Cooperative Self‐Assembly of a Quaternary Complex Formed by Two Cucurbit[7]uril Hosts, Cyclobis(paraquat‐p‐phenylene), and a “Designer” Guest

Abstract

The self-assembly in aqueous solution of the well-known cyclophane, cyclobis(paraquat-p-phenylene) (BB(4+) ), and two cucurbit[7]uril (CB7) hosts around a simple hydroquinol-based, diamine guest (GH2 (2+) ) was investigated by (1) H NMR and electronic absorption spectroscopies, electrospray mass spectrometry and DFT computations. The formation of a quaternary supramolecular assembly [GH2 (2+) ⋅BB(4+) ⋅ (CB7)2 ] was shown to be a very efficient process, which takes place not only because of the attractive forces between each of the hosts and the guest, but also because of the lateral interactions between the hosts in the final assembly. This complementary set of attractive interactions results in clear cooperative binding effects that help overcome the entropic barriers for multiple component assembly.

Portal Effects on the Stability of Cucurbituril Complexes

“…With assistance from the group of my Miami colleague, Prof. Rajeev Prabhakar, we carried our extensive computational work using DFT methods [M06-2X functional and 6-31G(d) basis set]. [27] These computational results confirm the presence of cooperativity in the binding of the two types of hosts to the guest and are, thus, very useful to rationalize the 1 H NMR and ESI MS data. Similarly, binding of CB7 to GH 2 2 + strongly favors the binding of BB 4 + .…”

“…We obtained a K value of 3.2 3 10 3 M À1 for the formation of the GH 2 2 + gCB7 complex in Figure 7. [27] The K values measured for the formation of these two binary complexes are moderate, revealing that the complexation processes are not quantitative at the millimolar concentration levels used in 1 H NMR experiments. The hydrogens shown correspond to those with hydrogen binding donor ability on one of the two sides of the cavity.…”

“…We also measured K = 6.4 3 10 2 M À1 for the formation of the GH 2 2 + gBB 4 + complex in the same medium. [27] The K values measured for the formation of these two binary complexes are moderate, revealing that the complexation processes are not quantitative at the millimolar concentration levels used in 1 H NMR experiments. Therefore, we were nicely surprised to obtain intense electrospray (ESI) mass spectrometric peaks at m/z values of 526.0378 (calcd.…”

“…Structure of the "blue box" receptor. [27] We also carried out numerous 1 H NMR spectroscopic titrations, which allowed us to conclude that the formation of the quaternary GH 2 2 + gBB 4 + g(CB7) 2 complex is essentially quantitative at millimolar concentration levels. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 50 mM sodium acetate, and could not collect any evidence for the formation of the ternary GH 2 2 + g(CB7) 2 complex.…”

“…631.0408) corresponding to the quaternary complex with 6 and 5 positive charges, respectively, from an aqueous solution containing all three components. [27] We also carried out numerous 1 H NMR spectroscopic titrations, which allowed us to conclude that the formation of the quaternary GH 2 2 + gBB 4 + g(CB7) 2 complex is essentially quantitative at millimolar concentration levels. Similarly, NMR diffusion coefficient measurements also indicate the formation of the quaternary complex.…”