The chemistry of isomers of icosaborane(26), B20H26: synthesis and nuclear magnetic resonance study of various isomers of platinahenicosaboranes and diplatinadocosaboranes, and the X-ray crystal and molecular structures of 7,7-bis(dimethylphenylphosphine)-nido-7-platinaundecaborane and 4-(2′-nido-decaboranyl)-7,7-bis(dimethylphenylphosphine)-nido-7-platinaundecaborane

Abstract

An improved synthesis of the known compound [(PMe₂Ph)₂(PtB₁₀H₁₂)] has been developed by deprotonation of B₁₀H₁₄ with NNN′N′-tetramethylnaphthalene-1,8-diamine followed by treatment with cis-[PtCl₂(PMe₂Ph)₂]. This reaction has been applied to the 2,2′, 2,6′, and 1,5′ isomers of (B₁₀H₁₃)₂ to prepare various isomeric platinahenicosaborane clusters [(PMe₂Ph)₂(PtB₁₀H₁₁–B₁₀H₁₃)] which differ either in the position of the conjuncto-linkage or the site of the platinum atom in the cluster. Appropriate modification of the reaction stoicheiometry in the case of 2,2′-(B₁₀H₁₃)₂ led to the isolation of cisoid and transoid diplatinadocosaboranes [{(PMe₂Ph)₂(PtB₁₀H₁₁)}₂]. The X-ray crystal structure of [(PMe₂Ph)₂(PtB₁₀H₁₂)] showed it to contain a platinaundecaborane cluster in which the tetrahapto B₁₀H₁₂ group is twisted by ca. 20° with respect to the PtP₂ plane. Similarly, the molecular structure of the isomer of [(PMe₂Ph)₂Pt(η⁴-B₁₀H₁₁–B₁₀H₁₃)] obtained from 2,2′-(B₁₀H₁₃)₂ is distorted by a twist of ca. 8°. A detailed n.m.r. study of a number of these clusters has been made, using the resonances of ¹H, ¹¹B, ³¹P, and ¹⁹⁵Pt. In addition to permitting structural assignments, the data reveal a novel mutual pseudo-rotation of the η⁴-B₁₀H₁₁X group (X = H or B₁₀H₁₃) and the (PMe₂Ph)₂ grouping about the central Pt atom. For [(PMe₂Ph)₂(PtB₁₀H₁₂)] the two sets of ¹H-{³¹P} methyl resonances at 100 MHz coalesce at 71.5 °C with an implied activation energy ΔG^‡ of 79 ± 5 kJ mol^–1 for the fluxional process. Similar activation energies were deduced for the various isomers of [(PMe₂Ph)₂(PtB₂₀H₂₄)].