Reactive Intermediates in the Thermal and ... - ACS Publications

Chapter 8

Reactive Intermediates in the Thermal and Photochemical Reactions of Trinuclear Ruthenium Carbonyl Clusters

Downloaded by NORTH CAROLINA STATE UNIV on May 7, 2015 | http://pubs.acs.org Publication Date: February 26, 1987 | doi: 10.1021/bk-1987-0333.ch008

Peter C. Ford, Alan E. Friedman, and Douglas J. Taube Department of Chemistry, University of California—Santa Barbara, Santa Barbara, CA 93106

Summarized are a series of investigations using both thermal and photochemical techniques to probe the reaction dynamics of intermediates formed in various reactions of triruthenium cluster complexes. The c h e m i s t r y o f m e t a l c a r b o n y l c l u s t e r s has been l a r g e l y dominated by s y n t h e t i c c h e m i s t s who have p r e p a r e d a r e m a r k a b l e a r r a y o f s t r u c t u r e s i n v o l v i n g wide v a r i e t i e s o f m e t a l cage s t r u c t u r e s and modes o f c o o r d i n a t i o n o f even s i m p l e l i g a n d s . I n t e r e s t i n such systems has r a n g e d from s i m p l e c u r i o s i t y i n what t y p e s o f systems c a n i n d e e d be c o n s t r u c t e d and whether t h e s e s p e c i e s w i l l d i s p l a y u n i q u e c h e m i c a l p r o p e r t i e s t o attempts t o use c l u s t e r s as models f o r l i g a n d i n t e r a c t i o n s w i t h m e t a l s u r f a c e s and m e t a l p a r t i c l e s . Perhaps w i t h t h e exception of ligand f l u x i o n a l i t y processes, q u a n t i t a t i v e mechanistic i n v e s t i g a t i o n s o f c l u s t e r r e a c t i o n s have l a g g e d b e h i n d t h e s y n t h e t i c advances. However, i n r e c e n t y e a r s t h e r e has been i n c r e a s i n g a t t e n t i o n t o mechanistic d e t a i l s , s t i m u l a t e d i n p a r t by i n t e r e s t i n the p o s s i b l e r o l e o f v a r i o u s c l u s t e r s i n t h e homogeneous c a t a l y t i c a c t i v a t i o n o f c a r b o n monoxide, d i h y d r o g e n and o t h e r s m a l l m o l e c u l e s . Our own i n t e r e s t i n t h e r e a c t i o n mechanisms o f t r i a n g u l a r and t e t r a h e d r a l c l u s t e r s a r o s e i n i t i a l l y from t h e d i s c o v e r y t h a t such s p e c i e s may be a c t i v e components o f homogeneous c a t a l y s t s f o r t h e water gas s h i f t r e a c t i o n ( 1 , 2 ) , b u t t h i s has expanded t o o t h e r c h e m i c a l p r o p e r t i e s i n c l u d i n g t h e t r a n s f o r m a t i o n s s t i m u l a t e d b y p h o t o e x c i t a t i o n . The g o a l o f t h e p r e s e n t m a n u s c r i p t i s t o r e v i e w our i n v e s t i g a t i o n s o f s e v e r a l t h e r m a l and p h o t o c h e m i c a l r e a c t i o n s o f t r i a n g u l a r t r i r u t h e nium c a r b o n y l complexes w i t h an emphasis on a t t e m p t s t o c h a r a c t e r i z e the q u a n t i t a t i v e r e a c t i v i t i e s o f h i g h energy, r e a c t i v e i n t e r m e d i a t e s a l o n g the t r a j e c t o r i e s o f these chemical t r a n s f o r m a t i o n s . Photoreactions

o f RU3(00)^2 a

s

The p h o t o c h e m i s t r y o f Ru3(CO)^2 ^ been i n v e s t i g a t e d i n o u r l a b o r a t o r y (3-5) and o t h e r s (6-11) and has been shown t o i n v o l v e b o t h p h o t o f r a g m e n t a t i o n o f t h e c l u s t e r ( E q u a t i o n s 1 and 2) and p h o t o l a b i l i z a t i o n of carbonyls to give s u b s t i t u t e d t r i n u c l e a r c l u s t e r s R u 3 ( C 0 ) L (Equation 3). 1 1

0097-6156/87/0333-0123$06.00/0 © 1987 American Chemical Society

In High-Energy Processes in Organometallic Chemistry; Suslick, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

124

H I G H - E N E R G Y P R O C E S S E S IN O R G A N O M E T A L L I C C H E M I S T R Y

hi/ Ru (CO)

1 2

Ru (CO)

1 2

Ru (CO)

1 2

3

+ 3 CO

• 3 Ru(CO)

(1)

5

hi/ 3

+

3

L

3

·"

Ru(C0) L

(2)

4

hi/ 3

+

L

> Ru (CO) 3

1 1

L + CO

(3)

The e l e c t r o n i c spectrum o f R u ( C O ) ^ ( F i g u r e 1) i s dominated by an i n t e n s e a b s o r p t i o n band c e n t e r e d a t 392 nm ( e - 7.7 x 1 0 H'^cm"^i n cyclohexane s o l u t i o n ) . P h o t o f r a g m e n t a t i o n i s i n d i c a t e d ' by a d e c r e a s e i n t h i s band's i n t e n s i t y w i t h o u t a s h i f t i n t h e A , while p h o t o s u b s t i t u t i o n by L i s i n d i c a t e d by s h i f t s i n t h i s band t o l o n g e r wavelengths. P h o t o l y s i s a t 405 nm i n the p r e s e n c e o f P ( O C H ) l e d t o p h o t o f r a g m e n t a t i o n o n l y , w h i l e p h o t o l y s i s a t s h o r t e r wavelengths gave s p e c t r a l s h i f t s i n d i c a t i v e o f the f o r m a t i o n o f s u b s t i t u t e d c l u s t e r s . Quantum y i e l d s f o r photofragmentâtion Φf and p h o t o s u b s t i t u t i o n Φ i n o c t a n e s o l u t i o n s c o n t a i n i n g 0.012 M P ( O C H ) a r e i l l u s t r a t e d i n F i g u r e 1. 3

