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1.2.5 Product Subclass 5: Palladium(III)-Containing Complexes
D. C. Powers and T. Ritter General Introduction
Compared with the chemistry of palladium in the 0, I, II, and IV oxidation states, organopalladium(III) chemistry is in its infancy, and complexes containing palladium in the III oxidation state are rare.[1–4] Recent studies have expanded the family of characterized palladium(III) complexes and have also begun to elucidate the potential roles of palladium(III) intermediates in catalysis. This section will review preparative methods for the synthesis of palladium(III) complexes and discuss reactions in which palladium(III) intermediates are proposed. SAFETY: The palladium complexes reported herein can be prepared using the standard precautions generally taken with other potentially hazardous substances found in a chemistry laboratory. Many of the reagents used to prepare palladium(III) complexes are strong oxidants, which can be particularly hazardous. 1.2.5.1 Synthesis of Palladium(III)-Containing Complexes
1.2.5.1.1 Mononuclear Palladium(III) Complexes Mononuclear palladium(II) complexes are typically square planar whereas mononuclear palladium(IV) complexes are typically octahedral.[5] Based on the molecular orbital diagram in ? Figure 1, mononuclear palladium(III) complexes are anticipated to be paramagnetic, low-spin d7, tetragonally distorted octahedral complexes, in which the unpaired electron resides predominantly in the orbital.[6] ? Figure 1 Molecular Orbital Diagram for Mononuclear Palladium(II), Palladium(III), and Palladium(IV) Complexes[5,6] Unlike complexes based on platinum(III),[7–15] compounds containing palladium(III) are rare. Several mononuclear coordination complexes, proposed to contain palladium(III), have been observed by electrochemical measurements as well as EPR spectroscopy.[16–25] The spin density in these complexes, either metal- or ligand-centered, is the source of continuing discussion.[26–29] The various methods that have been developed for the preparation of mononuclear palladium(III) complexes are presented in the following sections. 1.2.5.1.1.1 Method 1: Disproportionation of Palladium(II) Complexes Facially coordinating 1,4,7-triazacyclononane and 1,4,7-trithiacyclononane ligands have been used to stabilize mononuclear palladium(III) complexes.[30–33] Complex 2, in which two facially coordinating tridentate ligands compose the octahedral coordination environment of the palladium(III) center, has been prepared by disproportionation of palladium(II) (? Scheme 1). X-ray crystallographic characterization has established the distorted octahedral geometry of the palladium centers, as expected for low-spin, d7 palladium(III). Electrochemical and spectroscopic investigations have indicated that the unpaired electron in complex 2 resides predominantly in the orbital, consistent with the molecular orbital diagram in ? Figure 1.[34–39] ? Scheme 1 Synthesis of Mononuclear Palladium(III) Werner Complexes by Disproportionation of Palladium(II)[34] Bis(1,4,7-triazacyclononane-?3N)palladium(III) Hexafluorophosphate (2):[34] PdCl2 (0.50 g, 2.8 mmol, 1.0 equiv) was dissolved in deionized H2O (20 mL) and the soln was adjusted to pH 9 with NaOH. The soln was warmed to 50 °C. 1,4,7-Triazacyclononane (0.90 g, 7.0 mmol, 2.5 equiv) was added directly to the PdCl2 soln, in which it dissolved rapidly. Heating was continued for 1 h at this temperature, during which time the remaining solid PdCl2 dissolved, yielding a lemon-yellow soln with deposited Pd metal (0.13 g; 45% of total Pd); the metallic solid was removed by filtration. The yellow filtrate contained two species; the major constituent was the cation of complex 2 with a minor amount of the cation of complex 1. Addition of sat. NH4PF6 soln caused precipitation of 2 as a yellow powder. 