Polyfluoroalkoxyaluminates

Polyfluoroalkoxyaluminates (PFAA) are weakly coordinating anions many of which are of the form [Al(ORF)4]−.

[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.

[3][4] 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(Ohfpp)4]− 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.

Reaction of lithium aluminum hydride with four equivalents of polyfluoroalcohol overnight in refluxing toluene yields the desired PFAA's .

The colorless products can be precipitated from toluene in high yields on multi-gram scales by cooling at -20 °C for an hour.

Their silver analogs are much more soluble however, making AgPFAA's more desirable reagents for liquid phase reactivity.

[7] [H(OEt2)2]+[Al(Opftb)4]− is isolable as a white powder sensitive to air and water and stable at moderately high temperatures.

Ab initio calculations and crystallographic structural analysis of [H(OEt2)2]+[Al(Opftb)4]− indicate potential unequal sharing of the proton between the two diethyl ether molecules, and the authors propose a solid state structure in which [H(OEt2)2]+ 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.

Rigorous tetrahedral geometry of the Mn(NO)4+[F{Al(Opftb)3}2]− salt indicates a pseudo-gas phase environment about the cation due to the weakly coordinating behavior of the anionic PFAA.

[9] Oxidation of Cr(CO)6 by NO+[PFAA]−'s under cold vacuum for short reaction times yields the kinetic product [Cr(CO)6]•+[PFAA]− as a pale yellow crystalline solid.

Oxidation in a closed room temperature vessel for long reaction times yields the thermodynamic product [Cr(CO)5(NO)]+[PFAA]− as an orange crystalline solid.

Assignment of the thermodynamic and kinetic products was further supported by ab initio calculations.

Fluctional Jahn-Teller distortions at room temperature are indicated by the presence of a broad band in the Raman spectra of these compounds.

[9] Cationic cobalt(I) sandwich complexes of the form Co(arene)2+[PFAA]- can be prepared via two synthetic routes (arene = mesitylene, benzene, fluorobenzene, o-difluorobenzene & PFAA = [Al(Opftb)4]− and [F{Al(Opftb)3}2]−).

Structural analysis of Co(I)bz2+[Al{OC(CF3)}4]− reveals the sandwich complex is slightly staggered, twisted 6° from an eclipsed confirmation.

[16] In the solid phase the material is stable to air and moisture, but is sensitive to diatomic oxygen in solution.

EPR analysis reveals that 90% of the unpaired electron spin density is located on the nickel center.

[14] Arene ligand exchange results in partial electron spin delocalization onto the aromatic arene ligand, with 84-87% of the unpaired electron spin density located on the nickel center.

[17] The former complex exhibits nearly identical bonding to its analog AlCp*2+ while the Cp substituents in the later compound exhibit η1 bonding due to two diethyl ether substituents bound to the aluminum center.

[21] The first main-group homoleptic olefin compound isolable in bulk was synthesized using a stabilizing PFAA counter ion.

[18] Germyl cation Halide abstraction from BrGeR3 (R = [C6H3(OtBu)2]3) using Ag+[Al(Opftb)4]- yields the germyl cation Ge[C6H3(OtBu)2]3+, stabilized by bulky ligands and a weakly coordinating PFAA anion.

Due to the weakly coordinating nature of the PFAA anion, solid state structure of the salt reveals no ion-ion contacts between the germyl cation and the PFAA, giving rise to a very electrophilic germanium species.

The salt, Sn(dmap)42+[Al(Opftb)4]2− is prepared by a different synthetic route.

[11] In the multistep reaction, [P4NO]+ is a proposed intermediate from analysis of collision-induced dissociation (CID) experiments.

Complex coupling present in the 31P NMR spectra of P9+ allowed for the determination of its structure.

[11] Due to low polarizability, large charge delocalization, and high conformational flexibility, PFAA salts are potentially useful ionic liquids.

[24] Several PFAA salts, including those of [Al(Ohfip)4]−, possess melting points as low as 273 K or colder.

Walden Plots, which are created by plotting the logarithm of conductivity against the logarithm of inverse viscosity, indicate that several [Al(Ohfip)4]− ionic liquids are potentially better than the best commercially available ionic liquids.

Space-filled crystal structure of Al(OC(CF 3 ) 3 ) 4 − . Oxygen atoms are colored red, aluminum pink, carbon gray and fluorine yellow
Proposed resonance structure of protonated ether. [ 7 ] Recreated from source.
Crystal structure of Mn(NO) 4 + [Al(OC(CF 3 )) 4 ] − recreated from source. [ 12 ] Manganese atoms are colored purple, nitrogen blue, oxygen red, aluminum pink, carbon gray and fluorine yellow.
Crystal Structure of [Cr(CO)6] •+ [F{Al( pftb ) 3 } 2 ] − recreated from source. [ 9 ] Chromium atoms are colored purple, oxygen red, aluminum pink, carbon gray and fluorine yellow.
Synthesis of Co(I)bz 2 + [Al(OR F ) 4 ] − (-OR F = -OC(CF 3 )). [ 13 ]
Ligand substitution pathways to produce nickel(I) arene and phosphine cations. [ 14 ] [ 15 ]
Survey of main group cations stabilized by PFAAs. a.) AlCp 2 + [ 17 ] b.) Ga(COD) 2 + [ 18 ] c.) In 2 (PPh 3 ) 7 2+ [ 19 ] d.) Ge[C 6 H 3 (O t Bu) 2 ] 3 + [ 20 ] e.) Sn(dmap) 4 2+ [ 10 ]