Reactivity of Coordinated Dioxygen - American Chemical Society

18 Reactivity of Coordinated Dioxygen JOHN F. ENDICOTT and KRISHAN KUMAR

Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch018

Wayne State University, Department of Chemistry, Detroit, MI 48202

Contrasts between the reactivity of free and coordinated O are considered with a special focus 2

on the reactions of transient dioxygen adduct, A

=

Co([14]aneN )(OH )O 2+, observed in aqueous mixtures of O and Co([14]aneN )(OH ) 2+. The greater reactivity of A than O towards outer-sphere elec tron transfer reagents is in part due to A being the stronger oxidant. The preliminary observations also suggest a smaller intrinsic barrier for elec tron transfer to A than to O . Reactions of A which result in formation of μ-peroxo adducts tend to be favored over outer-sphere electron transfer. The rate advantage of the pathways forming adducts appears to arise almost entirely from thermodynam ic factors. The coordinated O of A is not a 4

2

2

2

4

2 2

2

2

2

reactive radical with respect to hydrogen atom abstraction, addition to olefins, etc. The possi bilities for oxygen atom transfer have only begun to be investigated. Some comparisons with the reactivity reported for μ-peroxo complexes are considered. Oxidations dependent on molecular oxygen are of fundamental importance to many chemical and biochemical processes . A large number of these processes require the presence of some transition metal ion to "activate" the oxygen. The initial step in many of these reactions is the coordination of 0^ and the nature of the coordinated dioxygen moiety has been of intense interest (1-12). Despite the extensive studies of the binding of 0 to transition metal complexes, relatively l i t t l e is known 9

0097-6156/82/0198-0425$07.50/0 © 1982 American Chemical Society Rorabacher and Endicott; Mechanistic Aspects of Inorganic Reactions ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

MECHANISTIC ASPECTS OF INORGANIC REACTIONS

426 about

the

r e a c t i v i t y o f coordinated 0^.

In f a c t

there i s

an

element o f p e c u l a r i t y to the " a c t i v a t i o n " o f 0^ by c o o r d i n a t i o n to m e t a l s , s i n c e the formation plexes must reduce the net free t i o n process (see Table I f o r meters o f 0 ^ ) . T h i s paper w i l l

o f even moderately s t a b l e com energy a v a i l a b l e i n the o x i d a p e r t i n e n t thermochemical p a r a focus on the chemical r e a c t i v i t y

of coordinated 0 . o


Table I Summary o f Dioxygen Thermochemistry 0

+

+ 4 H + 4e~

2

°2

+

e

î

2 H

E° = 1.23 V

2°

E° = -0.15 V (pH 7)

" t °2~ +

0 " + 2 H + e" 2

H0

0

Ο/ + H

t

2

E° = 0.87 V (pH 7)

t*2°2

+

pK = 4.88 * a

•> 2 0 ·

2

H 0 2

49 k J m o l "

-> 20Η·

2

1

146 k J m o l "

1

(a) Parameters from r e f . ( 4 ) ; L a t i m e r , W.M. "The O x i d a t i o n States of the Elements and t h e i r P o t e n t i a l s i n Aqueous S o l u t i o n s " ; 2nd E d . , P r e n t i c e - H a l l : Englewood C l i f f s , N J , 1952; Sawyer, D . T . , G i b i a n , M . J . Tetrahedron, 1979, 35, 1471. Many c o b a l t ( I I ) complexes are v e r y r e a c t i v e towards d i s s o l v e d oxygen. The reasonably complex k i n e t i c behavior o f these systems can u s u a l l y be i n t e r p r e t e d i n terms o f a simple two step mechanism (8, 10): n

Co L

5

CoL 0 5

2

+ 0

2

+ Co

l CoL 0 5

I 3 [

L

5

2

K ,k ,k

J L Co-0-0-CoL 5

1

5

1

(1)

- 1

* ,k ,k_ 2

2

2

2

For a v e r y l a r g e number o f c o b a l t complexes i t i s p o s s i b l e

Rorabacher and Endicott; Mechanistic Aspects of Inorganic Reactions ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

( ) to

18.

