Can Benzylic Amide [2]Catenane Rings Rotate on Graphite?

Abstract

The structure and dynamics of two benzylic amide [2]catenanes, bearing phenyl- or thiophenyl-1,3-dicarbonyl groups and physisorbed onto a graphite surface, have been investigated by means of molecular mechanics along with elementary calculations of kinetic rate constants. For the isophthaloyl-based catenane (1), a delicate balance between enthalpy and entropy effects is pointed out. On one hand, van der Waals interactions are enhanced in a first series of co-conformations (A), including the global energy minimum, which are characterized by a near-parallel deposition and flattening of both macrocycles onto the graphite layer. On the other hand, vibrational entropy is shown to favor a very different mode for physisorption (B), with one of the interlocked macrocycles being oriented perpendicular, whereas the other remains parallel to the graphite surface. Both kinds of layout should coexist, the energy difference between the two deposited structures being less than 2 kcal/mol. From the calculated energy barriers and the related rate constants, the A mode of adsorption on graphite is shown to strongly impede circumrotational actions. On the other hand, circumrotation of one of the macrocycles is at least partially allowed at rates comparable to those found in a nonpolar solution when catenane 1 adopts a co-conformation of type B. The thiophenyl-based catenane (2) is found to be already enthalpically driven to a deposition mode of the B-type, favoring -stackings and phenyl-phenyl T-shape complexes within the catenane at the expense of dispersion interactions with the substrate. Although sterically allowed, a full circumrotational process appears in this case to be strongly hindered by physisorption forces, intramolecular interactions, and entropy effects. For catenane 2, rates of the order of one macrocyclic ring rotation per second are nonetheless reached at moderate temperatures (~400 K).