Synthesis and Characterization of Cu(II)-Tetraphenylporphinate

The synthesis of metalloporphyrins, particularly copper(II) tetraphenylporphinate (CuTPP), holds great significance in the realm of coordination chemistry. This experiment consists of a two-fold aim: firstly, to synthesize meso-tetraphenylporphyrin (H2TPP) through the reaction between pyrrole (C4H4NH) and benzaldehyde (C6H5CHO) in refluxing propanoic acid (CH3CH2COOH); and secondly, to investigate the interaction between the ligand and copper acetate monohydrate in refluxing N, N-dimethylformamide, resulting in the formation of CuTPP.

In the initial phase of the experiment, the focus is on the synthesis of H2TPP.

Pyrrole, an aromatic amine, undergoes a reaction with benzaldehyde in the presence of refluxing propanoic acid. The choice of propanoic acid as a solvent is crucial, providing optimal conditions for the formation of meso-tetraphenylporphyrin. The reaction pathway involves the condensation of pyrrole and benzaldehyde, leading to the formation of a porphyrin ring.

The characterization of H2TPP involves the analysis of its 1H NMR spectrum, UV spectrum, and determination of its molar absorption coefficient.

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The 1H NMR spectrum provides insights into the proton environments within the molecule, aiding in the confirmation of the synthesized product. Meanwhile, the UV spectrum helps in understanding the electronic transitions occurring in the porphyrin ring. Additionally, the molar absorption coefficient serves as a quantitative measure, allowing for the determination of concentration in subsequent reactions.

To assess the success of the synthesis, percentage purity is calculated at each step. This provides a quantitative evaluation of the yield and purity of H2TPP, crucial for the subsequent steps of the experiment.

The second phase of the experiment involves the reaction between the synthesized H2TPP ligand and copper acetate monohydrate in refluxing N, N-dimethylformamide. This step aims to form the desired product, CuTPP, a copper(II) metalloporphyrin complex.

Copper acetate monohydrate serves as a suitable metal source, reacting with the H2TPP ligand to form a coordination complex. The choice of N, N-dimethylformamide as the solvent ensures favorable reaction conditions, facilitating the coordination of copper ions with the porphyrin ligand.

Characterization of CuTPP involves the examination of its UV spectrum and determination of the molar absorption coefficient. The UV spectrum aids in understanding the electronic transitions specific to the copper-porphyrin complex. The molar absorption coefficient, similar to the characterization of H2TPP, provides a quantitative measure for concentration, allowing for accurate analysis.

Experimental Characterization Techniques

The use of sophisticated analytical techniques enhances the precision of the experiment. Nuclear Magnetic Resonance (NMR) spectroscopy, specifically the 1H NMR spectrum, enables the identification of distinct proton environments in the synthesized compounds. UV spectroscopy provides valuable information about the electronic transitions occurring in the porphyrin ring, aiding in the characterization of both H2TPP and CuTPP.

Upon synthesis, the characterization of H2TPP reveals distinctive peaks in its 1H NMR spectrum, corresponding to different proton environments within the molecule. The UV spectrum exhibits absorption peaks indicative of electronic transitions within the porphyrin ring, further confirming the successful formation of H2TPP. The molar absorption coefficient, determined through careful analysis, provides a quantitative measure of concentration, crucial for subsequent reactions.

Moving to the synthesis of CuTPP, the UV spectrum of the final product displays characteristic peaks associated with the copper-porphyrin complex. The molar absorption coefficient, determined in a manner similar to H2TPP, enables accurate concentration measurements. This comprehensive characterization ensures the reliability and validity of the experimental results.

Theoretical amount of C44H30N4 (H2TPP) expected = (0.014414/ 4)(614.7358) = 2.2152 g
Experimental amount of H2TPP obtained = 0.8014 g
Percentage yield = experimental yield/ theoretical yield = 0.8014/ 2.2152 x 100 = 36.177%
Observation: Colourless solution turns yellow then orange and finally dark purple-brown upon dropwise addition of
C6H5CHO AND C4H4NH down the wall of condenser. Upon filtration, vibrant purple crystals found.
The UV Spectrum of H2TPP is attached as Appendix B, 1H NMR Spectrum of H2TPP as Appendix C.

