Conducting polymers incorporating tungsten-capped calixarenes.

Calixarenes are highly versatile scaffolds for the design of 3-D cavities,1 and have found wide utility in host-guest chemistry and molecular recognition.2 The ability of calixarenes to selectively bind various cations at their lower rim has been widely utilized in the design of both small molecule3 and polymeric sensory materials.4 However, despite the numerous examples of calixarene inclusion complexes with organic molecules in the hydrophobic pocket, this binding mode has largely been unexplored with regard to sensors.1,5,6 As part of our continued interest in conjugated polymer-based sensors7 we endeavored to make conducting polymeric materials containing conformationally rigid metallocalixarene repeat units. In pursuing this system we considered that the communication between the polymer and the calixarene moiety could be mediated via a transition metal center. Calixarenes 1a-c were prepared by literature methods.8,9 Compound 1c was chosen because of its ability to form a strong host-guest complex with p-xylene.9 When calixarenes 1a-c were refluxed with WOCl4 in toluene overnight followed by addition of 2 and 2 equiv of TMS-Cl, the corresponding tungsten catecholate complexes 3 were obtained in high yield as red crystalline solids (Scheme 1). As expected, the 1H NMR spectra of 3 exhibit two different sets of signals due to the nonequivalent aromatic protons of the calixarene moiety.10 As the geometry of the polymer precursors was essential for our studies, we determined the X-ray structure of complex 3c.11 The important feature of the obtained structure is that the four adamantyl groups form a large deep conical cavity with an ellipsoid base capped by the metal center. Stille coupling of complexes 3a-c with tributylstannyl ethylenedioxythiophene (Bu3Sn-EDOT) in refluxing toluene produced 4a-c in reasonable yields (Scheme 1). Complexes 4a-c are dark red-brown crystalline compounds stable in air in the solid state for months. The substitution of the iodine atoms by EDOT results in a downfield shift in the NMR signals of the catecholate protons by more than 0.7 ppm. The hydrogen atoms remote to the substitution site remain largely unaffected. Dark red crystals of 4a were obtained by slow diffusion of hexane into a solution of 4a in CH2Cl2. The X-ray structure of 4a (Scheme 1) clearly demonstrates that the tungsten atom has a distorted octahedral configuration. A particular feature is that the phenolic rings of the calixarene trans to the catecholate are substantially distorted from the usual cone conformation with the two phenyl rings being practically coplanar.13 The other two phenolic oxygens of the calixarene moiety occupy mutually axial positions. The O(4)W(1)-O(6) angle is 104.5(2)°, while the O(3)-W(1)-O(5) angle is 164.3(2)° as expected for mutually trans ligands in a sixcoordinate metal complex. The axial tungsten-oxygen bond distances of 1.951(6) Å (W(1)-O(3)) and 1.910(6) Å (W(1)O(5)) are longer than the equatorial bond distances of (W(1)O(4) ) 1.867(5) Å and W(1)-O(6) ) 1.848(5) Å. The nearlinear W-O-C angle of the equatorial phenolates indicates greater p-orbital hybridization of the oxygen lone pairs. Hence back-bonding is likely responsible for the shorter W-O bonds. The EDOT-phenylene torsion angles are 35.7(13)° and 18.4(15)°. Oxidative electrodeposition of complexes 4 was performed under nitrogen in CH2Cl2/0.1 M Bu4NPF6 using interdigitated microelectrodes and the potentials were referenced to external Fc/Fc+. Repeated cycling between -0.2 and 0.7 V (50 mV/s) of a 0.5 μmol solution of 4 resulted in the anodic deposition of polymeric films.14 While polymer poly4a deposited films readily (15-20 cycles), the same process with 4c required a higher number of scans (up to several hundred) due to