Periphery-Palladated Carbosilane Dendrimers:  Synthesis and Reactivity of Organopalladium(II) and -(IV) Dendritic Complexes. Crystal Structure of [PdMe(C₆H₄(OCH₂Ph)-4)(bpy)] (bpy = 2,2‘-Bipyridine)

A carbosilane dendrimer with 12 peripheral iodoarene groups, [Si{(CH₂)₃Si((CH₂)₃SiMe₂(C₆H₄CH₂OC₆H₄I-4))₃}₄] (G₁-ArI, 9), and the corresponding G₀ model compound [Si{(CH₂)₃SiMe₂(C₆H₄CH₂OC₆H₄I-4)}₄] (G₀-ArI, 8) have been prepared from [Si{(CH₂)₃Si((CH₂)₃SiMe₂(C₆H₄CH₂Br))₃}₄] (G₁-Br, 7) and the corresponding G₀ model compound [Si{(CH₂)₃SiMe₂(C₆H₄CH₂Br)}₄] (G₀-Br, 6). These dendritic species react with [Pd₂(dba)₃·dba/tmeda] (dba = dibenzylideneacetone, tmeda = N,N,N‘,N‘-tetramethylethylenediamine) to yield the periphery-palladated complexes [Si{(CH₂)₃SiMe₂(C₆H₄CH₂O(C₆H₄-4)PdI(tmeda))}₄] (G₀-ArPdI(tmeda), 10) and [Si{(CH₂)₃Si((CH₂)₃SiMe₂(C₆H₄CH₂O(C₆H₄-4)PdI(tmeda))₃}₄] (G₁-ArPdI(tmeda), 11). Complexes 10 and 11 react with LiMe and 2,2‘-bipyridine (bpy) to yield the air-stable [Si{(CH₂)₃SiMe₂(C₆H₄CH₂OC₆H₄PdMe(bpy))}₄] (G₀-PdMe(bpy), 12) and [Si{(CH₂)₃Si((CH₂)₃SiMe₂(C₆H₄CH₂OC₆H₄PdMe(bpy)))₃}₄] (G₁-ArPdMe(bpy), 13). Complexes 12 and 13 undergo oxidative addition with benzyl bromide to form species containing Pd(IV) centers. These complexes can undergo subsequent reductive elimination at ambient temperature involving both Me−Ar and Me−CH₂Ph coupling on decomposition. Iodoarenes that model the arms of carbosilane-based dendrimers have been synthesized, and procedures have been developed for maximizing yields of organopalladium(II) and -(IV) derivatives of the iodoarenes as part of a program directed toward the isolation and study of organopalladium functionalized dendrimers. The iodoarenes RC₆H₄(CH₂OC₆H₄I-4‘)-4 (R = H (1a), SiMe₃ (1b)) were obtained and found to undergo facile oxidative addition to [Pd₂(dba)₃·dba/tmeda] to form [PdI(Ar)(tmeda)] (2a,b), which react with LiMe to form [PdMe(Ar)(tmeda)] (3a,b). Bpy displaces tmeda to form [PdMe(Ar)(bpy)] (4a,b), and the latter complexes undergo oxidative addition with benzyl bromide to form the complexes [PdBrMeAr(CH₂Ph)(bpy)] (5a,b). The palladium(IV) complex 5a undergoes facile and clean reductive elimination at ambient temperature in CDCl₃ to form the coupling products Me-C₆H₄(OCH₂Ph)-4 (89%), PhCH₂-C₆H₄(OCH₂Ph)-4 (9%), and Me-CH₂Ph (2%). However, 5b undergoes more complex behavior to form Me-C₆H₄(OCH₂C₆H₄(SiMe₃)-4‘)-4 (87%), Me-CH₂Ph (6%), and PhCH₂-CH₂Ph (7%) together with [PdBr₂(bpy)]. The complex [PdMe(C₆H₄(OCH₂Ph)-4)(bpy)] (4a) has been characterized by X-ray diffraction. The asymmetric unit contains two similar but crystallographically independent molecules. Each molecule has square-planar geometry for palladium with the aryl ring tilted by 76.2(4) and 67.1(3)° to the coordination plane, respectively. The crystal examined by X-ray diffraction exhibits significant substitutional disorder at one site:  [PdX(C₆H₄(OCH₂Ph)-4)(bpy)] (X = Me (71%), Cl (29%)).