J. Phys. Chem. 1994,98, 13665-13668
13665
31PNMR Study of the Substitution Reaction of Trimethylphosphine with Mo(C0)6 in Y Zeolite Sanlin Hu and Tom Apple* Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180 Received: July 7, 1994; In Final Form: October 7, 1994@
The substitution reaction of trimethylphosphine with molybdenum hexacarbonyl in dehydrated Na-Y zeolite has been studied by 31Psolid-state NMR spectroscopy. The overall rate of the reaction is governed by the two-term rate law rate = kl[Mo(C0)6] -I-kz[Mo(CO)6][PMe3]. The reaction occurs predominantly via an SN2 mechanism in the temperature range from 298 to 338 K. At higher temperatures the reaction occurs largely via an SN1 mechanism. Transition entropies were determined for the reaction in the high- and lowtemperature regions. At low temperatures the entropy of transition is negative. This is consistent with the seven-coordinate transition state expected in an SN2 reaction. At higher temperatures the entropy of transition becomes positive, as expected for an SN1 reaction involving predissociation of CO. The reaction is activated in the cages of the zeolite, lowering the enthalpy of transition (AH*) relative to the reaction in solution. This allows the reaction to proceed at a significant rate at room temperature. Mo(C0)5PMe3 is the major product of the reaction; however, some Mo(C0)4(PMe3)2 is also formed at higher temperatures and long reaction times.
Introduction
Experimental Section
There has been considerable interest in recent years in the species formed by absorption of Mo(CO)~into zeolites. This interest stems from the potential these systems hold for providing well-defined catalysts and unique electronic materials. A number of reports have appeared which deal with the thermal decarbonylation, loading level and structure? ~tability,~ motion: and location5 of supported molybdenum carbonyls in zeolites. Less is known about the reactions these sorbed molecules undergo within the zeolite cage. We report here a kinetic study of the reaction of molybdenum hexacarbonyl, Mo(CO)~,with trimethylphosphine, PMe3, in Na-Y zeolite by solid-state 31P NMR. The substitution reactions of the group VI metal carbonyls with phosphine and phosphite ligands have been canied out by Angelici et a1.6 in decalin solvent. Their results indicate that the hexacarbonyl complexes of group VI metals react with L (where L is a phosphine or phosphite) at a rate which is pseudo-first-order govemed by a two-term rate law:
Solid trimethylphosphine-silver iodide was obtained from Aldrich and was used as a gas phase source of PMe3 by heating the complex to 473 K. Mo(CO)~was obtained from Aldrich and was used without further purification. Na-Y zeolite with unit cell composition Na56(A102)56(Si02)136.27oH~Owas used as obtained from Union Carbide. 13C0 (99%) was acquired from MSD Isotopes. About 70 mg of the Na-Y zeolite was loaded into a 5 mm thin-walled NMR tube, and ca. 50 mg of glass wool was placed on top of the zeolite in order to prevent the sample from drawing up the tube during evacuation. All samples were dehydrated under dynamic vacuum to a base pressure of Torr by heating them at 373 K for 4 h followed by heating at 673 K for at least 10 h. The temperature was controlled by an Omega Series CN-370 controller. Following dehydration, the sample was cooled to room temperature and exposed to a constant supply of Mo(CO)~at its equilibrium vapor pressure of 0.80 Torr. The degree of loading was controlled by the exposure time. About 100 h was required for Mo(CO)~to diffuse down the 1.5 cm length of the sample bed in the NMR tube, corresponding to a diffusion coefficient of about 1 x lop4cm2/ s. The vacuum line was then evacuated to remove the overpressure of Mo(CO)~vapor. For samples prepared for 13C NMR experiments, the sample was then exchanged with 13C0 for 2 h. For kinetics runs the trimethylphosphine-silver iodide complex was placed in a small glass vessel connected to the vacuum line, and the complex was heated gradually to produce a given amount of PMe3. The sample was then exposed to the PMe3. The PMe3 adsorption was conducted at room temperature. Samples were also prepared in the reverse manner with PMe3 loaded first followed adsorption of Mo(CO)~. PMe3 loadings were determined by uptake measurements, while the loading of Mo(CO)~was determined gravimetrically. All samples were flame-sealed at least 1 cm above the top of the sample after completion of the loadings. Reported loadings have an uncertainty of f0.05 moleculekage.
