Enhanced Rotation by Ground State Destabilization in Amphidynamic Crystals of a Dipolar 2,3-Difluorophenylene Rotator as Established by Solid State 2H NMR and Dielectric Spectroscopy

We report the synthesis and rotational dynamics of the pillared metal-organic framework Zn-2(F2BDC)(2)(DABCO) where F2BDC = 2,3-difluorobenzene-1,4-dicarboxylate acts as a rotating dipolar linker and DABCO = 1,4-diazabicyclo[2.2.2]octane acts as a spacer (F2MOF 1). The pillared structure of F2MOF 1 was confirmed by X-ray diffraction and CP-MAS C-13 NMR analyses. Using variable temperature solid state H-2 NMR and broadband dielectric spectroscopy, we characterized the rotational dynamics of the dipolar F2BDC linker in the solid state. Variable temperature (VT) quadrupolar echo H-2 NMR measurements revealed a rotational activation energy of E-a = 6.8 +/- 0.1 kcal/mol, which agreed well with temperature- and frequency-dependent dielectric measurements, indicating a barrier of E-a = 7.1 +/- 0.5 kcal/mol. Structural data from single crystal X-ray diffraction and quantum mechanical calculations (DFT) suggest that the rotational potential is determined by steric interactions between the dipolar rotator and the stator linkers such that fluorine atoms in the F2BDC linker reduce the activation energy by destabilization of the coplanar BDC ground state.