Jenalyn D. Osuna

Tris(oxalato) metallates (III) I. INTRODUCTION Coordination compound is a generic term that refers to a neutral complex or an ionic compound having one of its ions as a complex. It contains a transition metal at the center that is coordinated to one or more ligands. The metal centers involved in the formation of coordination compounds are usually transition metals. On the other hand, ligands are neutral or anionic species that can exist independently. They contain a pair of unshared electrons that is used in the bond formation. They are classified according to the number of available lone pairs that they are capable of donating. For instance, a ligand may be classified as monodentate if it is capable of donating a lone pair, bidentate if two lone pairs can be donated and so on. (House, 2008) Coordination is a kind of Lewis acid-base reaction where the metal center acts as a Lewis acid or electron pair acceptor and the ligand acts as a Lewis base or proton donor. These compounds are practically used in catalysis, photochemistry and synthesis. (House, 2008) In this exercise, the tris(oxalato) metallates of trivalent metal ions such as aluminum, chromium, iron and cobalt were studied. Oxalate ion is a bidentate ligand that can bind to metal ions in various ways as shown on the figure below: Figure 1. 1. Various ways in which oxalate ion binds to a metal ion. However, typical chelate complexes of the metal oxalates are usually octahedral in geometry. The tris(oxalato) metallates of trivalent metal ions mentioned above assumes this geometry. Figure 1.2 shows the structures of the tris(oxalato) metallates of the said metal ions. O O O Al3+ C C O O O O O C O O C O O C O O O C O O 3 H2 O C C 3 H2O O O O Fe3+ O C C O O Cr3+ C O C O O O O O O 3 K+ 3 K+ C O C O 3 H2O C C O C O 3 K+ O O C 3 K+ O O C 3 H 2O C C O O O Co3+ O O C O C O O O C O Figure 1. 2. Structures of tris(oxalato)metallates of Aluminum, Chromium, Iron and Cobalt. In order to explore the structural characteristics of these complexes, the magnetic susceptibility as well as the infrared spectra of these metal complexes were examined. Two approaches can be used to explain and rationalize the observed spectra as well as the obtained magnetic moment values for chromium, iron and cobalt complexes. These approaches are the Valence Bond Theory and the Crystal Field Theory. Valence Bond Theory can be utilized by using the hybridization schemes in order to describe the d-block metal complexes. In this theory, an empty orbital on the metal center can accept a pair of electrons from a ligand to form a σ-bond. On the other hand, Crystal Field Theory is an electrostatic approach that can be utilized by considering ligands as point charges and assuming that there is no metal-ligand covalent interactions. In this theory, the ligand electrons are used to create an electric field around the metal center. (Housecroft and Sharpe, 2005). The exercise aimed to synthesize salts of tris(oxalato) complexes of trivalent metal ions of aluminum, chromium, iron and cobalt and to study the properties of these complexes by analyzing their structural characteristics using magnetochemical studies and infrared spectroscopy. II. METHODOLOGY The exercise was divided into two parts. The first part involves the synthesis of the tris(oxalato) complexes while the other involves their characterization through the examination of the provided data which described their magnetochemical and infrared spectral properties. Part 1: Preparation of the Oxalates Synthesis of K3[Al(C2O4) 3]·3H2O First, 0.5 g KOH was dissolved in 3.5 mL distilled water and was allowed to react with 0.09 g of 1x1 cm aluminum foil. Consequently after the reaction, the mixture was decanted into a new beaker in order to separate any solid residues left from the liquid mixture . Next, 1.08 g of oxalic acid dihydrate was added to it and the resulting mixture was heat to boiling with occasional stirring until a clear colorless solution was obtained. Then, the solution was cooled and 3.5 mL ethanol was added to it. Afterwards, the white solid product was filtered and washed with 1:1(v/v) ethanol-water mixture. Finally, the product was air dried and weighed to obtain the percent yield. Synthesis of K3[Cr(C2O4 ) 3]·3H2O First, 0.75 g of oxalic acid dihydrate was dissolved in a 50-ml beaker using 1.70mL warm distilled water. Consequently, 0.25 g potassium dichromate was added in portions to the oxalic acid solution. After the subsidence of the reaction, the solution was heat to boiling. Next, 0.33 g potassium oxalate monohydrate was dissolved in the reaction mixture. Then, the resulting mixture cooled to room temperature and to further enhance its cooling, 0.70 mL ethanol was added dropwise. Afterwards, the solid product was collected through filtration and was washed with 1:1(v/v) mixture of ethanol and water. Finally, the