Routes for the gas-phase total synthesis of dihydroxy magnesium carboxylate anions, [(RCO2)Mg(OH)2]–(R = CH3 and CH3(CH2)14)

Dihydroxy magnesium carboxylates, [(RCO₂)Mg(OH)₂]^–, a previously unknown class of compounds, have recently been discovered in meteorites using electrospray ionisation (ESI) ultra high-resolution mass spectrometry (HRMS) and collision-induced dissociation (CID) experiments (A. Ruf et al., Proc. Natl. Acad. Sci. U.S.A. 114 (2017) 2819). Here we present two general strategies for the gas-phase synthesis of [(RCO₂)Mg(OH)₂]^– (where R = CH₃ and CH₃(CH₂)₁₄) in a linear quadrupole ion trap mass spectrometer using combinations of CID and ion-molecule reactions (IMR) with water. Both involve the same first step (Step 1), where [(RCO₂)Mg(O₂CCH₃)₂]^– precursor ions generated via ESI are mass selected and subjected to decarboxylation. In Route I the resultant organomagnesate [(RCO₂)Mg(CH₃)(O₂CCH₃)]^– is subjected to the following reactions: Step 2a involves decarboxylation via CID to give [(RCO₂)Mg(CH₃)₂]^– which undergoes an IMR in Step 3a to give [(RCO₂)Mg(CH₃)(OH)]^–. For Route II [(RCO₂)Mg(CH₃)(O₂CCH₃)]^– undergoes an IMR to give in Step 2b, [(RCO₂)Mg(OH)(O₂CCH₃)]^–, which in Step 3b fragments under CID to give [(RCO₂)Mg(CH₃)(OH)]^–. Both routes converge in the final Step 4, where the organometallic ion [(RCO₂)Mg(CH₃)(OH)]^– reacts with water to give the desired dihydroxy magnesium carboxylate, [(RCO₂)Mg(OH)₂]^–. All ion molecule reactions proceed rapidly and cleanly to produce the hydroxides. In contrast, the decarboxylation reactions are in competition with other pathways including loss of the carboxylate anion and loss of a neutral ligand to give a magnesium coordinated enolate of acetate. DFT calculations were used to examine the structures and energies of species involved in the competing reactions associated with each of the steps.