Synthesis and structural studies on [Mg3(O2CC≡CSiMe3)6(HMPA)4]:: Mechanistic implications for the formation of the μ4-η4 bonding type in carbamato-bridged Mg6 cage compounds

The insertion reaction Of CO2 with Mg(C= CSiMe3)(2) was characterized by NMR spectroscopic studies to give a cage compound [Mg-n(O2CC=CSiMe3)2n. (THF)(m)] 1a, and the addition of HMPA / THF (HMPA = hexamethylphosphoramide) to solutions of la produces a trimeric compound, [Mg-3(O2CC=CSiMe3)(6)(HMPA)(4)], 1b. X-ray diffraction studies of this showed 1b to contain a completely centrosymmetric linear array of magnesium atoms linked with mu(2)-eta(2)- and mu(2)-eta(3)-O2CC= CSiMe3. It was found to be a key compound in the mechanism leading to the formation mu(4)-eta(4) bonding in carbamato-bridged Mg-6 cage compounds.