The Al(OR^F)₃/H₂O/Phosphane [R^F = C(CF₃)₃] System – Protonation of Phosphanes and Absolute Brønsted Acidity

Abstract

The synthesis of the classical, neutral donor–acceptor adducts Ph2MeP–/Ph3P–/Ph3As–Al(ORF)3 and H2O–Al(ORF)3 [1, 2, 3, 4, ORF = OC(CF3)3] is reported. The intermediate H2O–Al(ORF)3 (4) was generated by substitution of PhF in PhF–Al(ORF)3 with H2O and was analyzed in a long‐term NMR study over 22 days. This Brønsted acidic system was used in orienting experiments to protonate phosphanes such as PMePh2, PPh3, PCy3, P(tBu)3, and PCy2[2,4,6‐(iPr)3C6H2]. Depending on the use of one or two equivalents of PhF–Al(ORF)3, the new weakly coordinating anions [(RFO)3Al(μ‐OH)Al(ORF)3]– or [HOAl(ORF)3]– were obtained. However, in dependence of the steric bulk of the phosphanes, stable and unreactive R3P–Al(ORF)3 adducts were also observed in the NMR experiments. The absolute acidity of the key H2O–Al(ORF)3 adduct was evaluated by the relaxed COSMO cluster‐continuum (rCCC, COSMO = conductor‐like screening model) model in fluorobenzene solution. For a 0.001 M solution of H2O–Al(ORF)3, the medium acidity resulted as –986 kJ mol–1 or a pHabs value of 173. Long‐term hydrolysis of H2O–Al(ORF)3 (4), probably to give HORF and HOAl(ORF)2 followed by trimerization, gave [HOAl(ORF)2]3 (10), which was identified by X‐ray diffraction.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

“…It is typically observed as the decomposition product of the [Al(OR F ) 4 ] À WCA after reaction with av ery strong electrophile. [144][145][146] Ac lear difference between bridged WCAsf eaturing Lewis acids based on aluminum and boron is that aluminum forms the most stable bridged anion with the hard fluoride ligand bridging,w hereas the softer cyanide ligand has been used for the formation of an umber of bridged WCAsb ased upon the B(C 6 F 5 ) 3 Lewis acid. Theinsitu generated "[PCl 2 ] + " cation (Ag[Al(OR F ) 4 ] + PCl 3 )was exploited for access to its silver salt.…”

“…[144,155] Ty pical examples include [( F RO) 3 Al-X-Al-(OR F ) 3 ) 2 ] À X = F, [142] Cl, [145] X = OH; [146] [155] and [Ph 3 [155] Fort he Synthesis of Reactive Cations:W ei nclude some recent and exemplary examples highlighting this approach for the formation of reactive cations.S imple halide abstraction from the ruthenium fluoride complexes [Ru(L) 2 (CO) 2 F 2 ] (where L = PPh 3 or NHC) by the Lewis acids BF 3 ,P F 5 ,a nd B(C 6 F 5 ) 3 gives the corresponding fluoride-containing WCAs. Moreover,t his approach is ideally suited to the formation of bridged WCAso ft he type [LA-X-LA] À ,a nd even higher aggregates of bridging species of the type [LA-X-LA-X-LA] À can be formed.…”

“…[154] Therelated aqua complex H 2 O!B(C 6 F 5 ) 3 has received attention in the literature [254,255] and has been shown to be capable of protonating (and hence oxidizing) metallocene M(Cp) 2 complexes (where M = Cr,F e, or Co). [146] Higher Brønsted acidity can be achieved through the coordination of the alcohols HOR F ,H OC 6 F 5 ,a nd menthol, with the complexation of HOR F forming the strongest Brønsted acid. [257] Further investigations into the formation of adducts involving carboxylic acids such as propionic or n-decanoic acid and B(C 6 F 5 ) 3 showed that such adducts are strong Brønsted acids capable of effecting B À Cb ond protonolysis as well as initiating the polymerization of isobutene.…”

“…[143] Related bridged anions of the type [( F RO) 3 Al(m-X)Al-(OR F ) 3 ] À (where X = Cl or OH) are also accessible,b ut are more prone to either exchange reactions and/or dissociation to yield the corresponding [XAl(OR F ) 3 ] À and Al(OR F ) 3 species. [144][145][146] Ac lear difference between bridged WCAsf eaturing Lewis acids based on aluminum and boron is that aluminum forms the most stable bridged anion with the hard fluoride ligand bridging,w hereas the softer cyanide ligand has been used for the formation of an umber of bridged WCAsb ased upon the B(C 6 F 5 ) 3 Lewis acid. Bochmann and co workers reported the cyanide-bridged [(F 5 C 6 ) 3 B-CN-B(C 6 F 5 ) 3 ] À as well as the related dianionic [M{CNBC 6 F 5 ) 3 } 4 ] 2À (M = Ni or Pd) species and demonstrated their ability to support extremely rapid ethene polymerization.…”

Absolute Brønsted Acidities and pH Scales in Ionic Liquids

“…Due to our theoretical [20,21,28] and experimental [26,[29][30][31] work on (super)acidity and our in-house use of bromoaluminate ILs Cat + [Al n Br 3n + 1 ] À (Cat + = typical IL cation such as imidazolium, pyrrolidinium; n = 0-3) as highly acidic IL media, we were interested in the extension of the absolute pH abs value concept to IL media. Thus, in this article we first develop the general principles to extend the pH concept to all ILs and turn next to how these examples can be used to reach the Cat + [Al n Br 3n + 1 ]…”