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Photoinduced catalytic reactions of hydridotetrakis(diethyl phenylphosphonite)cobalt(I)
Journal of Molecular Catalysis, 40 (1987)
289 - 294
289
PHOTOINDUCED CATALYTIC REACTIONS OF HYDRIDOTETRAKIS( DIETHY L PHENY LPHOSPHONITE )COBALT( I) * SHIGERO OISHI Radiation Laboratory,
University of Notre Dame, Notre Dame, IN 46556
(U.S.A.)
(Received July 14, 1986; accepted November 7, 1986)
Summary The mechanism of photochemical isomerization of 3-phenylpropene to 1-phenylpropene in the presence of hydridotetrakis( diethyl phenylphosphonite)cobalt( I) (HCoP,; P = P(OEt),Ph) has been claimed to be a ‘photoassisted reaction’, on the basis of the fact that the isomerjzation stopped immediately when the light was turned off. On the other hand, by means of laser flash photolysis, we have identified a transient as a coordinatively unsaturated species (HCoP,) which maintains the catalytic cycle of the double-bond migration, and have noticed that the species may be regenerated in the dark catalytic cycle, namely, the reaction may proceed via a ‘photoinduced catalytic’ mechanism. In order to clarify the role of HCoPs, we have measured quantum yields and examined deuterium incorporation using DCoP4. Quantum yields (366 nm) of both the double-bond migration and the geometrical isomerization were found to exceed unity, e.g., 2.0 and 5.5 at 0.2 M of the olefins, respectively. This is consistent with a ‘photoinduced catalytic reaction’ rather than a ‘photoassisted reaction’. Indeed, even in such a diluted concentration of 3-phenylpropene (2.7 mM) as to keep @ as low as 0.44, the double-bond migration using DCoP4 gave only 10% deuterated 1-phenylpropene, which means that DCoP,(HCoP,) maintained 18 turns of catalytic cycle. The mechanism of photochemical isomerization of 3-phenylpropene to 1-phenylpropene in the presence of hydridotetrakis(diethy1 phenylphosphonite)cobalt(I) (HCoP,; P = P(OEt),Ph) has been claimed to be a ‘photoassisted reaction’**, on the basis of the fact that the isomerization stopped immediately when the light was turned off [ 11. On the other hand, by means of laser flash photolysis, we have identified a transient as a co*The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2883 from the Notre Dame Radiation Laboratory. **The role of an elementary photochemical reaction in photocatalysis by transitionmetal coordination compounds has been discussed in some reviews [ 4, 51. The terms used here are based on [ 41 because of its unambiguous classification. 0304-5102/87/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
11 hv
P+
Ph/YcH3 COP3
-
HCoP,
PhG-kop H
’
Scheme 1. Photoassisted mechanism (A) and photoinduced catalytic mechanism (B). Backward reactions are not shown and the reaction of 1 to 2 is the rate-determining step [I, 21.
ordinatively unsaturated species (HCoP,) which maintains the catalytic cycle of the double-bond migration [ 2,3], and have noticed that the species may be regenerated in the dark catalytic cycle, namely, the reaction may proceed via a ‘photoinduced catalytic’ mechanism. In order to clarify the role of HCoP,, we have measured quantum yields and examined deuteri~ incorporation using DCoP,. We describe here our results, which are consistent with a ‘photoinduced catalytic’ mechanism rather than a ‘photoassisted’ mechanism. As represented in Scheme 1, up to the formation of u-complex 2, elementary reactions are the same in both mechanisms, that is, the absorption of light by HCoP4 gives rise to coordinately unsaturated HCoPs, which is saturated by the coordination of 3-phenylpropene to give xcomplex 1, and the double-bond inserts to Co-H to produce u-complex 2 [l - 31. In the photoassisted mechanism (A), a free P coordinates to 2 and the elimination of HCoP, completes the cycle to give 1-phenylpropene, so a new photon is required to start the next cycle. On the other hand, in the photo~duced catalytic mech~ism (B), o-complex 2 changes to Ecomplex 3 with the shift of a hydride from internal carbon to cobalt, and the dissociation of 3-phenylpropene regenerates HCoP,, which may start the next cycle. The fact that quantum yield, which is defined as molecules of product formed per photon absorbed, and/or turnover number, which is defined as moIecules of product formed per molecule of catalyst species concerned with the reaction, exceeds unity, is a sufficient condition for a photoinduced catalytic reaction. For the measurement of quantum yields of the double-bond migration, cyclohexane solutions (4 ml) of HCoP4 (4.9 mM), 3-phenylpropene (0.05 1.35 M) and cumene (18 - 96 mM) as an internal standard for gas chromatography were irradiated by the 366 nm line (- 6.4 X lOwa einstein s-l)* of *Light intensity was measured before and after the irradiation by a vacuum photodiode calibrated by ferrioxalate actinometry [ 61.
