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. 2019 Jun 10;9(32):18183-18190.
doi: 10.1039/c9ra02580d.

Structural exploration of rhodium catalysts and their kinetic studies for efficient parahydrogen-induced polarization by side arm hydrogenation

Marino Itoda  1 , Yuki Naganawa  1   2 , Makoto Ito  1 , Hiroshi Nonaka  1 , Shinsuke Sando  1   3
Affiliations

Structural exploration of rhodium catalysts and their kinetic studies for efficient parahydrogen-induced polarization by side arm hydrogenation

Marino Itoda et al. RSC Adv. .

Abstract

Parahydrogen-induced polarization (PHIP) is a rapid and cost-effective hyperpolarization technique using transition metal-catalysed hydrogenation with parahydrogen. We examined rhodium catalysts and their kinetic studies, rarely considered in the research of current PHIP. It emerged that rhodium complexes with electron-donating bisphosphine ligands, with a dicyclohexylphosphino group, appear to be more effective than conventional rhodium catalysts.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Scheme 1
Scheme 1. Experimental concept of PHIP.
Fig. 1
Fig. 1. (a) Structural exploration of conventional rhodium complex 1. (b) Rhodium complexes 2–5 selected based on the screening of ligands.
Fig. 2
Fig. 2. (a) Conditions of the PHIP experiment of VA. (b) Polarization level % P13C at carbonyl carbon of EA. (c) Conversion rate of VA to EA. (d) 13C NMR spectrum of natural abundant EA (ca. 40 mM) in thermal equilibrium state (30° pulse, 1 scan). (e) 13C NMR spectra (30° pulse, 1 scan) of EA produced upon PHIP (initial concentration of VA 50 mM) with catalyst 1 or 4. S/N means signal to noise ratio with standard deviation (n = 3). (f) Stacked 13C NMR spectra of the hyperpolarized EA. PHIP experiments were conducted under the following conditions: 20 mL min−1 parahydrogen (1 atm) was bubbled for 10 s into 700 μL of methanol-d4 containing 5 mM catalyst and 50 mM VA, followed by polarization transfer.
Scheme 2
Scheme 2. Two steps of rhodium-catalysed hydrogenation.
Fig. 3
Fig. 3. Measurement of activation rates. (a) UV-vis spectra of 0.5 mM of Rh-dcpb complex with and without nbd. (b) Determination of activation rates. Activation rates were determined by monitoring absorption around 470–480 nm of 0.5 mM catalyst solution at 1 atm of hydrogen.
Fig. 4
Fig. 4. TOF of Rh catalysts 1–5 in hydrogenation for vinyl acetate (VA). Determination of TOF for VA (cat 1–5). 5 mM of catalyst solution in methanol-d4 containing 1 M of VA was set into an NMR tube, and bubbled with normal hydrogen (20 mL min–1).
Fig. 5
Fig. 5. Two different catalytic cycles of hydrogenation. nbd = 2,5-norbornadiene, S = solvent, VA = vinyl acetate, EA = ethyl acetate, PP = bisphosphine ligand.
Fig. 6
Fig. 6. (a) Conditions of PHIP experiment of PA. (b) Conversion rate of PA to AA and n-PA. (c) Polarization level % P13C at carbonyl carbon of AA and n-PA. (d) 13C NMR spectrum of natural abundant AA (ca. 35 mM) in thermal equilibrium state (30° pulse, 1 scan). (e) 13C NMR spectra (30° pulse, 1 scan) of AA and n-PA produced upon PHIP (initial concentration of PA 50 mM) with catalyst 1 or 4. S/N means signal to noise ratio with standard deviation (n = 3). PHIP experiments were conducted under the following conditions: 20 mL min−1 parahydrogen (1 atm) was bubbled for 10 s into 700 μL of methanol-d4 containing 2.5 mM catalyst and 50 mM PA, followed by polarization transfer.
Fig. 7
Fig. 7. (a) Illustration of flow hydrogenation system used in this study. (b) Reaction conditions for PHIP of VA in (a).
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