2

3


m a x

m a x

3

3

3

3

3

Photofragmentâtion Mechanisms. Photolysis ( A ^ 405 nm) o f R u ( C O ) ^ i n h y d r o c a r b o n s o l v e n t s under CO gave Ru(C0)5 as t h e s o l e p r o d u c t ( E q u a t i o n 1 ) . The quantum y i e l d Φ£ p r o v e d markedly dependent on P and on the s o l v e n t ( T a b l e I ) , w i t h donor s o l v e n t s s u c h as THF g i v i n g much s m a l l e r v a l u e s . P h o t o f r a g m e n t a t i o n i n o c t a n e ( A ^ 405 nm, Pco - 1 atm) w i t h v a r i o u s c o n c e n t r a t i o n s o f c o s o l v e n t s added l e d t o s i g n i f i c a n t q u e n c h i n g o f φ£ by donor s o l v e n t s and gave l i n e a r S t e r n - V o l m e r t y p e p l o t s ( e . g . , φ £ ° / φ £ v e r s u s [THF]) w i t h s l o p e s ( K ) o f 34 ± 1, 26 ± 1 and 16 ± 1 M" f o r THF, diglyme and c y c l o h e x e n e , respectively. In contrast, p h o t o l y s i s i n 2,5-dimethyltetrahydrofuran l e d t o quantum y i e l d s comparable t o t h o s e o b s e r v e d i n h y d r o c a r b o n s o l u t i o n s , an o b s e r v a t i o n w h i c h r e i n f o r c e s t h e v i e w t h a t the a b i l i t y to c o o r d i n a t e may be i m p o r t a n t t o the q u e n c h i n g p r o c e s s . G i v e n the w e l l documented r o l e o f h o m o l y t i c c l e a v a g e o f m e t a l m e t a l bonds i n the p h o t o r e a c t i o n s o f d i m e r i c complexes (2), a logical h y p o t h e s i s would be f o r the p h o t o f r a g m e n t a t i o n s o f t r i n u c l e a r com p l e x e s t o f o l l o w a s i m i l a r p a t h , e.g., r r

3

2

c o

r r

s v

1

M

M

M

Μ

Μ·

·Μ

One d i a g n o s t i c t e s t f o r such h o m o l y t i c p h o t o f r a g m e n t a t i o n has been the t r a p p i n g o f the m e t a l r a d i c a l s M' by c h l o r o c a r b o n s t o g i v e the r e s p e c t i v e c h l o r i d e s M-Cl. P h o t o l y s i s (405 nm) o f R u ( C O ) ^ i n a 1 . 0 M C C I 4 s o l u t i o n i n o c t a n e under CO ( 1 . 0 atm) d i d i n d e e d g i v e a d i f f e r e n t p r o d u c t t h a n i n the absence o f C C I 4 , and t h i s was i d e n t i f i e d as a m i x t u r e o f two i s o m e r i c c h l o r o complexes R u ( 0 0 ) 5 0 1 4 ( 3 ) . However, the a d d i t i o n o f C C I 4 t o o c t a n e s o l u t i o n s o f R u ( C O ) ^ had l i t t l e i n f l u e n c e on φ£ v a l u e s measured under CO ( 1 . 0 atm) ( T a b l e I ) , and Φ£ measured under a r g o n i n C C ^ / o c t a n e was f o u n d t o be s e v e r a l o r d e r s o f magnitude s m a l l e r t h a n t h a t measured under CO i n the same mixed s o l v e n t , even though CO i s n o t r e q u i r e d i n the s t o i c h i o m e t r y f o r the f o r m a t i o n o f c h l o r o c a r b o n y l ruthenium p r o d u c t s . 3

2

2

3


2


8.

Trinuclear Ruthenium Carbonyl Clusters

FORD ET AL.

125

Figure 1. Spectrum of Ru3(CO)^ *- octane. Quantum y i e l d s for photosubstitution and p h o t o f r a g m e n t â t i o n i n 25°C argon flushed octane i n the presence i n 0.012 M Ρ(0013)3 represented as a function of i r r a d i a t i o n wavelength (from reference 5). n

2

Table I .

Photofragmentation Quantum Yields for the 405 nm Photolysis of R u ( C O ) Different Solutions ( 2 5 ° C ) i

3

P

Octane

1 0 0 0 1 0 1 0 0 0 1 0 0 25 0 10 0 0 0 0 0 0 0 0 1 0 1 .0 1 .0 0 .0 1 .0

Cyclohexane

THF Diglyme CCI4

2,5-Me THF 2

a

c

d

Product Total Ρ Product Product

is is is is

a

b

solvent

b

n

12

ό

other additives

C Q

10

— 0.5 M THF 1.0

M CCI4

1.0

M CCI4

— — — 0.001 0.005 0.010 0.10

M M M M

P(OCH P(OCH P(OCH P(OCH

— — — — —

3

3 3

3

) ) ) )

3

3 3

3

28 < 0. 1 1 7 24 0 2 18 4 4 2 2 3 2 11 0 18 3 42 0 3 5 0 7 13 0 .7 20

Ru(C0)5 except where noted. 1.0 atm, balance being N or A r . Ru (C0) Cl . Ru(CO)4(P(OCH ) ). 2

z

x

y

3

x Φf

3


+ 4 ± 0.0 ± 4 c ± 1 ± 1.3 + 0.1 ± 0.6 ± 1.4 ± 0.6 + 1.7 ± 0.7 ± 0.1 + 3 ± 0.1 + 2 e

d

d

d

d

e

e

126

HIGH-ENERGY PROCESSES IN ORGANOMETALLIC CHEMISTRY

These results were explained by the discovery that the chlororuthenium complexes are not the primary photoproducts under CO i n 1.0 M CCl^/octane s o l u t i o n . Instead Ru(C0>5 proved to be the i n i t i a l product even after nearly complete photofragmentation of the s t a r t i n g material, and the chlorocarbonyl ruthenium products to be the r e s u l t of a secondary, dark reaction between the R u ( C 0 > 5 and C C I 4 ( 3 ) :


Ru(C0)

5

+ CCI4

• Ru (C0) Cl Z

x

y

(4)

+ ?

