Platinum ethyl complexes with β-agostic Pt–H–C bonding

Abstract

Protonation with non-co-ordinating acids of the complexes [Pt(η2-C2H4)(L–L)][L–L =(H11C6)2P(CH2)2P(C6H11)2, 1a, But2P(CH2)2PBut2, 1b, (H11C6)2P(CH2)3P(C6H11)2, 1c, But2P(CH2)3PBut2, 1d or o-But2PCH2C6H4CH2PBut2, 1e] and [PtEt2(L–L)]2a–2d affords a series of cationic platinum(II) complexes 3a–3e which in the case of 3a–3c adopt a cis ethene/hydride ground state whereas in 3d and 3e the otherwise electron-deficient metal centre is stabilized by a two-electron, three-centre agostic interaction with the β-CH bond of the ethyl ligand. Complexes 1–3 were characterized by 1H, 13C and 31P NMR spectroscopy and for 2d and 3d by single-crystal X-ray crystallography. The influence of the chelating diphosphine ligand on the strength of the agostic bond was monitored by NMR spectroscopy. This revealed that the cations undergo two fluxional processes in solution: (a) agostic methyl rotation and (b)β-elimination/ethene rotation, a combination of which scrambles all five protons and both carbon atoms of the ‘C2H5’ moiety. The 31P nuclei, however, remain inequivalent at temperatures up to 300 K. The agostic interaction was displaced by a small two-electon donor molecule L to form the series of adducts [PtEt(L)(L–L)]+(L = acetonitrile or pyridine) in which the ‘normal’ ethyl complex is the first formed species. The adducts are unstable to loss of C2H4 by β-elimination to form the series of cationic hydrides [PtH(L)(L–L)]+. For comparison, the complex [PtH(O3SCF3){But2P(CH2)3PBut2}] was synthesised and characterized by 1H and 31P NMR spectroscopy and X-ray crystallography.