Small Azaparacyclophanes as Potential Selective Scavengers of Mercury. Crystal Structure of the Complex Hg2(L1)Cl4 (L1 = 16,17,19,20-Tetramethyl-2,6,9,13-tetraaza[14]paracyclophane)

Abstract

The ligational properties of the macrocycles 16,17,19,20-tetramethyl-2,6,9,13-tetraaza[14]paracyclophane (L1) and 2,6,9,13-tetraaza[14]paracyclophane (L2) and of its dibenzylated open-chain counterpart 1,12-dibenzyl-1,5,8,12-tetraazadodecane (L3) toward Hg2+ are reported. The equilibrium constants have been determined by potentiometry at 298.1 K in 0.15 mol dm-3 NaCl. Competition of the ligands with chloride anions for the coordination of Hg2+ was required for the determination of the stability constants due to the high stability of the complexes formed. L1 and L2 selectively complex Hg2+ over first transition series metal ions and particularly over Cu2+. NMR measurements prove that just three of the four nitrogen donors in ligands L1 and L2 are involved in the coordination to Hg2+; the remaining benzylic nitrogen atom is not coordinated. NMR spectra also show a strong diamagnetic shielding (ca. 2 ppm) for one of the protons in the central methylene groups of the propylenic chain linked to the uncoordinated benzylic nitrogen. These signals appear for [HgL1]2+ at 0.3 ppm and for [HgL2]2+ at −0.4 ppm. The crystal structure of [Hg2L1Cl4] agrees with the NMR studies. Crystals of [Hg2L1Cl4] (C20H36Cl4Hg2N4) are monoclinic space group P21/c, with a = 10.360(2) Å, b = 17.535(2) Å, c = 14.918(1) Å, β = 105.98(1)°, Z = 4, R1 = 0.0471, wR2 = 0.1064. The tetrahedral coordination polyhedra around the Hg2+ ions are formed in one case by the benzylic nitrogen atom and the two consecutive nitrogen atoms of the central part of the bridge and one chloride anion and, in the other case, by the remaining benzylic nitrogen and three chloride anions. Both coordination polyhedra are strongly distorted, particularly the one involving three nitrogen atoms of the bridge.