J. Am. Chem. SOC. 1993,115, 11823-11830
11823
Hydrolytically Stable Organic Triester Capped Polyoxometalates with Catalytic Oxygenation Activity of Formula [RC(CH20)3V3P2W15O59l6-(R = CH3, NO2, CH2OH) Yuqi Hou and Craig L. Hill’ Contribution from the Department of Chemistry, Emory University, Atlanta, Georgia 30322 Received July 12, 1993”
Abstract: The apparent instability of fully oxidized (do) redox active alkoxypolyoxometalates to hydrolysis inferred from literature studies would appear to obviate complexes involving covalent bonds between polyols (e.g. carbohydrates) and polyoxometalate complexes for applications in aqueous phase catalysis and medicine. The title capped complexes [RC(CH20)3VV3P2WV11sOs9]G ([1-R]: R = CH3, NO2, and CH2OH), prepared in very high yield (>90%) by the @ ~ ~ ]Q, = (nstoichiometric condensation of l,l,l-tris(hydroxymethy1)ethane derivatives with Q s H ~ [ V ~ P ~ W I where C4H9)4NI in CH3CN, however, have substantial kinetic stability with respect to hydrolysis (e.g. 7112 912 h for 40 mM [l-CH,] in D20 with pD = 0 at 100 “C). IH, 31P,and slV N M R spectra during and after the condensation synthesis of the triester complexes [1-R] indicate the absence of polyoxometalate intermediates in significant concentration during the reactions. These techniques and 183WNMR and IR confirm the only detectable product in each case is the C3” regioisomer involving replacement of the three 12-oxo edge-bridging oxygens of the V3 cap in the parent polyoxometalate, the most basic oxide ions in the complex, with the three 12-alkoxy groups of the triols. Two types of experiments, complementary to those addressing hydrolytic removal of the 1,1,1-tris(alkoxymethy1) “caps” of the title complexes, indicate both a kinetic and a thermodynamic preference for reaction of the more electron rich alcohols with the parent heteropolyanion, Q~H4[V3P2W15062],to form the title complexes. First, competitive kinetics methods indicate the relative reactivities of CH3OH, CH3C(CH20H)3, (HOCH2)C(CH20H)3, and (02N)C(CH2OH)3, respectively, with this polyanion were 6:6:5:1. Second, thermodynamic equilibration in cap-cap’ exchange (or triple transesterification) reactions, RC(CH20H)3 + [ R ’ C ( C H ~ O ) ~ H V ~ P ~ W ~ S O[RC(CH2O)3HV3P2W1sOs9lSS~]~+ R’C(CH2OH)3, where R, R’ = NO2 or CH2OH (DMF solution, 50 OC, 3 days), indicated a 35-fold preference for binding of the HOCH2C(CH2-)3 cap relative to the 02NC(CH2-)3 cap. While the title capped complexes showed stability upon reduction on the cyclic voltammetry time scale (nearly reversible voltammograms), preparative electrochemical or chemical reduction in slightly wet CH3CN resulted in immediate and complete hydrolysis to form the free 1,l ,I-tris(alkoxymethy1) caps and [v3P2w15062]9-. Potentiometric titration, elemental analysis, and chemical reactivity confirmed that the [1-CH3] complex could be prepared in two different protonation states, QsH[l-CH3] and Q2H4[1-CH3]. Several experiments with the thioether mustard gas analog tetrahydrothiophene (THT) demonstrated that the [ 1-R] complexes are capable of catalyzing oxygenation. Comparative kinetics methods gave the following order of reactivity for oxidation of T H T by tert-butyl hydroperoxide (TBHP): Q2H4[1-CH3I (fastest catalyst) > OV(O-iPr)3 (a monomeric Vv model for breakdown of [1-CH3] during turnover) > (n-BuqN)2W04 (a monomeric Wvl ] catalyst (slowest). These model) > Q5H4[V3P2W15062] (the parent complex) > QsH[l-CH3] > Q ~ [ P ~ W I &>~ no relative rates coupled with other kinetics measurements and the complete lack of degradation of [ l-CH31 examined after 30 turnovers (30 equiv of THTO from THT and TBHP) collectively provide strong evidence that kinetically significant fragmentation during catalysis by [l-CHj] is minimal and that [1-CH3] in varying protonation states is probably the active catalyst. One possible activated complex for the rate-determining transition state is given for T H T oxidation by TBHP catalyzed by [1-CH3] consistent with the unusual rate law: vo = k[THT] [TBHP] [[1-CH3]I2. The 10.1 kcal mol-’. Arrhenius plot is quite linear (R = 0.988) yielding E,
-
-.
-
Introduction Early transition metal oxygen anion clusters, or polyoxometalates for short, simultaneously exhibit several properties that make them attractive for applications in catalysis, medicine, and many other areas. Among these properties are an ability to be reversibly reduced by several electrons, resistance to thermal and oxidative degradation, and synthetic accessibility and alterability.l-I4 The potential utility of polyoxometalates in both Abstract published in Advance ACS Abstracts, November 15, 1993. (1) References 2-14are recent reviewsof polyoxometalates: 2 4 aregeneral reviews; 5 is on nomenclature; 6 and 7 are on heterogeneous catalysis; 8-12 are on homogeneous catalysis; and 13 and 14 are on photocatalysis. (2) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer-Verlag: Berlin, 1983. (3) Day, V. W.; Klemperer, W. G. Science 1985, 228, 533. (4) Pope, M. T.; Miller, A. Angew. Chem., Int. Ed. Engl. 1991,30,3448. ( 5 ) Jeannin, Y.; Fournier, M. Pure Appl. Chem. 1987, 59, 1529. (6) Misono, M. Catal. Rev.-Sci. Eng. 1987, 29, 269. (7) Misono, M.; Nojiri, N. Appl. Caral. 1990, 64, 1-30. 0
OOO2-7863/93/1515-11823$04.00/0
catalysis and chemotherapeutics could be considerably enhanced if a rational and flexible methodology existed for the formation (8) Kozhevnikov,I. V.; Matveev, K. I. Russ. Chem. Rev. (Engl. Transl.) 1982, 51, 1075-1088. (9) Kozhevnikov, I. V.; Matveev, K. I. Appl. Catal. 1983, 5, 135-150. (10) Hill, C. L. In Metal Catalysis in Hydrogen Peroxide Oxidations; Strukul, G.,Ed.; Kluwer Academic Publishers: Dordrecht, 1992; Chapter 8, pp 253-280. (11) Hill,C.L.;Khenkin,A.M.;Weeks,M.S.;Hou,Y.InACSSymposium Series on Catalyfic Selective Oxidation; Oyama, S . T., Hightower, J. W., Eds.; American Chemical Society: Washington, DC, 1993; Chapter 6, pp 67-80. (12) Hill, C. L.; Kim, G . 4 . ; Prosser-McCartha, C. M.; Judd, D. In
Polyoxometalates: From Platonic Solids to Anti-retrouiral Actiuiry; Pope,
M. T., Miiller, A., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1993; in press. (13) Papaconstantinou, E. Chem. SOC.Rev. 1989, 18, 1-31. (14) Hill, C. L.; Prosser-McCartha. C. M. In Photosensitizafion and Photocatalysis Using Inorganic and Organometallic Complexes; Kalyanasundaram, K., GrBtzel, M., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1993; Chapter 13, pp 307-330.
