Understanding substrate substituent effects to improve catalytic efficiency in the SABRE hyperpolarisation process

TY - JOUR

T1 - Understanding substrate substituent effects to improve catalytic efficiency in the SABRE hyperpolarisation process

AU - Stanbury, Emma

AU - Richardson, Peter Michael

AU - Duckett, Simon

N1 - © The Royal Society of Chemistry 2019

PY - 2019/8/7

Y1 - 2019/8/7

N2 - The use of parahydrogen based hyperpolarisation in NMR is becoming more widespread due to the rapidly expanding range of target molecules and low-cost of parahydrogen production. Hyperpolarisation via SABRE catalysis employs a metal complex to transfer polarisation from parahydrogen into a substrate whilst they are bound. In this paper we present a quantitative study of substrate–iridium ligation effects by reference to the substrates 4-chloropyridine (A), 4-pyridinecarboxaldehyde methyl hemiacetal (B), 4-methylpyridine (C) and 4-methoxypyridine (D), and evaluate the role they play in the SABRE catalysis. Substrates whose substituents enable stronger associations yield slower substrate dissociation rates (kd). A series of variable temperature studies link these exchange rates to optimal SABRE performance and reveal the critical impact of NMR relaxation times (T1). Longer catalyst residence times are shown to result in shorter substrate T1 values in solution as binding to iridium promotes relaxation thereby not only reducing SABRE efficiency but decreasing the overall level of achieved hyperpolarisation. Based on these data, a route to achieve more optimal SABRE performance is defined.

AB - The use of parahydrogen based hyperpolarisation in NMR is becoming more widespread due to the rapidly expanding range of target molecules and low-cost of parahydrogen production. Hyperpolarisation via SABRE catalysis employs a metal complex to transfer polarisation from parahydrogen into a substrate whilst they are bound. In this paper we present a quantitative study of substrate–iridium ligation effects by reference to the substrates 4-chloropyridine (A), 4-pyridinecarboxaldehyde methyl hemiacetal (B), 4-methylpyridine (C) and 4-methoxypyridine (D), and evaluate the role they play in the SABRE catalysis. Substrates whose substituents enable stronger associations yield slower substrate dissociation rates (kd). A series of variable temperature studies link these exchange rates to optimal SABRE performance and reveal the critical impact of NMR relaxation times (T1). Longer catalyst residence times are shown to result in shorter substrate T1 values in solution as binding to iridium promotes relaxation thereby not only reducing SABRE efficiency but decreasing the overall level of achieved hyperpolarisation. Based on these data, a route to achieve more optimal SABRE performance is defined.

U2 - 10.1039/C9CY00396G

DO - 10.1039/C9CY00396G

M3 - Article

SN - 2044-4753

SP - 3914

EP - 3922

JO - Catalysis Science and Technology

JF - Catalysis Science and Technology

IS - 9

M1 - CY-ART-02-2019-000396.R1

ER -