Photochemical Intermediates - ACS Symposium Series (ACS

3 Photochemical Intermediates J. J . T U R N E R and M. P O L I A K O F F

Downloaded by MONASH UNIV on April 22, 2013 | http://pubs.acs.org Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch003

University of Nottingham, Department of Chemistry, Nottingham NG7 2RD England

Few would deny the importance of photochemistry few would further deny that full exploitation of photochemistry demands an intimate knowledge of the identity, structure and behavior of the appropriate intermediates. In this article we review some of the methods used to obtain this information, and the techniques are illustrated with some detailed examples taken from Organometallic - mostly metal carbonyl - Photochemistry (1). Since this symposium looks towards the 21st century(!) a few speculations on future experiments are included. C l e a r l y , mechanistic i n v e s t i g a t i o n s can provide circumstanti a l evidence f o r the p a r t i c i p a t i o n of p a r t i c u l a r intermediates i n a r e a c t i o n but, here, we are concerned with the d e f i n i t i v e observat i o n of these s p e c i e s . I f the intermediates are r e l a t i v e l y s t a b l e then d i r e c t s p e c t r o s c o p i c observation of the species during a room-temperature r e a c t i o n may be p o s s i b l e . As a r a t h e r extreme example of t h i s , the zero-valent manganese r a d i c a l s , Mn(CO)3L2 (L = phosphine) can be photochemically generated from Mn2(CO)3L2, and, i n the absence o f O2 o r other r a d i c a l scavengers, are s t a b l e i n hydrocarbon s o l u t i o n f o r s e v e r a l weeks C 2 , 3 ) . However, we are u s u a l l y more anxious to probe r e a c t i o n s i n which unstable i n t e r mediates are p o s t u l a t e d . There are, broadly speaking, three approaches - continuous generation, instantaneous methods and matrix isolation. Continuous generation simply means that the intermediate i s continuously replenished by; some method and examined under pseudoe q u i l i b r i u m c o n d i t i o n s . For i n s t a n c e , Whyman (4) was able, u s i n g a s p e c i a l IR c e l l working at high pressure and temperature, t o monitor the behavior of s e v e r a l species of importance i n the thermal hydroformylation c a t a l y t i c c y c l e . S i m i l a r l y , Koerner von Gustorf and colleagues (5) have monitored the photochemical

0097-6156/83/0211-0035$07.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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CENTURY

production of s u b s t i t u t e d i r o n carbonyls by IR spectroscopy at -60OC. In the most popular instantaneous technique, f l a s h photolysis, a b u r s t o f l i g h t (or i n pulse r a d i o l y s i s , a pulse o f e l e c t r o n s ) generates a high concentration of intermediates whose disappearance i s monitored by k i n e t i c spectroscopy. There are very many a p p l i c a t i o n s o f t h i s technique to Organometallic Chemistry but the great problem i s that, using UV methods i n s o l u t i o n chemistry, i t i s almost impossible to i d e n t i f y an intermediate p o s i t i v e l y . Unfortunately, conventional IR spectroscopy, which could provide t h i s i d e n t i f i c a t i o n , i s much too slow to monitor r e a l l y f a s t r e a c t i o n s . There are, however, l i k e l y to be t e c h n i c a l developments which w i l l overcome t h i s problem. Fourier-Transform IR w i t h i t s a b i l i t y to acquire data q u i c k l y w i l l prove i n c r e a s i n g l y valuable f o r f a i r l y slow r e a c t i o n s . Thus, using FTIR, Chase and Weigert (6) have monitored the c a t a l y s t l i f e t i m e i n the Fe(C0)5~ c a t a l y z e d o l e f i n i s o m e r i z a t i o n , and Kazlauskas and Wrighton (7) have c h a r a c t e r i z e d the tfjorcodentate intermediate i n the photochemi c a l formation of t e t r a c a r b o n y l ( 4 , 4 - d i a l k y l - 2 ^ b i p y r i d i n e ) metal from M(C0)5« For genuinely f a s t r e a c t i o n s the recent experiments o f Schaffner and colleagues (8) look extremely promising; using f a s t IR detectors and a "point-by-point flash p h o t o l y s i s technique they have been able t o probe the s h o r t - l i v e d intermediates i n the s o l u t i o n chemistry of Cr(CO)£. There i s a l s o the p o s s i b i l i t y of e x p l o i t i n g pulsed Resonance Raman spectroscopy, but there w i l l be problems with p h o t o l y s i s and fluorescence being more s i g n i f i c a n t than Raman s c a t t e r i n g (9). Stopped flow and r e l a t e d techniques may be considered as a "slow form of instantaneous techniques. f

11

11

In both continuous generation and flash photolysis, the intermediate s t a r t s to r e a c t as soon as i t i s formed and i f i t i s very r e a c t i v e (e.g. "naked" Cr(C0)5, Ni(CO)3) i t may not be p o s s i b l e to detect. I f there i s a s i g n i f i c a n t a c t i v a t i o n energy f o r these react i o n s , r e d u c t i o n i n temperature may slow the r e a c t i o n r a t e to such an extent that the intermediate can be examined by more " l e i s u r e l y spectroscopic methods (e.g. IR), but simply lowering the temperatures w i l l be i n e f f e c t i v e f o r very r e a c t i v e species which have a c t i v a t i o n energies close to zero. However, i f the species can be i s o l a t e d i n a r i g i d matrix, then r e a c t i o n may be prevented a l t o gether by i n t r o d u c i n g a higjh a c t i v a t i o n energy f o r d i f f u s i o n . The technique of matrix isolation - e i t h e r i n frozen glasses a t 77K or i n frozen noble gases a t *\>4-20K - has provided s t r u c t u r a l informat i o n about many organometallic intermediates (10-13), but, of course, without any k i n e t i c or e n e r g e t i c data. In what f o l l o w s , we s e l e c t a number of systems o f current i n t e r e s t and t r y to i l l u s t r a t e the r e l a t i o n s h i p between the r e s u l t s obtained by the various techniques mentioned above.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

3.

