Palladium (II)–Salan Complexes as Catalysts for Suzuki–Miyaura C–C Cross-Coupling in Water and Air. Effect of the Various Bridging Units within the Diamine Moieties on the Catalytic Performance

Abstract

Water-soluble salan ligands were synthesized by hydrogenation and subsequent sulfonation of salens (N,N’-bis(slicylidene)ethylenediamine and analogues) with various bridging units (linkers) connecting the nitrogen atoms. Pd (II) complexes were obtained in reactions of sulfosalans and [PdCl4]2−. Characterization of the ligands and complexes included extensive X-ray diffraction studies, too. The Pd (II) complexes proved highly active catalysts of the Suzuki–Miyaura reaction of aryl halides and arylboronic acid derivatives at 80 °C in water and air. A comparative study of the Pd (II)–sulfosalan catalysts showed that the catalytic activity largely increased with increasing linker length and with increasing steric congestion around the N donor atoms of the ligands; the highest specific activity was 40,000 (mol substrate) (mol catalyst × h)−1. The substrate scope was explored with the use of the two most active catalysts, containing 1,4-butylene and 1,2-diphenylethylene linkers, respectively.

Copper(II) Complexes of Sulfonated Salan Ligands: Thermodynamic and Spectroscopic Features and Applications for Catalysis of the Henry Reaction

“…Variation of the linker group between the secondary nitrogen atoms in the salan molecules may give the possibility of fine-tuning the electronic properties of the complexed metal ion and also the steric characteristics of the metal complex with obvious consequences on the physical (e.g., spectroscopic) and chemical (e.g., catalytic) behavior of the molecule. With that aim in mind, a few studies of salan complexes included also variation of the bridging unit. − Among the metal ions not previously reviewed, copper­(II) is particularly useful for investigation of the formation and electronic and steric properties of its salan complexes because, in addition to the usual spectroscopic methods [UV–vis, NMR, mass spectrometry (MS)], such complexes can also be probed by electron paramagnetic resonance (EPR) and its related hyperfine techniques, a sensitive tool to study paramagnetic systems.…”

“…In our laboratories, we have prepared a series of sulfosalans with various bridging units between the secondary nitrogen atoms. ,,, Complex formation equilibria of some of these ligands were studied with palladium­(II) , and nickel­(II); however, most of our investigations were directed toward the catalytic properties of Pd II -salan complexes in C–C cross-coupling reactions in aqueous reaction mixtures. ,, It was assumed that knowledge of the distribution of various complex species as a function of the pH could help in finding the optimal reaction conditions and might be instructive for the use of similar complexes of earth-abundant (instead of precious) metals in homogeneous catalysis. Indeed, we succeeded in developing the first process of hydrogenation and redox isomerization of allylic alcohols catalyzed by a nickel­(II) complex with an HSS-type ligand .…”

“…Indeed, we succeeded in developing the first process of hydrogenation and redox isomerization of allylic alcohols catalyzed by a nickel­(II) complex with an HSS-type ligand . In another study, palladium­(II) complexes formed with an extensive series of sulfosalan ligands with various bridging units were applied for the Suzuki–Miyaura cross-coupling reaction of aryl halides with arylboronic acid derivatives. , …”

Nickel(II)-Based Building Blocks with Schiff Base Derivatives: Experimental Insights and DFT Calculations

“…Although the tetradentate chelating Schiff base ligands with the dianionic N 2 O 2 donor set, known as salen-like ligands [29,30], and their corresponding complexes continue to be the most popular and the most studied for their numerous applications (sensing [31], medicine [32,33], catalysis [14,32,34], as synthons for various supramolecular structures [35][36][37]), Schiff base ligand with lower denticity, namely tridentate, are also common [30,38]. Tridentate ligands offer three donor atoms (mostly O, N and S) and act as anionic pincer-type ligand to efficiently chelate the metal ions, thus generating versatile conformationally rigid [M(L A/D )] n+type metalloligands, where L A/D represents electron withdrawing (A) or electron donating (D) tridentate Schiff base ligands.…”

Advances in Cross-Coupling Reactions

“…The catalytic activity of palladium(II)-salan complexes in Suzuki-Miyaura cross-coupling reactions is studied by Udvardy, Joó et al as an alternative to classical phosphines, showing that salan ligands can be used in water and air to perform cross-coupling reactions [36]. In summary, cross-coupling reactions constitute one of the most relevant methods in modern organic chemistry and have allowed many new transformations in this science.…”