Effects of N-oxidation on the molecular and crystal structures and properties of isocinchomeronic acid, its metal complexes and their supramolecular architectures: experimental, CSD survey, solution and theoretical approaches

Abstract

Nine coordination complexes and polymer (M/L/X) based on Co, Ni, Zn, Cu (M), pyridine-N-oxide-2,5-dicarboxylic acid (H₂pydco) (L) and either isonicotinamide (Ina), piperazine (pipz), 2,2′-bipyridine (bipy) and 1,10-phenanthroline (phen) (X) were synthesized and characterized by elemental analyses, infrared spectroscopy and single crystal X-ray diffraction. The resulting empirical formulae of the prepared complexes are [Co(H₂O)₆][Co(pydco)₂(H₂O)₂]·2H₂O (1), [M(pydco)(H₂O)₄]₂ [M = Co (2), Ni (3), Zn (4)], [Co(pydco)(bipy)(H₂O)₂]·4H₂O (5), [Co(pydco)(phen)(H₂O)₂]·5.135(H₂O)·0.18(EtOH) (6), [Cu(Hpydco)(bipy)Cl]·2H₂O (7), [Cu(Hpydco)(bipy)Cl]₂·2H₂O (8), and {[AgCu(H₂O)₂(phen)(pydco)]NO₃}_n (9). With the exception of 9, which forms an extended structure via multiple coordination modes, all the complexes contain (H)pydco as a bidentate ligand coordinated to the metal ion via the N-oxide and the adjacent carboxylate group oxygen atom, creating a chelate ring. The metal centers exhibit either distorted octahedral (1–6) or square pyramidal (7–9) geometry. Our results demonstrate that, when acting cooperatively, non-covalent interactions such as X–H⋯O hydrogen bonds (X = O, N, C), C–O⋯π and π⋯π stacking represent driving forces for the selection of different three-dimensional structures. Moreover, in compounds 2–4, 1D supramolecular chains are formed where O⋯π–hole interactions are established, which unexpectedly involve the non-coordinated carboxylate group. The non-covalent interaction (NCI) plot index analysis reveals the existence of the O⋯π–hole interactions that have been evaluated using DFT calculations. The Cremer and Pople ring puckering parameters are also investigated. The complexation reactions of these molecules with M were investigated by solution studies. The stoichiometry of the most abundant species in the solution was very close to the corresponding crystals. Finally, the effect of N-oxidation on the geometry of complexes has been also studied using the Cambridge Structural Database. It shows that complexes containing N-oxidized H₂pydc are very rare.