No 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) 5d z 2 orbital to the vacant Tl(I) 6p z 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.

As part of our interest in the design and reactivity of P,O ligands, and because the insertion chemistry of small molecules into a metal alkyl bond is very dependent on the ancillary ligands, the behavior of Pt-methyl complexes containing the beta-phosphonato-phosphine ligand rac-Ph2PCH(Ph)P(O)(OEt)2 (abbreviated PPO in the following) toward CO insertion has been explored. New, mononuclear Pt(II) complexes containing one or two PPO ligands, [PtClMe(kappa2-PPO)] (1), [Pt{C(O)Me}Cl(kappa2-PPO)] (2), [PtMe(CO)(kappa2-PPO)]OTf (3 x OTf), [PtMe(OTf)(kappa2-PPO)] (4), trans-[PtClMe(kappa1-PPO)2] (5), [PtMe(kappa2-PPO)(kappa1-PPO)]BF4 (6 x BF4), [PtMe(kappa2-PPO)(kappa1-PPO)]OTf (6 x OTf), and [Pt{C(O)Me}(kappa2-PPO)(kappa1-PPO)]BF4 (7 x BF4) have been prepared and characterized. Hemilability of the ligands is observed in the cations 6 and 7 in which the terminally bound and chelating PPO ligands exchange their role on the NMR time-scale. The acetyl complexes 2 and 7 are stable in solution, but the former deinserts CO upon chloride abstraction. We also demonstrate the ability of PPO to behave as an assembling ligand and to stabilize a heterometallic Pt-Ag metal complex, [PtMe(kappa2-PPO){mu-(eta1-P;eta1-O)PPO)}Ag(OTf)(Pt-Ag)]OTf (8 x OTf), which was obtained by reaction of 5 with AgOTf to generate more reactive, cationic complexes. Whereas the first equivalent of AgOTf abstracted the chloride ligand, the second equivalent added to the cationic complex with formation of a Pt-Ag bond (2.819(1) A). The complexes 1, 2, 4, 5 x CH2Cl2, and (8 x OTf)2 have been structurally characterized by single-crystal X-ray diffraction. The latter has a dimeric nature in the solid state, with two silver-bound triflates acting as bridging ligands between two Pt-Ag moieties. In addition to the Ag-Pt bond, the Ag+ cation is stabilized by a dative O -->Ag interaction involving one of the PPO ligands.