Molecular Motion and Conformational Interconversion of Ir^I·COD Included in Rebek’s Self-Folding Octaamide Cavitand

Abstract

We report experimental and theoretical evidence of restrained axial rotation for heteroleptic L2·Ir(I)·1,5-cyclooctadiene (COD) complexes included in the aromatic cavity of Rebek's self-folding octaamide cavitand. At 298 K, the axial spinning motion of the included organometallic guests was slow on the (1)H NMR time scale and produced a proton spectrum for the bound host indicative of C2 symmetry. Signals corresponding to aromatic protons of the bound host coalesced at 323 K, indicating that the spinning process of the included guest became fast on the (1)H NMR time scale and that the complex approached C4 symmetry. Surprisingly, lowering the temperature of the solution to 193 K induced an additional splitting of the proton signals observed at room temperature for both the bound host and the included guest. We propose the emergence of a new element of chirality in the complexes, which was associated with a slow interconversion, on the (1)H NMR time scale between the two chiral twisted-boat conformers of the chelated COD included in the already chiral cavity of the container. This leads to the inclusion complexes existing in solution as pairs of two racemic diastereomers. We estimated that the racemization barrier for the two cyclochiral conformers of the Ir(I) chelated COD was 5 kcal mol(-1) higher as an included organometallic complex than as free in solution. Furthermore, we performed a van't Hoff plot and determined that the inclusion of the organometallic complex in the cavitand was endothermic and exclusively driven by entropy (ΔH = 5.9 kcal mol(-1) and ΔS = 33.9 cal mol(-1) K(-1)).

Dynamic and Persistent Cyclochirality in Hydrogen-Bonded Derivatives of Medium-Ring Triamines

“…Encouraged by this result, we questioned whether a hydrogen bond-accepting group such as an amide with an innately higher barrier to N–C­(O) bond rotation would increase even further the barrier to directionality reversal. Indeed, several dynamic, cyclochiral compounds are based on secondary amide hydrogen bond networks that run around the rim of their bowl-like structures, , but directionality reversal of such systems is not associated with inversion of the local amide geometry. ,,− The opportunity to integrate amide N–C­(O) bond rotation with directionality reversal appears to be unique to the present system.…”

Simple Model of Protein Energetics To Identify Ab Initio Folding Transitions from All-Atom MD Simulations of Proteins

“…Proteins and enzymes oversee all mechanical and chemical processes within cells. In recent years, the emergence of advanced genome manipulation techniques and the advent of directed evolution methods have spurred the development of new proteins with unprecedented properties as materials or enzymes that are capable of carrying out non-natural reactions in mild conditions, providing attractive alternatives to the use of solid-phase or homogeneous chemical catalysts. − …”

Noncovalent tailoring of coordination complexes by resorcin[4]arene-based supramolecular hosts

“…In the deep cavitands, effective noncovalent interactions from the walls restrict the configuration of flexible molecules and potentially stabilize reactive intermediates. 99 The monocationic Ir I complex, [Ir I (COD) 2 ] + (COD = 1,5-cyclooctadiene) is conformationally flexible, as displayed by a rapid interconversion process between the two chiral twisted-boat (TB) conformations of the coordinated COD ligand. When the Ir I complex was dissolved in THF, the dissociation of one of the COD ligands proceeded to give a heteroleptic Ir I complex [Ir I (COD) (thf ) 2 ] + .…”