J. Phys. Chem. 1081, 85, 549-556
20% of the total U(V1) in each of the 1,0 and 3,5 species, respectively. The close correspondence between the equilibrium distribution of U(V1) species at pH 3.8 and the relative intensities of the bands in Figure 2, curve F, led to the assignment of the shoulder at 836 cm-l as due to the symmetric stretching mode of the U(V1) in the trinuclear cluster, (UO,),(OH),+. One further striking aspect is that the intensity changes of the bands a t 836, 851, and 869 cm-’ (sequence A-F, Figure 2) follow the equilibrium distribution shown in Figure 3 for the 3,5,2,2, and 1,O species, respectively. (The pH values at which A-F of Figure 2 were measured8 are indicated on Figure 3.) Heretofore, no discussion of Raman spectra for uranyl clusters larger than the dinuclear species has been reported. The decrease in frequency of the UOZz+modes upon complexation of the ion has been attributed3s6largely to overlap of the nonbonding & and 6, uranium orbitals with the ligand orbitals, resulting in a considerable ligand-metal electron density shift. A piling up of charge on the uranium atom then results in the repulsion of the negative oxygen atoms and a “loosening” of the U=O bonds. Especially significant is the effect of halide and hydroxyl ligands which produce unexpectedly low frequencies for the uranyl v1 and v3 modes (3). It is thus anticipated that there will be some weakening of the uranyl bonds as the complex ion hydrolyzes and aggregates. The first of these hydrolyzed species is the dinuclear complex in which two uranyl ions are joined together by two hydroxyl bridges.s The second species is (8) These values were measured with a pH meter standardized against a NBS buffer, and therefore include the assumption that the Ht activity coefficients are similar in both media.
540
considerably more complex in its bonding since it is believed to consist of a ring of three UOzz+ions bridged by hydroxyl groups and then capped on the top and bottom by the two remaining hydroxyl ions which are separately bonded to each of the three uranium atoms.7 Since the uranium atoms in the trinuclear complex are even more extensively bound by hydroxyl ions than in the dinuclear species, it is reasonable to expect that the uranyl bonds will be further weakened and the frequency of the symmetric mode will be correspondingly reduced. The agreement between the intensity shift of these bands as a function of pH and the known equilibrium distribution of the uranium species confirms these conclusions. The observed shift in the v1 band in the aqueous solutions is only 15-18 cm-’ between adjacent hydrolysis species and the v3 band is unobserved while the v2 band is very weak. These observations indicate that the basic linear UOZ2+ structure is still dominant in the hydrolyzed molecules. A frequency range of 780-900 cm-l has been previously noted for v1 of U022+which is indicative of the degree of UOzZ+complexation by ligand groups in a plane perpendicular to the axial 0-U-0 direction. A still broader range of values could be anticipated as the UOzz+ion becomes more intimately complexed to a network structure such as that which prevails when the hydrolyzed actinides aggregate. Acknowledgment. L. M. Toth was sponsored by the Division of Chemical Sciences, US. Department of Energy under contract W-7405-eng-26 with the Union Carbide Corporation. G. M. Begun was sponsored by the Division of Material Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract W-7405-eng-26with the Union Carbide Corporation.
A Study of Supported Chlorocarbonylbls(triphenylphosphine)rhodlum(I) and Its Reactions with CO Armando Luchettl, Larry F. Wleserman, and Davld M. Hercules* Department of Chemistry, Universlly of Pittsburgh, Pittsburgh, Pennsylvanla 15260 (Received: JuW 14, 1980; In Final Form: September 22, 1980)
-
-
The supported analogue of RhCl(CO)(Ph,P)Zwas prepared by a two-step reaction: siloxane plus Rh precursor siloxane complex; siloxane complex plus support supported complex. Silica and alumina were used as supports. Infrared and X-ray photoelectron spectroscopy (ESCA) were used to study changes which occur when the supported complex reacts with CO or CO + H2.Similar behavior was found with and without Hp On silica, initially a square-planar dicarbonyl complex is formed from the starting monocarbonyl. Increasing severity of CO treatment produces a dimeric species. The dimeric species is unstable and reverts to the initial dicarbonyl species on standing. Subsequent treatment with CO restores the dimer, indicating reversibility. When the alumina-supportedcomplex is treated with CO, both square-planarand pentacoordinated dicarbonylsare formed. Dimeric species are formed from each kind of dicarbonyl and are distinguished by their infrared spectra. On standing, all species revert to the square-planar cis complex which, on subsequent CO treatment, gives a cis dimer as in the case of silica. Reactions of the supported complexes with CO are similar to the reactims of comparable homogeneous complexes in solution.
Introduction There has been recent attention to studies of supported Rh complexes, because of their activity as homogeneous catalysts in reactions such as hydrogenation and hydro0022-3654/81/2085-0549$01.25/0
formylation. Many studies have been carried out with complexes supported on organic materials1* while only a (1) Jarrell, M. 5.;Gates, B. C. J. Catal. 1978, 54, 81.
