Palladium complexes orthometallation

Cyclopalladated complexes, which are most conunonly prepared from tetrachloropaUa-date(II) and appropriate amines or imines, have drawn the attention of inorganic, coordination, and organometallic chemists since the first report in 1965 (for imines) and in 1968 (for amines) by Cope and co-workers. ° These palladacycle species can easily be transformed into chiral form by employing corresponding chiral amines or imines. Two representative examples of the chiral cyclopalladated species are illustrated in Figure 1. This section will describe three examples that represent applications of these chiral orthometallated palladium complexes in organic synthesis. 

In 1971, an interesting application of the chlorobridged Pd(II) complexes with orthometallated chiral amines was demonstrated by Otsuka and co-workers resolution of racemic chiral phosphincs. The binuclear species reacts with tertiary phosphines or arsines to form two equivalents of mononuclear complexes (Scheme 3). If both the phosphines and the orthometallated palladium complexes were chiral, the mononuclear products could be a mixture of diastereomers. With appropriate combinations of the chiral racemic phosphines and the enantiomerically pure orthometallated palladium species, one of the two enantiomers of the phosphines reacts with the palladium complex selectively to give a specific diastereomer of the mononuclear palladium complexes, leaving the other enantiomer of the phosphine unreacted. 

Palladium(II) complexes containing orthometalated chiral amines such as those shown in Figure 7 are useful reagents ... 

Figure 7 Palladium(II) complexes containing orthometalated chiral amines.

Gold(III) usually displays a square planar geometry, typically observed in d8 metallic complexes such as palladium(II), platinum(II), rhodium(I) and iridium (I), for which an enormous number of liquid crystals have been described [3-5], mainly as orthometallated compounds. However, only a gold(III) metallomesogen has been published. Since the first gold mesogen was reported in 1986, many other compounds have been described. 

Earlier, H-shaped orthometallated palladium(II) complexes were discussed which displayed nematic and smectic phases. However, Praefcke has reported related complexes where the number of chains on the periphery of the molecule is increased leading to columnar mesophases. The complex in Figure 91 displays a monotropic nematic discotic phase (No) and the structure has been confirmed as that in the diagram by single-crystal X-ray crystallography [170]. 

In 1997, Resolutions of Tertiary Phosphines and Arsines with Orthometallated Palladium(ll)-Amine Complexes, by Wild [3]... 

Decomposition products of tolylphosphines should give 3-methyl substituents if the orthometallation mechanism is operative. For palladium catalyzed decomposition of triarylphosphines this is not the case [16]. Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [17, 18]. Several rhodium complexes catalyze the exchange of aryl substiments at triarylphosphines [18] ... 

We bave previously prepared and described binuclear orthometallated complexes of palladium(II) which could be used as precursors to prepare potoitially mixed-valence compounds. Hie presence of unusual Pd-N amido bonds in these complexes adds further interest based cm the reactivity shown by amides of the platinum group metals. ... 

An example of a doubly orthopalladated aryl bis(phosphinite) ligand 95 has been reported. " Aryloxopalladium(ii) complexes were synthesized using the Herrmann-Beller palladacycle. Coordination of the 2-acylphenolates to palladium results in a significant perturbation of the delocalization within the aromatic ring. Another example involves a doubly orthometallated complex from o-G6H4(PBu )2 and Pd(OAc)2. ... 

Wild SB. Resolutions of tertiary phosphines and arsines with orthometallated palladium(II)-amine complexes. Coord. Chem.Rev. 1997 166 291-311. 

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