Transient species in the pulse radiolysis of cyclohexane solutions of group VI metal carbonyls

Radiat. Phys. Chem. 1979, Vol. 13, pp. 133-138 Pergamon Press Ltd., Printed in Great Britain

TRANSIENT SPECIES IN THE P U L S E RADIOLYSIS OF C Y C L O H E X A N E SOLUTIONS OF GROUP VI METAL CARBONYLS LUCIA FLAMIGNI C.N.R., Laboratorio di Fotochimica e Radiazioni d'alta enerl~, Via De'Castagnoli 1, 40126 Bologna, Italy (Received 15 June 1978)

Abstract~Pulse radiolysis of cyciohexane solutions of hexacarbonyis of chromium, molybdenum and tungsten yields transient visible absorptions with A.~, respectively, at 505 nm, 410 nm, 415 nm assigned by comparison with photochemical data, to the related pentacarbonyl. The following reactions: M(CO)6 + e~,~

) M(CO)~-

M(CO)6- + C6H ~2

, M(CO)5 + CO + C6I-I,2

have been identified as the main mechanism responsible for pentacarbonyl formation by pulse radiolysis. In argon saturated solutions pentacarbonyls decay by first order kinetics; the rates are independent of radiation doses and hexacarbonyl concentration but somewhat a~ected by the origin of cyclohexane used. Hypotheses on the product are advanced. Reaction of pentacarbonyls with cyclohexene, hydrogen, nitrogen and carbon oxide are detected and reaction rates measured. Spectra of transient M(CO)~.6Ha0 and M(CO)sN2 are reported.

INTRODUCTION THE PHOTOLYSIS of chromium, molybdenum and tungsten hexacarbonyls has been studied in hydrocarbon giassesC'-?~ and rare gas matrices. ~s~ The product has been identified, on the basis of its IR spectra, as the C ~ structure of the related pentacarbonyL The appearance of new IR bands on melting the glass has been initially attributed by Stolz et al. ¢I) to the D3h isomer; however recently this result has been explained by the formation of polynuclear species, c2'9~ Qualitative photolysis studies have been reported on cyclohexane solutions of these compounds (*'le~and more recently flash photolysis data have been published on Cr(CO)s cyclohexane solutions. In these experiments Nasielski et al. °) detected an absorption with AM, = 483 nm, decaying to form a further species with A=u = 438 nm. They assigned these absorptions respectively to the C4, and C2,, rather than D3,, structure of Cr(CO)s, b y analogy with the experiments in hydrocarbon glasses/~) More recently Von Gustorf e! af. "s~ using a flash photolysis apparatus with a

better time resolution detected a first absorption with A=u = 503nm decaying to form a species with A , ~ = 445 rim. They assigned the absorptions to Cr(CO)s, on which structure no hypothesis was made, and to the reaction product of the pentacarbonyl with a solvent impurity, supposed to be an oxygen containing compound. Data on the reactions of the photo-adduct Cr(CO)s with various iigands have been also reported. (n~ In some respects ionizing radiations are expected to give similar results to photoexcitation as already reported for other photochromic compounds. ~t3~The aim of the present work is to study the effect of electron irradiation on cyclohexane solutions of V I group metal c a r b o n y l s a n d to point out analogies and differences between the radiolysis and photolysis of these solutions. A complicating feature of our experiments in comparison with the photochemical ones is the production of a wide variety of cyciohexane radiolysis products, i.e. excited states, cyclohexene, hydrogen, cyclobexyl radical, bicyciohexyl°'~'2j> which are able to react with the solute and its intermediates. 133

L. FLAMIGN!

134

i

EXPERIMENTAL

i

!

!

Materials The carbonyls Cr(CO)s, W(CO)6 (Ventron) and Mo(CO), (Societ~ Intermedi Ospiate) were sublimed twice. Cyclohexane (Merck p.a.) was passed through a ! m column of freshly activated silica gel and stored under argon before use. Methanol (Merck) was purified by the method used by Baxendale and Wardman for 2-propanol.~'~ Carbon monoxide, argon, nitrogen and hydrogen were of the purest grade available and were used as purchased.