291
[Olefin]
/
M
Fig. 1. Quantum yields of the double-bond migration of 3-phenylpropene (0) and the geometrical isomerization of 1-phenylpropene (m) us. the concentrations of the olefins. Conversions were kept around 20% by adjusting irradiation time length. l-Phenylpropene formed by the double-bond migration consisted of 40% cis- and 60% trans-isomer at a very early stage of the reaction. At the photostationary state for 1-phenylpropene, it consisted of 5% cis- and 95% trans-isomer.
a high-pressure mercury lamp using a monochromator for 20 - 60 min with stirring under Ar. The reaction mixtures were eluted through a short alumina column with cyclohexane in the dark to eliminate HCoP,, and analyzed for 1-phenylpropene by gas chromatography (liquid phase: PEG 20M). The same procedures as above were followed for the isomerization of cis-l-phenylpropene. As shown in Fig. 1, quantum yields of both the double-bond migration and the geometrical isomerization exceed unity at concentrations of olefins higher than 0.07 and 0.04 M, respectively. This clearly indicates that HCoPs, which is generated initially by the absorption of a photon, is regenerated in the cycle and drives the next cycle more readily with increasing concentrations of olefins.* It should, however, be noted that @ does not necessarily *For the photostationary HCoP4 3 k-1
state:
HCoPs + P
HCoPs + olefin & n-complex k-2 ks n-complex + + HCoP3 + olefin’
(continued)
292
TABLE 1 Incorporation of deuterium duplicate runs
into the olefins using DCoP,;
results are the average of
o^“-Q-l+Q-+QJ0.226 mmol D%
0.231 mmol D%
0.014
0.034
0.176
IOf 2
IOk2
I If I
0.219 Oil
0.010 35f3
exceed unity even for a photoinduced catalytic reaction as shown at low concentrations of the olefins. The coordination of 3-phenylpropene to HCoP, was reported to be faster than that of cis-1-phenylpropene by a factor of 7 [3]. This does not contradict the fact that @ of 3-phenylpropene is smaller than that of c&-lphenylpropene, because the addition of Co-H at the step of 1 to 2 in the anti-Markownikoff direction (not shown in Scheme 1B) leads to the starting olefin, so this cycle is not included in the calculation of a’, even though this direction of addition is predominant as shown below. Incorporation of deuterium into the olefins was examined using DCoPq* (Table 1). The decrease in the deuterium content of DCoP4 with the proceeding reaction is negligible, because conversions were limited to low levels so that total deuterium incorporated amounts to 4.6% and 0.7% of starting DCoP, (0.52 mmol) for the double-bond migration and geometrical isomerization, respectively. Therefore, low deuterium contents in the resultant olefins exclude the photoassisted mechanism, because every cycle starts from DCoP4 in this mechanism. @ is proportional to kz[olefin]/k_l(k_z + ks)[P]. Bending of the curve for Cp of the double-bond migration with increasing [olefin] may be due to the increase in [PI. *Neither mass spectrometry nor NMR detected the presence of HCoP4 in DCOPJ. The deuterium content of DCoP4 can be estimated as >98%, since NaBD4 (98%) and EtOD (99.9%) were used for preparation [7]. The deuterium content of each olefin was determined by GC-mass spectrometry. Diluted solutions of the olefins (2.7 mM in n-pentane) were used for the photoreactions so as to keep @ low, namely, 0.44 and 0.19 for the double-bond migration and the geometrical isomerization, respectively. Otherwise contents of deuterium in the olefins were so low that measurements by mass spectrometry were not reliable.
293
Scheme 2. Deuterium incorporation into 1-phenylpentene accompanied by geometrical isomerization, assuming &-addition and &-elimination [ 8 1.
