Question 1 refers to the virtual experiment A model cobalt complex for oxygen uptake (delivered via the OpenScience Laboratory and found in Study Week 10). As well as testing your understanding of this experiment and associated material, this question will prepare you for the experimental write-up that you will be required to complete for TMA05, which forms a significant part of the examinable component for this module.
(a) Prior to completing an experimental procedure it is necessary to carry out a risk assessment. Refer to Activity 1.3 in the experiment and the material safety data sheets and risk and safety phrases to answer the following questions in your own words:
(i) Determine the Exposure Risk category for Co(II) nitrate hexahydrate, explaining your reasoning.
(ii) Would you perform the synthesis of Co(salen) complex on the open bench or in a fume-cupboard? Explain your reasoning. What personal protection should be worn during the procedure?
(iii) The first step in the experiment to monitor the uptake of O2 by the Co(salen) complex is to react the complex with dimethyl sulfoxide (DMSO). Discuss how you should dispose of any waste DMSO and explain why you would not simply dispose of it down the sink?
(b) Write an account of the online experiment, A model cobalt complex for oxygen uptake, under the headings given below and including the information asked for under each heading.
With reference to this figure, describe the method used to measure the uptake of oxygen. You should write this in your own words and avoid simply repeating the instructions given. You do not need to include the diagram of the apparatus in your answer.
Figure 1 Apparatus to determine the uptake of O2 by the Co(salen) complex
Results and discussion - Copy and paste your table of data and a plot of oxygen uptake against time into your answer. Prepare this section for the experimental report using the following questions as a guide.
How long did it take for the complex to become saturated with oxygen?
Optional question: The following question has no marks associated with it as it is not part of the assessment, however, if you wish to engage further with this topic by answering the question, feedback will be supplied.
(c) The Resonance Raman spectra of normal and enriched 18O-labelled [(DMSO)(Salen)Co]2O2 are shown in Figure 2. Considering the two peaks indicated with an asterix, what does this technique suggest about the nature of the coordinated oxygen species?
Figure 2 Resonance Raman spectrum of [(DMSO)Co(salen)]2O2 (bottom spectrum) and enriched 18O-labelled [(DMSO)Co(salen)]2O2 (top spectrum) [Hester R.E., 1981]
This question carries 15 marks for this assignment and tests learning outcomes KU2, KU3, CS1, CS2, KS2, KS3 and PS2.
This question will help prepare you for a question that assesses your understanding of NMR in TMA05.
NMR data for the 1H NMR spectra of salen (1) in CDCl3 containing TMS (tetramethylsilane) are given in Table 1. You will find it useful referring to the numbering system shown in the salen structure (1) in your discussion.
(a) Considering the structure of salen (1), discuss whether the number of proton signals observed in the 1H NMR spectrum and their relative integrals are what you would expect for the salen ligand. (You do not need to discuss multiplicity in your answer.)
(b) What can you deduce from the fact that the peak in the 1H NMR spectrum at 13.20 ppm disappears on shaking the solution with D2O?
(c) Assign the peaks in the 1H NMR spectrum for salen, noting that it is not possible to assign the aromatic signals unequivocally. You should, however, comment on how the multiplicity of these peaks can enable you to suggest two possibilities each for their assignment.
(d) Tenting is observed for the signals at 7.16 and 6.72 ppm. What does this suggest in terms of the relative arrangement of the protons from which these signals arise?