Research
Project I. Fabrication and Utilization of Tetrahedral "Cage-Hostage"
Systems:
[46]Adamanzanezinc(II)
and [36]Adamanzanezinc(II)
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The 1987 Nobel Prize in Chemistry was awarded to Jean-Marie Lehn, Donald J. Cram, and Charles J. Pederson for their development and use of molecules with structure-specific interactions of high selectivity. This is now known as supramolecular chemistry or "host-guest" chemistry. As shown above, both [46]Adamanzanezinc(II) and [36]Adamanzanezinc(II) could be considered an extreme example of a "host-guest" system, what could be termed a "cage-hostage" system. Both of these complexes should be relatively big, soft cations that could potentially contain various metal centers depending on the desired application.
It is envisioned that the metal ion will be trapped inside of the ligand cage due to the topology of the ligand. What results from such an arrangement is the marked reduction in the chemical reactivity of the metal ion, while maintaining a positive charge, a net magnetic moment, and potentially a source of radiation with the choice of another, more applicable metal. As the free complex, it will be useful as a phase transfer catalyst. By substituting other metals during the synthesis of the complex, the catalyst will be able to carry various charges or no charge. In addition, the complex would allow for the exploration of the coordination modes of metals when encapsulated in a tetrahedral arrangement of donor atoms.
The potential synergistic arrangements formed by tethering the complex to other moieties are very promising. For instance, a simple cationic surfactant could be synthesized by the addition of a lauryl chain. Other applications envisioned include an inert cationic resin for use in anion exchange, solid-state phase transfer catalysis, chromatographic separations, and data storage. By far the most exciting applications are medicinal. With a site or tumor specific tether, and careful selection of the entrapped metal, very sensitive imaging and/or very selective radioimmunotherapy may be accomplished in vivo, with limited side effects due to the chemical inertness of the caged metal ion.
Much of the actual work in this project will be synthetic in nature. Once a student is assigned a portion of the molecule to be synthesized, he/she will be expected to review and read the literature, help design synthetic strategies, setup reactions, purify products via standard methods (flash chromatography, recrystallization, etc.), characterize products with standard techniques (nuclear magnetic resonance, infrared spectroscopy, etc.), record data properly in lab notebooks, and finally write a research report following the American Chemical Society's Committee on Professional Training's guidelines.
Research Project II: Elucidation of the Basis of Chemical Shifts in the 1H and 13C Nuclear Magnetic Resonance Spectrum of Diamagnetic Transition Metal Coordination Complexes
The structure of a large number of organic compounds can be elucidated from the proton Nuclear Magnetic Resonance (NMR) spectrum using published chemical shift tables. This is not the case for the many transition metal complexes where the NMR spectrum is too complicated to allow for assignments based on tabulated data, instead the spectrum has been used as a fingerprint to confirm known complexes. With the advances in pulsed techniques and field strengths over the past 30 years, the resources to make chemical shift assignments are now available. With these assignments, the long-range interactions affecting chemical shift can now be explored using a library of simple, well-known coordination complexes. The long-term goals of this project are: 1) the elucidation of the major contributions to chemical shift in the NMR of coordination complexes, 2) the determination of any correlations between chemical shift and chemical reactivity, electromagnetic spectra, or other physical properties, 3) the prediction of chemical shifts in related complexes using applicable models and theories.
[Co(bipy)2CO3]Cl·4H2O was synthesized as a model complex. Synthesis of the desired complex was accomplished by a substitution reaction on a cobalt(III) precursor as shown above. Acid induced decomposition of the carbonate followed by aquation and eventual replacement with 2,2'-bypyridine led to the isolation of a red/orange product via recrystalization from absolute ethanol.
