WEAK-INTERACTIONS IN CRYSTALS - 4,4'-DI(TERT-BUTYL)-N-N'-BIPYRIDINIUM DIPERCHLORATE - A BIPHENYL ANALOG WITH PERPENDICULAR PYRIDINE RINGS

Oxidation of substituted N-aminopyridinium compounds yields bipyridinium salts. After anion exchange Br- --> ClO4-, it was possible to grow single crystals from the 4,4'-di(tert-butyl) derivative. The structure determined at 100 K shows a dihedral angle of 84 degrees between the molecular halves connected by a shortened N-N bond of 143 pm in length. This result is contrary to that expected from the isoelectronic correspondence ]C-C[<->]N+-N+[, which would suggest planarity, as has been observed experimentally both in solid biphenyl, and in tetraalkylhydrazine dications and radical cations. Lattice packing analysis, however, reveals the presence of hydrogen bonds C(H)... O between the phenyl rings and the perchlorate anions. AM 1 enthalpy hypersurface calculations for the isoelectronic series H(5)C(5)X-YC5H5 (X-Y = C-C, N-+-C, N-+-N+, B---N+, B---C, and B---B-) predict single-minimum potentials for both the dication ]N+-N+[ as well as the dianion ]B--B-[ with the molecular halves twisted perpendicular to each other. In detailed model calculations, counteracting effects of pi electron density delocalization vs. H/H repulsion of the oi tho ring hydrogens adjacent to the central bond seem to dictate the delicate balance of the biphenyl twisting. For further experimental confirmation, the structure of the isosteric molecule 4,4'-di(tert-butyl)biphenyl has been determined: in contrast to the unsubstituted pi hydrocarbon with a lattice-enforced dihedral angle of 0 degrees, close to the calculated valuer the two molecular halves are twisted by 40 degrees-as in biphenyl in the gas phase.