J . Am. Chem. SOC.1986, 108, 5036-5038
5036
carboxylic g r o ~ p . ~ The * ' ~ pK, values of le-g (3.80, 3.94, and 3.93, respectively) seem to reach a limiting value and become independent of the ring size. When the pKa determinations of lb, If, and l g were carried out in the presence of a 100-fold excess of urea, we found a small but significant enhancement of the pKa values of If and l g (both O.lO), an effect that hardly exists in the case of l b (ApK, 6 0.04). This increased pKa value can be attributed to a specific inclusion of urea,I0 a proposed complexation which is supported by the isolation of a crystalline If-urea (]:I) complex." The X-ray analysis of the If-urea complex'* (Figure 1) shows that the proton of the carboxylic group is coordinated -0distance to the urea oxygen atom via a short hydrogen bond (0. 2.54 A, 0-H.e 0 angle 167'). The total encapsulation of urea results in a complex with an apolar exterior, a property that allows for the solubilization of urea in an apolar solvent. A 0.10 M solution of l f in CDCI, solubilized 0.05 M urea (25 O C , 17 h). In the absence of l f no urea could be dissolved in CDC13. These results prove that the electrophilic moiety in crown ethers 1 can assist in the complexation of a neutral molecule, and, therefore, we assumed that other electrophiles should also be able to assist in this type of complexation.13 When a dilute solution of 2,6-pyrido-27-crown-9 (2e) and an equimolar amount of l i t h i u m perchlorate was treated with 1-2 equiv of urea in ethanol, a crystalline complex (l:l:2) precipitated.I4 X-ray structure analysis (Figure ])I5 revealed that one of the urea molecules is encapsulated in the crown ether cavity ( N - s . 0 distances in hydrogen bonds 2.99-3.17 A) in a similar way as in the If-urea complex with the Li+ cation replacing the hydrogen atom of the carboxylic group as a coordinating electrophile (Lis. distance 1.90 A, Li. .N distance 2.20 A). The tetrahedral coordination of the lithium cation is completed by a crown ether oxygen and the oxygen atom of a second urea molecule. These Li-..O distances (1.98 and 1.95 A) are in agreement with values reported for other 4-fold coordinated Li+ complexes. l6 From these results we conclude that electrophiles such as Li+ or H+can be vital bridging species in the cocomplexation of neutral guests by crown ethers.
Netherlands Foundation for Chemical Research (S.O.N.) with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.). Supplementary Material Available: Tables containing listings of positional and thermal parameters of the two structures (12 pages). Ordering information is given on any current masthead page.
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e
0
Acknowledgment. These investigations were supported in part by the Netherlands Technology Foundation (S.T.W.) and by the (9) 1d.H20 complex (1:l) was obtained from a methanol/water solution (mp 65-67 "C); X-ray analysis shows an encapulated complex in which three hydrogen bonds are formed between the host and the guest molecule. To be published. (10) Grwtenhuis, P. D. J.; Uitenvijk, J. W. H. M.; Reinhoudt, D. N.; van Staveren, C. J.; Sudhdter, E. J. R.; Bos, M.; van Eerden, J.; Klooster, W. T.; Kruise, L.; Harkema, S . J . Am. Chem. SOC.1986, 108, 780-788. (11) Yield 88% (mp 75-78 "C); ' H NMR (CDCI,) 6 3.65 (m, 32 H, CHzCH2),4.70( s , 4 H, Ar CHI), 5.6 (br s . 