Analysis of chemical exchange in iridium N-heterocyclic carbene complexes using heteronuclear parahydrogen-enhanced NMR

The signal amplification by reversible exchange process (SABRE) enhances NMR signals by unlocking hidden polarization in parahydrogen through interactions with to-be-hyperpolarized substrate molecules when both are transiently bound to an Ir-based organometallic catalyst. Recent efforts focus on optimizing polarization transfer from parahydrogen-derived hydride ligands to the substrate in SABRE. However, this requires quantitative information on ligand exchange rates, which common NMR techniques struggle to provide. Here, we introduce an experimental spin order transfer sequence, with readout occurring at <sup>15</sup>N nuclei directly interacting with the catalyst. Enhanced <sup>15</sup>N NMR signals overcome sensitivity challenges, encoding substrate dissociation rates. This methodology enables robust data fitting to ligand exchange models, yielding substrate dissociation rate constants with higher precision than classical 1D and 2D <sup>1</sup>H NMR approaches. This refinement improves the accuracy of key activation enthalpy ΔH<sup>‡</sup> and entropy ΔS<sup>‡</sup> estimates. Furthermore, the higher chemical shift dispersion provided by enhanced <sup>15</sup>N NMR reveals the kinetics of substrate dissociation for acetonitrile and metronidazole, previously inaccessible via <sup>1</sup>H NMR due to small chemical shift differences between free and Ir-bound substrates. The presented approach can be successfully applied not only to isotopically enriched substrates but also to compounds with natural abundance of the to-be-hyperpolarized heteronuclei.

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Publication Details

Journal
Communications Chemistry
Published
2024-12-03
DOI
https://doi.org/10.1038/s42004-024-01376-z
Citations
6
Primary Topic
Advanced NMR Techniques and Applications
Type
article
Field-Weighted Citation Impact
1.44

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article

Analysis of chemical exchange in iridium N-heterocyclic carbene complexes using heteronuclear parahydrogen-enhanced NMR

Alexander A. Auer, Kai Buckenmaier, Markus Plaumann, Rainer Herges et al.
6 citations
Communications Chemistry
Advanced NMR Techniques and Applications
1.44
article

Analysis of chemical exchange in iridium N-heterocyclic carbene complexes using heteronuclear parahydrogen-enhanced NMR

Alexander A. Auer, Kai Buckenmaier, Markus Plaumann, Rainer Herges, Igor V. Koptyug, Oleg G. Salnikov, Simon B. Duckett, Andrey N. Pravdivtsev, Jan‐Bernd Hövener, Arne Brahms, Xin Gui, Nikita V. Chukanov, Ivan V. Skovpin, Eduard Y. Chekmenev, Rainer Körber, Charbel D. Assaf
article en
6 citations

Abstract

The signal amplification by reversible exchange process (SABRE) enhances NMR signals by unlocking hidden polarization in parahydrogen through interactions with to-be-hyperpolarized substrate molecules when both are transiently bound to an Ir-based organometallic catalyst. Recent efforts focus on optimizing polarization transfer from parahydrogen-derived hydride ligands to the substrate in SABRE. However, this requires quantitative information on ligand exchange rates, which common NMR techniques struggle to provide. Here, we introduce an experimental spin order transfer sequence, with readout occurring at <sup>15</sup>N nuclei directly interacting with the catalyst. Enhanced <sup>15</sup>N NMR signals overcome sensitivity challenges, encoding substrate dissociation rates. This methodology enables robust data fitting to ligand exchange models, yielding substrate dissociation rate constants with higher precision than classical 1D and 2D <sup>1</sup>H NMR approaches. This refinement improves the accuracy of key activation enthalpy ΔH<sup>‡</sup> and entropy ΔS<sup>‡</sup> estimates. Furthermore, the higher chemical shift dispersion provided by enhanced <sup>15</sup>N NMR reveals the kinetics of substrate dissociation for acetonitrile and metronidazole, previously inaccessible via <sup>1</sup>H NMR due to small chemical shift differences between free and Ir-bound substrates. The presented approach can be successfully applied not only to isotopically enriched substrates but also to compounds with natural abundance of the to-be-hyperpolarized heteronuclei.

Communications ChemistryVol. 7(1)
Physikalisch-Technische Bundesanstalt (DE), Wayne State University (US), Christian-Albrechts-Universität zu Kiel (DE), The Barbara Ann Karmanos Cancer Institute (US), Max-Planck-Institut für Kohlenforschung (DE), Max Planck Institute for Biological Cybernetics (DE), International Tomography Center (RU), Clinical Research Center Kiel (DE), University of York (GB), Otto-von-Guericke-Universität Magdeburg (DE)
National Science Foundation, Government of the United Kingdom, UK Research and Innovation, Deutsche Forschungsgemeinschaft, Bundesministerium für Bildung und Forschung, Max-Planck-Gesellschaft, Russian Science Foundation, HORIZON EUROPE Framework Programme, European Regional Development Fund, Division of Chemistry
Clean water and sanitation
Openalex Percentile: Top 19%
Advanced NMR Techniques and Applications
1.44
DOIOpen Access PDFOpenAlex
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