Lanthanide triflates are triflate salts of the lanthanide family with many uses in organic chemistry as Lewis acid catalysts. The catalysts act similarly to aluminium chloride or ferric chloride, but are stable in water, which makes it possible to use water as a solvent instead of organic solvents.
Contents
Molecular structure
Lanthanide triflates consist of a lanthanide metal ion and three triflate ions. The lanthanides, or rare earth metals, are the elements from lanthanum to lutetium in the periodic table. Triflate is a contraction of trifluoromethanesulfonate; its molecular formula is CF3SO3, and is commonly designated ‘OTf’. Triflic acid is a ‘superacid’ so its conjugate base ions are very stable. Lanthanide triflates are normally nonahydrates, mostly commonly depicted as Ln(OTf)3·(H2O)9; however, in the solid state and in aqueous solution, the waters are bound to the lanthanide and the triflates are counteranions, so more accurately lanthanide triflate nonahydrate is written as [Ln(H2O)9](OTf)3. Anhydrous lanthanide triflates, Ln(OTf)3, are also easily obtained as described below. The metal triflate complex is strongly electrophilic, thus acts as a strong Lewis acid.
Synthesis
Lanthanide triflates are synthesized from lanthanide oxide and aqueous triflic acid. In a typical preparation, a 1:1 (v/v) solution of trilfic acid in water is added to a slight stoichiometric excess of lanthanide oxide. The mixture is stirred and heated at 100 °C for a few hours, and the excess lanthanide oxide is filtered off. The excess oxide ensures all of the triflic acid is consumed. The water is removed under reduced pressure (or simply boiled away) to leave a hydrated lanthanide triflate, Ln(H2O)9(OTf)3.
In simplified form the reaction is
Ln2O3 + 6HOTf → 2Ln(OTf)3 + 3H2O
Since the reaction takes place in aqueous solution, more accurately,
Ln2O3 + 6HOTf + 18H2O → 2[Ln(H2O)9](OTf)3 + 3H2O
Anhydrous lanthanide triflates can be produced by dehydrating their hydrated counterparts by heating between 180 and 200 °C under reduced pressure for 48 hrs. This is a major advantage of lanthanide triflates compared to lanthanide halides, whose anhydrous forms require more tedious synthetic procedures because they cannot be obtained by dehydrating their hydrates (because of oxyhalide formation).
[Ln(H2O)9](OTf)3 → Ln(OTf)3 + 9H2O (180-200 °C, ~10−2 - 10−4 torr, 48 hrs)
Lewis acid catalysis
Lewis acids are used to catalyse a wide variety of reactions. The mechanism steps are:
- Lewis acid forms a polar coordinate with a basic site on the reactant (such as an O or N)
- Its electrons are drawn towards the catalyst, thus activating the reactant
- The reactant is then able to be transformed by a substitution reaction or addition reaction
- The product dissociates and catalyst is regenerated
Common Lewis acids include aluminium chloride, ferric chloride and boron trifluoride. These reactions are usually carried out in organic solvents; AlCl3, for example, reacts violently with water. Typical solvents are dichloromethane and benzene.
Friedel-Crafts Reactions
Lanthanide triflates can replace conventional Lewis acids in various types of reactions. One important class is Friedel-Crafts acylations and alkylations, which are one of few ways to add C-C bonds to aromatics. The synthesized products are used in many products including pharmaceuticals and agrochemicals.
These reactions are usually carried out with AlCl3 as the catalyst, in an organic solvent. In the acylation reaction, AlCl3 complexes with the product. It must be added in large excess and is destroyed during product recovery, so atom efficiency is poor. The reaction is quenched with water, creating large volumes of corrosive aluminous, acidic waste- 3 mol HCl per mol AlCl3. In one example, Clark et al. estimate 0.9 kg of AlCl3 is wasted per kilogram of dimethyl acetophenone produced. Product separation can also be difficult.
Lanthanide triflates can dramatically cut the impact of these syntheses. They are able to achieve high conversion using small quantities. These catalysts are stable in water, so avoid the need for organic solvents; some reaction rates are even enhanced by aqueous systems. They don’t complex with products, so separation is simple, and the catalyst is easily recovered- in many cases the solution is simply reused.
