Chemical Kinetics in Amine Containing Monodentate and Bidentate Cobalt Ligands

Andrew McTammany

CoIIINH3H3N

H3N NH3

Cl

Cl

CoIIIH2OH3N

H3N NH3

Cl

NH3

CoIII

Cl

Cl

NH2

NH2

H2N

H2N

CoIIIH2O

ClNH2

NH2

H2N

H2N

+ H2O + Cl-

+ H2O + Cl-

Background

Dichloride Cobalt (III) compounds can be arranged in either a cis or trans configuration. The isomerization reaction can be studied between the two isomers.

By varying the ligands attached to the central cobalt atom, the changes in the isomerization kinetics can be elucidated.

Background

Temperature dependent NMR spectroscopy can be used to monitor the kinetic parameters of the aquation reaction between the trans and cis isomers of both compounds. Using line widths, absorption can be determined.

trans-Co(C2H8N2)2Cl2 + H2O cis-[Co(C2H8N2)2 (H2O)Cl]Cl2

trans-Co(NH3)4Cl2 + H2O cis-[Co(NH3)4(H2O)Cl]Cl2

Background

By varying the temperatures at which the kinetic data is obtained, the following equation can be used to determine the enthalpy and entropy of activation.

Ln( ) An Eryling plot of ln(k/T) versus 1/T yields this

formula.

Background

Elucidating the thermodynamic properties of the two cobalt compounds, molecular orbital theory can be employed to account for relative stabilities of these compounds.

Method

Trans dichloro-bis-ethylenediamine cobalt (III) was synthesized from CoCl2*6H2O. It was mixed 10% ethylenediamine before being heated in an evaporation dish to before 12M HCl was added to form the desired green crystals.

CoCl2*6H2O + 2C2H8N2 + HCl trans-[Co(C2H8N2)2Cl2 ]Cl

Method

[Co(NH3)4CO3]NO3 is first produced as an intermediate by adding (NH4)2 CO3 to Cobalt (II) Nitrate hexahydrate in concentrated ammonia. 30% hydrogen peroxide was then added slowly to the precipitate.

NH3, (NH4)2 CO3 , H2O2

Co(NO3)2*6H2O [Co(NH3)4CO3]NO3

Method

Concentrated HCl was then added to [Co(NH3)4CO3]NO3 to produce the trans crystals in a temperature dependent reaction.

[Co(NH3)4CO3]NO3 + 3 HCl trans-[Co(NH3)4Cl2]Cl + CO2 +HNO3

Results

The formation of trans-[Co(C2H8N2)2Cl2 ]Cl was successful, producing 72.4%, but attempts to take temperature dependent NMR spectra were less fruitful. Spectroscopic data from UV/Vis analysis provided a rate constant at room temperature for evaluation.

+ H2O + Cl-

CoIII

Cl

Cl

NH2

NH2

H2N

H2N

CoIIIH2O

ClNH2

NH2

H2N

H2N

Results

The reaction demonstrated first order kinetics, with a rate constant of 3.6*10 -5 ± .4*10-5 sec-1

Results

Only the cis-Co(NH3)4Cl2 was produced in 45.3% yield, and not the trans product while following the procedure in volume 31 of Inorganic Syntheses by Erdman.

Results

A second trial was performed, altering the procedure and conducting the experiment in acetone instead of water. This would lower the reaction temperature and eliminate the possibility of the aquation reaction. A blue product was produced and characterized using NMR spectroscopy.

Discussion

The rate found was not the true isomerization, but racemization, as substantial quantity of the initial trans isomer was still in solution, shown by the final UV/Vis spectrum. The value of 3.6*10 -5 ± .4*10-5 sec-1 compares somewhat comparably with the literature value of 4.9*10 -4 sec-1.

Discussion

The reasoning behind the failed formation of trans-Co(NH3)4Cl2 has to do with the formation of the intermediate, [Co(NH3)4CO3]NO3. This compound is in a cis conformation as shown in the diagram.

CoIIIOH3N

H3N NH3

O

NH3

C

O

Discussion

Amines form particularly strong bonds with Cobalt (III), d6, compounds. A molecular orbital diagram demonstrates this. It only occupies the t2g orbital, whereas a Co(II) compound would have unpaired electrons.

t1u* __ __ __

a 1g* __

eg* __ __

t2g __ __ __

Discussion

Since Co(NH3)4CO31+ is cis, the presence of heat

and excess chloride ions prevented the displacement of an amino group for a Cl, instead just displacing the CO3. This would result in the cis conformation.

+HCl CoIIIOH3N

H3N NH3

O

NH3

C

O

CoIIIClH3N

H3N NH3

Cl

NH3

Discussion

The trans was formed when the experiment was conducted in acetone with diluted HCl. However, after prolonged heating, the compound isomerized into the cis conformation.

+ HCl + heat CoIII

OH3N

H3N NH3

O

NH3

C

O

CoIIINH3H3N

H3N NH3

Cl

Cl

CoIIIClH3N

H3N NH3

Cl

NH3

Conclusion

To elucidate the kinetic and thermodynamic quantities, the experiment needs to be repeated. The trans-Co(NH3)4Cl2 should be synthesized using a lower temperature and lower concentration of acid. A mixture of HCl and H2SO4 could be used instead. From there the effect of different ligands can be evaluated.

Conclusion

The experiment did demonstrate the stability of Cobalt (III) compounds with amine ligands. It is interesting to note that the trans-[Co(C2H8N2)2Cl2 ]Cl was readily formed. This suggests that the energy of activation into the cis conformation is higher, since it was not as easily produced experimentally.

References• Orvis, Jeffery A. Journal of Chemical Education. 2003; 80, 803-805• Angelici, R.J. Synthesis and Techniques in Inorganic Chemisty; Saunders: Philadelphia, 1969; pp 25-30.• Borer, L. L. ; Erdman, H.W. In Inorganic Syntheses; Cowley, A.H., Ed; Wiley: New York, 1997; Vol 31, pp

270-271.• Bailar J.C. In Inorganic Syntheses; Wiley: New York, 1946 Vol 2 pp 222-225.• Shriver, Atkins, Langford. Inorganic chemistry; Oxford University Press: Oxford, 1994, pp 242-243• Holleman-Wiberg. Inorganic chemistry; Academic Press: San Diego , CA, 2001; p 1484• Sargeson, AM. Aust. J. Chem., 1963, 16, 352-5

• A special thanks to Wednesday’s Inorganic Chemistry Lab Section