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^{Articles > Inorganic Synthesis of Transition Metal Complexes}

_{Date Published: 15 May 2023}

Inorganic Synthesis of Transition Metal Complexes

^{Joon Ko and Junsung Oh}

Abstract

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Inorganic synthesis is a fundamental branch of chemistry focused on the design and creation of inorganic compounds through controlled chemical reactions. It encompasses the synthesis of a wide range of materials, including metals, metal complexes, ceramics, and semiconductors, among others. This abstract provides an overview of the key aspects and significance of inorganic synthesis.

The process of inorganic synthesis involves the manipulation of various elements and their compounds to produce new substances with desired properties and structures. It encompasses both traditional methods and advanced techniques, such as solvothermal synthesis, hydrothermal synthesis, co-precipitation, and chemical vapor deposition, among others. These methods enable the synthesis of materials with precise control over composition, particle size, morphology, crystallinity, and surface properties.

Inorganic synthesis plays a crucial role in numerous scientific and technological fields. It serves as the foundation for the development of functional materials with applications in catalysis, energy storage, sensors, optics, electronics, and medicine. By tailoring the synthesis parameters, researchers can optimize material properties to meet specific requirements, leading to improved performance and novel functionalities.

Furthermore, inorganic synthesis contributes to the advancement of fundamental scientific understanding. It allows researchers to explore the structure-property relationships of various inorganic compounds, elucidate reaction mechanisms, and uncover new phenomena. This knowledge not only enriches our understanding of the fundamental principles governing matter but also facilitates the development of innovative synthetic strategies and materials with enhanced performance.

In summary, inorganic synthesis is a diverse and interdisciplinary field that enables the creation of a wide range of functional materials. Through precise control over synthesis parameters, researchers can tailor the properties of these materials to suit specific applications. Moreover, the fundamental insights gained through inorganic synthesis contribute to scientific knowledge and drive further advancements in materials science and chemistry as a whole.

Introduction

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Inorganic synthesis plays a vital role in the development of functional materials with diverse applications. One area of significant interest within inorganic synthesis is the synthesis of cobalt complexes. Cobalt complexes exhibit unique electronic, magnetic, and catalytic properties, making them valuable in various fields, including catalysis, energy storage, and medicinal chemistry. This introduction provides an overview of the importance and applications of inorganic synthesis in the context of cobalt complexes.

Cobalt, a transition metal, possesses a rich chemistry due to its variable oxidation states and coordination preferences. By synthesizing cobalt complexes, researchers can explore the coordination chemistry of cobalt and manipulate its properties to design materials with desired characteristics. These complexes can be synthesized by various methods, such as ligand exchange reactions, template synthesis, and coordination polymerization.

One prominent application of cobalt complexes lies in catalysis. Cobalt-based catalysts have been extensively studied for a wide range of transformations, including hydrogenation, oxidation, and carbon-carbon bond formation. These complexes can exhibit high catalytic activity, selectivity, and stability, making them valuable in industrial processes and sustainable energy conversion technologies.

Furthermore, cobalt complexes play a crucial role in energy storage systems, particularly in the field of rechargeable batteries. Cobalt-based cathode materials, such as lithium cobalt oxide (LiCoO₂), are widely used in lithium-ion batteries due to their high energy density and cycling stability. Inorganic synthesis allows researchers to optimize the composition, crystal structure, and surface properties of cobalt-based materials, aiming for improved battery performance and reduced cost.

In the field of medicinal chemistry, cobalt complexes have shown promise as potential therapeutic agents. These complexes can exhibit unique coordination geometries and redox properties, which can be utilized for various applications, such as anticancer drugs, antimicrobial agents, and contrast agents for medical imaging. Inorganic synthesis enables the design and synthesis of cobalt complexes with specific ligands and structures, optimizing their biological activity and reducing toxicity.

Overall, the synthesis of cobalt complexes through inorganic methods provides a versatile platform for tailoring the properties of cobalt-based materials. These materials find applications in catalysis, energy storage, and medicinal chemistry. By understanding the coordination chemistry of cobalt and employing inorganic synthesis techniques, researchers can unlock the full potential of cobalt complexes, leading to advancements in various scientific and technological domains.