rate = kl[MO(CO)6]
+ kz[MO(CO)6][L]
(1)
This reaction has been studied in Y zeolites by Ozin et al.’ In that work kinetic studies were interpreted under the assumption that only disubstituted product was formed. Rates of reaction were sensitive to loadings of PMe3 and CO overpressure. Two pathways were observed, one which is retarded by CO and one which is not. We report here that the rate of the reaction of Mo(CO)~with PMe3 in Na-Y zeolite is also govemed by a two-term rate law, but the mechanism is temperature-sensitive. The reaction proceeds via an SN2 process, characterized by the second term in eq 1 in the lower temperature regime from 298 to 338 K. At higher temperatures the reaction pathway changes to predominantly SN1. We have observed both mono- and disubstituted species and can identify the major product as the monosubstituted species. @Abstractpublished in Advance ACS Abstracts, November 15, 1994.
0022-365419412098-13665$04.50/0 0 1994 American Chemical Society
Hu and Apple
13666 J. Phys. Chem., Vol. 98, No. 51, 1994 31Psingle-pulse NMR spectra were acquired at 8.45 T on a Chemagnetics CMX-360 spectrometer using a double-resonance probe. The 90" pulse width was 6 ,us. Recycle times between 1 and 10 s were used depending on the species being monitored. T I measurements were carried out at room temperature, using the inversion-recovery method8 with values in the range from 200 p s to 10 s. For kinetics studies the temperature was controlled by a gas flow cryostat using clean dry air as the heater gas. Typically, the 31P spectra for the kinetics studies were acquired with only about 10 min of signal averaging. Free induction decays were obtained at various time intervals from 5 to 60 min depending on the temperature of the kinetics run. Chemical shifts are reported relative to 85% H3P04, with negative shifts implying greater shielding. Reported chemical shifts are reported with a confidence of f0.5ppm.
1
I/ m m
Results and Discussion
3b0
Mo(CO)a Loading. When dehydrated Na-Y zeolite is exposed to Mo(CO)6 vapor, the impregnation of Mo(CO)~ proceeds with the formation of a light yellow band which widens progressively from the top toward the bottom of the 1.5 cm bed as adsorption takes place. The progress of the loading can be monitored easily by observing the yellow band front. The color change is probably due to partial decarbonylation of the Mo(CO)~during the impregnation: Na-Y
+ Mo(CO),
L
-
Na-Y-Mo(CO)S
+ CO
(2)
Mo(CO)s subcarbonyl species have been reported on alumina upon adsorption of Mo(CO)~. This species yields a yellow color? Further support for the validity of reaction 2 is provided by the observation that the pressure above the sample increases during adsorption to a value greater than the equilibrium vapor pressure of the Mo(CO)~.This pressure increase can only come from CO released upon formation of Mo(CO)5. Condensation of the Mo(CO)~with a dry icelacetone cold finger leaves a residual CO overpressure corresponding to 0.17 CO per adsorbed Mo species. Thus, approximately 1 in every 6 Mo(CO)~ species adsorbs with loss of CO. Following adsorption, the white color of the original zeolite sample can be recovered by exposing the sample to an overpressure of CO. The adsorbed Mo(C0)s returns to the white Mo(CO)~species. The I3C NMR spectrum of this sample shows a single motionally narrowed resonance at 206 ppm assignable to Mo(CO)~. We find the maximal loading of Mo(CO)~in dehydrated Na-Y zeolite to be two Mo(CO)~per supercage. This is consistent with a number of other observations.2C-e The framework of Na-Y zeolite creates a 3-dimensional network structure of large 13 %, diameter supercages. There are eight supercages per unit cell, with four 7.5 A 12-ring windows providing access to the supercage. Crystallographic studies of Na-Y zeolite have shown that the extraframework cations in the supercage are located at both sites I1 and III. The 6-ring site II is the most highly populated, accounting for 3235 cations per unit cell (or roughly four Na+ per supercage).10 These cations are tetrahedrally arranged in the supercage. This has led Ozin et a1.2e-g to postulate that the Mo(CO)~species form between the Na+ cations as Naf--0-C -Mo-C-0--Na+ in an orthogonal manner. This arrangement allows two Mo(CO)6 species per cage. PMe3 Loading. Our uptake and gravimetric measurements yield a saturation loading of 4 PMe3 per supercage in the dehydrated Na-Y zeolite, of 32 PMe3 per unit cell, in agreement with Ozin's group.7 This implies that the adsorption of PMe3
ab0
!bo
-'O
I""
-100
""I'
-do0
-300
NMR of adsorbed PMe3 (top) and coadsorbed Mo(CO)~ and PMe3 (bottom). Figure 1.
