Using signal amplification by reversible exchange (SABRE) to hyperpolarise ¹¹⁹Sn and ²⁹Si NMR nuclei

Abstract

The hyperpolarisation of the ¹¹⁹Sn and ²⁹Si nuclei in 5-(tributylstannyl)pyrimidine (A_Sn) and 5-(trimethylsilyl)pyrimidine (B_Si) is achieved through their reaction with [IrCl(COD)(IMes)] (1a) or [IrCl(COD)(SIMes)] (1b) and parahydrogen via the SABRE process. 1a exhibits superior activity in both cases. The two inequivalent pyrimidine proton environments of A_Sn readily yielded signal enhancements totalling ∼2300-fold in its ¹H NMR spectrum at a field strength of 9.4 T, with the corresponding ¹¹⁹Sn signal being 700 times stronger than normal. In contrast, B_Si produced analogous ¹H signal gains of ∼2400-fold and a ²⁹Si signal that could be detected with a signal to noise ratio of 200 in a single scan. These sensitivity improvements allow NMR detection within seconds using micromole amounts of substrate and illustrate the analytical potential of this approach for high-sensitivity screening. Furthermore, after extended reaction times, a series of novel iridium trimers of general form [Ir(H)₂Cl(NHC)(μ-pyrimidine-κN:κN')]₃ precipitate from these solutions whose identity was confirmed crystallographically for B_Si.