The X-ray Absorption Spectroscopic Model of the Copper(II) Imidazole Complex Ion in Liquid Aqueous Solution: A Strongly Solvated Square Pyramid

Cu K-edge extended X-ray absorption fine structure (EXAFS)
and
Minuit X-ray absorption near-edge structure (MXAN) analyses were combined
to evaluate the structure of the copper­(II) imidazole complex ion
in liquid aqueous solution. Both methods converged to the same square-pyramidal
inner coordination sphere [Cu­(Im)4Lax]2+ (Lax indeterminate) with four equatorial nitrogen atoms
at EXAFS, 2.02 ± 0.01 Å, and MXAN, 1.99 ± 0.03 Å.
A short-axial N/O scatterer (Lax) was found at 2.12 ±
0.02 Å (EXAFS) or 2.14 ± 0.06 Å (MXAN). A second but
very weak axial Cu–N/O interaction was found at 2.9 ±
0.1 Å (EXAFS) or 3.0 ± 0.1 Å (MXAN). In the MXAN fits,
only a square-pyramidal structural model successfully reproduced the
doubled maximum of the rising K-edge X-ray absorption spectrum, specifically
excluding an octahedral model. Both EXAFS and MXAN also found eight
outlying oxygen scatterers at 4.2 ± 0.3 Å that contributed
significant intensity over the entire spectral energy range. Two prominent
rising K-edge shoulders at 8987.1 and 8990.5 eV were found to reflect
multiple scattering from the 3.0 Å axial scatterer and the imidazole
rings, respectively. In the MXAN fits, the imidazole rings took in-plane
rotationally staggered positions about copper. The combined (EXAFS
and MXAN) model for the unconstrained cupric imidazole complex ion
in liquid aqueous solution is an axially elongated square-pyramidal
core, with a weak nonbonded interaction at the second axial coordination
position and a solvation shell of eight nearest-neighbor water molecules.
This core square-pyramidal motif has persisted through [Cu­(H2O)5]2+, [Cu­(NH3)4(NH3,H2O)]2+,, and now [Cu­(Im)4Lax)]2+ and appears to be the geometry
preferred by unconstrained aqueous-phase copper­(II) complex ions.