Inorganic Chemistry I (June 2005) NMR Methods in Inorganic Chemistry. Answer parts (a) AND (b) AND EITHER part (c) OR part (d).

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1 Inorganic Chemistry I (June 2005) Question 1 NMR Methods in Inorganic Chemistry Answer parts (a) AND (b) AND EITER part (c) OR part (d). (a) Answer ALL parts of this question. i) Show how the sensitivity of a particular nucleus in NMR is related to the natural abundance and the magnetogyric ratio of the nucleus. (2 marks) ii) Discuss the difficulties associated with recording NMR spectra of the following nuclei, compared with 1 NMR. Nucleus Spin number, I Natural Abundance / % Magnetogyric ratio (γ) / 10 7 rad.t -1.s / N 1/ Al 5/ S 3/ (6 marks) (b) Using the Popple notation, describe the spin systems displayed by the highlighted nuclei (in bold and underscored) in the following compounds. In one of your answers, identify nuclei that are chemically equivalent but magnetically inequivalent. PPh 3 Cl Rh PPh3 PPh 3 (iii) CMe 3 (9 marks) Page 1

2 c) The 19 (top) and 19 -{ 11 B} (bottom) NMR spectra of the anion [C 3 C 2 B 3 ] are shown in the figure below. The chemical shifts and coupling constants for [C a 3C b 2B c 3] are as follows: a a a b c B c c b _ δ( a ) = ppm δ( b ) = ppm δ( c ) = ppm 3 J a-b = 0.7 z 4 J a-c = 4.9 z 3 J b-c = 1.5 z 1 J c-b = 40.9 z 1 J b-b = 19.6 z Explain the appearance of the spectra by redrawing the peaks and label the couplings ( 19, I = 1/2, 100% natural abundance; 11 B, I = 3/2, % natural abundance, no coupling to other nucluei are observed). Note that in this particular case the 3 J - coupling constants are much smaller than the 4 J - coupling constant. (8 marks) d) Suggest a possible geometry for the Xe 5 + cation, based on the appearance of its 19 NMR spectrum given below. Give a full assignment, redraw the peaks and label the coupling constants. ( 19, I = ½, 100% natural abundance; 129 Xe, I = ½, 26.44% natural abundance) (8 marks) Page 2

3 Question 2 Introduction to Organometallic Chemistry Answer parts (a) and (b) AND EITER part (c) OR part (d). (a) With the aid of simple energy level diagrams, explain the basis of the 18-electron rule for compounds containing π-acidic ligands (include in your answer reference to σ-bonding ligands only). (9 marks) (b) Which of the following compounds obey the 18-electron rule? Classify each according to the MLXZ formalism. Assign the formal oxidation state and give the d electron count of the metal in each case. i) [Zn(N 3 ) 6 ] 2+ ii) ebr 2 (CO) 4 iii) IrCl(CO)(PPh 3 ) 2 iv) [Tc(CO) 4 Br] 2 (8 marks) (c) A mixture of e(co) 5 and Mn 2 (CO) 10 dissolved in hexane was irradiated for 45 minutes with ultraviolet light from a mercury lamp. After removal of the volatile liquids, the solid residue, A, was sublimed onto a cold finger as red crystals. A was found to have moderate solubility in organic solvents, and to absorb in the infrared at 2067, 2019 and 1987 cm -1. The mass spectrum of A showed a series of ions from m/e 166 to m/e 558; adjacent peaks in the series were separated by 28 mass units. Significant amounts of three other series of ions, beginning at m/e 55, 56 and 111 respectively, were observed: there was no series beginning at m/e 110. Deduce a structure for A, show that it conforms to the 18-electron rule, and relate it to the experimental observations. [The most abundant isotopes of manganese and iron are 55 Mn and 56 e, respectively] (8 marks) (d) Outline mechanisms for both of the following catalytic processes: i) Alkene hydrogenation using Wilkinson s catalyst ii) Methanol carbonylation using [Rh(CO) 2 I 2 ] (8 marks) Page 3

