Fixation of carbon dioxide by oxalic amidinato magnesium complexes: Structures and reactions of trimetallic magnesium carbamato and related complexes

Article abstract of DOI:10.1002/(sici)1099-0682(200005)2000:5<1055::aid-ejic1055>3.0.co;2-m

The reaction of oxalic amidines R¹-N=C(NHR²)-C(NHR²)=N-R¹ with CH₃MgX followed by uptake of CO₂ results in the formation of the trimeric carbamato complexes [R¹-N=C(NR²-COO)-C(NR²-COO)-C(NR²COO)=N- R¹]₃Mg₃(THF)₆ (2a: R¹ = R² = Ph; 2b: R¹ = R² = p-tolyl) as the thermodynamically stable final products of the reaction. Their X-ray crystal structures show that the three metal centres are in a linear arrangement. The central magnesium ion is octahedrally surrounded by six O-donor atoms of the μ₂-carbamato bridges, while both peripheral magnesium ions are facially coordinated by three O-donor atoms of the carbamato groups and three THF molecules. This coordination sphere can be considered as a structural model for the active centre in the ribulose-1,5-bisphosphate carboxylase/oxygenase enzyme. Compound 2a reacts with ZnCl₂ or CoBr₂, with CO₂ elimination, to form dimeric complexes of the type [X₂M(oxalamidinato)MX₂][Mg(DMF)₆] (M = Zn, Co; X = Cl, Br). X-ray crystal structure analyses show that the d-metals are tetrahedrally coordinated. The magnesium-bromide-containing intermediates in the formation of 2a and 2b are able to transfer CO₂ to acetophenone, thus simulating the CO₂ activation step in enzymatic biotin-dependent carboxylation reactions.