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Metal cluster photo catalysis: photoinduced hydrogenation of ethylene by the tetraruthenium cluster complex H4Ru4(CO)12
Journal of Molecular Catalysis, 19 (1983)
213
213 - 222
METAL CLUSTER PHOTOCATALYSIS: PHOTOINDUCED HYDROGENATION OF ETHYLENE BY THE TETRARUTHENIUM CLUSTER COMPLEX H4Ruq( CO) r2
YOSHIHARU
DOI, SHIGERU
TAMURA
and KUNIHIRO
Research Laboratory of Resources Utilization, Tokyo Nagatsuta-cho, Midori-ku, Yokohama 227 (Japan) (Received
September
KOSHIZUKA
Institute
of Technology,
4259
9, 1982)
Summary Near-ultraviolet irradiation of the H4Ruq(C0)i2 complex in heptane solution induces the catalytic hydrogenation of ethylene in the presence of hydrogen at 35 “C!. The 366 nm quantum yield for ethane formation is 1.8. Carbon monoxide inhibits the photo-induced hydrogenation of ethylene. Kinetics of the photocatalytic hydrogenation are presented as functions of H4Ruq(CO)it concentration, and of pressures of CzH4, H, and CO. In addiexchanges tion, photolysis of D4Ru, (CO) i2 induces hydrogen-deuterium between both H2 and D,, and between Dz and C,H,. The mechanism most consistent with the experimental facts involves photodissociation of CO from the catalyst precursor H4Ruq(C0)i2 as the fisrt step in the catalytic cycle. The properties of photogenerated catalytic species are compared with those of the active species in thermal catalysis by H4Ruq(C0)i2 at 72 “C.
Introduction Transition-metal cluster complexes have been used as homogeneous catalysts for isomerization and hydrogenation of olefins, and the catalysis has been investigated for both its mechanistic and synthetic values [l- 171. The majority of studies have focused on thermal catalysis, and relatively few reports on photocatalysis by metal clusters have appeared. Graff et al. [16, 171 have demonstrated that photolysis of iron and ruthenium cluster complexes initiates catalytic olefin isomerikation at lower temperatures than does thermal catalysis. However, the properties of photogenerated catalytic species are not well understood. In a previous paper [ 141 we investigated the thermal catalysis of ethylene hydrogenation by the tetraruthenium cluster H,Ru4(CO)r2 at 72 “C, and proposed a catalytic cycle involving H3Ru4(CO)il(CZH5) as an intermediate. Herein we report the results of photoinduced hydrogenation of ethyl0304-5102/83/$3.00
@ Elsevier
Sequoia/Printed
in The Netherlands
214
ene with the H,Ru4( CO) i2 complex at a low temperature of 35 “C. The properties of photogenerated catalytic species will be discussed in comparison with our mechanism proposed for thermal catalysis. A preliminary communication of this work has been published [ 151.
Experimental
section
H4Ruq( CO) i2 and its deuterated analogue were synthesized by a reported method [18]. Ethylene (Takashio ultra pure grade) was dried by passing it through a column of 4 A molecular sieve. Hydrogen, deuterium (purity 99.5%), and carbon monoxide were purchased from Takachio Co. and used without further purification. Heptane (Wako pure grade) was dried on 4 A molecular sieve and saturated with dry argon prior to use. The apparatus and procedure used for hydrogenation study are the same as those reported in a previous paper [14]. The photo-induced hydrogenation of ethylene with a heptane solution of H4Ruq(C0)i2 was carried out at 35 “C in a round-bottomed Pyrex vessel of cu. 257 ml with a Teflon-coated stirring bar. After 5 ml of heptane solution of H4Ruq(C0)i2 and prescribed amounts of hydrogen and ethylene were introduced, the reaction vessel was immersed in a water bath thermostatted at the reaction temperature. When required, carbon monoxide was admitted in the reaction vessel by a syringe through a side arm sealed with silicon rubber. Irradiation was conducted using a 450 W medium-pressure Hg lamp equipped with an appropriate Toshiba glass filter (A > 310 nm or h = 366 f 20 nm). Samples of the gases above the reaction solution were withdrawn at intervals by a syringe through the silicon rubber seal, and analyzed quantitatively on a Hitachi 163 FID gas chromatograph with a 2 m column of Porapak Q. For the reaction with deuterium, deuterated products were also separated by gas chromatography and analyzed by a Hitachi RMU-7M mass spectrometer. Infrared spectra of reaction solutions were recorded on a Hitachi 295 spectrometer. Electronic absorption spectra were recorded on a Shimazu MPS-5000 spectrophotometer.
Results Photoinduced stoichiometric hydrogenation The photochemical reaction of ethylene with H4Ruq(C0)i2 was examined at 35 “C in the absence of H,. The results are shown in Fig. 1. Before irradiation no detectable amounts of ethane are formed at 35 “C. When the light (X > 310 nm) is turned on, hydrogenation of ethylene takes place, yielding about two molecules of ethane per molecule of H4Ru4(C0)i2, indicating that all hydride ligands on the ruthenium cluster transfer to ethyl ene molecules. As shown in Fig. 1, the addition of carbon monoxide during the course of photochemical reaction suppresses the hydrogenation rate.
215
6
0
200
100
Time
300
400
(min)
Fig. 1. Yield of CzH6 produced during the photochemical reaction of CzH4 with H4Ru4= (CO)rz. Reaction conditions: 35 ‘C, PC H = 100 torr, 0.8 mM solution of H4Ru4(C0)r2 5 ml, 450 W medium-pressure Hg lamp,*P;rex filter, CO added in run B = 30 pmol.
