Ring Shift Isomerization Reaction of Monocyclopentadienylruthenium(II) Complexes of Rubrene. Kinetic and Thermodynamic Studies of Metal-Arene Binding Selectivity

The synthesis and characterization of two isomeric complexes containing a CpRu⁺ (Cp = η⁵-C₅H₅) moiety bound to rubrene (5,6,11,12-tetraphenylnaphthacene) are reported. Reaction of rubrene with 1 equiv of CpRu(CH₃CN)₃⁺ at room temperature yields the isomer with one CpRu⁺ moiety bound to an end naphthacene ring, while the analogous reaction at elevated temperatures yields the isomer with the CpRu⁺ group bound to a substituent phenyl group on the naphthacene core. The naphthacene-bound isomer undergoes slow conversion to the phenyl-bound isomer in acetone solution. Kinetic studies of arene complexation by CpRu⁺ (arene = anthracene, benzene) in acetone solution are reported and used to explain the selectivity exhibited by CpRu(CH₃CN)₃⁺ toward different arene rings in rubrene. The rate constant for the complexation of anthracene in acetone solution is ~4.9 (7) × 10₃ times greater than that for the corresponding complexation of benzene. This result is consistent with the ground-state destabilization of anthracene because of its lower resonance energy and with a proposed transition state that involves the partial disruption of the arene's aromaticity by the metal. Plots of fc_obsvd vs [CH₃CN]are consistent with mechanisms in which preequilibria result in the loss of two or three acetonitriles before the transition state in the formation of the CpRu(η⁶-arene)⁺ complex. For benzene the overall equilibrium greatly favors the arene complex (K_eq > 4 × 10³ M²) but for anthracene the equilibrium is much less favorable (K_eq ~ 5.7(9) M²). The interplay of kinetic and thermodynamic factors permits arenes with higher degrees of aromaticity (such as biphenyl) to replace kinetically favored arenes (such as anthracene) to yield the thermodynamic product.