0 1993 American Chemical Society
Hou and Hill
11824 J. Am. Chem. Soc., VoZ. 115, No. 25, 1993
of hydrolytically stable linkages between organic and biological R R = CH, NO, CHZOH groups and the surfaces of the complexes. Such surface groups could facilitate recognition and preassociation of catalytic substrates and biological targets while hydrolytic stability of the 3 edge-shared linkages would render the organic derivatized polyoxometalates VV ions stable in water. Water is not only a solvent of growing value for catalysis in an age of concern about toxic organic solvents and reagents and burgeoning biotechnology industries but also the solvent mandated by medical applications. At present two types = internal PV of linkages between polyoxometalate oxygen atoms and organic "tetrahedron" or organometallic groups have demonstrable long term hydrolytic stability in partial or completely aqueous systems, the organosilyl and related main group organometallic derivatives of Knoth and cyclopentadienyltitanium P ~ p e and ~ ~ the - ~ alkyl-substituted ~ = wVI ions (RCpTiI") polyoxometalate derivatives of Klemperer and Keana.27-31 A third group of organic derivatized polyoxometalates, the bis(organophosphony1) polyoxotungstates of formula Figure 1. The structures of the title complexes, [RC(CH20)3[CsHjP(0)]2XB+W1 1039(8-n)-(whereXn+ = P5+, Si4+)have limited V ~ P Z W I ~ ORS = ~ CH3, ] ~ , NOa, CHzOH ([ l-CH31, [l-N02], and [ 1-CH2OH], respectively), in polyhedral notation. In this notation the MO6 and hydrolytic stability with halflives of hours (not too short for internal Po4 units are represented as coordination polyhedra: octahedra chemotherapeuticefficacy but not too long to interfere with bodily and tetrahedra, respectively. The vertices of the polyhedra define the clearance) .32 The optimal "handles" for molecular recognition oxygen nuclei while the central metal or nonmetal ions are internal to for catalytically or physiologically active water soluble polyoxeach polyhedron. ometalates, however, would be the polyols. Carbohydrates are ubiquitous in biological systems, structurally diverse, and often complexes are not likely to be compatible with oxidative catalysis play key roles in selective molecular binding and consequent or physiological conditions as nearly all of them contain, in whole physiological activity. By extension carbohydrate-polyoxometor in part, reduced vanadium ions, which dictates, in large part, alate conjugates with adequate hydrolytic stability might facilitate their stability .38-40 targeting of polyoxometalates in antiviral ~ h e m o t h e r a p y . l ~ * ~ ~ - ~ ~We report here the synthesis and characterization of the first Unfortunately, definitive work including that of Day, Klemperer, alkoxypolyoxometalates with both hydrolytic stability and catand S ~ h w a r t zand ~ ~others has established that alkoxypolyoxoalytic oxygenation activity. These complexes of formula metalates (alternative nomenclature: organic esters of polyox[RC(CH20)3HV3P2W150j9l5-([1-R]: R=C ometalates) are notoriously labile to hydrolysis. While the group OH), illustrated in Figure 1,are the tris(hydroxymethy1) triestess of Zubieta has used hydrothermal conditions and the chelating of [ V ~ P ~ jO62l9-, WI a complex first reported by Pope, 'ITeze, and 'triesters to prepare a gallery of polyalkoxyvanadates, these their c o - w o r k e r ~and ~ ~subsequently ~~ characterized further by Finke and co-workers, who alluded to irreversible changes in the (15) Knoth, W. H. J. Am. Chem. SOC.1979, 101, 759-760. NMR spectra of solutions of the complex containing methanol, (16) Knoth, W. H. J. Am. Chem. SOC.1979, 101, 2211-2213. a point not further developed.44 The spectroscopic, hydrolytic, (17) Knoth, W. H. Ligated Transition Metal Derivatives of Heteropolyanions; E. I. Du Pont de Nemours and Company, 1980. redox, and catalytic oxygenation properties of the new title (18) Domaille, P. J.; Knoth, W. H. Inorg. Chem. 1983, 22, 818-822. complexes are described. (19) Barkigia, K. M.; Rajkovic, L. M.; Pope, M. T.; Quicksall, C. 0. J. Am. Chem. SOC.1975, 97,4146-4147. (20) Kwak, W.; Pope, M. T.; Scully, T. F. J. Am. Chem. SOC.1975, 97, 5735-573 8. (21) Kwak, W.; Rajkovic', L. M.; Pope, M. T.; Quicksall,C. 0.; Matsumoto, K. Y.; Sasaki, Y. J. Am. Chem. SOC.1977, 99, 6463-6464. (22) Wasfi, S. H.; Kwak, W.; Pope, M. T.; Barkigia, K. M.; Butcher, R. J.; Quicksall, C. 0.J. Am. Chem. Sac. 1978,100,7784-7787. (23) Zonnevijlle, F.; Pope, M. T. J. Am. Chem. SOC.1979, 101, 2731-
2732. (24) Sethuraman, P. R.; Leparulo, M. A.; Pope, M. T.; Zonnevijlle, F.; Brgvvard, C.; Lemerle, J. J. Am. Chem. SOC.1981, 103, 7665-7666. (25) Barkigia, K. M.; Rajkovic-Blazer,L. M.; Pope, M. T.; Quicksall, C. 0. Inorg. Chem. 1981,20, 3318-3323. (26) Chorghade, 6. S.;Pope, M. T. J. Am. Chem. SOC.1987,109,51345138. (27) Che,T. M.;Day,V. W.; Francesconi,L. C.; Fredrich,M. F.;Klemperer, W. G.; Shum, W. Inorg. Chem. 1985, 24,4055-4062. (28) Che, T. M.; Day, V. W.; Francesconi,L. C.; Klemperer,W. G.; Main, D. J.; Yagasaki, A.; Yaghi, 0. M. Inorg. Chem. 1992, 31, 2920-2928. (29) Keana, J. F. W.; Ogan, M. D. J. Am. Chem. SOC.1986,108,79517957. (30) Keana, J. F. W.; Ogan, M. D.; Lii, U.;Beer, M.; Varkey, J. J. Am. Chem. SOC.1986, 108, 7957-7963. (31) Keana, J. F. W.; Wu,Y.; Wu,G. J. Org. Chem. 1987,52,2571-2576. (32) Kim, G.-S.; Hagen, K. S.; Hill, C. L. Znorg. Chem, 1992,31,53165324. (33) The antiviral and anti-HIV properties of polyoxometalates are under considerable experimental scrutiny at present (see review in ref 12 and the papers in refs 34-36). (34) Inouye, Y.; Tokutake, Y.; Kunihara, J.; Yoshida, T.; Yamase, T.; Nakata, A.; Nakamura, S. Chem. Pharm. Bull. 1992,40, 805. (35) Inouye, Y.; Tokutake, Y.; Yoshida, T.; Yoshiko, S.; Hujita, H.; Dan, K.; Yamamoto, A.; Nishiya, S.; Yamase, T.; Nakamura, S. Antiuirul Res. 1993, 20,317-331. (36) Hill, C,; Weeks, M.; Schinazi, R. F. J. Med. Chem. 1992,35, 1216. (37) For a thorough discussion of steric versus electronic effects in the binding of organic groups to polyoxometalates, see Day, V. W.; Klemperer, W.G.; Schwartz, C. J. Am. Chem. Soc. 1987, 109,4030-6044.
