RESUMO
The goal of the "Opportunities for Catalysis Research in Carbon Management" workshop was to review within the context of greenhouse gas/carbon issues the current state of knowledge, barriers to further scientific and technological progress, and basic scientific research needs in the areas of H2 generation and utilization, light hydrocarbon activation and utilization, carbon dioxide activation, utilization, and sequestration, emerging techniques and research directions in relevant catalysis research, and in catalysis for more efficient transportation engines. Several overarching themes emerge from this review. First and foremost, there is a pressing need to better understand in detail the catalytic mechanisms involved in almost every process area mentioned above. This includes the structures, energetics, lifetimes, and reactivities of the species thought to be important in the key catalytic cycles. As much of this type of information as is possible to acquire would also greatly aid in better understanding perplexing, incomplete/inefficient catalytic cycles and in inventing new, efficient ones. The most productive way to attack such problems must include long-term, in-depth fundamental studies of both commercial and model processes, by conventional research techniques and, importantly, by applying various promising new physicochemical and computational approaches which would allow incisive, in situ elucidation of reaction pathways. There is also a consensus that more exploratory experiments, especially high-risk, unconventional catalytic and model studies, should be undertaken. Such an effort will likely require specialized equipment, instrumentation, and computational facilities. The most expeditious and cost-effective means to carry out this research would be by close coupling of academic, industrial, and national laboratory catalysis efforts worldwide. Completely new research approaches should be vigorously explored, ranging from novel compositions, fabrication techniques, reactors, and reaction conditions for heterogeneous catalysts, to novel ligands and ligation geometries (e.g., biomimetic), reaction media, and activation methods for homogeneous ones. The interplay between these two areas involving various hybrid and single-site supported catalyst systems should also be productive. Finally, new combinatorial and semicombinatorial means to rapidly create and screen catalyst systems are now available. As a complement to the approaches noted above, these techniques promise to greatly accelerate catalyst discovery, evaluation, and understanding. They should be incorporated in the vigorous international research effort needed in this field.
RESUMO
The reactions of Cp*TaMe3C1 (Cp* = qs-CsMeS) with a variety of alkali-metal alkoxide, alkylamide, and alkyl reagents have been examined. Reaction with LiNMe2 produces Cp*Ta(NMez)Me3, which decomposes at 25 OC to an imine (or metallaazirane) complex, Cp*Ta(CH2NMe)Me2. The decomposition is a first-order, unimolecular process with a large kinetic isotope effect (kH/kD = 9.7). Monoalkylamides (LiNHR) react with Cp*TaMe3C1 to form imido complexes Cp*Ta(NR)Mez. Reaction of Cp*TaMe3C1 with lithium diisopropylamide forms a bridging methylene complex, Cp*Me2Ta(pCH2)(p-H)zTaMezCp*. The alkoxide compounds Cp*Ta(OR)Me3 (R = Me, CHMe2, CMe3) are very stable and decompose only over 100 OC. Alkyl complexes are stable only if the alkyl group does not have j3-hydrogens. Treatment of Cp*TaMe3C1 with (2-methylally1)magneium bromide affords an unstable tantalum 2-methylallyl compound, which decomposes cleanly to the trimethylenemethanec omplex Cp*TaMeZ(q4-C(CH2),)T. he rates of hydrogen abstraction or elimination processes in this system correlate with the nature of the atom bound to tantalum: for reactions involving a /3-hydrogen transfer the order is C > N > 0, while the facility of a-hydrogen abstraction reactions appear to decrease in the reverse order N > C. These reactivity patterns appear to reflect the variance in T a x , Ta-N, and Ta-O bond energies in this series. Hydrogenation of the imido compounds (Cp*Ta(NR)Me2) in the presence of phosphine ligands yields new examples of imido hydride complexes Cp*Ta(NR)H,(L) (L = PMe3, PMe2(C6Hs); R = CMe3, CHzCMe3). A moderately stable alkyl hydride complex, Cp*Ta(CH2NMe)Me(PMe3)H, has also been prepared.
