Structures and Dynamic Behavior of Large Polyhedral Coordination Cages: An Unusual Cage-to-Cage Interconversion

The bis-bidentate bridging ligand L {alpha,alpha'-bis-[3-(2-pyridyl)pyrazol-1-yl]-1,4-dimethylbenzene}, which contains two chelating pyrazolyl-pyridine units connected to a 1,4-phenylene spacer via flexible methylene units, reacts with transition metal dications to form a range of polyhedral coordination cages based on a 2M:3 L ratio in which a metal ion occupies each vertex of a polyhedron, a bridging ligand lies along every edge, and all metal ions are octahedrally coordinated. Whereas the Ni(II) complex [Ni8L12]-(BF4)(12)(SiF6)(2) is an octanuclear cubic cage of a type we have seen before, the Cu(II), Zn(II), and Cd(II) complexes form new structural types. [Cu6L9](BF4)(12) is an unusual example of a trigonal prismatic cage, and both Zn(II) and Cd(II) form unprecedented hexadecanuclear cages [M16L24]X-32(X = ClO4 or BF4) whose core is a skewed tetracapped truncated tetrahedron. Both Cu6L9 and M16L24 cages are based on a cyclic helical M3L3 subunit that can be considered as a triangular panel, with the cages being constructed by interconnection of these (homochiral) panels with additional bridging ligands in different ways. Whereas [Cu6L9](BF4)(12) is stable in solution (by electrospray mass spectrometry, ES-MS) and is rapidly formed by combination of Cu(BF4)(2) and L in the correct proportions in solution, the hexadecanuclear cage [Cd16L24](BF4)(32) formed on crystallization slowly rearranges in solution over a period of several weeks to the trigonal prism [Cd6L9](BF4)(12), which was unequivocally identified on the basis of its H-1 NMR spectrum. Similarly, combination of Cd(BF4)(2) and L in a 2:3 ratio generates a mixture whose main component is the trigonal prism [Cd6L9](BF4)(12). Thus the hexanuclear trigonal prism is the thermodynamic product arising from combination of Cd(II) and L in a 2:3 ratio in solution, and arises from both assembly of metal and ligand (minutes) and rearrangement of the Cd-16 cage (weeks); the large cage [Cd16L24](BF4)(32) is present as a minor component of a mixture of species in solution but crystallizes preferentially.