Experimental Section Materials and Methods. Acetonitrile (Burdick & Jackson) was dried over 3A molecular sieves before use. The thioether substrates and oxidants were purchased from Aldrich. The complexes Q2w0427(Q = n-Bu4N), OV(O-iPr)3,45 and Q ~ H ~ [ V ~ P Z W I were ~ O ~prepared Z ] ~ ~ by literature procedures. The purity and composition of the latter polyoxometalate complex was determined to be very high based on IR, 31P,and 51VNMR and elemental analysis. 'H NMR spectra were recorded using a General Electric QE-300 instrument. 31P,51V,and 183WNMR spectra were recorded using an IBM WP-200SY NMR spectrometer at 81.01,52.58, and 8.34 MHz, respectively. All solvents for use in NMR were dried by storage over activated 3A molecular sieves for at feast a week prior to use. *H, 31P,51V,and 183WNMR chemical shifts were referenced to (CH&Si, 85% H3PQ4, 100% Vocl3, and 2.0 M Na2W04 in D20, respectively. In the case of 31P,51V,and 183WNMR spectra, referencing was achieved by the substitution method and chemical shifts downfield from the reference are reported as positive (+a). The probe temperature was 25 O C in all NMR experiments. Infrared spectra were obtained as (38) Chen, Q.; Liu, S.; Zubieta, J. Inorg. Chem. 1989, 28, 4433-4434. (39) Khan, M. I.; Chen, Q.; Goshorn, D. P.; Hope, H.; J. J. Am. Chem. SOC.1992,114, 3341-3346. (40) Chen,Q.;Goshorn, D. P.;Scholes,C. P.;Tan,X.-l.;Zubieta, J. J.Am. Chem. Soc. 1992, 114,4467-4681. (41) Mossoba, M. M.; OConnor, C. J.; Pope, M. T.; Sinn, E.; Heme, G.; TezzC, A. J , Am. Chem. SOC.1980,102,6864-6866. (42) Harmalker, S. P.; Pope, M. T. J. Am. Chem. SOC.1981,103,73817383. (43) Harmalker, S. P.; Leparulo, M. A.; Pope, M. T. J. Am. Chem. SOC. 1983,105, 4286-4292. (44) Finke, R. 6.;Rapko, B.; Saxton, R. J.; Domaille, P. J. J. Am. Chem. SOC.1986,108, 2947-2960. (45) Hillerns, F.; Rehder, D. Chem. Ber. 1991, 124, 2249-2254.
Organic Triester Capped Polyoxometalates
J. Am. Chem. SOC.,Vol. 115, No. 25, 1993
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elemental analyses were consistent with the formulation of the complex. KBr pellets with 2-5 wt % of the sample using a Nicolet 510M FTIR 'H NMR (CD3CN at 25 "C): 6 0.99 (t, 60H 3H of cap), 1.40 (sextet, spectrophotometer. Potentiometric titrations of [ l-CH3I in acetonitrile 40H), 1.67 (quintet, 40H). 2.90 (s, lH, OH), 3.16 (t, 40H), 5.54 (s, 6H, solution with 1.0 M methanolic (n-CdH&NOH to assess protonation -CHfl). IR (cm-I): 1088 (m), 1065 (w), 1023 (vw), 958 (s), 912 (s), states were carried out using a Model 240 Corning pH meter equipped 81 1 (vs), 794 (s), 732 (m). The elemental analysis was satisfactory. with a combination micro glass electrode at room temperature. The numbers of titratable protons in all cases agreed with the formulations Esterification of [v82wl&6Zp by Alcohols. The kinetic and of the polyoxometalate complexes based on all other data (uide infro). thermodynamic affinities of alcohols toward [v3P2w1S062]p- were Elemental analyses were conducted by Atlantic Microlab, Inc. (Norcross, assessed. In a competitive kinetics experiment, 0.2 mmol of OzNCGA) for C, H, and N and by E+R Microanalytical Laboratory (Corona, (CH20H)3 and 0.2 mmol of one of the alcohols, methanol, CH,C(CHzNY) for all other elements. OH)3, or (HOCH2)C(CHzOH)3, were dissolved in 10 mL of DMF. To thesolution wasaddedO.l mmolofQ~H4[V3P~Wl~O62],and Preparation of [ ( ~ C ~ ~ ) ~ N Y I [ C H ~ C ( C H Z O ) ~Q V ~a P1 ~W -I thissolution ~S~~ was stirred at 40 "C for 24 h. Aliquots were removed for analysis and CH3]. To 2.07 g (0.4 mmol) of Q S H ~ [ V ~ P ~ Win~50 S mL O ~ of ~ ]CH3CN added dropwise to ca. 200 mL of diethyl ether to precipitate the product. was added 0.06 g (0.5 mmol) of l,l,l-tris(hydroxymethy1)ethane. The The precipitate was collected by filtration, washed three times with diethyl resulting solution was stirred at room temperature for 24 h and a large ether, and dried in vacuo prior to analysis by IH NMR. Products were excess of ether was slowly added to the solution until a precipitate formed. quantified by 'H NMR. By this method, the relative reactivities of the The precipitate was separated by filtration, washed with ether, and dried alcohols for [V3P2Wls062]9'were determined to be 6:6:5:1 for CHoOH: in vacuo for 2 days, to yield about 2.0 g of [1-CH3] as an orange powder CH~C(CHZOH)~:(HOCH~)C(CH~OH)~:(O~N)C(CH~OH)~. (95% yield based on Q~H4[V3P2W15062]).The IH NMR (CD3CN) shows one characteristic resonance at 6 = 5.44 for the three equivalent Hydrolytic Stability of Qfll-CH,]. One hundred equivalents of D20 methylenegroups consistent with attachment to the threevanadium atoms were added to a 0.1 M CD3CN solution of the title compound and the on one cap or end of the C3" starting polyoxometalate complex. The resulting solution was kept at 60 OC for 72 h. The spectral properties resonance of the ester (triol) methyl group was coincident with the confirmedthat there was no apparent hydrolysis of the tris(hydroxymethy1) resonance of the terminal methyl of the tetra-n-butylammonium groups: "cap" from the complex: IH NMR: 6 5.47 (-CH&). 3 L P N M R-4.50 6 1.00 (t, 60H + 3H of cap), 1.43 (sextet, 40H), 1.65 (quintet, 40H), (Av1p = 8.4 f 0.4 Hz) and -9.80 ppm ( A q p = 8.9 f 0.5 Hz). 3.17 (t, 40H), 5.44 (s, 6H, CH2-0). 31P,slV, and lS3WNMR spectra Determination of Rate Constant for Hydrolysis of Lia[l-CH3]. confirm that effective symmetry is maintained in the capped-catalyst Solubility limitations compelled us to prepare the analogous lithium salt product, [1-CH3]. 31PNMR (0.01 M in CD3CN at 25 "C): 4 . 5 0 ppm P ~ W I S ~ ~for ~ ]conof [l-CHs], L ~ ~ H [ C H ~ C ( C H ~ O ) ~ V ~(LisH[l-CH3] (Av1p = 8.4 f 0.4 Hz) and-9.60 ppm (Av1p = 8.0 f 0.3 Hz). 51VNMR venience), for this study. The lithium salt was prepared in the following (0.01 M in CD3CN at 25 "C): -551.0 ppm (Av1/2 = 1243.8 f 9.3 Hz). manner. Fifteen milliliters of an acetonitrilesolution of 2.5 g (0.5 mmol) lS3WNMR (0.15 M in CD3CN at 25 "C, 11 000 acquisitions): -113.4 of QsH[l-CH3] was added dropwise to 20 mL 50/50 v/v CH,CN/HzO ppm (lW, Av1p = 3.1 f 0.1 Hz, 2J~-cm = 6.3 f 0.5 Hz), -153.6 ppm saturated solution of (CH3)4NBr with stirring. The resulting (CH&N+ (2W,Av1/2=3.5fO.l H z , ' J p a w = 1 . 