Observability of Paramagnetic NMR Signals at over 10 000 ppm Chemical Shifts

We report an experimental observation of 31P NMR resonances shifted by over 10 000 ppm (meaning percent range, and a new record for solutions), and similar 1H chemical shifts, in an intermediate‐spin square planar ferrous complex [ tBu(PNP)Fe‐H], where PNP is a carbazole‐based pincer ligand. Using a combination of electronic structure theory, nuclear magnetic resonance, magnetometry, and terahertz electron paramagnetic resonance, the influence of magnetic anisotropy and zero‐field splitting on the paramagnetic shift and relaxation enhancement is investigated. Detailed spin dynamics simulations indicate that, even with relatively slow electron spin relaxation (T 1 ≈10−11 s), it remains possible to observe NMR signals of directly metal‐bonded atoms because pronounced rhombicity in the electron zero‐field splitting reduces nuclear paramagnetic relaxation enhancement.
High‐resolution paramagnetic NMR resonances of a ferrous complex with an intermediate spin ground state with chemical shifts beyond ten thousand ppm were detected, the observability of which are attributed to a large rhombic zero‐field splitting resulting in an effective suppression of the dipolar relaxation mechanism.