4 H, NH2), 7.26 (s, 3 H, Ar H). (12) C25H@O1C H 4 N 2 0 ,monoclinic, space group P2,/n,a = 19.990 (2) A, b = 8.943 (1) c = 16.660 (1) A, @ = 94.54 (1)". V = 2969 (1) A I , Z = 4, D, = 1.29 g cm-', p(Mo Ka) = 1.0 cm-'. Measured 3887 unique reflections (3" < B < 22.5"). T = 159 K. Structure solved by direct methods. Full-matrix least-squares refinement of 1988 observed reflections (F: > 3u(F:)). Hydrogens from difference Fourier maps; carboxylic and urea hydrogens included in refinement; crown ether hydrogens calculated (C-H distance 0.96 A). Final R = 3.6%, R, = 3.8%, w = 4F,'/a2(F,Z), 382 variables. (13) (a) Pascard, C.; Riche, C.; Cesario, M.; Kotzyba-Hibert, F.; Lehn, J.-M. J . Chem. Soc., Chem. Commun. 1982,557-560. (b) Hosseini, M. W.; Lehn, J.-M. J . Chem. Soc., Chem. Commun. 1985, 1155-1157. (c) Lehn. J.-M. Science (Washington, D.C.) 1985, 227, 849-856. (14) Yield 80% (mp 107-109 "C); 'H NMR (CDCI,) 8 3.2-3.8 (m, 28 H, CH2CH2),4.73 (s, 4 H, Ar CHI), 7.2-7.9 (m, 3 H, Ar H). Anal. Calcd for C23H,,CILiN5014:C, 42.11; H, 6.61; N, 10.68. Found: C, 42.36; H, 6.59; N , 10.63. (15) C21H3SN08~2(CH4N20)~LiC104, triclinic, space group Pi,a = 15.687 (6) A, b = 11.797 (3) A, c = 9.280 (2) A, a = 103.64 (2)". /3 = 97.40 (3)'. y = 99.90 (l)", V = 1618 (2) A', 2 = 2,D, = 1.35 g cm-], p(Mo Ka) = 1.8 cm-'. 5660 Unique reflections (4" C B C 25"), T = 242 K. Solution by direct methods. Refinement of 3875 observed reflections, urea hydrogens included, crown ether hydrogens in calculated positions. Final R = 6.7%. R, = 9.8%, 450 variables. (16) (a) Verbist, J.; Meulemans, R.; Piret, P.; van Meerssche, M. Bull. SOC.Chim. Belg. 1970, 79, 391-396. (b) Suleimanov, Kh.; Sergienko, V . S.; Kipkalova, N.; Sulaimankulov, K. Koord. Khim. 1979, 11, 1732-1736. (c) Lebioda, L. Acta Crysfallogr.,Sect. B 1980,36, 271-275. (d) Groth, P. Acta Chem. Scand.. Ser. A 1982, 36, 109-1 15.
k,
Barrier to Reductive Elimination of Hydrogen from Dihydridobis( trimethylphosphine) platinum( 11) Diane L. Packett and William C. Trogler* D e p a r t m e n t of C h e m i s t r y , D - 0 0 6 U n i v e r s i t y of C a l i f o r n i a a t San D i e g o La Jolla, C a l i f o r n i a 92093
Receitied March 31, 1986
Oxidative addition and reductive elimination of H-H, C-H, and C-C bonds occur in many catalytic and synthetic r e a ~ t i 0 n s . I ~ Mechanistic studies of the reductive elimination of carbon-hydrogen5-' and carbon-carbons-'* bonds have been reported for several systems. Intermolecular reductive elimination' I can hamper the study of these reactions on a single metal center. The intramolecular reductive elimination of molecular hydrogen from a mononuclear platinum, palladium, or nickel complex has not been well characterized, although elimination of hydrogen from metal clusters has been suggested as a model for the behavior of hydrogen on surfaces. I Several theoretical studies have considered the oxidative addition and reductive elimination of molecular hydrogen from bis(phosphine)platinum(II) species'"17 to gain insight into the mechanism of processes that might occur on a platinum surface. These studies suggest reductive elimination proceeds via a "late" transition state that contains an $-dihydrogen ligand. Kinetic barriers predicted range between 18 and 42 kcal/mol. We report here the first experimental investigation of intramolecular, rate-determining reductive elimination of H, from a mononuclear dihydride complex containing a metal from the nickel triad, a series of metals frequently used as heterogeneous hydrogenation catalysts. (1) Collman, J. P.; Hegedus, L. S . Principles and Applications of Organotransifion Metal Chemistry; University Science Books: Mill Valley, CA, 1980; Chapter 4. (2) Collman, J. P. Arc. Chem. Res. 1968, I , 136-143. (3) Halpern, J . Arc. Chem. Res. 1970, 3, 386-392. (4) James, B. R. Homogeneous Hydrogenation; Wiley: New York, 1973. (5) Abis, L.; Sen, A.; Halpern, J . J . Am. Chem. SOC. 1978, 100, 291 5-291 6. (6) (a) Michelin, R. A.; Faglia, S.; Uguagliati, P. Inorg. Chem. 1983, 22, 1831-1834. (b) Buchanan, J . M.; Stryker, J . M.; Bergman, R. G . J . Am. Chem. SOC.1986, 108, 1537-1550. (7) Okrasinski, S. J.; Norton, J. R. J . Am. Chem. Soc. 1977, 99, 295-297. (8) Braterman, P. S.; Cross, R. J.; Young, G. B. J . Chem. SOC.,Dalton Trans. 