9048
J . Am. Chem. SOC.1989, 1 11, 9048-9056
Novel Ruthenium-Oxo Complexes of Saturated Macrocycles with Nitrogen and Oxygen Donors and X-ray Crystal Structure of trans-[Ru"( L)O( H20)][C10& Chi-Ming Che,* Wai-Tong Tang, Wing-Tak Wong, and Ting-Fong Lait Contribution from the Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong. Received November 7, I988
Abstract: Reaction of K,[RuCI,(H O)] with L (1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) in refluxing ethanol yielded t r ~ n s - [ R u ~ ~ ~ ( L ) C isolated l ~ ] ~ +as, chloride salt. Treatment of tr~ns-[Ru~~~(L)Cl,]+ with Ag+ in water which could be oxidized to t r ~ n s - [ R u ~ ~ ( L ) O ( H , 0 )and ] ~ +transled to the formation of tr~ns-[Ru~'~(L)(OH)(H,0)]~+, [RU~'(L)(O),]~+ by electrochemical and Ce(1V) oxidation, respectively. Magnetic susceptibility measurements on the perchlorate salts of trans- [Ru1I'( L) (OH)(H20)],+,trans- [Ru'"( L)(O)(H,O)] ,+,and trans- [ RuV1(L)( 0)2]2+ showed that their pcffvalues are 1.86, 2.87, and 0 wB, respectively, in accordance to their electronic formulation [Ru(III), (dx,J2(d, + d,J2(dX, - dyZ)l; Ru(lV), (dxy)2(dx2)1(dy2)1; Ru(VI), (d,),]. The Ru=O stretching frequencies for the Ru(IV) and Ru(V1) complexes are 845 and 865 cm-'. The X-ray crystal structure of t r a n s - [ R ~ ~ ~ ( L ) ( O ) ( H ,[C1O4],.2H,O 0)] has been determined: C21H34N2CI2Ol4Ru;M , 710.47; triclinic, space group Pi;a = 11.157 (9,b = 11.697 (3), c = 12.598 (2) A; a = 74.05 ( l ) , 8 = 61.55 (2), y = 71.33 (2)'; V1354.7 A3;Z = 2; d,,, = 1.742, dmcasd = 1.72 g ~ m - w(MO ~ ; Ka) = 8.4 cm-I. The coordinated oxo and aquo groups are trans to each other. The measured Ru=O distance of 1.739 ( 2 ) A is the shortest one ever reported for oxoruthenium(1V) complexes. The intermolecular separation of two Ru=O moieties is 2.8 A, which features significant nonbonded [Ru=O..O=Ru] interaction. At pH = 1.1, the E l values for the trans-[Ru"(L)(0)2]2+/t~ans-[R~IV(L)(0)(H20)]2+ and trans-[R~~~(L)O(H~0)]~~/trans-[Ru~~'(L)(OH)(H,O~]~~ couples are, respectively, 0.92 and 0.58 V vs SCE. Thus, f r a n s - [ R ~ ~ ~ ( L ) ( Ois) ~a] ~powerful + oxidant and is 0.26 V more oxidizing than t r a n ~ - [ R u ~ ~ ( T M C ) ( 0 ) ,(TMC ]~+ = 1,4,8,1I-tetramethyl-l,4,8,1l-tetraazacyclotetradecane).t r ~ n s - [ R u ~ ' ( L ) ( O ) ~is] ~capable + of oxidizing a wide variety of organic substrates, including alcohols, olefins, aromatic hydrocarbons, and adamantane under mild conditions. Electrocatalytic oxidation of benzyl alcohol and oxidation of organic substrates with PhIO are achieved with the [Ru"'(L)(OH)(H,O)]~+ or [Ru"'(L)(0)(H2O)l2+complex as catalyst.
