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One of the defining characteristics of complexes of the transition metals is their ability to exist in multiple oxidation states.  This flexibility underlies the importance of these compounds in mediating the chemical transformations that occur in the processing and use of petroleum-derived feedstocks (e.g., hydrogenation, partial oxidation), as well as those involved in the redox chemistry associated with alternative energy use (e.g., as electrocatalysts in fuel cells). 

Traditionally, the ligands bound to the transition metals have been employed to modulate the electronic and steric properties of the metals, but not to play a direct role in oxidation state changes.  It has, however, been acknowledged for some time that certain classes of ligands are "non-innocent" with regards to redox chemistry.  In other words, these particular ligands may undergo either ligand-localized changes in oxidation state, or may be so electronically intertwined with the metal center that any redox changes are delocalized between metal and ligand. 

While the existence of non-innocent ligands has been appreciated for some time, the area has been undergoing a renaissance in recent years as chemists have discovered that redox participation of ligands can lead to new modes of reactivity.  This symposium aims to bring together a diverse array of scientists in an effort to weave together the diverse strands of thought in the field.  In addition to presentations focusing on spectroscopic, structural, and theoretical investigations that play a critical role in elucidating the complex electronic structures of these species, the symposium will highlight the complexes' reactivity that makes use of these unusual electronic structures. 

The modes of reactivity explored by the chemists that presented in the symposium were heavily focused on energy production and petrochemical use (perhaps unsurprisingly given the importance of redox chemistry in these areas).  One session of the symposium was devoted to dioxygen activation, an essential step in any use of chemical fuels, and one for which improvements in efficiency are widely viewed as critical in improving the efficiency of fuel cells or in the production of fuels from solar-generated electricity.  The interactions of metal-dithiolene or -thiolate complexes with alkenes—where ligand-substrate, rather than metal-substrate, bond formation takes place—was also a featured topic.  Additional threads included the use of ligand redox chemistry as a mode of regulating materials properties or chemical reactivity, of storing redox equivalents in a sort of "molecular battery", and of using ligand-centered redox chemistry to extend reactivity with hydrocarbons and alternative fuels such as methanol to redox-inert metal centers (turning the usual mode of inorganic reactivity neatly on its head).

The non-North American speakers for travel support was provided played a key role in advancing the symposium's goals of cross-fertilization.  Karl Wieghardt and Wolfgang Kaim have been pioneers in the synthesis and structural and spectroscopic characterization of metal complexes of redox-active ligands.  Pierre Braunstein brought a perspective on the integration of the physical properties of redox-active ligands with reactivity studies such as olefin polymerization.  Finally, Frank Neese presented on utilizing the tools of modern computational chemistry to understand the electronic structure problems of redox-active ligand complexes.  The participation of these scholars in the discussions in the symposium, in addition to their presentations themselves, will contribute greatly to the intellectual fruitfulness of the symposium.

Program Summary.  The full roster of speakers at the symposium, with their affiliations and presentation titles, is given below.  Speakers for whom PRF support was provided are marked with an asterisk.

Session 1:  Quinonoid and related ligands

Karl Wieghardt (Max Planck Institut MŸlheim), "A new class of redox-non innocent ligands:  1,2-diketones and their 1,2-diketonate(1–) radical complexes"

*Wolfgang Kaim (UniversitŠt Stuttgart), "Competition between different non-innocent ligands in metal complexes"

Alan Heyduk (University of California, Irvine), "Harnessing amidophenolate ligands for multi-electron small molecule activation at d0 metals"

*Pierre Braunstein (UniversitŽ Louis Pasteur, Strasbourg), "Quinonoid ligands and their metal complexes"

A. B. P. Lever (York University), "o-Benzoquinonediimine: an exceptionally non-innocent ligand"

Session 2:  Nitrogen ligands

Dan Nocera (MIT), "Multi-electron mixed-valence of tetrapyrrole metal complexes"

Paul Chirik (Cornell), "Bis(imino)pyridine iron imide and alkylidene chemistry:  Guilty as charged"

Tom Vaid (Washington University), "Porphyrins and phthalocyanines as non-innocent ligands in main-group metal complexes"

Ken Caulton (Indiana University), "Redox chemistry of the amide ligand-to-metal bond in (tBu2PCH2SiMe2)2N- metal complexes"

Robin Hicks (University of Victoria), "Synthesis, coordination chemistry, and properties of new redox-active bridging ligands based on indigo"

Session 3:  Oxygen activation

Cortlandt Pierpont (University of Colorado), "Dopamine complexes of iron as sources of 'reactive oxygen species' associated with Parkinson's disease"

Jon Wilker (Purdue University), "A redox-active ligand in the ocean:  DOPA and iron in marine mussel adhesive"

Etsuko Fujita (Brookhaven), "Water oxidation catalysts: Ruthenium complexes with non-innocent quinone ligands"

Jake Soper (Georgia Tech), "Multielectron redox catalysis cycles based on oxygen activation by Group 7 metals with non-innocent ligands"

Mahdi Abu-Omar (Purdue University), "Dioxygen activation on zirconium(IV) by confiscating electrons from the ligand"

Session 4:  Sulfur ligands

Marcetta Darensbourg (Texas A&M University), "Redox Active Ligands as Related to the Active Site of [FeFe]-Hydrogenase"

Michael Jensen (Ohio University), "Oxidation of Ni(II) in N3S2 ligand fields inspired by nickel superoxide dismutase"

Kun Wang (Exxon-Mobil), "Teaching an old dog new tricks: Novel reactivity of nickel dithiolenes with olefins"

Ulrich Fekl (University of Toronto), "How redox-active ligands in dithiolene complexes lead to ligand-based bond-forming reactions"

Craig Grapperhaus (University of Louisville), "One way or reversible: Alkene addition to metal-coordinated thiyl radicals"

Session 5:  Aryloxide ligands

*Frank Neese (UniversitŠt Bonn), "Theoretical approaches to metal-radical systems"

Tom Rauchfuss (University of Illinois), "Effects of non-innocent ligands on organometallic transformations"

Dan Stack (Stanford University), "Investigation of the locus of oxidation and spectroscopic features of oxidized copper and nickel bis-phenoxide complexes"

Claudio Verani (Wayne State University), "Transition metal complexes of redox-active ligands as thin film precursors for molecular electronics"

Seth Brown (University of Notre Dame), "Modulating the interaction between substrates and redox-active ligands using titanium-oxygen bonding"