4 f O . l H ~ , ~ J w a w = 7 . 9 * salt was filtered, washed with three 15-mL portions of H20, and then 0.5 Hz), and -167.5 ppm (2W, Av1/2 = 8.0 f 0.3 Hz, z J ~= a 11.4~ dissolved in 100 mL of 50/50 v/v CH3CN/H20 and passed through a f 0.5 Hz). IR (cm-I): 1088 (m), 1067 (w), 1026 (vw), 956 (s), 909 (s), 50-mL bed of AG 5OW-X2 resin (BIO RAD) charged with Li+ at the 818 (vs),798 (s),733 (m). Potentiometrictitrationoftheisolatedcomplex rateof 2 mL/min. Theeluant was concentrated with a rotary evaporator indicated one proton consistent with the formulation QsH[I-CH3]. and the product was dried in vacuo at 50 OC for 2 days, yielding 2.5 g Anal. Calcd for C S S H ~ ~ O N S ~ ~C ~, 19.45; P ~ VH,~ 3.62; W ~N, ~ :1.33; (70%) of the hydroscopic product. IH NMR (DzO) shows two P, 1.18; V, 2.92; W, 52.60. Found: C, 19.20; H, 3.51; N, 1.38; P, 0.97; characteristic resonances at 6 = 1.07 and 5.51. 31PNMR (0.01 M in V, 3.19; W, 52.02. D20 at 25 OC): -6.56 (Av1/2 = 19.8 f 0.9 Hz) and -12.66 ppm (Av1p = 19.0 f 0.8 Hz). 51VNMR (0.01 M in D20 at 25 "C): -540.8 ppm Preparation of Q ~ ) ~ [ C H ~ C ( C H ~ O ) ~ V ~ P Z W The ~ &more ~ ] . acidic (Av1/2 = 8 9 1 . 8 f 9.3 H z ) . A n a l . C a l c d f o r form of [ l-CH,] was prepared by ion exchange using AG 50W-X2 resin. L~SH[CH~C(CH~O)~V~P~W Li,I S0.86. O ~ ~Found: ]: Li, 0.85. Ten grams (1.9 mmol) of [ I-CHs] were dissolved in 50 mL of CHjCN The hydrolysis was conducted as follows: To 0.5 mL of acidified D20 and the solution was passed through 20 g of the AG 50W-X2 (BIORAD) acidic cation exchange resin charged with H+ and pre-washed (1 N HCl, pD = 0), 0.0815 g (0.02 mmol) of LisH[l-CH3] and about with CH3CN. Several additional 5-mL portions of 50/50 v/v CH30.5 pL of DMSO, as an internal standard, were added. The solution was CN/H20 were required to wash all the complex off the column. The transferred to a NMR tube, which was placed in a 100 OC oil bath. The eluant was collected and solvent was removed at about 45 "C with a hydrolysis process was monitored by IH NMR. The rate constant for rotary evaporator. The solid was dried in vacuo at room temperature for hydrolysis was determined by either the rate of disappearance of the 2 days yielding 8.3 g (95% based on QsH[l-CH3]) of the product as an reactant 'H NMR signal or the appearance of the organic triol 'H NMR orange powder. IH NMR (CD3CN at 25 "C): 6 1.00 (t, 28H + 3H of signal to be koba = 7.6 X I P h-' at this temperature and pH. cap), 1.42 (sextet, 18H), 1.64 (quintet, 18H), 3.13 (t. 18H), 5.58 (s, 6H, Exchange of Tris(hydroxymethy1) "Cap" on [v3PZWl&ZP. In one -CH&). IIP NMR (0.01 M in CDJCN at 25 "C): -4.45 ppm (Av1p case, the relative thermodynamic stabilities of two different tris= 9.4 f 0.4 Hz) and -9.40 ppm (Av1p = 8.8 f 0.3 Hz). 51VNMR (0.01 (hydroxymethyl) derivatives as triesters for [ V ~ P ~ W ~ Scould O S ~be] ~ M in CDsCN at 25 "C) only one peak at -561.0 ppm (Av1/2 = 1143.8 assessed. This was done by using the triple transesterification capcap' f 10.3 Hz). IR (cm-I): 1089 (m), 1067 (sh), 1025 (vw), 959 (s), 912 exchange reaction involving the nitrocapped complex and the free (s), 814 (vs), 797 (s), 731 (m). Potentiometric titration of the isolated hydroxymethylcap, pentaerythritol, and the reverse reaction involving complexindicatedca. four protons, whichis consistent with the formulation the hydroxymethyl-capped complex and the free nitrocap, tris(hyQ2H4[ 1-CH31. The elemental analysis was satisfactory. droxymethy1)nitromethane. To 5.0 mL of DMF in a 10-mL roundbottom flask was added0.05 mmol of Q ~ H [ O ~ N C ( C H ~ O ) ~ V ~ P ~ W I S O S ~ ] Preparation of Q J I [ O Z N C ( C H ~ O ) ~ V S Z W [l-N02]. I ~ ~ ~ ~The , same and 0.05 mmol of HOCH~C(CH~OH)J. The flask was capped with a procedure was used as for [1-CH3] but the triol (02N)C(CHzOH)3 was rubber stopper and wrapped with aluminum foil to exclude light, and the used in place of CH3C(CH20H)3. The single IH NMR resonanceof the was stirred at 25 OC for 3 days. The solution was added to 100 was I S O S ~ solution ] capped polyanion moiety of Q ~ H [ O ~ N C ( C H ~ O ) ~ V ~ P ~ W mL of ether with stirring. The resulting precipitate was filtered, washed observed at 6 = 5.82. IR (cm-I): 1086 (m), 1066 (w), 1024 (vw), 954 (s), 909 (s), 820 (vs), 806 (s), 733 (m). The elemental analysis was with ether, and dried in vacuo for 24 h. 'H NMR showed that 35 f 5% of the original cap (02NC(CH2-)3) had been replaced by the new one satisfactory. (HOH2CC(CH2-)3). The same procedure and conditions were used Preparation of Q ~ H [ H O C H Z C ( C H ~ O ) ~ V ~ P[l-CH2OH]. ZWI~~~~ ~ S OOzNC(CH2S~] starting with Q s H [ H O H ~ C C ( C H ~ O ) ~ V ~ P Z Wand Pentaerythritol (0.08 g; 0.6 mmol; 1.5 equiv) and 2.07 g (0.4 mmol) of didI S O S ~ ] OH),. lH NMRshowed that Q s H [ H O H ~ C C ( C H ~ ~ ) ~ V ~ P ~ W QsH~[V~P~W were I S stirred ~ ~ ~ ]in 30 mL of DMF contained in a 100not react with 02NC(CH2OH), (detectability limit in analysis structure involves alkoxide formation (esterification) a t the three symmetry equivalent ~2-oxooxygens in the edge-bridging triad of three Vv ions that comprise the cap of the parent complex. The N M R parameters of the [ 1-R] complexes are similar but distinct from those of the parent complex in the same medium. Second, the numbers of peaks and the line widths are all consistent with proton exchange between water oxygen atoms and polyoxometalate oxygens that is slow on the N M R time scale. Slow proton exchange can lead to line broadening, as in the case of the [1-R] complexes, or even multiple peaks, as well as slight changes in chemical shifts in the N M R spectra of protonated polyoxometalate complexes in highly dried dipolar aprotic media.44 This is confirmed by addition of 100 equiv of DzO which sharpens the N M R peaks as proton exchange between water oxygen atoms and polyoxometalate oxygens approaches the fast exchange limit. For example, the 51Vresonance in dry CDsCN (6 = -551.0 ppm with Av1p = 1243.8 f 9.3 Hz) transforms upon addition of 100 equiv of D20 to a sharper single resonance (6 = -549.5 ppm with Av1p = 1057.2 f 9.5 Hz). The IR spectra also strongly suggest retention of the effective C30 structure of the parent polyoxometalate, [v3P2w15062]9-, and