,
There is a growing interest in the oxidation chemistry of ruthenium-oxo complexes,1,2 because of their potential application in catalytic and stiochiometric oxidation of organic substrates. Our recent studies have shown that ruthenium-oxo complexes in oxidation states of IV-VI can be stabilized with macrocyclic tertiary amine ligands, such as T M C (1,4,8,1 l-tetramethyl1,4,8,1 l-tetraazacyclotetradecane).2c These ruthenium macrocyclic complexes, which are stable toward demetalation, have been found to possess reversible electrochemistry3 and provide a good model system for mechanistic investigation of proton-coupled multielectron-transfer reactions. However, except for the t r a n ~ - [ R u ~ ( T M C ) 0 ( X ) (X ] ~ += CI, N3,and NCO) cations, which are generated in situ by electrochemical means,2a-fthe reported ruthenium( 1V)- and ruthenium(VI)-oxo complexes of TMC are not capable of oxidizing substrates a t an appreciable rate a t room temperature,,' thus hindering their uses as oxidative catalysts. W e are concerned with the synthesis of macrocyclic ruthenium-oxo complexes with sufficiently high redox potentials for C-H 'Correspondence on X-ray work should be addressed to Dr. Lai. (1)Recent works on Ru=O complexes by other researchers: (a) Rcecker, L.; Meyer, T. J. J . Am. Chem. SOC.19877,109,746.(b) Lau, T. C.; Kcchi, J. K. J . Chem. SOC.,Chem. Commun. 1987,748. (c) Griffith, W. P.; Ley, S. V.; Whiteombe, G. P.; White, A. D. J. Chem. SOC.,Chem. Commun. 1987, 1625. (d) El-Hendawy, A. M.; Griffith, W.P.; Piggott, B.; Williams, D. J. J . Chem. SOC.,Dalton Trans. 1988, 1983. (e) Marmion, M. E.; Takeuchi, K. J. J . Am. Chem. SOC.1988,110,1472.(f) Llobet, A.; Doppelt, P.; Meyer, T. J. Inorg. Chem. 1988,27,514. (g) Bailey, C. L.; Drago, R. S . J. Chem. Soc., Chem. Commun. 1987,179. (h) Nazeeruddin, M. K.; Rotzinger, F. R.; Comte, P.; Gratzel, M. J . Chem. SOC.,Chem. Commun. 1988,872. (2)(a) Che, C. M.; Wong, K. Y.; Mak, T. C. W. J . Chem. SOC.,Chem. Commun. 1985,546,988.(b) Che, C. M.; Wong, K. Y . ;Leung, W. H.; Poon, C. K. Inorg. Chem. 1986,25,345.(c) Che, C.M.; Lai, T. F.; Wong, K. Y . Inorg. Chem. 1987,26,2289.(d) Che, C. M.; Leung, W. H. J . Chem. soc., Chem. Commun. 1987. 1376. (e) Che. C. M.: Yam. V. W. W. J . Am. Chem. SOC.1987, 109, 1262. (f) Wong, K. Y.; Che, C.'M.; Anson, F. C. Inorg. Chem. 1987.26. 737. (3)(a) Che, C.M.; Wong, K. Y . ; Anson, F. C. J . Electroanal. Chem. Interfacial Electrochem. 1987,226,211.(b) Che, C. M.; Poon, C. K. Pure Appl. Chem. 1988,60,1201. (4) See also: Yam, V. W. W.; Che, C. M.; Tang, W. T. J . Chem. SOC., Chem. Commun. 1988, 100.
0002-7863 /89/15l1-9048$01.50/0
bond oxidation. Also, systematic variation of the redox potentials of ruthenium-oxo complexes in a given oxidation state would lead to a new class of compounds suitable for the study of driving force dependence in organic oxidation. In this respect, our methodology involves tuning the physical properties and redox potentials of the metal complexes through variation of the structure of nonlabile equatorial ligands4 W e describe here the generation of a new class of highly oxidizing ruthenium-oxo complexes, which effect chemical and electrochemical oxidation of organic substrates with a saturated macrocyclic ligand having nitrogen and oxygen donors, L (1,12-dimethyl-3,4:9,10-dibenzo-l , I 2-diaza-5,8-dioxacyclo-
U
Ligand L
pentadecane). The structure of the novel trans-[Ru"(L)O( H 2 0 ) I 2 + complex, a prototype example of trans-oxoaquoruthenium(1V) system, has been established by X-ray crystallography.
