Ligand control of agostic M ⋯ H ⋯ C three-centre, two-electron bonding in bicyclo[2.2.1]hept-2-yl complexes of platinum and palladium. X-Ray crystal structures of [Pt(η2-C7H10){But2P(CH2)2PBut2}] and [Pt(C7H11){But2P(CH2)2PBut2}][BPh4]

Abstract

The reaction of non-co-ordinating acids with the Pt⁰ and Pd⁰ alkene complexes [M(η²-C₇H₁₀)(L–L)][M = Pt or Pd; L–L =(C₆H₁₁)₂P(CH₂)₂P(C₆H₁₁)₂, 1a or 1f: Bu^t₂P(CH₂)₂PBu^t₂, 1b or 1g; and o-C₆H₄(CH₂PBu^t₂)₂, 1e or 1h; M = Pt, L–L =(C₆H₁₁)₂P(CH₂)₃P(C₆H₁₁)₂1c or Bu^t₂P(CH₂)₃PBu^t₂1d] affords a series of cationic bicyclo[2.2.1]hept-2-yl complexes 2a–2h in which the otherwise electron-deficient metal centre is stablized by a three-centre, two-electron (agostic) interaction with the exo-3-CH bond. The complexes were characterized by ¹H, ¹³C and ³¹P NMR spectroscopy and for 1b and 2b by single-crystal X-ray crystallography. In complex 1b the norborn-2-ene is bound to the platinum in a normal in-plane η²-mode with equal Pt–C distances [2.110(7), 2.108(8)Å] and Pt–P distances [2.273(2), 2.274(2)Å]. For 2b the crystallographic results reveal a long Pt–C_β contact of 2.309(5)Å which is bridged by a hydrogen atom forming the agostic bond, whereas the Pt–C_α bond is shortened to 2.096(4)Å. The Pt–P bond trans to the weak agostic bond is significantly shorter than the cis Pt–P bond [2.256(1) and 2.311(1)Å respectively], and this asymmetry in the co-ordination of the diphosphine is reflected in the ³¹P NMR spectrum of 2b for which ¹J(PtP_trans)¹J(PtP_cis). The extent of agostic interaction, as indicated by NMR parameters [¹J(PtH), ¹J(PtP_trans), etc.], depends on the bite angle of the diphosphine and the bulk of the substituents on phosphorus such that the smallest diphosphines induce the strongest M ⋯ H ⋯ C interaction. All the agostic complexes undergo a rapid intramolecular rearrangement on the NMR time-scale at room temperature involving β-elimination and alkene rotation. However, the ³¹P nuclei remain non-equivalent up to 300 K.