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

Abstract

Structural control among hexagonal (trimer), rhomboidal (dimer), and infinite-chain supramolecular complexes with three different supporting ligands of ethylenediamine (en), <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine (en*), and 1,2-bis­(diphenyl)­phosphinoethane (dppe) [(en)­Pd­(L)]₃(OTf)₆ <b>1t·OTf</b>, [(en*)­Pd­(L)]₂(PF₆)₄ <b>2d·PF</b>_<b>6</b>, and [(dppe)­Pd­(L)­(OTf)₂]_∞ <b>3·OTf</b> (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 (<b>1t·OTf</b>/<b>2t·PF</b>_<b>6</b>) and the molecular rhomboid (<b>1d·OTf</b>/<b>2d·PF</b>_<b>6</b>) was observed in the en/en* ligand systems, whereas <b>3·OTf</b> 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 <b>2t/2d·PF</b>_<b>6</b> 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 <b>2t·</b>[α-PW₁₂O₄₀]^3–, as confirmed by a combination of ¹H and ³¹P DOSY and CSI-MS spectral data