A library of 10 Mn-containing complexes capable of switching reversibly between the Mn(II) and Mn(III) oxidation states was prepared and evaluated for potential usage as MRI reporters of tissue redox activity. We synthesized N-(2-hydroxybenzy1)-N,N',N'-ethylenediaminetriacetic acid (HBET) and N-(2-hydroxybenzyl-N,N',N'-trans-1,2-cyclohexylenediaminetriacetic acid (CyHBET) ligands functionalized (-H, -OMe, -NO2) at the 5-position of the aromatic ring. The Mn(II) complexes of all ligands and the Mn(11) complexes of the 5-H and 5-NO2 functionalized ligands were synthesized and isolated, but the Mn(III) complexes with the 5-OMe functionalized ligands were unstable. H-1 relaxivity of the 10 isolable complexes was measured at pH 7.4 and 37 degrees C, 1.4 T. Thermodynamic stability, pH-dependent complex speciation, hydration state, water exchange kinetics of the Mn(B) complexes, and pseudo-first order reduction kinetics of the Mn(III) complexes were studied using a combination of pH-potentiometry, UV-vis spectroscopy, and H-1 and O-17 NMR measurements. The effects of ligand structural and electronic modifications on the Mn(II/III) redox couple were studied by cyclic voltammetry. The Mn(II) complexes are potent relaxation agents as compared to the corresponding Mn(III) species with [Mn-II(CyHBET)(H2O)](2-) exhibiting a 7.5-fold higher relaxivity (3.3 mM(-1) s(-1)) than the oxidized form (0.4 mM(-1) s(-1)). At pH 7.4, Mn(II) exists as a mixture of fully deprotonated (ML) and monoprotonated (HML) complexes and Mn(II) complex stability decreases as the ligands become more electron-releasing (pMn for 10 mu M [Mn-II(CyHBET-R'(H2O)](2-) decreases from 7.6 to 6.2 as R' goes from -NO2 to -OMe, respectively). HML speciation increases as the electron-releasing nature of the phenolato-O donor increases. The presence of a water coligand is maintained upon conversion from HML to ML, but the water exchange rate of ML is faster by up to 2 orders of magnitude (kin for H[Mn-II(CyHBET)(H2O)](-) and [Mn-II(CyHBET)(H2O)](2-) are 1.2 x 10(8) and 1.0 x 10(10) s(-1), respectively). The Mn(II/III) redox potential can be tuned over a range of 0.30 V (E-1/2= 0.27-0.57 V) through electronic modifications to the 5-substituent of the aromatic ligand component. However, care must be taken in tuning the ligand electronics to avoid Mn(II)-ligand autoredox. Taken together, these results serve to establish criteria for optimizing Mn(III) versus Mn(II) relaxivity differentials, complex stability, and Mn(II/III) redox potential.