Abstract
A library of unsymmetrical SCN pincer palladacycles, [ClPd{2-pyr-6-(RSCH2)C6H3}], R = Et, Pr, Ph, p-MePh, and p-MeOPh, pyr = pyridine, has been synthesized via C–H bond activation, and used, along with PCN and N’CN unsymmetrical pincer palladacycles previously synthesized by the authors, to determine the extent to which the trans influence is exhibited in unsymmetrical pincer palladacycles. The trans influence is quantified by analysis of structural changes in the X-ray crystal and density functional theory (DFT) optimized structures and a topological analysis of the electron density using quantum theory of atoms in molecules (QTAIM) to determine the strength of the Pd-donor atom interaction. It is found that the trans influence is controlled by the nature of the donor atom and although the substituents on the donor-ligand affect the Pd-donor atom interaction through the varied electronic and steric constraints, they do not influence the bonding of the ligand trans to it. The data indicate that the strength of the trans influence is P > S > N. Furthermore, the synthetic route to the family of SCN pincer palladacycles presented demonstrates the potential of late stage derivitization for the effective synthesis of ligands towards unsymmetrical pincer palladacycles.
Highlights
IntroductionPalladacycles have been extensively studied since their discovery in 1965 by Cope and Siekman [1]
Palladacycles have been extensively studied since their discovery in 1965 by Cope and Siekman [1].They have been widely used as catalysts or pre-catalysts in organic reactions, such as in Heck and Suzuki–Miyaura cross-couplings [2,3,4,5]
It has been shown that the trans influence plays a key role in the stability of unsymmetrical pincer palladacycles, with the bond strength, and the bond length of the Pd-donor atom interaction affected significantly when trans to a ligand exhibiting a strong trans influence
Summary
Palladacycles have been extensively studied since their discovery in 1965 by Cope and Siekman [1]. The pincer palladacycle structures areTheir stabilized by an an intramolecular coordination to the theby metal of the two donor atoms in the side arms. The attractive feature of pincer palladacycles is the possibility for fine-tuning the the catalytic catalytic activity activity by by varying varying the the two two side arms to modify the palladium environment, fine-tuning side arms arms to to modify modify the the palladium palladium environment, environment, fine-tuning the catalytic activity bytheir varying the two side by changing the donor atoms and substituents, providing the opportunity to alter hard/soft acid by changing the donor atoms and their substituents, providing the opportunity to alter hard/soft acid by changing the or donor atoms and their substituents, providing the steric opportunity to alter hard/soft acid base properties, by changing the ring size, giving rise to varying hindrance [12]. We have investigated the trans influence in both model and experimentallyexperimentally-characterized unsymmetrical pincer palladacycles, using DFT calculations and characterized unsymmetrical pincer palladacycles, using DFT calculations and quantum theory of quantum theory of atoms in molecules (QTAIM) analysis.
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