The Use of Ligand Substituents to Modify Photochemical Reactivities

16 The Use of Ligand Substituents to Modify

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Photochemical Reactivities Studies of Ruthenium(II) and Rhodium(III) Ammine Complexes

PETER

C.

FORD,

G.

MALOUF,

J.

D.

PETERSEN,

and

V.

A.

DURANTE

University of California, Santa Barbara, Calif. 93106

Quantum yield data demonstrate that substituents can be used to modify the photoaquation reactivities of the complexes Rh(NH )5L and Ru(NH ) L (L = meta- or para -substituted pyridine). For the Rh(III) complexes, these modifications involve only minor shifts in ligand field excited-state energies, and the relatively small changes in the photolability of L follow an order inverse to the ligands Brönsted basicities. For the Ru(II) complexes, these modi fications involve dramatic shifts in the energy of metal-to -ligand charge transfer excited states with the result that photoaquation quantum yields vary by three orders of magnitude. It is concluded that with the unreactive com plexes, the charge transfer state is lowest in energy whereas with the more reactive complexes, the ligandfieldexcited state is lowest in energy. 3+

2+

3

3 5

'

C u b s t i t u e n t s a t positions r e m o t e f r o m r e a c t i o n sites h a v e b e e n ^

used

extensively t o s t u d y o r g a n i c r e a c t i o n m e c h a n i s m s , b u t s u c h effects

h a v e b e e n e x a m i n e d t o a lesser degree i n t r a n s i t i o n m e t a l systems. T h e advantages of s u b s t i t u e n t studies is r e a d i l y a p p a r e n t f o r c e r t a i n systems. F o r π-unsaturated l i g a n d s (e.g., p y r i d i n e a n d other a r o m a t i c heterocycles), substituents i n m e t a - a n d p a r a - r i n g positions c a n b e u s e d t o generate r e l a t i v e l y subtle e l e c t r o n i c p e r t u r b a t i o n s that i n c l u d e changes i n l i g a n d b a s i c i t y a n d i n t h e energies o f π s y m m e t r y m o l e c u l a r o r b i t a l s . I n these positions, little o r n o steric i n t e r a c t i o n w i t h the c o o r d i n a t i o n site results. 187 King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

188

INORGANIC

COMPOUNDS WITH

UNUSUAL

PROPERTIES

T h e electronic p e r t u r b a t i o n s a p p e a r to be i d e a l l y s u i t e d for use i n the m o d i f i c a t i o n of p h o t o c h e m i c a l p r o p e r t i e s . P r e v i o u s l y r e p o r t e d w o r k d e m o n s t r a t e d that substituents c a n b e u s e d to tune the energies of e x c i t e d states r e s p o n s i b l e for t h e emission spectra of c e r t a i n g r o u p V I I I m e t a l complexes the a b s o r p t i o n spectra of complexes transfer ( M L C T ) b a n d s (2).

(1)

a n d to m o d i f y s i g n i f i c a n t l y

displaying metal-to-ligand charge

I n this p r e s e n t a t i o n , w e s u m m a r i z e some

recent attempts to use l i g a n d substituents i n o u r studies of t r a n s i t i o n Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch016

metal complex photochemical reaction mechanisms. T h e particular sub jects of interest are the m e t a l a m m i n e complexes M ( N H ) L \ w h e r e M 3

W

5

is R h ( I I I ) or R u ( I I ) a n d L is a m e t a - or p a r a - s u b s t i t u t e d p y r i d i n e . Spectral

Properties

T a b l e I lists the s p e c t r a l s i m i l a r i t i e s a n d differences

of

analogous

r u t h e n i u m ( I I ) a n d r h o d i u m ( I I I ) complexes. T h e h e x a a m m i n e complexes each h a v e the t w o l i g a n d field transitions ( Α ι - > Τ Ί a n d A ! - » T ) 2

p e c t e d for these l o w s p i n 4d o c t a h e d r a l complexes. 6

ex

F o r the p y r i d i n e c o m

plexes, the h i g h e r energy a b s o r p t i o n i n e a c h case is a l i g a n d - c e n t e r e d 7TL—7TL* b a n d analogous

to t h a t of free p y r i d i n e ; h o w e v e r , t h e

lower

energy b a n d s differ i n character. F o r the r h o d i u m ( I I I ) c o m p l e x , the l o n g est w a v e l e n g t h b a n d has a n energy a n d a n i n t e n s i t y c o m p a r a b l e to those of the l o w e r e n e r g y b a n d of the h e x a a m m i n e a n d is assigned to a l i g a n d field

transition

Ru(NH ) py 3

5

2 +

(3).

