Dipalladium II Compounds

As mentioned at the very start of the chapter, +2 is the most common oxidation state for Group 10 metals due to the d electron configuration. For palladium, Pd(II) species have played a pivotal role in elementary reactions in palladium-catalysis, for example, oxidative addition/reductive elimination, and thus have received extensive research interest historically. In the following section, the advancements of dipalladium(II) compounds with Pd(II) - Pd(II) bonds in the last decade will be summarized. 

As it has been discussed in Section 10.2.2.2.4), a dipalladium(I) precursor [Pd2(MeCN)g][BF4]2l can react with neutral arene to produce a dipalladium(I) compound with Pd(I)-Pd(I) bond bridged by a neutral arene ligand [80a]. A systematic study revealed that the outcome of the reaction was dependent on the arene/conjugated 2ilkene involved small arenes (benzene, anthracene) afford the Pd(I)-Pd(I) compound, while cyclophane gives the oxidative addition of Pd(I)-Pd(I) toward 

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In 2009, this possibility was realized by Ritter and coworkers. The two-electron oxidation of dipalladium(II) compound 148 at low temperature (-30 °C) afforded the dipalladium(lll) compound 149 with significant Pd-Pd distance contraction from 2.84A in 148 to 2.57A in 149 (Entry 1, Table 10.9) (Scheme 10.68) [108]. The existence of a Pd(III)-Pd(III) bond was further proven by the diamagnetism of 149, which was the result of spin pairing of two d Pd(III) centers. Warming 149 to ambient temperature led to bimetallic reductive elimination to form a C-Cl bond, along with unidentified Pd(II) species. This was the first clearly defined example of carbon-heteroatom reductive elimination from a binuclear transition metal complex, and created a new horizon of palladium organometallic chemistry based on synergetic Pd(III)-Pd(III) bond [113]. 

The compound referred to as dichloro-(l,4-butadiene)palla-dium(II) in Volume VI has been correctly formulated as dichlorobis(4-chlorobutenyl)dipalladium(II) by B. L. Shaw. The second equation on page 218 of Volume VI should read ... 

AllyI)( / -cyclopentadienyl)palladiuin(II), first prepared by B. L. Shaw, is a labile organopalladium compound useful for preparations of various Pd(0) complexes. The present preparation from bis(> -allyl)di )u-chIoro-dipalladium(II) follows the method of Shaw. 

These alkenylation products are easily utilized for the formation of heterocyclic compounds by cyclization reactions. For example, ethyl A -methyl-lV-(3,4-methylenedioxy)benzylglycinate is cyclopalladated regiospecifically at C(6) when treated with Li2PdCl4. The product, the di-p-chloro-bis(AOV-dialkylbenzylamine-6-C,iV)-dipalladium(II) complex 7.19, undergoes a substitution reaction via the insertion of methyl vinyl ketone between the palladium metal and the phenyl carbon atom. The resultant p-aryl-a,p-unsaturated ketone 7.20 is cyclized using anhydrous potassium carbonate in ethanol to the corresponding ethyl iV-methyl-1,2,3,4-tetrahydroisoquinolinium-3-carboxylate 7.21, as shown in Eq. (7.19) [76, 77]. 

The only stable Pd(II) compound with bis(/ r/-butyl)acetylene may be prepared by reaction of Bu CCBu with bis(benzonitrile)dichloropalladium(II) or di-/x-chloro-dichlorodi (ethylene )dipalladium (II) ... 

DipaUadium(O) compounds with p-P, P -coordinated diphosphane ligands [R2P(CH2) PR2] have been known for long time [49a] and have been studied for their potential to activate organic halides and hydrosilanes [49,56]. Recently, another example of this type of compound was reported from the reduction of Pd(II) dichloride with"BuLi (Scheme 10.41). The dipalladium(O) compound 88 features a Pd(0) - Pd(0) bond (2.8560(6) A) (Entry 7, Table 10.6), and can react with dichloromethane to produce dipaUadium(II) compound [55]. 

A corner sharing dipalladium(I) compound with PNP-pincer ligand (93) was synthesized by Ozerov and coworkers from photolysis of a Pd(II) tilkyl precursor (Scheme 10.44), in which the Pd(l)-Pd(l) bond length was reported as 2.5758(4) A (Entry 3, Table 10.7) and was validated as a Pd(I)-Pd(I) single o-bond [60]. The Pd(I)-Pd(I) bond in the compound was proven to have versatile reactivity toward organic smtJl molecules. A reactivity study revealed that C-X, H-H, and H-E (E = 0, N) bonds can add across the Pd(I)-Pd(I) bond to afford the corresponding Pd(II) compounds, which represents a new pathway of activation of Hj, H2O, and NH3. 

A tridentate ligand based on 1,8-naphthyridine (NP), which bears a ferroceneyl amide pendant arm, was synthesized and used to support dipalladium(I) and diruthenium(I) compounds [69]. The synthesis of the dipalladium(I) compound 104 started with a Pd(II) precursor, but the detail of the redox reaction was not discussed in the original article (Scheme 10.54). Complex 104 is diamagnetic and the Pd(I)-Pd(I) bond is short at 2.3952(8) A (Entry 14, Table 10.7). Compound 104 was proven to be an active catalyst for Suzuki and Heck coupling, and a bimetallic-synergy mechanism was proposed for the pivotal oxidative addition/reductive elimination steps. 

Until very recently, +3 was an overlooked oxidation state for fundamental palladium organometallic chemistry. Dipalladium(III) compounds were elusive, while the only report of Pd(III)-Pd(III) bond before 2005 was made by Cotton and coworkers (147 in Scheme 10.67 ) [107]. Significant distance contraction (0.16 A) upon oxidation from dipalladium(II) 146 to dipalladium(III) 147 was observed which was attributed to the result of removing two electrons from an antibonding Pd—Pd a orbital. 

The concept and methodology was extended to C-H bond hydroxylation with O, using dipaUa-dium(II) compound 146 as catalyst [110]. A dipalladium(III) compound 152 was individually prepared and suggested by UV-vis spectroscopy as a possible model for the Pd(III) intermediate of the catalysis (Scheme 10.70). 

Other dipalladium(III) compounds (155-156) were made and structurally characterized (Figure 10.13) [111, 112]. All of the compounds were made from the oxidation of diptJladium(II) precursor, and significant contract of intermettJlic distance upon oxidation were observed for tJl... 

Dipalladium(ii) complexes (8), and their platinum(ii) analogues, react rapidly with five-co-ordinate rhodium(i) compounds [RhCl2H(PR%)2] to give di-/ -chloro-species... 

Solvent effects on, and products from, reaction of styrene with ethylene in the presence of di-)ti-chloro-dichlorobis(styrene)dipalladium(n), [Pd-(Ph CH—CH2)Cl2]2, indicate a mechanism similar to (i)->(iv) above, with the addition of a preliminary equilibrium between the dimer and solvated monomers. The mechanism of reaction of styrene with vinyl compounds, catalysed by the same chloride-bridged dipalladium complex, has been studied using isotopic tracer (H, D) experiments. Palladium-acetate-catalysed reaction of styrene with benzene, also investigated using deuterium tracer experiments, involves no hydride shift, in contrast to the rather closely related Wacker process. The importance of intermediates with palladium-carbon n-bonds in palladium(ii)-catalysed alkylation and arylation of alkenes has been demonstrated. 

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