Rhodium-Catalyzed B–H Activation of 1,2-Azaborines: Synthesis and Characterization of BN Isosteres of Stilbenes

The first example of catalytic B–H activation of azaborines leading to a new family of stilbene derivatives through dehydrogenative borylation is reported. Ten 1,2-azaborine-based BN isosteres of stilbenes have been synthesized using this method, including a BN isostere of a biologically active stilbene. It is demonstrated that BN/CC isosterism in the context of stilbenes can lead to significant changes in the observed photophysical properties such as higher quantum yield and a larger Stokes shift. Direct comparative analysis of BN stilbene <b>3g</b> and its carbonaceous counterpart <b>6g</b> is consistent with a stronger charge-transfer character of the excited state exhibited by <b>3g</b> in which the 1,2-azaborine heterocycle serves as a better electron donor than the corresponding arene

Reactions of Lanthanide Atoms with Oxygen Difluoride and the Role of the Ln Oxidation State

Laser-ablated lanthanide metal atoms were condensed with OF₂ in excess argon or neon at 4 K. New infrared absorption bands were observed and assigned to the oxidative addition products OLnF₂ and OLnF on the basis of ¹⁸O isotopic substitution and electronic structure calculations of the vibrational frequencies. The dominant absorptions in the 500 cm^–1 region are identified as Ln–F stretching modes, which follow the lanthanide contraction. The Ln–O stretching frequency is an important measure of the oxidation states of the Ln and oxygen and the spin state of the complex. The OCeF₂, OPrF₂, and OTbF₂ molecules have higher frequency Ln–O stretching modes. The Ce is assigned to the IV oxidation state and the Pr and Tb are assigned to a mixed III/IV oxidation state. The remaining OLnF₂ compounds have lower Ln–O stretches, and the Ln is in the III oxidation state and the O is in the −1 oxidation state. For all of the OLnF compounds, the metal is in the III oxidation state, and the Ln–F bonds are ionic. In OCeF₂, OLaF, and OLuF, the bonding between the Ln and O is best described as a highly polarized σ bond and two pseudo π bonds formed by donation from the two 2p lone pairs on the O to the Ln. Bonding for the OLnF₂ compounds in the III oxidation state is predicted to be fully ionic. The bonding in OLnF₂ and OLnF is dominated by the oxidation state on the lanthanide and the spin state of the molecule. The observation of larger neon to argon matrix shifts for Ln–O modes in several OLnF molecules as compared to their OLnF₂ analogues is indicative of more ionic character in the OLnF species, consistent with the more formal negative charge on the oxygen in OLnF

Structures and Properties of the Products of the Reaction of Lanthanide Atoms with H₂O: Dominance of the +II Oxidation State

The reactions of lanthanides with H₂O have been studied using density functional theory with the B3LYP functional. H₂O forms an initial Lewis acid–base complex with the lanthanides exothermically with interaction energies from −2 to −20 kcal/mol. For most of the Ln, formation of HLnOH is more exothermic than formation of H₂LnO, HLnO + H, and LnOH + H. The reactions to produce HLnOH are exothermic from −25 to −75 kcal/mol. The formation of LnO + H₂ for La and Ce is slightly more exothermic than formation of HLnOH and is less or equally exothermic for the rest of the lanthanides. The Ln in HLnOH and LnOH are in the formal +II and +I oxidation states, respectively. The Ln in H₂LnO is mostly in the +III formal oxidation state with either LnO^–/LnH^– or Ln(H₂)⁻/LnO^2– bonding interactions. A few of the H₂LnO have the Ln in the +IV or mixed +III/+IV formal oxidation states with LnO^2–/LnH^– bonding interactions. The Ln in HLnO are generally in the +III oxidation state with the exception of Yb in the +II state. The orbital populations calculated within the natural bond orbital (NBO) analysis are consistent with the oxidation states and reaction energies. The more exothermic reactions to produce HLnOH are always associated with more backbonding from the O­(H) and H characterized by more population in the 6s and 5d in Ln and the formation of a stronger LnO­(H) bond. Overall, the calculations are consistent with the experiments in terms of reaction energies and vibrational frequencies

Electronic structure investigations of titanium oxide nanoclusters, boron-nitrogen heterocycles, and reaction products of lanthanides with oxygen difluoride and lanthanides with water

Advanced electronic structure methods on high performance computers have been used to predict the reactions of lanthanides, properties of liquid chemical hydrogen storage systems, and Fe doped TiO2 nanoclusters. Chapter 2 describes a detailed experimental matrix isolation and computational study of the reactions of lanthanide atoms with F2O. The experimental data is analyzed in terms of the results of density functional theory and CCSD(T) calculations. The products OlnF and OLnF2 are observed, with most Ln in the +III oxidation state for both products. The bonding in these molecules is strongly dependent on the oxidation state of the lanthanide. The coupling of the spin on the O with that on the Ln is important in determining the Ln-O frequency. Chapter 3 describes the reactions of the lanthanides with H2O. The dominant products are LnO + H2 and HLnOH with the Ln in the +II oxidation state. The difference in the reactions of F2O and H2O are due to the differences in the reactant and product bond strengths. Chapter 4 describes combined experimental and computational studies of the liquid chemical hydrogen storage systems based on substituting a C-C with a B-N. Experimental structural analysis and high level electronic structure calculations suggest that the aromaticity of the 1,3-dihydro-1,3-azaborine heterocycle is intermediate between that of benzene and that of 1,2-dihydro-1,2-azaborine. The development of the first reported parental BN isostere of cyclohexane featuring two BN units is thermally stable up to 150 °C with a H2 storage capacity of 4.7 weight% is described. High level computations have been used to predict the reaction energetics of the formation of two cage compounds from the H2 desorption reactions. The photophysical properties resulting from BN/CC isosterism for 10 1,2-azaborine-based BN isosteres of stilbenes have been explained by using high level electronic structure calculations. Chapter 5 describes computational and experimental evidence for facile charge transfer from the transition metal ion Fe(II) to titanium sites in nanoscale TiO2 and its oxynitride, TiO2-xNx. The transfer has been characterized through core level and valance band photoelectron spectroscopies and detailed electronic structure calculations. (Published By University of Alabama Libraries

Bis-BN Cyclohexane: A Remarkably Kinetically Stable Chemical Hydrogen Storage Material

A critical component for the successful development of fuel cell applications is hydrogen storage. For back-up power applications, where long storage periods under extreme temperatures are expected, the thermal stability of the storage material is particularly important. Here, we describe the development of an unusually kinetically stable chemical hydrogen storage material with a H₂ storage capacity of 4.7 wt%. The compound, which is the first reported parental BN isostere of cyclohexane featuring two BN units, is thermally stable up to 150 °C both in solution and as a neat material. Yet, it can be activated to rapidly desorb H₂ at room temperature in the presence of a catalyst without releasing other detectable volatile contaminants. We also disclose the isolation and characterization of two cage compounds with <i>S</i>₄ symmetry from the H₂ desorption reactions