The synthesis of dimethyldihydrocyclopent [a] pyrene, ion(1-) and its metal complexes: and the interpretation of their ¹H NMR data
Date
2018-06-26
Authors
Khalifa, Nasr A.
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Abstract
The synthesis of trans-11b,11c-dimethyl-11b,11c-dihydro-7H-cyclopent [a] pyrene, 109, from trans-10b,10c-dimethyl-10b,10c-dihydropyrene, 32, was achieved in six steps in an overall yield of 29%. Deprotonation of 109 gave the first annuleno-fused cyclopentadienide, trans-11b,11c-dimethyl-11b,11c-dihydrocyclopent[a] pyrene, ion(1-), 101. Experimental and theoretical proton NMR results for the anion in the presence and absence of the counter cation were analysed. The cyclopentadienyl anion, when fused to 32, has 53% of the effective bond-fixing ability of benzene fused to the same system. In terms of benzene resonance energy units, cyclopentadienyl anion has an effective resonance energy of 0.53.
Metal complexation of the cyclopent[a]dihydropyrene, 101, was investigated, and gave the first two cyclopent-fused large annulene metal complexes, (6a,7,8,9,9a- μ5]-trans-11b,11c-dimethyl-11b,11c-dihydrocyclopent[a] pyrene-pentamethylcyclopentadienylruthenium(II), 139, and (6a,7,8,9,9a- μ5) trans-11b,11c-dimethyl-11b,11c-dihydrocyclopent[a] pyrene-tricarbonylmanganese (I), 141. Experimental and theoretical 1H NMR results for the two complexes were analysed. Ruthenocene, when fused to 32, was found to be 1.38 times more bond-fixing than benzene itself. Similarly, cyclopentadienylmanganesetricarbonyl is 1.33 times more bond-fixing than benzene. In terms of benzene resonance energy units, the two complexes have effective experimental resonance energies of 1.42 and 1.36, respectively.
The diamagnetic susceptibility, X, of a cyclopentadienylruthenium moiety, with the center of anisotropy located at the metal atom, was calculated as -330x10-36 m3 per molecule. The same parameter for a manganese tricarbonyl moiety, with the center of anisotropy being located at 3.2 A° down from the manganese atom, was calculated as -635x10-36 m3 per molecule.
An X-ray structure determination of 32 was finally achieved some 25 years after its first synthesis. The structural data confirm the planarity and lack of bond alternation in the bridged annulene, indicating that it is aromatic.
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Keywords
Aromatic compounds, Synthesis