Molecular encapsulation via metal-to-ligand coordination in a Cu(I)-folded molecular basket

A molecular basket, composed of a semirigid C-3 nu 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 (K-a = 1.73 +/- 0.08 x 10(5) M-1) and driven by enthalpy (Delta H degrees = -7.2 +/- 0.1 kcal/mol, Delta S degrees = -0.25 eu). Variable temperature H-1 NMR studies, assisted with 2D COSY and ROESY investigations, revealed the existence of Cu(I)-folded basket 10(b) with a molecule of acetonitrile occupying its interior and coordinated to the metal. Interestingly, 10(b) is in equilibrium with Cu(I)-folded 10(a), whose inner space is solvated by acetone or chloroform. The incorporation of a molecule of acetonitrile inside 10(a) was found to be driven by enthalpy (Delta H degrees = -3.3 +/- 0.1 kcal/mol), with an apparent loss in entropy (Delta S degrees= -9.4 +/- 0.4 eu); this is congruent with a complete immobilization of acetonitrile and release of a loosely encapsulated solvent molecule during 10(a/b) interconversion. From an Eyring plot, the activation enthalpy for incorporating acetonitrile into 10(a) was found to be positive (Delta H-double dagger = 6.5 +/- 0.5 kcal/mol), while the activation entropy was negative (Delta S-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.