Monohalogenated carbamates where hydrogen bonding rules without halogen bonding: is there a link between poor carbamate crystal growth and Z′ > 1?

Abstract

Three series of monohalophenyl-N-pyridinylcarbamate isomer grids (CxxX) are reported (with x = para-, meta-, ortho; X = fluorine, chlorine, bromine), and Cx as the aminopyridinyl group (C₅NH₄NH–) and xX as –CO₂C₆H₄X. Nine crystal structures are presented with all nine structures aggregating via N–H⋯N intermolecular interactions as their primary hydrogen bond; as chains in CmpX, CmmX and as dimers in the CopX triad. Halogen bonding is absent and there is a preference for N–H⋯N and C–H/π⋯π stacking intermolecular interactions. Structural similarities abound with all three CmpX structures isomorphous in space group Pc and with CopF and CopCl isomorphous in P (No. 2). Differences between the monohalogenated CxxX carbamates and their CxxM and CxxOMe relatives (M, OMe = methyl, methoxy) are discussed, especially the reluctance of CxxX to form quality single crystals as most crystallise as filamentous-like fibres under a range of crystallisation conditions. Conformational analyses of the three [3 × 3] CxxX isomer grids of minimized structures in gas-phase are detailed for comparisons with the solid-state structures. Modelling results reveal inherent steric problems in CxoX associated with bulky pendant ortho-groups (i.e. bromine). Physicochemical analyses including melting point analysis and solid-state correlations are rationalised. A question is posed as to why the CxxX carbamates are reluctant to crystallise as quality single crystals and with a higher incidence of Z′ > 1 in comparison with related carboxamides.