Organometallics 1991, 10, 818-819
818
Pressure Dependence of Associative Substitution Reactions of (15-C,H,)Rh(CO), P. Vest,' J. Anhaus, H. C. Bajaj, and R. van Eldik" Institute for Inorganic Chemistty, University of Witten/Herdecke,Stockumer Strasse IO, 5810 Witten, FRG Received April 23, 1990
Summary: The substitution reactions of (q5-C5H,)Rh(C0)2 with PPh, and P(n-Bu), were studied as a function of nucleophile concentration, temperature, and pressure up to 150 MPa in toluene as solvent. The pressure dependence of the substitution reactions results in volumes of activation between -14 and -17 cm3 mol-' at 40 'C. The results are interpreted in terms of a ring-slippage mechanism (q5 17, $'), which facilitates the associative attack at the metal center. The results are discussed in reference to available literature information.
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It is generally accepted that substitution reactions of 18-electron metal complexes may proceed according to an associative reaction mode when the complex can delocalize a pair of electrons onto a ligand such as cyclopentadienyl (Cp) or nitrosyl and so make available a coordination site for the nucleophilic attack.z4 In other cases, such complexes usually undergo dissociative substitution via a 16electron intermediate, although two-term rate laws in favor of parallel associative and dissociative substitution reactions have also been r e p ~ r t e d . ~ - ~ Recent work in our laboratories on the substitution behavior of metal carbonyl complexes has clearly demonstrated that the assignment of a mechanism based on kinetic data a t ambient pressure only is not unequivocal.'O-'* The volume of activation for such reactions, determined from the pressure dependence of the rate constant, exhibits a strong mechanistic discrimination ability and assists the assignment of the intimate mechanism.12-20 Typical values for the rate-determining dissociation of CO are around +20 cm3 m0l-l.l2J7 No characteristic values are presently available for limiting associative substitution reactions of metal carbonyl complexes.m For this reason, we have studied the pressure dependence (1) Participated in this study at the Institute for Physical and Theoretical Chemistry, University of Frankfurt, Niederurseler Hang, 6000 Frankfurt/Main; Vest, P. Diplomarbeit, University of Frankburt, 1987. (2) Basolo, F. Znorg. Chim. Acta 1985, 100, 33. (3) Basolo, F. Israel J. Chem. 1986,27, 233. (4) Schuster-Woldan, H. G.; Basolo, F. J.Am. Chem. SOC.1966,88, 1657. (5) (6) (7) (8)
Rerek, M. E.; Basolo, F. Organometallics 1983, 2, 372. Cheong, M.; Basolo, F. Organometallics 1988, 7, 2041. Graham, J. R.; Angelici, R. J. J. Am. Chem. SOC.1966,88, 3658. Pardue, J. E.; Memering, M. M.; Dobson, G. R. J. Organomet. Chem. 1974, 71,407. (9) Schneider, K. J.; van Eldik, R. Organometallics 1990, 9, 92. (10) Macholdt, H.-T.; van Eldik, R. Transition Met. Chem. (London) 1985, 10, 323. (11) Macholdt, H.-T.; van Eldik, R.; Dobson, G. R. Znorg. Chem. 1986, 25, 1914. (12) Taube, D. J.; van Eldik, R.; Ford, P. C. Organometallics 1987,6, 125. (13) Van Eldik, R., Ed. Inorganic High Pressure Chemistry: Kinetics and Mechanisms; Elsevier: Amsterdam, 1986. (14) Kotowski, M.; van Eldik, R. Coord. Chem. Reu. 1989, 93, 19. (15) Van Eldik, R.; Asano, T.; le Noble, W. J. Chem. Reo. 1989,89,549. (16) Awad, H. H.; Dobson, G.R.; van Eldik, R. J. Chem. SOC.,Chem. Commun. 1987, 1839. (17) Schmidt, G.; Paulus, H.; van Eldik, R.; Elias, H. Znorg. Chem. 1988,27, 3211. (la! Awad, H. H.; Dobson, C. B.; Dobson, G. R.; Leipoldt, J. G.; Schneider, K.; van Eldik, R.; Wood, H. E. Znorg. Chem. 1989,28, 1654. (19) Anhaus, J.; Bajaj, H. C.; van Eldik, R.; Nevinger, L. R.; Keister, J. Organometallics 1989, 8, 2903. (20) Schneider, K. J.; van Eldik, R. Organometallics 1990, 9, 1235.
0276-7333/91/2310-0818$02.50/0
of the substitution of CO in Rh(Cp)(CO), by PPh3 and P(n-Bu),. We found indirect evidence for the participation of an intermediate species in the overall substitution process.
