Hydrogen-bonding interactions between linear bipyridinium cations and nitrate anions

A series of diprotonated bipyridium nitrates has been synthesised. The bipyridines considered are 4,4-bipyridine (bipy), trans-1,2-bis(pyridin-4-yl)ethene (bpe), 4,4- azobis(pyridine) (diazobpe), 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine(pytz), 3,6-bis(imidazol-1-yl)pyridazine (ipd) and 1,4-bis[(4,4-pyridyl) ethenyl] benzene (bpeb). A total of seven structures are discussed: two polymorphs of each of H(2)bipy(NO3)(2) and H(2)bpe(NO3)(2) and one crystal form of each of H(2)diazobpe(NO3)(2), H(2)bpeb(NO3)(2) and H(2)ipd(NO3)(2). Both N-H...O and C-H...O hydrogen-bonds are thought to be structure determining. The structural parameters of the N-H...O contacts fall into extremely narrow ranges: in contrast, the structural parameters for the C-H...O contacts differ widely and are considered to fall into three distinct types. Evidence for the importance of C-H...O hydrogen-bonds arises primarily from the different structures of the salts formed by [H(2)bpe](2+) and [H(2)diazobpe](2+) which differ solely in the ethene/diazo moiety linking the pyridine rings. The C-H donors of the ethene-link of the [H(2)bpe](2+) cation form hydrogen-bonds to the nitrate anion thereby generating a 2-D sheet structure. In the absence of these donors, the anions act as mu-O,O' bridging units linking neighbouring diazo-linked [H(2)diazobpe](2+) cations in adjacent unit cells. Secondary support for the significance of C-H...O interactions comes from the recurrence of particular hydrogen-bonding ring and chain motifs. Evidence for the delicate energy balance between different structures comes from the formation of the two pairs of polymorphs.