High Thermodynamic Stability and Extraordinary Kinetic Inertness of Copper(II) Complexes with 1,4,8,11‐Tetraazacyclotetradecane‐1,8‐bis(methylphosphonic acid): Example of a Rare Isomerism between Kinetically Inert Penta‐ and Hexacoordinated Copper(II) Complexes

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Abstract
In an aqueous solution at room temperature, 1,4,8,11‐tetraazacyclotetradecane‐1,8‐bis(methylphosphonic acid) (H4L1) and CuII form a pentacoordinated (pc) complex, pc‐[Cu(L1)]2−, exhibiting conformation I of the cyclam ring. At high temperature, the complex isomerises to a hexacoordinated isomer, trans‐O,O‐[Cu(L1)]2−, with a trans‐III conformation of the cyclam ring. In pc‐[Cu(L1)]2−, four ring nitrogen atoms and one phosphonate oxygen atom are arranged around CuII in a structure that is half‐way between a trigonal bipyramid and a tetragonal pyramid, with one phosphonic acid group uncoordinated. In the trans‐O,O‐[Cu(L1)]2− isomer, the nitrogen atoms form a plane and the phosphonic acid groups are in a mutually trans configuration. A structurally very similar ligand, 4‐methyl‐1,4,8,11‐tetraazacyclotetradecane‐1,8‐bis(methylphosphonic acid) (H4L2), forms an analogous pentacoordinated complex, pc‐[Cu(L2)]2−, at room temperature. However, the complex does not isomerise to the octahedral complex analogous to trans‐O,O‐[Cu(L1)]2−. Because of the high thermodynamic stability of pc‐[Cu(L1)]2−, (log β=25.40(4), 25 °C, I=0.1 mol dm−3 KNO3) and the formation of protonated species, CuII is fully complexed in acidic solution (−log [H+]≈3). Acid‐assisted decomplexation of both of the isomers of [Cu(H2L1)] takes place only after protonation of both uncoordinated oxygen atoms of each phosphonate moiety and at least one nitrogen atom of the cycle. The exceptional kinetic inertness of both isomers is illustrated by their half‐lives τ1/2=19.7 min for pc‐[Cu(H2L1)] and τ1/2 about seven months for trans‐O,O‐[Cu(H2L1)] for decomplexation in 5 M HClO4 at 25 °C. The mechanism of formation of pc‐[Cu(L1)]2− is similar to those observed for other macrocyclic complexes.For classification of the cyclam ring conformation in complexes, see B. Bosnich, K. Poon, M. Tobe, Inorg. Chem. 1965, 4, 1102.

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