Bond, Q, and controlled cleavage

US chemists have made an iron catalyst that can be used to rapidly break strong carbon-hydrogen bonds within molecules, up to one thousands times faster than other methods. The research could solve one of the great chemical challenges.

Writing in the fledgling journal Nature Chemistry, Larry Que of the University of Minnesota, in Minneapolis, and colleagues, Dong Wang, Erik Farquhar, and Audria Stubna, Eckard Muenck, explain how they have taken inspiration from nature to find a way to control the cleavage of C-H bonds. The process could have major implications for the petrochemical industry where the conversion of petroleum from its constituent compounds into more valuable products often relies on controlled cleavage of hydrocarbons. The team also suggests that modelling nature in this way might also provide enzymologists with new insights into how the active sites in nature's catalysts function.

In nature, methane is oxidized to methanol by a soluble methane monooxygenase enzyme that exploits an oxygen-bridged dinuclear iron intermediate in the rare IV oxidation state. This diiron(IV) complex, an intermediate known as Q, perhaps because of the Bond connection, is highly effective in natural systems. However, chemists have so far failed to come close to its potency in accelerating reactions until now. According to Que, the only two reported diiron(IV) complexes have activities towards C-H bonds that fall far short of the natural catalyst.

Chemists have previously made two models of this diiron(IV) species, but says Que, neither performed very effectively. In contrast, the new compound made by Que and his colleagues has chemical groups, ligands, that grab onto, or chelate, the iron atoms with four nitrogen-prongs and, in a first for iron complexes, can also attack strong oxygen-hydrogen bonds in small alcohol molecules.

The team explains how they have now modelled the chemistry of methane monooxygenase-Q (MMO-Q) we model the chemistry of MMO-Q without the "extraneous" protein scaffolding used in nature. They have generated an oxo-bridged diiron(IV) complex through a process of electrochemical oxidation. "This species is a more effective oxidant," they explain, "It can attack C-H bonds as strong as 100 kilocalories per mole, which are relatively strong bonds. It also reacts with cyclohexane 100 to 1,000 times faster than mononuclear (as opposed to dinuclear) FeIV=O complexes with closely related chemical groups.

Strikingly, this species can also cleave the strong O-H bonds of methanol and tertiary-butyl alcohol," instead of their weaker C-H bonds, representing the first example of O-H bond activation for iron complexes, the team explains. Of course, without nature's helping hand it remains several orders of magnitude less powerful as the MMO enzyme itself. Nevertheless, the research is a step in the right direction towards an artificial version of MMO that can catalyse the conversion of simple hydrocarbons to their oxygenated counterparts.