Palladium cyclometalated complexes

The ligands were prepared by phosphitylation of diphenols with 53 and its analogue obtained from (S)-(- -)-indolinemethanol. Compounds 57-60 were obtained in 81-86% yields in high optical purity (>90% de) except for 59, which was unexpectedly racemic. These compounds were successfully employed to form palladium cyclometallated complexes. 

Diphenylimidazole with palladium acetate forms the cyclometallated complex 80 (X = OAc) (97AOC491). The acetate group is replaced by chloride or bromide when 80 (X = OAc) reacts with sodium chloride or lithium bromide, respectively, to give 80 (X = C1, Br). Bromide with diethyl sulfide forms the mononuclear complex 81. Similar reactions are known for 1 -acetyl-2-phenylimidazole (96JOM(522)97). 1,5-Bis(A -methylimidazol-2-yl)pen-tane with palladium(II) acetate gives the cyclometallated complex 82 (OOJOM (607)194). 

P-chiral dibenzophosphole oxide (52a) (Scheme 14) shows liquid crystalline behaviour [52], a property that is of interest in the area of electro-optical displays [53]. Chiral resolution of (52a) was achieved by column chromatographic separation of the diastereoisomers obtained following coordination of the o -benzophosphole (52b) to chiral cyclometallated palladium(II) complexes [52]. Notably, the presence of a stereogenic P-centre is sufficient to generate a chiral cholesteric phase. 

The choice of an ionic liquid was shown to be critical in experiments with [NBuJBr (TBAB, m.p. 110°C) as a catalyst carrier to isolate a cyclometallated complex homogeneous catalyst, tra .s-di(ri-acetato)-bis[o-(di-o-tolylphosphino) benzyl] dipalladium (II) (Scheme 26), which was used for the Heck reaction of styrene with aryl bromides and electron-deficient aryl chlorides. The [NBu4]Br displayed excellent stability for the reaction. The recycling of 1 mol% of palladium in [NBu4]Br after the reaction of bromobenzene with styrene was achieved by distillation of the reactants and products from the solvent and catalyst in vacuo. Sodium bromide, a stoichiometric salt byproduct, was left in the solvent-catalyst system. High catalytic activity was maintained even after the formation of visible palladium black after a fourth run and after the catalyst phase had turned more viscous after the sixth run. The decomposition of the catalyst and the formation of palladium... 

Cyclometallated iridium complexes, for OLEDs, 12, 145 Cyclometallated palladium(II) complexes from amines and pyridines, 8, 280 with C,C-chelating ligands, 8, 291 enantioselective synthesis, 8, 296 ferrocene-based palladacycles, 8, 292 four-membered palladacycles, 8, 297 imine- and oxime-based complexes, 8, 285 with N-N and N=N bonds, 8, 288 palladacycle catalysis, 8, 297... 

Many of the reactions outlined in Section 4 for Pd-C bonds occur in cyclometalated complexes. However, the existence of the Pd-C bond in a chelate ring imposes some kinetic stability on the bond. In general, mild acids and bases and oxygen are unreactive with cyclometalated palladium complexes. More vigorous reagents will lead to useful reactions, however. 

Electrophilic reagents, including Br2, CI2, NCS-SCN, and I2, will react rapidly and in high yield to add across the Pd-C bond. Mineral acids will cleave the bond in cyclometalated complexes as well. Since cyclometalation with palladium halide complexes generates HCl, frequently a weak base, such as sodium acetate, is added to prevent reversal of the cyclometalation reaction. 

The formation of hydridopalladium complexes free of phosphine ligands (Figure 4) were observed when cyclometalated palladium chloride complexes were subjected to hydrogenolysis in DMF solutionsJ ... 

Cyclometallated complexes incorporating transition metals other than palladium react with alkynes and act as synthetic precursors to hetero-... 

The aim of this account was to show that cyclometallated complexes, the majority being palladium-containing derivatives, in stoichiometric or catalytic quantities, can certainly be part of the arsenal of the chemist seeking to synthesize cyclic compounds selectively and efficiently. One of the major themes of our research in this respect has been to study these reactions from an organometallic chemist s point of view, with the particular goal of defining the precise role of the metal in the hetero- or carbocyclization process. This has led to some unusual and fruitful mechanistic findings and some useful synthetic approaches to many novel hetero- and carbocyclic systems. 

Solid-state thermolysis of 1 -alkyl-2,4-bipyridinium palladium complexes gave cyclometallated complexes [PdCl2(2,4 -R-bipy-H)] (R = H, Me, Bu, CsHu, CgHi, or C10H21) in quantitative yield. ... 

Dinuclear cyclometallated palladium(ii) complexes 81 have been obtained from Schiff base ligands. Cyclopalladated complexes of oximes 82 and 83 and their ligand-exchange reactions have been studied. Ketoximes form palladacycle dimers such as 84. " ... 

V. I. Ovcharenko, S. V. Larionov, G. N. Mironova, and L. B. Volodarskii, Bull. Acad. Sci. USSR, Div. Chem. Sci., 1979,28, 599. Dimeric palladium(u) complexes with cyclometallated imidazoline nitrosyl radical ligands. 

The reversible reaction of tri-n-butylstannylfuran with the cyclometallated palladium complex 24 yields the ti C) coordinated 2-furyl complex 25 (98JA11016). 

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