Experimental Section Rh(Cp)(CO), was prepared from [Rh(C0)2C1]221and T1CpZ2 according to the method reported in the literatureB and the cited modifications.24* The yellow-orange oil was purified a few times via high vacuum freeze distillation, which resulted in a deep orange solution. The overall yield was 35% by weight after the third purification cycle. The IR spectrum recorded in a liquid cell (NaCl windows) was in good agreement with that in the l i t e r a t ~ r e . ~ , ~ ~ Chemical analysesz6 resulted in (theoretically expected values) % C, 37.4 (37.5); % H, 2.0 (2.2); % 0, 14.4 (14.3);and % Rh 46.1 (45.9). All reactions were performed in toluene (Merck, Uvasol), which was distilled over CaH, under N, atmosphere. All test solutions were prepared under Nz atmosphere using gas-tight syringes and Schlenk techniques. PPh3 (Janssen) was was ) ~distilled freshly recrystallized out of ethanol, whereas P ( ~ - B U under N2. Kinetic and spectral measuremnets were performed in a thermostated (h0.l "C) cell compartment of a Shimadzu UV-250 spectrophotometer. Kinetic experiments a t elevated pressure were performed in a thermostated (h0.1"C) highpressure cell2' attached to a Zeiss spectrophotometer. Solutions were placed in a pillbox optical cell" with the aid of a special filling system.29 IR spectra were recorded on a Nicolet 5 SX FT-IR instrument.
Results and Discussion The IR spectrum of (q5-Cp)Rh(CO),in toluene exhibits strong absorption bands a t 2049 and 1984 cm-', which are in excellent agreement with those reported e l ~ e w h e r e . ~ ~ ~ ~ During reaction 1, these carbonyl bands decrease and a (q5-Cp)Rh(CO)z+ P(n-Bu)a (q5-Cp)Rh(CO)P(n-Bu)3+ CO (1)
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new band occurs at 1944 cm-', which is in good agreement with a band at 1942 cm-' reported in the literature! Very similar spectral changes were recently reported for substitution reactions of (q5-C5H4X)Rh(C0)2.6 The UV-vis spectrum of ( Q ~ - C ~ ) R ~ (inCtoluene O ) ~ exhibits a characteristic shoulder at 430 nm (t = 262 M-l cm-'). Both spectroscopic methods indicated that such solutions are stable for a few days at 40 "C, in agreement with literature finding^.^ On addition of PPh3, the UV-vis spectrum of the final product is characterized by an absorbance maximum a t 445 nm. Repetitive scan spectra indicated two reaction steps; the first is accompanied by an absorbance decrease a t 410 nm and the second by an absorbance increase at 370 and 445 nm, respectively. The rates of the two reactions differ significantly such that they can be (21) Hieber, W.; Lagally, H. Z . Anorg. Allg. Chem. 1943, = I , 96. (22) Meister, M. Angew. Chem. 1957, 69, 533. (23) Fischer, E. 0.;Bittler, K. 2.Naturforsch. 1961, 16b, 225. (24) Knight, I.; Mays, M. I. J. Chwm. SOC.A 1970, 654. (25) Herrmann, W. A,; Kruger, C.; Bernal, I. J. Organomet. Chem. 1977, 140, 73. (26) Analytical Laboratory, Hoechat, Frankfurt Main, FRG. (27) Fleischmann, F. K.; Conze, E. G.;Stranks, R.; Kelm, H. Reo. Sci. Znstrum. 1974,45, 1427. (28) le Noble, W. J.; Schlott, R. Reu. Sci. Znstrum. 1976, 47, 770. (29) Wieland, S.; van Eldik, R. Reo. Sci. Znstrum. 1989,60,955.
b.
0 1991 American Chemical Society
Organometallics, Vol. 10, No. 3, 1991 819
Notes Table I. kob.as a Function of [PPhJ, Temperature, and Pressure for Reactiona 1 temp, [PPh3], lO5kOb,* lO'k,' AH*, AS * "C M S-1 M-' s-' kJ mol-' J K-' mol-' 35.0 0.10 1.6 f 0.1 1.55 f 0.09 75 f 3 -73 f 11 0.15 0.20 40.0 0.10 0.15 0.20 45.0 0.10 0.15 0.20 50.0 0.10 0.15 0.20 55.0 0.10 0.15 0.20 temp, O C
40.0
2.2 f 0.1 3.2 f 0.2 2.6 f 0.1 4.3 f 0.2 5.6 i 0.2 4.3 f 0.2 6.3 f 0.3 8.8 f 0.3 7.1 i 0.3 10.8 i 0.4 14.6 f 0.5 9.7 f 0.4 14.8 f 0.6 19.6 f 0.8 Pressure, MPa 5 50 100 150
(qs-Cp)Rh(CO),
30.0 2.8 f 0.1 35.0 4.3 f 0.1 40.0 7.2 f 0.1 45.0 9.8 f 0.1 50.0 105kOb: S-1
4.9 5.6 7.7 10.7
f 0.2 f 0.2 f 0.2 f 0.3
AV', cm3 mol-' -14.3 f 1.5
a [Rh] = 1.5 X M; reaction was followed a t 370 nm. *Mean value of at least three kinetic runs. CSecond-orderrate constant calculated as the mean value of kob/[PPh3]. dMean value of a t least three kinetic runs; [PPh,] = 0.15 M.