Experimental Section Materials. K2[RuC15(H20)](Johnson and Matthey) was used as received. Trifluoroacetic acid (99%, Aldrich) was purified by distillation under nitrogen atmosphere. Acetonitrile (Mallinkrodt chromAR) was purified by treatment with KMnO, and then distillation over CaH,. Tetrabutylammonium fluoroborate (Southwestern Analytical Chemicals, electrochemetric grade) for electrochemical studies was dried in a vacuum oven at 100 OC for 24 h. The organic substrates used in oxidation were purified by standard methods, and the purity was checked by gas chromatography. The ligand, 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane, was prepared by literature m e t h ~ d . ~ 0 1989 American Chemical Societv
Novel Ru-Oxo Complexes of Saturated Macrocycles
J . Am. Chem. SOC.,Vol. 11I , No. 25, 1989 9049
[Ru"'(L)CI2]CI. An ethanolic solution of the ligand L (1.5 mmol in Table I. Crystallographic Data for [RU(L)(O)(H,O)](CIO~)~.~H~O 200 mL) was added dropwise to a rigorously stirred ethanolic suspension ~Ru(N,0,)O(H,0~~(C10n),.2H,0 formula wt 710.47 - of K2[RuC15(H20)](0.5 g, 1.4 mmol) under refluxing condition. The a = 11115-7 (5) space group Pi addition process took 3 h for completion, and the mixture was further T=294& 1 K b = 11.697 (3) A refluxed for another 12 h. This was then filtered, and the filtrate was A = 0.71073 A = 12.598 (2j A evaporated to nearly dryness. The orange [Ru"'(L)Cl2]CI solid was pEalcd= 1.742 g cm-' a = 74.05 (1)' precipitated upon addition of water ( I O mL); yield 67%. Anal. Calcd pmcssd = 1.72 g p = 61.55 (2)' for [Ru(L)CI,]CI: C, 46.0; H, 5.1; N, 5.1. Found: C, 46.2; H , 5.0; N, p = 8.4 cm-I y = 71.33 (2)' (c) in CH,CN]: 379 (1540), 274 (4530), 218 nm 5.2. UV-vis ,A,[, R(FJ = 0.033 V = 1354.7 (8) A' (1 1 900). The following three compounds are isolated as perchlorate RJF,,) = 0.044 z=2 salts. Caution! Perchlorates are potentially shock-sensitive and should be handled in small quantity. trans-[R~~~'(L)(OH)(H~O)](Cl0~)~.2H,0. A mixture of [Ru"'(L)w/w SE-30 on Chromosorb W (8CrlOO mesh size), in. X 6 ft stainless CI2]C1(0.3 g) and silver(1) toluene-p-sulfonate (0.5 g) in distilled water steel column, with nitrogen as the carrier gas. Component identification (25 mL) was heated at 80 "C for 0.5 h until all the starting [Ru"'(L)was established by comparing the retention time with authentic sample C12]Cl solid had been dissolved. A greenish yellow solution was obtained. and by gas chromatographic-mass spectral analysis (GC-MS). QuanThis was filtered to remove the insoluble silver chloride. The yellow titation of individual gas chromatographic components was performed trans-[Ru"'(L)(OH)(H20)](CI04), solid was precipitated by the addiby the internal standard method employing a Shimadzu C-R3A election of NaCIO,. Crystallization was performed using hot trifluoroacetic tronic integrator. cis- and trans-stilbene oxides were quantified by 'H acid (0.2 M). Yield was quantitative. Anal. Calcd for trans-[Ru"'NMR spectroscopy using deoxybenzoin as the internal standard. Kinetic (L)(oH)(H~O)](CIO,)~.