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 (1) H and (19) F, it significantly reduces the signal-to-noise ratio in (31) P and (13) C 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 (31) P 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 (1) H and (31) P{(1) H} FlowNMR data quality in a Rh-catalysed hydroformylation reaction has been demonstrated. With the PRA added, (13) C relaxation times were reduced more than six-fold, allowing quantitative reaction monitoring of substrate consumption and product formation by (13) C{(1) H} FlowNMR spectroscopy at natural abundance.