Mechanistic Aspects of Inorganic Reactions - ACS Publications

15 Generation of Reactive Intermediates via Photolysis of Transition-Metal Polyhydride

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Complexes GREGORY L. GEOFFROY Pennsylvania State University, Department of Chemistry, University Park, PA 16802 The photochemical properties of several transition metal polyhydride complexes are de scribed. Irradiation of [MoH (dppe) ] (dppe = Ph PCH CH PPh ) and [MoH (PPh Me) ] gives loss of Η . Under N atmospheres the dinitrogen complexes [Mo(N ) (dppe) ] and [Mo(N ) (PPh Me) ] are formed in near quantitative yield. Irradiation of [ReH (dppe) ] with UV light leads to elimination of H with a 366-nm quantum yield of 0.07 ± 0.02. The i n i t i a l photoproduct is [ReH(dppe) ] or a solvated derivative, but this species is highly reactive and rapidly adds N , CO, C H , C H , and CO to give adduct complexes. NMR evidence indi cates that [ReH(dppe) ] undergoes rapid but rever sible ortho metalation and insertion into the C-H bonds of benzene. Irradiation of the complexes [ReH L ] (L = PMe Ph, PMePh , PPh ) and [ReH (PMe Ph) ] gives efficient loss of phosphine in the primary photochemical reaction. The 366 nm quantum yields for the [ReH L ] complexes range from 0.13-0.18; the 366 nm quantum yield for PMe Ph loss from [ReH (PMe Ph) ] is 0.4. Under an H atmosphere, [ReH (PMe Ph) ] is converted into [ReH (PMe Ph) ] upon photolysis; the pentahydrides in turn lose another equivalent of phosphine to 4

2

2

2

2

2

4

2

2

4

2

2

2

2

3

2

2

2

4

2

2

2

2

2

4

2

2

2

2

5

3

2

2

2

3

4

5

2

3

2

3

5

3

2

2

2

3

4

4

3

0097-6156/82/0198-0347$10.25/0 © 1982 American Chemical Society Rorabacher and Endicott; Mechanistic Aspects of Inorganic Reactions ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

348

MECHANISTIC ASPECTS OF INORGANIC REACTIONS

give the corresponding [ReH L ] complexes. The heptahydrides are themselves photosensitive and react to give a mixture of [Re H L ] and [Re H L ] dimers. The dinuclear compound [Pt H (dppm) ] PF , 4, (dppm = Ph PCH PPh ) and [Pt H Cl(dppm) ] PF , 5, have been found to lose H when irradiated in either the solid state or in solution with 366 nm quantum yields of 0.62 and 0.06, respectively. One mole of H is rapidly evolved upon photolysis of 4 in CH CN solution and the photoproduct has been identified as [Pt H(CH CN)(dppm) ]PF . H NMR data indicate a structure for the latter having a direct Pt-Pt bond with a terminal hydride bound to one Pt and the CH CN ligand to the other. These experiments demonstrate the feasibility of H loss upon photolysis of dinuclear d i - and polyhydrides when at least one of the hydrides is bound in a terminal fashion. 7

2

2

8

4

2

2

6

2

2

2

2


6

6

5

3

2

2

2

2

2

3

2

3

2

6

1

3

2

A number o f s t u d i e s have demonstrated t h a t p h o t o l y s i s o f t r a n s i t i o n metal d i - and p o l y h y d r i d e complexes can lead to the generation o f v e r y r e a c t i v e i n t e r m e d i a t e s , g e n e r a l l y v i a photo induced l o s s o f H ( 1 , 2 ) . Two such examples are shown i n eqs 1 (3) and 2 ( 4 - 8 ) . 9

Z

h

[IrH (PPh ) ] 3

3

3

^

H + [IrH(PPh ) ] 2

3

/

H Ir(PPh ) 3

P P h

2

H

+

first

(PPh ) ] 3

3

leads

V

H-Ir(PPh ) 3

(U>-

5

the

ë

2

[M(n-C H ) H ] In

2

5

2

2

example, to

loss

5

V

P P h

2

2

[M(H-C H ) ] 5

photolysis of H

2

(1)

3

and

of

5

2

+ H

thermally

(2)

2

stable

r

[I H ~ 3

formation o f [ I r H ( P P h ) ] . 3

3

This species i s extremely r e a c t i v e and does not p e r s i s t as such but i n s t e a d undergoes the o r t h o - m e t a l l a t i o n r e a c t i o n shown i n

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

15.

Photolysis of Transition-Metal

GEOFFROY

Polyhydride

Complexes

349

eq 1. This g i v e s another d i h y d r i d e complex which undergoes f u r t h e r photo-induced l o s s of h* t o g i v e [ I r ^ H ^ P P h p ( P P h ) l 2

3

as the f i n a l product. P h o t o l y s i s of the molybdenocene tungstenocene d i h y d r i d e s , eq 2, a l s o leads t o l o s s of H 2

2

and and


t o the formation of the corresponding metallocenes. These are extremely r e a c t i v e and e i t h e r add s u b s t r a t e molecules or decay by i n s e r t i o n i n t o C-H bonds o f the coordinated C^H^ l i g a n d s . Green and coworkers (5-8) have shown i n a s e r i e s of s t u d i e s t h a t photogenerated tungstenocene behaves as. an o r g a n o m e t a l l i c c a r bene and r e a d i l y i n s e r t s i n t o C-H bonds of a v a r i e t y of s o l v e n t molecules, e.g., eq 3 ( 5 ) . [W(n-C H ) H ] + C H 5

5

Photoinduced

2

2

6

5

6

V

[W(H-C H ) H(C H )] + H 5

5

2

6

5

(3)

2

r e d u c t i v e e l i m i n a t i o n o f h* from p o l y h y d r i d e com 2

plexes, Η Μ Ι χ

β

(χ > 3) can, i n p r i n c i p l e , l e a d t o the g e n e r a t i o n

of h i g h l y r e a c t i v e compounds t h a t should be capable of a c t i v a t i n g normally i n e r t s u b s t r a t e s such as N , C0 , and perhaps hy drocarbons. Multiple H l o s s can occur from p o l y h y d r i d e com plexes having an even number of hydride l i g a n d s to g i v e what may be c a l l e d doubly c o o r d i n a t i v e l y - u n s a t u r a t e d complexes and hence an extremely r e a c t i v e s p e c i e s . P h o t o l y s i s of complexes w i t h an odd number of hydride l i g a n d s can g i v e c o o r d i n a t i v e l y - u n s a t u r a ted complexes t h a t s t i l l have hydride l i g a n d s and, hence, cen t e r s of r e a c t i v i t y . To t e s t these n o t i o n s we have undertaken a s e r i e s of s t u d i e s of the photochemical p r o p e r t i e s of p o l y h y d r i d e complexes and summarize our r e s u l t s h e r e i n . 2

2

2

Photoinduced E l i m i n a t i o n of H and MoH (PPh Me) 4

2

2

from MoH (dppe) 4

2

4

We i n i t i a l l y i r r a d i a t e d the [MoH (dppe) ] (dppe = P h ~ PCH CH PPh ) and [MoH (PPh Me) ] complexes under an N atmos phere ( 9 ) , e x p e c t i n g to form the known b i s ( d i n i t r o g e n ) complexes i f photoinduced l o s s of a l l f o u r hydride l i g a n d s were t o occur. T h i s e x p e c t a t i o n was r e a l i z e d , and h i g h y i e l d s of t r a n s - [ M o ( N ) 4

2

2

2

4

2

2

2

4

2

2

(dppe) J

and

2

t r a n s - [ M o ( N ) ( P P h M e ) ] were obtained. 2

2

2

For

4

2

ex

ample, p h o t o l y s i s of a benzene s o l u t i o n of [MoH (dppe) ] a t 25°C 4

2

for

12 h w i t h 366-nm l i g h t under a continuous N

93%

yield

of

trans-[Mo(N )^(dppe) ], ?

0

eq

4.

2

The

purge gave a nondescript



350 [MoH (dppe) J 4

2

+ 2N

2

5

V 2

H

2

+ trans-[Mo(N ) (dppe) ] 2

2

(4)

2

s p e c t r a l changes which o b t a i n upon p h o t o l y s i s and the n e c e s s i t y of the N purge i n order t o d r i v e the r e a c t i o n s have precluded 2

accurate quantum y i e l d measurements, although the time s c a l e o f the p h o t o l y s i s i n d i c a t e s t h a t the quantum y i e l d s are low. The conversion shown i n eq 4 has been observed t o occur t h e r m a l l y (10) b u t a t a r a t e much slower than t h a t which o b t a i n s under photochemical c o n d i t i o n s . We a l s o found t h a t p h o t o l y s i s o f [Moh* (dppe) ] under a CO atmosphere g i v e s a mixture o f c i s - and Downloaded by UNIV OF TEXAS AT DALLAS on July 12, 2016 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch015

4

2

trans-[Mo(C0) (dppe) ]. 2

2

I r r a d i a t i o n under

and C 0

2

atmospheres leads t o complex a r r a y s o f products which have proven d i f f i c u l t t o separate and c h a r a c t e r i z e . An important q u e s t i o n i s what happens i n the absence o f a s u i t a b l e s u b s t r a t e t h a t can t r a p the photogenerated i n t e r m e d i a t e ( s ) : Are r e a c t i v e s p e c i e s such as [Mo(dppe) J and [Mo(PPh 2

Me)^]

2

photo-generated and do they p e r s i s t i n s o l u t i o n ?

Photo

l y s i s o f r i g o r o u s l y degassed benzene s o l u t i o n s o f [MoH (dppe) ] 4

2

w i t h 366-nm l i g h t g i v e s a c o l o r change from y e l l o w t o b r i g h t orange over a p e r i o d o f s e v e r a l hours. This c o l o r change i s accompanied by a steady decrease i n i n t e n s i t y o f the 1714-cm * Mo-H i i° * δ = -3.6 ppm hydride resonance char a c t e r i s t i c o f [MoH (dppe) ]. No new hydride resonances o r

V

v

D r a t

n

ana

t

n

4

e

2

metal hydride v i b r a t i o n s appear i n e i t h e r spectrum. The s i n g l e t 31 a t 83.5 ppm i n i t i a l l y present i n t h e Ρ NMR spectrum o f [MoH (dppe) ] i n benzene-d^ s o l u t i o n a l s o decreases i n i n t e n s i t y as the p h o t o l y s i s proceeds, and a new s i n g l e t a t 80.2 ppm ap pears and grows i n . Mass s p e c t r a l , gas chromatographic, and Toepler pump analy ses o f the gases above i r r a d i a t e d benzene s o l u t i o n s show the formation o f H w i t h an average o f 1.9 mole o f h* r e l e a s e d p e r 4

2

2

2

mole o f [MoH (dppe) ] i r r a d i a t e d . 4

2

E v a p o r a t i o n o f s o l v e n t from

these i r r a d i a t e d s o l u t i o n s g i v e s an extremely a i r - s e n s i t i v e orange s o l i d which shows no metal hydride v i b r a t i o n s i n i t s infrared

1

spectrum and no hydride resonances i n i t s H NMR 31 spectrum. The Ρ NMR spectrum o f t h i s compound i s complex, showing 12 separate resonances, i n c l u d i n g an i n t e n s e peak a t 13.2 ppm a t t r i b u t a b l e t o f r e e dppe. Repeated attempts a t r e c r y s t a l l i z a t i o n and p u r i f i c a t i o n o f t h i s compound f a i l e d t o g i v e a pure product. Elemental analyses o f compounds obtained from s e v e r a l d i f f e r e n t experiments d i d not agree w e l l w i t h each other nor w i t h any obvious f o r m u l a t i o n .


