Cannula techniques

The lilac colored reaction mixture is transferred into a second Schlenk flask (100 mL) using a double-ended stainless steel cannula. The second flask contains hexane previously cooled to — 78 °C (Dry Ice-acetone bath). A lilac colored solid precipitates. The solvent is siphoned off and the solid is washed three times with hexane (20mL) previously cooled to — 78 °C and transferred into the flask using the double ended cannula technique. The wash solvent is siphoned off and remaining solid is dried under vacuum (10-3torr, oil pump) at -40°C for 8h. Yields for MoCp(CO)3(FBF3) 282 319 mg (85-96%), for WCp(CO)3 (FBF3) 357-378 mg (85-90%). 

In a dry box, an oven-dried 500-mL three-necked round-bottomed flask equipped with an N2 gas inlet, a precision mechanical stirrer, and a stopper is charged with 39.66 g (100 mmol) of WC16 and 18 g (51 mmol) of W(CO)6. The sealed flask is removed from the dry box, connected to a source of prepurified N2, and bubbler for pressure release. The stopper is replaced with a rubber septum and 300 mL of purified chlorobenzene is added via cannula techniques. Carbon monoxide evolution starts immediately. 

Fig. 1.13. Variation on the cannula technique. The glass lube with fritted filter and the glass delivery tubes are attached to the glass standard taper joints by means of plastic adaptors (e.g., Kontes K-179800). The flexible plastic tubing connecting the two parts should be impervious to solvents. Teflon tubing is best and. to prevent contamination of the filtrate, Tygon tubing should be avoided when organic solvents are used.

Solvents may be dried in individual batches using conventional distillation apparatus (Chapter 11), but it is more convenient to dry common solvents such as dichloromethane, diethyl ether, and THF in continuous stills (Section 5.5). In either case the solvent must be protected from moisture using an inert atmosphere (nitrogen or argon). Rigorously dried solvents must be stored under an inert atmosphere and handled using syringe or cannula techniques (Chapter 6). 

Cannula techniques have several advantages over other approaches to the manipulation of very sensitive materials ... 

A bench can be equipped with the apparatus necessary for cannula techniques for a relatively modest sum. 

Cannula techniques are complementary to other techniques for handling air sensitive materials. In particular, while they may be used to prepare NMR samples of air sensitive materials and to manipulate solids, it Is normally more convenient to carry out both these operations in an Inert atmosphere glove box. In our experience cannula techniques are at least as good as any other technique for the exclusion of oxygen from a reaction mixture (in fact we find them to be superior to inert atmosphere glove boxes), but glove boxes and high vacuum manipulation lines provide better protection from adventitious water. 

Pump and Purge. A key objective of cannula techniques is to achieve very low partial pressures of oxygen without the use of time consuming high vacuum techniques, and this is achieved by repeated pump and purge cycles. Even a heavily used rotary pump will give an ultimate vacuum of 1 x 10 2 mm Hg, and will rapidly reduce the pressure in a small reaction vessel to 1 m Hg. In a typical example a 224 mL reaction vessel contains approximately 2 mmol of O2, and evacuation to 1 mm Hg reduces the quantity of 02 to 2 x 10 3 mmol. If the vessel is refilled and the cycle repeated the quantity of O2 reduces to 2 x 10 mmol, and it is clear that relatively poor vacuums are sufficient to provide very low effective partial pressures of O2 after 2 or 3 pump/purge cycles. 

Water Reduction. The principle weakness of cannula techniques is that small volumes of solution can be exposed to large surface areas of glass and steel. These are major sources of water contamination, and we routinely oven dry all transfer cannulae and glassware before use. 

Filtration. One of the most convenient aspects of the cannula techniques is that filtration does not require the use of fritted glass equipment. Small scale filtrations are simply carried out by transferring the liquid to be filtered from one Schlenk tube to another through a filter stick as shown in Figure 5. When the filtration is complete the filter paper is discarded, and the cannula washed with acetone to prepare it for the next filtration. The advantages of filter stick filtration over filtration through fritted glass apparatus maybe summarized as follows ... 

Recrystallization. The convenience of cannula techniques for the manipulation of solutions allows access to a wide variety of recrystallization techniques under inert atmosphere conditions. The general procedure in all recrystallizations is to extract a crude compound into a solvent with a polarity close to the minimum required to dissolve the material, filter the solution, and then reduce the polarity of the medium until the compound crystallizes out of solution. The product may then be collected by filtration, leaving small quantities of compounds with similar or higher solubilities in the solute mother liquor. 

We thank the National Science Foundation and the Office for Naval Research for financial support of our research program. We would also like to acknowledge the many colleagues and students who have contributed to the development of these techniques. We would particularly like to acknowledge our debt to Dr. Malcolm Green -many of the cannula techniques described are variations on techniques which were originally developed in his laboratories at Oxford University. 

Cannula techniques air-sensitive materials, 7 components, 6 double tip needles, 9 flexibility, 7 solution transfer, 6-23 speed,7... 

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