New polynuclear catalysts for atom-transfer radical addition reactions

Homo- and heterobimetallic complexes of the late transitions metals have been synthesized by metathesis reactions of the corresponding chloro-bridged dimers. These reactions have the advantage of being fast and giving rise to structurally defined products in quantitative yield. Complexes with three chloro-bridges of the general formula [LM(μ-Cl)3RuLm] (LM = (arene)Ru, CpIr, or CpRh; RuLm = RuCl(PPh3)2 or RuCp*) were investigated in detail and several of these were structurally characterized. Evidence is provided that triply bridged complexes of this kind can undergo fast exchange reactions. The complexes with the fragment {RuCl(PPh3)2} were found to catalyze atom-transfer radical addition (ATRA) reactions. Structurally related complexes with chelating 1,4-bis(diphenylphosphino)butane (dppb) or 1,4-bis(dicyclohexylphosphino)butane (dcypb) ligands instead of the two PPh3 ligands could likewise be generated. A total of 69 different combinations were prepared and tested in a combinatorial fashion for their ability to catalyze the ATRA of CCl4 to styrene. Two combinations were found to be remarkably active and the corresponding complexes [CpRh(μ-Cl)3RuCl(PPh3)2] and [{(tpc)Rh(μ-Cl)3Ru(dcypb)}2(μ-N2)] were identified and structurally characterized. They proved to be among the most active catalysts described so far. Other ATRA catalysts were investigated. The cationic complex [CpRu(PPh3)2-(CH3CN)]OTf was found to display a higher stability than the related neutral catalyst [Cp*RuCl(PPh3)2]. Total TONs of up to 890 for the addition of CHCl3 to styrene were obtained at a temperature of only 40°C. For the first time, ATRA reactions are reported using a mixture of [(arene)RuCl2]2 and PCy3 as the catalyst precursors. As a product of the reaction between [(C6H3iPr3)-RuCl2]2 and PCy3, the tetranuclear complex [{(C6H3iPr3)Ru(μ-Cl)3RuCl(PCy3)}2(μ-N2)] was isolated, which itself proved to be highly active. When the arene and the N2 ligands of the latter were exchanged for a cymene ligand and an ethylene ligand, respectively, the resulting binuclear complex [(cymene)Ru(μ-Cl)3RuCl(PCy3)(η2-C2H4)] was obtained. It proved to be even more efficient than the dinitrogen complex and allowed the Kharasch addition of CCl4 to 1-olefins to be carried out at a temperature as low as 0°C. The observed TOFs (1100 h-1 at 24°C and 1550 h-1 at 40°C) are comparable to those reported for the most active catalysts.