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(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.

Regioselective (site-selective) functionalisation of unsaturated halogenated nitrogen, oxygen and sulfur heterocycles by Pd-catalysed cross-couplings and direct arylation processes

“…Finally, catalyst loading, better referred to as [Pd] concentration, 8 can have a dramatic effect on whether coupling occurs or not-higher turnovers are usually seen at lower [Pd] concentrations. 9 The inverse correlation of catalytic activity and [Pd] concentration is rationalised by agglomeration of Pd and precipitation of Pd black-this explains why some couplings appear to turnover slowly, if at all. Moreover, the addition of further Pd catalyst/precatalyst likely hinders further turnover of substrate.…”

Application of Cyclopalladated Compounds as Catalysts for Heck and Sonogashira Reactions

“…For instance, sulfi nimine palladacycle 41 performs less effi ciently than cyclopalladated oximes when employed in the coupling of aryl iodides in triethylamine at 80 ° C, showing good yields [83] . In addition, palladacycle 81 , derived from benzoquinoline (Figure 8.17 ), has been found as the most effective from a series of related nitrogen complexes in the Sonogashira reaction of p -bromoacetophenone and phenylacetylene, using triethylamine as co -solvent at 100 ° C, although the presence of Cu(I) is essential to enhance the reaction rate [154] . In addition, palladacycle 81 , derived from benzoquinoline (Figure 8.17 ), has been found as the most effective from a series of related nitrogen complexes in the Sonogashira reaction of p -bromoacetophenone and phenylacetylene, using triethylamine as co -solvent at 100 ° C, although the presence of Cu(I) is essential to enhance the reaction rate [154] .…”

Catalytic activity of some palladium complexes of a phosphorus ylide and the structure of a 2-phenylaniline-based palladacycle complex

“…The use of nitrogen-based ligands has been reported to demonstrate the catalytic activity in Suzuki and Heck reactions [18][19][20][21][22][23][24][25][26][27][28][29], which drew our attention to study the catalytic activity of 1 ( Fig. 1) in homogenous media.…”