Paramagnetic Relaxation Agents for Enhancing Temporal Resolution and Sensitivity in Multinuclear FlowNMR Spectroscopy

Alejandro Bara Estaun, Marie C. Harder, Catherine L. Lyall, John P. Lowe, Elizaveta Suturina, Ulrich Hintermair

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3 Citations (SciVal)

Abstract

Sensitivity in FlowNMR spectroscopy for reaction monitoring often suffers from low levels of pre-magnetisation due to limited residence times of the sample in the magnetic field. While this in-flow effect is tolerable for high sensitivity nuclei such as 1H and 19F, it significantly reduces the signal-to-noise ratio in 31P and 13C spectra, making FlowNMR impractical for low sensititvity nuclei at low concentrations. Paramagnetic relaxation agents (PRAs), which enhance polarisation and spin-lattice relaxation, could eliminate the adverse in-flow effect and improve the signal-to-noise ratio. Herein, [Co(acac) 3], [Mn(acac) 3], [Fe(acac) 3] , [Cr(acac) 3] , [Ni(acac) 2] 3, [Gd(tmhd) 3] and [Cr(tmhd) 3] are investigated for their effectiveness in improving signal intensity per unit time in FlowNMR applications under the additional constraint of chemical inertness towards catalytically active transition metal complexes. High-spin Cr(III) acetylacetonates emerged as the most effective compounds, successfully reducing 31P T 1 values four- to five-fold at PRA concentrations as low as 10 mM without causing adverse line broadening. Whereas [Cr(acac) 3] showed signs of chemical reactivity with a mixture of triphenylphosphine, triphenylphosphine oxide and triphenylphosphate over the course of several hours at 80° C, the bulkier [Cr(tmhd) 3] was stable and equally effective as a PRA under these conditions. Compatibility with a range of representative transition metal complexes often used in homogeneous catalysis has been investigated, and application of [Cr(tmhd) 3] in significantly improving 1H and 31P{ 1H} FlowNMR data quality in a Rh-catalysed hydroformylation reaction has been demonstrated. With the PRA added, 13C relaxation times were reduced more than six-fold, allowing quantitative reaction monitoring of substrate consumption and product formation by 13C{ 1H} FlowNMR spectroscopy at natural abundance.

Original languageEnglish
Article numbere202300215
JournalChemistry - A European Journal
Volume29
Issue number38
Early online date17 May 2023
DOIs
Publication statusPublished - 6 Jul 2023

Bibliographical note

This work was supported by the Royal Society (UF160458; fellowship to UH), the EPSRC Dynamic Reaction Monitoring Facility (EP/P001475/1), the EPSRC Centre for Doctoral Training in Catalysis (EP/L016443; studentship to ABE), and the Erasmus+ programme of the European Commission (student mobility placement to MCH).10.1002/chem.202300215Accepted Manuscript Chemistry - A European Journal

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article

Keywords

  • chromium
  • flowNMR
  • multinuclear NMR spectroscopy
  • spin-lattice relaxation time and paramagnetic relaxation agents

ASJC Scopus subject areas

  • Chemistry(all)
  • Catalysis
  • Organic Chemistry

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