Challenges of Measuring Soluble Mn(III) Species in Natural Samples

Abstract

Soluble Mn(III)–L complexes appear to constitute a substantial portion of manganese (Mn) in many environments and serve as critical high-potential species for biogeochemical processes. However, the inherent reactivity and lability of these complexes—the same chemical characteristics that make them uniquely important in biogeochemistry—also make them incredibly difficult to measure. Here we present experimental results demonstrating the limits of common analytical methods used to quantify these complexes. The leucoberbelin-blue method is extremely useful for detecting many high-valent Mn species, but it is incompatible with the subset of Mn(III) complexes that rapidly decompose under low-pH conditions—a methodological requirement for the assay. The Cd-porphyrin method works well for measuring Mn(II) species, but it does not work for measuring Mn(III) species, because additional chemistry occurs that is inconsistent with the proposed reaction mechanism. In both cases, the behavior of Mn(III) species in these methods ultimately stems from inter- and intramolecular redox chemistry that curtails the use of these approaches as a reflection of ligand-binding strength. With growing appreciation for the importance of high-valent Mn species and their cycling in the environment, these results underscore the need for additional method development to enable quantifying such species rapidly and accurately in nature.

Hydrogen Peroxide Disproportionation Activity Is Sensitive to Pyridine Substitutions on Manganese Catalysts Derived from 12-Membered Tetra-Aza Macrocyclic Ligands

“…pH-potentiometric titrations were performed with Mn(II) ions under a N 2 atmosphere since Mn(III) species are known to both easily hydroxylate and disproportionate into Mn(II) and Mn(IV) species, concomitant with formation of MnO x species. 21 Thermodynamic stability (log K ML ) of the Mn(II) complexes of the ligand series (Table 2) shows that the PyN 3 scaffold forms the most stable complex regardless of the pyridine substitution on the 4-position, following the basicity trend observed for the protonation steps of the ligand alone: OH PyN 3 (10.96) > PyN 3 (10.11) > OMe PyN 3 (9.70). The effect of the number of pyridine rings within the ligand scaffold on the log K ML did not follow the trend observed with ligand basicity.…”

“…This approach significantly aids in the goal of preparing each manganese complex in situ. pH-potentiometric titrations were performed with Mn­(II) ions under a N 2 atmosphere since Mn­(III) species are known to both easily hydroxylate and disproportionate into Mn­(II) and Mn­(IV) species, concomitant with formation of MnO x species …”

Evidence of dissolved trivalent manganese in acidic forest soils

“…Because the complexation in all samples was not finished in approximately 90 s (e.g., Figure 1A), a discrimination between Mn 2+ and Mn 3+ was possible in forest floor solutions and soil solutions. More recently, the kinetic evaluation of the porphyrin method has been criticized (Kim et al., 2022). It was stated that the method should be more applicable to single ligand solutions showing the binding affinities of Mn to isolated organic molecules.…”