Secondary diphosphine and diphosphido ligands: synthesis, characterisation and group 1 coordination compounds

Abstract

Two types of secondary diphosphines, 1,8-(ArPH)₂C₁₄H₈ (1a: Ar = Tripp, 2,4,6-triisopropylphenyl; 1b: Ar = Mes, 2,4,6-trimethylphenyl) and 1,3-(^tBuPHCH₂)₂C₆H₄ (2), based on rigid 1,8-anthracene and flexible m-xylyl frameworks, respectively, have been synthesized using different strategies. Compounds 1a and 1b were formed by nucleophilic aromatic substitution of a potassium organophosphido salt onto 1,8-difluoroanthracene, while compound 2 was obtained by addition of the Grignard reagent [1,3-(ClMgCH₂)₂C₆H₄]_x to a dichloroorganophosphine, followed by reduction to the diphosphine. These compounds were isolated as ca. 1 : 1 mixtures of rac and meso diastereomers as determined by multinuclear NMR spectroscopy. Borane and selenide derivatives of 2, 1,3-(^tBuPH(BH₃)CH₂)₂C₆H₄ (3) and 1,3-(^tBuPH(Se)CH₂)₂C₆H₄ (4), were obtained. Preferential crystallization of one diastereomer of 3 and 4 was observed; X-ray crystallographic studies identified this as the rac isomer for diselenide 4. Metallation studies of compounds 1a and 2 yielded several alkali metal salts. The reaction of KH or K metal with 1a yielded the compounds 1,8-(TrippPK)₂C₁₄H₈·xTHF (5) and 1,8-(TrippPK)₂C₁₄H₁₀·xTHF (6), respectively; in complex 6 the central aromatic ring has been reduced to yield a bent dihydroanthracene backbone. A crown ether derivative of 6, [K(18-crown-6)(THF)₂]₂[1,8-(TrippP)₂C₁₄H₁₀] (7), was characterised crystallographically. Double deprotonation of compound 2 with ⁿBuLi/TMEDA (TMEDA = N,N,N′,N′-tetramethylethylenediamine) afforded the yellow dilithium complex 1,3-(^tBuPLiCH₂)₂C₆H₄·TMEDA (8), which crystallized as a dimer featuring lithium–arene π interactions.