UNIVERSITY OF EAST ANGLIA School of Chemistry Main Series UG Examination 2016-17 ADVANCED TOPICS IN INORGANIC CHEMISTRY CHE-7301Y Time allowed: 2 hours. Answer THREE of the following FOUR questions. You are advised to spend an equal amount of time on each question. All questions carry an equal number of marks. Answer EACH question in a SEPARATE answer book. The breakdown of marks within each question is indicated by the percentage figure in brackets on the right. The following is provided: Periodic Table Do not remove this question paper from the examinations room. Notes are not permitted in this examination. Do not turn over until you are told to do so by the Invigilator. (CHE-7301Y) Module co-ordinator: Simon Lancaster (CHE) Copyright of the University of East Anglia Version 2
2 1. Answer ALL parts (a) to (d). An equimolar mixture of tris(pentafluorophenyl)borane, B(C 6 F 5 ) 3, and tris(tertbutyl)phosphine, P(C 4 H 9 ) 3, is heated at 60 ºC in toluene under an atmosphere of H 2 gas for 2 hours. Upon cooling the reaction mixture to room temperature, the reaction product precipitates out of the solution as a white, crystalline, solid. Halfway through this experiment, an aliquot of the reaction mixture is removed and analyzed by multinuclear NMR spectroscopy. Selected resonances and the nuclei concerned are given in the table below. Nuclei Chemical shift / ppm Multiplicity 11 B +63 Broad singlet 11 B 25.6 Doublet (J = 87 Hz) 31 P +63 Singlet 31 P +59.6 Doublet (J = 430 Hz) 1 H +5.1 Doublet (J = 430 Hz) 1 H +4.5 Singlet 1 H +3.6 Quartet (J = 87 Hz) (a) (i) Write down a definition of Lewis acid and Lewis base. [10%] (ii) Write down a balanced chemical equation describing the overall reaction and identify the crystalline product formed. [10%] (b) Using the data above and your answer to part (a), assign each resonance to the reactant or product species involved. Explain why the 1 H resonance at +3.6 ppm appears as a quartet. [40%] (c) If the same reaction is repeated in the presence of the imine shown below as a substrate, where the R-group represents a bulky organic group, then a catalytic reduction of the imine can occur. Draw this catalytic reaction cycle, clearly showing the reactants and products in each step. In particular, clearly show the mechanism for the reduction of the imine and show the structure of the product that is formed. [30%] N R question 1 continues / (CHE-7301Y) Version 2
3 /question 1 continued (d) If the bulky R-group on the imine is replaced with a less bulky substituent, such as a methyl group (CH 3 ), and no tris(tert-butyl)phosphine is added to the reaction mixture, would catalytic reduction using tris(pentafluorophenyl)borane and hydrogen still occur? Explain your answer fully. [10%] (CHE-7301Y) TURN OVER Version 2
4 2. Answer ALL parts (a) to (e). (i) (ii) (iii) (iv) Zr Me 2 Si Zr 1 2 Me 2 Si Zr Me 2 Si Zr 3 4 (a) Name the four types of polypropene microstructure (or tacticity) represented by the projections (i) (iv). [10%] (b) How can pre-catalysts 1 4 be activated to give propene polymerization catalysts? [20%] (c) Which activated pre-catalyst would give rise to which microstructure? [20%] (d) How would you determine the tacticity of a given polymer sample? [10%] (e) Take one of the stereoregular microstructures and the pre-catalyst you matched it with in part (c). Explain the origin of the stereoselectivity. [40%] (CHE-7301Y) Version 2
5 3. Answer BOTH parts. (a) You are provided with a cylinder of chlorine, samples of hafnium metal, pentamethylcyclopentadiene, potassium hydride and any solvents and equipment you require. Explain, illustrating every step with a balanced chemical equation, how you would prepare a catalyst for the dehydrocoupling of dimethylstannane to give the polystannane (Me 2 Sn) n. [60%] (b) Draw the catalytic cycle for the dehydrocoupling of dimethylstannane. Label each of the mechanistic steps. [40%] 4. Answer ALL parts (a) to (c). (a) An accurate description of the atomic radius of gold requires that relativistic effects are taken into account. What