15.


GEOFFROY

The

o b s e r v a t i o n of l o s s

[MoH^Cdppe)^]

irradiated

of

and

Polyhydride

1.9

the

mol

Complexes

o f HL

accompanying

per mole o f 1 infrared,

31 and Ρ NMR s p e c t r a l changes are not i n c o n s i s t e n t w i t h the formation of [Mo(dppe) ] upon i r r a d i a t i o n of [MoH,(dppe) ] i n 31 degassed s o l u t i o n . However, the Ρ NMR spectrum of the s o l i d m a t e r i a l i s o l a t e d from these experiments c l e a r l y i n d i c a t e s t h a t t h i s s p e c i e s , i f formed, i s not s u f f i c i e n t l y s t a b l e f o r i s o lation. We suspect t h a t [Mo(dppe) ], p o s s i b l y s o l v a t e d , may i n i t i a l l y be photogenerated but t h a t i t subsequently decomposes or r e a c t s w i t h s o l v e n t when no o t h e r s u i t a b l e s u b s t r a t e i s available. Loss of H^ from [Mo^H^dppe^] presumably proceeds 9


H,

9

2

in

a stepwise f a s h i o n v i a i n i t i a l g e n e r a t i o n o f In

regard t o the q u e s t i o n of whether

[Moh* (dppe) ]. 2

o r not

2

[Mo(dppe) ] 2

i s photogenerated from [ M o H ^ d p p e ^ ] , other workers (11,12) have concluded from a s e r i e s of f l a s h p h o t o l y s i s s t u d i e s t h a t i r r a d i a t i o n of t r a n s - [ W ( N ) ( d p p e ) ] does g i v e t r a n s i e n t f o r mation o f the analogous [W(dppe) ] complex, p o s s i b l y as a s o l vated s p e c i e s . T h i s l a t t e r complex subsequently r e a c t s w i t h N^ t o regenerate t r a n s - [ W ( N ) ( d p p e ) ] and w i t h CO and H^ t o 2

2

2

2

2

give

[W(CO) (dppe) ] and 2

mation

of

2

2

[WH (dppe) ],

2

4

respectively.

2

The

for

[W(dppe) ] i n these r e p o r t e d s t u d i e s c l e a r l y makes 2

[Mo(dppe) ] a reasonable i n t e r m e d i a t e i n our experiments. 2

Photogeneration o f R e a c t i v e [ReH(dppe) of H

2

v i a Photoinduced Loss

from [ R e H ( d p p e ) J Although the chemistry of [ReH (dppe) ] has not been ex t e n s i v e l y examined, the complex i s known t o be t h e r m a l l y q u i t e s t a b l e [ c f . , ( 1 3 ) ] . I t shows no tendency t o l o s e H when heated to 180°C i n an evacuated C a r i u s tube (14) and i t w i l l not r e a c t to g i v e the known [ReH(N )(dppe) J d e r i v a t i v e when heated under 2

3

3

2

2

2

60 p s i o f N

(15).

2

2

I n c o n t r a s t t o most metal h y d r i d e s , t r e a t

ment of [ReH (dppe) ] w i t h HC1 gives simple p r o t o n a t i o n r a t h e r 3

2

than l o s s of H ,

eq 5 (14).

2

[ReH (dppe) ] + HC1 3

Although

[ReH (dppe) ]Cl

2

4

[ReH (dppe) ] q

9

2

does not undergo thermal l o s s


(5) of


352 H^,

i t does

readily

occur upon p h o t o l y s i s

t o generate [ReH-

(dppe)^] as a v e r y r e a c t i v e photoproduct (13). species r a p i d l y

adds s u b s t r a t e

As d e t a i l e d be

low,

this

molecules

(N^, CO,

C^H^,

C^H^, and CO^) and i n s e r t s i n t o C-H bonds of benzene s o l


vent and the phenyl groups o f the dppe l i g a n d s . P h o t o l y s i s o f [ReH^dppe)^] i n Degassed S o l u t i o n . Irradia t i o n o f degassed benzene s o l u t i o n s o f the complex w i t h 366-nm l i g h t g i v e s a c o l o r change from y e l l o w t o g o l d w i t h a c o r r e s ponding i n t e n s i f i c a t i o n and s h i f t i n the a b s o r p t i o n maximum from 320 t o 310 nm. I n the IR, as the i r r a d i a t i o n proceeds, the

1

1

v _ (1860 cm' ) and δ _ (850 cm" ) bands o f [ReH (dppe) ] M

H

Μ

Η

3

2

decrease i n i n t e n s i t y and no new bands appear i n the metal hy d r i d e r e g i o n . The p r o d u c t i o n o f H^ d u r i n g p h o t o l y s i s was v e r i f i e d by mass s p e c t r a l and gas chromatographic analyses o f the gases above i r r a d i a t e d s o l u t i o n s . I n three separate e x p e r i ments, the gases above e x h a u s t i v e l y p h o t o l y z e d (6-10 days) s o l u t i o n s were p e r i o d i c a l l y removed and q u a n t i t a t e d by Toepler pump techniques, g i v i n g an average v a l u e o f 0.94 ± 0.18 mol o f H^ released/mol o f complex i r r a d i a t e d . These r e s u l t s c l e a r l y demonstrate the e l i m i n a t i o n o f H^ and suggest the s t o i c h i o m e t r y shown i n eq 6. [ReH (dppe) ] 3

2

4

V

H

£

+ [ReH(dppe) ]

(6)

2

The 366-nm quantum y i e l d of H l o s s from [ReH~(dppe) ] i s 0.07 ± 0.02. Although the s t o i c h i o m e t r y o f the r e a c t i o n (eq 6) and the r e a c t i v i t y experiments d e t a i l e d below i n d i c a t e t h a t the p r i mary photoproduct from [ReH (dppe) ] i s almost certainly 0

0

L

3

[ReH(dppe) ] 2

2

o r a s o l v a t e d d e r i v a t i v e , t h i s s p e c i e s has proven

d i f f i c u l t t o d e t e c t and c h a r a c t e r i z e d i r e c t l y because o f i t s high r e a c t i v i t y . As shown by the f o l l o w i n g s e r i e s o f NMR and IR experiments, i t undergoes r e v e r s i b l e i n s e r t i o n i n t o the C-H bonds o f both benzene s o l v e n t and the phenyl groups o f the dppe ligands.

The *H NMR spectrum

of a C ^

solution

o f [Reh* 3

( d p p e ) ] shows a steady decrease i n i n t e n s i t y o f t h e δ -7.35 2

[ReH (dppe) ] 3

2

resonance as the sample i s i r r a d i a t e d , but no new

resonances appear anywhere i n the metal h y d r i d e r e g i o n (δ 0 •> -30). Exposure o f such s o l u t i o n s t o N does n o t y i e l d t h e 2


ô


GEOFFROY

15.

-9.5

resonance

characteristic

Polyhydride

of

the

Complexes

expected

353

[ReH(N )2

(dppe)^] product (see below); y e t the IR spectrum of the s o l i d m a t e r i a l obtained a f t e r e v a p o r a t i o n of s o l v e n t shows a strong a t 2006 cm c h a r a c t e r i s t i c of t h i s compound (16). The N=N l a c k of a H NMR resonance f o r the product thus i n d i c a t e s t h a t the d i n i t r o g e n complex a c t u a l l y formed i s the deuterated ana logue, [ReD(N )(dppe)^], and thus i t s s o l u t i o n p r e c u r s o r must be 1

V

2

[ReD(dppe) ].

F u r t h e r support f o r the


2

[ReD(N )(dppe)^] 2

for

m u l a t i o n comes from the presence o f a weak band i n i t s IR spec trum a t 1330 cm (dppe)^]

and

1

t h a t i s not present i n the spectrum of [ReH^-

which

may

be

attributed

to a V

R e

.

D

vibration.

The other s i g n i f i c a n t f e a t u r e s of the IR spectrum of t h i s m a t e r i a l are the weak three-band p a t t e r n a t 1542, 1557, and 1575

cm

characteristic

of

ortho-metalated arylphosphines

(13, 17) and a weak broad band centered a t 2260 cm The l a t t e r may be a t t r i b u t e d t o an aromatic C-D s t r e t c h (C^D^, Vç_n= 1

1

2277, 2267 cm" ; P ( 2 , 6 - C H D ) , v _ = 2254 cm" ) which i s p r e sumed t o a r i s e from ortho-deuterated dppe l i g a n d s . F u r t h e r e v i dence f o r the i n c o r p o r a t i o n of deuterium i n t o the dppe l i g a n d s 6

3

2

3

c

D

comes from the H NMR s p e c t r a l changes i n the ô 0-10 s p e c t r a l region. [ReH (dppe) J shows a broad s i n g l e t a t ô 7.5, a m u l t i p l e t a t ô 7.0, and a broad s i n g l e t a t δ 2.1. The r e l a t i v e i n t e n s i t i e s of these resonances are i n the r a t i o o f 2:3:1, and they are l o g i c a l l y a t t r i b u t e d t o the dppe o r t h o , meta/para, and methylene p r o t o n s , r e s p e c t i v e l y . Upon p h o t o l y s i s of [ReH ~ (dppe) J i n C^D^ s o l u t i o n , the δ 7.5 resonance completely d i s appears w h i l e the other resonances remain r e l a t i v e l y unchanged. At the same time, the s o l v e n t C^D_H resonance i n c r e a s e s i n i n o ο t e n s i t y , thus i n d i c a t i n g H/D exchange between s o l v e n t and co o r d i n a t e d dppe. The r e s u l t s d e s c r i b e d above can be accounted f o r by the sequence of r e a c t i o n s o u t l i n e d i n Scheme I . I n s e r t i o n i n t o the C-D bonds of C^D^ presumably y i e l d s the seven-coordinate com p l e x 1. This r e a c t i o n must be r e a d i l y r e v e r s i b l e s i n c e the NMR and IR experiments d e t a i l e d above i n d i c a t e t h a t i t leads t o H/D exchange t o produce [ReD(dppe) ] and C^D^H. The ready r e v e r s i b i l i t y o f t h i s r e a c t i o n presumably o b t a i n s because o f severe s t e r i c crowding i n 1. i n which there are nine phenyl groups l o cated around the c e n t r a l Re atom. [ReH(dppe) ] a l s o undergoes 3

