Pyridine Ligand Rotation in Self-Assembled Trigonal Prisms. Evidence for Intracage Solvent Vapor Bubbles

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• 作者：Jaroslav Vacek ; Douglas C. Caskey ; Dominik Horinek ; Richard K. Shoemaker ; Peter J. Stang ; Josef Michl
• 刊名：Journal of the American Chemical Society
• 出版年：2008
• 出版时间：June 18, 2008
• 年：2008
• 卷：130
• 期：24
• 页码：7629 - 7638
• 全文大小：3088K
• 年卷期：v.130,no.24(June 18, 2008)
• ISSN：1520-5126

文摘

The rate of interconversion of the two inequivalent edges of the pyridine rings in the trigonal prism 3c, self-assembled from 3 equiv of the star connector, tetrakis[4-(4-pyridylethynyl)phenyl]cyclobutadienecyclopentadienylcobalt, and 6 equiv of a platinum linker, cis-(Me₃P)₂Pt²⁺2TfO⁻, was determined by DNMR in nitromethane. It exhibits a highly unusual bilinear Eyring plot. In the low temperature regime, the activation enthalpy ΔH^‡ is ~12 kcal/mol and an activation entropy ΔS^‡ ranges from ~−15 to ~0 cal/mol·K as a function of the nature and concentration of the anions present. The reaction is attributed to hindered rotation of the pyridine rings about the Pt−N bond, facilitated by a tight pairing with a counterion. Above a counterion-dependent limiting temperature, ΔH^‡ and ΔS^‡ change abruptly to ~35 kcal/mol and ~60 cal/mol·K, respectively. The changes largely compensate, such that the reactions have comparable rates in the two regimes, both amenable to DNMR measurement, but their mechanisms clearly differ. Several kinetic models for the involvement of ion pairing equilibria fit the observed data nearly equally well, and they all contain a reaction step with high ΔH^‡ and ΔS^‡ values in the high-temperature regime. Its mechanism is proposed to involve a counterion-assisted reversible dissociation of one or two adjacent Pt−N bonds, followed by nearly free rotation of the terminal pyridine ring or rings and subsequent bond reclosure, which is similar to the last presumed step in the initial prism assembly. An interpretation of the very high ΔS^‡ value is suggested by molecular dynamics calculations: at equilibrium, there is a bubble of gaseous nitromethane solvent inside the prism, and it collapses when the prism opens as the transition state is reached. A simple calculation of the entropy of cavitation provides quantitative support for this tentative proposal. The presence of such voids might be generally important for the formation and properties of self-assembled cages.