@ 1981 American Chemlcal Society
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few have used inorganic support^.^-^ In most cases, workers have been more concerned with catalytic activity of the supported homogeneous catalysts than with the nature of the species on the support under different conditions. This stands in contrast to studies of comparable homogeneous complexes in s~lution.l"-'~ In the case of supported homogeneous catalysts, it has been shown that different structures and consequently different catalytic behavior can be obtained depending on the support, the precursor complex, the ligands used to link the complex to the support, and how the synthesis is carried o ~ t . ~ J ~ We report a study of a supported RhCl(CO)(Ph3P), analogue which is active for hydroformylation of olefins when supported on alumina and silica. We studied this complex as a function of temperature and pressure of CO and CO/H2, to elucidate the different species present under conditions close to those used for hydroformylation. We were particularly interested in seeing how these species vary with the support. Behavior was analogous to the homogeneous complexes in solution, although affected by the support. Species were observed that are unstable in the solid state and are likely intermediates in the catalytic reaction.
m
U
0 e
'E
. C
0
I-
I
l
l
I
2 2 0 0 2000 1900
Experimental Section RhC1(CO)(Ph3P)2(1) was synthesized by reaction of RhC1(Ph3P), (Alfa Ventron) with CO according to the method of Wilkinson.lG The complex, RhCl(C0)(PhzPCH2CH2Si(OEt)3)2 (2) [uco 1962 cm-', mp 96-101 OC (dec)] was prepared by reaction of [Rh(C0)2C1]2(Strem) and Ph2PCHzCH2Si(OEt)3 (3) (Strem) according to the procedure of the BP group.16 Complex 2 was supported on silica gel (surface area 700 m2/g, pore volume 1.0 cm3/g) (Alfa Ventron) and on y-alumina (surface area 200 m2/g, pore volume 0.6 cm3/g) (Harshaw),by the method of the BP group,17 modified as follows. Silica gel (5.0 g) dried in vacuo 24 h at 100 "C was suspended in benzene (30 mL) and 2 (1.0 g) in benzene (20 mL) was added. The mixture was refluxed for 20 h. During this time most of the benzene was distilled away. After cooling, the solid was transferred to a soxhlet apparatus and extracted with benzene for 22 h. Finally, the silica complex was dried in vacuo for 22 h at 60 "C (Rh content 2.0%). The method (2)Farrell, M. 0.; Van Dyke, C. H. J. Organometal. Chem., 1979,172, 367. (3)Allum, K. G.; Hancock, R. D.; Howell, I. V.; Pitkethly, R. C.; Robinson, P.J. J. Organometal. Chem. 1975,87,189. (4)Arai, H. J. Catal. 1978,51,135. (5)Pittman, C. U.,Jr.; Smith, L. R. J. Am. Chem. SOC.1975,97,1749. (6)Jarrell, M. S.;Gates, B. C.; Nickolson, E. D. J. Am. Chem. SOC. 1978,100,5727. (7)Allum, K. G.; Hancock, R. D.; Howell, I. V.; McKenzie, S.; Pitkethly, R. C.; Robinson, P. J. J. Organometal. Chem. 1975, 87, 203. (8)Bartholin, M.; Graillat, C.; Guyot, A.; Coudurier, G.; Bandiera, J.; Naccache, C. J.Mol. Catal. 1977/78,3,17. (9)Spek, T.G.; Scholten, J. J. F. J. Mol. Catal. 1977/78,3,81. (10)Vastag, S.;Heil, B.; Marko, L. J. Mol. Catal. 1979,5,180. (11)Hjortkjaer, J. J.Mol. Catal. 1979,5,377. (12)Turner, M.; Jouanne, J. V.; Brauer, H. D.; Kelm, H. J.Mol. Catal. 1979,5,425, 433,447. (13)Dickers. H. M.: Haszeldine, R. N.; Mather, A. P.: Parish, R. V. J. Organometal. Chem. 1978,161, 91. (14)Evans, D.; Yagupsky, G.; Wilkinson, G. J. Chem. SOC.A 1968, 2660.
(15)Allum, K. G.; Hancock, R. D.; McKenzie, S.; Pitkethly, R. C. In "Catalysis"; Hightower, J. W., Ed.; North Holland Amsterdam, 1973; 1, 477. (16)Osborn, J. A,; Jardine, F. H.; Young, J. F.; Wilkiion, G. J. Chem. SOC.A 1966,1711. (17)Allum, K. G.; Hancock, R. D.; Howell, I. V.; Lester, T. E.; McKenzie, S.; Pitkethly, R. C.; Robinson, P. J. J. Organometal. Chem. 1976,107,393.
I
ieoo
I
1700
Wovenumber ( cm-' )
Figure 1. Infrared spectra of supported chlorocarbonylbls(trlpheny1phosphlne)rhodium(I) catalysts. Top: silica support, (a) silica; (b) 2 days after preparatlon; (c) 30 days after prepatttion. Bottom: alumina support, (d) alumina: (e) lmmedhtely after preparation; (f) 7 days after preparatlon.
for alumina was the same except for the use of 0.25 g of complex and 1.25 g of alumina (Rh content 1.6%). Our modification of the BP procedure gave complexes having higher Rh content. All preparations were carried out in a dry, oxygen-free atmosphere. Solvents were degassed with nitrogen before use. CO and Hz used were purchased from Union Carbide. The bulk Rh content of the catalysts was determined by X-ray fluorescence. Reactions of the supported complexes with CO and CO Hz were carried out in an autoclave (Parr reactor) equipped with automatictemperature control and pressure indicator. All samples were evacuated 2 h at 50 "C before the CO was introduced. Infrared spectra were obtained by using a Digilab FTS-1OM spectrometer. Samples were suspended in Nujol and measured by sandwiching between KBr plates. ESCA spectra were taken on an AEI-ES2OOA spectrometer by using an A1 anode. Surface area measurements were obtained by using an MS-3 Quantachrome Corp. Monosorb single-point BET analyzer.
+
Results Our preparation of 1, for use as a reference compound, gave a yellow complex showing a CO stretching vibration at 1964 cm-' in agreement with literature results.1a-20 The reaction of p,p'-dichloro-tetracarbonyl-dirhodium(1) [Rh(CO)2C1]2(4) with triphenylphosphine results in the formation of different compounds depending on the quantity of triphenylphosphine used.20i21 The reaction of 3 with 4, on the other hand, appears to form a single complex, tr~ns-RhCl(CO)(Ph~PCH~CH~si(0Et)~),'6 (21, having a CO stretching vibration at 1962 cm-l. 2 is readdy reacted with (18) Vallarino, L. J. Chem. SOC.A 1957,2287. (19)Chatt, J.; Shaw, B. L. J. Chem. SOC.A 1966,1437.