0.04,

~o

0.03

"o

~O.~,

Sample preparation and irradiations Solutions were prepared under argon in dark flasks fitted with greaseless taps. Before the irradiation they were flushed for 30min with argon if not otherwise specified. Gases were introduced into the solutions through a bubbler filled with the solvent. The irradiation vessel was a 2 cm quartz cell, filled through an automatically operated tap. Transient absorption measurement were carried out at room temperature using a 12 M o V Vickers accelerator as electron source and a detection system described elsewhere. ~u~ Analysing light was filtered with a 5 mm Pyrex filter to cut off radiation with X < 365 urn to prevent photolysis of the solutes. Pulse doses were measured by a charge collector calibrated against the thiocyanate dosimeter using G ~ 0 = 2, 15 x 104. Doses delivered to cyclohexane solutions were calculated using a simple density factor.

PRODUCTION OF M(CO)s Irradiation by electron pulses of argon saturated Cr(CO)6, Mo(CO)6, W(CO)6 cyclohexane solutions

I

!

400

4k,

!

I

0.04

O.03

o 0.01

X,nm

~

FIG. 1. Optical absorption spectra detected in pulse radiolysis of a 3 x 10-3 mol dm -3 cyclohexane solution of Cr(CO)6 saturated with argon. Pulse length= lOOns; dose = 2,9x 10~eVdm -3. ©, O.D. at the end of the pulse; A, O.D. 300 p.s after the end of the pulse; D, O.D. I ms after the end of the pulse.

0.01,

i

35O

i

4OO X, nm

i

45O

1

5O0

FIG. 2. Optical absorption spectra detected in pulse radiolysis of a 3 x 10-~ tool dm -3 cyclohexane solution of Mo(CO),, saturated with argon. Pulse length= lOOns; d o s e - - 2 , 9 x 10=eVdm -;. ©, O.D. at the end of the pulse; A, O.D. 3 ms after the end of the pulse; r], O.D. 10ms after the end of the pulse.

yields transient visible absorption, with A , , , respectively, at 505 rim, 410 rim, 415 rim. The spectra detected at the end of a lOOns pulse ( 2 , 9 x 102°eVdm-') for a solute concentration of 3 x 10-3 mol dm -3 are reported in Figs. 1-3. The band detected in Mo(CO)6 solution (Fig. 2) shows a shoulder around 470 rim, due to a species decaying with a rate of (3, l _ 0 , 3 ) x 103s -I, probably formed by a secondary reaction. A comparison with photochemical data both in matrix and in solution, allow us to attribute the visible band to the related pentacarbonyl (Table 1). While for W(CO)~ and Mo(CO)5 there is a good agreement with matrix data, for Cr(CO)5 in solution, as previously r e p o r t e d , " " the absorption maximum is at higher wavelength. Saturation with N20, an efficient electron scavenger "~' and cyclohexane excited state quencher, ~j4"~ removes completely the pentacarbonyl peak. The mechanism we propose for the radiolytic production of pentacarbonyl is the following

(I)

(2)

Cell,2

' C6H~2, e~'ol~,C61-I'2

M(CO)6 + e ~ ,

) M(CO)6-

Transient species of group VI metal carbonyls I

t

0.04

0.03 ' ou

u o.0~ K 0

0.0!

i

3~o

45o

4O0

r~o

X, nm

l~o. 3. Optical absorption spectra detected in pulse radiolysis of a 3 x 10-3 mol dm -3 cyclohexane solution of W(CO)s, saturated with argon. Pulse length= lOOns; dose = 2,9x 10ZeeVdm-3. C), O.D. at the end of the pulse; A, O.D. lOOms after the end of the pulse; D, O.D. I s after the end of the pulse.

(3) M(CO)s- + C~H~2

, M(CO)5-t- CO + C t H i 2

(4) CtHI*2+M(CO)6

) M(CO)s+CO+CsHn.

