Metallaheteroborane chemistry. Part 9. Syntheses and spectroscopy of platinum and palladium phosphine complexes containing η5-{As2B9}-based cluster ligands. Crystal structures of [3,3-L2-closo-3,1,2-PtAs2B9H9](L = PPh3 or PMe2Ph) and [3-Cl-3,8-(PPh3)2-closo-3,1,2-PdAs2B9H8]

Abstract

The reaction between [Pt(PPh₃)₄] and closo-1,2-As₂B₁₀H₁₀ in ethanol gives [3,3-(PPh₃)₂-closo-3,1,2-PtAs₂B₉H₉]1 in low yield (9%). Reaction of [nido-7,8-As₂B₉H₁₀]^– and [MCl₂(PR₃)₂] in tetrahydrofuran(thf) in the presence of a ten-fold excess of NEt₃ affords [3,3-(Pr₃)₂-closo-3,1,2-MAs₂B₉H₉](M = Pt, R₃= Me₂Ph 2; M = Pd, R₃= Ph₃3 or Me₂Ph 4) and [3-Cl-3,8-(PR₃)₂-closo-3,1,2-PdAs₂B₉H₈](R₃= Ph₃5; or R₃= Me₂Ph, 6) in low-to-moderate (20–45%) yields. X-Ray diffraction analyses of 1, 2 and 5 confirm the closo icosahedral MAs₂B₉ cage structures and the presence of a MAs₂ triangulated face in each of these molecules. Both compounds 1 and 2 crystallise in the monoclinic space group P2₁/n with Z= 4. Cell dimensions: 1, a= 1230.0(2), b= 1736.5(4), c= 1881.0(3) pm and β= 91.44(2)°; 2, a= 1308.6(3), b= 1058.0(2), c= 1934.7(3) pm and β= 104.45(2)°. The structure of 1 was refined to a final R of 0.0574 (R′ 0.0532) for the 4160 reflections with I 2.0σ(I). One of the As atoms is disordered over two sites. Calculated Pt-As distances are 259.1(5), 255.7(5) and 264.0(6) pm, As–As 251.5(6) and 243.5(5) pm. Structure 2 had a final R of 0.0384 (R′ 0.0376) for the 3778 reflections with I 2.0σ(I). The Pt–As distances are 265.5(4) and 254.5(4) pm and As–As is 249.7(3) pm. Compounds 5 crystallises in the triclinic space group P with Z= 2, a= 1037.8(2), b= 1321.9(3), c= 1712.9(3) pm, α= 103.67(1), β= 93.19(1) and γ= 110.86(1)°; final R= 0.0392 (R′= 0.0408) for 4688 reflections with I 2.0σ(I). The Pd-As distances are 256.3(4) and 253.9(4) pm and As–As is 247.7(3) pm. Compounds 1–6 were characterised by multielement NMR spectroscopy. Variable-temperature ¹H studies of 2, 4 and 6 show the metal to As₂B₉ ligand bonding is fluxional at room temperature with ΔG‡₂₉₈ < ca. 30 kJ mol^–1 for 2 and 4 and possibly slightly higher for 6 at ca. 32 kJ mol^–1. An analysis of the frontier molecular orbitals of the model ligand P₂B₉H₉ and their possible interactions with a Pt(PH₃)₂ group shows no clear preference between the two possible conformations that the PtP₂ unit could adopt above the P₂B₃ face to which it bonds.