J. Phys. Chem. 1986, 90, 4882-4887
4882
Molecular Oxide-Supported Rhenium Carbonyl Complexes: Synthesis and Characterization by Vibrational Spectroscopy P. S. Kirlin, F. A. DeThomas, J. W. Bailey, H. S. Gold, C. Dybowski, and B. C. Gates* Center f o r Catalytic Science and Technology, Departments of Chemical Engineering and Chemistry, Uniaersity of Delaware, Newark, Delaware 19716 (Receiaed: March 14, 1986; In Final Form: May 9, 1986)
Supported rhenium catalysts were prepared by bringing the rhenium cluster [H3Re3(CO),,] in hexane solution in contact with metal oxide supports 7-A1203and MgO. The resulting surface species were characterized by UV-vis, infrared, Raman, and inelastic electron tunneling spectroscopy. On 7-A1203,the rhenium cluster was initially physisorbed; on MgO, it was deprotonated to give the surface-bound anion [H2Re3(CO),,]-. On exposure to air the surface clusters broke up, leading to formation of surface-bound rhenium subcarbonyls identified by the spectra as [Re(C0)3(0-S}(L-S12] (S = AI or Mg of the support and L = OH or possibly fi-oxo). The vibrational spectra provide detailed evidence of the surface structure, including the C-0, Re-OH, Re-0, and Re-C bonds.
Introduction Supported metal catalysts are among the most important used in technology, but their structures are difficult to elucidate because they are highly nonuniform. Progress in the understanding of supported metal catalysts has been accelerated by investigations of simple models, single crystals of metal with systematically varied surface structures.’ The single crystals are useful in representing the different geometric arrangements of metal atoms that occur in supported-metal crystallites, but they do not provide an inroad into the problem of metal-support interactions. Our attempts to understand the structures of supported-metal catalysts are complementary to those involving single crystals of metal. We have prepared supported metals with isolated metal centers having structures so similar to those of molecular analogues that they can be determined precisely by surface spectroscopic methods; these supported “molecular” catalysts even offer the prospect of direct characterization of the metal-support bonds. Rhenium is the metal of choice for several reasons: (1) it is stable on metal oxide supports in a variety of forms, ranging from mononuclear carbonyl c o m p l e x e ~ ~to- ~metal oxide clustersS to oxide layers6to metal crystallites;’ (2) it is an important component in technological catalysts for reactions including alkene metathesis6 and reforming of petroleum naphtha;* and (3) the catalytic performance is sensitive to the oxidation state, ligand environment, and degree of aggregation of the rhenium. Oxide-bound carbonyls of transition metals have been formed by numerous method^,^ including surface reactions formally described as the oxidative addition of hydroxyl groups across the metal-metal bonds of metal These oxide-bound subcarbonyls can be represented as [ M(CO)jGS)k(L-S}m],where M is the transition metal (e.g., Mo, W, Re, Fe, Ru, Os, Rh), S is the surface cation of the metal oxide support (e.g., AI, Mg, Si, Ti), L is a weakly coordinated ligand (e.g., OH, ,u-oxo), and j , k , and m are integers ranging from 0 to 5.2.’(t’3 Much effort ( 1 ) Somorjai. G. A. Adc. Catal. 1977, 26, 1.
(2) Kirlin. P. S.; DeThomas. F. A.; Bailey, J. W.; Moller, K.: Gold, H. S.; Dybowski, C.: Gates, B. C. Surf. Sci., in press. (3) Nicolaides, C. P.; Gates, B. C. J . Mol. Catal., 1986, 35, 391. (4) McKenna, W . P.; Higgins, B. E.; Eyring, E. M. J . Mol. Catal. 1985, 31. 199. ( 5 ) Akimoto. M.; Shima, H. 1.; Echigoya, E. J . Caral. 1984, 86, 173. (6) Kerkhof, F. P. J. M.; Moulijn. J. A.; Thomas, R. J . Catal. 1979, 56. 219. (7) Szyrnura, J. A.; Paryjczak, T. React. Kine?. Caral. Left. 1979, 12, 219. (8) Sinfelt, J. H. Bimetallic Catalysis: Discoveries, Concepts, and Applications; Wiley: New York, 1983. (9) Gates, B. C., Guczi, L., Kniizinger, H., Eds. Metal Clusters in Catalysis; Elsevier: Amsterdam, in press. ( I O ) Basset, J. M.; Choplin, A. J . Mol. Catal. 1983, 21, 95. (11) Hucul, D. A.; Brenner, A. J . Phys. Chem. 1981, 85, 496. (12) Kuznetsov, V. L.; Bell, A. T.; Yermakov, Y. I. J . Cafal. 1980,65, 374. (13) van’t Blik, H . F. J.: van Zon, J . B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J . A m . Chem. Soc. 1985. 107, 3139.