In theory, the pyridine rings were all expected to be nearly magnetically equivalent yielding four 1H resonances six 13C resonances. This was based on experience with organic molecules. In reality, the NMR analysis showed eight 1H resonances and eleven 13C resonances. In [Co(bipy)2CO3]+, the two bipy ligands are equivalent as expected due to a C2 rotation axis of symmetry, but the two pyridine rings within each bipy ligand are not equivalent. Shown below is the assignment of resonances to specific protons and carbons, which was accomplished using two-dimensional NMR techniques, specifically COSY, NOESY, and HETCOR. The second set of resonances 8.62 - 7.55 is assigned to the f - j ring based on the previous work of Kanekar in 1969 and confirmed by our lab. Using [Co(bipy)3]3+ we have found similar downfield shifts in the 1H NMR spectrum.
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1H
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13C
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a
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9.07
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152.8
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b
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8.21
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132.5
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c
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8.65
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145.8
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d
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8.76
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127.5
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e
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--
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159.8
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f
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--
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159.7
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g
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8.62
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127.5
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h
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8.29
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144.7
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i
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7.53
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131.1
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j
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7.55
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155.4
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k
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--
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168.7
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It is remarkable that while protons a and j are magnetically equivalent on the free ligand, they have clearly different chemical shifts in [Co(bipy)2CO3]+. Protons a - d are all shifted from 1.52 ppm to 0.14 ppm downfield as compared to protons g - j, suggesting an increase in shielding. The greater shielding may be due to donation of electron density from the carbonate trans to the affected rings.
One complex is not enough to fully explore trends in chemical shifts. What is proposed is to study a representative library of cobalt(III) complexes based on [Co(bipy)2X2]+, where X is a representative sample of monodentate or bidentate ligands. This project is ideal for introductory undergraduate research. It is easily broken down into small, but inclusive mini-projects. A student will be assigned one or two complexes to synthesize, purify, characterize {Infrared Spectroscopy, Ultraviolet-visible Spectroscopy, Magnetic Susceptibility, and NMR (including 2-dimentional techniques)}, and write a report of their findings. The student will not only experience the entire synthetic process, but will also see how their mini-project fits into the bigger picture of the long term research efforts.
Research Project III: Resolution of Isomers and Elucidation of Properties of Cobalt(III) Complexes Containing 1,8-Diamino-3-thia-6-azaoctane, a Ligand with the Donor Sequence NNSN
In 1893, Alfred Werner published a landmark paper entitled "Beitrag zur Konstitution anorganischer Verbindungen," in which he introduced the concept of coordination bonding in a series of cobalt(III) compounds. Over a century later we are continuing his research asking simple questions such as: "How are atoms around a metal center arranged in space? And, why do they arrange themselves the way they do?" Cobalt(III) forms many stable, colored, diamagnetic complexes, which makes it an ideal candidate for studying geometric and stereo isomers of ligands when complexed. Potential applications of the complexes include the stereospecific binding and activation of optically active amino acids and other carbonyl containing moieties such as amino acid esters, amides or peptides.
Sargeson and Searle, in 1967, found that there were three distinct geometric isomers formed when triethylenetetramine (NNNN) was coordinated to cobalt(III). All three of these geometric isomers, α-cis, β-cis, and trans, exhibited an octahedral coordination mode with regards to the cobalt(III) center. Worrell and Busch reported the synthesis of cobalt(III) complexes derived from 1,8-diamino-3,6-dithiaoctane (NSSN) in 1969. Surprisingly, they found that NSSN was only able to coordinate to cobalt(III) in the α-cis geometry. It is believed that this geometric specificity is due to the atomic size and the bond angle constraints experienced by the two in-plane sulfur linkages. Due to the asymmetry of NNSN, there are four possible geometric isomers for its cobalt(III) complexes (shown below). It was hypothesized that the α-cis isomer and β1-cis isomer should be the only two allowable geometric isomers expected for NNSN cobalt(III) complexes.
Coordination of NNSN to cobalt(III) was accomplished using two separate methods. The first method yielded only one isomer. The second method yielded three distinct isomers. With the unexpected discovery of a third isomer, it seems that with only one thia linkage the complex is pliable enough to accommodate the size angle and bond angle restrictions of the thia linkage. Future work on this project includes purification and identification of optical isomers followed by the investigation of catalytic activity.
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