It i s therefore clear that a d i r a d i c a l s u f f i c i e n t l y l o n g - l i v e d to be trappable i n the manner seen i n metal dimer systems i s not formed with RU3(CO)^2- Thus, the dominant feature of the photofragmentation pathways i s the role of two electron donors. Ligands which are π - a c i d s such as CO, ethylene or phosphorous donors P R 3 , each gave p h o t o f r a g m e n t â t i o n while l i t t l e photoactivity occurs for longer λ ^ ^ . when L i s a harder donor such as cyclohexene, THF, diglyme or 2-me thy1te t r a h y d r ο furan. Such c h a r a c t e r i s t i c s led to the proposal ( 3 , 8 ) that the mechanism for the fragmentation pathway must involve the formation of a reac tive intermediate, an isomer of RU3(CO)^2 capable of f i r s t order return to the i n i t i a l cluster or of capture by a two electron donor. Scheme 1 i l l u s t r a t e s the proposed mechanism for photofragmentation.

hi/ Ru (CO) 3

I

12

1

» [Ru (CO) *] 3

• Ru (CO) 3

k

• I

(5)

6

()

12

2

I + L -j

> Ru (CO) L 3

(7)

12

*3

k

12

1'

+2L,fast

4

I '

> R u ( C 0 ) L + Ru (CO)

I'

• Ru (CO)

4

3

2

12

8

• 3 Ru(C0) L 4

+ L

(8)

(9)

SCHEME 1

A possible formulation for I i s i l l u s t r a t e d below. This could be formed by the h e t e r o l y t i c cleavage of a Ru-Ru bond an corresponding movement of a carbonyl from a terminal s i t e to a bridging one to maintain the charge n e u t r a l i t y of both Ru atoms. The r e s u l t would be to leave one ruthenium atom electron deficient (a 16 electron species) and capable of coordinating a two electron donor to give another intermediate I ' .


8.

127


FORD ET AL.

(CO) Ru 4

§ I Flash photolysis studies were therefore conducted with the goal of probing for the presence of such intermediates i n the photofrag mentation (4,5). Flash photolysis ( A > 395 nm) of Ru3(CO)^2 cyclohexane solution under CO (1.0 atm) showed some net photoreact i o n , but no transients were detectable with lifetimes > 30 /is. Neither observable transients nor net photochemistry resulted from a s i m i l a r flash experiment under argon. In contrast, a CO e q u i l i b r a t e d cyclohexane solution of Ru3(CO)^2 containing THF (1.0 M) displayed transient bleaching i n the spectral region 380-460 nm which decayed exponentially ( k - 20 ± 5 s" ) to give a f i n a l absorbance consistent with a small amount of net photoreaction. The same transient behav ior with an i d e n t i c a l value was noted for an analogous THF/cyclohexane solution under argon with the exceptions that consid erably more bleaching was apparent immediately after the f l a s h and that no net photochemistry was seen. These results are consistent with formation of a species such as I' which decays l a r g e l y back to Ru3(CO) Similar transient bleaching at 390 nm followed by exponential decay to a f i n a l absorbance was seen i n argon e q u i l i b r a t e d cyclohex ane solutions containing cyclohexene, PPI13, or P ( O C H 3 ) 3 . Flash photolysis with added cyclohexene led to j u s t small net photoreac t i o n , but photolysis with added PPI13 or Ρ ( 0 ^ 3 ) 3 gave net c l u s t e r fragmentation. For these ligands, the flash photolysis k i n e t i c s were more conveniently investigated at 480 nm, where transient absorbance increases were seen (Figure 2). For P(OCH3)3» v a r i a t i o n of t h i s ligand's concentration (0.005 to 0.05 M) d i d not affect k but d i d affect the amount of transient formed and the extent of net photoreaction. The k^ values determined for the various donor ligands follow the order THF < cyclohexene < PPh < P(OCH )3 (Table I I ) . If k 3 « k + k 5 , the φ£ i s determined by three pairs of competitive processes. The f i r s t i s the formation of I from R u 3 ( C O ) ^ 2 * i n competition with decay to Ru3(CO)^2 occurs with an e f f i c i e n c y Φ^. The second i s the competition between decay of I back to Ru3(CO)^2 (rate constant k^) and capture of I by L to give I ' ( K 2 ) . The t h i r d i s the competition between Equation 9 to reform Ru3(CO)I2 fragmentation v i a Equation 8 to give products. Analysis of the various quantum y i e l d (5,8) and f l a s h photolysis (5) experiments i n terms of Scheme I have l e d to the following conclu sions: 1) The l i m i t i n g quantum y i e l d for photofragmentation ( A ^ 405 nm) i n hydrocarbon solutions would be Φ^, which was determined to be about 0.05 moles/einstein. 2) Trapping of I to give I' i s r e l a t i v e l y insensitive to the nature of L , r e l a t i v e values of K 2 being 1.6, 1.1 and 1.0 for CO, Ρ(ΟΟΗ )3 and PPh , respectively. These values are consistent with the nature proposed for X, i . e . , a coordinatively unsaturated species which i s e s s e n t i a l l y unselective i n reacting with available ligands. 3) The apparent values of k (fragmentation of I' to products of lower nuclearity) f a l l into the sequence: CO, C H 2 - C H 2 » Ρ ( 0 ^ 3 ) 3 > ΡΡΙΊ3 » cyclohexene > THF, an I

N


i r r

1

d

1 2

d

3

3

4

a

a

n

n

d

d

r r

3

3

4



128


Figure 2. Absorbance ( A - 480 nm) vs time trace for the longer wavelength flash photolysis ( A ^ > 395 nm) of a cyclohexane solution of R u ( C O ) plus P(OCH ) (0.010 M) (from reference 5). m o n

r r

3

Table I I .

