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

The tetradentate ligand Lnaph contains two N-donor bidentate pyrazolyl−pyridine units connected to a 1,8-naphthyl
core via methylene spacers; L*45 and L*56 are chiral ligands with a structure similar to that of Lnaph but bearing
pinene groups fused to either C4 and C5 or C5 and C6 of the terminal pyridyl rings. The complexes [Cu(Lnaph)](OTf)
and [Ag(Lnaph)](BF4) have unremarkable mononuclear structures, with CuI being four-coordinate and AgI being two-coordinate with two additional weak interactions (i.e., “2 + 2” coordinate). In contrast, [Cu4(Lnaph)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(L*45)](BF4) and [Ag(L*45)](ClO4) having similar mononuclear crystal
structures and with the ligands being tetradentate chelates. In contrast, [Ag4(L*56)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(L*45)](ClO4)
and [Ag4(L*56)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 CoII, CuII, and CdII, Lnaph generates a series of unusual dodecanuclear
coordination cages [M12(Lnaph)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 L*45 forms the simpler tetranuclear, tetrahedral
coordination cage [Zn4(L*45)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(L*45)18](ClO4)24 can be detected
in solution by ESMS. Formation of [Zn4(L*45)6](ClO4)8 is diastereoselective, with the chirality of the pinene groups
controlling the chirality of the tetranuclear cage.