Stable CAAC‐Triazenes: A New Nitrogen Ligand System With Donor and Conformational Flexibility, and With Application in Olefin Activation Catalysis

Abstract

AbstractN‐heterocyclic imines such as pyridylidene amines impart high catalytic activity when coordinated to a transition metal, largely imposed by their electronic flexibility. Here, this donor flexibility has been applied for the first time to CAAC‐based systems through the synthesis of CAAC‐triazenes. These new ligands offer a larger π‐conjugation that extends from the N‐heterocyclic carbene through three nitrogens rather than just one, as observed in N‐heterocyclic imines. We demonstrate the straightforward synthesis of three new CAAC‐triazenes containing different substituents on the terminal triazene nitrogen. These compounds are remarkably stable up to 120 °C without loss of N2 as typically observed with similar triazenes. E‐to‐Z isomerization within the triazene is instigated by UV light and is partially reversible dependent on the triazene substituent. The quinoline‐substituted CAAC‐triazene 1‐Q has been applied as an L,L’‐type ligand in the synthesis of [PdCl2(1‐Q)], [PdCl(Me)(1‐Q)] and [Pd(Me)(H2O(1‐Q)]+. E‐to‐Z ligand isomerization also occurs when coordinated to PdCl2, providing access to on‐metal manipulation. The cationic complex [PdMe(H2O)(1‐Q)]+ is a precatalyst for oligomerization of ethylene to form initially 2‐butene and subsequently linear and branched C8−C12 products from butene activation. Moreover, isomerization of 1‐hexene takes place efficiently with exceptionally low catalyst loading (10 ppm) and up to 74,000 turnover numbers.