is linked to the number of the cation sites within the supercage. This involves an interaction between the lone pair on PMe3 and the cation sites of the type ZO- Naf*.PMe3. The 31P NMR spectrum of this sample shows a single resonance at -59 ppm, close to the resonance of neat PMe3 liquid, which has a chemical shift of -61 ppm. The signal at -59 ppm is completely removed when the sample is evacuated to 1 x Torr with or without heating of the sample, although heating greatly accelerates the desorption. Both the small chemical shift change upon sorption and the disappearance of the peak upon outgassing indicate that the interaction between PMe3 and the dehydrated Na-Y is a weak one. We, therefore, consider this species to be physisorbed PMe3. Due to the weak bonding and high mobility of the PMe3, we will not separate the [PMe3] in our kinetics analysis into those PMe3 molecules contained in the same cage as a Mo species and those in "empty" cages. This procedure was used in ref 7 (although we believe that it was done incorrectly)." Coadsorption of MO(CO)~ and PMe3. Following saturation loading of PMe3, the maximum loading of Mo(CO)~decreases from 2 to 1 Mo(C0)dsupercage. This is probably due to crowding in the supercage. When a sample which was loaded initially with 2 Mo(CO)~per cage is exposed to PMe3 vapor at room temperature, the maximum loading of PMe3 is decreased from 4 to 2 PMeslcage. The adsorption rate is also greatly reduced compared to that in the empty cage. Kinetics of the Reaction of Mo(CO)a with PMe3. The kinetics study of reaction of PMe3 with Mo(CO), in the Na-Y zeolite was carried out by 31P solid-state NMR spectroscopy. If only PMe3 is loaded into the dehydrated Na-Y zeolite, just one peak is observed at -59 ppm. No resonance at -2 ppm, assignable to the[(CH3)3P-H]+ complex, is observed.12 For kinetics runs Mo(CO)~is adsorbed initially to a loading far lower than the concentration of PMe3, typically being set at 0.25 Mo(CO)6 molecule per cage. This loading ensures pseudo-firstorder behavior. For the sample initially loaded with Mo(CO)~ and exposed to PMe3, we observe a new peak in the 31PNMR spectrum at -21 ppm with a broader line width than the peak assigned to physisorbed PMe3 (Figure 1). We assign this resonance to the reaction product Mo(C0)5(PMe3). The intensity of this peak increases as the reaction proceeds. At long reaction times and at higher temperatures we observe a shoulder at -17 ppm in the 31P spectrum. We assign this shoulder to the disubstituted product Mo(C0)4(PMe3)2.
