Pyridine ligand rotation in self-assembled trigonal prisms. Evidence for intracage solvent vapor bubbles.

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)2Pt(2+) 2 TfO(-), was determined by DNMR in nitromethane. It exhibits a highly unusual bilinear Eyring plot. In the low temperature regime, the activation enthalpy DeltaH(double dagger) is approximately 12 kcal/mol and an activation entropy DeltaS(double dagger) ranges from approximately -15 to approximately 0 cal/mol x 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, DeltaH(double dagger) and DeltaS(double dagger) change abruptly to approximately 35 kcal/mol and approximately 60 cal/mol x 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 DeltaH(double dagger) and DeltaS(double dagger) 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 DeltaS(double dagger) 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.

Coordination-driven face-directed self-assembly of trigonal prisms. Face-based conformational chirality.

The coordination-driven self-assembly of four different trigonal prisms from 3 equiv of one of four different tetrapyridyl star connectors and 6 equiv of a platinum linker dication in nitromethane is presented. This face-directed approach affords high yields without template assistance. The prisms have been characterized by multinuclear and DOSY NMR and dual ESI-FT-ICR mass spectrometry. The use of a conformationally chiral star connector leads to a conformationally chiral prism when connector arm ends attached to a vertex have a strongly correlated twist sense and chirality is communicated across polyhedral faces, edges, and vertices. Molecular mechanics results suggest that in the smallest prism 3d collective effects dominate and the all-P and all-M conformers are strongly favored. NMR data prove that the two edges of the pyridine rings in the triflate salts of 3a-3d are distinct. An Eyring plot of rates obtained from line-shape analysis and 1-D EXCHSY NMR yields an activation enthalpy DeltaH(double dagger) of approximately 12 kcal/mol and activation entropy DeltaS(double dagger) of approximately -15 cal/mol x K for the edge interconversion process, compatible with pyridine rotation around the Pt-N bond. For 3c, this behavior is observed only up to approximately 318 K. At higher temperatures, the Eyring plot is again linear but follows a very different straight line, with a DeltaH(double dagger) of approximately 35 kcal/mol and DeltaS(double dagger) of approximately 60 cal/mol x K. This highly unusual result is further investigated and discussed in the following companion paper.

Toward self-assembled surface-mounted prismatic altitudinal rotors. A test case: trigonal and tetragonal prisms.

[structure: see text] A self-assembly path toward prismatic molecular rotors based on transversely reactive terminally metalated molecular rods and pyridine-terminated star connectors has been extended. The concept has been tested on the assembly of trigonal and tetragonal prisms from the biphenyl rod, [Ph2P(CH2)3PPh2]Pt+ -C6H4-C6H4-Pt+ [Ph2P(CH2)3PPh2], and the star-shaped connectors, 1,3,5-tris(4-ethynylpyridyl)benzene and [tetrakis(4-pyridyl)cyclobutadiene]cyclopentadienylcobalt, respectively. The prisms have been fully characterized by NMR and MS, including diffusion-ordered NMR and collision-induced dissociation, and their chiral structures optimized by molecular mechanics are discussed.

Toward self-assembled surface-mounted prismatic altitudinal rotors. A test case: molecular rectangle.

[structure: see text] A self-assembly path toward prismatic molecular rotors based on transversely reactive terminally metalated molecular rods and pyridine-terminated star connectors is outlined. The concept is tested on the assembly of the biphenyl rod [Ph(2)P(CH(2))(3)PPh(2)Pt(+)-C(6)H(4)-C(6)H(4)-Pt(+)[Ph(2)P(CH(2))(3)PPh(2)] and 4,4'-bipyridyl into a molecular rectangle, fully characterized by NMR and MS, including diffusion-ordered NMR and collision-induced dissociation MS.

Computational studies of aliphatic amine basicity.

Computational studies have been used to examine the structural and energetic effects of adding small numbers of water molecules to ammonia, methylamine, dimethylamine, and trimethylamine, and their respective ammoniums ions using the effective fragment potential method. Distinct structural effects with only a few fragment water molecules are revealed. The complexity of structures increases with the number of water fragments with the water fragments forming complex networks. Structural and energetic effects are used to probe the so-called anomalous basicity effect of ammonia and the methylamines on going from the gas phase to aqueous solution.