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Unique Guest‐Inclusion Properties of a Breathing Ionic Crystal of K 3 [Cr 3 O(OOCH) 6 (H 2 O) 3 ][ α ‐SiW 12 O 40 ]⋅16 H 2 O
Author(s) -
Uchida Sayaka,
Mizuno Noritaka
Publication year - 2003
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200304874
Subject(s) - polyoxometalate , molecule , crystallography , crystallization , ionic bonding , chemistry , chemical polarity , inclusion compound , crystal (programming language) , methylene , ion , crystal structure , stereochemistry , organic chemistry , computer science , programming language , catalysis
A microstructured ionic crystal, K 3 [Cr 3 O(OOCH) 6 (H 2 O) 3 ][ α ‐SiW 12 O 40 ]⋅16 H 2 O ( 1 ) was synthesized by the complexation of the Keggin‐type polyoxometalate of [ α ‐SiW 12 O 40 ] 4− with a macrocation of [Cr 3 O(OOCH) 6 (H 2 O) 3 ] + . Compound 1 possessed a straight channel, with an opening of approximately 0.5×0.8 nm, which contained the water of crystallization. The use of the macrocation with large size (0.7 nm) and small charge (+1) reduced the anion–cation interaction and was essential for the channel formation. The molecular structures of the polyoxometalate and the macrocation in 1 were retained under vacuum at 473 K. Analogues of 1 were synthesized with [ α ‐PVW 11 O 40 ] 4− or [Fe 3 O(OOCH) 6 (H 2 O) 3 ] + . The water of crystallization in 1 was removed under vacuum at room temperature to form the closely packed guest‐free phase 2 . Compound 2 reversibly and repeatedly included water and polar organic molecules with two carbon atoms or less. Guest inclusion was highly selective and a difference of even one methylene group in the organic guest molecule was discriminated by the host. Polar organic molecules with longer methylene chains and nonpolar molecules such as dinitrogen and methane were completely excluded. The guest‐inclusion properties could be explained by the ion–dipole interaction between the host and the guest, which is proportional to the dipole moment of the guest molecule and inversely proportional to the ion–dipole (host–guest) distance. Thus, small polar molecules were selectively absorbed. These distinctive guest‐inclusion properties were successfully applied to the oxidation of methanol from a mixture of C 1 and C 2 alcohols. These results show unique guest inclusion and catalysis by rationally designed ionic crystals.