Origin of the Zero-Field Splitting in Mononuclear Octahedral Mn^IV Complexes: A Combined Experimental and Theoretical Investigation

Abstract

The aim of this work was to determine and understand the origin of the electronic properties of Mn(IV) complexes, especially the zero-field splitting (ZFS), through a combined experimental and theoretical investigation on five well-characterized mononuclear octahedral Mn(IV) compounds, with various coordination spheres (N6, N3O3, N2O4 in both trans (trans-N2O4) and cis configurations (cis-N2O4) and O4S2). High-frequency and -field EPR (HFEPR) spectroscopy has been applied to determine the ZFS parameters of two of these compounds, MnL(trans-N2O4) and MnL(O4S2). While at X-band EPR, the axial-component of the ZFS tensor, D, was estimated to be +0.47 cm(-1) for MnL(O4S2), and a D-value of +2.289(5) cm(-1) was determined by HFEPR, which is the largest D-magnitude ever measured for a Mn(IV) complex. A moderate D value of -0.997(6) cm(-1) has been found for MnL(trans-N2O4). Quantum chemical calculations based on two theoretical frameworks (the Density Functional Theory based on a coupled perturbed approach (CP-DFT) and the hybrid Ligand-Field DFT (LF-DFT)) have been performed to define appropriate methodologies to calculate the ZFS tensor for Mn(IV) centers, to predict the orientation of the magnetic axes with respect to the molecular ones, and to define and quantify the physical origin of the different contributions to the ZFS. Except in the case of MnL(trans-N2O4), the experimental and calculated D values are in good agreement, and the sign of D is well predicted, LF-DFT being more satisfactory than CP-DFT. The calculations performed on MnL(cis-N2O4) are consistent with the orientation of the principal anisotropic axis determined by single-crystal EPR, validating the calculated ZFS tensor orientation. The different contributions to D were analyzed demonstrating that the d-d transitions mainly govern D in Mn(IV) ion. However, a deep analysis evidences that many factors enter into the game, explaining why no obvious magnetostructural correlations can be drawn in this series of Mn(IV) complexes.

Experimental and Theoretical Identification of the Origin of Magnetic Anisotropy in Intermediate Spin Iron(III) Complexes

“…The origin of the magnetic anisotropy, and even the trend in values, differs from our earlier studies on the same ligand system with Co III ( S= 1),10e and different complexes with S= 3/2 (Mn IV ),13c which is a clear indication that any change induces different magnetic behavior depending on the spin system and analogies cannot be drawn a priori with systems that are yet to be investigated. Understanding of these effects and the potential for DFT methods to predict them, opens new opportunities in the rational design of magnetic materials with desired properties.…”

“…Although predicted D values are in good agreement with the experimental data, the calculated E/D values display significant deviation, which is often observed in aD FT framework. [13] Generally,t he main factor driving the magnetic anisotropy is the spin-orbit coupling (SOC). Interestingly,t he trend in magnetic anisotropy is seen in which D is decomposed into its SOC (D SOC )a nd spin-spind ipolar contribution (D SSC )t erms using CP-DFT (Table 3).…”

Near-infrared 2Eg → 4A2g and visible LMCT luminescence from a molecular bis-(tris(carbene)borate) manganese(IV) complex

“…S1 [32][33][34][35] and Mn(IV). 31,[36][37][38][39] This result indicates that there is nothing unusual about the spin quartet ground state of the title complex.…”

Dinuclear manganese(iii) complexes with bioinspired coordination and variable linkers showing weak exchange effects: a synthetic, structural, spectroscopic and computation study

“…Although a negative EPR result at 77 K is not proof of the inability to electrochemically generate Mn IV species in solution, we note that a variety of bona fide Mn IV coordination complexes exhibit EPR spectra (even in some cases as magnetically undiluted solids) at temperatures as high as ambient. [65][66][67] However, there are small differences in the voltammetric responses found for 1-3. The highest oxidation potential was found for 2 with only one oxidation peak demonstrating the (electro)chemical equivalence of the two Mn III sites and the lack of measurable interaction between them, consistent with the magnetometry and HFEPR.…”