Steric and solvent effects on the CO2 fixation of magnesium compounds

RMgBr (R = Ph,Pr-i) and MgY2 (Y = Pr-i C equivalent to CPh, NMe2, NEt2, NPh2) react with carbon dioxide, yielding different bridging carboxylato and carbamato magnesium compounds: [BrMg(mu -O2CPh)(THF)(2)](2) (1) and [Mg-n(O2CY)(2n)(Sol)(m)] [n = 3, m = 2, Y = Pr-i, Sol = THF (2); HMPA (HMPA = hexamethylphosphoramide)(3); n = 3, m = 2, Y = C equivalent to 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 [Mg-2(O2CNPh2)(4)(HMPA)(2)] (9). Bubbling carbon dioxide into the THF solution of Mg((NPr2)-Pr-i)(2), we unexpectedly obtained two novel compounds: [Mg-6(mu (4)-O)(O2CNi-Pr-2)(10)] (10) and [Mg-5(mu (5),eta (6)-CO3)((O2CNPr2)-Pr-i)(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 carbonate. These compounds were characterized using H-1, C-13 NMR, and IR spectroscopy, mass spectrometry, and elemental analyses.