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

Abstract

In an aqueous solution at room temperature, 1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic acid) (H(4)L(1)) and Cu(I) (I) form a pentacoordinated (pc) complex, pc-[Cu(L(1))](2-), exhibiting conformation I of the cyclam ring. At high temperature, the complex isomerises to a hexacoordinated isomer, trans-O,O-[Cu(L(1))](2-), with a trans-III conformation of the cyclam ring. In pc-[Cu(L(1))](2-), four ring nitrogen atoms and one phosphonate oxygen atom are arranged around Cu(I) (I) 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(L(1))](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) (H(4)L(2)), forms an analogous pentacoordinated complex, pc-[Cu(L(2))](2-), at room temperature. However, the complex does not isomerise to the octahedral complex analogous to trans-O,O-[Cu(L(1))](2-). Because of the high thermodynamic stability of pc-[Cu(L(1))](2-), (logbeta=25.40(4), 25 degrees C, I=0.1 mol dm(-3) KNO(3)) and the formation of protonated species, Cu(I) (I) is fully complexed in acidic solution (-log [H(+)] approximately 3). Acid-assisted decomplexation of both of the isomers of [Cu(H(2)L(1))] 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 tau(1/2)=19.7 min for pc-[Cu(H(2)L(1))] and tau(1/2) about seven months for trans-O,O-[Cu(H(2)L(1))] for decomplexation in 5 M HClO(4) at 25 degrees C. The mechanism of formation of pc-[Cu(L(1))](2-) is similar to those observed for other macrocyclic complexes.

Metal Complexes of 4,11‐Dimethyl‐1,4,8,11‐tetraazacyclotetradecane‐1,8‐bis(methylphosphonic acid) – Thermodynamic and Formation/Decomplexation Kinetic Studies

“…[29,30] The main advantage of the ligand is an immediate complex formation (to the pc isomer) at pH Ͼ 4 and room temperature. [26] The isomeric ligand, 1,4-H 4 te2p (H 4 L 4 , Scheme 1), [31] forms also very stable complexes and is even more selective for copper(II); an analogous isomerism was observed for the complexes. [32] Similarly, the monophosphonic acid cyclam derivative H 2 te1p (Scheme 1) forms a thermodynamically highly stable complex with copper(II) having a cyclam ring in the trans-III conformation and the phosphonic acid group in the apical position; the complex is also kinetically inert.…”

“…With copper(II), two highly stable isomeric forms of [Cu(H 2 L 1 )] were isolated. [26] The low-temperature kinetic isomer is pentacoordinate (pc isomer) with one phosphonic acid pendant arm non-coordinated and cyclam ring in the conformation I (according to Bosnich's nomenclature [27] ). On heating, it isomerizes to the thermodynamic octahedral isomer with trans arrangement of phosphonic acid moieties and trans-III [27] cyclam ring conformation.…”

“…Both isomers are kinetically inert, and the slow acid-assisted decomposition may be a consequence of an overall positive charge of complex species upon the full protonation of non-coordinated phosphonate oxygen atoms. [26] Among divalent metal ions, copper(II) is preferably complexed from thermodynamic as well as kinetic points of view. [28] We also showed that the complex-ation selectivity can be employed for an efficient analytical determination of copper.…”

Studies on Cage‐Type Tetranuclear Metal Clusters with Ferrocenylphosphonate Ligands

“…Most studies to date have been carried out on phenylphosphonic and alkylphosphonic acids. [23][24][25] Phosphonic acids with other organic functional groups such as azacrown ethers, [26][27][28] amines, [29][30][31] sulfonate/sulfonic acid [32,33] and carboxylate/carboxylic acid groups [34][35][36][37] have been found to be better ligands that can form many complexes with novel structures and properties. Motivated by the current work on functional coordinate materials bearing ferrocene (Fc) groups, we have synthesized a ferrocenylphosphonic acid, and subsequently used it for coordination with metal salts, thus yielding materials displaying both ferrocene and metal-phosphonate properties in conjunction with robust inorganic backbones.…”

Copper(II) Complexes of a Hexadentate Mixed‐Donor N3S3 Macrobicyclic Cage: Facile Rearrangements and Interconversions

“…Such a tetragonal distortion opposes the threefold symmetry of sar-type ligands, and indeed this lowering of symmetry has been observed and reported on a number of occasions with different cage ligands in complexes with Cu II . [11,30,31] Removal of labile divalent Cu [32] and Hg [33] from ligands that belong to the sar family is remarkably slow in comparison with the processes that occur on nonoctahedrally encapsulating polyamines, [34,35] but it is able to be observed and studied under less extreme conditions than those required for demetalation of Co cage complexes. The rate of demetalation is also dependent on the nature of the apical substituents.…”