Computational Investigations of the Electronic Structure and Properties of Bimetallic Biomimetic Complexes

Abstract

Enzymes have evolved to perform specific functions with high efficiency and selectivity. It is estimated that approximately a quarter to a third of all proteins contain an essential metallic cofactor that enables catalysis of reactions that would be difficult to perform otherwise. Certain classes of enzymes have evolved to incorporate multiple metals within their active sites to control their reactivity such as iron-iron hydrogenase, nickel-iron hydrogenase, and carbon monoxide dehydrogenase. The multiple metal centers can help position substrates, fine tune the potential for electrochemical transformations, or allow for more complicated chemical transformations than would be possible with a single metal. Density functional theory can expand upon experimental models of these enzymes by calculating potential intermediates and investigating possible mechanisms where experimental methods are insufficient or too impractical.

Computational investigations of a series of nickel-iron hydrogenases give insight into the increased oxygen tolerance of the selenium variant of the [NiFe]-Hydrogenase. Selenium forms a weaker terminal bond to oxygen disfavoring the formation of doubly oxygenated species and instead preferentially forming a singly oxygenated complex. This singly oxygenated complex is then capable of repairing the oxygen damage through a more accessible reduction process.

In another study, a dimanganese complex’s assembly in solution was shown to proceed through a series of intramolecular rearrangements promoting loss of carbon monoxide ligands. The exchange of labeled ¹³CO was found to go through an associative interchange mechanism where an incoming CO breaks the bond between the manganese center and either a sulfur or amidic oxygen ligand followed by scrambling through a trigonal prismatic transition state.

Finally, the magnetic properties an iron-nitrosyl/nickel-dithiolene with four isolated radicals were interpreted by calculations to possess strong antiferromagnetic coupling of the central two radicals and weak ferromagnetic coupling between the two outer ones leading to a narrow gap between the ground state singlet and a low-lying triplet excited state. The electronic structure of this system was further described by using a linear H4 model and full configuration interaction calculations to model the additional excited states.