Fourth-order zero-field splitting parameters of [Mn(cyclam)Br2]Br determined by single-crystal W-band EPR

Abstract

Single-crystal W-band (95 GHz) electron paramagnetic resonance (EPR) studies have been performed at 20 K and at room temperature on a tetragonal Mn(III) compound with potential application as a building block for high-spin clusters. The observed EPR spectra correspond to an anisotropic high-spin S = 2 ground state and have been attributed to equivalent centers related by fourfold symmetry. Accurate values for the spin Hamiltonian parameters were obtained from the analysis of the data at both temperatures. At 20 K the contribution of fourth-order zero-field splitting terms was shown to be significant, with parameter values Bá = 0.0009(3) cm -', B = 0.0006(2) cm-' and B = 0.0017(3) cm -', to be considered together with the second-order parameters D = -1.1677(7) cm -' and E = -0.0135(6) cm -'.

Anisotropic Hyperfine Interaction in the Manganese(III) Hexaaqua Ion

“…The parameters E and B are somewhat negatively correlated, and a precise definition requires data from a range of frequencies, including X-band measurements, which are tractable only if the EPR linewidths are sufficiently narrow. The similar values of D suggests that the structure and bonding in the [Mn(OD 2 ) 6 ] 3+ is similar in the two systems.…”

“…Information concerning the spatial property of the hyperfine interaction matrix requires the use of higher frequencies, as shown in a study of manganese(iii)-doped rutile. [12] In this communication we report single-crystal and powder EPR spectra of the [Mn(H 2 O) 6 ] 3+ ion at high fields and multiple frequencies. To minimize line broadening due to spin-spin dipolar interactions, the [Mn(H 2 O) 6 ] 3+ ion was doped into the diamagnetic cesium gallium alum, Cs-[Ga(H 2 O) 6 ](SO 4 ) 2 ·6 H 2 O, thus enabling the precise determination of the metal hyperfine interaction parameters.…”

“…Each electronic transition is doubled but this doubling is not found in the powder spectra, representative examples of which are shown in Figure 2. Therefore, the doubling must be due to either crystal twinning, a small misalignment when orienting the crystal, or a result of magnetically nonequivalent [Mn(OH 2 ) 6 ] 3+ ions that differ in terms of the orientation of the threefold axes for the sites on which they lie. Rotation of the magnetic field less than 108 away from the fourfold axis has the effect of rendering all sites magnetically nonequivalent, and the complexity of the spectrum increases enormously.…”

“…The single-crystal EPR spectra exhibited several magnetically nonequivalent species, reflecting the space group of the host and the propensity of the [Mn(OH 2 ) 6 ] 3+ ion to distort along a Jahn-Teller active coordinate. There are four tervalent cations in the cubic unit cell, each occupying a site of S 6 symmetry on one of the four unique threefold axes of the unit cell.…”

“…[6] (cyclam = 1,4,8,11-tetraazacyclotetradecane). An analysis of this result, however, is not relevant in the present context and will not be discussed here.…”