(20)Var shausky, Y. 5.; Cherkasova, T. G.; Buzina, N. A. J. Organometal. Chem. 1973,56,375. (21) Rollmann, L. D. h o g . Chim. Acta 1972,6,137.
The Journal of Physical Chemistry, Vol. 85,No. 5, 198 1 55 1
Supported Rh Complexes
TABLE I: Carbonyl-Stretching Frequencies for SiRh Samples Treated with CO or CO
+ H, IR freq,"cm"
sample SiRh 2a SiRh 2b SiRh 2c SiRh 2d sample c 2e sample d a
temp, "C
press., psi
terminal CO
bridge CO
1983w 2090m, 2020m, 1985sh 2082s. 2012s 2078d,vs, 2024w 2085m, 2022m 2078d,vs, 2022w
1804d,s
time, h
after preparation 50 14.7 100 1200 100 1200 after 30 days 100 1400
1 4 66
70
1804d,s
vs, very strong; s, strong; m, medium; w, weak; sh, shoulder; d, doublet.
TABLE 11: ESCA Binding Energies of Homogeneous Complexes Supported on Silica binding energies,a eV ~~
~
Rh complex RhCl( CO(PPh,),
a
P
3d,,, 3d,,2 2s 308.7 313.4 189.2 RhCl(CO)(Ph,PCH,CH,Si(OEt),), 308.5 313.2 188.8 SiRh; after preparation 308.3 312.9 189.8 308.2 312.8 190.2 SiRh; 50 "C; 14.7 psi 1h SiRh; 100 'C; 1200 psi; 66 h 308.1 312.6 190.4 SiRh;b 150 "C; 1200 psi; 26 h 307.8 312.5 Relative to C 1s = 285.0 eV. Rho (powder) BE 3d,, = 307.9 eV, 3d,, =
a surface containing OH groups. The sequence of reactions used to link 2 with a suitable support is the only one that produces a monocarbonyl species when phosphorus ligands are used' and produces a complex having a well-defined Rh:P ratio. Figure 1 shows the IR spectra of the supported complex. The observed CO stretching frequencies are 1983 cm-' when supported on silica (spectrum b) and 1978 cm-' for an alumina support (spectrum e). As can be seen from Figure 1,after allowing the sample to stand for a period of time the CO stretching vibration on silica disappears and on alumina decreases significantly. The supported complexes were treated with CO and with a 1:l mixture of CO + H2at different temperatures and pressures for different periods of time. The species obtained do not depend on the presence of hydrogen. Therefore we shall not make any formal distinction between CO and CO + Hztreatments. Silica Support. Figure 2 shows IR spectra for the rhodium complex supported on silica, treated with CO and CO + H2under increasingly severe conditions. Under mild conditions (spectrum 2a) two bands appear at 2090 (sym) and 2020 cm-l (asym). These bands are characteristic of a carbonyl species having two terminal CO groups linked to a rhodium atom.22J3 On the lower frequency band, a shoulder appears at approximately 1985 cm-'. This probably arises from a small quantity of the starting monocarbonyl. The maximum intensity of the two 20002100-cm-' bands is observed for samples treated at 100 "C, 1200 psi CO for 4 h (spectrum 2b). The band positions shift a little, depending on the extent of CO treatment, The ranges are roughly 2090-2080 and 2020-2010 cm-'. Similar changes have been observed in terminal CO groups for Rh carbonyls.' Under more severe CO treatment, the lower frequency band weakens and a narrow doublet appears at approximately 1800 cm-l (spectrum 212). The position of this latter band is characteristic of a CO bridging two Rh atoms.% At the same time the 2080-cm-' band appears as a narrow doublet. The transformation (22)Yang,A. C.;Garland, C. W. J. Phys. Chem. 1967, 61, 1504. (23)Arai, H.;Tominaga, H. J. Catal. 1976, 43,131. (24)Yates, J. T., Jr.; Duncan, T. M.; Worley, S. D.; Vaughan, R. W. J. Chem. Phys. 1979, 70,1219.
2P 131.6 131.6 132.5 132.7 132.8 132.9 312.7 eV.
c1 2p 198.7 198.5 198.7 198.4 198.3
0 1s 533.3 532.5 533.0 533.1 533.2 533.0
Si 2p 102.4 103.6 103.4 103.5 103.5
TABLE 111: ESCA Intensity Ratios of SiRh Samples
sample SiRh: after preparation SiRh; 50 "C; 14.7 psi, 1h SiRh:lOO"C: 1200 psi, 66 h SiRh; 150 'C; 1200 psi, 26 h
c1+ Rh/ P P 2sl P Si 2p/Rh Rh 2plSi 0.18 0.14 0.22 0.025
c1 +
P 2sI Si 0.040
0.16
0.15
0.29 0.023 0.044
0.10
0.22
0.37
0.07
0.39
0.021
0.036
0.027
to this stage is almost complete at 100 "C, 1200 psi CO and 66 h of exposure. When the sample giving spectrum 2c is allowed to age for 10 days, spectrum 2d results. Note that this is qualitatively equivalent to spectrum 2b. Thus the species formed under very drastic conditions is not stable and reverts to the dicarbonyl species. If one gives a severe treatment with CO to the sample giving spectrum 2d, spectrum 2e results. This is identical with spectrum 2c, and indicates that the transformation between the two species is reversible. Table I summarizes the CO stretching frequencies and approximate intensities for all of the silica-supported Rh catalysts. A sample treated at 150 "C and 1200 psi CO for 24 h has a grey appearance and the IR bands are very weak. All other samples appear to be yellow to gold in color and have been analyzed by using ESCA. Table I1 shows the ESCA binding energy data for the elements in the different samples. The most important result from the ESCA study is that Rh remains in the +1 oxidation state during all the CO-induced changes. For the sample treated at 150 "C, ESCA indicates that some Rh metal is formed. Table I11 shows ESCA intensity ratios for the various silica-Rh samples. It is somewhat difficult to interpret these data because of problems with deconvolutingthe C12p and P 2s spectra. However, it is clear from the data of Table I11 that the sum of the P 2s and C12p intensities relative to the Si 2p intensity remains essentially constant throughout all the CO treatments. The Rh/Si intensity ratio decreases with increasing temperature. The ESCA peak shape for Rh indicates that rhodium metal is forming at higher
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------l
u 0) C
O
r .-c
$ I!
