Synthesis and characterization of new cyclometallated Pd(II) complexes with bridging or terminal imidato ligands. Crystal structures of [{Pd(μ-succinimide)(phpy)}2] and [Pd(azb)(succinimide)(PPh3)] (phpy = 2-phenylpyridine; azb=azobenzene)

On the mechanism of palladium-catalyzed aromatic C-H oxidation

The mechanism of Pd-catalyzed aromatic C-H oxidation chemistry continues to be vigorously discussed. Historically, Pd(II)/Pd(IV) catalysis cycles have been proposed. Herein, we present a detailed study of Pd(OAc)(2)-catalyzed aromatic C-H chlorination and propose dinuclear Pd(III) complexes as intermediates. We have identified a succinate-bridged dinuclear Pd(II) complex, which self-assembles during catalysis, as the catalyst resting state. In situ monitoring of catalysis has revealed that chlorination proceeds with turnover-limiting oxidation of a dinuclear resting state, and that acetate ions, liberated during the formation of the catalyst resting state, catalyze the bimetallic oxidation. Informed by reaction kinetics analysis, relevant dinuclear Pd(III) complexes have been prepared and observed to undergo selective C-Cl reductive elimination. Based on the combination of kinetic data obtained during catalysis and explicit structural information of relevant intermediates, we propose a Pd(II)(2)/Pd(III)(2) catalysis cycle for Pd(OAc)(2)-catalyzed aromatic C-H chlorination.

Mono- and binuclear cyclometallated palladium(ii) complexes containing bridging (N,O-) and terminal (N-) imidate ligands: Air stable, thermally robust and recyclable catalysts for cross-coupling processes

Novel dinuclear cyclometallated palladium complexes [{Pd(mu-NCO)(C circumflex accent N)}(2)], containing asymmetric imidato -NCO- bridging units have been synthesised [C circumflex accent N = 7,8-benzoquinolyl; -NCO- = succinimidate (1c), phthalimidate (1a-3a) or maleimidate (3c)]. The reaction of these complexes, and the previously reported analogous imidate precursors containing a phenylazophenyl (1a-3a) or 2-pyridylphenyl (1b-3b) backbone, with tertiary phosphines provides novel mononuclear N-bonded imidate derivatives of the general formula [Pd(C circumflex accent N)(imidate)(L)][L = PPh(3), P(4-F-C(6)H(4))(3) or P(4-MeO-C(6)H(4))(3)]. The single crystal structures of [Pd(azb)(phthalimidate)(P(4-MeO-C(6)H(4))(3))](9a) and [Pd(bzq)(phthalimidate)(PPh(3))](7c) have been established. Dinuclear complexes (1a-3a, 1b-3b, 1c-3c) demonstrate outstanding thermal stability in the solid-state, as shown by thermoanalytical techniques. A marked influence of bridging imidate groups on the initial decomposition temperature is observed. The dinuclear and mononuclear derivatives are shown to be active catalysts/precatalysts for the Suzuki-Miyaura cross-coupling reactions of aryl bromides with aryl boronic acids, and the Sonogashira reactions of aryl halides with phenyl acetylene (in the presence and absence of Cu(I) salts). The conversions appear to be dependent, to some extent, on the type of imidate ligand, suggesting a role for these pseudohalides in the catalytic cycle in both cross-coupling processes. Lower catalyst loadings in 'copper-free' Sonogashira cross-couplings favour higher turnover frequencies. We have further determined that these catalysts may be recycled using a poly(ethylene oxide)(PEO)/methanol solvent medium in Suzuki-Miyaura cross-coupling. Once the reaction is complete, product extraction into a hexane/diethyl ether mixture (1 : 1, v/v) gives cross-coupled products in good yields (with purity > 95%). The polar phase can then be re-used several times without appreciable loss of catalytic activity.