Reactivity of Coordinated

ENDICOTT A N D KUMAR

Dioxygen

427

i s o l a t e and p u r i f y the r e l a t i v e l y s t a b l e 1:2 μ-peroxo dimers. I t i s o f t e n p o s s i b l e t o manipulate r e a c t i o n c o n d i t i o n s , such as solvent or temperature, i n order to d e t e c t , o r sometimes even i s o l a t e , the more t r a n s i e n t 1:1 adducts. Direct investigation of the chemistry of r e a c t i v e 1:1 CoLÔ^ adducts i s g e n e r a l l y hampered by t h e i r very short l i f e t i m e s and, consequently, small concentrations. We have found some m a c r o c y c l i c tetraamine com plexes o f c o b a l t ( I I ) to r e a c t r a p i d l y with 0^ to form r e l a t i v e l y i n t e n s e l y absorbing 1:1 adducts whose formation and decay may be monitored on a stopped-flow time s c a l e . Thus f o r [l4]aneN^ = Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch018

1,4,8,11-tetraazacyclotetradecane 3

1

= (8 ± 2) χ 1 0 M" , k 5

1

χ 1 0 M" , and k

(cyclam), a t 25C (μ = 0.1),

5

= (4.9 ± 0.4) χ 10 M*^"

£

1

= (5.0 ± 1.5) χ 1 0 M ' V , K 5

1

1

(13).

2

= (8 ± 3)

We have been

using t h i s system to explore the r e a c t i v i t y o f the coordinated oxygen molecule. Enough i n f o r m a t i o n has been accumulated t h a t we may make some p r e l i m i n a r y comparisons between f r e e 0^, monomeric dioxygen plexes .

complexes,

and peroxo

bridged

b i n u c l e a r com

P o s s i b l e Reactions o f Coordinated Dioxygen F i v e c l a s s e s o f r e a c t i o n s o f coordinated 0^ may be u s e f u l l y distinguished: 1.

Outer-sphere e l e c t r o n t r a n s f e r i n which formation o f a chemical bond between the reductant and 0 i s s t e r i c ally difficult: 2

M0 2.

2

+ M0 H + S 2

(3)

2

+ S- -» M0 S 2

(4)

Hydrogen atom a b s t r a c t i o n : M0

4.

+

Formation o f metastable μ-peroxo complexes ( a l s o ad duct formation, n u c l e o p h i l i c substitution, radical coupling, etc.) M0

3.

+ S" + H

2

+ SH -* M0 H + S 2

(5)

Oxygen atom t r a n s f e r : M0

2

+ S

MO + SO


(6)

MECHANISTIC ASPECTS OF

428 5.

INORGANIC REACTIONS

H e t e r o l y t i c or homolytic displacement: (7) (8)

We have been seeking t o i l l u s t r a t e each of these r e a c t i o n s c l e a r l y w i t h a d i r e c t l y observed r e a c t i o n of the Co([14]aneN,)2+ (0H )0 complex. 2

2


Outer-Sphere E l e c t r o n T r a n s f e r Reactions of 0^ M o i e t i e s A l l known dioxygen m o i e t i e s r e a d i l y r e a c t w i t h outer-sphere e l e c t r o n t r a n s f e r reagents, provided the thermochemical r e q u i r e ments are met. A r e p r e s e n t a t i v e c o l l e c t i o n of data i s presented i n Table I I . These outer sphere r e a c t i o n s can be used t o make a comparison of the i n t r i n s i c r e a c t i v i t i e s of the dioxygen moi eties. Thus, one may o b t a i n f r e e energy independent r e a c t i v i t i e s by c o r r e c t i n g f o r the redox e q u i l i b r i u m c o n s t a n t s , or one may c o r r e c t f o r both the e q u i l i b r i u m constant and the counter reagent self-exchange r a t e constant (14) to o b t a i n apparent self-exchange r a t e constants^ of the dioxygen m o i e t i e s themselves (i.e., for 0 + *0 " J 0 ~ + *0 , etc.). While the data i n 2

