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The purpose of this investigation was to determine the structural features responsible for the unusual purple color exhibited by a TpMo(CO)2(η3-allyl) complex (Figure 1). The compound was synthesized in a nine-step reaction sequence with the structures of each step confirmed by 1H NMR, 13C NMR, and IR data. The electronic and structural properties of the purple compound were also analyzed by UV-Vis spectroscopy and X-ray crystallography, to determine the differences between it and the orange molybdenum compounds previously synthesized (Figure 1).
For decades, chemists have been synthesizing increasingly complex molecules for therapeutic, biological, and other purposes. Many biological processes are controlled by enantiomers of specific molecules. Enantiomers are isomeric molecules that are non-superimposable on their mirror images. Biological systems can usually differentiate between enantiomers. For example, the active site of an enzyme usually recognizes one enantiomer but not the other. For numerous drugs and medicines, only one enantiomer produces the desired effects needed for the body, while the other does something completely different.
For the past 10 years, the Liebeskind lab has been synthesizing complex enantiomerically pure heterocyclic compounds using organometallic scaffolds. These scaffolds consist of a molybdenum atom and its ligands bound to structurally simple unsaturated ligands. Transition metals are used in catalysis in the chemical industry and in biological processes due to the ability to function as electron reservoirs, donating and accepting electrons as necessary. The research group focuses on chiral scaffolds of TpMo(CO)2(η3-allyl) complexes (Tp = hydridotris(pyrazolyl)borate, Figure 2). The scaffolds are the building blocks to synthesizing highly functionalized alkaloids. Alkaloids are biologically active amines and are found in many drugs, such as morphine. Also, the molybdenum complexes are of prominent interest because they are both air- and moisture-stable, allowing them to be used in the lab with routine bench-top techniques.
The main focus of this project is an investigation of the relationship between structure and chromophore for a purple complex formed from a commonly used molybdenum scaffold in the Liebeskind lab. The purple compound is of interest because most molybdenum complexes previously synthesized have a brilliant marigold-like color. This project will determine the structural and electronic variations found in the purple TpMo(CO)2(η3-allyl) complex relative to the yellow-orange compounds. A dramatic color change reflects a significant change in the molecule chromophore. A chromophore is a collection of atoms that gives a molecule color. This change in chromophore should be related to a structural variation (change in bond length and/or angle distortion). Figure 3 shows the predicted reason for the difference in color between the two groups of molecules. The purple complex has electron donation into the ring while the orange molybdenum complex has donation into the protecting group. These structural differences can be evaluated by X-ray crystallography. X-ray crystallography requires a single crystal of a compound and provides a three-dimension representation of the molecule, complete with bond lengths and angles. Ultraviolet-visible (UV-Vis) spectroscopy is a second method used to investigate the relationship between structure and chromophore. From UV-Vis measurements, chemists can determine energy level spacing between molecular orbitals. Molecular orbital spacing differences between two similar compounds can explain differences in color and reactivity.
1. Synthesized the molybdenum and Tp source (Scheme 1).
2. Synthesized the purple TpMo(CO)2(η3-allyl) compound (Scheme 2)
3. The product of each step was purified by silica gel column chromatography.
4. Characterized each step with 1H NMR, 13C NMR, and IR.
5. Grew single crystals of the TpMo(CO)2(η3-allyl) compound.
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6. Submitted the crystals for X-ray crystallography study
7. Investigated the TpMo(CO)2(η3-allyl) with UV-Vis spectroscopy.
The purple TpMo(CO)2(η3-allyl) complex was successfully synthesized from hydrogenolysis of the Cbz group using a sample previously made by the group. UV-Vis measurements (Table 1) show that the purple complex contains larger spacing within its molecular orbital than the orange complex. The UV-Vis values are consistent with the colors of the compounds because purple complexes absorb near 500 nm in UV-Vis measurements and orange compounds absorb near 400 nm. Single crystals of the purple NH complex were submitted for analysis by X-ray crystallography, but data is still being processed.
From the UV-Vis measurements it can be seen that the TpMo(CO)2(η3-allyl) purple complex differs in electronic properties. There is a larger energy gap between the molecular orbitals of the purple complex in comparison to the orange complex.
The next step for this project is to obtain the crystal structure measurements for the purple compound. The bond lengths and angles for the ring will be compared to the bond lengths of the Cbz protected molybdenum compound. It may also be interesting to synthesize the other purple molybdenum compound (Figure 5). By seeing the relationship for the two compounds it may be easier to understand why there is such a difference in color from the orange compounds.
The last two steps before the hydrogenolysis of the purple complex in the nine-step synthesis reaction needs to be completed in order to finish the synthetic sequence.
It may also be interesting to do further studies on the electronic properties of the two purple molybdenum complexes. From the UV-Vis studies it may be possible to design a molecular orbital diagram for both the orange and purple molybdenum complexes.
Research funded by grant #GM52238, awarded by the National Institute of General Medical Sciences, DHHS. Research also funded by Howard Hughes Medical Institute grant #52003727. Many thanks to the Liebeskind lab group for providing help and guidance.
The purpose of this research was to investigate structure and electronic differences between a purple molybdenum complex and an orange molybdenum complex. The purple molybdenum complex had an ultraviolet-visible spectroscopy signal of 486 nm which means that it has higher electronic energy shell splitting within the d-orbitals of its molecular orbital structure. The orange compound had an UV-Vis measurement of 408 nm, which means that it has a smaller energy shell splitting then the purple complex. The structural features of the molecule were investigated by X-ray crystallography which measures the bond lengths and angles of a structure. It is predicted that the bond lengths within the structure will shorten with the free amine because the electrons from the nitrogen are donating to the ring rather then the protecting group seen in the other molybdenum complexes.
Silica gel column chromatography, X-ray crystallography, NMR, IR, UV-Vis spectroscopy
Purification, Synthesis, Structural and Electronic effects, Characterization
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