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TAML Technology Overview

Catalyst Design Strategies

In producing a catalyst for activating hydrogen peroxide, for most purposes one must avoid so-called "Fenton Chemistry"—the chemistry that occurs when metal ions such as ferrous or ferric in water interact with peroxide. This chemistry is characterized by the liberation of oxygen accompanied by the production of a flux of hydroxyl radicals. The hydroxyl radical is a potent H-atom abstractor because the O-H bond of water is particularly strong, 119.6 kcal.mol-1, stronger than almost all C-H bonds. Thus, Fenton chemistry tends to ravage organic matter and is not selective enough to be useful for most reaction chemistry applications.

One wants instead for a metal ion to produce another type of reactive intermediate. One such intermediate could be a metal-oxo complex. Another could be a peroxo complex of which there a number of variations. The oxo complex arises when the metal ion abstracts an oxygen atom from hydrogen peroxide and discards water making a two-electron oxidized metal-oxo species. Such chemistry is found in the cytochrome P-450 enzymes. We think that to induce this reaction manifold over Fenton chemistry one must bind to the metal ion electron releasing ligands that favor the two-electron oxidation at the metal that accompanies metal-oxo formation.

The ligand system employed must also be resistant to oxidation. If the metal-oxo complex oxidatively degrades its own ligand system, then the metal ion will be released from its special electronic environment and Fenton chemistry might return with all its lack of selectivity. For toxicity reasons, one really wants to employ iron which is common in the Earth's crust and is used by Nature for many redox catalysts. However, because iron readily produces Fenton chemistry, the ligand system employed must be resistant to oxidative degradation for a prolonged period if a useful catalyst is going to be achieved. We have followed the iterative design strategy for two decades to achieve viable and improved hydrogen peroxide activators.

The iterative design strategy proceeds as follows.

  1. One starts with a polydentate ligand thought to be suitable and a metal is coordinated in a low valent state.
  2. The complex is chemically or electrochemically oxidized by one or more electrons in an inert medium until the oxidized complex decomposes.
  3. The degraded system is carefully studied. Complete mass balance is sought to assist in understanding the fate of the ligand and the metal. This process identifies the sensitive group on the ligand where the oxidative degradation process started.
  4. The vulnerable group is replaced with a substitute thought to be less sensitive and the cycle is repeated.

By following this design loop we have developed a series of catalysts with different lifetimes, from seconds to hours, and different activities.