2

2

2

Table I I are s e l e c t i v e , they a r e , n e v e r t h e l e s s , reasonably r e p r e s e n t a t i v e . Self-exchange parameters obtained from k i n e t i c data reported i n the l i t e r a t u r e are w i d e l y s c a t t e r e d (Table I I ) f o r both the 0 / 0 and p-superoxo/p-peroxo couples. Despite 2

2

v a r i o u s problems of d e t a i l , the i n t r i n s i c b a r r i e r s to e l e c t r o n t r a n s f e r are g e n e r a l l y l a r g e r f o r the p-superoxo/p-peroxo reac t i o n s than f o r the 0 / 0 r e a c t i o n s (~ 100 ± 5 k J mol compared -1 to ~ 45 ± 5 k J mol ). Much, but not a l l , of t h i s d i f f e r e n c e i n i n t r i n s i c b a r r i e r s must a r i s e from the d i f f e r e n c e s i n charge type of the r e a c t i o n s considered. The remaining c o n t r i b u t i o n s may o r i g i n a t e i n s o l v a t i o n e f f e c t s s i n c e the changes i n 0-0 bond l e n g t h accompanying e l e c t r o n t r a n s f e r make r e l a t i v e l y s m a l l c o n t r i b u t i o n s (< 5 k J m o l " ; 13,15,16) to the a c t i v a t i o n bar rier. The r e a c t i o n s of the Co([14]aneN^)(0H )0 complex present 1

?

9

1

2 +

2

2

us w i t h some s l i g h t l y d i f f e r e n t problems. F i r s t of a l l , the r e a c t i o n s are k i n e t i c a l l y complex s i n c e r e a c t i o n s (3) occur i n c o m p e t i t i o n w i t h ( 2 ) . One must t h e r e f o r e perform r e l a t i v e l y thorough k i n e t i c s t u d i e s , v a r y i n g counter-reagent c o n c e n t r a t i o n s over a_ s u f f i c i e n t range t h a t the r a t e becomes p s e u d o - f i r s t order i n [S ] , i n order t o s o r t out the r e a c t i o n s of i n t e r e s t . A c h a r a c t e r i s t i c c o l l e c t i o n of data i s presented i n F i g u r e 1. A matter of some concern has been the e s t i m a t i o n of a r e -



3

5

2

[(NH ) Co] 0

2

0 "

2

5 +

2 +

6

5

2+

3

5

2 +

2+

3

Ru(NH )

Fe

6

Co(phen) 2+

Co(bpy)

3

2

+

2 +

2+

3

3+ Ru(NH ) (isn)

Ru(en)

3

2+ Ru(NH ),(phen)

3

Ru(NH ) (isn)

3

Ru(NH ) 2+

Co(sep)

0

2

Counter Reagent

n°2

M

2

63*

1.0 M LiOAc

3

0.05 Μ KN0

4

0.1 M LIC10,

2.0 M L i C 1 0

1.0 M L i C l O ^

3

0.05 Μ KN0

0.1 M NaHC0

1.0 M LiOAc

0.1 M HTFMS

8

4

f

2

£

£

£

£

£

£

0.95^

0.95^

0.95^

0.95J0.95i

0.15

-0.15

-0.15

-0.15

-0.15

-0.15

V

(M0 )

E

on next page)

6

2 E

3.7xl0 ï

2.8xl0"

9.6xl0 !5

188^

380Ï

2.2xl0 ^

36«

7.7x10"^

0.108-

45^

0.1 M HTFMS

1

M-V

k , ab

0.2 M NaCl

Medium

f

0.05-

0.74-

-0.226-

0.39^

0.319-

0.387-

0.172-

0.515^

Ο. 387-

-0.Οδ-

-0.3*

V

E (Counter reagent)

Table I I C o n t r a s t i n Outer-Sphere E l e c t r o n - T r a n s f e r R e a c t i v i t y o f Dioxygen Complexes-


1

3

6

5 s

1

3

3 £

4χ10 ί

4.0a

3xl0"

40

20-

4.7xl0

4

5 s

3.6xl0 -

3.2xl0 ^

4.7xl0

4χ10 ί

5.1*

M-V

reagent)

exch (Counter f n

?

