J. Am. Chem. SOC.1991, 113, 5305-531 1
5305
Activation of Methane and Toluene by Rhodium( 11) Porphyrin Complexes Bradford B. Wayland,* Sujuan Ba, and Alan E. Sherry Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323. Received October 12, 1990
Abshrct Thermodynamic and kinetiemechanistic studies are reported for reactions of (tetramesitylporphyrhato)rhodium(II) monomer, (TMP)Rh', and (tetraxylylporphyrinato)rhodium(II) dimer, [ (TXP)Rh],, with methane that produce hydride and methyl derivatives. A Rh'LRh" bond energy of 12 kcal mol-' in [(TXP)Rh], was determined by IH NMR line broadening and found to dominate differences in the thermodynamic and kinetic parameters for reactions of methane with Rh(I1) porphyrins. The sum of the Rh-H and Rh-CH3 energies is found to be 117 kcal in both the (TMP)Rh and (TXP)Rh derivatives. Rate laws, activation parameters, and deuterium isotope effects suggest that a four-centered linear transition state (Rh-H,C.-H--Rh) provides a relatively low activation enthalpy route for methane reacting with two metalloradicals. Comparative studies demonstrate that rhodium(I1) porphyrins react with toluene exclusively at the benzylic C-H bond, and kinetic studies suggest that this reaction proceeds through a transition state related to that for the methane reactions. Aromatic C-H bond reactions are kinetically excluded for rhodium(I1) porphyrins due to steric effects in the transition state.
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Introduction A variety of metal complexes have been observed to react with the C-H unit of methane by oxidative addition to a single metal center,," u-bond metathesis of M-X units,&* and addition to M-X groups? We have recently reported a somewhat different type of methane reaction where oxidative addition occurs to two independent metalloradical units.I0 (Tetramesitylporphyrinato)rhodium(II), (TMP)Rh', reacts selectively with methane in C6D6 solvent to form the hydride and methyl derivatives (eq 1 ) . k
2(TMP)Rh'
+ C H 4 & (TMP)Rh-H + (TMP)Rh-CH3 k-l
(1)
Reaction 1 proceeds by concerted formation of products through a transition state that contains two metalloradicals and methane [(TMP)RhCH,.Rh(TMP)]. This paper presents a more complete description of the kinetiemechanistic and thermodynamic features of reaction 1 , extends the methane reaction studies to (tetraxylylporphyrinato)rhodium(II) dimer, [(TXP)Rh],, presents comparative studies for alkyl C-H bond reactions of toluene, and explores the origins of the unusual preference of rhodium(I1) porphyrins to react with alkane relative to aromatic C-H bonds.
Results Reactions of Rhodium(II) Porphyrins with Methane in Benzene. (TMP)RL'. Benzene solutions of (TMP)Rh' react with methane to form equal quantities of (TMP)Rh-CH3 and (TMP)Rh-H in accord with eq 1 Qualitative evidence for the reversibility of eq 1 is obtained from the observation that benzene solutions containing (TMP)Rh-H and (TMP)Rh-CH3 at 353 K give reductive elimination to form (TMP)Rh' and CH,. Reaction 1 proceeds to ' H N M R observable equilibrium positions at a methane pressure of 1 atm and temperatures of 353, 373, and 393 K. Temperature dependence of the equilibrium constant (K,)determined by integration of the ' H N M R spectra (K,(353 K) = I
(1) (a) Hoyano, J. K.; McMaster, A,, Graham, W. A. G. J . Am. Chem. Soc. 1983, 105, 7190. (b) Rest, A. J.; Whitewell, I.; Graham, W. A. G.; Hoyano, J. K.; McMaster, A. D.J . Chem. Soc., Chem. Commun. 1984,624. (c) Ghosh, C. K.; Graham, W. A. G.J . Am. Chem. SOC.1983, 109, 4726. (2) Janowicz, A. H.; Bergman, R. G. J . Am. Chem. Soc. 1983,105,3929. (3) Wenzel, T. T.; Bergman, R. G. J . Am. Chem. Sm. 1986, 108,4856. (4) Hackett, M.; Whitesides, G.M. J . Am. Chem. Soc. 1988,110, 1449. (5) Harper, T. G.; Shinomoto, R. S.; Deming, M. A,; Flood, T. C. J . Am. Chem. SOC.1988, 110, 7915. (6) Watson, P.J . Am. Chem. SOC.1983, 105, 6491. (7) (a) Fendrick, C. M.; Marks, T. J. J . Am. Chem. Soc. 1984,106,2214. (b) Fendrick, C. M.; Marks, T. J. J . Am. Chem. Soc. 1986, 108, 625. (8) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J . Am. Chem. Soc. 1987. 109. 8109. (9) Cummins, c. C.; Baxter, S. M.; Wolczanski, P.T. J . Am. Chem. Soc. 1988, 110, 8731. (10) Sherry, A. E.; Wayland, B. 8. J . Am. Chem. Soc. 1990, 112, 1259.
