Abstract

1. Applications of Two-Dimensional NMR to the Study of Cobalt(III) Complexes Containing Tris(2-aminoethyl)amine and Ethylenediamine Dr. Mark McClure and Stephanie Baker University of North Carolina at Pembroke Department of Chemistry and Physics COSY (Correlated Spectroscopy): COSY is a two-dimensional experiment used to establish proton-proton correlations. The one-dimensional H-1 spectrum appears across the axis and the cross peaks reveal coupled protons. COSY spectra were acquired at a resolution of 512 x 256 bytes, over sweep widths of 1700 Hz, 1200 Hz, 720 Hz, 360 Hz, 300 Hz, and 240 Hz. The smaller sweep widths gave higher resolution but increased the acquisition time. The optimal width appeared to be somewhere between 360 and 300 Hz. The spectrum can be broken down into general regions. The most noticeable, a singlet, at 2.7 ppm which shows no cross peaks with any other regions of the spectrum can be assigned as ethylenediamine. While multiple other cross peaks are observed, these regions represent overlap of multiplets and therefore could not be resolved at the individual proton level. HETCOR HETCOR is a two-dimensional experiment used to establish proton-carbon correlations. The one-dimensional H-1 spectrum appears across the vertical axis while the C-13 spectra appears across the horizontal axis. HETCOR spectra were acquired at a resolution of 4096 x 256 bytes, over sweep widths of 720 Hz in the proton dimension. Narrowing the sweep width resulted in a better spectrum than previously obtained. Our carbon spectrum showed six peaks at 64.643 ppm (large), 61.962 ppm (small), 46.380 ppm (large), 45.555 ppm (small), 45.106 ppm (small), and 44.472 ppm (small). Based on chemical shift the signals in the vicinity of 61-64 ppm can be assigned to carbon atoms adjacent to the central nitrogen; those in the vicinity of 45-46 ppm are adjacent to the terminal nitrogen donors. The relative size of the carbon peaks also aids in assignment since two of the arms are equivalent in a mirror plane. Based on above consideration the signal at 64.643 ppm is assigned to C1, the signal at 61.962 is assigned to C3, and the signal at 46.380 ppm is assigned to C2. The HETCOR allowed for assignments of the other carbon signals. The two carbon signals at 45.106 and 44.472 carbon peaks clearly showed interaction with the ethylenediamine peak at 2.87 ppm allowing these to be assigned to C5 and C6. This allows for the peak at 45.555 ppm to be assigned to C4. Conclusion By narrowing the spectral width we were able to acquire higher-resolution two-dimensional spectra that previously obtained for this compound. Sweep widths of 300-360 Hz appeared to be optimal for this compound. The ethylenediamine was found to give rise to a singlet (not what was expected) at 2.7 ppm – this did not show cross peaks to any other region in the COSY spectrum. The HETCOR allowed for assignments of all the C-13 signals and while some correlation could be made with the proton spectrum, assignment at the individual-proton level was not possible due to the complexity of the spectrum. • Abstract • Cobalt (III) complexes have been widely studied by one-dimensional NMR techniques. However there has been very little application of two-dimensional NMR to these systems. The complexity of the 1-D spectrum necessitates the use of 2-D NMR to establish through-bond and through-space correlations. This research involved the application of 2-D NMR to the study of a cobalt (III) complex containing tris(2-aminoethyl)amine and ethylenediamine. The 2-D experiments preformed include COSY and HETCOR. The COSY, which shows through-bond interactions, did not show cross peaks between the ethylenediamine and the rest of the spectrum. The HETCOR was used to establish a number of proton-carbon correlations and aided in the assignment of the C-13 spectrum. While narrowing the spectral width produced higher-resolution two-dimensional spectra than previously obtained, its complexity prevented analysis of the spectrum at the individual-proton level. • What is NMR? • NMR (Nuclear Magnetic Resonance) spectroscopy involves the absorption of radio-frequency radiation by the nuclei of atoms. In the absence of an external magnetic field nuclei have random orientations. In the presence of an external magnetic field the nuclei have two orientations; aligned with or against the field. The energy difference lies in the radio-frequency region of the electromagnetic spectrum. Absorption of energy occurs when the energy of the radiation matches the energy difference between these two states. • Expected Splitting Patterns: • The tren ligand contains three ethylene linkages: • Two ethylene linkages are reflected through a mirror plane and are therefore equivalent • The hydrogen atoms in these ethylene linkages are all nonequivalent; each is coupled to the remaining three • A mirror plane bisects the third ethylene linkage • The hydrogen atoms in this ethylene linkage are divided into two sets of two equivalent atoms • The mirror plane also bisects the ethylenediamine molecule • These hydrogen atoms also divided into two sets of two equivalent atoms Structural Illustration of [Co(tren)en]Cl3 COSY Spectrum Figure 1: Without Magnetic Field Figure 2: With Magnetic Field HETCOR Spectrum References [1] M. R. McClure and J. Holcombe, "Synthesis and NMR Characterization of Cobalt(III) Complexes with Triethylenetetramine, 2,2-Bipyridine, and 1,10-Phenanthroline". Journal of Coordination Chemistry, 2004, 57, 907-915.[2] Poster, "NMR Spectroscopy of Cobalt(III) Complexes Containing the Tripodal Tetradentate Ligand Tris(2-aminoethyl)amine." M. R. McClure and Natasha Oris-Thomas. Presented at the ninth annual NC-LSAMP conference for undergraduate research on March 18, 2005. [3] Poster, "H-1 and C-13 NMR Spectroscopy of Cobalt(III) Complexes Containing the Tripodal Tetradentate Ligand Tris(2-Aminoethyl)amine.” M. R. McClure and Johnithan White. Presented at the second annual State of North Carolina Undergraduate Research Symposium on November 18, 2006.