Crystal chemistry of tetraradial species. Part 9. The versatile BPh4- anion, or how organoammonium H(N) atoms compete for hydrogen bonding

Abstract

In organoammonium cations containing two or more N atoms, the H(N) atom introduced by the protonation may engage in an N→H(N)...Y bond to an external acceptor (anion or solvent molecule); in an intra-cation (intra-annular) N→H(N)...N' bond; or in a branched N→H(N)...N',Y bond(s), simultaneously intra-cation and to one or more external acceptors. The outcome of the competition for these bond types is determined by various factors, e.g., the size of the ring to be closed by insertion of the H(N) atom, steric accessibility of H(N), and the H-bond-forming strength of Y. Some of these are discussed in considerable detail and are exemplified by the hitherto unreported crystal structures of the tetraphenylborates (solvated or unsolvated) of the 2,2':6',2"-terpyridinium (1), 2,3-bis(2-pyridyl)pyrazinium (2), 2,2'-dipyridylammonium (3), 6,7-dihydro[b,j][1,10]phenanthrolinium (9), 2,4,7-triamino-6-pteridinium (triamterenium, 10), proton-sponge (1-Me2N-8-Me2NH-naphthalene, 11, 12), and 9-amino-1,2,3,4-tetrahydroacridinium (tacrinium, 13) cations. The crystal structures of the comparison compounds 2,3-bis(2-pyridyl)pyrazine (4) and its mono- (2A) and diprotonated (3) chlorides, and of 2-phenylpyridinium (6) and 7,8-benzoquinolinium (7, 8) tetraphenylborates, have also been determined. The many interesting features of these crystal structures, such as the X→H(X)...phenyl bonding (X = N, O, C), are commented upon and crystallographic comparisons with numerous literature compounds are offered. Included in the discussion are the monoprotonated intrabridgehead cations of the bicyclic diamines of Alder et al., the N-H(N)-N' angle of which has been shown, analytically, to be dominated by the size of the smallest of the rings in these tricyclic systems.Key words: crystal structures, hydrogen bonding, intrabridgehead cations, proton sponges, tetraphenylborates.