A chiral pentadecanuclear metallamacrocycle with a sextuple twisted Möbius topology.

Construction of cyclic molecules has received much attention in recent years because of both scientific curiosity and their potential use in applications such as magnetic materials, sensors for small molecules, and new classes of catalysts.1 Although the possibility of forming molecules with a twisted cyclic band structure (Möbius molecules) was predicted theoretically among organic molecules such as polyenes and other conjugated aromatic systems,2 only a few of such molecules have been realized experimentally.3 Recently, an example of an inorganic Möbius strip of NbSe3 crystal based on the coordination chemistry of inorganic elements has been reported.4 A suitable design of the ligands and an appropriate choice of metal ions acting as connecting nodes can also lead to twisted cyclic architectures with a Möbius topology. The only metallamacrocycle of Möbius topology was obtained from a self-assembly of [Li2(pyridyl-2,6-diphenyl)(tmeda)2] (tmeda ) N,N,N′,N′-tetramethylethylene diamine) and a AuI-diphosphane complex.5 While several strategies exist to introduce multiple twists in conjugated cyclic polyenes or other aromatic systems,6 none of these exhibits a twist greater than double twists (360° twists), which is probably due to the large strain that the cyclic system needs to accommodate. Metallamacrocycles that may experience lower strains around the metal centers due to the twist may be a convenient alternative for the generation of multiple twisted cyclic systems. The pentadentate N-acylsalicylhydrazide can serve as an asymmetric ditopic bridging ligand between octahedral metal ions using its tridentate and bidentate binding modes to form metallamacrocycles (Scheme 1).7 Depending on the influence of the N-terminal steric domain during self-assembly, the ligand chooses either Λ or ∆ configuration around each metal center (Figure S1). In addition, it is also possible to modify the nuclearity of the macrocycle8 and the stereochemistry of the metal centers.9,10 A decrease in the steric volume near the CR-Câ region may increase the possibility of three or more successive metal centers having the same chiral configuration in the macrocycle that can induce multiple twists in the cycle. Herein, we report on the self-assembly and characterization of a unique manganese-based pentadecanuclear metallamacrocycle from a manganese ion and a pentadentate ligand, N-phenylpropiolyl salicylhydrazide (H3L), which has the smallest steric volume near the CR-Câ region (Scheme 1). X-ray quality single crystals of the complex were grown from a DMF solution of the ligand, N-phenylpropiolyl salicylhydrazide, and manganese acetate by layering with diethyl ether. The pentadecanuclear metallamacrocycle with C3 symmetry, [Mn15L15S15] 1 (Figure 1), where S is either a DMF or a water molecule, crystallizes in the chiral space group P63, and the asymmetric unit contains five ligand units bound to five metal centers (Figure S2). The steric requirements of the ethynyl phenyl group in the Nterminal steric domain restrain the chiral sequence to ‚‚‚∆∆∆∆Λ‚‚‚ around the metal center. The occurrence of successive metal centers of the same ∆ configuration produces double twists (360° twists) in the metallamacrocycle along the direction of the propagation (Figure 2 and Figure S3). To obtain an insight into the nature and extent of the twist and its contribution to the overall geometry of the metallamacrocycle, we calculated the twist resulting from the chiral configuration at each metal center with respect to the ligand orientation. The consecutive twist angles calculated using the dihedral angle between the neighboring ligand planes were +90, +81, +99, +82, and -92°, where the direction and the extent of an individual twist is determined by the chiral sequence of four consecutive metal centers (Table S1). The five consecutive (+ + + + -) twists contribute a net twist of 360° (double twist) along the direction of propagation.11 The macrocycle makes a sharp turn at the metal center with a Λ configuration (Figure 2). Three such turns in the backbone lead to a C3 symmetric pentadecanuclear metallamacrocycle. This results in a sum of six 180° twists in the overall cycle, creating a sextuple twisted metallamacrocycle (Figure S4).12 The double twist in the asymmetric unit orients the three salicyl moieties toward the inner core of the macrocycle on one face of the cycle (Figure S5). The chiral nature of the C3 symmetric macrocycle13 is exemplified by the fact that the inner core of the other face of the macrocycle has different residues, that is, three ethynyl phenyl groups. Depending on the nature of the N-acyl residue, the chiral sequence of the metal centers is determined to minimize the unfavorable steric repulsion and to maximize the favorable weak interactions, such as the C-H‚‚‚π and π‚‚‚π stacking interactions. The macrocyclic assembly has a total of nine intramolecular C-H‚‚‚O interactions between the N-terminal aromatic C-H’s and oxygen atoms in the bridging domains of the different ligand units (Table S2). Three intramolecular C-H‚‚‚π interactions between † Hanyang University. ‡ Korea University. Scheme 1 Published on Web 10/26/2007