Theoretical amount of CuC44H28N4 (CuTPP) expected = (0.00017259)(676.265920)= 0.11672 g
Experimental amount of CuTPP obtained = 0.0250 g
Percentage yield = 0.0250/ 0.11672 = 21.419 %
Observation: Solution turns from purplish-blue to red solution upon reflux. At separatory funnel, a light blue-green
layer is above a dark red layer. In column, broad red layer is eluted faster than a sharply-defined tarry layer. Red
solution eluted is rotary evaporated to obtain reddish purple crystals.
The UV Spectrum of CuTPP is attached as Appendix D.
Part C

For UV spectroscopy analysis of H2TPP:
Dilution Factor for series dilution = 100/1 = 100
Concentration of initial solution = (0.0141 g/ 614.7358gmol-1
) x (1000/ 100 cm3
) = 2.2937 x 10-4 M
Concentration of final solution = 2.2937 x 10-4 M/ 100 = 2.2937 x 10-6 M

For UV spectroscopy analysis of CuTPP:
Dilution Factor for series dilution = 100/1 = 100
Concentration of initial solution = (0.0038 g/ 678.2859 gmol-1
) x (1000/ 100 cm3
) = 5.6023 x 10-5 M
Concentration of final solution = 5.6023 x 10-5 M/ 100 = 5.6023 x 10-7 M
Table 3: Calculated molar absorptivity coefficients (in 4 significant figures) from the wavelength and absorbance for
each compound. Its accuracy is limited by the number of significant values of the Absorbance

General Discussion
4 C6H5CHO + 4 C4H4NH + 3/2 O2  C44H30N4 + 7 H2O
H2TPP + Cu(O2CCH3)2(H2O)  Cu(TPP) + 2 HO2CCH3 + H2O

In the realm of coordination chemistry, the synthesis and characterization of metalloporphyrins play a pivotal role. This essay delves into the detailed process of synthesizing meso-tetraphenylporphyrin (H2TPP) using the Alder-Longo method, followed by the metallation of the porphyrin to form copper(II) tetraphenylporphinate (CuTPP). The Alder-Longo method, employed in Part A and C of the experiment, initiates with the protonation of carbonyl oxygen, facilitating nucleophilic addition by pyrrole.

Part A: Synthesis of H2TPP Using Alder-Longo Method

The Alder-Longo method involves the protonation of carbonyl oxygen, creating a conducive environment for nucleophilic addition by pyrrole. This nucleophilic addition occurs in an acidic medium, specifically propanoic acid, promoting the reaction with benzaldehyde or its methanol analogue. Protonation of benzaldehyde is succeeded by the release of a proton to stabilize positively-charged Nitrogen, while protonation of the methanol analogue results in the formation of water as a byproduct. The reaction progresses until the meso-tetrasubstituted porphyrin ring is completed.

It's noteworthy that propanoic acid is chosen over acetic acid due to its higher yield (20-25% more). This preference is attributed to the crystallization challenges posed by acetic acid, owing to its high pKa. The addition of boiling chips ensures smooth boiling, preventing product loss through the condenser. Notably, this reaction is carried out in open air to facilitate the necessary oxidation of porphyrinogen to the porphyrin ring by the loss of 6 electrons.

Following reflux, the purple crystals are collected through suction filtration and washed with methanol to eliminate polar organic impurities and residual propanoic acid. To minimize yield loss during transfer, the crystals are placed on a weighted filter paper before measuring the overall weight. This meticulous procedure, repeated for both Part A and C, ensures minimal yield loss.

Characterization of H2TPP

The extended conjugation within the system significantly increases the absorption wavelength, promoting electrons from π to π*. This arises from the overlapping p-orbitals of each Carbon and Nitrogen in the system. The resulting absorption wavelength falls in the range of 560 nm to 600 nm, yielding a violet complementary color. Additionally, a less significant color contribution stems from the n to π* transition, though its occurrence is forbidden and its effect is overshadowed by the allowed π to π* transition.

The experimental yield for H2TPP is 36.177%, surpassing the literature value of 20%. Loss in yield is attributed to incomplete reactions and transfer losses during the process. The equilibrium nature of the reactions, except for oxidation, contributes to the incomplete reaction. The enthalpy of stabilization when Nitrogen loses its positive charge drives the spontaneous loss of a proton to the solution. However, the purity of the yield is questioned due to the possible presence of meso-tetraphenylchlorin, a contaminant identical to H2TPP but with differing double bond locations.

