DOSY NMR, X-ray Structural and Ion-Mobility Mass Spectrometric Studies on Electron-Deficient and Electron-Rich M₆L₄ Coordination Cages

A novel modular approach to electron-deficient and electron-rich M₆L₄ cages is presented. From the same starting compound, via a minor modulation of the synthesis route, two C₃-symmetric ligands L1 and L2 with different electronic properties are obtained in good yield. The trifluoro-triethynylbenzene-based ligand L1 is more electron-deficient than the well-known 2,4,6-tri(4-pyridyl)-1,3,5-triazine, while the trimethoxy-triethynylbenzene-based ligand L2 is more electron-rich than the corresponding benzene analogue. Complexation of the ligands with cis-protected square-planar [(dppp)Pt(OTf)₂] or [(dppp)Pd(OTf)₂] corner-complexes yields two electron-deficient (1a and 1b) and two electron-rich (2a and 2b) M₆L₄ cages. The single crystal X-ray diffraction study of 1a and 2a confirms the expected octahedral shape with a ca. 2000 ų cavity and ca. 11 Å wide apertures. The crystallographically determined diameters of 1a and 2a are 3.7 and 3.6 nm, respectively. The hydrodynamic diameters obtained from the DOSY NMR in CDCl₃:CD₃OD (4:1), and diameters calculated from collision cross sections (CCS) acquired by ion-mobility mass spectrometry (IM-MS) were for all four cages similar. In solution, the cage structures have diameters between 3.3 to 3.6 nm, while in the gas phase the corresponding diameters varied between 3.4 to 3.6 nm. In addition to the structural information the relative stabilities of the Pt₆L₄ and Pd₆L₄ cages were studied in the gas phase by collision-induced dissociation (CID) experiments, and the photophysical properties of the ligands L1 and L2 and cages 1a, 1b, 2a, and 2b were studied by UV-vis and fluorescence spectroscopy.