Structurally Diverse Diazafluorene-Ligated Palladium(II) Complexes and Their Implications for Aerobic Oxidation Reactions

Abstract

4,5-Diazafluoren-9-one (DAF) has been identified as a highly effective ligand in a number of Pd-catalyzed oxidation reactions, but the mechanistic basis for its utility has not been elucidated. Here, we present the complex coordination chemistry of DAF and palladium(II) carboxylate salts. Multiple complexes among an equilibrating mixture of species have been characterized by 1H and 15N NMR spectroscopy and X-ray crystallography. These complexes include monomeric and dimeric PdII species, with monodentate (κ1), bidentate (κ2) and bridging (μ:κ1:κ1) DAF coordination modes. Titration studies of DAF and Pd(OAc)2 reveal the formation of two dimeric DAF/Pd(OAc)2 complexes at low [DAF] and four monomeric species at higher [DAF]. The dimeric complexes feature two bridging acetate ligands together with either a bridging or non-bridging-κ1 DAF ligand coordinated to each PdII center. The monomeric structures consist of three isomeric Pd(κ1-DAF)2(OAc)2 complexes, together with Pd(κ2-DAF)(OAc)2 in which the DAF exhibits a traditional bidentate coordination mode. Replacing DAF with the structurally related, but more-electron-rich derivative 9,9-dimethyl-4,5-diazafluorene (Me2DAF) simplifies the equilibrium mixture to two complexes, a dimeric species in which the Me2DAF bridges the two Pd centers and a monomeric species with a traditional κ2-Me2DAF coordination mode. The use of DAF in combination with other carboxylate ligands (CF3CO2− or tBuCO2−) also results in a simplified collection of equilibrating PdII–DAF complexes. Collectively, the results highlight the ability of DAF to equilibrate rapidly among multiple coordination modes, and provide valuable insights into the utility of DAF as a ligand in Pd-catalyzed oxidation reactions.

4,5-Diazafluorene and 9,9’-Dimethyl-4,5-Diazafluorene as Ligands Supporting Redox-Active Mn and Ru Complexes

“…In their work, Song and co-workers have found that the acidic C-H bonds of daf can undergo deprotonation that results in follow-up reactivity [16][17][18][19][20]. More broadly, Stahl [21][22][23] and several other groups [24][25][26] have developed a number of catalyst systems supported by diazafluorene ligands. In all these cases, daf and its derivatives seem to play a decisive role in enabling unique chemistry, confirming the usefulness of the ligands as a counterpoint to the more common R bpy family.…”

Detection of Palladium(I) in Aerobic Oxidation Catalysis

“…(1) and (2)] [9,10] and that DAF adopts diverse coordination modes, including κ 1 , κ 2 , and bimetallic bridging (μ) ligation, under the catalytic conditions. [11,12] …”

“…[10,11] For example, the ortho and para protons of mononuclear κ 2 -DAF-ligated Pd II complexes overlap, [10] while the corresponding peaks of the new species are well resolved (Δδ > 0.5 ppm). Dimeric Pd II species with bridging DAF ligands have similarly well resolved chemical shifts for the ortho and para protons; however, the o -C H chemical shifts appear much further downfield (δ 9.9–10.6 ppm) than those in the new species (δ 8.6–8.7 ppm).…”

Arylation of N‐Methyl‐2‐oxindole with Arylboronic Acids in Water Catalyzed by Palladium(II) Pincer Complexes with a Low Catalyst Loading

“…At the end of the cycle, the catalyst Pd II L was regenerated by the oxidation of intermediate I by O 2 . , The same reaction performed under a N 2 atmosphere afforded the expected product.…”