Polyfluoroalkoxyaluminates (PFAA) are weakly coordinating anions many of which are of the form [Al(OR<sub>F</sub>)<sub>4</sub>]−.[1] Most PFAA's possesses an Al(III) center coordinated by four −ORF (RF = -CPh(CF3)2 (hfpp), -CH(CF3)2 (hfip), -C(CH3)(CF3)2 (hftb), -C(CF3)3 (pftb)) ligands, giving the anion an overall -1 charge. The most weakly coordinating PFAA is an aluminate dimer, [F{Al(O''pftb'')<sub>3</sub>}<sub>2</sub>]−, which possess a bridging fluoride between two Al(III) centers.[2] The first PFAA, [Al(O''hfpp'')<sub>4</sub>]−, was synthesized in 1996 by Steven Strauss, and several other analogs have since been synthesized, including [Al(O''hfip'')<sub>4</sub>]−, [Al(O''hftb'')<sub>4</sub>]−, and [Al(O''pftb'')<sub>4</sub>]− by Ingo Krossing in 2001. These chemically inert and very weakly coordinating ions have been used to stabilize unusual cations, isolate reactive species, and synthesize strong Brønsted acids.
Work by Strauss demonstrated that the synthesis of Li+[Al(O''hfpp'')<sub>4</sub>]− could be achieved from the reaction of lithium aluminum hydride and HOhfpp.[3] Analogous metal PFAA salts (MPFAA's) were later synthesized by Krossing using a similar synthetic pathway.[4]
While Li+[Al(O''hfpp'')<sub>4</sub>]− is readily soluble in hydrocarbon solvents, presumably due to aryl substituents, Li+[Al(O''hfip'')<sub>4</sub>]−, Li+[Al(O''hftb'')<sub>4</sub>]−, and Li+[Al(O''pftb'')<sub>4</sub>]− are only sparingly soluble in common organic solvents including dichloromethane (DCM), toluene, and hexane. Their silver analogs are much more soluble however, making AgPFAA's more desirable reagents for liquid phase reactivity.
Ag+[Al(O''hfip'')<sub>4</sub>]−, Ag+[Al(O''hftb'')<sub>4</sub>]−, and Ag+[Al(O''pftb'')<sub>4</sub>]− can be synthesized via salt metathesis reactions; ultrasonication of a suspension of Li+[PFAA]− and an excess of AgF at 40 °C for 12 hours produces the final colorless products in high yields on multigram scales. Analogous M+[Al(O''pftb'')<sub>4</sub>]−, M = Na, K, Rb, Cs, salts can also be prepared via the same synthetic route, from the metathesis reactions of Li+[Al(O''pftb'')<sub>4</sub>]− with the corresponding MCl salt.[5] [6]
Strong Brønsted acids, [H(OEt<sub>2</sub>)<sub>2</sub>]+[Al(O''pftb'')<sub>4</sub>]− and [H(THF)<sub>2</sub>]+[Al(O''pftb'')<sub>4</sub>]−, can be prepared via the reaction of Li+[Al(O''pftb'')<sub>4</sub>]− with two equivalents of Lewis base, Et2O or THF, and strong acid, HX (X = Cl, Br).[7] [H(OEt<sub>2</sub>)<sub>2</sub>]+[Al(O''pftb'')<sub>4</sub>]− is isolable as a white powder sensitive to air and water and stable at moderately high temperatures. [H(THF)<sub>2</sub>]+[Al(O''pftb'')<sub>4</sub>]− can be isolated as a crystalline solid from a brown oily reside, presumably containing polymerized THF products formed upon addition of strong acid.
Ab initio calculations and crystallographic structural analysis of [H(OEt<sub>2</sub>)<sub>2</sub>]+[Al(O''pftb'')<sub>4</sub>]− indicate potential unequal sharing of the proton between the two diethyl ether molecules, and the authors propose a solid state structure in which [H(OEt<sub>2</sub>)<sub>2</sub>]+ is described as a diethyl ether molecule acting as a hydrogen bond acceptor from an ethanol molecule which stabilizes an ethyl cation as a Lewis base in one resonance structure.
Nitrosonium salts, NO+[Al(O''hfpp'')<sub>4</sub>]− and NO+[Al(O''pftb'')<sub>4</sub>]−, can be prepared via an exchange reaction of the respective lithium salt with nitrosonium hexafluoroantimonate.[8]
The NO+[Al(O''pftb'')<sub>4</sub>]− salt can be obtained in much higher yields than the analogous hfpp salt and can be used to oxidize several transition metal and main group element complexes.