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There are many different binding modes of ligands to transition metal atoms. In past experiments with Co(II) and Co(III), you observed that a ligand such as Chloride or amine can coordinate in a monodentate fashion. You also observed that carbonate occupies two sites of a transition metal and is known as a bidentate ligand. In this experiment, you will observe the reaction of a ? -bond in an olefin with a molybdenum metal center to form a piano-stool shaped molecule.
The molybdenum metal retains its octahedral shape with three carbonyl’s serving as the “legs” of the stool and the mesitylene is the “seat” of the stool and is position on one face of the octahedral. This form of coordination is relatively common with olefins and metal atoms, the most famous being ferrocene, (cyclopentadiene)2Fe. It is composed of two 5-membered rings situated directly above and below the metal center. It looks like an iron sandwich. The reaction of molybdenum hexacarbonyl and mesitylene is unstable with respect to air.
Because inorganic chemists desire to work with compound that have some component that is air sensitive, they have devised a couple of ways to manipulate air-sensitive materials. We will focus our efforts on the use of a Schlenk line in this laboratory. A Schlenk line is a piece of equipment that is designed with the purpose of conducting experimental operations and techniques under an inert air environment. You will be shown how to use a Schlenk line during lab. Experimental Metal carbonyls can be very toxic and should be handled carefully and weighed in a hood.
Metal carbonyls composed of light metals, such as Ni(CO)4 can have high volatilities (43 °C). Fortunately, we are working with Mo(CO)6, which has a higher sublimation point. In addition, the reaction will produce a bit of CO(g) which, at high levels can be fatal. Since your reaction vessels will be vented into a hood, you will not need to be concerned about asphyxiation. Week 1 – synthesis and Schlenk line techniques. (1,3,5-C6H3(CH3)3)Mo(CO)3, Mesitylene Tricarbonylmolybdenum(0) I Place 1 g of Mo(CO)6 and 10 mL mesitylene in a small round bottom flask equipped with a stirbar.
Place a condenser on the r. b. flask and a gas adapter on the top of the condenser. (Mo(CO)6 will sublime and stick to the walls of the condenser. It can be washed back to the round bottom flask by the mesitylene if the condenser is not cooled by water – the temperature of the air around the condenser will be sufficient to condense the mesitylene. ) Flush the solution with N2 from the Schlenk line for a couple of seconds then close the joints and boil the solution under a N2 atmosphere for 30 minutes. Turn off the heat and let the solution cool.
When the solution has cooled dismantle the apparatus and add ~15 mL of hexanes. A yellow precipitate should appear. Filter the precipitate over a medium frit and wash it with 5 mL of hexanes. The solid should have yellow and black elements to it. The yellow is the product and the black is Mo metal. Dissolve the yellow powder in a minimum amount of CH2Cl2 (~5-10 mL) and filter the solution. Combine the dichloromethane solution with 25 mL of hexanes and filter off the yellow product. Wash with hexanes (2 x 5 mL). Calculate the percent yield. Caution – the product can be sublimed at high temperatures and low pressures, so do not use the vacuum oven to take off solvent). The product is unstable to light and air over a period of time, so it should be stored in a vial flushed with N2 in you drawers or cabinets. Week 2 Take an I. R. of your sample and of the Mo(CO)6 starting material. Note the stretching frequencies for the carbonyl (C? O) near 2000 cm-1. Also note where additional stretches occur which originate from the mesitylene ligand. Perform a 1H-NMR on your (1,3,5-C6H3(CH3)3)Mo(CO)3 product and also mesitylene in d-Chloroform.
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