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 LiC6H3-2,6-Mes(2) (Mes = C6H2-2,4,6-Me-3) or LiC6H3-2,6-Trip(2) (Trip = C6H2-2,4,6-Pr-i(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 H-1 and C-13 NMR spectroscopy as well as by P-31 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= PC6H3-2,6-Trip(2), 12, and the phosphafluorene species, 1-(2,4,6-triisopropylphenyl)-5,7-diisopr phafluorene, 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=EC6H3- 2,6-Mes(2) (E = As, 8; Sb, 9; Bi, 10) and 2,6-Trip(2)H(3)C(6)E=EC6H3-2,6-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)C6H3-2,6-Trip(2) (14b). The compounds, which displayed high thermal stability, were characterized by H-1, C-13, and P-31 NMR and W-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 pi overlap is quite important in the fifth- and sixth-period elements of this group.