To address potential impurities, particularly meso-tetraphenylchlorin, column chromatography is employed in Part C. Although the impurity is later removed, a UV-Vis spectrum obtained for H2TPP indicates an absorbance peak at 649 nm, higher than expected for the last Q-band. This discrepancy raises concerns about overestimation in reported yields. Additionally, significant loss of porphyrin during adsorption to the stationary phase underscores the importance of maximizing yield in the early stages.

An improvised experimentation suggestion involves using 2,3-Dichloro-5,6-Dicyanobenzoquinone (DDQ) to oxidize chlorin to H2TPP. Monitoring the progress of the reaction could be achieved by observing a leftward shift of the Q-band to 646 nm and a reduced absorbance for the peak. This approach aims to enhance the yield and purity of H2TPP.

In conclusion, the synthesis and characterization of Cu(II)-tetraphenylporphinate involve intricate steps, utilizing the Alder-Longo method for H2TPP formation and subsequent metallation. The detailed procedures, along with the challenges faced in ensuring purity and yield, shed light on the nuances of coordination chemistry experiments. Further exploration and refinement of experimental techniques are suggested to maximize the yield and purity of metalloporphyrins, expanding their potential applications in various scientific domains.

Scheme 1: Synthesis of H2TPP through Nucleophilic Addition and Protonation

In Scheme 1, the synthesis of meso-tetraphenylporphyrin (H2TPP) unfolds via nucleophilic addition to the carbonyl carbon, simultaneously accompanied by the protonation of the carbonyl oxygen. The sequential steps involve nucleophilic addition to benzaldehyde, followed by proton loss, and nucleophilic addition to a methanol analogue, followed by the loss of water. The schematic representation allows for the possibility of the methanol analogue continuing to react with two benzaldehyde molecules, leading to the same product. This scheme was crafted using ACD/ChemSketch Version 11.02.

Part B: Cu(II) Incorporation into H2TPP and Crystallization Process

In this phase, copper(II) incorporation into H2TPP occurs in the presence of catalytic acetate ions in N, N-dimethylformamide (DMF). DMF serves as the solvent due to its slight dissociation effect on Cu(COOCH3)2∙H2O. The addition of monohydrate acetate to DMF initiates dissociation, involving the reactive CuAc+ and non-reactive dimer Cu2Ac4. The monohydrate form, experimentally observed, accelerates the reaction rate due to increased acetate ion availability from water, which facilitates dissociation. The mechanism remains unestablished in literature, but suggestions include an association between metal ions and porphyrin, leading to the loss of two inner protons. The rate-determining step is potentially linked to the deformation of the rigid porphyrin plane. Reflux provides the necessary energy for the reaction, resulting in a sitting-atop complex of Cu(II) on the distorted porphyrin plane.

Following reflux, the reaction mixture undergoes quenching in an ice bath, inducing crystallization of CuTPP. Water is added to dissolve any unreacted copper acetate. Subsequent washing with dichloromethane in a separatory funnel, owing to its nonpolar nature compared to DMF, facilitates the removal of CuTPP, which settles at the bottom layer. The separated solution is then rotary evaporated to eliminate excess solvent, leveraging the vacuum environment to vaporize solvents efficiently.

Part C: Column Chromatography for Impurity Removal

In the final stage, column chromatography is employed to eliminate impurities. A mixture of hexane and toluene serves as the mobile phase, reducing polarity and favoring the elution of nonpolar CuTPP. The elution speed is influenced by the polarity of the mobile phase, with the equal-volume mixture achieving an intermediate polarity. Prior to chromatography, the crude product is dissolved in minimal mobile phase solvent to prevent dilution during elution, ensuring a more concentrated sample.

Impurities, such as polar pyrrolic oligomers and meso-tetraphenylchlorin, are removed through this chromatographic process. The eluent, containing the desired product, is collected, rotary evaporated, and washed with methanol to yield reddish-purple crystals. The incorporation of copper into the porphyrin ring alters the delocalization observed in H2TPP, resulting in a slightly reduced absorption wavelength. The complementary color observed, based on the color wheel model, falls between violet and red due to the absorption at a higher frequency between green and yellow wavelengths (540 nm to 580 nm).

