Reports: ND350971-ND3: Electrocatalytic Water Oxidation by Manganese Pyridinophane Complexes

Jeremy M. Smith, New Mexico State University

1. Synthesis, characterization and reactivity of (Py2NR2)Mn2+ complexes

Fig. 1. A. (Py2NR2)Mn2+ complexes under investigation. B. X-ray crystal structure of (Py2NR2)Mn(NCMe)22+; PF6- counterions not shown.
Metallation of the pyridinophane ligands is readily achieved by reaction of the appropriate macrocycle with MnBr2. The resulting colorless (Py2NR2)MnBr2 (R = H, Me, tBu) complexes have good solubility and stability over a wide pH range. Conductivity measurements reveal the 
Fig. 2. X-ray crystal structure of [(Py2NH2)MnIII(m-O)2MnIV(Py2NH2)]3+; counterions not shown.
complexes to be fully ionized in neutral aqueous solution.

In the solid state, all the complexes are air stable, however in aqueous solution, the complex with R = H shows a slight change in color over the course of a few days, possibly due to aerobic oxidation. Anion metathesis with nitrate increases the rate of coloration, eventually forming the green complex [(Py2NH2)MnIII(m-O)2MnIV(Py2NH2)]3+, which has been characterized by X-ray crystallography (Fig. 2). This complex has similar structural features as reported [MnIII(m-O)2MnIV] dimers. The same air sensitivity is not observed for R = Me, tBu, likely because the bulkier R groups prevent the formation of the [Mn(m-O)2Mn] diamond core.

   Fig. 3. Kinetics of O2 formation from H2O2 disproportionation, (Py2NMe2)Mn2+ catalyst. Initial pH 4.4, 1 mM catalyst, maximum yield of O2 is 120 mL. Inset: (Py2NH2)Mn2+ catalyst. 2. H2O2 disproportionation

We have investigated the manganese pyridinophane complexes as hydrogen peroxide disproportionation catalysts (i.e. catalase mimics) due to the potential relationship between catalase and water oxidation reactivity. The complexes with R = H, Me are found to be good hydrogen peroxide disproportionation catalysts in aqueous solution. There is no catalase reactivity when R = tBu. Preliminary kinetics investigations using volumetric measurements of O2 formation reveal that when R = H, the complex has moderate activity, but limited robustness, having a turnover number (TON) of 830 at pH 4 (Fig. 3, inset). The complex is active under acidic and basic conditions, with the greatest activity and longevity at pH = 9 (initial rate = 40 mmol O2 /s, TON 1940). When R = Me, the catalyst is more robust (TON > 66000), albeit slower (Fig. 3). The activity of this particular catalyst is remarkable because: (1) the TON is several orders of magnitude greater than any other reported catalase mimics; and (2) it is active in aqueous solution, conditions under which most catalase mimics are not stable. We are currently undertaking a more comprehensive investigation of the kinetic behavior of these complexes to more fully delineate their catalase activity, as well as other likely oxidative behavior (e.g. peroxidase reactivity). This will allow for better comparison with previously reported catalase mimics.
   Fig. 4. CV showing catalytic current for (Py2NtBu2)Mn2+. Inset: Controlled potential electrolysis (+1.63 V) of 1 mM (Py2NtBu2)Mn2+, pH 12.9, glassy carbon electrode. The background current has been subtracted.3. Electrochemical water oxidation

The cyclic voltammograms of (Py2NR2)Mn2+ (R = Me, tBu) in basic buffer (pH 12.9) both reveal catalytic waves with an overpotential for water oxidation of ca. 830 mV (Fig. 4). Bulk electrolysis experiments provide further evidence for catalysis, with good stability when R = tBu (Fig. 4, inset). The formation of gas is observed at the working electrode is observed. Control experiments to verify the homogeneous catalytic water oxidation reactivity of this complex are in progress. Our working hypothesis is that the bulkier tBu substituents prevent the formation of dead-end [Mn(m-O)2Mn] diamond core structures and divert the reactivity away from H2O2 disproportionation towards water oxidation. Mechanistic and computational studies to better understand the reaction mechanism will be conducted in the upcoming year.
  4. Electrochemical water reduction

Fig. 5. Catalytic waves for (Py2NH2)Mn2+ and (Py3NH3)Mn2+. Inset: Controlled potential electrolysis (-1.47 V) of 1 mM catalyst solution, pH 6.0, glassy carbon electrode. The background current has been subtracted.
Although (Py2NH2)Mn2+ does not catalyze water oxidation, we were pleasantly surprised to observe a catalytic wave with an overpotential of ca. 1.4 V for water reduction (Fig. 5). Bulk electrolysis (pH 6) reveals the complex to be unstable under these conditions, with complete loss of activity within 3 h of electrolysis (Fig. 5, inset). Despite this setback, this is a very exciting result as we are unaware of any reports of water reduction by a manganese complex under aqueous conditions. The other pyridinophane complexes, (Py2NR2)Mn2+ (R = Me, tBu) decompose with the deposition of metallic manganese under the same conditions. The macrocycle N-H protons are therefore critical to the observed reactivity, possibly acting as proton donors.

Since it is likely that only one coordination site is necessary for proton reduction, we decided to investigate the water reduction reactivity of the  Fig. 6. Proposed structure of (Py3NH3)Mn2+ in aqueous solution.related complex, (Py3NH3)Mn2+. The macrocycle in this complex is proposed to act as a pentadentate ligand (Fig. 6). This binding mode provides a free amine donor with sufficient flexibility to act as a proton donor. Gratifyingly, this complex has a larger catalytic current at slightly lower overpotential (Fig. 5) and substantially greater stability under catalytic conditions (Fig. 5, inset). The formation of a gas at the working is electrode is also observed during bulk electrolysis, which we are in the process of detecting and quantifying. We plan to extend this investigation to other metals that are easier to reduce.
  5. Career Impact and Student Impact

The PI is a member of a team from three campuses in the state (NM Tech, NMSU and UNM) that is developing methods for the use of solar fuels. The results from the ACS-PRF supported project have been used as preliminary data as part of a solar fuels sub-project ($2 million) of the New Mexico EPSCoR renewal application.

A graduate student is currently employed on the project, beginning August 2012, and will continue to be employed for the upcoming year. This research assistant support will allow him to devote more time to the project.