A heptane solution of H,Ru,(C0)r2 exhibits an intense maximum at 366 nm (E, 17 800) in the near-UV region. The yellow-orange solution of H4Ru,(C0)r2 turned red during the course of the photoinduced stoichiometric hydrogenation of ethylene. Figure 2 shows the absorption spectral changes during the photochemical reaction of C&H, with H4Ruq(C0)r2. The production of ethane during the reaction results in gradual and irreversible spectral changes. When the stoichiometric hydrogenation is complete, the reaction solution exhibits a broad band at 360 nm and shoulders at 430 and 500 nm. However, the ruthenium products arising from the photochemical reaction of C,H4 with H,Ru4(C0)r2 have not been identified. Graff and Wrighton [17] have found the photochemical reaction of 1-pentene with
360
400
440
480 Wavelength
520
560
600
640
(nm)
Fig, 2. Electronic absorption spectral changes during the photochemical reaction of CzH4 with HQRu~(CO)~~. Reaction conditions: 35 “C, PC H = 400 torr, 0.08 mM solution of H,Ru+(CO),, = 2.5 ml, 450 W medium-pressure Hg tar&p, Pyrex filter, reaction times; a = 0 min, b = 20 min, c = 40 min, and d = 125 min.
216
0
30
60
90
120
150
l&I
Time(min)
Fig. 3. Changes in the yield of &He during the catalytic hydrogenation of CzH4 under sequential thermal and photochemical conditions at 35 “C. Reaction conditions: PC,H = = 100 torr, 0.21 mM solution of H~Ru~(CO),~ = 5 ml, 450 W medium-pressure kg % lamp, Pyrex filter.
H,Ru,(C0)i2 to yield H2Ru4(CO),, as the main ruthenium product. In contrast, Johnson et al. [ 191 have demonstrated that irradiation of H,Os,(CO) i2 in the presence of RCH=CH, yields H,Os,(CO)i,(HC,HR). Photoinduced catalytic hydrogenation In the presence of excess hydrogen, the H4Ru,(C0)i2 acts as a powerful catalyst for the photoinduced hydrogenation of ethylene. Figure 3 shows changes in the rate of ethane formation during the catalytic hydrogenation of ethylene under sequential thermal and photochemical conditions at 35 “C. Before irradiation the rate of ethane formation is negligible. When the light (X > 310 nm) is turned on the rate of ethane formation increases gradually and then attains a stationary value. The existence of an induction period in the ethane formation indicates clearly the photochemical generation of catalytically active species from the tetraruthenium cluster precursor H4Ru,(CO),,. After the light is turned off, the rate of ethane formation decreases gradually toward zero. When the catalytic activity had disappeared under dark, the infrared and electron absorption spectra of the catalyst solution were recorded. The detectable Ru compound was H4Ruq(C0)i2, and its recovery was found to be almost 100% within experimental error. Figure 4 shows the relation between the stationary rate of hydrogenation under irradiation and the concentration of H4Ru,(C0)i2. As shown in Fig. 4B, the stationary rate is proportional to the H4Ruq(C0)i2 concentration in the range below 0.01 mM. In contrast, this rate is independent of the H,Ru,above 0.05 mM, where H,Ru,(C0)i2 complex absorbs (CO),, concentration all photons irradiated into the catalyst solution. At 0.11 mM of H,Ru,(CO)i, concentration the quantum yield for ethane formation was measured using 366 nm light (1 X lo7 einstein/min) at 35 “C under 100 torr C,H,, and 100 torr HZ. The quantun yield was found to be 1.8 in excess of unity, which indicates that the ethane formation is catalytic with respect to the number of photons absorbed.
217
A
. 11-
.
.
7
Fig. 4. Relation between hydrogenation rate and H4Ru4(C0)r2 concentration under irradiation. Fig. 4B expands the region of H~Ru~(CO)~~ concentrations below 0.01 mM. solution = 5 ml, 450 W Reaction conditions: 35 ‘C, PC& = PH2 = 100 torr, heptane medium-pressure
Hg lamp,
Pyrex
filter.
Figure 5 shows the relation between the hydrogenation rate and the ethylene pressure. The hydrogenation rate attains a maximum value at 20 torr, followed by a gradual decrease with increasing ethylene pressure. Figure 6 shows the dependence of hydrogenation rate on hydrogen pressure. The hydrogenation rate increases to a constant value with increasing hydrogen pressure. The rate at 0 torr hydrogen pressure corresponds to the maximum rate of the stoichiometric hydrogenation under irradiation. The presence of Hz accelerates the rate of hydrogenation.
0
200
100
100
Q4(T~rr)
Py (Tcrr)
Fig. 5. Relation between hydrogenation rate and ethylene Reaction conditions: 35 ‘C, PH = 100 torr, 8.4 /JM solution W medium-pressure Fig. 6. Relation Reaction pressure
Hg lamp, between
conditions: Hg lamp,
35
Pyrex
Pirex
filter;
irradiation. = 5 ml, 450
filter.
hydrogenation “C, 8.4
pressure under of H4Rq(CO)r2
PM
rate and hydrogen solution
pressure
of H~Ru~(CO)~~
under
irradiation.
= 5 ml, 450
A, P~,H, = 20 torr and B, Pc,H, = 100 torr.