PhypoP3
_/
+
phy&op3 -
f -
[%
Ph w, DCoP,
Ph/‘p;x
-
D
1-z
ko,,
CH,D \
Ph
--+(l-rf
/y cop,
Ph+,.f+HzD HiOPS
Scheme 3. Deuterium incorporation in the reaction of 3-phenylpropene. fraction of the addition in the ~ti-M~kownikoff direction.
x stands for the
In the case of the geometrical isomerization, the turnover number of DCoPs (HCoPs) is easily calculated as at least 2.9 (= 100/35), referring to Scheme 2. According to Scheme 3 for the double-bond migration, the deuterium content of I-phenylpropene and the ratio of amounts of deuterated 3-phenylpropene to deuterated l-phenylpropene are represented as (l~n)~(x/2)” -I and 2(1- X)/X, respectively, where n stands for turnover number. From the data of Table 1, x and n are estimated as 0.89 and 18, respectively. Predominance of the addition in the anti-Markownikoff direction, implying severe steric requirements for forming u-complexes, resulted in Q! as low as 0.44, even though the cycle turned 18 times. For both the geometrical isomerization and the double-bond migration, turnover numbers exceeded unity, this being consistent with the photoinduced catalytic mechanism. Although a turnover number larger than unity is necessary and sufficient for a photoinduced catalytic reaction, it is important to recognize the possibility of Q, exceeding unity in this reaction.
Acknowledgement I wish to thank Dr. Robin Edidin for the meas~emen~ content by GC-mass spectrometry.
of deuteri~
294
References 1 M. Onishi, K. Hiraki, M. Matsuda and T. Fukunaga, Chem. Lett. (1983) 261. 2 S. Oishi, K. Tajime, A. Hosaha and I. Shiojima, J. C&em. Sot., Chem. Commun., (1984) 60’7. 3 S. Oiihi, N. Kihara and A. Hosaka, J. Organometall. Chem., 276 (1984) C33. 4 H. Henning, D. Rehorek and R. D. Archer, Coord. Chem. Rev., 61 (1985) 1. 5 L. Moggi, A. Juris, D. Sandrini and M. F. Manfrin, Rev. Chem. Intermediates, 4 (1981) 171. 6 C. G. Hatchard and C. A. Parker,fioc. R. Sot. London, Ser. A, 235 (1956) 518. 7 D. Titus, A. A. Orio and H. B. Gray, Znorg. Synth., 13 (1977) 117. 8 Cf., e.g., F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th edn., Wiley-Interscience, New York, 1980; pp. 1252 - 1255.
289 - 294
289
PHOTOINDUCED CATALYTIC REACTIONS OF HYDRIDOTETRAKIS( DIETHY L PHENY LPHOSPHONITE )COBALT( I) * SHIGERO OISHI Radiation Laboratory,
University of Notre Dame, Notre Dame, IN 46556
(U.S.A.)
(Received July 14, 1986; accepted November 7, 1986)
Summary The mechanism of photochemical isomerization of 3-phenylpropene to 1-phenylpropene in the presence of hydridotetrakis( diethyl phenylphosphonite)cobalt( I) (HCoP,; P = P(OEt),Ph) has been claimed to be a ‘photoassisted reaction’, on the basis of the fact that the isomerjzation stopped immediately when the light was turned off. On the other hand, by means of laser flash photolysis, we have identified a transient as a coordinatively unsaturated species (HCoP,) which maintains the catalytic cycle of the double-bond migration, and have noticed that the species may be regenerated in the dark catalytic cycle, namely, the reaction may proceed via a ‘photoinduced catalytic’ mechanism. In order to clarify the role of HCoPs, we have measured quantum yields and examined deuterium incorporation using DCoP4. Quantum yields (366 nm) of both the double-bond migration and the geometrical isomerization were found to exceed unity, e.g., 2.0 and 5.5 at 0.2 M of the olefins, respectively. This is consistent with a ‘photoinduced catalytic reaction’ rather than a ‘photoassisted reaction’. Indeed, even in such a diluted concentration of 3-phenylpropene (2.7 mM) as to keep @ as low as 0.44, the double-bond migration using DCoP4 gave only 10% deuterated 1-phenylpropene, which means that DCoP,(HCoP,) maintained 18 turns of catalytic cycle. The mechanism of photochemical isomerization of 3-phenylpropene to 1-phenylpropene in the presence of hydridotetrakis(diethy1 phenylphosphonite)cobalt(I) (HCoP,; P = P(OEt),Ph) has been claimed to be a ‘photoassisted reaction’**, on the basis of the fact that the isomerization stopped immediately when the light was turned off [ 11. On the other hand, by means of laser flash photolysis, we have identified a transient as a co*The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2883 from the Notre Dame Radiation Laboratory. **The role of an elementary photochemical reaction in photocatalysis by transitionmetal coordination compounds has been discussed in some reviews [ 4, 51. The terms used here are based on [ 41 because of its unambiguous classification. 0304-5102/87/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
11 hv
P+
Ph/YcH3 COP3
-
HCoP,
PhG-kop H
’
Scheme 1. Photoassisted mechanism (A) and photoinduced catalytic mechanism (B). Backward reactions are not shown and the reaction of 1 to 2 is the rate-determining step [I, 21.
