Organometallic Chemistry Name:____Kim Jong Un___________ CHEM 534/334 Final Exam December 13, 2011 Prof. Malachowski 1H NMR chemical shifts CHE-334 questions CHE-534 questions Part I: 5 of 7 questions (110 pts.) Part II: 3 of 5 questions (45 pts.) Total exam: 8 of 12 for 155 pts. Part I: 6 of 7 questions (120 pts.) Part II: 3 of 5 questions (45 pts.) Total exam: 9 of 12 for 165 pts.2 Part I: Questions from Class Presentations CHE-334: Answer 5 of 7 questions. The first two are worth 25 points and the remaining questions are worth 20 points. CHE-534: Answer 6 of 7 questions. All questions are worth 20 points. 1. The proposed mechanism for the recent seminal work on C-C bond cleaving of an unstrained aromatic ring is shown below (Aaron Sattler et al. Nature2010, 463, 523-526). Answer 5 of the 6 parts of question 1. a) The equilibria between 2, 3 and 4 is a critical entry point to the ultimate goal of reaching complex 8. In analyzing these two equilibria, 2 to 3 and 2 to 4, what is the same about these reactions? What makes these processes different? Identify one unifying and one differentiating characteristic. (Do NOT make this more complicated than it is!) Both equilibria are C-H oxidative addition reactions, but the reactions occur with a different C-H bond of the aromatic (quinoxaline) ring (C-3 C-H in 4; C-5 C-H in 3). b) Some complexes in the mechanism above have a 2 designation while others have a 2. What does 2 and 2 describe? 2 is a denticity designation and 2 deals with hapticity. Denticity describes the number of atoms from a particular ligand that are coordinated to a metal. These are not consecutive atoms, i.e. atoms that are also bonded to each other. Hapticity deals with the number of consecutive atoms from one ligand that are bonded to a metal. c) The W reagent, 1, could also exist as the trigonal bipyramidal structure on the left below. Provide a reason that the structure on the right is favored.3 PMe3WPMe3PMe3Me3PPMe3WPMe3CH2PMe3Me3PMe3PMe3PH1 The structure on the left is a d6 W with five ligands for a total electron count of 16. The structure on the right is a d4 W with seven ligands for a total electron count of 18. The higher electron count and one that matches the “18 electron rule” probably make the structure on the right more stable or favorable. For more information see: Hexakis(trimethylphosphine)tungsten(0): synthesis, structure, and reactivity, Daniel Rabinovich, Gerard Parkin, J. Am. Chem. Soc., 1990, 112 (13), pp 5381–5383 As reported in the article, the key novel reaction, the C-C bond cleaving step, occurs with the conversion of 6 to 8. d) How did the authors prove experimentally that the transformation of 6 to 7 was important for the formation of 8? The authors ran the reaction under an atmosphere of H2 and that caused a decrease in the yield of 8. This proved that the release of H2 in the reductive elimination was a critical step in the formation of 8. e) Why might the process to form 7 be important before the W performs the critical C-C bond cleavage to form 8? The transformation of 6 to 7 is a reductive elimination and it probably provides the W metal with the necessary electron density to backbond into the C-C bond and eventually cleave it. f) The authors reported the following experimental information for 8. 13C NMR IR, CN stretch complex 8 211.4 ppm 1709-1698 cm-1 normal range for M-CN-R (isocyanide): 151.8-238.7 ppm 2310-1670 cm-1 What does this say about the bonding relationship between the W-C? Briefly explain. The relatively high NMR chemical shift and the lower IR stretch suggest that there is considerable backbonding occurring between the W and the C. With more electron donation from the W, the C has a lower bond order with the N and this limits the amount of energy in the C=N bond, thereby lowering the stretching frequency in IR. Likewise a lower C=N bond order limits the shielding of the C from the electrons in N. 2. The article entitled, “A Predictably Selective Aliphatic C–H Oxidation Reaction for Complex Molecule Synthesis” by M. Christina White et al. (Science2007, 318(5851), 783-7) described oxidation reactions with the complex shown below.4 a) What is the oxidation state of the Fe? Fe+2 b) Identify the X and L type ligands on the complex. All ligands are L type, including the pyridine N‟s, the pyrrolidine N‟s and the acetonitrile N‟s. c) What is the total electron count of the Fe in 4? 18 e-„s (d6 + 6 x 2 e- ligands) d) What is the coordination number and molecular geometry of the Fe in 4? 6, octahedral. e) Draw the electronic orbital diagram for the Fe d orbitals. Include the correct number of electrons based on your answer to part a of this question. Label the d and d orbital groups. egor dt2gor dFeL62+E f) Although the mechanism of the oxidation processes catalyzed by 4 was unknown, if the substrate and/or oxidant did interact directly with the iron, i.e. inner sphere reactions, what would be the mechanism of ligand substitution and what ligand(s) would be most likely to substitute? Why would this ligand be most likely to leave? Since 4 is an 18 e- complex, it would most likely react by a dissociative mechanism. The acetonitrile ligands (CH3CN) would be the ligand most likely to leave. Since all the other N ligands are part of a tetradentate ligand, they would not be easily replaced since the chelate effect would keep them bound to the iron. 3. The authors of the recent article entitled, “A Synthetic Model of the Mn3Ca Subsite of the Oxygen-Evolving Complex in Photosystem II” (Theodor Agapie et al. Science2011, 333(6043), 733-736), detailed the synthesis of the metal cluster complex 4. Answer 5 of the 6 parts in question 3.5 a) The presenters of the article in class showed a method for calculating the approximate formal charge on each Mn in the cluster. Determine the charge by showing that calculation. X-type ligand analysis: four oxo (-2) ligands = -8 three alkoxy (-1) ligands = -3 three acetate (-1) ligands = -3 total negative charges = -14 Ca is +2, so there is a charge of -12 remaining for the three Mn atoms. Therefore each Mn must have an approximate formal charge of +4. b) What analytical tool did the researchers use to determine the oxidation state of the Mn atoms in the metal cluster? How did they use this tool to determine the oxidation state? The researchers used X-ray