TURNER

AND

Photochemical

POLIAKOFF

Intermediates

37

Cr(CO)s...X and Related Species I t i s g e n e r a l l y b e l i e v e d (1) that the photochemical t i o n r e a c t i o n s of Cr(CO)£ can be summarised: Cr(C0)

hv 6

Cr(C0)5 + CO — C r ( C 0 )

5

L

substitu-

+ CO

However, i t w i l l become apparent that the process i s more complex than t h i s simple scheme suggests. The o b j e c t i v e i s t o determine the s t r u c t u r e of the intermediate, examine i t s behavior and to unravel the photochemical processes. This can only be achieved by a combination of techniques.


f

M a t r i x S t u d i e s . In the e a r l y 60 s Sheline and co-workers (14,15) used IR spectroscopy to f o l l o w the photochemical behavior of f r o z e n s o l u t i o n s of M(CO)fc i n glasses at 77K. C l e a r evidence f o r the generation of carbonyl fragments was obtained but the conclusions were n e c e s s a r i l y somewhat s p e c u l a t i v e . Perutz and Turner (16) d e f i n i t i v e l y e s t a b l i s h e d the s t r u c t u r e of the primary photoproduct, Cr(C0)5, by UV p h o t o l y s i s of Cr(CO)£ i n noble gas and methane matrices at 20K. Figure 1 i l l u s t r a t e s both IR and v i s i b l e s p e c t r a . C e r t a i n features o f t h i s work are worth n o t i n g . The stoichiometry, symmetry and even bond angles of the carbonyl fragments c o u l d be determined r e l i a b l y , u s i n g i s o t o p i c l^CO Cl8o enrichment combined with the simple frequency f a c t o r e d force f i e l d (17) and simple i n t e n s i t y arguments (18). Thus, the primary photoproduct i s Cr(C0)5 with C4 symmetry, i n agreement with t h e o r e t i c a l predictions.(19) [Preparing i s o t o p i c a l l y enriched carbonyls and r e l a t e d species i s not always a t r i v i a l problem. We have r e c e n t l y developed (20) a method which looks p a r t i c u l a r l y promising i n some cases. CW CO2 l a s e r i r r a d i a t i o n of a gas phase mixture c o n t a i n i n g S F 5 as an energy t r a n s f e r agent can promote thermal chemistry without complications due to w a l l r e a c t i o n s , e.g. V

Fe(C0)

5

+ 13co + SFe

C02

laser

Fe(12 0) - (13co) C

5

x

x

The i s o t o p i c a l l y enriched carbonyl can be separated from the CO and S F 5 i n a c i r c u l a t i n g vacuum system.] I t was observed that p h o t o l y s i s could be reversed by i r r a d i a t i o n of the matrix w i t h l i g h t at a wavelength corresponding to the v i s i b l e absorption band of Cr(C0)5.

Although the IR spectrum of Cr(C0)5 was only s l i g h t l y dependent upon the matrix, the p o s i t i o n of the v i s i b l e band of Cr(C0)5 was extremely s e n s i t i v e to matrix (21,22). Figure 2 i l l u s t r a t e s t h i s e f f e c t . Mixed matrix experiments were used to prove that t h i s e f f e c t a r i s e s from i n t e r a c t i o n between the matrix species and the




cm Figure 1.

T O W A R D T H E 21 ST

CENTURY

nm

1

Photolysis of Cr(CO)

6

in CH at 20 Κ (IR and visible spectra). k

Key: top, deposition of Cr(CO)$ (Τι* mode marked 6); middle, 15 s photolysis with unfiltered Hg arc showing production of Cr(CO)s and molecular CO [Cr(CO), has three IR bands (marked A A and E) and a visible band (marked 5), and the UV band of Cr(CO)$ is not shown]; and bottom, 2 min photolysis with Hg arc + λ >375-nmfilter,showing regeneration of Cr(CO) . The spectra above 2050 cm' and the visible spectra are taken with about five times as much material as the spectra below 2050 cm' . (Reproduced from Ref. 21. Copyright 1975, American Chemical Society.) h

h

1

s

1

Ne SF CF

6

4

Ar Kr Xe CH

4

glass

ι!

1 i 1.1 500

550

600 nm

INCREASING STRENGTH OF Cr(CO) ...X INTERACTION 5

Figure 2. Diagrammatic representation of the position of the visible band of Cr(CO) in different matrices (plot is linear in cm' ). (Reproduced from Ref. 21. Copyright 1975, American Chemical Society.) 1

5


3.

