Hiizu Iwamura

High-spin Organic Molecules and Spin Alignment in Organic Molecular Assemblies

Publisher Summary This chapter discusses organic molecules with more than two coupled spins ( S ≥ 3/2). They are arbitrarily called “high-spin molecules.” Molecular solids in which more than two unpaired electrons are aligned in parallel among neighboring molecules are also discussed. The chemistry of high-spin organic molecules and highly ordered spin alignment in organic molecular assemblies are expected to open up a new field in science and are considered also to be the best area in which interesting magnetic properties from organic materials can be looked for. Ferromagnetism is found in some minerals, inorganic compounds, metals, and alloys, but has never been associated with organic compounds. The chapter establishes the molecular designs that will permit the alignment of spins in organic molecules and organic molecular solids. On the basis of rigorous molecular design guided by theory and model experiments, a number of high-spin organic molecules have been synthesized and spin alignment in molecular assemblies has been advanced with reasonable success. The chapter introduces two approaches—that is, spin alignment within a molecule and among molecules is complementary to each other in expanding our understanding of chemical bonds and organic molecules and in developing novel organic magnetic materials.

Preparation and ESR detection of a ground-state nonet hydrocarbon as a model for one-dimensional organic ferromagnets

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

HIGH-SPIN POLYNITROXIDE RADICALS AS VERSATILE BRIDGING LIGANDS FOR TRANSITION METAL COMPLEXES WITH HIGH FERRI/FERROMAGNETIC TC

) 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

Magnetic behavior of nonet tetracarbene as a model for one-dimensional organic ferromagnets.

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.

Magnetic properties of the crystals of p-(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)benzoic acid and its alkali metal salts

n-Conjugated di- and trinitroxide radicals with triplet and quartet ground states, respectively, were allowed to react with manganese@) bis(hexafluoroacety1- acetonate) to give polymer complexes having well-defined structures. While a 1:l 1- D complex from the rn-phenylenebis(nitr0xide) 5 was found to be a metamagnet (Tc = 5.5 K), 2:3 2-D and 3-D complexes from the trinitroxides 7- 9 became ferri/ferro- magnets with Tc in the range 3.4 - 46 K. Design of these self-assembled heterospin systems with tailored dimensions and the sign and magnitude of the exchange cou- pling will serve as a useful strategy for exploring higher Tc molecule-based magnets.

Electron spin resonance line shapes of randomly oriented molecules in septet and nonet states by a perturbation approach

Tetracarbene (1) was generated by photolysis of the corresponding tetradiazo compound (2) in a 2-methyltetrahydrofuran glass or a single crystal of benzophenone at cryogenic tempreatures. The temperature dependence of paramagnetic susceptibility revealed its nonet spin multiplicity in the ground state. A behavior suggesting the presence of antiferromagnetic intermolecular interaction was also found when generated in the glass. The field dependence of magnetization of 1 was analyzed in terms of the Brillouin function and the experimental data fit closely to that of the theoretical value for J = */*. This is independent evidence for the nonet spin multiplicity of 1. The characteristic saturation behavior of magnetization in 1 was found to be due to its high spin multiplicity. Thus 1 may be regarded as a molecular superparamagnet. In the polycarbenes, the localized n spins a t carbenic centers are aligned all in parallel through the electron correlation between the mobile K spins, resulting in the intramolecular ferromagnetic spin ordering. Although the mechanism of spin ordering of a spins in polycarbenes is entirely different from that in metallic ferromagnets, the interaction between n and K spins resembles s-d interaction in dilute alloys. The potentiality of polycarbenes as a microdomain in macroscopic ferromagnets will be discussed. The explosive growth in the number of reports on organic conductors and superconductors has arisen from genunie interests in mobility of electrons in organic molecular crystals, components of which are insulators in themse1ves.I On the other hand, molecular design of organic ferromagnets which is becomming the current topic of theoretical interest in organic material science deals with the behavior of electron spins in organic molecules.* One of the strategies toward this goal is to construct organic molecules with high-spin multiplicity as domains in ferromagnets and to introduce ferromagnetic intermolecular (interdomain) interaction among them. High-spin organic molecules are accessible when nonbonding molecular orbitals are present due to the symmetry of the alternant hydrocarbon skeleton. Examples of the above category are m-phenylenebi~(phenylmethylene),~ its higher homologues, 3,6-dimethyleneanthracenediyl1,8-dioxyI

Intramolecular interaction between a hydroxyl group and π-electrons—XXII: Relationship between conformation and frequency shift by intramolecular O—H ⋯ π interaction in olefinic alcohols☆

