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Project description and research plan "We may perhaps produce chemical fuels The 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 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 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 (a synthon), as for example in the ,

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 (). It may then react with an electrophile (a 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 , 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 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 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 leading to CO2 elimination finds many practical uses in 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. For a free acid the substituent also acts as a proton acceptor. CO2 loss is also observed upon heating of carboxylate ionic liquids. Gas phase decarboxylation of carboxylate anions may take place according to equation (4), provided that the corresponding R. has positive electron affinity. 11 – – RCO2 → R + CO2 (4) Electrospray ionization (ESI) provides a straightforward and efficient method for producing carboxylate from spraying carboxylic acid solutions. 12 By combining ESI and collisional

Scheme 1 activation it is possible to form arenide ions from deprotonated substituted benzoic acids, with higher yields of R– the more electron withdrawing the substituents are. The reverse of decarboxylation is the addition of an R– group to carbon dioxide. The classical way of providing R– synthons is by Grignard reagents in the form of R-Mg-X (X = halide). This and other reductive schemes for using CO2 as carbon feedstock in chemical synthesis, for example by electrochemistry, currently receive a lot of attention. The reactivity of prototypical Grignard – reagents of the type CH3MgL2 (L = Cl or O2CCH3), formed by decarboxylation of MgL2 adducts of acetate by collision induced dissociation (CID) were studied in the gas phase by O’Hair et al. 13 – Under the reaction conditions used it was not possible to trap the Grignard adduct of CH3MgL2 with water, methanol, and , and small aldehydes. Due to kinetic reasons instead CH4 formation was observed to occur. Similarly, formation of H2 is observed in reactions between − magnesium hydride anions, HMgL2 (L = Cl and HCO2) and formic acid. As a part of our suggested survey of the very interesting reactivity landscape between Grignard chemistry and decarboxylation reactions, also with biological and industrial CO2 fixation in mind, it would be necessary to apply a broader perspective by investigating the unimolecular and bimolecular chemistry of magnesium or zinc complexes of a wider range of carboxylic acids and + metal ions M(II) in the form of ML (M = Cu, Zn and perhaps other metals; L = OH, Cl, Br, RCO2). 5 In addition, we propose to investigate a series of di- and triacids, giving rise to intramolecularly linked ligands with the potential of loosing several CO2 in consecutive dissociations. A further perspective, not yet investigated, will be to study the requirements for CO2 exchange within – CH3CO2ML2 using isotope-labelled species.

(c) Models of dipolar and ionic addition of H2 to CO2, in water clusters. Electrolytic reduction of CO2 is a complex process, involving short-lived surface and solution species that often are difficult to identify and characterize. Matters are much simplified in the gas phase, and studying suitable model systems allows for elucidation of key features of the elementary reaction steps that lead to hydrogenation.

For example, the addition of H2 to CO2 in the isolated state, provided the existence of nearby hydride donor and proton donor sites, can be considered a two-step process giving rise to formic acid; addition of – – + hydride (AH + CO2 → HCO2 + A) followed by proton transfer (BH – + HCO2 → HCOOH + B). The hydride and proton donating sites require sources of hydrogen. In electrolysis, hydride is most likely present at the cathode. In contrast, concerted addition of an intact H2 molecule (already formed at the cathode) to a CO2 molecule is symmetry forbidden according to the Woodward-Hoffmann rules and consequently has a high-energy barrier, making this mechanism irrelevant. In other words, it may be rewarding to identify the requirements for each of the two elementary steps separately. The first step, hydride transfer, may be obtained from a wide of range of potential hydride donors, AH–, provided the hydride affinity of CO2 is the higher than that of A. In addition to the thermochemical requirement defined by the difference in hydride affinity between CO2 and M, one may expect that electronic effects associated with the hydride transfer are operative, giving rise to an inherent – + activation energy. The final proton transfer step HCO2 + BH will depend on the relative proton – affinity of HCO2 and B but is more trivial in the sense that proton transfer reactions have negligible inherent activation energy. - It is well established that the radical anion CO2 ! is present during electrochemical reduction, and - good evidence for mechanisms involving CO2 ! as a key intermediate, either in solution near the 8 electrode or as an electrode surface species, has been presented. However, it seems unclear how - further reaction between CO2 ! and H2O occurs. Intermediate radical species proposed to exist along this route, including HCO2!, are unstable toward spontaneous dissociation and it may be questioned if these species have sufficiently lifetime to play any significant role during reaction. Also in this case, studying well- selected model systems may be rewarding in telling the actual course of electrochemical reduction of CO2. The ionic or dipolar nature of these reactions is clearly affected by the medium, in this case water. Clusters of water molecules, (H2O)n, constitute particularly attractive small-scale models for bulk water, and by introducing ionic or neutral molecules into a cluster, it becomes possible to investigate solvation in water at a fundamental level. The interaction between a small number of water molecules and polar or charged particles is in this respect essential. The nano-solvation environment found in clusters containing one or several “solute” molecules is an ideal model of solvation in bulk. 6 (d) Electrocatalysis

