Mössbauer, Electron Paramagnetic Resonance, and Crystallographic Characterization of a High-Spin Fe(I) Diketiminate Complex with Orbital Degeneracy

The synthesis and X-ray structure of the low-coordinate, high-spin Fe^I compound LFe(HCCPh) (L = HC(C[^tBu]N[2,6-diisopropylphenyl])₂]^-), 1, are reported. Low-temperature Mössbauer and electron paramagnetic resonance (EPR) spectroscopies reveal that the electronic ground state is a Kramers doublet with uniaxial magnetic properties (effective g values g_x = 8.9, 0 < g_y, g_z < 0.3) that is well isolated from the excited states. The observation of a large and positive magnetic hyperfine field, B_int = +68.8(3) T, demonstrates that the orbital angular moment is essentially unquenched along one spatial direction. Relaxation rates obtained from variable-temperature Mössbauer spectra were fit to an Orbach process, yielding Δ = 130−190 cm^-1 for the energy gap (“zero-field splitting”) between the two Kramers doublets of the S = ³/₂ multiplet. Density functional theory (DFT) and time-dependent DFT calculations give insight into the electronic structures of the ground and excited states. The oxidation state of the iron and the bond order of the phenylacetylene ligand in complex 1 are analyzed using DFT, showing a substantial back-bonding interaction. Spin−orbit coupling acting in the subspace of quasi-degenerate z² and yz orbitals provides a consistent description of both the zero-field splitting and magnetic hyperfine parameters that fits the EPR and Mössbauer data for 1. Interestingly, the spin−orbit coupling involves the same two orbitals (z², yz) as in the analogous three-coordinate Fe^II compounds, because back-bonding significantly lowers the energy of the xy orbital, making it the lowest doubly occupied d orbital. Despite the different oxidation state and different number of atoms in the first coordination sphere, the electronic structure of LFe^I(HCCPh) can be interpreted similarly to that of three-coordinate Fe^II diketiminate complexes, but with a substantial effect of back-bonding. To our knowledge, this is the first detailed Mössbauer and EPR study of a structurally characterized high-spin Fe^I complex.