Further Studies of the Spectroscopic Properties and Photochemistry of

19 Further Studies of the Spectroscopic Properties and Photochemistry of Binuclear Rhodium(I)

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 16, 2016 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch019

Isocyanide Complexes KENT

R.

MANN

and

HARRY

B. G R A Y

A r t h u r Amos Noyes Laboratory of C h e m i c a l Physics, California Institute of Technology, Pasadena, CA 91125

The position of the lowest allowed electronic transition (1a --> 2a in D ) in Rh L 2+ (L = binucleating isocyanide) depends on the rotameric configuration of the complex [553 nm inRh (bridge) 2+(eclipsed); 515 nm in Rh (TM4bridge)42+ (30° staggered)]; the Rh(I)-Rh(I) distances are 3.26 and 3.25 Å, respectively. Reaction of Rh (bridge) 2+ (orRh 2+)with H in dilute aqueous acidic solutions yields Rh H 4+(λmax= 780 nm). The Rh H 4+ species also is pro duced by low-temperature (—60°C) irradiation (λ > 520 nm) of 6M HCl solution of HRh Cl (λ = 578 nm). The possible role of Rh H 4+ as an intermediate in the H -producing photoreaction of HRh Cl (HRh Cl Rh Cl 2+ + H ) is discussed. 2u

1g

4h

2

2

4

4

2

2

2

4

2

4

2

4

Our

2

2

2+

max

2

2

2

2

4

2

2+

2

2+

2

interest i n the e l e c t r o n i c structures a n d spectroscopic p r o p e r t i e s o f square p l a n a r complexes o f d

8

ions w i t h

π-acceptor

l i g a n d s dates

b a c k 15 years ( 1 ) . T h e w o r k r e p o r t e d i n this c h a p t e r g r e w o u t o f studies of t h e spectroscopic p r o p e r t i e s o f R h ( I ) isocyanides w h i c h w e i n i t i a t e d i n 1973. O n e d i s c o v e r y w e m a d e early i n these studies w a s that R h (CNR)4

+

complexes aggregate i n s o l u t i o n , y i e l d i n g discrete

binuclear,

t r i n u c l e a r , a n d e v e n h i g h e r o l i g o m e r s (2,3). W e b e c a m e i n t r i g u e d w i t h the

spectroscopic

decided

properties

to p r e p a r e

(bridge =

o f these o l i g o m e r i c R h ( I ) species a n d

a particular

1,3-diisocyanopropane),

binuclear

complex,

R h ( bridge )4 2

f o r d e t a i l e d s t u d y (4).

2+

Both the

0-8412-0429-2/79/33-173-225$05.00/0 © 1979 American Chemical Society King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

226

INORGANIC

spectroscopic

properties

COMPOUNDS

a n d the redox

t u r n e d o u t to b e i n t e r e s t i n g

WITH

UNUSUAL

chemistry

PROPERTIES

II

of R h ( b r i d g e ) 2

4

2 +

and w e have continued our

(4,5,6,7),

investigations of this system as d e s c r i b e d h e r e i n . Structural

and Spectroscopic

Properties

T h e prototypal complex, R h (bridge) 2

4

2 +

, a n d s i m i l a r complexes are


s y n t h e s i z e d easily b y s l o w a d d i t i o n of the b i n u c l e a t i n g i s o c y a n i d e l i g a n d to a d i c h l o r o m e t h a n e s o l u t i o n of [ R h ( C O D ) C l ] to g i v e , i n t h e case of 2

[Rh(COD)Cl]

2

+ 4 bridge - » R h ( b r i d g e ) C 1 2

4

bridge, a dark blue precipitate, R h ( bridge ) C 1 . 2

m a d e b y m e t a t h e t i c a l reactions. Rh (bridge) (BPh ) 2

4

4

2

+ 2COD

(1)

O t h e r salts c a n b e

2

T h e x-ray c r y s t a l structure analysis of

r e v e a l e d t h e p r e s e n c e of discrete

cations a n d B P h " anions (8). 4

4

2

Rh(bridge)

4

A v i e w of t h e structure of R h ( b r i d g e )

4

is s h o w n i n F i g u r e 1. E a c h R h C

2

4

2 +

2 +

u n i t is v e r y closely square p l a n a r , a n d

the l i g a n d b o n d angles a n d b o n d lengths are q u i t e close to values f o u n d f o r other

Rh(I)

i s o c y a n i d e complexes

(3).

T h e R h - R h distance

is

3.264 A . T h e close contact of t h e t w o p l a n a r R h ( C N R ) v e r y i n t e r e s t i n g spectroscopic p r o p e r t i e s .

