Synthesis, Spectroscopy, and Hydrogen/Deuterium Exchange in High-Spin Iron(II) Hydride Complexes

Very
few hydride complexes are known in which the metals have a
high-spin electronic configuration. We describe the characterization
of several high-spin iron­(II) hydride/deuteride isotopologues and
their exchange reactions with one another and with H2/D2. Though the hydride/deuteride signal is not observable in NMR spectra,
the choice of isotope has an influence on the chemical shifts of distant
protons in the dimers through the paramagnetic isotope effect on chemical
shift. This provides the first way to monitor the exchange of H and
D in the bridging positions of these hydride complexes. The rate of
exchange depends on the size of the supporting ligand, and this is
consistent with the idea that H2/D2 exchange
into the hydrides occurs through the dimeric complexes rather than
through a transient monomer. The understanding of H/D exchange mechanisms
in these high-spin iron hydride complexes may be relevant to postulated
nitrogenase mechanisms.