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Oxygen evolution: An inorganic example
INORG,
NUCL.
CHEM.
LETTERS
Vol. 9,
pp. 331-335,
OXYGEN
EVOLUTION:
1973. Perlamon
AN INORGANIC
Press.
Printed
in
Great
l~itain.
EXAMPLE
Joseph E. Farley and Hossein Razavi Department of Chemistry, Georgetown University Washington, D . C . 20007 (Rece ived 9 October 1972J
We previously established
(1) t h a t a q u e o u s s o l u t i o n s of
Ru(NH3) 5 0 Ru(NH 3) 4 0 Ru(NH 3) s 7+ ( h e r e a f t e r I ) r a p i d l y o x i d i z e OH- with t h e p r o d u c t i o n o f m o l e c u l a r o x y g e n a n d t h e c o r r e s p o n d i n g 6+ c a t i o n ( " r u t h e n i u m red"*, hereafter II).
I + OH-
--
II +
1/2 H 2 0
+ I/4 02
(I
W e n o w report evidence which shows that there is a fairly stable intermediate (hereafter designated IV) formed in this reaction.
T w o separate lines of evi-
dence support this conclusion. Previously, reaction I w a s followed spectrophotometrically at the w a v e length of greatest absorbance-difference between reactant and product. measurements
These
showed that the main spectral change w a s completed in a few
milliseconds w h e n [GH-] w a s near .I M .
We
n o w report that spectral measure-
ments in the region of relatively low U V absorption peaks s h o w that a second, m u c h slower, change occurs following the initial reaction. an intermediate is involved in reaction 1.
This indicates that
The second reaction has no effect
on the visible spectrum, indicating that the intermediate has a visible spectrum quite similar to that of II. If an acidic solution of I is m a d e basic and then, after the short time needed for the main spectral change to occur, is reacidified, the spectrum of the final mixture demonstrates the presence of up to 25% of I, everl though the spectrum of I had been destroyed by the addition of O H - .
A solution of II m a d e from re-
*This c a t i o n , a m o n g o t h e r s , is found in t h e c o m m e r c i a l b i o l o g i c a l s t a i n of t h e same name.
331
332
OXYGEN EVOLUTION
Vol. 9, No.3
c r y s t a l l i z e d a u t h e n t i c II i s s t a b l e i n b a s e a n d a l s o (in t h e a b s e n c e of o x i d a n t s ) in a c i d .
The r e a c t i o n o f II in a c i d w i t h s u c h o x i d a n t s a s 0 2 or H 2 0 2 i s s l o w .
In c o n t r a s t , t h e r e f o r m a t i o n o f I from t h e i n i t i a l p r o d u c t o f r e a c t i o n 1 i s r a p i d . The a m o u n t of I r e f o r m e d on r e a c i d i f i c a t i o n d e c r e a s e s w i t h i n c r e a s i n g i n t e r v a l b e t w e e n a d d i t i o n o f OH- a n d of I-I+ .
This line of evidence is completely separate
from t h e s p e c t r o p h o t o m e t r i c m e a s u r e m e n t s d e s c r i b e d in t h e p r e v i o u s p a r a g r a p h and confirms the existence of an intermediate, and adds the information that the i n t e r m e d i a t e y i e l d s I on r e a c t i o n w i t h a c i d . The r a t e of d e c a y o f t h e i n t e r m e d i a t e i n b a s i c s o l u t i o n c a n b e f o l l o w e d e i t h e r b y s p e c t r o p h o t o m e t r i c m e a s u r e m e n t s in t h e UV a s f i r s t d e s c r i b e d , reacidification followed by spectrophotometric assay in the visible.
or b y
Both t h e s e
m e t h o d s s h o w t h a t b e t w e e n . 0 2 M a n d 2M [ O H - ] , d e c a y o f t h e i n t e r m e d i a t e i s a f i r s t - o r d e r p r o c e s s a n d b o t h m e t h o d s g i v e t h e s a m e r a t e c o n s t a n t for t h i s p r o cess.