é l

4.9x10"

8

6

2χ1θ" -4 1x10 7χ1θ"

H

3

4

3

3

3

1.6xlO

3.4xl0

3.8xl0

1.7xl0

l.OxlO

9.9xl0

1.3

1

M M " sc

(M0 )

êxch


?

2

4

2

J

2

}

4

2

(0H )0

2

}

{Co([14]aneN )

2

(0H )0

2 +

4

{Co([14]aneN )

2

(0 ,NH ) '}

3

{[(NH ) Co)

2

(0 ,ΝΗ )"* }

τ

3

3

4

2+

2+

Ru(NH ) (phen)

„2+

Co(sep)

Ru(NHj 3'6

2

2+ Co(phen) 3 R u ( b p y ) 3+

Co(bpy)

4

{[(NH ) Co]

3

Counter Reagent

n°2

M

3+

3

A

4

0.1 M LiCIO

0.1 M HC10,

0.1 M HC10

0.2 M NaCIO,

0.1 M NaCIO, (pH 2)

1.0 M HC1

0.05 M KN0„

0.05 Μ KN0

Medium

s

5

~9xl0 -

8

5

1.8xl0 -

5

4.7xl0 -

5

2.4xl0 -

3.4xl0 -

32^-

90^

M

ab

0.22-

0.22-

0.22-

0.22-

0;75J-

0.75^

0.75J-

2

(M0 )

0.515^

-0.226-

-0.3^

0.05-

1.26-

0.39-

0.319-

V

(Counter reagent)

E

Table I I (cont.) Contrast i n Outer-Sphere E l e c t r o n - T r a n s f e r R e a c t i v i t y o f Dioxygen Complexes-


1

3

3.2xl0 -

6

3xl0~ -

5.1*

3

4χ10 ί

.9h 2.0x10 -

40"-

20-

M'V

k

ex h (CounEer reagent)

-5

5

1.6xl0

8xl0

1.9x10

6

6

-6

4.9xl0

1.2x10

4.8x10"

6.7x10

9

(M0 ) - l -1 M s M

exch


M

2

4

}

2

3

}

2

2 +

}

4

(0H2)02 }

{Co([l4]aneN )

2

(0H )0

4

{Co([l4]aneN )

2

(0 ,NH )

3

{[(NH ) Co]

2

(0 ,NH )

4

2

2+

3

4

2+

2+

6

Ru(NH ) (phen)^

„2+

Co(sep)

3

Ru(NH )

2+ Co(phen) 3 3+ Ru(bpy)

Co(bpy)

4

U(NH ) Co]

3

Counter Reagent

n°2

3+

4

0.1 M L i C 1 0

0.1 M HC10,

0.1 M HC10

0.2 M NaCIO

0.1 M NaCIO (pH 2)

4

5

5s

8

~9xl0 -

5

1.8xl0 -

5

4.7xl0 -

2.4xl0 -

3.4x10

1.0 M HC1

s1

90^ 32

3

- l -1 M s M

ab

0.05 Μ KN0„

0.05 Μ KN0

Medium

0.22-

0.22-

0.22-

0.22-

0;75^-

0.75J-

0.75^

(M0 2 )

6

-3o 3.2xl0 ^

3x10

5.1*

3

4χ10 ί

.9h 2.0x10 -

40»-

20-

exch (Counter reagent) M s

k

s

9

(M0 )

1.6X10

8x10"

1.9x10

6

6

-6

4.9xl0

1.2x10

4.8χ1θ"

6.7x10

M

exch

(Continued on next page)

0.515^

-0.226-

-0.3-

0.05-

1.26»-

0.39-

0.319-

(Counter reagent)

E

Table II (cont.) Contrast i n Outer-Sphere E l e c t r o n - T r a n s f e r R e a c t i v i t y o f Dioxygen Complexes-



ρ

5

§

Η

ο

ο2 *

>

g

Krishnamurthy,

Davies, R.; Sykes, A. G. J . Chem. Soc. (A) 1968, 2831.

K. C ; Wahl, A. C. J . Am. Chem. Soc. 1958, 80, 5921.

g

Latimer, W. M. " O x i d a t i o n P o t e n t i a l s " : 2nd Ed., P r e n t i c e - H a l l : Englewood C l i f f s , N. J . , 1952.

