Steric and Solvent Effects on the CO₂ Fixation of Magnesium Compounds

RMgBr (RPh,iPr) and MgY2 (Y = iPr, C⋮CPh, NMe2, NEt2, NPh2) react with carbon dioxide, yielding different bridging carboxylato and carbamato magnesium compounds: [BrMg(μ-O2CPh)(THF)2]2 (1) and [Mgn(O2CY)2n(Sol)m] [n = 3, m = 2, Y = iPr, Sol = THF (2), HMPA (HMPA = hexamethylphosphoramide) (3); n = 3, m = 2, Y = C⋮CPh, Sol = THF (4); n = 3, m = 2, Y = NMe2, Sol = THF (5), HMPA (6); n = 6, m = 0, Y = NEt2 (7), and NPh2 (8)]. The addition of HMPA/THF to hexamer 8 resulted in a deaggregation reaction, yielding the dimer [Mg2(O2CNPh2)4(HMPA)2] (9). Bubbling carbon dioxide into the THF solution of Mg(NiPr2)2, we unexpectedly obtained two novel compounds: [Mg6(μ4-O)(O2CNiPr2)10] (10) and [Mg5(μ5,η6-CO3)(O2CNiPr2)8(HMPA)2] (11), which were isolated from THF/hexane and HMPA/toluene solvents, respectively. X-ray crystallographic determinations were performed on compounds 1, 3, and 6−11, which were found to be either linear or cage compounds, depending upon the steric size of Y and the bonding mode between magnesium atoms and the bridging carboxylato, carbamato, or carbonato. These compounds were characterized using 1H, 13C NMR, and IR spectroscopy, mass spectrometry, and elemental analyses.