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 fcobsvd 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 (Keq > 4 × 103 M2) but for anthracene the equilibrium is much less favorable (Keq ~ 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. © 1990, American Chemical Society. All rights reserved.