The nature of the bonding in symmetrical pincer palladacycles

Sarote Boonseng,John Spencer,Gavin W Roffe,Hazel Cox

doi:10.1039/c5dt00031a

Sarote Boonseng, John Spencer + Show 2 more

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https://doi.org/10.1039/c5dt00031a

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Abstract

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.

Highlights

Palladacycles, where a Pd–C bond is intramolecularly stabilised by donor atoms typically from a sulfur, nitrogen or phosphorus donor, are an interesting class of compound
Pincer complexes are a type of palladacycle whereby two intramolecular donor atoms chelate to the palladium along with the Pd–C bond
These pincers can be symmetrical with identical side arms, such as PdNCN (e.g. [ClPd{2,6-(Me2NCH2)2C6H3}]) and PdSCS (e.g. [ClPd{2,6-(MeSCH2)2C6H3}] examples (Fig. 1), or more seldom, unsymmetric with different donor atoms or groups such as SCN or PCN types.[7,8]

Summary

Introduction

Palladacycles, where a Pd–C bond is intramolecularly stabilised by donor atoms typically from a sulfur, nitrogen or phosphorus donor, are an interesting class of compound The discovery that they could display extremely high catalytic activity in Suzuki–Miyaura coupling and Heck olefination by Herrmann and Beller et al.[1,2] developed the field into a vibrant research area, with several reviews showing the recent developments.[3,4,5,6] Pincer complexes are a type of palladacycle whereby two intramolecular donor atoms chelate to the palladium along with the Pd–C bond. Determining the nature of the bonding in symmetrical pincer palladacycles is a vital first step to understanding the bonding and reactivity of unsymmetric pincer palladacycles

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