12

3

3

F i r s t Order Rate Constants for Decay of Transients Seen by Longer Wavelength ( A ^ > 390 nm) Flash Photolysis of Ru (C0)]o i - Cyclohexane Solutions with Various Added Ligands r r

n

3

a

Ligand

Concentration(M)

none CO H C=CH P(OCH ) PPh cyclohexene THF/CO THF

0.0084 M d 0.005-0 05 M 0.005-0 01 M 0.01 M [THF] - 1.0 M 1.0 M

2

2

3

3

3

c

Comments > 5 > 5 > 5 900 200 59 20 20

X X X

+ + ± ± ±

10* b 10 b 10 b 30 30 10 5 5 4

4

e

no net photoreaction net p h o t o f r a g m e n t â t i o n net photofragmentation net photofragmentation net photofragmentation no net photoreaction small net photofrag. no net photoreaction

Τ « 25°C, Table from reference 5. No transient seen, estimated rate i s lower l i m i t . P - 1.0 atm. 1.0 atm. c o

C

H

" 2 4 e

Identical behavior noted i n neat THF solution.


8.

129


FORD ET AL.

order which q u a l i t a t i v e l y p a r a l l e l s the π - a c i d i t y of L . A possible explanation i s that the a c t i v a t i o n b a r r i e r for i n i t i a l fragmentation (to give Ru(C0) L plus Ru2(C0)g?), which would involve the bridging CO —• terminal CO transformation, may be lower for a π - a c i d L owing to the more electron withdrawing nature of the bridging CO. 4

Photosubstitution mechanisms. Continuous photolysis of t h i s cluster i n the presence of PPh or P(0CH ) and at wavelengths shorter than 405 nm led to spectral changes i n d i c a t i n g formation of substituted clusters (4). The marked wavelength dependence of the photosubsti tution quantum y i e l d s i s consistent with the d i r e c t reaction from an upper l e v e l excited state p r i o r to i n t e r n a l conversion to the state(s) responsible for fragmentation. Unlike the fragmentation pathway, the photosubstitution quantum y i e l d s were l i t t l e affected by solvent; therefore, Φ /Φ£ r a t i o s were much higher i n THF solutions than i n hydrocarbon solutions. Flash photolyses of Ru (CO)^2 shorter wavelengths ( A ^ > 315 nm) were c a r r i e d out both i n THF and cyclohexane solutions. No transients were noted i n the l a t t e r solvent, but i n THF under excess CO, transient absorbance i n the wavelength range 480 to 550 nm, which decayed exponentially back to the s t a r t i n g spectrum with a [CO] dependent k ^ , was observed. Similar f l a s h photolysis of Ru (CO)^2 i n argon flushed THF solution with excess PPh or P(OCH ) also gave i n i t i a l transient absorptions at these monitoring wavelengths s i m i l a r to those noted under CO. However, i n these cases, the system was shown to undergo further absorbance increases exponentially to a f i n a l product spectrum consistent with net reaction to give, p r i n c i p a l l y , the substituted clusters R U ( C 0 ) ; Q Plots of k ^ vs [L] or [CO] were curved, but the double r e c i p r o c a l plots ( k ^ vs were l i n e a r i n each case (Figure 3). These data are i n t e r p r é t a b l e i n terms of a reaction scheme where the primary photoreaction is the d i s s o c i a t i o n of CO to give, f i r s t a R u ( C 0 ) n intermediate (II), then the solvated species Ru (C0)^^S (IT).


3

3

3

8

a

t

t

n

e

3

0

r r

s

3

3

3

3

l

3

0

Q

3

s

s

3

hi/

Ru (CO) 3

• Ru (C0)

12

3

1:L

+ CO

(10)

II k

s

II + S !

» Ru (C0) S 3

u

(11)

II' k

co

II + CO

> Ru (CO) 3

k

II + L

12

(12)

L >Ru (C0) L 3

n

SCHEME 2


(13)

130


The r e l a t i v e solvent independence of Φ supports the view that the f i r s t step i s CO d i s s o c i a t i o n rather than an associative displacement by solvent or another ligand. The transient seen by flash photolysis i n THF i s proposed to be I I ' (S - THF), since no transient precursor to substitution with a l i f e t i m e > 30 /is was seen i n cyclohexane despite the comparable Φ i n both solvents. According to Scheme 2, when L - CO or [L] » [CO] the following relationship would hold true: 8

5

k k. [L] obs - k + k l L j L


k

s

s

(14)

