A high-resolution XAS study of aqueous Cu(II) in liquid and frozen solutions: pyramidal, polymorphic, and non-centrosymmetric.

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.