Phosphaborane chemistry. Syntheses and calculated molecular structures of mono- and di-chloro derivatives of 1,2-diphospha-closo-dodecaborane(10)

Abstract

The reaction between nido-B₁₀H₁₄ and PCl₃ in the presence of proton sponge (PS = 1,8-bis(dimethylamino)naphthalene) in CH₂Cl₂ yielded a mixture of icosahedral diphosphaboranes from which 1,2-closo-P₂B₁₀H₁₀ (1a) and its mono- and di-chloro derivatives 3-Cl-1,2-closo-P₂B₁₀H₉ (2a), 4-Cl-1,2-closo-P₂B₁₀H₉ (2b), 3,6-Cl₂-1,2-closo-P₂B₁₀H₈ (3a), and 3,4-Cl₂-1,2-closo-P₂B₁₀H₈ (3b) were isolated together with 4-Cl(CH₂)₄O-1,2-closo-P₂B₁₀H₁₀4via preparative HPLC. Individual compounds were characterized by NMR (¹H, ¹¹B, ³¹P) spectroscopy and, except for 4, geometry optimised at the (RMP2(fc)) level with a 6-31G* basis set. The optimisation disclosed considerable distortions of the icosahedral cage as documented by the narrowing of the pentagonal B–P–B angles (ca. 93°) and the longer P–P separations (>2.31 Å). In this context, the RMP2(fc)/6-31G* geometries of 1,7-closo-P₂B₁₀H₁₀ (1b) and that of the non-isolated 4,5-Cl₂-1,2-closo-P₂B₁₀H₈ isomer (3c) were also derived. The geometries were used as a basis for GIAO-SCF/II calculations of the ¹¹B NMR chemical shifts which were in good agreement with experimental δ (¹¹B) values. Moreover, the molecular structure of 3a was determined by an X-ray diffraction analysis to demonstrate satisfactory agreement with the theoretical geometry.