Dynamic effects on ligand field from rapid hydride motion in an iron(ii) dimer with an S=3 ground state

Hydride complexes are important in catalysis and in iron-sulfur enzymes like nitrogenase, but the impact of hydride mobility on local iron spin states has been underexplored. We describe studies of a dimeric diiron(ii) hydride complex using X-ray and neutron crystallography, Mossbauer spectroscopy, magnetism, DFT, and ab initio calculations, which give insight into the dynamics and the electronic structure brought about by the hydrides. The two iron sites in the dimer have differing square-planar (intermediate-spin) and tetrahedral (high-spin) iron geometries, which are distinguished only by the hydride positions. These are strongly coupled to give an S-total = 3 ground state with substantial magnetic anisotropy, and the merits of both localized and delocalized spin models are discussed. The dynamic nature of the sites is dependent on crystal packing, as shown by changes during a phase transformation that occurs near 160 K. The change in dynamics of the hydride motion leads to insight into its influence on the electronic structure. The accumulated data indicate that the two sites can trade geometries by rotating the hydrides, at a rate that is rapid above the phase transition temperature but slow below it. This small movement of the hydrides causes large changes in the ligand field because they are strong-field ligands. This suggests that hydrides could be useful in catalysis not only due to their reactivity, but also due to their ability to rapidly modulate the local electronic structure and spin states at metal sites.

Automated summary

We conducted studies on a dimeric diiron(ii) hydride complex using various techniques, including X-ray and neutron crystallography, Mossbauer spectroscopy, magnetism, DFT, and ab initio calculations. Our findings provide insights into the dynamics and electronic structure changes induced by hydrides. The hydride positions determine the differing iron geometries, which in turn give rise to a coupled ground state with substantial magnetic anisotropy. The ability of hydrides to rapidly modulate the local electronic structure and spin states at metal sites suggests their potential importance in catalysis.