Symmetry Breaking in Solution-Phase [Cu(tsc)₂(H₂O)₂]²⁺: Emergent Asymmetry in Cu–S Distances and in Covalence

Abstract

The structure of aqueous Cu­(II)-bis-thiosemicarbazide, [Cu­(tsc)2]2+, is reported following EXAFS and MXAN analyses of the copper K-edge X-ray absorption (XAS) spectrum. The rising K-edge feature at 8987.1 eV is higher energy than those of crystalline models, implying unique electronic and structural solution states. EXAFS analysis (k = 2–13 Å–1; 2 × Cu–N = 2.02 ± 0.01 Å; 2 × Cu–S = 2.27 ± 0.01 Å; Cu–Oax = 2.41 ± 0.04 Å) could not resolve 5- versus 6-coordinate models. However, MXAN fits converged to an asymmetric broken symmetry 6-coordinate model with cis-disposed TSC ligands (Cu–Oax = 2.07 and 2.54 Å; Cu–N = 1.94 Å, 1.98 Å; Cu–S = 2.20 Å, 2.41 Å). Transition dipole integral evaluation of the sulfur K-edge XAS 1s → 3p valence transition feature at 2470.7 eV yielded a Cu–S covalence of 0.66 e–, indicating Cu1.34+. The high Cu–S covalence and short Cu–S bond in aqueous [Cu­(tsc)2(H2O)2]2+ again contradict the need for a protein rack to explain the unique structure of the blue copper active site. MXAN models of dissolved Cu­(II) complex ions have invariably featured broken centrosymmetry. The potential energy ground state for dissolved Cu­(II) evidently includes the extended solvation field, providing a target for improved physical theory. A revised solvation model for aqueous Cu­(II), |[Cu­(H2O)5]·14H2O|2+, is presented.