the tris(hydroxymethy1) group in the product [1-R] complexes. Two distinguishing features exhibited by the [1-R] complexes that are not present in [v3P2w15062]9- are the VC-O from the tris(hydroxymethy1) cap (-1027 cm-I) and a slight splitting in one of the V M - ( F M stretching fundamentals (800-820 cm-I). The more highly protonated complex Q2H4[l-CH31, readily prepared by ion exchange, is much more electrophilic and active as an oxygenation catalyst than Q5H[l-CH3] (vide infra) yet appears to be indefinitely stable in CH3CN solution at ambient temperature. Potentiometric titration in CH3CN, the solvent chosen for catalytic studies reported below, elemental analysis, and chemical reactivity indicate the more highly protonated formulation. All the physical and spectroscopic data (Experimental Section) collectively indicate that the formation of covalently or strongly bonded dimers or oligomers of the capped complexes, [ 1-R], in solution is negligible. Furthermore, this data coupled with kinetics and other data discussed below do not indicate that weak intermolecular interactions between these complexes in solution are of sufficient magnitude to have an experimentally significant effect on the redox behavior of these complexes. Esterification Chemistry of [V3P2W150~2]~-with Alcohols. Despite a number of definitive papers published recently that address the formation and chemistry of alkoxypolyoxometalates (organic esters of polyoxometalates), there is paucity of actual kinetic or thermodynamic information on these processes and the complexes. Three types of experiments were conducted to assess the kinetic and thermodynamic aspects of polyoxometalate esterification, depicted by eq 1, and the reverse reaction, hydrolysis of the capped complexes, depicted principally by the reverse of eq 1. Full details (conditions of reaction, analytical methods, etc.) for each of the subsequently described experiments are given in the Experimental Section. In the first experiment, equimolar quantities of a representative monodentate primary alcohol, methanol, and one of the three triols CH3C(CH20H)3, (HOCH2)C(CH2OH)3, or (02N)C(CH2OH)3were allowed to compete with each other for reaction with [v3P2w15062]9-.The products weredetermined to be stableunder the reaction conditions and the relative quantities of the capped products, [1-R], were proportional to the relative reactivities of the different alcohols. The relative reactivities for [v3P2w15062]9were determined to be 6:6:5:1 for CH3OH, CHsC(CH20H)3,
J. Am. Chem. SOC.,YO/. 115, No. 25, 1993 1 1827
P
I
v
0.0 0
I
/
1
100
200
I
300
time (hrs) Figure5 HydrolysisofLi5H[l-CH3]to l,l,l-tris(hydroxymethy1)ethane and [V3P2W15062]9-, the reverse of eq 1. Conditions: 40 mM LiSH[lCHJ], pD = 0; 100 O C . Conversion = y = (CO- C)/Cb, where Co and C = initial concentration and concentration at time 1, respectively, of
L~~H[I-CHJ]. (HOCH2)C(CHzOH)s, and (02N)C(CH20H)3, respectively. The more electron rich alcohols clearly have a kinetic preference for esterification relative to the more electron poor alcohols. This is consistent with the do metal centers being electrophilic and alcohols nucleophilic and the fact that polyoxometalate esterifications take place under neutral or acidic conditions. In a second experiment, the kinetics of hydrolysis of [ l-CHs] was conducted using the highly water soluble LisH salt of [l-CHj] which was prepared by two successive cation exchange procedures from analytically pure Q5H[1-CH3Ia The observed hydrolytic process was the effective reverse of eq 1 and the free l,l,l-tris(hydroxymethy1)ethane “cap” and the parent heteropolytungstate, [v3P2w&,2]9-, were formed cleanly. The rate, monitored by IH N M R under pseudo-first-order conditions at pD = 0 and 100 OC is given as a conversion plot in Figure 5. The kobs = 7.6 X 1 P h-1 (or 2.1 X le7 s-I), which translates to a. half-life of 912 h under these fairly brutal hydrolytic conditions! Although the rate of forward reaction (i.e. eq 1) could not be assessed under the same conditions in order to experimentally evaluate a thermodynamic binding constant for eq 1, K = kl/k-l, it is clear that K is orders of magnitude greater than 1. The rate data indicate that some types of alkoxypolyoxometalates clearly have the requisite stability in water to be compatible with medical applications and environmentally attractive catalytic processes in water. With the myriad of polysaccharides currently of interest in biotechnology and other research areas, the possibilities for functional covalent polysaccharide-polyoxometalate conjugates would seem considerable. A third experiment permitted an evaluation of the thermodynamic affinities of triol caps for the parent heteropolytungstate, [v3P2w15062]9-. Capcap’ exchange (or triple transesterification) reactions, eq 2, were conducted in DMF. Reaction with 1.Oequiv
,+ [R’C (CH20),HV,P2W, 50,,]’-
-
DMF
RC ( C H 2 0 H )
[RC(CH,0)3HV3P,W,,05,15-+ R’C(CH20H), (2) R = CH,OH, R’ = NO, and R = NO,, R’ = CH,OH (0.05 mmol) of Q5H[02NC(CH20)3V3P2W15059] and 1.Oequiv of (HOCHz)C(CH20H)3 in D M F at 25 OC for 3 days produced the free 02NC(CHzOH)3 cap and Q ~ H [ H O C H ~ C ( C H Z O ) ~ V3P2W15059] in 35 & 5% yield based on the starting complex. In contrast, the reverse cap-cap’ exchange reaction involving Q5H[ H O C H ~ C ( C H ~ ~ ) J V J P ~ and W I 02NC(CH20H)3 ~O~~] under identical conditions produced no detectable triple transesterification products. This indicates that the more electron rich tris(hydroxymethyl) compounds form thermodynamically more stable caps on the [v3P2w15062]9- than the electron poor tris(hydroxymethyl) compounds. Not surprisingly then, the reaction
Hou and Hill
11828 J. Am. Chem. SOC.,Vol. I 1 5, No. 25, 1993
Table I. Oxidation of Tetrahydrothiophene (THT) by Three Oxidants Catalyzed by Q2H4[1-CH31°
I
THT oxidant conversion (turnovers)b product (selectivity)c THTO (>99%) TBHP 32 (18) H202 15 (43) THTO (>99%) THTO (>99%) PhIO 23 (13) " Q = n-BudN+. Conditions: 5 X 10-6 mol of QzH4[1-CH,], 0.28 mmol of THT, and 0.28 mmol of oxidant in 5.0 mL of CH&N under Ar for 2 h at 25 OC. Conversion = [ 1 - (moles of substrate at the end of the reaction/moles of substrate initially)] X 100. Turnover = moles of THTO product/moles of catalyst. Selectivity = (moles of THTO product/moles of all detectable THT-derived products) X 100.