T U R N E R A N D POLiAKOFF

Photochemical

39

Intermediates

empty 6th c o o r d i n a t i o n s i t e o f Cr(C0)5. Of these species the weakest i n t e r a c t i o n w i l l c l e a r l y i n v o l v e Cr(C0)5-..Ne and the strongest Cr(C0>5...CH4, so although the s h i f t i n i s due mostly to p e r t u r b a t i o n o f o r b i t a l s only populated i n the excited state (12,13), the trend i n X a l s o r e f l e c t s grotmd state i n t e r a c t i o n (23). This i n t e r a c t i o n i s o f very great importance i n understanding the s o l u t i o n behavior o f Cr(C0)5 - i n p a r t i c u l a r , i t suggests that no solvent w i l l be completely innocuous towards "naked Cr(C0)5 and that t r a c e i m p u r i t i e s may be extremely important i n the photochemistry. An important technique f o r probing the photochemical mechanism involves the combination o f matrix i s o l a t i o n and the use o f plane p o l a r i z e d l i g h t f o r both p h o t o l y s i s and IR/UV-vis spectroscopy (24-26). The p o l a r i z e d p h o t o l y s i s can generate p r e f e r e n t i a l l y o r i e n t e d molecules i n the matrix which d i s p l a y d i c h r o i c absorption of l i g h t . In the absence of photochemical s t i m u l a t i o n , the molecules are h e l d r i g i d l y i n the matrix cage and so the photochemically induced dichroism can subsequently be probed by s p e c t r o s c o p i c techniques. [This method i s somewhat akin t o the use of nematic solvents by Gray and colleagues (27) f o r the assignment o f UV s p e c t r a , e.g. Mn2(CO)io-J In t h i s way i t was shown that the v i s i b l e absorption band i n Cr(C0)5...X has a t r a n s i t i o n moment of Ε symmetry and the d e t a i l e d photochemical behavior could be explained by the scheme shown i n Figure 3. I t w i l l be noted that t h i s mechanism i n v o l v e s the i n i t i a l generation from Cr(C0)£ o f C4 Cr(C0)5 i n an e x c i t e d s i n g l e t s t a t e ( Ε ) ; t h i s matches Hay's c a l c u l a t i o n s , (28) although i t has caused some s u r p r i s e (29). The e x c i t e d C4 molecule decays via a s t r u c t u r e t o ground s t a t e C4 0-k]) and subsequent photochemically induced i n t e r c o n v e r s i o n between Cr(C0)5 i n d i f f e r ent o r i e n t a t i o n s proceeds v i a t h i s same U 3 intermediate. Thus the regeneration o f Cr(C0)£ from Cr(C0)5 a r i s e s because the Cr(CO)> fragment i s " s t i r r e d " by the i r r a d i a t i o n u n t i l the empty coordina t i o n s i t e encounters the o r i g i n a l l y e j e c t e d CO. This mechanism i s r e l e v a n t to s o l u t i o n chemistry and perhaps f o r t u i t o u s l y p r e d i c t s a quantum y i e l d f o r Cr(CO)£ _ J Û L ^ C r ( C 0 ) 5 ..X o f h which i s i n exact agreement with a recent determination (30) o f the quantum y i e l d f o r the s o l u t i o n r e a c t i o n o f Cr(C0)6 w i t h pyridine. m a x

tf

fl


11

V

Ί

V

V

n

:

The matrix experiments thus r e v e a l some complex photochemistry of relevance t o s o l u t i o n chemistry but the experiments do not provide information about k i n e t i c s . For t h i s we need a f l u i d medium e.g. gas o r l i q u i d , and we consider such experiments i n the next two s e c t i o n s . F l a s h p h o t o l y s i s suggests i t s e l f as the technique f o r d e t e c t i n g a species as r e a c t i v e as Cr(C0)5 but before d e s c r i b i n g these experiments we show what can be achieved from low-temperature s o l u t i o n s . Low-temperature S o l u t i o n s . The matrix s p e c t r o s c o p i c data f o r Cr(C0)5...X (Figure 2) suggest that the i n t e r a c t i o n between Cr(C0)5 and s a t u r a t e d hydrocarbons may be q u i t e s u b s t a n t i a l ,


40

CHEMISTRY:

TOWARD

T H E 21ST

CENTURY


INORGANIC

CO

CO

Figure 3. Scheme of the photochemical behavior of M(CO) in a mixed N /Ar matrix via a trigonal bipyramidal intermediate. Key: O, the matrix cage; and *, *E excited state of Cr(CO) . (Reproduced from Ref. 25. Copyright 1978, American Chemical Society.) 5

5


t


3.

T U R N E R A N D POLiAKOFF

Photochemical

Intermediates

41

perhaps large enough to s t a b i l i z e Cr(C0)5...(hydrocarbon) species at low temperatures i n f l u i d s o l u t i o n s . T y l e r and P e t r y l a k (31) have indeed shown, using IR spectroscopy, that i n e x t r a o r d i n a r i l y w e l l p u r i f i e d methylcyclohexane the species M(C0)5...MCH (M = Cr, Mo, W; MCH = methylcyclohexane) can be photochemically generated w i t h a l i f e t i m e of about an hour at -780C. The degree of p u r i t y of MCH r e q u i r e d i s extremely h i g h s i n c e even t r a c e concentrations of unsaturated hydrocarbons promote the production of TT-olefin complexes. Although there i s l i t t l e doubt that T y l e r and P e t r y l a k produced Cr(C0)5...MCH, a more s e n s i t i v e t e s t of these weak i n t e r a c t i o n s i s U V - v i s i b l e spectroscopy (21) and i t i s hoped such measurements w i l l be made. We have r e c e n t l y been i n v e s t i g a t i n g an a l t e r n a t i v e approach, liquid noble gases. These seem to be i d e a l solvents f o r i n v e s t i g a t i n g organometallic photochemistry; the noble gases are complete l y transparent over a very wide s p e c t r a l range (hence long pathlengths are p o s s i b l e ) . By working i n a s p e c i a l l y designed pressure c e l l (32) i t i s p o s s i b l e to cover the temperature range ^80K to 240K, using Ar, Kr o r Xenon, and these noble gas s o l v e n t s , p a r t i c u l a r l y Ar (Figure 2 ) , are l i k e l y to be weakly i n t e r a c t i n g . S u r p r i s i n g l y , metal carbonyls are s o l u b l e i n these l i q u i d s e.g. Cr(C0)6 i n l i q u i d Xe, and Fe(C0)5 even i n l i q u i d Ar. Our i n i t i a l experiments have i n v o l v e d N2 complexes, s i n c e these are expected to be moderately s t a b l e , and i n the case o f Cr(C0)5N2, allow comparison w i t h both matrix i s o l a t i o n and room temperature f l a s h p h o t o l y s i s experiments. We have generated (33,34) the species C r ( C O ) 5 - ( N 2 ) (x = 1 to 5) by p h o t o l y s i s of Cr(C0)6 i n liquid xenon at -80°C doped w i t h N2. Figure 4 shows both the p h o t o l y t i c production of Cr(CO)5N2 and also photochemical regene r a t i o n of Cr(C0)6, i . e . e x a c t l y mimicking the matrix behavior (22). Prolonged p h o t o l y s i s o f Cr(C0)6 i n l i q u i d xenon w i t h h i g h e r concentration of added N2 generates more h i g h l y ^ - s u b s t i t u t e d s p e c i e s . Figure 5 shows how the s t a b i l i t i e s of the d i f f e r e n t C r ( C O ) 6 - ( N 2 ) species depend on the degree of N2 s u b s t i t u t i o n and suggests that k i n e t i c parameters might be o b t a i n a b l e . For thermally less s t a b l e compounds, s e q u e n t i a l p h o t o l y s i s and s p e c t r a l a n a l y s i s are inadequate because the compounds decompose before they are detected. However, using a s p e c i a l 4-way c e l l we have been able to generate the very unstable species Ni(00)3^, by p h o t o l y s i s o f N i (CO) 4 i n l i q u i d Kr doped with N2 at -170OC, and record the IR spectrum during UV i r r a d i a t i o n . On switching o f f the UV lamp the IR spectrum o f Ni (00)3^ decays and by monitoring the r a t e of t h i s decay we have measured k i n e t i c and a c t i v a t i o n x

