Organometallics 1982, 1, 1726-1728
1726 x/AoIAo -
xj
vs.
hydrogenation6 of thorium v2-acylsoccurs. Thus, eq 4 is readily understandable in terms of hydrogenolytic interception of D (Th-C bond hydrogenolysis is generally facile6J6)and is analogous to the final step in the thorium hydride catalyzed hydrogenation of actinide $-acyls (eq
t
5).6
lo\
P\
Th-H
M-:CR
M
(5)
CH2R
HZ
In summary, the present kinetic and chemical evidence strongly implicate organoactinide formyls as key branch points in carbonylation processes which lead either to enediolates (CO-CO coupling) or to the production of methoxide functionalities (CO hydrogenation). Further efforts to quantify and manipulate these processes are in progress. Acknowledgment. We thank the National Science Foundation (Grant CHE8009060)for generous support of this research. Registry No. A (R = CH[C(CH,),],, 79932-08-4; K (R = CH[C(CH3)3]2, 83398-67-8.
B
~
~
trations of CO were maintained. The quantitative analysis consisted of integrating the Th-H resonance vs. the toluene-d, aromatic resonance as a function of time. Plots were fit by standard linear regression techniques, using a Hewlett-Packard HP1000-F data system and associated graphic peripherals. (21) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.; 1981,103,6650-6667. Marks, T. J. J. Am. Chem. SOC. (22) (a) The vapor pressure of toluene under these conditions, ca. 20 torraZzb is relativelv small and constant. (b) "CRC Handbook of Chemi s t j and Physics";GOth ed.; CRC Press: Boca Raton, FL, 1979; p D-209. (23) An authentic sample was prepared by reaction of Th[(CH3),C5]z(OCH[c(CH,)3]zJH with methanol: 'H NMR (C6D6)6 1.00 (8, 9H), 1.91 (s, 30 H), 3.68 ( 8 , 1H), 3.75 (8. 3 H); IR (Nuiol mull, cm-' 1390 (m). 1364 (m). 1113 ( 8 ) . 1057 ( 8 ) . 1009 (SI. 960 (w). Si9 (w). 765 (w). 722 (w),'656 (8). Anal. Calcd for CjbH,,ThO;: C, 53.'24; H,7.74. Found: C, 53.28; H,7.66.
-.--. -.
%
m
2
0
100
200
300
Pco
400
500
600
700
800
(torr)
Figure 2. A. Kinetic plots for the reaction of Th[(CH3)&&-
{OCH[C(CH3)3]zJHwith CO at various constant CO preasures. B. Dependence of the observed rate constant on CO pressure. (20) In a Vacuum Atmoepheres glovebox equipped with a recirculating Dri-Train, 2.5-mL portions of a 0.060 M solution of recrystallized Th[(CH,)5Cq]z(OCH[C(CHs)a]zJH*a in toluene-de (dried over Na/K) were syringed into the NMR tube portion of the reaction vessel (Figure 1) which had previously been flamed under high vacuum (lo4 torr). The apparatus was then removed to a vacuum line and the thorium hydride solution freeze-thaw degassed. Next, the system was charged with the desired ressure of CO (passed through MnO and 4A molecule sieve columnsR) by using a mercury U-tube manometer.= The vessel was next sealed and the solution-containing portion of the NMR tube placed in a 30 f 0.4 OC thermostated bath, shielded from light. Rapid stirring wm then initiated with a magnetic stirrer placed under the bath (control experiments established that under the present reaction conditions, variations in the stirring rate did not detectably affect the measured reaction rates). Kinetic measurements were performed by quickly removing the reaction vessel from the constant temperature bath,transferring the stirring bar to the holding cup with a magnet, and inserting the NMR tube into the probe (30 "C) of a JEOL FX-9OQ NMR spectrometer. The instrument had been previously tuned to maximize field homogeneity under nonspinning conditions. Proton spectra were recorded on disk under previously optimized conditions (nonspinning),the NMR tube removed from the probe, the stirring bar returned to the reaction solution, and the reaction veasel returned to the thermostated bath. Typically this operation required 2.0-2.5 min, and the kinetic results were found to be insensitive to whether this time was or was not included in the total reaction time. The kinetic measurements were typically carried out for 4 or more half-lives. Even for the lowest pressure CO run (155 torr), it was calculated that the CO pressure decreased by no more than 4% during the course of the run, ensuring that essentially constant concen-
0276-733318212301-1726$01.25/0
An Alternate Blnuclear Mechanlsm for Aldehyde Formation In the Reppe-Modlfled Hydroformylatlon Reactiont James C. Barborak' Department of Chemistry University of North Carolina at Greensboro Greensboro, North Carolina 274 12
Kevln Cann' Union Carbide Corporation BoundBrook, New Jersey 08805 Received June 2, 1982
Summary: The isolable intermediates from the Reppemodified hydroformylation reaction, H-Fe(CO), and R-Fe(CO),, are shown to react together to provide aldehyde product. This result suggests that a binuclear intermediate may be responsible for aldehyde formation.
