Towards Weakly Coordinating Anions with the Extremely Electron Withdrawing Perfluoropyridinoxy Ligand –OC₅F₄N

Abstract

The extreme electron withdrawing properties of the perfluoropyridinoxy ligand –OC5F4N were used for the preparation of new (weakly) coordinating borate and aluminate anions of the type [E(OC5F4N)4]– (E = B or Al). These new anions are based on the potent parent Lewis acids E(OC5F4N)3, which possess exceptionally high calculated fluoride ion affinities (FIAs) of 500 and 587 kJ mol–1 for E = B and Al respectively. For aluminum, this extreme Lewis acidity dominates the chemistry and from mixtures of the neutral polymeric Lewis acid [Al(OC5F4N)3]n, the five‐ and six‐coordinate complexes Al(OC5F4N)3(OEt2)2 (1) and [Al(OC5F4N)2(µ‐OC5F4N) (NCMe)2]2 (2) were crystallized upon addition of ether or MeCN. The aluminate salts M[Al(OC5F4N)4] (M = Li or K) were prepared from the reaction between the alcohol 4‐HO–C5F4N and either LiAlH4 or K[AlEt4] respectively. The aluminate anion [Al(OC5F4N)4]– remains Lewis acidic coordinating small donor molecules forming [Al(OC5F4N)4(L)]– (L = THF or NMe3) and even supports formation and structural characterisation of the aluminum dianion containing salt [Na(OEt2)2][Na][Al(OC5F4N)5] (8). The from NaBH4 and 4‐HO–C5F4N accessible borate salt Na[B(OC5F4N)4] shows increased kinetic stability in comparison to the aluminum analogue.

Synthesis of a Perfluorinated Phenoxyphosphorane and Conversion to Its Hexacoordinate Anions

“…More detailed analysis revealed that the impact of FIA dampening on a Lewis acid increases with molecular size and therefore inclusion of a solvent model in such calculations is imperative for large molecules, even when they are neutral. Given that some recent reports of FIA and GEI calculations for large neutral Lewis acids have seemingly been performed without consideration of solvent effects, the values presented should be regarded as upper limits . Our results also suggest that trends in GEI may not predict reactivity outcomes when ionic species are involved.…”

“…Several considerations suggested this might be a fruitful line of inquiry (Scheme d): i) the Lewis acidity of B(OC 6 F 5 ) 3 is comparable to that of B(C 6 F 5 ) 3 , illustrating the similar group electronegativity of the OC 6 F 5 and C 6 F 5 substituents; ii) the related polyfluorinated aryloxy groups OC 5 F 4 N and OC(C 6 F 5 ) 3 have very recently been used to generate aluminum Lewis superacids, iii) the octahedral [M(OC 6 F 5 ) 6 ] − (M=Nb, Ta) anions have been shown to be comparable to the [B(C 6 F 5 ) 4 ] − anion in their ability to stabilize electrophilic metallocenium ion catalysts; iv) the proposed octahedral [P(OC 6 F 5 ) 6 ] − anion would feature the same number of electron‐withdrawing fluorine and oxygen atoms as the robust [M(OTeF 5 ) 6 ] − anions (M=As, Sb); and v) our FIA calculations revealed that a free P(OC 6 F 5 ) 5 molecule would indeed be a Lewis superacid (FIA=497 kJ mol −1 at the BP‐86/SV(P) level).…”

Insights on the Lewis Superacid Al(OTeF5)3: Solvent Adducts, Characterization and Properties

“…. [25] This reflects well on the similar Lewis acidity of the two compounds. An established method for experimentally gauging the acidity of a Lewis acid is the Gutmann-Beckett method, in which the 31 P NMR chemical shift of triethylphosphine oxide OPEt3 in a Lewis acid complex is analyzed in respect to free OPEt3.…”

“…[10] To further validate the high fluoride ion affinity of these solvent adducts, [Al(OTeF5)3(SO2ClF A similar ligand scrambling of the anion was reported by Riddlestone et al when they treated the Lewis acid Al(OC4F4N)3 with trityl chloride. [25] In an attempt to obtain halogen-bridged adducts of the form [Al(OTeF5)3(Me3SiF)] and [Al(OTeF5)3(Me3SiCl)] [29] in analogy to literature-known Al(OC(CF3)3)3(Me3SiF) and [Al(OC(CF3)3)3(Me3SiCl)] [11] , we treated [Al(OTeF5)3(SO2ClF)2] with the respective trimethylsilyl halides. Instead of the desired reaction, the formation of the species Me3SiOTeF5 is observed in both cases.…”

Oxygen‐Bridged Ga2(Et)3(OTeF5)3 and the Weakly Coordinating Anions [Ga(Et)(OTeF5)3]− and [Ga(OTeF5)4]−

“…Especially Ag + salts as oxidizer [11] and [CPh 3 ] + (trityl) salts as hydride or carbanion abstraction source [12] find application in generating reactive cations. More recent examples such as [FAl(N(C 6 F 5 ) 2 ) 3 ] À , [13] [((CF 3 ) 3 CO) 3 AlÀFÀ Al(OC(CF 3 ) 3 ) 3 ] À , [14] [FAl(OC(C 6 F 5 ) 3 ) 3 ] À , [15] and[ Al(OC 5 F 4 N) 4 ] À [16] illustratet hat there is still an increasing interest in new WCAs. Our group described the tetrakis(pentafluoroorthotellurato) aluminate [Al(OTeF 5 ) 4 ] À and demonstrated its versatile applications with reactive cationss uch as [CPh 3 ] + ,[ NO] + ,p rotonated benzene [C 6 H 7 ] + ,o rp rotonated white phosphorus[ P 4 H] + .…”