product was air dried and weighed to obtain the percent yield. Synthesis of K3[Fe(C2O4 ) 3]·3H2O First, 2.0 g potassium oxalate was dissolved in 2.5 ml hot water in a 50-ml beaker. Second, 0.80 ml of 0.6 g/mL aqueous solution of FeCl 3 was added to the potassium oxalate solution, which was maintained in an ice bath for about 30 mins in order to induce crystallization. Next, the solution was decanted to obtain the solid crystals. Then, the product was redissolved in 3.5 mL warm water, was cooled in an ice bath and was filtered to collect the final product. Afterwards, the green crystals was washed with 1:1(v/v) mixture of ethanol and water and then with ethanol. Finally, the product was air dried and weighed to obtain the percent yield. Synthesis of K3[Co(C2O4) 3]·3.5H2 O First, 1.03 g oxalic acid dihydrate and 3.07 g potassium oxalate monohydrate were dissolved in 20 ml boiling water. Second, 1.0 g cobalt carbonate was added in small portions to the solution and the resulting solution was cooled at about 35oC and 6.0 g lead dioxide was added to it. Next, 2.0 mL of 1:1(v/v) glacial acetic acid and water was added dropwise to the reaction mixture for 10 minutes. Then, the reaction mixture was filtered and the collected filtrate was slowly added with 20 mL ethanol using burette in a dropwise manner. Afterwards, the mixture was filtered to collect the solid product and was recrystallized by dissolving in 20 mL water and slowly adding 23.3 mL of ethanol while occasionally stirring. Finally, the mixture was filtered again and the product was air dried and weighed to obtain the percent yield. Part 2: Magnetochemical studies and Infrared Spectroscopy The magnetic susceptibility balance and infrared spectroscopy were not available in the laboratory; thus, theoretical data for the magnetochemical studies and IR were provided for each complex to be able to deduce the electronic configuration. III. RESULTS AND DISCUSSION: Synthesis of K3[Al(C2O4) 3]∙3H2O Potassium hydroxide was used as an oxidizing agent. Through its excessive presence, it has provided a basic environment for the solution so that Al(OH) 3 , which is insoluble at neutral pH, can be dissolved. Aluminum foil served as the source of Al 3+. The oxidation of Al to Al3+ was facilitated by the basic conditions through the virtue of the following reaction: Al(OH)3 + -OH → [Al(OH) 4]As the oxidation reaction progressed, bubbling occurred which indicates the evolution of H 2 gas according to the following reaction: KOH + 3H2O + Al (s) → KAl(OH)4 + 3/2H2(g) Oxalic acid dihydrate served as the source of oxalate ligand and excess addition of it was done in order to ensure its coordination to the metal center. The reaction involved is described by the following equation: 2KOH + KAl(OH) 4 + 3H2C2O4∙2H2O → K3[Al(C2O4) 3] ∙3H2O + 9H2O The overall reaction is 3KOH + H2C2 O4∙2H2O + Al (s) → K3[Al(C2O4) 3] ∙3H2O + 6H2O + 3/2H2(g) The product obtained is white powdery solid and the percent yield for this reaction is 43.98%. Although Aluminum is not a transition metal, it was capable of forming a tris(oxalato) complex due to the fact that it has low-lying 3d-orbitals where the electrons can go to form a complex. However, it lacks d-d electron transition so that it was not allowed to obtain colors at the visible range of spectra. Hence, the white coloration of this complex is attributed to these reasons. Synthesis of K3[Cr(C2O4) 3]∙3H2O Potassium dichromate is an oxidizing agent that served as the source of Cr3+. It was added in portions since it causes vigorous reaction. Oxalic acid dihydrate facilitated the reduction of Cr 6+ to Cr3+. The reaction involved is described by the following equation: H2 C2O4∙2H2 O + H2O → C2O42- + 2H3 O+ C2O4 2- + 14H3O+ + Cr2O72- → 2Cr3+ + 21H2O + 6CO2(g) Heating supplied energy to the system. On the other hand, potassium oxalate monohydrate served as the source of oxalate ligand as well as the source of K +, according to the following reaction: 2Cr3+ + 6 K + + 6C2O42- + 6H2O → K3[Cr(C2O4) 3] ∙3H2O The overall reaction is 9C2O42- + 14H3O+ + Cr2O7 2- + 6K+ → 2K3 [Cr(C2O4) 3] ∙3H2O + 15H2O + 6CO2(g) The product obtained is a color changing crystals that is blue-green under daylight and magenta under fluorescent light. The percent yield for this reaction is 61.59%. K3[Cr(C2O4 )3]∙3H2O is a strongly dichroic substance. Dichroism refers to anisotropy in absorption of light wherein different colors of the material can be observed depending on the polarization state of the light travelling through it. The dichroic effect is caused by the occurrence of two levels of excitation so that two colors are emitted as a result. For the case of this compound, blue-green and magenta was observed from the normal light and artificial light, respectively. Synthesis of K3[Fe(C2O4)3]∙3H2O Potassium oxalate served as the source of both the ligand and K+ while the ferric chloride solution served as the source of Fe3+. Synthesis of K 3[Fe(C2O4)3]∙3H2O is an example of metathesis reaction, which is a doubledisplacement reaction involving ion interchange. Furthermore, it is a process that involved exchange in bonds between two reacting chemical species resulting to the creation of products having similar or identical bonding affiliations such that the by-product does not co-precipate with the insoluble product in the solution. The metathesis reaction in the formation of the Fe complex is described by the following equation where the potassium and iron metals exchanged their corresponding anions (oxalate and chloride, respectively): 3 K2C2O2 + FeCl 3 + H2O  K3[Fe(C2O2) 3]. 