I n contrast, the l o w e r e n e r g y b a n d of

aqueous

has a v e r y h i g h e x t i n c t i o n coefficient a n d , b e c a u s e of the

sensitvity of the b a n d to r i n g substituents (vide b a n d w a s d e s i g n a t e d M L C T i n character Table I.

infra)

a n d solvent, the

(2).

S p e c t r a l D a t a f r o m Some R u ( I I ) a n d R h ( I I I ) Complexes i n Aqueous Solution ^max

Complex Rh(III) Rh(NH y+ 3

Rh(NH ) py 3

Ru(II) Ru(NH ) 3

5

6

2 +

Ru(NH ) py 3

3 +

6

2 +

nm

Log e

Assignment

305 255 302 259

2.13" 2.00 2.20 3.44

LF LF LF

385 275 407 244

1.59 2.83 3.89 From Ref. 2. 6

King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

(?)

16.

Modifying

FORD E T A L .

Photochemical

189

Reactivities


40r

Figure 1. Energy level diagram for photoreaction of Rh(NH ) . k : aquation of reactive excited state to give photoproducts; k and k : nonradiative and radiative deactivation to ground state, respectively. 3+

3 6

n

Photochemistry

p

r

of Rhodium (III)

P h o t o l y s i s i n aqueous complexes R h ( N H ) X 3

5

2 +

Complexes

solution of t h e r h o d i u m ( I I I )

or R h ( N H ) L 3

3 +

5

pentaammine

( w h e r e X " is a h a l o g e n a n i o n

a n d L is a n u n c h a r g e d base s u c h as N H , H 0 , o r p y r i d i n e ) a l w a y s leads 3

2

to p h o t o a q u a t i o n p a t h w a y s w h e n t h e l o w e r e n e r g y L F b a n d s a r e i r r a d i a t e d (4,5).

T h e l i g a n d field e x c i t a t i o n w a s i n t e r p r e t e d as r e s u l t i n g i n a n

efficient i n t e r s y s t e m c r o s s i n g / i n t e r n a l c o n v e r s i o n to a l o w e s t e n e r g y L F state f o l l o w e d b y n o n r a d i a t i v e d e a c t i v a t i o n to g r o u n d state, b y a q u a t i o n to p r o d u c t s ( i n a m b i e n t t e m p e r a t u r e fluid s o l u t i o n ) , o r b y r a d i a t i v e d e a c t i v a t i o n ( i n l o w t e m p e r a t u r e glasses) ( 6 ) . T h e s e processes a r e i l l u s t r a t e d i n F i g u r e 1 f o r the h e x a a m m i n e complexes w h e r e the l o w e s t e x c i t e d state is r p r e s e n t e d as t h e t r i p l e t T . 3

s e v e r a l r e c e n t articles (7,8,9)

lg

( I t s h o u l d b e noted, however, that i n

i t was emphasized that spin orbit c o u p l i n g

constants o f the h e a v i e r metals are o f s u c h m a g n i t u d e t h a t s p i n m u l t i p l i c i ties m a y n o t b e a m e a n i n g f u l w a y t o l a b e l t h e r e a c t i v e l o w - e n e r g y state. ) I f the l o w e s t state is p r e s u m e d to b e r e s p o n s i b l e f o r t h e p h o t o r e a c t i o n s , the q u a n t u m y i e l d f o r aquation is then represented b y E q u a t i o n 1 w h e r e


190

INORGANIC

Φίο is the efficiency

COMPOUNDS WITH

UNUSUAL

PROPERTIES

of i n t e r c o n v e r s i o n of h i g h e r states to t h e r e a c t i v e

state ( i t w a s s h o w n to h a v e a v a l u e of o n e i n p r e v i o u s studies w i t h t h e halopentaammines

(5)).