are relativistic effects? Describe the consequences of relativistic effects on: (i) the relative energies of electrons in the 5d and 6s orbitals of gold (ii) the atomic radius of gold compared to silver. [20%] (b) Show and discuss the geometric structures of the following complexes: (i) (Me 2 S)Au (ii) [Au(C 6 F 5 ) 2 ] (iii) [Au 4 ] (iv) Me 2 AuBr (v) [OAu 3 L 3 ] + (L = PPh 3 ) [30%] (c) When (Ph 3 P)Au is reacted with 1 molar equivalent of AgSbF 6 in the presence of 3-hexyne, a chloride-free ionic complex A results which possesses an Au:Sb ratio of 1:1. On the other hand, if (Ph 3 P)Au is reacted with 0.5 molar equivalent of AgSbF 6 in the absence of 3-hexyne, a product B results with an Au:Sb ratio of 2:1 and a P: ratio of 2:1. Give balanced equations for these two reactions, show the structures of the reaction products, and comment on the bonding in each case. [50%] END OF PAPER (CHE-7301Y) TURN OVER Version 2
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Feedback for CHE-7301Y 16-17 Overall the marks for the examination were very high. Average: 78.93 Standard deviation: 19.42 Q1 In general question 1 was done very well. Average: 87.02 Standard deviation: 10.07 Often marks were lost for minor errors such as equilibrium arrows written as irreversible, and curly arrows indicating electron movement in mechanisms drawn in the wrong direction - which is concerning for 4th Year masters level chemists. The mechanistic recall of most students was excellent, with the correct sequence of major reaction steps given. Too many students did not specifically assign chemical species to NMR assignments. Instead of B(C 6F 5) 3, preferring "reactant B" or "product H-P". If the meaning was clear the benefit of the doubt was given, but this level of pseudo-chemical nomenclature usage is very disappointing at this level. A very few students assigned the singlet for H 2 as the alkyl groups on the phosphine, despite having a chemical shift well clear of alkyl protons, but in general the multinuclear NMR assignment was done well. Q2 Average: 80.25 Standard deviation: 16.00 Parts (a) and (c) were essentially perfectly answered. A few students failed to identify a mechanism for alkylation and cation generating in part (b). A chloro cation or neutral dialkyl would not be an active catalyst. (d) was covered in the lecture course. A full answer discussed 13 C NMR analysis at the pentad level. (e) The answers to this part were remarkably good. There was sometimes confusion regarding the key stereoselective step, which is alkyl migration to a specific pro-chiral face of the coordinated propene. Q3 Average: 82.30 Standard deviation: 15.74 (a) Given the average, this question was clearly done very well. Recurring errors included selecting reagents (such as n-buli) which were not provided. The other cause for concern was the failure to distinguish between pentamethylcyclopentadienyl (the ligand) and pentamethylcyclopentadiene (the protonated precursor) and ensuing issues with equation balancing.
(b) This was essentially recall of the dehydrocoupling mechanism discussed in the lecture course. Most students correctly described a sigma-bond metathesis followed by alpha-hydride elimination mechanism. Full credit was given for two sigma-bond metathesis steps. The greatest case for concern was apparent confusion regarding the significance of a transition state and the use of reaction type labels. The label sigma-bond metathesis is best given to the curved arrow in a catalytic cycle indicating a reaction between two intermediates or (as in this case) to the four-membered transition state most students drew. That is, sigma-bond metathesis involves the formation of the transition state, it is not the process by which the transition state becomes the product or intermediate. So, given the catalytic scheme most students drew, the label should have been applied not to the arrows but to the transition state. Q4 The question was generally well answered, and no consistent problems were apparent. Average: 62.25 Standard deviation: 18.14