Synthesis and structures of dilithium (1,2-diphosphido)-benzenes

Abstract

Treatment of the appropriate (1,2-diphosphino)benzenes C₆H₃(PHR)₂-1,2-R′-4 (R′= Me or H; R = H, Ph or SiMe₃) with LiBuⁿ and tmen [tmen = Me₂N(CH₂)₂NMe₂] in hexane afforded the dilithium diphosphides [Li(tmen)]₂[C₆H₃(PR)₂-1′,2-R′-4](R′= H, R = Ph 1; R′= H, R = SiMe₃2; R′= Me, R = Ph 3; R′= H = R 4). Crystal structures of 1 and 2 show that each phosphorus atom is bonded equivalently to the two lithium atoms (and conversely each Li to the two P atoms), the two lithium atoms being on opposite sides of the C₆H₄P₂ plane. Solution multinuclear NMR spectral studies of the parent diphosphines, their dilithio-derivatives 1–3 and the phosphole 10[of which C₆H₄(PHSiMe₃)₂-1,2 6 and C₆H₃(PHPh)₂-1,2-Me-4 7, and the phosphole C₆H₃[{([graphic omitted]Ph]Me-4 10 and 1–3 are new compounds] have been carried out. The diphosphine C₆H₄(PHPh)₂5, like 6 and 7, was a ca. 1 : 1 mixture of diastereoisomers, 6 having a much larger (9.2 ppm) chemical shift difference between P and P′ than 5(0.66 ppm). Virtual coupling from both phosphorus atoms was observed for the SiMe₃¹H, ¹³C and ²⁹Si NMR signals of 6. Variable-temperature ³¹P-{¹H} and ⁷Li-{¹H} NMR spectra for 1 and 2 revealed dynamic behaviour; the low-temperature limiting spectra for each of 1 and 2 showed septets [³¹P-{¹H}] and triplets [⁷Li-{¹H}], consistent with the structures of the crystalline molecules.