Metal-bound hydrogen atom with extreme NMR shift discovered

For the first time, a 3d metal-bound hydrogen atom was detected by paramagnetic solution nuclear magnetic resonance (NMR) spectroscopy. With the help of computational modelling, chemists have pinned it down at –3560ppm – the most extreme upfield proton chemical shift ever found.

Most chemists are used to the sharp peaks and refined spin–spin coupling patterns in diamagnetic proton (¹ H) NMR. But spectra of paramagnetic compounds – those with unpaired electrons like radicals or some metals – look different: they have broad signals and a chemical shift range of around 200ppm.

For example, the hydrogen atoms in diamagnetic ferrocene have a chemical shift of around 4ppm, but paramagnetic dimethylnickelocene’s protons can be found at 200 and –200ppm. Hydrogen atoms bound directly to a paramagnetic metal have even more extreme shifts, and until now hydrides bound to 3d paramagnetic centres had never been detected by solution NMR.

Chemists have now found a hydrogen atom bound to iron in a square-planar complex at –3560ppm – the most shifted proton signal recorded to date. The signal’s position was also extremely temperature dependent, shifting by 2000ppm between –50 and 110°C.

In order to target the right chemical shift range, the researchers had turned to density functional theory calculations. The simulations had predicted the H at –3770ppm.

Paramagnetic metal compounds like their planar iron complex can catalyse reductions, the researchers point out, which is why characterising them is important.

Note: this article was amended on 3 January 2019. The original erroneously reported that no hydrogen atoms bound to paramagnetic metal centres have ever been detected by NMR