High-Frequency and -Field EPR Spectroscopy of Tris(2,4-pentanedionato)manganese(III):  Investigation of Solid-State versus Solution Jahn−Teller Effects

Abstract

High-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy of a classical coordination complex, Mn(acac)(3) (Hacac = 2,4-pentanedione), has been performed on both solid powder and frozen solution (in CH(2)Cl(2)/toluene, 3:2 v/v) samples. Parallel mode detection X-band EPR spectra exhibiting resolved (55)Mn hyperfine coupling were additionally obtained for frozen solutions. Magnetic susceptibility and field-dependent magnetization measurements were also made on powder samples. Analysis of the entire EPR data set for the frozen solution allowed extraction of the relevant spin Hamiltonian parameters: D = -4.52(2); |E| = 0.25(2) cm(-1); g(iso) = 1.99(1). The somewhat lower quality solid-state HFEPR data and the magnetic measurements confirmed these parameters. These parameters are compared to those for other complexes of Mn(III) and to previous studies on Mn(acac)(3) using X-ray crystallography, solution electronic absorption spectroscopy, and powder magnetic susceptibility. Crystal structures have been reported for Mn(acac)(3) and show tetragonal distortion, as expected for this Jahn-Teller ion (Mn(3+), 3d(4)). However, in one case, the molecule exhibits axial compression and, in another, axial elongation. The current HFEPR studies clearly show the negative sign of D, which corresponds to an axial (tetragonal) elongation in frozen solution. The correspondence among solution and solid-state HFEPR data, solid-state magnetic measurements, and an HFEPR study by others on a related complex indicates that the form of Mn(acac)(3) studied here exhibits axial elongation in all cases. Such tetragonal elongation has been found for Mn(3+) and Cr(2+) complexes with homoleptic pseudooctahedral geometry as well as for Mn(3+) in square pyramidal geometry. This taken together with the results obtained here for Mn(acac)(3) in frozen solution indicates that axial elongation could be considered the "natural" form of Jahn-Teller distortion for octahedral high-spin 3d(4) ions. The previous electronic absorption data together with current HFEPR and magnetic data allow estimation of ligand-field parameters for Mn(acac)(3).

EPR Spectroscopic Characterization of the Manganese Center and a Free Radical in the Oxalate Decarboxylase Reaction

“…Manganic ion often exhibits parallel mode EPR signals at apparent free electron g-values near 8 -9 (ϳ800 G at X-band), as can be seen in the spectra of salicylidenato Mn(III) and acetylacetonato Mn(III) (28,29), and in the spectrum of oxidized manganese-dependent superoxide dismutase (30). When OxDC is mixed anaerobically with oxalate, such that no turnover may occur, a variety of lines in the parallel mode spectrum with the characteristic sextet splitting of manganese are evident below fields of ϳ1500 G (Fig.…”

Multi-frequency and high-field EPR study of manganese(III) protoporphyrin IX reconstituted myoglobin with an S= 2 integer electron spin

“…In the Mn(III)Mb, the ZFS parameters obtained from the simulated MFEPR spectra do not explain the parallel mode X-band EPR signal originated from the transition between the closely spaced ground non-Kramers doublet (DM s = 4 transition), especially its rhombicity, i.e., E 2 /D (see Appendix). The second-order perturbation theory for the X-band EPR analysis in the low magnetic field region should not be applied and the cubic fourth-order ZFS terms may be considered as described in recent reports [9,25,26]. Thus, traditional experiments are good for detecting the given species, but pretty bad in determining dependable parameters.…”

EMR Data on Mn(III; S=2) Ions in MnTPPCl Complex Modelled by Microscopic Spin Hamiltonian Approach

“…Note that the SOC constant λ in crystal is reduced from the free ion value λ 0 : λ = k × λ 0 , k ≤ 1, due to the orbital (covalency) reduction [16,17,19]. The values λ 0 available in literature range from: 83.4 cm −1 [29], 88 cm −1 [6,30,31], 89 cm −1 [32], 90.4 cm −1 [16], to 110 cm −1 [28]. To take into account the contributions to the ZFSPs and g-factors arising from the higher-lying 3 H multiplet states (see Fig.…”