Frank Patrick, Szilagyi Robert K, Gramlich Volker, Hsu Hua-Fen, Hedman Britt, Hodgson Keith O
Department of Chemistry, Stanford University , Stanford, California 94305, United States.
Stanford Synchrotron Radiation Lightsource, SLAC, Stanford University , Stanford, California 94309, United States.
Inorg Chem. 2017 Feb 6;56(3):1080-1093. doi: 10.1021/acs.inorgchem.6b00991. Epub 2017 Jan 9.
Sulfur K-edge X-ray absorption spectroscopy (XAS) spectra of the monodentate sulfate complexes [M(itao)(SO)(HO)] (M = Co, Ni, Cu) and [Cu(Metren)(SO)] 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)(SO)] is featureless. The sulfur K-edge XAS of [Cu(itao)(SO)] but not of [Cu(Metren)(SO)] 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 [M(SO)(HO)] (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.
单齿硫酸根配合物[M(itao)(SO₄)(H₂O)]（M = Co、Ni、Cu）和[Cu(Metren)(SO₄)]的硫K边X射线吸收光谱（XAS）在2479.4、2479.9、2478.4和2477.7 eV处分别呈现出明确的预边跃迁，尽管不存在直接的金属 - 硫键，而[Zn(itao)(SO₄)]的XAS预边则没有特征。[Cu(itao)(SO₄)]而非[Cu(Metren)(SO₄)]的硫K边XAS在2472.1 eV处独特地呈现出一个弱跃迁，比上升的K边的第一个拐点低8.7 eV。预边跃迁也出现在晶体[M(SO₄)(H₂O)]（M = Fe、Co、Ni和Cu，但不包括Zn）的硫K边XAS以及高价早期过渡金属的硫酸盐中。基态密度泛函理论（DFT）和含时密度泛函理论（TDDFT）计算表明，从配位硫酸根到顺磁性晚期过渡金属的电荷转移产生自旋极化，使硫酸根配体的自旋向上（α）和自旋向下（β）自旋轨道发生不同程度的混合，导致硫酸根硫处的自旋密度为负。基态DFT计算表明，硫3p特征随后混入金属4s和4p价轨道以及配体反键轨道的各种组合中，产生可测量的硫XAS跃迁。TDDFT计算证实了在硫K边能量上升以下0.5 - 2 eV处存在XAS预边特征。当能量低于具有S 3p特征的前沿占据轨道的轨道与铜（II）电子空穴混合时，出现2472.1 eV的特征。自旋极化以及自由基特征通过硫与电子空穴之间的几个键的传递，为在没有任何直接金属 - 吸收体键的情况下过渡金属含氧阴离子配体的XAS光谱中预边跃迁的反直觉出现提供了一种新机制。2472. eV的跃迁是铜（II）进一步自由基化的证据，这种自由基化延伸穿过硫酸根与itao配体之间的氢键桥，并且涉及能量低于前沿能级的轨道。这种电子结构特征为氧化还原活性金属蛋白的通过键电子转移机制提供了直接的光谱证实。
