The Renaissance of Base Metal Catalysis Enabled by Functional Ligands

The replacement of precious metals by base metals in homogeneous catalysis is of great interest due to their high natural abundance and generally lower toxicity. However, in many cases the reactivity of second-and third-row transition metals cannot easily be transferred to first-row metals due to fundamental differences in electronic structure and bonding, which affect key elementary steps of homogeneous catalysis, like oxidative addition or reductive elimination. These formally multielectron redox steps are often less favorable for 3d metal complexes for various reasons, such as a preference for higher spin states resulting in lower metal-ligand bond dissociation energies. This tutorial review covers the recent efforts to make up for these problems and establish first-row metal homogeneous catalysts with “noble metal-like” reactivity by the use of functional ligands. Such ligands enable the replacement of challenging, metal-centered multielectron redox steps, e.g., by ligand-centered redox activity or alternative, redox-neutral ways of substrate activation, such as heterolytic H–E cleavage. These concepts will be discussed in the context of recent milestones, which also emphasize that the alternative mechanistic scenarios do not only allow for use of base metals in homogeneous catalysis but offer unprecedented reactivities and selectivities.