PI: Gregory S. Boebinger, Director National High Magnetic Field Laboratory Supported by NSF (No. DMR ), and State of Florida Figure: The High frequency EPR spectra of [Pd 2 (DAniF) 4 ]+ Note the increased resolving power of higher magnetic fields, which requires higher frequency spectroscopy. J. F. Berry, E. Bill, E. Bothe, F. A. Cotton, N. S. Dalal, S. A. Ibragimov, N. Kaur, C. Y. Liu, C. A. Murillo, S. Nellutla, J. M. North and D. Villagr, Journal of the American Chemical Society 129, 2007, Finding Unpaired Electrons in Palladium Paddle-wheel s= ½ complex Palladium compounds are of fundamental and industrial interest for applications in catalysis and hydrogen-storage. Of the known palladium “paddle wheel” compounds (top left), only two contain paramagnetic (Pd 2 ) +5 (s = ½) cores (top right). Conflicting reports regarding the location of the unpaired electron hamper our understanding of the mechanisms underlying the behavior and functionality of the complex. Previously-published Electron Paramagnetic Resonance (EPR) at 9.6 GHz, the highest commercially available frequency, does not have enough spectral resolution to identify the molecular orbital that contains the unpaired electron. Increasing magnetic field pushes the EPR resonance to higher frequency: 34 GHz to 106 GHz to 211 GHz. Only then are the g-tensor components resolved. The measured g-tensor spread of ~0.03 implies that the unpaired electron is on the centrally-located metal-based molecular orbital, in contrast to the earlier low-field results that suggested the orbital to be localized on the outlying ligand. This information will assist in developing a first understanding of catalysis at the molecular level. 2007

Palladium is one of the most important metals for use as a catalyst to speed up chemical reactions. High magnetic fields enable scientists to directly measure the location of the electron responsible for the desired catalytic behavior. PI: Gregory S. Boebinger, Director National High Magnetic Field Laboratory Supported by NSF (No. DMR ), and State of Florida A wide variety of palladium molecules, including the “paddle wheel” compound above, are being studied for use as new catalysts. Catalysts find extensive industrial use for their ability to nucleate and speed up chemical reactions. Although X-rays can determine the location of all the atoms in a molecule, catalysis depends instead on the location of the electrons, in particular the location of the unpaired (hence chemically active) electron. Electron paramagnetic resonance (EPR), the flipping of an electron’s intrinsic magnetic field, is the technique of choice for settling whether the unpaired electron is spread over the central metallic core or the peripheral parts of the molecule (the ‘ligand’). The resolution of commercially available spectrometers is often not adequate, in which case high magnetic fields and unique ultra-high frequency spectrometers (right) are required to locate the unpaired electron. These quantitative studies of complex palladium molecules are opening a new window on an exciting frontier of modern chemistry: the understanding of catalysis on a molecular level. Finding Unpaired Electrons in Palladium 2007