Spectroscopic Studies of a Caged Cobalt Complex

Emma Morrison

Advanced Inorganic Laboratory

•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-octaazabicyclo[6.6.6]eicosane

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

Methods--Syntheses• Synthesis of tris(ethylenediamine) cobalt(III) chloride1:

– Mix CoCl2•6H2O 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-->orange-->yellow-orange needles

• Synthesis of cobalt(III) sepulchrate diethyldithiocarbamate2,4:– Create aqueous suspension of [Co(en)3]Cl3 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

• 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)3]Cl3 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)3]Cl3 in D2O; [Co(sep)][S2CNEt2]3 in C6D6 with small amount of (CD3)2CO to increase solubility; [Co(sep)]Cl3 in D2O

Figure 1. Mechanism of cage formation using aqueous solutions of formaldehyde and ammonia6.

[Co(en)3]3+

[Co(sep)]3+

2

Figure 3. Electronic Spectrum of [Co(sep)]Cl3

Figure 2. Electronic Spectrum of [Co(en)3]Cl3

343nm 464nm

342nm 475nm

3+

3+

(Literature values for maximum absorption are 338nm and 466nm6)

(Literature values for maximum absorption are 340nm and 472nm4)

3.62eV

2.61eV

2.67eV

3.63eV

Results

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

Figure 5. 1H NMR Spectrum of [Co(sep)][S2CNEt2]3

3+

Benzene--solvent peak

CH2 of S2CNEt2

(quartet) Cage 1Hs (overlapping)

CH3 of S2CNEt2

H2O

Acetone (different degrees of deuteration)

Figure 6. 1H NMR Spectrum of [Co(sep)]Cl3

3+

H2O--solvent peak

Methylene protons of caps

methylene protons of en

acetone

Figure 7. Compare with 1H NMR spectrum from literature4: •Chemical shift axis is shifted

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

• Structural analysis using 1H NMR:– 1H NMR spectrum of [Co(en)3]Cl3

• 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 --> 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)][S2CNEt2]3

• 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 AA’BB’ splitting pattern is unresolved as a multiplet at ~2.6ppm

– 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 AA’BB’ 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)3]Cl3 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

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

Conclusion• The electronic spectrum is not changed significantly upon

the transformation of the template [Co(en)3]Cl3 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

References1Angelici 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.