/------I
v +1 .o
I -1 .o
0.0 EN
Figure 6. Cyclic voltammograms of Q5H[l-CH3] and the precursor polyoxometalate. (A) [l-CH,]; (B) the precursor complex, Q5H4[v3P2wlSo62] (Q = n-BudN+). Conditionsin both voltammograms:
-
1 mM complex; 0.1 M (n-BudN)PF6 supporting electrolyte in CH3CN;
25 OC at glassy carbon working electrode; potentials relative to Fc+/Fc; scan rate 200 mV s-I.
after reoxidation. The conditions and details of these reactions are given in the Experimental Section. Catalyzed Oxygenation of an Organic Substrate, Tetrahydrothiophene (THT), by [l-a~]. The oxidation of thioethers by oxygen donors was chosen to evaluate potential catalytic oxygenation activity by an exemplary tris(hydroxymethy1) capped complex, [l-CH3], for three reasons: (i) it is of practical interest in decontamination (selectiveoxidation of the prominent thioether chemical warfare agent mustard or H D is a major goal)48and other areas, (ii) it is generally simple, facile, and amenable to ready quantitation, and (iii) it has been investigated for regular nonderivatized polyoxometalate catalysts.49 The particular reaction examined is illustrated in eq 3. The addition of a catalytic
of a more electron rich alcohol with [V3P2W15062l9- is both kinetically and thermodynamically favored. This result should be substantially extrapolatable to dopolyoxometalates in general (THV 0 as these compounds, as a class, are all electron poor and (THTO) electrophilic. It is appropriate to note that steric and not just DO = oxygen donor oxidant (TBHP, H202,and PhIO used) electronic contributions are important to esterification of polyox om eta late^.^^ The research of Day, Klemperer, and Schwartz on the alkylation and esterification of C2v-Nb2W4019k with a amount (1 equiv) of [l-CHs] to 57 mM (57 equiv) of both THT variety of electrophiles, RX (X = chlorides, sulfonates, and substrate and one of three oxidants, tert-butyl hydroperoxide triflates), or alcohols, respectively, to yield the Nb2W4019R3(TBHP), hydrogen peroxide, or iodosylbenzene, in CHjCN leads complexes provides one elegant experimental example of the to the rapid and selective oxidation of THT to the corresponding importance of steric effects in binding aliphatic organic groups sulfoxide, tetrahydrothiophene oxide (THTO), in effectively to polyoxometalates. For both alkylation and esterification quantitative yields with all three oxidants (Table I). More reactions, the less sterically hindered groups ( R = CH3, CH3significantly, the catalyst, [1-CH3], recovered after 20 to 30 CH2) end up on the more sterically hindered but more nucleophilic turnovers is still intact in all reactions. doubly bridging oxygens while the more sterically hindered groups The recovery of a catalyst after reaction does not, however, say (R = (CH3)2CH, (CH&Si, or [(CH&C](CH&Si) end up on anything definitive regarding its actual role in catalysis. Catalystthe less sterically hindered but less nucleophilic terminal oxygen^.^^ derived species isolated during or after reaction often represent Redox Stability of [1-CH3]. The cyclic voltammograms of QsHkinetic c u l - d e - s a c ~that , ~ ~ is, they do not lie along the principal [ l-CH31 and the precursor polyoxometalate Q ~ H ~ [ V ~ P Z W I ~ Ocatalysis ~ ~ ] reaction coordinate. The actual catalyst is often an at 1.0 mM in CH3CN are given Figure 6. The following altered form or a fragment of the initial species added at the experimentally reproducible points regarding the voltammograms outset of the reaction. To address the issue of the active form are noted (formal potentials (E?, = (E, Epa)/2 versus Fc+/ of [1-CH3] in catalysis of the THT oxidation by TBHP we Fc): (1) There are two quasi-reversible redox processes for QsHexamined the time course of product generation by several [ l-CH31, a 2-electron process at -0.3 1 V and a 2-electron process complexes and determined the rate law for oxidation by the most at -0.72 V. The number of electrons in each redox process was active catalyst of those complexes evaluated. The complexes confirmed independently by CeIV titration. (2) Two analogous examined as potential catalysts were Q5H [l-CHs], the more highly redox processes are seen in Q s H ~ [ V ~ P ~ W(-0.31 I ~ O and ~ ~ -0.60 ] protonated form, Q2H4[l-CHs], ( ~ - B U ~ N ) ~and W OV(O-iPr)3, O~ V). That the processes are analogous is a point consistent with models for possible Wvl and Vv monomeric fragments of [ l-CH31, a minimal change in the electronic structures of the most anodic respectively, the parent complex, Q5H4[V3P2W15062Irand the portion of the molecules, the V3OI3caps, upon replacing the formal isostructural all-Wvl Wells-Dawson complex, Q ~ [ P Z W , ~ O ~ ~ ] . ~ 2 - 0 ions ~ - with the formal r2-RO- ions during the esterification, Figure 7 shows the rates of THTO production from oxidation of THT by TBHP in the presence of the above complexes and in eq 1. In contrast to the apparent implied stability of the reduced the absence of any catalyst. The rates are in the following order: forms of [1-CH3] on the cyclic voltammetry time scale (tens of Q2H4[1-CH3] (fastest) > OV(O-iPr)3 > ( ~ - B u ~ N ) ~>WQ5H4O~ seconds), these forms are neither thermodynamically nor kinet(48) Yang, Y.-C.; Baker, J. A,; Ward, J. R. Chem. Rev. 1992,92, 1729ically stable on slightly longer time scales in this medium (CH31743. C N with a few equivalents of H20). Controlled potential (49) Hill, C. L.; Faraj, M. In Proceedings of the 1989 US.Army Chemical Research, Development and Engineering Center Scientific Conference on reduction after the first reduction wave at -0.31 V leads to Chemical Defense Research; US Army Armament, Munitions & Chemical decomposition in minutes. In both cases the dominant products Command, 1989; pp 131-134. are those of hydrolysis, 1,1,1-tris(hydroxymethyl)ethane and (50) For example, see: Chan, A. S. C.; Pluth, J. J.; Halpern, J. J. Am. Chem. SOC.1980, 102, 5952-5954. reduced [V3P2W15062]9-asconfirmed by lH, 31P,and 51VN M R
+
Organic Triester Capped Polyoxometalates
J . Am. Chem. SOC.,Vol. 1 1 5, No. 25, 1993
11829
4
1,'. ; ,
,
,
+
0 0
2
I
,;,
.
