Mining 2:2 Complexes from 1:1 Stoichiometry: Formation of Cucurbit[8]uril–Diarylviologen Quaternary Complexes Favored by Electron-Donating Substituents

Abstract

A 1:1 binding stoichiometry of a host-guest complex need not consist of a single host and guest. Diarylviologens containing electron-donating substituents complexed with cucurbit[8]uril (CB[8]) in a 1:1 stoichiometry exhibit abnormally large binding enthalpies compared to typical enthalpy changes observed for 1:1 binary complexes. Here, several CB[8]-mediated host-guest complexes, which were previously reported as 1:1 binary complexes, are verified to be 2:2 quaternary complexes by a combination of isothermal titration calorimetry, H, NOESY, and ROESY NMR, and ion mobility mass spectrometry, clearly indicating a binding motif of two partially overlapping diarylviologens held in place with two CB[8] molecules. Formation of 2:2 quaternary complexes is favored by electron-donating substituents, while electron-withdrawing substituents typically result in 1:1 binary complexes. The stacking of two highly conjugated diarylviologens in one quaternary motif affords the complexes enhanced conductance when considered as a single-molecular conductor. Moreover, an additional conducting signal previously observed for this "supramolecular" conductor can be readily understood with our 2:2 complexation model, corresponding to a parallel conductance pathway. Therefore, a 2:2 quaternary complex model grants a greater understanding of such supramolecular complexes, enabling the design of engineered, hierarchical structures and functional materials.

Directional Self‐Sorting with Cucurbit[8]uril Controlled by Allosteric π–π and Metal–Metal Interactions

“…In all cases we surveyed, charged substituents are distributed between both CB [8] rims as evenly as possible. While Scherman's guests 4a [24] and our Pt complexes 6a-6c and 7b form quaternary complexes with unevenc harged istributionsa tt he CB [8] rims, one can argue that stacked 2:2a ssemblies are typically more stable than unstacked ones (see Figure 10 a), as long as encapsulating the free moiety of one of the ditopic guests (labeled Gi nF igure 10 a) into the water-filled cavity of the binary complex (labeled H) is more favorable than switching from an even to an unevend istribution of charges. An interaction between ap ositive group and aC B[n]r im corresponds to an approximate 4kcal mol À1 gain in total free energy (at least in the case of CB [7]), [45,87] and the free binding energy of ag uest such as berberine( 1)i nto binary complex 1·CB [8]i sa pproximately 8-9kcal mol À1 .…”

“…The authors concluded that enhanced interactions between the U‐shaped guest and the inner wall of CB[8], as well as limited exposure of the alkyl chain to water, advantageously compensate conformational strain and Coulombic repulsion between the ammonium groups . Very recently, Scherman and co‐workers showed that electron‐rich diarylviologens 4 a (X=H, CH 3 , OCH 3 , SCH 3 , and N(CH 3 ) 2 , NHCOCH 3 ) form discrete 2:2 complexes with CB[8] instead of the expected dynamic oligomers (see Figure ) . The balance of positive charges between the carbonylated rims is again drastically uneven, with two pyridinium units interacting with one pair of portals, and none with the other pair.…”

“…[23] Very recently,S cherman and co-workerss howed that electron-rich diarylviologens 4a (X = H, CH 3 ,O CH 3 ,S CH 3 , and N(CH 3 ) 2 ,N HCOCH 3 )f orm discrete 2:2c omplexes with CB [8] insteado ft he expected dynamic oligomers (see Figure 1). [24] The balance of positive charges between the carbonylated rims is again drastically uneven, with two pyridinium units interacting with one pair of portals, and none with the other pair.T he authors justify the 2:2b inding motif by proposing (1) favorable Coulombici nteractions between the electron-rich aryl substituents of one guest with the electron-deficient pyridinium unit of the other,a nd (2) ar educed electrostatic repulsion between both positive methylviologen groups as the positive charges are weakened by the neighboring electron-rich units. As imilar2:2 complexation had been unequivocally identified ay ear earlier by the same group with CB [8] and azobenzene 4b.…”

Chemical Sensors Based on Cucurbit[n]uril Macrocycles

“…For instance, a hypsochromic shift of the (usually enhanced) emission is often found for 1 : 1 CBn-dye complexes, pointing towards polarity differences between bulk water and the CBn cavity. [128] Besides, chromophore deaggregation (preventing aggregation-caused quenching, ACQ) often leads to significant emission enhancement and absorbance spectra changes. [42,118,119,120,121,122,123,124,125,126,127] Pronounced colourswitching upon 2 : 2 complex formation with CB8 was found for aryl-viologen chromophores.…”

2:2 Complexes from Diphenylpyridiniums and Cucurbit[8]uril: Encapsulation‐Promoted Dimerization of Electrostatically Repulsing Pyridiniums

“…In this context, severale legant examples have been reported. [7][8][9] However,a ll complexes have been investigated only in solution,a nd X-ray crystal structures have never been available, although they are crucially important for providing direct evidencef or the binding stoichiometry and refined structuralp arameters in the solid state.…”