I-
I
l
I
I
l
I
1
2 2 0 0 2000 1900 1800 1700 Wovenumber ( cm" ) I
l
l
I
I
I
U o,
O
r .c
E v)
C
I!
c
I
l
l
I
I
2 2 0 0 2000 1900 1800 Wovenumber ( cm-'
I 1700
Figure 2. Infrared spectra of chlorocarbonylbis(trIphenylphosphine)rhodium(1) catalysts supported on silica: (a) 50 'C; 14.7 psi CO; 1 h; (b) 100 "C;1200 psi CO; 4 h; (c) 100 " C 1200 psi CO -I-H, (5050); 66 h; (d) sample c (Figure 2) after 30 days; (e) sample d at 100 "C; 1400 psi CO; 70 h.
temperatures, which correlates to the intensity decrease. This result is expected if Rh metal clusters are formed or if Rh migrates away from the surface. Extraction with benzene was used for all samples and indicated that the complex remains strongly linked to the silica support. Alumina Support. The alumina-supportedRh complex was treated with CO under conditions similar to those used for silica. Substantial differences in the infrared spectra were observed. Figure 3a shows the infrared spectrum for mild treatment with CO. It is obvious that the spectrum is more complicated than the corresponding silica spectrum. Five instead of three bands are present in the same region. The lowest frequency band (1981 cm-') is due to the starting monocarbonyl. Assignments of the four higher
2 2 0 0 2000 1900 1800 Wovenumber ( cm-'
1700
)
Flgure 3. Infrared spectra of chiorocarbonylbis(triphenylphosphine)rhodlurn(1) catalysts supported on aiumlna: (a) 50 'C 20 psi, 4 h with CO; (b) 120 'C: 1400 psi; 52 h with C O (c) sample b after 15 days: (d) sample c at 100 'C; 1400 psi CO; 20 h.
frequency bands cannot be made just from their positions and relative intensities. With additional studies, however, reasonable structures of the carbonyl complexes have been established. Under more severe treatment, the 2022-cm-' band increases in intensity and a large band in the bridged CO region appears having a low-frequency shoulder, as shown in Figure 3b. The high-frequencyband has remained the same, while the others appear as shoulders on the 2020cm-' band. The dominant bands are indicative of bridging and terminal carbonyls. Since the terminal carbonyl frequency is lower and the bridging carbonyl frequency is higher than for the cis complex (7), a dimer similar to 7 is indicated where the terminal CO is trans to the chlorine.n* Incomplete conversion to this structure is obvious from the persistant bands at 2080,2052, and 1800 cm-l. After several days the spectrum changes. The 1840- and 2020-cm-l bands disappear simultaneously and then the 1800-cm-' band disappears. The trans structure of dimer 7 apparently is less stable than the cis form. After 10 days the spectrum appears as shown in Figure 3c which is very similar to the spectrum of the silica-supported catalyst for both initial and for aged samples. The modest shift toward lower frequency is likely due to the support. Therefore we have assigned the species present under these conditions to a cis-dicarbonyl structure (6) similar to the one on silica (2080,2000 cm-'). By treating the sample under more severe conditions, we would expect to see the same behavior as for silica. After more severe treatment, a narrow doublet appears at approximately 1800 cm-', and the 2080-cm-' band turns into a doublet, while the intensity of the 2000-cm-' band decreases. These changes are seen (26) Uguagliati, P.;Deganello, G.;Bwtto, L.; Belluco, U. Inorg. Chem. 1969, 8, 1626. (26) Smith, G. C.;Chajnacki, T. P.; Dasgupta, S. R.; Iwetate, K.; Watters, K.L.Inorg. Chem. 1976,14,1419. (27) Chatt, J.; Shaw, J. J. Chem. SOC.A 1966, 1437. (28) Blum, J.; Oppenheimer, E.; Bergmann, E. D. J. Am. Chem. SOC. 1967,89, 2338.
