4) and Cd(4) cages to give a statistical mixture of Co(4), Co(3)Cd, Co(2)Cd(2), CoCd(3) and Cd(4) cages takes months in solution at room temperature.