The Chemistry of CO2 Activation and Fixation Professor Einar Uggerud

Project description and research plan "We may perhaps produce chemical fuels The chemistry of CO2 activation and fixation directly from sunlight, CO2, and water" Professor Einar Uggerud, University of Oslo Richard A. Kerr and Robert F. Service Professor Knut Børve, University of Bergen in One of the 125 grand challenges Science magazine (2005). Summary Our objective is to describe the essential molecular factors, at the most fundamental level, that govern how CO2 forms covalent bonds to hydrogen and carbon, mediated by electrons and catalysed by specific metals. This knowledge is of high relevance to large-scale processes in which carbon dioxide may be converted to commodity chemicals and polymers, and to biological CO2 fixation. This will be achieved by employing state-of-the-art experimental techniques of accurately defined gas phase reaction systems and key species, infrared action spectroscopy and advanced mass spectrometric and ion storage techniques. In addition, large-scale quantum chemical modelling for accurate simulation of spectral properties, reaction mechanisms and dynamics will be applied, thus significantly enhancing the interpretation of the experimental findings. This unique combination of experimental and computational methodology, alongside with the well-documented ability of the PIs in producing original and high-quality research, warrants highly significant outcome from the project, to be published in the best journals. The three-year project will hire one postdoctoral fellow and two Ph.D. students, who will benefit from the combined expertise of two research groups, and their local, national and international network of top-level collaborators. Costs for experimental campaigns in Oslo, Berlin, Paris and Stockholm are included. 1. Relevance relative to the call for proposals The increasing atmospheric level of CO2 poses one of the most serious challenges to mankind, and is of worldwide concern. Besides reducing the total combustion of hydrocarbons, there exist two potentially viable strategies for counteracting the increasing CO2 levels. One is to capture CO2 from effluent gases from fossil-fuel power plants and deposit the catchment into geological formations (CO2 catchment). The other is to incorporate CO2 into chemical production as a feedstock for 2 1,2 synthetic fuels, commodity chemicals or polymers like polycarbonates (CO2 fixation). In the latter strategy, either a carbon-carbon bond or a carbon-hydrogen bond between CO2 and a second substrate molecule has to be established in the reaction, and normally energy input is required, typically by transfer of electrons, either to activate CO2 or the other substrate (reductive coupling). When CO2 fixation is integrated with a solar cell (photovoltaics), this is often referred to as artificial photosynthesis. Progress in the utilization of carbon dioxide as a reactant in chemical industry, and better insight into biological and artificial photosynthetic fixation of CO2, requires detailed knowledge of the mechanism in terms of each of the elementary reaction steps and the molecular factors that govern reactivity in each of these steps. In this respect, understanding the reaction mechanisms is central to all research in this fast growing field. 3 4 There is an articulated understanding that the fundamental understanding in this field is insufficient. 5 In C–C bond forming reactions, CO2 in its pristine form acts as an electrophile, meaning that the second substrate molecule has been activated by electron transfer in the first place, forming a carbon nucleophile (a carbanion synthon), as for example in the Grignard reaction, CO2 + RMgX → RCO2H (1). Alternatively, the CO2 molecule can be activated by electron transfer, to promote reversal of its electric character, turning it into a nucleophile (umpolung). It may then react with an electrophile (a carbocation synthon or a proton) to form a C–C (or C–H) bond. To exemplify the two approaches to CO2 reactivity, in electrolytic reduction of CO2 with water, to form formic acid, formaldehyde or methanol, a hydride ion is transferred to CO2 to form a C–H bond. Alternatively, CO2 can be activated by electron transfer, and a C–H bond is eventually formed by subsequent transfer of a proton. For both C–C and C–H bond formation, we therefore realize a reaction dichotomy, distinguished by whether electrons first are supplied to CO2 or to the other substrate. In any given case it may be difficult to determine which of the two scenarios are in operation, since the intermediates are often very short-lived. Nature may serve as a model and source for inspiration. In photosynthesis H2O is converted to O2 and hydrogen, the latter in the form of the reduced species NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate, NADP+). This occurs in the thylakoid membranes of chloroplasts of green plants, algae and cyanobacteria that contain chlorophyll and the associated photosystem units. The reduced species, NADPH, in turn is transferred to the stroma of the 6,7 chloroplast, where reaction with CO2 occurs. The net reduction reaction, ignoring the involvement of other species present, is + 6CO2 + 12NADPH → C6H12O6 (glucose) + 6NADP (2). This reaction involves the magnesium centred Rubisco enzyme in a complex series of elementary reactions that is often called the Calvin cycle or the dark reaction, eventually leading to the carbohydrate product. It is generally considered that CO2 acts as an electrophile in the dark reaction of photosynthesis. However, the complexity of the dark reaction makes it difficult to sort out the detailed order of elementary steps, including ruling out mechanisms in which activated CO2 may act as the nucleophile. This idea also opens up the possibility for inventing schemes for artificial photosynthesis working according to the umpolung scheme. 