Rf = [1]
Following TLC, column chromatography was employed to realize separation of the bulk sample. In column chromatography, a glass column is packed with a slurry composed of a polar stationary phase, typically alumina or silica gel, in an organic solvent (Landgrebe, 2005). The level column is then loaded with the bulk sample, and then eluted with the solvent or solvent mixture that achieved the best separation in the TLC experiment (Landgrebe, 2005). Similar to TLC, the more polar the analyte species, the more strongly it is adsorbed to the more polar stationary phase. That is, the less polar the analyte, the more quickly it is eluted from the column, thus, achieving the desired separation. Following elution of the less polar species, the column can then be eluted with a more polar solvent, removing the remaining analyte species from the column.
To fully characterize the Cu(II)TPP species, the technique of electron paramagnetic resonance (EPR) was employed. In this technique, a paramagnetic sample is placed in a uniform magnetic field and subsequently irradiated with microwaves (Shriver et al., 2006). Using this technique, the number of unpaired electrons, electron configuration, and symmetry can be elucidated. In this experiment, H2TPP and Cu(II)TPP were synthesized from benzaldehyde, propionic acid, pyrrole and copper (II) acetate. The subject compounds were then separated using TLC and column chromatography. Then, the subject compounds were subjected to UV-Vis spectroscopy, EPR, and HNMR.
Experimental
To achieve the synthesis of the subject compounds, a Mettler AE 200 analytical balance was used to make all mass measurements. Additionally, Kimax 100 ± 0.6 mL, 50 ± 0.3 mL and 10 ± 0.1 mL graduated cylinders were used as deemed appropriate for the volume of reagent needed. For the UV-Vis spectrochemical data, an Ultraspec 2100 UV – Vis spectrophotometer with a PC interface was used. In addition, HNMR and EPR were performed, however, the instruments used were not provided.
The synthesis of H2TPP entailed adding 1.8 mL of pyrrole, a brown liquid, and 1.6 mL of benzaldehyde, a yellow liquid, to 40 mL of propionic acid in a 100 mL round bottom flask. The solution was allowed to reflux for 30 minutes, after which time, the solution had turned black in color. The solution was cooled to room temperature and then placed in an ice bath for 15 minutes. Following that, the solution was filtered using a Büchner funnel, at which time bright purple crystals appeared. The crystals were washed with cold methanol, and then sucked dry for 10 minutes. The dry H2TPP crystals were transferred to a weigh boat and the mass was found to be 0.2196 g.
Following the above procedure, 0.1018 g of H2TPP was added to 40 mL of dichloromethane in a round bottom flask, along with 2 boiling chips. A solution of 0.2405 g or copper (II) acetate monohydrate in 12.0 mL of methanol was prepared and added to the round bottom flask. The solution was heated to reflux for 15 minutes, after which time the round bottom was removed from the heating mantle and placed into an ice bath.
Once cool, the solution was transferred to a 125 mL Kimax separatory funnel and extracted with 4, 15 mL volumes of distilled water. The organic (dichloromethane) layer was collected in a tarred 100 mL round bottom flask and then rotovapped to dryness. The round bottom flask was then reweighed and a mass of 94.3 mg of Cu(II)TPP was calculated by difference. The product was then dissolved in 5 mL of dichloromethane and placed in a capped and sealed six dram vial for one week.
The resulting Cu(II)TPP solution was then spotted onto five silica gel TLC plates along with a solution of H2TPP in dichloromethane. The TLC plates were then developed in using five different mobile phases: 1:1 hexanes, dichloromethane; 1:1 toluene, dichloromethane; hexanes; dichloromethane; acetone. The Rf values for each of the analytes in each of the solvent systems were calculated, using Equation 1, and recorded.
It was subsequently determined that 1:1 hexanes, dichloromethane was the optimal solvent system to achieve the desired separation of H2TPP and Cu(II)TPP. Thus, a column was packed with a slurry of silica gel and hexanes. The Cu(II)TPP solution was loaded into the column and eluted using the appropriate solvent system. After isolation of the Cu(II)TPP in a 100 mL round bottom flask, the solution was rotovapped to dryness. The resulting product was solvated in 2 mL of dichloromethane, to which was added 10 mL of methanol. After said addition, the resulting solution was placed in an ice bath for 15 minutes where Cu(II)TPP solids were observed to form.
