Determination of zero-field splitting parameters for a MnIV center using variable-temperature, variable-field magnetic circular dichroism spectroscopy: Comparison to electron paramagnetic resonance spectroscopy

The ground and excited state properties of [Mn(OMe)3(Me3TACN)]+ (Me3TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane), which features a mononuclear MnIV center, were investigated using electronic absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field (VTVH) MCD spectroscopies. Due to small zero-field splittings (ZFSs), MnIV centers are difficult to study using X-band electron paramagnetic resonance (EPR) spectroscopy. In this work, VTVH MCD spectroscopy was explored as an alternative method. [Mn(OMe)3(Me3TACN)]+ served as a test complex, because accurate ZFS parameters are available from high-field, high-frequency EPR (HF-EPR) data [33]. Using an E/D of 0, as determined from X-band EPR data, the VTVH MCD data collected for [Mn(OMe)3(Me3TACN)]+ yield an axial ground-state ZFS parameter (D) of similar accuracy, but lower precision, than that obtained from HF-EPR spectroscopy (D = +0.2(1) and +0.250(5) cm−1 from VTVH MCD and HF-EPR data, respectively). Thus, when combined with X-band EPR spectroscopy, the VTVH MCD method can serve as an alternative to HF-EPR spectroscopy even for systems with small ZFSs. In addition, analysis of electronic absorption and MCD spectral data collected for [Mn(OMe)3(Me3TACN)]+ permit the extraction of excited state parameters 10Dq and B (∼18 490 and 595 cm−1, respectively), which are comparable to those of other MnIV complexes.