Chemical Kinetics in Amine Containing Monodentate and Bidentate Cobalt Ligands Andrew McTammany

1
Chemical Kinetics in Amine Containing
Monodentate and Bidentate Cobalt LigandsAndrew
McTammany
H2O ? Cl- H2O ? Cl-
2
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.

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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)ClCl2 trans-Co(NH3)4Cl2 H2O?
cis-Co(NH3)4(H2O)ClCl2
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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.

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Background

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

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Method

• Trans dichloro-bis-ethylenediamine cobalt (III)
  was synthesized from CoCl26H2O. It was mixed
  10 ethylenediamine before being heated in an
  evaporation dish to before 12M HCl was added to
  form the desired green crystals.
• CoCl26H2O 2C2H8N2 HCl ? trans-Co(C2H8N2)2Cl2
  Cl

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Method

• Co(NH3)4CO3NO3 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)26H2O ? Co(NH3)4CO3NO3

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Method

• Concentrated HCl was then added to
  Co(NH3)4CO3NO3 to produce the trans crystals
  in a temperature dependent reaction.
• Co(NH3)4CO3NO3 3 HCl ? trans-Co(NH3)4Cl2Cl
  CO2 HNO3

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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-

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Results
The reaction demonstrated first order kinetics,
with a rate constant of 3.610 -5 .410-5 sec-1
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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.

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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.

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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
  3.610 -5 .410-5 sec-1 compares somewhat
  comparably with the literature value of 4.910 -4
  sec-1.

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Discussion

• The reasoning behind the failed formation of
  trans-Co(NH3)4Cl2 has to do with the formation of
  the intermediate, Co(NH3)4CO3NO3. This
  compound is in a cis conformation as shown in the
  diagram.

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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 __ __ __
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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 ?

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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 ?

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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.

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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.

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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 Wednesdays Inorganic
  Chemistry Lab Section