*r Silverman, J . ; Dodson, R. W. J . Phys. Chem. 1952, 56, 846. - Hand, T. D.; Hyde, M. R.; Sykes, A. G. Inorg. Chem. 1975, 14, 1720. s Chandrasekaran, K.; Natàrajan, P. J . Chem. Soc. Dalton Trans. 1981, 478. t Kumar, Κ.; E n d i c o t t , J . F., work i n progress.

£

H

^

>

g °

Hoffman, A. B.; Taube, H. Inorg. Chem. 1968, 7, 1971.

1 McLendon, G.; Mooney, W. F. Inorg. Chem. 1980, 19, 2. - Yee, E. L.; Cave, R. J . ; Guyer, K. L.; Tyma, P.D.; Weaver, M.J. J . Am. Chem. Soc. 1979, 101, 1131. 1 Chou, M. ; C r e u t z , C ; S u t i n , N. J . Am. Chem. Soc. 1977, 99, 5615.

25

- Lavalee, C ; L a v a l l e e , D. K.; Deutsch, E. A. Inorg. Chem. 1978, 17, 2217. —

w

£

4*

- Brown, G. M.; S u t i n , N. J . Am. Chem. Soc. 1979, 101, 883.

* Brown, G. M.; K r e n t z i e n , H. J . ; Abe, M.; Taube, H. Inorg. Chem. 1979, 18, 3374.

- Meyer, T. J . ; Taube, H. Inorg. Chem. 1968, 7, 2369.

- Lim, H. S.; B a r c l a y , D. J . ; Anson, F. Inorg. Chem. 1972, 11, 1460.

- Stanbury, D. M.; Hass, 0.; Taube, H. Inorg. Chem. 1980, 19, 518.

- I l a n , Υ. Α.; C z a p s k i , G.; M e i s e l , D. Biochim. Biophys. Acta 1976, 430, 209.

- Creaser, I . I . ; H a r r o w f i e l d , J . McB.; H e r l t , A. J . ; Sargeson, A. M.; Springborg, J . ; Geue, R. J . ; Snow, M. R. J . Am. Chem. Soc. 1977, 99, 3181.

- 25°

Notes f o r Table I I .




ENDICOTT A N D K U M A R

"3

433

Dioxygen

"3 5 ζ Γ CRu(NH ^*D X10 , M 4

3

Figure 1. Competition kinetics for the Ru(NH ) reduction of Co([14]aneN )(OH )0 \ Reactions at 25°C, pH 2, and μ = 0.1(NaClO ). Individual pseudofirst-order rate constants were determined from the exponential (to four half-lives) decay of Co([14]aneN,,)(OH )0absorbance at 360 nm. Reactions were per formed by mixing a solution containing Ru(NH ) * and Co([14]aneN\)(OH ) *(1 X 10 M) with a solution saturated in 0 (1.2 χ 10~ M) in an Aminco stoppedflow system. 2¥

s

2

6

4

2

2

h

2

3

3

2

2

6

2

3


2

2

MECHANISTIC ASPECTS OF

434

INORGANIC REACTIONS

2+ + r e d u c t i o n p o t e n t i a l f o r the C o ( [ l 4 ] a n e N ^ ) ( 0 H ) 0 ' Geiger and Anson (17) have observed an i r r e v e r s i b l e 2+

versible couple.

2

oxidation

of

Co( [ l4]aneN^) (OH )0 H 2

at

2

~

1 V and

2

have argued

t h a t t h i s i s a reasonable p o t e n t i a l f o r the h a l f - r e a c t i o n Co([14]aneN )(0H )0 + H + e" J Co([14]aneN )(OH )0 H 2 +

4

2

+

(9): (9)

