Research performed at beamline Balder at MAX IV gives new insights into the arrangement of solvent molecules around metal ions. In this case, the ion studied was copper(II). The knowledge gained furthers the understanding of metal-containing biomolecules in our bodies, and materials used for catalysis.
Many molecules in our bodies contain metal ions. An example is the protein haemoglobin which contains iron that binds to and transports the oxygen in our blood. To understand how these biomolecules work, one has to take into account how they interact with the water molecules surrounding them. Another example is the materials used for catalysis where the reacting molecules often are in solution.
Many different theories
To understand the chemistry of metal ions in solution one needs to have detailed knowledge on how the solvent molecules are bound to it. For most metal ions and aqueous solutions, this has been known for a long time. However, for the copper(II) ion, important in many areas of industrial processes as well as biochemistry, this has been intensely debated in the chemical literature for the last 20 years, professor Ingmar Persson, Swedish University of Agricultural Sciences, Uppsala, says.
There have been many different theories on how the water molecules of the solution arrange themselves around the copper(II) ion. The number of molecules from the solution that arrange themselves around the metal ion is called coordination number.
It was for a long time assumed that the hydrated copper(II) ion is six-coordinate with four strongly bound water molecules in a square-plane and with another two water molecules perpendicular to the square-plane (axial positions) at longer distance in a so-called Jahn-Teller distorted octahedral structure. A large majority of all reported hydrated copper(II) ion in the solid state from crystallographic studies has been reported to have this structure. Twenty years ago this view was challenged for aqueous solutions, and it was proposed that only one water molecule was present perpendicular to the square plane of water molecules. A third view put forward but not proven experimentally was that the two water molecules at longer Cu-O bond distance were at different distances, professor Persson continues.
EXAFS answers the question
At beamline Balder, one of the methods used is called Extended X-ray Absorption Fine Structure, EXAFS. In this method, the X-ray energy is varied while measuring how much is absorbed by the sample and the resulting graph is a “fingerprint” that contains information of an atom and the molecules surrounding it.
For the copper(II) ion, there is a long series of publications using a wide range of structural, physico-chemical, and theoretical methods to sort out how the solvent molecules are arranged. EXAFS is one of the methods which should be able to answer this question, but this requires high-quality data in a wide energy range. This was not possible to perform at MAX-lab, but the properties of the Balder beamline at MAX IV allows it, Persson explains.
To understand more of the properties of the copper(II) ion in solution, several different solvents were used. Some solvents have bulkier molecules which means that fewer of them will fit around the copper(II) ion.
It could be expected as some solutions are light blue as water, while in solvents which consist of large bulky molecules are green, Persson says.
The researchers measured how oxygen-containing molecules, such as water, arrange themselves around copper in both solid and liquid form. In this way, it is possible to test how well earlier reported studies have been able to measure the arrangement of molecules around metal ions as compared to the highly resolved EXAFS method available at beamline Balder.
Results for copper(II) in solution
The results show that the copper(II) ion in eight of the studied solvents, including water, binds four solvent molecules at short Cu-O bond distance, 1.96 Å in a square-plane, and two solvent molecules in axial positions at different Cu-O bond distances, approximately 2.15 and 2.32 Å. Thanks to the good data quality and large energy range of the collected X-ray absorption spectra these distances could be resolved. In the remaining two solvents with large solvent molecules there is only room for four molecules in the square-plane, Persson says.
Results of crystallographic studies
This study also included five solid copper(II) compounds with only water molecules binding to copper. The crystallographic studies of these compounds have shown that the Cu-O bond distances are pair-wise identical, they are so-called centrosymmetric. As described above, this not the case in an aqueous solution where they are non-centrosymmetric. By comparing EXAFS spectra of copper(II) in aqueous solution and solid compounds it became evident that the structure of copper(II) only binding to water molecules is the same. This shows that the crystallographic studies give an incorrect picture of the structure. This can be explained by the different Cu-O bond distances in the axial positions are randomly oriented in the individual unit cells in the solid, and that crystallography gives a centrosymmetric mean value of non-centrosymmetric complexes. This strongly indicates that about 90 % of all copper(II) compounds in the solid state reported to be centrosymmetric in fact are non-centrosymmetric. This shows that to get a correct structure description of e.g. copper(II) compounds in the solid state, information from crystallography and EXAFS needs to be combined.
Header image: Glass vials containing copper(II) ions in different solvents. The colour of the solution reflects the arrangement of solvent molecules around the copper ion.
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Ingmar Persson, Daniel Lundberg, Éva G. Bajnóczi, Konstantin Klementiev, Justus Just, and Kajsa G. V. Sigfridsson Clauss
EXAFS Study on the Coordination Chemistry of the Solvated Copper(II) Ion in a Series of Oxygen Donor Solvents
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