Reaction Intermediates DOI: 10.1002/ange.201108477 Formation and Characterization of Gaseous Adducts of Carbon Dioxide to Magnesium, (CO 2 )MgX  (X = OH, Cl, Br) HØloïse Dossmann (Soldi-Lose), Carlos Afonso, Denis Lesage, Jean-Claude Tabet, and Einar Uggerud* There is great current interest in the properties and reactivity of carbon dioxide and its activated forms owing to the relevance to photosynthetic CO 2 uptake [1] and to sequestra- tion of CO 2 from flue gases formed upon the burning of fossil fuels; [2] the former process is essential to life on Earth, and the latter is important in sustaining it under balanced condi- tions. [3] Efficient fixation of carbon dioxide by covalent C  C bond formation requires two extra electrons [Equation (1)]: RX þ CO 2 þ 2e  ! RCO 2  þ X  ð1Þ This general two-electron reduction is applicable to a range of different reactions. In plant cells, the formation of a bond between the carbon atom of CO 2 and one carbon atom of the other substrate, ribulose-1,5-bisphosphate (RuBP), occurs at the magnesium-centered active site of the enzyme RuBisCO, [4] the electrons being obtained from chlorophyll via NADPH 2 . In electrochemical and photoelectrochemical CO 2 fixation, the electrons are supplied by coupling to an electrochemical process that is mediated by various metals. [5] Synthetic ways to fixate CO 2 are very often involving Grignard reaction. [6] This reaction proceeds with metals, traditionally magnesium, directly providing the electrons. In this context, we have undertaken the study of some elusive [Mg + CO 2 ] complexes to characterize them and study their ability to form new C  C bonds. Mass spectrometry was an ideal tool as it makes possible to produce, isolate, and probe the structures and reactivities of elusive intermediates in the rarefied gaseous state; that is, in an chemical environment free from interference of neighboring molecules, including the effects of a solvent, which otherwise would make such intermediates too short-lived for observation. [7] In the follow- ing, we present the chemical characterization and reactivity of a novel and fragile structural form of carbon dioxide bonded to magnesium. The species in question are conveniently prepared by electrospray ionization of solutions containing magnesium(II) salts and oxalic acid. In parallel to the experiments, quantum-chemical calculations have been per- formed to gain more insights into the structures and mechanisms of the species under study. Electrospray ionization (ESI) in the negative-ion mode was applied to mixtures of oxalic acid (Ox) and magnesium salts MgCl 2 and MgBr 2 dissolved in CH 3 OH/H 2 O (90:10 v/v) From the recorded mass spectra we observe signals that are due to MgC 2 O 5 H  , MgClC 2 O 4  , and MgBrC 2 O 4  ions. Upon collisional activation, these ions (see the Supporting Infor- mation) dissociate by loss of CO 2 , 2CO 2 , and MgC 2 O 4 . The ions resulting from carbon dioxide loss, MgCO 3 H  , MgClCO 2  , and MgBrCO 2  , are presented herein. Accurate mass measurements and isotopic patterns confirm each of the molecular formulae. Three possible molecular structures for the [MgCO 2 X]  ion have been considered: XMg(h 2 -O 2 C)  , XMg(h 2 -CO 2 )  , and Mg(O 2 CX)  (X = OH, Cl, Br). Quan- tum-chemical calculations for X = OH show that of these isomeric forms, the former is lowest in potential energy (Figure 1) and that the electronic triplet is 131 kJ mol 1 higher in energy than the singlet. A potential energy diagram (Supporting Information) shows possible mechanisms of formation of these ions from the starting oxalic acid adduct. The application of decarboxylation as a method of forming Figure 1. Relative energies [kJ mol 1 ] for HOMg(h 2 -O 2 C)  , HOMg(h 2 - CO 2 )  , and Mg(OCO 2 H)  in their electronic singlet ( 1 Mg) and triplet forms ( 3 Mg). Data obtained from G4-optimized structures. [*] H. Dossmann (Soldi-Lose), C. Afonso, D. Lesage, J.-C. Tabet Institut Parisien de Chimie MolØculaire UniversitØ Pierre et Marie Curie-Paris 6, UMR 7201- FR2769 Place Jussieu, 75252 Paris Cedex 05 (France) C. Afonso UniversitØ de Rouen, UMR CNRS 6014 COBRA 1 rue Tesnire, 76130 Mont-Saint-Aignan (France) E. Uggerud Massespektrometrilaboratoriet og Senter for teoretisk og bereg- ningsbasert kjemi (CTCC), Kjemisk institutt, Universitetet i Oslo Postboks 1033 Blindern, 0315 Oslo (Norway) E-mail: einar.uggerud@kjemi.uio.no Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201108477. . Angewandte Zuschriften 7044  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2012, 124, 7044 –7047