Dynamic Behavior, Redistribution Reactions, and Intermetallic Distances of Dinuclear Bis(μ-pyrazolato)rhodium(I) Complexes

Abstract

Replacement of the olefin in the complexes [{Rh(μ-RPz)(C 2 H 4 ) 2 } 2 ] (RPz = pyrazolate (Pz) ( 1 ), 3-methylpyrazolate (MePz) ( 2 ), 3,5-dimethylpyrazolate (Me 2 Pz) ( 3 )) by tert -butyl isocyanide gives [{Rh(μ-RPz)(CNBu t ) 2 } 2 ] ( 4, 5, 6 ) respectively. Complex 4 can alternatively be prepared from [{Rh(μ-Pz)(cod)} 2 ] (cod = 1,5-cyclooctadiene) ( 7 ) leading to a chemical equilibrium between 4, 7, and the intermediate [(cod)Rh(μ-Pz) 2 Rh(CNBu t ) 2 ] ( 8 ), which has been also isolated. Kinetic studies on this apparent ligand redistribution reaction, leading to 8, show that it follows a second order rate, giving the activation parameters Δ H ≠ = 21.8 kcal·mol - 1, ΔS ≠ = −7.4 eu, and Δ G ≠ 298 = 24.0 kcal·mol - 1, suggesting that the dinuclear complexes are the active species, and no fragmentation seems to occur. The molecular structures of complexes 3, 4, 8, and [{Rh(μ-Pz)(CO) 2 } 2 ] ( 9 ) determined by X-ray diffraction show discrete dinuclear complexes with the six-membered “Rh(μ-Pz) 2 Rh” ring showing a boatlike conformation. Complexes 3 and 4 present the shortest and the largest intermetallic nonbonding separations, 3.0961(2) and 3.8995(6) Å, respectively, so far reported in di-μ-pyrazolato-dirhodium(I) complexes. In addition, complex 3 shows the shortest olefinic C C distance found in ethylene rhodium complexes and accordingly a very low activation energy, 10.0 kcal·mol - 1, for the rotation of the ethylene ligands. Complex 8 undergoes two independent intramolecular fluxional processes associated to the ring inversion of the six-membered “Rh(μ-Pz) 2 Rh” metallacycle and to a σ-1,2-metallotropic shift showing activation parameters Δ H ≠ = 15.0 kcal·mol - 1, Δ S ≠ = −1.7 eu and Δ H ≠ = 19.7 kcal·mol - 1, Δ S ≠ = 4.8 eu, respectively. The first movement is influenced by medium effects and may be restricted by the ancillary ligands and the substituents on the pyrazolate rings in other bis(μ-pyrazolato) complexes. Finally, the intermetallic distance in dinuclear pyrazolato complexes is analyzed in terms of steric and electronic factors.