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

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posted on 22.02.2016, 05:28 by Patrick Frank, Maurizio Benfatto, Britt Hedman, Keith O. Hodgson
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.