ELECTRON-TRANSFER AND ION-PAIR FORMATION .33. THE SINGLE-ELECTRON REDUCTION OF TETRAPHENYL-P-BENZOQUINONE BY ALKALI-METALS - ENDOR SPECTRA OF CONTACT ION-PAIRS AS WELL AS TRIPLE IONS IN SOLUTION AND SINGLE-CRYSTAL STRUCTURES OF BOTH THE NEUTRAL COMPOUND AND ITS SODIUM-SALT

Tetraphenyl-p-benzoquinone, according to its single crystal structure, shows some steric congestion: its quinone ring is distorted by 7 degrees to a chair conformation, and its phenyl substituents are twisted around their CC axes between 46 degrees and 72 degrees. The half-wave reduction potentials of -0.57 and -1.25 V in acetonitrile confirm negligible pi interaction of the phenyl substituents. Addition of alkalimetal tetraphenylborate salts lowers the second reduction potential due to contact ion formation, which can be confirmed by UV/VIS spectra recorded under aprotic conditions. Extensive ESR/ENDOR investigations prove the formation of the following species in THF solution: Tetraphenyl-p-benzosemiquinone radical anion contact ion pairs [M(.-) Me(solv)(+)](.) (Me(+): Li+, Na+, Rb+, Cs+) and contact triple ion radical cations both with identical cations [M(.-) (Me(solv)(+))(2)](.+) (Me(+): Li+, Na+, Cs+) and different cations [M(.-) (Li-solv(+))(Me(solv)(+))](.+) (Me(+): Na+, Cs+). Addition of crown ethers can lead to external solvation of the Me(+) counter cations, whereas cryptands form internal solvation complexes. The radical anion of 2,6-diphenyl-p-benzosemiquinone adds cations at its phenyl-free molecular half. The radical anion salt [tetraphenyl-p-benzosemiquinone(.-) (Na+(tetrahydropyrane)(2))] could be crystallized and its structure determined at 200 K. In agreement with the Hirota sign rules for contact radicals in solution, the Na+ ion is found 62 pm above the pi plane and 29 degrees outside the axis of the CO bound, which is elongated due to one-electron reduction by 5 pm to 127 pm.