Abstract
High-resolution EXAFS (k = 18 Å(-1)) and MXAN XAS analyses show that axially elongated square pyramidal [Cu(H2O)5](2+) dominates the structure of Cu(II) in aqueous solution, rather than 6-coordinate JT-octahedral [Cu(H2O)6](2+). Freezing produced a shoulder at 8989.6 eV on the rising XAS edge and an altered EXAFS spectrum, while 1s → 3d transitions remained invariant in energy position and intensity. Core square pyramidal [Cu(H2O)5](2+) also dominates frozen solution. Solvation shells were found at ∼3.6 Å (EXAFS) or ∼3.8 Å (MXAN) in both liquid and frozen phases. However, MXAN analysis revealed that about half the time in liquid solution, [Cu(H2O)5](2+) associates with an axially non-bonding 2.9 Å water molecule. This distant water apparently organizes the solvation shell. When the 2.9 Å water molecule is absent, the second shell is undetectable to MXAN. The two structural arrangements may represent energetic minima of fluxional dissolved aqueous [Cu(H2O)5](2+). The 2.9 Å trans-axial water resolves an apparent conflict of the [Cu(H2O)5](2+) core model with a dissociational exchange mechanism. In frozen solution, [Cu(H2O)5](2+) is associated with either a 3.0 Å axial non-bonded water molecule or an axial ClO4(-) at 3.2 Å. Both structures are again of approximately equal presence. When the axial ClO4(-) is present, Cu(II) is ∼0.5 Å above the mean O4 plane. This study establishes [Cu(H2O)5](2+) as the dominant core structure for Cu(II) in water solution, and is the first to both empirically resolve multiple extended solution structures for fluxional [Cu(H2O)5](2+) and to provide direct evidence for second shell dynamics.
Highlights
Ever since the opening provided by the neutron diffraction and molecular dynamics study of Pasquarello et al.,1 the solution structure of [Cu(aq)]2+ has come under renewed and intensive scrutiny.2–14 All studies agree on the stable presence of four equatorial water ligands, which exhibit a uniform Cu–Ow distance of about 1.96 Å
Combined EXAFS and MXAN studies found the complex ions of Cu(II) in aqueous ammonia or imidazole solutions to be strongly solvated axially elongated square pyramids, each with a single axial water ligand at a distance of about 2.18 Å or 2.13 Å, respectively
Samples of 0.10 M Cu(ClO4)2 in 1M HClO4 were prepared for measurement as a liquid solution at room temperature or as a frozen solution at 10 K
Summary
Ever since the opening provided by the neutron diffraction and molecular dynamics study of Pasquarello et al., the solution structure of [Cu(aq)]2+ has come under renewed and intensive scrutiny. All studies agree on the stable presence of four equatorial water ligands, which exhibit a uniform Cu–Ow distance of about 1.96 Å. Studies of the very rapid solution dynamics have typically reported difficulties in detecting the number and even presence of the axial water ligands.. Combined EXAFS and MXAN studies found the complex ions of Cu(II) in aqueous ammonia or imidazole solutions to be strongly solvated axially elongated square pyramids, each with a single axial water ligand at a distance of about 2.18 Å or 2.13 Å, respectively. These were accompanied in the transverse axial position by a non-bonded ∼3 Å distant N/O scatterer that could only be described as a localized solvating molecule, most likely water. In the case of the ammonia complex, the solvation shell contributed strong multiple scattering features to the rising K-edge
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call