Synthesis and Characterization of Molecular Hexagons and Rhomboids and Subsequent Encapsulation of Keggin-Type Polyoxometalates by Molecular Hexagons

Structural control among hexagonal
(trimer), rhomboidal (dimer), and infinite-chain supramolecular complexes
with three different supporting ligands of ethylenediamine (en), N,N,N′,N′-tetramethylethylenediamine (en*), and 1,2-bis­(diphenyl)­phosphinoethane
(dppe) [(en)­Pd­(L)]3(OTf)6 1t·OTf, [(en*)­Pd­(L)]2(PF6)4 2d·PF6, and [(dppe)­Pd­(L)­(OTf)2]∞ 3·OTf (OTf = trifluoromethane sulfonate;
L = 1,3-bis­(4-pyridylethynyl)­benzene) in the solid and solution states
was investigated. The encapsulation of a large Keggin-type polyoxometalate
[α-PW12O40]3– by these
complexes was also examined. As the steric bulkiness of the supporting
ligands increased in the order of en < en* < dppe, the hexagonal,
rhomboidal, and infinite-chain structures were obtained, as confirmed
by X-ray crystallography. In solution, equilibrium between the molecular
hexagon (1t·OTf/2t·PF6) and the molecular rhomboid (1d·OTf/2d·PF6) was observed in
the en/en* ligand systems, whereas 3·OTf with the
dppe ligand did not exhibit equilibrium and instead existed as a single
species. These phenomena were established by cold-spray ionization
mass spectroscopy (CSI-MS) and 1H diffusion ordered NMR
spectroscopy (DOSY). The addition of the highly negatively charged
Keggin-type phosphododecatungstate [α-PW12O40]3– to a solution of 2t/2d·PF6 resulted in the encapsulation of the tungstate
species in the cavity of the molecular hexagon to form {[(en*)­Pd­(L)]3[⊃α-PW12O40]}­(PF6)3 2t·[α-PW12O40]3–, as confirmed by a combination of 1H and 31P DOSY and CSI-MS spectral data.