Experimental Procedure

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A. Pentaamminechlorocobalt(III) Chloride, [Co(NH₃)₅Cl]Cl₂

1. A 250 mL Erlenmeyer flask was prepared with a cork stopper.
2. 15 M ammonia (20 mL) was added to the 250 mL Erlenmeyer flask under the hood.
   • 
3. Ammonium chloride (3.0 g) was prepared and added to the Erlenmeyer flask.
4. The ammonium chloride was dissolved in the ammonia solution with a little gentle swirling.
5. Cobalt (II) chloride hexahydrate (5 g) was placed into the Erlenmeyer flask, with a constant swirling of the flask.
   • The reacting mixture should be a yellow-brown slurry.
   • 
6. In the hood, 30% hydrogen peroxide (6.0 mL) was added to the flask slowly with constant swirling.
   • This produced a vigorous reaction with the effervescence of gaseous oxygen.
7. When the effervescence ceased, 12 M HCl (20 mL) was slowly added to the reaction mixture in the flask.
   • During the addition of HCl, the temperature of the reaction mixture rose, and the purple product precipitated leaving a blue supernatant liquid.
   • 
8. The mixture was heated for about 15 minutes on a hot plate.
   • The temperature was regulated carefully to within 75-85 ℃.
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9. The flask was cooled in an ice bath.
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10. The mixture was filtered using a Büchner funnel.
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11. After the filtered mixture was dried, it was scraped and transferred to a test tube.
    • 

B. Hexaamminecobalt(III) Chloride, [Co(NH₃)₆]Cl₃

1. A hot water bath was set up on a hotplate, using a 1000 mL beaker containing water (200 mL).
   • The hotplate was turned on at its minimum setting.
2. A clean 250 mL beaker with gradations was selected from the lab drawer.
3. Ammonium chloride (10.0 g) was added to the 250 mL reaction beaker.
4. Deionized water (40. mL) was added to the reaction beaker, and the beaker was stirred until most of the ammonium chloride dissolved.
5. Cobalt (II) chloride hexahydrate (8.0 g) was added to the reaction beaker.
   • The reaction beaker was stirred until most of the solids dissolved.
   • 
6. In the hood, 15 M ammonia (40. mL) was added to the reaction beaker.
   • Ammonia was pumped slowly and smoothly so that none spattered.
   • Caution: This is a concentrated basic reagent with a strong odor.
   • The reaction mixture was stirred with a stirring rod for about thirty seconds.
   • 
7. Activated charcoal (0.8 g) was added.
   • The activated charcoal serves as a catalyst for the reaction that forms the bonds between NH₃ and Co. It also catalyzes the transformation of Co²⁺ into Co³⁺ by H₂O₂.
   • 
8. In the hood, 10 % mass hydrogen peroxide (50. mL) was added, one pump at a time, stirring between each pump.
   • Caution: Be careful not to spill or spatter this reagent on the skin: use gloves.
   • The purpose of the hydrogen peroxide is to convert the cobalt from Co²⁺ to Co³⁺.
9. Once all bubbling stopped, the 250 mL reaction beaker was placed in a 1000 mL hot water bath, with a temperature of roughly 60 ℃.
   • The volume of the reaction mixture should be 135 ± 5 mL.
   • The color of the solution should be almost deep maroon.
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10. The reaction beaker was left in the water bath for 30-40 minutes while maintaining the temperature of the water between 50 ℃ and 70 ℃.
    • The solution was stirred periodically.
11. Place the 250 mL reaction beaker out of the water bath.
12. Using a 400 mL beaker, an ice bath was prepared using tap water.
13. The reaction mixture was cooled below 10 ℃ in the ice bath.
    • The cooling of the solution promotes crystallization.
    • 
14. A Buchner funnel apparatus was set up, and the reaction mixture was filtered through it.
15. The contents of the filter cup were scraped and put back into the reaction beaker.
    • The filter cup was rinsed with deionized water from the squirt bottle into the reaction beaker.
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16. The used filter paper was discarded in the trash, and the filtrate (liquid) was discarded into the aqueous cobalt waste container.
17. The sides of the reaction beaker were rinsed with a stirring rod and deionized water.
18. Deionized water was added to the reaction beaker until the total solution volume was approximately 100 mL.
19. In the hood, 12 M HCl (5 mL) was added to the reaction beaker.
20. The mixture was heated directly on the hot plate with frequent stirring.
21. The Büchner funnel was rinsed with deionized water.
22. The reaction mixture was filtered using a Büchner funnel.
    • The filtrate should be orange, and the dissolved product is contained in it.
23. The reaction beaker was rinsed to remove any residual carbon.
24. The filtrate was transferred to the clean reaction beaker.
    • Try to transfer any solid present in the filtrate by swirling the sidearm flask.
25. In a hood, 12 M HCl (15 mL) was added to the reaction beaker and stirred.
26. The reaction beaker was placed in the ice bath with stirring.
    • The reaction mixture was cooled below 10 ℃.
27. The filter cup of the filtering apparatus was cleaned, and the filtering apparatus was reassembled.
28. The cold precipitated product was filtered through the filtering apparatus.
29. The product was scraped and transferred to a test tube.
    • 