ordinatively unsaturated species (HCoP,) which maintains the catalytic cycle of the double-bond migration [ 2,3], and have noticed that the species may be regenerated in the dark catalytic cycle, namely, the reaction may proceed via a ‘photoinduced catalytic’ mechanism. In order to clarify the role of HCoP,, we have measured quantum yields and examined deuteri~ incorporation using DCoP,. We describe here our results, which are consistent with a ‘photoinduced catalytic’ mechanism rather than a ‘photoassisted’ mechanism. As represented in Scheme 1, up to the formation of u-complex 2, elementary reactions are the same in both mechanisms, that is, the absorption of light by HCoP4 gives rise to coordinately unsaturated HCoPs, which is saturated by the coordination of 3-phenylpropene to give xcomplex 1, and the double-bond inserts to Co-H to produce u-complex 2 [l - 31. In the photoassisted mechanism (A), a free P coordinates to 2 and the elimination of HCoP, completes the cycle to give 1-phenylpropene, so a new photon is required to start the next cycle. On the other hand, in the photo~duced catalytic mech~ism (B), o-complex 2 changes to Ecomplex 3 with the shift of a hydride from internal carbon to cobalt, and the dissociation of 3-phenylpropene regenerates HCoP,, which may start the next cycle. The fact that quantum yield, which is defined as molecules of product formed per photon absorbed, and/or turnover number, which is defined as moIecules of product formed per molecule of catalyst species concerned with the reaction, exceeds unity, is a sufficient condition for a photoinduced catalytic reaction. For the measurement of quantum yields of the double-bond migration, cyclohexane solutions (4 ml) of HCoP4 (4.9 mM), 3-phenylpropene (0.05 1.35 M) and cumene (18 - 96 mM) as an internal standard for gas chromatography were irradiated by the 366 nm line (- 6.4 X lOwa einstein s-l)* of *Light intensity was measured before and after the irradiation by a vacuum photodiode calibrated by ferrioxalate actinometry [ 61.
291
[Olefin]
/
M
Fig. 1. Quantum yields of the double-bond migration of 3-phenylpropene (0) and the geometrical isomerization of 1-phenylpropene (m) us. the concentrations of the olefins. Conversions were kept around 20% by adjusting irradiation time length. l-Phenylpropene formed by the double-bond migration consisted of 40% cis- and 60% trans-isomer at a very early stage of the reaction. At the photostationary state for 1-phenylpropene, it consisted of 5% cis- and 95% trans-isomer.
a high-pressure mercury lamp using a monochromator for 20 - 60 min with stirring under Ar. The reaction mixtures were eluted through a short alumina column with cyclohexane in the dark to eliminate HCoP,, and analyzed for 1-phenylpropene by gas chromatography (liquid phase: PEG 20M). The same procedures as above were followed for the isomerization of cis-l-phenylpropene. As shown in Fig. 1, quantum yields of both the double-bond migration and the geometrical isomerization exceed unity at concentrations of olefins higher than 0.07 and 0.04 M, respectively. This clearly indicates that HCoPs, which is generated initially by the absorption of a photon, is regenerated in the cycle and drives the next cycle more readily with increasing concentrations of olefins.* It should, however, be noted that @ does not necessarily *For the photostationary HCoP4 3 k-1
state:
HCoPs + P
HCoPs + olefin & n-complex k-2 ks n-complex + + HCoP3 + olefin’
(continued)
292
TABLE 1 Incorporation of deuterium duplicate runs
into the olefins using DCoP,;
results are the average of
o^“-Q-l+Q-+QJ0.226 mmol D%
0.231 mmol D%
0.014
0.034
0.176
IOf 2
IOk2
I If I
0.219 Oil
0.010 35f3