The Journal of Physical Chemistty, Vol. 85, No. 5, 198 1 553
Supported Rh Complexes
TABLE IV: Carbonyl-Stretching Frequencies for AlRh Samples Treated with CO or CO + H, IR freq,a cm" sample
temp, "C press., psi time, h
terminal CO
bridge CO
1978s 2080m, 2052s, 2022m, 2000s, 1981m 2080m, 2052sh, 2020vs 2080m, 2000m 2078d,vs, 2052m, 2022sh, 2000s
1840m,1800sh
AlRh after preparation 50 20 4 3a AlRh 120 1400 52 3b AlRh 3c sample b after 15 days 100 1400 70 3d sample c vs, very strong; s, strong; m, medium; sh, shoulder; d, doublet. TABLE V: ESCA Binding Energies of Homogeneous Complexes Supported on Alumina binding energies,a eV
I 0-Si-CH CH PPh I 2 2\2
Y-Co
I 0-Si-CH2CH2PPh2 I
Rh sample
3d,,, 3d,,, P 2p C1 2p A1 2p 308.7 313.4 189.2 198.7 308.5 313.2 188.8 198.5
RhCl(CO)(PPh,), RhCI(C0)(Ph,PCH,CH,Si( OEth 11 308.8 AlRh ; after preparation AlRh; 50 "C; 309.0 20 psi; 4 h A1Rh;lBO"C; 308.6 1400 psi; 52 h a Relative to C 1s = 285.0
A
'
B
2 c1
313.4 189.9 199.0 75.0 313.5 190.3 199.2 75.2
I I 0-Si-CH CH PPh2-Rh-CO I 2 2 /\
i I
I
I
O-SiCH2CH2PPh2
I
313.1 190.4 199.2 75.1
I,
eV.
1800d,vs
C1
E
Figure 4. Structures of supported rhodium carbonyl complexes.
TABLE VI: ESCA Intensity Ratios of AlRh Samples AlRh; after preparation AlRh; 50 "C;20 psi; 4 h AlRh; 120 "C;1400 psi; 52 h
Rh/Al Cl/Rh Cl/Al 0.48 0.29 0.14 0.47 0.23 0.11 0.41 0.23 0.10
in Figure 3d. In addition, there are two weak bands at 2052 and 2022 cm-'. The two weak bands are due to the pentacoordinated dicarbonyl complex (8). Dominating bands in Figure 3d are due to the cis dimer (7) which is analogous to the complex formed on silica. Infrared data for the alumina-Rh samples are summarized in Table IV. ESCA results for the alumina-supported Rh complexes are similar to those for Rh-silica. Table V summarizes the ESCA binding energy data for the Rh-alumina complexes. It is clear that the +1 oxidation state of Rh is present in all samples. The intensities for rhodium, chlorine, and phosphorus remain relatively constant for Rh-alumina as shown from the data of Table VI. The samples show the same yellow or yellow-gold colors. Similar to silica, benzene does not extract any Rh species from the alumina-supported complex.
Discussion Silica Support. The complex RhCl(C0)(PhzPCH2CHzSi(OEt)3)z links both to silica and alumina retaining its trans configuration. This produces a supported complex having structure 5 in Figure 4. This species gives a single CO stretching band observed at 1983 cm-l on silica and 1978 cm-' on alumina. When structure 5 is treated with 1200 psi of CO at 100 "C for 4 h, it forms a cis-dicarbonyl monophosphine rhodium complex (6). This assignment is the same as made by Allum et ala7 for the reaction of [Rh(CO),Cl], with silica-bound phosphines. Rhodium coordination with one silica-bound phosphine ligand is replaced by coordination with CO and is responsible for the two infrared bands at 2090 and 2020 cm-l. The same cis-dicarbonyl complex was obtained by the BP group with a different method of synthesis and shows infrared bands having the same frequencies7 (see Table VII).
Under more extreme conditions (Figure 2c), absorption bands at 2078,2024 cm-l (terminal dicarbonyls) and 1804 cm-l (bridging carbonyl) are observed. In general, carbonyl groups absorb a t 2100-2000 cm-' while bridging carbonyl groups absorb at 1900-1800 cm-'. The infrared absorption frequencies for various metal-carbonyl complexes are listed in Table VII. Wilkinsonl* observed that the reaction of RhH(CO)(PPh3)3in solution with CO gave first the dicarbonyl RhH(C0)2(PPh3)zand then the dimer [Rh(C0)z(PPh3)2]z.Early studies showed that carbonyls normally did not form a bridged structure if halogens were present. With the synthesis of binuclear complexes, known as "A-frame" complexes,3' a metal-metal bond can be simultaneously bridged by halogen and carbonyl ligands. With additional CO, the halogen bridge is displaced by a carbonyl group. Mague and Sanger3"used 13C0labeling to study the CO exchange process. The absence of initial enrichment of 13C0at the bridging position, relative to the terminal position, indicates that CO attack is on one rhodium with conversion of the terminal carbonyl originally on the rhodium center into a bridging ligand. The infrared absorption bands of 2c and 2e SiRh samples are similar to the spectrum obtained for unsupported Rh&.Xl)16. When Rh6(CO)16is supported on silica and exposed to 1atm of CO, the absorption band frequencies (29) Farone, F.; Ferra, G.; Rotondo, E. J. Organometal. Chem. 1971, 33, 221. (30) Mague, J. T.; Sanger, A. R. Inorg. Chem. 1979,18,2060. (31) Chaston, S. H. H.; Stone, F. G. A. J. Chem. SOC.A 1969, 500. (32) Smith, G. C.; Chojnacki, T. P.; Dasgupta, S. R.; Iwatate, K.; Watters, K. L. Inorg. Chem. 1975, 14, 1419. (33) Conrad, H.; Ertl, G.; Knozinger, H.; Kuppers, J.; Latta, E. E. Chem. Phys. Lett. 1976,42, 115. (34) Anderson, J. R.; Elmes, P. S.;Hower, R. F.; Mainwaring, D. W. J. Catal. 1977,50, 508. (35) Booth, B. L.; Else, M. J.; Fields, R.; Haszeldine, R. N. J. Organometal. Chem. 1971,27, 119. (36) Yao, H. C.; Rothschild, W. G. J. Chem. Phys. 1978, 68, 4774. (37) Kubiak, C. P.; Eisenberg, R. J. Am. Chem. SOC.1977, 99, 5129. (38) Chini, P.; Martinengo, S. Inorg. Chim. Acta 1969,3,315. Chem. Commun. 1967,251. (39) Morris, D. E.; Tinker, H. B. Chemtech. Sept 1972, 554.
