Current Projects
Research in the Hayes group involves the synthesis of inorganic molecules for application in new chemical transformations and catalysis. Four unique projects directly address this goal.
Novel Catalysts for the Synthesis of Green Polymers
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One research avenue being pursued is to tackle the challenge of preparing new materials that are both biodegradable and biocompatible. A particularly attractive alternative to conventional polyolefin materials is polylactide, PLA, which is generated by the ring-opening polymerization of lactide, the cyclic ester dimer of lactic acid. PLA is desirable because of its processability and useful macroscopic properties. The synthetic precursors to PLA are found in inexpensive renewable resources, such as beets and corn. Furthermore, the resultant polymers are biodegradable and have found applications in such areas as bulk packaging materials and the agricultural industry. It has also garnered interest from the medical community for its use in adsorbable sutures, as matrices for the slow release of pharmaceuticals, and polymer scaffolds for tissue engineering.
Despite these benefits, the industrial process for PLA synthesis involves a melt polymerization protocol in the presence of a metal containing catalyst. This method offers very little control over the molecular weights and stereochemistry of the final product; also, the metal catalyst remains embedded within the polymer. Our strategy focuses upon development of discrete homogeneous lactide polymerization catalysts, specifically, alkaline earth metal alkyl, alkoxide and amido species.[1] Homogeneous systems are highly advantageous because they allow rational fine tuning of the steric and electronic environment of the metal centre.
Initial forays into this avenue of investigation involved the preparation of zinc complexes of a neutral phosphinimine ligand featuring a dibenzofuran scaffold, which yielded complexes catalytically active in the polymerization of l-lactide at 100 °C.[2]
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More recently, we have demonstrated that highly active cationic magnesium complexes of a neutral bis(phosphinimine) ligand were suitable for the preparation of poly(ε-caprolactone) at ambient temperature in several minutes.[3].
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The preparation of analogous cationic organozinc complexes has recently been achieved[4].
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Further studies will provide the opportunity to produce materials of defined molecular weights containing specific macroscopic properties, while ultimately serving as a platform for the discovery of new materials. As a better understanding of the catalyst systems unfold, the steric environment of the ancillary ligand set will be fine tuned for stereospecific polymerization by introduction of chiral functionalities. Stereochemistry of the polymer is particularly important because it determines its mechanical and physical properties, and also, its rate of chemical and biological degradation.
In summary, this project explores the virtually uncharted organometallic chemistry of non-Cp supported alkaline earth metals with the possibility of contributing significantly to the fundamental understanding of the production of environmentally friendly materials.
Polyphosphazenes
A second polymer project in the Hayes group studies inorganic polymers using solid-state NMR spectroscopy.[1] This is a collaboration between the Hayes group and Dr. Paul Hazendonk. The work focuses specifically on a class of polymers known as poly(phosphazenes), which are hybrid inorganic-organic materials of great interest due to their potential applications in novel electronics, biomaterials and biodegradable plastics.
We have demonstrated that by using 31P magic-angle spinning (MAS) solid-state (SS) NMR spectroscopy, the complicated thermal ring-opening polymerization of hexachlorocyclotriphosphazene can be quantitatively probed to give information about reaction progress, chain propagation and cross-linking of the resultant polymer.[2]
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Deconvolution analysis of the SS 31P NMR spectra of a reaction mixture aliquots.
In a separate contribution, it was shown that the detailed morphology of another polymer, poly[bis(trifluoroethoxy)phosphazene, could be probed using MAS SS NMR spectroscopy.[3] For instance, the degree of crystalline versus amorphous domains in a given sample can be determined with a specialized pulse sequence. Furthermore, it was demonstrated that SS NMR is a viable technique for assigning specific crystalline domains (e.g. α-, β- or γ-phases) and may prove to be a valuable complementary technique to X-ray diffraction methods.
Synthesis and Reactivity of Rare Earth Complexes with Unusual Bonding Motifs
Another thrust in the Hayes research group involves the preparation of novel monoanionic pincer ligands to support metal main-group multiple bonding (M=E; E = N, P, Si) within lanthanide and group 3 complexes. Although there are several examples of rare earth complexes containing bridging and dimeric imido and phosphinidene ligands, there are no structurally characterized examples of a 4f element involved in an unconstrained multiple bond with any of the main group elements N, P or Si. Contrary to the 4f elements, several examples of scandium complexes containing a terminal imido functionality have recently been reported. Therefore it is possible that the absence of the analogous 4f compounds from the literature lies not in thermodynamic limitations, but rather, a lack of kinetic accessibility.
Using a novel bis(phosphinimine)carbazole ligand, we have prepared a thermally sensitive dialkyl lutetium(III) complex.[1]
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At temperatures above 0 °C, this compound undergoes two sequential intramolecular ortho-metallation processes.[1]
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Further reactivity with substrates such as the bulky aniline Mes*NH2 affords a mixed anilide / aryl complex. This complex again is quite reactive and undergoes an intramolecular rearrangement to a different structural isomer. The nature of this rearrangement has been probed to determine if it proceeds through an unprecedented transient lutetium imido (LLu=NR) intermediate. Kinetic and mechanistic (deuterium labelling) studies have demonstrated that the intramolecular rearrangement occurs via a unique direct metalation exchange process.[2]
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It is expected that, with further studies in this area, complexation of our ligands (which will be designed to impart thermal stability in the product) will generate new platforms for obtaining rare bonding motifs which will lead to the discovery of unique reaction behaviour and catalytic activity. For example, complexes exhibiting metal main-group mulitiple bonds may be implicated in industrially relevant transformations such as the metathesis of imines, aldehydes and carbodiimides, group transfer reactions and bond activation processes. Such complexes are likely to be of fundamental interest, and also useful from an applications perspective. Species containing metal main-group mulitiple bonds may also serve as reasonable models for intermediates in various catalytic transformations; thus, providing a unique glimpse at otherwise fleeting structures.
Catalytic Functionalization of Hydrocarbons
A final direction in the Hayes group research program is the development of catalytic systems for converting hydrocarbons into useful organic compounds. Such derivatization is especially desirable for smaller hydrocarbons, such as methane, which contribute significantly to the production of greenhouse gases. The metal-mediated coupling of an alkane with an alkene is an example of such a useful transformation.
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This research utilizes phosphido ancillary ligands to synthesize reactive early (groups 3-5) and late (groups 8-10) transition metal complexes for the activation of chemically inert small molecules. A specific emphasis is placed upon the selective functionalization of hydrocarbons produced as undesirable side products in the petrochemical industry.
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The isolation of complexes with this ligand motif should provide insight into fundamental questions regarding bonding and structure, as well as topics with more immediately tangible benefits, such as rational improvements of current catalytic systems and the development of entirely novel reactions. The short term objectives of this project are to generate metal complexes with unusual structures and oxidation states, with an eye to the ultimate goal of achieving catalytic functionalization of hydrocarbons and other small molecules.
