An Inclusion Complex of Diquat in the Concave Cavity - Social Problems | SOC 2004, Study notes of Conflictology

Download An Inclusion Complex of Diquat in the Concave Cavity - Social Problems | SOC 2004 and more Study notes Conflictology in PDF only on Docsity! Chapter 12 An Inclusion Complex of Diquat in the Concave Cavity Provided by Two Dibenzo-24-Crown-8 Hosts 12.1. INTRODUCUTION The chemistry of inclusion complexes has been extensively investigated by scientists all over the world with diverse objectives. Some recent examples include improvement of the solubility of fullerenes in water,1 stabilization of a dye molecule,2 microencapsulation of biologically active compounds,3 drug delivery,4 preparation of a 3D porous magnet,5 and fabrication of a molecular elevator.6 Dibenzo-24-crown-8 (1) and diquat (2) derivatives are commonly used hosts and guests in this field.7,8 Though Pospisil et al. first studied the complexation between 1 and 2 in methanol in 1988,8c-d the complexation mechanism is still not clear up to now. Furthermore, no new complexes based on these species have been reported in the last 16 years. Here we report proton NMR and mass spectrometric characterizations and X-ray ananlysis of this complex, which was found to have a interesting conformation in the solid state. NN 2 + + 2 PF6 - H1 OOOO OOOO 1 195 12.2. RESULTS AND DISCUSSION When 1 was mixed with an equivalent of 2 in CD3COCD3, a yellow color was observed immediately due to charge transfer interactions between the electron-rich aromatic rings of 1 and electron-poor 2. A Job plot9 (Fig. 1) demonstrated that the complex between 1 and 2 was of 1:1 stoichiometry in solution. The apparent assoiciation constant (Ka) of 1•2 calculated based on the proton NMR data was 2.0 (± 0.2) × 102 M-1 in acetone.† The complex between 1 and 2 was also characterized by the electrospray ionization mass spectrometry (Fig. 2). The base peak at m/z 471.4 was assigned to [1 + Na]+. Two peaks corresponding to the 1:1 stoichiometry were found: m/z 777.5 (25%) [1•2 - PF6]+ and 316.5 (60%) [1•2 - 2PF6]2+. Two unexpected peaks related to the 2:1 stoichiometry were also found: m/z 1225.8 (2%) [12•2 - PF6]+ and 540.7 (5%) [12•2 - 2PF6]2+. A single crystal of 2 for X-ray analysis was grown by vapor diffusion of pentane into an acetone solution of 2.# As shown by the obtained crystal structure (Fig. 3), the two pyridinium rings of 2 are twisted with an angle of 18.5° and a centroid-centroid distance of 4.23 Å. The complex between 1 and 2 has a 2:1 stoichiometry in the solid state as shown by its crystal structure (Fig. 4) obtained from the X-ray analysis of a single crystal grown by vapor diffusion of pentane into an equimolar solution of 1 and 2 in acetone.# 12•2 is stabilized by C−H···O hydrogen bonding, face-to-face π-stacking and CH/π interactions.13 There are totally ten hydrogen bonds between the guest 2 and the two hosts (B-K, Figs. 4a and 4b). Four of eight pyridinium hydrogens of diquat are involved in these hydrogen bonds (B-D, H, I). What is more important here is that all four methylene hydrogens of diquat are hydrogen bonded to the two hosts (E, F, G, K, J), while the methyl hydrogens of the closely related paraquat are not involved in interactions with the host in most of reported paraquat complexes.14 One phenylene ring of each host is π-stacked to the same pyridinium ring of 2. The two host molecules are connected by one aromatic C−H···O hydrogen bond (L) and an aliphatic C-H/π 196 Figure 3. Two views of the solid-state structure of 2 as determined by X-ray crystallography. Two PF6 counter ions and hydrogens have been omitted for clarity. The angle and centroid-centroid distance between two pyridinium rings of 2 (deg and Å): 18.5 and 4.23. 199 Figure 4. Stick (a) an stick (c) and space-fil provided by the two h ions and hydrogens e a A E F B D H G IC J K L b c d d space-filling (b) representations of the X-ray structure of 12•2 and ling (d) representations of a part of it showing the concave cavity osts. (a and b) The hosts are red and yellow and 2 is blue. Four PF6- xcept the ones on 2 or involved in hydrogen bonding or the CH/π 200 interaction have been omitted for clarity. CH/π interaction parameters: distance between the hydrogen and the closest carbon on the aromatic ring (A) 2.85 Å; hydrogen-centroid distance 2.72 Å; carbon-centroid distance 3.62 Å; C-H···centroid angle: 150.8°. Hydrogen-bonding parameters: H···O distances (Å), C-H···O angles (deg), and C-O distances (Å) B = 2.55, 131, 3.26; C = 2.40, 127, 3.07; D = 2.43, 154, 3.31; E = 2.41, 145, 3.27; F = 2.38, 144, 3.23; G = 2.51, 154, 3.43; H = 2.34, 141, 3.13; I = 2.46, 143, 3.27; J = 2.48, 133, 3.23; K = 2.51, 124, 3.17; L = 2.40, 160, 3.31. Face-to-face π-stacking parameters: centroid-centroid distances (Å) and dihedral angles (deg) 3.63 and 10.2, 3.69 and 8.90. The dihedral angle and the centroid-centroid distance between two pyridinium rings (deg and Å): 14.9 and 4.23. 12.3. CONCLUSIONS In a summary, we report an inclusion complex of diquat in the concave cavity provided by two dibenzo-24-crown-8 hosts. 12.4. ACKNOWLEDGEMENTS This work was supported by the National Science Foundation (DMR0097126, HWG). The purchase of the diffractometer Xcalibur2 (VPISU) was also supported by the National Science Foundation (CHE-131128). REFERENCES AND NOTES † 1H NMR characterizations were done on solutions with constant [1]0 and varied [2]0. Based on these NMR data, ∆0, the difference in δ values for H1 of 1 in the uncomplexed and fully complexed species, was determined using the Benesi-Hildebrand method.10 Then Ka was calculated from Ka = (∆/∆0)/{{1-(∆/∆0)}{[2]0-(∆/∆0)[1]0}}. # Crystal data of 2: plate, colorless, 0.065 × 0.172 × 0.176 mm3, C12H12F12N2P2, FW 474.18, monoclinic, space group P21, a = 6.3160(12), b = 14.707(3), c = 9.0800(16) Å, β 201