Alkyne gem‐Hydrogenation: Formation of Pianostool Ruthenium Carbene Complexes and Analysis of Their Chemical Character

Parahydrogen (p‐H₂) induced polarization (PHIP) NMR spectroscopy showed that [Cp^XRu] complexes with greatly different electronic properties invariably engage propargyl alcohol derivatives into gem‐hydrogenation with formation of pianostool ruthenium carbenes; in so doing, less electron rich Cp^X rings lower the barriers, stabilize the resulting complexes and hence provide opportunities for harnessing genuine carbene reactivity. The chemical character of the resulting ruthenium complexes was studied by DFT‐assisted analysis of the chemical shift tensors determined by solid‐state ¹³C NMR spectroscopy. The combined experimental and computational data draw the portrait of a family of ruthenium carbenes that amalgamate purely electrophilic behavior with characteristics more befitting metathesis‐active Grubbs‐type catalysts.     

Parahydrogen-Induced Radio Amplification by Stimulated Emission of Radiation

Radio amplification by stimulated emission of radiation (RASER) was recently discovered in a low-field NMR spectrometer incorporating a highly specialized radio-frequency resonator, where a high degree of proton-spin polarization was achieved by reversible parahydrogen exchange. RASER activity, which results from the coherent coupling between the nuclear spins and the inductive detector, can overcome the limits of frequency resolution in NMR. Here we show that this phenomenon is not limited to low magnetic fields or the use of resonators with high-quality factors. We use a commercial bench-top 1.4 T NMR spectrometer in conjunction with pairwise parahydrogen addition producing proton-hyperpolarized molecules in the Earth's magnetic field (ALTADENA condition) or in a high magnetic field (PASADENA condition) to induce RASER without any radio-frequency excitation pulses. The results demonstrate that RASER activity can be observed on virtually any NMR spectrometer and measures most of the important NMR parameters with high precision.     

Heterogeneous Microtesla SABRE Enhancement of 15N NMR Signals

The hyperpolarization of heteronuclei via signal amplification by reversible exchange (SABRE) was investigated under conditions of heterogeneous catalysis and microtesla magnetic fields. Immobilization of [IrCl(COD)(IMes)], [IMes=1,3‐bis(2,4,6‐trimethylphenyl), imidazole‐2‐ylidene; COD=cyclooctadiene] catalyst onto silica particles modified with amine linkers engenders an effective heterogeneous SABRE (HET‐SABRE) catalyst that was used to demonstrate a circa 100‐fold enhancement of ¹⁵N NMR signals in ¹⁵N‐pyridine at 9.4 T following parahydrogen bubbling within a magnetic shield. No ¹⁵N NMR enhancement was observed from the supernatant liquid following catalyst separation, which along with XPS characterization supports the fact that the effects result from SABRE under heterogeneous catalytic conditions. The technique can be developed further for producing catalyst‐free agents via SABRE with hyperpolarized heteronuclear spins, and thus is promising for biomedical NMR and MRI applications.