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{(CH 2) 3Si((CH 2) 3-SiMe 2(C 6H 4CH 2OC 6H 4I-4)) 3} 4] (G 1-ArI, 9), and the corresponding G 0 model compound [Si-{(CH 2) 3SiMe 2(C 6H 4CH 2OC 6H 4I-4)} 4] (G 0-ArI, 8) have been prepared from [Si{(CH 2) 3Si((CH 2) 3-SiMe 2(C 6H 4CH 2Br)) 3} 4] (G 1-Br, 7) and the corresponding G 0 model compound [Si{(CH 2) 3SiMe 2-(C 6H 4CH 2Br)} 4] (G 0-Br, 6). These dendritic species react with [Pd 2(dba) 3·dba/tmeda] (dba = dibenzylideneacetone, tmeda = N,N,N′,N′-tetramethylethylenediamine) to yield the periphery-palladated complexes [Si{(CH 2) 3SiMe 2(C 6H 4CH 2O(C 6H 4-4)PdI(tmeda))} 4] (G 0-ArPdI(tmeda), 10) and [Si{(CH 2) 3Si((CH 2) 3SiMe 2(C 6H 4CH 2O(C 6H 4-4)PdI(tmeda))3}4] (G 1-ArPdI(tmeda), 11). Complexes 10 and 11 react with LiMe and 2,2′-bipyridine (bpy) to yield the air-stable [Si-{(CH 2) 3SiMe 2(C 6H 4CH 2OC 6H 4PdMe(bpy))} 4] (G 0-PdMe(bpy), 12) and [Si{(CH 2) 3Si((CH 2) 3-SiMe 2(C 6H 4CH 2OC 6H 4PdMe(bpy))) 3} 4] (G 1-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-CH 2Ph 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 RC 6H 4(CH 2OC 6H 4I-4′)-4 (R = H (1a), SiMe 3 (1b)) were obtained and found to undergo facile oxidative addition to [Pd 2(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(CH 2Ph)(bpy)] (5a,b). The palladium-(IV) complex 5a undergoes facile and clean reductive elimination at ambient temperature in CDCl 3 to form the coupling products Me-C 6H 4(OCH 2Ph)-4 (89%), PhCH 2-C 6H 4(OCH 2Ph)-4 (9%), and Me-CH 2Ph (2%). However, 5b undergoes more complex behavior to form Me-C 6H 4-(OCH 2C 6H 4(SiMe 3)-4′)-4 (87%), Me-CH 2Ph (6%), and PhCH 2-CH 2Ph (7%) together with [PdBr 2(bpy)]. The complex [PdMe(C 6H 4(OCH 2Ph)-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-(C 6H 4(OCH 2Ph)-4)(bpy)] (X = Me (71%), Cl (29%)).