~H~O: C, 35.40; H, 4.9; N, 3.9; CI, 9.9. Found: measurements on the oxidation of organic substrates by trans- [RuV'C, 35.4; H, 4.8; N, 3.8; CI, 9.6. IR (Nujol mull): v(H,O) 3350 cm-'. (L)(O),] [C104]2were carried out spectrophotometrically with a Unicam UV-vis ,A,[, (e) in H20]: 320 (1510), 275 (2680), 223 nm (7200). SP8000 spectrophotometer. The reaction temperature was maintained trans-[R~'~(L)O(H~O)](C10~)~~2H~0. This was prepared electroat fO.l OC by circulating water through the cell holder using a Lauda chemically by constant potential electrolysis of a I mmol solution (0.1 Model WB-20/R thermostat bath and circulator. The reaction was M C F,COOH/C F,COONa) of trans- [Ru"'( L) (OH)(H20)](CIO,) at followed by monitoring the decrease in absorbance of the Ru(V1) coma potential of 0.8 V vs SCE. The t r a n s - [ R ~ ~ ~ ( L ) O ( Hcomplex ~ 0 ) ] ~was ~ plex at 385 nm. The pseudo-first-order rate constant kat, was determined precipitated out from the solution by addition of NaC104. Yield was from the plot of -In ( A - A,) against time, where A is the absorbance C, 35.5; quantitative. Anal. Calcd for [Ru'V(L)O(OH2)](CI0,)2~2H20: at , , A of the reactant complex and A , is the experimentally determined H, 4.7; N, 3.9; CI, 9.9. Found: C, 35.6; H, 4.7; N, 3.9; CI, 9.8; UV-vis infinity point. The pK, measurements were performed using spectro,A,[, ( e ) in H20]: 540 (120), 265 (2400), 232 nm (8300). photometric method. tran~-[Ru~'(L)(O)~](ClO,)~. trans-[R~~(L)(0H)(H,O)](Cl0~)~ (0.1 X-ray Crystal Structure of [RU(C~~H~~N~~~)(~)(H~O)](CIO~) g) was dissolved in deionized water (20 mL). To the stirred solution was X-ray diffraction data were collected on an Enraf-Nonius CAD4 difadded excess [NH,],[Ce(NO,),] (0.2 8). Upon standing, [RuV'(L)fractometer with graphite-monochromated Mo Ka radiation (A = (O),] gradually precipitated out as an orange solid, yield 80%. 0.71073 A). The unit cell dimensions were obtained from a least-squares C, 37.5; H, 4.2; N, 4.2; Anal. Calcd for tran~-[Ru~'(L)(O),](ClO~)~: fit of 25 reflections in the range of 19' < 28 < 25'. The data were CI, 10.5. Found: C, 37.2; H, 4.3; N, 4.0; CI, 10.4. UV-vis ,A,[, ( e ) in corrected for Lorentz and polarization effects, but no absorption corH20]: 385 (2510), 230 nm (8400). IR (Nujol mull): v,,(Ru02) 865 rection was made. Three check reflections, monitored every 2 h, showed cm-I. no significant variation in intensity. Crystal and structure determination Physical Measurements. Elemental analyses of the newly prepared data are summarized in Table I. Calculations were carried out on a complexes were performed by Butterworth Laboratories Ltd. IR spectra MicroVax I1 computer using the Enraf-Nonius SDP programs6 were measurd in Nujol mull using Perkin Elmer 577 spectrophotometer The position of the ruthenium atom was obtained from a Patterson (4000-200 cm-I). Electronic absorption spectra of freshly prepared synthesis, and the rest of the non-hydrogen atoms were recovered from solutions were measured with a Shimadzu UV240 spectrophotometer. a subsequent Fourier map. After several cycles of full-matrix leastMagnetic susceptibilities of solid samples were measured at room temsquares refinement, the hydrogen atoms were revealed in a difference perature by the Guoy method using mercury tetrathiocyanatocobaltateFourier map; however, in the structure factor calculation, only the hy(11) as the calibrant. drogen atoms of the water molecules were taken from the difference map Cyclic voltammograms were obtained with Princeton Research (PAR) while all the others were generated geometrically (C-H = 0.95 A). All instruments model 175 (universal programmer) and model 173 (ponon-hydrogen atoms were refined anisotropically, and the hydrogen attentiostat-galvanostat). Working electrodes are pyrolytic graphite and oms with assigned isotropic thermal factors were not refined. glassy carbon. Before each cyclic voltammetric scan, the