The X-ray absorption spectroscopic model of the copper(II) imidazole complex ion in liquid aqueous solution: A strongly solvated square pyramid

摘要

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) _4L _(ax)] ~(2+) (L _(ax) indeterminate) with four equatorial nitrogen atoms at EXAFS, 2.02 ± 0.01 ?, and MXAN, 1.99 ± 0.03 ?. A short-axial N/O scatterer (L _(ax)) was found at 2.12 ± 0.02 ? (EXAFS) or 2.14 ± 0.06 ? (MXAN). A second but very weak axial Cu-N/O interaction was found at 2.9 ± 0.1 ? (EXAFS) or 3.0 ± 0.1 ? (MXAN). In the MXAN fits, only a squarepyramidal 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 ? 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 ? 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(H _2O) _5] ~(2+), [Cu(NH _3) _4(NH _3,H _2O)] ~(2+), ~(1,2) and now [Cu(Im) _4L _(ax))] ~(2+) and appears to be the geometry preferred by unconstrained aqueous-phase copper(II) complex ions.