In the title molecule, C7H6N4O3, the bicyclic ring system is planar with the carboxymethyl group inclined by 81.05 (5)° to this plane. In the crystal, corrugated layers parallel to (010) are generated by N—H...O, O—H...N and C—H...O hydrogen-bonding interactions. The layers are associated through C—H...π(ring) interactions. A Hirshfeld surface analysis indicates that the most important contributions to the crystal packing are from H...O/O...H (34.8%), H...N/N...H (19.3%) and H...H (18.1%) interactions. The volume of the crystal voids and the percentage of free space were calculated to be 176.30 Å3 and 10.94%, showing that there is no large cavity in the crystal packing. Computational methods revealed O—H...N, N—H...O and C—H...O hydrogen-bonding energies of 76.3, 55.2, 32.8 and 19.1 kJ mol−1, respectively. Evaluations of the electrostatic, dispersion and total energy frameworks indicate that the stabilization is dominated via dispersion energy contributions. Moreover, the optimized molecular structure, using density functional theory (DFT) at the B3LYP/6–311G(d,p) level, was compared with the experimentally determined one. The HOMO–LUMO energy gap was determined and the molecular electrostatic potential (MEP) surface was calculated at the B3LYP/6–31G level to predict sites for electrophilic and nucleophilic attacks.

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The asymmetric unit of the title compound, C10H12N4O2S, contains two molecules differing slightly in the orientations of the methyl groups. In the crystal, a sandwich-type structure extending parallel to the ab plane is formed by weak C—H...O and C—H...N hydrogen bonds together with slipped π-stacking interactions. A Hirshfeld surface analysis of the crystal structure indicates that the most important contributions to the crystal packing are from H...H (43.5%), H...O/O...H (17.9%) and H...N/N...H (17.4%) interactions. The molecular structure optimized by density functional theory (DFT) at the B3LYP/ 6–311 G(d,p) level is compared with the experimentally determined structure in the solid state. Further calculations include the HOMO–LUMO energies and molecular electrostatic potential (MEP) surfaces.

The substituted cyclopentadienyl ring in the title molecule, [Fe(C5H5)(C18H13ClN)], is nearly coplanar with the phenyl-1-(4-chlorophenyl)methanimine substituent, with dihedral angles between the planes of the phenylene ring and the Cp and 4-(chlorophenyl)methanimine units of 7.87 (19) and 9.23 (10)°, respectively. The unsubstituted cyclopentadienyl ring is rotationally disordered, the occupancy ratio for the two orientations refined to a 0.666 (7)/0.334 (7) ratio. In the crystal, the molecules pack in `bilayers' parallel to the ab plane with the ferrocenyl groups on the outer faces and the substituents directed towards the regions between them. The ferrocenyl groups are linked by C—H...π(ring) interactions. A Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the crystal packing are from H...H (46.1%), H...C/C... H (35.4%) and H...Cl/Cl...H (13.8%) interactions. Thus C—H...π(ring) and van der Waals interactions are the dominant interactions in the crystal packing.

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