a
A
4
6
0
time(h) Figure 7. Relative rates of formation of tetrahydrothiophene oxide (THTO) in the oxidation of tetrahydrothiophene (THT) by tert-butyl hydroperoxide(TBHP) catalyzed by [ 1-CH3Jin two different protonation states,Q2&[1-CH3] (B) andQ~H[l-CH3](e),by theuncappedprecursor Vv (0) and Wvl (A) complex, Q S H ~ [ V ~ P ~ W (0), I ~ Oby ~ Zmonomeric ] degradation model compounds (wo42-and OV(O-iPr)s, respectively), and by the Wells-Dawson heteropolytungstate, Q6[P~W18062](A).The control oxidation reaction with no catalyst is also shown (+).
[ V ~ P ~ W I> SO Q d~[ ~l - C ~ H 3 ] > Q6[P2wi8062] > no Catalyst (slowest). These data provide four mechanistically useful inferences or observations: (1) All the species exhibit a catalytic 1.0 1.2 1.4 1.6 1.8 effect; the ratio of catalyzed to uncatalyzed processes, kcatlkuncat, log [TBHP] (mM) ranges from ca. 8.7 (for the least active complex, Q ~ [ P ~ W I S O ~ ~ ] ) Figure 8. log-log plots for the initial rates of reaction of THT with to ca. 833 (for the most active complex, Q2H4[1-CH3]). (2) TBHP catalyzed by QzH~[I-CH~]: (A) dependence on THT; (B) Catalysis by [1-CH3] is acid dependent with the tetraprotonated dependence on TBHP. For conditions see the Experimental Section. form (from elemental analysis and potentiometric titration), being ca. 87 times more active than the monoprotonated form. (3) The most active of the catalysts is Q2H4[1-CH3]. It is significantly more active than all the likely fragments that might form in small quantities in situ from it. (4) At parity of protonation state (4 titratable protons), [1-CH3] is more active than the parent uncapped heteropolyanion, [V3P2W15062]9-. These data and the spectroscopic data delimiting the extent of decomposition of [ 1-CH31under turnover conditions (negligible) collectively imply that kinetically significant fragmentation during catalysis by [l-CH3] is minimal and that [I-CHj] in varying protonation states is most likely the principal catalyst. .. . A plausible mechanism for the process depicted in eq 3 involves - 0 . 4 -0.3 - 0 . 2 -0.1 0.0 0.1 0.2 the direct oxidation of THT by the oxidized form of the log [1-CH3] (mM) polyoxometalate catalyst, eq 4, followed by reoxidation of the Figure 9. Concentration dependence on QzH4[1-CHo] for the initial THT oxidized polyoxometalate H,O rates of reaction of THT with TBHP catalyzed by Q Z H ~ [ ~ - C H ~ ] . Conditions given in the Experimental Section. THTO + 2H' reduced polyoxometalate (4)
+
+
+
+
+
-
-
reduced polyoxometalate 2H' OX RED oxidized polyoxometalate
+
+ H,O
(5)
OX = oxidant (TBHP in this study) and R E D = corresponding reduced form (t-BuOH = TBA in this study) reduced form of the catalyst, eq 5, given the considerable number of organic substrate oxidations catalyzed by strongly oxidizing vanadium-containing polyoxometalates that probably proceed by this general mechanism.8~9~49~51-ss Furthermore, the oxidation of thioethers by TBHP catalyzed by vanadium-substituted heteropolytungstic acids, and in particular, H~PV2Mo10040,had been (51) Neumann, R.; Lissel, M. J. Org. Chew. 1989, 54, 46074610.
(52) Neumann, R.;Levin, M. J . Org. Chem. 1991, 56, 5707-5710. (53) Neumann, R.;Assael, I. J. Chem. SOC.,Chem. Commun. 1988,12851287. (54) Ali, B. E.;Brtgeault, J.-M.;Mercier, J.; Martin, J.;Martin, C.;Convert, 0. J. Chem. SOC..Chem. Commun. 1989, 825-826. (55) Ali, B. E.; Brtgeault, J.-M.; Martin, J.; Martin, C. New J. Chem. 1989, 13, 173-175.