exceed unity even for a photoinduced catalytic reaction as shown at low concentrations of the olefins. The coordination of 3-phenylpropene to HCoP, was reported to be faster than that of cis-1-phenylpropene by a factor of 7 [3]. This does not contradict the fact that @ of 3-phenylpropene is smaller than that of c&-lphenylpropene, because the addition of Co-H at the step of 1 to 2 in the anti-Markownikoff direction (not shown in Scheme 1B) leads to the starting olefin, so this cycle is not included in the calculation of a’, even though this direction of addition is predominant as shown below. Incorporation of deuterium into the olefins was examined using DCoPq* (Table 1). The decrease in the deuterium content of DCoP4 with the proceeding reaction is negligible, because conversions were limited to low levels so that total deuterium incorporated amounts to 4.6% and 0.7% of starting DCoP, (0.52 mmol) for the double-bond migration and geometrical isomerization, respectively. Therefore, low deuterium contents in the resultant olefins exclude the photoassisted mechanism, because every cycle starts from DCoP4 in this mechanism. @ is proportional to kz[olefin]/k_l(k_z + ks)[P]. Bending of the curve for Cp of the double-bond migration with increasing [olefin] may be due to the increase in [PI. *Neither mass spectrometry nor NMR detected the presence of HCoP4 in DCOPJ. The deuterium content of DCoP4 can be estimated as >98%, since NaBD4 (98%) and EtOD (99.9%) were used for preparation [7]. The deuterium content of each olefin was determined by GC-mass spectrometry. Diluted solutions of the olefins (2.7 mM in n-pentane) were used for the photoreactions so as to keep @ low, namely, 0.44 and 0.19 for the double-bond migration and the geometrical isomerization, respectively. Otherwise contents of deuterium in the olefins were so low that measurements by mass spectrometry were not reliable.
293
Scheme 2. Deuterium incorporation into 1-phenylpentene accompanied by geometrical isomerization, assuming &-addition and &-elimination [ 8 1.
PhypoP3
_/
+
phy&op3 -
f -
[%
Ph w, DCoP,
Ph/‘p;x
-
D
1-z
ko,,
CH,D \
Ph
--+(l-rf
/y cop,
Ph+,.f+HzD HiOPS
Scheme 3. Deuterium incorporation in the reaction of 3-phenylpropene. fraction of the addition in the ~ti-M~kownikoff direction.
x stands for the
In the case of the geometrical isomerization, the turnover number of DCoPs (HCoPs) is easily calculated as at least 2.9 (= 100/35), referring to Scheme 2. According to Scheme 3 for the double-bond migration, the deuterium content of I-phenylpropene and the ratio of amounts of deuterated 3-phenylpropene to deuterated l-phenylpropene are represented as (l~n)~(x/2)” -I and 2(1- X)/X, respectively, where n stands for turnover number. From the data of Table 1, x and n are estimated as 0.89 and 18, respectively. Predominance of the addition in the anti-Markownikoff direction, implying severe steric requirements for forming u-complexes, resulted in Q! as low as 0.44, even though the cycle turned 18 times. For both the geometrical isomerization and the double-bond migration, turnover numbers exceeded unity, this being consistent with the photoinduced catalytic mechanism. Although a turnover number larger than unity is necessary and sufficient for a photoinduced catalytic reaction, it is important to recognize the possibility of Q, exceeding unity in this reaction.
Acknowledgement I wish to thank Dr. Robin Edidin for the meas~emen~ content by GC-mass spectrometry.
of deuteri~
294
References 1 M. Onishi, K. Hiraki, M. Matsuda and T. Fukunaga, Chem. Lett. (1983) 261. 2 S. Oishi, K. Tajime, A. Hosaha and I. Shiojima, J. C&em. Sot., Chem. Commun., (1984) 60’7. 3 S. Oiihi, N. Kihara and A. Hosaka, J. Organometall. Chem., 276 (1984) C33. 4 H. Henning, D. Rehorek and R. D. Archer, Coord. Chem. Rev., 61 (1985) 1. 5 L. Moggi, A. Juris, D. Sandrini and M. F. Manfrin, Rev. Chem. Intermediates, 4 (1981) 171. 6 C. G. Hatchard and C. A. Parker,fioc. R. Sot. London, Ser. A, 235 (1956) 518. 7 D. Titus, A. A. Orio and H. B. Gray, Znorg. Synth., 13 (1977) 117. 8 Cf., e.g., F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th edn., Wiley-Interscience, New York, 1980; pp. 1252 - 1255.