Two spin-containing fragments connected by a two-electron one-center heteroatom π spacer. A new open-shell organic molecule witha singlet ground state

Abstract

The synthesis of 4,4-iminobis(2,2′,2″,4′,4″,6,6′,6″-octachlorotriphenylmethyl) diradical3, a stable organic magnetic molecule consisting of two spin-containing fragments linked by a nitrogen atom, is reported. Electron paramagnetic resonance (EPR) analysis in methyltetrahydrofuran solution (∼10^–3 m) at low temperatures showed a typical fine structure (g_xx=2.0042;g_yy=2.0048;g_zz=2.0027) of Δm_s=±1transition( | D/hc |=0.0071 cm^–1; | E/hc |=0.0006 cm^–1) as well as a broad (ΔH_pp=7.5 G) and weak signal of Δm_s=±2 transition (g=4.124), due to an asymmetric and excited triplet state corresponding to an intramolecular spin–spin interaction which diminishes with decreasing temperature. It also showed a pair of small peaks, that might be associated with a weaker dipole–dipole interaction which diminishes with increasing temperature, emerging at both sides of a central and single peak due to a doublet state resonance corresponding to 2,2′,2″,4′,4″,6,6′,6″-octachloro-4-{3,5-dichloro-4-[bis(2,4,6-trichlorophenyl)methylene]cyclohexa-2,5-dienylideneamino}triphenylmethyl radical 4, obtained by smooth oxidation of 3. From magnetic susceptibility measurements of the sample in the solid, a linear four-spin model was applied to establish that the singlet is the ground state of the molecule and that two triplets (J_intra=–286±30 K, J′_inter=–160±50 K) were the low-lying excited states. Organic solutions of 3 in air slowly oxidize to give 4, a much more persistent monoradical which is also obtained by a smooth oxidation of 3 with AgNO₃ in CHCl₃ . Cyclic voltammograms for the reduction of 3 and 4 in dimethylformamide (DMF) with tetra-n-butylammonium perchlorate exhibited three consecutive redox couples with standard potentials of –0.23, –0.58 and –0.74 V vs. SCE, indicating a reasonable stability of anions 3^–and3^2– in DMF solution.

References

1. For reviews see: J. S. Miller, A. J. Epstein and W. M. Reiff, Chem. Rev., 1988, 88, 201; Acc. Chem. Res., 1988, 21, 114; H. Iwamura, Adv. Phys. Org. Chem., 1990, 26, 179; D. A. Dougherty, Acc. Chem. Res., 1991, 24, 88; H. Iwamura and N. Koga, Acc. Chem. Res., 1993, 26, 346; H. Kurreck, Angew. Chem., Int. Ed. Engl., 1993, 32, 1409; J. S. Miller and A. J. Epstein, Angew. Chem., Int. Ed. Engl., 1994, 33, 385; A. Rajca, Chem. Rev., 1994, 94, 871.
2. D. Griller and K. U. Ingold, Acc. Chem. Res., 1976, 9, 13.
3. M. Ballester, Acc. Chem. Res., 1985, 18, 380 and references cited therein M. Ballester, Adv. Phys. Org. Chem., 1989, 25, 267 and references cited therein.
4. J. Veciana, C. Rovira, O. Armet, V. M. Domingo, M. I. Crespo and F. Palacio, Mol. Cryst. Liq. Cryst., 1989, 176, 77; J. Veciana, C. Rovira, M. I. Crespo, O. Armet, V. M. Domingo and F. Palacio, J. Am. Chem. Soc., 1991, 113, 2552; J. Carilla, L. Juliá, J. Riera, E. Brillas, J. A. Garrido, A. Labarta and R. Alcalá, J. Am. Chem. Soc., 1991, 113, 8281; J. Veciana, C. Rovira, N. Ventosa, M. I. Crespo and F. Palacio, J. Am. Chem. Soc., 1993, 115, 57; V. M. Domingo, J. Castañer, J. Riera and A. Labarta, J. Org. Chem., 1994, 59, 2604; R. Chaler, J. Carilla, E. Brillas, A. Labarta, Ll. Fajarí, J. Riera and L. Juliá, J. Org. Chem., 1994, 59, 4107; M. Ballester, I. Pascual, C. Carreras and J. Vidal-Gancedo, J. Am. Chem. Soc., 1994, 116, 4205.
5. (a) J. Carilla, Ll. Fajarí, L. Juliá, J. Riera and Ll. Viadel, Tetrahedron Lett., 1994, 35, 6529; (b) S. López, J. Carilla, Ll. Fajarí, L. Juliá, E. Brillas and A. Labarta, Tetrahedron, 1995, 51, 7301; (c) J. Carilla, Ll. Fajarí, L. Juliá, J. Sañé and J. Rius, Tetrahedron, 1996, 52, 7013.
6. L. Teruel, Ll. Viadel, J. Carilla, Ll. Fajarí, E. Brillas, J. Sañé, J. Rius and L. Juliá, J. Org. Chem., 1996, 61, 6063.
7. (a) P. M. Lahti, A. S. Ichimura and J. A. Berson, J. Org. Chem., 1989, 54, 958; (b) J. Org. Chem., 1991, 56, 3030; (c) C. Ling, M. Minato, P. M. Lahti and H. van Willigen, J. Am. Chem. Soc., 1992, 114, 9959; (d) M. Minato, P. M. Lahti and H. van Willigen, J. Am. Chem. Soc., 1993, 115, 4523; (e) N. Yoshioka, P. M. Lahti, T. Kaneko, Y. Kuzumaki, E. Tsuchida and H. Nishida, J. Org. Chem., 1994, 59, 4272.
8. O. Armet, J. Veciana, C. Rovira, J. Riera, J. Castañer, E. Molins, J. Rius, C. Miravitlles, S. Olivella and J. Brichfeus, J. Phys. Chem., 1989, 91, 5608.
9. Z. Galus, Fundamentals of Electrochemical Analysis, Harwood, Chichester, 1976, ch. 7, p. 9; H. Lund and M. M. Baizer, Organic Electrochemistry. An Introduction and a Guide, Marcel Dekker, New York, ch. 2, p. 3.
10. H. R. Falle, G. R. Lukhurst, A. Horsefield and M. Ballester, J. Chem. Phys., 1969, 50, 258.
11. D. C. Reitz and S. I. Weissman, J. Chem. Phys., 1960, 33, 700.
12. The distance between the two α-carbons has been calculated from standard angles and bond lengths, and assuming that the C–N bond length is 1.426 Å and the C–N–C angle is 108.0°.
13. A. M. Trozzolo, R. W. Murray and E. Wasserman, J. Am. Chem. Soc., 1962, 84, 4990.
14. T. Mitsumori, K. Inoue, N. Koga and H. Iwamura, J. Am. Chem. Soc., 1995, 117, 2467.
15. R. L. Carlin, Magnetochemistry, Springer-Verlag, Berlin, Heidelberg, 1986, p. 21.