A strategy for the propagation of hydrogen-bonding in bicyclic guanidinium salts

Abstract

Protonation of the {6,6}- and {5,5}-bicyclic guanidines 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) and 1,4,6-triazabicyclo[3.3.0]oct-4-ene (Htbo), respectively, to afford the hydrochloride salts [hppH₂][Cl] (1a) and [HtboH][Cl] (2) was achieved using [NEt₃H][Cl]. Anion exchange involving 1a and NaBPh₄ generated the borate salt [hppH₂][BPh₄] (1c). Crystal structure analysis of 1a and the analogous hydrobromide salt [hppH₂][Br] (1b) showed two NH⋯X hydrogen-bonds between the cation and the anion, and association of the [hppH₂][X] ion-pairs into dimeric units. In salt1c the [hppH₂]⁺ cation was shown to be located within a cavity defined by phenyl substituents from the [BPh₄]⁻ anion, with no hydrogen bonding present. Reducing the size of the heterocyclic rings from {6,6}- to {5,5}- in the [HtboH]⁺ salt promotes formation of extended structures due to a wider angle between the projected hydrogen-bonds to the halide anion. Results from analysis of the bond parameters within the guanidinium cations are used to explain the distribution of π-electron density throughout the bicyclic framework.