The Influence of Alkali and Alkaline Earth Metals on Transition-metal Catalyzed Reactions
Alkali and alkaline earth metals are useful co-catalysts in many inorganic and organometallic reactions. The interactions of these metals to transition-metal complexes modulate the overall structures. The alkali and alkaline earth metal ions withdraw electron density from transition metal center and tune the reduction potentials. The stabilization of anion intermediates by alkali and alkaline earth metal ions reduces transition-state energy and enhance the reactivity kinetically. In homogeneous catalysis, alkali and alkaline earth metals have been shown to bind to electron-rich anionic ligands that then stabilize the formation of heterobimetallic complexes. This is especially important when key transition states are stabilized by such coordination as this can lead to greatly enhanced catalytic efficiency. These metal ions can also collaboratively bind to crown-ether installed ligands and control metal-ligand cooperativity. In addition, reaction selectivity could also be enhanced by using different sizes of alkali or alkaline earth metal ions. Thus, incorporation of alkali and alkaline earth metals to transition-metal catalysts can modulate or completely change the reactivity.