Substitution Reaction of PMe3 with Mo(CO)~
J. Phys. Chem., Vol. 98,No. 51, 1994 13667
In benzene solution the monosubstituted product and the disubstituted product resonate about 3 ppm apart with the monosubstituted peak farther upfield (-17.3 and -14.5, respectively).’ Our shifts in Na-Y appear to be in close agreement to those in solution with an approximate 3 or 4 ppm downfield shift for both resonances. While Angelici et ale6observed only monosubstituted product in decalin solvent, Ozin et aL7 reported only the disubstituted product for the reaction in Na-Y. This assignment was based primarily upon IR and SEM-EDX measurements. They showed some similarities between the IR spectrum of the product and bulk Mo(C0)4PMe2; however, the IR spectrum of Mo(CO)5PMe3 was not shown. The SEM-EDX measurements provided similar Mo:P ratios for the kinetic product and bulk standards having a 2:l ratio. In that work it was not described how the intensity from the kinetic product was separated from that due to unreacted species in the cages. The 31PNMR shifts reported by Ozin do not support the assignment of the product to disubstituted Mo(C0)4(PMe3)2. Their observed shift of -20.1 ppm actually falls closer to that of the monosubstituted product. It is also not clear why only disubstituted product should appear when the monosubstituted product much be formed fist. We are fortunate enough to observe both species in our experiments, and their shifts are consistent with those in the liquid state. The fact that the peak at - 17 ppm appears at long reaction times is further support for its assignment to the disubstituted form. The rate constants of the reaction are obtained by monitoring the integral intensity of the product species. In order to obtain quantitative results, the pulse repetition rate was set greater than 5 times the spin-lattice relaxation time (TI) of the peaks being measured. The peak at -59 ppm has a TI of 200 ms while that at -21 ppm is 1 s. The kinetic studies of group VI metal carbonyl complexes with P-donor nucleophiles in decalin solvent have shown that the reactions proceed via both CO dissociative and ligand associative paths. A CO dissociation pathway dominates for M o . ~Those studies found that the two-term rate law (1) holds for the reaction of Mo(CO)~with PMe3 in decalin. With [MI representing the concentration of Mo(CO)~,[L] representing PMe3 concentration, and [PI representing the product Mo(C0)5PMe3, eq 1 becomes (3) R = k2LI [MI + k, [MI Under pseudo-first-order conditions with [PMe3] >> [Mo(C0)6]
R = k,,[MI
(4)
where
R = d[P]/dT = -d[M]/dt = keff[M]
(6)
[PI + [MI = [MI,
(8)
one obtains
therefore
5 T 4.5
4 h
8
s. &
3.5
Y c
C
2 4 0
I 50
100
150
200
250
300
Time (min)
Figure 2. Plot of In{ 1 - [P]/[P]-} vs time for the reaction of PMe3 with Mo(C0)6 at various temperatures (in K): 0,298; 323; 0, 333; A, 348; x, 358; 0, 373; A, 383.
+,
and
We measure the extent of reaction at very long times and find it to be greater than 95% complete at all temperatures; thus
and
leading to ln{l - [P]/[PlW}= -ked (13) Figure 2 shows a plot of ln{l - [P]/[P]=} versus time obtained by integration of the product resonance in the temperature range from 298 to 383 K. Pseudo-fiist-order rate constants k e = ~ kl kz[PMe3] are obtained at all temperatures used in this study. A dramatic change in the slope of the plot of ln(kdT) versus 1/T is observed in moving from the lower temperature range to the higher temperature range (Figure 3). Non-Arrhenius behavior of this type is often associated with a mechanism change. The reaction was examined as a function of ligand concentration. Table 1 shows the effect of PMe3 ligand concentration on rate of the reaction. From Table 1 it is clear that the reaction rate is sensitive to the initial concentrations of PMe3 in the lower temperature range, but not in the higher temperature range. This indicates that the reaction is dominated by an associative mechanism (ligand concentration-dependent) in the lower temperature range from 298 to about 338 K. It is dominated by a dissociative mechanism in the higher temperature range from about 338 to 383 K. To further explore the reaction mechanism in the two temperature regimes, the reaction was investigated with and without an overpressure of CO. Figure 3 shows the effect of an external pressure 500 Torr of CO on the reaction rate. The reaction is retarded only in the higher temperature range, providing further evidence for a dissociative (CO inhibited) mechanism in this regime. The activation parameters for the reaction were calculated for the dissociative and associative mechanisms from the
