TY - JOUR
T1 - Paramagnetic Relaxation Agents for Enhancing Temporal Resolution and Sensitivity in Multinuclear FlowNMR Spectroscopy
AU - Bara Estaun, Alejandro
AU - Harder, Marie K.
AU - Lyall, Catherine L.
AU - Lowe, John P.
AU - Suturina, Elizaveta
AU - Hintermair, Ulrich
N1 - 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 ManuscriptChemistry - A European Journal
PY - 2023/5/17
Y1 - 2023/5/17
N2 - 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.
AB - 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.
KW - chromium
KW - flowNMR
KW - multinuclear NMR spectroscopy
KW - spin-lattice relaxation time and paramagnetic relaxation agents
UR - http://www.scopus.com/inward/record.url?scp=85159579474&partnerID=8YFLogxK
U2 - 10.1002/chem.202300215
DO - 10.1002/chem.202300215
M3 - Article
C2 - 36946535
SN - 0947-6539
JO - Chemistry - A European Journal
JF - Chemistry - A European Journal
M1 - e202300215
ER -