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Edited by Michael D. Fayer, Stanford University, Stanford, CA, and approved October 29, 2018 (received for review May 31, 2018)
Significance
We demonstrate that the transient intermediate structure, which is too fast for traditional techniques, e.g. NMR or X-ray diffraction, to resolve in a thermally driven reaction can be determined in situ with the vibrational cross-angle method. The principle of the method illustrated in the work is general for other organic reactions and the method is expected to bring profound impacts for the studies of chemical reactions. The reaction mechanism of a well-defined catalytic system with performance to generate H2 from formic acid is elucidated.
Abstract
The molecular structure of a catalytically active key intermediate is determined in solution by employing 2D IR spectroscopy measuring vibrational cross-angles. The formate intermediate (2) in the formic acid dehydrogenation reaction catalyzed by a phosphorus–nitrogen PN3P–Ru catalyst is elucidated. Our spectroscopic studies show that the complex features a formate ion directly attached to the Ru center as a ligand, and a proton added to the imine arm of the dearomatized PN3P* ligand. During the catalytic process, the imine arms are not only reversibly protonated and deprotonated, but also interacting with the protic substrate molecules, effectively serving as the local proton buffer to offer remarkable stability with a turnover number (TON) over one million.
Footnotes
↵1Y.Z. and X.C. contributed equally to this work.
↵2Present address: Beijing National Laboratory for Condensed Matter Physics, CAS Key Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences, 100190 Beijing, China.
- ↵3To whom correspondence may be addressed. Email: hkw{at}kaust.edu.sa or junrong{at}pku.edu.cn.
Author contributions: Y.Z., K.-W.H., and J.Z. designed research; Y.Z., X.G., H.C., and J.Z. performed research; B.Z., Y.P., H.L., S.M., C.G., and K.-W.H. contributed new reagents/analytic tools; Y.Z., X.C., H.C., K.-W.H., and J.Z. analyzed data; and Y.Z., X.C., K.-W.H., and J.Z. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1809342115/-/DCSupplemental.
Published under the PNAS license.
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Structural analysis of transient reaction intermediate in formic acid dehydrogenation catalysis using two-dimensional IR spectroscopy
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