electrodes were Final agreement factors are shown in Table I. Atomic coordinates of pretreated by procedures as described previously., Saturated calomel non-hydrogen atoms are listed in Table 11. Selected bond distances and electrode (SCE) was used as the reference electrode. angles are given in Table 111. Tables of hydrogen atomic parameters, Controlled potential coulometry was performed using a PAR Model anisotropic thermal parameters, some distances and angles, some least377A coulometric cell and a PAR Model 179 digital coulometer. The squares planes, and structure factors are available as supplementary working electrode was a glassy-carbon cup (Atomergic Chemical V25material. 12). Catalytic reactions between the ruthenium complexes and various Results and Discussion organic substrates (0.3 mmol) in the presence of iodosylbenzene (0.2 The ligand L, a saturated macrocycle having nitrogen and mmol) were performed by dissolving both the ruthenium complex (0.002 oxygen donors, was previously reported by Lindoy and ~ u w o r k e r s . ~ mmol) and substrate in acetonitrile (2 mL). Stoichiometric oxidation of So far, its coordination chemistry is mainly confined to the comwas performed by organic substrates by trans-[R~~'(L)(O),](C10,)~ plexes of first-row transition When compared to the dissolving the ruthenium(V1) complex (0.05 mmol) in acetonitrile (1 mL) containing the dissolved substrate (0.5 mmol). The reaction mixture was N,-donor macrocycles such as TMC, it has a weaker ligand field stirred with a magnetic stirrer under degassed and dark conditions and strength [ lODq for T M C and N,Oz are 11 3OOs and 9615 ~ m - ~ , ~ ~ the temperature maintained at room temperature in a water bath. respectively]. While it reacts with K,[RuCIS(H20)] to give Controlled experiment in the absence of the ruthenium complex was [Ru(L)Cl,]+ in reasonable yield, an attempt to synthesize performed for each reaction. The products were analyzed by gas chrotrans[0s"'(L)Cl2]+ following a similar procedure as that for matography, 'H NMR, mass spectrometry, and UV-visible spectroscopy. trans- [OS~'~(TMC)CL,]+ was unsuccessful. Because of the hyGas chromatographic analyses were conducted using a Varian model drophobic nature of the two benzene rings, its metal complexes 940 gas chromatograph with flame ionization detector. Gas chromatohave low solubilities in water. For this reason, isolation of the graphic columns used include a 10% w/w Carbowax 20M on Chromoperchlorate salts of trans-[R~~~*(L)(OH)(H,0)]~' (1) and in. X 6 ft stainless steel column, a 10% sorb W (80-100 mesh size),
-
( 5 ) (a) Armstrong, L. G.; Grimsley, P. G.; Lindoy, L. F.; Lip, H. C.; Morris, V. A,; Smith, R. J. Inorg. Chem. 1978, 17, 2350. (b) Adam, K. R.; Anderegg, G.; Lindoy, L. F.; Lip, H. C.; McPartlin, M.; Rea, J. H.; Smith, R. 5.; Tasker, P. A. Inorg. Chem. 1980, 19, 2956. (c) Ekstrom, A,; Lindoy, L. F.; Lip, C. H.; Smith, R. J.; Goodwin, H. J.; McPartlin, M.; Tasker, P. A.
J . Chem. SOC.,Dalton Trans. 1919, 1027.
(6) Frenz, B. A. Enraf-Nonius Structure Determination Package SDP User Guide; Enraf-Nonius: Delft, The Netherlands, 1985. (7) (a) Adam, K. R.; Lindoy, L. F.; Smith, R. J. J . Chem. SOC.,Chem. Commun. 1979, 812. (b) Gocdwin, H. J.; Henrick, K.; Lindoy, L. F.; McPartlin, M.; Tasker, P. A. Inorg. Chem. 1982, 21, 3261. (8) Wagner, F.; Barefield, E. K. Inorg. Chem. 1976, IS, 408.
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Che et al.
9050 J . Am. Chem. SOC.,Vol. 111, No. 25, 1989
lobo!
