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Palladium(II)-catalyzed oxidation reactions exhibit broad utility in organic synthesis; however, they often feature high catalyst loading and low turnover numbers relative to non-oxidative cross-coupling reactions. Insights into the fate of the Pd catalyst during turnover could help to address this limitation. Herein, we report the identification and characterization of a dimeric Pd I species in two prototypical Pd-catalyzed aerobic oxidation reactions: allylic C-H acetoxylation of terminal alkenes and intramolecular aza-Wacker cyclization. Both reactions employ 4,5-diazafluoren-9-one (DAF) as an ancillary ligand. The dimeric Pd I complex, [Pd I (μ-DAF)(OAc)] 2 , which features two bridging DAF ligands and two terminal acetate ligands, has been characterized by several spectroscopic methods, as well as single-crystal X-ray crystallography. The origin of this Pd I complex and its implications for catalytic reactivity are discussed., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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 (1)H and (15)N NMR spectroscopy and X-ray crystallography. These complexes include monomeric and dimeric Pd(II) species, with monodentate (κ(1)), bidentate (κ(2)), and bridging (μ:κ(1):κ(1)) 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 nonbridging (κ(1)) DAF ligand coordinated to each Pd(II) center. The monomeric structures consist of three isomeric Pd(κ(1)-DAF)2(OAc)2 complexes, together with Pd(κ(2)-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 κ(2)-Me2DAF coordination mode. The use of DAF in combination with other carboxylate ligands (CF3CO2(-) or tBuCO2(-)) also results in a simplified collection of equilibrating Pd(II)-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.

We have prepared and thoroughly characterized, using X-ray crystallographic, spectroscopic, and computational methods, the diazide adduct of [Fe(III)(dapsox)(H2O)2](+) [dapsox = 2,6-diacetylpyridinebis(semioxamazide)], (1), a low-molecular weight, functional analogue of iron superoxide dismutase (FeSOD). The X-ray crystal structure of the dimeric form of 1, (Na[Fe(III)(dapsox)(N3)2]·DMF)2 (2) shows two axially coordinated, symmetry inequivalent azides with differing Fe-N3 bond lengths and Fe-N-N2 bond angles. This inequivalence of the azide ligands likely reflects the presence of an interdimer hydrogen bonding interaction between a dapsox NH group and the coordinated nitrogen of one of the two azide ligands. Resonance Raman (rR) data obtained for frozen aqueous solution and solid-state samples of 2 indicate that the azides remain inequivalent in solution, suggesting that one of the azide ligands of 1 engages in an intermolecular hydrogen bonding interaction with a water molecule. Density functional theory (DFT) and time-dependent DFT calculations have been used to study two different computational models of 1, one using coordinates taken from the X-ray crystal structure of 2, and the other generated via DFT geometry optimization. An evaluation of these models on the basis of electronic absorption, magnetic circular dichroism, and rR data indicates that the crystal structure based model yields a more accurate electronic structure description of 1, providing further support for the proposed intermolecular hydrogen bonding of 1 in the solid state and in solution. An analysis of the experimentally validated DFT results for this model reveals that the azides have both σ- and π-bonding interactions with the Fe(III) center and that more negative charge is located on the Fe-bound, rather than on the terminal, nitrogen atom of each azide. These observations are reminiscent of the results previously reported for the azide adduct of FeSOD and provide clues regarding the origin of the high catalytic activity of Fe-dapsox for superoxide disproportionation.

In (η⁶-p-cymene)(difluorophosphinato-κO){2-[(1H-pyrazol-1-yl)methyl-κN²]pyridine-κN}ruthenium(II) 0.85-hexafluorophosphate 0.15-tetrafluoroborate, [Ru(PO₂F₂)(C₁₀H₁₄)(C₉H₉N₃)](PF₆)0.85(BF₄)0.15, (I), the [PO₂F₂]⁻ ligand exhibits positional disorder due to one F atom and one O atom sharing the same two positions related by a mirror reflection across the O-P-F plane. The correct composition of this coordinated anion was successfully determined to be [PO₂F₂]⁻ by refining the complex with various tetrahedral anions in which terminal atoms have similar atomic form factors. The noncoordinated counter-ion is compositionally disordered between [PF₆]⁻ and [BF₄]⁻. The difficulty in determining the correct composition of this anion illustrates the importance of a crystallographer remaining impartial and open to encountering unexpected moieties in the process of elucidating a structure.

A modular synthetic method has been developed for the preparation of Ru polypyridyl complexes bearing a terminal alkyne. This method proceeds through a readily accessible intermediate with a silyl-protected alkyne and allows access to a variety of five- and six-coordinate Ru complexes. These complexes can be easily attached to azide-functionalized electrode surfaces with only slight perturbation of the redox properties of the parent complex.

The title compound, [PdCl(2)(C(21)H(17)N(3))], is a member of a sequence of Pd, Pt and Co dichloride complexes bearing polysubstituted (pyrazol-1-ylmethyl)pyridine ligands. It is shown that there is a correlation between the steric bulkiness of the bidentate (pyrazol-1-ylmethyl)pyridine ligands and the Pd-N(pyrazole) distances, i.e. the larger the ligand, the longer the bond. In contrast, no trend is observed between the steric properties of the ligand and the Pd-N(pyridine) bond lengths.

The Pd(OAc)(2)/pyridine catalyst system is one of the most convenient and versatile catalyst systems for selective aerobic oxidation of organic substrates. This report describes the catalytic mechanism of Pd(OAc)(2)/pyridine-mediated oxidation of benzyl alcohol, which has been studied by gas-uptake kinetic methods and (1)H NMR spectroscopy. The data reveal that turnover-limiting substrate oxidation by palladium(II) proceeds by a four-step pathway involving (1) formation of an adduct between the alcohol substrate and the square-planar palladium(II) complex, (2) proton-coupled ligand substitution to generate a palladium-alkoxide species, (3) reversible dissociation of pyridine from palladium(II) to create a three-coordinate intermediate, and (4) irreversible beta-hydride elimination to produce benzaldehyde. The catalyst resting state, characterized by (1)H NMR spectroscopy, consists of an equilibrium mixture of (py)(2)Pd(OAc)(2), 1, and the alcohol adduct of this complex, 1xRCH(2)OH. These in situ spectroscopic data provide direct support for the mechanism proposed from kinetic studies. The catalyst displays higher turnover frequency at lower catalyst loading, as revealed by a nonlinear dependence of the rate on [catalyst]. This phenomenon arises from a competition between forward and reverse reaction steps that exhibit unimolecular and bimolecular dependences on [catalyst]. Finally, overoxidation of benzyl alcohol to benzoic acid, even at low levels, contributes to catalyst deactivation by formation of a less active palladium benzoate complex.