Application of Organometallic Chemistry – Breaking the Inert C-H Bond Literature Special Topic Date Created by [John Lee, University of Tennessee Chattanooga, John-Lee@utc.edu] and posted on VIPEr (www.ionicviper.org) on [July 16, 2012], Copyright [July, 16, 2012]. This work is licensed under the Creative Commons [Attribution-NonCommercial-ShareAlike 3.0 Unported] License. To view a copy of this license visit http://creativecommons.org/about/license/

You want to break which bond! Example Organic Molecules -Why do we draw line structures for Organic molecules? -High bond energy methane 100 kcal/mol benzene 110 kcal/mol -High pKa on the order of >40 This slide uses line structures vs. full organic structure to remind the student that we normally do not draw the C-H bond out of convenience, but that convenience is because the C-H bond is unreactive (in most cases). An astute student could ask “But hexane has three different C-H bonds?” The answer is beyond the scope but you could direct them to studies that have shown H/D exchange reactions show all bonds are accesiblie (kinetically) but the final thermodynamic product is the stronger methyl C-H bond. **,but…Nature uses the C-H bond: See methane monooxygenase and vitamin-b12

In organic chemistry…. Generic Substitution Reaction (SN1 shown) A very brief (possibly too brief?) reminder that in organic chemistry the reaction usually involves some sort of reactive functional group. Leaving Groups (LG) TsO-, NH3 > I-, H2O > Br- > Cl- > HO-, NH2-, RO- Carey, F. A. Organic Chemistry, 4th ed. 2001, McGraw-Hill, Chapter 8.

In organometallic chemistry…. Cross-Coupling Reactions This slide could be removed, and is very busy if used. To remind students of the Pd cross coupling reactions that were recently awarded a Nobel Prize. Heck, R. F.; Negishi, E.; Suzuki, A. palladium catalyzed cross couplings in Organic Synthesis: Great Art in a Test Tube. http://nobelprize.orglnobeljrizes/chemistry/laureatesl}O10/press.html (March 17, 2011). but wait I said C-H not C-X activation?

If it ain’t broke why fix it? -Nearly everything comes from coal, oil or natural gas Coal: 1,000 – 2,000 years Oil: 50 – 150 years Natural gas: 207 – 590 years -However this is not sustainable for population growth Why? -We are a good 10-20 (optimistic) years from solar -For the interim time we should use our resources to the best of our ability How? The why are we interested in this slide. I took a sustainable future approach. Lewis, N. S.; Nocera, D. G. Proc. Nat. Acad. Sci. 2006, 103, 15729-15735.

How? Rationale design and synthesis Metal + Ligands C-donor, N-donor, O-donor, P-donor, or S-donor etc… Most complexes that do C-H activation must by synthesized. This is a fun slide using a blender instead of the round bottom flask. It ends with, remember it is about how the ligands affect the d orbitals. What it is really all about! Shriver and Atkins Inorganic Chemistry, 5th ed. 2001, Freeman, Chapter 1.

Some typical mechanisms Spend some time here going through each one. I tell the students that we will skip the last two. As a side note the figure on this slide is a figure I generated and did not come from the text of the paper. If you would like the chemdraw in order to make edits please email me and I will send the file. Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507-514.

Example 1: Oxidative addition Qualitative MO Diagram -The metal’s oxidation state increases by two units Must be accessible -Requires an electron rich metal Low oxidation state Electron donating ligands -The metal’s coordination # increases by two -Many d8 metals meet these criteria: Primarily RhI and IrI Sometimes PdII and PtII Both words and a very qualitative MO diagram (only showing the C-H – M interaction) is shown. Highlight the pi interaction.

Example of oxidative addition: Alkane dehydrogenation An example of oxidative addition in catalysis is given. This is taken from the Nature paper; however, see references from A. Goldman for further information. I used propane as a theoretical example.

Example 2: s-Bond metathesis Qualitative MO Diagram -Early transition metals that do not have a n+2 oxidation state available -Late transition metals that do not have an accessible n+2 oxidation state -Many d0 metals meet these criteria: For example, ScIII, ZrIV, TaV Both words and a very qualitative MO diagram (only showing the C-H – M interaction) is shown. Highlight there is no pi interaction because there are no d electrons.

Example of s-bond metathesis: Hydromethylation of propylene An example of sigma bond metathsis in catalysis is given. This is taken from above cited paper.. Sadow, A. D.; Tilley, T. D. J. Am. Chem. Soc. 2003, 125, 7971-7977.

Example #3: Electrophilic substitution Qualitative MO Diagram A very qualitative MO diagram (only showing the C-H – M interaction) is shown. Highlight that this is usually involves later transition metals where the d orbitals have dropped in energy. A chance to quiz on periodic trends, and an example of a crystal structure is given for a more advanced topic Milstein, D. et al. J. Am. Chem. Soc. 1998, 120, 12539-12544

Example of electrophilic substitution: Methane oxidation An example of electrophilic mechanism in catalysis. See the referenced paper for more information Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T; Fuji, H. Science 1998, 280, 560-564.

C-H Functionalization: A challenge C-C Bond Formation – Olefin Insertion This last slide is a deep thought question. Many C-C bond forming reactions in organometalic chemistry involve olefin insertion; however, olefin insertion prefers somewhat electron deficient metals (but not so much to do polymerization) and oxidative addition prefers electron rich metals. Foley, N. A.; Lee, J. P.; Ke, Z.; Gunnoe, T. B.; Cundari, T.. R. Acc Chem. Res. 2009, 42, 585-597.