1.2.5.1.1.2 Method 2: Oxidation of Palladium(II) Complexes with Perchloric Acid Mononuclear palladium(III) complex 4 has been prepared by chemical oxidation of mononuclear palladium(II) complex 3 with perchloric acid (? Scheme 2).[30] Experimental details of the oxidation of 3 with perchloric acid are unavailable. ? Scheme 2 Preparation of a Mononuclear Palladium(III) Complex by Oxidation of a Mononuclear Palladium(II) Complex with Perchloric Acid[30] 1.2.5.1.1.3 Method 3: Electrochemical Oxidation of Palladium(II) Complexes In 2010, controlled potential electrolysis (CPE) was used to prepare the first mononuclear organometallic complexes of palladium(III) (complexes 6, ? Scheme 3).[40] One-electron oxidation of complexes 5 results in the formation of mononuclear palladium(III) complexes 6, in which the palladium(III) centers are stabilized by chelating tetradentate ligands. ? Scheme 3 Preparation of Mononuclear Palladium(III) Complexes by Controlled Potential Electrolysis of Mononuclear Palladium(II) Complexes[40] R1 X- Conditions Yield (%) Ref Me BF4- Bu4NBF4, CH2Cl2 78 [40] Me PF6- Bu4NPF6, THF 63 [40] Me ClO4- Bu4NClO4, THF 86 [40] Ph ClO4- Bu4NClO4, THF 52 [40] Chloro[3,7-di-tert-butyl-3,7-diaza-1,5(2,6)-dipyridinacyclooctaphane-?4N]methylpalladium(III) Tetrafluoroborate (6, R1 = Me; X = BF4); Typical Procedure:[40] CPE of 5 (R1 = Me) was performed in a two-compartment bulk electrolysis cell in which the auxiliary electrode was separated from the working compartment by a medium-frit glass junction. The electrolysis was carried out in a 100-mL electrolysis cell using a reticulated vitreous carbon working electrode. A stirred soln of mononuclear Pd(II) complex 5 (R1 = Me; 90.0 mg, 177 µmol, 1.00 equiv) in deaerated 0.1 M Bu4NBF4 in CH2Cl2 (70 mL) was electrolyzed at a potential of 0.600 V at 20 °C. The electrolysis was stopped after the charge corresponding to one-electron oxidation had been transferred. The dark green soln resulting from electrolysis was stored at -20 °C overnight. The resulting green fine-crystalline precipitate of mononuclear Pd(III) complex 6 (R1 = Me; X = BF4) was collected by filtration from the cold soln and washed with both Et2O and pentane; yield: 78%. The product was recrystallized (layering a MeCN soln of the product with Et2O at -20 °C) to give 6 (R1 = Me; X = BF4)•MeCN as a dark blue-green solid; yield: 78.9 mg (70%); 1H NMR (CD3CN, d): 12.3 (br s), 10.0, 8.6, -3.2; µeff = 1.80 µB (Evans method, CD3CN soln); UV-vis (MeCN) ? (?): 723(1.1 × 103), 545 (sh, 4.9 × 102), 368 (3.3 × 103), 263 nm (1.2 × 104). 1.2.5.1.1.4 Method 4: Oxidation of Palladium(II) with Single-Electron Oxidants One-electron oxidation of mononuclear palladium(II) complex 7 with either ferrocenium hexafluorophosphate or thianthrenyl hexafluoroantimonate affords mononuclear palladium(III) complex 8 (? Scheme 4).[40] Electrochemical and chemical oxidations (? Sections 1.2.5.1.1.3 and 1.2.5.1.1.4, respectively) allow access to complementary substrate classes; electrochemical oxidation of 7 failed to provide access to mononuclear palladium(III) complex 8. ? Scheme 4 Preparation of a Mononuclear Palladium(III) Complex from a Mononuclear Palladium(II) Complex Using a One-Electron Oxidant[40] [3,7-Di-tert-butyl-3,7-diaza-1,5(2,6)-dipyridinacyclooctaphane-?N4]dimethylpalladium(III) Perchlorate (8):[40] A soln of ferrocenium hexafluorophosphate (58.7 mg, 177 µmol, 1.00 equiv) in MeCN (3 mL) was added dropwise to a stirred suspension of 7 (86.8 mg, 177 µmol, 1.00 equiv) in MeCN (7 mL) at rt in a N2-filled drybox. The mixture was stirred for 20 min, and then the solvent was removed under reduced pressure. The solid residue was redissolved in MeCN (2 mL) and the soln was filtered through a cotton plug. A solid sample of LiClO4 (56.7 mg, 533 µmol, 3.01 equiv) was added to the filtrate causing precipitation of a dark green crystalline solid. The suspension was stored at -30 °C for 30 min. The resulting dark green...
