Multicomponent Crystals with Two Fast Reorienting Constituents Over Perpendicular Noncovalent Axes

Here, we report two new multicomponent crystals (1 and 2) using 2,7-diphenyl-9H-carbazole as the static fragment and two highly mobile components: 1,4-diazabicyclo[2.2.2]octane (DABCO) and tetrahydrofuran (THF). Our findings indicate that their motion does not take place in an isotropic and multiaxial manner, as it occurs in many inclusion compounds, but rather, two independent hydrogen bonds act as specific rotational axes within the crystals. Highly symmetric DABCO rotates over an N-H center dot center dot center dot N axis, while THF preferentially rotates around a weak C-H center dot center dot center dot O axis that is perpendicular to that of DABCO. Dynamic characterization of 1 by VT H-2 NMR experiments supports that DABCO experiences rapid jumps of 120 degrees (threefold) with a k(rot) > 10 MHz at 300 K and 5 MHz at 150 K At the same time, THF experiences 180 degrees jumps in the fast regime, even at 175 K. Complementarily, H-1(T-1) relaxometry studies show that DABCO has an E-a = 3.5 kcal/mol, while for THF, an E-a = 2.3 kcal/mol was computed by periodic DFT. Non-isotropic mobility of two constituents within crystals is extremely rare and could have strong implications for future applications of multicomponent solids with regulated dynamics.

Automated summary

The study reveals the non-isotropic motion of two constituents within crystals, with DABCO rotating rapidly over an N-H center dot N axis and THF preferentially rotating around a weak C-H center dot O axis perpendicular to that of DABCO. Dynamic characterization shows rapid jumps and rotations, indicating potential implications for future applications of multicomponent solids with regulated dynamics.