Palladium dimer

The platinum complex is square planar, while the palladium dimer also has planar 4-coordination (for other examples of mercaptide bridges see section 3.8.3) [116]. 

The hexakis(methyl isocyanide) dimers, [Pt2(CNMe)6], undergo photolytic cleavage of the Pt—Pt bond to give 15-electron radicals, Pt(CNMe)3.94 Mixtures of platinum and palladium dimers give rise to heteronuclear complexes under photolytic conditions. Mixtures of normal and deuterium-labeled methyl isocyanide complexes reveal that the metal-ligand bonds undergo thermal redistribution.94... 

In high polarity solvents, such as acetonitrile, dimethyl sulfoxide, methyl acetate, and methanol, the branched dimer, 4-methyl azelate precursor, is formed in high yield. In methanol, a methoxy dimer (CH302CCgH] 4(0013)0020113) is also formed in moderate yield. Heavies contain both an acyclic methyl, 4-pentadienoate trimeric product and high molecular weight methyl, 4-pentadienoate homopolymer. Polymerization in the absence of air(48) appears to be catalyzed by traces of the tertiary phosphine which is used to prepare the palladium dimerization catalyst. 

For both the dipalladium and palladium-platinum complexes the metal-metal bond is unusually reactive and a number of small molecules undergo an insertion reaction with (9) to give (10 equation 8). The corresponding sulfide-bridged dipalladium dimer can be prepared from the reaction of S8 or MeCHCH2S with (9).83 A mixed rhodium-palladium dimer can also be prepared from (9) (see Scheme 6).84... 

Studies of the reactivity of transition metal cyclopropenylium complexes are negligible. A single example on the acid hydrolysis of the hexachloroantimonate palladium dimer complex [(i-Pr2N)2C3(PdCl)]2-2SbCl6, and its demetallation using SbCl5, is outlined in equation 293. 

Carbonylation of (n-allyl)palladium dimers. Reaction of carbon monoxide in methanol with (7t-allyl)palladium dimers requires high pressure and results in modest yields of products. However, addition of the sodium salt of a carboxylic acid permits carbonylation at 25" under low pressure of CO and results in high, yields of fi.y-... 

Figure 4. Geometry of methane interaction with palladium dimer (left panel) and PdO molecule (right panel) a) adsorbed methane and b) adsorbed methyl and hydrogen after C-H bond scission.

The analysis of the DOS profile fully confirms this hypothesis. In Figure 7 contributions of metal s and d shells to the total density of states for the host Pd2, Rh2 and PdO clusters are given. The contribution of a 5s shell to the occupied part of the spectrum of the palladium dimer is much smaller than for other systems. In the case of 4d shells (Figure 7b) it is the rhodium dimer molecule where the large part of the d spectrum lies above the Fermi level while for the palladium dimer spectrum the main d contribution keeps well below the Fermi level. 

The dimeric amido complexes underwent reductive elimination after cleavage to form two monomeric, 3-coordinate, 14-electron amido complexes. In the case of the anilido dimer 20, a half-order rate dependence in the palladium complex showed that the reductive elimination occurred after reversible cleavage of the dimer to form two monomers. In the case of the f-butylamido complex 21, rapid reductive elimination occurred after irreversible dimer cleavage. This conclusion was supported by reaction rates that were first order in palladium dimer and by the lack of crossover during the reductive elimination reactions containing two doubly-labeled dimers. 

A number of systems consist of a palladium salt, typically PdCb or Pd(OAc)2, with abase. For example, PdCb-NaOAc catalyzes the aerobic oxidation of secondary alcohols in ethylene carbonate under nuld conditions. Sheldon has carried out mechanistic investigations on a number of related Pd systems and shown that water-soluble complexes of Pd(II) with phenanthrohnes are stable, recyclable catalysts for the selective aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic liquid liquid system. The active catalyst is a dihydroxy-bridged palladium dimer. 

Silver-NHC complexes have been shown as excellent NHC delivery agents. Few examples of transmetallation using distinct late TMs have been reported. Tungsten pentacarbonyl NFIC complex (203) was found efficient to transfer easily its NHC ligand to palladium (equation 35) and platinum (equation 36) yielding, respectively, palladium dimer (204) and platinum complex (205) with carbonyl ligand transfer. This is to be noted, as chromium and molybdenum analogs of (203) exhibited similar reactivity. 

Vilar and coworkers reported the isolation of two new dinuclear Pd(I) dimers containing P(Bu-f)2Bph (Bph = biphenyl) (equation 48) as ligand168. The complexes were prepared by the same conproportionation used to prepare the palladium dimers with trialkylphosphines. The complexes contain a single phosphine, and both metals are coordinated to the terminal phenyl group of the biphenyl unit in a p2 — rf -iy fashion. These complexes were tested for the animation of aryl halides with arylamines. The reaction of 4-chlorotoluene with p-tolylamine catalyzed by 52a or 52b formed di-p-tolylamine in 78 to 81% yield within 3 h at room temperature, whereas the same reaction conducted with a mixture of Pd2(dba)3 and P(Bu-f)2Bph required 80 °C to obtain 90% yield. 

Halopyridines are typically poor substrates for Heck reactions due to the possibility of pyridyl-bridged palladium dimers preventing additional reactions. Improved reaction conditions, of catalyst and the use of DMA as cosolvent, have allowed greater functional group tolerance [136], This was reduced to practice in the transformation of 388 into 389. 

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