B e t w e e n 15 ° a n d 30 ° a t [(3H-] = 0 . 1 2 5 M ,
t o a ,%H* o f 23 k c a l / m o l e a n d a ZIS* o f 9 e . u .
this rate constant corresponds At [ O H - ] l e s s t h a n . 0 2 M , d e c a y
of the intermediate follows more complicated kinetics.
O
4.0
3.0 '0
2.0
0
I
5o
~'~]1 x (IO'"M) FIG. 1
L i n e a r i z a t i o n o f K i n e t i c D a t a A c c o r d i n g to E q u a t i o n 2
I
Vol. 9, No. 3
OXYOEN EVOLUTION
333
The r a t e of d e c a y of t h e i n t e r m e d i a t e d e c r e a s e s w i t h i n c r e a s i n g [OH-] and follows the rate law k'
= k[H+]/K+
(2
[I-I +]
w h e r e k = 5 x l 0 - 3 s e c -~ a n d K a = 1 x 1 0 - 1 4 M a t 2 5 ° C . The form of 2 s u g g e s t s t h a t a c o n j u g a t e a c i d of IV i s r e a c t i v e . The o b s e r v e d f i r s t - o r d e r k i n e t i c s s u g gest that r e a c t i o n s such as dimerization are not r a t e - d e t e r m i n i n g .
The s i m i -
l a r i t y of t h e m a i n f e a t u r e s of t h e s p e c t r u m of II a n d t h a t of IV i n d i c a t e s t h a t both s p e c i e s h a v e c l o s e l y r e l a t e d c h r o m o p h o r e s . The f o l l o w i n g m e c h a n i s m ( c o o r d i n a t e d a m m o n i a s o m i t t e d ) i s c o n s i s t e n t w i t h t h e s e results: OH-
+ R u O R u O R u ?+
OH-
+ I
2 R u O R u ( O H ) ORu 6+ 2 III*
Ru(OC~RuORu
8÷ + H ÷ + H+
IV
- - R u O R u ( O H ) O R u 6+ slow --- III*
(3
~ R u O R u O R u 6+ + Ru(OO)RuORu 6+ + H 2 0 fast -II + IV + H20
(4
_. R u O R u ( O H ) O R u 7÷ slow -V
(S
In acid: Ru O R u ( O H ) O R u
?÷
V 4 HO 2
-- ½ R u O R u O R u ~÷ + ½ R u O R u O R u ?÷+ ½HO2+ Tr. Cat - - 1 / 2 I I + 1 / 2 I + 1 / 2 HO 2 + 1 / 2 --- 2 H 2 0 fast
+ 3 02
½ I-I+ (6
(7
In base: 2 Ru O Ru(OH) O Ru ?+ 2V
-~ -~
2RuORuORu 2 II
e+
+
21-I+
+21-I +
+
02
+
02
(8
This model predicts that adding base to an acidic solution of I (reactions 3,4) followed by rapid reacidification (reactions 5,6,7 ) should yield 25% of I. The maximum
yield w e have obtained is within experimental error of this amount.
If only base is added to I (reactions 3, 4, S, 8), the final products are II and 02 . Elsewhere (I), w e present evidence that reaction 1 involves nucleophilic attack of O H -
on the central Ru of I. III* is either a peroxide ( - O O H )
or a species with a seven-coordinate ruthenium atom, in both cases the un-
334
OXYGEN EVOLUTION
Vol. 9. No. 3
p a i r e d e l e c t r o n of I would be d i s p l a c e d from a n o n , bonding ~ o r b i t a l a n d o c c u p y a n a n t i b o n d i n g ~ o r b i t a l (2).
D i s p r o p o r t i o n a t i o n of III* v i a 3 l e a d s to the
p r o d u c t i o n of a p e r o x i d e IV in w h i c h t h e ~r-bonding m a n i f o l d h a s t h e s a m e e l e c tronic population as the filled-orbital species II.
Simple i n o r g a n i c a n d o r g a n i c
p e r o x i d e s f r e q u e n t l y d e c o m p o s e by b i m o l e c u l a r m e c h a n i s m s (3), but d i s p r o p o r t i o n a t l o n of
cobalt
t h i s p a t t e r n (4).
