[Cu(aq)]2+ is structurally plastic and the axially elongated octahedron goes missing

Abstract

High resolution (k = 18 Å or k = 17 Å) copper K-edge EXAFS and MXAN (Minuit X-ray Absorption Near Edge) analyses have been used to investigate the structure of dissolved [Cu(aq)] in 1,3-propanediol (1,3-P) or 1,5-pentanediol (1,5-P) aqueous frozen glasses. EXAFS analysis invariably found a single axially asymmetric 6-coordinate (CN6) site, with 4×O = 1.97 Å, O = 2.22 Å, and O = 2.34 Å, plus a second-shell of 4×O = 3.6 Å. However, MXAN analysis revealed that [Cu(aq)] occupies both square pyramidal (CN5) and axially asymmetric CN6 structures. The square pyramid included 4×HO = 1.95 Å and 1×HO = 2.23 Å. The CN6 sites included either a capped, near perfect, square pyramid with 5×HO = 1.94 ± 0.04 Å and HO = 2.22 Å (in 1,3-P) or a split axial configuration with 4×HO = 1.94, HO = 2.14 Å, and HO = 2.28 Å (in 1,5-P). The CN6 sites also included an 8-HO second-shell near 3.7 Å, which was undetectable about the strictly pyramidal sites. Equatorial angles averaging 94° ± 5° indicated significant departures from tetragonal planarity. MXAN assessment of the solution structure of [Cu(aq)] in 1,5-P prior to freezing revealed the same structures as previously found in aqueous 1M HClO, which have become axially compressed in the frozen glasses. [Cu(aq)] in liquid and frozen solutions is dominated by a 5-coordinate square pyramid, but with split axial CN6 appearing in the frozen glasses. Among these phases, the Cu-O axial distances vary across 1 Å, and the equatorial angles depart significantly from the square plane. Although all these structures remove the d , d degeneracy, no structure can be described as a Jahn-Teller (JT) axially elongated octahedron. The JT-octahedral description for dissolved [Cu(aq)] should thus be abandoned in favor of square pyramidal [Cu(HO)]. The revised ligand environments have bearing on questions of the Cu(i)/Cu(ii) self-exchange rate and on the mechanism for ligand exchange with bulk water. The plasticity of dissolved Cu(ii) complex ions falsifies the foundational assumption of the rack-induced bonding theory of blue copper proteins and obviates any need for a thermodynamically implausible protein constraint.

EXAFS Study on the Coordination Chemistry of the Solvated Copper(II) Ion in a Series of Oxygen Donor Solvents

“…The pattern of bond distances and Debye–Waller coefficients and the fact that the EXAFS spectra of solid [Cu(H 2 O) 6 ](ClO 4 ) 2 , Cu(NO 3 ) 2 ·2.5H 2 O, [Cu(H 2 O) 6 ](BrO 3 ) 2 , and [Cu(H 2 O) 6 ]SiF 6 and the aqueous solutions of copper(II) superimpose ( Figure 1 ) strongly indicate the hydrated copper(II) ion and the solvated copper(II) ion in methanol and the six amide solvents bind six solvent molecules in a noncentrosymmetric tetragonally elongated octahedral geometry. This model is in full agreement with the observation using EPR that the hydrated copper(II) ion is noncentrosymmetric in aqueous solution 6 , 7 and is in full agreement with the EXAFS studies reported by Frank et al 13 , 22 , 25 The survey of structure determinations of solid compounds containing a hexaaquacopper(II) ion or a pentaaqua(O-ligand)copper(II) complex described in the Introduction strongly indicates that individual copper(II) complexes in fact are six-coordinate and noncentrosymmetric. It is therefore very important to apply a non-lattice-dependent method such as EXAFS to determine the structures of such compounds.…”

“… 9 This study was followed a by series of publications using EXAFS and XANES spectroscopy, X-ray and neutron scattering on liquids, and DFT and QM/MD simulations on different levels to study the coordination chemistry of the hydrated copper(II) ion in aqueous solution. 10 − 25 A majority of these studies propose a tetragonally elongated square-pyramidal configuration as the most likely or stable structure, 9 , 10 , 13 , 15 , 16 , 25 but several studies have proposed dynamic equilibria among four-, five-, and/or six-coordinated hydrate complexes, 14 , 19 , 20 , 22 , 23 as well as a six-coordinate Jahn–Teller distorted-octahedral configuration. 11 , 12 , 18 , 21 This shows that the discussion of the structure of the hydrated copper(II) ion aqueous solution is still a matter of debate.…”

“… 22 Studies on frozen samples of 1,3-propanediol and 1,5-pentanediol aqueous glasses revealed asymmetric six-coordination with Cu–O eq , Cu–O ax1 , and Cu–O ax2 bond distances of 4 × 1.94, 2.22, and 2.34 Å, respectively, from EXAFS data, while MXAN data revealed copper sites with two kinds of six-coordination, asymmetric Jahn–Teller distorted-octahedral (4 × 1.94, 1 × 2.14, and 2.28 Å) and capped-square-pyramidal configurations (5 × 1.94 and 2.22 Å), and five-coordinated sites in a square-pyramidal configuration (4 × 1.95 and 2.23 Å). 25 D’Angelo and co-workers have studied the solvated copper(II) ion methanol, dimethyl sulfoxide and acetontrile solution by combined XANES and EXAFS, showing that tetragonally elongated square-pyramidal coordination is preferred in these solvents. 26 , 27 …”

A model for the effect of ion pairing on an outer sphere electron transfer

“…In the following, we consider only the fivefold complex. The calculated Cu-O bond lengths are in good agreement with experimental data [25]; the difference amounts to 0.05Å.…”

“…Experimentally, a five-fold coordinated Cu ++ has also been observed by neutron diffraction [24] and X-ray absorption spectroscopy [23,[25][26][27]. Further evidence in favor of this structure also follows from Car-Parrinello MD [28] and ab initio molecular dynamics [29,31] simulations.…”