'
'
' ' ' ' sbo
cm-1
1700
'
I
803 " 1
1000
8 I
cn-l
Table 11. Fractional Coordinates and Thermal Parameters' of Non-Hydrogen Atoms and Their ESD's for [Ru(CZIH~~N~O~)(O)(HZO)I (C104)2.2H20 atom X Y z E,, 8,' Ru 0.12577 (2) 0.31727 (2) 0.32617 (2) 1.943 (4) ci(i) 0.23190 (9j -0.1600 (i)' 0.88023 (sj 4.34 (2)' Cl(2) 0.41335 (8) 0.18778 (8) 0.66170 (7) 3.68 (2) 2.59 (5) 0 0.0805 (2) 0.3911 (2) 0.4469 ((2) 2.65 (5) 0.1457 (2) 0.4329 (2) O(1) 0.1570 (2) 2.74 (5) 0.2940 (2) 0.2878 (2) O(2) 0.3414 (2) 3.24 (5) 0.2070 (2) 0.1798 (2) O(3) 0.2093 (2) 6.16 (8) 0.2103 (5) 0.0004 (3) O(4) 0.1466 (3) 7.4 (1) 0.0801 (4) 0.0901 (3) O(5) 0.4775 (3) 5.65 (9) O(11) 0.2667 (3) -0.1516 (3) 0.7544 (3) 8.9 (1) O(12) 0.1766 (5) -0.0397 (4) 0.9125 (3) 10.6 (1) -0.2313 (4) 0.9541 (4) O( 13) 0.1345 (4) 7.1 (1) O(14) 0.3541 (3) -0.2087 (4) 0.9007 (3) 5.53 (8) 0.0847 (3) 0.6230 (3) O(21) 0.4163 (3) 6.59 (9) 0.1537 (3) 0.7756 (3) O(22) 0.4240 (3) 8.2 (1) 0.2358 (4) 0.5738 (3) O(23) 0.5302 (4) 9.1 (1) 0.2743 (4) 0.6725 (4) O(24) 0.2896 (4) 2.45 (6) 0.4779 (2) 0.2014 (2) N(l) 0.1237 (2) 2.21 (5) 0.3134 (2) 0.3796 (2) N(2) -0.0828 (2) 2.60 (7) 0.1456 (3) 0.5546 (3) C(l) -0.0799 (3) 3.43 (9) 0.1301 (3) 0.6670 (3) C(2) -0.1901 (3) 4.1 (1) 0.0963 (4) 0.7731 (3) C(3) -0.1705 (4) 0.0796 (3) 0.7666 (3) 3.89 (9) C(4) -0.0393 (4) 3.22 (8) 0.0956 (3) 0.6553 (3) C(5) 0.0731 (3) 2.54 (7) 0.1269 (3) 0.5520 (3) C(6) 0.0508 (3) 3.41 (8) 0.1014 (3) 0.4070 (3) C(7) 0.3051 (3) 3.35 (8) 0.2038 (3) 0.3864 (6) C(8) 0.3664 (3) 2.88 (7) 0.4005 (3) 0.2580 (3) C(9) 0.3913 (3) 3.50 (8) 0.4140 (3) 0.3126 (3) C(10) 0.4670 (3) 0.5218 (4) 0.2760 (4) 4.23 (9) C(11) 0.5142 (3) 4.3 (1) 0.61 15 (3) 0.1911 (4) C(12) 0.4852 (4) 3.73 (9) 0.5967 (3) 0.1387 (3) C(13) 0.4069 (4) 0.4909 (3) 0.1712 (3) 3.00 (8) C(14) 0.3593 (3) 0.4775 (3) 0.1136 (3) 2.91 (8) C(15) 0.2742 (3) 0.5933 (3) 0.2572 (3) 2.94 (7) C(16) 0.0599 (3) 3.02 (7) 0.4804 (3) 0.1252 (3) C(17) 0.0513 (3) 3.17 (7) 0.4648 (3) 0.1980 (3) C(18) -0.0985 (3) 2.86 (7) 0.3411 (3) 0.2721 (3) C(19) -0.1166 (3) 2.87 (7) 0.3973 (3) 0.4698 (3) C(20) -0.1889 (3) 2.70 (7j 0.1840 (3) 0.4384 (3) C(21) -0.0982 (3) "Anistropically refined atoms are given in the form of the isotropic equivalent displacement parameter defined as E, = 4/,[~i~,&ai.aj].