Elucidation of the primary mechanistic cycles involved in homogeneous reactions catalyzed by organotransitionmetal complexes has required that attention be focused on the chemical reactivity of the supposed intermediates in these processes. In the course of such investigations, a number of binuclear reactions have been identified which +
Dedicated to the memory of Professor Rowland Pettit
0 1982 American Chemical Society
Organometallics, Vol. I , No. 12, 1982 1727
Communications Scheme I
Scheme I1
1
5
C H 2 =C H p
Fe(CO)4
7
CH3CH2C(=O)FeH(C@)4
co
4
C H 3C H 2 F e H ( C O ) 4
CH3CH2CFe(C@13
CHsCH2CHO
-
HCo(C0)d + CH,C(=O)Co(CO), CH3CHO + C02(C0)8 (1) unimolecular process.' More recently, a model for the aldehyde-forming step in the hydroformylation reaction has been suggested by Jones and Bergman, on the basis of the observation that CpMoH(CO), reacts with CpMoR(CO), to produce aldehyde and a molybdenum dimer.2 This result was believed to obtain through interactions of the metal hydride with an unsaturated acyl species in equilibrium with the metal alkyl. Although model studies are useful in suggesting reaction sequences in catalytic cycles in which the intermediates are transient and usually too unstable to isolate, it is particularly rewarding to investigate the reactivities of species which are actual intermediates in bona fide homogeneous processes. Isolable intermediates can be obtained in the Reppe modification of the hydroformylation reaction: the principal catalytic cycle for which has been resolved by Pettit and co-workers,4using Fe(CO)5to effect reaction. With base, Fe(C0)5 is converted to the tetracarbonylhydridoferrate anion 1, which is subsequently protonated in the reaction medium to form tetracarbonyldihydridoiron (2)) as is indicated in Scheme I. The dihydridoiron species 2 coordinates the olefin and the reaction proceeds to give aldehyde in a mechanism analogous to that of the oxo process. In this catalytic cycle, metal carbonyl hydrides, alkyls, and acyl species are present in varying concentrations, allowing for the possibility of occurrence of binuclear interactions similar to the model reactions described above. Although H2Fe(C0), (2) was proposed as the intermediate which initiates the hydroformylation ~ e q u e n c eit, ~ could not be found in appreciable concentrations. Its presence was implicated by demonstrating that 1,5cyclooctadiene rearranged rapidly to the conjugated diene in the reaction medium, and H2Fe(C0), is known to bring about such rearrangements. The dominant species under the reaction conditions was the anion 1, as shown by infrared analysis. In our own investigations, the reactivity of this anion with the conjugate base of the alkyliron tetracarbonyl complex 3 was examined; in the basic medium of the Reppe reaction deprotonation of 3, as well as (1) Breslow, D. S.; Heck, R. F. Chem. Ind. 1960, 467. 1979,101,5447. (2) Jones, W. D.; Bergman, R. G. J . Am. Chem. SOC. (3) Reppe, W.; Vetter, H. Justus Liebigs Ann. Chem. 1953,582,133.
(4).Kang,H.; Mauldin, C. H.; Cole, T.; Slegeir, W.; Cann, K.; Pettit, R. J . Am. Chem. SOC. 1977, 99, 8323.