7300 f 700; Kl (373 K) = 3300 f 400; K1 (393 K) = 1100 f 200) provides thermodynamic values for reaction 1 ( M I o = -13.0 f 1.5 kcal mol-'; SIo = -19 f 5 cal K-l mol-') (Figure 1). The equilibrium constant at 296 K (PCH4= 1 atm) was too large to be determined by ' H N M R . Linearity of kinetic plots of [(TMP)Rh']-l versus time for more than 3 half-lives at conditions where the methane concentration is held constant and reaction 1 proceeds to virtual completion ( T = 296 K; PcH4= 10 atm; [CH4] = 2.35 X lo-' M; [(TMP)Rh*], = 7.0 X lo" M) indicates that the forward reaction is second order in (TMP)Rh' (Figure 2). Good agreement between the rate constants evaluated on the basis of a third-order process at methane pressures of 1.0 atm ( k , (296 K) = 0.132 L2 mol-2 s-l) and 10.0 atm ( k , (296 K) = 0,122 L2 mol-2 S-I) indicates that the reaction is first order in methane (Figures 2 and 3). Plots of [(TMP)Rh']-l versus time for reaction 1 at 353 and 393 K (PCH,= 1 atm) are shown in Figure 3. Rate constants a t 353 K ( k , (353 K) = 0.83 L2 mol-2 s-I) and 393 K ( k , (393 K) = 3.68 L2 mol-2 s-l) were determined from the linear portion of these plots, which occur during the early stages of the reaction. Deviations from linearity become pronounced for the kinetic data at 393 K (Figure 3) as expected for a process approaching equilibrium. Temperature dependence of the forward rate constants ( k , ) yields activation parameters for reaction 1 (AH,'= 7.1 f 1.0 kcal mol-I; AS,' = -39 f 5 cal K-' mol-I). (TMP)Rh' fails to react with neat C6D6 or C6H6over a period of months at 353 K. Solutions containing ( T M P ) R h - C 6 H ~and (TMP)Rh-H in contact with H2 (PH,= 200 Torr) failed to produce any evidence for reductive elimination of benzene over a period of months at 353 K. [(TXP)Rhh. Benzene solutions of [(TXP)Rh], react reversibly with methane (Pch = 1-5 atm) ( T = 353-393 K) in accord with eq 2. Proton N M R spectra used in following the progress of reaction 2 are illustrated in Figure 4. Reaction 2 proceeds to an observable equilibrium position (Pch = 0.8-1.0 atm; T = 353, 393 K), which permitted evaluation of equilibrium constants by integration of the IH N M R spectra (K2 (353 K) = 0.03 0.01; K2(393 K) = 0.04f 0.01). The insensitivity of K2to temperature
*
[(TXP)Rh]z
+ CH4
k k-1
(TXP)Rh-H
+ (TXP)Rh-CH,
(2)
changes indicates that AH? is approximately zero (AH?= 0 kcal mol-,; ASlo = -7 f 3 cal K-' mol-,). Kinetic data for reaction 2 were obtained by following the loss of [(TXP)RhI2 at 353 and 393 K (Figure 5). The concentration dependence of the rate can be fitted to a reaction that approaches equilibrium with a rate = law in accord with the stoichiometry of reaction 2 k2[[(TXP)Rh]2][CH,]; K2 k2/k-2) (k2 (353 K) = 0.44 X 101-' L mol-' s-'; K2 (353 K) = 0.034; k2 (393 K) = 6.1 X lW3 L mol-'; s-'; K 2 (393 K) = 0.028) (Figure 5). Temperature dependence
0002-7863/91/1513-5305%02.50/0 0 1991 American Chemical Society
Wayland et al.
