Inorg. Chem. 1996, 35, 869-875
869
Surface Organometallic Chemistry on Oxides: Reaction of Trimethylphosphine with Bis(allyl)rhodium Grafted onto Silica Susannah L. Scott, Pascal Dufour, Catherine C. Santini,* and Jean-Marie Basset* Laboratoire de chimie organome´tallique de surface, UMR CNRS-CPE 9986, Ecole Supe´rieure de Chimie-Physique-Electronique de Lyon, 43 blvd du 11 novembre 1918, 69616 Villeurbanne Cedex, France ReceiVed May 23, 1994X
The reaction of Rh(η3-C3H5)3 with the surface hydroxyl groups of partially dehydroxylated silica leads to the formation of the surface organometallic complex (tSiO)(tSiOX)Rh(η3-C3H5)2, 1 (where X is H or Sit), with evolution of propene. The reaction of 1 with PMe3 was examined, and two major pathways were discovered. Reductive elimination of two allyl ligands as 1,5-hexadiene and coordination of PMe3 give the surface product (tSiO)Rh(PMe3)3, 2, which was characterized by elemental analysis, IR and 31P MAS NMR spectroscopy. We also prepared 2 independently from CH3Rh(PMe3)3 and partially dehydroxylated silica. The second major reaction pathway is the elimination of propene to give (tSiO)2Rh(η1-C3H5)(PMe3)3, 3. The presence of the σ-bound allyl ligand was inferred from its characteristic IR spectrum. The reactivity of 3 toward CO was examined: insertion of CO into the Rh-C bond followed by reductive elimination of the silyl ester tSiOC(O)C3H5 produces (tSiO)Rh(CO)(PMe3)2, 4. In static vacuum, 3 decomposes to give allyl alcohol, which is slowly decarbonylated by 2.
Introduction Hydroxyl groups present on oxide surfaces are likely to play a role in the chemistry of oxide-supported metal catalysts. Our investigations of the surface organometallic chemistry of rhodium have revealed two possible reaction pathways. When rhodium contains no hydroxyl groups in its coordination sphere, its reaction chemistry is strictly comparable to that of molecular analogues. However, surface hydroxyl group(s) are coordinated to some surface organometallic rhodium fragments, and may participate in the reactions. In the latter case, molecular chemistry does not adequately model the surface reactivity. The well-defined bis(allyl)rhodium surface complex is our starting point, since the coordination sphere of the surface organometallic fragment is well-defined. It is prepared on partially dehydroxylated silica according to the reaction stoichoimetry shown in eq 1.1,2 The direct coordination of the
tSi-OH + Rh(η3-C3H5)3 f (tSiO)Rh(η3-C3H5)2 + C3H6 1 (1) rhodium to a surface oxygen atom has been observed by Raman spectroscopy1 and, on the titania surface, by EXAFS.3 Three microenvironments have been proposed for 1 on the silica surface, based on molecular analogies,4 IR evidence2 and theoretical calculations.5 A sharp IR band at 3636 cm-1 was assigned to the ν(OH) vibration of a rigid silanol group perturbed by coordination to Rh (species 1b), Table 1, whereas a broad X Abstract published in AdVance ACS Abstracts, January 1, 1996. (1) Ward, M. D.; Harris, T. V.; Schwartz, J. J. Chem. Soc., Chem. Commun. 1980, 357-359. (2) Dufour, P.; Houtman, C.; Santini, C. C.; Ne´dez, C.; Basset, J. M.; Hsu, L. Y.; Shore, S. G. J. Am. Chem. Soc. 1992, 114, 4248-4257. (3) Iwasawa, Y. AdV. Catal. 1987, 35, 187-264. (4) Tanaka, I.; Jin-No, N.; Kushida, T.; Tsutui, N.; Ashida, T.; Suzuki, H.; Sakurai, H.; Moro-Oka, Y.; Ikawa, T. Bull. Chem. Soc. Jpn. 1983, 56, 657. (5) Halet, J. F.; Hoffmann, R. J. Am. Chem. Soc. 1989, 111, 3548-3559.
