Spectroscopic Studies of a Caged Cobalt Complex

1
Spectroscopic Studies of a Caged Cobalt Complex

• Emma Morrison
• Advanced Inorganic Laboratory

2

• The purpose of this experiment was to synthesize
  a caged cobalt(III) complex and compare the
  electronic and nuclear magnetic resonance spectra
  against those of the un-caged template complex.
• Complex of interest cobalt(III) sepulchrate
  sepulchrate sep 1,3,6,8,10,13,16,19-octaazabic
  yclo6.6.6eicosane

3
Introduction

• Cryptates are caged complexes in which the
  central transition metal ion is coordinated
  inside of a macrobicyclic ligand system
• Ligand system is highly stable
• Reducing a cobalt(III) cryptate and then
  reoxidizing the cobalt results in unchanged
  chirality and ligation, suggesting that the cage
  remains completely formed during the process6
• Supports outer-sphere electron transfer mechanism
• Negligible ligand substitution over extended time
  periods in the typically labile Co(II) reduced
  state, as shown through 60Co isotopic labeling
  experiments4
• The high stability of the cryptates suggests
  their application as inert oxidizing and reducing
  agents3
• The cryptates are formed through the
  polymerization reaction of formaldehyde and a
  chosen molecule as caps for the three
  ethylenediamine ligands

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Methods--Syntheses

• Synthesis of tris(ethylenediamine) cobalt(III)
  chloride1
• Mix CoCl26H2O with a four times equivalent of
  ethylene dihydrochloride salt in an aqueous
  solution
• Raise the pH and add a dilute solution of
  hydrogen peroxide in order to promote ligand
  substitution
• Isolate product using suction filtration
• Follow by color changes pink--gtorange--gtyellow-or
  ange needles
• Synthesis of cobalt(III) sepulchrate
  diethyldithiocarbamate2,4
• Create aqueous suspension of Co(en)3Cl3 and
  Li2CO3, which acts as a base
• Simultaneously, add separate dilute aqueous
  solutions of formaldehyde and ammonia dropwise
• Three formaldehyde molecules react with three
  nitrogens of the ethylenediamine ligands and are
  capped by the ammonia (see figure 1)
• Precipitate the cryptate out by adding an aqueous
  solution of sodium diethyldithiocarbamate
• Exploit that cobalt dithiocarbamate salts are
  insoluble in water to avoid need of column
  chromatographic separation2
• Isolated product is bright red powder

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• Conversion to cobalt(III) sepulchrate chloride
• Suspend dithiocarbamate salt in acetonitrile
• Add concentrated HCl until all solid dissolves to
  form an orange solution
• Concentrate by heating, cool to crystallize out
  trichloride salt
• Methods--Spectroscopy
• UV/Vis Spectroscopy for analysis of electronic
  spectra
• Instrument Hewlett Packard 8453 UV/Vis
  spectrometer
• Samples Co(en)3Cl3 in dH2O Co(sep)Cl3 in
  dH2O
• Blank and scan over range of 250nm-800nm
• 1H NMR Spectroscopy for structural analysis
• Instrument 200 MHz Varian NMR Spectrometer
• Samples Co(en)3Cl3 in D2O Co(sep)S2CNEt23
  in C6D6 with small amount of (CD3)2CO to increase
  solubility Co(sep)Cl3 in D2O

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Figure 1. Mechanism of cage formation using
aqueous solutions of formaldehyde and ammonia6.
Co(en)33
2
Co(sep)3
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Figure 3. Electronic Spectrum of Co(sep)Cl3
Results
Figure 2. Electronic Spectrum of Co(en)3Cl3
(Literature values for maximum absorption are
338nm and 466nm6)
3
3.62eV
2.67eV
343nm
464nm
(Literature values for maximum absorption are
340nm and 472nm4)
3
3.63eV
2.61eV
342nm
475nm
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Figure 4. 1H NMR Spectrum of Co(en)3 Cl3
3
Methylene protons (12 1H)
Protons bonded to nitrogen--broadened due to
exchange with D2O solvent
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Figure 5. 1H NMR Spectrum of Co(sep)S2CNEt23
Acetone (different degrees of deuteration)
CH2 of S2CNEt2 (quartet)
Cage 1Hs (overlapping)
3
Benzene--solvent peak
H2O
CH3 of S2CNEt2
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Figure 6. 1H NMR Spectrum of Co(sep)Cl3
methylene protons of en
Methylene protons of caps
H2O--solvent peak
acetone
3
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Figure 7. Compare with 1H NMR spectrum from
literature4 Chemical shift axis is shifted
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Discussion

• Electronic Transitions
• The electronic transition energies of the caged
  complex are only slightly shifted
• The lower energy transition in further shifted
  towards lower energies
• The higher energy peak of the cryptate is more of
  a shoulder, suggesting the possibility of
  metal-to-ligand charge transfers
• However, the sensitivity below 300nm is decreased
• Without the introduction of a conjugated system
  within the ligand or a change in the identity of
  the atoms bound directly to the metal center, the
  d-d transition energies should not experience a
  significant change
• Since the colbat is coordinated directly to 6
  nitrogens in both the caged and uncaged complex
  and the structure of the ligands is similar, the
  d-d electronic transitions, which are the
  observed transitions, are not altered
  significantly
• If a spectrum had been recorded for the
  dithiocarbamate salt of the sepulchrate, the d-d
  transitions would have been largely hidden by the
  charge transfer within the diethyldithiocarbamate
  anion2

