Spin-Polarization-Induced Preedge Transitions in the Sulfur K-Edge XAS Spectra of Open-Shell Transition-Metal Sulfates: Spectroscopic Validation of σ-Bond Electron Transfer

Abstract

Sulfur K-edge XAS spectra of the monodentate sulfate complexes [MII(itao)(SO4)(H2O)0,1] and [Cu(Me6tren)(SO4)] exhibit well-defined pre-edge transitions at 2479.4 eV, 2479.9 eV, 2478.4 eV, and 2477.7 eV, respectively (M = Co, Ni, Cu), despite having no direct metal-sulfur bond, while the XAS pre-edge of [Zn(itao)(SO4)] is featureless. The sulfur K-edge XAS of [Cu(itao)(SO4)] but not of [Cu(Me6tren)(SO4)] uniquely exhibits a weak transition at 2472.1 eV, an extraordinary 8.7 eV below the first inflection of the rising K-edge. Pre-edge transitions also appear in the sulfur K-edge XAS of crystalline [MII(SO4)(H2O)] (M = Fe, Co, Ni, Cu, but not Zn) and in sulfates of higher-valent early transition metals. Ground state density functional theory (DFT) and time-dependent DFT (TDDFT) calculations show that charge transfer from coordinated sulfate to paramagnetic late transition metals produces spin polarization that differentially mixes the spin-up (α) and spin-down (β) spin-orbitals of the sulfate ligand, inducing negative spin density at sulfate sulfur. Ground state DFT calculations show that sulfur 3p character then mixes into metal 4s and 4p valence orbitals and various combinations of ligand anti-bonding orbitals, producing measureable sulfur XAS transitions. TDDFT calculations confirm the presence of XAS pre-edge features 0.5 to 2 eV below the sulfur rising K-edge energy. The 2472.1 eV feature arises when orbitals at lower energy than the frontier occupied orbitals with S 3p character mix with the Cu(II) electron hole. Transmission of spin polarization and thus of radical character through several bonds between the S and the electron hole provides a new mechanism for the counter-intuitive appearance of pre-edge transitions in the XAS spectra of transition metal oxoanion ligands in the absence of any direct metal-absorber bond. The 2472.1 eV transition is evidence for further radicalization from Cu(II), which extends across an H-bond bridge between sulfate and the itao ligand and involves orbitals at energies below the frontier set. This electronic structure feature provides direct spectroscopic confirmation of the through-bond electron transfer mechanism of redox-active metalloproteins.

Supramolecular Assembly of U(IV) Clusters and Superatoms with Unconventional Countercations

“…Of the four Zn-sites, two are fully coordinated to water, and the other two bind inside the U 70 ring (Figure b). The 3d 10 electron configuration of Zn II minimizes covalent linkage to the sulfate ligands, as recently described in X-ray absorption studies . In this prior study, pre-edge features in sulfur K-edge X-ray absorption spectra were observed for sulfate complexes of Co­(II), Ni­(II), and Cu­(II) but not Zn­(II).…”

“…The 3d 10 electron configuration of Zn II minimizes covalent linkage to the sulfate ligands, as recently described in X-ray absorption studies. 57 In this prior study, pre-edge features in sulfur K-edge X-ray absorption spectra were observed for sulfate complexes of Co(II), Ni(II), and Cu(II) but not Zn(II). The authors attributed this to ligand (sulfate) contributions to the 3d LUMO of the divalent transition metal, which is not possible for closed-shell Zn II .…”

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

“…The two XAS spectra exhibit a similar rising K-edge, while the XAS of the copper complex also exhibits a very strong pre-edge feature of 0.68 normalized intensity units. Prior ligand K-edge XAS studies have shown that ligand pre-edge features most often arise from delocalization of the 3d hole of a first row transition metal into a filled ligand n p shell. ,,− The pre-edge feature then reflects a vertical 1s → n p electronic transition into the newly unfilled ligand valence n p state. The intensity of the ligand pre-K-edge transition is a direct measure of ligand–metal covalency. − …”

Supramolecular Assembly of U(IV) Clusters and Superatoms

“…The 3d 10 of Zn II configuration minimizes covalent linkage of the sulfate ligands, as recent in X-ray absorption studies. 43 Small Angle X-ray Scattering. The extensive U70-Mn II -U70 and minimal U70-Zn II -U70 connectivity observed in the solid-state compounds was also evidenced upon dissolution.…”