Terminal Hydride Complex of High-Spin Mn

The iron–molybdenum cofactor of nitrogenase (FeMoco)
catalyzes
fixation of N2 via Fe hydride intermediates. Our understanding
of these species has relied heavily on the characterization of well-defined
3d metal hydride complexes, which serve as putative spectroscopic
models. Although the Fe ions in FeMoco, a weak-field cluster, are
expected to adopt locally high-spin Fe2+/3+ configurations,
synthetically accessible hydride complexes featuring d5 or d6 electron counts are almost exclusively low-spin.
We report herein the isolation of a terminal hydride complex of four-coordinate,
high-spin (d5; S = 5/2) Mn2+. Electron paramagnetic resonance and electron–nuclear double
resonance studies reveal an unusually large degree of spin density
on the hydrido ligand. In light of the isoelectronic relationship
between Mn2+ and Fe3+, our results are expected
to inform our understanding of the valence electronic structures of
reactive hydride intermediates derived from FeMoco.