2-
I l l-
3
could be taking place as part of these catalytic cycles. Combinations of transition-metal hydrides with other organometallic species have yielded a number of examples of binuclear processes which might serve as models for specific steps in hydrogenation, Fischer-Tropsch, and hydroformylation reactions. For example, Heck and Breslow have shown that two assumed intermediates in the oxo process could react together bimolecularly to yield aldehyde (eq 1)rather than through the usually proposed
6
-
L
CH3CH2CHO
HFe(C@14]
of 2 and 4, certainly occurs, at least to some e ~ t e n tas ,~ shown in eq 2. B
CH3CH2FeH(C0),ZCH3CH2-Fe(CO),+ BH+ 3 5
(2)
The alkyliron tetracarbonyl anion 5 was prepared by the reaction of ethyl iodide in anhydrous tetrahydrofuran at 0 OC with the tetracarbonyliron dianion;6the dianion was generated by deprotonation of Et4N+H-Fe(CO), with butyllithium.' H-Fe(CO), 1
BuLi
-
2-Fe(C0)4
Et1
CH3CH2-Fe(CO), 5
Addition of 1 molar equiv of Et4N+H-Fe(C0), to a tetrahydrofuran solution of 5, generated as described above, led to rapid formation of propionaldehyde.6 The H-Fe(CO), 1
+ CH3CH2-Fe(C0), 5
A
u
o n
-b
CH3CH2CH0 65-70%
formation of propionaldehyde rather than ethane indicates that the anion 1 is reacting with the unsaturated acyliron anion 6, which is in equilibrium with 5, but not detectable by infrared analysis (Scheme 11). The absence of ethane is significant; hydrocarbon products were found in several reactions of alkylmanganese species and metal hydride^.^ Aldehyde production in this reaction clearly shows that in the Reppe process, alternative binuclear mechanisms may also account for products formed. To determine if the unsaturated acyl intermediate 6 is necessary for coordination of the metal hydride prior to product elimination,we examined the reaction of the anion 1 with a saturated analogue of 6. Triphenylphosphine H-Fe(CO), 1
-
+ CH3CH2C(=0)-FePPh3(CO), 7
2h
CH,CH,CHO 35 % coordinates readily with 5 to produce 7;6 upon addition of 1molar equiv of H-Fe(CO), in THF to a solution of 7 at 0 "C, slow formation of propionaldehyde was detected. A significantly slower rate of aldehyde formation was found (5) Walker, H. W.; Kresge, T.; Ford, P. C.; Pearson, R. J. Am. Chem. SOC.1979, 101, 7428. (6) Collman, J. P. Acc. Chem. Res. 1975, 8, 342. (7) Pettit, R., unpublished results. (8) Determined by gas chromatographic analysis by comparison with an internal standard (toluene). A blank reaction of species 5 showed it to be stable under reaction conditions yielding only trace amounts of aldehyde and ethylene which formed in the injection port of the gas chromatograph via thermal decomposition. Yields greater than 70% were difficult to achieve as it was found that 5 consumed propionaldehyde under reaction conditions but at a slower rate than observed for aldehyde production. The organometallic byproduct of this reaction was not identified. The infrared spectrum of the byproduct in THF has three major absorptions at 1875, 1920, and 1950 cm". (9) (a) Halpern, J. Pure Appl. Chem. 1979, 51, 2171. (b) Hoxmeier, R.; Deubzer, B., Kaez, H. D. J. Am. Chem. SOC.1971,93, 536.
1728
Organometallics 1982, I , 1728-1730
in this case when compared with the analogous reaction of 1 with 5; only a 35% yield of aldehyde was realized after a 2-h period even though a substantially higher concentration of the acyl species was initially present. This experiment attests to the importance of an unsaturated acyl intermediate in the production of aldehyde. Acidification of the mixture from the reaction of 1 with 7 after 2 h with excess acetic acid resulted in a 95% yield of aldehyde, indicating that species 7 was not consumed by other processes and was intact after a 2-h period. Attempted hydrogenation of 5 in THF with 1atm of H2 at 0 OC yielded a small amount (15%) of aldehyde after a 2-h period. Formation of aldehyde by oxidative addition CH3CHz-Fe(C0)4+ CH3CH2C(=O)-Fe(C0)3
Hz 2h
CH3CH2CHO 15% of H2 to an unsaturated metal acyl species is the product-forming step in the hydroformylation reaction.1° In this case, the rate of oxidative addition of 1 to 6 must be much faster than to H2because the reductive elimination step would be expected to be rapid in either case. In an attempt to understand how the hydride 1 interacts with 5 (or 6), the reaction of 5 with more traditional hydride sources was performed. Addition of either NaBH411 or LiAlH4 in excess to THF solutions of 5 failed to produce evidence of any reaction having taken place. Infrared analysis showed that 5 remained intact over a 2-h period during which no ethane, propionaldehyde, or propanol formation could be detected by gas chromatographic analysis. Although both of these reducing agents are considerably stronger hydride sources than 1, neither would be expected to coordinate (or oxidatively add) to species 6. When the rates of reactivity of 1 with both 5 and 7 are contrasted, another potential mechanism by which aldehyde could be produced can be eliminated. The hydride 1 could act as an acid5 which transfers a proton to 6 or 7 with subsequent elimination of aldehyde. However, were this to occur, the more basic of the two species, anion 7, would yield aldehyde at a faster rate, and this was not observed. Protonation of the alkyliron anion 5 would yield the neutral alkyliron hydride 3, an intermediate proposed in the Reppe process. This species has been shown to eliminate alkane in the absence of C0.l2 We found that even under 100 psi of CO pressure this unstable intermediate, prepared in situ from 1 and ethyl iodide at ambient temperature generates ethane and not aldehyde. Therefore, at least under the conditions employed in these studies Et4N+H-Fe(C0)4+ Et1
CO (100 psi) THF, 25 oc
C2H6+ Fe(CO),
aldehyde production is not due to a mononuclear elimination process. Whether the mechanism of aldehyde formation from the binuclear species shown in Scheme I1 is an elimination process from only one of the two iron atoms or the result of reductive elimination from adjacent metal centers cannot be distinguished on the basis of the present work. Finally, the reactivity of the acyl species 8 was contrasted with that of the alkyl intermediates 5 . The mixture of (10)Heck, R. F.;Breslow, D. S. J. Am. Chem. Soc. 1961, 83, 4023. (11)Treatment of a neutral alkylmanganese carbonyl species with NaBHl has been shown to yield the homologous alcohol, see: Fischer, E. 0.; Aumann, R. J. Organomet. Chem. 1967,8,1. (12)Alper, H.Tetrahedron Lett. 1975,2257.
