Photoproduction of Hydrogen from Water Catalysed
by Metal Sulfur Chelates
Rosangela Battaglia, Rainer Henning, and Horst Kisch*
I n s t i t u t f ü r S t r a h l e n c h e m i e i m
M a x - P l a n c k - I n s t i t u t f ü r K o h l e n f o r s c h u n g , S t i f t s t r a ß e 3 4 - 3 6 , D - 4 3 3 0 M ü l h e i m a . d . R u h r
Z . N a t u r f o r s c h . 3 6 b , 3 9 6 - 3 9 7 ( 1 9 8 1 ) ; r e c e i v e d D e c e m b e r 1 , 1 9 8 0
P h o t o r e d u c t i o n o f W a t e r , C a t a l y s i s , M e t a l S u l f u r C h e l a t e s
M e t a l d i t h i o l e n e s a n d o t h e r s u l f u r c o n t a i n i n g m e t a l c o m p l e x e s c a t a l y s e t h e p h o t o p r o d u c t i o n o f h y d r o g e n f r o m w a t e r i n t h e p r e s e n c e o f t e t r a h y d r o - o r d i h y d r o f u r a n s .
Catalysed photochemical generation of hydrogen from water in homogeneous solution is known to occur only in the presence of a reducing agent and a multicomponent catalyst system composed of a sensitizer, e.g. tris(2,2'-bipyridine)Ru
2+, an electron relay species, e.g. N,N'-(fimethyl-4,4'-bipyridinium- chloride, and of a redox catalyst like colloidal platinum [1-6]. The two latter components may be omitted on the expense of catalytic activity [7]. We recently discovered simple and very efficient cata- lytic systems consisting of a metal dithiolene cata- lyst and a reducing agent only [8 a]. In the present communication we report on the ability of a large number of metal sulfur chelates to catalyse the photoproduction of hydrogen from water in homo- geneous solution [8 b].
In a typical experiment 0.06 mmol of the zinc complex of type I (Fig. 1) (1: M = Zn, R = CN, n = 2, z = —2, 7i-Bu4N
+as the counter ion) were
dissolved in 60 ml of tetrahydrofuran (THF) followed by addition of 60 ml of water. After 20 h of irradia- tion (immersion lamp apparatus, lamp: Philips HPK 125 W, X > 254 nm) 200 ml of hydrogen were formed; if THF is replaced by 2,5-dihydrofuran, the rate of H2-evolution is enhanced by a factor of ten and the formerly homogeneous solution turns into an emulsion. Under the latter reaction condi- tions the catalyst is active during four to five days and up to five liters of hydrogen are obtained;
maximum turnover numbers are in the range of 2000 mmol H2/mmol catalyst. The cleavage of water was demonstrated by using D2O for the same kind of experiment and the composition of the gas evolved was determined to be 87% D
2, 10% HD and 3 % H
2.
Fig. 1 summarizes the types of complexes in- vestigated [9], metal dithiolenes (I) exhibit the best catalytic properties. Among the numerous com- pounds of type I studied - M = Ti, V, Cr, Mo, W, Mn, Re, Fe, Os, Co, Ni, Pd, Pt, Cu, Au, Zn, Cd;
R = H, CH
3, Ph, CN; R - R = CS
3, C
20
2; n = 2 , 3 ; 2 = 0, — 1 , —2 - w e found that the zinc complexes give the highest activities and the best D2/HD/H2 ratios. All other complexes show either no (III:
M = W, M' = Ni; V, VI) or strongly reduced activity (II: M=Fe, Co; III: M=W, M ' = Fe; IV: R = P h N H , M = Cr, F e , Co, Z n ; R = O-CH3-C
6H
4, M = Ni, Cu) and generate predominantly HD if irradiated in D2O/THF. The zinc complex 1 was therefore used to study the mechanism of ^-production.
The UV-VIS spectrum of 1 in ethanol at room
temperature exhibits two broad absorption bands
at 267 (e = 23,620) and 383 nm (£ = 21800). Ex-
citation at each of these wavelengths gives rise to a
very weak emission at room temperature and a
rather strong one at 77 K , both centered at 446 nm
in ethanol. This is the first time that emission has
been observed from a metal dithiolene.
UV-VIS absorption spectroscopy reveals that 1 is transformed at early reaction stages to species absorbing at higher energy than the starting com- plex. This conversion easily occurs upon irradiation at 267 nm whereas other products are formed if 1 is excited at 383 nm. A similar wavelength dependence is found for the quantum yield of H2-production (9>(H 2 ) * 0.03 at 267 nm, < 10~ 5 at 366 nm) [10].