F o r the p y r i d i n e c o m p l e x , a q u a t i o n of the u n i q u e l i g a n d is t h e p r e dominant photoreaction Rh(NH ) py


3

A

5

3 +

+

p a t h w a y for 313-nm irradiation (Reaction H 0 2

h

v

(

3

1

3

n m

}

Rh(NH ) H 0 + 3

5

2

3

+

py

2). (2)

q u a n t u m y i e l d of 0.14 m o l e / e i n s t e i n w a s m e a s u r e d f o r E q u a t i o n 2

b y a s p e c t r a l t e c h n i q u e w h i l e t h e q u a n t u m y i e l d of c o m p e t i t i v e N H

3

p h o t o a q u a t i o n c a n b e e s t i m a t e d f r o m p H changes as h a v i n g a n u p p e r l i m i t of 0.02. S i m i l a r or s m a l l e r l i m i t s for Φ Η Ν

3

c a n be estimated for the

other R h ( I I I ) p y r i d i n e c o m p l e x e s d i s c u s s e d here. P h o t o a q u a t i o n q u a n t u m y i e l d s for s e v e r a l p y r i d i n e c o m p l e x e s are summarized w i t h absorption

a n d emission spectral data i n T a b l e I I .

T h e s e d a t a i n d i c a t e the f o l l o w i n g .

T h e lowest energy absorption b a n d ,

l i g a n d field i n character, is l i t t l e affected b y a 4 - m e t h y l o r 3-chloro s u b stituent, a n d i t appears at t h e s a m e w a v e l e n g t h f o r a l l three p y r i d i n e

Table II.

Photochemical and Photophysical Properties of [ R h ( N H ) L ] [ C l 0 ] 3 3

5

4

hv Ru(NH,).py«- +

H 0 2

Reaction 3).

5

photoand py

T h e M L C T excited

>Ru(NH ) H 0 + 3

3

2

2

+

py

(3)

^ J ^ Y \

»Ru(NH ) (H 0)py + + 3

4

2

2

N H

3

state c a n b e c o n c e p t u a l i z e d as h a v i n g a n o x i d i z e d m e t a l i o n a n d a r a d i c a l i o n l i g a n d i n a c o o r d i n a t i o n c o m p l e x , e.g. M L C T * .

I t appears u n l i k e l y

(NH^Ru^'N^Q^ MLCT* that M L C T * w o u l d b e l a b i l e ; i n p a r t i c u l a r , it appears u n l i k e l y t h a t i t w o u l d b e l a b i l e t o w a r d N H - a q u a t i o n . T h e e l e c t r o n p r o m o t e d f r o m the 3

l o w s p i n R u ( I I ) has π s y m m e t r y w i t h respect to the m e t a l - l i g a n d b o n d , a n d the σ m e t a l a m m o n i a b o n d s a n d the σ c o m p o n e n t of t h e R u - p y b o n d s h o u l d be little affected or p e r h a p s e v e n e n h a n c e d b y t h e m o r e p o s i t i v e n a t u r e of the r u t h e n i u m i n t h e e x c i t e d state. T h e π c o m p o n e n t R u - p y b o n d n o d o u b t is less i m p o r t a n t i n M L C T * .

of t h e

H o w e v e r , since π

b a c k b o n d i n g is r e l a t i v e l y i n s i g n i f i c a n t i n the s t a b i l i t y of R u ( I I I )

com

plexes, i t is i m p r o b a b l e t h a t M L C T * w o u l d b e n o t i c e a b l y m o r e r e a c t i v e t h a n the c o r r e s p o n d i n g R u

1 1 1

c o m p o u n d w h i c h is n o t l a b i l e u n d e r p h o t o l y

sis c o n d i t i o n s . O n the basis of these considerations, one m i g h t c o n c l u d e that i f one e x c i t e d state is r e s p o n s i b l e for t h e a q u a t i o n of b o t h N H

3

and py from


192

INORGANIC

Ru(NH ) py 3

5

analogous

, i t s h o u l d b e a l i g a n d field state.

5

UNUSUAL PROPERTIES

T h i s state w o u l d

be

to the r e a c t i v e L F state r e s p o n s i b l e f o r p h o t o a q u a t i o n

of

2 +

Ru(NH ) CH CN 3

COMPOUNDS WITH

3

2 +

( w h e r e the l o w e s t a b s o r p t i o n b a n d is L F i n c h a r -

acter ( 9 ) ) a n d of R h ( N H ) L 3

5

3 +

E s t i m a t e s of t h e energies

(vide supra).

of the l o w e s t M L C T a n d L F states suggest t h a t f o r R u ( N H ) p y 3

l o w e s t state m a y b e L F i n c h a r a c t e r

5

2 +

the

(10).