2

3

2

2

2


MECHANISTIC

354

ASPECTS OF INORGANIC REACTIONS

r e v e r s i b l e i n t r a m o l e c u l a r i n s e r t i o n i n t o t h e C-H bonds o f the phenyl groups o f the dppe l i g a n d s , i . e . , o r t h o - m e t a l a t i o n . I f H/D exchange w i t h s o l v e n t has p r e v i o u s l y occurred t o y i e l d [ReD(dppe) ], t h i s r e a c t i o n leads t o i n c o r p o r a t i o n o f deuterium 2

i n t o t h e dppe l i g a n d s Scheme I I .

v i a t h e sequence

o f r e a c t i o n shown i n

Ok


Scheme I [ReH (dppe) ] 3

2

Η Ν [ReH(N )(dppe) ] -

[ReD(dppe) ] 2

.P [ReD(N )(dppe) ]

Scheme I I [ReH(dppe) ] +

t [R H(D)(C^)(dppe) ]

2

e

2

[ReH(D)(C D )(dppe) ] t [ReD(dppe) ] + C ^ H 6

5

[ReD(dppe) ] «- H — R e 2

2

2

Ρ

«-

(

^Re Re



GEOFFROY

15.

Polyhydride

Complexes

355

P h o t o l y s i s of [ReH^(dppe) ] i n the Presence of Reactant Gases 2

Irradiation N^, CO, and C^H^ plexes

of

benzene s o l u t i o n s of

atmospheres leads to the r e s p e c t i v e adduct com

[ReH(N )(dppe) ], 2

(dppe) ]

[ReH^Cdppe)^] under

[ReH(CO)(dppe) ],

2

III).

and

[ReH(D H )2

4

compounds 1 were i d e n t i f i e d p r i m a r i l y by comparison of t h e i r IR and H NMR s p e c t r a to reported values (16). Although the i n f r a r e d s p e c t r a of the [ReH(L)(dppe) ] adduct -1 complexes do not show the c h a r a c t e r i s t i c 1860- and 850-cm 9

(Scheme

2

These p r e v i o u s l y d e s c r i b e d


z

9

1

bands of [ReHg(dppe) ], the H NMR

s p e c t r a o f the photoproducts

2

show a q u i n t e t a t δ -7.35 sidual trihydride.

(Jp_

H

=

17.8

Hz)

i n d i c a t i v e of r e

A comparison of the r e l a t i v e i n t e n s i t i e s of

the hydride resonances of [ReHîdppe^] and [ReH(L) ( d p p e ) ] i n d i c a t e s t h a t approximately 50% conversion to the corresponding [ReH(L)(dppe) ] complexes occurs d u r i n g 20-24 h i r r a d i a t i o n . Increased conversion occurs w i t h increased i r r a d i a t i o n time, but i t i s d i f f i c u l t t o d r i v e these photoreactions e n t i r e l y t o completion. Adduct complexes prepared by t h i s method are i n v a r i a b l y contaminated w i t h r e s i d u a l [ReHg(dppe) ]. 2

2

2

The lene. C H 2

2

[ReH(dppe) ] intermediate can a l s o be trapped by acety 2

I r r a d i a t i o n of benzene s o l u t i o n s of [ReH^(dppe) ] under a 2

atmosphere gives a slow c o l o r change from y e l l o w t o

light

brown, and a brown s o l i d can be i s o l a t e d by s o l v e n t evaporation from the i r r a d i a t e d s o l u t i o n s . This m a t e r i a l shows a weak band i n i t s IR spectrum a t 1690

cm

1

and a q u i n t e t a t

δ -4.35

(Jp_H

= 20 Hz) i n i t s *H NMR spectrum, i n a d d i t i o n to a q u i n t e t a t δ -7.35 due to r e s i d u a l [ReH^(dppe) ]. The product i s t h e r m a l l y 2

unstable and s o l u t i o n s darken c o n s i d e r a b l y upon standing under N , e v e n t u a l l y d e p o s i t i n g an o i l y brown r e s i d u e a f t e r s e v e r a l -1 days. The IR band a t 1690 cm i m p l i e s the presence of an alkyne l i g a n d i n the photoproduct, although Vç*=ç i n alkyne complexes v a r i e s c o n s i d e r a b l y w i t h the nature o f the complex and w i t h the alkyne s u b s t i t u e n t s (18, 19, 20). The u p f i e l d hydride resonance a t δ -4.3 i s i n the range of the hydride resonances of the other adduct complexes (15) and thus f o r m u l a t i o n of the product as [ R e H ( C H ) ( d p p e ) ] i s i n d i c a t e d . 9

2

The

2

photogenerated

2

[ReH(dppe) ] 2

intermediate

is


also

Rorabacher and Endicott; Mechanistic Aspects of Inorganic Reactions ACS Symposium Series; American Chemical Society: Washington, DC, 1982. 2

2

2

2

-H 0

2

[ReH(C H )(diphos) ]

hv,

[ReH(C0)(diphos) ]

2

M

2

[ReH(diphos) ]

2

3

[ReH (diphos) ]

[ReH(N )(diphos) ]

Scheme I I I

C0„

2

4

2

[ReH(C H )(diphos) ]

2

[Re(0 CH)(diphos)


2

15.


GEOFFROY

r e a d i l y scavenged by CO^.

Polyhydride

Complexes

357

I r r a d i a t i o n of benzene s o l u t i o n s of

[ReH^Cdppe)^] i n the presence of CO^

gives a slow c o l o r change

from y e l l o w to orange, and a b r i g h t orange s o l i d can be i s o l a ted by e v a p o r a t i o n of s o l v e n t from the i r r a d i a t e d s o l u t i o n s . The IR spectrum of t h i s m a t e r i a l shows two new bands a t 1554


and

1356

cm

1

which are not present i n the spectrum of [ReH«13 (dppe)^]. S u b s t i t u t i o n of CO^ i n the p h o t o l y s i s experiment gives an orange product w i t h an i n f r a r e d spectrum which shows -1 12 13 bands a t 1515 and 1334 cm as expected f o r the C -> C sub s t i t u t i o n , and thus CO^ has c l e a r l y been incorporated i n t o the v

molecule. No _ j j v i b r a t i o n i s apparent i n the metal h y d r i d e r e g i o n of the IR spectrum. A group of bands i s centered a t 1960 cm \ but these appear t o be due to the coordinated dppe l i gands. The *H NMR spectrum of a benzene s o l u t i o n o f the orange s o l i d shows no new u p f i e l d resonances, although the ever present q u i n t e t a t δ -7.35 due to r e s i d u a l [ReH (dppe) ] i s e v i d e n t . 13 1 13 The C{ H} NMR spectrum o f the CO^ enriched product shows a R e

q

9

s i n g l e resonance a t δ 171.9 which s p l i t s i n t o a doublet w i t h ^C-H "*" P p l d spectrum. The product o f the r e a c t i o n w i t h CO^ i s s e n s i t i v e t o =

n t n e

r o t o n

c o u

e

thermal s u b s t i t u t i o n of carbon d i o x i d e . Storage of a benzene s o l u t i o n of the complex under an N^ atmosphere r e s u l t s i n a slow c o l o r change from orange to y e l l o w . Removal of s o l v e n t from the y e l l o w s o l u t i o n gives y e l l o w c r y s t a l s of [ReH(N )(dppe)^], 2

1

c h a r a c t e r i z e d by i t s strong v ^ ^ a t 2006 cm .

This r e a c t i o n

=

appears

t o be q u a n t i t a t i v e as evidenced by the complete

appearance of the 1554- and 1356-cm ment of the orange product w i t h

1

IR bands. gives

dis

Similar treat

[ReH^Cdppe)^].

The

orange product s l o w l y decomposes when heated to 80-100°C, and mass s p e c t r a l and gas chromatographic analyses of the gases above decomposed samples show the presence of CO^. Quantita t i v e Toepler pump a n a l y s i s of the evolved gases upon decomposi t i o n of the h i g h e s t p u r i t y m a t e r i a l t h a t we have obtained gave a v a l u e of 0.97 mol of CO^ g i v e n o f f / m o l of complex. The s p e c t r a l p r o p e r t i e s of the product i s o l a t e d from photo lysis

of

[ReH (dppe) ] 3

2

i n the presence of C 0

2

are most con

s i s t e n t w i t h i t s f o r m u l a t i o n as a formate d e r i v a t i v e . cally,

the IR bands a t 1554 and 1356 cm

1

may be

Specifi

respectively



358 attributed

t o the v(CO,J and v ( C 0 ) 2'asym 2'sym

vibrations of a b i -

o

v

dentate

formate

whose b i d e n t a t e fraction

ligand.

F o r comparison,

[RuHiO^CH)(PPh^)^],

structure

has been e s t a b l i s h e d by X-ray

( 2 1 ) , shows bands a t 1565 and 1340 cm

[IrClH(0 CH)(PPh ) ] 2

3

with

2

dif

Similarly,

a presumed b i d e n t a t e

s t r u c t u r e has

1

IR bands a t 1550 and 1345 cm (22). I n c o n t r a s t , complexes w i t h u n i d e n t a t e formate l i g a n d s g e n e r a l l y show t h e v ( C 0 ) ζ asym


o

above 1600 cm" the

(22, 23, 24).

above-mentioned

For example, a d d i t i o n o f CO t o

[IrClH(0 CH) ( P P h ^ ]

gives

2

[IrClH(0 CH)2

(C0)(PPhg) ] w i t h a u n i d e n t a t e formate l i g a n d , and the c o r r e s 2

1

ponding IR bands are a t 1687 and 1362 cm" (22). trum o f [Re(0 CH)(dppe) ] 2

2

The IR spec

shows no evidence f o r a V ^ J J v i b r a 1

t i o n b u t does show a weak V~ „ band a t 2825 cm

t h a t i s not

L-n

present i n [ReHg(dppe) ] and which may be a t t r i b u t e d t o the formate C-H v i b r a t i o n . The most d e f i n i t i v e evidence f o r the formate f o r m u l a t i o n f o r [Re(0 CH)(dppe) ] i s the doublet a t δ 171.9 ( J _ = 202 Hz) 13 in i t s C NMR spectrum. Both t h e chemical s h i f t and t h e c o u p l i n g constant f a l l w i t h i n the ranges e s t a b l i s h e d f o r o r g a n i c formates (25). The chemical s h i f t s o f e t h y l formate and formic a c i d , f o r example, a r e δ 160.7 and 166.7, r e s p e c t i v e l y (25). 13 The C-H c o u p l i n g constants f o r t h e formate carbon g e n e r a l l y l i e between 170 and 230 Hz w i t h HC0 showing a c o u p l i n g con s t a n t o f 194.8 Hz (25). The rhenium d i t h i o f o r m a t e complex [ R e ( S C H ) ( C 0 ) ( P P h ) ] , prepared by F r e n i , e t a l . (26), v i a the r e a c t i o n shown i n eq 7, 2

2

2

C

H

9

1

2

[ReH(CO) (PPh ) ] + C S 2

3

3

2

2

3

2

£

[Re(S CH)(CO) (PPh ) ] + PPh 2

2

3

2

(7)

3

has been c h a r a c t e r i z e d by X-ray d i f f r a c t i o n and shown t o have the s t r u c t u r e 2 (27). T h i s complex i s i s o e l e c t r o n i c w i t h [Re( 0 C H ) ( d p p e ) ] and, t h u s , by analogy t o [ R e ( S C H ) ( C 0 ) ( P P h ) ] , 2

2

the most reasonable s t r u c t u r e shown i n 3.