0022-3654/86/2090-4882$01.50/0 , , ,
has been devoted to characterizing these surface-bound carbonyl c o m p l e ~ e s ,but ~ the species present under catalytic reaction conditions (e.g., for hydrocarbon conversion) are typically not known; the group VI11 (groups 8-10)s3 metals usually are reduced and aggregated under catalytic conditions, and the advantages of structural simplicity are lost. From the published work we may infer several criteria for preparation of stable and catalytically active metal subcarbonyls on metal oxide supports: (1) a nonreducible metal-oxygen bond is required to tether the metal complex to the support; (2) one or more weakly coordinated (and consequently easily displaced) surface ligands may be required to allow bonding of reactants to the metal center; and (3) a metal center with a low oxidation state is often needed, at least for hydrocarbon conversions, since reactions such as H 2 dissociation and C-H insertion are oxidative additions and are favored by high electron density on the metal center.14 The goal of this research was to prepare simple mononuclear rhenium carbonyl complexes anchored to metal oxide supports and to determine their structures and catalytic properties. Here we report the structure determination by vibrational spectroscopy (infrared, Raman, and inelastic electron tunneling spectroscopies) and UV-vis spectroscopy. Further structural details determined by EXAFS and catalytic data are to be reported e1~ewhere.l~
Results and Discussion Catalyst Preparation. Catalysts were prepared by bringing the organometallic precursor [H3Re3(C0)12]in hexane solution in contact with the metal oxide support (y-A1203or MgO). The resulting organometallic surface species were characterized spectroscopically, and the rhenium loadings (1-6 wt %) were determined by X-ray fluorescence spectroscopy. Infrared and Electronic Spectra. The spectra indicate that physisorption of the organometallic precursor is predominant on y-Alz03(Figure la,b); we find close agreement between the broad envelope in the carbonyl absorptions in the infrared spectrum of the surface species (Figure l a ) and that of [H3Re3(C0),2]in cyclohexane (Figure 1b). The corresponding UV-vis spectra (Figure 2a,b) agree to the same extent. The coalescence of the two major peaks in the UV-vis spectrum of the supported rhenium species and the shoulder at 410 nm suggest that a small fraction of the adsorbed [H3Re3(CO),,] may have been deprotonated by the basic sitesI2on the y-A1203surface; this suggestion is supported by the corresponding features in the IR spectrum (Figure la). In contrast, the agreement of the spectrum of [H3Re3(CO),,] supported on M g O (Figure 2c) with that of (14) Collman, J. P.; Hegedus, L. S. Principles and Applications of Organotransifion Metal Chemisfry; University Science Books: Mill Valley, CA, 1980. ( I 5) Kirlin, P. S.; Gates, B. C., to be published
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4883
Oxide-Supported Rhenium Carbonyl Complexes
n
I
2200
I
2100 2000 1900 WAVENUMBERS, c r n - '
1800
Figure 1. Infrared spectra in the carbonyl stretching region of (a) sample prepared from [H,Re3(C0),2]and A1203(1.3 wt % Re); (b) [H3Re,(CO),,] in cyclohexane; (c) sample prepared from [H,Re,(CO),,] and MgO (2.2 wt % Re); and (d) [(C6H5)4As]+[H2Re3(CO)12]in acetone.
[(C6H5)4As]+[H2Re3(C0)12]in CH2C12(Figure 2d) indicates that [H3Re3(CO),,] is deprotonated by the basic Mg0I6 surface (undergoing dissociative chemisorption). The infrared spectra (Figure lc,d) confirm this conclusion; the shifts in the infrared spectrum of the supported species relative to that of [(C6H5)4As]+[H2Re3(C0)12]in acetone are attributed to the interaction of the carbonyl ligands of the monoanion with the Mg2+ ions of the surface.I6 Extraction of the MgO-supported sample with a solution of [(C6H5)4AsCl]in CH2C12(acetone reacts with MgO) gives a solution with a spectrum indistinguishable from that of Figure Id, demonstrating that, once formed, the anion can be extracted via cation metathesis into solution. Exposing the supported trirhenium clusters to air at 25 "C for 18 h or heating them in H2 at 225 "C for 4 h resulted in the breakup of the trirhenium framework and the formation of mononuclear rhenium subcarbonyls on the metal oxide surfaces. A comparison of the electronic absorption spectra of Figures 2 and 3 shows that the peaks characteristic of the sorbed clusters (Figure 2) disappear when the samples are heated (Figure 3). On the basis of a comparison with reported s p e ~ t r a , ' ~the ~ ' remaining ~ absorption (262 nm) for both the y-A1203-and MgO-supported samples (Figure 3) is assigned to a metal-ligand charge-transfer band in the mononuclear carbonyl complex. The infrared spectra clearly indicate the fragmentation of the supported clusters to give mononuclear complexes (Figure 1 vs. 4). The spectra measured after cluster breakup are nearly the same as those reported for mononuclear complexes having the formula [Re(CO),L2X] (where L is a donor ligand and X is a halogen).I9 These complexes of Re(1) have C, symmetry, and their infrared spectra agree closely with that of the surface species in the number, relative intensity, and position of the carbonyl bands (16) Lamb, H.H.;Gates, B. e. J . A ~Chem. . sot. 1986, 108, 81 and references cited therein. (17) Blakney, G . B.; Allen, W. F. Inorg. Chem. 1971, 10, 2763. (18) Gray, H. B.; Billig, E.; Wojeicki, A,; Farons, M. Can.J. Chem. 1963, 41, 1281. (19) Sartorelli, U.; Canziani, F.; Zingales, F. Inorg. Chem. 1966, 5,2233.
200 400 600 WAVELENGTH, nm Figure 2. Electronic absorption spectra of (a) sample prepared from [H3Re3(C0)12]and A1203 (1.3 wt 7% Re); (b) [H,Re,(CO),,] in cyclohexane; (c) sample prepared from [H,Re,(CO),,] and MgO (2.2 wt % Re); and (d) [(C6H5)4As]+[H2Re3(CO)12]in CH2C1,.
+
(2A' A"). On the basis of this agreement, it has been proposed3 that [Re(CO)3{O-Al)(HO-A1)2] is formed from [Re(CO),Br] on y-A1203(here the braces { ) denote surface groups). We infer that [Re(CO)3(O-A1){HO-A1]2] and [Re(CO)3(O-Mg](HO-Mg]2] (Figure 5) were similarly formed from [H3Re3(C0),2]on the surfaces of y-A1203and MgO, respectively. However, the donor ligands are not identified specifically, and other possible structures such as [Re(CO)3{OAl){O
