Coordination Chemistry of Tetradentate N-Donor Ligands Containing Two Pyrazolyl−Pyridine Units Separated by a 1,8-Naphthyl Spacer: Dodecanuclear and Tetranuclear Coordination Cages and Cyclic Helicates

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Abstract

The tetradentate ligand L(naph) contains two N-donor bidentate pyrazolyl-pyridine units connected to a 1,8-naphthyl core via methylene spacers; L45 and L56 are chiral ligands with a structure similar to that of L(naph) but bearing pinene groups fused to either C4 and C5 or C5 and C6 of the terminal pyridyl rings. The complexes [Cu(L(naph))](OTf) and [Ag(L(naph))](BF4) have unremarkable mononuclear structures, with Cu(I) being four-coordinate and Ag(I) being two-coordinate with two additional weak interactions (i.e., "2 + 2" coordinate). In contrast, [Cu4(L(naph))4][BF4]4 is a cyclic tetranuclear helicate with a tetrafluoroborate anion in the central cavity, formed by an anion-templating effect; electrospray mass spectrometry (ESMS) spectra show the presence of other cyclic oligomers in solution. The chiral ligands show comparable behavior, with [Cu(L45)](BF4) and [Ag(L45)](ClO4) having similar mononuclear crystal structures and with the ligands being tetradentate chelates. In contrast, [Ag4(L56)4](BF4)4 is a cyclic tetranuclear helicate in which both diastereomers of the complex are present in the crystal; the two diastereomers have similar gross geometries but are significantly different in detail. Despite their different crystal structures, [Ag(L45)](ClO4) and [Ag4(L56)4](BF4)4 behave similarly in solution according to ESMS studies, with a range of cyclic oligomers (up to Ag9L9) forming. With transition-metal dications Co(II), Cu(II), and Cd(II), L(naph) generates a series of unusual dodecanuclear coordination cages [M12(L(naph))18]X24 (X- = ClO4- or BF4-) in which the 12 metal ions occupy the vertices of a truncated tetrahedron and a bridging ligand spans each of the 18 edges. The central cavity of each cage can accommodate four counterions, and each cage molecule is chiral, with all 12 metal trischelates being homochiral; the crystals are racemic. Extensive aromatic stacking between ligands around the periphery of the cages appears to be a significant factor in their assembly. The chiral analogue L45 forms the simpler tetranuclear, tetrahedral coordination cage [Zn4(L45)6](ClO4)(8), with one anion in the central cavity; the steric bulk of the pinene chiral auxiliaries prevents the formation of a dodecanuclear cage, although trace amounts of [Zn12(L45)18](ClO4)24 can be detected in solution by ESMS. Formation of [Zn4(L45)6](ClO4)8 is diastereoselective, with the chirality of the pinene groups controlling the chirality of the tetranuclear cage.

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