Planar Three-Coordinate High-Spin Fe^II Complexes with Large Orbital Angular Momentum:  Mössbauer, Electron Paramagnetic Resonance, and Electronic Structure Studies

Mössbauer spectra of [LFe^IIX]⁰ (L = β-diketiminate; X = Cl^-, CH₃^-, NHTol^-, NHtBu^-), 1.X, were recorded between 4.2 and 200 K in applied magnetic fields up to 8.0 T. A spin Hamiltonian analysis of these data revealed a spin S = 2 system with uniaxial magnetization properties, arising from a quasi-degenerate M_S = ±2 doublet that is separated from the next magnetic sublevels by very large zero-field splittings (3|D| > 150 cm^-1). The ground levels give rise to positive magnetic hyperfine fields of unprecedented magnitudes, B_int = +82, +78, +72, and +62 T for 1.CH₃, 1.NHTol, 1.NHtBu, and 1.Cl, respectively. Parallel-mode EPR measurements at X-band gave effective g values that are considerably larger than the spin-only value 8, namely g_eff = 10.9 (1.Cl) and 11.4 (1.CH₃), suggesting the presence of unquenched orbital angular momenta. A qualitative crystal field analysis of g_eff shows that these momenta originate from spin−orbit coupling between energetically closely spaced yz and z² 3d-orbital states at iron and that the spin of the M_S = ±2 doublet is quantized along x, where x is along the Fe−X vector and z is normal to the molecular plane. A quantitative analysis of g_eff provides the magnitude of the crystal field splitting of the lowest two orbitals, |ε_yz − ε_z²| = 452 (1.Cl) and 135 cm^-1 (1.CH₃). A determination of the sign of the crystal field splitting was attempted by analyzing the electric field gradient (EFG) at the ⁵⁷Fe nuclei, taking into account explicitly the influence of spin−orbit coupling on the valence term and ligand contributions. This analysis, however, led to ambiguous results for the sign of ε_yz − ε_z². The ambiguity was resolved by analyzing the splitting Δ of the M_S = ±2 doublet; Δ = 0.3 cm^-1 for 1.Cl and Δ = 0.03 cm^-1 for 1.CH₃. This approach showed that z² is the ground state in both complexes and that ε_xz − ε_z² ≈ 3500 cm^-1 for 1.Cl and 6000 cm^-1 for 1.CH₃. The crystal field states and energies were compared with the results obtained from time-dependent density functional theory (TD-DFT). The isomer shifts and electric field gradients in 1.X exhibit a remarkably strong dependence on ligand X. The ligand contributions to the EFG, denoted W, were expressed by assigning ligand-specific parameters:  W_X to ligands X and W_N to the diketiminate nitrogens. The additivity and transferability hypotheses underlying this model were confirmed by DFT calculations. The analysis of the EFG data for 1.X yields the ordering W_{N(diketiminate)} < W_Cl < W_N‘HR, W_CH3 and indicates that the diketiminate nitrogens perturb the iron wave function to a considerably lesser extent than the monodentate nitrogen donors do. Finally, our study of these synthetic model complexes suggests an explanation for the unusual values for the electric hyperfine parameters of the iron sites in the Fe−Mo cofactor of nitrogenase in the M^N state.