Activation of Thiophenes by Superacids: Protonation and Polymerization

Two aspects of thiophene chemistry are of current interest in materials chemistry. Polythiophenes are under intense study for possible applications in electronics and photonics,1 and metal-thiophene interactions are relevant to the catalytic hydrodesulfurization (HDS) process.2,3 This communication presents studies at the interface of these otherwise distinct topics, resulting from an examination of the interaction of TiCl4 with thiophenes. We have found that CH2Cl2 solutions of HCl and TiCl4 readily protonate tetramethylthiophene to give salts of [2-HC4Me4S]. 1H and 13C NMR analysis of these solutions indicate that protonation occurs at the 2-position.4,5 In the 1H NMR spectrum, all four CH3 signals are well resolved, two of which are coupled to the unique proton in accord with similar results using more conventional superacids.6 From these solutions we obtained single crystals of [2-HC4Me4S][Ti2Cl9], thus allowing the first structural analysis of a protonated thiophene (Figure 1). The reflection data refined well, allowing for the identification of the unique H-atom position.7 The [2-HC4Me4S] cation features a tetrahedral carbon, C2, connected to C3 and S via bond lengths closer to values for single bonds, 1.474(7) and 1.804(5) Å, respectively. The C2-C3-C4-S sequence displays a diene-like alternation in bond lengths. The C4-S distance of 1.673(5) Å is shorter than in thiophene (1.71 Å), indicative of an enhanced S-C π-interaction. The Ti2Cl9 anion is unexceptional, consisting of a cofacial bioctahedral arrangement. There is one short Cl‚‚‚H interaction between a terminal chloride on the anion and the unique hydrogen. To better understand the TiCl4/HCl/C4Me4S reaction, we examined the interaction of C4Me4S and TiCl4 in the absence of acids. Floriani and co-workers had shown that hexamethylbenzene reacts with TiCl4 to give [(η6C6Me6)TiCl3][Ti2Cl9]. NMR studies suggest that the analogous [(η-C4Me4S)TiCl3] is formed when CH2Cl2 † Department of Chemistry, University of California, Davis, CA. (1) (a) Katz, H. E.; Torsi, L.; Dodabalapur, A. Chem. Mater. 1995, 7, 2235. (b) Shi, G.; Jin, S.; Li, C. Science 1995, 267, 994. (c) Torsi, L.; Dodabalapur, A.; Lovinger, A. J.; Katz, H. E.; Ruel, R.; Davis, D. D.; Baldwin, K. W. Chem. Mater. 1995, 7, 2247. (d) Roncali, J. Chem. Rev. (Washington, D.C.) 1992, 72, 771. (e) Faid, K.; Fretchette, M.; Ranger, M.; Mazerolle, L.; Levesque, I.; Leclerc, M.; Chen, T.; Rieke, R. D. Chem. Mater. 1995, 7, 1390. (2) (a) Gates, B. C. Catalytic Chemistry; John Wiley & Sons: New York, 1992. (b) Rauchfuss, T. B. Prog. Inorg. Chem. 1996, 8, 945. (3) Angelici, R. J. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed; John Wiley & Sons: New York, 1994; Vol. 3, p 1433. (4) Yuan, Y.; Gunig, E.; Miller, L. L. Chem. Mater. 1995, 7, 255. (5) 1H NMR (CD2Cl2) δ 5.05 (m, 1H, SC3Me3CHMe), 3.09 (d, J ) 3.42 Hz, 3H, SC3Me3CHMe), 2.57 (s, 3H, SC3Me3CHMe), 2.27 (s, 3H, SC3Me3CHMe), 1.87 (d, J ) 7.81 Hz, 3H, SC3Me3CHMe). (6) Hogeveen, H. Recueil 1966, 85, 1072. (7) For C8H13Cl9STi2: monoclinic P21/n, a ) 9.441(5), b ) 11.708(5), c ) 18.214(13) Å, R ) γ ) 90°, â ) 91.18(4)°, Z ) 4, V ) 2013(2) Å3, T ) 130 K, Fcalcd ) 1.835 Mg/m3, for 3539 independent reflections (Mo KR; Rint ) 1.45%; 184 parameters refined)Rw (based on F) ) 3.72% (0.0° e 2θ e 50.0°). Figure 1. Structure of [2-HC4Me4S][Ti2Cl9]. Principal bond lengths (Å): S-C(1), 1.804 (5); C(1)-C(2), 1.474 (7); C(2)-C(3), 1.370 (7); C(3)-C(4), 1.405 (7); C(4)-S, 1.673 (5). VOLUME 9, NUMBER 3 MARCH 1997