5 10 15 Intensity ( arb. units) 2 θ θ (degree) Re-heating to 300 K 3% contraction 3% expansion 80 KPa O 2 3% contraction Cooling from 300 to 90 K Drying (reduced pressure, 373 K) 0.0 0.5 1.0 1.5 × 10 5 D i r e c t O b s e r v a t i o n o f O 2 M o l e c u l e s A d s o r b e d i n a N a n o p o r o u s M e t a l - O r g a n i c S o l i d D i r e c t O b s e r v a t i o n o f O 2 M o l e c u l e s A d s o r b e d i n a N a n o p o r o u s M e t a l - O r g a n i c S o l i d Figur e 1 One-dimensional regular assembly of dioxygen molecules, which cannot be realized under normal c o n d i t i o n s , h a s l o n g b e e n o n e o f t h e m o s t f a s c i n a t i n g t a r g e t s i n c h e m i s t r y a n d p h y s i c s b e c a u s e o f t h e k e e n i n t e r e s t i n i t s m a g n e t i c a n d p h o t o p h y s i c a l p r o p e r t i e s t h a t a r e c h a r a c t e r i s t i c o f l o w d i m e n s i o n a l i t y [ 1 ] . O n e a p p r o a c h t o t h e fo rm at io n of a on e - di me ns io na l sp ec if ic as se mb ly o f d i o x y g e n m o l e c u l e s i s t o u s e a u n i f o r m n a n o - s i z e d c h a n n e l i n a m i c r o p o r o u s c o m p o u n d . M o l e c u l e s t e n d t o b e c o n d e n s e d i n a n a n o - s i z e d ch an ne l by a ty pe of co nf in em en t ef fe ct re su lt in g f r o m d i s p e r s i o n - t y p e a t t r a c t i v e i n t e r a c t i o n s o f c o n t i g u o u s p o r e w a l l s , w h i c h e v e n t u a l l y f o r m a n o r d e r e d a s s e m b l y w e l l s u i t e d t o r e g u l a t e d p o r e g e o m e t r y . A c l u e t o a r e g u l a r o n e - d i m e n s i o n a l assembly is the creation of well-regulated channel s t r u c t u r e s . C r y s t a l l i n e m i c r o p o r o u s c o o r d i n a t i o n p o l y m e r s p o s s e s s i n g u n i f o r m l o w - d i m e n s i o n a l channels are relevant for a host framework where gu es t mol ec ul es are co nfi ne d in the ir ch an ne ls [2- 4 ] . H e r e w e r e p o r t t h e f i r s t d i r e c t o b s e r v a t i o n o f dioxygen molecules physisorbed in nano-channels of a mi cr op or ou s co pp er co or di na ti on po ly me r by t h e M E M ( M a x i m u m E n t r o p y M e t h o d ) / R i e t v e l d Method using in situ high-resolution synchrotron X- ray powder diffraction measurements on beamline BL02B2 [5]. For this purpose, we employed a microporous c o p p e r c o o r d i n a t i o n p o l y m e r w i t h p i l l a r e d l a y e r s t r u c t u r e ( C P L - 1 ) , w h i c h p o s s e s s e s o n e - d i m e n s i o n a l n a n o - c h a n n e l s w i t h a c r o s s s e c t i o n o f 4 × 6 Å 2 . shows in situ powde r X-ray d i f f r a c t i o n p a t t e r n s o f a s - s y n t h e s i z e d C P L - 1 a n d a n h y d r o u s C P L - 1 w i t h o x y g e n a t 8 0 K P a o v e r a t e m p e r a t u r e r a n g e o f 3 0 0 t o 9 0 K . T h e p o w d e r d i f f r a c t i o n p a t t e r n c h a n g e s s i g n i f i c a n t l y ( i ) a f t e r he at in g un de r re du ce d pr es su re to re mo ve wa te r molecules, ( ii ) during the cooling process between 1 3 0 K a n d 1 5 0 K , a n d ( i i i ) d u r i n g t h e r e h e a t i n g Fig. 1. Temperature-dependence of synchrotron XRD patterns of CPL-1 at 80 KPa of O 2 . 300 K 90 K 110 K 130 K 150 K 200 K 300 K 300 K 64 Fig. 2(a) F i g u r e 2 ( b ) References [1] W. Mori et al. , Mol. Cryst. Liq. Cryst. 306 (1997) 1. [ 2 ] R . K i t a u r a , K . S e k i , G . A k i y a m a , S . K i t a g a w a , A n g e w . C h e m . I n t . E d . 