7055
J. Phys. Chem. 1989, 93, 7055-7056
Motions of Adsorbed Rh(CO)* in Y Zeolites P.Molitor and T.Apple* Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304 (Received: June 21, 1989)
NMR line shapes are calculated for adsorbed Rh(C0)2 species undergoing a 180' flipping motion about the C2axis. The resulting line shapes are compared to published NMR spectra for dicarbonyls of Rh formed in Y zeolites. The calculations predict the observed temperature dependence of the line shapes and indicate that adsorbed Rh dicarbonyls in zeolites undergo flipping with an activation energy of 12 kJ. Free rotation about the C2 axis does not occur.
Introduction There is a great deal of interest in the adsorption properties of C O on supported group 8-10 metals. It has-been firmly established that C O bonds to supported rhodium in three modes, linear, bridged, and dicarbonyl forms. 0
0 I
C
/ \
0
c',
0
majority of C O was present as the dicarbonyl.
Theory Spectra were calculated according to the method given by Mehring8 as
r;'-
M
d b
M
linear
bridged
dicarbonyl
A good deal of evidence suggests that the dicarbonyl form of Rh occurs exclusively on isolated Rh atoms' and that these atoms are in the +1 oxidation statee2 Isolated hydroxyl groups have been implicated as the oxidizing agent in the formation of Rh(I).' In a recent study we have shown that the dicarbonyl of rhodium in zeolites behaves as a two-spin pair and that, in agreement with previous work on Rh on A12034the C-Rh-C bond angle is 90°.5 Several solid-state N M R studies of adsorbed CO on Rh have been reported on amorphous supports6 and Y zeolite^.^ A question concerning the motional properties of adsorbed dicarbonyls remains. We report N M R line-shape calculations of Rh(C0)2 species bonded to a support and undergoing a 180' flipping motion about the C2 axis which bisects the C-Rh-C angle. Spectra calculated with this motional model agree well with published spectra and predict the temperature dependence of the line shapes which have been observed for dicarbonyl species formed in Rh-Y zeolites.' Motional models based upon anisotropic free rotation about the C2axis do not agree with the experimentally observed spectra at any temperature.
Experimental Method N M R spectra of the dicarbonyl were calculated using a 90' C-Rh-C bond angle as found by Yates et aL4 for C O on Rh/ A1203 and recently in our group for CO on Rh/Y zeolite.s In the calculations C O molecules exchange positions by a rapid flip about the C2axis which bisects the C-Rh-C bond angle. Since the ratio of 7, the chemical shift asymmetry parameter, to Au, the chemical shift anisotropy, is =0.05, an axially symmetric shift tensor was used for CO. For our calculations we chose values of q3= -50 ppm, corresponding to the field along the C-O bond and u I 1= uZ2= 300 ppm perpendicular to the C-0 direction. Shifts are relative to T M S with positive shifts referring to lower shielding. Comparison will be made between calculated N M R line shapes and the N M R spectra of rhodium dicarbonyls obtained by Takahashi et a].' We present their catalyst preparation procedure here. Those workers treated NaY zeolite with RhC13 at 343 K for 12 h followed by washing and then drying overnight at 373 K. The catalyst was then subjected to evacuation at 393 K for 3 h followed by cooling to room temperature. The catalyst was then exposed to CO at 393 K for 3 h, evacuated, and reexposed for two more cycles, the last of 6 h duration. The catalyst was then evacuated for 1 min at 393 K and placed in an N M R tube. IR spectra coupled with thermal desorption verified that the *Author to whom correspondence should be sent.
0022-3654/89/2093-7055$01.50/0
where 1 is a "one-vector", Wis the site occupancy probability = (1/2,1/2) and
+
i(u-1~1) K
-K i(a-u2)
+K
1
(2)
where K = 1 / and ~ 7 is the mean residence time and ul and u2 are the resonance frequencies in sites 1 and 2 between which the flipping occurs. Evaluation of (1) yields
The frequencies ul and u2 depend upon the orientation (a) of the dicarbonyl in the magnetic field. As pointed out by Mehring,8 evaluation of (3) for a powder can lead to numerical instabilities; therefore the Fourier transform of (3) was calculated as G(t,Q) = exp(-Kt)[(K/R) sinh Rt
+ cosh R t ]
(4)
where R = [F? - (alEvaluation of eq 4 for a powder leads to two cases K < (ul - u2)/2 and K > (ul - u2)/2. For K < (uI - u2)/2, R is imaginary and the magnetization oscillates relative to the reference frequency set at (ul u2)/2.
