C-C Bond Formation of Mg- and Zn-Activated Carbon Dioxide.

Gas-phase activation of CO2 by chloride tagged metal atoms, [ClM]- (M=Mg, Zn), has been investigated by mass spectrometry and high-level quantum chemistry. Both metals activate CO2 with significant bending of the CO2 moiety to form complexes with the general formula [ClM,CO2 ]- . The structure of the metal-CO2 complex depends on the method of formation, and the energy landscapes and reaction dynamics have been probed by collisional induced dissociation and thermal ion molecule reactions with isotopically labeled species. Having established these structural relationships, the gas-phase reactivity of [ClM(κ2 -O2 C)]- with acetaldehyde (here considered a carbohydrate mimic) was then studied. Formation of lactate and enolate-pyruvate complexes are observed, showing that CO2 fixation by C-C bond formation takes place. For M=Zn, even formation of free pyruvate ([C3 H3 O3 ]- ) is observed. Implications of the observed CO2 reactivity for the electrochemical conversion of carbon dioxide, and to biochemical and artificial photosynthesis is briefly discussed. Detailed potential energy diagrams obtained by the quantum chemical calculations offer models consistent with experimental observation.

Dissociation of Mg(ii) and Zn(ii) complexes of simple 2-oxocarboxylates - relationship to CO2 fixation, and the Grignard and Barbier reactions.

Three deprotonated 2-oxocarboxylic acids, glyoxylate, pyruvate, and 2-oxobutyrate (RCOCO2-, R = H, CH3, CH3CH2) have been associated with MgCl2 and ZnCl2 to generate [RCOCO2MCl2]- (M = Mg, Zn) complexes. Upon collision-induced dissociation these complexes all undergo efficient eliminations of CO2 and CO, via an intermediate [RCOMCl2]- product, to ultimately give [RMCl2]- products. The pyruvate and 2-oxobutyrate complexes also undergo efficient elimination of HCl to produce the enolate-metal complexes [H2C[double bond, length as m-dash]COCO2MCl]- and [H3CHC[double bond, length as m-dash]COCO2MCl]-. These enolate complexes have binding motifs reminiscent of the active centres in some CO2-fixating enzymes and the CO2 reactivity of these enolate complexes was therefore investigated, but only adduct formation could be observed. Quantum chemical calculations predict the magnesium complexes to decarboxylate without reverse barriers to carboxylation, and the zinc complexes to decarboxylate with considerable reverse barriers. The subsequent CO loss occurs with reverse barriers in all cases. The HCl loss is also predicted to occur overall without reverse barriers for both metals.

Spectroscopic identification of a bidentate binding motif in the anionic magnesium-CO2 complex ([ClMgCO2 ](-)).

A magnesium complex incorporating a novel metal-CO2 binding motif is spectroscopically identified. Here we show with the help of infrared photodissociation spectroscopy that the complex exists solely in the [ClMg(η(2) -O2 C)](-) form. This bidentate double oxygen metal-CO2 coordination has previously not been observed in neutral nor in charged unimetallic complexes. The antisymmetric CO2 stretching mode in [ClMg(η(2) -O2 C)](-) is found at 1128 cm(-1) , which is considerably redshifted from the corresponding mode in bare CO2 at 2349 cm(-1) , suggesting that the CO2 moiety has a considerable negative charge (∼1.8 e(-) ). We also employed electronic structure calculations and kinetic analysis to support the interpretation of the experimental results.

The dissociation of glycolate-astrochemical and prebiotic relevance.

On the basis of mass spectrometric experiments and quantum chemical calculations, including detailed kinetic and dynamics calculations, we report the unimolecular dissociation of an isolated glycolate anion. The dominating processes are: loss of formaldehyde; loss of carbon monoxide; loss of carbon dioxide; and loss of a hydrogen molecule, with the latter having the lowest energetic threshold. At higher energies, CO loss is the dominating reaction. The loss of CO may be followed by a second CO loss, leading to the H(-)H2O complex in close mechanistic relationship to the Nibbering reaction. The results provide valuable insights into possible mechanisms for interstellar and prebiotic formation of glycolate via the reverse of the unimolecular dissociation reactions. We propose that the addition of the complex of OH(-) and CO to CH2O is the most feasible route to gas phase synthesis of glycolate, since all species are abundant in interstellar space.

Unimolecular dissociation of anions derived from maleic acid (MaH2) in the gas phase: MaH- and MaMgCl--- relationship to Grignard chemistry and reductive CO2 fixation.

We have conducted collision induced dissociation experiments on the hydrogen maleate anion (MaH(-), m/z = 115) and the anionic maleate MgCl complex (MaMgC(-), m/z = 173). In addition, we have computationally investigated the observed fragmentation reactions. We find that both anions readily undergo two consecutive decarboxylations resulting in product ions at m/z = 71 and 27 for MaH(-), and at m/z = 129 and 85 for MaMgCl(-). The first decarboxylation is more facile for MaMgCl(-) than for MaH(-), while loss of CO(2) from Ma(-CO(2))H(-) is more facile than for Ma(-CO(2))MgCl(-). We also find that MaH(-) loses water, and we propose a mechanism for this loss. No first-generation fragmentation product other than Ma(-CO(2))MgCl(-) is seen for MaMgCl(-). Based on the observed unimolecular chemistry, we discuss some of its implications on reductive CO(2)-fixation and Grignard chemistry.