Conformational polymorphism and fluxional behaviour of M(PR3)2 units in closo-twelve-atom metallaheteroboranes with MX2B9(X = C or As) and MZB10 cages (Z = S, Se or Te)

Abstract

The compounds [3,3-(PEt₃)₂-closo-3,1,2-PtAs₂B₉H₉]1 and [2,2-(PMe₂Ph)₂-closo-2,1-PtSB₁₀H₁₀]2 were synthesised and characterised by NMR spectroscopy and X-ray crystallography. Crystals of 1 exhibit conformational polymorphism with five conformers of the Pt(PEt₃)₂ unit above the As₂B₃ face; those of 2 contain single conformers. The free energy of the barrier to rotation of the Pt(PMe₂Ph)₂ unit above the SB₁₀ cage ligand in 2 has been determined in chloroform solution and is compared with data from other platinum and palladium metallaheteroboranes containing C₂B₉H₁₁, As₂B₉H₉, SeB₁₀H₁₀ and TeB₁₀H₁₀ ligands. A mechanism for the rotation of M(PR₃)₂ units above heteroborane ligand faces is suggested. It involves shifting the M(PR₃)₂ unit viaηⁿ-bonded species (n < 5) with a concomitant twisting of the M(PR₃)₂ unit about an axis passing approximately through the metal atom and the antipodal B atom.