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With the steady decline in availability of non-renewable resources and escalating costs of energy worldwide,1,researchers have begun to harness solar energy by delving into photovoltaic cells and artificial photosynthesis. Despite our best efforts, efficiency for devices such as these has not yet surpassed the output of fossil fuels.

Plants convert solar energy to stored chemical energy through photosynthesis with the help of heterocylic organic compounds known as porphyrins in chlorophyll2 all the time:

The theory behind artificial photosynthesis is to use some porphyrin enzyme mimic modeled after chlorophyll to facilitate the dissociation of water into H2 and O2 gas.3 Porphyrins in these biological contexts—as in cytochrome c and hemoglobin—serve as electron transporters.4 Designing metalloporphyrin photocatalysts in a similar vein would allow for hydrogen gas production through water splitting.

Combustion of hydrogen gas yields water, so the scientific community has a vested interest in pursuing sources of energy that are both renewable and environmentally responsible. Designing cars fueled by hydrogen, for example, is of interest because hydrogen’s energy content per mass is approximately two and a half times that of gasoline.1

In previous studies,5 the Schiff-base Copper(II) complex, [CuLN4](ClO4)2, has not shown any photocatalytic properties. It also is unreactive with strong oxidants like nitric acid. In this study, we seek to further characterize the complex by observing the kinetics of [CuLN4](ClO4)2 with a strong reductant through UV-visible spectroscopy.

Synthesis and Recrystallization Discussion

Redox Kinetics of [CuLN4](ClO4)2Spencer Collinga, Michelle Leatherwooda,

Jeanette A. Krauseb, Justin Stacea*

References

A four-coordinate Copper(II) complex, [CuLN4](ClO4)2 (LN4 = N,N’-bis-(1-pyridin-2-yl-ethylindene)-propane-1,3-diamine) has been synthesized and characterized for potential application as a photocatalyst mimicking the role of chlorophyll. The ligand was synthesized via a condensation reaction. Cu(ClO₄)₂ was added as a chromophore. The dark blue crystalline precipitate was recovered by vacuum filtration and then recrystallized by slow diffusion of diethyl ether into a concentrated acetonitrile solution. [CuLN4](ClO4)2 was then characterized by X-ray crystallography.

1. Fukuzumi, S., Yamada, Y., Karlin, K.D., 2012. Hydrogen peroxide as a sustainable energy carrier: Electrocatalytic production of hydrogen peroxide and the fuel cell. Electrochimica Acta 82, 493-511.

2. Hederstedt, L., 2012. Heme A biosynthesis. Biochimica et Biophysica Acta 1817, 920-927.

3. Bottari, G., Trukhina, O., Ince, M., Torres, T., 2012. Towards artificial photosynthesis: Supramolecular, donor-acceptor, porphyrin- and phthalocyanine/carbon nanostructure ensembles. Coordination Chemistry 256, 2453-2477.

4. Tomo, T., Shinoda, T., Chen, M., Allakhverdiev, S.I., Akimoto, S., 2014. Energy transfer processes in chlorophyll f-containing cyanobacteria using time-resolved fluorescence spectroscopy on intact cells. Biochimica et Biophysica Acta 1837, 1484-1489.

5. Stace, Justin. Research Project Michelle Leatherwood. April 2012. 

6. Gabrielli, C., Beitone, L., Mace, C., Ostermann, E., Perrot, H., 2007. On the behaviour of copper in oxalic acid solutions. Electrochimica Acta 52, 6012-6022.

a: Belmont University, Nashville, TNb: University of Cincinnati, Cincinnati, OH*: Corresponding Author: justin.stace@belmont.edu

Balancing the ionic strength—while allowing for exploration of the rate law—may complicate kinetics of reducing agents and the [CuLN4](ClO4)2 complex. Spectral data collected to identify the rate law suggests the presence of intermediate(s), and the biexponential behavior in the decay band may indicate the generation of multiple products.

Best fits for the products of the reaction between the CuLN4](ClO4)2 complex and oxalic acid suggest that some organometallic complex and an unknown colorless product are formed. Previous literature6 also leads us to infer a complexation between the Cu2+ and some partially or fully depronated form of oxalic acid. The colorless product may represent the dissociated LN4 ligand as some of this complexation occurs. Mechanistic proposals have not yet included other anions in solution—namely, chloride and perchlorate ions.

Introduction

Future StudiesThe rate law and mechanism of the [CuLN4](ClO4)2

complex and oxalic acid will be further explored in order to decipher the rich kinetics.

Further mechanistic studies will pursue identification of the products formed by this reaction to solidify a mechanism.

The thermodynamic parameters of the equilibrium between the [CuLN4](ClO4)2 complex and oxalic acid, such as ∆G, ∆S, and ∆H, shall be determined.

Comparative studies between reactions of other reductants like ascorbic acid and the [CuLN4](ClO4)2 complex will be performed.

Studies to identify binding constants for LN4 ligand and other ions in solution can also elucidate rich kinetics.

Electrochemical studies will be performed to further characterize the [CuLN4](ClO4)2 complex.

Spectroscopic AnalysisAn 8.26 mM stock solution of the [CuLN4](ClO4)2 complex in a 55 mM solution of NaCl was combined at constant temperature

(48.2 degrees Celsius) with various dilutions of 60 mM oxalic acid via the Agilent SFA-20 Rapid Mixing Accessory. UV-visible spectra from these reactions showed a decrease in intensity over time at the complex’s reactant band (622 nm) and an increase at the product band (960 nm). The extinction coefficient of the [CuLN4](ClO4)2 complex in NaCl was found to be ε622 = 105 M-1cm-1.

After identifying the reactant and product bands, 8.26 mM of the [CuLN4](ClO4)2 complex in NaCl solution was mixed with 60.55 mM of oxalic acid, and absorbance readings at the two bands were taken over a period of 20 minutes at 48.2°C. The readings reflected a decrease in intensity at the reactant band and an increase in the intensity at the product band. Various dilutions of 8.26 mM of the [CuLN4](ClO4)2 complex were mixed with 60.55 mM of oxalic acid to further characterize the decay band.

Pro-Kineticist software was used to analyze both the single-point (622 nm, 960 nm) and full-spectrum data for mechanism modeling. Preliminary modeling for the reaction between the [CuLN4](ClO4)2 complex and oxalic acid led to a mechanism wherein two colored species—assigned as [CuLN4]2+ and Cu(Ox), (HCu(Ox))+, or (Cu(Ox)2) 2—and one colorless species are formed by the reaction.

N

N N

N

"LN4"

NH2NH2 N

O

+ 2

1,3-diaminopropane 2-Acetylpyridine

CH3OHReflux, 3h

+ Cu(ClO4)2CH3OH1h

N

N N

NCu

2+

2 ClO4-

[CuLN4](ClO4)2

Time-Resolved Vis-NIR Spectra: [CuLN4](ClO4)2 + H2C2O4

Distribution of Complexed SpeciesIn a Copper Oxalate Solution

UV-visible spectrum of decay band (622 nm)