4

+

units results i n some

T h e electronic absorptions of

l o w e s t energy c a n b e i n t e r p r e t e d satisfactorily u s i n g the m o l e c u l a r o r b i t a l

Figure 1. View of the structure of Rh (bridge) ; the Rh-Rh distance is 3.264 A (from a crystal structure analysis of Rh (bridge) (BPh^) ) (7) 2

2

4

2+

i

2

King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

19.

M A N N

Binuclear

A N D GRAY

Rh(I) Isocyanide

227

Complexes

_2a (orb ) 2u

u

X

2u.

"2a

dz*(a Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 16, 2016 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch019

2

l g

lg

a

2u

(or a,)

)^^

dz*(a ) lg

"Ia (or a,) 1g

IVIL4

IM]

IM]

\J

\J

Ml_4

Journal of the American Chemical Society

4h

Figure 2. Relative energies of selected molecular orbitals in binuclear Rh(I) complexes of D or D symmetry (2) 4

D

4h

D

4 h ( 4cP D

D

4

h

d

e n e r g y l e v e l d i a g r a m s h o w n i n F i g u r e 2. T h e strongest

intermonomer

electronic interactions i n v o l v e t h e orbitals that extend p e r p e n d i c u l a r to the m o l e c u l a r p l a n e , n a m e l y , a (d *) lg

I n t h e m o n o m e r i c units a a c t i o n of t h e t w o a

2u

lg

2u

the L U M O .

Inter(la ) lg

set. S i m i l a r s p l i t t i n g o f t h e u p p e r set o c c u r s to

2u

a n d a n t i b o n d i n g (2a )

lg

ir*(CNR)].

z

orbitals causes t h e i r s p l i t t i n g i n t o a b o n d i n g

a n d a n t i b o n d i n g (la ) g i v e b o n d i n g (2a )

a n d a [p ,

z

is t h e H O M O a n d a

lg

orbitals. I n b o t h cases t h e

2u

u p p e r a n d l o w e r sets c o n t a i n orbitals of t h e same s y m m e t r y , a l l o w i n g c o n s i d e r a b l e m i x i n g that leads to s t a b i l i z a t i o n of t h e l o w e r set relative t o t h e u p p e r set (2a ,

la ) 2u

a

partial

bonding

interaction

S i n c e t h e l o w e r set is

2a ).

lg

2u

exists

between

(la , lg

filled,

the t w o R h ( C N R )

4

+

fragments. T h e lowest allowed electronic transition i n the binuclear is la

2u

complex

-> 2a , w h i c h is p r e d i c t e d t o b e at s u b s t a n t i a l l y l o w e r energy t h a n lg

the c o r r e s p o n d i n g t r a n s i t i o n i n R h ( C N R ) t i o n a n d e m i s s i o n spectra

2

(e =

a b s o r p t i o n s p e c t r u m is assigned A 1

n m e m i s s i o n p e a k is A

2u

monomers. Electronic absorp-

for R h (bridge)

s h o w n i n F i g u r e 3. T h e intense 1

+

4

-» A 1

lg

lg

in C H C N

2 +

4

3

s o l u t i o n are

14,500) b a n d at 553 n m i n t h e

-» A 1

2u

(la

2u

- » 2a ), lg

a n d t h e 656

( T ^ 2 nsec).

S p e c t r a l d a t a f o r several b i n u c l e a r R h ( I ) c o m p l e x e s

are g i v e n i n

T a b l e I. T h e d a t a s h o w that l e n g t h e n i n g t h e c a r b o n c h a i n i n t h e l i g a n d b a c k b o n e shifts t h e b a n d to h i g h e r energy. ( 8 ) o f t h e structure of R h ( T M 4 - b r i d g e ) 2

2,5-diisocyanohexane)

shows

that

4

2 +

O u r recent determination

(TM4-bridge =

the rotameric

2,5-dimethyl-

c o n f i g u r a t i o n of t h e



228

INORGANIC

30

28

26

24

COMPOUNDS

22

UNUSUAL

20 -3

v x 10

WITH

16

PROPERTIES

14

II

12

-1 (cm

)Journal of the American Chemical Society

Figure 3.

Absorption ( ) and emission ( ) spectra of (BPhj,) in acetonitrile solution at 25 ± 2°C (6)

Rh (bridge)^2

2

l i g a n d s is a n i m p o r t a n t i n f l u e n c e o n t h e p o s i t i o n of t h e l o w e s t a l l o w e d e l e c t r o n i c t r a n s i t i o n . I n this c o m p l e x t h e t w o l i g a n d planes a r e r o t a t e d a w a y f r o m a n eclipsed configuration b y about 30° ( F i g u r e 4 ) , b u t the R h - R h distance is n e a r l y t h e same as f o u n d i n R h ( b r i d g e ) 2

T h e b l u e shift of t h e la

2u

- » 2a

±

4

2 +

(3.254 A ) .

transition i n R h ( T M 4 - b r i d g e ) 2

4


2 +

to 515

19.

