Atom-transfer radical reactions catalyzed by a coordinatively unsaturated diruthenium amidinate, [(η⁵-C₅Me₅) Ru(μ₂-i-PrNC(Me)Ni-Pr)Ru(η⁵-C₅Me ₅)]⁺

Atom-transfer radical cyclization (ATRC) and addition (ATRA) catalyzed by a coordinatively unsaturated diruthenium amidinate complex 4, [(η⁵-C₅Me₅)Ru(μ₂-i-PrNC(Me) Ni-Pr)Ru(η⁵-C₅Me₅)]⁺, are investigated, and their features are compared with those of atom-transfer radical polymerization (ATRP). As an example of ATRC, a cationic diruthenium amidinate 4 is found to exhibit excellent catalytic reactivity for the cyclization of N-allyl α-halogenated acetamides including an alkaloid skeleton at ambient temperature. A catalytic species generated in situ from a halide complex, (η⁵-C₅Me₅) Ru(μ₂-i-PrNC(Me)Ni-Pr)Ru(η⁵-C₅Me ₅)(X) [X=Cl, Br] and sodium salts of weakly coordinating anions such as NaPF₆ and NaBPh₄ also shows high catalytic activity; this actually provides a solution for a problematic instability of 4 as the practical catalyst. The in situ-generated catalyst species 4 is also active towards the intermolecular ATRA of α,α,γ-trichlorinated γ-lactam with alkenes at rt to afford the corresponding α-alkylated γ-lactams in moderate yields. Examination of ATRP of methyl methacrylate (MMA) showed that both the isolated 4 [Y=PF₆] and in situ-generated 4 [Y=PF₆] are effective for the polymerization of MMA in the presence of 2-bromoisobutylate as the initiator. Use of the isolated catalyst results in controlled polymerization at initial stage of the reaction; in contrast, the polymerization with in situ-generated catalyst produces poly(MMA) with wide molecular weight distribution. The isolated catalyst 4 is powerful for the activation of a C-Br bond of macromolecule initiators; BrCMe₂CO ₂[O(CH₂)₄]_n-n-Bu (M _n=3800; M_w/M_n=1.2) initiated ATRP of MMA even at 25°C to afford the poly(THF)-poly(MMA) block copolymer of M _n=26,000 and M_w/M_n=1.2 with the aid of 4. The roles of the coordinatively unsaturated ruthenium species for these reactions are discussed.