Lewis Acid-Base Pair Assists Enzyme Mimic

In the search for better catalysts, chemists often try to emulate an enzyme’s ability to efficiently bind, activate, and functionalize a reactant molecule. In the latest twist, a University of Michigan research team has designed a transition-metal complex featuring a ligand with functional appendages that mimic the secondary coordination sphere of a metalloenzyme’s binding pocket. Oscar Tutusaus, Chengbao Ni, and Nathaniel K. Szymczak synthesized a vanadium terpyridine complex with one pyridine ring bearing a boronic ester Lewis acid and another pyridine ring bearing a morpholine Lewis base (J. Am. Chem. Soc., DOI: 10.1021/ja400962h). Their design is reminiscent of so-called frustrated Lewis pair systems, which have pent-up catalytic reactivity because of the inability of the acid-base pair, held apart by bulky substituents, to completely combine to form a neutral adduct. The Michigan researchers tested their new complex through its reaction with hydrazine (N₂H₄). They found that the nitrogen atom of the Lewis base and the boron atom of the Lewis acid help hold hydrazine in place for vanadium to pull apart the N–N bond (shown), with the Lewis pair acting in the same way that peripheral amino acids in an enzyme’s binding site help hold a substrate. Chemically reducing vanadium slices the N–N bond to form ammonia, a key action of nitrogenase enzymes and an important intermediate step in the industrial process for making ammonium-based fertilizers. The synergistic transition-metal/Lewis pair interaction could become a general methodology for developing new catalysts, Szymczak says.