1
NSF/NHMFL Research Experiences for Undergraduates (REU) (RE) ALTADENA Effect on Fluoro-Olefins Ryan Quiñones 1 , Ronghui Zhou 1 , Evan Zhao 1 , Wei Cheng 2 , Luke Neal 2 , Helena E. Hagelin-Weaver 2 , Clifford R. Bowers 1 1 Department of Chemistry, University of Florida, Gainesville, FL 32611 2 Department of Chemical Engineering, University of Florida, Gainesville, FL 32611 Olefin hydrogenation catalysis on a dispersed metal surface continues to serve as an important synthetic reaction. The use of deuterium revealed the intricacies of this deceptively simple reaction, leading to the Horiuti-Polanyi model (Figure 1). Products formed with D 2 yielded saturated alkanes with varying numbers of deuterons, which indicates non-pair-wise addition and proton/deuteron exchange on the metal surface. Alkyl-alkene exchange allows for the sequential, step-wise addition of deuterium on the metal surface for product formation. However, an excessive amount of C 2 H 2 D 2 is found in the reaction products from the hydrogenation of ethene that could not be explained solely by sequential addition. This lead to the proposal of a distinct, direct addition pathway. Special acknowledgements go to Prof. Russ Bowers, for graciously granting me the opportunity to work in his group. Dr. Ronghui Zhao and Mr. Evan Zhao for serving as excellent mentors throughout my REU experience. Prof. Hagelin-Weaver and her group in the Department of Chemical Engineering for their collaborative efforts and contributions. The rest of my friends in the Bowers Group for their warm welcome, and guidance whenever needed. Dr. Dolbier for generously contributing 3,3,3-trifluoropropene gas, hexafluoropropene gas, as well as helpful discussions in handling fluorine. NSF/NHMFL REU Program for this opportunity and their support. Figure 1: Horiuti-Polanyi Mechanism (1) activation of H 2 by oxidative addition on a metal surface; (2) alkene adsorption; (3) hydrogen migration to the β-carbon of the alkene with formation of σ-bond between the metal and α-carbon; (4) reductive elimination of free alkane Background Parahydrogen Induced Polarization (PHIP) NMR Figure 2: Temperature dependence of hydrogen spin states PHIP NMR utilizes the pure singlet nuclear spin order of parahydrogen to result in intense signals via symmetry breaking hydrogenation reactions. PHIP comes in two variations, PASADENA and ALTADENA. The primary requirement for PHIP is the pair-wise addition of parahydrogen, which proves to be crucial in our experiments. Hydrogen gas can be enriched to ~50% parahydrogen by cooling using liquid nitrogen, with temperature dependence shown in Figure 2. S ignal A mplification B y R eversible E xchange (SABRE) Effect ALTADENA allows for the sharing of singlet spin orders in the strong coupling regime found at low magnetic fields, which forms the basis for the SABRE effect. Hydrogenation of the substrate does not occur, resulting in a non-hydrogenative form of ALTADENA that can conceivably hyperpolarize a wide array of substrates using parahydrogen. Traditional SABRE effect has been demonstrated by Duckett et al. using an Ir complex to simultaneously bind both pyridine and H 2 to transfer hyperpolarization to the pyridine nuclei via a sharing mechanism on the complex – effectively functioning as a spin transfer catalyst. Figure 3 : NMR from hydrogenation of propene (PE) and 3,3,3-trifluoropropene (TFPE) Thermally polarized spectra (blue; Figure 2) follows the expected 1 H ratios in both the unreacted propene (PE) substrate and it’s hydrogenation product, propane (PA). 3,3,3-Trifluoropropene (TFPE) exhibits a lower conversion rate, so it’s corresponding thermal spectrum is indiscernible due to signals at or below the level of noise (Figure 2C, D). Attempts to use hexafluoropropene as a substrate ultimately failed due to significantly low reactivity. Temperature dependence of the conversion rate between catalysts is shown in Figure 4A. Calculations were done using spectral peak areas in experiments using normal hydrogen gas (n-H 2 ). Spectra acquired with 50% parahydrogen (p-H 2 ) exhibit intense signals due to the ALTADENA effect. Pure ALTADENA spectra (red; Figure 3A) was obtained by subtracting n-H 2 spectra from p-H 2 spectra. Interestingly, we observe anomalous ALTADENA signals on the alkene substrates, indicating hyperpolarization without undergoing a hydrogenation reaction – identical to the original SABRE effect. GC/MS analysis rules out pair-wise addition to trace amounts of propyne. Full temperature dependence of ALTADENA signals for both substrates and catalysts is shown in Figure 4B. Figure 4: Temperature dependence of (a) conversion and (b) ALTADENA signals Figure 5: Illustration of setup 3 mass flow controllers, controlled by the LabView program, allow for accurate control of reagent gases. Gas flows into reactor tube containing the catalyst, with variable temperature capabilities. Gas then flows into spectrometer for data acquisition. This work is supported by DMR1157490 and ACS-PRF #52258-ND5 C C C C D D C C C C C C or or 1 2 3 4 Introduction Results & Discussion References The evidence provided here indicates a new aspect of the HP mechanism – pair-wise exchange. Producing a net effect identical to the SABRE effect, yet functioning through a distinct chemical exchange process, we dub this phenomenon reversible exchange (RE) ALTADENA. The direct exchange process is emphasized in Figure 6. Figure 6: Modified HP Mechanism Proposed direct exchange pathway circumvents alkyl intermediate. Allows for non-hydrogenative hyperpolarization by p- H 2 (RE) ALTADENA has the potential to significantly broaden the array of substrates that can be hyperpolarized, with applications in medical imaging as well. We also report the first use of fluoro-olefins in the hydrogenative studies of PHIP NMR. The electronegativity of fluorine shifted the chemical shift equilibrium, and allowed for greater observation of the (RE) ALTADENA effect. Acknowledgements 1. Horiuti, I. & Polanyi, M. Exchange reactions of hydrogen on metallic catalysts. T Faraday Soc 30, 1164‐ 1172, doi:Doi 10.1039/Tf9343001164 (1934). 2. Bowers, C. R. & Weitekamp, D. P. Transformation of Symmetrization Order to Nuclear Spin Magnetization by Chemical Reaction and Nuclear Magnetic Resonance. Physical Review Letters 57, 2645‐ 2648, doi:10.1103/ PhysRevLett.57.2645 (1986). 3. Bowers, C. R. & Weitekamp, D. P. Para‐Hydrogen and Synthesis Allow Dramatically Enhanced Nuclear Alignment. Journal of the American Chemical Society 109, 5541‐5542, doi:Doi 10.1021/Ja00252a049 (1987). 4. Chen, J. Y. (2008). Spin Isomers of Molecular Hydrogen and Improved Sensitivity for NMR and MRI, (4), 134– 136. 5. Adams, R. W. et al. Reversible Interactions with para‐Hydrogen Enhance NMR Sensitivity by Polarization Transfer. Science 323, 1708‐1711, doi:DOI 10.1126/science.1168877 (2009). Conclusions Instrumentation
