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

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.

Scheme 1. Experimental concept of PHIP.

Fig. 1. (a) Structural exploration of conventional rhodium complex 1. (b) Rhodium complexes 2–5 selected based on the screening of ligands.

Fig. 2. (a) Conditions of the PHIP experiment of VA. (b) Polarization level % P_¹³C 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. Two steps of rhodium-catalysed hydrogenation.

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. 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. Two different catalytic cycles of hydrogenation. nbd = 2,5-norbornadiene, S = solvent, VA = vinyl acetate, EA = ethyl acetate, PP = bisphosphine ligand.

Fig. 6. (a) Conditions of PHIP experiment of PA. (b) Conversion rate of PA to AA and n-PA. (c) Polarization level % P_¹³C 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. (a) Illustration of flow hydrogenation system used in this study. (b) Reaction conditions for PHIP of VA in (a).