The reaction leading to M(CO)s ÷ via electron abstraction by Cell;'2 is energetically favourable, the ionization potentials of carbonyls ¢2s) being about 1,5 eV lower than the value reported for cyclohexane. ¢=s~) Nevertheless electron capture b y the solute (reaction 2) is a faster process than solute positive ion formation because of the high electron mobility. Hence we can assume that at the concentration we used the recombination process involved in pentacarbonyl production is practically between solute anion and solvent cation. If the pentacarbonyl were produced by a step involving solute cation, it should have a formation rate dependent on solute concentration; the failure to detect even at very low concentration of solute the formation of pentacarbonyl absorption

135

on a nanosecond timescale is a good support for the previous assumption. In the overall mechanism direct dissociation of M(CO)s by radiation is too small to be taken into account. No direct detection of reactions 2 and 3 was possible in our experimental conditions due to the short lifetime and absorption spectra of solvated electrons in liquid hydrocarbons c=s~and to the high recombination rates in solvents with low dielectric constants. We detected the electron scavenging by carbonyls (reaction 2) in methanol, following the electron decay in pure solvent and in a wide range of M(CO)6 concentrations. The measured rate constants were: (5, 5 - 0, 5) x 109 tool-' dm 3 s -I for Cr(CO)6, (8, 8 -+ 0, 5) x 109 mol -I dm s s -I for W(CO)6 and (1,3--+0,2)xl0SOmoi -s dm s s - ' for Mo(CO)s. In the case of Mo(CO)6 we were able to detect the formation, with the same rate as the electron reaction, of a product absorbing around 480 nm. It decayed b y second order kinetics to a spectrum very similar to Mo(CO)s in cyclohexane solutions. The absorption at 480 nm was assigned to the anion Mo(CO)6- which in a polar solvent is stabilized by solvation and decays by reaction with CH3OH2 ÷ to yield a pentacarbonyl strongly solvated by methanol. These data exclude the possibility of M(CO)~- dissociation, to give directly the pentacarbonyl and C O - , prior to recombination. In a solvent with low dielectric constant such as cyclohexane, reaction 2 should follow non homogeneous kinetics, c~Je~ An indication that M(CO)s is formed via the mechanism proposed is the applicability of the expression for non homogeneous kinetics correlating the yield of the product to the scavenger concentration a°~

(5)

G ( P ) = Gj + G=~ '~/(a[S])

l + "V(a[S])

where G~ and G=~ are the yields expressed in G units of free and geminate ions, [S] is the

TAaL~ 1. A~,(nm) IN VISIBLEBANDSOF ]~[(CO)y

Cr(CO)s Mo(CO)s W(CO)5

M.T.H.F. Glass

Arson matrix

Methane matrix

Cyclohexane solutions

This work

483~ 410~2) 416(z~

542"), 533c~) 435(°, 429~6) 440~4),437~6)

489~6) 411(6) 413(6)

503" i)

505 ± 5 410 - 5 415 ± 5

414(~

L. FLAMIONI

136

t

scavenger concentration and a is an empirical parameter related to the scavenging efficiency of the solute in a particular solvent. Equation (5) can be modified for our purposes to the form

i

i

i

GO T

(6)

O.D.

-

(O.D.)s = (O.D.)~

+

[S]-'r~

where O.D. is the optical density of the pentacarbonyl at a chosen wavelength, (O.D.)s is the optical density of the pentacarbonyl at the same A when only free ions are scavenged, i.e. at very low concentration of solute and (O.D.),~ is the optical density of pentacarbonyl at the same A due to scavenging of geminate ions, i.e. the O.D. at infinite concentration of scavenger minus (O.D.),. At the concentrations used (O.D.), can be ignored and plots of (O.D.) -1 vs [M(CO)~] - m should be straight lines. In Fig. 4 are reported O.D. -~ measured at the end of a 100 ns pulse (2,9 x 10~' eV dm -3) at A , ~ vs [M(CO)6] - ~ for the three carbonyls; the good linearity suggests that a non homogeneous mechanism is operating. F r o m the graph any quantitative evaluation of parameters of equation (6) is uncertain, due to the approximation introduced above and to the big error in the evaluation of the intercept, but the following scale of electron scavenging e i c i e n c i e s , i.e. a, providing a similar intercept for the three compounds, can be derived:

Mo(CO)~ > W ( C O k > Cr(COk.