3H2O + 3 KCl The product obtained is apple-green solid crystals and the percent yield for this reaction is 77.31%. Synthesis of K3[Co(C2O4) 3]∙3.5H2 O Oxalic acid dihydrate served as the source of ligand while potassium oxalate monohydrate served as the source of K +. On the other hand, cobalt carbonate served as the source of Co 2+. Lead dioxide was used as an oxidizing agent for the conversion of Co 2+ to Co3+. It is a milder oxidizing agent that is selective in terms of the formation of the desired complex. Thus, formation of interferences from other forms of cobalt oxide was avoided. Glacial acetic acid was used to dissolve lead dioxide. This acid was used instead of sulfuric acid or hydrochloric acid because it prevents the precipitation of Pb ions with sulfate or chloride ions. In the synthesis of K3[Co(C2 O4) 3]∙3.5H2O, the reactions involved are the following: 2CoCO3 + H2 C2O4∙2H2 O + 2H2O  [Co(C2O4)(H2O) 4] + CO2 2[Co(C2O4)(H2O)4] + 9H3O+ + PbO2 2[Co(C2O4)(H2O) 4]+ + PbO + 3H2 O 2[Co(C2O4)(H2O) 4]+ + 4K2C2 O4∙2H2O  2K3[Co(C2O4) 3]∙ 3.5 H2O + 2K+ And the net reaction is: 2CoCO3 + 2H2 C2O4 ∙2H2 O + 2H3O+ + PbO2 + 4K2C2O4∙2H2O  2K3[Co(C2O4 ) 3]∙3.5 H2 O + H2O + CO2 + PbO + 2K+ The product obtained is green-colored solid crystals and the percent yield for this reaction is 32.90%. Among the other tris(oxalato) metallates studied in this exercise, the water of crystallization of the Co complex is different in terms of stochiometry. In the formation of cobalt complex, the ligand was added first before the oxidizing agent. Otherwise, CoCO3 will be directly oxidized by PbO2 before the attachment of the ligand and that may result to the formation of a different cobalt complex. For the Co complex, it is 3.5 per mole of compound since the production of the complex exists in 2 moles. Hence, 7 moles of water gives a mole of Co complex with 3.5 moles water. InfraRed Spectroscopy Infrared spectroscopy is an analytical technique that reveals information about the presence or absence of a certain functional group in a given compound. In this technique, samples are irradiated with infrared light in order to trigger molecular vibrations. Table 1.1 shows the observed IR frequency for each corresponding band assignment. Table 1.1. Observed frequency and peak description for each corresponding band assignments in IR spectra. Band Assignment Observed Frequency, cm-1 Peak Description- Strong, broad peak O H- Strong, sharp peak C O- Medium, sharp peak C O 800-900 Weak, sharp peak C C 500-700 Medium to weak, sharp peak M O From the given IR spectra, it was observed that the intensity of the peaks of each complexes are almost the same. The information gathered from the infrared spectra provided was summarized in Table 1.2. This table shows the band assignment for the observed frequencies of each metal complexes. It can be noticed that the metal complexes has the same set of observed band assignments. Also, the values of the observed frequencies are very near for each metal complexes. This observation proves that the complexes has the same basic octahedral structure. However, high spin complexes are observed to give more intense peaks as compared to those having low spins. Table 1.2. Determination of the band assignments of the observed frequencies for each metal complex. Complex Observed Frequency, cm-1 Band Assignment 3416.54 O-H stretch 1704.98 C=O stretch 1411.49 C-O stretch Al complex 909.91 C-C stretch- M-O bond stretch 493.70 Ring deformations 3439.73 O-H stretch 1710.39 C=O stretch- C-O stretch 1261.50 Cr complex- C-C stretch- M-O bond stretch- Ring deformations 3424.09 O-H stretch 1712.29 C=O stretch- C-O stretch Fe complex- C-C stretch- M-O bond stretch 503.28 Ring deformations 3545.14 O-H stretch 1705.84 C=O stretch- C-O stretch Co complex- C-C stretch- M-O bond stretch Magnetochemical studies The study of the magnetic properties of materials whether they attach to or repelled by a magnet is referred to as magnetochemisty. There are three possible sources of magnetic fields in an atom and these are those created by nuclear spin, electron spin and electron orbital motion. There are five different forms in which a substance may exhibit