x

x

x

energy parameters (35) f o r Ni(C0)3N + CO •Ni(CO)4 + N2. UV p h o t o l y s i s of Cr(CO)£ i n pure l i q u i d xenon ( i . e . i n the absence of N2) produced IR bands of a t r a n s i e n t species ( t i ^ 2 sec at -78°C) which may w e l l be unstable Cr(C0)5...Xe. We b e l i e v e that such experiments w i l l provide an important e x t r a window on s o l u t i o n k i n e t i c s monitored by f l a s h techniques to which we now t u r n . 2



ΙΑ C σ

5

2

J ι ι 2250 2230

Cr(CO) N

(a)

5

Wavenumber/ cm"

i

g 2

1940

t

6

Figure 4. Absorption spectra of Cr(CO) N and Cr(CO) dissolved in liquid Xe at —79°C. Key: · · ·, before photolysis; , after 3.3 min irradiation with an unfilteredH g arc lamp placed 28 cm from the center of the cryostat; and , after irradiation for about 12 min with the Hg arc lamp and Balzers 367-nm filter about 14 cm from the center of the cryostat. (Where the curves are indistinguishable, only the dotted curve is shown.) For clarity, the noise has been omitted from the left curve. The feature marked with an asterisk (*) is assigned to Cr(CO) (N ) . (Reproduced with permission from Ref. 33. Copyright 1980, Royal Society of Chemistry.)

60
-C5H5)W(CO)3R i n p a r a f f i n matrices. With R = CH3 the intermediate i s b l u e , but when R contains $ hydrogens the i n t e r mediate i s yellow; by analogy w i t h the Cr(C0)5 i n t e r a c t i o n data (21) i t i s concluded that t h i s yellow intermediate adopts a c y c l i c s t r u c t u r e i n which a $-H i s coordinated back to the metal centre. The most potent species f o r a c t i v a t i n g hydrocarbons are l i k e l y to be naked t r a n s i t i o n metal atoms. Margrave (62) and Ozin (63) have shown that cocondensation of Fe atoms o r Cu atoms w i t h methane at ^10K followed by UV i r r a d i a t i o n produces d e f i n i t e s p e c t r o s c o p i c evidence f o r the i n s e r t i o n of the metal atoms i n t o the C-H bond. I t i s l i k e l y that f u r t h e r experiments along these l i n e s w i l l open up new c a t a l y t i c p o s s i b i l i t i e s . Rearrangements. I t w i l l have been obvious from the Cr(C0)5-.. X d i s c u s s i o n above that the matrix technique i s p a r t i c u l a r l y w e l l s u i t e d to the examination o f photochemically induced rearrangements and i s o m e r i z a t i o n s i n c e the matrix holds the p a r t i c i p a t i n g species i n f i x e d p o s i t i o n s and hence allows s p e c t r o s c o p i c study of each, y e t allows ready photochemical i n t e r c o n v e r s i o n . One of the more d e t a i l e d s t u d i e s of t h i s type (£4) has i n v o l ved the u n r a v e l l i n g o f the rearrangement modes o f the c o o r d i n a t i v e l y unsaturated species W(C0)4CS, which was generated by UV p h o t o l y s i s of W(C0)5CS; the experiments i n v o l v e d IR/UV s p e c t r o s copy, wave-selective p h o t o l y s i s and a n a l y s i s based on ^ C 0 subs t i t u t i o n . The r e s u l t s are summarised i n Figure 7. These experiments also c o n f i r m that W(C0)4CS i s i n i t i a l l y formed i n an e x c i t e d s t a t e , as p r e d i c t e d by the scheme o u t l i n e d i n Figure 3. Even more complex experiments have been performed on matrix i s o l a t e d Fe(C0)4, generated by UV p h o t o l y s i s of Fe(C0)5. Isotopic l a b e l l i n g coupled w i t h CW-CO l a s e r pumping (65) o f the CO s t r e t c h i n g v i b r a t i o n s (\>1900 c u r l ) showed that the rearrangement mode o f Fe(C0)4 follows an i n v e r s e Berry pseudo-rotation as shown i n Figure 8. S i m i l a r l y , i n experiments w i t h Fe(fi0) (generated i n a matrix from Fe(C0)4L; L = NMe3) , the C 3 and C forms of the species can be i n t e r c o n v e r t e d w i t h l i g h t o f appropriatE wavelength(66). V

s


50

INORGANIC C H E M I S T R Y : TOWARD T H E 2 1 S T C E N T U R Y

S

c x

max ~

300 nm

3

0

5

n

m

300 nm 48Sf

S C


432 nm 489 nm

CS

"max 470 nm

max 445 nm /v

Figure 7. Summary of the photochemistry of W(CO) CS in an Ar matrix. (Reproduced from Ref. 64. Copyright 1976, American Chemical Society.) 5

1919 1898

1902

1894

1894

1881

1901 1894 1881

12

î6

13

18

Figure 8. The observed IR-laser induced isomerizations of Fe( C 0) -j( C 0) species in an Ar matrix. X represents the C 0 group, and the numbers represent the wavenumbers of the CO laser lines that induce the particular isomerizations. h

13

18


x

3.