~ - p e r o x y s p e c i e s on t h e a d d i t i o n of a c i d do not f o l l o w
S i n c e the n a t u r e of the b r i d g e b e t w e e n t h e ruthenium a t o m s i s
of r e l a t i v e l y minor (2) s i g n i f i c a n c e in t h e b o n d i n g in s u c h i o n s , it i s not u n r e a s o n a b l e to p r o p o s e t h a t s u b s t i t u t i o n of a ~ - p e r o x y group for a ~ - o x i d e ion s h o u l d h a v e a s m a l l e f f e c t on t h e c h r o m o p h o r e .
Slow p r o t o n a t i o n r e a c t i o n s l i k e
4 a r e known in w e l l - c h a r a c t e r i z e d ~ - p e r o x y s y s t e m s ( 5 , 6 ) .
Peroxide reactions
a r e n o t o r i o u s l y s u b j e c t to t r a c e c a t a l y s i s a n d r e a c t i o n 5 g i v e s some e v i d e n c e of t h i s ( r e f o r m a t i o n of I on r e a c i d i f i c a t i o n o c c u r s by f i r s t - o r d e r k i n e t i c s , but t h e r a t e i n c r e a s e s a s the i n t e r v a l b e t w e e n a d d i t i o n of b a s e a n d a d d i t i o n of a c i d i n creases).
R e a c t i o n 8 o c c u r s in b a s i c s o l u t i o n s i n c e I p r o d u c e d by 6 i s u s e d up
by 3 a s f a s t a s it i s formed. The f a c i l e o x i d a t i o n of OH- by I [ h i g h l y u n u s u a l for i n o r g a n i c o x i d a n t s (7)] i s f a v o r e d by s e v e r a l c i r c u m s t a n c e s :
non-labile, non-acidic ligands
o c c u p y a l l o r d i n a r y c o o r d i n a t i o n p o s i t i o n s of ruthenium a n d p r e v e n t normal OHc o o r d i n a t i o n , t h i s m e t a l h a s a t e n d e n c y to form s p e c i e s of c o o r d i n a t i o n number 7 (8), t h e r e a r e l o w - l y i n g antibonc~ing o r b i t a l s of t h e m e t a l c l u s t e r w h i c h c a n a c c e p t e l e c t r o n d e n s i t y from OH-.
T h e s e f a c t o r s c o m b i n e to f o s t e r a t t a c k of OH-
on t h e m e t a l ion; the p r e s e n c e of a v i c i n a l o x y g e n f a v o r s formation of a d i o x y g e n l i g a n d , a n d a l s o e n h a n c e s the s t a b i l i t y of t h e i n t e r m e d i a t e s p e c i e s . It s e e m s l i k e l y t h a t a s e q u e n c e of r e a c t i o n s l i k e t h a t d e s c r i b e d h e r e m a y be i n v o l v e d in t h e c a t a l y s i s of o x y g e n e v o l u t i o n a t e l e c t r o d e s by d i s s o l v e d r u thenium c o m p o u n d s (9).
A l s o , t h e Ru O Ru O Ru grouping may be r e g a r d e d a s a
m o d e l of p l a t i n u m - m e t a l o x i d e s s u r f a c e l a y e r s i n v o l v e d in o x y g e n e v o l u t i o n from a n o d e s (10).
The s i g n i f i c a n c e of t h e s e r e a c t i o n s for p h o t o s y n t h e t i c o x y g e n e v o -
l u t i o n i s o u t l i n e d s e p a r a t e l y (11).
W e are grateful for support under National Science Foundation Grant (GP 32188 ).
Vol. 9, No. 3
OXYGEN EVOLUTION
335
References
1)
J.E. EARLEYana T. FEALEY, Chem. Commun., 331 (1971) Inorg. Chem., 12, 0000 (1973) ( in press ).
2) 3)
C.K. J~)RGENSEN and L.E. ORGEL, Mol. Phys., 4, 215 (1961). e.g.E.KOUBEC, G. LEW and J.O. EDWARDS, Inorg. Chem., 3, 1331 (1964).
4)
R. DAVIES and A.B. SYKES, J. Chem. Soc. (A) 1968, 2840.
5) 6) 7)
e.g.M.
MOKI and J.A. WEIL, J. Amer. Chem. Soc., 89, 3732 (1967).
