Dipalladium III Compounds

The milestone discovery by Cotton and coworkers opened a new horizon not only for fundamental palladium organometallic chemistry but also for palladium catalysis. As the cornerstone of modern organic synthesis, palladium cattilysis was widely accepted to be based on Pd(II)/Pd(IV) monosite two-electron redox process. The two-electron oxidation of dipaUadium(II) 146 into diptJladium(III) 

147 raised the possibility of intermetallic synergistic redox pair Pd(II) -Pd(II)/Pd(III) -Pd(III) to challenge the traditional Pd(II)/Pd(IV) mechanism of palladium catalysis. 

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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... 

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]. 

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