Non‐Coordinated Phenolate Anions and Their Application in SF6 Activation

Abstract The reaction of the strong monophosphazene base with the weakly acidic phenol leads to the formation of a phenol–phenolate anion with a moderately strong hydrogen bond. Application of the more powerful tetraphosphazene base (Schwesinger base) renders the isolation of the corresponding salt with a free phenolate anion possible. This compound represents the first species featuring the free phenolate anion [H5C6‐O]−. The deprotonation of phenol derivatives with tetraphosphazene bases represents a great way for the clean preparation of salts featuring free phenolate anions and in addition allows the selective syntheses of hydrogen bonded phenol‐phenolate salts. This work presents a phosphazenium phenolate salt with a redox potential of −0.72 V and its capability for the selective activation of the chemically inert greenhouse gas SF6. The performed two‐electron reduction of SF6 leads to phosphazenium pentafluorosulfanide ([SF5]−) and fluoride salts.

Phenol represents the simplest aromatic alcohol, and thus has been in the focus of numerous theoretical calculations [1,2,3,4] as well as practical applications. [5,6] Especially sodium phenolate has emerged as ah ighly important bulk chemical for the industrial production of salicylic acidi nt he Kolbe-Schmitt process. [7,8] Fundamental reactions in the biosphere are strongly dependent on phenolic species. The amino acid tyrosine (p-hydroxyphenylalanine) is crucial fort he successo fp hotosynthesis, as tyrosine is photo-oxidized in the oxygen evolvingc omplex (OEC) of the photosystem II via ap roton coupled electron transfer (PCET) reaction with ah ydrogen bonded histidine. [9] Hydrogenb onds of phenol are strongly governingt he acidity of OH functions, which turned out to be crucial in several biological and chemical systems. [3-5, 10, 11] With regard to the great importance of phenol,i ti ss urprising that the molecular structure and the characteristics of the non-coordinated phenolatea nion have not been unambiguously documented.
Phenol with ap K BH + value of 9.98 [1,12] is weakly acidic andi s easily deprotonated by alkali hydroxides or hydrides to yield the corresponding metal phenolates. [5,7,13] Fraser et al. reported on the separation of sodium and potassiumc ations from phenolates [15] and phenol-phenolate salts [16] by meanso fc rown ethers. For the latter they reported short hydrogen bonds with OÀOd istances of 247.1(3) pm to 248(1) pm. The strongt endency of hydrogen bondingi sa lso observed in the imidazolium salt of Clyburne and co-workers ( Figure 1, right),w hich exhibits strongc ation-anion interactions. [14] Reetz et al. used tetra-n-butylammonium hydroxide for the deprotonation of phenol to generate af ree [H 5 C 6 -O] À anion withoutc ation-anion interactions. Alla ttempts to isolate the phenolate anion were thwarted by the selectivef ormation of the phenol-phenolate adduct (Figure 1, left). [11] Davidson applied phosphonium ylides for the deprotonation of phenols resulting in salts featuring short cation-anion CÀH···O À hydrogen bonds. [17,18] In addition to that, numerous substituted phenol derivatives containing electron-withdrawing groups, thus featuring an increased acidity,w ere investigated. [6,19] However,n oe xample of the non-coordinated phenolate anion [H 5 C 6 -O] À was reporteds of ar.T he structuralc haracteristics of mono-and tetraphosphazene bases like 1 and 2, [20] presented in Scheme 1a nd Scheme2,s eem promising for the design of systems featuring the free phenolate anion,a sw ell as phenolate derivativesc ontaining electron-donating groups.
The deprotonation of phenol with equimolar quantities of the commerciallya vailablep yrrolidino phosphazene 1 in diethyl ether leads to the precipitation of al ight browno il. [21] The 31 PNMR spectrum of the supernatant shows the signal of the free base at d = À10.3 ppm. Thus,t he basicity of the pyrrolidino phosphazene 1 is not sufficient for the complete deproto- Figure 1. Structures of the phenol-phenolate anion [11] and an NHC adduct of ap henol derivative. [14] nation of phenol and solely affords ap henol-phenolate adduct (Scheme 1, Figure 2). [22] Salt [1H][PhO(HOPh)] crystallizes from the reaction mixture at À28 8Ci na n8 4% yield. [21] In the 31 PNMR spectrum of the product as ignal at d = 22.2 ppm is observed, which is due to the protonated phosphazene [1H] + + .
The reaction of equimolar quantities of phenola nd 2 leads to the precipitation of the expectedp henolate [2H][PhO] ( Figure 3) as colorless crystals in yields up to 95 %. [21] The product is highly air sensitivea nd decomposes above 75 8C. The decomposition of the product in [D 1 ]chloroform and [D 3 ]acetonitrile solution was observed by 13 CNMR spectroscopy and led to deep blue and strong yellow solutions, respectively,whose color eventually faded. [21] Salt [2H][PhO] is the first example of the non-coordinated phenolate anion. The anion is disordered in ar atio of 94:6. [22] In the major representative the closest CÀH···O À contact of cation and anion( O1BÀC31) was determined to 325.9(2) pm, which is in the range of CÀH···O À hydrogen bonds. [18,26] The CÀ Ob ond length of the anion in [2H][PhO] amountst o 128.7(2) pm and is thus significantly shortenedi nc omparison to the CÀOb onds of coordinated anions as present in sodium phenolate (133(1) pm) [27] or in [1H][PhO(HOPh)] (131.9(2) pm). This bond shortage points to as ignificant resonance stabilization of the negative charge, which is also confirmed by a strongu pfield shift (d = 5.5 ppm) of the signal of the para positioned protoni nt he 1 HNMR spectrum (Figure 4, top). The CÀC distances in free [PhO] À are slight elongated (138.6(1) pm to 143.6(1) pm) comparedt os odium phenolate (138(1) Scheme2.Synthesis of phenolate salts using phosphazene 2.   . [21] Hydrogen bonding brings about downfield shifts of the aromatic protons in the 1 HNMR spectruma nd clean couplings (Figure 4, bottom).
Deprotonation of this phenol with 2 clearly furnished the corresponding phenolate salt [2H][ MeOtBu2 PhO] (Figure 7). [21] The salt is significantly more air sensitivet han the previously discussed phenolate. Air contact effects aq uick color change from yellow to red-brown.