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• Structural analysis using 1H NMR
• 1H NMR spectrum of Co(en)3Cl3
• It is likely that the scale of the chemical shift
  is not centered correctly
• The 12 methylene protons of the ethylnediamine
  ligands are chemically equivalent --gt produce the
  sharp peak that should be located closer to
  3.5ppm
• The 12 amine protons give a broad peak due to the
  hydrogen bonding with the solvent, which
  increases the chemical shift range
• 1H NMR spectrum of Co(sep)S2CNEt23
• The peaks of interest are weak compared to the
  solvent peaks and noise level because the
  solubility in benzene is very low (literature
  suggests high solubility in solvents such as
  chloroform2)
• The ethyl groups of the ditiocarbamate anion give
  a quartet (CH2 protons split by CH3 protons) and
  a triplet (CH3 protons split by CH2 protons)
• Indistinguishable complex multiplet from the
  cryptate ligands
• The doublet of doublets of the cap methylene
  protons in only resolved as a doublet at 3.6ppm
• The AABB splitting pattern is unresolved as a
  multiplet at 2.6ppm

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• 1H NMR spectrum of Co(sep)Cl3
• The scale of the chemical shift axis is not
  centered correctly
• The doublet of doublets corresponds to the 12
  methylene protons of the caps and should be
  centered at 4ppm4
• The 12 methylene protons of the ethylenediamine
  ligands has a more complex AABB splitting
  pattern that is not resolved well and should be
  centered at 3.2ppm4
• The cobalt(III) complexes will become
  N-deuterated in the NMR sample tube because the
  hydrogen bonding with the D2O causes proton
  exchange
• The amine protons were only seen in the
  Co(en)3Cl3 spectrum because this spectrum was
  recorded the immediately after dissolving the
  compound and because this compound was at a much
  higher concentration, making the exchange time
  longer

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Future Directions

• Record proton decoupled 13C NMR spectra to see
  how the symmetry and equivalent carbons might
  change with the caged complex
• Reduce the Co(III) center to Co(II) using zinc
  dust3,4.
• Compare the electronic and 1H NMR spectra of the
  Co(III) and Co(II) sepulchrates (note that Co(II)
  is paramagnetic and will cause line broadening)
• Carry out kinetic study of the oxidation of
  Co(II) to Co(III) in the presence of an oxygen
  atmosphere using UV/Vis spectroscopy to confirm
  that the rate law is second order5
• Carry out the syntheses and spectroscopic
  analyses of other cobalt cryptates and
  subsequently compare the structure and stabilities

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Conclusion

• The electronic spectrum is not changed
  significantly upon the transformation of the
  template Co(en)3Cl3 into the caged complex
  Co(sep)Cl3
• d-d transitions are not altered since the
  identity of the bound atoms is not altered
• Caging does not affect electronic transitions
• The chemical equivalence of the ligand protons is
  broken when the complex is transformed into a
  caged complex due to the different environments
  of the cap and ethylenediamine methylene protons
• Still high symmetry (D3), but methylene protons
  of caps are more shielded than methylene protons
  of ethylenediamine
• Complex splitting patterns arise when the protons
  are no longer chemically equivalent

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References

• 1Angelici RJ, Girolami GS, Rauchfuss TB.
  Synthesis and Technique in Inorganic Chemistry A
  Laboratory Manual, 3rd Ed.
• 2Gahan, Lawrence R. Healy, Peter C. Patch,
  Graeme J. Synthesis of cobalt(III) cage
  complexes A twist on an old theme in the
  inorganic laboratory. J. Chem. Edu. 1989, 66,
  445.
• 3Creaser II, Harrowfield J MacB, Herlt AJ,
  Sargeson AM, Springborg J, Geue RJ, Snow MR.
  Sepulchrate a macrobicyclic nitrogen cage for
  metal ions. J. Am. Chem. Soc. 1977, 99,
  3181-3182.
• 4Creaser II, Geue RJ, Harrowfield J MacB, Herlt
  AJ, Sargeson AM, Snow MR, Springborg J.
  Synthesis and reactivity of aza-capped
  encapsulated Co(III) ions. J. Am. Chem. Soc.
  1982, 104, 6016-6025.
• 5Bakac A, Espenson JH, Creaser II, Sargeson A.
  Kinetics of the superoxide radical oxidation of
  cobalt sepulchrate(2). A flash photolytic
  study. J. Am. Chem. Soc. 1983, 105, 7624-7628.
• 6Harrowfield J MacB, Lawrance GA, Sargeson AM.
  Facile synthesis of a macrobicyclic hexaamine
  cobalt(III) complex based on tris(ethylenediamine)
  cobalt(III). J. Chem. Educ. 1985, 62, 804-806.