La(OTf)3 catalysts can also reduce the number of processing steps and use greener reagents; Walker et al. reported successful acylation yields using carboxylic acid directly, rather than acyl chloride. Their process generates only a small volume of aqueous sodium bicarbonate waste. Similar results have been cited for the direct acetylation of alcohols.
Other C-C bond-forming reactions
La(OTf)3 catalysts have been used for many other carbon-carbon bond forming reactions, such as Diels-Alder, aldol, and allylation reactions. Some reactions require a mixed solvent, such as aqueous formaldehyde, although Kobayashi et al. have developed alternative surfactant-water systems.
Michael additions are another very important industrial method for creating new carbon-carbon bonds, often with particular functional groups attached. Addition reactions are inherently atom efficient, so are preferred synthesis pathways. La(OTf)3 catalysts not only enable these reactions to be carried out in water, but can also achieve asymmetric catalysis, yielding a desired enantio-specific or diastereo-specific product.
C-N bond-forming reactions
Lewis acids are also used to catalyse many C-N bond-forming reactions. Pyridine compounds are common in biology and have many applications. Normally, pyridine is synthesized from acetaldehyde, formaldehyde and ammonia under high temperatures and pressures. Lanthanide triflates can be used to synthesize pyridine by catalysing either the condensation of aldehydes and amines, or the aza Diels-Alder reaction catalytic synthesis. Again, water can be used as a solvent, and high yields can be achieved under mild conditions.
Nitro compounds are common in pharmaceuticals, explosives, dyes, and plastics. As for carbon compounds, catalysed Michael additions and aldol reactions can be used. For aromatic nitro compounds, synthesis is via a substitution reaction. The standard synthesis is carried out in a solution of nitric acid, mixed with excess sulfuric acid to create nitronium ions. These are then substituted on to the aromatic species. Often, the para-isomer is the desired product, but standard systems have poor selectivity. As for acylation, the reaction is normally quenched with water, and creates copious acidic waste. Using a La(OTf)3 catalyst in place of sulfuric acid reduces this waste considerably. Clark et al. report 90% conversion using just 1 mol% of ytterbium triflate in weak nitric acid, generating only a small volume of acidic waste.
La(OTf)3 catalysts have also been used for cyanations, and three-component reactions of aldehydes, amines & nucleophiles.
Advantages
The substitution of organic solvents by water reduces the amount of waste and the metals are recoverable and hence reusable.
Generally, the benefits of these catalysts include:
Green catalysts
Lanthanide triflates are one of the most promising green chemistry catalysts. Unlike most conventional catalysts, these compounds are stable in water, so avoid the need for organic solvents, and can be recovered for reuse. Since leading researcher Kobayashi’s 1991 paper on their catalytic effect in water, the range of researched applications for La(OTf)3 catalysts has exploded. The commercialisation of these techniques has the potential to significantly reduce the environmental impact of the chemical industries.
Disadvantages
The main disadvantages of these new catalysts compared with conventional ones are less industrial experience, reduced availability and increased purchase cost. As they contain rare metals and sulfonate ions, the production of these catalysts may itself be a polluting or hazardous process. For example, metal extraction usually requires large quantities of sulfuric acid. Since the catalyst is recoverable, these disadvantages would be less over time, and the cost savings from reduced waste treatment and better product separation may be substantially greater.
The toxicity of individual lanthanides vary. One vendor MSDS lists safety considerations including dermal/eye/respiratory/GI burns on contact. It also lists possible hazardous decomposition products including CO, CO2, HF and SOx. The compounds are hygroscopic, so care is required for storage and handling. However, these considerations also apply to the more common catalysts.
These possible disadvantages are difficult to quantify, as essentially all public domain publications on their use are by research chemists, and do not include Life Cycle Analysis or budgetary considerations. Future work in these areas would greatly encourage their uptake by industry.
Recent developments
Researchers are continually finding new applications where it can replace other less efficient, more toxic Lewis acids. Recently it has been tested in synthesizing epoxies and other polymerisation reactions, and in polysaccharide synthesis. It has also been trialled in green solvents other than water, such as ionic liquids and supercritical carbon dioxide. To enhance recovery, researchers have developed La(OTf)3 catalysts stabilised by ion exchange resin or polymer backbones, which can be separated by ultrafiltration. Solvent-free systems are also possible with solid-supported catalysts.