2+

2

4

2

2

We have found the outer sphere reductions to be n e a r l y pH independent, which argues that p r o t o n a t i o n occurs a f t e r e l e c t r o n


transfer

and

that

sequently, we

pK

+ 3.65

[14]aneN

4

+0.42

6.8

- 9.17

- 2.37

Me [14]l,lldieneN, 4

+0.384

> 5

-10.39

> - 5.39

Me [l4]4,lldieneN, 4

+0.564

2.6

- 4.30

- 1.70

2

6

Collman may w e l l have entered the " r e g i o n o f i n t e r e s t " w i t h h i s f a c e - t o - f a c e p o r p h y r i n ; i t seems t o be i n a r e g i o n where the peroxide i s i n a c o n s i d e r a b l y d i f f e r e n t s t a b i l i t y regime from anything t h a t has been looked a t so f a r . So i t i s q u i t e conceivable he does have something s p e c i a l going on. DR. HENRY TAUBE (Stanford U n i v e r s i t y ) : Another i n t e r e s t i n g p o i n t about Collman s complex should be noted. L e t us presume t h a t a peroxide intermediate i s generated i n h i s case. In a l l of Dr. E n d i c o t t * s examples, the remarkable o b s e r v a t i o n i s t h a t i t i s very d i f f i c u l t t o reduce the 0-0 bond k i n e t i c a l l y i n the b i n u c l e a r μ-peroxo complexes. I n Cullman*s case, t h i s r e d u c t i o n 1

American Chemical Society Library 1155 16th St. N. W.

Rorabacher and Endicott; Mechanistic Aspects of Inorganic Reactions Washington. D. C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


448

occurs a t p o t e n t i a l s as p o s i t i v e as +0.7 V. That i s a p u r e l y k i n e t i c phenomenon which we would l i k e t o understand. I t h i n k t h a t t h i s d i f f e r e n c e i n behavior between C o l l m a n s system and the systems s t u d i e d by E n d i c o t t and by M a r t e l l i s q u i t e remark able.


1

DR. ALBERT HAIM ( S t a t e U n i v e r s i t y o f New York a t Stony Brook): Dr. E n d i c o t t has discussed the r e a c t i o n s o f bound ox ygen. I would l i k e t o comment on the outer-sphere r e a c t i o n s o f f r e e oxygen. I n approaching t h i s problem I thought I would do something 2+ u s e f u l w i t h t h e r e a c t i o n between e x c i t e d s t a t e Ru(bpy)^ 2+ and paraquat (PQ ). I f you leave a l i t t l e oxygen i n t h e system, the paraquat r a d i c a l ^ f o r m e d i n the quenching step r e a c t s w i t h oxygen and produces 0 . This i s a very n i c e way o f generating 0 radical. The PQ / 0 r e a c t i o n ( r a t e constant o f 9-1-1 + 3+ about 10 M s ) competes e f f e c t i v e l y w i t h the PQ /Ru(bpy) 2

2

2

3

reaction. *Ru(bpy) directly

2+ 3

Consequently, f o l l o w i n g t h e e f f i c i e n t quenching o f 2+ + by PQ and t h e PQ - 0 r e a c t i o n , one may observe 2

the recombination o f 0

2

and ruthenium ( I I I )

a t 450 nm.

The r e a c t i o n r a t e may be s t u d i e d as a f u n c t i o n of pH t o produce a k i n e t i c t i t r a t i o n curve. The l i m i t i n g r e a c t i o n a t h i g h pH 3+ i n v o l v e s Ru(bpy)^ tion

and 0 2

t o the extent

The r e a c t i o n r a t e slows i n propor

of protonation

of 0 2

Using

Bielski's

[ B i e l s k i , B. H. J . , Photochem. P h o t o b i o l . 1978, 28, 645] value for the pK o f H0 , we i n f e r t h a t the 0 r e a c t i o n i s d i f f u s i o n a ζ ζ c o n t r o l l e d and the H 0 species i s e s s e n t i a l l y u n r e a c t i v e . Thus, even i n a c i d s o l u t i o n , the l i t t l e b i t of 0 i n e q u i l i b r i u m w i t h o

o

2

2

H0

2

carries the reaction.

