Dr. François Gabbaï, Chemistry Department, Texas A&M University, is supported by the Chemical Structure, Dynamics and Mechanisms Program to explore novel, cationic, main group Lewis acids as receptors for small anions including fluoride, cyanide and azide. This research focuses on the synthesis and study of boron, silicon and antimony compounds whose anion affinity is increased by a combination of Coulombic, hydrophobic, and/or chelate effects. These new, cationic Lewis acids are studied as acceptors in charge transfer processes. This reactivity can be harnessed for the design of turn-on anion sensors. In addition to investigating the synthesis and structure of these novel main group compounds, special attention is devoted to their anion binding selectivity, their compatibility with aqueous environments, and the magnitude of the photophysical response induced by anion binding.
This research advances the fundamental understanding of anion recognition processes. Cyanide is widely available in both research and industrial settings even though its potential release into the environment is a source of concern; Cyanide is a toxic anion which can bind to and deactivate enzymes in the body, sometimes with fatal consequences. While fluoride is often added to drinking water and toothpaste because of its beneficial effects in dental health, high doses of this anion can be lead to dental fluorosis. Anion receptors capable of binding and/or sensing potentially toxic anions in water aid in both environmental remediation and medicine. The graduate and undergraduate students involved in this research program receive excellent training and research experiences in synthetic inorganic chemistry and photophysics. By being exposed to a project concerning the capture of toxic anions in water, this project increases the students' awareness of current environmental and health challenges. Efforts to translate some of this research into undergraduate Inorganic Chemistry Laboratory courses provide a vector for the integration of research and undergraduate education.
Cyanide is a toxic anion which binds to and deactivates the cytochrome-c oxidase enzyme with sometimes fatal consequences. Because cyanide is widely available in both research and industrial settings, its use for harmful purposes or its release in the environment are sources of concern. Another important nucleophilic anion is the fluoride anion. This anion is often added to drinking water and toothpaste because of its beneficial effects in dental health. It is also administered in the treatment of osteoporosis. High doses of this anion are, however, dangerous and can lead to dental or skeletal fluorosis. Thus, as for cyanide, the development of methods that can sense this anion in water has become a topical objective. Ideally, such methods should be sufficiently sensitive to allow for the detection of these anions near or below their drinking water maximum contaminant level set at 4 ppm (210 μmol) for fluoride and 0.2 pm (7.7 μmol) for cyanide by the Environmental Protection Agency. In this project, we have investigated the synthesis and study of cationic main group Lewis acids which can be used for the complexation and sensing of the above mentioned toxic anions, namely fluoride and cyanide. Central to our approach has been the use of organoboron derivatives that are decorated by positively charged functionalities. The presence of these functionalities provides an electrostatic drive for the capture of the negatively charged anion and facilitates the formation of boron-anion bonds. This simple approach has allowed us to design boron-based molecules capable of sensing fluoride and cyanide at ppm or ppb concentrations, in water. By controlling the structure of the boron compounds, we have discovered that hydrophobic effects can be used to enhance the anion affinity of these compounds. We have also been able to access derivatives that signal the presence of the anion by a colorimetric or a fluorescence turn-on response. Our work has also generated some unanticipated results. In particular, we have discovered that elements other than boron and including antimony and tellurium can be used as the basis of our anion sensor design. We have also developed strategies for the capture and controlled release of fluoride anions for applications in organic synthesis. This project has contributed to the training of 14 graduate students. Four of these students, who have already obtained their PhD, are currently employed as postdoctoral fellows, as researchers in the chemical industry or as professors in academic institutions. All other students are currently enrolled in the Gabbaï group and will graduate in the next 1-5 years. Our project has also benefited from the contributions of visiting scholars from China, Egypt, France and Korea thus contributing to the internationalization of our research activities. In addition to contributing to the training of highly qualified personnel, this work has allowed us to establish both domestic and international collaborations. Finally, this project has provided an impetus for a number of service and outreach activities ranging from the organization of symposia at professional meetings to lectures and scientific presentations in K-4 classrooms.