1977, 1892-1897. (9) Moravskiy, A.; Stille, J. K. J . Am. Chem. SOC.1981, 103. 4182-4186. (10) Gillie, A.; Stille, J. K. J . Am. Chem. SOC.1980, 102, 4933-4941. (11) Norton, J . R. Acc. Chem. Res. 1979, 12, 139-145. (12) Tamaki, A.; Magennis, S. A.; Kochi, J. K. J . Am. Chem. SOC.1974, 96, 6 140-6 146. (13) Bavaro, L. M.; Montangero, P.; Keister, J. B . J . Am. Chem. SOC. 1983, 105,4977-4981. (14) Noell, J . 0.; Hay, P. J. J . Am. Chem. SOC.1982, 104, 4578-4584. (15) Low, J. J.; Gcddard, W. A., 111 J . Am. Chem. SOC.1984, 106, 6928-6937. Low, J . J.; Goddard, W . A,, 111 Organometallics 1986, 5, 609-622. (16) Obara, S.; Kitaura, K.; Morokuma, K. J . Am. Chem. SOC.1984,106, 7482-7492. (17) Balazs, A. C.; Johnson, K. H.; Whitesides, C .M. Inorg. Chem. 1982, 21. 2162-2174.
0002-7863/86/ 1508-5036%0lSO10 0 1986 American Chemical Society
J . Am. Chem. Soc., Vol. 108, No. 16, 1986 5037
Communications to the Editor Table I. Kinetic Data for Reductive Elimination of H2 from Dihvdridobisl trimethvlDhosuhine)Dlatinum(II)
solvent THF k,(21 "C), S-'
AH*,^
(7.3 f 0.4) x 10-4' 9.4 1
*
2.5-Me7THF (1.8 f 0.06) X
(3.0 & 0.01) X
2,2,5,5-MedTHF
14.0 f 0.6
20.0 f 0.5
-23 f 2
-2 f 2
kcal/mol A S , b
-41 f 3
cal/(mol K) lop A'
4.3 8.4 12.8 All errors represent the standard deviation obtained from a linear least-squares analysis. From an Eyring plot. Preexponential factor from an Arrhenius plot. ~
The complex trans-dihydridobis(trimethy1phosphine)platinum(11), (1) was prepared by reduction of ~ i s - P t C l ~ ( P M with e~)~ sodium naphthalide under a H2 atmosphere as described previo ~ s l y . ' In ~ solution, 1 exists as an equilibrium mixture of cis and trans isomers. Solutions of cis- and trans-1 are stable under hydrogen, but when exposed to nitrogen they decompose with evolution of H 2 to yield Pt(PMe3), and platinum metal (eq 1 )
kl
f r a n ~ - P t H ~ ( P M e ~ ) ~c i ~ - P t H ~ ( p M e ~ ) ~ Pt(PMe3), + Pt
+ H 2 (1)
according to a first-order rate law koM [PtH2(PMe3)J. It was not possible to trap the intermediate Pt(PMe3)2species by reaction with an olefin or acetylene since solutions of 1 react with such substrates (e.g., tert-butylethylene) by an associative pathway.I9 At temperatures above -10 OC, solutions of cis- and trans-1 decompose at a rate conveniently measurable by FTIR spectroscopy.20g2' Since cis-trans isomerization occurs within minutes at -80 OCI8 this process is fast21,22compared to reductive elimination. The reaction was followed by monitoring the decrease ~ 2030 cm-I) or in absorbance of peaks from cis-1 ( u ~ =- 1985, trans-1 (uR-" = 1720 These IR data show that cis-1 exists as a dihydride rather than an q2-dihydrogen complex as found recently for several metal system^.^^,^^ Further confirmation of the dihydride structure derives from the absence of lJH-D 30 Hz, expected24 for q2-deuterium hydrogen, in the ' H N M R spectrum of c i ~ - p t H D ( P M e ~ ) ~ . First-order plots for the disappearance of 1 were linear for several half-lives of the reaction. Rate data and activation parameters are given in Table I. The rate constant kl was unaffected by the initial concentration of 1 (0.1-0.3 M) or by the presence of colloidal platinum in the solution. Comparison of the rate of