TURNER

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Photochemical

POLIAKOFF

51

Intermediates

Photochemical Routes. F i n a l l y , two sets of experiments from Rest's laboratory which demonstrate the s u b t l e i m p l i c a t i o n s of q u i t e complex matrix photochemistry experiments. The f i r s t (67) i n v o l v e s (r) -C5H5)Co(C0)2 and can be summarised 5

(n5c H )Co(CO)


5

5

290

m

/

C

Q

»*

t T i

l

(n3-c H )Co(CO) 5

5

3

nm

[There was no evidence f o r production of (n^-Cs*^)Co(C0) i n Ar matrices, presumably because of very ready recombination w i t h CO i n the matrix cage. Whether a matrix i s o l a t e d carbonyl molecule can e j e c t CO without automatic recombination probably requires the photochemical path to lead to a ground s t a t e fragment i n which the empty coordination s i t e i s o r i e n t e d away from the photoejected CO - see the Cr(C0)5 photochemical scheme (Figure 3) .] In a CO matrix, therefore, the primary product involves an expand ed c o o r d i n a t i o n number ("ring-slippage") and i t i s argued that such a species i s c o n s i s t e n t with the a s s o c i a t i v e mechanism prop osed f o r room temperature s u b s t i t u t i o n reactions of (r|5-C5H5)Co(C0) . By c o n t r a s t , when (n -C5H5)Fe(C0)2(C0R) i s photolysed (fcâ), even i n CO m a t r i c e s , only CO loss i s observed with (r|5-C5H5)Fe(C0)(C0R) being formed en route to (n5-C5H5)Fe(CO)2R. 2

5

Dinuclear Carbonyls. One o f the c h a r a c t e r i s t i c features i n the UV s p e c t r a of metal-metal bonded d i n u c l e a r carbonyls and s u b s t i t u t e d species i s an intense band i n the 350 nm region (1). For example, f o r Mn2(CO)io t h i s band, at 336 nm, i s assigned t o a t r a n s i t i o n from the f i l l e d σ M-M bonding molecular o r b i t a l to the corresponding antibonding o r b i t a l . Confirmation of t h i s assignment comes from an elegant experiment (27) based on the argument that such a t r a n s i t i o n must be p o l a r i z e d along the Mn-Mh axis ( i . e . of t r a n s i t i o n moment B2) confirmed by examining the d i c h r o i c p r o p e r t i e s of IR and UV bands of MXI2(CO);LO i n a nematic solvent. A great deal of photochemistry of d i n u c l e a r carbonyls i s c o n s i s t e n t with the generation of r a d i c a l s f o l l o w i n g M-M bond schism on i r r a d i a t i o n i n t o the σ — • o*band. However, even f o r the much studied Mri2(CO)io the p i c t u r e i s s t i l l obscure. For i n s t a n c e , pulse r a d i o l y s i s (65) and f l a s h p h o t o l y s i s (70-72) combine to suggest that i n a d d i t i o n to generation of two Mn(C0)5 r a d i c a l s other photochemical routes may i n v o l v e bridged-Mn2(CO)^o> Mn2(CO)9,Mn2(CO)3; however, i n view of the enormous importance o f solvent i n t e r a c t i o n s and i m p u r i t i e s (see Cr(C0)5 above) one wonders what the c o r r e c t explanation i s going to be. Matrix i s o l a t i o n has already provided a v a l u a b l e i n s i g h t i n t o the behavior o f d i n u c l e a r carbonyls. In the f i r s t experiments of t h i s k i n d we were able (ZD to show that, on p h o t o l y s i s i n s o l i d Ar, Fe2(C0)g loses CO to form Fe2(C0)s i n both b r i d g e d and unbridged forms and the behavior of these fragments was s t u d i e d ;


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i n t e r e s t i n g l y Hoffmann (74) has r e c e n t l y drawn a t t e n t i o n to t h i s Fe2(C0)3 species and to Stone's experiments i n v o l v i n g polynuclear complexes of Fe2(CO>3. More r e c e n t l y , Sweany and Brown (75) have shown that UV p h o t o l y s i s of 002(00)3 i n argon generates unbridged Co2(CO)7, while i n CO matrices there i s evidence f o r Co(CO)4. Thus, there i s evidence that matrix studies could help with Mri2(CO)i0> unfortunately, such experiments have so f a r been rather unrevealing. Presumably the Cage E f f e c t encourages the recombination o f any Mn(C0)5 r a d i c a l s generated by p h o t o l y s i s . Indeed the best way of generating Mn(C0)5 photochemically i s by UV p h o t o l y s i s of H*fci(C0)5 i n a CO matrix (54). The s t r u c t u r e o f Mn(C0)5 as a C4 fragment has now been e s t a b l i s h e d (54) by i s o t o p i c IR and, more r e c e n t l y , confirmed by ESR (£6). I t i s , however, our b e l i e f that subtle wavelength-dependent p h o t o l y s i s experiments, p a r t i c u l a r l y using p o l a r i z e d l i g h t , w i l l e v e n t u a l l y unravel the problem of Mri2(CO)io* Another d i n u c l e a r carbonyl which presents i n t e r e s t i n g problems i s [(n -C5H5)Fe(C0)2] 2· photochemistry proceed e x c l u s i v e l y through homolytic f i s s i o n to produce two (1^-05115) Fe(C0)2 r a d i c a l s or by other p o s s i b l e routes? The d i s c u s s i o n of t h i s r e a c t i o n has involved mechanistic and s y n t h e t i c studies (77), f l a s h p h o t o l y s i s (78) and low-tempe rature p h o t o l y s i s (22.) - the l a t t e r work, i n THF or e t h y l c h l o r i d e at -78°C, invokes an i n t e r mediate i n which the Fe-Fe d i r e c t bond i s broken but the two halves of the molecule are h e l d together by a CO b r i d g e . C l e a r l y such an i n t r i g u i n g problem merits more d e t a i l e d i n v e s t i g a t i o n s .


