Cu(II) and Ni(II) metallacycles derived from barbituric acids: Synthesis with solvent/agent effects in crystallization and structural characterization with non-covalent interaction analysis

It is well known that solvent molecules and additional agents significantly impact crystal structures. In relation to this, the barbituric acid-based 1,3-dimethyl-2,4,6-trioxo-hexahydro-pyrimidine-5-(2,6-dimethylphenylamino)methylene (HL1) as ligand and three novel copper(II) complexes, 2[Cu(L1)2] (1), 2[Cu(L1)2(4,4′bpy)] (2), Cu(L1)2(CH3)2SO (3), (where H is the deprotonatable hydrogen) were synthesized and characterized by elemental analysis, FT-IR spectroscopy. As a comparison, a potentially tridentate barbituric acid-based ligand, 1,3-dimethyl-2,4,6-trioxo-hexahydro-pyrimidine-5-(8-quinolylamino)methylene (HL2), and its three new copper(II) and nickel(II) complexes, Cu(L2)2 (4), Cu(L2)2 (5) and Ni(L2)2 (6), were also synthesized and characterized by elemental analysis, FT-IR spectroscopy. The crystal structures of the ligands and their complexes were determined based on an X-ray diffraction study. X-ray diffraction studies have shown that the ligands HL1 and HL2 exist in keto-enamine form. The crystal structure of complexes of HL1 revealed that Cu center is distorted square-planar in 2[Cu(L1)2] (1), while it is found in distorted square-pyramidal in relation to additional agent (4,4′ bpyridine) and solvent (DMSO) conditions in 2[Cu(L1)2(4,4′bpy)] (2) and Cu(L1)2(CH3)2SO (3), respectively. In the crystal structure, [M(L2)2] (M = Cu (4 and 5), Ni (6)), metal ions possess a distorted octahedral environment, and the HL2 ligands coordinate with a meridional geometry. Structures of all complexes reveal a tendency to form C–H···O, C–H···π and π···π interactions, except for molecule Cu(L1)2(CH3)2SO (3) in terms of C–H···O bonds which was not observed. Exploring noncovalent interactions indicates similar molecular structures in [M(L2)2] (M = Cu (4 and 5), Ni (6)) suggesting the difference in packing due to the solvent of crystallization (CH2Cl2) in the lattice of Cu(L2)2 (5) and the different metal ion in the molecular structure of Ni(L2)2 (6). The packing in the structures of HL1 complexes is found to be strongly dependent on the additional agent (4,4′ bpyridine) and solvent (DMSO) fused in molecular structure with different supramolecular assemblies such as 1D chains, π···π dimers, 2D sheets, etc. In addition, Hirshfeld surface analysis and 2D fingerprint plots were employed to verify intermolecular interactions in both ligands and their complexes