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(18) Packett, D. L.; Jensen, C. M.; Cowan, R. L.;Strouse,C. E.;Trogler, W. C. fnorg. Chem. 1985, 24, 3578-3583. (19) Packett, D. L.; Trogler, W. C., unpublished results. (20) Reactions were carried out in Schlenk tubes (under a N2 atmosphere) placed in a thermostat4 bath. At timed intervals, aliquots were withdrawn by syringe and transferred into CaFz liquid sample cells. It was important to exchange the atmosphere frequently to prevent the buildup of Hzin the reaction flask, which slows down the reaction. Cells were flushed with nitrogen before use and placed in an IBM 32 FTIR spectrometer. For measurements below 20 O C , the cells were precooled by flushing with nitrogen that was passed through a cooling coil in an NJpentane slush bath at -130 OC. (21) It was not possible to monitor the reaction by NMR spectroscopy, because the dihydride resonances broaden into the base line at temperatures above -30 OC where the reactions occur. (22) Decay of the IR absorbances attributed to cis- and trans-I proceeds at the same rate, proving that the isomerization tram-1 == cis-1 is rapid compared to reductive elimination. The absorbances that were monitored obeyed Beer's Law over the concentration range of interest. According to the scheme of eq 1 kw = k , ( K / K + 1). Equilibrium wnstants were determined in each solvent by NMR spectroscopy at temperatures between -40 and -80 OC and extrapolated to the temperatures of this study. While the accuracy of the multiplicative factor K / ( K + I ) effects the absolutes rates it has little influence on the values of activation parameters. (23) (a) Sweany, R. L. J . Am. Chem. SOC.1985, 107, 2374-2379. (b) Crabtree, R. H.; Lavin, M. J. Chem. Soc., Chem. Commun. 1985, 1661-1662. (24) (a) Morris, R. H.; Sawyer, J. F.; Shiralian, M.; Zubkowski, J. D. J. Am. Chem. SOC.1985, 107, 5581-5582. (b) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. J . Am. Chem. SOC.1984, 106,451-452.
decomposition of PtD2(PMe3)2revealed an inverse kinetic isotope effect of 0.72 f 0.06 (standard deviation for three determinations). These results are consistent with a mechanism involving unimolecular rate-determining reductive elimination of H2. An inverse isotope effect is expected for the breaking of a low frequency bond to hydrogen (Pt-H) and formation of a high frequency bond (H-H) in the transition state.25 Alternatively an q2-dihydrogen complex could form reversibly in a preequilibrium step as an intermediate. The equilibrium isotope effect would favor q2-D2 over q2-H2since the zero point energy of the first species would be significantly less than the second. This could also lead to the observed inverse kinetic isotope effect.26 It was impossible to investigate the effect of added phosphine ligands on the reaction rate; free phosphines react with 1 to produce the known five-coordinate species PtH2(PMe3)3.27 A mechanism involving phosphine dissociation as the rate-determining step is, however, difficult to reconcile with the inverse kinetic isotope effect. Furthermore, dissociation of phosphine was not observed for reductive elimination in the electronically similar complex C ~ ~ - P ~ H ( C H ~ ) ( P P ~ ~ The inhibiting effect of added H 2 on the reaction suggests that an intermediate forms reversibly en route to Pt(PMe,), and Pt products because addition of H2 after complete reaction does not reverse the process. Although PtH2L2species catalyze isotope exchange2' between H2 and D2, crossover experiments at low conversions were possible. Both PtH2(PMe3)2and PtD2(PMe3)2were dissolved in T H F solvent a t room temperature (under a N2 atmosphere) and the atmosphere above the solution was analyzed by gas chromatography2*near the beginning of the reaction. The evolved gas showed H2 and D2, but no detectable (