M A N N

Binuclear

A N D GRAY

Table I .


2 2 2 2

4

2 t

4

Data"

Lowest Absorption Band (nm)

(bridge) (4-bridge) (TM4-bridge) (CYCL05-bridge) 2 +

4

2 t

4

2 +

229

Complexes

Absorption and Emission Spectral

Complex Rh Rh Rh Rh

Rh(I) Isocyanide

Emission (nm)

553 526 515 423

656 634 614 583

N

N

II

II

C

C

(CYCL05-bridge) "25 ± 2°C in acetonitrile solution.

CME4 C6

CME3

CMEI CME2

Figure 4. View of the structure of Rh (TM4bridge)^; the Rh-Rh distance is 3.254 A (from a crystal structure analysis of Rh (TM4-bridge) 2

2

Jf


230

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

n m is best e x p l a i n e d i n terms o f t h e smaller s p l i t t i n g o f t h e 2a , lg

2a

2u

II

set.

L e s s s p l i t t i n g w o u l d b e e x p e c t e d because o f t h e m u c h s m a l l e r o v e r l a p o f the C N R TT* c o m p o n e n t s of 2a

a n d 2a

lg

Thermal

2u

i n the observed configuration.

Reactions

The R h ( bridge ) 2

4

complex (abbreviated R h

2 +

2

2 +

) a n d its d e r i v a t i v e s


u n d e r g o a v a r i e t y o f v e r y i n t e r e s t i n g t h e r m a l reactions, e.g., Cl Rh

2

2

>ClRh Cl

2 +

2

2 +

A

m a x

— 338 n m

(2)

A

m a x

= 574 n m

(3)

yellow HCl Rh

2

>H R h C l

2 +

2

2 +

blue H 2HRh Cl 2

blue

2 +

2

>R h H 4

2

4 +

+ 2HC1

A

m a x

— 780 n m

super reduced

C2C

the Rh-Rh

distance is 2.84 A


(4)

19.

M A N N

A N D GRAY

Binuclear Rh(I) Isocyanide

231

Complexes

I n R e a c t i o n 2, b o t h R h ( I ) atoms are o x i d i z e d to a species that c o n tains a R h - R h single b o n d , w i t h t h e a d d i t i o n of t w o l i g a n d s i n a trans arrangement. Rh (bridge) 2

Several X - a n d X Y - t y p e molecules 4

i n this w a y ( 4 ) . T h e R h ( b r i d g e ) C 1 2

c r y s t a l l i z e d f r o m aqueous

4

2

2 +

c o m p l e x has b e e n

H C l s o l u t i o n as a C l " salt a n d s t r u c t u r a l l y

c h a r a c t e r i z e d b y x-ray t e c h n i q u e s

( 8 ) . A v i e w of t h e c o m p l e x c a t i o n is

shown

bridge

i n Figure

5.

T h e four

e c l i p s e d , as i n R h ( b r i d g e ) 2


o x i d a t i v e l y a d d to

2

2 +

4

2 +

ligands

are again

rigorously

, b u t a shorter R h - R h d i s t a n c e (2.84 A ) is

o b s e r v e d , as e x p e c t e d i n a c o m p l e x possessing a R h - R h single b o n d ( 9 ) . R e a c t i o n 3 c o u l d also b e f o r m u l a t e d as a n o x i d a t i v e a d d i t i o n a l t h o u g h earlier w e p o i n t e d o u t ( 5 ) that electronic spectroscopic properties of t h e dark blue H R h C l 2

2 +

species are n o t i c e a b l y different f r o m those of R h C l 2

and related R h ( I I ) - R h ( I I )

complexes.

intense v i s i b l e a b s o r p t i o n b a n d spectrum of H R h C l 2

i n d -d 7

7

2 +

(A

m a x

2

2 +

T h e m a i n p o i n t w a s that t h e =

574 n m , e =

52,700) i n t h e

falls at l o w e r energy t h a n most a - » o-* transitions

b o n d e d complexes (e.g., t h e RhoClo

blue

2+

+ H

(5)

2

yellow

I r r a d i a t i o n ( 5 4 6 n m ) of H C l solutions of t h e b l u e H R h C l 2

p l e x leads to t h e p r o d u c t i o n of R h C l 2

2

2 +

2 +

com-

a n d H gas ( R e a c t i o n 5 ) . T h e 2

b a c k t h e r m a l r e a c t i o n is r e l a t i v e l y s l o w , t a k i n g several days to r e t u r n t h e system to t h e i n i t i a l state.