CO2 may undergo 2, 4, 6, and even 8e reduction, accompanied by the addition of protons. This opens for a wide range of target products, such as CO, HCOOH, (COOH)2, HCHO, CH3OH, C2H4, C2H5OH, and CH4, with the electrocatalyst as a key to achieving useful turnover frequency as well as selectivity. A problem common to most implementations of these reactions is that they involve high-energy reaction steps that require large overpotentials to become feasible. The task of the electrocatalyst is to reduce the overpotential for the desired reaction by stabilizing high-energy intermediates by bond-formation to the catalyst. Focusing on the 2e reduction, there are Fe(0) catalysts that show exceptionally high activity, low overpotential and nearly quantitative faradaic yield of CO, albeit in a non-aqueous solvent14 the Fe(0) state may be electrogenerated, while - phenolic substituents on the porphyrin ligand stabilize the initial Fe(I)CO2 adduct through H- bonding and also favor proton transfers. Largely reduced overpotential and almost quantitative yield of CO was obtained under aqueous conditions through stabilization of the formate radical by addition of an ionic-liquid-forming salt, 1-Et-3-Me-imidazolium tetrafluoroborate to high concentration. 15 There are strong indications that the rôle of the imidazolium salt is not merely to stabilize the formate radical but also involves specific interactions with the electrode (Ag, Pt). Similarly, adding protonated to the solution shows promising effects on the reduction to formic acid and methanol.8 Further, Brookhart et al.16 reported on an Ir-PNP pincer complex able to reduce CO2 with high selectivity to formate, with water as solvent. Still, it is desirable to pursue catalytic systems based on abundant elements and several recent studies (see 17 and references therein) report on promising activity and selectivity for formate production over Sn/SnOx nanocatalysts. Catalyst stability is favored at mildly alkaline conditions under which bicarbonate serves as a reactant reservoir for formate formation. 17 The formation of formate over Sn(112) was very recently explored by DFT modeling, although without considering the bicarbonate species as a possible starting point.18

Approaches, hypotheses and choice of method Academic research in this area provides both highly interesting opportunities and very demanding challenges. For our laboratories, with background and expertise in chemical reactivity, spectroscopy and theoretical methodology (computational quantum chemistry, reaction dynamics and reaction kinetics), it is natural to aim our activities at the fundamental aspects of the problem in terms of characterization and thermochemistry of reactants, products and intermediates at the atomic level, as well as providing detailed reaction mechanisms of the elementary reaction steps — in other words conducting a full survey of the energy landscapes of the reactions, taking full advantage of an arsenal of precise spectroscopic and spectrometric methods for characterization, currently with extraordinary spectroscopic, temporal and positional precision. State-of-the-art experimental and theoretical molecular techniques will be used to explore a set of carefully selected reaction systems of reduced dimensionality, as described above. Most experiments will be conducted in Oslo using the Fourier transform ion cyclotron resonance (FT- ICR) and QTOF mass spectrometers equipped with EI/CI, electrospray ionization and supersonic expansion cooling cluster sources. However, for the most demanding experimental challenges, we need to collaborate with international experts who master highly advanced techniques using top- level research infrastructures. In particular, we will involve our collaborators professors Mats Larsson (Stockholm) and Knut Asmis (Leipzig/Berlin). In Stockholm, the brand new DESIRE 7 facility offers unprecedented opportunities for advanced mass spectrometric experiments, while the recently opened free electron laser (FEL) facility at the Fritz-Haber Institute in Berlin gives access to intense highly resolved infrared radiation for spectroscopic characterization of stored ions. In 2 – fact, the already quoted work on Mg(η -O2C)Cl gave rise to the very first publication from the FEL in Berlin. These two individuals support the initiative, and oblige themselves to participating in the project. In this manner the current project supports our long time goal in strengthening experimental physical chemistry in Norway, with emphasis on the groups in physical chemistry in Bergen and Oslo, by extensive international collaboration. Quantum-mechanical modeling forms an essential part of the project, both as complementary approach to the experimental cluster studies and as a stand-alone source of information about selected metal/metaloxide-based systems for electrocatalytic reduction of CO2. For the first part, the Gaussian package of quantum mechanical programs will be the main tool, focusing on equilibrium structures and energies of water-based clusters with dissolved molecules, ions or an electron. For the second part, slab models as described with DFT and plane-wave bases will become a valuable tool, augmented with Car-Parrinello calculations and multireference benchmarks to ascertain electronic states in complicated cases. These calculations will require extensive computational resources form the national NOTUR consortium for high-performance computations.

