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

Abstract

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

“…35 We propose that they pertain to the first steps of cage unraveling and thus the last steps in the mechanism of its self-assembly, and to the structure of the solvent around and/or inside the cage.…”

Supramolecular Coordination: Self-Assembly of Finite Two- and Three-Dimensional Ensembles

“…Theoretical studies showed that largest trifacial box, 404 , displayed a highly unusual kinetic behavior during the pyridine ligand rotation. 464 …”

Superchiral Pd3L6 Coordination Complex and Its Reversible Structural Conversion into Pd3L3Cl6 Metallocycles

“…[2] Thec oordination geometry of the metal ions and the number of the binding sites of the ligands and overall structure of the ligand define geometry of the coordination assembly.The use of divalent or tetravalent Pd II of cis-, trans-, or square-planar binding geometry,t ogether with various rigid or flexible pyridyl ligands,has proven to be asuccessful strategy in obtaining numerous supramolecular assemblies.In general, the ligands used for their preparation are mostly symmetrical, relatively rigid, and very often achiral. [6,7] TheM 3 L 6 assembly is represented by work of Fujita et al, where 608 8 donor,1 ,2-bis(ethynylpyridine)benzene, in combination with Pd II ,y ielded solvato-controlled assemblies of Pd 3 L 6 and Pd 4 L 8 . [4] Focusing on the chiral concave coordination assemblies resembling the structures presented and starting from the smallest ones,several examples of chiral metallo-supramolecular rhomboids have been prepared by self-assembly of chiral bis-pyridyl-substituted ditopic ligands with 908 8 cis-blocked Pd II ,o ften with [(enPd)(NO 3 ) 2 ].…”