Paramagnetism and antiaromaticity: singlet-triplet equilibrium in doubly charged benzenoid polycyclic systems

Abstract

Fused benzenoid systems such as anthracene, phenanthrene, 3,4-benzophenanthrene, and chrysene were reduced with lithium, sodium, and potassium in various solvents to the antiaromatic doubly charged species. The NMR and patterns of the dianions revealed a strong dependence on the countercation, solvent, and temperature. This dependence is interpreted by the existence of an equilibrium process between the singlet ground state and a thermally accessible excited triplet state of the antiaromatic dianions. The direction and extent of the equilibrium is determined by the energy gaps between the LUMO and HOMO of these charged species, which depends, in turn, on the topology of the hydrocarbon and on the solvation properties of the obtained salt. Conjugated benzenoid polycyclic hydrocarbons undergo a 2-fold reduction process.* Scrutinized magnetic resonance studies, to be described below, performed on the resulting dianions reveal some hitherto unencountered phenomena that concern fundamental properties of antiaromatic charged systems. These phenomena point toward the existence of an equilibrium between the singlet ground state of the doubly charged benzenoid polycycles and a low-lying, thermally accessible excited triplet state. The equilibrium is found to have a substantial influence on the spectral patterns of these systems. HMO considerations differentiate between two classes of (4n + 2) *-conjugated polycyclic species: systems endowed with C3 or higher axial symmetry for which the highest occupied and lowest unoccupied orbitals appear in pairs vs. systems with lower axial (1) (a) The Hebrew University of Jerusalem. (b) The Weizmann Institute of Science, Rehovot. (2) (a) Streitwieser, A.; Suzuki, S. Tetrahedron 1961, 16, 153-168. (b) Cox, R. H.; Terry, H. W.; Harrison, L. W. Tetrahedron Let f . 1971, 50, 4815-4817. (c) For general review, see: Bates, R. B. In Comprehensive Carbanion Chemistry; Elsevier: New York, 1980; Part A, pp 1-54, and references cited therein. symmetry in which no such orbital degeneracies exist.3 The difference between the two classes becomes crucial when the polycycles are reduced to the corresponding (4n) 7 dianions. In the first group the two additional electrons populate two different degenerate orbitals, which may lead to a triplet ground state. This is the case in systems such as the triphenylene dianion (1),4 1,3,5-triphenylbenzene dianion (2),5 and under strong solvating conditions the coronene dianion system (3).6 It should be noted that dianions of C3 or a higher symmetry do not adopt by necessity a triplet ground state. Calculations have shown that configuration (3) (a) Wert, J. E.; Bolton, J. R. ESR Elementary Theory and Practical Applications; McGraw-Hill: New York, 1972; pp 232-246. (b) Breslow, R. Pure Appl. Chem. 1982, 54, 927-938. (4) (a) Willigen, H. v.; Broekhoven, J. A. M.; Boer, E. Mol. Phys. 1967, 12, 533-548. (b) Sommerdijk, J. L.; Boer, E. J . Chem. Phys. 1969, 50, ( 5 ) (a) Jesse, R. E.; Biloen, P.; Prins, R.; Voorst, J. D. W.; Hoijtink, G. J. Mol. Phys. 1963,6, 633-635. (b) Broekhoven, J. A. M.; Sommerdijk, J. L.; Boer, E. Ibid. 1971, 20, 993-1003. (6) (a) Glasbeek, M.; Voorst, J. D. W.; Hoijtink, G. J. J . Chem. Phys. 1966, 45, 1852. (b) Glasbeek, M.; Visser, A. J. W.; Maas, G. A,; Voorst, J. D. W.; Hoijtink, G. J. Chem. Phys. Lef t . 1958, 2, 312-314. 4771-4774. 0002-7863/83/1505-2164$01.50/0 1