THE N-H...PHENYL(PLANE) INTERACTION HAS ALL
THE PERSISTENCE OF A FIVE-YEAR-OLD IN A CANDY STORE. Stan Cameron, Pradip K. Bakshi, Kathy N. Robertson & O. Knop; Department of Chemistry, Dalhousie University, Halifax N.S. Canada B3H 4J3.
When an aqueous solution of an acidified organic amine is added to an aqueous solution of sodium tetraphenylborate, an immediate thick (often white) precipitate is formed. It does not seem to matter what amine is used or whether the concentration is 0.1 M or 70 ppm, the result is the same. While the anion (RnH4-n)+ / (BPh4)- cation interaction is probably the main energy term in the lattice energy which produces these intensely water-insoluble materials, at the heart of every structure is a 'hydrogen bond' where the N-H bond (or bonds) of the ammonium ion form an attractive interaction to the plane (or planes) of a phenyl ring of the tetraphenylborate cation. This system is remarkably robust and some aliphatic imminium ions with solution lifetimes of fractions of a second have been stabilised for years as crystalline tetraphenylborate salts.
The hydrogen bonds are formed with a number of strategies, but in all the many samples we have studied so far[1], where there is at least one N-H bond in the organo-ammonium ion, a hydrogen to ring-plane bond is always formed. The overwhelming preference of the tetraphenylborate ion is for a 1:1 ratio with the cation even when the amine is di- or multi-functional.
The general overview of the strategies used by this system to form the N-H...phenyl(plane) will be given. Our attempts to persuade the tetraphenylborate ion to take on di-charged (and multi-charged) organodiammonium ions will be described together with the response of the tetraphenylborate ion which invariably decisively outmanoeuvers us. Only a few of the crystal samples survive cooling, but the high-angle, low-temperature diffraction studies of the N-H...Phenyl interaction in the few samples that do survive low temperatures will be outlined.
1 Knop,O., Cameron,T.S. and others, Cand. J. Chem. (1980), 58, 1355; (1993), 71, 1495; (1994), 72, 1273