Y. Teki, T. Takui, K. Itoh
Jun 1, 1983
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Journal of the American Chemical Society
Abstract
The spin state and molecular conformation of a novel alternant hydrocarbon, m-phenylenebis[(diphenylmethylen-3-yl)methylene] (I) , have been studied by electron-spin resonance. The tetracarbene 1 was generated by the photolysis of the corresponding tetradiazo compound, m-phenylenebis[m-(a-diazobenzyl)phenyldiazomethane] (2), which was synthesized as follows: The reaction of m-tolylmagnesium bromide with isophthalonitrile produced 1,3-di-(m-toIuoyl)benzene, which was oxidized in two steps to isophthalophenone-3,3’-dicarboxylic acid. The Friedel-Crafts reaction of the bis(acid chloride) in benzene gave 3,3’-dibenzoylisophthalophenone. The corresponding tetrahydrazone was oxidized with active MnO, to give 2. The eight-line ESR spectrum due to 1 was obtained when benzophenone single crystals doped with 2 were irradiated with the 405-nm mercury line at 4.2 K. The relative separation and integrated intensities of the lines are in accord with the AM, = f l allowed transitions between the fine-structure sublevels of the nonet spin manifold in the high-field limit. The resonance fields and signal intensities observed at the K band (25 GHz) were well-reproduced by a third-order perturbation calculation based on the spin Hamiltonian 7f = g@SH + D[Sz2 -S(S + 1)/3] + E(Sx2 S$) with g = 2.002, D = +0.031 61 cm-I, E = 0.003 94 cm. I , and .S = 4, proving 1 to be in the nonet state. Only the nonet spectrum was observed after photolysis. The temperature dependence of its total signal intensity in the range 1.8-56 K showed the observed nonet state to be the ground state, while the other triplet, quintet, and septet states also expected from the eight parallel spins are located at least 300 cm-’ above the ground state. When the sample was warmed, the spectrum showed irreversible spectral transitions at 64 K to two sets of new nonet signals and again at 92 K to a fourth set of nonet signals, which finally decayed out at 160 K. The transitions are ascribable to molecular conformation changes, which led to four nonet isomers. A semiempirical calculation of their fine-structure tensors has been carried out assuming the dipole-dipole interaction between the electron spins and the one-center n-T interactions on the divalent carbon atoms to be predominant. By fitting them to the observed fine-structure tensors, we obtained the most probable conformations for each of the four nonet isomers. Evidence for a one-photon process in the photodissociation of 2 into 1 was also obtained. The novel hydrocarbon described in this paper has the highest spin multiplicity so far reported among organic as well as inorganic molecules. This unusually high-spin multiplicity results from the topological symmetry. Such a high-spin state is relevant to the design of organic ferromagnets. Most known organic molecules have singlet ground states and are therefore usually diamagnetic. Paramagnetic organic compounds, less frequently, are usually free radicals. Their doublet ground state stems from an odd number of electrons. These magnetic properties contrast with those of inorganic compounds for which high-spin multiplicity in the ground state is not unusual. Until the early 1960s, this difference was held to reflect the low symmetry of organic molecules: from the group theoretical point of view, at most triply degenerate molecular orbitals could be found. Therefore, Hund’s rule predicts a maximum spin of 3 / 2 . In fact, organic molecules with the maximum spin consistent with their symmetry have not been detected yet. The pentachlorocyclopentadienyl cation’ and the cyclopentadienyl cation2 have such degenerate orbitals. They have been synthesized and they have been proven to have a triplet ground state. Due to their C, symmetry, their highest occupied molecular orbitals are doubly degenerate, hence the parallel spins. It should be noted that such degeneracy can be lifted by Jahn-Teller distortion as observed with the pentaphenylcyclopentadienyl cation, 4 ringlet in the ground ~ t a t e . ~ . ~ For organic molecules, therefore, high-spin multiplicity may not be expected from the degeneracy due to geometrical symmetry. Higuchi did the early theoretical work on organic high-spin molecules in 1963. He calculated the fine-structure parameters due to electron spin-spin interactions for several aromatic hydrocarbons being hypothetical a t that time.5*6 The first high-spin molecule was reported by one of us (K. I.)’ and subsequently by Wasserman et aI.* in 1967. This aromatic hydrocarbon, mphenylenebis(phenylmethylene), is a quintet in the electronic ground state. Its fine-structure parameters obtained by electron-spin resonance (ESR) were in reasonable agreement with ‘Osaka City University. tlnstitute for Molt.cular Science. 0002-7863/86/ 1508-2147$0l.50/0 the values predicted by H i g ~ c h i . ~ , ~ This hydrocarbon was a prototype for the series of high-spin hydrocarbons detected thereafter: m-phenylenebis(methy1ene) (S = 2),* benzene1,3,5-tris(phenyImethylene) ( S = 3),1° biphenyl-3,3’-bis(phenylmethylene) (S = 0, 1, 2),” 1,3,5-benzenetriyltris[bis(biphenyl-4-yl)methyl] ( S = 3 / 2 ) , 1 2 and 3,3’-diphenylmethylenebis(phenylmethy1ene) (S = 3).13 In addition, quintet and septet nitrenes isoelectronic with the above-mentioned quintet and septet hydrocarbons have been detected since.*J4 Recently, we published a preliminary report of the detection by single-crystal ESR of an aromatic hydrocarbon, mphenylenebis[(diphenylmethylen-3-yl)methylene] (1) with nonet spin multiplicity (S = 4) in the electronic ground state.I5 Static ( I ) Breslow, R.; Hill, R.; Wasserman, E. J . Am. Chem. SOC. 1364, 86, 5349-5350. (2) Saunders, M.; Berger, R.; Jaffe, A.; McBride, J. M.; O’Neill, J.; Breslow, R.; Hoffman, J. M., Jr.; Perchonock, C.; Wasserman, E.; Hutton, R. S.; Kuck, V. J . J . A m . Chem. SOC. 1973, 95, 3017-3018. (3) Breslow, R.; Chang, H. W.; Yager, W. A. J . Am. Chem. SOC. 1963, 85, 2033-2034. (4) Breslow, R.; Chang, H. W.; Hill, R.; Wasserman, E. J. Am. Chem. SOC. 1967,89, 1 1 12-1 119. (5) Higuchi, J. J . Chem. Phys. 1963, 38, 1237-1245. (6) Higuchi, J. J . Chem. Phys. 1963, 39, 1847-1852. (7) Itoh, K. Chem. Phys. Left . 1967, 1, 235-238. (8) Wasserman, E.; Murray, R. W.; Yager, W. A,; Trozzolo, A. M.; Smolinsky, G . J . Am. Chem. SOC. 1967, 89, 5076-5078. (9) Higuchi, J. Bull. Chem. SOC. Jpn. 1970, 43, 3773-3779. (IO) Takui, T.; Itoh, K. Chem. Phys. Lef f . 1973, 19, 120-124. (11) Itoh, K. Pure Appl. Chem. 1978, 50, 1251-1259. (12) Reibisch, K.; Kothe, H.; Brickmann, J. Chem. Phys. Letf . 1972, 17, 86-89. Brickmann. J.: Kothe. G. J . Chem. Phw. 1973. 59. 2807-2814. (13) Teki, Y.; Ta’kui; T.; Yagi, H.; Itoh, K.; Iwimura, ti. J.’Chem. Phys. (14) Wasserman, E.; Scheller, K.; Yager, W. A. Chem. Phys. Lef t . 1968, 1985, 83, 539-547.