Harnessing molecular motion to reversibly control macroscopic properties, such as shape and size, is a fascinating and challenging subject in materials science. Here we design a crystalline cobalt(II) complex with an n-butyl group on its ligands, which exhibits a reversible crystal deformation at a structural phase transition temperature. In the low-temperature phase, the molecular motion of the n-butyl group freezes. On heating, the n-butyl group rotates ca. 100° around the C–C bond resulting in 6–7% expansion of the crystal size along the molecular packing direction. Importantly, crystal deformation is repeatedly observed without breaking the single-crystal state even though the shape change is considerable. Detailed structural analysis allows us to elucidate the underlying mechanism of this deformation. This work may mark a step towards converting the alkyl rotation to the macroscopic deformation in crystalline solids.

Ten types of neutral charge transfer (CT) complexes of coronene (electron donor; D) were obtained with various electron acceptors (A). In addition to the reported 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex of 1:1 stoichiometry with a DA-type alternating π column, TCNQ also afforded a 3:1 complex, in which a face-to-face dimer of parallel coronenes (Cor-As) is sandwiched between TCNQs to construct a DDA-type alternating π column flanked by another coronene (Cor-B). Whereas solid-state (2)H NMR spectra of the 1:1 TCNQ complex formed with deuterated coronene confirmed the single in-plane 6-fold flipping motion of the coronenes, two unsynchronized motions were confirmed for the 3:1 TCNQ complex, which is consistent with a crystallographic study. Neutral [Ni(mnt)2] (mnt: maleonitriledithiolate) as an electron acceptor afforded a 5:2 complex with a DDA-type alternating π column flanked by another coronene, similar to the 3:1 TCNQ complex. The fact that the Cor-As in the [Ni(mnt)2] complex arrange in a non-parallel fashion must cause the fast in-plane rotation of Cor-A relative to that of Cor-B. This is in sharp contrast to the 3:1 TCNQ complex, in which the dimer of parallel Cor-As shows inter-column interactions with neighboring Cor-As. The solid-state (1)H NMR signal of the [Ni(mnt)2] complex suddenly broadens at temperatures below approximately 60 K, indicating that the in-plane rotation of the coronenes undergoes down to approximately 60 K; the rotational rate reaches the gigahertz regime at room temperature. Rotational barriers of these CT complexes, as estimated from variable-temperature spin-lattice relaxation time (T1) experiments, are significantly lower than that of pristine coronene. The investigated structure-property relationships indicate that the complexation not only facilitates the molecular rotation of coronenes but also provides a new solid-state rotor system that involves unsynchronized plural rotators.

For the purpose of investigating the correlation between host gas adsorption ability and structural flexibility, the combination of metal benzoate complexes [M(II)(2)(bza)(4)] (M(II) = Rh (a), Cu (b); bza = benzoate) and pyrazine derivatives (pyz = pyrazine (1), 2-mpyz = 2-methylpyrazine (2), 2,3-dmpyz = 2,3-dimethylpyrazine (3)) yields a series of one-dimensionally assembled complexes. The study of the adsorption properties of this series was examined for CO(2), H(2), N(2), O(2), and Ar gases at 195 K (CO(2)) or at 77 K (all others). The adsorption manners of these complexes are similar for each gas, while the pressure at which adsorption started or rapidly grew increased with a rise in the number of methyl groups in the case of adsorbable gases. The maximum amount of adsorption was a positive integer, e.g., 3 molecules per M(2) unit for 1 and 2 and 2 molecules per M(2) unit for 3 in the case of CO(2) adsorption for all complexes at 0.1 MPa of adsorbable gases. Structural transformation was observed accompanying gas adsorption. This transformation was observed when the adsorption amount reached 1 molecule per M(2) unit, suggesting a correlation of the adsorption amount and dynamic adsorption behavior. Single-crystal X-ray analyses of nonincluded crystals and CO(2) inclusions for all hosts (1-3) revealed that large structural changes occurred through CO(2) adsorption to increase the inner space for adsorption gases, depending on the substituents on the pyrazine ring. These facts were confirmed as a transition by DSC measurements using a mixed CO(2)/N(2) gas atmosphere. Solid-state (1)H and (2)H NMR studies of the crystalline sample of 1a and its partially deuterated samples of 1a' (deuterated phenyl group) and 1a'' (deuterated pyrazine) revealed rapid 180 degree-flip motions of the aromatic rings of the host skeletons, which form the walls of the channels. These "rotating" motions would help the diffusion of CO(2) molecules through a narrow channel at relatively low pressure. Indeed, the motions of phenyl groups and methyl-substituted pyrazine moieties of phenyl deuterated 3a were confirmed to be very slow by solid-state (1)H and (2)H NMR spectra, where the amount of adsorbed gas molecules was small for 3a at 0.1 MPa of CO(2).

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