The Nature of the Electron Transfer Agent in the Dimmer Photolysis - Research Paper Example

Summary of Review Article Introduction: Organometallic radicals that do not follow the conventional 18-electron norm exist, and this has been established. This has specific relevance to the 17-electron organometallic complexes of chromium, molybdenum, and tungsten, which have been the subject of several studies. Over the last ten years a lot of significant information has been gathered on the chemistry of the 17-electron organometallic complexes. However, most of this information relates to the mechanistic aspects, and structural information on the chemistry of the organometallic complexes. For a better understanding of the chemical reactivity of the organometallic complexes, it is necessary to look into the thermodynamic data of the chemistry of organometallic complexes. This paper attempts to provide a perspective of the thermodynamic aspects through electrochemical analysis. Aims: The aims of this study include providing additional information on the nature of the electron transfer agent in the dimmer photolysis. One of the options is the 17-electron species. Recent studies have suggested that the 19-electron radical CpM(CO)3NCMe is more likely to be the species that is being oxidized, and this has arisen from the cyclic voltammetry studies of the dimmer Cp2M2(CO)6. Making use of a recently emergent electro-chemical technique, known as the photo modulated voltammetry the study aims at better evaluation of the electrochemical nature of the organometallic complexes. Through these investigations, the study also aims at providing additional information on the more stable odd-electron complexes like CpCr(CO)3. The study also expects that as a result of the frequency domain nature of PMV, it would be possible to get information on the kinetics of the of the intermediate that is under investigation. The study also aims to provide a better understanding of the nature of the electro active species, through the analysis of the observations of the electrochemical potentials in combination with the preliminary kinetic information. Finally the study aims at looking at the suitability of photo-modulated voltammetry in the study of odd-electron inorganic complexes. Methodology: Using appropriate chemical techniques materials required for the study like cyclopentadienyl ligand, and methylcyclopentadienyl ligand were prepared. The oxidized surface of Sodium (Fisher) was taken off before making use of it. Re-crystallization of the metal hexacarbonyls M(CO)6 in acetone was done prior to their use in the study. Cr(CO)6 was stored under nitrogen and used as received. Tetrabutylammonium hexaflourophosphate was used in all the experiments, used as received. Spectrograde solvents were used in all the spectroscopic and electrochemical measurements, and included acetonitrile, dimethyl formamide, methylene chloride, and tetrahydrofuran. All these were stored under inert atmosphere and used as received. Precursor dimmers were prepared as per earlier procedures. All the reactions were conducted under nitrogen, using N2-bubble-deoxygenated solvents. Recordings of the UV-Vis spectra were done with the help of a Perkin-Elmer Lambda 2s UV-vis spectrometer, using 1 cm matching quartz cells. Recordings of the infrared spectra were done with the help of a Nicolet 20SXC FTIR spectrometer, using a 1 mm path length sodium chloride solution. Conventional electrochemical measurements were all performed with the help of an EG&G PAR 273A potentiostat that was coupled to a personal computer. Using an N2-degassed 0.001M solution of dimmer in 0.1-0.2 M TBAH/(CHCN, DMF, CH2CL2, THF) solution a typical electrochemical reaction was conducted. When the electrochemical reaction was conducted using the chromium dimmer, cyclic voltammetry was conducted in a vacuum atmosphere, in a N2 glove box. The three-electrode system that was used in all the electrochemical experiments consisted of a platinum button-working electrode, a coiled platinum wire of 20 gauge as counter electrode, and a freshly prepared silver/silver chloride (22 gauge) reference electrode. The scan rates use for cyclic voltammetry ranged from 50 to 500 mV/s. In the case of photo modulated voltammetry the 488nm continuous wave line emitted by an argon ion laser was modulated with the help of a low voltage electrooptic light modulator. A variable 10 MHz function generator was used to drive the power supply/amplifier for the light modulator. The sinusoidally modulated light then becomes incident on the electrochemical flow cell. A comparison was effected on the ac component of the electro chemical signals received by a PAR 273A potentiostat and the reference signal of the function generator from an EG&G PAR 5210 lock in amplifier. By measuring the signal from a fast photodiode assembly with reference to the signal of the function generator with the help of a Tektronix digitizing oscilloscope, the determination of the phase shift for lifetime analysis was corrected. All potentials were reported in comparison to the saturated calomel electrode. In the case of typical photo modulated voltammetry experiments, the argon ion laser was set to 488nm with a power that was incident on the cell, of 0.250-0.350 W watts that was measured with the help of a power meter. The modulated frequency was fixed and measured at a specific frequency that ranged from 100Hz to 10Khz. The observed potentials were maintained independent of the excitation frequency over a range of measured kHz. For every experiment a solvent solution of the analyte and supporting electrolyte was N2 degassed for a period of fifteen minutes in a closed system. With regard to the chromium dimmer the solvent was prepared in a glove box and stored within a sealed system that was protected from light. A positive pressure of N2 gas was used to on the solution that was delivered through an electrochemical flow cell, so that a flow rate of approximately 5mL/min was achieved and maintained. In order to ensure that a voltammetric steady was achieved and maintained the scan rates of 4-10mV/s were made use of. For every experiment, cyclic voltammetry was conducted on an internal ferrocene standard, in order that it would be possible to determine the actual potentials observed in comparison to the silver/silver chloride electrode that was being used as a reference. Electro-chemical measurements conducted on the dimeric precursors to the 17-electron photoproducts provided the purity of the materials used in the study. Every reduction or oxidation process followed an ECE mechanism. Solutions of the M=Cr system are known to contain a substantial and steady state of 17-electron radicals. The one to ten percent solutions were adequate for direct observation using electrochemical methods and suitable for measurements of dimerization constants. A comparison of the literature results with the potential determined for the reduction of the 17-electron radical (n5-C5H5)Cr(CO)3. The comparison showed a very suitable agreement between the two. The monomethylated dimmers that were studied also displayed adequate quantities of the 17-electron radical (n5-C5H5)Cr(CO)3. The reversible couple at E1/2 =-o.34 volts was assigned to the reduction of the 17-elctron radical (n5-C5H5)Cr(CO)3. At more negative potentials reduction of the anion was observed. Conclusion: On the basis of the findings of the study the several conclusions were arrived at. A critical element to the complete understanding and dynamic reactivity of the odd-electron complex is the assignment of accurate electrochemical potentials to these odd-electron complexes. The results obtained by this study demonstrate for the first time that in the study of inorganic radical, photo modulated voltammetry provides a very adequate and additional option. The electrochemical results that this study has thrown up give direct support to the quality of the many of the previous estimates of these potentials, and as such this study extends the possibility of more studies with greater depth into similar intermediates. The lifetime information as a result of this study are crude, when compared to the more established spectroscopic determination, and yet offer an insight into the nature of the electro active species in intermolecular electron transfer reactions as studied previously. As a final conclusion the study recommends that there is a need for further studies to evaluate the utility of photo modulated voltammetry in the case of kinetic measurements. (Barbini. et al. 1996). Works Cited 1. Barbini. et al. 1996. Direct Electrochemical Investigations of 17-Electron Complexes of CpM(CO)3 (M = Mo, W, and Cr). Inorganic Chemistry 35: 4017-4022. Read More