title: Utilizing proton transfer to produce molecular salts in bromanilic acid substituted-pyridine molecular complexes - predictable synthons?
abstract: Controlled introduction of proton transfer into the design of a series of molecular complexes is described, delivering the systematic production of ionic molecular complexes (molecular salts). The controlled production of molecular salts has relevance as a potential strategy in the design of pharmaceutical materials. In nine molecular complexes consisting of bromanilic acid with the N-heterocyclic compounds 2-, 3- and 4-picoline [bis(2/3/4-methylpyridinium) 2,5-dibromo-3,6-dioxocyclohexa-1,4-diene-1,4-diolate, 2C6H8N(+)·C6Br2O4(2-)], 2,3-, 2,4-, 2,5- and 3,5-lutidine [2,3/2,4/2,5/3,5-dimethylpyridinium 2,5-dibromo-4-hydroxy-3,6-dioxocyclohexa-1,4-dien-1-olate, C7H10N(+)·C6HBr2O4(-)], and 3-bromo-4-methylpyridine [3-bromo-4-methylpyridinium 2,5-dibromo-4-hydroxy-3,6-dioxocyclohexa-1,4-dien-1-olate, C6H7BrN(+)·C6HBr2O4(-)] and 2-bromo-3-methylpyridine [2-bromo-3-methylpyridine-2,5-dibromo-3,6-dihydroxycyclohexa-2,5-diene-1,4-dione (1/1), C6H6BrN·C6H2Br2O4], proton transfer occurs readily between the bromanilic acid molecule and the N heteroatom of the pyridine ring, in all cases producing a charge-assisted bifurcated N-H...O hydrogen bond. This reinforces the value of this motif as a design tool in the crystal engineering of such complexes. The protonation state (and stoichiometry) significantly affect the supramolecular synthons obtained, but 1:2 stoichiometries reliably give rise to PBP synthons and 1:1 stoichiometries to PBBP synthons (where P indicates a methylpyridine co-molecule and B a bromanilic acid molecule). The influence of halogen interactions on the wider crystal packing is also discussed, with C-H...Br and Br...O interactions the most prevalent; only one Br...Br interaction is found.
authors.affiliation: Department of Chemistry, University of Bath, Bath BA2 7AY, England.
authors.forename: Lynne H
authors.initials: LH
authors.lastname: Thomas
authors.forename: Martin S
authors.initials: MS
authors.lastname: Adam
authors.forename: Andrew
authors.initials: A
authors.lastname: O'Neill
authors.forename: Chick C
authors.initials: CC
authors.lastname: Wilson
mesh_terms: D018360:Crystallography, X-Ray; D006571:Heterocyclic Compounds / Q000737:chemistry*; D006842:Hydrocarbons, Brominated / Q000737:chemistry*; D006860:Hydrogen Bonding; D015394:Molecular Structure; D010847:Picolines / Q000737:chemistry*; D011522:Protons; D011725:Pyridines / Q000737:chemistry*; D012492:Salts / Q000737:chemistry*
chemical_list: D006571:Heterocyclic Compounds; D006842:Hydrocarbons, Brominated; D010847:Picolines; D011522:Protons; D011725:Pyridines; D012492:Salts
keywords: bromanilic acid; crystal engineering; crystal structure; hydrogen bonding; lutidine salts; picoline salts; proton transfer