Lanthanide trifluoromethane sulfonates

The dehydration of lanthanide perchlorates to obtain the anhydrous salt has been studied [13-15]. Lighter lanthanide perchlorate lose the water of hydration readily at 200°C under vacuum while the heavier lanthanide salts produced insoluble basic salts. Anhydrous heavier lanthanide perchlorates have been obtained by extraction with anhydrous acetonitrile. Utmost precaution should be exercised in the purification of lanthanide perchlorate, since the mixture of lanthanide perchlorate and acetonitrile can lead to an explosion. An alternate approach involves the addition of triethylorthoformate to the mixture or refluxing the solvent through a Soxhlet extractor packed with molecular sieves [3], In view of the hazardous nature of perchlorates, alternate materials such as lanthanide trifluoromethane sulfonates have received some attention. Lanthanide triflates are thermally stable, soluble in organic solvents, unreactive to moisture and are weak coordinating agents. Triflic acid is stronger than perchloric acid [17]. Lanthanide perchlorates and triflate have the same reduction potentials in aprotic solvents and the dissociation of the triflates is less than the perchlorates in acetonitrile [17],... 

More recently, interesting results have been achieved, such as the use of cafalyfic amounts of lanthanide trifluoromethane sulfonates (triflates) alone or microencapsulated on polyacrylonitrile as reusable catalysts moreover ecocompatible methods have been set up based on the use of graphite as a solid catalysti or even exploiting the possibility to perform the reaction without any catalyst. ... 

The aqueous aza-Diels-Alder reaction of an aldehyde and an amine hydrochloride with a diene is catalyzed by lanthanide(III) trifluoromethane sulfonates (Ln(OTf)3, triflates [24]). Some examples are reported in Schemes 6.12 and 6.13. With respect to uncatalyzed reactions, the lanthanide catalyst allows milder reaction conditions, increases the reaction yield and does not affect the diaster-eoselectivity of the reaction, but influences the regiochemistry as in the cycloaddition of 25 with 1,3-dimethyl-1,3-butadiene (Schemes 6.10 and 6.12). These results have been applied [24b-d] to the synthesis of azasugars (Scheme 6.14). 

Trifluoromethane sulfonate salts of lanthanides are used as starting materials for the preparation of other lanthanide complexes instead of perchlorates for safety reasons. 

Lanthanide(III) trifluoromethane sulfonates in the hydrated form can be prepared by reacting the rare earth oxide with triflic acid in aqueous solution. The product is hydrated salt which is heated under vacuum to produce an anhydrous rare earth trifluoromethane sulfonate. The anhydrous salts are used in organic reactions. Some of the advantages of using rare earth triflates over conventional Lewis acids are (i) they act as catalysts rather than stoichiometric reagents, (ii) they are effective catalysts in aqueous solutions and (iii) they can be easily recovered without any loss of catalytic activity for subsequent use. 

Grey anhydrous scandium triflate, [Sc(03SCF3)3] (triflate = trifluoromethane sulfonate), has been obtained by dehydration of the hydrate at 190 200 °C the hydrated salt was itself obtained from the reaction of hydrated scandium chloride and dilute triflic acid. [Sc(03SCF3)3], in which triflate is believed to act as a bidentate ligand (similar to perchlorate in 80(004)3), is not isomorphous with the lanthanide analogues. 

The hexagonal lattices which are isostructural with lanthanide hydroxides, trichlorides, tribromides, ethylsulfates and trifluoromethane sulfonates, contain two magnetically equivalent lanthanide ions per unit cell. The structure is sueh that the nearest and next-nearest neighbours are significantly closer than other neighbours and the most effective... 

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