T h e a b o v e d i s c u s s i o n c a n be s u m m a r i z e d as suggesting t h a t the l o w e s t L F e x c i t e d state is l a b i l e t o w a r d a q u a t i o n w h i l e M L C T states are not. T h e r e f o r e the r e a c t i v i t y of R u ( N H ) p y Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch016

3

5

2 +

is a t t r i b u t e d to a n e n e r g y

l e v e l scheme s u c h as that d e p i c t e d i n F i g u r e 2 w h e r e t h e i n i t i a l l y p o p u l a t e d M L C T states i n t e r c o n v e r t to the r e a c t i v e L F state w h i c h is also the lowest e n e r g y state. P r e s e n t d a t a d o n o t d i s c r i m i n a t e w h e t h e r n o n r a d i a ive d e a c t i v a t i o n d i r e c t l y f r o m M L C T states also occurs.

F i g u r e 2, h o w -

ever, does suggest the f o l l o w i n g e x p e r i m e n t a l e v a l u a t i o n of this i n t e r p r e t a t i o n . T h e d a t a for the R h ( I I I ) systems i n d i c a t e t h a t L F excited-state energies are n o t w i l d l y p e r t u r b e d b y p y r i d i n e l i g a n d substituents. A t the same t i m e , h o w e v e r , i t is k n o w n that p y r i d i n e substituents h a v e d r a m a t i c effects o n the energies of M L C T a b s o r p t i o n b a n d m a x i m a (2)

and pre-

s u m a b l y s i m i l a r effects o n the energies of a l l M L C T states.

Therefore

a p p r o p r i a t e use o f substituents w o u l d t u n e the M L C T e n e r g y to a p o i n t w h e r e the l o w e s t state is M L C T i n character. I f i n i t i a l e x c i t a t i o n is f o l l o w e d b y efficient i n t e r c o n v e r s i o n to the l o w e s t e n e r g y e x c i t e d state, the -

'CT

hv

Figure 2. Excited state diagram for proposed mechanism for photoaquation of Ru(NH ) py . k„: reaction leading to photoproducts. s 5

2+


16.

Modifying

FORD E T A L .

Photochemical

193

Reactivities

o b s e r v e d p h o t o r e a c t i v i t y s h o u l d reflect the r e a c t i v i t y of t h e l o w e s t

MLCT

state, n o t t h a t of the l o w e s t L F state. W i t h r e g a r d to p h o t o a q u a t i o n , these p r e d i c t i o n s a p p e a r to b e o u t b y the d a t a i n T a b l e I I I . I r r a d i a t i o n at t h e M L C T A

borne

of a v a r i e t y

m a x

of p y - X complexes i n p H 3 aqueous s o l u t i o n gives L v a l u e s of 0 . 0 2 - 0 . 0 5 m o l e / e i n s t e i n for A Amax > ~ 490.

Ru(NH ) H 0 * + L v

3

5

2 t

2

3

L Χ CI

2

2

10

3

2+

2+

(NH,) RuN^Q^—Ô—ΝΗι

(NH,).RuN


5

t

Cl irr, nm 405 449 480 500 520 546

38. ± 42. ±

1. 2.

43. ± 48. ±

1. 2.

4.5 1.5 1.1 0.37 0.35 0.30

6 6

± 0.1 ± 0.1 db 0.1 ± 0.02 ± 0.02 ± 0.02

Spectroscopic quantum yields were measured at 296°K in p H 3 aqueous solution; μ = 0.2M (NaCl). Not determined. α

6

MLCT states

E M

prod.

• prod.

Figure 3. Excited-state representations revised to account for tempera ture and Xfrr dependence of aquation quantum yields for Ru(NH ) L quantum yields in aqueous solution, (a) Diagram for photoreactive com plex, e.g. Ru(NH ) (3,5-dichloropyndine)? ; and (b) diagram for a rela tively unreactive complex, Ru(NH ) (isonicotinamide)? . s 5

s 5

+

s 5

+


2+

16.

Modifying

FORD E T A L . Table V .

Photochemical

Reactivities

E f f e c t of T e m p e r a t u r e o n Q u a n t u m Y i e l d s " o f L Aquation f r o m R u ( N H ) L * 3


β

2

5

2

λΐΓΓ> nm

moles/ einstein X 10*

296 328

500 500

0.37 1.3

296 328

449 449

T, NH

197

°K

E (app), kcal/mole a

42. 82.

