Changing Lanes from Concerted to Stepwise Hydrogenation: The Reduction Mechanism of Frustrated Lewis Acid–Base Pair Trapped CO2 to Methanol by Ammonia–Borane

Abstract

Unabated anthropogenic release of carbon dioxide (CO2) is contributing to global climate change and represents a colossal environmental predicament. Furthermore, increasing demand for fossil fuel resources has raised concerns about the stability of the global energy supply. These problems have led the scientific community to look for renewable fuel alternatives. In nature, plants and algae trap and utilize CO2 in photosynthesis, but this process is not sufficient to combat the rapid rise of CO2 concentration in the atmosphere. The trapping of CO2 and its subsequent reduction has surfaced as a chemical challenge of great interest because this transformation could be a viable route for renewable carbonbased fuels. However, the limited reactivity of CO2 has slowed progress in developing efficient reduction methods. Primarily, CO2 reduction can be achieved by using electrocatalysts or heterogeneous photocatalysts that involve transition-metal containing complexes and materials. Recently, Stephan and Menard demonstrated the trapping of CO2 by a frustrated Lewis acid base pair (FLP) along with the subsequent reduction of the trapped CO2 to methanol by ammonia–borane. This is a rare instance where CO2 reduction to a liquid fuel has been achieved without the use of a transition metal. Additionally, this reaction has unfolded a new dimension to FLP-facilitated chemistry. Although ammonia–borane is a popular chemical hydrogen storage material, it has been recently shown to function as a hydrogenating agent for imines in a concerted fashion through simultaneous proton and hydride transfer from ammonia–borane to imines. Earlier theoretical investigations by Paul and co-workers suggested that ammonia– borane releases hydrogen in a similar fashion to transitionmetal complexes and N-heterocyclic carbenes. However, dehydrogenation of ammonia–borane is also known to initiate through stepwise routes, via N H activation, and in some cases B H activation. Thus, the FLP-CO2 reduction involves two interesting aspects: a) the mechanism of reduction of CO2 to methanol at room temperature and atmospheric pressure and b) the hydrogenation pathway by ammonia–borane for this particular substrate. A detailed understanding of the mechanistic features of this remarkable sequence of chemical reactions would provide valuable insights for developing strategies of CO2 reduction. In our current endeavor, we have used hybrid density functional theory to unravel the molecular pathways for the reduction of FLP trapped CO2 to methanol by ammonia–borane. Our computational investigation characterizes the crucial transition states and intermediates that are encountered along the reaction path of this intriguing reaction. Furthermore, we show the chameleon-like nature of ammonia–borane as a reducing agent by showing that the hydrogenation pathways change with similar substrates in different electronic environments. In the current study we have focused on unfolding the mechanistic details of the reduction of FLP-trapped CO2. The optimized molecular geometries of PMes3–AlCl3 and the FLP–CO2 adduct exhibit overall satisfactory agreement with the molecular structures obtained from X-ray crystallographic studies by Stephan and Menard. We find the trapping of CO2 by FLP is energetically favorable by 31.0 kcalmol , which is in good agreement with the experimental finding that the FLP–CO2 complex is stable at 80 8C. Scheme 1 displays the predicted route for the multistep reduction process of FLP–CO2 by ammonia–borane. Our computations show FLP–CO2 binds ammonia–borane through a weak stabilizing interaction (in the solution phase this is predicted to be less than 1 kcalmol , without zeropoint correction) between a hydridic hydrogen on the borane of ammonia–borane and the carbon of the trapped [a] L. Roy, Prof. Dr. A. Paul Raman Centre for Atomic, Molecular and Optical Sciences Indian Association for the Cultivation of Science 2A & 2B Raja S. C. Mullick Road, Kolkata-700032 (India) E-mail : rcap@iacs.res.in [b] Dr. P. M. Zimmerman College of Chemistry, University of California at Berkeley Berkeley, CA 94720. (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem201002282.