4 2 (2003) 428. [ 3 ] R . K i t a u r a e t a l . , A n g e w . C h e m . I n t . Ed. 41 (2002) 133. [4] M. Kondo et al. , Angew. Chem. Int. Ed. 38 (1999) 140. [ 5 ] R y o K i t a u r a , S u s u m u K i t a g a w a , Y o s h i k i K u b o t a , T a t s u o C . K o b a y a s h i , Koichi Kindo, Yoshimi Mita, Akira Matsuo, M i c h i h i r o K o b a y a s h i , H o - C h o l C h a n g , T a d a s h i C . O z a w a , M e g u m i S u z u k i , Mak oto Sak ata , Mas aki Tak ata , Sci enc e 298 (2002) 2358. p r o c e s s f r o m 9 0 K t o 3 0 0 K . N o c h a n g e w a s ob se rv ed wi th ou t an y ox yg en mo le cu le s ov er th e whole temperature range studied. Crystal structure of anhydrous CPL-1 at 90 K w i t h o u t O 2 m o l e c u l e s d e t e r m i n e d b y R i e t v e l d a n a l y s i s r e v e a l e d t h a t t h e p o r o u s s t r u c t u r e w a s i d e n t i c a l t o a s - s y n t h e s i z e d C P L - 1 w i t h s l i g h t structure distortion. MEM/ Rietveld analysis proved s i g n i f i c a n t l y t h a t n o w a t e r m o l e c u l e e x i s t s i n t h e nano-channel ( ). Cell parameters of the anhydrous CPL-1 with O 2 o f 8 0 K P a a t 9 0 K ( s p a c e g r o u p P 2 1 / c ) w e r e d e t e r m i n e d b y R i e t v e l d a n a l y s i s a s f o l l o w s : a i s 4.68759(4) Å, b 20.4373(2) Å, c 10.9484(1) Å, and β 96.9480(6) . T h e r e l i a b i l i t y ( R ) f a c t o r b a s e d o n t h e B r a g g intensities R I and the weighted profile R factor R wp o f t h e f i n a l R i e t v e l d f i t t i n g w e r e 3 . 9 % a n d 2 . 1 % , respectively. The final electron densities, obtained b y t h e m a x i m u m - e n t r o p y m e t h o d w i t h r e l i a b i l i t y f a c t o r R F o f 1 . 5 % , c l e a r l y r e v e a l t h e 3 - D p i l l a r e d - l a y e r s t r u c t u r e , w h i c h i s c o n s i s t e n t w i t h s i n g l e - c r y s t a l d a t a . s h o w s t h e p e r s p e c t i v e view of the electron density distribution map along the a-axis; front view and side view. The earthnut- s h a p e d e l e c t r o n d e n s i t i e s , w h i c h a r e p r e s u m a b l y due to O 2 molecules, are clearly recognized in the m i d o f c h a n n e l s . T h e r e l a t i v e l y s m a l l v a l u e o f isotropic displacement parameter (B 4.1(2)) and no disorder of O 2 molecules indicate that O 2 molecules a d s o r b e d i n t h e n a n o - c h a n n e l i s l i k e s o l i d s t a t e rather than liquid state at 90K which is much higher than the freezing point of O 2 , 54.4 K at atmospheric p r e s s u r e . I n t e r e s t i n g l y , t w o O 2 m o l e c u l e s a l i g n p a r a l l e l w i t h e a c h o t h e r a l o n g t h e a - a x i s w i t h intermolecular distance of 3.28(4) Å which is much smaller than the one corresponding to the minimum of the Lennard-Jones potential (Re = 3.9 Å). This i n t e r m o l e c u l a r d i s t a n c e i s c o m p a r a b l e t o t h e d i s t a n c e be tw ee n th e tw o ne ar es t mo le cu le s in so li d α -O 2 , wh ic h is st ab le be lo w 24 K at at mo sp he ri c pr es su re . T h i s i n d i c a t e s t h a t O 2 m o l e c u l e s a d s o r b e d i n t h e na no -c ha nn el fo rm va n de r Wa al s di me rs (( O 2 ) 2 ), w h o s e s u c c e s s f u l s t r u c t u r a l c h a r a c t e r i z a t i o n h a s not been reported yet. Each dimer aligns along the a-axis to form 1-D ladder-like structure. Susumu Kitagawa Kyoto University E-mail: kitagawa @ sbchem.kyoto-u.ac.jp Fig. 2. MEM electron density maps of (a) anhydrous CPL-1 without O 2 molecules at 90 K and (b) CPL-1 w i t h a d s o r b e d O 2 a t 9 0 K a s a n e q u a l – d e n s i t y contour surface. The equicontour level is 1.0 e Å -3 . (a) (b) 65
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