0276-7333/82/2301-1728$01.25/0
CH,CH,C(=O)-Fe(CO), 8
-
+ H-Fe(C0)4
1 essentially no reaction
8 with H-Fe(CO), (1) produced only traces of aldehyde after 2 h. Quenching of the reaction mixture with HOAc produced 57 % propionaldehyde, indicating that a substantial amount of 8 was present at the end of this period. The principal catalytic cycle of the Reppe-modified hydroformylation reaction is very similar to the oxo process, differing only in the manner in which the initial metal hydride is formed. Further parallels can be developed in reactions of intermediate species in both systems, which demonstrates that binuclear mechanisms are alternate potential pathways in a number of catalytic systems. Registry No. 1, 18716-80-8;5, 44966-61-2; 7, 83476-35-1;8, 45048-27-9;CH&H&HO, 123-38-6.
Role of Homogeneous Formate Complexes In the Water Gas Shift Reaction Catalyzed by the Group 6 Metal Carbonylst William A. R. Slegelr," Rlchard S. Saplenra, Richard Rayford, and Lllllan Lam Energy Technology Programs Department of Energy and Environment Brookhaven National Laboratory Upton, New York 11973 Received October 7, 1982
Summary: The mechanism of the homogeneous water gas shift reaction catalyzed by the group 6 metal carbonyls involves the intermediacy of a formate complex. Catalytic pressure reactions as well as stoichiometric photolysis and pyrolysis experiments provide support for this mechanism.
Metal formates have not been generally regarded as key intermediates in catalytic reactions of carbon oxides. However, their intermediacy has been observed in a number of catalysis-related reactions. Metal formates have been isolated in many carbon dioxide reductions.' Zinc formate may be pyrolyzed to methanol and formaldehyde.2 Formates have been suggested in connection with the function of promoters with iron and cobalt catalysts for the Fischer-Tropsch ~ynthesis.~ Formate intermediates have also been observed on magnesia, alumina, and ironchromia catalysts for the water gas shift reaction (WGSR): The role of formates in zinc oxide catalyzed WGSR, carbon dioxide reduction, and methanol decomposition has been studied at varying surface coverages and temperatures by means of infrared spectro~copy.~ Our interest in formate intermediates is related to the oxide theory6 which emphasizes the importance of oxygen-bound species in catalytic reactions. Within this Dedicated to the memory of the late Professor Rolly Pettit, a friend and a teacher. (1)Haynes, P.; Slaugh, L. H.; Kohnle, J. F. Tetrahedron Lett. 1970, 365.
(2)Hofman, K.; Schibsted, H. Chem. Ber. 1918,51, 1389, 1398. (3) Storch, H.; Golumbic, N.; Anderson, R. B. "The Fischer-Tropsch and Related Syntheses"; Wiley: New York, 1951. (4)Rubene, N.A.; Davydon, A. A.; Kravstov, A. V.; Ursheva, N. V.; Smolyaninov, S. I. Kinet. Katal. 1976,17, 465. (5)Tamaru, K.'Dynamic Heterogeneous Catalysis"; Academic Press: New York, 1978,pp 115-29. (6)Sapienza, R. S.;Sansone, M. J.; Spaulding, L. D.; Lynch, J. F. "Fundamental Research in Homogeneous Catalysis"; Tsutsui, M., Ed.; Plenum Press: New York, 1979;Vol. 3, pp 179-97.
0 1982 American Chemical Society
