Oxidative-addition of organic monochloro derivatives to dinuclear rhodium complexes: Mechanistic considerations

The addition of RCH2Cl to the complex [{Rh(mu -Pz)(CNBut)(2)}(2)] (Pz = pyrazolate, 1) occurs under mild conditions to yield the bis(alkyl)rhodium(III) complexes [{Rh(mu -Pz)(eta (1)-CH2R)(CNBut)(2)}(2)(mu -Cl)Cl (R = Ph, CH=CH2, CO2Me, COMe). These reactions proceed through two separate steps, as evidenced by the observation of the intermediate mixed-valence complex [(CNBut)(2)Rh-I(mu -Pz)(2)Rh-III(eta (1)-CH2Ph)(Cl)(CNBut)(2)] resulting from the trans-addition of PhCH2Cl at a single metal center in 1. Similar Rh(I)-Rh(III) complexes [(cod)Rh(mu -Pz)(2)Rh(eta1-CH2R)(Cl)(CNBut),1 (R = Ph (7), CH=CH2, CO2Me) result from the addition of RCH2Cl to the mixed-ligand complex [(cod)Rh(mu -Pz)(2)Rh(CNBut)(2)] (6). These exist in benzene and toluene as two interconverting conformers, differentiated by the location of the RCH2 group either between the two metals in the pocket of the complexes (endo conformer) or at the trans position (exo conformer). An intramolecular boat-to-boat inversion accounts for this exchange. The thermodynamic parameters for complex 7 were DeltaH(not equal) = 16.2 kcal.mol(-1), DeltaS(not equal) = 4.9 eu, and DeltaG(293)(not equal) = 14.9 kcal.mol(-1). Primary alkyl bromides RCH2Br react with 1 and with 6 to give the Rh(III) complexes [{Rh(mu -Pz)(eta (1)-CH2R)(CNBut)(2)}(2){mu -Br)}Br (R = Ph, CO2Me) and the mixed-valence complex [(cod)Rh{mu -Pz)(2)Rh(eta (1)-CH2Ph)(Br)(CNBut}(2)]. However, the secondary alkyl bromide PhCH(Me)Br reacts with 1 and with 6 to give the dirhodium(II) complexes [{Rh(mu -Pz)(Br)(CNBut)(2)}(2)] and [(cod)(Br)Rh(mu -Pz)(2)Rh(Br)(CNBut)(2)], respectively, along with 2,3-diphenylbutane and traces of styrene and ethylbenzene, suggesting free-radical pathways for these reactions. The thermal decomposition of the bis(alkyl)chloro complexes gives free isobutene and the cyanide complexes [(CNBut)(2)(eta (1)-CH2R)Rh(mu -Pz)(2)(mu -Cl)Rh(eta (1)-CH2R)(CN)(CNBut)] (R = Ph, COMe, CO2Me). Decomposition of the bis(alkyl)bromo derivatives occurs under sunlight irradiation and anaerobic conditions to give bibenzyl, cis- and trans-stilbene, and toluene.