A.G. SYKESand J.A. WEIL, Frog. Inorg. Chem., 13, 1 (1970). G. NORD and O. WERNBORG, J. Chem. Soc., Dalton, 866 (1972).
8)
M.T. FLOOD, R.F. ZIOLO, J.E. FARLEY and H.B. GRAY, submitted to J. Amer. Chem. Soc.
9)
L.D. BURKE, T.O. O'MEARA, J. Electroanal. Chem., 36 (1972).
1o)
J.P. HOARE, Adv. Electrochem. Electrochem. Eng., 6, 201 (1967). L.D. BURKE, T.O. O'MEARA, J. Chem. Soc., Faraday Trans., 839 (1972)
11)
I.E. EARLEY, Inorg. Nucl. Chem. Letters
NUCL.
CHEM.
LETTERS
Vol. 9,
pp. 331-335,
OXYGEN
EVOLUTION:
1973. Perlamon
AN INORGANIC
Press.
Printed
in
Great
l~itain.
EXAMPLE
Joseph E. Farley and Hossein Razavi Department of Chemistry, Georgetown University Washington, D . C . 20007 (Rece ived 9 October 1972J
We previously established
(1) t h a t a q u e o u s s o l u t i o n s of
Ru(NH3) 5 0 Ru(NH 3) 4 0 Ru(NH 3) s 7+ ( h e r e a f t e r I ) r a p i d l y o x i d i z e OH- with t h e p r o d u c t i o n o f m o l e c u l a r o x y g e n a n d t h e c o r r e s p o n d i n g 6+ c a t i o n ( " r u t h e n i u m red"*, hereafter II).
I + OH-
--
II +
1/2 H 2 0
+ I/4 02
(I
W e n o w report evidence which shows that there is a fairly stable intermediate (hereafter designated IV) formed in this reaction.
T w o separate lines of evi-
dence support this conclusion. Previously, reaction I w a s followed spectrophotometrically at the w a v e length of greatest absorbance-difference between reactant and product. measurements
These
showed that the main spectral change w a s completed in a few
milliseconds w h e n [GH-] w a s near .I M .
We
n o w report that spectral measure-
ments in the region of relatively low U V absorption peaks s h o w that a second, m u c h slower, change occurs following the initial reaction. an intermediate is involved in reaction 1.
This indicates that
The second reaction has no effect
on the visible spectrum, indicating that the intermediate has a visible spectrum quite similar to that of II. If an acidic solution of I is m a d e basic and then, after the short time needed for the main spectral change to occur, is reacidified, the spectrum of the final mixture demonstrates the presence of up to 25% of I, everl though the spectrum of I had been destroyed by the addition of O H - .
A solution of II m a d e from re-
*This c a t i o n , a m o n g o t h e r s , is found in t h e c o m m e r c i a l b i o l o g i c a l s t a i n of t h e same name.
331
332
OXYGEN EVOLUTION
Vol. 9, No.3
c r y s t a l l i z e d a u t h e n t i c II i s s t a b l e i n b a s e a n d a l s o (in t h e a b s e n c e of o x i d a n t s ) in a c i d .
The r e a c t i o n o f II in a c i d w i t h s u c h o x i d a n t s a s 0 2 or H 2 0 2 i s s l o w .
In c o n t r a s t , t h e r e f o r m a t i o n o f I from t h e i n i t i a l p r o d u c t o f r e a c t i o n 1 i s r a p i d . The a m o u n t of I r e f o r m e d on r e a c i d i f i c a t i o n d e c r e a s e s w i t h i n c r e a s i n g i n t e r v a l b e t w e e n a d d i t i o n o f OH- a n d of I-I+ .
This line of evidence is completely separate
from t h e s p e c t r o p h o t o m e t r i c m e a s u r e m e n t s d e s c r i b e d in t h e p r e v i o u s p a r a g r a p h and confirms the existence of an intermediate, and adds the information that the i n t e r m e d i a t e y i e l d s I on r e a c t i o n w i t h a c i d . The r a t e of d e c a y o f t h e i n t e r m e d i a t e i n b a s i c s o l u t i o n c a n b e f o l l o w e d e i t h e r b y s p e c t r o p h o t o m e t r i c m e a s u r e m e n t s in t h e UV a s f i r s t d e s c r i b e d , reacidification followed by spectrophotometric assay in the visible.
or b y
Both t h e s e
m e t h o d s s h o w t h a t b e t w e e n . 0 2 M a n d 2M [ O H - ] , d e c a y o f t h e i n t e r m e d i a t e i s a f i r s t - o r d e r p r o c e s s a n d b o t h m e t h o d s g i v e t h e s a m e r a t e c o n s t a n t for t h i s p r o cess.
