Electronic Structure of Tetrahedral,S= 2, [Fe{(EPⁱPr₂)₂N}₂], E = S, Se, Complexes: Investigation by High-Frequency and -Field Electron Paramagnetic Resonance,⁵⁷Fe Mössbauer Spectroscopy, and Quantum Chemical Studies

Abstract

In this work, we assessed the electronic structures of two pseudotetrahedral complexes of Fe II , [Fe{(SP i Pr 2 ) 2 N} 2 ] (1) and [Fe-{(SeP i Pr 2 ) 2 N} 2 ] (2), using high-frequency and -field EPR (HFEPR) and fielddependent 57 Fe Mossbauer spectroscopies. This investigation revealed S = 2 ground states characterized by moderate, negative zero-field splitting (zfs) parameters D. The crystal-field (CF) theory analysis of the spin Hamiltonian (sH) and hyperfine structure parameters revealed that the orbital ground states of 1 and 2 have a predominant d x 2 −y 2 character, which is admixed with d z 2 (∼10%). Although replacing the S-containing ligands of 1 by their Se-containing analogues in 2 leads to a smaller |D| value, our theoretical analysis, which relied on extensive ab initio CASSCF calculations, suggests that the ligand spin−orbit coupling (SOC) plays a marginal role in determining the magnetic anisotropy of these compounds. Instead, the d x 2 −y 2 β → d xy β excitations yield a large negative contribution, which dominates the zfs of both 1 and 2, while the different energies of the d x 2 −y 2 β → d xz β transitions are the predominant factor responsible for the difference in zfs between 1 and 2. The electronic structures of these compounds are contrasted with those of other [FeS 4 ] sites, including reduced rubredoxin by considering a D 2 -type distortion of the [Fe(E−X) 4 ] cores, where E = S, Se; X = C, P. Our combined CASSCF/DFT calculations indicate that while the character of the orbital ground state and the quintet excited states' contribution to the zfs of 1 and 2 are modulated by the magnitude of the D 2 distortion, this structural change does not impact the contribution of the excited triplet states.

Magnetic anisotropy and structural flexibility in the field-induced single ion magnets [Co{(OPPh2)(EPPh2)N}2], E = S, Se, explored by experimental and computational methods

“…It should be noted that for both CoE 4 and CoO 2 E 2 complexes, the small effect of the nature of the E donor atom (either S or Se) on the magnitude of D in these Co( ii ) systems is consistent with similar findings for other 3d-metal-based complexes, such as tetrahedral Fe( ii ) S = 2 FeE 4 , E = S, Se, 55 and [Fe{(EPPh 2 ) 2 N} 2 ], E = S, 75 Se, 78 as well as octahedral Ni( ii ) ( S = 1) [Ni{(OPPh 2 )(EPPh 2 )N} 2 (sol) 2 ], E = S, Se; sol = dmf, thf, 79 dmso. 80 …”

“…The other spin Hamiltonian parameters, such as E/D and the g-values used to simulate the turning point branches, were taken from the HFEPR investigation of the CoE 4 complexes. 74 It should be noted that for both CoE 4 and CoO 2 E 2 complexes, the small effect of the nature of the E donor atom (either S or Se) on the magnitude of D in these Co(II) systems is consistent with similar findings for other 3d-metal-based complexes, such as tetrahedral Fe(II) S = 2 FeE 4 , E = S, Se, 55 and [Fe {(EPPh 2 ) 2 N} 2 ], E = S, 75 Se, 78 as well as octahedral Ni(II) (S = 1) [Ni{(OPPh 2 )(EPPh 2 )N} 2 (sol) 2 ], E = S, Se; sol = dmf, thf, 79 dmso. 80 High-resolution EPR spectroscopy at 9, 95, and 275 GHz CoO 2 S 2 complex.…”

“…40 The experimental methods that are employed for thisrather demanding -task, have been recently reviewed, 41 and mainly include DC magnetometry, 40 high-frequency and -field EPR spectroscopy (HFEPR), [42][43][44] far-infrared magnetic spectroscopy (FIRMS), [45][46][47] inelastic neutron scattering, [48][49][50] magnetic circular dichroism [51][52][53] and Mössbauer spectroscopy. [54][55][56][57] Each experimental method has its advantages and limitations. 41 Among these methods, direct current (DC) magnetometry has been a conventional method of estimating the magnitude of the zfs D and E components.…”

Well-Defined Iron Sites in Crystalline Carbon Nitride

“…However, excellent fits were also possible using smaller values of D by including antiferromagnetic coupling from neighboring Fe 2+ ions (| J | = 1–5 cm –1 ; Figure S18). For tetrahedral high-spin iron­(II) sites in coordination compounds, typical values lie in the range −10 to +15 cm –1 . − We surmise that this PTI-bound Fe 2+ site is similar since PTI/FeCl 2 showed no X-band EPR spectrum at 10 K, suggesting that D is greater than the X-band frequency.…”

Spectroscopic and Theoretical Investigation of High-Spin Square-Planar and Trigonal Fe(II) Complexes Supported by Fluorinated Alkoxides

“…Even though a comparable behavior is often described when AC magnetometry is employed to investigate the spin-dynamics of molecular magnets, due to the considerably shorter time scale, such effects are rarely observed using Mossbauer spectroscopy. 53,54 Interestingly, the orientation-dependent change in the relaxation rate observed for four-coordinate LFe II (μ-Cl) 2 Li-Figure 7. Field-and temperature-dependent Mossbauer spectra recorded for S. The solid red lines are simulations obtained using eq 2, and the parameters listed in Tables 1 and 2.…”