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TABLE VII: Infrared AbsorDtion Freauencies of Metal-Carbonvl Comdexes IR, cm'' compound [Rh(!J-Cl)(CO1 2 1 2 trans-RhCI( CO)(PPh, )z Cis-RhCl(CO)(PPh,), trans-Rh Cl(C0 L ( PPh L . I
11
'4
Rh6(C0
116
Rh,(CO),,/SiO, + CO, 1atm CO adsorbed on Rhu or R h / y - A 1 , 0 3 2 ~ ~ o\\Rnp~o c/
species I (twin carbonyls)
species 11, (single linear)
terminal 2102m, 2089s, 2075w, 2 0 3 2 ~ , 2 0 0 0 w , ~ 2105m, 2089s, 2090w, 2 0 3 5 s , 2 0 0 3 w ~ 1965s) 1 9 1 8 ~1961" ,~ 1978" 19804' 2088, 200242 198219 1980br 2017m, 1992s 1962." 1966' 2090; 2018' 1997s) 1978vsJo 1992s, 1977vs 1962 vs,br 1984sh, 1968vs 2047m. 1998vs 2075s, 2070s, 2062ah,2044m, 2016w, 1978w 2115w, 2090s, 2070vs, 2065s, 2055s, 2045m. 2028m. 2021m 2105w, 2070s, 2047w, 2022m, 2020m 1984.1965.1938 2082sh, 2069vs, 2052s) 2080, 20003' 2060, 2010 2078sh, 2070vs, 2052s, 2043m, 2028m, 2020m 2095,2060,2035
bridging
1863s" 1850s,br3' 1 8 1 6 ~ ~ 1759~~' 188551 1796vs,br 1833w, 179332 1779" 179 5vs,bPiC 18053* 1796v~,bP?~ 18603'
2100, 2030 2070
Rh'
8
species IIIa, (bridging)
1870
species IIIb
1900
Rh Rh
a
This work.
Prepared according to ref 34.
DPM = bis(dipheny1phosphino)methane ((C6H,),PCH,P(C,H,),).
TABLE VIII: ESCA Binding- Energies - of Rhodium ESCA BE,a eV compound
RhCl( CO RPPh,), Rh(CO)Cl(PPh,CH,CH,Si(OEt),), SiRh, after preparation AIR& after preparation RhCl,*XH,O a Relative to C 1s = 285.0 eV.
Rh 3d,,, 307.1 307.7 308.0 307.7 308.2 308.7 308.6 308.3 308.8 309.4
are shifted significantly (see Table VII) and are not similar to those obtained for samples SiRh 2c and 2e. Chini and M a r t i n e n g ~showed ~ ~ that Rh6(CO)16is synthesized from [Rh(C0)2C1]2at 25 OC and 1atm of CO in a polar solvent (Le., methanol) with 5% water present. Since all solvents were dried before use and reactions were carried out under nitrogen, significant formation of Rb(CO)16is not likely. In addition, it is not expected that Rh6(CO)16is capable of reverting back to the silica-bound rhodium(1) dicarbonyl chloride on sitting at 25 OC for 30 days. Structure 7 is proposed as the most likely structure of the rhodium complex present for 2c and 2e SiRh for several
Rh 3d,,, 311.9 312.5 312.7 312.5 312.8 313.4 313.2 312.9 313.4 314.0
c Is
Si 2 ~ , , ,
531.8, 530.2 533.6 530.6 532.8 532.5 533.3 532.5 533.0
103.5
Al2P 75.0
102.4 103.6 75.0
reasons. First, on the basis of ESCA results, the Rh remains in the +1 oxidation state (see Table VIII) and the chlorine and phosphorus intensities remain relatively constant. Second, the infrared data indicate that both terminal and bridging carbonyls are present. By contrast when two carbonyls are cis on a single rhodium atom, they have two equally intense infrared bands a t -2095 and -2027 cm-1.22 This is in complete agreement with structure 6. Third, desorption of CO at elevated temperature indicates that the strength of the metal carbonyl bond order is bridging carbonyls > single linear carbonyl > twin carbonylsa (see Table VII, species I, 11, and IIIa).
