Ligand Steric Effects of α-Diimine Nickel(II) and Palladium(II) Complexes in the Suzuki–Miyaura Cross-Coupling Reaction

Abstract

Nickel catalysts represent a low cost and environmentally friendly alternative to palladium-based catalytic systems for Suzuki–Miyaura cross-coupling (SMC) reactions. However, nickel catalysts have suffered from poor air, moisture, and thermal stabilities, especially at high catalyst loading, requiring controlled reaction conditions. In this report, we examine a family of mono- and dinuclear Ni(II) and Pd(II) complexes with a diverse and versatile α-diimine ligand environment for SMC reactions. To evaluate the ligand steric effects, including the bite angle in the reaction outcomes, the structural variation of the complexes was achieved by incorporating iminopyridine- and acenaphthene-based ligands. Moreover, the impact of substrate bulkiness was investigated by reacting various aryl bromides with phenylboronic acid, 2-naphthylboronic acid, and 9-phenanthracenylboronic acid. Yields were the best with the dinuclear complex, being nearly quantitative (93–99%), followed by the mononuclear complexes, giving yields of 78–98%. Consequently, α-diimine-based ligands have the potential to deliver Ni-based systems as sustainable catalysts in SMC.

Mono- and Dinuclear α-Diimine Nickel(II) and Palladium(II) Complexes in C–S Cross-Coupling

“…9−13 Moreover, reductive elimination is promoted by greater orbital overlap due to a wider bite angle. 34,41 Accordingly, Pd A-1, possessing a higher %V bur and ligand bite angle, affords better yields than Ni A-1. For understanding the improved performance of Pd B-1, similar observations are relevant.…”

“…The ligands and complexes were synthesized by adopting a reported procedure. 34 Iminopyridine-(1a, 1b, and 1d) and acenaphthene-based (1c) ligands were prepared through condensation reactions of primary arylamines, 2-pyridine carboxaldehyde, and acenaphthoquinone, respectively. Two equivalents of the ligand 1a with NiCl 2 (DME) afforded the mononuclear bis-ligated complex, Ni A-1, whereas 1 equiv of ligand 1d generated the dinuclear monoligated complex, Ni B-1.…”

“…34−36 In addition, electronic and steric features, including the ligand bite angle, are adjustable by incorporating various functional moieties in the ligand structure. 34,37,38 In a recent publication, we have demonstrated a series of αdiimine Ni(II) and Pd(II) complexes for C−C bond formation through the Suzuki−Miyaura cross-coupling reaction. 34 Both iminopyridine-and acenaphthene-based ligand frameworks were implemented to synthesize mono-and dinuclear Ni(II) and Pd(II) complexes for investigating the steric features in the catalytic performance.…”

“…34,37,38 In a recent publication, we have demonstrated a series of αdiimine Ni(II) and Pd(II) complexes for C−C bond formation through the Suzuki−Miyaura cross-coupling reaction. 34 Both iminopyridine-and acenaphthene-based ligand frameworks were implemented to synthesize mono-and dinuclear Ni(II) and Pd(II) complexes for investigating the steric features in the catalytic performance. Also, sterically challenging boronic acids were incorporated with a wide variety of aryl halides containing reactive functionalities.…”

Design, Synthesis and Characterization of Aurone Based α,β‐unsaturated Carbonyl‐Amino Ligands and their Application in Microwave Assisted Suzuki, Heck and Buchwald Reactions

“…To overcome these drawbacks, extensive research work has been done and synthetic chemists have explored a number of phosphine‐free palladium complexes effective for these coupling reactions [5] . Recently, various phosphine‐free ligands have been used in the palladium catalyzed coupling reactions, such as diimine, [6] N‐heterocyclic carbenes, [7] diamines, [8] amines, [9] amides, [10] imidazole, [11] pyridine, [12] bipyridine, [13] 1,10 phenanthroline, [14] Chitosan, [15] and N, O donor site ligands [16] . Among these, the ligands based on nitrogen and oxygen donor sites are unique owing to their convenient synthetic method, high stability in different redox condition and high catalytic activity in catalysis reactions [17] .…”