Crystal and Solution Structure of Two Dinuclear Hexadentate Macrocyclic Complexes of Palladium (II)

Two dinuclear macrocyclic complexes of palladium with hexadentate ligands have been synthesized. The crystal structures of Pd₂([18]aneN₆)Br₄·4H₂O ([18]aneN₆ = 1,4,7,10,13,16-hexaazacyclooctadecane) ([formula omited], a = 8.092 (2) Å, b = 11.533 (2) Å, c = 7.699 (2) Å, α = 72.85 (2)°, β = 100.52 (3)°, γ = 112.20 (2)°, V = 634.3 ų, Z = 1, R = 0.066, R_w = 0.069) and Pd₂([20]- aneN₆)Br₄·H₂O ([20]aneN₆ = 1,4,7,11,14,17-hexaazacycloeicosane) (Cmcm, a = 9.375 (4) Å, b = 13.992 (5) Å, c = 18.845 (7) Å, V = 2472 ų, Z = 4, R = 0.041, R_w = 0.045) were determined, and the palladium ions were each found to be coordinated to three amines of the ligand and a bromine anion. The halide atoms are disposed trans and cis in the two complexes, respectively. A metal-metal distance of 3.018 Å was found in the Pd₂([18]aneN₆)Br₂²⁺ cation. For both complexes, NMR spectroscopy suggests that the rigidity of the crystalline structures is maintained in solution under neutral or acidic conditions. However, fluxionality is observed in basic media. Preliminary kinetic studies demonstrate the effects of the disposition of the metal centers in the two complexes upon their ligand substitution behavior. The Pd₂([20]aneN₆)Br₂²⁺ cation undergoes substitution by a mechanism similar to that observed for the Pd₂(dien)Br⁺ analogues. For the Pd₂([18]aneN₆)Br₂²⁺ cation, a dissociative mechanism may be postulated from the observed kinetics.