Structures and conformational analysis of a 3 × 3 isomer grid of nine N-(fluorophenyl)pyridinecarboxamides

Abstract

A 3 × 3 isomer grid of N-(fluorophenyl)pyridinecarboxamides is reported and integrating crystal structure analyses, ab initio optimisation calculations (gas phase and solvated forms in CH₂Cl₂, H₂O) and conformational analyses. The nine NxxF isomers (x = 4-, 3- or 2-substitution on N and F) are investigated and compared to determine and correlate factors underpinning (a) the roles of the F/N atom substituents on molecular conformation and overall supramolecular aggregation, (b) competition between intermolecular amide⋯amide (in NppF) or intra-/intermolecular amide⋯pyridine hydrogen bond formation and (c) structural and physico-chemical properties. Crystal structure analyses of the NxxF isomers reveal different primary aggregation processes as either N–H⋯N or N–H⋯OC and with NmpF forming an unusual cyclic N–H⋯N hydrogen bonded tetrameric assembly [as a R⁴₄(24) ring]. Compounds NpmF and NpoF are isomorphous and the latter is also disordered. Conformational analysis of the NxxF molecular structures from DFT calculations differs from the crystal structure results for several isomers and highlighting the co-operative effects of intra-/intermolecular interactions in the solid state.