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

摘要

The bis-bidentate bridging ligand L {a,a'-bis- [3-(2-pyridyl)pyrazol-1-yl]-1,4-dimethyIbenzene}, 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 [Ni_8Li_(12)]- (BF_4)_(12)(SiF_6)_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. [Cu_6L_9](BF_4)_(12) is an unusual example of a trigonal prismatic cage, and both Zn(II) and Cd(II) form unprecedented hexadecanuclear cages [M_(16)L_(24)]X_(32)(X = ClO_4 or BF_4) whose core is a skewed tetracapped truncated tetrahedron. Both Cu_6L_9 and M__(16_L_(24) cages are based on a cyclic helical M_3L_3 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 [Cu_6L_9] (BF_4)_(12) is stable in solution (by electrospray mass spectrometry, ES- MS) and is rapidly formed by combination of Cu(BF_4)_2 and L in the correct proportions in solution, the hexadecanuclear cage [Cd_(16)L_(24)](BF_4)_(32) formed on crystallization slowly rearranges in solution over a period of several weeks to the trigonal prism [Cd_6L_9] (BF_4)_(12), which was unequivocally identified on the basis of its ~1H NMR spectrum. Similarly, combination of Cd(BF_4)_2 and L in a 2:3 ratio generates a mixture whose main component is the trigonal prism [Cd_6L_9](BF_4)_(12). Thus the hexanuclear trigonal prism is the thermodynamic product arising from combination of Cd(Il) 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 [Cd_(16)L_(24)] (BF_4)_(32) is present as a minor component of a mixture of species in solution but crystallizes preferentially.