Cyclopalladated azobenzenes

Pseudo-stilbenes may emit fluorescence that is, contrary to true stilbenes, generally weak at room temperature and often weak even at low temperatures. Protonated azobenzene-type molecules and many protonated azo dye molecules emit strong fluorescence in sulfuric acid at 77 K with quantum yields of about 0.1. Inclusion of azobenzene in the channels of AIPO4-5 crystals provides complexation of the n-electrons and space confinement. This leads to emission by protonated azobenzene at room temperature. For their cyclopalladated azobenzenes, Ghedini et al. " report quantum yields of ca. 1T0 and lifetimes of ca. 1 ns. In contrast, donor/acceptor pseudo-stilbenes, if emitting at low temperatures or when adsorbed to surfaces, are weak emitters. In textile chemistry, it has long been known that azo dyes adsorbed to fibers may show fluorescence. ... 

Figure 2.13 Deeply coloured dinuclear cyclopalladated azobenzene complexes. The colour depends on the bridging group...

There are, however, indications that electronic effects may in some cases control the carbopalladation step. For example, the reaction of methyl 3-phenylpropynoate with an ionic cyclopalladated azobenzene tetrafluoroborate complex gives rise to the formation of the 2,4-diphenyl isomer of the cinnolinium salF (Scheme 12). In this case, the conjugating effect of the ester group apparently exerts more influence in directing the carbopalladation than the phenyl does. 

A cyclometallated azobenzene eomplex 91 was identified as a new class of highly efficient photorefractive materials.Gyclopalladated azobenzenes with N,0 chelating ligands and cyclic voltammetric studies were reported.Mesomorphic materials exhibiting a nematic phase were derived from a cyclopalladated azo compound.Mesogenic cyclopentadienyl cyclopalladated azobenzene eomplexes were also made. " Reactions of cyclopalladated azobenzenes were studied with xanthates and dithiocarbamates yielding ternary complexes. " The products were studied electroehemically. 

Oxygenation takes place with peracids. The cyclopalladated benzylamine complex 466 is converted into the salicylaldamine complex 504 by the treatment with MCPBA[456] or /-BuO H[457]. Similarly, azobenzene is oxidized with MCPBA at the ortho position[458]. 

Photorefractivity is a property exhibited by some materials in which the redistribution in space of photogenerated charges will induce a nonuniform electric space-charge field which can, in turn, affect the refractive index of the material. In a new material the active species is a highly efficient cyclopalladated molecule97,98 shown in Figure 5. The palladium-bonded azobenzene molecule is conformationally locked, and gratings derived from cis—trans isomerizations can be safely excluded. 

Cyclopalladations, that is, C-H and C-C activation mediated by Pd(II) species allowing selective functionalizations, are highly important synthetic reactions. In fact, the first examples of aromatic C-H activation were reported by Cope and Siekman in 1965."" " Reacting azobenzenes with PdCb or K2[PtCl4], they observed the formation of dimeric products that were easily transformed into monomeric complexes with a five-membered ring structure [Eq. (6.71)] ... 

Recently Rouike has reported a series of cyclopalladated imine complexes with either acetylacetonate or cyclopentadienyl as the other ligand, as in Figure 8 [29]. Unlike Ghedini s azobenzene complexes these are isomerically pure but still display mesophases at similar temperatures. The square planar 16e acetylacetonate complexes display both nematic and Sa phases with extended mesophase ranges compared to the free ligands. Not surprisingly, the formally trigonal bipyramidal 18e cyclopentadienyl complexes show a nematic phase at much lower temperatures and tend to decompose above c. 180°C. 

Cyclopalladation of 4 -(functionalized)-6 -phenyl-2)2,-bipyridine ligands (HL) afforded several new [PdLCl] complexes liquid crystalline complex 50 exhibits thermotropic nematic mesomorphism, which represents the first example of an organometallic mesogen with a terdentate o-C deprotonated ligand. Complex 50 is photoluminescent at 77 K [48]. Other researchers have reported the applications of orf/m-platinated or -palladated complexes bearing azobenzene or imine ligands as liquid crystalline materials [49]. 

By palladium (O)-catalyzed carbonylation of l,2-diaza-l,3-buta-dienes. Boeckman et al. (01OL3651) (Scheme 42) reported that stable l-(l,2-diphenylethenyl)-2-phenyldiazene 180 when treated with 10 or 1 mol% of Pd(Ph3P)4 or Pd(dppe)2 catalysts, respectively, in toluene under 1 or 2 atm of carbon monoxide, at room temperature or 100 °C for 0.25-30 h, afforded pyrazol-3-one 184 in excellent yields. Although no intermediates were detected, by analogy to the previously studied cyclopalladation of azobenzene, the mechanism shown in Scheme 42 was proposed. The mechanism involves formation of a cr-complex 181 between 180 and Pd(0)-catalyst, cycloaddition by n—>-n isomerization to... 

Azobenzenes and heteroaromatic nitrogen cyclopalladated complexes (palladacycles) 06CCR(250)1373. 

Aromatic compounds 413, bearing hetero atom-containing groups at positions suitable for forming mainly five-membered or sometimes six-membered chelating rings, undergo cyclopalladation at an ort/io-position to form a cr-arylpalladium bond as in 414 by virtue of the stabilization due to the chelation of these hetero atoms. The ort/io-palladation products 414 are stable and can be isolated [173]. After the first report on the preparation of the azobenzene and A, A-dimethylbenzy-lamine complexes 415 and 416 [174], numerous complexes have been prepared. 

The reaction of Scheme 11 is validated for RX = Br2 by the NMR detection of all four species in the proposed cycle and illustrates again a system where aryl halide involvement is readily explained in the same way but with the severe reservations expressed above for other systems. Facile cyclopalladation of azobenzene, oxidation of Pd(II) by chlorine (e.g., formation of 16), and reductive elimination via C—Cl coupling... 

The 2-aryl-3-indazohnes 7.39 that are synthesized by reaction of the azobenzene cyclopalladated complexes with carbon monoxide are also obtained from the cleavage products 7.69 by reaction with monoisocyanides and carbon monoxide in methanol at 40 °C under a pressure of 35 kPa/cm [71]. 

An atypical example exists whereby the cationic azobenzene cyclopalladated complex 30 reacts with methyl-3-phenylpropynoate to afford a cinnolium salt 31 which was assumed to result from a 1,4-Mi-... 

Numerous azobenzenes and imines have been used to prepare cyclopalladated ctmiplexes, and the liquid crystal properties of some of die products have been investigated. 1 Cyclometallated derivatives have also been prepared from 2-phenylpyiidines, > 2,6-diphenylpyridine, 6-phenyl bipy. O and 6-(2-thienyl) bipy.509 Other substrates subjected to cyclopalladation include amidines, thio- and selenoamides, ll> l N-phenylsulftmyl glycine, hydroxyquinoline derivatives, the drugs diazepam and prazepam, and PBu 3. Doubly cyclopalladated complexes have been prepared from 1,3-diacetylbenzene dioxime and also from N,N-dialkyl benzene-l,3-dicarbaldimines. The 2D NMR spectra of cyclopalladated 8-methylquinoline and benzo-(h)-quinoline show that a CH bond occupies a fifth coordination position above the square plane.520... 

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