Different packing motifs mediated by hydrogen bonds in the hydrochloride salts of two pyridoxal <i>N</i>-acylhydrazone derivatives

The crystal structures of two hydro-chloride salts of pyridoxal-<i>N</i>-acyl-hydrazone-Q (Q = heterocyclic aromatic ring) derivatives, <i>viz</i>. (<i>E</i>)-3-hy-droxy-5-(hy-droxy-meth-yl)-2-methyl-4-{[(pyridin-4-ylformamido)-imino]-meth-yl}pyridin-1-ium chloride dihydrate, C₁₄H₁₅N₄O₃ ⁺·Cl^-·2H₂O, (<b>I</b>), and (<i>E</i>)-3-hy-droxy-5-(hy-droxy-meth-yl)-2-methyl-4-{[(pyrimidin-2-ylformamido)-imino]-meth-yl}pyridin-1-ium chloride dihydrate, C₁₃H₁₄N₅O₃ ⁺·Cl^-·2H₂O, (<b>II</b>) are described. The cations, which are protonated at the pyridine N atom of the pyridoxal ring, have similar overall conformations: the dihedral angles between the pyridoxal ring and the terminal aromatic ring are 12.63 (12) and 6.11 (15)° for (<b>I</b>) and (<b>II</b>), respectively. Each cation features an intra-molecular O-H⋯N hydrogen bond, which closes an <i>S</i>(6) ring, but a difference arises in the conformation of the C-C-C-O fragment terminated by the the ring carbon atom bonded to the side chain and the O atom of the hy-droxy-methyl group: <i>gauche</i> for (<b>I</b>) and <i>anti</i> for (<b>II</b>). The extended structures of (<b>I</b>) and (<b>II</b>) feature numerous strong (N-H and O-H donors) and weak (C-H donor) hydrogen bonds. In (<b>I</b>), the NH_p (pyridine) grouping links to the terminal N atom of the pendant unprotonated pyridine ring of an adjacent cation to generate [010] chains, whereas the NH_h (hydrazide) and OH_hm (hy-droxy-meth-yl) moieties link to chloride ion acceptors. In (<b>II</b>), the NH_p and OH_hm groupings bond to chloride anions whereas NH_h bonds to a water mol-ecule. Hydrogen-bonded chains of water mol-ecules occur in (<b>I</b>) and centrosymmetric tetra-mers in (<b>II</b>). The Hirshfeld surfaces of (<b>I</b>) and (<b>II</b>) are computed and the structures of related compounds are briefly compared.