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

Robert S. Koefod, Kent R Mann

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Abstract

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.

Original languageEnglish (US)
Pages (from-to)7287-7293
Number of pages7
JournalJournal of the American Chemical Society
Volume112
Issue number20
DOIs
StatePublished - Feb 1990

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