4 Question 3 Bioinorganic Chemistry Answer part (a) AND EITER part (b) OR part (c) (a) Answer ALL parts of this question Explain briefly the meaning of the terms primary, secondary and tertiary when applied to protein structures. (3 marks) What is the entatic state in the context of metalloprotein function? (3 marks) (iii) Describe the structure of the metal-binding sites in the blue copper proteins plastocyanin and azurin. ow is the redox behaviour of these proteins influenced by the copper coordination environment and the protein structure? (7 marks) (b) Answer ALL parts of this question (iii) Briefly describe or illustrate the coordination sphere of the iron centre in the haem ring of the deoxy form of aemoglobin and Myoglobin. State the oxidation state of the iron in this deoxy form and draw its d-electron configuration. (3 marks) Describe how dioxygen can bind to the iron centre in Myoglobin. Include in your answer a full description of what orbitals are used on the dioxygen and discuss any changes in the d-electron configuration of the iron centre. (6 marks) Brief discuss the allosteric (co-operative O 2 binding) effect in aemoglobin. (3 marks) (c) Answer ALL parts of this question Draw the balanced equation for the hydroxylation of an aliphatic organic molecule R- by cytochrome P450. (2 marks) Draw the full catalytic cycle for this process showing clearly the oxidation state and the coordination sphere of the metal centre in each step of the reaction. (10 marks) Page 4

5 Answers Question 1 NMR Methods in Inorganic Chemistry (a) Sensitivity = (γ N /γ ) 3 x natural abundance (2 marks) 15 N: low abindance and low magnetogyric ration leads to low sensitivity.(2 marks) 27 Al: line broadening due to quadrupole moment and lower sensitivity (2 marks) 33 S: line broadeing and much lower sensitivity (2 marks) (b) AA BB AX 2 ABB XX 3 marks each, 1 mark is awarded to correct identification of (in)equivalence in or (iii). (3 x 3 marks) (c) a B b c : C 3 4 J - resulting in a quartet. 3 J - too small to be observed clearly. (2 marks) : C 2 : 2 J B- resulting in 4 equal intensity lines, which reduces to a singlet in the 11 B decoupled spectrum. 3 J - too small to be observed clearly. (2 marks) : B 3 : 4 J - resulting in a quartet, 1 J B- resulting in 4 equal intensity quartets. (2 marks) 3 J - too small to be observed clearly in the coupled spectrum, but in the 11 B decoupled spectrum a quartet of triplets can be observed. (2 marks) (d) AX 4 spin system, square pyramidal structure (1 mark) - 1 axial fluorine ( a, quintet); and 4 equatorial fluorines ( b, doublet) (2 marks), and 129 Xe satellites (2 marks). 2 J( a e ), 1 J(Xe a ) and 1 J(Xe e ) couplings labelled correctly (3 marks). Question 2 Introduction to Organometallic Chemistry (a) With the aid of simple energy level diagrams, explain the basis of the 18-electron rule for compounds containing π-acidic ligands (include in your answer reference to σ-bonding only ligands). σ-bonding only energy level diagram (4 marks) energy level diagram for π-acidic ligands (5 marks) (9 marks total) (b) Which of the following compounds obey the 18-electron rule? Classify each according to the MLXZ formalism, and assign the formal oxidation state and give the d electron count of the metal in each case. Page 5

