Palladium is an extensively studied transition element in organometallic chemistry as a versatile promoter of various stoichiometric and catalytic organic transformations. [1] Although palladium generally prefers low oxidation states 0, ΓΎ 1, and ΓΎ 2, the involvement of organopalladium(iv) intermediates have been claimed in important catalytic processes. [2, 3] In contrast to platinum, which has an extensive organometallic chemistry in the ΓΎ 4 oxidation state, [4] organopalladium(iv) complexes had hardly been studied until the first unequivocal trialkylpalladium(iv) complex, [PdIMe 3 (bpy)], was isolated and characterized by X-ray analysis. [5] Since this report, various polyalkylpalladium(iv) complexes 2 were synthesized from dialkylpalladium(ii) precursors 1 by oxidative addition of alkylhalides, or oxidation by H 2 O, halogens, dichalcogenides, etc., and most of them are thermally labile with the reductive elimination of an alkane readily occuring at or below room temperature to produce the corresponding palladium(ii) fragments 3 [Eq. ( 1)]. [6] In striking contrast, we found that the reaction of a readily available palladium(0) complex, [Pd 2 (dba) 3 (dba ΒΌ trans,trans-dibenzilideneacetone), with tetrachloro-1,2-benzoquinone (o-chloranil) and bicyclo[2.2.1]hept-2-ene (norbornene) directly produced a novel spirocyclic palladium(iv) dialkyl complex 4 in high yield [Eq. ( 2)].
Quinones have received considerable attention in coordination chemistry as redox-active, strongly electron-accepting ligands. Various transition metals have been shown to coordinate to p-quinones at their localized ring olefinic bonds. [7] On the other hand, o-quinones usually form complexes containing semiquinone or hydroquinone chelate ligands with the concomitant oxidation of transition-metal centers. [8] This was the case for palladium. The reaction of the Pd 0 Β±phosphane complex [Pd(PPh 3 ) 4 ] with o-chloranil has already been reported to furnish a palladium(ii) catecholate complex [(Ph 3 P) 2 Pd(O 2 C 6 Cl 4 )]. [9] Remarkably, the treatment ZUSCHRIFTEN 3814