Homologous Series of Heavier Element Dipnictenes 2,6-Ar₂H₃C₆E=EC₆H₃-2,6-Ar₂ (E = P, As, Sb, Bi; Ar = Mes = C₆H₂-2,4,6-Me₃; or Trip = C₆H₂-2,4,6-iPr₃) Stabilized by m-Terphenyl Ligands

The synthesis and characterization of several m -terphenyl heavier main group 15 (P, As, Sb, or Bi) dihalides, together with their reduction to give a homologous series of double-bonded dipnictenes, are reported. Reaction of LiC 6 H 3 -2,6-Mes 2 (Mes = C 6 H 2 -2,4,6-Me 3 ) or LiC 6 H 3 -2,6-Trip 2 (Trip = C 6 H 2 -2,4,6- i Pr 3 ) with the appropriate trihalide affords 2,6-Mes 2 H 3 C 6 ECl 2 (E = As, 1; Sb, 2; Bi, 3 ) and 2,6-Trip 2 H 3 C 6 ECl 2 (E = P, 4; As, 5; Sb, 6; Bi, 7 ). The compounds 1 − 7 were characterized by 1 H and 13 C NMR spectroscopy as well as by 31 P NMR spectroscopy in the case of 4 . In addition, the structures of 3, 5, and 6 were determined. Reduction of the phosphorus species 4 with potassium in hexane gives a mixture of the diphosphene 2,6-Trip 2 H 3 C 6 P PC 6 H 3 -2,6-Trip 2, 12, and the phosphafluorene species, 1-(2,4,6-triisopropylphenyl)-5,7-diisopropyl-9-phosphafluorene, 11 . The compound 11, which results from the insertion of a phosphorus into a C(Ar)−C(i-Pr) bond was synthesized in higher yield by the reduction of 4 with magnesium. The simple reduction of 1 − 4, 6, and 7 with potassium, and of 5 with magnesium, yielded the new series of dipnictenes, 2,6-Mes 2 H 3 C 6 E=EC 6 H 3 -2,6-Mes 2 (E = As, 8; Sb, 9; Bi, 10 ) and 2,6-Trip 2 H 3 C 6 E=EC 6 H 3 -2,6-Trip 2 (E = P, 12; As, 13; Sb, 14a; Bi, 15 ), as well as the partially reduced species 2,6-Trip 2 H 3 C 6 (Cl)SbSb(Cl)C 6 H 3 -2,6-Trip 2 ( 14b ). The compounds, which displayed high thermal stability, were characterized by 1 H, 13 C, and 31 P NMR and UV−vis spectroscopy. The structures of 8 − 11, 13, 14a, and 14b were determined. These compounds constitute the first homologous series of dipnictene structures for all the heavier group 15 elements. The E−E bond shortenings observed for the heaviest antimony or bismuth derivatives lead to the conclusion that π overlap is quite important in the fifth- and sixth-period elements of this group.