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

The homoleptic magnesium carbamato complex Mg-6(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 M-6(R2NCO2)(12) structure and is a double helix of MgOx (x = 5, 6) coordination polyhedra with Delta or Delta stereochemistry arising from the configuration around the six-coordinate Mg2+ cations. It crystallized in the orthorhombic space group Ccca with two molecules of Delta1 and two of Delta1 per unit cell (a = 21.548 Angstrom, b = 25.094 Angstrom, c = 15.4485(11) Angstrom, alpha = beta = gamma = 90 degrees). Extensive solution characterization of 1 by 1-dimensional proton and C-13 NMR spectroscopy and by two-dimensional H-1-{C-13} 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.