The purple, powdery solids were then filtered via suction filtration with a Büchner funnel and rinsed several times with cold methanol. The Cu(II)TPP product was dried under vacuum and transferred to a tarred six dram vial and weighed. The mass of the product was found to be 10.3 mg. The synthesized H2TPP and Cu(II)TPP were then dissolved in toluene and subjected to UV-Vis and EPR analysis. H2TPP was analyzed using HNMR.
Results
The percent yield of H2TPP was found to be 2.27%. This was found by dividing the actual yield of 0.2196 g by the theoretical yield of H2TPP, 9.69 g. To calculate the theoretical yield, the following equations (2 & 3) were used, with 1.6 mL of benzaldehyde as the limiting reagent.
[2]
[3]
Similarly, the percent yield of Cu(II)TPP was found to be 0.10%, using 15.76 mmol of H2TPP as the limiting reagent.
The UV-Vis spectra for the Cu(II)TPP and H2TPP are shown in Figures 3 and 4, respectively. Red arrows are used to indicate the absorbance peaks.
Figure 3:
Figure 3: Absorbance vs. Wavelength of Cu(II)TPP in toluene
Figure 4:
Figure 4: Absorbance vs. Wavelength of H2TPP in toluene.
The energies of the various peaks are summarized in Table 1. The energies given in Table 1 were calculated using the formula shown in Equation 4, and, as an example, the values for the peak at 415 nm in both spectra:
J [4]
For the thin layer chromatography experiment, a sample plate is shown in Figure 5:
Figure 5:
Table 1:
Table 1: Peak designator, absorbance, wavelength and calculated energy of
UV-Visible absorbance spectra shown in Figures 3 and 4.
The calculated Rf values are shown for the respective spots in Figure 5, specifically, H2TPP on the left, and Cu(II)TPP on the right. The displayed values were calculated using Equation 1, as shown in this example for the H2TPP spot in Figure 5:
Rf = [5]
The values for distances traveled by the analyte compounds (spot distance), distances traveled by the respective solvent systems (solvent front), calculated Rf values, and the five solvent systems are summarized in Table 2:
Table 2:
Table 2: Solvent system, analyte spot distances, solvent front distance and calculate Rf values for the five solvents used in TLC.
Figure 6:
Figure 6 shows the HNMR for H2TPP. Notice the peaks a 7.75 – 7.80, 8.24 – 8.25, 8.88, and –2.73. In addition, Figures 7 and 8 show the EPR spectra for Cu(II)TPP at room temperature and Cu(II)TPP at 77 oK, respectively.
Figure 7:
Figure 8:
Discussion
The complete balanced reactions for the synthesis of the subject compounds for this experiment are shown in Equations 6 and 7.
4 Benzaldehyde + 4 Pyrrole + 4 Propionic acid → tetra-phenyl porphyrin [6]
tetra-phenyl porphyrin + Cu(II)(OAc)2 · H2O → Cu(II)TPP + H2O [7]
In the first reaction, Equation 6, the macro molecule tetra-phenyl porphyrin was synthesized from four benzaldehyde, which provided the meso phenyl goups, 4 pyrrole, which were the nitrogen containing five membered rings, and 4 propionic acid, which composed the meso bridges. In the second reaction, the two nitrogens are reduced and then copper takes a position in the center of the porphyrin ring in a d9, square planar, four coordinate system. Because this is a 17 electron system (9 from copper and 2 from each nitrogen) and a d9 complex, the t2g and eg orbitals undergo Jahn – Teller distortion. Thus, the frontier orbital splitting is shown in Figure 9:
Figure 9:
This hypothesis is confirmed by Figures 8 and 9, in which the EPR spectra are analogous to an axially symmetrical molecule. This fits the geometry of the d9 square planar complex, which has undergone tetragonal Jahn – Teller distortion to such a high degree that the axial ligands have disappeared. Additionally, the EPR spectra reveal that the SOMO is d x2— y 2, as the multiplicity, calculated using Equation 8, yields 4 (n = 1 & I = 3/2), thus we see 4 peaks meaning that the unpaired electron is coupling with the nuclear spin of the copper most of the time.
M = 2nI + 1 [8]
The HNMR spectrum in Figure 6 confirms the identity of the subject compound as H2TPP. Due to the high degree of symmetry of the porphyrin ring, only 5 different hydrogens are expected: those on the pyrrole moiety, those at the pyrrole – propionic acid moiety junction, two unique hydrogens on the phenyl moieties and those on the central nitrogens. We can assign the peaks at 8.88 ppm as those hydrogens on the pyrrole moiety; the peaks at 8.24, 8.25 ppm to those hydrogens at the pyrrole – propionic acid moiety junction; those at between 7.75 and 7.80 ppm as hydrogens at the meta and para positions on the phenyl rings; and the peak at -2.73 ppm to the hydrogens on the nitrogens in the center of the porphyrin ring.
