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 = pyr azolate, 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 e videnced by the observation of the intermediate mixed-valence complex [(CNB ut)(2)Rh-I(mu -Pz)(2)Rh-III(eta (1)-CH2Ph)(Cl)(CNBut)(2)] resulting from th e 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 compl ex [(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 R CH2 group either between the two metals in the pocket of the complexes (end o conformer) or at the trans position (exo conformer). An intramolecular bo at-to-boat inversion accounts for this exchange. The thermodynamic paramete rs for complex 7 were DeltaH(not equal) = 16.2 kcal.mol(-1), DeltaS(not equ al) = 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 mix ed-valence complex [(cod)Rh{mu -Pz)(2)Rh(eta (1)-CH2Ph)(Br)(CNBut}(2)]. How ever, the secondary alkyl bromide PhCH(Me)Br reacts with 1 and with 6 to gi ve the dirhodium(II) complexes [{Rh(mu -Pz)(Br)(CNBut)(2)}(2)] and [(cod)(B r)Rh(mu -Pz)(2)Rh(Br)(CNBut)(2)], respectively, along with 2,3-diphenylbuta ne and traces of styrene and ethylbenzene, suggesting free-radical pathways for these reactions. The thermal decomposition of the bis(alkyl)chloro com plexes gives free isobutene and the cyanide complexes [(CNBut)(2)(eta (1)-C H2R)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 ir radiation and anaerobic conditions to give bibenzyl, cis- and trans-stilben e, and toluene.