V

5

D

o

e

s

t l i e

Polynuclear C l u s t e r s . The photochemistry of polynuclear c l u s t e r s i s a very a c t i v e f i e l d , p a r t l y because of the p o t e n t i a l c a t a l y t i c importance of these compounds. I t i s s t i l l not c l e a r how to p r e d i c t the conditions which w i l l lead t o d e c l u s t e r i f i c a t i o n as opposed to i n t e r n a l rearrangement or s u b s t i t u t i o n (1). There i s an absence of good d e f i n i t i v e evidence f o r s p e c i f i c intermediates and we c l o s e with a h i n t . Some years ago i n rather crude unpublished work (80) we showed that the p h o t o l y s i s of Fe3(CO)i2 i s o l a t e d i n a matrix l e d to production of some CO, but more p a r t i c u l a r l y , complete disappearance of IR bands due to the b r i d g i n g CO groups and the appearance of new terminal CO IR bands. We b e l i e v e that c a r e f u l studies of t h i s k i n d , t a k i n g advantage of s o p h i s t i c a t e d FTIR methods, w i l l provide valuable i n s i g h t i n t o even complex photochemical intermediates. Acknowledgments We thank a l l those who have had discussions with us and are g r a t e f u l to the S.E.R.C. f o r generous support of our work. We wish to acknowledge the help of a l l our colleagues i n Nottingham, p a r t i c u l a r l y Mr P.W. Lemeunier and Dr M.A. Healy.


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Literature Cited

1. For an excellent recent review see Geoffroy, G.L.; Wrighton, M.S. "Organometallic Photochemistry", Academic Press: New York, 1979. 2. Kidd, D.R.; Cheng, C.P.; Brown, T.L. J. Am. Chem. Soc. 1978, 100, 4103. 3. McCullen, S.B.; Brown, T.L. J. Am. Chem. Soc. in press 4. Whyman, R. J. Organomet. Chem. 1975, 94, 303, and earlier references quoted therein. 5. Koerner von Gustorf, E.A.; Leenders, L.H.G.; Fischler, I; Perutz, R.N. Adv. Inorg. Rad. Chem. 1976, 19, 65. 6. Chase, D.B.; Weigert, F.J. J. Am. Chem. Soc. 1981, 103, 977. 7. Kazlauskas, R.J.; Wrighton, M.S. J. Am. Chem. Soc. in press. 8. Hermann, H.; Grevels, F.W.; Henne, Α.; Schaffner, Κ., to be published. 9. Brus, L.E.; personal communication. 10. Moskovits, M.; Ozin, G.A. Eds.; "Cryochemistry"; Wiley: New York, 1976 11. Barnes, A.J.; Müller, Α.; Orvilie-Thomas, W.J. Eds. "Matrix Isolation Spectroscopy"; Deidel, 1981. 12. Turner, J.J.; Burdett, J.K.; Perutz, R.N.; Poliakoff, M. Pure and Appl. Chem. 1977, 49, 271. 13. Burdett, J.K. Coord. Chem. Rev. 1978, 27, 1. 14. Stolz, I.W.; Dobson, G.R.; Sheline, R.K. J Am. Chem. Soc. 1962, 84, 3589. 15. Stolz, I.W.; Dobson, G.R.; Sheline, R.K. J Am. Chem. Soc. 1963, 85, 1013. 16. Perutz, R.N.; Turner, J.J. Inorg. Chem. 1975, 14, 262. 17. Burdett, J.K.; Poliakoff, M.; Timney, J.Α.; Turner, J.J. Inorg. Chem. 1978, 17, 948. 18. Burdett, J.K. Inorg. Chem. 1981, 20, 2607. 19. Burdett, J.K. "Molecular Shapes"; Wiley-Interscience: New York, 1980. 20. Ryott, G.J. Ph.D., Thesis, Nottingham University, Nottingham, 1982. 21. Perutz, R.N.; Turner, J.J. J. Am. Chem. Soc. 1975, 97, 4791. 22. Burdett, J.K.; Downs, A.J.; Gaskill, G.P.; Graham, M.A.; Turner, J.J.; Turner, R.F. Inorg. Chem. 1978, 17, 523. 23. Demuynck, J.; Kochanski, E.; Veillard, A. J. Am. Chem. Soc. 1979, 101, 3467. 24. Burdett, J.K.; Perutz, R.N.; Poliakoff, M.; Turner, J.J. J.C.S. Chem. Commun. 1975, 157. 25. Burdett, J.K.; Grzybowski, J.M.; Perutz, R.N.; Poliakoff, M; Turner, J.J.; Turner R.F. Inorg. Chem. 1978, 17, 147. 26. Baird, M.S.; Dunkin, I.R.; Hacker, N.; Poliakoff, M.; Turner, J.J. J. Am. Chem. Soc. 1981, 103, 5190. 27. Levenson, R.A.; Gray, H.B.; Ceasar, G.P. J. Am. Chem. Soc. 1970, 92, 3653. 28. Hay, P.J. J. Am. Chem. Soc. 1978, 100, 2411.