5306 J. Am. Chem. SOC.,Vol. 113, No. 14, 1991
I
9.5 9.0
t =0
ppqL7-4
8.5
y' e
-
8.0
2
L
-H
7.5 7.0
I
I
6.5 2.5
2.6
2.8
2.7
2.9
IJT x to3
Figure 1. Van't Hoff plot for the reaction of (TMP)Rh' with CHI to solution (K,(353 K) form (TMP)Rh-H and (TMP)RhCH3 in 7300; K I(373 K) = 3300;KI (393 K) 1100).
1-
L
io. ' A
an
12000 10000
8000 6000
I
II
I
4000 2000
0 4 . 0
,
i o
.
, 2 0
.
, 30
40
.
, 5 0
.
I
6 0
t(hour) Figure 2. Second-order plot for the reaction of (TMP)Rh' with CHI in C6D6at 296 K. ([(TMP)Rh'], = 7.02 X la'M; 10 atm; PcH,= IO atm; [CH,] = 0.235 M; kl = 0.122 L2 mol-' s-l).
Figure 4. Proton NMR spectra for [(TXP)Rh]' (4.48X IO-' M) with CHI (PCH,= 5.18 atm, [CH,] = 0.1270 M) in C6D6: (A) initial spectrum ( I = 0), (B)after heating for 16 h at 353 K for (a) (TXP)Rh-H, (b) (TXP)Rh-CH3, and (c) [(TXP)RhI2. [(TXP)Rh]2
+ CHq a
(TXP)Rh.H c (TXP)Rh.CHg
14000
-=
P, n.
E
Y Y \
12000 1woo 8000
6000
Y
4000
1
2000
0
5
10
15
20
25
30
t(hour) Figure 3. Kinetic studies for reaction 1 in C6D6 at a seriea of temperatures. Points on the graph represent experimental data. Solid lines are calculated for a precess that is second order in (TMP)Rh' and first crdcr in CHI approaching equilibrium. (e393 K, [(TMP)Rh']' = 7.90X la' M, [CH,] = 2.35 X 1W2 M, k l (393 K) = 3.68Lzmol-' s-', K,(393 K) IOOO,A, 353 K, [(TMP)Rh'], 7.80X la'M, [CH,] = 2.45 X IO-' M, k, (353 K) = 0.832 L2 mol-2s-', K I (353 K) = 7300, 0 , 296 K, [(TMP)Rh'], 8.85 X IO-'M, [CH,] = 2.46 X IO-' M, & I (296 K) 0.132 L2 mol-2 s-l). of the forward rate constants yields values for the activation parameters (AH,'= 17 f 3 kcal mol-'; AS,*= -25 i 7 cal K-' mol-'). Benzene solutions of (TXP)Rh-H and (TXP)Rh-CH3 that were prepared by independent routes were observed to reductively eliminate methane with formation of [(TXP)Rh], and CHI at 353 K. Benzene solutions of [(TXP)RhI2 are indefinitely stable
0
I 2
4
6
8
10
12
t(hour)
Figure 5. Representative kinetic studies for reaction 2 in CsD6 at 353 and 393 K. Points on the graph represent experimental data. Solid lines are calculated for a process that is first order in both [[(TXP)RhI2and CH, approaching equilibrium. (A,393 K, [[(TXP)Rh],], = 2.25 X la' M,[CH,] = 5.36 X IF2M, k2 (393 K) = 6.1 X lO-) L mol-' s-l, K2(393 K) 0.028;e 353 K, [[(TXP)Rh],], 4.48 X IO-' M, [CH,] 0.1270 M, k2 (353 K) = 0.44 X IO-' L mol-' s-I, K2 (353 K) = 0.034). at 353 K, and solutions containing (TXP)Rh-H and (TXP)RhC6HSfail to eliminate benzene over a period of several months at 353 K. [(OEP)RhI2. A reexamination of the reaction of [(OEP)RhI2 with methane in C6D6 resulted in observation of the reversible formation of a small quantity of (OEP)Rh
