Phosphine-Free Bis(Pyrrolyl)pyridine Based NNN-Pincer Palladium(II) Complexes as Efficient Catalysts for Suzuki-Miyaura Cross-Coupling Reactions of Aryl Bromides in Aqueous Medium

Palladium complexes containing NNN pincer type ligands and their activities in Suzuki-Miyaura cross coupling reaction

Five new NNN pincer-type ligands and their palladium complexes were successfully synthesised and characterised by FT-IR, ¹H NMR, ¹³C NMR, and UV-vis analyses. TEM analysis was used to observe the morphological character of the black residues obtained from the fourth cycle of the reusability test. Furthermore, suitable crystals of the N²,N⁶-bis(2-tert-butylphenyl)pyridine-2,6-dicarboxamide and its palladium complex were elucidated with the X-ray single crystal diffraction method. Both the ligand and its palladium complex crystallise in a monoclinic system with space group P2₁/c for the H₂L⁴ and C2/c for the palladium complex. The structure of the pincer ligand and its palladium complex were stabilised by intramolecular and intermolecular C-H⋅⋅⋅O, C-H⋅⋅⋅N, and N-H⋅⋅⋅N contacts. A Suzuki-Miyaura cross-coupling reaction between aryl halides and phenylboronic acid was used to assess the catalytic abilities of the palladium pincer complexes. All of the prepared complexes exhibited considerable catalytic activity. However, complexes 4 (Acetonitrile-N²,N⁶-bis(2-tert-butylphenyl)pyridine-2,6-dicarboxamidopalladium(II)) and 5 (Acetonitrile-N²,N⁶-bis(2-nitrophenyl)pyridine-2,6-dicarboxamidopalladium(II)) provided almost 100% conversion with nearly 100% yield in the reaction between 4-bromotoluene and phenylboronic acid. Furthermore, these active complexes catalysed the reaction of the sterically hindered and deactivated substrates (1-Bromo-4-izobutylbenzene and 2-bromo-6-methoxynaphthalene) with phenylboronic acid, and complete conversion and yields up to 100% were achieved in a short time with the 2-bromo-6-methoxynaphthalene.

Substituted pyrrolyl-cyanopyridines on the platform of acylethynylpyrroles via their 1 : 2 annulation with acetonitrile under the action of lithium metal

A facile one-pot synthesis of 2-(pyrrolyl)-4-aryl(hetaryl)-5-cyano-6-methylpyridines in 63–87% yield via the cyclization of available acylethynylpyrroles with two molecules of acetonitrile under the action of lithium metal at room temperature has been developed.

Highly Functionalized Pyrrolylpyridines from 2-(Acylethynyl)-pyrroles

A novel family of pharmaceutically prospective, densely functionalized (aryl-, hetaryl-, acyl-, and vinyl-substituted) pyrrolylpyridines has been assembled in up to 95% yields via the amination (NH4Cl/K2CO3/DMSO system, 90 °C, 16 h) of 2-(acylethynyl)pyrroles, followed by cyclization (MeOH, reflux, 6 h, up to 90% yield) of the formed intermediate 1-(pyrrol-2-yl)-1-aminoenones with acetylenic ketones (Bohlmann–Rahtz reaction). The intermediate pyrrolyl aminodienones, prospective building blocks for organic synthesis, can be separately synthesized (DMSO, 90 °C, 6 h) in 70–86% yields. A novel facet of this chemistry is stereoselective synthesis of C-vinylated pyridines, (E)-3-[5-acyl-2-aryl-6-(pyrrol-2-yl)pyridin-3-yl]-1-arylprop-2-en-1-ones, by involving the second molecule of acylacetylene into the reaction.

Nickel(II) carbonyl, ammonia, and aceto-nitrile complexes supported by a pyridine dipyrrolide pincer ligand

The synthesis, isolation and crystal structures of nickel(II) carbonyl, aceto-nitrile and ammonia complexes supported by a dianionic, pyridine dipyrrolide pincer ligand [pyrr2py]2-, namely, carbonyl[2,2'-(pyridine-2,6-di-yl)bis-(3,5-di-p-tolyl-pyrrolido-κN)]-nickel(II), [Ni(C41H33N3)(CO)], ammine[2,2'-(pyridine-2,6-di-yl)bis-(3,5-di-p-tolyl-pyrrolido-κN)]nickel(II), [Ni(C41H33N3)(NH3)], and (aceto-nitrile-κN)[2,2'-(pyridine-2,6-di-yl)bis-(3,5-di-p-tolyl-pyrrolido-κN)]nickel(II), [Ni(C41H33N3)(CH3CN)], as well as the free ligand 2,6-bis-(3,5-di-p-tolyl-pyrrol-2-yl)pyridine, C41H35N3 or [pyrr2py]H2 are reported. The nickel complexes are four-coordinate and adopt a square-planar geometry. The CO stretch of the nickel-bound carbon monoxide ligand of [pyrr2py]Ni(CO) has been observed at 2101 cm-1. The ammonia and aceto-nitrile complexes, [pyrr2py]Ni(NH3) and [pyrr2py]Ni(NCMe) feature all-nitro-gen coordination spheres around nickel consisting of different N-donor ligand types.

Synthesis and Electronic Structure of Neutral Square-Planar High-Spin Iron(II) Complexes Supported by a Dianionic Pincer Ligand

Two square-planar high-spin Fe^II complexes bearing a dianionic pyridine dipyrrolate pincer ligand and a diethyl ether or tetrahydrofuran ligand were synthesized and structurally characterized, and their electronic structures were elucidated by a combined spectroscopic and computational approach. In contrast to previous examples, the S = 2 ground states of these square-planar Fe^II complexes do not require an overall anionic charge of the compounds or incorporation of alkali metal cations. The tetrahydrofuran complex exhibits an equilibrium between four- and five-coordinate species in solution, which was supported by ¹H NMR and ⁵⁷Fe Mössbauer spectroscopy and comparison to a structurally characterized five-coordinate pyridine dipyrrolate iron bis-pyridine adduct. A detailed computational analysis of the electronic structures of the four- and five-coordinate species via density functional theory provides insight into the origins of the unusual ground state configurations for Fe^II in a square-planar ligand field and explains the associated characteristic spectroscopic parameters.