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29. Adamson, A.W.; personal communication. 30. Nasielski, J.; Colas, A.J. Organomet. Chem. 1975, 101, 215. 31. Tyler, D.R.; Petrylak, D.P. J. Organomet. Chem. 1981, 212, 289. 32. Beattie, W.H.; Maier, W.B.; Holland, R.F.; Freund, S.M.; Stewart, B. Proc. SPIE (Laser Spectroscopy) 1978, 158, 113. 33. Maier, W.B.; Poliakoff, M.; Simpson, M.B.; Turner, J.J. J.C.S. Chem. Commun. 1980, 587. 34. Maier, W.B.; Poliakoff, M.; Simpson, M.B.; Turner, J.J. Inorg. Chem., in press. 35. Maier, W.B.; Poliakoff, M.; Simpson, M.B.; Turner, J.J. to be published. 36. Bonneau, R.; Kelly, J.M. J. Am. Chem. Soc. 1980, 102, 1220. 37. Lees, A.J.; Adamson, A.W. Inorg. Chem. 1981, 20, 4381. 38. Breckenridge, W.H.; Sinal, N. J. Phys. Chem. 1981, 85, 3557. 39. Tumes, W.; Gitlin, B.; Rosan, A.M.; Yardley, J.T. J. Am. Chem. Soc. 1982, 104, 55. 40. Nathanson, G.; Gitlin, B.; Rosan, Α.; Yardley, J.T. J. Chem. Phys. 1981, 74, 361, 370. 41. Perutz, R.N.; Turner, J.J. J. Am Chem. Soc. 1975, 97, 4800. 42. Poliakoff, M.; Turner, J.J. J.C.S. Faraday Trans. II, 1974, 70, 93. 43. Mahmoud, Κ.A.; Narayanaswamy, R.; Rest, A.J. J. Chem. Soc., Dalton, 1981, 2199. 44. Kazlauskas, R.J.; Wrighton, M.S. J. Am. Chem. Soc. 1980, 102, 1727. 45. Boxhoorn, G.; Stufkens, D.J.; Oskam, A. Inorg. Chim. Acta, 1979, 33, 215. 46. Boxhoorn, G.; Schoemaker, G.C.; Stufkens, D.J.; Oskam, Α.; Rest, A.J.; Darensbourg, D.J. Inorg. Chem. 1980, 19, 3455. 47. Geoffroy, G.L. Prog. Inorg. Chem. 1980, 27, 123. 48. Green, M.L.H. Pure Appl. Chem. 1978, 50, 27. 49. J.C.S. Grebenik, P.; Downs, A.J.; Green, M.L.H.; Perutz, R.N. Chem. Commun. 1979, 742. 50. Chetwynd-Talbot, J.; Grebenik, P.; Perutz, R.N. J.C.S. Chem. Commun. 1981, 452. 51. Sweany, R.L. X Cong. Organomet. Chem., Toronto, 1981, Abstract IC05, 44. 52. Poliakoff, M; Turner, J.J. J.C.S. Dalton, 1974, 2276. 53. Poliakoff, M.J.C.S. Dalton, 1974, 210. 54. Church, S.P.; Poliakoff, M.; Timney, J.A.; Turner, J.J. J. Am. Chem. Soc. 1981, 103, 7515. 55. Sweany, R.L. Inorg. Chem. 1982, 21, 752. 56. Wermer, P.; Ault, B.S.; Orchin, M. J. Organomet. Chem. 1978, 162, 189. 57. Sweany, R.L. Inorg. Chem. 1980, 19, 3512. 58. Parshall, G.W. "Homogeneous Catalysis"; Wiley Interscience: New York, 1980. 59. Janowicz, A.H.; Bergman, R.G. J. Am. Chem. Soc. 1982, 104, 352.



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60. Poliakoff, M. Chem. Soc. Rev. 1978, 7, 528. 61. Kazlauskas, R.J.; Wrighton, M.S. J. Am. Chem. Soc. in press. 62. Billups, W.E.; Konarski, M.M.; Hauge, R.H.; Margrave, J.L. J. Am. Chem. Soc. 1980, 102, 7394. 63. Ozin, G.A.; McIntosh, D.F.; Mitchell, S.A.; Garcia-Prieto, J. J. Am. Chem. Soc. 1981, 103, 1574. 64. Poliakoff, M. Inorg. Chem. 1976, 15, 2022, 2892. 65. Poliakoff, M.; Turner, J.J. "Chemical and Biochemical Applications of Lasers", Moore, C.B. Ed; Academic Press; New York, 1980; Vol. 5, p.175. 66. Boxhoorn, G.; Cerfoutain, M.B.; Stufkens, D.J.; Oskam, A. J.C.S. Dalton, 1980, 1336. 67. Crichton, O.; Rest, A.J.; Taylor, D.J. J.C.S. Dalton, 1980, 167. 68. Fettes, D.J.; Narayanaswamy, R.; Rest, A.J. J.C.S. Dalton, 1981, 2311. 69. Waltz, W.L.; Hackelberg, O.; Dorfman, L.M.; Wojcicki, A. J. Am. Chem, Soc. 1978, 100, 7259. 70. Hughey, J.L.; Anderson, C.P.; Meyer, T.J. J. Organomet. Chem. 1977, 125, C49. 71. Wegman, R.W.; Olsen, R.J.; Gard, D.R.; Faulkner, L.R.; Brown, T.L. J. Am. Chem. Soc. 1981, 103, 6089. 72. Yasufuku, K.; Yesaka, H.; Kobayashi, T.; Yamazaki, H.; Nagakura, S. Proc. 10th Conf. Organomet. Chem., Toronto, 1981, Abstract IC02, p41. 73. Poliakoff, M.; Turner, J.J. J. Chem. Soc. A 1971, 2403. 74. Hoffmann, R. Nobel Lecture, 1981. 75. Sweany, R.; Brown, T.L. Inorg. Chem. 1977, 16, 421. 76. Symons, M.C.R.; Sweany, R.L. Organometallics, in press. 77. Abrahamson, H.B.; Palazzotto, M.C.; Reichel, C.L.; Wrighton, M.S. J. Am. Chem. Soc. 1979, 101, 4123. 78. Caspar, J.V.; Meyer, T.J. J. Am. Chem. Soc., 1980, 102, 7794. 79. Tyler, D.R.; Schmidt, M.A.; Gray, H.B. Inorg. Chem. 1979, 14, 2753. 80. Poliakoff, M. Ph.D., Thesis, Cambridge,University, Cambridge, England, 1972. 81. Welch, J.Α.; Peters, K.S.; Vaida, V. J. Phys. Chem., 1982, 86, 1941. RECEIVED August 3, 1982


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Discussion


A.W. Adamson, U n i v e r s i t y o f S o u t h e r n C a l i f o r n i a : You spoke o f an e x c i t e d s t a t e C^ p r o d u c t . We have n e v e r been a b l e t o o b s e r v e s o l u t i o n p h o t o c h e m i s t r y of a c o o r d i n a t i o n compound i n w h i c h l i g a n d d i s s o c i a t i o n p r o d u c e s an e x c i t e d s t a t e p r o d u c t . F u r t h e r , w h i l e c h e m i l u m i n e s c e n t r e a c t i o n s a r e known, t h e ones i n v o l v i n g c o o r d i n a t i o n compounds p r o d u c e l i g h t o n l y i n q u i t e low y i e l d , i n d i c a t i n g t h a t t h e p a t h t h r o u g h an e x c i t e d s t a t e i s n o t a f a v o r e d one. JDo you know o f any d i r e c t e x p e r i m e n t a l e v i d e n c e f o r t h e [C^ ] p r o d u c t ?