investigated recently and established to proceed by the above mechanism (eqs 4 and 5).49 It was of some surprise and considerable interest that the control reaction involving the direct oxidation of THT by Q2H4[1-CH3] was several orders of magnitude slower (71/2 of months) than the rate of oxidation in the complete reaction system (THT/TBHP/QzHd[ 1-CH3]). Clearly, eqs 4 and 5 are not an important mechanism for THT oxidation by TBHP catalyzed by Q2H4[1-CH3]. A number of rate measurements on this reaction were made and the log-log plots for THT and TBHP are given in Figure 8. Not surprisingly, the rate is first order with respect to both THT and TBHP. More interesting and unusual is the second order dependence on the catalyst, Q2H4[1-CH3] (Figure 9), resulting in the rate law in eq 6. The y-intercepts in log-log plots from kinetics studies are
uo = k[THT] [TBHP] [ [1-CH3]I2
(6)
often marginally meaningful and usually not mentioned. In the
11830 J. Am. Chem. SOC.,Vol. 115, No. 25, 1993 case here, the strikingly close values on they intercepts of the two first order terms, THT and TBHP in eq 6 (Figure 8), indirectly confirm the rate law (see Appendix). Independent calculation of the rate constant from Figures 8A, 8B, and 9 respectively mM” min-I. yields 7.8, 6.7, and 7.5 X Both component do ions in the catalyst, Wvl and Vv, were previously hown11,56-61 and demonstrated in this study (Figure 7) to catalyze heterolytic electrophilic oxidations using TBHP. These reactions as a class are very well s t ~ d i e dand ~ ~all, ~proceed ~ via doperoxometal intermediates without a change in the oxidation state of the do transition metal center. It is more likely that Vv than Wvl is directly involved in the THT oxidation by TBHP catalyzed by Q2H4[1-CH3] for three reasons. First, the Vv sites are more labile than the Wvl sites. Second, Vv species are usually more active as catalysts in the electrophilic oxygenation of organic substrates than analogous Wvl which was found to be the case for monomeric Vv and Wvl model compounds examined in this study (Figure 7). Third and most importantly, the Vv sites are the most basic and thus the p2-oxygens between two Vv sites are most likely to be protonated first. Although considerableliterature on do transition metal catalyzed oxygenation by peroxides and the data here would implicate formation of a peroxovanadium(V) intermediate that attacks substrate (THT),64 the exact structure of this intermediate cannot be inferred from the data. A final experimental datum of mechanistic value is the temperature dependence of the reaction, THT by TBHP catalyzed byQZH4[1-CH3]. The Arrheniusplotwasquitelinear (correlation coefficient = 0.988) and gave an activation energy of 10.1 kcal! mol. The rate law necessarily requires the involvement of one molecule each of THT and TBHP and two molecules of [1-CH3] prior to or including the rate limiting activated complex. Whereas more than one well precedented activated complex could be drawn for a rate law first order in substrate, oxidant, and catalyst, and other rate law, the one operable here, eq 6, does not lend itself to any well-delineated activated complex. One of many possible activated complexes we favor is shown in Figure 10. As drawn, this model of the activated complex does not specify exact bond connectivity around the Vv centers. For example, the substrateattacking peroxyvanadium and protonated Vv centers cannot be ~ - p2-RO- is specifically specifically identified and neither ~ 2 - 0 nor implicated. We visualize this as effectively a bimolecular nucleophilic attack of THT on a complex of one molecule of TBHP and two molecules of [1-CH3] and not a mechanistically unreasonably quadramolecular process. In the TBHP-([ l-CH3])2 complex, the principal role of the first [1-CH3] is to activate TBHP by binding to a Vv center, while the principal role of the second [ 1-CH31 is to further electrophilically activate the bound TBHP group via H-bonding as shown. The p2-v1-OOR binding mode of alkylperoxides is p r e ~ e d e n t e dand ~ ~ the electrophilic (56) Sheldon, R. A.; Kochi, J. K. Metal- Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 198 1, Chapter 3. (57) Parshal1,G.W.;Itte1,S. D. HomogeneousCatalysis. The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes, 2nd ed.;
Wiley-Interscience: New York, 1992, pp 151 and 261. (58) Sheldon, R. A. CHEMTECH 1991, 566-576. (59) Jergensen, K. A. Chem. Rev. 1989,89, 431-458. (60) Jergensen, K. A.; Schiett, B. Chem. Reu. 1990, 90, 1483-1506. (61) Drago, R. S. Coord. Chem. Reu. 1992, 117, 185-213. (62) For an example of a particularly thorough study, see this and the following reference: Woodward, S. S.;Finn, M. G.; Sharpless, K. B. J . Am. Chem. Soc. 1991, 113, 106113. (63) Finn,M.G.;Sharpless,K.B . J . Am. Chem. SOC.1991,113,113-126. (64) The conventional wisdom based on considerable data is that electrophilic heterolyticoxygenationcatalyzed by dotransition metal species takes place through the intermediacy of peroxometal species (see ref 56).
Hou and Hill
+-
Figure 10. One model for the activated complex in the rate determining transition state for THTO production via TBHP oxidation of THT catalyzed by [1-CH3] that is consistent with the rate law (eq 6), other kinetic and spectroscopic data, and literature investigations. This is effectively a bimolecular nucleophilic attack of THT on a complex of TBHPand twomoleculesof [l-CH,], whereone [l-CH,] activatesTBHP via bonding to Vv and the other [l-CH>]activates by H bonding to the bound TBHP. As drawn, this model of the activated complex does not specify exact bond connectivity around the Vv centers. For example, the Vv centers ( 1 2 - 0 2 - or @2-RO-) associated with protonation or with formation of the peroxyvanadium intermediate cannot be unequivocally identified.
activation roles of both the Vv center and the protonated oxygen of the second [l-CH,], to facilitate attack by the reasonably nucleophilic THT sulfur atom, are reasonable. This activated complex is consistent with all the rate, spectroscopic, acid-base, and other data in this manuscript. Although one can argue that protons associated with one of the [1-CH3] complexes can bind THT via a salt bridge (formation of the protonated THTH+ would function as a countercation to the two negatively charged [ I-CHs] complexes) or via hydrogen bonding, both scenarios are unlikely. Either protonation of sulfur or hydrogen bonding to sulfur would deactivate this atom with respect to receiving an electrophilic oxygen atom. Furthermore, the pK, values of THTH+ (- -5.4)66 versus TBHPH+ ( -1 .7)67argue against proton activation of the former and for proton activation of the latter.
-
Acknowledgment. C.L.H. thanks the US.Army Research Office (DAAL03-87-K-013 1) for support.
Appendix From the rate law, eq 6, one can write eqs 7-9, which isolate log u1 = log (k[TBHP][[l-CH,]]*) log ug = log (L[THT][[1-CHJIZ) log ~3 = log (k[THT] [TBHP])
+ log [THT]
(7)
+ log [TBHP]
(8)
+ 2 log [ [I-CHJ]
(9)
[THT], [TBHP], and [1-CH3], respectively. From theseequations it is not only apparent that the slopes of these plots, yield the orders, are 1, 1, and 2 respectively for THT, TBHP, and [ l-CH31, but as the concentrations of [TBHP] and [THT] were the same in the experiments, they intercepts should also be similar in the first two plots and distinct in the third plot. This is the case (Figures 8 and 9). This simple treatment illustrates one reason it can be more informative to evaluate orders of reactions in kinetics investigations by not using pseudo-first-order conditions (flooding all concentrations but the limiting one) but instead keeping the concentrations of all reactants in a comparable regime. (65) Furin, G. G.; Rezvukhin,A. I.; Fedotov, M. A.; Yakobson, G. G. J. Fluorine Chem. 1983, 22, 231. (66) Kemp, D. S. Organic Chemistry; Worth: New York, 1980, pp 1079. (67) Curci, R.; Edwards, E. 0. In Catalytic Oxidations with Hydrogen Peroxide as Oxidant; Strukul,G., Ed.;Kluwer: Dordrecht,The Netherlands, 1992, Chapter 3.
