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.

Non‐coordinated and Hydrogen Bonded Phenolate Anions as One‐Electron Reducing Agents

“…The potential of the anion in [1H][(PhO) 2 H] with a value of E Ox =+0.22(1) V vs. SCE experienced an even stronger anodic shift. This trend is confirmed by calculated ionization potentials at the BP86/6–311+g(3df,2p) level of theory, [21] according to which the non‐coordinated anion [PhO] − ( E i =228.69(1) kJ mol −1 ) is significantly influenced by hydrogen bonding to a water molecule in [PhO(H 2 O)] − ( E i =267.42(1) kJ mol −1 ) [10] . The hydrogen bond donation capability in [(PhO) 2 H] − is characterized by a further increase of E i to 314.90(1) kJ mol −1 .…”

“…As described before, the oxidation potential of salt [1H][PhO] was determined to E Ox =−0.12(1) V vs. SCE [10] . This value is significantly cathodically shifted compared to the reported literature data in acetonitrile solution (+0.24 V vs. SCE) [12]…”

“…As discussed previously, [10] salt [1H][PhO] exhibits the first non‐coordinated phenolate [H 5 C 6 ‐O] − anion as building block. The 13 C NMR resonance of the C‐O − carbon atom in [PhO] − ( δ =175.0 ppm) is significantly deshielded in comparison to the anions [C 10 H 7 O] − ( δ =173.7 ppm), [ Me t Bu2 PhO] − ( δ =170.2 ppm), [ t Bu3 PhO] − ( δ =170.3 ppm) and [ MeO t Bu2 PhO] −[10] ( δ =168.0 ppm).…”

“…Salt [1H][(PhO) 2 H] incorporates the anion with an asymmetric, moderately strong hydrogen bond [18] with d(O1‐O8)=243.7(2) pm and therefore the 13 C NMR resonance of the C‐O − carbon atoms of δ =167.2 ppm is shifted upfield in comparison to free [PhO] − ( δ =175.0 ppm) [10] …”

“…Thus, in the absence of cation‐anion interactions, the effect of hydrogen bonding on the redox properties of phenolate anions can be investigated in detail. The presence of water also effects the oxidation potential of phenol, [10] which casts doubt on the reported phenolate redox data from the literature, which were obtained from phenolates generated by deprotonation with tetraalkylammonium hydroxide hydrates in acetonitrile solution [11, 12] . The elucidation of the influence of hydrogen bonding requires uncharged phosphazene bases for the deprotonation of phenols to create a definite design of hydrogen bonded phenol‐phenolate adducts or phenolate hydrates.…”