L

A p l o t of k b " l vs [ L ] ~ l i s thus predicted to be l i n e a r with slope = k / k _ k and a nonzero intercept k . " ^ as seen i n Figure 3. From the average intercept of (0.9 ± 0.4) x 10" s, a k _ value of about 1.1 χ 10 s" can be estimated for the d i s s o c i a t i o n of THF from I I ' . The r e l a t i v e values of k^ can be determined from r a t i o s of the slopes given that k / k _ should be ligand independent. These r e l a t i v e values are 8, 1.5 and 1.0 for CO, Ρ ( 0 ^ 3 ) 3 and PPI13, respectively. It i s i n t e r e s t i n g to note that the photosubstitution intermedi ate XX appears to be s i g n i f i c a n t l y more s e l e c t i v e toward reaction with various two electron donor substrates than i s the photofragmen t a t i o n intermediate χ . One speculative r a t i o n a l i z a t i o n of this i s that the R u 3 ( C 0 ) ^ ^ intermediate has the opportunity to "delocalize" i t s unsaturation by having one CO bridge an edge of the metal t r i a n g l e with concomitant formation of a multiple metal-metal bond. A s i m i l a r rearrangement is not accessible to χ . 0

s

s

s

L

s

3

s

3

1

s

s

Summary. Figure 4 i l l u s t r a t e s the proposed p h o t o f r a g m e n t â t i o n and photosubstitution mechanisms Ru3(CO)^2 ( £ ) · Quantum y i e l d s for the l a t t e r process are markedly wavelength dependent, very small or undetectable at A ^ > 400 nm but dominant for UV e x c i t a t i o n . The photofragmentation pathway, which i s dominant for longer A ^ , i s quenched by donor ligands such as THF, but φ£ values are l i t t l e affected by the presence of C C I 4 . Thus, i t i s proposed that frag mentation occurs v i a a nonradical isomer of the s t a r t i n g cluster having an unsaturated ruthenium center which reacts rather nonselect i v e l y with two electron donors L to give Ru3(CO)^2^» precursor to the photofragmentation. The photosubstitution pathway i s proposed to proceed by CO d i s s o c i a t i o n to give the intermediate R u 3 ( C 0 ) ^ ^ , which i s trapped by THF to give another transient, R u 3 ( C 0 ) ^ ^ S . Analyses of both CW quantum y i e l d and k i n e t i c flash photolysis data lead to the conclusion that XX i s s i g n i f i c a n t l y more s e l e c t i v e than is X toward reactions with ligands, the greater s e l e c t i v i t y of II suggested to be a consequence of a greater a b i l i t y to delocalize the unsaturation. Recent low temperature photochemical studies by Bentsen and Wrighton (12), who used FTIR to characterize intermediates and prod ucts, appear to confirm key q u a l i t a t i v e features of the models pro posed for the photofragmentation and photosubstitution mechanisms i n Schemes I and II and i n Figure 4 (5). Short wavelength e x c i t a t i o n (313 nm) of Ru3(CO)^2 *- 90 Κ alkane glass was shown to give f i r s t a R U 3 ( C 0 ) I ; L species with only terminal CO's which then rearranged to an isomeric form of R u 3 ( C 0 ) ^ ^ having a bridging CO. In the presence of r r

r r

t

n

e

n



8.

FORD ET AL.

131


F i g u r e 3. Double r e c i p r o c a l p l o t s o f the k i n e t i c s d a t a o b t a i n e d f o r the decay o f the t r a n s i e n t s s e e n f o r the s h o r t w a v e l e n g t h flash photolysis ( A > 315 nm) o f THF s o l u t i o n s o f ^ 3 ( 0 0 ) ^ 2 i n the p r e s e n c e o f v a r i o u s l i g a n d s L (from r e f e r e n c e 5 ) . i r r

F i g u r e 4. Q u a l i t a t i v e model f o r the p h o t o r e a c t i o n s in solution.

o f Ru3(CO)^2


132


various L , these intermediates reacted to form the Ru3(C0)^^L photosubstitution product. Longer wavelength e x c i t a t i o n ( A ^ > 420 nm) of Ru3(CO)i2 i n 195 Κ alkane solution containing excess CO was shown to give Ru(C0) and Ru2(C0)o as the i n i t i a l products. Similar reaction i n the presence of PPI13 gave an intermediate formulated as Ru3(CO)i2( h3), which displayed an IR band (1791 cm" ) characteris t i c of a bridging CO as proposed above for I ' . However, under these conditions (195 Κ ) , this intermediate did not fragment but underwent CO or PPh loss to give RU3(CO)^(ΡΡηβ) or Ru3(CO)^2L a s t l y , i t i s appropriate to comment on the relationships between the intermediates seen i n photochemical studies and possible reactive intermediates along the reaction coordinates of related thermal transformations. E a r l i e r k i n e t i c s studies (13) of the reactions of Ru3(CO)^2 i t h various phosphorous ligands PR3 have found evidence for both f i r s t order and second order pathways leading to substitu t i o n plus some c l u s t e r fragmentation. The f i r s t order path was pro posed to proceed v i a reversible CO d i s s o c i a t i o n to give an intermedi ate analogous to I I . r r

5

pp

1


3

w

Ru (CO) 3

R u 3 ( C 0 ) + CO

12

-6 k

Ru (C0) 3

1 1

(15)

n

k

+ L

7 • Ru (C0) L 3

(16)

11

5

1

The kg value was determined to be about 6.9 χ 10" s" independent of the nature of L i n 50°C decalin (ΔΗ* - 31.8 kcal mol" ; AS* - +20.2 cal mol" Κ " ) . Competition r a t i o s k.g/ky equal to 3 and 5 were determined for L - P(0Pti3)3 and PPI13, respectively under the same conditions. The second order pathway was proposed to occur v i a nucleophilic attack of L on the c l u s t e r , and an intermediate with a formulation the same as 11' was suggested, without supporting e v i dence of i t s existence, as a possible i n i t i a l product of this nucleophilic attack. However, since fragmentation was only a minor side reaction of the substitution reactions with L - PPh3, i t i s quite u n l i k e l y that the photofragmentation and second order thermal substitution reactions occur v i a a common intermediate. 1

1

1

Photoisomerization of

HR^fCO^Qfu-^-COC^) i

,

1

C O C H

Photolysis of the methylidyne cluster H R u ( C O ) ( / " ' ' 3) i n cyclohexane solution leads to an unprecedented oxygen-to-carbon a l k y l migration to form the bridging acyl complex HRu3(CO) (M-r7 -C(O)CH3) (B) : 3

1 0

2

10

(17)


8.


FORD ET AL.