Reaction 4 is the energy transfer process from the cyclohexane excited singiet° ' 1 ~ to the solute, leading to the formation of an excited state able to undergo dissociation, as known from photochemistry. The overlap of solvent emission and solute absorption spectra do not allow confirmation by measurements based on cyclohexane fluorescence intensity and the short lifetime of cyclohexane excited state ° " presents problems for the measure of lifetimes in the presence of solutes. We believe, however, that this process takes place and contributes to a small extent in the formation of M(CO)s.

R E A C T I O N S O F M(CO)~ In argon saturated solutions the pentacarbonyls

'u

40

2O

3O

[M(CO)s]-Vz mol

4O d m -3

FIG 4. Variation of the reciprocal of the optical density of M(CO)5 measured at A ~ at the end of the pulse, with the reciprocal of square root of M(COk concentration. Pulse length = 100 ns; dose = 2,9 × 10~ eV dm -3. A, O.D. "t of Cr(CO)5 measured at `1 = 505 nm; D, O.D. -~ of W(CO)s measured at `1 = 415 nm; C), O.D. -t of Mo(CO)s measured at ,I = 410 nm.

decay by first order kinetics to yield further species absorbing at lower wavelength (Figs. I-3). Dose and concentration effects on the rate constants are reported in Table 2. As data were somewhat affected by the origin of cyclohexane used, to allow comparison measurements were made on solutions made up with the same batch of solvent. From data reported in Table 2 it can be concluded that reactions of M(CO)s with solvent radiolysis products do not take place in our experimental conditions. W(CO)~ data are subject to a large error and need confirmation using a more suitable detection system. The possibilities for M(CO)5 reaction in argon saturated solutions are: (a) isomerization, Co) coordination of a solvent molecule, (c) reaction with solvent impurities. The last possibility has been reported for Cr(CO)~ in photochemical experiments ~u~ and is supported by changes in rate constants with the origin of the solvent but it can be only a secondary reaction in our experimental conditions since too high a concentration of impurities would be required for first order kinetics. Possibility (a) and (b) are both plausible if we admit reaction (c) takes place to a small extent, giving a product absorbing in the same range of wavelength of the product of the main reaction; we will then refer to the reaction product of M(CO)5 as M(CO)s X which could be an isomer of the pentacarbonyl or a carbonyl containing the solvent in the sixth coordination place. Cr(CO)~ X shows a maximum around 465 nm,

T r a n s i e n t s p e c i e s of group VI metal c a r b o n y l s TABLE

2.

DOSE

AND CONCENTRATION

EFFECTS

ON DECAY RATES OF M(CO)

