Experimental X-ray and DFT Structural Analyses of M12L8 Poly-[n]-catenanes Using exo-Tridentate Ligands

ABSTRACT: Despite their potential applications in host-guest chemistry, there are only five reported structures of poly-[n]-catenanes self-assembled by elusive M12L8 icosahedral nanocages. This small number of structures of M12L8 poly-[n]-catenanes is because selfassembly of large metal-organic cages (MOCs) with large windows allowing catenation by means of mechanical bonds is very challenging. Structural reports of M12L8 poly-[n]-catenanes are needed to increase our knowledge about the self-assembly and genesis of such materials. Poly-[n ]-catenane (1 center dot p-CT) self-assembly of interlocked M12L8 icosahedral cages (M = Zn(II) and L = 2,4,6-tris-(4-pyridyl)benzene (TPB)) including a new aromatic guest (p-chlorotoluene (p-CT)) is reported by single-crystal XRD. Despite the huge internal M12L8 voids (> 2500 angstrom 3), p-CT is ordered, allowing a clear visualization of the relative host-guest positions. DFT calculations have been used to compute the electrostatic potential of the TPB ligand, and various aromatic guests (i.e., o-dichlorobenzene (o-DCB), p-chloroanisole (p-CA), and nitrobenzene (NBz)) included (ordered) within the M12L8 cages were determined by single-crystal XRD. The computed maps of electrostatic potential (MEPs) allow for the rationalization of the guest's inclusion seen in the 3D X-ray structures. Although more crystallographic X-ray structures and DFT analysis are needed to gain insights of guest inclusion in the large voids of M12L8 poly-[n]-catenanes, the reported combined experimental/DFT structural analyses approach can be exploited to use isostructural M12L8 poly-[n]-catenanes as hosts for molecular separation and could find applications in the crystalline sponge method developed by Fujita and co-workers. We also demonstrate, exploiting the instant synthesis method, in solution (i.e., o-DCB), and in the solid-state by neat grinding (i.e., without solvent), that the isostructural M12L8 poly-[n]-catenane self-assembled with 2,4,6-tris-(4-pyridyl)pyridine (TPP) ligand and ZnX2 (where X = Cl, Br, and I) can be kinetically synthesized as crystalline (yields Pe. 60%) and amorphous phases (yields Pe. 70%) in short time and large quantities. Despite the change in the aromatic nature at the center of the rigid exo-tridentate pyridine-based ligand (TPP vs TPB), the kinetic control gives the poly-[n]-catenanes selectively. The dynamic behavior of the TPP amorphous phases upon the uptake of aromatic guest molecules can be used in molecular separation applications like benzene derivatives.