Development of a PNN Pincer Ligand and Its Iron and Cobalt Complexes for Selective Catalytic Amide Synthesis

Abstract

The formation of amide bonds from carboxylic acids and amines has been the commercially available route for decades, enabling the synthesis of various compounds used in industries ranging from pharmaceuticals to polyurethanes. However, these conventional methods of synthesis have significant sustainability challenges, as they generate copious amounts of waste, produce harmful by-products, and rely on the use of toxic chemicals. Given the increasing demand for amide-containing compounds and the environmental drawbacks associated with traditional synthesis routes, researchers have been prompted to explore more sustainable approaches to amide bond formation, aligning with the principles of Green Chemistry. [1], [2], [3]

In this paper, we report the design and synthesis of cobalt (II) and iron (II) complexes with a new type of PNN pincer ligand to facilitate selective amide formation during an alcohol and amine coupling reaction. We propose two new pincer ligands, PhPNHN and iPrPNHN, that are designed to allow for modes of metal-ligand cooperativity (MLC) in order to expand on the versatility of their chemical functions. The metal complexes provide an opportunity for a fundamental understanding of the key mechanistic steps in selective amide formation in addition to structure-reactivity relationships to improve future catalyst design. With the support of single crystal X-ray crystallography, 31P NMR, 1H NMR, elemental analysis, and high-resolution mass spectrometry we report the successful synthesis of a new type of PhPNHN pincer ligand and two new organometallic complexes.