Analysis of the multimerization of Cu2+ complexes in aqueous solution by 1H NMR and Evans' method

Ceramic high temperature superconductors are near-perfect conductors for electricity and promising materials for applications in magnets, generators and transformers in the context of green energy generation and energy saving industrial devices. To allow their integration in conducting wires, these brittle materials are deposited on flexible substrates using precursor-containing chemical solutions. The development of such precursor solutions is a time-consuming and complicated task that would greatly benefit from being able to predict the solution’s stability as a function of factors such as pH and concentrations of metal ions and additives1. Triethanolamine (TEA) is often used to stabilize metal ions in aqueous YBa2Cu3O7-x (YBCO) precursor solutions. However, the nature of the complexation of Cu2+ ions with TEA is still a matter of debate. To increase the accuracy in predicting the solution’s stability, clarification on this issue is of course desirable. Although crystal structures of dimeric and tetrameric complexes have already been described, there exists no data concerning the existence of such complexes in solution. Here, we present NMR evidence of the pH-dependent formation of TEA complexes containing at least two Cu2+ ions in solution. By measuring the magnetic moment per Cu2+ ion using 1H- NMR spectroscopy and Evans’ method2,3, we demonstrate that multi-nuclear copper-triethanolamine complexes form in aqueous solution at pH 4 or higher. This information can be obtained accurately in complex mixtures with large counterion and background species concentrations by making use of diamagnetic corrections from reference measurements and tabulated data. Our approach will be most useful to further improve and develop speciation models that simulate the stability of precursor solutions for the deposition of Cu-based ceramic films. 1. Lommens, P., Feys, J., Vrielinck, H., De Buysser, K., Herman, G., Callens, F. and Van Driessche, I. Dalton Trans. 2012, 41, 3574-3582. 2. Evans, D.F. J. Chem. Soc. 1959, 400. 3. De Buysser, K, Herman, G.G., Bruneel, E., Hoste, S. and Van Driessche, I. Chem; Phys. 2005, 315, 286-292