W medium-
218
Effect of additives In order to elucidate the properties of active species, the effects of various additives on the hydrogenation rate were examined at 35 “C. Two types of chlorocarbons, CCL, (5.2 mmol) and CH,C& (6.4 mmol), were added to the catalyst solution, but neither affected the catalytic activity under irradiation. In contrast, the addition of carbon monoxide suppressed the hydrogenation rate. The rate decreased upon increasing the amount of CO added. As Fig. 7 shows, the inverse rate of hydrogenation is linear with respect to the CO pressure.
1 0
. 0.05
0.10
0.15
Pco(Torr)
Fig. 7. Plot of [rate]-’ us. CO pressure. Reaction conditions; 35 ‘C, PC+, = &, = 100 tort-, 8.4 /JM solution of H~Ruq(C0)12 = 5 ml, 450 W medium-pressure Hg lamp, Pyrex filter.
Pho tochemical reaction with deu terium Deuterium-labeling experiments were carried out at 35 “C under irradiation (2 310 nm) by using D4Ruq(C0)i2 in place of H4Ruq(C0)i2. When the mixture of Dz (50 torr) and H, (50 torr) was exposed to irradiation for 50 min, a gas composition of 37% H,, 26% HD and 37% D, was observed. The formation of HD indicates that D4Ruq(C0)i2 catalyzes the photochemical exchange between D, and Hz. During this reaction a small amount of CO was detected in the mass spectra of the gas components. Table 1 summerizes the results for the photoinduced hydrogenation of ethylene with Dz at 35 “C!. When Dz was used in place of H,, the rate of ethane formation decreased and the kinetic isotope effect, hu/kn, was 1.19. A similar small isotope effect has been observed for the thermal catalytic hydrogenation of ethylene (kH/kD = 1.22) at 72 “C under dark with the same tetraruthenium cluster complex D4Ruq( CO) i2 [ 141. The deuterium distribution of ethylene shown in Table 1 indicates a hydrogen-deuterium exchange between C2H, and Dz. Ethylene-d1 (C,H,D) is detected as the main product in the initial stage of reaction. The rate of hydrogen-deuterium exchange between reactants is faster than the rate of ethane formation by one order of magnitude.
219 TABLE
1
Deuterium Time (min)
0 25 50 100
distribution
of ethylene Ethylene
Conv.b (%)
0 0.3 1.0 2.8
in the photochemical
reaction
of &Ha with D, at 35 “Ca
(%)
do
d,
d2
d3
d4
100 93 83 73
0 7 15 21
0 0 2 5
0 0 0 1
0 0 0 0
aReaction conditions: Pc,H, = 100 torr, PH, = 100 torr, (CO),, = 5 ml. bConversion of ethylene hydrogenated to ethane.
and 0.4 mM solution
of DaRuq-
Discussion The photochemistry of metal-metal bonded complexes has been extensively investigated [ 16, 17, 19 - 281. For dinuclear and trinuclear clusters [ 16, 20 - 261 the dominant photochemical reaction pathway for the lowest cluster-localized excited state is the cleavage of metal-metal bonds resulting in fragmentation. Ru,(CO)~~, for example, has been shown to yield monomeric Ru(CO)~L complex upon photolysis in the presence of appropriate ligand L [22, 231. For larger clusters where one metal is bonded to more than three other metal atoms [ 17, 19, 27, 281, reformation of the metalmetal bond inhibits clusterphotofragmentation, and ligand photosubstitution is a common reaction pathway. For example, H,Ru,(CO)~~ has been shown to yield H4R~4(CO)llL upon photolysis of 366 nm light in the presence of ligand L as P(OCH,), or P(C!,H,)a [ 171. Then, it is likely that photodissociation of CO from the catalyst precursor H,Ru,(C0)r2 leads to the formation of active species H4R~4(CO)11 without breaking up the tetraruthenium cluster framework, as proposed by Graff and Wrighton [ 171. hv(366 H,Ru,(COh
nm) ’
<
H,Ru,(CO)~~
+ CO
(1)
k-1 The following experimental facts support a first step comprising CO photodissociation : (i) reversible changes in the hydrogenation rate under dark and irradiation (Fig. 3), and the recovery of H4Ru4(C0)r2 after photocatalytic hydrogenation; (ii) the evolution of CO on exposing hydrogen to a heptane solution of H4Ru4( CO) 12 under irradiation; and (iii) CO inhibition of the photochemical reaction (Figs. 1 and 7).
