Dinuclear Palladium(II) and -(III) Compounds with O,O-Chelating Ligands. Room-Temperature Direct 2‑Phenylation of 1‑Methylindole

New dinuclear palladium­(III) compounds of general formula Pd₂[(C₆H₄)­PPh₂]₂[O–O]₂Cl₂, O–O being chelating phenolates C₆H₄OC­(O)­R (R = CH₃, <b>3a</b>; R = C₂H₅, <b>3b</b>; R = OPh, <b>3c</b>) or acetylacetonates RC­(O)­CHC­(O)­R (R = CH₃, <b>4a</b>; R = CF₃, <b>4b</b>; R = C­(CH₃)₃, <b>4c</b>), have been obtained by oxidation with PhICl₂ of the corresponding palladium­(II) compounds. The stability of the new compounds has been studied by ³¹P NMR spectroscopy from 200 to 298 K. DFT calculations of the stability of the complexes have also been performed. In agreement with these calculations, only compound Pd₂[(C₆H₄)­PPh₂]₂[(CF₃C­(O)­CHC­(O)­CF₃]₂Cl₂, <b>6b</b>, showed the highest thermal stability. <b>6b</b> was characterized by X-ray diffraction methods, presenting the longest Pd–Pd distance, 2,6403(6) Å, observed among the already described discrete Pd₂⁶⁺ compounds. The isolated palladium­(II) and -(III) compounds have been tested at room temperature in the catalytic 2-phenylation of 1-methylindole with [Ph₂I]­PF₆. With <b>3a</b> as precatalyst the reaction was completed in 2 h with a 93% isolated yield. The results were compared with those obtained with other orthometalated dinuclear and mononuclear palladium compounds

Dinuclear Palladium(II) and -(III) Compounds with O,O-Chelating Ligands. Room-Temperature Direct 2‑Phenylation of 1‑Methylindole

New dinuclear palladium­(III) compounds of general formula Pd₂[(C₆H₄)­PPh₂]₂[O–O]₂Cl₂, O–O being chelating phenolates C₆H₄OC­(O)­R (R = CH₃, <b>3a</b>; R = C₂H₅, <b>3b</b>; R = OPh, <b>3c</b>) or acetylacetonates RC­(O)­CHC­(O)­R (R = CH₃, <b>4a</b>; R = CF₃, <b>4b</b>; R = C­(CH₃)₃, <b>4c</b>), have been obtained by oxidation with PhICl₂ of the corresponding palladium­(II) compounds. The stability of the new compounds has been studied by ³¹P NMR spectroscopy from 200 to 298 K. DFT calculations of the stability of the complexes have also been performed. In agreement with these calculations, only compound Pd₂[(C₆H₄)­PPh₂]₂[(CF₃C­(O)­CHC­(O)­CF₃]₂Cl₂, <b>6b</b>, showed the highest thermal stability. <b>6b</b> was characterized by X-ray diffraction methods, presenting the longest Pd–Pd distance, 2,6403(6) Å, observed among the already described discrete Pd₂⁶⁺ compounds. The isolated palladium­(II) and -(III) compounds have been tested at room temperature in the catalytic 2-phenylation of 1-methylindole with [Ph₂I]­PF₆. With <b>3a</b> as precatalyst the reaction was completed in 2 h with a 93% isolated yield. The results were compared with those obtained with other orthometalated dinuclear and mononuclear palladium compounds

Pyrazole and Pyrazolate as Ligands in the Synthesis and Stabilization of New Palladium(II) and (III) Compounds