slower film growth. This slow rate can be attributed to the bulky adamantyl groups in poly4c that create steric repulsions, large voids, and higher solubility oligomers. Complex 4b showed deposition properties intermediate between the other two analogues. As expected for a highly delocalized polymer, the cyclic voltammograms of poly4a-c in the oxidative region exhibit broad waves.15,16 Poly4a-c also display a tungsten-centered reduction wave at ca. -1.1 V. This redox activity is unaffected by the polymerization process as evident from the cyclic voltammetry data of the monomeric and polymeric forms (cf. 4a: E1/2 ) -1.130 V vs poly4a: E1/2 ) -1.117 V).17 The current of the tungsten reduction wave relative to the polymer oxidation wave depends on the film thickness. Thicker films display a reduction in the tungsten-centered electroactivity relative to that of the polymer. This effect has been previously observed when the metal center’s electroactivity is not matched to that of the polymer.18 Slow electrochemical kinetics limit the diffusion of charge within (1) (a) Böhmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713. (b) Gutsche, C. D. Calixarenes; The Royal Society of Chemistry: Cambridge, England, 1989. (2) (a) Ikeda, A.; Shinkai, S. Chem. ReV. 1997, 97, 1713. (b) de Namor, A. F. D.; Cleverly, R. M.; Zapata-Ormachea, M. L. Chem. ReV. 1998, 98, 2495. (3) For the latest review see: Ludwig, R. Fresenius’ J. Anal. Chem. 2000, 367, 103. (4) Marsella, M. J.; Newland, R. J.; Carrol, P. J.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 9842. (5) For size-selective recognition see: (a) Shinkai, S.; Araki, K.; Manabe, O. J. Chem. Soc., Chem. Commun. 1988, 187. (b) Shinkai, S. J. Inclusion Phenom. 1989, 7, 193. (6) For sensory monolayers containing calixarenes see: (a) Schierbaum, K. D.; Weiss, T.; Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N.; Gopel, W. Science 1994, 265, 1413. (b) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. AdV. Mater. 2000, 12, 1315. (c) Dermody, D. L.; Crooks, R. M.; Kim, T. J. Am. Chem. Soc. 1996, 118, 11912. (d) Cygan, M. T.; Collins, G. E.; Dunbar, T. D.; Allara, D. L.; Gibbs, C. G.; Gutsche, C. D. Anal. Chem. 1999, 71, 142. (7) (a) Swager, T. M. Acc. Chem. Res. 1998, 31, 201. (b) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537. (8) Arduni, A.; Casnati, A. In Macrocyclic Synthesis: A Practical Approach; Parker, D., Ed.; Oxford: New York, 1996, p 145. (9) Khomich, A. N.; Shokova, E. A.; Kovalev, V. V. Synlett 1994, 12, 1027. (10) (a) Corazza, F.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Inorg. Chem. 1991, 30, 4465. (b) Xu, B.; Carroll, P. J.; Swager, T. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 2094. (c) Xu, B.; Miao, Y.-J.; Swager, T. M. J. Org. Chem. 1998, 63, 8561. (11) X-ray structure data for 3c‚CHCl3: C78H70Cl12I2O6W, I2/a, a ) 26.574(4) Å, b ) 10.2177(15) Å, c ) 28.363(7) Å, R ) γ ) 90°, â ) 92.702(2)°, V ) 7693(2) Å3, Z ) 4. See Supporting Information for full details. (12) X-ray structure data for 4a‚CH2Cl2: C48H36Cl4O10S2W, P1h, a ) 12.279(3) Å, b ) 12.373(4) Å, c ) 15.036(4) Å, R ) 77.306(5)°, â ) 85.491(5)°, γ ) 78.231(5)°, V ) 2180.3(10) Å3, Z ) 2. (13) For relevant crystal structures see: Zanotti-Gerosa, A.; Solari, E.; Giannini, L.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Chem. Soc., Chem. Commun. 1996, 119. See also ref 10b. (14) (a) Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J. Am. Chem. Soc 1984, 106, 7389. (b) Ofer, D.; Crooks, R. M.; Wrighton, M. S. J. Am. Chem. Soc. 1990, 112, 7869. (15) Handbook of Conducting Polymers; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds; Dekker: New York, 1998. (16) For extensive electrochemical studies of p-phenylene-EDOT polymers see: (a) Irvin, J. A.; Reynolds, J. R. 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