133

T h i s t r a n s f o r m a t i o n was demonstrated (14) by e v a l u a t i n g changes i n the UV, IR and NMR s p e c t r a and comparing t h e s e t o the s p e c t r a o f a u t h e n t i c samples o f each c l u s t e r (15,16). Quantum y i e l d s f o r the p h o t o i s o m e r i z a t i o n d e p i c t e d i n E q u a t i o n 17 were f o u n d t o be n o t a b l y dependent b o t h on the CO c o n c e n t r a t i o n and on the A ^ . A l t h o u g h the r e s u l t i n g o p t i c a l changes were the same f o r d i f f e r e n t A ^ , the quantum y i e l d s i n CO s a t u r a t e d c y c l o h e x a n e ranged from < 10"^ a t 405 nm t o 4.9 x 1 0 " a t 313 nm. Furthermore, Φ v a r i e d l i n e a r l y from 1.2 x 1 0 ' a t P - 0.0 t o 4.9 x 1 0 ' a t P - 1 . 0 atm f o r 313 nm p h o t o l y s i s i n cyclohexane. r r

r r

2

2

4

2


c o

c o

No c l u s t e r f r a g m e n t a t i o n was o b s e r v e d i n i t i a l l y , a l t h o u g h l o n g term p h o t o l y s i s (313 nm) o f Β i n CO s a t u r a t e d c y c l o h e x a n e e v e n t u a l l y d i d l e a d t o f r a g m e n t a t i o n t o Ru(C0)5 p l u s a c e t a l d e h y d e : CH

3

1

(C0) Ru^y— RuCCO)4 3

?

+

hv

0

ϋ

- —

H

5

(

l

g

)

H^RuIcO), T h i s p h o t o r e a c t i o n was s t u d i e d q u a n t i t a t i v e l y u s i n g a u t h e n t i c samples o f HRu3(CO) (/i-r7 -C(O)CH ) , and a quantum y i e l d o f 1.1 x 1 0 ' m o l e s / e i n s t e i n was d e t e r m i n e d . Any p r o p o s e d mechanism f o r the u n p r e c e d e n t e d t r a n s f o r m a t i o n d e s c r i b e d by E q u a t i o n 18 must a c c o u n t f o r the p r o m o t i o n o f t h i s pho t o i s o m e r i z a t i o n by CO, a l t h o u g h CO i s n o t r e q u i r e d by the s t o i c h i o m etry. A p o s s i b l e i n i t i a l s t e p would be s i m i l a r t o t h a t f o r the Ru3(CO)^2 f r a g m e n t a t i o n (Scheme 1 ) . I n t h i s a Ru-Ru bond i s b r o k e n c o n c o m i t a n t w i t h the movement o f a CO from a t e r m i n a l t o a b r i d g i n g s i t e t o form an u n s a t u r a t e d i n t e r m e d i a t e analogous t o χ. A specula t i v e p r o p o s a l a l o n g t h e s e l i n e s i s p r e s e n t e d i n F i g u r e 5. The key f e a t u r e o f t h i s p r o p o s a l would be the f o r m a t i o n o f I I I w i t h one u n s a t u r a t e d ruthenium, which c o u l d be c a p t u r e d by CO t o promote the subsequent s t e p s l e a d i n g from the /i-rç^-methylidyne t o the ^ - r / - a c y l complex. 2

10

3

3

2

I f such a scheme i n d e e d were r e s p o n s i b l e f o r the above i s o m e r i z a t i o n t h e n E q u a t i o n 17 s h o u l d a l s o be f a c i l i t a t e d by o t h e r twoe l e c t r o n donors c a p a b l e o f c a p t u r i n g I I I . I n t h i s c o n t e x t , i t i s n o t a b l e t h a t use o f THF r a t h e r t h a n c y c l o h e x a n e as the s o l v e n t (under argon) g i v e s a much l a r g e r quantum y i e l d 1.4 x 1 0 " . Thus, u n l i k e the p h o t o f r a g m e n t a t i o n o f Ru3(CO)^2, which i s quenched by the donor s o l v e n t , the l i g a n d i s o m e r i z a t i o n i s promoted by THF, p r o b a b l y because CO i s n o t r e q u i r e d i n the o v e r a l l s t o i c h i o m e t r y o f the l a t t e r transformation. P r e l i m i n a r y f l a s h p h o t o l y s i s experiments are a l s o c o n s i s t e n t w i t h t h i s view. Flash photolysis ( A ^ > 313 nm) o f A i n a r g o n d e a e r a t e d THF gave a l o n g l i v e d ( r > 1 s) t r a n s i e n t w h i l e r e p r o d u c i b l e t r a n s i e n t s w i t h l i f e t i m e s g r e a t e r t h a n 30 /*s c o u l d n o t be o b s e r v e d when analogous experiments were c a r r i e d o u t i n c y c l o h e x ane. Thus we c o n c l u d e t h a t a key i n t e r m e d i a t e i n the p h o t o i s o m e r i z a t i o n o f A i s an u n s a t u r a t e d c l u s t e r such as I I I w h i c h c a n be t r a p p e d by the two e l e c t r o n donor THF. Although p r e l i m i n a r y r e s u l t s suggest t h a t subsequent rearrangement t o Β by the THF a d d u c t may be l e s s e f f i c i e n t t h a n from the p r o p o s e d CO adduct, the former a p p a r e n t l y can f u n c t i o n i n t h i s manner. 3

r r



H I G H - E N E R G Y PROCESSES IN O R G A N O M E T A L L I C CHEMISTRY

F i g u r e 5. P r o p o s e d scheme f o r t h e p h o t o i s o m e r i z a t i o n o f HRu3(CO) (^-r - -COCH ) t o H R u 3 ( C O ) ( / i - r - C ( O ) C H ) . L

10

7

2

3

10

7

3


8.

135


FORD ET AL.