220
In a previous paper [14] we have concluded that the first step in the thermal catalysis for ethylene hydrogenation at 72 “C! is loss of CO from H4Ru4(C0)i2. In fact, the properties of photogenerated catalytic species are very similar to those of active species in thermal catalysis, as described below: (i) similar kinetic isotope effect (hn/hn = 1.19 in photocatalysis and 1.22 in thermal catalysis); (ii) in both thermal and photochemical catalyses CO inhibits the ethylene hydrogenation, and the rate of hydrogen-deuterium exchange between C2H4 and Dz is faster than the rate of ethane formation by one order of magnitude; and (iii) similar kinetics of catalytic hydrogenation of ethylene. The mechanism proposed for the catalytic hydrogenation of ethylene is outlined in eqns. 2 - 5: Kz H4RMCO)ii
+
H4Ru4(COhl
+ C2H4
H2
I
H&u4(C0)11
(2)
K3 z
(3)
H,Ru,(CO)1dC,Hs)
k4 J%Ru4(CO)l,(C,W
+ H2
-
H,Ru4(CO)ll
+ C2H,
(4) (5)
In the above scheme hydrogen and ethylene molecules react competitively with the coordinately unsaturated complexes H,Ru,( CO) I I and HsRu4( CO) 1I(C2HS). This mechanism is supported by the following experimental facts: (i) A rapid H2/D2 exchange reaction to give HD can be rationalized in terms of hydride exchange on the ruthenium cluster framwork through reaction 2. (ii) In the reaction of C2H4 and D2, the hydrogen-deuterium exchange between reactants occurs more rapidly than the ethane formation. It is reasonable to assume that an ethyl-tetraruthenium intermediate, H3Ru4(CO),,(C,H,), is present in the catalytic cycle and that the reverse reaction of eqn. 3 to give a free ethylene molecule is substantially faster than the formation of ethane via reaction 4. (iii) The acceleration of hydrogenation rate by the presence of H, (see Fig. 6) suggests that the hydrogenolysis reaction 4 is the predominant final step in the catalytic cycle of ethylene hydrogenation. In the stoichiometric hydrogenation of ethylene with H4Ru4(C0)i2, ethane must be formed through a slow transfer reaction of coordinated hydride to the ruthenium-ethyl bond, resulting in the decomposition of H4Ru4(C0)i2. In the presence of excess hydrogen, the rate of the hydride transfer reaction is apparently negligible compared to the rate of hydrogenolysis reaction 4. (iv) The suppression of the hydrogenation rate by excess ethylene (> 20 torr, see Fig. 5) is accounted for by reaction 5.
221
On the basis of the catalytic cycle outlined by steps 1 - 5, the rate of ethane formation in the presence of hydrogen is given by eqn. 6:
d[W61 dt
U-U [Ru,l, = 1 + (k-,/4,) [CO] + K,[H,] + K3[C2H,] + K&s[C,H,]’ k4K3[C2H41
(6)
where $i, k_l and k4 are the rate constants for photodissociation of CO, for the reverse reaction of eqn. 1 and for hydrogenolysis reaction 4, respectively, andK2, K3 and K, are equilibrium constants of the respective steps 2, 3 and 5; [RUG] T denotes the total concentration of tetraruthenium clusters, and [CO], [H,] and [ C,H,] refer to the free concentrations of the respective substrates. First-order dependence of the hydrogenation rate on the cluster conby the results in Fig. 4B. Assuming that centration [ Ru,] r is confirmed substrate concentrations in the catalyst solution are proportional to the respective partial pressures, eqn. 6 predicts that the respective plots of us. PH2-’ should be [RQJT (d[C2& lldt)-’ vs. PCO and [RLQ] (d[C,H,]/dt)-’ linear, which have been verified by using the results of Figs. 6 and 7. In addition, eqn. 6 predicts that (d[C,H,] /dt)[Ru,],’ exhibits a maximum with increasing PCzH,, which is confirmed by the results in Fig. 5. Thus, our kinetic data support the scheme outlined by steps 1 - 5. Steps 2 - 4 are the same as those proposed in the thermal catalysis of ethylene hydrogenation at 72 “C with H4Ruq(C0)i2 [ 141. In the thermal catalysis, the presence of reaction 5 was not appreciated. At the high temperature of 72 “C! the value of KS may be relatively small compared to the value of K3.
Acknowledgements We wish to thank Professors discussions.
Tominaga
Keii and Yoshio Ono for valuable
References J. Norton, D. Valentine and J. P. Collman, J. Am. Chem. Sot., 91 (1969) 7537. G. Pregaglia, A. Andreetta, G. Ferrari and R. Ugo, J. Organometal. Chem., 33 (1971) 73. J. B. Keister and J. R. Shapley, J. Organometal. Chem., 85 (1975) C29;J. Am. Chem. Sot., 98 (1976) 1056. (a) M. Valle, D. Osella and G. A. Vaglio and M. Valle, Inorg. Chim. Acta, 20 (1976) 213. (b) P. M. Lansarot, G. A. Vaglio and M. Valle, Inorg. Chim. Acta, 36 (1979) 213. R. C. Ryan, C. U. Pittman, Jr. and J. P. O’Conner, J. Am. Chem. Sot., 99 (1977) 1986. P. Frediani, Y. Matteoli, M. Bianchi, F. Piacenti and G. Menchi, J. Organometal. Chem., 150 (1978) 273. A. J. Sivak and E. L. Muetterties, J. Am. Chem. Sot., 101 (1979) 4878. S. A. Fouda and G. L. Rempel, Inorg. Chem., 18 (1979) 1.
