Strategies for the Hyperpolarization of Acetonitrile and Related Ligands by SABRE

We report on a strategy for using SABRE (signal amplification by reversible exchange) for polarizing ¹H and ¹³C nuclei of weakly interacting ligands which possess biologically relevant and nonaromatic motifs. We first demonstrate this via the polarization of acetonitrile, using Ir­(IMes)­(COD)Cl as the catalyst precursor, and confirm that the route to hyperpolarization transfer is via the J-coupling network. We extend this work to the polarization of propionitrile, benzylnitrile, benzonitrile, and trans-3-hexenedinitrile in order to assess its generality. In the ¹H NMR spectrum, the signal for acetonitrile is enhanced 8-fold over its thermal counterpart when [Ir­(H)₂(IMes)­(MeCN)₃]⁺ is the catalyst. Upon addition of pyridine or pyridine-d₅, the active catalyst changes to [Ir­(H)₂(IMes)­(py)₂(MeCN)]⁺ and the resulting acetonitrile ¹H signal enhancement increases to 20- and 60-fold, respectively. In ¹³C NMR studies, polarization transfers optimally to the quaternary ¹³C nucleus of MeCN while the methyl ¹³C is hardly polarized. Transfer to ¹³C is shown to occur first via the ¹H–¹H coupling between the hydrides and the methyl protons and then via either the ²J or ¹J couplings to the respective ¹³Cs, of which the ²J route is more efficient. These experimental results are rationalized through a theoretical treatment which shows excellent agreement with experiment. In the case of MeCN, longitudinal two-spin orders between pairs of ¹H nuclei in the three-spin methyl group are created. Two-spin order states, between the ¹H and ¹³C nuclei, are also created, and their existence is confirmed for Me¹³CN in both the ¹H and ¹³C NMR spectra using the Only Parahydrogen Spectroscopy protocol.