NNN type pincer Pd (II) complexes of pyridine-2,6-dicarboxamides: Catalytic activity and supramolecular formation

Abstract

Pyridine-2,6-dicarboxamide, NNN pincer type pro-ligands, and their palladium complexes were synthesized. The prepared pro-ligands and complexes were characterized with several characterization techniques (FT-IR, ¹H NMR, ¹³C NMR, and UV–vis analyses). The molecular structure of the acetonitrile-N²,N⁶-bis(2-bromophenyl)pyridine-2,6-dicarboxamidopalladium(II) complex has been determined by single crystal XRD measurement. This complex contains intramolecular and intermolecular H-bonds, CH⋅⋅⋅π and face-to-face π⋅⋅⋅π interactions. In addition, two synthons were found in the crystal lattice that have been generated through hydrogen-bond interactions. Homo-synthons ($R_2^2\left(12\right)$), which forms through CH⋅⋅⋅O intermolecular interactions (2.816 Å for C15H15⋅⋅⋅O1 and 2.603 Å for C13H13⋅⋅⋅O2) and hetero-synthons ($R_2^1\left(8\right)$), that generated via CH⋅⋅⋅N intermolecular interactions (C21H21C⋅⋅⋅N4, 3.439 Å). Furhermore, homo-synthons ($R_3^3\left(11\right)$ and $R_2^2\left(9\right)$) through intermolecular contacts were determined between the palladium complex and the acetonitrile molecules. Furthermore, a quantitative analysis of the noncovalent interactions within the structure was performed using Hirshfeld surface and two-dimensional fingerprint plot analyses. The three-dimensional arrangement of the crystal packing was assessed during the energy-framework calculations, revealing that the electrostatic energy framework had a considerable influence on the dispersion energy framework. The [PdL⁵] complex provided a conversion of more than 98 % and a yield of more than 98 % in the shortest time among other catalysts, in which the Suzuki–Miyaura CC cross-coupling reaction (SMR) was carried out with the 4-bromotoluene substrate. The activity of [PdL⁵] complex was then examined in the reactions of two other substrates, 1-bromo-4-isobutylbenzene and 2-bromo-6-methoxynaphthalene. Although a yield of over 97 % and a conversion of over 97 % were observed in the reaction of 1-bromo-4-isobutylbenzene after two hours, the reaction of 2-bromo-6-methoxynaphthalene achieved complete conversion and a yield of over 99 % within just one hour.