222 E. Band, A. Kokorin, W. R. Pretzer and M. G. Thomas, Inorg. Chem., 19 (1980) 1552. S. Slater and E. L. Muetterties, Inorg. Chem., 19 (1980) 3337. W. Reinman, W. Abboud, J. M. Basset, R. Mutin, G. L. Rempel and A. K. Smith, J. Mol. Catal., 9 (1980) 349. M. Bianchi, U. Matteoli, G. Menchi, P. Frediani, S. Pratesi and F. Piacenti, J. Organometal. Chem., 198 (1980) 73. G. Balavoine, J. Dang, C. Eskenazi and H. B. Kagan, J. Mol. Cotal., 7 (1980) 531. Y. Doi, K. Koshizuka and T. Keii, Znorg. Chem., 21 (1982) 2732. Y. Doi, S. Tamura and K. Koshizuka, Inorg. Chim. Acta, 65 (1982) L63. J. L. Graff, R. D. Sanner and M. S. Wrighton, J. Am. Chem. Sot., 101 (1979) 273. J. L. Graff and M. S. Wrighton, J. Am. Chem. Sot., iO2 (1980) 2123. S. A. R. Knox, J. W. Koepke, M. A. Andrews and H. D. Kaesz, J. Am. Chem. Sot., 97 (1975) 3942. (a) B. F. G. Johnson, J. W. Kelland, J.$Lewis and S. K. Rehani, J. Organometal. Chem., 113 (1976) C42. (b) S. Bhaduri, B. F. G. Johnson, J. W. Kelland, J. Lewis, P. R. Raithby, S. Rehani, G. M. Sheldrick, K. Wong and M. McPartlin, J. Chem. Sot., Dalton Trans., (1979) 562. M. S. Wrighton and D. S. Ginely, J. Am. Chem. Sot., 97 (1975) 2065. R. W. Wegman, R. J. Olsen, D. R. Gard, L. R. Faulkner and T. L. Broun, J. Am. Chem. Sot., 103 (1981) 6089. B. F. G. Johnson, J. Lewis and M. V. Twigg, J. Organometal. Chem., 67 (1974) C 75. F.- W. Grevels, J. G. A. Reuvers and J. Takats, J. Am. Chem. Sot., 103 (1981) 4069. D. R. Tyler, M. Altobelli and H. B. Gray, J. Am. Chem. Sot., 102 (1980) 3022. D. R. Tyler and H. B. Gray, J. Am. Chem. Sot., 103 (1981) 1683. R. A. Epstein, T. R. Gaffney, G. L. Geoffroy, W. L. Gladfelter and R. S. Henderson, J. Am. Chem. Sot., 101 (1979) 3847. J. L. Graff and M. S. Wrighton, J. Am. Chem. Sot., 102 (1981) 2225. H. C. Foley and G. L. Geoffroy, J. Am. Chem. Sot., 103 (1981) 7176.
9 E. L. Mutterties,
10 11 12 13 14 15 16 17 18 19
20 21 22 23 24 25 26 27 28
213
213 - 222
METAL CLUSTER PHOTOCATALYSIS: PHOTOINDUCED HYDROGENATION OF ETHYLENE BY THE TETRARUTHENIUM CLUSTER COMPLEX H4Ruq( CO) r2
YOSHIHARU
DOI, SHIGERU
TAMURA
and KUNIHIRO
Research Laboratory of Resources Utilization, Tokyo Nagatsuta-cho, Midori-ku, Yokohama 227 (Japan) (Received
September
KOSHIZUKA
Institute
of Technology,
4259
9, 1982)
Summary Near-ultraviolet irradiation of the H4Ruq(C0)i2 complex in heptane solution induces the catalytic hydrogenation of ethylene in the presence of hydrogen at 35 “C!. The 366 nm quantum yield for ethane formation is 1.8. Carbon monoxide inhibits the photo-induced hydrogenation of ethylene. Kinetics of the photocatalytic hydrogenation are presented as functions of H4Ruq(CO)it concentration, and of pressures of CzH4, H, and CO. In addiexchanges tion, photolysis of D4Ru, (CO) i2 induces hydrogen-deuterium between both H2 and D,, and between Dz and C,H,. The mechanism most consistent with the experimental facts involves photodissociation of CO from the catalyst precursor H4Ruq(C0)i2 as the fisrt step in the catalytic cycle. The properties of photogenerated catalytic species are compared with those of the active species in thermal catalysis by H4Ruq(C0)i2 at 72 “C.
Introduction Transition-metal cluster complexes have been used as homogeneous catalysts for isomerization and hydrogenation of olefins, and the catalysis has been investigated for both its mechanistic and synthetic values [l- 171. The majority of studies have focused on thermal catalysis, and relatively few reports on photocatalysis by metal clusters have appeared. Graff et al. [16, 171 have demonstrated that photolysis of iron and ruthenium cluster complexes initiates catalytic olefin isomerikation at lower temperatures than does thermal catalysis. However, the properties of photogenerated catalytic species are not well understood. In a previous paper [ 141 we investigated the thermal catalysis of ethylene hydrogenation by the tetraruthenium cluster H,Ru4(CO)r2 at 72 “C, and proposed a catalytic cycle involving H3Ru4(CO)il(CZH5) as an intermediate. Herein we report the results of photoinduced hydrogenation of ethyl0304-5102/83/$3.00
@ Elsevier
Sequoia/Printed
in The Netherlands
214
ene with the H,Ru4( CO) i2 complex at a low temperature of 35 “C. The properties of photogenerated catalytic species will be discussed in comparison with our mechanism proposed for thermal catalysis. A preliminary communication of this work has been published [ 151.