6 i) [Zn(N 3 ) 6 ] 2+, ML 2+ 6, Zn 2+, d 10 (2 marks) ii) ebr 2 (CO) 4, ML 4 X 2, e 2+, d 6 (2 marks) iii) IrCl(CO)(PPh 3 ) 2, ML 3 X, Ir 1+, d 8 (2 marks) iv) [Tc(CO) 4 Br] 2, [ML 4 X] 2, Tc 1+, d 6 (2 marks) (8 marks total) (c) A mixture of e(co) 5 and Mn 2 (CO) 10 dissolved in hexane was irradiated for 45 minutes with ultraviolet light from a mercury lamp. After removal of the volatile liquids, the solid residue, A, was sublimed onto a cold finger as red crystals. A was found to have moderate solubility in organic solvents, and to absorb in the infrared at 2067, 2019 and 1987 cm -1. The mass spectrum of A showed a series of ions from m/e 166 to m/e 558; adjacent peaks in the series were separated by 28 mass units. Significant amounts of three other series of ions, beginning at m/e 55, 56 and 111 respectively, were observed: there was no series beginning at m/e 110. Deduce a structure for A, show that it conforms to the 18-electron rule, and relate it to the experimental observations. [The most abundant isotopes of manganese and iron are 55 Mn and 56 e, respectively] Ans. i) irradiation with UV light leads to loss of CO ligands, ii) infrared spectroscopy indicates that all the carbonyl ligands are terminal, iii) solubility and volatility imply a neutral, rather than a charged, species, iv) mass spectral data indicate that emn species are present, but not Mn 2. v) A trinuclear complex which satisfies the eighteen electron rule, and the other experimental observations, is the linear species (CO) 5 Mn e(co) 4 Mn(CO) 5. (8 marks total) (d) Outline mechanisms for the following catalytic processes: iii) ydrogenation using Wilkinson s catalyst iv) Methanol carbonylation using [Rh(CO) 2 I 2 ] Ans: i) Catalytic cycle for hydrogenation (4 marks) ii) Catalytic cycle for methanol carbonylation (4 marks) (8 marks total) Question 3 BIOINORGANIC CEMISTY Primary structure - the sequence of covalent peptide bonds formed between the individual amino acids. (1 mark) Secondary structure - the local conformation over groups of adjacent amino acids. eg -bonded α- helices, β-plaited sheets. (1 mark) Page 6

7 Teriary structure - the overall 3D arrangement/folding of the protein polymer (1 mark) The specific 3D conformation of the metalloprotein places the metal in an environment such that it is poised to undertake its biological function. A clear manifestation of this effect is in metalloenzymatic catalysis in which the protein provides a coordination geometry for the metal centre that is closer in energy to the transition state of the catalysed reaction (figure of reaction profile). (3 marks) (iii) Description of plastocyanin structure - the Cu centre lies within a pocket formed by the protein chain and is bound by a cysteine, a methionine and two histidine residues (distorted tetrahedral or trigonal pyramidal geometry). The Cu-S(Met) bond is considerably elongated (igure) (2 marks). Description of azurin structure - The binding of the Cu centre resembles that of plastocyanin ie by a cysteine, a methionine and two histidine units but with an additional weak O donor provided by an adjacent glycinyl residue (figure). (2 marks). The donor ligands in both proteins are a compromise between those favoured by both Cu(I) and Cu(II). Electron transfer occurs via a transition state which is intermediate between Cu(I) and Cu(II) states and oxidation/reduction occurs with minimal disruption to the coordination environment and protein structure. The metal-free apoproteins also display very little structural change indicating that the protein and binding sites are preorganised prior to metal coordination ie good examples of entatic states. (3 marks) (b) Coordinates to 4 N in planar haem ring and one axial histidine N and e sits below the plane of the haem ring [1]. e oxidation state II [1]. igh spin (draw splitting) [1] O 2 binds in a bent mode [1] using a sigma interaction from its sigma* orbital (draw) [2]. Electron transfer occurs from the e to the O 2 sigma* weakening the O 2 bond [1] giving high spin e(iii) draw splitting [1] e moves into the plane of the haem ring since d(x2-y2) is now empty [1] (iii) b contains four subunits each with a haem centre and capable of binding O 2 [1]. Once one O 2 is bound subsequent O 2 binding is easier (graph?). [1] This is due to the e moving into the plane of the haem on O 2 binding which pulls the is group with it resulting in a conformational change in the protein. [1] (c) C + O e - C O + 2 O [2] Page 7

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