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 [Mg_n(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.