UVM Theses and Dissertations
Format:
Print
Author:
LeSuer, Robert J.
Dept./Program:
Chemistry
Year:
2003
Degree:
Ph. D.
Abstract:
An exhaustive electrochemical study of bis(maleonitriledithiolato)gold(II) has been conducted under a variety of supporting electrolyte and solvent conditions. The reduction of the formally gold(III) starting material consists of two discrete reversible one-electron transfers. The first reduction to the formally Au(II) complex is chemically and electrochemically reversible and has been the subject of several previous investigations. Further reduction to the presumably Au(I) species is complicated by slow electron transfer and a chemical follow-up reaction. Spectroscopic evidence and literature precedence help to formulate the cause of the slow charge transfer as a square-planar to tetrahedral structural change. The chemical follow-up reaction has been shown to be dependent on solvent and supporting electrolyte. The electrochemical behavior of bis(maleonitriledithiolato)gold(III) in tetrahydrofuran has been modeled as an EECrev mechanism with the Crev step being a medium effect involving ion pairing and solvation.
The fate of the electrochemically generated Au(I) is dependent on solvent parameters and consists of competition between an ion-pairing and solvation interactions with the nucleophilic sulfur atoms of the ligand. Oxidation to the neutral Au(IV) species is chemically and electrochemically reversible on the CV timescale, however, attempts to generate the neutral complex in the bulk of solution are complicated by auto-reduction and poor solubility. Under the appropriate conditions of low concentration and sub-ambient temperatures, one can generate solutions of the neutral complex. ESR spectra of the complex show a significant quadrupole effect which manifests itself in a field-dependent hyperfine splitting pattern. These results have been used to speculate on the structure of the neutral complex, which is likely to form a weak dimer in solution.
The use of supporting electrolytes containing the weakly coordinating anion tetrakis(pentafluorophenyl)borate [TFAB]- has been of recent interest. Supporting electrolytes containing this anion have been used in low-polarity solvents such as tetrahydrofuran and dichloromethane to address issues such as reactivity and solubility of polycations as well as a general increase in the conductivity of these solvents. This work provides a quantitative analysis of the conductivity of tetrabutylammonium tetrakis(pentafluorophenyl)borate in dichloromethane, tetrahydrofuran and benzotrifluoride with a comparison to "traditional" electrolytes that typically contain hexafluorophosphate or tetrafluoroborate anions. Association constants calculated using the paired ion model by Fuoss indicate that salts containing [TFAB]- are dissociated at least two orders of magnitude more than their "traditional" anion analogues. This work also demonstrates a conductometric method that can be readily incorporated into research projects that would be complemented by conductivity data but do not have conductivity as a main focus.
Fluorous biphasic systems (FBS) have been recently used to facilitate various chemical transformations. This work presents the development of a methodology with which to conduct electrochemistry in highly fluorous media. Use of an imidazolium salt containing fluorous groups on the anion and cation provides the solubility and conductivity necessary to perform electrochemistry in a 1:1 by volume mixture of perfluoromethylcyclohexane:benzotrifluoride. Progress has also been made in the use of FBS to facilitate the electrochemical synthesis of a fluorinated (benzene)chromiumdicarbonylphosphine complex. An important result from this work is the facile separation of a nearly 100-fold excess of supporting electrolyte from the electrochemically generated product by utilizing the solubility properties and thermally regulated miscibility of fluorous:organic mixtures.
The fate of the electrochemically generated Au(I) is dependent on solvent parameters and consists of competition between an ion-pairing and solvation interactions with the nucleophilic sulfur atoms of the ligand. Oxidation to the neutral Au(IV) species is chemically and electrochemically reversible on the CV timescale, however, attempts to generate the neutral complex in the bulk of solution are complicated by auto-reduction and poor solubility. Under the appropriate conditions of low concentration and sub-ambient temperatures, one can generate solutions of the neutral complex. ESR spectra of the complex show a significant quadrupole effect which manifests itself in a field-dependent hyperfine splitting pattern. These results have been used to speculate on the structure of the neutral complex, which is likely to form a weak dimer in solution.
The use of supporting electrolytes containing the weakly coordinating anion tetrakis(pentafluorophenyl)borate [TFAB]- has been of recent interest. Supporting electrolytes containing this anion have been used in low-polarity solvents such as tetrahydrofuran and dichloromethane to address issues such as reactivity and solubility of polycations as well as a general increase in the conductivity of these solvents. This work provides a quantitative analysis of the conductivity of tetrabutylammonium tetrakis(pentafluorophenyl)borate in dichloromethane, tetrahydrofuran and benzotrifluoride with a comparison to "traditional" electrolytes that typically contain hexafluorophosphate or tetrafluoroborate anions. Association constants calculated using the paired ion model by Fuoss indicate that salts containing [TFAB]- are dissociated at least two orders of magnitude more than their "traditional" anion analogues. This work also demonstrates a conductometric method that can be readily incorporated into research projects that would be complemented by conductivity data but do not have conductivity as a main focus.
Fluorous biphasic systems (FBS) have been recently used to facilitate various chemical transformations. This work presents the development of a methodology with which to conduct electrochemistry in highly fluorous media. Use of an imidazolium salt containing fluorous groups on the anion and cation provides the solubility and conductivity necessary to perform electrochemistry in a 1:1 by volume mixture of perfluoromethylcyclohexane:benzotrifluoride. Progress has also been made in the use of FBS to facilitate the electrochemical synthesis of a fluorinated (benzene)chromiumdicarbonylphosphine complex. An important result from this work is the facile separation of a nearly 100-fold excess of supporting electrolyte from the electrochemically generated product by utilizing the solubility properties and thermally regulated miscibility of fluorous:organic mixtures.