8 Also in electrolytic CO2 reduction the order of events may be unclear, taking formation of formic acid as an example: 3 + – CO2 + 2H + 2e → HCOOH (3). –. Is the electron first transferred to the CO2 molecule, giving hydrated CO2 , which may act as a carbon nucleophile, or is the electron first transferred to a proton in the vicinity of the cathode giving rise to adsorbed hydrogen atoms, which in turn may react with pristine CO2? The actual mechanism will depend on the nature of the electrode. Furthermore, depending on conditions and in particular the catalytic selectivity of the electrode, adsorbed hydrogens may form H2 rather than formic acid. 2. Aspects relating to the research project Background and status of knowledge (a) Metal CO2 activation. The properties and reactivity of metal–CO2 complexes is intimately related to how the metal is coordinated to CO2. In neutral complexes containing a single metal atom monodentate coordination 1 2 M(η -CO2) or bidentate coordination M(η -CO2) are most common, corresponding to structures (A) and (B) in the upper box, respectively. Linear “end-on” coordination (C) is also seen, but is less common. Work on isolated electron-rich anionic metal–CO2 complexes is scarce, but such complexes also prefer the coordination modes A and B, retaining a bent CO2 moiety as a result of the partial negative charge transfer into the π* orbital of CO2. In contrast, cationic metal–CO2 complexes have so far shown to exclusively coordinate “end-on” to the metal Comparison of vibrational predissociation (C) due to the electron deficiency resulting in electrostatic − spectra of ClMgCO2 •D2 to simulated IR bonding between the positively charged metal atom and 2 – spectra for the (1) [ClMg(η -O2C)] (III), 2 – the partial negative charge on oxygen in CO2. In addition and (2) [ClMg(η -CO2)] isomers (IV). to these, we have recently been able to synthesize and 2 characterize single metal anionic complexes with bidentate double oxygen Mg(η -O2C) coordination (D), comprising a novel kite-formed binding motif, which are best characterized as being salts of dihydroxycarbene (HOCOH). 9,10 The magnesium is essentially Mg(II) in these compounds, which are extremely reactive, and can only be isolated in the gas phase. The carbene 2 character is reflected in the ability of Mg(η -O2C) to act as a carbon nucleophile by forming C–C bonds in addition and substitution reactions. This is a key point since metal activation in this manner leads to umpolung of the otherwise electrophilic carbon of CO2. Irrespective of whether this reductive mode of binding CO2 plays a role as a short-lived intermediate in the Calvin cycle or not, the very concept offers a new lead in the development of artificial photosynthesis. Clearly, a periodic-table-wide exploration of the stability and properties of such complexes is highly desirable, applying experimental and computational methods available. It should also be mentioned that complexes of this kind react vigorously with water and that the unusual structural arrangement was recently verified through infrared photodissociation experiments of the magnesium complexes, see also lower box above. From the literature and our own explorative work we have indications that elements of groups 1 and 2 2 are most likely to form M(η -O2C) complexes, but it is also likely that low oxidation states of 4 some transition metals have this ability. Our strategy is first to systematically explore the periodic plethora by computational quantum chemistry, of the structure and stability of the various complex motifs (A – D) for these elements, and then try to form them using electrospray ionization of solutions of the metal oxalate salts (or similar precursors), and thereby study their reactivity and stability using mass spectrometric techniques. (b) Decarboxylation and carboxylation, the Grignard perspective. Thermolysis of carboxylic acids leading to CO2 elimination finds many practical uses in organic chemistry and decarboxylases are essential for catalysing elimination of CO2 from amino acids in vivo, for example for the transfer of L-DOPA to dopamine — the latter essential to synaptic transmission. Decarboxylation may either occur from the acid itself or from its salt. In both cases the reaction is facilitated by the presence of electron withdrawing substituents near the reaction centre.

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