Throughout history, chemists have developed new reactions to enable the construction of complex molecules. Among these, C–C cross-coupling reactions are particularly important due to their central role in the synthesis of molecular building blocks, pharmaceuticals, and agrochemicals.[1] Air- and moisture-stable palladium pre-catalysts are widely used because of their high selectivity and broad functional group tolerance. However, the increasing cost and demand for noble metals have motivated the search for alternative catalytic systems.[2] Iron, the second most abundant metal in the Earth’s crust, represents an attractive alternative due to its low cost, wide availability, and minimal environmental and biological toxicity.[3] In particular, iron complexes featuring a half-sandwich motif have shown excellent stability.[4] Nevertheless, these systems typically allow the generation of only a single coordination site.
In this work, we focus on a modular ligand design that incorporates a hemilabile donor, capable of reversibly coordinating to the metal centre. One class of ligands consists of 1,2-PPh₂–EPh–C₆H₄ frameworks, in which the phosphine acts as a strong donor and the E atom (O, S, or Se) as a weaker donor. A second class comprises N-heterocyclic carbene (NHC) ligands, where the carbene carbon serves as the strong donor and the E atom as the weaker donor. This design enables the transient generation of an additional coordination site during the catalytic cycle, thereby allowing a “classical” oxidative addition/reductive elimination mechanism analogous to that observed in palladium catalysis.
Herein, we present the synthesis and stability of the first examples of these iron complexes, supported by computational studies that compare the stability of the individual steps along the reaction pathway. In addition, the newly developed complexes and ligands are evaluated in catalytic reactions, in order to assess their influence on alternative iron-based catalytic systems.