The p o t e n t i a l s o f t h e oxygen and

ruthenium couples are s u f f i c i e n t t o make s i n g l e t oxygen, so one must a c t u a l l y determine whether s i n g l e t o r t r i p l e t oxygen i s produced. L e t us r e c o n s i d e r the o r i g i n a l *Ru quenching r e a c t i o n . Why i s paraquat r e q u i r e d as a mediator i n order t o b r i n g about the e l e c t r o n t r a n s f e r from *Ru t o oxygen t o produce 0 which i s then 2

followed

by the r e a c t i o n o f 0

2

w i t h ruthenium(III)?

f

Why c a n t

t h i s r e a c t i o n take p l a c e d i r e c t l y ? The e x p l a n a t i o n f o r t h i s i s found i n the 1973 r e p o r t by Demas [Demas, J . N.; Diementi, D. ; H a r r i s , E. W. J . Am. Chem. Soc. 1973, 95, 6865.] i n which he noted t h a t the r e a c t i o n of *Ru



18.

ENDICOTT A N D K U M A R


with

oxygen does not produce CL.

rate

constant

Dioxygen

449

Rather oxygen quenches *Ru 9-1-1 w i t h a r a t e constant ( i n aqueous s o l u t i o n ) o f 3.3 χ 10 M s But there i s no net r e a c t i o n , only energy t r a n s f e r . Demas demonstrated t h a t by p u t t i n g t r a p s i n methanol, n o t i n water. He obtained s i n g l e t oxygen i n h i g h y i e l d s . The r e s u l t i n g i n t e r p r e t a t i o n i s t h a t the r e a c t i o n o f *Ru w i t h oxygen produces a s i n g l e t oxygen d i r e c t l y . In 1976 Dr. S u t i n [ L i n , C.-T.; S u t i n , N. J . Phys. Chem. 1976, 80, 77] placed a footnote i n one o f h i s a r t i c l e s _ s u g g e s t i n g the p o s s i b i l i t y t h a t *Ru r e a c t s w i t h 0^ t o make 0^ , which then turns around and back r e a c t s w i t h ruthenium(III) t o produce s i n g l e t oxygen. Drs. Stanbury and Taube [Stanbury, D. M. ; Mulac, W. Α.; S u l l i v a n , J . D.; Taube, H. Inorg. Chem. 1980, 19, 3735] have a l s o commented on the Demas study. They have used a r a t e con s t a n t f o r the self-exchange i n v o l v i n g O^-O^ t o c a l c u l a t e the f o r the *Ru oxygen r e a c t i o n . They obtained a 9 -1 -1 value o f ~2 χ 10 M s , which i s very c l o s e t o the value o f 9 -1 -1 3.3 χ 10 M s which Demas measured. They have noted t h a t these r e s u l t s suggest t h a t Demas arguments may not be very con c l u s i v e . I would l i k e t o suggest t h a t i t i s not p o s s i b l e t o d i s t i n g u i s h between the energy t r a n s f e r and t h e e l e c t r o n t r a n s 2+ f e r mechanism f o r the r e a c t i o n o f *Ru(bpy)^ w i t h 0^ without doing a complete k i n e t i c a n a l y s i s o f t h e systems w i t h and w i t h out paraquat. The p e r t i n e n t experiment i s t o take the system where we know t h a t we a r e making 0^ from t h e paraquat system and deter mine what k i n d o f oxygen we a r e o b t a i n i n g from the recombina t i o n r e a c t i o n s . We can then repeat Demas experiment i n aqueous s o l u t i o n and compare the r e s u l t s . U n f o r t u n a t e l y , i t i s very d i f f i c u l t t o f i n d s o l u b l e t r a p s f o r s i n g l e t oxygen i n aqueous s o l u t i o n . So one has t o u t i l i z e one o f these "soups". The soup which I have used contains 0^, DPBF, DÔ, paraquat, and a de tergent. This composition was s e l e c t e d on the b a s i s o f a paper i n 1979 by L i n d i g and Rodgers [ L i n d i g , D. Α.; Rodgers, M. A. J . Phys. Chem. 1979, 83, 1683] i n which they s t u d i e d the l i f e time o f the s i n g l e t oxygen s t a t e i n aqueous s o l u t i o n by s o l u b i l i z i n g t h i s t r a p by means o f an aqueous m i c e l l a r d i s p e r s i o n . The r a t e constant f o r the r e a c t i o n o f the t r a p w i t h s i n g l e t oxygen i s very high. I n D 0 the l i f e t i m e o f the s i n g l e t oxygen i s r e l a t i v e l y l o n g , 53 microseconds. Thus, the c r i t i c a l e x p e r i ment c o n s i s t s o f p l a c i n g e x c i t e d s t a t e r u t h e n i u m ( I I ) , paraquat, oxygen, and the t r a p i n DÔ, and observing whether the t r a p d i s appears a t the r a t e which L i n d i g and Rodgers reported o r , a l t e r 1

