Bimetallic reactivity of dirhodium compounds leading to functionalized methylene-bridged compounds

Reactions of [{Rh(mu -Pz)(CNBut)(2)}(2)] (Pz = pyrazolate, 1) with dichloromethane, alpha,alpha -dichlorotoluene, 1,1-dichloroacetone, and methyldichloroacetate gave the functionalized methylene-bridged compounds [{Rh(mu -Pz)(Cl)(CNBut)(2)}(2)(mu -CHR)] (R = H, Ph, COMe, CO2Me), respectively. The molecular structure of [{Rh(mu -Pz)(Cl)(CNBut)(2)}(2){mu -CH(CO2Me)}] was determined by an X-ray diffraction study. Similarly, the thiolate complex [{Rh(mu -SBut)(CNBut)(2)}(3)] was reacted with dichloromethane to give [{Rh(mu -SBut)(Cl)(CNBut)(2)}(2)(mu -CH2)]. The gem-dichloroalkanes are formally broken in three fragments that become a methylene-bridging group and two terminal chloride ligands in these two-center four-electron oxidative-addition reactions. Decreasing the nucleophilicity of the metals via steric effects, while keeping the basicity of the metals constant, leads to a different type of product, as shown by reaction of dichloromethane with [{Rh(mu -Me(2)Pz)(CNBut)(2)}(2)] to give [{Rh(mu -Me(2)Pz)(Cl)(CNBut)(2)}(2)]. A deeper insight into these reactions is provided by reactions of the complex [(cod)Rh(mu -Pz)(2)Rh(CNBut)(2)] with methyl dichloroacetate and dichloromethane to give the mixed-valence Rh(I)-Rh(III) complex [(cod)Rh(mu -Pz)(2)Rh(Cl){eta (1)-CHCl(CO2Me)}(CNBut)(2)] and the methylene-bridged complex [(cod)(Cl)Rh(mu -Pz)(2)(mu -CH2)Rh(Cl)(CNBut)(2)], respectively. The former evidences that the above rections take place in two steps and that the second step involves an internal oxidative-addition reaction following an S(N)2 mechanism. The latter reaction is very suggestive of how cooperative effects can act in a dinuclear complex to induce an unusual reactivity.