This International Collaborations in Chemistry award funded by the Inorganic, Bioinorganic, and Organometallic Chemistry Program and the Office of International Science and Engineering supports work by Professor Guy Bertrand of the University of California at Riverside to carry out fundamental/basic studies on the use of stable carbenes. These novel carbenes are used for stabilizing highly reactive species and for activating stable molecules. The synthesis of carbene-stabilized monoatomic and diatomic carbon(0) compounds is being investigated. These compounds are of considerable importance in biology and in synthetic chemistry. Synthetic cycles using carbenes to activate P4 and the subsequent addition of organic fragments to the ensuing highly reactive mono- and diatomic phosphorus species are being developed. Indeed, processes using white phosphorus, but avoiding chlorine, are highly desirable to meet the growing demand for phosphorus derivatives under increasingly stringent environmental regulations. The carbene-induced de-oligomerization of BNH containing polymers also under investigation. The greatest technical barrier for a practical application of ammonia borane as a hydrogen-storage material for fuel-cell-powered vehicles lies in the discovery of energy-efficient regeneration routes from the polymers that are formed in the dehydrogenation process. Calculations to accurately describe these novel compounds are being performed by Professor Bertrand's German collaborator, Professor Gernot Frenking of Phillipps-Universitaet Marburg. This German work is separatately supported by the Deutsche Forschungsgemeinschaft (DFG).
This project is at the interfaces between organic, inorganic, and physical chemistry and is well-suited for the multidisciplinary education of graduate coworkers. The graduate students of the U.S. and German groups involved in this collaborative effort have the opportunity to learn both computational and experimental techniques through reciprocal visits of three months. This project addresses several issues that are of critical importance for society, such as the study of substances with potential biological applications (para-benzynes), the environmentally friendly synthesis of bulk chemicals (phosphorus derivatives), and the hydrogen economy (ammoniaborane).
Shortly after their discovery at the end of the 80s, stable singlet carbenes have been recognized as excellent ligands for transition metal based catalysts, and as organo-catalysts in their own right. At the end of the 2000s, it has been shown that they can coordinate main group elements in their zero oxidation state, and even activate small molecules. We have found novel applications for carbenes. We have reported that they allow for the isolation of phosphorus-centered radicals, species that had been postulated as transient intermediates in several chemical reactions, but never structurally characterized. Phosphorus centered radicals are particularly interesting for spin-labeling experiments since the orientation dependence of the large hyperfine coupling with the phosphorus nucleus could provide detail concerned with much faster molecular movements than that available from the widely used nitroxides. Thanks to the stabilizing effect of carbenes, we have isolated a tricoordinated neutral boron derivative, which is isoelectronic with amines. In other words, in contrast to the usual boranes, which are prototypical Lewis acids, our new boron compound is a Lewis base, and a potential L ligand for transition metals. We have discovered the first nitrene, a nitrogen analogue of carbenes, which is stable at room temperature. This compound, namely a phosphinonitrene, has been fully characterized. It is related to transition metal nitrido complexes (metallonitrenes, LnMN), which have extensively been studied due to their implications in biological nitrogen fixation by the nitrogenase enzymes, and the industrial Haber-Bosch hydrogenation process of N2 into NH3. We have shown that the phosphinonitrene can as well act as a nitrogen atom transfer agent. Thanks to the presence of two p-accepting cyclic (alkyl)(amino)carbenes (CAACs), we have isolated the first complexes featuring one and two atoms of gold in the formal zero oxidation state. The preparation of these compounds constitutes the first step towards the synthesis of ultra-small gold nanoparticles, which have applications ranging from care diagnostics to advanced methods of controlled intracellular delivery. These results have been obtained thanks to the collaboration with the German group of Professor Frenking, who has done all the computational investigations. Most interactions between the two groups have been done by Internet. Additionally, three American students spent three months in Germany learning computational techniques, which will ideally prepared them for their careers in Academia or Industry. Gerald Manuel, a Chicano-Latino undergraduate who was working on this project, has been accepted in several Graduate Programs, and will join UC Irvine. David Ruiz, a Hispanic student, the first of his family to earn an undergraduate degree was accepted into UC Riverside’s Mentoring Research Intership Program (MSRIP) in 2009, and now in the Graduate Program of UCSD where he has been selected to receive a GAANN fellowship (Graduate Assistance in Areas of National Need).
