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

Abstract

Mössbauer spectra of [LFe(II)X](0) (L = beta-diketiminate; X = Cl(-), CH(3)(-), 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.CH(3), 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.CH(3)), 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 z(2) 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, /epsilon(yz) - epsilon(2)(z)/ = 452 (1.Cl) and 135 cm(-1) (1.CH(3)). A determination of the sign of the crystal field splitting was attempted by analyzing the electric field gradient (EFG) at the (57)Fe 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 epsilon(yz) - epsilon(2)(z). The ambiguity was resolved by analyzing the splitting Delta of the M(S) = +/-2 doublet; Delta = 0.3 cm(-1) for 1.Cl and Delta = 0.03 cm(-)(1) for 1.CH(3). This approach showed that z(2) is the ground state in both complexes and that epsilon(yz) - epsilon(2)(z) approximately 3500 cm(-1) for 1.Cl and 6000 cm(-1) for 1.CH(3). 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(CH(3)) 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.

Neutral and cationic Fe(II) β-diketiminate complexes

“…The ironcarbon bond lengths (2.0414(18) Å in 1 and 2.0222(18) Å in 2) lie well in the range of Fe-C distances observed for three-coordinate b-diketiminate iron alkyls [7,8,10,11]. The b-diketiminate ligands coordinate in the normal j 2 (N,N) chelating mode and the aryl substituents on the nitrogen atoms are oriented roughly orthogonal to the coordination plane.…”

“…They seem perfectly suitable for the stabilisation of low coordination number iron(II) alkyl species. Indeed, in recent years, the Holland group [7,8] has reported 12-valence electron, 3-coordinate iron(II) monoalkyls, initially using the 2,5-bis(tert-butyl) b-diketiminate ligand tBu BDK. Subsequent efforts by us [9] and the Holland group [10,11] [12] that could be conveniently generated in situ by reaction of ( Me BDK)H with n-BuLi in THF, followed by reaction with FeCl 2 or FeCl 2 (THF) 1.5 .…”

Surface-Template Assembly of Two-Dimensional Metal−Organic Coordination Networks

“…It is worth pointing out that the metal centers that are coordinated in these different environments may possess distinct electronic properties, which might result in novel magnetic or catalytic properties. 44,45 …”

The Reactivity Patterns of Low-Coordinate Iron−Hydride Complexes

“…19,29 Magnetic Mössbauer spectra measured at 4.2 K with applied fields up to 7 T show that the major component is diamagnetic, suggesting antiferromagnetic coupling between the two iron(II) ions. In sum, the spectroscopic data support the formulation of 1a as an exchange-coupled pair of high-spin iron(II) ions with a ground state of S ) 0.…”

“…18a Extremely bulky bidentate -diketiminate ligands ( Figure 2) have π-donating nitrogen atoms that lead to a weak ligand field. 19 The bulky diisopropylphenyl groups maintain a low coordination number that mimics the ligand-poor environment of the iron atoms in the FeMoco. Using these ligands, we have isolated the only examples of iron-hydride complexes with a coordination number less than fiVe ( Figure 2).…”