C. trans-Dichlorobis(ethylenediamine)cobalt(III) Chloride, [CoCl₂(en)₂]Cl (en = ethylenediamine)

1. Water (about 300 mL) was heated in a 600 mL beaker and maintained at a moderate boil.
2. In an evaporating dish, cobalt (II) chloride hexahydrate (4.0 g) was combined with distilled water (10 mL). 
3. 10 % ethylenediamine (15 mL) was added to the evaporating dish. 
4. The evaporating dish was placed on top of the beaker of boiling water.
   • The mixture was stirred over the steam bath for 40 minutes. 
5. The volume of the solution was maintained at about 20 mL by occasionally adding small portions of water.
   • During this process, the Co²⁺ is oxidized to Co³⁺ by the oxygen in the air. 
   • Good agitation is necessary to promote solvation of the oxygen. 
   • 
6. Concentrated 12 M HCl (12 mL) was added. 
7. The heating and stirring of the solution continued, without the addition of water, until a thin slurry of crystals was formed.
   • Stopping the evaporation at the right time is critical to the success of the synthesis.
   • Stopping too soon will give a poor yield of the product.
   • Stopping too late will give an impure product.
8. The slurry was cooled to room temperature by setting the evaporating dish on the lab bench.
   • It was occasionally stirred for 15 minutes.
9. The mixture was filtered using a Buchner funnel with a side-arm flask attached to an aspirator.
   • When the draining from the Buchner funnel essentially stopped, 6 M HCl (5 mL) was added to the funnel.
     • Do not use pure water to wash your product. If you do, the complex will decompose.
   • The mixture was gently stirred up with a spatula. 
   • If the product was still brown or blue, the product was rewashed with 6 M HCl.
   • If the product is pure green, the washing was skipped to avoid loss of product.
10. The moist crystals of the product were transferred to a clean watch glass.
    • The product was dried and transferred to a test tube using a spatula.
    • 

D. Pentaammineaquacobalt(III) Chloride, [Co(NH₃)₅(H₂O)]Cl₂

1. [Co(NH₃)₅Cl₂]Cl₂ (1.0100 g, 4.03 * 10^-3 mol) was dissolved in 5 % NH_{3 (aq)} (15.15 mL) in a 100 mL conical flask to produce a purple solution.
2. The solution was stirred and heated for about 15-30 minutes.
3. The mixture was dissolved and a purple color solution appeared.
4. The mixture was cooled to 10 ℃ in an ice bath.
   • 
5. 12 M HCl_(aq) (2.7 mL) was added, and a red slurry was formed.
6. The mixture was cooled to about 3 ℃.
7. The product was filtered off using a Buchner funnel.
   • 
8. Ice-cold ethanol (6.7 mL) was added to wash the product.
9. The product was dried at room temperature to give a red powder of [Co(NH₃)₅(H₂O)]Cl₂.

Data and Data Work-up

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Picture #1: Pentaamminechlorocobalt(III) Chloride, [Co(NH₃)₅Cl]Cl₂

Picture #2: Hexaamminecobalt(III) Chloride, [Co(NH₃)₆]Cl₃

Picture #3: trans-Dichlorobis(ethylenediamine)cobalt(III) Chloride, [CoCl₂(en)₂]Cl (en = ethylenediamine)

Picture #4: trans-Dichlorobis(ethylenediamine)cobalt(III) Chloride, [CoCl₂(en)₂]Cl (en = ethylenediamine), When the Amount Was Doubled in Procedure

Picture #5: 0.04 M Cobalt (II) Chloride Solution (Left) and 0.04 M Pentaamminechlorocobalt(III) Chloride (Right)

Graph #1: Wavelength of Cobalt (II) Chloride Pentahydrate from Visible Spectrophotometer

Graph #2: Wavelength of [Co(NH₃)₅Cl]Cl₂ from Visible Spectrophotometer

Results & Discussion

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Through the given procedures, each inorganic cobalt complex was synthesized. Cobalt (II) chloride, CoCl₂ · 6(H₂O), was used as the reactant for each cobalt complex. Pentaamminechlorocobalt(III) chloride, [Co(NH₃)₅Cl]Cl₂, was a purple compound, hexaamminecobalt(III) chloride, [Co(NH₃)₆]Cl₃, was a yellow compound, trans-Dichlorobis(ethylenediamine)cobalt(III) chloride, [CoCl₂(en)₂]Cl (en = ethylenediamine), was a green compound, and pentaammineaquacobalt(III) chloride, [Co(NH₃)₅(H₂O)]Cl₂ was a red compound.

