Electronic Structure and Vibrational Signatures of the Delocalized Radical in Hydrated Clusters of Copper(“II”) Hydroxide CuOH⁺(H₂O)_0–2

Abstract

The copper hydroxide ion, CuOH+, serves as the catalytic core in several recently developed water-splitting catalysts, and an understanding of its chemistry is critical to determining viable catalytic mechanisms. In spite of its importance, the electronic structure of this open-shell ion has remained ambiguous in the literature. In particular, computed values for both the thermodynamics of hydration and the vibrational signatures of the mono- and dihydrates have shown prohibitively large errors compared to values from recent experimental measurements. In this work, the source of this discrepancy is demonstrated to be the propensity of this ion to exist between traditional Cu­(I) and Cu­(II) oxidation-state limits. The spin density of the radical is accordingly shown to delocalize between the metal center and surrounding ligands, and increasing the hydration serves to exacerbate this behavior. Equation-of-motion coupled-cluster methods demonstrated the requisite accuracy to resolve the thermodynamic discrepancies. Such methods were also needed for spectral simulations, although the latter also required a direct simulation of the role of the deuterium “tag” molecules that are used in modern predissociation spectroscopy experiments. This nominally benign tag molecule underwent direct complexation with the open-valence metal ion, thereby forming a species akin to known metal–H2 complexes and strongly impacting the resulting spectrum. Thermal populations of this configuration and other more traditional noncovalently bound isomers led to a considerable broadening of the spectral lineshapes. Therefore, at least for the CuOH+(H2O)0–2 hydrates, these benchmark ions should be considered to be delocalized radical systems with some degree of multireference character at equilibrium. They also serve as a cautionary tale for the spectroscopy community, wherein the role of the D2 tag is far from benign.

Persistence of a Delocalized Radical in Larger Clusters of Hydrated Copper(II) Hydroxide, CuOH+(H2O)3–7

“…Figure displays the evolution of the NBO partial charges on the copper ion (blue) and hydroxyl/hydroxide ligand (red), as well as the spin density at each cluster size, in analogy to the analysis provided earlier for n = 3. In the isolated CuOH + ion, copper exhibits a charge of only +0.97, which is consistent with our previous assignment of this electronic structure as behaving more like Cu­(I) . The radical in this reference ion resides mainly on the OH ligand, yet a small amount of spin density is also located on the copper ion.…”

“…The second-shell water molecule is a double hydrogen-bond acceptor from the metal-bound water ligands in this isomer. Given the known strong ligand binding energies for this complex, 22,58 the low relative energy of 3B suggests a particularly stable hydrogen-bond arrangement that resembles other known cation-water complexes. 59−64 Other higherenergy, 3-coordinate hydrogen-bonding arrangements were observed in water-bound isomer 3C (5.31 kcal/mol) and OHbound isomer 3D (11.85 kcal/mol).…”

“…In the isolated CuOH + ion, copper exhibits a charge of only +0.97, which is consistent with our previous assignment of this electronic structure as behaving more like Cu(I). 22 The radical in this reference ion resides mainly on the OH ligand, yet a small amount of spin density is also located on the copper ion. Upon hydration, the ionic Cu 2+ •••OH − contribution is preferentially stabilized and begins to mix into the groundstate electronic structure, although the role of this configuration does not begin to saturate until the square-planar coordination shell is completed at n = 3.…”

“…Ab initio quantum chemistry simulations, combined with recent experimental advances in cold-ion predissociation spectroscopy and mass spectrometry methods, 20 provide insight into the inner-sphere redox transformations that are often inaccessible to traditional electrochemistry methods. 21 The first of these studies 22 (hereafter "Paper I)" examined the electronic structure of small hydrates of copper(II) hydroxide, CuOH + (H 2 O) 0−2 , and the present follow-up investigation seeks to determine whether this small-cluster behavior evolves with larger cluster sizes. The main qualitative discovery of the first investigation was that CuOH + natively exists between Cu 1+ •••OH • and Cu 2 +• •••OH − limiting electronic configurations and accordingly exhibits both multireference and delocalized-radical characters.…”

Molecular Motion in the Interconverting σ-H2, Di-, and Tri-hydride Regimes: Mo(PH3)5H2

“…With an all-electron basis set and traditional valence definition for phosphorous and hydrogen, a total of 29 orbitals were, therefore, frozen in the correlation calculations. (A brief tutorial covering frozen-core considerations in transition metals can be found in the Supporting Information of ref .) Single-point energy calculations using the cc-pVDZ geometries were then computed with a cc-pVTZ basis set.…”