J. Am. Chem. SOC.1995,117, 806-816
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Macrocyclic x-Conjugated Carbopolyanions and Polyradicals Based upon Calix[4]arene and Calix[3]arene Rings Andmej Raja,* Suchada Rajca, and Shailesh R. Desai Contribution from the Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304 Received August 17, 1994@
Abstract: Calix[4]arene- and calix[3]arene-based polyether precursors to polyradicals are synthesized. n-Conjugated carbanions, such as calix[4]arene-based tetraanion and calix[3]arene-based trianion, are prepared and studied using NMR spectroscopy and voltammetry. A 4-fold-symmetric conformer for the tetraanion and two non-interconverting conformers (3-fold- and 2-fold-symmetric) for the trianion are found on the NMR time scale. Oxidation of the tetraanion gives the corresponding calix[4]arene-based S = 2 tetraradical. However, ESR spectroscopy suggests that the predominant product from oxidation of calix[3]arene-based trianion is the corresponding triradical dimer. The related calix[3]arene-based S = 1 diradical is found to be monomeric. Additional characterization of octaradical 1* and pentaradical 2s', which were described in a preliminary communication, is presented.
Introduction Synthesis of very-high-spin mesoscopic-size molecules is a challenging and important problem related to the recent topic of organic magnetism.'S2 In order to obtain very-high-spin molecules, strong intramolecular ferromagnetic coupling between "unpaired" electrons must be maintained. The necessity for the multisite interaction creates formidable synthetic difficulties such as the following: (1) the yield for site generation (p) has to be ultra-high for even a modest yield of molecules with all sites intact, e.g., the yields of intact molecules with 4, 8, and 90 sites forp = 0.95 (95% yield per site) would be 81%, 66%, and 1%; (2) defects, which we define as failures to generate an interacting site, may pose an obstacle to an effective interaction between the remaining site^.^,^ The second problem is especially relevant to strongly spin coupled polyradicals, where the sites (with "unpaired" electrons) are formally incorporated in the coupling paths connecting the sites.les5 In the examples below, the sites, paths, and defects are represented with dots, bars, and open circles, respectively.
uncoupled parts with lower spin, (ii) spin coupling is preserved in a macrocyclic polyradical with 5 1 defect? Polyradicals in (i) and (ii) are labeled as 0-proof and 1-proof, respectively.le In the preliminary communication, macrocyclic octaradical 18' and its acyclic pentaradical analogue 2" were reported. Magnetization measurements for 1* indicated S = 4 ground state and negligible interruption of spin coupling by defects6
A defect at an inner site affects the spin coupling, depending on the connectivity between the sites: (i) spin coupling is interrupted by dividing a linear or dendritic polyradical into
The goal of of the present work is to determine the spin coupling in the calix[n]arene-based (n = 3, 4) polyradicals. In addition to further characterization of 1* and 2s*, the parent calix[4]arene-based mconjugated system, tetraradical 3", the related calix[3]arene-based diradical 4-Hh, and the dimer of triradical4" are reported. The calix[3]arene-based polyradicals are fragments of the Mataga polymer, which was proposed in 1968;' this remarkable n-conjugated two-dimensional honeycomb lattice is 5-pr0of.l~
Abstract published in Advance ACS Abstracts, January 1, 1995. (1) Recent reviews on spin-coupled organic molecules: (a) Dougherty, D. A. Acc. Chem. Res. 1991,24,88, (b) Iwamura, H.; Koga, N. Acc. Chem. Res. 1993.26,346, (c) Iwamura, H. Adv. Phys. Org. Chem. 1990,26,179, (d) Borden, W. T.; Iwamura, H.; Berson, J. A. Acc. Chem. Res. 1994,27, 109, (e) Rajca, A. Chem. Rev. 1994,94, 871. (2) Reviews on mesocopic-size molecules: Diederich, F. Nature 1994, 369,199. Mekelburger,H.-B.; Jaworek, W.; Vogtle, F. Angew. Chem., Znt. Ed. Engl. 1992,31, 1571. Newkome, G. R., Ed. Advances in Dendritic Macromolecules; JAI Press: Greenwich, 1994; Vol. I. (3) Rajca, A.; Utamapanya, S. J. Am. Chem. SOC. 1993,115, 10688.
(4) Iwamura's nonacarbene is an example of a high-spin molecule with very high p: Nakamura, N.; Inoue, K.; Iwamura, H. Angew. Chem., Znt. Ed. Engl. 1993,32,872. (5) Attaching spin sites to a chain (spin-coupling path) as pendants gives only weak coupling so far; a defect at the inner site is expected to further weaken the coupling: Nishide, H.; Kaneko, T.; Igarashi, M.; Tsuchida, E.; Yoshioka, N.; Lahti, P. M. Macromolecules 1994,27, 3082. Sasaki, S.; Iwamura, H. Chem. Len. 1992,1759. (6) Rajca, A.; Rajca, S.; Padmakumar, R. Angew. Chem., Znt. Ed. Engl. 1994,33,2091. (7) Mataga, N. Theor. Chim. Acta 1968,10, 372.
s=1/2
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0002-7863/95/1517-0806$09.00/00 1995 American Chemical Society
J. Am. Chem. SOC., Vol. 117, No. 2, 1995 807
Macrocyclic JG- Conjugated Carbopolyanions and Polyradicals
Mataga Polymer
Results and Discussion A. Synthesis of Polyethers. Synthesis of the tetraether 3-(OMe)4, triether 4-(OMe)3, and diether 4-H-(OMe)2,which are precursors to tetra-, tri-, and diradicals 3*, 4*, and 4-H2', is outlined in Figure 1. The macrocyclic rings are constructed by condensation of the bis(aryllithiums), which are generated in situ from 5-OMe or 5-H by B r L i exchange, with diketones such as 6 or 1,3-phenylenebi~(4-tert-butylphenyl)methanone;~ the resultant macrocyclic diols are isolated in 30-50% yield^.^ Two isomers, cis and trans ether-diol, are isolated in the case of calix[3]arene rings; etherification proceeds with retention of configuration giving the corresponding cis and trans triethers 4-(OMe)3. Calix[4]arene-based diether-diol and tetraether are isolated as isomeric mixtures. Synthesis of the polyether precursors to octaradical 1%and pentaradical z5*,which follows the methodology analogous to that in Figure 1, is described in the supplementary material.6 Notably, for more sterically hindered calix[4]arene derivatives, such as hexaether-diol l-(OMe)dOH)2and octaether 1-(OMe)s, one 2-fold-symmetric isomer has been isolated.6
Bl
Figure 1. Synthesis of polyethers: A, n-BuLi, ether,
