Iridium trihydride and tetrahydride complexes and their role in catalytic polarisation transfer from <i>para</i>hydrogen to pyruvate

This work details how the unusual iridium tetrahydride [Ir(H)₄(IMes)(sulfoxide)]Na and trihydride [Ir(H)₃(IMes)(sulfoxide)₂] can be formed in a solvent dependent reaction of [IrCl(COD)(IMes)] with sulfoxide (dimethyl or methylphenyl), base, and H₂. In the case of dimethyl sulfoxide, the four hydride ligands of the tetrahydride are equivalent, and the IMes and sulfoxide ligands mutually <i>trans</i>. However, for phenyl methyl sulfoxide, this isomer of the tetrahydride forms alongside its <i>cis</i> counterpart where the remarkable symmetry breaking effect of the sulfoxide leads to it presenting four chemically distinct hydride ligands. These products and their ligand arrangements are characterised and the reaction pathways leading to their formation probed using NMR spectroscopy and <i>para</i>hydrogen-hyperpolarised methods. Subsequently, they form as previously unidentified low concentration by-products in the important SABRE-catalysed hyperpolarisation of pyruvate, and their concentration should be minimised for efficient polarisation transfer.