Ultra-fast Rotors for Molecular Machines and Functional Materials via Halogen Bonding: Crystals of 1,4-Bis(iodoethynyl)bicyclo[2.2.2]octane with Distinct Gigahertz Rotation at Two Sites

Abstract

As a point of entry to investigate the potential of halogen-bonding interactions in the construction of functional materials and crystalline molecular machines, samples of 1,4-bis(iodoethynyl)bicyclo[2.2.2]octane (BIBCO) were synthesized and crystallized. Knowing that halogen-bonding interactions are common between electron-rich acetylenic carbons and electron-deficient iodines, it was expected that the BIBCO rotors would be an ideal platform to investigate the formation of a crystalline array of molecular rotors. Variable temperature single crystal X-ray crystallography established the presence of a halogen-bonded network, characterized by lamellarly ordered layers of crystallographically unique BIBCO rotors, which undergo a reversible monoclinic-to-triclinic phase transition at 110 K. In order to elucidate the rotational frequencies and the activation parameters of the BIBCO molecular rotors, variable-temperature (1)H wide-line and (13)C cross-polarization/magic-angle spinning solid-state NMR experiments were performed at temperatures between 27 and 290 K. Analysis of the (1)H spin-lattice relaxation and second moment as a function of temperature revealed two dynamic processes simultaneously present over the entire temperature range studied, with temperature-dependent rotational rates of k(rot) = 5.21 × 10(10) s(-1)·exp(-1.48 kcal·mol(-1)/RT) and k(rot) = 8.00 × 10(10) s(-1)·exp(-2.75 kcal·mol(-1)/RT). Impressively, these correspond to room temperature rotational rates of 4.3 and 0.8 GHz, respectively. Notably, the high-temperature plastic crystalline phase I of bicyclo[2.2.2]octane has a reported activation energy of 1.84 kcal·mol(-1) for rotation about the 1,4 axis, which is 24% larger than E(a) = 1.48 kcal·mol(-1) for the same rotational motion of the fastest BIBCO rotor; yet, the BIBCO rotor has three fewer degrees of translational freedom and two fewer degrees of rotational freedom! Even more so, these rates represent some of the fastest engineered molecular machines, to date. The results of this study highlight the potential of halogen bonding as a valuable construction tool for the design and the synthesis of amphidynamic artificial molecular machines and suggest the potential of modulating properties that depend on the dielectric behavior of crystalline media.

Molecular Rods: Facile Desymmetrization of 1,4‐Diethynylbicyclo[2.2.2]octane

“…The published synthesis of the diyne 1 works well and the reported overall yield is reproducible. Three comments on our results are called for:…”

“…Hydrocarbon 1 is relatively easily accessible in multigram quantities from diethyl succinate ( 4 ) by following a published procedure (Scheme ) . The average overall yield of this nine‐step synthesis usually reaches 11 % …”

“…They are also essential motifs in systems designed for studying various physical phenomena such as singlet energy or electron, transfer. Efforts to construct regular two‐dimensional and three‐dimensional arrays of rotors frequently rely on the use of bridgehead substituted bicyclo[1.1.1]pentanes (BCP),, , , bicyclo[2.2.2]octanes (BCO),, and triptycenes as key building blocks. A few of the BCO‐based systems already showed ultra‐fast rotation, in the solid state and interesting phenomena such as correlated motion in pairs of neighboring rotators, as well as second harmonic generation responses due to local space‐inversion symmetry breaking in centrosymmetric crystals , …”

1,3‐Diethynylbicyclo[1.1.1]pentane, a Useful Molecular Building Block

“…The diiododiyne 33 was also synthesized. In this case, however, we started from the disilyl derivative 7 applying the procedure used probably for the first time by Jacobson and Carothers14 and recently by others,15 who treated the starting alkynes with potassium hydroxide and iodine in methanol. We used 7 in that procedure to accomplish the deprotection and subsequent iodination in a one‐pot reaction (Scheme b).…”

Precessional Motion in Crystalline Solid Solutions of Ionic Rotors

“…Therefore the ability to combine type and strength of the interactions around the chosen molecular fragment is crucial for a successful design of materials with “tuneable” dynamic processes. To this scope several approaches have been proposed: covalent modification of the fragment with bulky substituents, trapping within molecular macrocages or porous frameworks, and synthesis of co‐crystals based on halogen bonding interactions …”