Periphery-palladated carbosilane dendrimers : Synthesis and reactivity of model organopalladium(II) and (IV) complexes : Crystal structure of [PdMe(C6H4(OCH2Ph)-4)(bpy)] (bpy=2,2'-bipyridine

Abstract

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%)).