Inside or outside a ligand cleft? Synthetic, structural, and kinetic inertness studies of zinc, cadmium, and mercury complexes of cross-bridged cyclam and cyclen

Abstract

Ethylene cross-bridging of the popular tetraazamacrocyclic ligand cyclam has led to metal complexes with enhanced kinetic inertness. The synthesis and spectral characterization of zinc(II), cadmium(II), and mercury(II) complexes of cross-bridged cyclam (L¹) as well as cross-bridged cyclen (L²) are reported along with the details of our synthetic route to L². X-ray structural studies revealed that all Zn(II) and Cd(II) cations are fully κ⁴N-coordinated inside the respective ligand's molecular cleft with L¹ providing the better fit for Zn(II). While Hg(II) is similarly coordinated to L², it has been found to complex L¹ outside the ligand cleft in a novel exo-κ²N-mode. Solution NMR data of the κ⁴N complexes are consistent with the presence of only a single cis-folded isomer in each case. Ligand ¹H and ¹³C coupling to both ^111,113Cd and ¹⁹⁹Hg in their complexes can be clearly discerned. The relative kinetic inertness of representative cross-bridged complexes in acidic aqueous solution has been assessed and found to be in the following order: Zn(II) > Cd(II) ≫ Hg(II). The data also reaffirm that cross-bridged cyclam ligand L¹ forms a substantially more inert complex with zinc(II) than either the smaller cyclen analogue L² or the unbridged 1,4,8,11-tetramethyl–cyclam L³.