Palladium complexes nitrogen donor atoms

The inclusion of the nitrogen donor atom in an aromatic heterocycle allows the possibility of -bonding with the metal centre, giving these ligands some similarities to tertiary phosphines. Thus pyridyl and bipyridyl ligands stabilize organometallic complexes of palladium (see companion volume).2 ... 

Palladium will form complexes with the great majority of organic compounds containing nitrogen donor atoms. Some representative Pd-N complexes containing mono- and bidentate nitrogen ligands are summarized in Table 5. 

A logical extension of the condensation reactions which yield multidentate nitrogen donors is the formation of macrocyclic ligands. The preference of palladium for square planar coordination makes it an ideal metal for the formation of complexes of these ligands. Thus palladium porphyrins are very stable and resistant to demetallation.281 Both 14- and 16-atom macrocycles have been used to form complexes. The ligand l,8-dihydro-5,7,12,14-tetramethyl-dibenzo[b,i][l,4,8,ll]tetraaza[14]annulene reacts with [PdCl2(PhCN)2] to yield the complex (46). 

Cyclometalation is one of the most reliable and versatile methods for the synthesis of stable Pd-C a-bonds. In this reaction, a palladium (or other metal) complex is reacted with a ligand possessing a donor atom. The metal is directed by the donor atom to insert into a C-H bond, forming a chelate ring. This donor is usually the nitrogen of a tertiary amine, heterocyclic amine, or imine, or a phosphorus in a tertiary phosphine or phosphite. Other donor atoms, such as O and S, are occasionally encountered. The chelate ring size is normally five. ... 

The tripeptide Gly-L-His-Gly coordinates to paUadium(II) via the nitrogen atoms of the amino group, deprotonated peptide, and the imidazole (Figure 10). The coordination of the imidazole nitrogen over the second peptide lutrogen is expected, due to the fact that paUadium(II) favors soft donor atoms (like aromatic nitrogens) over harder donor atoms. This same coordination is also seen in the tripeptide Gly-L-His-Lys. The fourth coordination site of this complex is occupied by a chloride ion. In the 1 2 palladium(II)-Gly-Gly-L-Cys complex, the palladium... 

A series of l-(pyridin-2-yl)benzimidazolium salts were synthesized by Chianese et al. and used as precursors to rigid, five-membered chelated biden-tate NHC-pyridine ligands [57]. The ligands were constructed with either very small (H, Me) or very bulky (2,6-diisopropylphenyl, 2,4,6-triisopropylphenyl) groups adjacent to the nitrogen and carbon donor atoms. Cationic palladium-allyl complexes were synthesized under mild conditions and used as catalysts for the allylic substitution reaction between allylic carbonates and either sodium dieth)f-2-methylmalonate or Af-methylbenzylamine. 

Schemes 58-62. A new non-rigid phosphine ligand was synthesized and reacted with Fe(CO)5 to form the mononuclear iron complex (Equation (81)). Phosphino-oxazoline ligands were used as assembling ligands for hetero-metallic complexes, where the phosphorus atom binds to iron and the nitrogen atoms act as donor atoms to copper, cobalt, or palladium (Scheme 58). The copper complex catalyzes cyclopropanation and Diels-Alder reactions. When 2-(A -diphenylphosphinomethyl-A -cyclohexyl)aminopyridine (NNP) reacts with Fe(CO)5 in ethanol, /ra .r-(OC)3Fe(NNP)2 is formed (Scheme 59). This monometallic complex can then be reacted with a copper salt in CH2GI2 to form a complex having an Fe-Cu dative bond. The complex was demonstrated to be an efficient catalyst for the cyclopropanation of styrene by ethyl diazoacetate and for the Diels-Alder reaction of cyclopentadiene and methacrolein. No other heterometallic complexes have been shown to have such reactivity. Previously known...

Cyclopalladated complexes of 2-(acetylpyridine)phenylhydrazone have been r twted the coordination sphere contains two nitrogen donors in addition to the aryl and halide ligands. Displacement of coordinated water from the palladium and platinum catimis [M(3-sulphonyl-pyridine)(Me2NC6H4)(OH2)] where the ortiio- metallated carbon atom is irons to the water ligand proceeds with a rate enhanconent of 10 to 10 relative to the cations [ML(OH2)] where L is a pentasubstituted triamine. Detailed kinetic studies suggest that the M-C bond labilises the water... 

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