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)(3)Si ((CH2)(3)-SiMe2(C6H4CH2OC6H4I-4))(3)}(4)] (G(1)-ArI, 9), and the correspond ing Go model compound [Si ((CH2)(3)SiMe2(C6H4CH2OC6H4I-4)}(4)] (G(0)-ArI, 8 ) have been prepared from [Si{(CH2)(3)Si{(CH2)(3)-SiMe2(C6H4CH2Br))(3)}(4)] (G(1)-Br, 7) and the corresponding G(0) model compound [Si{(CH2)(3)SiMe2-( C6H4CH2Br)}(4)] (G(0)-Br, 6). These dendritic species react with [Pd-2(dba) (3). dba/tmeda] (dba = dibenzylideneacetone, tmeda = N,N,N',N'-tetramethyle thylenediamine) to yield the periphery-palladated complexes [Si{(CH2)(3)SiM e2(C6H4CH2O(C6H4-4)PdI(tmeda))}(4)] (G(0)-ArPdI(tmeda) 10) and [Si{(CH2)(3) Si((CH2)(3)SiMe2(C6H4CH2O(C6H4-4)PdI))(3)}(4)] (G(1)-ArPdI(tmeda), 11). Com plexes 10 and 11 react with LiMe and 2,2'-bipyridine (bpy) to yield the air -stable [Si{(CH2)(3)SiMe2(C6H4CH2OC6H4PdMe(bpy))}(4)] (G(0)-PdMe(bpy), 12) and [Si{(CH2)(3)Si((CH2)(3)-SiMe2(C6H4CH2OC6H4PdMe(bpy)))(3)}(4)] (G(1)-ArP dMe(bpy), 13). Complexes 12 and 13 undergo oxidative addition with benzyl b romide to form species containing Pd(N) centers. These complexes can underg o 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) deriv atives of the iodoarenes as part of a program directed toward the isolation and study of organopalladium functionalized dendrimers. The iodoarenes RC6 H4(CH2OC6H4I-4')-4 (R = H (1a), SiMe3 (1b)) were obtained and found to unde rgo facile oxidative addition to [Pda(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 u ndergo oxidative addition with benzyl bromide to form the complexes [PdBrMe Ar(CH2Ph)(bpy)] (5a,b). The palladium(IV) complex 5a undergoes facile and c lean reductive elimination at ambient; temperature in CDCl3 to form the cou pling products Me-C6H4(OCH2Ph)-4 (89%), PhCH2-C6H4(OCH2Ph)-4 (9%), and Me-C H2Ph (2%). However, 5b undergoes more complex behavior to form Me-C6H4-(OCH 2C6H4(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 charac terized by X-ray diffraction. The asymmetric unit contains two similar but crystallographically independe nt molecules. Each molecule has square-planar geometry for palladium with t he aryl ring tilted by 76.2(4) and 67.1(3)degrees to the coordination plane , respectively. The crystal examined by X-ray diffraction exhibits signific ant substitutional disorder at one site: [PdX-(C6H4(OCH2Ph)-4)(bpy)] (X = M e (71%), Cl (29%)).