Authors: Emily Y. Tsui, Dr. Michael W. Day, Prof. Theodor Agapie
Journal: Angewandte Chemie International Edition
Affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology
Cartoon of the Trinuclear Copper Complex: The nitrogen donors are 2-pyridyl moities. Note: Variable capping ligand which coordinates to all three coppers is not pictured.
Many energy relevant chemical transformations of current interest, including the reduction of N2, O2, H+, and CO2 or the oxidation H2O and CH4, require between two and eight electrons. Traditional transition metal catalysts capable of performing multi-electron processes on a single metal atom, such as palladium (Pd) and Rhodium (Rh), are rare and therefore expensive. In most cases, cheaper alternatives, namely the first row transition metals such as iron (Fe) and cobalt (Co) primarily react by transitioning between one-electron oxidations and reductions. Nature solves this dilemma of needing to perform multi-electron redox reactions and only having access to earth-abundant transition metals able to donate or accept one electron by placing multiple metal centers in close proximity, allowing for multi-metal cooperation.
Drawing inspiration from nature, the authors of this paper attempt to synthesize coordination complexes that contain multiple first-row transition metals with the hope that they will be able to perform multi-electron processes without use of expensive metals. The eventual goal is to identify active, inexpensive catalysts which will lower the cost of fuel cells, clean power storage, and clean fuel production.
This publication focuses on the synthesis of a multi-dentate ligand capable of supporting simultaneous coordination to three copper (Cu) atoms. The synthesis of the ligand is elegant and concise, starting from an inexpensive 2-bromoacetophenone. In the presence of base for deprotonating the three oxygen (O) binding sites, Cu(II) could be inserted into the ligand framework using a few different starting materials. When a phosphate (PO43-) salt of Cu(II) was used, the isolated complex contained one phosphate trianion coordinated to each of the Cu atoms through an O. When Cu(II) triflate (OTf) was used, one triflate anion coordinates to all three Cu atoms in the same fashion as the phosphate, while the others remain uncoordinated. When a Cu(I) source was added to the ligand, each Cu(I) atom coordinated only to two pyridyl nitrogen, while the alcohol remained protonated.
Reaction of the Cu(I) containing ligand with excess O2 in the presence of a base converted the complex to the Cu(II) complex, demonstrating its ability to act as a source of three electrons. The identity of the product of oxygen reduction could not be determined, but no H2O2 was observed. The Cu(II) complex could be converted back into the Cu(I) complex with a chemical reductant. Reaction of the Cu(II)OTf complex with tetrabutylammonium bromide or iodide switched out the OTf for a halide, bridging all three Cu atoms.
Because each Cu(II) has one unpaired electron, the 3Cu(II) complex is expected to have a spin of 3/2. Due to antiferromagnetic coupling however, two of the spins pair and the complex has a spin of 1/2. Through the combination of magnetic analysis and comparison to reported 3Cu(II) complexes with bridging oxides and halides, the authors were able to determine that the Cu centers “communicate” through the bridging alkoxides.
Future work will focus on determining the products of O2 reduction, the effect of switching the metals for other first row transition metals, and other small molecules with which the complexes can react.
Edit 3/2/2011: Another paper from the Agapie group was just published on the stucture and properties of other first row transition metals complexed by this ligand. http://pubs.rsc.org/en/Content/ArticleLanding/2011/CC/C0CC05608A