Table 111. Selected Bond Lengths and Angles with ESD's in Parentheses Bond Lengths, 8, Ru-0(3) 2.199 (3) Ru-0 1.739 (2) Ru-N(1) 2.094 (2) Ru-O(1) 2.115 (2) Ru-N (2) 2.103 (3) R~-0(2) 2.153 (2)
trans-[R~~~(L)(O)(H~0)]~+ (2) from aqueous medium becomes
1973, 1315.
feasible. Oxidation of trans- [RulI1(L)(OH)(H,O)] 2+ by Ce( IV)
O-Ru-O( 1) O-Ru-0(2) O-Ru-0(3) O-Ru-N( 1) O-Ru-N( 2) O( 1)-R~-0(2) O( 1)-R~-0(3) O( l)-Ru-N(l)
Angles, deg 90.6 (1) O(l)-Ru-N(2) 87.6 (1) 0(2)-R~-0(3) 171.4 (1) 0(2)-R~-N(l) 94.6 (2) 0(2)-Ru-N(2) 92.9 (2) 0(3)-Ru-N(I) 80.1 (1) 0(3)-Ru-N(2) 83.5 (1) N(l)-Ru-N(2) 171.7 (1)
91.2 ( 1 85.3 (1 93.6 (1 171.3 ( 1 90.6 ( 1 93.4 (2 95.1 (1
gave t r ~ n s - [ R u ~ ~ ( L ) ( O ) ,(3) ] ~ +in quantitative yield. H 2 0 2has also been found to effect the oxidation; however, the oxidized Ru(V1) product prepared by this way is usually contaminated with some unknown species, which are difficult to remove. trans[ R U ' ~ ( L ) ( O ) ( H ~ O ) can ] ~ +be most conveniently prepared in pure form by electrochemical oxidation of its Ru(II1) precursor a t a potential of 50 mV more positive than the E l l , of the Ru(IV)/ Ru(II1) couple. The newly prepared ruthenium complexes are air-stable solids. The dioxoruthenium(V1) complex is diamagnetic as expected for d2 dioxometal c ~ m p l e x e s . * The ~ ~ measured room-temperature F~~~values for the Ru(II1) and Ru(1V) complexes are 1.86-1.90 and 2.87 pB,respectively, corresponding to the spin-only values of one and two unpaired electrons [Ru(III), (d,)2(d, + dy,)2(d, - dy,)2; Ru(IV), (dxy)2(dxz)1(dyz)1]. Similar perfvalues of other oxoruthenium( IV) complexes have been reported previously.k The trans configurations of 1 and 3 are inferred from the X-ray crystal structure of trans-[R~~~(L)(O)(H~0)]~' (2), since these three complex cations are rapidly interconverted into each other by electrochemical oxidation and reduction in aqueous solutions. For [Ru111(L)C12]+,its UV-vis spectrum shows only one intense p,(C1) d,(Ru) charge-transfer band at 380 nm, which is characteristic of the trans-dichlorotetraammineruthenium(II1) system." A direct comparison of the I R spectra of the ruthenium(III), ruthenium(IV), and ruthenium(V1) complexes allows the identifi-
-
(9) See also: Griffith, W. P.; Pawson, D.J . Chem. SOC.,Dalron Trans.
(10) Walker, D. D.; Taube, H.fnorg. Chem. 1980, 20, 2828.