0020-1669/96/1335-0869$12.00/0
IR band in the same region was ascribed to a less rigid coordinated silanol such as in 1a.2 We have found that the surface chemistry of 1 is strongly influenced by the presence and quantity of surface hydroxyl groups.6 Silanol groups were also found to be essential for the reductive carbonylation of silica-supported RhCl3 to [Rh(CO)2Cl]2 and Rh6(CO)16.7 These processes require surface mobility of the organometallic fragments, which is favored by the presence of silanols. The extent of nucleation appears to depend directly on the surface water content.8 The subject of this report is the reaction of 1 with PMe3, which affords two different silicasupported phosphine complexes of Rh, whose relative amounts depend on the silanol content of the silica. Results Adsorption of PMe3 on Silica. The addition of PMe3 (100 Torr) to silica3009 causes the nearly complete disappearance of the free silanol groups (tSi-OH) as judged by the decrease in intensity of the band at 3747 cm-1. At the same time, a broad band attributed to hydrogen-bonded silanols appears at 3300 cm-1. The solid-state proton-decoupled 31P MAS spectra of this material gave a sharp resonance at -61.3 ppm (liquid-like (6) Dufour, P.; Scott, S. L.; Santini, C.; Lefebvre, F.; Basset, J. M. Inorg. Chem. 1994, 33, 2509-2517. (7) Roberto, D.; Psaro, R.; Ugo, R. Organometallics 1993, 12, 22922296. (8) Psaro, R.; Roberto, D.; Ugo, R.; Dossi, C.; Fusi, A. J. Mol. Catal. 1992, 74, 391-400. (9) The notation silica300 means that silica was dehydroxylated under dynamic vacuum at 300 °C.
© 1996 American Chemical Society
870 Inorganic Chemistry, Vol. 35, No. 4, 1996
Scott et al.
Table 1. Identification of Surface Species and Comparison with Molecular Analogues infrared bands compound formula
no. 3-C
(tSiO)(tSiOH)Rh(η
3H5)2
(tSiO)Rh(PMe3)3
frequency (cm-1)
assignment
1
3636 3048, 2992 1492, 1462 1391
ν(O-H) ν(C-H) δ(CH2) ν(C-C-C)
2
2967, 2906 1422, 1295
ν(C-H) δ(CH3)
2980, 2900 1435, 1425, 1290
ν(C-H) δ(CH3)
ClRh(PMe3)3
(tSiO)Rh(CO)(PMe3)2
a
ref
3.1
-2.5, -10.6
this work
-0.38 (td), -11.31 (dd)a
16
6.80 (d), -20.2 (br m) 3
4
ClRh(CO)(PMe3)2
tSiOC(O)CH2CHdCH2
chem shift, δ (ppm) 2
CH3Rh(PMe3)3 (tSiO)2Rh(η1-C3H5)(PMe3)3
31P
elem anal. P/Rh
5
3072 2967, 2905 1612 1422, 1295
ν(dC-H) ν(C-H) ν(CdC) δ(CH3)
2974, 2908 1975 1422, 1295
ν(C-H) ν(CO) δ(CH3)
2970, 2910 1960 1425, 1420, 1275
ν(C-H) ν(CO) δ(CH3)
3072 1765 1643
ν(dC-H) ν(CO) ν(CdC)
3.1
-13.2, 24.3
this work
1.9
-9.3
this work
-10.22 (d)b
16 6
In toluene-d8. b In C6H6-C6D6 (10%).
PMe3) and a very weak, broad resonance at 43.6 ppm (adsorbed OdPMe3).10 The free silanol groups are completely regenerated by evacuation of the IR cell at room temperature. This result was confirmed by volumetric adsorption and desorption measurements: PMe3 which is adsorbed on silica300 is recovered quantitatively by desorption to a liquid N2 trap. Thus PMe3 is reversibly adsorbed on partially dehydroxylated silica. This finding is consistent with previous studies.11 Reaction of (tSiO)Rh(η3-C3H5)2, 1, with PMe3. The formation of 1 by the reaction of Rh(η3-C3H5)3 with partially dehydroxylated silica (pretreated at 200-550 °C) has been described in detail.1,2,12 One equivalent of propene per chemisorbed Rh is evolved, giving a yellow supported complex with two allyl ligands bound to Rh, eq 1. It has also been shown that when the reaction takes place on deuterated silica, the gaseous product is exclusively propene-d1.13 The IR spectrum of 1, Figure 1, contains bands at 1492, 1462, and 1391 cm-1 characteristic of allyl ligands coordinated in an η3-fashion. Qualitative and Quantitative Analysis. The reaction of 1 with 0.1 Torr of PMe3 caused the sample to change color from dark yellow to light yellow. 1,5-Hexadiene and propene were liberated into the gas phase, and were identified by GC. The relative amounts of the two gaseous products depend on the degree of dehydroxylation of the silica, Table 2. On silica dehydroxylated at g400 °C, both gaseous products were observed. On silica dehydroxylated at