THF is essential for the catalytic system and seems to be the reducing agent since dihydrofurans
and furan are detected by gas chromatography of the liquid reaction phase. The use of open chain aliphatic ethers instead of THF prevents catalytic H2-production. Air has no influence on the rate of H2-evolution whereas CO gives rise to a long induc- tion period. This points to competition of CO, THF and/or H2O for an empty coordination site.
W e a r e i n d e b t e d t o W . S c h l a m a n n , B . U l b r i c h , J . B ü c h e l e r , a n d N . Z e u g f o r h e l p f u l a s s i s t a n c e .
[ 1 ] B . V . K o r j a k i n , T . S . D s a b i e v , a n d A . E . S h i l o v , D o k l . A k a d . N a u k , S S S R 2 2 9 , 1 2 8 ( 1 9 7 6 ) . [ 2 ] J . - M . L e h n a n d J . - P . S a u v a g e , N o u v . J . C h i m i e 1 ,
4 4 9 ( 1 9 7 7 ) .
[ 3 ] A . M o r a d p o u r , E . A m o u y a l , P . K e l l e r , a n d H . K a g a n , i b i d . 2 , 5 4 7 ( 1 9 7 8 ) .
[ 4 ] K . K a l y a n a s u n d a r a m , J . K i w i , a n d M . G r ä t z e l , H e l v . 6 1 , 2 7 2 0 ( 1 9 7 8 ) .
[ 5 ] M . K i r c h , J . - M . L e h n , a n d J . - P . S a u v a g e , i b i d . 6 2 , 1 3 4 5 ( 1 9 7 9 ) a n d r e f e r e n c e s c i t e d t h e r e i n .
[ 6 ] K . K a l y a n a s u n d a r a m a n d M . G r ä t z e l , i b i d . 6 8 , 4 7 8 ( 1 9 8 0 ) a n d r e f e r e n c e s c i t e d t h e r e i n ; J . K i w i , E . B o r g a r e l l o , E . P e l i z z e t t i , M . V i s c a , a n d M . G r ä t z e l , A n g e w . C h e m . 9 2 , 6 6 3 ( 1 9 8 0 ) .
[ 7 ] P . J . D e L a i v e , B . P . S u l l i v a n , T . J . M e y e r , a n d D . G . W h i t t e n , J . A m . C h e m . S o c . 1 0 1 , 4 0 0 7 ( 1 9 7 8 ) ; K . K a l v a n a s u n d a r a m , N o u v . J . C h i m i e 3 , 5 1 1 ( 1 9 7 8 ) .
[ 8 ] a ) R . H e n n i n g , W . S c h l a m a n n , a n d H . K i s c h , A n g e w . C h e m . 9 2 , 6 6 4 ( 1 9 8 0 ) ;
b ) B o o k o f A b s t r a c t s , p . V I - 2 , I I I . I n t e r n a t . C o n f . o n t h e P h o t o c h e m i c a l C o v e r s i o n a n d S t o r a g e o f S o l a r E n e r g y , B o u l d e r 1 9 8 0 .
[ 9 ] F o r p r e p a r a t i o n a n d p r o p e r t i e s o f c o m p l e x e s I , I I s e e : G . N . S c h r a u z e r , A d v . C h e m . S e r . 1 1 0 , 7 3 ( 1 9 7 2 ) ; J . A . M c C l e v e r t y , P r o g r . I n o r g . C h e m . 1 0 , 4 9 ( 1 9 6 8 ) ; G . S t e i m e c k e , H . - J . S i e l e r , R . K i r m s e , a n d E . H o y e r , P h o s p h o r u s S u l f u r 7 , 4 9 ( 1 9 7 9 ) ; D . C o u c o u v a n i s , D . G . H o l a h , a n d F . J . H o l l a n - d e r , I n o r g . C h e m . 1 4 , 2 6 5 7 ( 1 9 7 5 ) ; V : A . M ü l l e r , W . - O . N o l t e , a n d B . K r e b s , A n g e w . C h e m . 9 0 , 2 8 6 ( 1 9 7 8 ) ; I I I : A . M ü l l e r a n d S . S a r k a r , A n g e w . C h e m . 8 9 , 7 4 8 ( 1 9 7 7 ) ; I V : B . B . K a u l a n d K . B . P a n d e y a , J . I n o r g . N u c l . C h e m . 4 0 , 1 0 3 5 , 2 2 9 ( 1 9 7 8 ) ; V I : T h . B . R a u c h f u s s a n d D . M . R o u n d - h i l l , J . A m . C h e m . S o c . 9 7 , 3 3 8 6 ( 1 9 7 5 ) .