3. The project plan, project management, organisation and cooperation Work packages

WP 1. Metal CO2 activation 2 (i) Computational study to identify stable M(η -O2C) species, by a partial study of the periodic table (Groups 1 and 2 and selected transition metals). 2 (ii) Formation of stable M(η -O2C) species (see point above) and characterization of their nucleophilic reactivity in formation of C–C bonds using electrospray ionization of mass spectrometry and ion trapping techniques. 2 (iii) Spectroscopic characterization of selected M(η -O2C) species by photodissociation (action spectroscopy; IR, UV and X-ray), see points above. Project-funded staff: One Ph.d. student (Oslo).

WP 2. Decarboxylation and carboxylation, the Grignard perspective (i) Formation of Grignard type of reagents M = (Mg, Zn, Cu, Ag, Au) by CID mass spectrometry: - - - - RCO2MXn → RMXn + CO2 in competition with carboxylate formation RCO2MXn → RCO2 + MXn. - (ii) Monitoring the reactivity of RMXn towards Lewis acids for alkyl anion transfer. - - (iii) Exchange of isotopically labelled CO2: RCO2MXn + CO2 → RCO2MXn + CO2

Project-funded staff: One postdoctoral fellow (Oslo).

8 WP 3. Electrolytic CO2 reduction

This WP combines (i) quantum chemical modeling of the catalytic reduction of CO2 over Sn/SnO2(s) and (ii) experimental and theoretical cluster studies of the interaction between selected intermediates and water. This allows for coupling of quantum chemical modeling of electrocatalytic reactions and the exploration of reactivity of solvated electrons in finite clusters that offer well- defined elemental composition and free of side reactions that remove the solvated electron.

(i) The state of the nanocatalyst will be modeled at slightly alkaline pH. Adsorption and reduction of HCO3- to HCOO- will be explored on Sn and SnO2, including stable crystallographic planes and the impact of defect sites. Both slab and cluster models will be used for studying electronic, steric and thus energetic profiles of individual reaction steps. The computational hydrogen electrode (CHE) model proposed by Nørskov et al. 19 and previously applied to CO2 reduction 20,21 will be applied to calculate the free energy change between two electrochemical steps involving a proton and an electron transfer. We will also explore the approach to electrode-potential-dependent activation barriers for inner-sphere reactions are proposed in 22.

(ii) In single-electron transfer the activation energy is largely due to solvent reorganization and thus difficult to compute. This makes experimental studies of the capture of water-solvated electrons by carbon oxygenates as well as electron and/or proton transfer of great value. Moreover, utilizing the principle of micro reversibility, we probe the reverse of electrolytic hydrogenation, namely dehydrogenation of formic acid, by reacting size-selected anionic water clusters, which can conveniently be made using the equipment in Oslo, as follows: – – CO2 (H2O)n + HCOOH and e (H2O)n + HCOOH (5). The experimental studies will be accompanied by high level quantum chemical calculations to survey key features of the respective potential energy surfaces, and possibly also incorporate trajectory calculations (direct reaction or Car-Parrinello dynamics).

Project-funded staff: One Ph.d. student (Bergen).

