Metallaheteroborane chemistry. Part 5. Synthesis of closo-[2,2-(PR3)2-1,2-SePtB10H10](R3= Et3, Bun3 or Me2Ph) and arachno-[9,9-(PMe2Ph)2-6,9-SePtB8H10] and their characterisation by nuclear magnetic resonance techniques: the crystal and molecular structures of closo-[2,2-(PEt3)2-1,2-SePtB10H10] and arachno-[9,9-(PMe2Ph2),-6,9-SePtB8H10]

Abstract

The close compounds [2,2-(PR₃)₂-1,2-SePtB₁₀H₁₀], where R₃= Et₃(1) or Buⁿ₃(2), have been isolated in low-to-moderate yield from the reaction between cis-[PtCl₂(PR₃)₂] and nido-[7-SeB₁₀H₁₁]^– in tetrahydrofuran. Both closo-[2,2-(PMe₂Ph)₂-1,2-SePtB₁₀H₁₀](3) and arachno-[9,9-(PMe₂Ph)₂-6,9-SePtB₆H₁₀](4) were isolated from the reaction with R₃= Me₂Ph. N.m.r. spectroscopy confirms the general structures of the compounds, and variable-temperature ¹H-{³¹P} experiments on (3) show the rotational metal-to-heteroborane bonding fluxionality to have ΔG₃₂₈^‡ca. 58 kJ mol^–1. An X-ray diffraction study of compound (1) shows the orthorhombic crystals belong to the space group Fdd2, with Z= 16 and cell dimensions a= 55.052(6), b= 17.842(5), and c= 10.259(4)Å, R= 0.042 for 2 551 observed reflections. Cage interatomic distances involving Pt or Se are Pt–Se 2.676(2), Pt–B 2.23(2)–2.32(2), and Se–B 2.08(2)–2.20(2)Å. Crystals of (4) are orthorhombic, space group P2₁nm with a= 6.935(1),b= 10.048(2), and c= 17.413(2)Å, R= 0.022 for 1 375 observed reflections. Interatomic distances include Pt–B 2.147(10)–2.230(7) and Se–B 2.062(15)–2.113(8)Å. Both the X-ray crystallographic and n.m.r. results for (4) indicate this compound belongs to the arachno class. As a model for (4), the bonding of the PtP₂ unit to the heteroborane SB₈H₁₀ ligand has been analysed in a frontier-orbital approach.