Substitution Reactions of the Hydride Cluster H R U ( C O ) ; Q 3


The t r i n u c l e a r ruthenium hydride ion HRu3(C0)^^~ has drawn considerable recent attention as a prominent species i n homogeneous catalysts for the water gas s h i f t reaction (1) and for the h y d r o g é n a t i o n , hydroformylation, and h y d r o s i l a t i o n of alkenes (17,18). In the course of investigating the r e a c t i v i t y of HRu3(C0)^^"" and i t s p o t e n t i a l roles i n such c a t a l y t i c cycles, this ion was found (19) to be remarkably more l a b i l e toward ligand substitution (Equation 19) than the parent neutral carbonyl Ru3(CO)^2 which was described above as being rather slow to undergo substitutions. H R u ( C O ) " + PPh ! 3

11

> HRu (CO) (PPh )~' + CO

3

3

10

(19)

3

A systematic investigation (19) of this reaction i n THF has shown the k i n e t i c s to be consistent with the following scheme:

HRu (CO) 3

-

τ

1 1

• HRu (CO) ~ + CO 3

(20)

10

IV k

9

IV + PPh -,

• HRu (CO) (PPh )

3

3

k.

10

_

(21)

3

9

This mechanism predicts that (at low [CO]) a p l o t of k vs [PPI13] w i l l approach the condition where k^PPt^] » k.g[C0] and k reaches a l i m i t i n g value equal to kg, the rate of CO d i s s o c i a t i o n from H R u 3 ( C 0 ) i i ~ . This has been shown to be the case for several d i f f e r e n t [CO] (Figure 6), and k ( l i m i t i n g ) - 2.1 s" ± 0.1 has been determined at 25°C and ambient pressure (19). Thus, the rate data are consistent with this model and argue against reaction of PPI13 with H R u 3 ( C 0 ) i i ~ i n an associative or interchange pathway to displace CO. However, an alternative mechanism by which CO i s displaced by the nucleophilic attack of solvent was not excluded by these k i n e t i c s r e s u l t s , e s p e c i a l l y given that the a c t i v a t i o n parameters ΔΗ* - 16.0 ± 1.7 kcal/mol and AS* - -1.9 ± 3.0 c a l m o l Κ" for k ( l i m i t i n g ) (19) would indeed appear to be more consistent with an associative type mechanism than with the d i s s o c i a t i v e path described above. This problem was addressed by measuring the stopped-flow k i n e t i c s of Equa t i o n 19 under l i m i t i n g conditions ( k - kg) at various pressures (20) . A p l o t of l n ( k ) vs Ρ (Figure 7) gave the a c t i v a t i o n volume AVt - +21.2 ± 1.4 cm* mol" ). Quantitative p r e d i c t i o n of the expected AV^'s for these models i s r e s t r i c t e d by the absence of p a r t i a l molar volume data i n THF for the various reactants and i n t e r mediates. However, one may assume that H R u 3 ( C 0 ) j j ~ and the dissoc iated intermediate IV have s i m i l a r V's and that V(C0) i n t h i s solvent i s close to that of l i q u i d CO (about 23 cm /mol). Thus, the measured AV$ value strongly supports the concept of a l i m i t i n g d i s s o c i a t i v e mechanism. Further consistent with t h i s view i s the observation (19) that k i s e s s e n t i a l l y independent of whether the solvent i s THF or the more s t e r i c a l l y demanding 2,5-dimethyl-THF. o b s

o b s

1

o b s

- 1

1

o b s

O D S

o b s

1

a v e

3

O D S


136


2.51

> HRu (C0) (PPh ) 3

i0

3

+ CO

1 atm N->

2. 0·


ν·

1. 5

1. 0

0. 5

0. 02 CPPh 3 Q

F i g u r e 6. P l o t s o f k vs [PPh ] f o r the r e a c t i o n o f H R u ( C 0 ) ~ p l u s P P h i n THF under v a r i e d P a t 25°C ( c u r v e s drawn f o r i l l u s t r a t i v e p u r p o s e s o n l y ) (from r e f e r e n c e 1 9 ) . o

3

1 1

F i g u r e 7.

b

s

3

3

Plots of ( k

c o

o b s

)

H R u ( C 0 ) ~ + PPh 3

n

vs pressure f o r the r e a c t i o n 3

• H R u ( C O ) ( P P h ) " + CO 3

1 0

3

R e a c t i o n r u n i n 2 5 ° , N f l u s h e d THF w i t h l i m i t i n g of PPh ( O - 0.053 M P P h ; Δ - 0.086 M P P h ) . 2

3

3

concentrations

3


8.

FORD E T A L .

137


The above k i n e t i c s studies of the thermal reactions provide powe r f u l i n d i r e c t evidence for the operation of a l i m i t i n g d i s s o c i a t i v e mechanism i n this solvent and for the formation of a reactive intermediate such as IV. Such studies also allow one to evaluate the r e l a t i v e r e a c t i v i t i e s of that intermediate with d i f f e r e n t substrates. For example, k . g / k ç , the r a t i o of the rate constants for reaction of IV with CO or PPh i n 25° THF, was determined to have the value 15 ± 4 by analysis of the rate data presented i n Figure 7. However, under favorable conditions, i t should be possible to use flash photolysis to observe the reactive intermediate d i r e c t l y and to measure absolute rate constants for i t s various reactions. In this context the photochemistry of the HRu (CO)n~" cluster anion has been b r i e f l y explored i n these laboratories. Continuous photolysis ( λ ^ ) of a solution of [Bu N][HRu (CO) ] ( À 387 nm, c - 6,900 L mol" cm" ) i n CO saturated or argon flushed THF led to no observable photochemistry; i . e . no fragmentation was seen. (Photosubstitution with CO would be undetectable and thermal reactions with other ligands are too rapid for convenient investigation of the photochemical analogs.) However, f l a s h photolysis ( A > 315 nm) of this s a l t (1 x 10" mol L " ) i n argon flushed THF led to observable transient bleaching i n the 370-440 nm wavelength region and transient absorption i n the 450-540 nm region i n d i c a t i n g the formation of new species. The absorbance changes at a l l wavelengths detectable decayed to the s t a r t i n g spectrum v i a second order k i n e t i c s and with the same l i f e t i m e . When a small but known concentration of CO (1.3 x 10" mol L " ) was introduced to the reaction solution, the same transient was observed, but the decay k i n e t i c s became f i r s t order. This observation c l e a r l y suggests that the transient formed i n this experiment i s the r e s u l t of CO photodissociation.


