Conformation control of a flexible tetratopic ligand by modulation of the length of the rigid bridging ligand is achieved during the self-assembly of metallacycles. The ability to control the conformation of the flexible ligand using an ancillary rigid ligand provides a new method for the preparation of novel metallacycles that contain both flexible and rigid modules with highly accurate prediction of the final structures.Precise control of supramolecular metallacycles requires a rational design of molecular components, because the information that determines the specific assembly should be encoded in the molelcular architecture. 1 Recently, the use of flexible motifs to construct metallacycles has increased because of the advantages associated with their use, such as adaptive recognition properties and breathing ability in the solid state. [2][3][4][5] Flexible organic components are generally less predictable during self-assembly and have a tendency to generate [2]catenanes 2 or oligomers upon reaction with metal ions. Prior research in this area has focused on the use of di-and tritopic flexible N-donors to construct both ionic and neutral metallacycles. [2][3][4][5][6] The synthesis and use of semirigid tetratopic N-donors as structural components for metallacycles is rare because of their higher conformational flexibility, which must be restricted to obtain well-preorganized subunits. 7 Herein, we report an effective rigiditymodulated approach for the design of neutral metallacycles. The rigid anionic linker responsible for determination of M 3 3 3 M separation allows for conformational control of the flexible motif with simultaneous use of a rigid bis(chelator), tetratopic flexible N-donors, and fac-Re(CO) 3 cores during self-assembly (Scheme 1).In this study, the flexible ligand 1,2,4,5-tetrakis(5,6-dimethylbenzimidazol-1-ylmethyl)benzene (TXyBim) and two rigid moieties, 2,2 0 -bis(benzimidazolyl) (H 2 -Bim) 8 and chloranilic acid (H 2 -CA), were explored as basic building units. In the case of TXyBim, four benzimidazoles are connected via flexible methylene groups to an arene core. This flexibility permits TXyBim to adopt several possible conformations because of the orientation of the four benzimidazolyl arms (Chart S1 in the Supporting Information). 9 Compound 1 was assembled from Re 2 (CO) 10 , TXyBim, and H 2 -Bim in toluene in 68% yield under solvothermal conditions (Scheme 2). 10 The resulting yellow product is airand moisture-stable and slightly soluble in polar solvents. The IR spectrum of 1 exhibited strong bands at 2018, 1914, and 1900 cm -1 , characteristic of fac-Re(CO) 3 . The structure of compound 1 was determined by X-ray crystallographic analysis, which revealed that metallacycle 1 contains four fac-Re(CO) 3 cores, two Bim groups, and one TXyBim moiety, as shown in Figure 1. 10 The coordination geometry around the Re centers is a distorted octahedron with a C 3 N 3donor environment. The dianionic Bim is coordinated in a