B e t w e e n 15 ° a n d 30 ° a t [(3H-] = 0 . 1 2 5 M ,
t o a ,%H* o f 23 k c a l / m o l e a n d a ZIS* o f 9 e . u .
this rate constant corresponds At [ O H - ] l e s s t h a n . 0 2 M , d e c a y
of the intermediate follows more complicated kinetics.
O
4.0
3.0 '0
2.0
0
I
5o
~'~]1 x (IO'"M) FIG. 1
L i n e a r i z a t i o n o f K i n e t i c D a t a A c c o r d i n g to E q u a t i o n 2
I
Vol. 9, No. 3
OXYOEN EVOLUTION
333
The r a t e of d e c a y of t h e i n t e r m e d i a t e d e c r e a s e s w i t h i n c r e a s i n g [OH-] and follows the rate law k'
= k[H+]/K+
(2
[I-I +]
w h e r e k = 5 x l 0 - 3 s e c -~ a n d K a = 1 x 1 0 - 1 4 M a t 2 5 ° C . The form of 2 s u g g e s t s t h a t a c o n j u g a t e a c i d of IV i s r e a c t i v e . The o b s e r v e d f i r s t - o r d e r k i n e t i c s s u g gest that r e a c t i o n s such as dimerization are not r a t e - d e t e r m i n i n g .
The s i m i -
l a r i t y of t h e m a i n f e a t u r e s of t h e s p e c t r u m of II a n d t h a t of IV i n d i c a t e s t h a t both s p e c i e s h a v e c l o s e l y r e l a t e d c h r o m o p h o r e s . The f o l l o w i n g m e c h a n i s m ( c o o r d i n a t e d a m m o n i a s o m i t t e d ) i s c o n s i s t e n t w i t h t h e s e results: OH-
+ R u O R u O R u ?+
OH-
+ I
2 R u O R u ( O H ) ORu 6+ 2 III*
Ru(OC~RuORu
8÷ + H ÷ + H+
IV
- - R u O R u ( O H ) O R u 6+ slow --- III*
(3
~ R u O R u O R u 6+ + Ru(OO)RuORu 6+ + H 2 0 fast -II + IV + H20
(4
_. R u O R u ( O H ) O R u 7÷ slow -V
(S
In acid: Ru O R u ( O H ) O R u
?÷
V 4 HO 2
-- ½ R u O R u O R u ~÷ + ½ R u O R u O R u ?÷+ ½HO2+ Tr. Cat - - 1 / 2 I I + 1 / 2 I + 1 / 2 HO 2 + 1 / 2 --- 2 H 2 0 fast
+ 3 02
½ I-I+ (6
(7
In base: 2 Ru O Ru(OH) O Ru ?+ 2V
-~ -~
2RuORuORu 2 II
e+
+
21-I+
+21-I +
+
02
+
02
(8
This model predicts that adding base to an acidic solution of I (reactions 3,4) followed by rapid reacidification (reactions 5,6,7 ) should yield 25% of I. The maximum
yield w e have obtained is within experimental error of this amount.
If only base is added to I (reactions 3, 4, S, 8), the final products are II and 02 . Elsewhere (I), w e present evidence that reaction 1 involves nucleophilic attack of O H -
on the central Ru of I. III* is either a peroxide ( - O O H )
or a species with a seven-coordinate ruthenium atom, in both cases the un-
334
OXYGEN EVOLUTION
Vol. 9. No. 3
p a i r e d e l e c t r o n of I would be d i s p l a c e d from a n o n , bonding ~ o r b i t a l a n d o c c u p y a n a n t i b o n d i n g ~ o r b i t a l (2).