+
Hu and Apple
13668 J. Phys. Chem., Vol. 98, No. 51, 1994
that the zeolite greatly lowers the enthalpy of transition for formation of the activated complex in the associative regime. This implies that the favorable interactions between the complex and the surroundings are maintained to a greater extent than those in solution during formation of the activated complex. It is this effect which allows the reaction to proceed at room temperature in the zeolite, while higher temperatures were required for reaction in decalin. At higher temperatures the reaction proceeds more readily by the predissociation of a CO group from Mo(CO)~: that is, the first-order term (SN1) dominates (k2[L]lkl 1, Table 1). The activation entropy for the associative mechanism is negative, as expected for a seven-coordinate activated complex. Due to the constraints of the zeolite cage, the value of A&* in the zeolite is considerably more negative than that found in decalin solvent. The reaction at low temperatures, therefore, involves a nucleophilic attack by PMe3 upon the Mo(CO)~.This results in a seven-coordinate transition state. It is interesting to note
Society: Washington, DC: 1977; p 144. Yong, Y. S.; Howe, R. F. J . Chem. Soc., Faraday Trans. 1 1986,82,2887. Okamoto, Y.; Maezawa, A.; Kane, H.; Imanaka, T. J . Catal. 1988, 112, 585. (2) (a) Ozin, G.; Ozkar, S.; McIntosh, D. J. Chem. SOC., Chem. Commun. 1990, 841. (b) Komatsu, T.; Namba, S.; Yashima, T.; Domen, K.; Onishi, T. J. Mol. Catal. 1985, 33, 345. (c) Komatsu, T.; Yashima, T. J. Mol. Catal. 1987,40, 83. (d) Yong, Y. S.; Howe, R. F.; Hughes, A. E.; Jaeger, H.; Sexton, B. A. J. Phys. Chem. 1987, 91, 6331. (e) Ozkar, S.; Ozin, G.; Moller, K.; Bein, T. J . Am. Chem. SOC.1990, 112, 9575. (0 Moller, K.; Bein, T.; Ozkar, S.; Ozin, G. A. J. Phys. Chem. 1991, 95, 5276. (g) Ozin, G. A.; Ozkar, S.; Macdonald, P. J . Phys. Chem. 1990, 94, 6939. (3) (a) Okamoto, Y.; Maezawa, A.; Kane, H.; Imanaka, T. In Proceedings of 9th Intemational Congress on Catalysis, Calgary; Phillips, M. J., Teman, M., Eds.; Chem. Inst. Canada: Ottawa, Ontario, 1988; Vol. 1, p 11. (b) Okamoto, Y.; Maezawa, A.; Kane, H.; Mitsushima, I.; Imanaka, T. J . Chem. Soc., Faraday Trans. 1 1988, 84, 851. (c) Ward, M. B.; Lunsford, J. H. In Proceedings of 6th Intemational Zeolite Conference, Reno, W,Olsen, D., Bisio, A., Eds.; Butterworth: London, 1984; p 405. (4) Shirley, W. M.; Powers, C. A.; Tway, C. L. Colloids Surf. 1990, 45, 57. ( 5 ) Tway, C. L.; Apple, T. M. Inorg. Chem. 1992, 31, 2885. (6) Graham, J.; Angelici, R. Inorg. Chem. 1967, 11, 2082. Angelici, R.; Graham, J. J . Am. Chem. SOC.1966, 88, 3658. (7) Pastore, H.; Ozin, G.; Poe, A. J.Am. Chem. SOC. 1993, 115, 1215. (8) Farrar, T. C.; Becker, E. D. Pulse and Fourier Transfoim NMR; Academic Press: New York, 1971. (9) Hanson, B. E.; Wagner, G. W.; Davis, R. J.; Motell, E. Inorg. Chem. 1984, 23, 1635. (10) Breck, D. W. Zeolite Molecular Sieves; Kieger Publishing: Malabar, 1984. (1 1) In ref 7 the authors assume that every cage containing a Mo(CO)~ species will bind two PMe3 species at the vacant cation sites and that the remaining PMe3 molecules are distributed evenly between the remaining sites in the "vacant" cages. There is, however, no reason to assume that the cages containing a Mo(CO)~should preferentially bind two PMe3 species (100% occupancy in those cages-less in the remaining cages). The PMe3 molecules should be distributed equally among all available cation sites. (12) Lunsford, J.; Rothwell, W.; Shen, W. J.Am. Chem. SOC.1985,107, 1540. Rothwell, W.; Shen, W.; Lunsford, J. J. Am. Chem. SOC.1984, 106, 2452.
