Synthesis and reactions of dinuclear palladium complexes containing methyls and hydride on adjacent palladium centers: Reductive elimination and carbonylation reactions

The transmetalation reaction of trimethylaluminum with the palladium chloride dimer Pd/sub 2/Cl/sub 2/(..mu..-dppm)/sub 2/ (1) (dppm = bis(diphenylphosphino)methane) at -78/degrees/C gave an intermediate, Pd/sub 2/ClMe(..mu..-dppm)/sub 2/ (2), which disproportionated at /approximately/ 10/degrees/C to yield the trans-face-to-face palladium dimer Pd/sub 2/Cl/sub 2/Me/sub 2/(..mu..-dppm)/sub 2/ (3) and a palladium dimer Pd/sub 2/Cl/sub 2/(..mu..-CH/sub 2/)(..mu..-dppm)/sub 2/ (5). The use of excess trimethylaluminum at -40/degrees/C gave the dimethyl complex, Pd/sub 2/Me/sub 2/(..mu..-dppm)/sub 2/ (7). When 2 and 7 were protonated, a stable A-frame chloro-bridged dimer (Pd/sub 2/HMe(..mu..-Cl)(..mu..-dppm)/sub 2/)/sup +/ (6) and a hydride-bridged dimer (Pd/sub 2/Me/sub 2/(..mu..-H)(..mu..-dppm)/sub 2/)/sup +/ (10) that rearranged to the acyl complex (Pd/sub 2/H(COCH/sub 3/)(..mu..-Cl)(..mu..-dppm))/sup +/ (11) and then on warming eliminated acetaldehyde. The carbonylation of 8 (1 atm) proceeded stepwise to give first the mono- and then the diacyl complexes. The diacyl complex (Pd/sub 2/(COCH/sub 3/)/sub 2/(..mu..-H)(dppm)/sub 2/)/sup +/ (13) underwent the reductive elimination of acetaldehyde at ambient temperatures. 23 references, 3 figures, 5 tables.