Experimental
section
H4Ruq( CO) i2 and its deuterated analogue were synthesized by a reported method [18]. Ethylene (Takashio ultra pure grade) was dried by passing it through a column of 4 A molecular sieve. Hydrogen, deuterium (purity 99.5%), and carbon monoxide were purchased from Takachio Co. and used without further purification. Heptane (Wako pure grade) was dried on 4 A molecular sieve and saturated with dry argon prior to use. The apparatus and procedure used for hydrogenation study are the same as those reported in a previous paper [14]. The photo-induced hydrogenation of ethylene with a heptane solution of H4Ruq(C0)i2 was carried out at 35 “C in a round-bottomed Pyrex vessel of cu. 257 ml with a Teflon-coated stirring bar. After 5 ml of heptane solution of H4Ruq(C0)i2 and prescribed amounts of hydrogen and ethylene were introduced, the reaction vessel was immersed in a water bath thermostatted at the reaction temperature. When required, carbon monoxide was admitted in the reaction vessel by a syringe through a side arm sealed with silicon rubber. Irradiation was conducted using a 450 W medium-pressure Hg lamp equipped with an appropriate Toshiba glass filter (A > 310 nm or h = 366 f 20 nm). Samples of the gases above the reaction solution were withdrawn at intervals by a syringe through the silicon rubber seal, and analyzed quantitatively on a Hitachi 163 FID gas chromatograph with a 2 m column of Porapak Q. For the reaction with deuterium, deuterated products were also separated by gas chromatography and analyzed by a Hitachi RMU-7M mass spectrometer. Infrared spectra of reaction solutions were recorded on a Hitachi 295 spectrometer. Electronic absorption spectra were recorded on a Shimazu MPS-5000 spectrophotometer.
Results Photoinduced stoichiometric hydrogenation The photochemical reaction of ethylene with H4Ruq(C0)i2 was examined at 35 “C in the absence of H,. The results are shown in Fig. 1. Before irradiation no detectable amounts of ethane are formed at 35 “C. When the light (X > 310 nm) is turned on, hydrogenation of ethylene takes place, yielding about two molecules of ethane per molecule of H4Ru4(C0)i2, indicating that all hydride ligands on the ruthenium cluster transfer to ethyl ene molecules. As shown in Fig. 1, the addition of carbon monoxide during the course of photochemical reaction suppresses the hydrogenation rate.
215
6
0
200
100
Time
300
400
(min)
Fig. 1. Yield of CzH6 produced during the photochemical reaction of CzH4 with H4Ru4= (CO)rz. Reaction conditions: 35 ‘C, PC H = 100 torr, 0.8 mM solution of H4Ru4(C0)r2 5 ml, 450 W medium-pressure Hg lamp,*P;rex filter, CO added in run B = 30 pmol.
A heptane solution of H,Ru,(C0)r2 exhibits an intense maximum at 366 nm (E, 17 800) in the near-UV region. The yellow-orange solution of H4Ru,(C0)r2 turned red during the course of the photoinduced stoichiometric hydrogenation of ethylene. Figure 2 shows the absorption spectral changes during the photochemical reaction of C&H, with H4Ruq(C0)r2. The production of ethane during the reaction results in gradual and irreversible spectral changes. When the stoichiometric hydrogenation is complete, the reaction solution exhibits a broad band at 360 nm and shoulders at 430 and 500 nm. However, the ruthenium products arising from the photochemical reaction of C,H4 with H,Ru4(C0)r2 have not been identified. Graff and Wrighton [17] have found the photochemical reaction of 1-pentene with
360
400
440
480 Wavelength
520
560
600
640
(nm)
Fig, 2. Electronic absorption spectral changes during the photochemical reaction of CzH4 with HQRu~(CO)~~. Reaction conditions: 35 “C, PC H = 400 torr, 0.08 mM solution of H,Ru+(CO),, = 2.5 ml, 450 W medium-pressure Hg tar&p, Pyrex filter, reaction times; a = 0 min, b = 20 min, c = 40 min, and d = 125 min.
216
0
30
60
90
120
150
l&I
Time(min)
Fig. 3. Changes in the yield of &He during the catalytic hydrogenation of CzH4 under sequential thermal and photochemical conditions at 35 “C. Reaction conditions: PC,H = = 100 torr, 0.21 mM solution of H~Ru~(CO),~ = 5 ml, 450 W medium-pressure kg % lamp, Pyrex filter.
H,Ru,(C0)i2 to yield H2Ru4(CO),, as the main ruthenium product. In contrast, Johnson et al. [ 191 have demonstrated that irradiation of H,Os,(CO) i2 in the presence of RCH=CH, yields H,Os,(CO)i,(HC,HR). Photoinduced catalytic hydrogenation In the presence of excess hydrogen, the H4Ru,(C0)i2 acts as a powerful catalyst for the photoinduced hydrogenation of ethylene. Figure 3 shows changes in the rate of ethane formation during the catalytic hydrogenation of ethylene under sequential thermal and photochemical conditions at 35 “C. Before irradiation the rate of ethane formation is negligible. When the light (X > 310 nm) is turned on the rate of ethane formation increases gradually and then attains a stationary value. The existence of an induction period in the ethane formation indicates clearly the photochemical generation of catalytically active species from the tetraruthenium cluster precursor H4Ru,(CO),,. After the light is turned off, the rate of ethane formation decreases gradually toward zero. When the catalytic activity had disappeared under dark, the infrared and electron absorption spectra of the catalyst solution were recorded. The detectable Ru compound was H4Ruq(C0)i2, and its recovery was found to be almost 100% within experimental error. Figure 4 shows the relation between the stationary rate of hydrogenation under irradiation and the concentration of H4Ru,(C0)i2. As shown in Fig. 4B, the stationary rate is proportional to the H4Ruq(C0)i2 concentration in the range below 0.01 mM. In contrast, this rate is independent of the H,Ru,above 0.05 mM, where H,Ru,(C0)i2 complex absorbs (CO),, concentration all photons irradiated into the catalyst solution. At 0.11 mM of H,Ru,(CO)i, concentration the quantum yield for ethane formation was measured using 366 nm light (1 X lo7 einstein/min) at 35 “C under 100 torr C,H,, and 100 torr HZ. The quantun yield was found to be 1.8 in excess of unity, which indicates that the ethane formation is catalytic with respect to the number of photons absorbed.
