Abstract
We report the synthesis of [Mn(tacud)(2)](OTf)(2) (1) (tacud = 1,4,8-triazacycloundecane), [Mn(tacd)(2)](OTf)(2) (2) (tacd = 1,4,7-triazacyclodecane), and [Mn(tacn)(2)](OTf)(2) (3) (tacn = 1,4,7-triazacyclononane). Electrochemical measurements on the Mn(III/II) redox couple show that complex 1 has the largest anodic potential of the set (E (1/2) = 1.16 V vs NHE, ΔE (p) = 106 mV) compared to 2 (E (1/2) = 0.95 V, ΔE (p) = 108 mV) and 3 (E (1/2) = 0.93 V, ΔE (p) = 96 mV). This is due to the fact that 1 has the fewest 5-membered chelate rings and thus is least stabilized. Magnetic studies of 1-3 revealed that all complexes remain high spin throughout the temperature range investigated (2 - 300 K). X-band EPR investigations in methanol glass indicated that the manganese(II) centers for 2 and 3 resided in a more distorted octahedral geometric configuration compared to 1. To ease spectral interpretation and extract ZFS parameters, we performed high-frequency high-field EPR (HFEPR) at frequencies above 200 GHz and a field of 7.5 T. Simulation of the spectral data yielded g = 2.0013 and D = -0.031 cm(-1) for 1, g = 2.0008, D = -0.0824 cm(-1), |E/D| = 0.12 for 2, and g = 2.00028, D = -0.0884 cm(-1) for 3. These results are consistent with 3 possessing the most distorted geometry. Calculations (PBE0/6-31G(d)) were performed on 1-3. Results show that 1 has the largest HOMO-LUMO gap energy (6.37 eV) compared to 2 (6.12 eV) and 3 (6.26 eV). Complex 1 also has the lowest HOMO energies indicating higher stability.