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Weak Interactions in Crystals: 4,4′‐Di( tert ‐butyl)‐ N‐N ′‐bipyridinium Diperchlorate—A Biphenyl Analogue with Perpendicular Pyridine Rings
Author(s) -
Bock Hans,
Nick Sabine,
Näther Christian,
Bensch Wolfgang
Publication year - 1995
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.19950010810
Subject(s) - biphenyl , pyridine , perpendicular , chemistry , crystallography , medicinal chemistry , stereochemistry , photochemistry , organic chemistry , geometry , mathematics
Oxidation of substituted N ‐aminopyridinium compounds yields bipyridinium salts. After anion exchange Br − → CIO 4 − , 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° between the molecular halves connected by a shortened NN bond of 143 pm in length. This result is contrary to that expected from the isoelectronic correspondence 〉CC〈↔〉NN〈, 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–YC 5 H 5 (X–Y = CC, + NC, + NN + , − BN + , BC, and BB − ) predict single‐minimum potentials for both the dication 〉NN〈 as well as the dianion 〉BB〈 with the molecular halves twisted perpendicular to each other. In detailed model calculations, counteracting effects of π electron density delocalization vs. H/H repulsion of the ortho 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 π hydrocarbon with a latticeenforced dihedral angle of 0°, close to the calculated value, the two molecular halves are twisted by 40°—as in biphenyl in the gas phase.