Synthesis and Characterization of Palladium(II) Complexes of Cross-Bridged Tetraazamacrocycles

Ethylene cross-bridged tetraazamacrocycles have been used to stabilize first-row transition metal complexes for applications under harsh conditions, such as biomedical imaging and aqueous oxidation catalysis. We have applied these ligands to the synthesis of complexes of a larger second row transition metal, namely, square pyramidal Pd2+ complexes, which may be useful for future catalytic processes. We now report the synthesis and crystal structure determination of four novel Pd2+ complexes of the general formula [PdLCl]PF6, where L is the dimethyl ethylene cross-bridged derivative of 12aneN4 (cyclen), 13aneN4 (homocyclen), or 14aneN4 (cyclam), or the dibenzyl ethylene cross-bridged derivative of 14aneN4 (cyclam). Solid-state structures of all four complexes can be described as a distorted square pyramidal with τ5 values ranging from 0.01 to 0.20, with three macrocycle nitrogen atoms and one chloride in the square base and the fourth ligand nitrogen constrained by the cross-bridge to the axial position. Cyclic voltammetry of the complexes in acetonitrile shows stabilization of the Pd3+ oxidation state by all four complexes. Electronic spectroscopy reveals the typical behavior for square pyramidal Pd2+.