J . J . Turner. U n i v e r s i t y of Nottingham: The short answer i s No(!); but the f i r s t point to make i s that we don't claim that C4v Cr(C0>5 i n an excited s t a t e i s a 'product' - undoubtedly the 'product' i s C4 Cr(C0>5 i n the ground s t a t e . What the experiments t e l l us i s t h i s . The p o l a r i z e d p h o t o l y s i s / s p e c t r o s c o p i c experiments ( r e f . 24,25 of above) prove that the Cr(C0>5X species are ' s t i r r e d ' by v i s i b l e l i g h t , f i r s t v i a an e x c i t e d s t a t e of the C4 Cr(C0>5X and then, almost c e r t a i n l y , v i a the D3 intermediate s t a t e (see f i g u r e 9 i n r e f . 25), i . e . a l l the pathways i n f i g u r e 3 (above) are e s t a b l i s h e d except that from Cr(C0)5 to D3 Cr(C0)5 v i a the e x c i t e d C4 fragment. However, i n matrix experiments, my colleague, Martyn P o l i a k o f f ( r e f . 64 above), showed that p h o t o l y s i s of s t e r e o s p e c i f i c a l l y ^ço-labelled trans( C0)W(C0)4CS y i e l d s , as the major product, square-pyramidal cis-(' C0)W(C0)3CS with the CS group i n the a x i a l p o s i t i o n , i . e . the p r i n c i p a l path involves l o s s of e q u a t o r i a l CO and rearrangement of the remaining CS and four CO groups. Whether t h i s i s imagined as CO loss followed by rearrangement ( i . e . analogous to the e x c i t e d s t a t e path i n f i g u r e 3)or as a concerted process does not a f f e c t the argument - we c e r t a i n l y don't consider the e x c i t e d C4 fragment as a ' t h e x i ' s t a t e , we j u s t f i n d i t e a s i e r to p i c t u r e the 6 > 5 process v i a the e x c i t e d fragment. V

V

n

n

V

13

L3

V

G.A. O z i n , U n i v e r s i t y of T o r o n t o ; I n our Cr/CO m a t r i x c o c o n d e n s a t i o n e x p e r i m e n t s (Angew. Chem., I n t . E d . Eng. 1975, 14, 2 9 2 ) , we r e p o r t e d e v i d e n c e f o r t h e f a c i l e f o r m a t i o n of a binuclear chromium c a r b o n y l complex Cr (CO)i or Cr (CO)x which c o u l d be d e s c r i b e d a s s q u a r e p y r a m i d a l C r ( C O ) weakly i n t e r a c t i n g w i t h e i t h e r a Cr(CO) or Cr(CO) moiety i n the v a c a n t ( s i x t h ) s i t e . As a r e s u l t , t h e i n f r a r e d s p e c t r u m of t h i s " w e a k l y - c o u p l e d " b i n u c l e a r s p e c i e s c l o s e l y r e s e m b l e d t h a t of the mononuclear f r a g m e n t C r ( C 0 ) . I w o u l d l i k e t o ask y o u , w h e t h e r o r n o t y o u h a v e any e v i d e n c e f o r t h e e x i s t e n c e of such a binuclear s p e c i e s i n y o u r C r ( C O ) /Xe cryogenic solutions following various photolysis treatments. 2

0

2

5

5

6

5

6


x


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J . J . Turner, u n i v e r s i t y o f Nottingham: You are q u i t e correct that species such as Cr(C0>5...X, where X i s a weakly c o o r d i n a t i n g 'ligand* such as Ar, Xe, CH4, Cr(C0>6, show b a r e l y distinguish able IR s p e c t r a i n the carbonyl region. However, the v i s i b l e band i s extremely s e n s i t i v e t o X whether measured i n matrices (Ar, 533 nm; Xe, 492 nm; CH4, 489 nm (see r e f . 21 above); Cr(CO)£, ^460 nm ( J . Amer. Chem. S o c , 1975, 97^ 4805)) o r i n s o l u t i o n (cyclohexane, 510 nm; Cr(C0)6, 485 nm (see r e f . 36 above)). The p o s i t i o n of t h i s band r e f l e c t s the strength of Cr(C0)5/X i n t e r a c t i o n which suggests that Cr(C0)£ and Xe w i l l i n t e r a c t s i m i l a r l y with Cr(C0)5; given that i n l i q u i d Xe, Cr(C0)6 i s extremely d i l u t e (^1 ppm) i t seems u n l i k e l y that one would observe Cr(C0)5... Cr(C0)6 i n preference to Cr(C0)5...Xe. Nevertheless, proof of t h i s must await UV data which we are i n the process o f o b t a i n i n g . Two f u r t h e r r e l a t e d comments. F i r s t l y , s i n c e Ν2 i s a "good" l i g a n d ( X , 364 nm) , w i t h N2~doped Xe there i s no trace even of Cr(C0)5...Xe s i n c e Cr/C0/N2 species predominate. Secondly, during experiments w i t h Ni(CO)4/N2/liquid Kr (see above), p h o t o l y s i s i n the complete absence of d i s s o l v e d N2 l e d t o the appearance o f a t r a n s i e n t carbonyl species w i t h IR bands s i m i l a r t o those assigned to NÎ2(C0)7 (J.E. Hulse and M. Moskovits; unpublished data) presumably the i n t e r a c t i o n of Ni(CO)3 with Ni(CO)4 i s considerably stronger than w i t h Kr. m a x


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