Molecular Encapsulation via Metal-to-Ligand Coordination in a Cu(I)-Folded Molecular Basket

Abstract

A molecular basket, composed of a semirigid C3v symmetric tris-norbornadiene framework and three pyridine flaps at the rim, has been shown to coordinate to a Cu(I) cation and thereby fold in a multivalent fashion. The assembly was effective (Ka = 1.73 +/- 0.08 x 10(5) M(-1)) and driven by enthalpy (DeltaH(o) = -7.2 +/- 0.1 kcal/mol, DeltaS(o) = -0.25 eu). Variable temperature (1)H NMR studies, assisted with 2D COSY and ROESY investigations, revealed the existence of Cu(I)-folded basket 10b with a molecule of acetonitrile occupying its interior and coordinated to the metal. Interestingly, 10b is in equilibrium with Cu(I)-folded 10a , whose inner space is solvated by acetone or chloroform. The incorporation of a molecule of acetonitrile inside 10a was found to be driven by enthalpy (DeltaH(o) = -3.3 +/- 0.1 kcal/mol), with an apparent loss in entropy (DeltaS(o) = -9.4 +/- 0.4 eu); this is congruent with a complete immobilization of acetonitrile and release of a "loosely" encapsulated solvent molecule during 10a/b interconversion. From an Eyring plot, the activation enthalpy for incorporating acetonitrile into 10a was found to be positive (DeltaH(double dagger) = 6.5 +/- 0.5 kcal/mol), while the activation entropy was negative (DeltaS(double dagger) = -20 +/- 2 eu). The results are in agreement with an exchange mechanism whereby acetonitrile "slips" into an "empty" basket through its side aperture. In fact, DFT (BP86) calculations are in favor of such a mechanistic scenario; the calculations suggest that opening of the basket's rim to exchange guests is energetically demanding and therefore less feasible.

Gated molecular baskets

“…Importantly, 5B contains Cu(I) cation at its rim coordinating to sp 2 nitrogen atoms of the pyridine gates and also a molecule of CH 3 NC occupying the host's cavity. [75][76][77] The use of external chemical stimulus (Cu(I)) therefore caused a disruption of three N-HÁ Á ÁN hydrogen bonds in 5A, reorganization of its gates and finally exchange of guests to give rise to 5B. To reverse these chemical changes, we used Na 2 S which coordinated to Cu(I) and thereby triggered the conversion of 5B back into 5A.…”

Controlling the dynamics of molecular encapsulation and gating

“…Three phthalimides extend this structure into a bowl-shaped platform. Aromatic rings containing hydrogen-bonding donor and acceptor sites (or transition-metal chelating groups) 90 are attached to the rim of the platform via CH 2 ''hinges'' to act as gates that, by controlling the portal size, regulate the encapsulation kinetics (Fig. 17).…”

Self-assembly of a water-soluble endohedrally functionalized coordination cage including polar guests

“…The [1a•Pd] 2+ CC might be also considered as a mono-nuclear metal-coordinated cavitand. The use of coordination metals in controlling the conformations and binding properties of covalent organic hosts have also been reported by the groups of Badjić 26,27,28 and Rebek. 29,30,31,32,33 The [1a•Pd] 2+ CC features two different and converging polar binding sites 34 defined by the four pyrrole NHs of the C [4]P unit and the four inwardlydirected α-CH protons of the pyridyl groups coordinated to Pd(II).…”