Binding Forms Previously known side-on (η2) peroxide complex (left) and new end-on (η1) superoxide complex.

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An international collaboration has discovered and characterized an unprecedented mode of binding of oxygen in Pd complexes with N-heterocyclic carbene (NHC) and/or PR 3 (trialkyl- or triarylphosphine) ligands (J. Am. Chem. Soc., DOI: 10.1021/ja1103348).

The work is a fundamental advance in organometallic chemistry. But it could also have practical implications by aiding the development of new oxidation catalysts because Pd(NHC)(PR 3 ) complexes are closely related to homogeneous catalysts for reactions that use molecular oxygen as an oxidant.

In a study of the kinetics and thermodynamics of O 2 binding to Pd centers, Carl D. Hoff of the University of Miami and coworkers isolated and obtained the crystal structure of a new type of Pd(NHC) 2 complex—a bis-superoxide complex with two end-on (η1) O 2 ligands. In such complexes, only one of the two oxygen atoms in each superoxide group is linked to Pd. The researchers also found that this superoxide complex could undergo a hydrogen-atom transfer reaction to form a bis-hydroperoxide complex.

“The idea that a Pd(0) center reacts with two equivalents of O 2 to form a stable bis-superoxide complex is remarkable,” says Shannon S. Stahl, a catalysis expert at the University of Wisconsin, Madison. That Hoff and coworkers “were able to get evidence for this—much less proceed to get a crystal structure and form the bis-hydroperoxide—there’s no way I would have believed this could happen.”

Stahl believes that the bis-superoxide also helps resolve a mechanistic controversy. He and his coworkers earlier prepared a Pd(NHC) 2 side-on (η2) peroxide complex in which both oxygen atoms are bound to Pd and bonded to each other. On the basis of a computational study, they proposed that the side-on peroxide forms via an end-on (η1) intermediate. But this hypothesis conflicts with an alternative proposal that the reaction is a single-step process in which both oxygen atoms bind Pd simultaneously. The discovery of the end-on bis-superoxide provides the first experimental evidence for the end-on-intermediate hypothesis, Stahl says.

Hoff and coworkers find that the size of NHC and PR 3 ligands in the Pd complexes appears to play a key role in determining the nature of oxygen binding. They propose that small ligands result in fast kinetics and the addition of a single O 2 to form a side-on peroxide and that large ligands cause slow kinetics, providing extra time for the addition of both an initial end-on O 2 and a second one, resulting in formation of the new end-on bis-superoxide.

Team member and inorganic chemistry professor Steven P. Nolan of the University of St. Andrews, in Scotland, says that the observation of novel end-on peroxide and hydroperoxide binding modes “may assist in designing ligand environments better suited for palladium-mediated oxidation catalysis.”