Electrophilic Activation: Unexpected Metal−Metal Bond-Assisted Tl+ Chelation by a Pt-Benzyl Moiety Instead of Chloride Abstraction

Abstract

One of the activation procedures most frequently used in late transition metal chemistry consists in generating cationic metal complexes by halide abstraction from the metal center in the presence of a weakly coordinating anion. We report here on the major effect of replacing Ag(I) with Tl(I) salts, although they are often used indiscriminately as halide abstractors. The Pt(II) complex [Pt(CH2Ph)Cl(PCH2-ox)] (1) (PCH2-ox = κ2-P,N-(oxazolinylmethyl)diphenylphosphine) yielded the expected metathesis product [Pt(CH2Ph)(OTf)(PCH2-ox)] (2) when treated with 1 equiv of AgOTf (OTf = SO3CF3) in CH2Cl2. In a coordinating solvent such as acetonitrile, chloride displacement readily afforded [Pt(CH2Ph)(NCCH3)(PCH2-ox)]X (3), irrespective of the nature of the halide abstractor (M+ = Ag+ or Tl+) and counterion (X- = OTf-, BF4-, PF6-) used. Reaction of 1 in CH2Cl2 with only half an equivalent of AgBF4 afforded the new, chloride-bridged dinuclear complex [{Pt(CH2Ph)(PCH2-ox)}2(μ-Cl)]BF4 (5·BF4), which results from trapping of the cation [Pt(η3-CH2Ph)(PCH2-ox)]+ (4) by unreacted 1. Similarly, the Pt/Pd heterometallic, single-chloride bridged complex [{Pt(CH2Ph)(PCH2-ox)}(μ-Cl){PdMe(PCH2-ox)}]BF4 (6·BF4) was obtained by reaction of 4 with [PdClMe(PCH2-ox)] in a 1:1 ratio. When 1 was reacted in CH2Cl2 with Tl+ instead of Ag+, formation of 4 was not observed and the main product was an unexpected adduct of Tl+ to 1 whose X-ray analysis established the formation of both a Pt−Tl bond and a η6-benzyl−Tl interaction. This bimetallic complex, [(PCH2-ox)ClPtTl{μ-(η1-CH2;η6-C6H5)CH2Ph}(Pt−Tl)]PF6 (7·PF6), is to our knowledge the first metal−metal bonded Tl−Pt−Cl complex to be fully characterized. The coordination geometry around Pt(II) is square-pyramidal, with Tl(I) in the apical position. The Pt−Tl distance of 3.0942(9) Å corresponds to a metal−metal bond that results mainly from donation of electron density from the Pt(II) 5dz2 orbital to the vacant Tl(I) 6pz orbital. The Pt−Tl bond is not exactly orthogonal to the Pt(II) square-plane (angle of 70(3)°), but parallel to the C(1)−C(2) bond, thus allowing better π-donation from the benzyl ligand to Tl+. When the corresponding benzoyl complex [Pt{C(O)Ph}Cl(PCH2-ox)] (9) was reacted with MX (M+ = Ag+, Tl+) in CH2Cl2, only chloride abstraction and CO deinsertion occurred. Our findings explain why halide abstraction to generate a cationic metal complex with enhanced (catalytic) reactivity may not come to full completion or fail owing to trapping of the cationic complex or “capture” of Tl+ by the neutral precursor acting, in our case, as an unprecedented chelate through metal−metal bond formation and benzyl coordination. The crystal structures of 1, 5·BF4·0.5CH2Cl2, 7·PF6, 9·0.5CH2Cl2, and 10·PF6·0.75C4H8O have been determined by X-ray diffraction.