Presentation on theme: "The effect of substituents on a vanadium-containing coordination complex was evaluated using linear free energy relationships and NMR spectroscopy. Substituents."— Presentation transcript:
The effect of substituents on a vanadium-containing coordination complex was evaluated using linear free energy relationships and NMR spectroscopy. Substituents (H, OH, Cl, NH 2 and NO 2 ) on the tridentate ligand, 2,6-pyridinedicarboxylic acid in the 4-position allowed the generation of a Hammett plot. 1 The data for the deprotonation reaction of the ligand resulted in a non-linear plot of log(K X /K H ) as a function of the Hammett constant. When the 1 H NMR chemical shift data (d X -d H ) was plotted as function of the Hammett constant a similar non-linear plot was generated for both ligand and vanadium(V) complex. However, by using 51 V NMR chemical shift data ( X - H ) and plotting against the Hammett constant, a linear correlation plot was obtained for dipicolinatooxovanadium(V) with an r 2 of 88%. This data demonstrate the effectiveness of NMR chemical shifts for examining linear free energy relationships in metallo-organic compounds. Substituent Effects on Dipicolinatooxovanadium(V) evaluated by NMR Spectroscopy Alejandro M. Trujillo, Jerome Burke, Debbie C. Crans* Department of Chemistry, Colorado State University, CO. 80523-1872, USA email: email@example.com Figure 1: Chemical structure of pyridine-2,6-dicarboxylate and dipicolinatoxovanadium(V). The substituents used for Hammett plots are of varying electronegativity (H, OH, Cl, NH 2 and NO 2 ) at the 4-position of the pyridine ring. Figure 2: A plot of the Hammett meta constant plotted against the –logKa (logK X /K H ) converted from pKa data from Smee et al forming 2,6-pyridinedicarboxylate. ABSTRACT: INTRODUCTION: CONCLUSIONS: REFERENCES: 2,6-pyridinedicarboxylate: Hammett meta vs. log(K X /K H ) Dipicolinatooxovanadium(V): Hammett ortho vs. 1 H NMR The use of 2,6-pyridinedicaroxylate (dipic) as a ligand for metal coordination complexes is widespread. 2,3,4,5 The resulting metal containing complexes have been used as anti-diabetic agents when chelated to V(V) and Zn(II). 4,5 Here we analyze the substituent effects on the ligand, 2,6- pyridinedicaroxylate and the V(V)-containing coordination complex, dipicolinatooxovanadium(V). Specifically we used a variety of Hammett plots and to evaluate the substituent effects and the linear free energy relationships (LFERs) of the ligand and metal complex. The 4-position of the pyridine ring of dipic has been referred to as the most significant position to study the substituent effects in dipic. 6 Will the ligand and complex have similar LFERs? The slope 0.5 observed in figure 2 indicates an increase in the reaction rate and pulling e- away from the ring by the e- donating groups. 11 The poor linear fit r 2 = 4 % indicates the potential for a differing mechanism or the presence of resonance structures. The addition of a terms did not significantly improve the fit to the data (r 2 = 4 % for x 2 ). The red lines represent a possible alternative fit, where two competing mechanisms are present in the system. To support this interpretation, additional substituents would need to be evaluated. 2,6-pyridinedicarboxylic acid: Hammet ortho vs. 1 H NMR Hammett plots have been generated previously using 1 H and 13 C NMR chemical shifts. 12 However this methodology is not probing a reaction as in figure 2, but the effect of substituents on the resulting chemical shifts. The y-axis of figure 3 was generated by subtracting the substituent chemical shift X from that of the parent H (Δ = X – H ). The slope c -0.12 is consistent between figure 2 and 3 (- ), however the fit r 2 = 6 % is also poor. The large r 2 illustrates the limitations observed by Salmon et al. indicating Hammett vs. ppm plots can produces a poor fit in certain systems. 