Structural Illustrations of [Co(tren)pn]Cl 3

1. Applications of Two-Dimensional NMR to the Study of Cobalt(III) Complexes Containing Tris(2-aminoethyl)amine and Propylenediamine Dr. Mark McClure and Stephanie Baker University of North Carolina at Pembroke Department of Chemistry and Physics • Abstract • We present two-dimensional NMR data for the cobalt complex [Co(tren)(pn)]Cl3 where tren represents the tripodal tetradentate ligand tris(2-aminoethyl)amine and pn represents the bidentate ligand propylenediamine. The two-dimensional NMR techniques employed included COSY and HETCOR. Through the COSY experiment, we were able establish general correlations between different regions of the spectrum, particularly those arising from the propylenediamine ligand. The HETCOR experiment was used to establish connectivities between the proton and carbon spectra. • Expected Splitting Patterns • The tren ligand contains three ethylene linkages: • Two ethylene linkages are reflected through a mirror plane and are therefore equivalent, but the hydrogen atoms in these ethylene linkages are all nonequivalent and each is coupled to the remaining three. • A mirror plane bisects the third ethylene linkage; these hydrogen atoms are divided into two sets of two equivalent atoms. • The mirror plane also bisects the propylenediamine molecule. • The protons on the terminal carbon atoms should be equivalent if perfect symmetry is maintained. • The protons on the center carbon atom should also be equivalent, but the splitting will be complex as these split by the protons on the terminal carbon atoms. • COSY (Correlated Spectroscopy): • COSY is a two-dimensional experiment used to establish proton-proton correlations. The one-dimensional H-1 spectrum appears along the axes and the cross peaks reveal coupled protons. • COSY spectra were acquired at a resolution of 512 x 256 bytes, over sweep widths of 720 Hz, 480 Hz, and 360 Hz. The optimal width appeared to be around 360 Hz. • Most significant are the signals arising from the propylenediamine. Since the propylenediamine contains a carbon which is not adjacent to a nitrogen the nuclei are shifted downfield. The peaks at 2.763 ppm (peak C), at 2.561 ppm (peak B), and at 1.913 ppm (peak A) are believed to have arose from the propylenediamine. Peak A is believed to be the center proton because it is shifted furthest upfield. Structural Illustrations of [Co(tren)pn]Cl3 HETCOR(Heteronuclear Correlation) HETCOR spectra were acquired at a resolution of 4096 x 1024 bytes, over sweep widths of 720 Hz in the proton dimension. Our carbon spectrum showed six peaks at 63.101 ppm (small), 61.773 ppm (large), 44.869 ppm (small), 45.973 ppm (large), 39.414 ppm (large), and 25.518 ppm (small). Based on chemical shift the signals in the vicinity of 61-63 ppm can be assigned to carbon atoms adjacent to the central nitrogen; those in the vicinity of 44-46 ppm can be assigned to carbon atoms 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 considerations the signal at 61.773 ppm is assigned to C1, the signal at 63.101 is assigned to C3, the signal at 45.973 ppm is assigned to C2, and the signal at 44.869 is assigned to C4. The two carbon signals at 39.414 and 25.518 belong to the propylenediamine. Although three signals were expected from the propylenediamine, only two were obtained. The signal at 25.518 was assumed to arise from C6 based upon chemical shift; the signal at 39.414 was assumed to arise from a combination of C5 and C7. The HETCOR confirms these assignments. The two peaks on the H-1 spectrum that appear as triplets both correlate to the signal at 39.414 ppm, confirming this signal arises from a combination of C5 and C7. The peak at 1.913 ppm showed a correlation to the carbon signal at 25.518 ppm confirming that this is the C6 carbon. Conclusion The inclusion of the propylenediamine ligand did cause the spectrum to be spread into a wider spectral width, which was the intent. In the corresponding ethylenediamine complex, all of the ethylenediamine protons gave rise to a singlet. In this complex, the propylenediamine ligand was divided into three sets: two triplets and a multiplet. In the COSY spectrum the triplets were shown to actually be a doublet of doublets. The protons on the C3 and C4 carbon atoms have the same symmetry as the protons on the C5 and C7 carbon atoms and should experience the same splittings; therefore this provided a glimpse of what would be observed for the protons on the C3 and C4 carbons in the absence of overlapping signals. While the HETCOR confirmed some general correlation between the proton and carbon spectra, assignment at the individual-proton level was not possible due to the complexity of the proton spectrum. COSY Spectrum Figure 2 (above) was ran at a resolution of 512 x 512 with a sweep width of 720 Hz. Figure 3 (left) was ran at a resolution of 512 x 512 with a sweep width of 360 Hz. 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.