4. Key perspectives and compliance with strategic documents Relevance and benefit to society In its extension, the project is part of a massive international effort to develop the technology necessary to operate a modern society without net release of CO2 to the environment. More specifically, an efficient route to formic acid from CO2 represent a promising approach to storing renewable energy in a form that may be used in mobile units, for instance in combination with fuel cells. Such a system may draw on the present grid for gasoline and thus alleviate the need to develop a worldwide distribution net for a gaseous energy-carrier such as hydrogen. Alternatively, formic acid may be used as an industrial starting point for synthesizing value-added compounds.

Environmental impact The long-term environmental impact of the successful completion of the project is thus both large, positive and may be viewed as part of the implementation of internationally recognized goals for limiting the ongoing change in global climate.

Ethical perspectives 9 Both universities are committed to high ethical standards, and have ethics policies in place. Moreover, the project employees will be engaged in ongoing activities concerning good practice in research, such as the course NANO310 Nanoethics at the University of Bergen. The research will be conducted and presented in agreement with national Guidelines for Research Ethics in Science and Technology.

Gender issues (Recruitment of women, gender balance and gender perspectives) Recruitment of PhD students in chemistry is quite well balanced genderwise – the challenge lies in recruiting and educating female postdocs for a further scientific career. Both universities exercise an active recruitment policy that evens out unequal gender selection and indirect discrimination that the share of women and men as professors and associate professors and in academic posts reflects the gender distribution in the recruitment base. The project employees will be offered access to CTCC’s programs for gender equality and career development, including participation in the successful series of Fem-Ex meetings (https://www.ctcc.no/events/conferences/2014/femex- oslo/). Human resources and recruitment policies The PIs, who both belong to research groups being highly rated in the evaluation report “Basic Chemistry Research in Norway”, have worked together in the Nordic Network for Ionic Clusters In The Atmosphere, NICITA, and have also successfully collaborated in a joint NFR FRIPRO project entitled "Nano-solvation in Hydrogen-Bonded Structures", 2011-2015. The main applicant, E.U., is head of both the Physical chemistry group and the Mass spectrometry laboratory at The Department of Chemistry, University of Oslo and the NFR Centre of Excellence, "The Centre of Theoretical and Computational Chemistry, CTCC", 2007-2017. These affiliations ensure ample experimental and computational resources for most of the experiments outlined above, as well as providing very good links to the mass spectrometric community nationwide and internationally. In order to carry out the project to full extent, we need to hire one PhD students and one Postdoc in Oslo and one PhD student in Bergen. One of these should have competence in experimental mass spectrometry and will be heavily involved with the experimental part of module 1, which is based on in-house instrumentation in Oslo. The other candidate should also be well versed in molecular modelling. The PhD in Bergen should have a strong background in electron-structure calculations and will develop experimental skills in ion-beam technology and spectroscopy. Moreover, the Norwegian CoE Centre of Theoretical and Computational Chemistry (CTCC) supports the project and is prepared to host the Post Docs and Ph.D.s associated with this project, to offer them access to our programs for gender equality and career development, and to integrate them in centre activities including seminars, courses and work shops. In addition, they will be exposed to a truly international research environment including regular visits to and visits from laboratories abroad. In particular, both of the PhD students will have extended research stays at abroad as indicated in support letters. Management and liaison Due to the diversity of the project, including technical and geographical issues, it will be mandatory to work out effective management procedures. We have identified the following important tasks: (i) The two project leaders will meet on a regular basis, with biweekly telephone meetings as the main 10 channel for communication with one common project meeting once a semester, (ii) The core group, including the two international colleagues, will meet on a regular basis, for project update and decision making. (iii) The local, national and international networks mentioned above, will provide a friendly environment for the project employees to discuss research ideas and hypotheses with competent scientists outside their local groups. These networks do provide funding for student exchange among the nodes in the network.

5. Dissemination and communication of results In addition to publishing scientific articles (as described elsewhere) and contributing at scientific conferences the PI’s are highly engaged in the popularization of science, both via radio and pod- casting (E.U; NRK’s Abels tårn) and monthly magazines (K.B.; on the editorial board of Universitetsforlaget’s Naturen). The project is well suited for presentation to a wider audience.

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