2

2

f o r [Re(0 CH)(dppe) ] 2

2


3

2

i s that

15.


GEOFFROY

Polyhydride

pph

Complexes

359

r^p

^Re^

"CH

JTRe^

oc^ J

PPh

P 3

p^|

ô*

l ^ p

2

!CH

3

The formation of [ReCO^CH)(dppe)^] presumably proceeds v i a Downloaded by UNIV OF TEXAS AT DALLAS on July 12, 2016 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch015

initial

a d d i t i o n o f CO^

t o photogenerated

[ReH(dppe) ]

to give

2

an adduct complex s i m i l a r to those obtained w i t h N^, CO, and C^H^ (eq 8 ) . Hydride m i g r a t i o n t o CO^ would then generate the

observed

formate d e r i v a t i v e

!ReH(dppe) ] + C0 2

[ReH(C0 )(dppe) 2

tions

must be

p l a c e d by N

2

J

2

t

2]

(eq 9 ) .

Both of these reac-

[ReH(C0 )(dppe) ] 2

(8)

[Re(0 CH)(dppe) ]

(9)

2

2

2

r e v e r s i b l e , however, s i n c e C0

and H

t o y i e l d [ReH(N )(dppe) ]

£

2

r e s p e c t i v e l y , and C 0

2

i s readily

2

and

dis

[ReH (dppe) ], 3

2

i s l i b e r a t e d upon thermal decomposition of

2

[Re(0 CH)(dppe) ]. 2

2

P h o t o d i s s o c i a t i o n of PR

Ligands from [ R e H ( P R ) ] and

3

3

3

4

{ R e H ( P R ) ] Complexes 5

3

3

The above r e s u l t s l e d us t o c o n s i d e r whether m u l t i p l e hy drogen m i g r a t i o n t o a bound C 0 or N l i g a n d might occur i f the 2

2

photogenerated i n t e r m e d i a t e possessed two or more h y d r i d e s . Photo-induced l o s s of H from [ R e H ^ ( P R ) ] , f o r example, would 2

give

3

3

[ R e H ( P R ) ] w i t h t h r e e hydrides which c o u l d p o t e n t i a l l y 3

3

3

transfer. However, we have found t h a t these pentahydrides do not undergo e l i m i n a t i o n o f H i n the primary photochemical event 2

but, i n s t e a d , e f f i c i e n t l y

l o s e a PR

3

ligand.

These compounds

thus c o n s t i t u t e the f i r s t c l a s s of monomeric d i - and p o l y h y d r i d e complexes i n which the dominant p h o t o r e a c t i o n has d e f i n i t e l y been shown t o be something other than H l o s s . The t r i h y d r i d e 2

complex

[ReH (PMe PH) ] has 3

2

undergo e f f i c i e n t d i s c u s s e d i n view which l o s e s H -

4

PR

3

of

also

been examined and

elimination. those

noted

These l a t t e r above

for

found

results

to are

[ReH (dppe) ] 3

2


2


360 Photo-induced

Loss

o f PR^ from [ReH,. ( P R , ^ ) ]

Complexes.


[ReH^CPMe^Ph)^] i s t h e r m a l l y q u i t e s t a b l e , showing no d e t e c t a b l e r e a c t i o n when heated i n a degassed i s o o c t a n e s o l u t i o n f o r 15 h at 80°C. However, 366 nm p h o t o l y s i s o f an i s o o c t a n e s o l u t i o n o f the complex r e s u l t s i n a r a p i d decrease i n i n t e n s i t y o f i t s e l e c t r o n i c a b s o r p t i o n band a t 338 nm. However, as the p h o t o l y s i s proceeds, the s p e c t r a l changes become more complex as sec ondary photochemical and/or thermal r e a c t i o n s occur. Irra d i a t i o n o f concentrated s o l u t i o n s o f the complex g i v e s an i n i t i a l change from c o l o r l e s s t o p a l e orange and the f o r m a t i o n o f an orange p r e c i p i t a t e , i d e n t i f i e d as [Re^HgiPMe^h)^] as i n d i cated by i t s H NMR spectrum which shows a q u i n t e t a t δ -6.31 ppm w i t h J _ = 9.52 Hz. F o r comparison, [ R e H g ( P E t P h ) ] 1

p

H

2

shows a q u i n t e t a t δ -6.59 ppm w i t h J _ p

H

2

4

= 9.28 Hz (28). Upon

continued p h o t o l y s i s t h i s p r e c i p i t a t e r e d i s s o l v e s t o g i v e a dark red s o l u t i o n . T h i s c o l o r a t i o n p e r s i s t s u n t i l one mole o f H per mole o f i n i t i a l [ReH,.(PMe^JPh)^] has been r e l e a s e d . Ex h a u s t i v e p h o t o l y s i s then r e s u l t s i n a green c o l o r a t i o n and e v e n t u a l l y , a f t e r 1.5 e q u i v a l e n t s o f H have been e v o l v e d , a a white f l o c c u l e n t p r e c i p i t a t e d e p o s i t s . E v a p o r a t i o n o f s o l v e n t at t h i s p o i n t g i v e s a green o i l which has a s t r o n g odor o f f r e e PMe Ph. ^ 1 The metal-hydride r e g i o n H NMR spectrum o f a sample t h a t had been i r r a d i a t e d up t o the p o i n t o f the red c o l o r a t i o n i s shown i n F i g u r e l a . T h i s spectrum shows t h e f o r m a t i o n o f 2

2

[ReH (PMe Ph) ] ( 7

of

2

2

ô R e H

3

= 20.2 H z ) , a t r a c e amount

H

2

4

ReH

p

H

= 20.2 H z ) , and a broad

( J „ = 10.8 Hz) which may be a t t r i b u t e d t o n

[Re H^(PMe Ph)^] dimer which has been s t r u c t u r a l l y charac 2

terized

2

by Caul ton,

[ReH (PMe Ph) ] 3

spectra 2.9

p

[ReH (PMe Ph) ] (ô "6.74 q t , J _

s e x t e t a t Ô -8.23 the

"5.11 t , J

2

4

et a l .

products

of authentic

( 2 9 ) . The [ReH (PMe Ph) ] and 7

were

identified

samples.

by

2

2

comparison t o

A doublet a t δ 1.05 (Jp_g

Hz) due t o uncoordinated PMe Ph 2

i s also

=

present i n t h e

spectrum but not shown i n F i g u r e l a . The p h o t o r e a c t i o n i s markedly i n h i b i t e d by the presence o f excess PMe Ph. F o r example, p h o t o l y s i s o f [ReH,.(PMe Ph) ] i n t h e presence o f a 100-fold excess o f PMe Ph gave no de t e c t a b l e r e a c t i o n a f t e r 8 h although a s i m i l a r l y prepared con t r o l sample w i t h o u t excess PMe Ph showed s u b s t a n t i a l r e a c t i o n after 1 h irradiation. 2

2

2

2


3

GEOFFROY


Polyhydride

Complexes


15.

solution (b).


361


362

The generation of free PMe^Ph and the suppression of the photoreaction by excess PMe^Ph both point to loss of PMe^Ph from [ReH^CPMe^Ph)^]

i n the primary V

[ReH (PMe Ph) ] 5

2

&

3

photochemical

event,

eq 10.

[ReH (PMe Ph) ] + PMe Ph 5

2

2

(10)

2

However, the presumed [ReH^(PMe Ph) ] intermediate must under-


2

2

go further reaction to give the mix of products described above. The overall reaction i s much cleaner when the irradiation is conducted i n the presence of H as evidenced by the UV-VIS 2

spectral changes, Figure 2, which show a smooth, rapid decrease i n the 338 nm absorption band as the irradiation proceeds. No suppression of the rate of reaction i s observed under an H atmosphere, i n direct contrast to the situation 2

found for those hydride complexes where the primary photoreaction i s H loss. The i n i t i a l product of photolysis under an 2

H

2

atmosphere i s [ReH (PMe Ph) ] (eq 11). ?

2

[ReH (PMe Ph) ] + H 5

2

3

h 2

2

¥

[ReH (PMe Ph) ] + PMe Ph ?

2

2

(11)

2

Consistent with the spectral changes shown i n Figure 2, [ReH^(PMe Ph) J 2

shows no absorption maximum below 272 nm (Table I ) .

2

*H NMR spectra from low conversion (~35%) photolysis experiments showed resonances attributable to [ReH,. (PMe^JPh)^], [ReH^_ (PMe Ph) ], 2

2

a

trace

of

[Re H (PMe Ph) ], and free 2

6

2

PMe Ph.

5

2

Furthermore, irradiation of 0.5 g of [ReH^.(PMepJPh)^] under an H

2

atmosphere, led to the isolation

(PMe Ph) J and [Re H (PMe Ph) ]. 2

2

2

degradation (PMe Ph) J 2

product

8

2

2

The latter i s a known thermal

4

of [ReH (PMe Ph) ] y

2

-> [Re Hg(PMe Ph) ]

2

of a mixture of [ReH^-

2

(30) and this

2

[ReH ?

conversion has also been shown

4

to be photoaccelerated (29). The quantum yield of PMe Ph loss 2

from

[ReH (PMe Ph) ] under an H 5

2

3

atmosphere i s 0.16 ± 0.02.

2

As noted above, photolysis i n degassed solutions does lead to evolution of H and formation of [Re H (PMe Ph) ] and [ R e ^ 2

(PMe Ph)~

3

[ReH L ] 5

[ReH L ] ?