magnetism. These include paramagnetism, diamagnetism, ferromagnetism, ferrimagnetism and antiferromagnetism. Paramagnetism is a phenomenon wherein the magnetic moments tend to align with the applied magnetic field due to the presence of unpaired electrons. In general, these substances has a positive values of Xg and Xm. Diamagnetism is a phenomenon wherein the magnetic moments are repelled by the applied magnetic field. It happens when all electrons present are paired. In contrast to paramagnetic substances, diamagnetic substances has negative values of X g and Xm. (Cox, 2004) Ferromagnetism is a phenomenon that happens when a high degree of magnetization between individual domains is present. When there is no external magnetic field present, these domains are randomly oriented. However, when an external magnetic field is applied, the magnetic moments in all domains are aligned in a larger degree. Some examples of metals that exhibit this characteristic includes iron, nickel and cobalt. Ferrimagnetism occurs to materials that has unpaired electrons so that these materials are capable of being attracted to magnets. In contrast to ferromagnetic materials, ferrimagnetic substances are less strongly attracted to magnets. This is due to ferrimagnetic coupling wherein the electron spins are aligned but not in the same direction. For this type of materials, magnetism is manifested as a result of imbalance in the number of spins having opposite directions. On the other hand, antiferromagnetism is a phenomenon that arises when coupling between electron spins of adjacent paramagnetic ions occurs. Oxo-bridged complexes is an example. (Cox, 2004) To determine the form of magnetism a substance exhibits, a magnetic susceptibility balance may be utilized. The principle behind its use exploits that fact that the measure of the force exerted by the field on a unit mass of the specimen is related to the number of unpaired electrons present per unit weight or per mole of the sample. The mechanism of the process involves the application of an external magnetic field on the substance so that induced circulation of electrons is produced. Any apparent change in the sample weight indicates a net outcome of the paramagnetic term. The sample protrudes from the field because its force is proportional to the magnetic gradient. (Cox, 2004) An example of a good method in the determination of the magnetochemical prope rties of compounds using magnetic susceptibility balance is Evans method. It is of the same configuration as the Gouy method. The only difference is that instead of measuring the force which the magnet exerts on the sample, it is the equal and opposite force which the sample exerts on the suspended permanent magnet that is measured. The set up involves a balanced system consisting of two pairs of magnets that are placed at the opposite ends of a beam and a coil mounted between the poles of the balance beam. When a sample is introduced between the poles of a pair of magnets, optical deflection of the beam may be detected. Passing current through the coil will brought back the balance to equilibrium and the amount of current used is proportional to the force exerted by the sample. (Cox, 2004) The parameters that would be obtained in performing Evans Method are the calibration constant (C), reading from the tube containing the sample (R), reading from the empty tube (R 0), sample length (l), sample mass (m), molar mass of the sample (MM) and temperature (T). These parameters are expressed in cgs units. There are a series of steps before the effective magnetic moment ( ) is determined. First, the calibration constant (equation 1) is determined. Next, the mass susceptibility of the sample (equation 2). Then, the diamagnetic susceptibility (equation 3). Afterwards, the corrected molar susceptibility (equation 4). Finally, the effective magnetic moment (equation 5). (equation 1) (equation 2) (equation 3) (equation 4) (equation 5) (equation 6) The spin-only magnetic moment can also be obtained (equation 6, where n is the numbe r of unpaired electrons). The spin-only magnetic moment is directly related to the number of unpaired electrons and through its computation, the preferred configuration of the molecule can be easily determined by considering the value of closest to . In the data provided, the calibrant that was chosen to be used is Hg[Co(SCN)4 ] for three reasons: (1) it is stable in moist air at room temperature, (2) its magnetic properties at the said conditions are known and (3) with these, comparison can be easily made. The data provided for the magnetochemical studies of the synthesized complexes was processed and summarized in Table 1.3 for the determination of the effective magnetic moment of each metal complexes. On the other hand, the preferred configuration of the metal complexes were determined in Table 1.4 by comparing the spin-only