When 0

2

is present,

Rh Cl 2

2

2 +

is p r o d u c e d

w i t h higher q u a n t u m yields ( T a b l e I I ) , indicating preferential reduction of 0

2

( r a t h e r t h a n r e d u c t i o n o f 2 H to H ) . T h e n a t u r e o f t h e d e p e n d e n c e +

of t h e q u a n t u m y i e l d f o r H

2

2

p r o d u c t i o n i n degassed H C l solutions o n a

H

Table I I . Quantum Yields for the Photooxidation of Rh (bridge) in H C l Solutions at 2 9 ° C 2

4

2+

10 ® (degassed)

10 ® saturated) 2

2

(M)

[HCl]

12.8 12.1 11.1 10.1 9.1 8.1 6.0 1.0

6.0 2.9 1.1 4.3 1.8 7.8 1.3 1.6

c

(air

a

X 10 X 10 x

io

4 4

4

X 10 X 10 X 10 X 10 X 10°

3

3

2

2

0

b

0.79 0.56 0.26 0.083 0.028 0.010 < 0.002 < < 0.001

4.3 b

b

5.2 b

b

2.2

° Based on measurements of the appearance of R h ( b r i d g e ) C 1 . N o t measured. For comparison, data for the photoreaction i n 9M H B r (giving H + R h (bridge) Br in degassed solution) are as follows: a = 7.8 X 10 , 10 * (degassed) =z 4.4, and 10 * (air saturated) = 8.8. 2

4

2

2+

b

c

2

4

2

2+

H

+

3

2

2


2

+


19.

M A N N

Binuclear

A N D GRAY

Rh(I)

Isocyanide

233

Complexes

NM Figure 7. Electronic absorption spectra of HRh Cl in 6M HCl solution upon irradiation (> 520 nm) at —60°C; 0, initial spectrum of HRh Cl ; 1, after 20 min irradiation; 2-7, dark reaction in intervals (total time of ca. 1 hr) 2+

2

2

(Table II)

2+

suggests that t h e m e c h a n i s m of R e a c t i o n 5 is q u i t e c o m p l i -

c a t e d , p e r h a p s i n v o l v i n g several species not o b s e r v e d i n r o o m t e m p e r a t u r e steady state i r r a d i a t i o n experiments. L o w temperature

(—60°C)

i r r a d i a t i o n ( > 520

6 M H C l solutions ( n o net p r o d u c t i o n of H 25°C)

of H R h C l 2

2 +

2

nm)

of

degassed

is o b s e r v e d i n 6 M H C l at

y i e l d s the spectral changes s h o w n i n F i g u r e 7.

It is

c l e a r that a p h o t o c h e m i c a l l y i n d u c e d d i s p r o p o r t i o n a t i o n r e a c t i o n occurs, R e a c t i o n 6:

3HRh Cl 2

blue

X > 520 nm 2 +

>Rh Cl 2

6 M HC1(-60°C)

2

2 +

yellow

+ HCl+

Rh H 4

2

(6)

4 +

super reduced

T h e s t o i c h i o m e t r y of R e a c t i o n 6 w a s d e t e r m i n e d f r o m t h e k n o w n values of the species.

m o l a r e x t i n c t i o n coefficients

of the r h o d i u m - c o n t a i n i n g

I n the d a r k at — 6 0 ° C , the y e l l o w species ( R h C l 2

slowly (hours) w i t h R h H 4

2

4 +

2

2 +

)

reacts

to regenerate the i n i t i a l c o m p l e x ( H R h C l ) . 2

2 +

T h e d a r k r e a c t i o n is k i n e t i c a l l y first o r d e r i n e a c h of the r h o d i u m - c o n t a i n i n g reactants w i t h k =

10" M " s e c 1

F l a s h k i n e t i c spectroscopic HRh Cl 2

2 +

that R h H 4

1

-1

at -

experiments

60°C in 6 M H C l . o n 0 . 1 M H C l solutions of

at 2 5 ° C are n o w b e i n g p e r f o r m e d . I n i t i a l results c l e a r l y s h o w 2

4 +

and R h C l 2

2

2 +

are f o r m e d several m i l l i s e c o n d s after the


flash;

234

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

II

these species b a c k r e a c t t o y i e l d t h e i n i t i a l b l u e c o m p l e x i n a f e w seconds (k ^ 1 0 M " sec" a t 2 5 ° C ) 4

1

( I I ) . S e v e r a l s h o r t e r - l i v e d transients a r e

1

o b s e r v e d i n t h e flash e x p e r i m e n t s , a n d these a r e p r o b a b l y t h e p r e c u r s o r s of R h H 4

2

and R h C l

4 +

2

2

2 +

.