The UV-Vis spectra in Figures 3 and 4 reveal a rather interesting phenonmenon. The strong absorbance peak at 415 nm is clearly both Laporte and spin allowed. Therefore, one would suspect that the transition would be either MLCT or LMCT. However, because the peak appears in both spectra, that is impossible. That is, it is impossible to have a charge transfer process that involves a metal if there is no metal present. The only conclusion, therefore, is that the charge transfer is a ligand – ligand charge transfer, an LLCT, if you will. Because the transition is spin allowed, there must be no change in multiplicity, and similiarly, the transition is Laporte allowed, so the transition is from g to u, or from u to g. The transition is likely between an electron in a bonding or non bonding orbital on the large, highly conjugated prophyrin ring and an antibonding orbital on one of the phenyl rings. Peaks 2 and 3 on the Cu(II)TPP spectrum are likely the result of a d-d or similar transition on the porphyrin ring as they are Laporte forbidden transitions. A similar conclusion can be drawn about peaks 5, 6, 7, and 8 in Figure 4, the H2TPP spectrum.
When separating the Cu(II)TPP with the separatory funnel it was necessary to use four, 15 mL volumes of distilled water rather than one 60 mL volume. The reason for this is two-fold: first, the volume of the separatory funnel was too small to accommodate such a large volume of water. Second, because dichloromethane is sparingly soluble in water, you increase the chance that you will solvate some of the dichloromethane in water when large volumes are used. If dicholormethane becomes dissolved in water, then any product dissolved in the dicholomethane will be lost to the aqueous layer, thereby losing product. By using several a small volumes of water, there was less opportunity for the dichloromethane to dissolve in water.
The solvent system used for the column chromatography was selected based on the difference in the Rf values, from TLC, for the different solvents. Clearly, the largest difference in Rf will have the greatest separation between the subject compounds. The developing solvent used was a 1:1 mixture of hexanes and dichloromethane, which had a difference in Rf of 0.22, by far the largest difference for the solvents analyzed. Once the proper solvent had been selected, the column was run in order to remove any unreacted H2TPP from the Cu(II)TPP solution. Because many of the techniques used for analysis are very sensitive, they require a high degree of sample purity.
Conclusion:
To a large degree this experiment was a success. The greatest problem was the absolutely terrible percent yields. However, because in first reaction, the synthesis of H2TPP, entropy is very negative, Gibbs free energy will be made more positive. In other words, during this synthesis one is fighting entropy the entire time. That being said, there is absolutely no excuse for the yield in the Cu(II)TPP synthesis. It should be noted that there was a lot of unreacted H2TPP left over in the reaction vessel when that portion of the experiment was discontinued.
In this experiment many different techniques were utilized, both during the synthesis and also during the characterization. TLC and column chromatography are both effective techniques for separation of species with different polarities. EPR, HNMR, and UV-Vis all contributed to the analysis of the identity of the subject compounds as well as to the electronics of the subject compounds.
References
1. Shriver, D. F., Atkins, P. W., Overton, T. L., Rourke, J. P., Weller, M. T., & Armstrong, F. A. (2006). Inorganic Chemistry, Fourth Ed. New York: W. H. Freeman and Company. p. 459 – 464.
2. Castro, C., (1971) Theory of Hemeprotein Reactivity. J. theor. Biol., 33, 475 – 490.
3. Adler, A. D., Varadi, V., Wilson, N., (1975). Porphyrins, Power, and Pollution. Annals of the New York Academy of Sciences, 244, 685 – 694.
4. Wolberg, A. & Manassen, J. (1969) Electrochemical and Electron Paramagnetic Resonance Studies of Metalloporphyrins and Their Electrochemical Oxidation Products. J. Am. Chem. Soc., 92 (10), 2982 – 2991.
5. Paliteiro, C. & Sobral, A. (2004). Electrochemical and spectroelectrochemical characterization of meso-tetra-alkyl porphyrins. Electrochimica Acta, 50, 2445 – 2451.
6. Landgrebe, J. A. (2005). Theory and Practice in the Organic Laboratory with Microscale and Standard Scale Experiments Fifth Ed. Belmont, CA: Brooks/Cole—Thomson Learning. p. 60 – 70.
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