2

[ReH L ]

t R e

• ^ - ^ -3H„

H

2

5

3

*l -H„

L

2 8 4>

>

ÂV

The photogenerated [ReHîPR^)^] complex could undergo b i n u c l e a r r e a c t i o n s w i t h [ R e H ( P R > ] , 1-3 5

d i r e c t l y give the

3

and

3

[ R e ^ H g i P R ^ ] and

1-4

i n Scheme IV, to

[ R e ^ C F R ^ ] dimers.

Con

s i s t e n t w i t h t h i s proposal i s the immediate o b s e r v a t i o n of these dimers a t the onset of p h o t o l y s i s and Norton's (38) s t u d i e s of the d i m e r i z a t i o n of HÔsiCO)^ which proceeds by a s i m i l a r b i molecular pathway. w i t h H^, yield

Reaction of the photogenerated [ReH,. ( P R , ^ ]

r e l e a s e d i n the course of r e a c t i o n s 1-3 and 1-4,

[ReH (PR > ]. 7

3

2

[ R e H ( P R ) ] t h e r m a l l y and 7

3

2

would

photochemically

decomposes to g i v e the r e s p e c t i v e [ R e H ( P R ) ] dimers which, i n 2

g

t u r n , are known to r e a c t w i t h excess PR ( P R ) ^ ] dimers (29, 30). 3

3

3

4

to y i e l d the

In our hands, t h i s l a t t e r

occurs a t room temperature immediately agents .

[Re^^-

conversion

upon combining the r e



370

Another p o s s i b l e route t o [ R e H ^ P R ^ ^ ] i s v i a the d i s p r o p o r t i o n a t i o n o f photogenerated

[ReH (PR ) ] 5

3

with

2

[ReH (PR ) ], 5

3

3

eq 17. [ReH (PR ) ] 5

3

+ [ReH (PR ) ]

2

5

3

[ R e h y P R ^ ] + [ R e h y P R ^ ] (17)

3

This r e a c t i o n would a l s o y i e l d

[ R e H ( P R ) ] which could add the 3

3

3

photoreleased phosphine t o g i v e [ R e H ( P R ) ] , a product observed Downloaded by UNIV OF TEXAS AT DALLAS on July 12, 2016 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch015

3

3

4

i n s i g n i f i c a n t q u a n t i t y f o r L = P P h , eq 18. 3

[ReH (PR ) ] 3

3

3

+ PR + [ R e H ^ P R ^ ]

(18)

3

I t i s s i g n i f i c a n t t h a t [ReH (PMe Ph) J 3

2

4

l o s e s PMe Ph i n i t s 2

primary p h o t o r e a c t i o n , whereas f o r [ReH (dppe) ] 3

r e a c t i o n i s c l e a r l y h* e l i m i n a t i o n

(13).

2

2

t h e observed

Note, however, t h a t

the quantum y i e l d s f o r these processes are v a s t l y d i f f e r e n t . E l i m i n a t i o n o f h* from [ReH (dppe) ] occurs w i t h a quantum 2

yield

3

o f 0.07 whereas

occurs w i t h

loss

2

o f PMe Ph 2

from

[Reh* (PMe Ph) ]

φ = 0.4. We suspect t h a t phosphine

3

2

4

l o s s and H

2

e l i m i n a t i o n a r e c o m p e t i t i v e p h o t o r e a c t i o n s i n these rhenium hydride complexes, w i t h phosphine l o s s the more e f f i c i e n t o f the two. I n the case o f [Reh* (dppe) ], the c h e l a t i n g dppe l i gands prevent n e t phosphine l o s s and the o n l y observed reac tion i s H loss. 3

2

2

Photoinduced E l i m i n a t i o n of H [Pt H (dppm)JPF 2

3

6

2

from the D i n u c l e a r Complexes

and [ P t H C l ( d p p m ) ] P F 2

2

2

6

Since p h o t o l y s i s o f monomeric d i - and p o l y h y d r i d e t r a n s i t i o n metal complexes o f t e n leads t o e l i m i n a t i o n o f H as t h e 2

dominant p h o t o r e a c t i o n , the q u e s t i o n a r i s e s as t o whether o r not H e l i m i n a t i o n w i l l occur from d i - and p o l y n u c l e a r h y d r i d e 2

complexes i n which the hydride l i g a n d s are bound t o d i f f e r e n t metals o r b r i d g e two o r more metal c e n t e r s . Such d i n u c l e a r e l i m i n a t i o n r e a c t i o n s could be important i n s o l a r energy con v e r s i o n schemes u s i n g s o l u b l e complexes f o r producing H from 2

H

T

h

e

f

e

w

n u c

e a r

2° * — > — P°ly l hydridocarbonyl c l u s t e r complexes which have had t h e i r photochemistry examined do n o t show H l o s s b u t i n s t e a d e i t h e r CO l o s s o r metal-metal bond 2

cleavage

(37-40).

I n search o f such H

2

elimination reactions



GEOFFROY

15.

Polyhydride

Complexes

371

we have examined the d i n u c l e a r hydride complexes [ P t H ( d p p m ) ] 2

PF , 6

4, (dppm = Ph PCH PPh ) and 2

2

3

2

[ P t H C l ( d p p m ) ] P F , 5, both of

2

2

2

2

6

which have been w e l l c h a r a c t e r i z e d by Brown, Puddephatt, and co-workers (41, 42, 43). The s t u d i e s reported h e r e i n show t h a t both of these complexes r e a d i l y l o s e h* upon p h o t o l y s i s and 2


demonstrate the f e a s i b i l i t y of such r e a c t i o n from d i - and nuclear hydride complexes. .CH

2 N

+

PPiu

Ph P 2

Pt'

Ph P 2

^

poly-

'Pt.

PPh„

N

'CH„

Degassed a c e t o n i t r i l e s o l u t i o n s of 4 show l i t t l e or no decomposition upon prolonged h e a t i n g a t 80°C, but p h o t o l y s i s (λ > 300 nm) of thoroughly degassed Ch^CN s o l u t i o n s of [ P t ^ (dppm) ]PF^ gives v i s i b l e gas e v o l u t i o n and a r a p i d change from 2

c o l o r l e s s to red. The UV-VIS s p e c t r a l changes which occur are shown i n F i g u r e 4, and the maintenance of the i s o s b e s t i c p o i n t s a t 337 nm and 372 nm i n d i c a t e s a c l e a n conversion t o products. The red c o l o r a r i s e s as a r e s u l t of the weak v i s i b l e a b s o r p t i o n by the photoproduct ( ε ^

0

1

= 130 M

cm * ) .

One

mole of h*

2

per

mole of complex i r r a d i a t e d i s r a p i d l y evolved (~30 min), a l though prolonged p h o t o l y s i s (> 48 h) leads t o secondary photo chemical r e a c t i o n s and f u r t h e r slow e v o l u t i o n of H^. A t o t a l of 1.42

moles of H

t r a t i o n and

were evolved

2

i n one

66 h experiment.

Concen

c o o l i n g of CH^CN s o l u t i o n s i r r a d i a t e d to the

point

of e v o l u t i o n of 1 mole e q u i v a l e n t of h* gives a red s o l i d which 2

has

been

Calcd. C,

characterized

for

46.01%; H,

as

C H F NP Pt : 5 2

4 g

6

3.59%).

5

2

The

[Pt H(CH CN)(dppm) ]PF -

(Anal.

C,

Found:

2

3

2

46.39%;

reaction

H,

6

3.57%.

shown i n eq

19


i s thus


a

î ι

WAVELENGTH

i

(nm)

ι

S

î

ι

t

s

i

t

9

Figure 4. VV-visible spectral changes during 366-nm photolysis of a CH CN solution of [Pt H (dppm) \PF . Arrows indicate the direction of the spectral changes after beginning photolysis; and the spectra were successively recorded after 0, 0.5, 1, 1.5, 2, 3, and 5 min total irradiation time.

ι~ι


î

15.


GEOFFROY

i n d i c a t e d and photoproduct.

spectroscopic

[Pt H (dppm) ] 2

3

+

+ CH CN

2

3

5

data

V

H

Polyhydride

imply

Complexes

structure

6 f o r the

+ [Pt H(Ch* CN)(dppm) ]

2

2

373

3

+

(19)

2

6 The

IR spectrum

1

o f 6 shows a ν . „ v i b r a t i o n a t 2033 cm , Ώ

"~


indicative weak v

rt"ii

o f a t e r m i n a l r a t h e r than b r i d g i n g h y d r i d e , 1

C N

1

s t r e t c h a t 2258 cm .

The hydride resonance i n the H

Ph P Ζ ι

PPh ι Z

0

H

and a

0

Pt ι a

Pt^ i b

Ph P

PPh

2

NCCH

0

3

2

CH^

NMR

spectrum

o f 6 i n CD^CN s o l u t i o n appears a t δ -8.9 as a 3 1 2

pseudoquartet due t o c o u p l i n g

t o the f o u r

Ρ n u c l e i ( Jp.jj

3

J D „ = 9 Hz) w i t h two s e t s o f 1/4 i n t e n s i t y s a t e l l i t e s from ' 195 1 c o u p l i n g t o the two i n e q u i v a l e n t P t atoms ( J p t _ = 975 Hz; 2 " J p t _ = 73 H z ) . As Puddephatt (44, 45, 46) has shown, t h i s H

a

H

b s a t e l l i t e pattern i s c h a r a c t e r i s t i c of a terminal hydride, whereas a 1:8:18:8:1 q u i n t e t p a t t e r n i s expected f o r a b r i d g i n g hydride. This *H NMR resonance p a t t e r n i s s i m i l a r t o t h a t ob served f o r [Pt H(CO)(dppm) ]PF (45) and [ P t H ( d p p m ) ] P F (46) 2

2

6

2

3

6

which have s t r u c t u r e s analogous t o t h a t proposed f o r 6. The *H NMR spectrum o f 6 a l s o shows a s i n g l e t a t δ 1.95 due t o the methyl

group

photoproduct pressure,

to

o f the Ch^CN l i g a n d . reacts

with

CO

The

and H ,

respectively yield

2

[Pt H(CH CN)(dppm) ] 2

3

+

2

both a t 25°C and 1 atm

[Pt H(C0)(dppm) J 2

2

(45) and


374


+

[Pt H (dppm) ] , 2

3

providing

2

further

support

for

t h e proposed

s t r u c t u r e and a l s o suggesting a r i c h d e r i v a t i v e chemistry f o r t h i s complex. P h o t o l y s i s o f 4 i n THF and acetone s o l u t i o n s gives s i m i l a r r e s u l t s although the photoproducts, which are presumably analo gous t o 6, a r e n o t as completely c h a r a c t e r i z e d . The s p e c t r a l p r o p e r t i e s o f t h e r e d s o l i d i s o l a t e d from p h o t o l y s i s o f THF s o l u t i o n s o f 4 are c o n s i s t e n t w i t h the f o r m u l a t i o n [Pt H(THF)9


1

(dppm) ]PF . 2

H NMR monitoring

6

s o l u t i o n s showed t h e formation and

[Pt H Cl(dppm) ] 2

H

2

2

+

2

of [Pt HCl (dppm) ] 2

2

2

+

2

(δ -12 m)

(δ -17 m) i n an approximate 7:3 r a t i o .

