Formation of a unique zinc carbamate by CO2 fixation: implications for the reactivity of tetra-azamacrocycle ligated Zn(II) complexes.

The macrocyclic ligand [13]aneN4 (L1, 1,4,7,10-tetra-azacyclotridecane) was reacted with Zn(II) perchlorate and CO2 in an alkaline methanol solution. It was found that, by means of subtle changes in reaction conditions, two types of complexes can be obtained: (a) the μ3 carbonate complex 1, {[Zn(L1)]3(μ3-CO3)}(ClO4)4, rhombohedral crystals, space group R3c, with pentacoordinate zinc in a trigonal bipyramidal enviroment, and (b) an unprecedenced dimeric Zn(II) carbamate structure, 2, [Zn(L2)]2(ClO4)2, monoclinic crystals, space group P21/n. The ligand L2 (4-carboxyl-1,4,7,10-tetra-azacyclotridecane) is a carbamate derivative of L1, obtained by transformation of a hydrogen atom of one of the NH moieties into carbamate by means of CO2 uptake. In compound 2, the distorted tetrahedral Zn(II) coordinates to the carbamate moiety in a monodentate manner. Most notably, carbamate formation can occur upon reaction of CO2 with the [ZnL1]2+ complex, which implicates that a Zn−N linkage is cleaved upon attack of CO2. S...