The versatility of pyrazole/pyrazolate as ligands has allowed the synthesis and the structural characterization of four different types of new orthometalated palladium compounds, for which DFT calculations have been performed in order to investigate their relative stabilities. [Pd₂{μ-(C₆H₄)­PPh₂}₂{μ-(R,R′₂pz)}₂] (R = R′ = H, <b>2a</b>; R = Br, R′ = H, <b>2b</b>; R = CH₃, R′ = H, <b>2c</b>; R = H, R′ = CH₃, <b>2d</b>; R = Br, R′ = CH₃, <b>2e</b>) compounds with <i>exo</i>-bidentate pyrazolatos are the first paddlewheel dinuclear palladium­(II) compounds with pyrazolato bridging ligands described and characterized in the literature. In the process of the synthesis of <b>2a</b>, a new tetranuclear intermediate compound, [Pd₄{μ-(C₆H₄)­PPh₂}₄(μ-pz)₂(μ–OH)₂] (<b>3a</b>), has been isolated and structurally characterized. Compounds of the general formula [Pd₂{μ-(C₆H₄)­PPh₂}₂Br₂(R,R′₂pzH)₂] (R = R′ = H, <b>4a</b>; R = Br; R′ = H, <b>4b</b>; R = CH₃; R′ = H, <b>4c</b>; R = H; R′ = CH₃, <b>4d</b>; R = Br; R′ = CH₃, <b>4e</b>) with pyrazoles as monodentate ligands have also been obtained, in which, according to the QTAIM analysis, additional Br···HN_pz weak interactions stabilize their structure. The tetranuclear Pd₂Ag₂ compounds, [Pd₂{μ-(C₆H₄)­PPh₂}₂{μ-(R,R′₂pz-Ag-R,R′₂pz)}₂] (R = R′ = H, <b>5a</b>; R = Br; R′ = H, <b>5b</b>; R = CH₃, R′ = H, <b>5c</b>), showed a distorted tetrahedron disposition of the metal atoms. The QTAIM analysis revealed an enhanced stability because of additional metal–metal interactions. New palladium­(III) compounds, [Pd₂{μ-(C₆H₄)­PPh₂}₂{μ-(R,R′₂pz)}₂Cl₂] (R = R′ = H, <b>6a</b>; R = Br, R′ = H, <b>6b</b>) were also synthesized by oxidation of compounds <b>2</b> with PhICl₂. DFT calculations highlighted their greater stability compared to that of similar compounds with N,N-donor ligands, such as formamidinatos and triazenidos

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

New orthometalated dinuclear triazenide palladium­(II) compounds of the general formula Pd₂[(C₆H₄)­PPh₂]₂[R–N–N–N–R]₂ (R = C₆H₅, <b>3a</b>; <i>o</i>-BrC₆H₄, <i>o</i>-<b>3b</b>; <i>o</i>-MeOC₆H₄, <i>o</i>-<b>3c</b>; <i>o</i>-MeC₆H₄, <i>o</i>-<b>3d</b> ; <i>p</i>-BrC₆H₄, <i>p</i>-<b>3b</b>; <i>p</i>-MeOC₆H₄, <i>p</i>-<b>3c</b>; <i>p</i>-MeC₆H₄, <i>p</i>-<b>3d</b>) 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 <i>o</i>-<b>3b</b> and <i>o</i>-<b>3c</b> underwent a reduction process giving dinuclear palladium­(I) compounds, Pd₂[R–N–N–N–R]₂ (R = <i>o</i>-BrC₆H₄, <i>o</i>-<b>4b</b>; R = <i>o</i>-MeOC₆H₄, <i>o</i>-<b>4c</b>). DFT calculations verified the importance of the mostly noncovalent Pd···Br or Pd···OMe axial interactions on the stability of these compounds. Under cyclic voltammetric conditions, compounds <b>3</b> with the only exception of compound <i>o</i>-<b>3b</b>, were found to undergo a first reversible wave that is assigned as the Pd₂⁵⁺/Pd₂⁴⁺ pair. Compounds <i>p</i><b>-3b</b>-<b>d</b> 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–Pd­(III)–Pd­(III)–Cl 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–N–N–N–R]₂Cl₂ (R = C₆H₅, <b>5a</b>; R = <i>p</i>-BrC₆H₄, <i>p</i>-<b>5b</b>), 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 <b>3a</b> 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 <i>o</i>-<b>3d</b> 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–Ph interactions