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6. Summary
Group 2 alkaline earth metals (magnesium, calcium, strontium, and
barium) are utilized in a wide range of applications. Examples include synthetic
reagents in organic chemistry, polymerization initiators, high temperature
superconductors, computer memory and metal-organic frameworks for gas
storage and separation. Until relatively recently, the coordination chemistry of the
Group 2 metals was relatively unexplored, in contrast to the well-studied
transition metals such as zinc, iron, manganese, etc. The highly reactive nature of
alkaline earth metal complexes and weak metal-ligand bonding they exhibit have
been major factors hindering the further exploration of their properties. The
metals themselves, as well as many of their organometallic complexes, will react
with air and moisture to produce insoluble oxides or hydroxides. Recently,
techniques have been developed and refined to allow for the efficient synthesis
and handling of these compounds, and the body of knowledge concerning these
compounds is rapidly increasing. This project is designed to explore novel
compounds which expand the utility of the alkaline earth metals and increase their
potential applications.
One of the primary uses for Group 2 compounds is as precursors for
Chemical Vapor Deposition (CVD). CVD is a process by which a thin metal film
can be deposited on a substrate surface. Alkaline earth metal materials obtained
via CVD range from Dynamic Random Access Memory (DRAM) circuits to high
temperature superconductors. CVD is a preferred method for the formation of thin
films due to its ability to provide coverage of non-uniform substrates and the ease
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with which film characteristics such as thickness and elemental composition can
be manipulated. Ideal CVD precursors must be volatile, however, they must also
strike a balance between stability and reactivity. Precursors must be volatile to
allow the compound to vaporize, yet they must be stable up to the delivery
temperature. The compound then must be reactive enough to decompose under
relatively mild heating in the CVD reactor.
Current CVD precursors have severe limitations, and a large body of
research is underway in the search for novel precursors with improved properties.
One potential problem is that alkaline earth metals are prone to aggregation due to
their high coordinative requirements. These aggregates are unsuitable for CVD as
they typically display low volatility. In order to reduce aggregation, neutral
chelating donors can be utilized to saturate the coordination sphere of the metal.
This strategy has been demonstrated to yield monomeric species, however, the
weak nature of the metal-donor bond often leads to premature donor loss and
consequently, compound decomposition and a loss of volatility. Although
precursors designed in this manner display monomeric structures, their utility is
limited because they decompose instead of entering the gas phase.
This project aims to overcome this challenge by designing ligands that can
stabilize the metal center without the use of neutral donors. This could potentially
lead to the formation of monomeric, volatile compounds which will not be
affected by premature donor loss. To accomplish this, the pyrazolate ligand
system has been explored. Pyrazolates are advantageous over other currently
employed ligand systems because they are easy to synthesize and functionalize,
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and do not incorporate fluorine or oxygen, which can be detrimental to thin film
formation during CVD. The strategy for this project was to design so-called
“pendant-arm pyrazolates,” which are the typical pyrazole backbone with bulky
substituents present at the 3 and 5 positions in the ring. These bulky substituents
will contain nitrogen atoms, phenyl rings, and/or methyl groups, all of which can
stabilize the metal center through various mechanisms. The substituents will also
be sufficiently bulky so as to encourage interaction with the open coordination
sites of the metal center.
The synthesis of the pendant arm pyrazolates ligands proved to be
challenging. A primary reason for the difficult synthesis is that although the
ligands themselves are not air- and moisture-sensitive, several of the reagents
used in their synthesis could decompose if exposed to the atmosphere,
necessitating the use of Schlenk line and glove box techniques. In many synthetic
attempts oily product mixtures that were difficult to separate were obtained.
Furthermore, due to difficulty in purification of intermediates and products and
the low yield of some steps of the synthesis, the product oils often present a
complex 1H NMR spectrum that is difficult to assign. In several instances, the 1H
NMR spectrum indicates the likelihood of product formation, however, attempts
to isolate the product have been unsuccessful.
Another important use for Group 2 metals is in the emerging field of
metal-organic frameworks. MOFs, as they are also known, are microporous
polymeric materials consisting of metal ions linked together by polydentate
organic bridging ligands. MOFs have been utilized in applications ranging from
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selective catalysis to high surface area adsorbents for gas separation and storage.
Research has intensified in this area due to the potential for metal-organic
frameworks to act as an on-board storage medium for hydrogen gas in
transportation applications.
Hydrogen fuel cell technology is one of the most promising alternatives to
fossil fuels for transportation, however, one major barrier to adoption is the
current lack of safe on-board storage techniques. The U. S. Department of Energy
(DoE) set a 2010 standard of 6.0 weight percent uptake of hydrogen for
transportation applications. This is an ambitious goal, however, the DoE also
stipulated that this must be accomplished under mild conditions (ambient
temperatures and less than 100 bar pressure). Furthermore, the storage material
must be rechargeable within 3 minutes. Current storage technology consists of
high-pressure cylinders, which is undesirable due to the risk of explosion in an
accident. Chemical storage as boranes or hydrides has also been explored due to
their high energy density, however, these methods typically are difficult to
regenerate, making reuse impractical. Metal-organic frameworks have shown
promise as high surface area adsorbents, however, current materials fall short in
meeting the DoE 2010 goals. To overcome this obstacle, a new approach is
needed.
Current metal-organic frameworks rely almost exclusively on transition
metals, which have higher molecular weights than the lighter alkaline earth metals
magnesium and calcium. Incorporation of lower molecular weight elements will
reduce the overall weight of the framework and yield a higher weight percent
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uptake of hydrogen. Furthermore, the majority of the reported metal-organic
frameworks are based on transition metal carboxylate or tetrazolate systems.
These studies have not yielded any materials which meet the DoE standard. The
pyrazole ligand system offers many of the same advantages as the currently
studied ligand systems, however, their unique structural features may allow for
the creation of superior metal-organic framework materials.
The defining characteristic of these compounds is their poor solubility.
The ligands employed are insoluble in most of the solvents that are commonly
used in Group 2 chemistry – THF, diethyl ether, chloroform, toluene, benzene,
hexane, and acetonitrile. A novel pyrazolate ligand was synthesized to help
overcome these solubility issues. Ultimately, the solubility was not improved, but
the design of the new ligand may also provide the potential for stabilizing
interactions in a compound synthesized with this ligand. Due to the poor
solubility of these compounds, the highly polar N,N-dimethylformamide (DMF)
was used as a solvent. Unfortunately, although DMF has been used as a solvent in
the literature, it was determined to be unsuitable for use with the pyrazole ligands.
Since DMF is has a weakly acidic proton in the aldehyde position that is likely
more acidic than the very weakly acidic protons of the pyrazolate ligands, which
may interfere with the reaction routes employed. Dimethylsulfoxide (DMSO) was
selected to replace DMF because the ligands are similarly soluble in the two, and
DMSO is aprotic. Work with DMSO has been promising to this point but is still
in progress. In acid-base reactions, DMSO has not yet been utilized as a solvent.
When it is applied, the influence of an acidic proton in the solvent will be absent,
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theoretically allowing the reactions to proceed as intended without the generation
of undesirable side products.
Metathesis has shown promise as a reaction route toward alkaline earth
metal organic frameworks. Analysis of the first step of the reaction shows the
deprotonation of the pyrazole ligand, and further work is underway to crystallize
and analyze this intermediate, and also to proceed directly to the second step of
the synthesis. Future work on this project will include further exploration of
DMSO as a solvent, which may remove many of the obstacles encountered in the
synthesis of these compounds.