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

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Abstract
Mössbauer spectra of [LFeIIX]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 MS = ±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, Bint = +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 geff = 10.9 (1.Cl) and 11.4 (1.CH3), suggesting the presence of unquenched orbital angular momenta. A qualitative crystal field analysis of geff 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 MS = ±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 geff provides the magnitude of the crystal field splitting of the lowest two orbitals, |εyz − εz2| = 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 εyz − εz2. The ambiguity was resolved by analyzing the splitting Δ of the MS = ±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 εxz − εz2 ≈ 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: WX to ligands X and WN 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 WN(diketiminate) < WCl < WN‘HR, WCH3 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 MN state.

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