Supported Rh Complexes
During oxidation, however, the linear and bridged CO react easily while "twin-type" CO reacts with oxygen only at elevated temperature^.^^ At elevated temperatures, in the absence of oxygen, bridging carbonyls are more stable than the dicarbonyl species. Excess CO can force bridging halogens to terminal positions and form carbonyl-bridging structure^.^^^^^ When structure 6 experiences higher temperatures and CO pressures, it is quite reasonable that structure 7 should result. On standing, complex 7 reverts to the cis-dicarbonyl (Figure 2d) which requires only simple cleavage of the CO bridge bonds to produce 6. The silica dimer (7) was suspended in benzene/ethanol and bubbled with N2at room temperature. In a short time, only the dicarbonyl species (6) was observed. An interesting parallel can be drawn with Wilkinson's dimer.14 The latter is formed from the dicarbonyl species with loss of hydrogen, so the reaction cannot be reversed without external hydrogen. When Wilkinson's dimer in solution is bubbled with N2it decomposes in benzene, but undergoes a solvent substitution reaction in a donor solvent (ethanol, dichloromethane). However, our dimer is formed without loss of ligands so the reaction can be reversed by itself and the solvent dimer is not formed. When the supported Rh complex is heated to 150 "C, ESCA showed the presence of Rh metal, indicating that the complex is not stable at this temperature and decomposes. Alumina Support. Five CO bands appear in the spectrum of the alumina-supported Rh complex, as opposed to three for silica under the same conditions. The spectra are shown in Figure 3. In spectrum 3a, the 1981-cm-' band can be attributed to some starting monocarbonyl remaining after CO treatment (5 in Figure 4). Observing the changes from 3a to 3c, it is evident that under the appropriate conditions the spectrum on alumina is similar to the one on silica. Spectra 2b, 2d, and 3c correspond to the cisdicarbonyl complex (6) which accounts for the 2080- and 2000-cm-' bands. The differences in the CO-stretching frequencies from the silica data can be attributed to the difference in support. Since the entire rhodium complex on alumina can be converted to the cis-dicarbonyl form (spectrum 34, it is unlikely that steric restrictions are responsible for the additional bands at 2052 and 2022 cm-'. The trans form of 6 would have a single major band near 1980 cm-l.15 Therefore we propose a pentacoordinated dicarbonyl complex (8 in Figure 4) which is formed by coordination of an additional CO ligand with the starting square-planar monocarbonyl complex 5. A similar complex was postulated for CO attack on the square-planar hydride species, RhH(C0) (PPh3)2,in Wilkinson's@hydroformylation reaction. Approximately, a 1:l molar ratio of RhH(C0)2(PPh3I2 and Rh2(C0)4(PPh3)4forms from RhH(C0)(PPh3I2at 25 "C and 1 atm of C02/Hz.39 Structure 8, RhC1(C0)2(PPh2Ch2Ch2Si-)2, requires that both phosphine groups remain coordinated to the rhodium when the complex is supported on alumina. This is consistent with the experimental observation that the 1981-cm-' band of the starting monocarbonyl, which has both phosphine groups coordinated, is stronger and persists longer on CO treatment for alumina than for silica. Other pentacoordinated rhodium(1) complexes have been reported widely in the l i t e r a t ~ r e . ' ~ . ~ ~ In summary, the spectrum on alumina in Figure 3a can be accounted for by three carbonyl species. First, a square-planar trans-monocarbonyl complex (5) accounts (40) Evans, D.; Osborn, J. A.; Wilkinson, G. J. Chem. SOC. A 1968,
3133.
The Journal of Physlcal Chemlstty, Vol. 85, No. 5, 1981 555
for the band at 1981 cm-'. Second, a square-planar cisdicarbonyl coomplex (6) accounts for the bands at 2080 and 2000 cm-l, Third, a pentacoordinated dicarbonyl complex (8) is present, accounting for the bands at 2052 and 2022 cm-l. The peaks in Figure 3b arise from the following. Complex 6 forms the dimer 7 accounting for the weak bands at 2080 and 1800 cm-l. Complex 8 forms a dimer with bands at 2020 and 1840 cm-' by losing a phosphorus ligand. The latter dimer is similar to structure 7 except that the terminal CO is trans to the chlorine. This postulate is supported by the lower frequency for the terminal C027~28p41142 and higher frequency of the bridging CO compared with the CO frequencies for complex 7. The trans form of complex 7 is more stable initially than the cis form since bands at 2020 and 1840 cm-' dominate spectrum 3b. Both dimers are unstable and by simple cleavage of the CO bridge they give the cis square-planar dicarbonyl complex (6). The trans dimer is thermodynamically less stable than the cis dimer since the intensity of the band at 1840 cm-' decreases faster than the band at 1800 cm-l over several days. Subsequent treatment of the cis-dicarbonyl complex (6) with CO yields the cis form of dimer 7 after 20 h. This accounts for 2078d,vs, 2000s, and 1800d,s cm-' bands in spectrum 3d. The presence of two bands at 2052m and 2022sh cm-l are from the formation of the pentacoordinated dicarbonyl complex (8). A similar CO treatment of complex 6 supported on silica produced only the cis dimer (7). The question arises as to why alumina would give the pentacoordinated complex whereas silica would not. The silica used had a surface area of 700 m2/g and a pore volume of 1cm3/g; the alumina had a surface area of 200 m2/g with a pore volume of 0.6 cm3/g. Since the rhodium content is almost the same on both supports, the alumina surface is more highly covered than the silica surface and the phosphine ligands are closer to each other. ESCA measurements show the standardized rhodium intensity is 2.2 times larger on alumina than on silica. Therefore the strain on the diphosphine complex will be greater for silica and thus the RhP bond will be more easily broken on treatment with CO. The frequencies of spectrum 3d are quite similar to the spectrum obtained for Rh6(CO)16(see Table VII). After treatment with 1 atm of CO, the bands in the CO region have significantly different frequencies than spectrum 3d. Also, it is unlikely that Rh6(CO)16is formed from the cis-dicarbonyl species without a significant amount of water present.38 The ESCA data show that rhodium remains in the +1 oxidation state throughout all of the treatment conditions (see Tables V and VIII).
Conclusions We have reported a study of the behavior of supported rhodium catalysts under conditions close to those used for hydroformylation reactions. We have shown that the species produced by reaction with CO vary with treatment, conditions, and support. Behavior of the supported homogeneous catalysts is comparable to that observed for homogeneous complexes in s01ution.l~The observation of unstable rhodium CO species and the tendency to produce different structures under different conditions indicates the possibility of many different species during hydroformylationreactions of these catalysts. These species are (41) Uguagliati, P.; Degianello, G.; Busseto, L.; Belluso, U. h o g . Chem. 1969,8, 1625. (42) Poilblanc, R.; Gallay, J. J. Organometal. Chem. 1971, 27, c53.
558
J. Phys. Chem. 1981, 85, 556-561
largely determined by the availability of phosphine groups connected to the central metal atoms. Because the rhodium CO species observed in this study are transient, it is likely that they are intermediates in the hydroformylation mechanism and consequently influence the course of the reaction and the selectivity of the catalysts.