etc. Recently we have generated tetracarbene, m-phenylenebis((diphenylmethylen-3-y1)methylene) (1) and proved its nonet spin multiplicity in the ground state by analyzing ESR fine structures of the sample oriented in a host single crystal of ben~ophenone.~ Now we have studied the magnetic behavior of this highly important tetracarbene 1 by means of magnetic susceptibility measurements.6 The magnet was composed of a main coil and a pair of reverse Helmholtz coils for the field gradient. The main field gradient, the temperature of a sample, and the weight change were carefully calibrated to enable the absolute magnetic susceptibility measurement. Inhomogeneity of the main magnetic field (HM) was leis than 0.2 T/m at the center of the main coil with the accuracy of lo-’. The accuracy of the field gradient (aHG/az) is better than IO-’ which was calibrated by a GaAs Hall device. The temperature of the cryostat (300-20 K) was regulated by the microcomputer, and data were recorded when the temperature stabilized within O.l%/min. The temperatures between 20 and 4 K were controlled manually by adjusting the flow of liquid helium, and lower temperatures than 4 K were obtained by pumping liquid helium. A platinum-resistance thermometer was used to read temperatures higher than 25 K and a carbon-resistance thermometer for lower temperatures. These temperature readings were calibrated in the whole temperature range with a magnetic thermometer with use of paramagnetic Cr(NH3)I Sculz, H. Adu. Phys. 1982, 31, 299. (b) Torrance, J . B. Arc. Chem. Res. 1979, 12, 79. (2) (a) McConnell, H. M. J . Chem. Phys. 1%3,39, 1910. (b) McConnell, H. M. Proc. Robert A . Welch Found. ConJ Chem. Res. 1967,1I, 144. (c ) Mataga, N. Theor. Chim. Acra 1968,10,372. (d) Ovchinnikov, A. A. Dokl. Akad. Nauk SSSR 1977, 236, 928. (e) Ovchinnikov, A. A. Theor. Chim. Acra 1978,47,297. (f) Buchachenko, A. L. Dokl. Akad. Nauk SSSR 1979, 244, 1146. (8) Breslow, R.; Juan, B.; Kluttz, R. Q.; Xia, C. 2. Terrahedron 1982,38,863. (h) Iwamura, H.; Sugawara, T.; Itoh, K.; Takui, T. Mol. Crysr. Liquid Cryst. 1985, 125, 251. (i) Breslow, R. Ibid. 1985, 125, 261. Q) Ooster, S.; Torrance, J. B.; Scumaker, R. R. The 1984 International Chemical Congress of Pacific Basin Societies, Abstract of Papers 07E28, Honolulu, Hawaii, December 1984. (3) (a) Itoh, K. Chem. Phys. Left. 1967,1, 235. (b) Itoh, K. Pure Appl. Chem. 1978, 50, 1251. (c) Wasserman, E.; Murray, R. W.; Yager, W. A.; Trozzolo, A. M.; Smolinsky, G. J. J . Am. Chem. SOC. 1967, 89, 5076. (4) (a) Seeger, D. E.; Berson, J . A. J . Am. Chem. SOC. 1983, 105, 5144. (b) Seeger, D. E.; Berson, J. A. Ibid. 1983, 105, 5146. (5 ) (a) Teki, Y.; Takui, T.; Itoh, K.; Iwamura, H.; Kobayashi, K. J . Am. Chem. SOC. 1983, 105, 3722. (b) Teki, Y . ; Takui, T.; Itoh, K.; Iwamura, H.; Kobayashi, K., to be published. ( 6 ) (a) Sugawara, T.; Bandow, S.; Kimura, K.; Iwamura, H.; Itoh, K. J . Am. Chem. SOC. 1984, 106, 6449. (b) Kimura, K.; Bandow, S. Solid State Phys. 1984, 20, 467. 0002-7863/86/ 15O8-0368

High-Spin Polycarbenes as a Model for Organic Ferromagnets

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Approaches from superhigh-spin molecules to organic ferromagnets

Abstract The crystals of p-(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)benzoic acid and its alkali metal (Li, Na, and K) salts were prepared. Dimer structures were found in the crystals of the free acid 1 and the lithium salt 1·Li· 1 2 MeOH by X-ray crystal structure studies. A dimer model was fitted to the observed magnetic susceptibility data of 1 and 1·Li· 1 2 MeOH in the whole temperature range studied. The optimized exchange coupling parameters 2J/kB in the dimers were −6.0 and +31.8 K in 1 and 1·Li· 1 2 MeOH, respectively. The sodium and potassium salts were paramagnetic and analyzed in terms of the Curie-Weiss relation to have weak Weiss fields of θ = −0.4 and −1.0 K, respectively.

Synthesis and Magnetic Properties of Bis(Hexafluoroacetylacetonato)Copper(II) Complex with 5-Bromo-1,3-Phenylenebis(N-tert-Butylaminoxyl) as a Bridging Ligand

The electron spin resonance line shapes of randomly oriented molecules in septet and nonet states are analyzed in terms of the formulas derived from a perturbation treatment to third‐order in the fine‐structure energy. The method is applied to a ground‐state septet hydrocarbon, 3, 3’‐diphenylmethylenebis (phenylmethylene), and a ground‐state nonet hydrocarbon, m‐phenylenebis[(diphenylmethylen‐3‐yl)methylene], randomly oriented in mixed polycrystalline powders of benzophenone. It is shown that the g factor and the fine‐structure parameters are determined from the line shapes of a K‐band spectrum with nearly the same accuracy as in a single‐crystal experiment. Extra lines have been observed in addition to the canonical lines corresponding to the external magnetic field along the principal axes of the fine‐structure tensor. The appearance of extra lines in the spectra of septet and nonet molecules is discussed relative to the third‐order perturbation theory.