Iridium N-heterocyclic carbene (NHC) complexes catalyse the para-hydrogen-induced hyperpolarization process, Signal Amplification by Reversible Exchange (SABRE). This process transfers the latent magnetism of para-hydrogen into a substrate, without changing its chemical identity, to dramatically improve its nuclear magnetic resonance (NMR) detectability. By synthesizing and examining over 30 NHC containing complexes, here we rationalize the key characteristics of efficient SABRE catalysis prior to using appropriate catalyst-substrate combinations to quantify the substrate’s NMR detectability. These optimizations deliver polarizations of 63% for 1H nuclei in methyl 4,6-d2-nicotinate, 25% for 13C nuclei in a 13C2-diphenylpyridazine and 43% for the 15N nucleus of pyridine-15N. These high detectability levels compare favourably with the 0.0005% 1H value harnessed by a routine 1.5 T clinical MRI system. As signal strength scales with the square of the number of observations, these low cost innovations offer remarkable improvements in detectability threshold that offer routes to significantly reduce measurement time.

Signal Amplification by Reversible Exchange (SABRE) is used to switch on the latent singlet spin order of para-hydrogen (p-H2) so that it can hyperpolarize a substrate (sub = nicotinamide, nicotinate, niacin, pyrimidine, and pyrazine). The substrate then reacts reversibly with [Pt(OTf)2(bis-diphenylphosphinopropane)] by displacing OTf– to form [Pt(OTf)(sub)(bis-diphenylphosphinopropane)]OTf. The 31P NMR signals of these metal complexes prove to be enhanced when the substrate possesses an accessible singlet state or long-lived Zeeman polarization. In the case of pyrazine, the corresponding 31P signal was 105 ± 8 times larger than expected, which equated to an 8 h reduction in total scan time for an equivalent signal-to-noise ratio under normal acquisition conditions. Hence, p-H2 derived spin order is successfully relayed into a second metal complex via a suitable polarization carrier (sub). When fully developed, we expect this route involving a second catalyst to successfully hyperpolarize many classes of substrates that are not amenable to the original SABRE method.

Manganese‐catalyzed C−H bond activation chemistry is emerging as a powerful and complementary method for molecular functionalization. A highly reactive seven‐membered MnI intermediate is detected and characterized that is effective for H‐transfer or reductive elimination to deliver alkenylated or pyridinium products, respectively. The two pathways are determined at MnI by judicious choice of an electron‐deficient 2‐pyrone substrate containing a 2‐pyridyl directing group, which undergoes regioselective C−H bond activation, serving as a valuable system for probing the mechanistic features of Mn C−H bond activation chemistry.

Reaction of [Ir(IMes)(COD)Cl] with pyridazine (pdz) or phthalazine (phth) and H2 results in the formation of the para-hydrogen magnetisation transfer catalysts [Ir(H)2(IMes)(pdz)3]Cl and [Ir(H)2(IMes)(phth)3]Cl.

Tripalladium clusters of the type [Pd 3 (PPh 3 ) 4 ] 2+ , wherein three linearly connected Pd atoms are stabilized by phosphine and arsine ligands, have been detected and isolated as intermediates during the reduction of well-defined mononuclear [Pd(OTf) 2 (XPh 3 ) 2 ] (X = P and X = As, respectively) to Pd nanoparticles (PdNPs). The isolated [Pd 3 (PPh 3 ) 4 ] 2+ cluster isomerizes on broad-band UV irradiation to form an unexpected photoisomer, produced by a remarkable change in conformation at one of the bridging PPh 3 ligands. A catalytic role for these [Pd 3 (XPh 3 ) 4 ] 2+ species is exemplified in Suzuki−Miyaura crosscoupling (SMCC) reactions, with high activity seen in the arylation of a brominated heterocyclic 2-pyrone. Use of the [Pd 3 (PPh 3 ) 4 ] 2+ cluster enables a switch in site selectivity for SMCC reactions involving 2,4-dibromopyridine from the typical C2bromide site (seen previously for mononuclear Pd catalysts) to the atypical C4-bromide site, thereby mirroring recently reported cyclic Pd 3 clusters and PdNPs. We have further determined that the thermal isomer and photoisomer of [Pd 3 (PPh 3 ) 4 ] 2+ are similarly catalytically active in the Pd-catalyzed hydrogenation of phenylacetylene to give styrene. Our findings link the evolution of mononuclear Pd(II) salts to PdNPs via the intermediacy of linear [Pd 3 (XPh 3 ) 4 ] 2+ clusters.