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

The macrocyclic ligand [13]aneN(4) (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 mu(3) carbonate complex 1, {[Zn(L1)](3)(mu(3)-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 P2(1)/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. Since complexes of tetra-azamacrocycles and Zn(II) are routinely applied for enzyme model studies, this finding implies that the Zn-azamacrocycle moiety generally should no longer be considered to play always only an innocent role in reactions. Rather, its reactivity has to be taken into account in respective investigations. In the presence of water, 2 is transformed readily into carbonate 1. Both compounds have been additionally characterized by solid-state NMR and infrared spectroscopy. A thorough comparison of 1 with related azamacrocycle ligated zinc(II) carbonates as well as a discussion of plausible reaction paths for the formation of 2 are given. Furthermore, the infrared absorptions of the carbamate moiety have been assigned by calculating the vibrational modes of the carbamate complex using DFT methods and the vibrational spectroscopy calculation program package SNF.