COORDINATION CHEMISTRY OF BIS(BENZYL)PHOSPHINATE

The work presented in this dissertation deals with the coordination chemistry of the bis(benzyl)phosphinate ligand with vanadium, tungsten and cobalt. The long term goal of this project was to produce and physically characterize high oxidation state transition metal oxide phosphinate compounds with potential catalytic applications. The reaction of bis(benzyl)phosphinic acid with VO(acac)2 in the presence of water or pyridine leads to the synthesis of trimeric vanadium(IV) clusters (V3(µ3-O)O2)(µ2-O2P(CH2C6H5)2)6(H2O) and (V3(µ3-O)O2)(µ2-O2P(CH2C6H5)2)6(py). In contrast, when diphenylphosphinic acid or 2-hydroxyisophosphindoline-2-oxide were reacted with VO(acac)2, insoluble polymeric compounds were produced. The trimeric clusters were characterized using FTIR, elemental analysis, single crystal diffraction, room temperature magnetic susceptibility, thermogravimetric analysis and differential scanning calorimetry. The variable-temperature, solid-state magnetic susceptibility was measured on (V3(µ3-O)O2)(µ2-O2P(CH2C6H5)2)6(py). The polymeric compounds were characterized using FTIR, powder diffraction and elemental analysis. Two different cubane clusters made of tungsten(V) and vanadium(V) were stabilized using bis(benzyl)phosphinate. The oxidation of (V3(µ3-O)O2)(µ2-O2P(CH2C6H5)2)6(H2O) with tBuOOH led to the formation of V4(µ3-O)4(µ2-O2P(Bn)2)4(O4). W4(µ3-O)4(µ2-O2P(Bn)2)4(O4) was produced by heating W(CO)6 in a 1:1 mixture of EtOH/THF at 120 ˚C. Both compounds were characterized using single crystal diffraction, FTIR, 31P-NMR, 1H-NMR and elemental analysis. W4(µ3-O)4(µ2-O2P(Bn)2)4(O4) was also characterized using UV-vis. Cobalt(II) reacted with bis(benzyl)phosphinate to produce three different dinuclear complexes. [(py)3Co(µ2-O2P(Bn)2)3Co(py)][ClO4], (py)3Co(µ2-O2P(Bn)2)3Co(Cl) and (py)(µ2-NO3)Co(µ2-O2P(Bn)2)3Co(py) were all characterized using single crystal diffraction, elemental analysis and FTIR. Room temperature magnetic susceptibility measurements were performed on [(py)3Co(µ2-O2P(Bn)2)3Co(py)][ClO4] and (py)3Co(µ2-O2P(Bn)2)3Co(Cl). The variable-temperature, solid-state magnetic susceptibility was also measured on [(py)3Co(µ2-O2P(Bn)2)3Co(py)][ClO4].

NANOSCALE ELECTROCHEMISTRY BY IN-SITU TRANSMISSION ELECTRON MICROSCOPY

Nanoscale research in energy storage has recently focused on investigating the properties of nanostructures in order to increase energy density, power rate, and capacity. To better understand the intrinsic properties of nanomaterials, a new and advanced in situ system was designed that allows atomic scale observation of materials under external fields. A special holder equipped with a scanning tunneling microscopy (STM) probe inside a transmission electron microscopy (TEM) system was used to perform the in situ studies on mechanical, electrical, and electrochemical properties of nanomaterials. The nanostructures of titanium dioxide (TiO2) nanotubes are characterized by electron imaging, diffraction, and chemical analysis techniques inside TEM. TiO2 nanotube is one of the candidates as anode materials for lithium ion batteries. It is necessary to study their morphological, mechanical, electrical, and electrochemical properties at atomic level. The synthesis of TiO2 nanotubes showed that the aspect ratio of TiO2 could be controlled by processing parameters, such as anodization time and voltage. Ammonium hydroxide (NH4OH) treated TiO2 nanotubes showed unexpected instability. Observation revealed the nanotubes were disintegrated into nanoparticles and the tubular morphology was vanished after annealing. The nitrogen compounds incorporated in surface defects weaken the nanotube and result in the collapse of nanotube into nanoparticles during phase transformation. Next, the electrical and mechanical properties of TiO2 nanotubes were studied by in situ TEM system. Phase transformation of anatase TiO2 nanotubes into rutile nanoparticles was studied by in situ Joule heating. The results showed that single anatase TiO2 nanotubes broke into ultrafine small anatase nanoparticles. On further increasing the bias, the nanoclusters of anatase particles became prone to a solid state reaction and were grown into stable large rutile nanoparticles. The relationship between mechanical and electrical properties of TiO2 nanotubes was also investigated. Initially, both anatase and amorphous TiO2 nanotubes were characterized by using I-V test to demonstrate the semiconductor properties. The observation of mechanical bending on TiO2 nanotubes revealed that the conductivity would increase when bending deformation happened. The defects on the nanotubes created by deformation helped electron transportation to increase the conductivity. Lastly, the electrochemical properties of amorphous TiO2 nanotubes were characterized by in situ TEM system. The direct chemical and imaging evidence of lithium-induced atomic ordering in amorphous TiO2 nanotubes was studied. The results indicated that the lithiation started with the valance reduction of Ti4+ to Ti3+ leading to a LixTiO2 intercalation compound. The continued intercalation of Li ions in TiO2 nanotubes triggered an amorphous to crystalline phase transformation. The crystals were formed as nano islands and identified to be Li2Ti2O4 with cubic structure (a = 8.375 Å). This phase transformation is associated with local inhomogeneities in Li distribution. Based on these observations, a new reaction mechanism is proposed to explain the first cycle lithiation behavior in amorphous TiO2 nanotubes.