Sexidentate co-ordination of the pendant-arm macrocycle 6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine (L1) to zinc(II). Crystal structure of [ZnL1][ClO4]2·H2O

Abstract

The pendant-arm macrocycle 6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine (L¹) co-ordinates as a sexidentate ligand to zinc(II) in neutral aqueous solution. The complex [ZnL¹][ClO₄]₂·H₂O crystallises in the monoclinic space group P2₁/c, a= 9.488(4), b= 14.736(9), c= 16.037(4)Å, β= 97.95(3)° and Z= 4. The Zn–N bond lengths in [ZnL¹]²⁺ are, on average, the shortest known for a hexaaminezinc(II) complex. The stability constant for formation of [ZnL¹]²⁺ was determined to be log K 15.0 ± 0.1, from potentiometric titrations. Five of the six successive protonation constants for the hexaamine L¹ were measured similarly and values pK_a 2.9, 5.5, 6.3, 9.9 and 11.0 found.