2

loss also

of photolysis o f 4 i n CD C1

occurs when 4 i s i r r a d i a t e d i n the

solid-

s t a t e . White s o l i d samples o f 4 i n vacuo o r under N r a p i d l y t u r n red upon exposure t o s u n l i g h t , f l u o r e s c e n t room l i g h t , o r UV i r r a d i a t i o n (λ > 300 nm). Mass s p e c t r a l a n a l y s i s o f gases above i r r a d i a t e d s o l i d samples showed the presence o f H ; 2

2

q u a n t i t a t i v e measurements gave an average value o f 0.90 moles of H evolved per mole o f complex i r r a d i a t e d . The r e d s o l i d 2

obtained

from such s o l i d - s t a t e p h o t o l y s i s shows a broad IR band 1

a t 2145 cm ( N u j o l ) , i n d i c a t i v e o f a t e r m i n a l hydride. Ex posure o f t h i s m a t e r i a l t o CO r a p i d l y gave the formation o f [Pt H(CO)(dppm) ]PF ; reaction with H slowly regenerated 2

2

6

[Pt Hg(dppm) ]PF£. 2

2

2

These v a r i o u s

r e s u l t s i n d i c a t e the s o l i d -

s t a t e r e a c t i o n shown i n eq 20.

[Pt H (dppn,) ]PF 2

3

2

f

oUâ

6

H

2 +

[Pt H(dppm) ]PF 2

2

(20)

6

state The exact nature o f the [Pt H(dppm)^]PF^ photoproduct i s pre s e n t l y unclear although the 2145 cm IR band i m p l i e s a t e r m i n a l , r a t h e r than b r i d g i n g , hydride. Similar results obtain for photolysis of solutions of 5 although the r e a c t i o n s occur much more s l o w l y . Photolysis of 5 i n THF gives a c o l o r change from c o l o r l e s s t o y e l l o w and e v o l u t i o n o f H^. A y e l l o w p r e c i p i t a t e can be i s o l a t e d from the 2

photolyzed s o l u t i o n and t h i s m a t e r i a l r a p i d l y r e a c t s w i t h CO i n C H C 1 s o l u t i o n t o give the known [ P t C l ( C 0 ) ( d p p m ) ] P F com p l e x (43). A r e a c t i o n analogous t o t h a t i n eq 19 i s thus i n d i cated b u t w i t h [ P t C l ( T H F ) ( d p p m ) ] P F as t h e photoproduct. 2

2

2

9

9

2

A


6

15.

GEOFFROY

Photolysis of Transition-Metal Polyhydride Complexes

P h o t o l y s i s o f s o l i d samples o f 5 a l s o leads t o

375

e v o l u t i o n and

an orange s o l i d , although the p h o t o r e a c t i o n i s not n e a r l y as e f f i c i e n t as f o r [ P t H ( d p p m ) ] P F . 2

The

3

2

6

366 nm quantum y i e l d s o f H

2

l o s s from CH^CN s o l u t i o n s

of 4 and 5 are 0.62 and 0.06, r e s p e c t i v e l y , c o n s i s t e n t w i t h the q u a l i t a t i v e observations noted above. E l i m i n a t i o n o f H


2

from the t r i h y d r i d e complex i s presumably more e f f i c i e n t be cause each P t center i s l i g a t e d by two hydrides and concerted h* e l i m i n a t i o n can r e a d i l y occur from one metal. E l i m i n a t i o n 2

of H

2

from

+

[ P t H C l ( d p p m ) ] , which has the two hydrides i n 2

2

2

t e r m i n a l p o s i t i o n s on d i f f e r e n t P t atoms i n the ground s t a t e , could occur v i a i n t r a m o l e c u l a r hydride m i g r a t i o n t o g i v e an a c t i v a t e d i n t e r m e d i a t e w i t h both hydrides bound t o the same Pt atom, such as 7 o r 8, f o l l o w e d then by concerted e l i m i n a t i o n of H . 0

7

8

A l t e r n a t i v e l y , l o s s o f H from 5 c o u l d occur v i a an intermo l e c u l a r pathway i n v o l v i n g two molecules o f 5, although bimolec u l a r r e a c t i o n s have not been observed f o r the v a r i o u s monomeric hydride complexes which undergo photoinduced l o s s o f H ( 1 , 2 ) , 2

2

or v i a c o u p l i n g o f the two hydrides across the two metal centers o f 5. This study has shown t h a t H e l i m i n a t i o n can be photo induced from d i n u c l e a r complexes i n which a t l e a s t one o f the hydrides i s bound i n a t e r m i n a l f a s h i o n . I t s t i l l remains t o be determined whether H e l i m i n a t i o n can occur from d i - and 2

2

p o l y n u c l e a r complexes when both hydrides occupy b r i d g i n g p o s i -


376


t i o n s o r from d i n u c l e a r complexes where the hydrides are t e r m i n a l , b u t no reasonable mechanism e x i s t s f o r simultaneously p l a c i n g both hydrides on the same metal.


General Experimental

Procedures

I r r a d i a t i o n Procedures. Samples were i r r a d i a t e d w i t h a 450 w Hanovia medium pressure Hg a r c lamp, a 100 w Blak-Ray B100A lamp equipped w i t h a 366 nm f i l t e r , o r on an o p t i c a l bench equipped w i t h a water cooled lamp housing (Photochemical Research A s s o c i a t e s , I n c . , Model ALH215), a 100 w high-pressure Hg a r c lamp (Osram HBO 100 w/2), a monochromator (Photochemical Research A s s o c i a t e s , I n c . , Model B102), and a thermostated c e l l h o l d e r . Quantum y i e l d s were determined u s i n g the l a t t e r apparatus, and l i g h t i n t e n s i t i e s were measured u s i n g f e r r i o x a l a t e actinometry (47). Samples were i r r a d i a t e d i n 1-cm quartz UV-VIS spectrophotometer c e l l s s e a l e d t o Kontes 4 mm q u i c k - r e l e a s e t e f l o n v a l v e s f o r attachment t o a vacuum l i n e . Samples were degassed by s e v e r a l freeze-pump-thaw c y c l e s and then p l a c e d under 1 atm o f H^. A f t e r i r r a d i a t i o n a t 366 nm, samples were t r a n s f e r r e d t o 0.1-cm quartz UV-VIS spectrophotometer c e l l s , and t h e decrease i n i n t e n s i t y o f t h e a b s o r p t i o n band maximum was measured. A l l determinations were performed i n triplicate. Samples f o r NMR experiments were prepared e i t h e r i n standard NMR tubes s e a l e d under vacuum or i n a degassable NMR tube equipped w i t h a stopcock and a 17/25 female S ground g l a s s j o i n t t o a l l o w s o l u t i o n s t o be degassed and then p l a c e d under an H^ atmosphere. Gases above i r r a d i a t e d s o l u t i o n s were q u a n t i t a t e d by standard Toepler pump techniques and were analyzed by mass spectrometry. S p e c t r a l Measurements. The f o l l o w i n g instruments were em ployed i n t h i s study: UV-VIS - Cary 17 or Hewlett-Packard HP 8450A; IR - Perkin-Elmer 580; Mass Spectra - AEI-MS-902; X

NMR - V a r i a n A-60A, JEOL PS-100 FT, Bruker WH 200. H NMR s p e c t r a were referenced e x t e r n a l l y t o TMS o r i n t e r n a l l y t o t h e 31 s o l v e n t , g e n e r a l l y benzene. Ρ NMR s p e c t r a were referenced t o e x t e r n a l 8 5 % H^PO^ and downfield chemical s h i f t s a r e r e p o r t e d as p o s i t i v e . Acknowledgments The r e s e a r c h d e s c r i b e d h e r e i n was supported by the N a t i o n a l Science Foundation (CHE-7728387). GLG g r a t e f u l l y acknowledges the C a m i l l e and Henry Dreyfus Foundation f o r a Teacher-Scholar Award (1978-1983), t h e A l f r e d P. Sloan Foundation f o r a Re search F e l l o w s h i p (1978-1981), and the many coworkers who con t r i b u t e d t o the research d e s c r i b e d h e r e i n .



15.

GEOFFROY


Polyhydride

Complexes

377

Literature Cited 1. Geoffroy, G. L.; Wrighton, M. S. "Organometallic Photo chemistry"; Academic Press: New York, 1979. 2. Geoffroy, G. L. Prog. Inorg. Chem. 1980, 27, 123. 3. Geoffroy, G. L.; Pierantozzi, R. J. Am. Chem. Soc. 1976, 98, 8054. 4. Geoffroy, G. L.; Bradley, M. G. Inorg. Chem. 1978, 17, 2410. 5. Giannotti, C.; Green, M. L. H. J. Chem. Soc., Chem. Commun. 1972, 1114. 6 Elmitt, K.; Green, M. L. H.; Forder, R. Α.; Jefferson, I.; Prout, K. J. Chem. Soc., Chem. Commun. 1974, 747. 7. Farrugia, L.; Green, M. L. H. J. Chem. Soc., Chem. Commun. 1975, 416. 8. Green, M. L. H.; Berry, M.; Couldwell, C.; Prout, F. Nouv. J. Chim. 1977, 1, 187. 9. Pierantozzi, R.; Geoffroy, G. L. Inorg. Chem. 1980, 19, 1821. 10. Archer, L. J . ; George, T. A. Inorg. Chem. 1979, 18, 2079. 11. Thomas, R. J. W.; Lawrence, G. S.; Diamantis, A. N. Inorg. Chim. Acta 1978, 30, L353. 12. Caruona, Α.; Kisch, H. Int. Conf. Organomet. Chem. 9th 1979, Abstract D10. 13. Bradley, M. G.; Roberts, D. Α.; Geoffroy, G. L. J. Am. Chem. Soc. 1981, 103, 379. 14. Freni, M.; Demichelis, R.; Giusto, D. J. Inorg. Nucl. Chem. 1967, 29, 1433. 15. Ginsberg, A. P.; Tully, M. E. J . Am. Chem. Soc. 1973, 95, 4745. 16. Tully, M. E.; Ginsberg, A. P. J . Am. Chem. Soc. 1973, 95, 2042. 17. Bennett, Μ. Α.; Milner, D. L. J. Am. Chem. Soc. 1969, 91, 6983. 18. Chisholm, M. H.; Clark, H. C. Inorg. Chem. 1971, 10, 2557. 19.