Acknowledgment. The authors thank M. B. Carvalho for his assistance and Professor F. A. Miller for reading the manuscript. We thank Professor Donald E. Leyden of the University of Denver for performing the X-ray analyses. This work was supported by the U.S.Army Research Office under Grant No. DAAG-77-G-0165.
* O-***H+N
Influence of the Polarity of the Environment on Easily Polarizable OH***N Hydrogen Bonds Johannes Frltsch” and Georg Zundel
Instnut fiir Physlkallsche Chemie der Universnat Miinchen, D-8000 Miinchen 2, West Qermeny (Received: August 22, 1980)
The intermolecular hydrogen bonds between chloro-substituted phenols and N-methylpiperidineand the intramolecular hydrogen bond in N-piperidinevaleric acid are studied by IR spectroscopy with respect to the influence of different organic solvents. The proton transfer equilibrium in the OH. .N + 0-..H+N bond is determined. It shifts with increasing polarity of the solvent in favor of the polar proton limiting structure. A linear relation is found between In K ~ and T (e - 1 ) / ( 2 ~+ l),demonstratingthat this shift is caused by the interaction of the dipole moment of the hydrogen bond with the reaction field arising from the polarization of the environment by the dipole. The shift of the proton transfer equilibrium is less pronounced with the intramolecular bond, since it is partially screened against the solvent. Furthermore, it is shown that, with carboxylic acid groups as donors, specific interaction effects of the hydrogen-bond donor with protic solvents may increase the transfer. The influence of the reaction field on B1H. .B2 * B1-s -.H+B2bonds is compared with the influence on B+H...B + B...H+B bonds by using literature results. In all systems, intense IR continua are found, indicating that the hydrogen bonds are easily polarizable. The intensity of these continua shows only a weak dependence on the degree of asymmetry of the proton potential, as long as it is not too asymmetrical. This result is discussed under the aspect of the particularly great proton polarizability of the bonds.
Introduction The characteristic property of hydrogen bonds of the type BIH. .B2 + B1-- -H+B2is the existence of a proton transfer equilibrium within these bonds, showing that a double minimum proton potential is present. Hydrogen bonds with such proton potentials are easily polarizable, and the proton polarizability of these hydrogen bonds is indicated by continuous absorptions in the IR ~pectra.l-~ The position of the proton transfer equilibria can be determined from characteristic bands of B1 or BP4 From several studies, using UV,69 IR,1*12 and dielectric meth~~~~~
~
(1) Weidemann, E. G.; Zundel, G. 2.Naturforsch. A 1970, 25, 627. (2) Janoschek, R.; Weidemann, E. G.; Pfeiffer, H.; Zundel, G. J. Am. Chem. SOC.1972,94, 2387. (3) Zundel, G. In “The Hydrogen Bond, Recent Developments in Theory and Experiments”;Schuster, P., Zundel, G., Sandorfy, C., Eds.; North-Holland Publishing Co.: Amsterdam, 1976; Vol. 11, Chapter 15. (4) Lindemann, R.; Zundel, G. J.Chem. Soc., Faraday Trans. 2 1977, 73. 788. -(5) Scott, R.; de Palma, D.; Vinogradov, S. J . Phys. Chem. 1968, 72, 3192. (6) Hudson, R. A.; Scott, R. M.; Vinogradov, S. N. Biochim. Biophys. Acta 1969,181, 353. (7) Beier, R. Dissertation, Universitiit Wien, 1975. (8) Baba, H.; Matsuyama, A.; Kokubun, H. Spectrochim. Acta, Part A 1969,25, 1709. (9) Schreiber, M.; Koll, A,; Sobczyk, L.Bull. Acad. Pol. Sci., Ser. Sci. Chim. 1978,26,651. (10) Lindemann,R.; Zundel, G. Biopolymers 1977,16,2407; 1978,17, 1285. (11) Zundel, G.; Nagyrevi, A. J. Phys. Chem. 1977,82, 685. (12) Pawlak, Z.; Magofiski, J. J. Solution Chem., in press. - I
0022-3654/81/2085-0556$01.25/0
0ds,13J4it is known that the proton transfer equilibria BIH- .B2* B p . .H+B2are dependent on the environment of these hydrogen bonds. Various authors13J6stressed the importance of the reaction field for the position of the proton transfer equilibrium and thus for the mean dipole moment of the hydrogen bonds. This reaction field was introduced by Onsagerls and is related to the dielectric permittivity by the Onsager parameter (e - 1)/(2e + 1). In the following, the influence of the environments on proton transfer equilibria is investigated systematically with OH-..N 0-.-H+Nbonds, whereby phenol-amine systems with intermolecular hydrogen bonds and Npiperidinevaleric acid with an intramolecular hydrogen bond are studied. The environments represented by the various solvents used are in the range of permittivities 2-10, since only in this region does the Onsager parameter change considerably.
-
Results and Discussion Intermolecular Hydrogen Bonds. The 2,4,6-trichlorophenol (pK, = 6.0)17-N-methylpiperidine (pK, = 10.1)’* (13) JadByn, J.; Mqecki, J. Acta Phys. Pol. A 1972,41,599. (14) Sobczyk, L. In “The Hydrogen Bond, Recent Developmenta in Theory and Experiments”; Schuster, P.; Zundel, G.; Sandorfy, C., Ed.; North-Holland Publishing Co.: Amsterdam, 1976; Vol. 111, Chapter 20. (15) Pfeiffer, H.; Zundel, G.; Weidemann, E. G. J.Phys. Chem. 1979, 83, 2544. (16) Onsager, L. J. Am. Chem. SOC.1936,58, 1486. (17) Drahonovski, J.; Vacek, Z. Collect. Czech. Chem. Common. 1971, 36, 3431.
0 1981 American Chemical Society