T h e r o l e o f these s h o r t e r - l i v e d i n t e r m e d i a t e s

in the photochemistry of H R h C l 2

is n o w b e i n g s t u d i e d .

2 +

T h e photoreactions of H R h C l


2

3HRh Cl

>Rh H

2

4

2

4 +

with H

2 +

+ Rh Cl 2

2 +

2

+

and 0

2

l i k e l y p r o c e e d as

+ HCl

(7)

A

Rh H

2

Rh H

2

Rh H

2

4

4

4

4 +

+ Rh Cl

4 +

+ 2 H C 1 ^± 2 H R h C l

4 +

+ 0

2

2

2 +

+ HCl

>3HRh Cl 2

2 +

2

2

+ H

(8)

2 +

(9)

2

+ 3HC1 - » H R h C P + R h C l 2

2

2

+ 2H 0

2 +

(10)

2

o u t l i n e d i n R e a c t i o n s 7, 8, 9, a n d 10. T h e p h o t o - i n d u c e d r e d o x d i s p r o portionation (Reaction 7 ) yields R h C l 2

containing product, a n d t h e reactive, T h e fate o f R h H 4

2

4 +

2

2 +

, w h i c h i s t h e final r h o d i u m -

reducing intermediate

H

+

, Rh H 4

2

4 +

b a c k reacts w i t h R h C l 2

r e g e n e r a t e t h e s t a r t i n g c o m p l e x ( R e a c t i o n 8 ) ; at h i g h a w i t h protons to give H 0 2

4

2

4 +

.

is d e t e r m i n e d b y t h e n a t u r e o f t h e r e a c t i o n m e d i u m ;

i n degassed s o l u t i o n at l o w a

but i f a

Rh H

2

H

a n d t w o molecules of H R h C l 2

+

2 +

,Rh H 4

2

4 +

2

2 +

to

reacts

(Reaction 9 ) ,

is also h i g h , o x i d a t i o n o c c u r s m a i n l y b y p a t h w a y ( 1 0 ) , o w i n g

to t h e v e r y r a p i d r e a c t i o n o f R h H 4

2

4 +

with 0 . 2

Acknowledgment T h i s research w a s supported b y t h e N a t i o n a l Science (CHE75-19086).

Foundation

M a t t h e y - B i s h o p , I n c . is a c k n o w l e d g e d f o r a generous

loan of r h o d i u m chloride. Literature Cited 1. Gray, H. B., Ballhausen,C.J.,J.Am. Chem. Soc. (1963) 85, 260. 2. Mann, K. R., Gordon, J. G., II, Gray, H. B.,J.Am. Chem. Soc. (1975) 97, 3553. 3. Mann, K. R., Lewis, N. S., Williams, R. M., Gray, H. B., Gordon, J.G.,II, Inorg. Chem. (1978) 17, 828. 4. Lewis, N. S., Mann, K. R., Gordon, J. G., II, Gray, H. B., J. Am. Chem. Soc. (1976) 98, 746. 5. Mann, K. R., Lewis, N. S., Miskowski, V. M., Erwin, D. K., Hammond, G. S., Gray, H. B., J. Am. Chem. Soc. (1977) 99, 5525. 6. Miskowski, V. M., Nobinger, G. L., Kliger, D. S., Hammond, G. S., Lewis, N. S., Mann, K. R., Gray, H . B., J. Am. Chem. Soc. (1978) 100, 485. 7. Gray, H . B., Mann, K . R., Lewis, N. S., Thich, J. A . , Richman, R. M., ADV. C H E M . SER. (1978) 168, 44.


19.

M A N N AND GRAY

Binuclear

Rh(I) Isocyanide

Complexes

235

8. Mann, K. R., Thich, J. A., Bell, R. A., Coyle, C. L., Gray, H . B., unpublished data. 9. Norman, J. G., Jr., Kolari, H. J., J. Am. Chem. Soc. (1978) 100, 791. 10. Levenson, R. A., Gray, H. B.,J.Am. Chem. Soc. (1975) 97, 6042. 11. Miskowski, V. M., Milder, S., Mann, K. R., unpublished data.


RECEIVED March 20, 1978. Contribution No. 5812 from the Arthur Amos Noyes Laboratory of Chemical Physics.


Further Studies of the Spectroscopic Properties and Photochemistry of

Recommend Documents