magnetic moment with the effective magnetic moment of the metal complexes. Table 1.3. Data for the determination of the effective magnetic moment of each metal complexes. Parameter Values Hg[Co(SCN) 4] Metal complex Al complex Cr complex (calibrant) mass of the sample, g- length of the sample tube, cm- reading on empty sample tube (R0 ) -110 -110 -110 reading on tube with sample (R) 586 -117 210 temperature, K- molar mass, g/mol- calibration constant (C- mass susceptibility (Xg-E-05 -E-E-05 diamagnetic correction (Xdia) n/a -1.587E-04 -1.587E-04 molar magnetic susceptibility (Xm) n/a -E-E-03 effective magnetic moment (μeff) BM imaginary number- Table 1.4. Determination of the preferred configuration of the transition metal complexes. Cr complex Fe complex Parameter High spin Low spin High spin Low spin magnetic spin, BM- Preferred (t2g)3(e g)0 (t2g)3(e g) 2 configuration Fe complex Co complex - -E-05 -1.587E-E- - -110 - -E-07 -1.652E-E- Co complex High spin Low spin- (t2g)6(e g)0 The preferred configuration of Al complex was not determined due to the fact that it is not a transition metal and thus, does not have a d-orbital. However, since it has a negative value for χg and χM, it exhibits diamagnetic properties. There is only one value for the spin-only magnetic moment of Cr complex which is an indication that its preferred configuration is absolute and that is high-spin only, (t2g) 3 (e g) 0. It was found out that the preferred configuration of Fe complex is the high-spin configuration, (t2g) 3(e g)2 , while for the Co complex it is the low-spin configuration, (t2g) 6(e g)0. Since the configuration of both Cr and Fe complexes are high-spin, then these substances exhibits paramagnetic properties. On the other hand, Co complex having low-spin configuration, exhibits diamagnetic properties. IV. SUMMARY AND CONCLUSION In this experiment, tris(oxalato) complexes of Al 3+, Fe 3+, Cr3+, and Co3+ were synthesized. Different colors were observed for each solid complexes. For Al complex, it is a white powder that provided 43.98 % yield; for Cr complex, it is blue green crystals that provided 61.59 % yield; for Fe complex, it is apple green crystals that provided 77.31 % yield; and lastly for Co complex, it is green crystals that provided 32.90 % yield. These color differences were associated with the d-d transitions. Although Al is not a transition metal, it was able to form a complex with oxalate because it has a low-lying d orbital that can hybridize and be occupied with the electrons from the ligand. However, due to the absence of d electron transitions in Al metal, the complex produced is plain white and was not as colorful and interesting as those with the transition metals. The properties of the metal complexes were studied using IR spectroscopy and magneto chemical properties. The data for these characterization techniques were provided. The IR peaks helped in the structure elucidation through the confirmation of the bonds present. The IR peaks for the complexes were interpreted based in the literature values for IR peak assignments. It was identified that these peaks were signals pertaining to the presence of C=O bonds, C-O bonds, M-O bonds and ring deformations. On the other hand, computational analysis of the magneto chemical parameters were used to explain the observed magnetic states of the complexes. According to the results, it was found out that Fe (III) and Cr (III) oxalates were paramagnetic, while the Co (III) complex was diamagnetic as shown in the values of the effective magnetic moments obtained. However, since Al 3+ is not a transition metal, it does not have µeff and µs however it is diamagnetic since its χg and χM is negative. It is therefore concluded that the concepts of orbital hybridization and d-orbital splitting based on the valence bond theory and the crystal field theory are indispensable knowledge that can be used to explain the properties of transition metals including their reactivities, stability, metal complex color produced, electronic spectra and magnetic properties. Although the experimental procedure was successful in producing considerably high yielding products, actual characterization of these were not done possibly due to instrument unavailability. In order to further understand and appreciate the applications of the chemical principles behind this experiment, it is strongly recommended that the students should have a hands-on practical experience to actual infrared spectroscopy and magneto chemical experiments. V. REFERENCES 1. Cox, P. A. (2004). Inorganic Chemistry. 2nd ed. Abingdon, UK: Garland Science/BIOS Scientific Publishers. pp-. House, J. E. (2008). Inorganic Chemistry. Canada: Elsevier, Inc. pp-. Housecroft, C.E. and Sharpe, A. G. (2005). Inorganic Chemistry. 2nd ed. Essex, England: Pearson Education Ltd. pp-. Laboratory Manual Chemistry 120.1 Inorganic Chemistry Laboratory