ABSTRACT
Owing to its high natural abundance compared to the commonly used transition (precious) metals, as well as its high Lewis acidity and ability to change oxidation state, aluminium has recently been explored as the basis for a range of single-site catalysts. This paper aims to establish the ground rules for the development of a new type of cationic alkene oligomerisation catalyst containing two Al(III) ions, with the potential to act co-operatively in stereoselective assembly. Five new dimers of the type [R2Al(2-py')]2 (R=Me, iBu; py'=substituted pyridyl group) with different substituents on the Al atoms and pyridyl rings have been synthesised. The formation of the undesired cis isomers can be suppressed by the presence of substituents on the 6-position of the pyridyl ring due to steric congestion, with DFT calculations showing that the selection of the trans isomer is thermodynamically controlled. Calculations show that demethylation of the dimers [Me2Al(2-py')]2 with Ph3C+ to the cations [{MeAl(2-py')}2(µ-Me)]+ is highly favourable and that the desired trans disposition of the 2-pyridyl ring units is influenced by steric effects. Preliminary experimental studies confirm that demethylation of [Me2Al(6-MeO-2-py)]2 can be achieved using [Ph3C][B(C6F5)4].
ABSTRACT
Single-source precursors are ubiquitous in a number of areas of chemistry and material science due to their ease of use and wide range of potential applications. The development of new single-source precursors is essential in providing entries to new areas of chemistry. In this work, we synthesize nine new structurally related bimetallic metal-zirconium alkoxides, which can be used as single-source precursors to zirconia-based materials. Detailed analysis of the structures of these complexes provides important insights into the main factors influencing their aggregation. Investigation of the thermal decomposition of these species by TGA, PXRD, SEM, and EDS reveals that they can be used to produce bimetal oxides, such as Li2ZrO3, or a mixture of metal oxides, such as CuO and ZrO2. Significantly, these studies show that thermodynamically unstable forms of zirconia, such as the tetragonal phase, can be stabilized by metal doping, providing the promise for targeted deposition of zirconia materials for specific applications.
ABSTRACT
Methods for measuring enantiomeric excess (ee) of organic molecules by NMR spectroscopy provide rapid analysis using a standard technique that is readily available. Commonly this is accomplished by chiral derivatisation of the detector molecule (producing a chiral derivatisation agent, CDA), which is reacted with the mixture of enantiomers under investigation. However, these CDAs have almost exclusively been based on carbon frameworks, which are generally costly and/or difficult to prepare. In this work, a methodology based on the readily prepared inorganic cyclodiphosph(iii)azane CDA ClP(µ-N t Bu)2POBorn (Born = endo-(1S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl) is shown to be highly effective in the measurement of ee's of chiral amines, involving in situ reaction of the chiral amines (R*NH2) with the P-Cl bond of the CDA followed by quaternization of the phosphorus framework with methyl iodide. This results in sharp 31P NMR signals with distinct chemical shift differences between the diastereomers that are formed, which can be used to obtain the ee directly by integration. Spectroscopic, X-ray structural and DFT studies suggest that the NMR chemical shift differences between diastereomers is steric in origin, with the sharpness of these signals resulting from conformational locking of the bornyl group relative to the P2N2 ring induced by the presence of the P(v)-bonded amino group (R*NH). This study showcases cheap inorganic phosphazane CDAs as simple alternatives to organic variants for the rapid determination of ee.
