Cyclic Palladium Formamidinate: Synthesis, Structure, Reactivity, and Application to Suzuki-Miyaura Cross-Coupling

The deprotonation of acyclic palladium amidine chloride (1) with potassium tert-butoxide in tetrahydrofuran results in palladium bis(formamidinate) (2). 2 undergoes a nucleophilic addition reaction with acetonitrile in the presence of PdCl₂ or Pd(OAc)₂ (OAc = acetate) to give dinuclear cyclic six-membered (triazapentadiene)palladium complexes (4a and 4b). These compounds are also prepared from cyclic six-membered (tap)PdCl₂ (5; tap = triazapentadiene) or formamidinium salts (6a-6c) with Pd(OAc)₂/NaOAc in acetonitrile, whereas the direct reaction of 2 with acetonitrile or acrylonitrile resulted in palladium black or an acyclic C-N-coupled product (3). A comparison of structure 4 from 2 suggests a possible intermediate dinuclear palladium complex whose structure was identified through theoretical calculations. Further, Suzuki-Miyaura cross-coupling reactions were carried out under different solvents notably in an ethanol/water medium at room temperature.

Tin(IV) Compounds with 2-C6F4PPh2 Substituents and Their Reactivity toward Palladium(0): Formation of Tin-Palladium Complexes via Oxidative Addition

The tin(IV) compounds Me_xSn(2-C₆F₄PPh₂)_4-x (1, x = 1; 2, x = 2) and ClSn(2-C₆F₄PPh₂)₃ (3) were obtained from the reactions of 2-LiC₆F₄PPh₂ with MeSnCl₃ (3:1), Me₂SnCl₂ (2:1), or SnCl₄ (3:1), respectively. The reactions of 2-LiC₆F₄PPh₂ with SnCl₄ in different stoichiometric ratios (4:1-1:1) gave 3 as the main product. Compound Cl₂Sn(2-C₆F₄PPh₂)₂ (4) was formed in the transmetalation reaction of 3 and [AuCl(tht)] but could not be isolated. 1 and 2 react with palladium(0) sources {[Pd(PPh₃)₄] and [Pd(allyl)Cp]} by the oxidative addition of one of their Sn-C_Aryl bonds to palladium(0) with formation of the heterobimetallic complexes [MeSn(μ-2-C₆F₄PPh₂)₂Pd(κC-2-C₆F₄PPh₂)] (5) and [Me₂Sn(μ-2-C₆F₄PPh₂)Pd(κ²-2-C₆F₄PPh₂)] (6) featuring Sn-Pd bonds. The reaction of 3 with palladium(0) proceeds via the oxidative addition of the Sn-Cl bond to palladium(0), thus furnishing the complex [Sn(μ-2-C₆F₄PPh₂)₃PdCl] (7) featuring a Sn-Pd bond and a pentacoordinate Pd atom. Transmetalation of Me_xSn(2-C₆F₄PPh₂)_4-x (x = 1-3) with [Pd(allyl)Cl]₂ gave Me_xClSn(2-C₆F₄PPh₂)_3-x and [Pd(allyl)(μ-2-C₆F₄PPh₂)]₂. For x = 1, the compound MeClSn(2-C₆F₄PPh₂)₂ (generated in situ) reacted with another 1 equiv of [Pd(allyl)Cl]₂ by the oxidative addition of the Sn-Cl bond to palladium(0) and the reductive elimination of allyl chloride, thus leading to [MeSn(μ-2-C₆F₄PPh₂)₂PdCl] (8). The reductive elimination of allyl chloride was also observed in the reaction of 3 with [Pd(allyl)Cl]₂, giving [Sn(μ-2-C₆F₄PPh₂)₃PdCl] (7). All compounds have been characterized by means of multinuclear NMR spectroscopy, elemental analysis, single-crystal X-ray diffraction, and selected compounds by ¹¹⁹Sn Mössbauer spectroscopy. Computational analyses (natural localized molecular orbital calculations) have provided insight into the Sn-Pd bonding of 5-8.