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) ((CN)-N-boolean AND)}(2)], containing asymmetric imidato -NCO- bridging units have been synthesised [(CN)-N-boolean AND = 7,8-benzoquinolyl; -NCO- = succinimidate (1c), phthalimidate (2c) 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((CN)-N-boolean AND) (imidate) (L)] [ L = PPh3, P(4-F-C6H4)(3) or P(4-MeO-C6H4)(3)]. The single crystal structures of [Pd(azb) (phthalimidate) (P(4-MeO-C6H4)(3))] (9a) and [Pd(bzq) (phthalimidate) (PPh3)] (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.