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.