D i s p r o p o r t i o n a t i o n of III* v i a 3 l e a d s to the
p r o d u c t i o n of a p e r o x i d e IV in w h i c h t h e ~r-bonding m a n i f o l d h a s t h e s a m e e l e c tronic population as the filled-orbital species II.
Simple i n o r g a n i c a n d o r g a n i c
p e r o x i d e s f r e q u e n t l y d e c o m p o s e by b i m o l e c u l a r m e c h a n i s m s (3), but d i s p r o p o r t i o n a t l o n of
cobalt
t h i s p a t t e r n (4).
~ - p e r o x y s p e c i e s on t h e a d d i t i o n of a c i d do not f o l l o w
S i n c e the n a t u r e of the b r i d g e b e t w e e n t h e ruthenium a t o m s i s
of r e l a t i v e l y minor (2) s i g n i f i c a n c e in t h e b o n d i n g in s u c h i o n s , it i s not u n r e a s o n a b l e to p r o p o s e t h a t s u b s t i t u t i o n of a ~ - p e r o x y group for a ~ - o x i d e ion s h o u l d h a v e a s m a l l e f f e c t on t h e c h r o m o p h o r e .
Slow p r o t o n a t i o n r e a c t i o n s l i k e
4 a r e known in w e l l - c h a r a c t e r i z e d ~ - p e r o x y s y s t e m s ( 5 , 6 ) .
Peroxide reactions
a r e n o t o r i o u s l y s u b j e c t to t r a c e c a t a l y s i s a n d r e a c t i o n 5 g i v e s some e v i d e n c e of t h i s ( r e f o r m a t i o n of I on r e a c i d i f i c a t i o n o c c u r s by f i r s t - o r d e r k i n e t i c s , but t h e r a t e i n c r e a s e s a s the i n t e r v a l b e t w e e n a d d i t i o n of b a s e a n d a d d i t i o n of a c i d i n creases).
R e a c t i o n 8 o c c u r s in b a s i c s o l u t i o n s i n c e I p r o d u c e d by 6 i s u s e d up
by 3 a s f a s t a s it i s formed. The f a c i l e o x i d a t i o n of OH- by I [ h i g h l y u n u s u a l for i n o r g a n i c o x i d a n t s (7)] i s f a v o r e d by s e v e r a l c i r c u m s t a n c e s :
non-labile, non-acidic ligands
o c c u p y a l l o r d i n a r y c o o r d i n a t i o n p o s i t i o n s of ruthenium a n d p r e v e n t normal OHc o o r d i n a t i o n , t h i s m e t a l h a s a t e n d e n c y to form s p e c i e s of c o o r d i n a t i o n number 7 (8), t h e r e a r e l o w - l y i n g antibonc~ing o r b i t a l s of t h e m e t a l c l u s t e r w h i c h c a n a c c e p t e l e c t r o n d e n s i t y from OH-.
T h e s e f a c t o r s c o m b i n e to f o s t e r a t t a c k of OH-
on t h e m e t a l ion; the p r e s e n c e of a v i c i n a l o x y g e n f a v o r s formation of a d i o x y g e n l i g a n d , a n d a l s o e n h a n c e s the s t a b i l i t y of t h e i n t e r m e d i a t e s p e c i e s . It s e e m s l i k e l y t h a t a s e q u e n c e of r e a c t i o n s l i k e t h a t d e s c r i b e d h e r e m a y be i n v o l v e d in t h e c a t a l y s i s of o x y g e n e v o l u t i o n a t e l e c t r o d e s by d i s s o l v e d r u thenium c o m p o u n d s (9).
A l s o , t h e Ru O Ru O Ru grouping may be r e g a r d e d a s a
m o d e l of p l a t i n u m - m e t a l o x i d e s s u r f a c e l a y e r s i n v o l v e d in o x y g e n e v o l u t i o n from a n o d e s (10).
The s i g n i f i c a n c e of t h e s e r e a c t i o n s for p h o t o s y n t h e t i c o x y g e n e v o -
l u t i o n i s o u t l i n e d s e p a r a t e l y (11).
W e are grateful for support under National Science Foundation Grant (GP 32188 ).
Vol. 9, No. 3
OXYGEN EVOLUTION
335
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