Tweezers for Parahydrogen: A Metal-Free Probe of Nonequilibrium Nuclear Spin States of H2 Molecules

To date, only metal-containing hydrogenation catalysts have been utilized for producing substantial NMR signal enhancements by means of parahydrogen-induced polarization (PHIP). Herein, we show that metal-free compounds known as molecular tweezers are useful in this respect. It is shown that ansa-aminoborane tweezers QCAT provided. (20-30)-fold signal enhancements of parahydrogen-originating hydrogens in H-1 NMR spectra. Nuclear polarization transfer from the polarized hydrogens to B-11 nuclei leads to a 10-fold enhancement in the B-11 NMR spectrum. Moreover, our results indicate that dihydrogen activation by QCAT and CAT tweezers is carried out in a pairwise manner, and PHIP can be used for understanding the activation mechanism in metal-free catalytic systems in general.