Fixation of Carbon Dioxide by Oxalic Amidinato Magnesium Complexes: Structures and Reactions of Trimetallic Magnesium Carbamato and Related Complexes

摘要

The reaction of oxalic amidines R~1-N = C(NHR~2)=N-R~1 with CH_3MgX followed by uptake of CO_2 results in the formation of the trimeric carbamato complexes [R~1-N=C(NR~2-COO)-C(NR~2COO)=N-R~1]_3Mg_3(THF)_6 (2a: R~1 = R~2 = Ph; 2b: R~1 = R~2 = 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 μ_2-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 ZnCl_2 or CoBr_2, with CO_2 elimination, to form dimeric complexes of the type [X_2M(oxalamidinato)MX_2][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 CO_2 to acetophenone, thus simulating the CO_2 activiation step i enzymatic biotin-dependent carboxylation reactions.