Cr4+ in Tetrahedral Coordination of Oxidic Solids: A Spectroscopic and Structural Investigation

Synthesis, characterization, and spectroscopic characteristics of chromium(6+) and -(4+) silicalite-2 (ZSM-11) materials

The systematic incorporation of Cr ions into a phase-pure silicalite-2 lattice was accomplished through hydrothermal synthesis using 3,5-dimethylpiperidinium as a templating agent. The Cr ions, after calcination to remove the template, were in the 6+ oxidation state, with their incorporation into the lattice verified by the systematic expansion of the unit cell as a function of Cr loading. The structures of these materials as revealed by electronic spectroscopy and X-ray absorption near-edge spectroscopy (XANES) were consistent with the dioxo structure typically exhibited by Cr(6+) in an amorphous silica matrix. These materials were highly luminescent, with the emission spectra showing an unusually well-resolved vibronic structure characteristic of an emissive site with little inhomogeneous broadening. The site was reduced under flowing CO to Cr(4+), as characterized by XANES. The reduction of Cr from 6+ to 4+ resulted in unit-cell volumes that are systematically smaller than those observed with Cr(6+), even though the ionic radius of Cr(4+) is larger. This is attributed to the fact that the Cr(6+) site is not a simple metal ion but a significantly larger [CrO(2)](2+) unit, requiring a larger lattice expansion to accommodate it. Through analysis of the XANES preedge and assignment of the ligand-field spectrum of the Cr(4+) ions, it is possible to establish isomorphic substitution into the silicalite lattice.

Ruthenium(VI) Doped into Single Crystals with the BaSO(4) and beta-K(2)SO(4) Structures: Optical Absorption Spectra of RuO(4)(2)(-)

By establishing suitable high-temperature preparation conditions we were able to grow the first RuO(4)(2)(-)-doped crystals of K(2)CrO(4), K(2)SeO(4), Cs(2)MoO(4), BaSO(4), BaCrO(4), and BaSeO(4). Their polarized absorption spectra at low temperatures are reported and discussed. These are very different from the ruthenate(VI) spectra in the literature, and the lack of resemblance between calculated and measured spectra of ruthenate(VI)-assumed to be RuO(4)(2)(-) in the past-is easily understood. The (3)A(2) --> (1)A(1) spin-flip transition of RuO(4)(2)(-) peaks at approximately 7100 cm(-)(1). Stronger bands due to (3)A(2) --> (3)T(2) and (3)T(1) are observed in the vis around 13 500 and 16 500 cm(-)(1), respectively, giving rise to the green-blue color of most samples. In the BaSO(4) host the (1)A(1) absorption line is split into three differently polarized components exhibiting a strong temperature dependence. This can be analyzed in terms of a Boltzmann population with three levels at 0, 13, and 18 cm(-)(1), corresponding to the three spinor components of the (3)A(2) ground state. In order to better understand the major changes which occur on going from the 3d(2) to the 4d(2) electron configuration the spectroscopic data of RuO(4)(2)(-) are compared with those of CrO(4)(4)(-), MnO(4)(3)(-), and FeO(4)(2)(-).

Synthesis, Characterization, and Electrochemistry of sigma-Bonded Cobalt Corroles in High Oxidation States

The synthesis, electrochemistry, spectroscopy, and structural characterization of two high-valent phenyl sigma-bonded cobalt corroles containing a central cobalt ion in formal +IV and +V oxidation states is presented. The characterized compounds are represented as phenyl sigma-bonded cobalt corroles, (OEC)Co(C(6)H(5)) and [(OEC)Co(C(6)H(5))]ClO(4), where OEC is the trianion of 2,3,7,8,12,13,17,18-octaethylcorrole. The electronic distribution in both molecules is discussed in terms of their NMR and EPR spectroscopic data, magnetic susceptibility, and electrochemistry.