3

3

Γ Γ 1

4

3

11

1

m a x

5

1

i r r

4

1

hi/ HRu (C0) " 3

11

•

HRu (CO) "" + CO 3

(22)

10

The rate constant for the exponential relaxation of the l a t t e r system to the s t a r t i n g system was calculated to be 1.4 χ 10 s" . From this value, an approximate second order rate constant of 1.0 x 10 L m o l ' s" was calculated for the reaction between IV and CO. Given the above determination of the l i m i t i n g rate constant for CO d i s s o c i a t i o n of 2 s" , the equilibrium constant for thermal CO d i s s o c i a t i o n from H R u ( C 0 ) ~ i n THF to give IV can be calculated from the r a t i o of the forward and back rate constants (kg/k.g) to be 2 x 10" mol L " . 3

1

7

1

1

1

3

11

7

1

Concluding remarks In this a r t i c l e we have summarized the use of both photochemical and more c l a s s i c a l thermal k i n e t i c s techniques to deduce the nature of intermediates i n the ambient temperature, f l u i d solution chemistry of several triruthenium c l u s t e r s . In some cases the photochemically generated intermediates appear to be the same as those proposed to be formed along thermal reaction coordinates, while i n other cases unique pathways are the results of electronic e x c i t a t i o n . The use of pulse photolysis methodology allows d i r e c t observation, and the meas urement of the reaction dynamics of such transients and provides quantitative evaluation of the absolute r e a c t i v i t i e s of these species. In some cases, detailed complementary information regarding


138


p h o t o r e a c t i o n i n t e r m e d i a t e s c a n be deduced a l s o b y t r a p p i n g t h e s e s p e c i e s a t low temperatures and c h a r a c t e r i z i n g t h e i r s p e c t r o s c o p i c p r o p e r t i e s ( 1 2 ) . The examples d e s c r i b e d h e r e i l l u s t r a t e t h e power o f a comprehensive a p p r o a c h u s i n g a l l o f t h e above t e c h n i q u e s t o i n v e s t i g a t e t h e c h e m i s t r i e s o f t h e s e h i g h energy s p e c i e s .


Acknowledgments T h i s work was s u p p o r t e d by g r a n t s from t h e N a t i o n a l S c i e n c e Founda t i o n (INT83-04030; CHE-8419283) and t h e US Department o f Energy, O f f i c e o f B a s i c Energy S c i e n c e s (DE-FG03-85ER13317). Key a s p e c t s o f the e x p e r i m e n t a l s t u d i e s d e s c r i b e d h e r e were c a r r i e d o u t i n t h e s e l a b o r a t o r i e s b y Marc D e s r o s i e r s and D a v i d Wink, whose i n t e l l e c t u a l contributions are greatly appreciated.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Ford, P. C. Accounts of Chem. Research 1981, 14, 31-37. Gross, D. C.; Ford, P. C. J. Am. Chem. Soc. 1985, 107, 585-593. Desrosiers, M. F.; Ford, P. C. Organometallies 1982, 1, 1715-1716. Desrosiers, M. F.; Wink, D. Α.; Ford, P. C. Inorg. Chem 1985, 24, 1-2. Desrosiers, M. F.; Wink, D. Α.; Trautman, R.; Friedman, A. E.; Ford, P. C. J. Am. Chem. Soc. 1986, 108, 1917-1927. Johnson, B. F. G.; Lewis, J.; Twigg, M. V. J. Organomet. Chem. 1974, 67, C75-76. Geoffroy, G. L.; Wrighton, M. S. "Organometallie Photochemis try"; Academic Press: New York, 1979. Malito, J.; Markiewicz, S.; Poë, A. Inorg. Chem. 1982, 21, 4335-4338. Burke, M. R.; Takats, J.; Grevels, F.-W.; Reuvers, J. G. A. J. Am. Chem. Soc. 1983, 105, 4092-4093. Leopold, D. G.; Vaida, V.; J. Am. Chem. Soc. 1983, 105, 6809-6811. Austin, R. G.; Paonessa, R. S.; Giordano, P. J.; Wrighton, M. S. ACS Adv. Chem. Ser. 1978, 168, 189-214. Bentson, J. G.; Wrighton, M. S., submitted for publication, private communication from M. S. Wrighton. Poë, Α.; Twigg, M. V. J. Chem. Soc. Dalton Trans. 1974, 1860-1866. Friedman, A. E.; Ford, P. C. J. Am. Chem. Soc., in press. Keister, J. B.; Payne, M. W.; Muscatella, M. J. Organometallics 1983, 2, 219. Boag, N. M.; Kampe, C. E.; Lin, Y. C.; Kaesz, H. D. Inorg. Chem. 1982, 21, 1704-1708. Süss-Fink, G. Angew. Chem. Int. Ed. Engl. 1982, 21, 73-74. Süss-Fink, G.; Reiner, J. J. Mol. Catal. 1982. 16, 231-242. Taube, D. J.; Ford, P. C. Organometallics 1986, 5, 99-104. Taube, D. J.; van Eldik, R.; Ford, P. C. Organometallics, in press.

RECEIVED November 12, 1986


Reactive Intermediates in the Thermal and ... - ACS Publications

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