Metal ion scrambling in hexanuclear M₆(Et₂NCO₂)₁₂ complexes (M = Co, Mg). Synthesis, solid state structure, and solution dynamics of heteronuclear Co_nMg_6-n(Et₂NCO₂)₁₂ complexes

Heteronuclear diethylcarbamato complexes of the form Co_nMg_6-n(Et₂NCO₂)₁₂ were prepared from the isostructural homonuclear precursors Mg₆(Et₂NCO₂)₁₂, 1, and Co₆(Et₂NCO₂)₁₂, 2, via a solvothermal methodology. Two materials were selected for single-crystal X-ray diffraction analysis: Co_1.6Mg_4.4(Et₂NCO₂)₁₂ and Co_2.7Mg_3.3(Et₂NCO₂)₁₂. Both compounds crystallize in the orthorhombic space group Ccca, as do 1 and 2. The molecular structure is best described as two trinuclear M₃ units cross-linked by diethylcarbamate ligands and twisted about one another, so that the complex has overall D₂ symmetry and is chiral. Each trinuclear unit consists of two terminal pentacoordinate metal ions and one central hexacoordinate metal ion. The X-ray diffraction data were unambiguous that the Co²⁺ ions migrate exclusively to the pentacoordinate sites in the heteronuclear complexes, thus demonstrating that metal ion scrambling at the molecular level must occur. The composition of individual crystals can be continuously varied for Co²⁺ mole fractions X_Co < 0.5, and the a and c unit cell distances are linearly related to X_co. This indicates that the compounds behave as solid solutions. There appears to be either a chemical or crystallographic phenomenon inherent in the synthetic methodology that prevents isolation of heteronuclear materials having X_Co > 0.5. Solution electronic spectroscopy and molecular weight measurements show that 2 can dissociate in chloroform and cyclohexane solution to give a dimeric complex 2′. This behavior contrasts with the stability of 1 in solution, as shown by NMR. The kinetic rate profile for formation of Co_nMg_6-n(Et₂NCO₂)₁₂ reveals saturation kinetics and is consistent with direct attack by 2′ on 1 to give the heteronuclear complex via a higher nuclearity intermediate. This study illustrates a general method for the preparation of solids based on heteronuclear Werner-type complexes of the M₆(Et₂NCO₂)₁₂ structure type, and the mechanism by which such compounds can be formed from isostructural homonuclear precursors.