Structural, spectroscopic, electrochemical, and magnetic properties for manganese(II) triazamacrocyclic complexes

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, Δ E p  = 106 mV) compared to 2 ( E 1/2  = 0.95 V, Δ E p  = 108 mV) and 3 ( E 1/2  = 0.93 V, Δ E 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.