Carboxylation of the Ni–Me Bond in an Electron-Rich Unsymmetrical PCN Pincer Nickel Complex

Abdelrazek H Mousa,Ola F Wendt,Josefine Hansson,Alexey V Polukeev

doi:10.1021/acs.organomet.9b00817

Abstract

The synthesis of a new unsymmetrical PCN ligand bearing tert-butyl groups on the phosphorus atom and isopropyl groups on the nitrogen donor atom is presented. It reacts with the commercially availa...

Highlights

The tridentate pincer ligand has been shown to be a useful platform for studying the activation of small molecules, in insertion reactions.[1−3] Due to the rigid tridentate chelation of the pincer ligand to the metal center, the thermal stability of complexes with e.g. hydride, hydroxide, and alkyl ligands, which play a crucial role in the activation of small molecules, has been greatly enhanced in comparison to those based on other labile ligands.[1−6] In addition, the fourth ligand is influenced by a strong trans donor, giving it an increased reactivity
We and others have found that complexes based on unsymmetrical pincer ligand scaffolds display unique chemical reactivities in comparison to the symmetrical scaffolds.[30,33,37,39−46] We recently reported the first example of an unsymmetric aromatic precedent with the corresponding (PCN) pincer nickel methyl complex and inter alia studied its reactivity toward CO2.37 this complex showed low reactivity toward CO2 and only 75% conversion was achieved at 150 °C and decomposition was observed during the carboxylation reaction
A slight modification of the PCN ligand scaffold by incorporating more bulky isopropyl groups on the nitrogen donor atom instead of the small methyl groups should enhance the stability of the methyl complex and increase the electron density of the nickel center, which in turn is expected to facilitate the carboxylation reaction

Summary

Introduction

The tridentate pincer ligand has been shown to be a useful platform for studying the activation of small molecules, in insertion reactions.[1−3] Due to the rigid tridentate chelation of the pincer ligand to the metal center, the thermal stability of complexes with e.g. hydride, hydroxide, and alkyl ligands, which play a crucial role in the activation of small molecules, has been greatly enhanced in comparison to those based on other labile ligands.[1−6] In addition, the fourth ligand is influenced by a strong trans donor, giving it an increased reactivity. We report the synthesis of a new PCNi‐Pr pincer nickel methyl complex and show that it undergoes carboxylation with CO2 under substantially milder reaction conditions.

Results

Conclusion