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) 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.

Halide Addition/Abstraction in Phosphido Derivatives: Isolation of the Thallium and Silver Intermediates

“…Analogous decreases in the 1 JPt,P values have been observed in phosphino derivatives after complexation with Tl + . 23,29,34,62 No coupling between 31 P and thallium was observed in the phosphido derivative 7, which is consistent with the finding of other authors. 18,23,29,33,34,62 In the spectrum of 8, the signal due to the P 5 atom appears as two doublets ( 107 Ag and 109 Ag isotopes, I ) 1 / 2 ) somewhat broadened at the base due to the poorly resolved coupling of P 5 with the Pt 2 atom.…”

“…All of these geometric parameters are in agreement with a Pt-Tl bond. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] A noteworthy feature of this structure is the presence of o-F · · · Tl contacts, with distances Tl-F (15) ) 2.951(7) Å and Tl-F(20) ) 3.014(7) Å, which are in the range found in platinum(II) complexes containing Pt-Tl bonds, 19,22,23,27 or thallium complexes containing intra- [37][38][39][40] or intermolecular F · · · Tl contacts. 26,29,37,[41][42][43][44][45] These o-F · · · Tl contacts are also detected in solution in the 19 F NMR spectra of 5 (below).…”

Organometallic palladium and platinum complexes with strongly donating alkyl coligands – Synthesis, structures, chemical and cytotoxic properties

“…1, Table 3). However, an inspection of a big number of benzyl palladium(II) or platinum(II) complexes reveals angles varying from about 103 [31] to 120 [32] for palladium(II) and 112 [33] to 119 [34] for platinum(II). Thus all values lie in the typical range.…”

Thallium(I) as a Coordination Site Protection Agent: Preparation of an Isolable Zero‐Valent Nickel Tris‐Isocyanide

“…Indeed, the electronic structure combination of coordinative unsaturation, coupled with strongly p-acidic ligands has, outside of gas-phase matrix isolation studies and the highly reduced carbonyl metalates championed by Ellis, [3] received limited attention. For neutral d 10 [Ni(CO) 3 ], analogues possessing strongly s-donating phosphines, such as [Ni-(PR 3 ) 3 ], have been widely investigated, [4] whereas recent attention has been given to three-coordinate species featuring NHC ligands. [5,6] With respect to strongly p-acidic isocyanide ligands, studies by Otsuka [7] on [Ni(CNtBu) 4 ] are noteworthy in that homoleptic [Ni(CNtBu) 3 ] is proposed to be the reactive entity in solution.…”

“…Whereas the formation of Tl I À M interactions has been well-studied, [10] considerably less attention has been paid to the behavior or persistence of these units in subsequent chemical reactions. [11] Furthermore, given the insolubility of Tl I halides in organic solvents with low dielectric constants, we reasoned that such a protecting group could be readily removed upon simple addition of external halide ions after the stepwise installation of three CNAr (5) , thereby suggesting that interactions from the CNAr Mes 2 ligand periphery are not responsible for holding the Tl atom in close proximity to the Ni center.…”