Cobalt (II) chloride, CoCl₂ · 6(H₂O) was oxidized from Co²⁺ to Co³⁺ to form pentaamminechlorocobalt(III) chloride, [Co(NH₃)₅Cl]Cl₂. Since the colors of the two compounds were similar, the visible spectrum for each compound was run to test whether it went through the oxidation from Co²⁺ to Co³⁺. For the visible spectroscopy, 0.04 M of each solution was prepared (0.5 g of CoCl₂ · 6(H₂O) in 50 mL solution and 0.05 g of [Co(NH₃)₅Cl]Cl₂ in 50 mL solution). The wavelength of CoCl₂ · 6(H₂O) showed a peak at a wavelength of 511 nm, and [Co(NH₃)₅Cl]Cl₂ showed two peaks at wavelengths of 363 nm and 530 nm. The difference in their spectra proved the oxidation from Co²⁺ to Co³⁺.

The synthesis of trans-Dichlorobis(ethylenediamine)cobalt(III) chloride, [CoCl₂(en)₂]Cl (en = ethylenediamine), was performed twice in order to produce a larger amount. For the first time, the process in the procedure was performed, and for the second time, the amounts were doubled in the second process. The first synthesis through the regular process resulted in a small amount of pure green compound; however, the second process with doubled procedure resulted in a dark green compound with some yellowish-orange impurities. 

Pentaammineaquacobalt(III) chloride, [Co(NH₃)₅(H₂O)]Cl₂, was filtered poorly, due to the relatively high solubility of the compound. Since the solution was too dilute, the solution in a beaker was put in the vacuum oven, at at temperature of 50 degrees celsius. While evaporating water from the solution, the temperature was mistakenly set up for high temperature, so the compound was burned with purple color.

Conclusion

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The synthesis of pentaamminechlorocobalt(III) chloride, hexaamminecobalt(III) chloride, trans-dichlorobis(ethylenediamine)cobalt(III) chloride, and pentaammineaquacobalt(III) chloride demonstrates the versatility and importance of inorganic synthesis in manipulating the properties of cobalt complexes. These compounds serve as prime examples of how controlled synthesis techniques can yield diverse coordination geometries and ligand environments, resulting in materials with distinct chemical and physical properties.

The synthesis of these cobalt complexes showcases the significance of inorganic synthesis in tailoring the properties and structures of coordination compounds. By carefully selecting the reactants, stoichiometry, and reaction conditions, researchers can manipulate the ligand environment and coordination geometry around the cobalt ion, resulting in compounds with distinct properties. These cobalt complexes find applications in various fields, including catalysis, magnetism, and biological sciences.

In conclusion, the synthesis of pentaamminechlorocobalt(III) chloride, hexaamminecobalt(III) chloride, trans-dichlorobis(ethylenediamine)cobalt(III) chloride, and pentaammineaquacobalt(III) chloride exemplifies the power of inorganic synthesis in designing cobalt complexes with diverse structures and properties. The ability to control the ligand environment and coordination geometry enables researchers to explore the fundamental aspects of coordination chemistry and opens avenues for innovative applications in different scientific disciplines.

Works Cited

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Experiment 6 preparation of an inorganic cobalt complex: Co(NH3 NCL3. (n.d.).
https://web.williams.edu/wp-etc/chemistry/epeacock/EPL_CHEM_153/153-LABMAN_PDF_05/6-PrepCoboltCompl.pdf 

Synthesis, characteristics and analysis of Co(III) complexes of the. studylib.net. (n.d.).
https://studylib.net/doc/5876518/synthesis–characteristics-and-analysis-of-co-iii–comple. 

Synthesis of hexaammine cobalt (III) chloride. Share and Discover Knowledge on SlideShare. (n.d.).
https://www.slideshare.net/saimkhalid04/synthesis-of-hexaammine-cobalt-iii-chloride 

The synthesis of trans-dichlorobis(ethylenediamine)cobalt(iii) chloride. (n.d.-b).
https://shms-prod.s3.amazonaws.com/media/editor/145607/SYNTHESIS_of_trans-dichlorobis_ethylenediamine_cobalt_III_Chloride.pdf