2,5-Dihydroxy-1,4-benzoquinones Appended with −P(═O)(OR) 2 (R = Me or Et) Groups and Their Ammonium and Lithium Salts: Structural, Spectroscopic, and Electrochemical Properties

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High Resolution Image Download MS PowerPoint Slide Synthetic methods yielding 2,5-dihydroxy-1,4-benzoquinones appended with dialkylphosphonato groups were established. Reactions of 1,4-dichloro-2,5-dihydroxybenzene with dialkylchlorophosphates ClP(═O)(OR) 2 (R = Me or Et) produced 1,4-dichloro-2,5-bis(dimethylphosphato)benzene ( 1a ) and 1,4-dichloro-2,5-bis(diethylphosphato)benzene ( 1b ). Low-temperature reactions of 1a – b with lithium diisopropylamide (2 equiv) proceeded with double anionic phospho-Fries rearrangement, yielding 2,5-dichloro-3,6-bis(dialkylphosphonato)-1,4-hydroquinones 2a – b . Oxidations of 2a – b with K 2 S 2 O 8 under basic conditions proceeded with the nucleophilic displacement of both chlorides by the hydroxy groups. Acidification of the reaction mixtures led to the isolation of 2,5-dihydroxy-1,4-benzoquinones appended with two dimethylphosphonato (H 2 3a ) or diethylphosphonato (H 2 3b ) groups. Reactions of H 2 3a and H 2 3b with excess NH 3 (aq) or with LiOH yielded salts (NH 4 ) 2 3a, (NH 4 ) 2 3b, Li 2 3a, and Li 2 3b . All compounds were characterized by spectroscopic (FT-IR, FT-NMR ( 1 H, 13 C, and 31 P)) methods and by high-resolution mass spectrometry or elemental analyses. Both 2,5-dihydroxy-1,4-benzoquinones H 2 3a – b and their ammonium and lithium salts were investigated by cyclic voltammetry. Compounds 2a, H 2 3a, H 2 3b, and salts (NH 4 ) 2 3a ·H 2 O, (NH 4 ) 2 3b ·2H 2 O, Li 2 3a ·2H 2 O, and Li 2 3b were characterized by single-crystal X-ray diffraction methods. Compounds H 2 3a /H 2 3b and their salts represent the first examples of anillic acids appended with −P(═O)OR 2 groups. Phosphonato functionalities were shown to be important factors in increasing the solubility of free acids as well as providing coordination/hydrogen bonding sites in their salts.

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