Assembly of the inorganic double helix Mg₆(Et₂NCO₂)₁₂ by fixation of carbon dioxide: Crystal and solution structure

The homoleptic magnesium carbamato complex Mg₆(Et₂NCO₂)₁₂, 1 (Et₂NCO₂^- = diethylcarbamato anion), was prepared by the reaction of dibutylmagnesium with diethylamine, followed by carboxylation using gaseous carbon dioxide. Crystallographic characterization demonstrated that 1 has the standard M₆(R₂NCO₂)₁₂ structure and is a double helix of MgO_x(x = 5, 6) coordination polyhedra with δ or ?? stereochemistry arising from the configuration around the six-coordinate Mg²⁺ cations. It crystallized in the orthorhombic space group Ccca with two molecules of δ1 and two of ??1 per unit cell (a = 21.548 Å, b = 25.094 Å, c = 15.4485(11) Å, α = β = λ = 90°). Extensive solution characterization of 1 by 1-dimensional proton and ¹³C NMR spectroscopy and by two-dimensional ¹H-{¹³C} NMR correlation techniques verified that the helical structure is maintained in solution. Moreover, these measurements indicated that the intramolecular dynamics of 1 relating to motions of the ethyl groups was substantially hindered in solution. Correlation of the crystallographic and NMR structural studies indicated that this arises from a combination of hindered rotation about the carbamato C-N bond and efficient packing of the ethyl groups around the Mg₆O₂₄ core. The result is an inverted-micelle-like structure for 1 in which the hydrophobic ethyl groups form a sheath largely restricting access to the hydrophilic Mg₆O₂₄ core.