Probing Hydrogen Activation in a Dimetal Dihydride Complex by Symmetric Exchange with Parahydrogen

We report an investigation
into the hydrogen exchange mechanism
in a dinickel(II) dihydride complex with C2v symmetry. To obtain atomic level information on the low-concentration
intermediate species, we use parahydrogen in combination with nuclear
magnetic resonance (NMR). Unexpectedly, despite the chemical equivalence
of the two hydride sites imposed by the mirror symmetry of the model,
we observe spontaneous conversion of the nuclear singlet order into
in-phase longitudinal magnetization–something that normally
requires sophisticated pulse sequences. The magnetization is retained
upon reductive elimination of dihydrogen from the dihydride species
and observed as an enhanced hydrogen NMR signal. This is explained
by a chain of nuclear relaxation interference processes that do not
require asymmetric intermediates; we confirm this in two ways: using
analytical relaxation theory and with brute-force numerical simulations.