+
(1) Yates, Jr., J. T.; Duncan, T. M.; Worley, S. D.; Vaughan, R. W. J . Chem. Phys. 1979, 70, 1219. (2) (a) Van't Blik, H. F. J.; Van Zon, J. B. A. D.; Koningsberger, D. C.; Prins, R. J . Mol. Caral. 1984, 25, 379. (b) Van't Blik, H. F. J.; Van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Konigsberger, D. C.; Prins, R. J . Am. Chem. SOC.1985,107, 3139. (c) Cavanagh, R. R.; Yates, Jr., J. T. J. Chem. Phys. 1981,74,4150. (d) Knozinger, H.; Thomton, E. W.; Wolf, M. J. Chem. Sor., Faraday Trans. 1 1979, 75, 1898. (e) Bilhou, J. L.; Bilhou-Bougnol, V.; Graydon, W. F.; Basset, J. M.; Smith, A. K.; Zonderighi, G. M.; Ugo, R. J. Organomet. Chem. 1978, 153, 73. (3) Basu, P.; Panayotov, D.; Yates, Jr., J. T. J. Phys. Chem. 1988, 91, 3133. (4) Yates, Jr., J. T.; Kolasinski, K. J . Phys. Chem. 1983, 79, 1026. 1 5 ) Molitor. P.: Shoemaker. R.; Apple, T. J. Phys. Chem. 1989, 93, 2891. . I.1., I c r r e ~Jr., , J. T.; Vaughan, R.W. J. Chem. Phys. Duncan, T. M.; Yates, Jr., J. T.; Vaughan, R. W. J . Robbins, J. L. J. Phys. C J. Phys. Chem. 1989,93, 2583. (7) Takahashi, N.; Miura, K.; Fukui, H. J . Phys. Chem. 1986,90, 2797. ( 8 ) Mehring, M. High Resolution N M R of Solids, 2nd ed.; SpringerVerlag: Berlin, 1983. (9) This is incorrectly printed in ref 8 as R = [J? - u , u ~ ] ~ / ~ .
0 1989 American Chemical Society
7056
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
Letters A small amount of N M R intensity (5% of the total) appears in the high-temperature spectra near the isotropic value of 180 ppm. With the exception of this small amount of intensity, the simulated spectra reproduce the observed behavior. The flipping motion has a distinct and very different effect on the NMR line shape from that calculated by a free-rotation model. Free rotation about the Cz axis implies a very small energy barrier, E, (ul - u2)/2, R is real and the magnetization decays. Following spherical integration of (4) with P ( 0 ) = sin 0 d e the free induction decays were tapered and Fourier transformed.
Results and Discussion The spectral simulations of hopping dicarbonyls are shown in Figure 1. The spectra are plotted as a function of flipping rate given as the ratio of the flipping rate K to the anisotropy defined as u33- uim. Figure 1 also shows the N M R spectra obtained by Takahashi et al.7 for comparison. The calculated spectra of Figure 1 show that at low flipping rates the line shape is identical with the static powder pattern. As the hopping rate increases to 5% of the shift anisotropy, intensity begins to grow in the central portion of the chemical shift tensor. The upfield edge of this new intensity is exactly halfway between u33and uI1.With a greater flipping rate intensity continues to increase at the midpoint of the tensor but the upfield edge is retained until a relative flipping rate of about (u33- ulso)/3 is reached. In the limit of high flipping rate the N M R powder pattern is inverted and decreased in anisotropy by '/* relative to the static powder pattern. Thus, in the low-temperature limit the Rh(C0)' species yields a chemical shift tensor with principle values u33 = -50, uI1= uZ2= 300, and qm= 183 ppm, while in the high-temperature limit one observes u33= oZ2= 125, u l l = 300, and uiso= 183 ppm, where u33 is measured along the Rh-C-0 bond and u I Iis measured in the direction perpendicular to the C-Rh-C plane. The simulated spectra account very well for the line-shape changes observed as a function of temperature by Takahashi et aL7 At 295 K a static powder pattern with a small amount of intensity halfway between uj3 and u l lis observed. When the temperature is raised to 373 K intensity continues to grow at the midpoint of the shift tensor. At 423 K the upfield edge of the tensor is lost and the full anisotropy has been reduced by one-half.
(5)
where @ is the angle between the rotation axis and the principal axis of the chemical shift tensor. In this case, the bisector of the C-Rh-C angle is the rotation axis and @ is 45'. Equation 5 predicts a decrease of the anisotropy to 114 its original value with no change in direction under conditions of fast rotation. This results in a powder pattern in the rapid motion limit with discontinuities at u = 212 ppm and u = 115 ppm, given the lowtemperature shielding parameters. Takahashi et ale7proposed that the line-shape changes induced at high temperature were due to the following reversible reaction Rh(I)(OH)(CO), zeo-O-Rh(I)(CO)2 + H 2 0 zeo-0-H (6) where zeo-0 represents the zeolite framework. Because the Rh species would have only weak interaction with the zeolite framework, the authors suggested that motions on the N M R time scale would be expected to occur. Our calculations suggest a different picture. The hydroxylated dicarbonyl species proposed by Takahashi et al.' would not be expected to be restricted to 180' flips about the axis bisecting the dicarbonyls. Instead, we propose that the carbonyl groups of Rh(CO)2 undergo a thermally activated flipping motion. Comparison of the experimental spectra with the calculated spectra yields flipping rates of 325, 650, and 1300 f 50 H z a t 295, 373, and 423 K, respectively. From this data an activation energy of 11.8 f 2.0 kJ is calculated. It is certainly possible that the small (
