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

Abstract

Novel dinuclear cyclometallated palladium complexes [{Pd(µ-NCO)(C⁁N)}₂], containing asymmetric imidato –NCO– bridging units have been synthesised [C⁁N = 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(C⁁N)(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.