137 5 AND FORMATION

RATES OF M(CO)sX [Cr(CO)s] × mol -~ dm 3 8,5.10 ~

1,6.10_3 2,9.10_ 3 [Mo(CO)6] × mol-ldm3 9.10_4 1,7;10_ 3

2,9.10_3 [W(COk]xmoi-tdm 3

dose × eV -t dm 3 × 10 -t9

kDE¢× S × 10 -3

kFORM× S × 10 -3

4,9

2,2±0,2

2,7±0,3

14,6 4,9 14,6 4,9 14,6

2,1±0,2 2,3±0,3 2,5±0,2 2,8±0,4 2,3±02

2,2±0,2 2,1±0,1 2,1±0,2 2,1±0,2 2,2±0,3

kDEC× S × 10-2

kFORM× S × 10-2

1,8±0,2 2,5±0,2 2,2±0,2 1,9±0,1 2,3±03 1,9±0,1

1,9±0,3 2,4±0,2 1,9±0,2 2,1±0,2 2,1±0,2 2,2±0,3

dose × eV -s d m 3 ×

10 -19

4,9 14,6 4,9 14,6 4,9 14,6 dosexeV-ldm3xl0

1,5.10_ 3

4,9

2,7.10_3

14,6 4,9 14,6

MO(CO)s X and W(CO)s X overlap the hexacarbonyl absorption. These products decay with rates of (1,5--+0,2)× 102s -~ for Cr(CO)sX, (2, 1 _+0,2) s -I for Mo(CO)5 X; no value could be measured for W(CO)s X because of its long lifetime. The disappearance is attributed to reaction with CO to reform the hexacarbonyis. This is supported by very small changes in hexacarbonyl spectra by prolonged y irradiation of argon saturated solutions of hexacarbonyls. The reactivity of M(CO)5 has been reported to be very high. °'"'n~ In this work reactions with CeHm, H2, N2, CO were detected. Reaction with cyclohexene, C6HIo, yields transient absorptions attributed to M(CO)sC6Hm with A. . . . respectively, at 430nm for Cr(CO)sC6Hm, 400nm for Mo(CO)sC~Hm and 395 nm for W ( C O ) s C 6 H I o . The rate constants have been determined by following the decay of pentacarbonyls and the growth of M(CO)sC6Hm in 3 x 10-3 mol dm -3 carbonyl solutions with CeHm concentration ranging from 1 x 10-4 mol dm -3 to 5 x 10-4 tool dm -~. The rate constants of reaction with hydrogen have been determined from the decay rate of pentacarbonyl absorption in solutions saturated with hydrogen, assuming [H2] = 4,7 x 10-3moldm -3, the value reported for n-heptane. ~n~ No formation of an3/ product could be

-t9

kD~xs

k~RuXs

5,5±I

6~±0,7

6,2±0,6 5,3±1 6,8±0,8

4,8±! 5,3±0,8 5,9±1

detected. Nitrogen and carbon monoxide react with M(CO)s yielding, respectively, an UV absorbing product, attributed to M(CO)sN,, shown in Fig. 5 and the hexacarbonyl. In both cases a small

!

t

$

400

450 X, nm

500

0.04

m w

o.o3

o~ Q.

O002.

FIo 5. Optical absorption spectra detected in pulse radiolysis of a 3 × 10 -3 tool d m -3 cyciohexane solution of Cr(CO)6, saturated with nitrogen (7× 10-3moldm-3). Pulse length = 100 ns; dose = 2,9 × 10~eeV dm -3. C), O.D. at the end Of the pulse; 1"7,O.D. 300 ~s after the end of the pulse.

