title: 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 X-ray absorption spectroscopy (XAS) spectra of the monodentate sulfate complexes [MII(itao)(SO4)(H2O)0,1] (M = Co, Ni, Cu) and [Cu(Me6tren)(SO4)] exhibit well-defined preedge transitions at 2479.4, 2479.9, 2478.4, and 2477.7 eV, respectively, despite having no direct metal-sulfur bond, while the XAS preedge 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. Preedge transitions also appear in the sulfur K-edge XAS of crystalline [MII(SO4)(H2O)] (M = Fe, Co, Ni, and 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 the 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 antibonding orbitals, producing measurable sulfur XAS transitions. TDDFT calculations confirm the presence of XAS preedge features 0.5-2 eV below the rising sulfur 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 copper(II) electron hole. Transmission of spin polarization and thus of radical character through several bonds between the sulfur and electron hole provides a new mechanism for the counterintuitive appearance of preedge 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 copper(II), which extends across a hydrogen-bond bridge between sulfate and the itao ligand and involves orbitals at energies below the frontier set. This electronic structure feature provides a direct spectroscopic confirmation of the through-bond electron-transfer mechanism of redox-active metalloproteins.
authors.affiliation: Department of Chemistry, Stanford University , Stanford, California 94305, United States.|Stanford Synchrotron Radiation Lightsource, SLAC, Stanford University , Stanford, California 94309, United States.
authors.forename: Patrick
authors.identifier: http://orcid.org/0000-0001-5343-197X
authors.initials: P
authors.lastname: Frank
authors.affiliation: Department of Chemistry and Biochemistry, Montana State University , Bozeman, Montana 59717, United States.|MTA-ELTE "Momentum" Chemical Structure/Function Laboratory , Budapest 1117, Hungary.
authors.forename: Robert K
authors.initials: RK
authors.lastname: Szilagyi
authors.affiliation: Laboratorium fuer Kristallographie, ETH-Zentrum , Sonneggstrasse 5, No. G 62, CH-8092 Zürich, Switzerland.
authors.forename: Volker
authors.initials: V
authors.lastname: Gramlich
authors.affiliation: Department of Chemistry, National Cheng-Kung University , Tainan City 701, Taiwan.
authors.forename: Hua-Fen
authors.initials: HF
authors.lastname: Hsu
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC, Stanford University , Stanford, California 94309, United States.
authors.forename: Britt
authors.initials: B
authors.lastname: Hedman
authors.affiliation: Department of Chemistry, Stanford University , Stanford, California 94305, United States.|SLAC National Accelerator Laboratory, Stanford University , Menlo Park, California 94025, United States.
authors.forename: Keith O
authors.initials: KO
authors.lastname: Hodgson
mesh_terms: D004579:Electron Transport; D008024:Ligands; D009942:Organometallic Compounds / Q000737:chemistry*; D011789:Quantum Theory; D013431:Sulfates / Q000737:chemistry*; D013455:Sulfur / Q000737:chemistry*; D028561:Transition Elements / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D008024:Ligands; D009942:Organometallic Compounds; D013431:Sulfates; D028561:Transition Elements; D013455:Sulfur