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

• Published in: Journal of organic chemistry 2008-07, Vol.73 (13), p.5100-5109
• Main Authors: Rieth, Stephen, Yan, Zhiqing, Xia, Shijing, Gardlik, Matthew, Chow, Albert, Fraenkel, Gideon, Hadad, Christopher M, Badjić, Jovica D
• Format: Article
• Language: English
• Subjects: Chemistry; Coordination compounds; Copper - chemistry; Exact sciences and technology; Heterocyclic compounds; Heterocyclic compounds with only one n hetero atom and condensed derivatives; Hydrocarbons, Cyclic - chemistry; Inorganic chemistry and origins of life; Kinetics and mechanisms; Ligands; Models, Molecular; Molecular Structure; Organic chemistry; Preparations and properties; Reactivity and mechanisms
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo800748k

A molecular basket, composed of a semirigid C 3v 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 × 105 M−1) and driven by enthalpy (ΔH° = −7.2 ± 0.1 kcal/mol, ΔS° = −0.25 eu). Variable temperature 1H 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 (ΔH° = −3.3 ± 0.1 kcal/mol), with an apparent loss in entropy (ΔS° = −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 (ΔH ⧧ = 6.5 ± 0.5 kcal/mol), while the activation entropy was negative (ΔS ⧧ = −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.