Despite meticulous procedures, the percentage yield for CuTPP is modest at 21.419%. Factors contributing to the loss in yield include the high energy barrier for the reaction, some CuTPP retention in the column due to adsorption, and the intentional exclusion of the tail end of the product to avoid eluting impurity. These factors collectively contribute to the acknowledged random errors in the experiment.

Table 4: The assignment of chemical shifts observed from the given 1H NMR spectrum

Understanding the absorption spectra of porphyrins is essential in elucidating their electronic structure and reactivity. The Gouterman Four Orbital Model provides valuable insights into the absorption bands in porphyrin systems, specifically the transition between two Highest Occupied Molecular Orbitals (HOMOs) and two Lowest Unoccupied Molecular Orbitals (LUMOs). This model's application to Cu(II)-Tetraphenylporphinate (CuTPP) reveals intriguing changes in absorption features linked to the metal center and substituents.

CuTPP, characterized by D4h symmetry, possesses nondegenerate HOMOs (a1u and a2u) and a set of degenerate LUMOs (eg). Transitions between these orbitals give rise to two excited states, both of 1Eu character. The ground state electronic configuration of porphyrin is 1A1g (a1u²a2u²). The lowest singlet excited configurations are 1(a2u¹eg¹) and 1(a1u¹eg¹). The near degeneracy of these states results in a resonance, producing a lower energy 1Eu state with a small oscillator strength known as the Q(0,0) band and a higher energy 1Eu state with a large oscillator strength, termed the Soret B(0,0) band. The Q band, being forbidden, is observed solely due to vibronic coupling to the Soret band.

Upon metallation of H2TTP with copper, a change in symmetry from D2h to D4h occurs, leading to a disappearance of the peak at 647 nm and a reduction in the number of observed Q bands. The increased symmetry results in fewer absorption bands, transitioning from 4 Q bands in H2TPP to none in CuTPP.

Comparing the Soret band wavelengths between H2TPP and CuTPP reveals a hypsochromic shift from 419 nm to 416 nm. This shift arises from the loss of delocalization around the four Nitrogen atoms in Tetraphenylporphyrin (TTP) and a significant metal-to-ligand charge transfer (MLCT) from the d orbitals of Cu²⁺ to the π* anti-bonding orbital of the porphyrin ligand. The large absorptivity value of 142,300 M⁻¹cm⁻¹ indicates that this transition is Laporte-allowed and spin-allowed, dominating over Ligand-to-Metal Charge Transfer (LMCT) and d-d transitions.

The reduction in molar absorptivity coefficient of the Soret band from 318,500 M⁻¹cm⁻¹ to 142,300 M⁻¹cm⁻¹ suggests that CuTPP has a diminished ability to absorb light compared to H2TPP. This property is crucial, especially in applications like catalysis, where the photocatalytic activity of H2TPP on chloroform is reduced upon metallation.

Precautions and Experimental Considerations

The experimental process includes precautionary measures to ensure safety and accurate results. Wearing gloves throughout the experiment is imperative due to the corrosive nature of chemicals used, including toluene in elution and chloroform in preparing KBr discs. The fumehood cover is employed to avoid inhaling vapors of propanoic acid, dichloromethane, toluene, and hexane. Boiling chips are strategically placed to prevent refluxing solvent spillage through the condenser.

For UV spectroscopy, thorough dissolution of H2TPP and CuTPP is ensured to prevent scattering caused by undissolved particles. Quick processing of H2TPP is conducted due to its photocatalytic activity on chloroform. Cuvettes are meticulously handled, wiping the sides to minimize radiation scattering and possible oil presence from fingerprints. Rinsing cuvettes with solvent and sample solution before filling them up is a standard practice to minimize sample carry-over.

In conclusion, the synthesis and characterization of CuTPP involve a deep analysis of the absorption spectra using the Gouterman Four Orbital Model. Changes in symmetry, transition energies, and absorptivity coefficients shed light on the electronic structure alterations upon metallation. The hypsochromic shift in the Soret band and the reduced molar absorptivity coefficient in CuTPP have significant implications for its applications, particularly in catalysis. The experimental precautions highlight the importance of safety and precision in conducting such intricate coordination chemistry experiments. The percentage yield data of 36.177% for H2TPP and 21.412% for CuTPP, along with the detailed UV spectra analysis, contribute to a comprehensive understanding of the synthesized compounds. Further research and refinement in experimental techniques hold promise for advancing the field of metalloporphyrin chemistry.