Tang-Wong, K. L.; Thomas, J. L.; Brintzinger, H. H. J. Am. Chem. Soc. 974, 96, 3694.

20. Nakamoto, K. "Infrared and Raman Spectra of Inorganic and Coordination Compounds"; 3rd Ed., Wiley: New York, 1978; p 387. 21. Kolomnikov, I. S.; Gusev, A. I.; Aleksandrov, G. G.; Lobeeva, T. S.; Struchkov, Y. T.; Vil'pin, M. E. J. Organo met. Chem. 1973, 59, 349. 22. Smith, S. Α.; Blake, D. M.; Kubota, M. Inorg. Chem. 1972, 11, 660. 23. Johnson, B. F. G.; Johnston, R. D.; Lewis, J . ; Williams, I. G. J. Chem. Soc., Dalton Trans. 1971, 689.



378

MECHANISTIC ASPECTS O F INORGANIC REACTIONS

24. Nakamoto, K. "Infrared and Raman Spectra of Inorganic and Coordination Compounds"; 3rd Ed., Wiley: New York, 1978; pp 232-233. 25. Levy, G. C.; Nelson, G. L. "Carbon-13 Nuclear Magnetic Resonance for Organic Chemists"; Wiley: New York, 1972. 26. Freni, J.; Giusto, D.; Romiti, P. J. Inorg. Nucl. Chem. 1971, 33, 4093. 27. Albano, V. D.; Bellon, P. L.; Ciani, G. J. Organomet Chem. 1971, 31, 75. 28. Bau, R.; Carroll, W. E.; Teller, R. G.; Koetzle, T. F. J. Am. Chem. Soc. 1977, 99, 3872. 29. Green, Μ. Α.; Huffman, J. C.; Caulton, K. G., submitted for publication. 30. Chatt, J.; Coffey, R. S. J. Chem. Soc. A 1969, 1963. 31. Giusto, D. Inorg. Chim. Acta Rev. 1972, 6, 91. 32. Freni, M.; Valenti, V. Gass. Chim. Ital. 1961. 91. 1357. 33. Norton, J. R. Accts. Chem. Res. 1979, 12, 139. 34. Balzani, V.; Moggi, L.; Manfrin, M. F.; Bolletta, F.; Gleria, M. Science 1975, 189, 852. 35. Lewis, N. S.; Mann, K. R.; Gordon, J. G.; Gray, Η. B. J. Am. Chem. Soc. 1976, 98, 7461. 36. 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. 37. Epstein, R. Α.; Gaffney, F. R.; Geoffroy, G. L.; Gladfelter, W. L.; Henderson, R. S. J. Am. Chem. Soc. 1979, 101, 3847. 38. Foley, H. C.; Epstein, R. Α.; Geoffroy, G. L., submitted for publication. 39. Johnson, B. F. G.; Lewis, J.; Twigg, M. V. J. Organomet. Chem. 1974, 67, C75. 40. Graff, J. L.; Wrighton, M. S. J. Am. Chem. Soc. 1980, 102, 2123. 41. Brown, M. P.; Puddephatt, R. J.; Rashidi, M.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1978, 516. 42. Brown, M.P.; Puddephatt, R. J.; Rashidi, M1; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1977, 951. 43. Brown, M. P.; Puddephatt, R. J.; Rashidi, M.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1978, 1540. 44. Puddephatt, R. J. ACS Symp. Ser. 1981, 155, 187. 45. Brown, M. P.; Fisher, J. R.; Mills, A. J.; Puddephatt, R. J.; Thompson, M. Inorg. Chim. Acta 1980, 44, L271. 46. Brown, M. P.; Fisher, J. R.; Muir, L. M.; Muir, K. W.; Puddephatt, R. J.; Thompson, Μ. Α.; Seddon, K. R. J. Chem. Soc., Chem. Commun. 1979, 931. 47. Hatchard, C. G.; Parker, C. A. Proc. Soc. London Ser. A 1956, 235, 518. RECEIVED April 27, 1982.


General Discussion—Generation of Reactive Intermediates via Photolysis of Transition-Metal Polyhydride Complexes Leader: Guillermo Ferraudi


DR. GUILLERMO FERRAUDI (Notre Dame U n i v e r s i t y ) : I was p u z z l e d by your p h o t o - e j e c t i o n of hydrogen. I wonder i f you have evidence or i f you can speculate as t o whether t h a t process i s a s i n g l e step or whether you expect to f i n d p r e c u r s o r s f o r your r e a c t i v e i n t e r m e d i a t e . DR. GEOFFROY: In the few cases i n which we have conducted mechanistic s t u d i e s , u s i n g l a b e l i n g techniques the r e s u l t s have shown f a i r l y c o n c l u s i v e l y t h a t the hydrogen comes o f f i n a concerted f a s h i o n . The two hydride l i g a n d s simply come o f f coupled together as they leave the metal t o produce hydrogen. We see no evidence f o r r a d i c a l s or the p r o d u c t i o n of i o n i c s p e c i e s . DR. FERRAUDI: So one can t h i n k of the r e a c t i o n as forming a three-atom arrangement a t some p o i n t , w i t h the rhenium and the two hydrogens forming a l o c a l i z e d type bond? DR.

GEOFFROY:

Right.

DR. JACK NORTON (Colorado S t a t e U n i v e r s i t y ) : One i n t e r e s t i n g r e s u l t t h a t has j u s t appeared i s Ray Sweany s r e p o r t of photochemical hydrogen l o s s from i r o n t e t r a c a r b o n y l d i h y d r i d e [Sweany, R. L. J . Am. Chem. Soc. 1981, 103, 2410]. This system i s v e r y c l o s e to the case i n which I claimed some years ago t h a t t h i s process d i d not occur. F i r s t of a l l , do you have any p o s s i b l e e x p l a n a t i o n s f o r the d i f f e r i n g behavior? Why i s Sweany's r e s u l t d i f f e r e n t from the r e s u l t of Tony Rest? Sec ond, i f hydrogen e l i m i n a t e s i n a concerted process such as you have j u s t d e s c r i b e d , why don't other species such as c i s - m e t h y l groups do l i k e w i s e ? !

DR. GEOFFROY: L e t me answer the l a s t q u e s t i o n f i r s t . I don't f u l l y understand c i s - m e t h y l groups. When complexes con t a i n i n g those have been i r r a d i a t e d , one g e n e r a l l y doesn't ob serve the formation of ethane. I n s t e a d , where the chemistry i s w e l l - d e f i n e d , i t looks as i f s o l v o l y s i s simply occurs t o y i e l d radicals. There a r e , however, some cases of d i a r y l complexes which, when i r r a d i a t e d , w i l l couple two phenyl l i g a n d s to g i v e b i p h e n y l as a product. Your f i r s t q u e s t i o n i s one which we have d i s c u s s e d p r e v i o u s l y . You know t h a t the r e p o r t by Rest i s simply c i t e d i n a footnote somewhere. I t has never been presented i n d e t a i l . I t h i n k t h a t what r e a l l y counts i s the s t a b i l i t y of the organom e t a l l i c product which i s l e f t behind. Certainly iron tetra carbonyl i s an o r g a n o - m e t a l l i c fragment which i s produced i n many r e a c t i o n s of i r o n c a r b o n y l s . But, as you know, osmium t e t r a c a r b o n y l i s apparently of such h i g h energy t h a t i t s forma-


MECHANISTIC

380

ASPECTS OF INORGANIC REACTIONS


t i o n i s n o t favored, and t h e compound decomposes v i a other means. I should a l s o add t h a t we do not understand why p h o t o l y s i s induces l o s s o f hydrogen from these d i h y d r i d e complexes. When you look a t t h e l i s t o f compounds I showed, there were many d i f f e r e n t metals and many d i f f e r e n t l i g a n d s e t s , w i t h many d i f f e r e n t e l e c t r o n i c c o n f i g u r a t i o n s . Hydrogen l o s s was observed i n a l l cases. Dr. T r o g l e r has p o i n t e d out t h a t there i s strong c o u p l i n g between t h e e x c i t e d s t a t e s and t h e v i b r a t i o n s which l e a d t o hydrogen l o s s . But we don't understand t h e e x c i t e d s t a t e s , so I r e a l l y can't answer your question p r o p e r l y . DR. WILLIAM WOODRUFF ( U n i v e r s i t y o f Texas): I would l i k e to make a comment not so much on your paper as on m e c h a n i s t i c photochemistry i n g e n e r a l . I t h i n k most o f us would agree t h a t i f we are going t o draw m e c h a n i s t i c c o n c l u s i o n s , we r e a l l y need to know what the s t r u c t u r e s o f the reactants and products are. One o f t h e problems i n photochemistry i s t h a t we g e n e r a l l y do not know t h e s t r u c t u r e o f t h e r e a c t a n t , which i s t h e e x c i t e d state. There aren't v e r y many s t r u c t u r e - s p e c i f i c probes i n s o l u t i o n , i n f a c t , none below about the m i l l i s e c o n d time s c a l e where e s r and NMR cease t o be a p p l i c a b l e . I n our l a b o r a t o r y , we have been able t o o b t a i n the resonant spectra o f e x c i t e d s t a t e s . I n two o f t h e three kinds o f systems t h a t we have observed so f a r , the s t r u c t u r e s o f the e x c i t e d s t a t e s are not p r e d i c t a b l e i n a s t r a i g h t f o r w a r d way, e i t h e r from the ground s t a t e s t r u c t u r e s or from c a l c u l a t i o n s . For example, t r a n s f e r o f the e x c i t e d s t a t e s o f Ru(bpy)^ III 3

to

Ru

(bpy) £ ( b p y

)

cannot

be p r e d i c t e d

in a

+

c l e a r and

s t r a i g h t f o r w a r d way. DR. DAVID McMILLIN (Purdue U n i v e r s i t y ) : As you probably know, Wrighton has shown t h a t i n c e r t a i n rhenium hydrides there i s a l a r g e deuterium e f f e c t on the e x c i t e d s t a t e l i f e t i m e [ G r a f f , J . L.; Wrighton, M. S. J . Am. Chem. Soc. 1981, 103, 2225]. So perhaps the hydrogen i s s p e c i f i c a l l y r e a c t i v e because a l o t o f t h e energy gets channeled i n t o these high-frequency modes. DR. GEOFFROY: The compounds i n which Wrighton saw t h e deuterium i s o t o p e e f f e c t were metal c l u s t e r compounds which do not l o s e hydrogen upon i r r a d i a t i o n . Those are a s p e c i a l s e t o f compounds. Some c o l l a b o r a t i v e work w i t h which we are c u r r e n t l y i n v o l v e d i n c l u d e s a s e r i e s o f tungsten molybdenum t e t r a h y d r i d e complexes and t e t r a d e u t e r i d e s which photo-eliminate hydrogen and a l s o luminesce. However, we do not see any e f f e c t on the l u m i nescence o r the quantum y i e l d o f l o s s o f hydrogen as we r e p l a c e hydrogen w i t h deuterium.