217
A
. 11-
.
.
7
Fig. 4. Relation between hydrogenation rate and H4Ru4(C0)r2 concentration under irradiation. Fig. 4B expands the region of H~Ru~(CO)~~ concentrations below 0.01 mM. solution = 5 ml, 450 W Reaction conditions: 35 ‘C, PC& = PH2 = 100 torr, heptane medium-pressure
Hg lamp,
Pyrex
filter.
Figure 5 shows the relation between the hydrogenation rate and the ethylene pressure. The hydrogenation rate attains a maximum value at 20 torr, followed by a gradual decrease with increasing ethylene pressure. Figure 6 shows the dependence of hydrogenation rate on hydrogen pressure. The hydrogenation rate increases to a constant value with increasing hydrogen pressure. The rate at 0 torr hydrogen pressure corresponds to the maximum rate of the stoichiometric hydrogenation under irradiation. The presence of Hz accelerates the rate of hydrogenation.
0
200
100
100
Q4(T~rr)
Py (Tcrr)
Fig. 5. Relation between hydrogenation rate and ethylene Reaction conditions: 35 ‘C, PH = 100 torr, 8.4 /JM solution W medium-pressure Fig. 6. Relation Reaction pressure
Hg lamp, between
conditions: Hg lamp,
35
Pyrex
Pirex
filter;
irradiation. = 5 ml, 450
filter.
hydrogenation “C, 8.4
pressure under of H4Rq(CO)r2
PM
rate and hydrogen solution
pressure
of H~Ru~(CO)~~
under
irradiation.
= 5 ml, 450
A, P~,H, = 20 torr and B, Pc,H, = 100 torr.
W medium-
218
Effect of additives In order to elucidate the properties of active species, the effects of various additives on the hydrogenation rate were examined at 35 “C. Two types of chlorocarbons, CCL, (5.2 mmol) and CH,C& (6.4 mmol), were added to the catalyst solution, but neither affected the catalytic activity under irradiation. In contrast, the addition of carbon monoxide suppressed the hydrogenation rate. The rate decreased upon increasing the amount of CO added. As Fig. 7 shows, the inverse rate of hydrogenation is linear with respect to the CO pressure.
1 0
. 0.05
0.10
0.15
Pco(Torr)
Fig. 7. Plot of [rate]-’ us. CO pressure. Reaction conditions; 35 ‘C, PC+, = &, = 100 tort-, 8.4 /JM solution of H~Ruq(C0)12 = 5 ml, 450 W medium-pressure Hg lamp, Pyrex filter.
Pho tochemical reaction with deu terium Deuterium-labeling experiments were carried out at 35 “C under irradiation (2 310 nm) by using D4Ruq(C0)i2 in place of H4Ruq(C0)i2. When the mixture of Dz (50 torr) and H, (50 torr) was exposed to irradiation for 50 min, a gas composition of 37% H,, 26% HD and 37% D, was observed. The formation of HD indicates that D4Ruq(C0)i2 catalyzes the photochemical exchange between D, and Hz. During this reaction a small amount of CO was detected in the mass spectra of the gas components. Table 1 summerizes the results for the photoinduced hydrogenation of ethylene with Dz at 35 “C!. When Dz was used in place of H,, the rate of ethane formation decreased and the kinetic isotope effect, hu/kn, was 1.19. A similar small isotope effect has been observed for the thermal catalytic hydrogenation of ethylene (kH/kD = 1.22) at 72 “C under dark with the same tetraruthenium cluster complex D4Ruq( CO) i2 [ 141. The deuterium distribution of ethylene shown in Table 1 indicates a hydrogen-deuterium exchange between C2H, and Dz. Ethylene-d1 (C,H,D) is detected as the main product in the initial stage of reaction. The rate of hydrogen-deuterium exchange between reactants is faster than the rate of ethane formation by one order of magnitude.
219 TABLE
1
Deuterium Time (min)
0 25 50 100
distribution
of ethylene Ethylene
Conv.b (%)
0 0.3 1.0 2.8
in the photochemical
reaction
of &Ha with D, at 35 “Ca
(%)
do
d,
d2
d3
d4
100 93 83 73
0 7 15 21
0 0 2 5
0 0 0 1
0 0 0 0
aReaction conditions: Pc,H, = 100 torr, PH, = 100 torr, (CO),, = 5 ml. bConversion of ethylene hydrogenated to ethane.
