Hybrid NCS palladium pincer complexes of thiophosphorylated benzaldimines and their ketimine analogs

Thiazole-based non-symmetric NNC–palladium pincer complexes as catalytic precursors for the Suzuki–Miyaura C–C coupling

New non-symmetric pincer palladacycles, containing [N,N,C] tridentate ligands are synthesized and their catalytic activities in the Suzuki–Miyaura cross coupling reaction are tested using water under aerobic conditions and IR irradiation.

A synthetic, catalytic and theoretical investigation of an unsymmetrical SCN pincer palladacycle

The SCN ligand 2-{3-[(methylsulfanyl)methyl]phenyl}pyridine, 1, has been synthesized starting from an initial Suzuki-Miyaura (SM) coupling between 3-((hydroxymethyl)phenyl)boronic acid and 2-bromopyridine. The C-H activation of 1 with in situ formed Pd(MeCN)4(BF4)2 has been studied and leads to a mixture of palladacycles, which were characterized by X-ray crystallography. The monomeric palladacycle LPdCl 6, where L-H = 1, has been synthesized, and tested in SM couplings of aryl bromides, where it showed moderate activity. Density functional theory and the atoms in molecules (AIM) method have been used to investigate the formation and bonding of 6, revealing a difference in the nature of the Pd-S and Pd-N bonds. It was found that S-coordination to the metal in the rate determining C-H bond activation step leads to better stabilization of the Pd(II) centre (by 13-28 kJ mol(-1)) than with N-coordination. This is attributed to the electron donating ability of the donor atoms determined by Bader charges. The AIM analysis also revealed that the Pd-N bonds are stronger than the Pd-S bonds influencing the stability of key intermediates in the palladacycle formation reaction pathway.

Non-symmetric pincer ligands: complexes and applications in catalysis

Pincer ligands have become ubiquitous in organometallic chemistry and homogeneous catalysis. Recently, new varieties of pincer ligands with non-symmetrical backbones and/or ligating groups have been reported and their application in transition metal complexes has been exploited in a variety of catalytic transformations. This non-symmetric approach vastly increases the structural and electronic diversity of this class of ligand. This approach has proven beneficial in a variety of ways, such as the use of a single weakly coordinating moiety, which can dissociate and thereby create a vacant coordination site to increase the catalyst activity. Additionally, this provides further access to chiral ligands and complexes for asymmetric induction. This perspective highlights recent, important examples of non-symmetric pincer ligands, which feature aryl or pyridine backbones, and the synthesis and use of subsequent complexes in catalytic transformations, and discusses the future potential of this type of ligand system.

The nature of the bonding in symmetrical pincer palladacycles

The accuracy of DFT-optimised geometries of the symmetrical pincer palladacycles PdNCN and PdSCS, [ClPd{2,6-(Me2NCH2)2C6H3}] and [ClPd{2,6-(MeSCH2)2C6H3}] respectively, has been evaluated by investigating the performance of eight commonly used density functionals with four combinations of basis set, in reproducing their X-ray crystal structures. It was found that whilst the ωB97XD functional performed best over all, the PBE and TPSS functionals performed best when considering the palladium coordination geometry. The role of the donor atom in the stability and reactivity of the symmetric palladacycles, PdYCY, Y = N, S, or P, has been determined using Bader's Atoms in Molecules method to elucidate the nature of the bonding, and using a model formation reaction, which involves the C-H activation of the pincer ligand YCY by PdCl2. The calculations reveal distinct differences in the bond strength and nature of the interaction of Pd with the donor atoms Y, which support differences in the thermodynamic stability of the palladacycles.

Solid-phase cyclopalladation in S,C,S′-pincer systems: rising alternative for synthesis in solution

Cyclopalladation of pincer ligands can proceed efficiently in the solid state, serving as a powerful alternative for synthesis in solution.