studied separately. We performed a detailed kinetic and spectroscopic study of both steps,' and our data for the second step are in good agreement with those reported in the l i t e r a t ~ r e .Our ~ IR measurements indicated only one reaction step. We also ran a series of 'H spectra on a Bruker 400-MHz NMR instrument but could not find evidence for the formation of an intermediate species. Our inability to detect the first step by IR or NMR techniques led to the conclusion that we may be dealing with the reaction of a minor colored impurity that is unrelated to reaction 1. In addition, only one reaction step was observed when P(n-Bu), was used as the entering ligand in reaction 1. The investigated reactions exhibit excellent first-order behavior in the presence of an excess PPh3 or P ( ~ - B U ) ~ . The observed rate constants as a function of entering ligand concentration, temperature, and pressure, along with the calculated activation parameters, are summarized in Tables I and 11. Our rate constant of (2.8 f 0.1) X lo4 M-' s-l for the reaction with PPh3 at 40 "C is in excellent M-' s-' agreement with a literature value of 2.9 X under these condition^.^ A similar agreement exists for M-ls-' at 40 the reaction with P(n-Bu),, viz., 3.5 X OC compared to 3.3 X M-'s-' in the l i t e r a t ~ r e .The ~ higher basicity of P ( ~ - B Uand ) ~ its smaller cone angle and size in terms of steric hindrance should account for the higher substitution rate. Both reactions are significantly accelerated by pressure. Plots of In kobs versus pressure are linear within the experimental error limits, and the volumes of activation, AV*, were calculated from the slope of these plots (=-AV*/RT) in the usual way. The reported rate and activation parameters, especially the significantly negative A S * and AV' values, strongly support the operation of a limiting associative (A) mechanism for both reaction^.^ This suggestion is in line with the general tendency of cyclopentadienyl ligands to undergo ring slippage (qs q3 q5) in order to accept an electron pair from the metal center and to vacate a metal orbital for associative attack by the entering n~cleophile.~ A more detailed analysis of the AV* data is appropriate. In the type of substitution reactions investigated, no major
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Table 11. kob,as a Function of [P(n-Bu)a],Temperature, and Pressure for the Reactiona
0.10 0.15 0.20 0.10 0.15 0.20 0.10 0.15 0.20 0.10 0.15 0.20 0.10 0.15 0.20 temp, O C
+ P(n-Bu), 1.45 f 0.04 2.25 f 0.06 3.1 f 0.1 2.0 f 0.1 3.3 f 0.1 4.2 f 0.1 3.4 f 0.1 5.2 f 0.1 7.0 f 0.2 5.5 f 0.2 8.4 f 0.2 11.0 f 0.3 8.9 f 0.3 13.5 f 0.4 17.9 f 0.5 pressure, MPa
40.0
5
50 100 150
+
( ~ 5 - C p ) R h ( C O ) P ( n - B ~ ) ,CO
1.50 f 0.05
71 f 3
-64 f 10
2.1 f 0.1
3.5 f 0.1 5.5 f 0.1 9.0
f
0.1 AV*, cm3 mol-' -17.0 f 0.7
104kOh! S-'
6.6 f 0.3 9.2 f 0.3 12.1 f 0.3 17.3 f 0.4
[Rh] = 1.5 X lo-, M wavelength = 365 nm. bMean value of at least three kinetic runs. Second-order rate constant calculated as the mean value of kob/[P(n-Bu),]. dMean value of at least three kinetic runs; [P(n-Bu)J = 0.18 M.
change in solvation is expected to affect the value of AV', and the observed effects must result from intrinsic volume changes, i.e., changes associated with changes in bond lengths and angles.13J5 The values of -14 and -17 cm3 mol-' reported for the volume of activation indicate that significant bond formation (ie., volume collapse) occurs on forming the transition state. The overall observed effect may be partially offset by the volume increase expected for the q5 to q3 ring slippage of the Cp ligand. We conclude that this study has provided evidence for the general req3 q5) postulated before5 and action sequence (v5 outlined in (2). It furthermore supports the general
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dh
oc/ 'co
Ril 0C'~'w
Rh
L'
'co
L = PPh3, P(rrBU)s
concept of a ring-slippage mechanism and the associative nature of the substitution process. The AV* values of between -14 and -17 cm3 mol-' f6und in this study demonstrate the order of magnitude that this parameter can have for such processes. In this respect, it is interesting to note that AV' values of between -6 and -20 cm3 mol-' were recently reported for associative substitution reactions of octahedral carbonyl complexes of Mo and W.18*"930 In general, a AV' value of -15 cm3 mol-' will result in doubling the observed rate constant on increasing the pressure from ambient to 100 MPa. Such effects can be measured with great accuracy by using relatively unsophisticated eq~ipment.'~ Acknowledgment. We gratefully acknowledge financial support from the Deutvche Forschungsgemeinschaft, Fonds der Chemischen Industrie, Max Buchner Forschungsstiftung and Volkswagen-Stiftung, as well as a DAAD stipend to H.C.B. (30) Wieland, S.;van Eldik, R. Organometallics, submitted for publication.