7.6 ±

0.4

4.1 ±

0.3

Measured in p H 3 aqueous 0.2M N a C l solution.

e x p l a n a t i o n is a t t r a c t i v e , n a m e l y t h a t e v e n the s m a l l r e a c t i v i t y n o t e d f o r Airr greater t h a n 500 n m w i t h the i s o n i c o t i n a m i d e c o m p l e x is the r e s u l t of t h e r m a l b a c k p o p u l a t i o n of t h e l o w e s t L F state. E v i d e n c e i n s u p p o r t of this c o n c e p t comes f r o m e x a m i n i n g the t e m p e r a t u r e - d e p e n d e n t Φι, v a l u e s ( T a b l e V ) . P h o t o l y s i s of t h e i s o n i c o t i n a m i d e c o m p l e x at 500 n m d i s p l a y s a n a p p a r e n t a c t i v a t i o n e n e r g y (7.6 k c a l ) t h a t is s u b s t a n t i a l l y greater t h a n t h a t o b s e r v e d for p h o t o l y s i s of the 3 , 5 - d i c h l o r o p y r i d i n e c o m p l e x a t 449 nm

(4.1 k c a l ) . T h i s difference (3.5 k c a l ) c o u l d e a s i l y represent t h e d i f

f e r e n c e i n energy b e t w e e n t h e l o w e s t L F * a n d the l o w e s t M L C T *

for

the isonicotinamide complex. Flash Photolysis Studies A p r e v i o u s flash p h o t o l y s i s s t u d y ( 1 2 )

of aqueous

Ru(NH )5py 3

2 +

r e p o r t e d transient b l e a c h i n g of t h e M L C T b a n d f o l l o w e d b y r e l a t i v e l y s l o w d e c a y to substrate a n d a q u a t i o n p r o d u c t s at a r a t e , i n p a r t , i n v e r s e l y p r o p o r t i o n a l to [ H ] . S i n c e t h e t r a n s i e n t b l e a c h i n g w a s too l o n g - l i v e d to +

r e p r e s e n t the b e h a v i o r of a n e x c i t e d state, this o b s e r v a t i o n w a s i n t e r p r e t e d

(NH ) Ru 3

6

I I I

N

2+

+

H+^=±(NH ) Ru 3

5

I I I

-N

(4) TH+


3+

198

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

i n terms of a n i n t e r m e d i a t e h a v i n g a free r a d i c a l c o o r d i n a t e d to a R u ( I I I ) center w i t h t h e p y r i d i n e n i t r o g e n a s s u m i n g a n i n s u l a t i n g t e t r a h e d r a l c o n figuration

c a p a b l e o f r e v e r s i b l e p r o t o n a t i o n ( 12)

(see R e a c t i o n 4 ) . T h e

effect o f [ H ] o n t h e transient b l e a c h i n g p a r a l l e l s t h e p H effect o n t h e +

quantum

y i e l d of photoaquation

t i n u o u s p h o t o l y s i s studies.

reported

previously

(10)

for

con

H o w e v e r , the proposed mechanism for for

m a t i o n of Τ i n v o l v e s r e a c t i o n o f a c h a r g e transfer state s u c h as M L C T * a n d i m p l i e s t h a t a charge

transfer state is p r e c u r s o r

to t h e p h o t o


a q u a t i o n of p y r i d i n e ( 1 2 ) . T h e c o n t i n u o u s p h o t o l y s i s experiments u s i n g

T a b l e V I . P r e l i m i n a r y Results f o r the Flash Photolysis of the Complexes R u ( N H ) L i n Deaerated Aqueous Solutions 3

nm 408

5

2 +

moles/ einstein X 10* 45.

Flash Photolysis Observations' 1

transient bleaching; pH-dependent decay t o starting material a n d products 6

320 nm. β

~10r*Af.

b


0

16.

Modifying

FORD E T A L .

Photochemical

v a r i o u s s u b s t i t u t e d p y r i d i n e s (vide

supra)

Reactivities

199

clearly demonstrate that an

M L C T state is n o t the p r e c u r s o r to the p h o t o a q u a t i o n of L i n t h e p H 3 aqueous solutions u s e d i n those studies. I n a d d i t i o n , n o e n h a n c e m e n t

of

The Use of Ligand Substituents to Modify Photochemical Reactivities

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