Redox Data of Tris (polypyridine)manganese(II) Complexes

: Very little cyclic voltammetry data for tris (polypyridine)manganese(II) complexes, [Mn II (N^N) 3 ] 2+ , where N^N is bipyridine (bpy), phenanthroline (phen) or substituted bpy or phen ligands, respectively; are available in the literature. Cyclic voltammograms were found for tris (4,7 ‐ diphenyl ‐ 1,10 ‐ phenanthroline)manganese(II) perchlorate only. In addition to our recently pub ‐ lished related research article, the data presented here provides cyclic voltammograms and corre ‐ sponding voltage ‐ current data obtained during electrochemical oxidation and the reduction of four [Mn II (N^N) 3 ] 2+ complexes, using different scan rates and analyte concentrations. The results show increased concentration and scan rates resulting in higher Mn(II/III) peak oxidation potentials and increased peak current ‐ voltage separations of the irreversible Mn(II/III) redox event. The average peak oxidation and peak reduction potentials of the Mn(II/III) redox events stayed constant within 0.01 V. Similarly, the average of the peak oxidation and reduction potentials of the ligand ‐ based reduction events of [Mn II (N^N) 3 ] 2+ were constant within 0.01 V.

The relevance of scan rate studies is to establish how redox behavior changes with scan rate, as well as to analyze the type of redox processes observed [10,11]. The present article provides, in addition to data provided in the related research article [3], redox data of four [Mn II (N^N)3] 2+ complexes containing the bidentate ligands N^N = bipyridine, 4,4′di-methoxy-2,2′-dipyridyl, 4,4′-di-tetra-butyl-2,2′-dipyridyl and 3,4,7,8-tetramethyl-1,10phenanthroline, each coordinated through two N atoms to Mn (see Scheme 1). Cyclic voltammograms were obtained at scan rates ranging over more than two orders of magnitude (0.05 to 5.12 Vs −1 ) and concentrations varying over one order of magnitude (0.001 to 0.010 mol dm −3 ), using acetonitrile as a solvent. The redox data of the tris(polypyridine)manganese(II) complexes were added to the published redox data of related complexes of Fe [12], Ru [13], Os [14] and Co [15,16]. The information herein is helpful to researchers who design complexes with specific redox properties, for example, as may be required in studies of redox mediators in dye-sensitized solar cells [17,18], antimicrobial activity [19,20] or molecular catalysts for CO2 and/or H2O reduction [21]. Scheme 1. Complex numbering and structure of Mn(II) polypyridine complexes.

Data Description
Results obtained from voltage-current data of the cyclic voltammograms (Figures 1-4 for oxidation, Figure 5 for reduction) of four Mn(II) complexes are presented in Tables  1-8 of this article. These four complexes, 1-4, contain different polypyridine ligands, namely, bipyridine, substituted bipyridine and substituted phenanthroline ligands (see Scheme 1). The cyclic voltammograms presented herein provide more detailed electrochemical data for 1-4 (i.e., at different scan rates and concentrations) than presented in the related research article [3], wherein only data from one scan rate (0.10 V s −1 ) and concentration (0.005 mol dm −3 ) were presented. A full electrochemical dataset of the related [Mn(phen)3](PF6)2 complex is available in the related research article [3].  [3] are representative of the Mn(II/III) redox process. The CV data illustrate increased concentrations and scan rates resulting in increased peak oxidation potentials of the irreversible Mn(II/III) redox transitions. The average of the peak oxidation (Epa) and peak reduction potentials (Epc), namely ½(Epa + Epc) of the Mn(II/III) redox couple, however, stayed within 0.01 V of the average value, irrespective of the increase in the peak current voltage separations upon increases in the scan rate and analyte concentration (e.g., see Figure 1a). The large peak current voltage separations (ΔEp) and small peak current ratios clearly indicate the Mn(II/III) oxidation process to be chemically and electrochemically irreversible [10,11].  Table 1. The Epa and Epc of the 5.12 V/s scan and the average E1/2 value are indicated in (a). Table 1. Electrochemical oxidation data (potential in V vs. Fc/Fc + ) obtained from the CV of tris(bipyridine)manganese(II) hexafluorophosphate (1), in acetonitrile (CH3CN) as solvent at the indicated scan rates ν (Vs −1 ) and concentrations C (mol dm −3 ).   Table 2. Average E1/2 value is indicated in (a).  (2), in acetonitrile (CH3CN) as solvent at the indicated scan rates ν (Vs −1 ) and concentrations C (mol dm −3 ).   Table 3. Average E1/2 value is indicated in (a).   Table 4. Average E1/2 value is indicated in (a).  Figure 5 shows the CVs obtained during the electrochemical reduction of complexes 1-4, using different scan rates. Tables 5-8 list the electrochemical data of the reduction of the complexes corresponding to Figure 5a-d, respectively. The reduction is ligand-based [3]. Complexes 1-3 each show three reduction peaks, corresponding to consecutive reductions in the three coordinated N^N ligands. Data of only the first 2 reduction peaks of 4 are shown, since the reduction of 4 is irreversible. The data illustrate that with the increased scan rate, the peak current voltage separations increase for 1-3, but the average of the peak oxidation (Epa) and peak reduction potentials (Epc) of each reduction peak, namely ½(Epa + Epc), stayed within 0.01 V of the average value.   a Data of 5.120 Vs −1 scan not included in average for peak 2. Table 6. Electrochemical reduction data (potential in V vs. Fc/Fc + ) obtained from the CV of tris(4,4′di-methoxy-2,2′-dipyridyl)manganese(II) hexafluorophosphate (2), in acetonitrile (CH3CN) as solvent at the indicated scan rates ν (Vs −1 ). Analyte concentration = 0.0033 mol dm −3 .

Methods
Complexes 1-4 were synthesized, purified and characterized as described in the related research article [3]. The cyclic voltammetry (CV) scans were obtained under similar conditions as described therein, as well as in our previous work [22,23]. The CV experiments were performed on a BAS100B Electrochemical Analyzer connected to a desktop computer containing the BAS100W version 2.3 software. The software provides currentvoltage and peak current--voltage data. The obtained current-voltage data were exported to Excel for further evaluation and visualization.
The analyte solution in a three-electrode cell was purged with argon gas before and throughout the CV experiment. Care was taken to keep the volume of the analyte solution constant, since the obtained current-voltage data depends on the analyte concentration as well as scan rate.
For the purpose of obtaining reproducible results, firm polishing of the working electrode before each CV scan was necessary, since electrode poisoning occurs during CV scans. Polishing of the working electrode was done on a Bühler polishing mat, moving the flat tip of the electrode in a figure-eight motion, at first using 1-micron diamond paste. The electrode was then rinsed with ethanol, water and acetonitrile, and dried. The polishing procedure was then repeated using ¼-micron diamond paste.