A "dendritic effect" in homogeneous catalysis with carbosilane-supported arylnickel(II) catalysts: Observation of active-site proximity effects in atom-transfer radical addition

Transmetalation of polylithiated, carbosilane (CS) dendrimers functionalize d with the potentially terdentate ligand [C6H2(CH2NMe2)(2)-2,6-R-4](-) ( NC N) with NiCl2(PEt3)(2) produced a series of nickel-containing dendrimers [G O]-N-4 (4), [G1]-Ni-12 (5), and [G2]-Ni-36 (7) in moderate to good yields. The metallodendrimers 4, 5, and 7 are catalytically active in the atom-tran sfer radical addition (ATRA) reaction (Kharasch addition reaction), viz. th e 1:1 addition of CCl4 to methyl methacrylate (MMA). The catalytic data wer e compared to those obtained for the respective mononuclear compound [NiCl( C6H2{CH2NMe2}(2)-2,6-SiMe3-4)] (2). This comparison indicates a fast deacti vation for the dendrimer catalysts beyond generation [GO]. The deactivation of [G1]-Ni-12 (5) and [G2]-Ni-36 (7) is caused by irreversible formation o f catalytically inactive Ni(III) sites on the periphery of these dendrimers . This hypothesis is supported by results of model studies as well as ESR s pectroscopic investigations. Interestingly, the use of two alternative nick elated [G1] dendrimers [G1]*-Ni-12 (11) and [G1]-Ni-8 (15), respectively, i n which the distance between the Ni sites is increased, leads to significan tly improved catalytic efficiencies which approximate those of the parent d erivative 2 and [GO]-Ni-4 (4). Preliminary membrane catalysis experiments w ith [GO]-Ni-4 (4) and [G1]-Ni-12 (5) show that 5 can be efficiently retaine d in a membrane reactor system. The X-ray crystal structure of the Ni(III) complex [NiCl2(C6H2{CH2NMe2}(2)-2,6-SiMe3-4)] (16), obtained from the react ion of 2 with CCl4, is also reported.