and 0.4 mM solution
of DaRuq-
Discussion The photochemistry of metal-metal bonded complexes has been extensively investigated [ 16, 17, 19 - 281. For dinuclear and trinuclear clusters [ 16, 20 - 261 the dominant photochemical reaction pathway for the lowest cluster-localized excited state is the cleavage of metal-metal bonds resulting in fragmentation. Ru,(CO)~~, for example, has been shown to yield monomeric Ru(CO)~L complex upon photolysis in the presence of appropriate ligand L [22, 231. For larger clusters where one metal is bonded to more than three other metal atoms [ 17, 19, 27, 281, reformation of the metalmetal bond inhibits clusterphotofragmentation, and ligand photosubstitution is a common reaction pathway. For example, H,Ru,(CO)~~ has been shown to yield H4R~4(CO)llL upon photolysis of 366 nm light in the presence of ligand L as P(OCH,), or P(C!,H,)a [ 171. Then, it is likely that photodissociation of CO from the catalyst precursor H,Ru,(C0)r2 leads to the formation of active species H4R~4(CO)11 without breaking up the tetraruthenium cluster framework, as proposed by Graff and Wrighton [ 171. hv(366 H,Ru,(COh
nm) ’
<
H,Ru,(CO)~~
+ CO
(1)
k-1 The following experimental facts support a first step comprising CO photodissociation : (i) reversible changes in the hydrogenation rate under dark and irradiation (Fig. 3), and the recovery of H4Ru4(C0)r2 after photocatalytic hydrogenation; (ii) the evolution of CO on exposing hydrogen to a heptane solution of H4Ru4( CO) 12 under irradiation; and (iii) CO inhibition of the photochemical reaction (Figs. 1 and 7).
220
In a previous paper [14] we have concluded that the first step in the thermal catalysis for ethylene hydrogenation at 72 “C! is loss of CO from H4Ru4(C0)i2. In fact, the properties of photogenerated catalytic species are very similar to those of active species in thermal catalysis, as described below: (i) similar kinetic isotope effect (hn/hn = 1.19 in photocatalysis and 1.22 in thermal catalysis); (ii) in both thermal and photochemical catalyses CO inhibits the ethylene hydrogenation, and the rate of hydrogen-deuterium exchange between C2H4 and Dz is faster than the rate of ethane formation by one order of magnitude; and (iii) similar kinetics of catalytic hydrogenation of ethylene. The mechanism proposed for the catalytic hydrogenation of ethylene is outlined in eqns. 2 - 5: Kz H4RMCO)ii
+
H4Ru4(COhl
+ C2H4
H2
I
H&u4(C0)11
(2)
K3 z
(3)
H,Ru,(CO)1dC,Hs)
k4 J%Ru4(CO)l,(C,W
+ H2
-
H,Ru4(CO)ll
+ C2H,
(4) (5)
In the above scheme hydrogen and ethylene molecules react competitively with the coordinately unsaturated complexes H,Ru,( CO) I I and HsRu4( CO) 1I(C2HS). This mechanism is supported by the following experimental facts: (i) A rapid H2/D2 exchange reaction to give HD can be rationalized in terms of hydride exchange on the ruthenium cluster framwork through reaction 2. (ii) In the reaction of C2H4 and D2, the hydrogen-deuterium exchange between reactants occurs more rapidly than the ethane formation. It is reasonable to assume that an ethyl-tetraruthenium intermediate, H3Ru4(CO),,(C,H,), is present in the catalytic cycle and that the reverse reaction of eqn. 3 to give a free ethylene molecule is substantially faster than the formation of ethane via reaction 4. (iii) The acceleration of hydrogenation rate by the presence of H, (see Fig. 6) suggests that the hydrogenolysis reaction 4 is the predominant final step in the catalytic cycle of ethylene hydrogenation. In the stoichiometric hydrogenation of ethylene with H4Ru4(C0)i2, ethane must be formed through a slow transfer reaction of coordinated hydride to the ruthenium-ethyl bond, resulting in the decomposition of H4Ru4(C0)i2. In the presence of excess hydrogen, the rate of the hydride transfer reaction is apparently negligible compared to the rate of hydrogenolysis reaction 4. (iv) The suppression of the hydrogenation rate by excess ethylene (> 20 torr, see Fig. 5) is accounted for by reaction 5.
221
On the basis of the catalytic cycle outlined by steps 1 - 5, the rate of ethane formation in the presence of hydrogen is given by eqn. 6:
d[W61 dt
U-U [Ru,l, = 1 + (k-,/4,) [CO] + K,[H,] + K3[C2H,] + K&s[C,H,]’ k4K3[C2H41
(6)
where $i, k_l and k4 are the rate constants for photodissociation of CO, for the reverse reaction of eqn. 1 and for hydrogenolysis reaction 4, respectively, andK2, K3 and K, are equilibrium constants of the respective steps 2, 3 and 5; [RUG] T denotes the total concentration of tetraruthenium clusters, and [CO], [H,] and [ C,H,] refer to the free concentrations of the respective substrates. First-order dependence of the hydrogenation rate on the cluster conby the results in Fig. 4B. Assuming that centration [ Ru,] r is confirmed substrate concentrations in the catalyst solution are proportional to the respective partial pressures, eqn. 6 predicts that the respective plots of us. PH2-’ should be [RQJT (d[C2& lldt)-’ vs. PCO and [RLQ] (d[C,H,]/dt)-’ linear, which have been verified by using the results of Figs. 6 and 7. In addition, eqn. 6 predicts that (d[C,H,] /dt)[Ru,],’ exhibits a maximum with increasing PCzH,, which is confirmed by the results in Fig. 5. Thus, our kinetic data support the scheme outlined by steps 1 - 5. Steps 2 - 4 are the same as those proposed in the thermal catalysis of ethylene hydrogenation at 72 “C with H4Ruq(C0)i2 [ 141. In the thermal catalysis, the presence of reaction 5 was not appreciated. At the high temperature of 72 “C! the value of KS may be relatively small compared to the value of K3.
Acknowledgements We wish to thank Professors discussions.
Tominaga
Keii and Yoshio Ono for valuable
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