Spectroscopic, structural and DFT studies of luminescent Pt(II) and Ag(I) complexes with an asymmetric 2,2′-bipyridine chelating ligand

A new unsymmetrically substituted 2,2′-bipyridine ligand, 5-methyl-5′-carbomethoxy-2,2′-bipyridine (L) was isolated from the dry distillation of the copper(II) complex, mono-aqua-bis(trans-5-methyl-pyridine-2-carboxylato-N,O)copper(II). The ligand was fully characterized. The spectroscopic and single-crystal X-ray diffraction (SCXRD) studies of the coordination compounds of the ligand with platinum(II) and silver(I); cis-Pt(L)Cl2 (1) and [Ag(L)2]PF6 (2), respectively are reported. In 1, the Pt centre coordinates to tertiary N atoms of the ligand and two chloride ions to form a neutral square-planar coordination sphere, while in 2, the Ag(I) centre is coordinated by two ligands through N atoms to generate a cationic flattened tetrahedron geometry in which two mean planes intersect each other at 50.93°. The pyridine rings are nearly coplanar as revealed by the torsion angle of N2-C7-C6-N1 1.32(5)°. In both complexes, L acts as a chelating ligand through pyridyl N atoms. In 1, the molecular units are stacked in a head-to-tail fashion with a Pt···Pt separation of 3.5 Å. Supramolecular self-assembly of the molecular units by extensive intermolecular contacts through C‒H···Cl and C‒H···O between the adjacent units results in an infinite two-dimensional flattened-out herringbone structure in the crystalline state. In 2, the molecular units are interconnected with each other by C‒H···O contacts between the adjacent units running parallel to each other. Both complexes are fluorescent in solution and have emission maxima in the UV-Vis regions, which is a very important property for optoelectronic applications. DFT (density functional theory) and TD-DFT (time-dependent-DFT) calculations were performed at B3LYP/6-311+G(d,p)/LANL2DZ level to explore structural, electronic, and spectroscopic properties to compare with the experimental results. The molecular orbitals (MOs) were carried out with DFT at the same level.