138

L. FLAMIGNI

POLIAKOFF, A. J. REST, J. J. TURNER and R. F. TURNER, J. Am. Chem. Soc. 1975, 97, 4805. 9. M. A. GRAHAM, R. N. PERu'rz, M. POLIAKOFF and J. k × tool dm -3 s × I0 -6 J. TURNER, J. Organometal. Chem. 1972, 34, C 34. 10. J. A. MCINTIRE, J. Phys. Chem. 1970, 74, 2403. Cr(CO)5 Mo(CO)~ W(CO)s ! I. J. M. KELLY, H. HERMANN and E. K. VON GUSTORF, C6Hto 30-+3 81 + 10 18-+2 Chem. Comm. 1973, 105. 12. J. M. KELLY, D. V. BENT, H. HERMANN, D. H2 4,2-+0,2 1,1 ±0,3 0,90---0,03 SCHULTE-FROHLINDE and E. K. VON GUSTORF, J. N2 1,8-+0,2 3,1 -+0,2 0,53-+0,05 OrganometaL Chem. 1974, 69, 259. CO 3,0-+0.2 2,3 -+0,3 1,5 -+0,2 13. Y. HIRSHBERG, J. Chem. Phys. 1957. 27. 758. 14. F. BUSl, L. FLAMIGNi and G. ORLANDI, Radial. Phys. Chem., in press. 15. J. WALTER and S. LIPSKY, Int. J. Radial. Phys. Chem. 1975, 7, 175. a m o u n t o f M(CO)s X is still f o r m e d . S a t u r a t i o n 16. F. Busl, L. FLAMIGINI and A. RODA, Int. J. Radial. concentrations assumed were, respectively, 7× Phys. Chem. 1975, 7, 589. 17. P. J. DYNE, Can. J. Chem. 1965, 43, 1080. 10 -~ mol d m - 3 f o r N2 O3) a n d 1, 1 × 10 -2 tool d m -3, 18. M. G. ROBXNSONand G. R. FREEMAN, J. Chem. Phys. t h e v a l u e r e p o r t e d in n - h e p t a n e , ~u~ f o r CO. 1968, 48, 983. R e a c t i o n r a t e s a r e s h o w n in T a b l e 3. T h e v a l u e 19. A. S. BLAre and N. H. SAGERT, Can. J. Chem. 1967, o b t a i n e d f o r r e a c t i o n o f Cr(CO)5 w i t h C O is in 45, 1351. a g r e e m e n t w i t h t h e o n e p r e v i o u s l y r e p o r t e d . "'~ 20. J. H. WARMANN, K. D. ASMUS and R. H. SCHULER, J. Phys. Chem. 1969, 73, 931. M(CO)sN2 a n d M(CO)sC6HIo d e c a y o n a t i m e s c a l e 21. G. R. FREEMAN and E. D. STOVES, Can. J. Chem. of s e c o n d s , p o s s i b l y b y r e p l a c i n g t h e n e w ligand 1968, 46, 3235. with the expelled CO and reforming the hexacar22. J. H. BAXENDALE and P. WARDMAN, J. Chem. Soc. bonyl. Farday I 1973, 69, 584. 23. A. HUT~N, G. ROF~ and A. MARTELLI, Quaderni dell'Area di Ricerca dell'Emilia-Romagna, 1974, $, Acknowledgements--The author thanks Dr. F. Busi for 67. having introduced the problem, Dr. J. H. Baxendale for 24. F. HIRAYAMA and S. LIPSKY, San Francisco Conf. useful discussion and colleagues for invaluable assisLiquid Scintillators 1970. lance. ?3. R. D. LLOYD and E. W. SCHLAG, Inorganic Chemistry 1969, 8, 2544. REFERENCES 26. M. I. AL-JANROURY and D. W. TURNER, J. Chem. Soc. 1964, 4434. 1. I. W. STOLZ, G. R. DOBSON and R. K. SHELINE, J. 27. K. WATANABE, T. NAKAYAMA and J. ]VIA'IWE, .i. Am. Chem. Soc. 1963, 8& 1013. Quanl. Spectmsc. Radial. Trans. 1962, 2, 369. 2. M. J. BOYLAN, P. S. BRATERIVlANand A. FULLARTON, 28. J. H. BAXENDALE, C. BELL and P. WARDMAN, jr. J. Organometal. Chem. 1971, 31, C 29. Chem. Soc. Faraday L 1973, 69, 776. 3. J. NASlELSKI, P. KIRSH and L. WILLPUTTE-STEXNERT, 29. A. HUMMEL, :7. Chem. Phys. 1968, 48, 3268. J. Organometal. Chem. 1971, 29, 269. 4. M. A. GRAHAM, M. POLIAKOFF and J. J. TURNER, J. 30. G. R. FREEMAN, Int. J. Radial. Phys. Chem. 1972, 4, Chem. Soc. (A) 1971, 2939. 237. 31. S. DELLONTE, E. GARDINI, F. BARIGELLETrl and G. 5. R. N. PERUTZ and J. J. TURNER, Inorganic Chemistry, ORLANDI, Chem. Phys. Letters 1977, 49, 596. 1975, 14, 262. 32. M. W. COOK, U.S. Atomic Energy Comm. UCRL 6. R. N. PERUTZ and J. J. TURNER, J. Am. Chem. Soc. 2459, 1954, 116. 1975, 97, 4791. 33. J. C. GJALDBACKand J. H. HILDEHARD, J. Am. Chem. 7. R. N. PERUTZ and J. J. TURNER, J. Am. Chem. Soc. Soc. 1949, 71, 3147. 1975, 97, 4800. 8. J. K. BURDETr, M. A. GRAHAM, R. N. PERUTZ, M. 34. J. C. GJALDRACK, Acta Chem. Scand. 1952, 6, 623. TABLE 3. REACTIONRATECONSTANTSOF M(CO)~