Indigodiimine: A Highly Fluxional Molecule that Tautomerizes via Double Proton Transfers

TL;DR: In this paper, it was shown that indigodiimine adopts a highly fluxional structure even in its electronic ground state, which implies a rapid intraor intermolecular tautomerization to the degenerate form 2b′, either as stepwise single proton transfers via 2a or 2c or as a concerted double proton transfer.

Abstract: Proton transfers in the ground and excited state continue to draw interest from many branches of chemistry.4 In particular, the photochemistry and photophysics of indigo (1) (Scheme 1) and its derivatives have been a matter of long-standing interest.5 In this paper we show by dynamic NMR that its isoelectronic nitrogen analog indigodiimine (2) adopts a highly fluxional structure even in its electronic ground state. For indigo in the electronic ground state only the centrosymmetric trans-structure 1a, characterized by a trans configuration with respect to the central double bond and two intramolecular (CdO‚‚‚H-N) hydrogen bonds, has been observed.5,6 Semiempirical quantum-mechanical calculations7 have confirmed that the degenerate forms 1b and 1b′ have substantially higher energy than 1a, and 1c higher energy than 1b and 1b′. However, as acid-base properties are often significantly altered in electronic excited states8 phototautomerization of 1a to 1c has been postulated as a radiationless pathway which effectively quenches fluorescence of 1.9 By contrast, the quantum-mechanical calculations7 indicate preferential formation of 1b over 1c in the excited state. An extremely weak fluorescence in 1 has since been detected and a combination of fluorescence and infrared studies indicates the absence of proton transfers in the first excited state of 1.10 By contrast, IR measurements of the NH-stretching band of 2 and its 15N-labeled isotopomers indicate that indigodiimine adopts the tautomeric form 2b.11 This implies a possible rapid intraor intermolecular tautomerization to the degenerate form 2b′, either as stepwise single proton transfers via 2a or 2c or as a concerted double proton transfer. Additionally, 2b may be subject to an internal rotation of the NH2 group. The combination of proton tautomerization and NH2 group rotation leads potentially to eight interconverting molecular states according to Scheme 2. By performing variable-temperature 1H NMR experiments on indigodiimine (2)12 in a mixed freon solvent,13 we were able to confirm Scheme 2 and report preliminary rate constants for both processes. Some typical superposed experimental and calculated spectra are shown in Figure 1. The predominance of 2b is verified by four singlets at ν1 ) 10.7, ν2 ) 9.8, ν3 ) 7.1 (partly obscured by aromatic proton signals), and ν4 ) 5.1 ppm. The signal assignment was obtained by 15N-labeling at the exocyclic nitrogen sites and by inference from changes in the line shape. As temperature is increased from 140 K the two high-field signals first broaden and then coalesce to a broadened singlet at (ν3 + ν4)/2 ) 6.1 ppm from which the value of ν3 could be inferred. The rate constant kr ≈ 1010.2(0.5 exp(-5.0 ( 0.3 kcal mol-1/RT), 140 K e T e 190 K, for the NH2 rotation14 could be obtained by fitting the theoretical to the experimental spectra.15 As the temperature is further increased all remaining three singlets broaden and coalesce into a single line at 8.1 ppm, indicating the onset of the tautomerization between 2b and 2b′. The associated rate constant kpt ≈ 109.6(0.4 exp(-5.8 ( 0.3 kcal mol-1/RT), 160 K e Te 270 K, for this double proton transfer16 was obtained again by line shape analysis. The aromatic proton signals also exhibit temperature-dependent changes that confirm (1) Santa Clara University. (2) Universitat Gottingen. (3) Freie Universitat Berlin. (4) For numerous examples see Michael Kasha special issue: J. Phys. Chem. 1991, 95, 10215-10524. For a more recent example see: Chudoba, C.; Lutgen, S.; Jentzch, T.; Riedle, E.; Woerner, M.; Elsasser, W. Chem. Phys. Lett. 1995, 240, 35. (5) For a review of the properties of indigo and its derivatives see: (a) Haucke, G.; Patzoldt, R. NoVa Acta Leopoldina Suppl. 1978, 11, 7 (photophysics and photochemistry). (b) Susse, P.; Steins, M.; Kupcik, V. Z. Kristallographie 1988, 184, 269 (crystallography). (c) Tatsch, E.; Schrader, B. J. Raman Spectrosc. 1995, 26, 476 (vibrational spectroscopy). (d) Klessinger, M.; Luttke, W. Chem. Ber. 1966, 99, 2136 (electronic structure). (6) Brode, W. R.; Pearson, E. G.; Wyman, G. M. J. Am. Chem. Soc. 1954, 76, 1034. (7) Suhnel, J.; Gustav, K. Mol. Photochem. 1977, 8, 437. Suhnel, J.; Gustav, K.; Patzold, R.; Fabian, J. Z. Phys. Chem. 1978, 259, 17. (8) Weller, A. Discuss. Faraday Soc. 1965, 183. (9) Wyman, G. M.; Zarnegar, B. M. J. Phys. Chem. 1973, 77, 1204. (10) Elsasser, T.; Kaiser, W.; Luttke, W. J. Phys. Chem. 1986, 90, 2901. (11) Luttke, W.; Sieghold, H. Angew. Chem. 1975, 87, 63; Angew. Chem., Int. Ed. Engl. 1975, 14, 52. (12) Indigodiimine was prepared from indole according to the literature methods: Madelung, W. Liebigs Ann. Chem. 1914, 58, 405. Sieghold, H. Dissertation, Universitat Gottingen, 1973. (13) The NMR solvent for these studies, a 1:5:5 mixture of CDCl3, CDCl2F, and CDClF2, is liquid down to 130 K. It was prepared according to: Siegel, J. S.; Anet, F. A. L. J. Org. Chem. 1988, 53, 2629. (14) ∆Hq ) 4.7 ( 0.3 kcal mol-1, ∆Sq ) -13 ( 2 cal mol-1 K-1. Scheme 1