12 Using the same experimental methodolgy as in figure 3, figure 4 illustrates the linear relationship of VO 2 [dipic-X] -. The 1 H NMR for the complex illustrates an increase in linear fit with an r 2 = 38 % when plotted against the values indicating an increase in the stability of the system. The + c (c = 0.48) may indicate that there is stabilization of a negative charge centered around the oxovanadium group and stabilization of the complex by e- withdrawing substituents. The creation of a Hammett plot using 51 V NMR chemical shifts as shown in figure 5, has not been previously attempted to the best of our knowledge. The y-axis was produced by subtracting the 51 V chemical shift of the substituent X from the chemical shift of the parent complex H. The slope of c = -6.6 indicates the effect of stabilization with e - donation. This is a significantly different result from the 1 H NMR study of figure 4. The 51 V NMR establishes a significant increase in the linear fit r 2 = 88 %. By using a curved line (x 2 ) the fit can be increased further to r 2 = 99 %. The deprotonation of 2,6-pyridinedicaroxylate does not produce a linear relationship to the Hammett values deviating from that of benzoic acid and other systems. 1 The scatted data creates a system where differing reaction mechanisms may be present. The fit to two lines suggests the substituents exhibit differing degrees of resonance stabilization and differing LFERs. Metal chelation of the dipic ligand significantly increased the linear fit to the Hammett values using NMR spectroscopy. The findings using 51 V NMR characterize the nature of the chelating pyridine nitrogen. This work illustrates a stabilization effect in 2,6-pyridinedicaroxylate through the V-N coordinate bond. EXPERIMENTAL: Figure 5: Plot of the Hammett para constant plotted against the 51 V NMR chemical shifts of dipicolinatoxovanadium(V). Samples contained 20 mM dipicolinatoxovanadium(V) VO 2 [dipic-X] - in D 2 O at pH ~3. The samples were referenced to internal DSS on a 400 MHz NMR. Figure 3: A scatter plot where the Hammett ortho constants are plotted against the 1 H NMR chemical shifts of solutions containing 20 mM of 2,6-pyridinedicarboxylate (dipic-X) in D 2 O at ~pH 7. The samples were referenced to internal DSS. Table 1: Information used for this poster. The Hammett constants are provided for the ortho, meta and para positions. The –logKa, 1 H and 51 V chemical shifts are tabulated. Figure 4: Plot of the Hammett ortho constant plotted against the 1 H NMR of 20 mM solution of dipicolinatoxovanadium(V) (VO 2 [dipic-X] - ) in D 2 O at ~pH 3. The spectra were referenced against internal DSS. Note: The NH 2 complex formed precipitates during acquisition; the resulting concentration is lower that 20 mM. 1.Hammett, L. P. J. Am. Chem. Soc., 1935, 59(1), 96. 2.Lis, S., Hnatejko, Z., and Elbanowski, M. Bull. Pol. Acad. Sci. Chem., 1994, 42(1), 49. 3.Germaine, G. R., and Murrel, W. G. Photochem. Photobiol., 1973, 17, 145. 4.Enyedy, E. A., Lakatos, A., Horvath, L., Kiss, T. J. Inorg. Biochem., 2008, 102, 1473. 5.Crans, D. C., Trujillo, A. M., Pharazyn, P. S., Cohen, M. D. Coord. Chem. Rev. 2011, 255, 2178. 6.Sakurai, H. ACS Symposium Series, 2007, 974, 110. 7.Hansch, C., and Leo, A., Wiley-Interscience, N Y., 1979. 8.Smee, J.J. et al. J. Inorg. Biochem., 2009, 103, 575. 9.Smee, J.J. et al. J. Inorg. Chem., 2007, 46, 9827. 10.Ooms, K. J. et. al. J. Inorg. Chem., 2007, 46, 9285. 11.Hopkinson, A. C. J. Chem. Soc. B 1969, 203. 12.Salmon, M., Jiminez, A., Salazar, I., and Zawadzki, R. J. Chem. Ed., 1973, 50(5), 370 – 371. Dipicolinatooxovanadium(V): Hammett para vs. 51 V NMR ACKNOWLEDGMENTS : Table 1 illustrates the values used for the following plots in this poster. The Hammett values used for this preliminary study are literature values for the pyridine ring structure. 7 The values may differ for the system due to the carboxyl groups at the 2 and 6 positions. The logKa values represent the deprotonation reaction observed for the carboxyl groups (pKa2) illustrated in figure 2. 8,9,10 The 1 H and 51 V NMR chemical shift data are experimental data for samples 20 mM dipic-X and VO 2 [dipic-X] - in D 2 O at on a 400 MHz NMR. The authors would like to thank the support received from the Crans lab and the support from the staff at the CSU CIF. This work was funded in part by the CRC-NSF CHE 0244181 grant. Hammett equation: log(K X /K H ) = NMR adaptation tof the Hammett equation: Δ = X – H = c
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