1

2



450

n a t i v e l y , whether there i s no consumption. Then the experiment i s repeated i n the absence of paraquat. F o r t u n a t e l y , we obtained f o u r measurements, two f o r each one o f these systems. So I b e l i e v e they a r e good r e s u l t s . In the case o f the system c o n t a i n i n g paraquat, we are cer t a i n t h a t there i s 0^ . The r e a c t i o n of 0^ and ruthenium(III) consumes the s i n g l e t t r a p . Therefore, we conclude t h a t the r e a c t i o n produces s i n g l e t oxygen. Repeating Demas experiment, but now i n aqueous s o l u t i o n , t h e t r a p disappears a t t h e r a t e which L i n d i g and Rodgers reported and i n the amount r e q u i r e d f o r h i g h y i e l d s o f s i n g l e t oxygen i n t h i s r e a c t i o n . So we conclude t h a t t h e e x c i t e d s t a t e ruthenium(II)-oxy gen r e a c t i o n produces s i n g l e t oxygen, as does the one i n v o l v i n g


1

ruthenium(III)

and 0^"*

DR. KENNETH KUSTIN (Brandeis U n i v e r s i t y ) : I would l i k e t o b r i n g t o your a t t e n t i o n a developing f i e l d which I b e l i e v e should be g e t t i n g more n o t i c e from mechanistic i n o r g a n i c chem i s t s , namely, t h e f i e l d o f o s c i l l a t i n g r e a c t i o n s . Up u n t i l r e c e n t l y , t h e only o s c i l l a t o r s t h a t had been discovered were discovered by s e r e n d i p i t y . But now we are i n v o l v e d i n a s p e c i f i c program o f t r y i n g t o design new o s c i l l a t i n g systems based on chemical p r i n c i p l e s r a t h e r than r e l y i n g on a c c i d e n t a l d i s coveries. And we appear t o be having c o n s i d e r a b l e success. Our progress i n t h i s f i e l d depends on two techniques t h a t have not been mentioned p r e v i o u s l y a t t h i s conference. F i r s t o f a l l , those o f us who learned something about r e l a x a t i o n w i l l a p p r e c i a t e the f a c t t h a t one can use conventional equipment f o r c a r r y i n g out r e l a x a t i o n experiments. When one has a complicated waveform, as i s t h e case i n o s c i l l a t i n g r e a c t i o n s , there i s s t i l l no reason why t h a t waveform cannot be perturbed by, f o r example, i n j e c t i n g i n t o the system a pulse o f some r e a c t a n t and then observing the change ( t h a t i s , the r e l a x a t i o n ) t h a t ensues. This p e r t u r b a t i o n and t h e ensuing r e l a x a t i o n can then be ana l y z e d i n the t y p i c a l f a s h i o n . Thus, one can s t i l l use a concen t r a t i o n jump experiment i n an o p e r a t i n g system. The other technique which has poved v a l u a b l e i n t h i s area i s computer s i m u l a t i o n . When the k i n e t i c data become very com p l i c a t e d , as w i t h o s c i l l a t i n g r e a c t i o n s i n v o l v i n g two elementary s t e p s , i t i s s t i l l p o s s i b l e t o o b t a i n r a t e constants from the data by doing computer s i m u l a t i o n . That i s a c t u a l l y not as out l a n d i s h as i t might appear. I t i s r e a l l y i n the same category as the F o u r i e r transform approach. I t h i n k t h i s i s an area t h a t w i l l make a c o n s i d e r a b l e impact upon i n o r g a n i c k i n e t i c s t u d i e s i n the f u t u r e .


Reactivity of Coordinated Dioxygen - American Chemical Society

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