Triazenides as Suitable Ligands in the Synthesis of Palladium Compounds in Three Different Oxidation States: I, II, and III

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• 作者：Susana Iba帽ez ; Larisa Oresmaa ; Francisco Estevan ; Pipsa Hirva ; Mercedes Sana煤 ; M^a Angeles 脷beda
• 刊名：Organometallics
• 出版年：2014
• 出版时间：October 13, 2014
• 年：2014
• 卷：33
• 期：19
• 页码：5378-5391
• 全文大小：575K
• ISSN：1520-6041

文摘

New orthometalated dinuclear triazenide palladium(II) compounds of the general formula Pd₂[(C₆H₄)PPh₂]₂[R鈥揘鈥揘鈥揘鈥揜]₂ (R = C₆H₅, 3a; o-BrC₆H₄, o-3b; o-MeOC₆H₄, o-3c; o-MeC₆H₄, o-3d ; p-BrC₆H₄, p-3b; p-MeOC₆H₄, p-3c; p-MeC₆H₄, p-3d) have been synthesized and structurally characterized. The characteristics of these compounds were compared with the isoelectronic formamidinate derivatives. These triazenide compounds have been suitable starting products in the synthesis of new not so common dinuclear palladium(I) compounds and new unusual palladium(III) ones. In the presence of an excess of the triazenide ligand, compounds o-3b and o-3c underwent a reduction process giving dinuclear palladium(I) compounds, Pd₂[R鈥揘鈥揘鈥揘鈥揜]₂ (R = o-BrC₆H₄, o-4b; R = o-MeOC₆H₄, o-4c). DFT calculations verified the importance of the mostly noncovalent Pd路路路Br or Pd路路路OMe axial interactions on the stability of these compounds. Under cyclic voltammetric conditions, compounds 3 with the only exception of compound o-3b, were found to undergo a first reversible wave that is assigned as the Pd₂⁵⁺/Pd₂⁴⁺ pair. Compounds p-3b-d also showed at more positive potentials a second reversible wave, Pd₂⁵⁺/Pd₂⁶⁺ pair. When the electrochemical oxidation was performed in the presence of chloride, the Cl鈥揚d(III)鈥揚d(III)鈥揅l species were detected. By chemical oxidation of the palladium(II) complexes with PhICl₂, two new relatively stable dinuclear palladium(III) compounds, Pd₂[(C₆H₄)PPh₂]₂[R鈥揘鈥揘鈥揘鈥揜]₂Cl₂ (R = C₆H₅, 5a; R = p-BrC₆H₄, p-5b), were synthesized and spectroscopically characterized at low temperature. DFT calculations have been performed to study the stability of all the palladium complexes. The isolated palladium(III) and -(II) compounds have been tested as precatalysts at room temperature in the catalytic 2-phenylation of indole with [Ph₂I]PF₆. With 3a the reaction was complete in the shortest reaction time, 7 h, with a 88% isolated yield. The highest yield (99%) but with higher reaction time, 24 h, was obtained with o-3d when CH₂Cl₂ was added to the reaction medium with the aim of solving the palladium complex. These catalytic results were compared with those obtained with other orthometalated palladium compounds with isoelectronic ligands: formamidinate and carboxylates. In the search for a possible first step in this catalytic oxidation process we have also performed DFT calculations exploring the potential Pd(III) intermediate formed by axial Pd鈥揚h interactions.