15.

GEOFFROY


Polyhydride

Complexes

381


DR. McMILLIN: When you were t a l k i n g about the dppe com p l e x , you invoked the c h e l a t e e f f e c t . I t seems t o me t h a t opening and c l o s i n g the c h e l a t e r i n g would be a good channel f o r n o n r a d i a t i v e decay. I don't see why t h a t would help promote the p r o d u c t i o n of hydrogen. I am wondering i f there might be some other s t r u c t u r a l f a c t o r s i n v o l v i n g the t r a n s i t i o n s t a t e which could account f o r the observed b e h a v i o r . DR. GEOFFROY : We o b v i o u s l y don't know why the photochem i s t r y changes. We know t h a t i t i s reasonable t h a t phosphine d i s s o c i a t i o n does not occur when we have a c h e l a t i n g diphosphine ligand. But we don't understand why we see hydrogen l o s s i n t h i s system when we don't see i t i n the other cases. DR. MICHAEL BERGKAMP (Brookhaven N a t i o n a l Lab): Do you have any d i r e c t evidence which i n d i c a t e s t h a t when you i r r a d i a t e these compounds the c h e l a t e r i n g a c t u a l l y opens up and c l o s e s ? DR. GEOFFROY:

No.

That i s pure s p e c u l a t i o n .

DR. BERGKAMP: When you i r r a d i a t e these compounds a t 366 nm, do you know e x a c t l y what type of s t a t e you are i r r a d i a t i n g ? DR. GEOFFROY: The 366 nm i r r a d i a t i o n s are c l e a r l y i n t o the lowest a b s o r p t i o n band. I t i s o f t e n a t a i l i n these complexes. U n f o r t u n a t e l y , the e l e c t r o n i c s t r u c t u r e s of these compounds, as w i t h most a l l o r g a n o - m e t a l l i c systems, are not w e l l - u n d e r s t o o d or w e l l - d e f i n e d . We could i n t e r p r e t the a b s o r p t i o n spectrum i n s e v e r a l d i f f e r e n t ways, and we could r a t i o n a l i z e why we see hydrogen, depending on how we i n t e r p r e t the a b s o r p t i o n spectrum. No matter what we do, we can r a t i o n a l i z e hydrogen l o s s . But i t i s not v e r y s a t i s f y i n g , because we do not know what the nature of the e x c i t e d s t a t e s i s . That i s an area which needs c o n s i d e r able study, t o d e f i n e the e l e c t r o n i c s t r u c t u r e s of these kinds of compounds. DR. ANTHONY POË ( U n i v e r s i t y of Toronto): As Dr. G e o f f r o y mentioned, there i s a f a i r b i t of work b e i n g done on d i n u c l e a r metal-metal bonded carbonyls but r a t h e r l e s s on metal c l u s t e r s [Geoffroy, G. L.; Wrighton, M. S., "Organometallic Photochem i s t r y , " Academic P r e s s : New York, 1979]. We have been i n t e r e s t ed f o r some time i n the thermal fragmentation of metal c l u s t e r s , and have r e c e n t l y looked a t some photochemical r e a c t i o n s as w e l l . I would l i k e to p r e s e n t some r e s u l t s here today which are very preliminary. Previous r e s u l t s i n d i c a t e d t h a t quantum y i e l d s f o r reac t i o n s of metal c l u s t e r s are low [ G r a f t , J . L. ; Sanner, R. D. ; Wrighton, M. S. J . Am. Chem. Soc. 1979, 101, 273; T y l e r , D. R. ; A l t o b e l l i , M. ; Gray, H. B. i b i d . 1980, 102, 3022]. That seemed to us a s u r p r i s i n g o b s e r v a t i o n . We a l s o n o t i c e d when we looked



382


at the l i t e r a t u r e t h a t almost none o f the photochemical s t u d i e s reported had i n v e s t i g a t e d competing k i n e t i c pathways and we decided t o study the dependence o f quantum y i e l d s on concen t r a t i o n s . I t seems a f a i r l y obvious t h i n g t o do, but i t hadn't been done p r e v i o u s l y . When we took ruthenium dodecacarbonyl and s t u d i e d i t s photochemical r e a c t i o n s w i t h t r i p h e n y l phosphine, we observed an i n c r e a s e i n the quantum y i e l d w i t h i n c r e a s i n g t r i p h e n y l phos phine c o n c e n t r a t i o n . The p l o t i s curved and appears t o be approaching a l i m i t i n g v a l u e o f φ. T h i s i m p l i e s t h a t t h e r e i s a r e a c t i v e i n t e r m e d i a t e which can undergo c o m p e t i t i v e r e a c t i o n , e i t h e r i n the forward d i r e c t i o n (where the r a t e term i s k ^ [ L ] ) or i n the reverse d i r e c t i o n (where the i n t e r m e d i a t e r e v e r t s back to the r e a c t a n t w i t h a r a t e constant k ). Thus, r ' φ

=

φαίπΟ^Ι]/^

1 + k [L]/k The two k i n e t i c parameters (φ(ΐίιη) and k^/k^) can be r e solved q u i t e e a s i l y by means o f a simple i n v e r s e p l o t . The l i m i t i n g quantum y i e l d f o r L = PPh« i s q u i t e s u b s t a n t i a l , having a v a l u e of about 0.15 ± 0.03. The l i m i t i n g quantum y i e l d s f o r L = PPh^ or P(OPh)^ are not f

r

dependent on whether the s o l v e n t i s cyclohexane or benzene. I f t h a t i s g e n e r a l l y t r u e , then we f i n d t h a t the c o m p e t i t i o n para meter i s v e r y dependent on s o l v e n t . That may not be s u r p r i s i n g . I t a l s o appears t h a t the l i m i t i n g quantum y i e l d may a c t u a l l y be dependent on the nature o f the l i g a n d . That i s s u r p r i s i n g . We must study more l i g a n d s t o determine whether t h a t c o n c l u s i o n i s correct. There i s c l e a r l y a wavelength dependence as w e l l . Going from 436 nm t o 313 nm, the quantum y i e l d f o r L = CO i n creases s i g n i f i c a n t l y . For the r e a c t i o n o f R u ( C 0 ) ( P - n - B u > w i t h P-n-Bu^ under 3

9

3

3

argon, we observe a decrease i n quantum y i e l d w i t h i n c r e a s i n g [P-n-BUg] w i t h φ reaching a lower l i m i t i n g v a l u e o f approx. 0.01. Under carbon monoxide, we o b t a i n a v e r y low quantum y i e l d indeed. Thus, i n t h i s system, the photochemical fragmen t a t i o n i s preceded e i t h e r by d i s s o c i a t i o n o f phosphine o r by d i s s o c i a t i o n o f CO, and not by some other process. The whole q u e s t i o n o f what the i n t e r m e d i a t e s a r e , how they r e a c t , how s o l v e n t - s e n s i t i v e they a r e , and what determines t h e l i m i t i n g quantum y i e l d s i s u n c l e a r a t t h i s time and t h e r e i s much s y s t e m a t i c work y e t t o be done. DR. WILLIAM TR0GLER (Northwestern U n i v e r s i t y ) : I wish t o make a b r i e f p r e s e n t a t i o n o f some r e s u l t s which r e l a t e t o Dr. Geoffroy's t a l k . We have been employing a d i f f e r e n t approach t o generate c o o r d i n a t i v e l y unsaturated c e n t e r s , u t i l i z i n g a reduc-



15.

GEOFFROY


Polyhydride

Complexes

383

t i v e e l i m i n a t i o n r e a c t i o n . The system i n v o l v e s the o x a l a t e ligand. I t i s well-known i n the photochemistry o f Werner com p l e x e s c o n t a i n i n g o x a l a t e t h a t one can p h o t o r e d u c t i v e l y e l i m i nate o x a l a t e . The f e r r i o x a l a t e actinometer i s a c l a s s i c ex ample. The photochemical r e a c t i o n involves a one-electron r e d u c t i o n o f the metal w i t h o x a l a t e , generating o x a l a t e anion r a d i c a l as an i n t e r m e d i a t e , which can then t r a n s f e r a second e l e c t r o n t o another metal. The t r i c k i s t o use a metal t h a t can undergo a t w o - e l e c t r o n r e d u c t i o n . T h i s was a c t u a l l y f i r s t observed by Blake and Nyman [Blake, D. M. ; Nyman, C. J . J . Chem. S o c , Chem. Commun. 1969, 483] about a decade ago, when they d i d some work on the P t ( I ^ ) C 0 ^ complex. I t appeared t h a t t h e i r r e s u l t s could be 3

2

2

r a t i o n a l i z e d by formation o f a Ρί(ΡΦ«) i n t e r m e d i a t e , which f o r m a l l y i s a species t h a t possesses two vacant c o o r d i n a t i o n sites. We went back and looked a t t h a t system, because we thought i t had q u i t e a b i t o f promise. I t turns out t h a t , w i t h t r i p h e n y l phosphine, platinum(O) w i l l a t t a c k t h e phosphorous phenyl bonds t o produce r e d polymers which are phosphido-bridged p l a t i num s p e c i e s . But we were more i n t e r e s t e d i n the s m a l l a l k y l phosphines, because we expect those t o make the metal center even more r e a c t i v e (being s t e r i c a l l y unhindered and more b a s i c ) . These l a t t e r systems, i n f a c t , work very w e l l . F o r exam p l e , when P t i P E t ^ ^ ^ O ^ ) i s i r r a d i a t e d a t 313 nm, we q u a n t i t a t i v e l y extrude 2 moles o f CO^ and generate a r e a c t i v e P t C P E t ^ ) ^ 2

fragment. Thus f a r , we have s t u d i e d about 15 d i f f e r e n t reac t i o n s i n which t h i s type o f intermediate i s generated. One o f the s i m p l e s t species which i l l u s t r a t e s t h i s p o i n t i s the Pt(H C=CH )(PEt^) complex where the i n t e r m e d i a t e has been 2

2

2

q u a n t i t a t i v e l y trapped as the zero v a l e n t ethylene complex. In summary, i t appears t h a t we are able t o i n i t i a t e much i n t e r e s t i n g chemistry by u s i n g photoreductive e l i m i n a t i o n o f coordinated o x a l a t e t o generate a metal center having two vacant coordination s i t e s .


Mechanistic Aspects of Inorganic Reactions - ACS Publications

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