Novel Ru-Oxo Complexes of Saturated Macrocycles
J . Am. Chem. Soc., Vol. I l l , No. 25, 1989 9051
a cation of Ru=O stretches, as illustrated in Figure 1. The respective ua,(RuV1O2)and v(Ru"=O) stretches at 865 and 845 cm-l are higher than those values found for the Ru(TMC)(O) analogues, t r a n s - [ R ~ " ~ ( T M C ) ( 0 )850 ~ ] ~cm-l; + , trans-[RuIv(TMC)(O)(X)]"+, 810-790 cm-l)," indicating stronger metal-oxo bonds in the present system. In fact, the Ru=O stretching frequency of complex 2 is the highest when compared to those values of 790-810 cm-I found in other Rul"=O complexes.1e,2c,11This is also in agreement with the X-ray result as described in the following section. The UV-vis absorption spectra of complexes 1-3 in 0.1 M C F 3 S 0 3 H are shown in parts a - c of Figure 2. Since the optical spectra of complex 2 in acetone, water, and perchloric acid (0.1 M ) are the same, the metal complex retains the (Ru=O) moiety in aqueous solutions, and the equilibrium constant for the reaction, trans-[ RuiV(L ) ( 0 ) ( H 2 O ) l 2 + trans- [ R u ~ ~ ( L ) ( O H ) ~if] ~it+ , exists, should be very small. The spectrum of 1in the 300-500-nm region is similar to that for trans-[R~~~'(bpy)~(OH)(H~O)]~+, X / nm showing one broad peak a t 320 nm, which is assigned to the p,(OH) d,(Ru) charge-transfer transition. For the monooxoruthenium(1V) system, the allowed p,(O) d,* (d,*. = d,, 1 b dyJ charge-transfer band is expected.2c In the case of 2, this band probably occurs at 300-260 nm; however, the assignment is complicated by the intraligand A K* transition of the benzene rl I rings in L, which also occurs in the high UV region. In Figure 5 2b, there is a weak absorption peak at 542 nm. The e, of this band is low (ca. 150 mol-' dm3 cm-l), suggesting that it is probably 0.6 rl d,* transition band. due to the low-energy d Irl it has been found that trans-dioxoIn previous P ruthenium(V1) complexes of neutral amines show characteristic \n \ vibronic structured (d,), (dxy)l(dT*)Itransitions a t 380-400 I 0 nm. In the present Ru(V1) complex, there is also an absorption rl peak a t 385 nm (Figure 2c), but this is unlikely to be a d - d band x because of its large emax of 2510 mol-] dm3 cm-l. It is likely that 0_* 4_ W the (d,)z (dxY)'(d,*)l transition sits underneath some lowenergy charge-transfer bands, and further spectroscopic studies are necessary before any definitive assignment can be drawn. Complexes 1 and 2 show average pK, values of 5.3 f 0.3 and 0.0 5.4 f 0.3, respectively, using spectrophotometric method. Parts 200 300 400 5b0 600 ,bo a and b of Figure 3 show the optical spectra of the two independent k / nm wavelengths, and the results obtained are in good agreement with each other. The similar pK, values of 1 and 2 are in accordance C with the electrochemical work (described in the later section) I where the plot of E l j 2versus pH for the Ru(IV)/Ru(III) couple is linear with a slope of 58 mV/pH unit from pH 1 to 7 . They d laO are also significantly low compared to the pK, value of 10.2 for Ir [R~~'(bpy)~(py)(OH,)]~+ (py = pyridine), which is also a dipsitive m cation.Ilb The result is not unexpected given the fact that p,(OH,) 0 E and d,*(Ru) n-bonding interaction, which will lower the pK, value rl Id of the coordinated H 2 0 , is not significant in the electron-rich 2 Ru(1I) center. Structure Description of trans-[Ruiv(L)O(H20)IC10,]2.2H20. Although the structures of monooxoruthenium(1V) complexes have previously been established in several cases, there has been no definitive structural evidence for ruthenium(1V) complexes [ R U ' ~ ( O ) ( H ~ Ohaving )] both coordinated oxo and aquo groups in the trans configuration. The [RU'~(O)(H,O)]complex, which is believed to be the product resulting from two-electron reduction of trans-dioxoruthenium(V1) ( [RuV1(O),]) or one-electron oxidation of trans-aquohydroxyruthenium(II1)[ R U ' ~ ~ ( O H ) ( H , O ) ] in aqueous solutions is difficult to distinguish from trans-dih / nm hydroxyruthenium( IV) ([ RuiV(OH),]) by ordinary electrochemFigure 2. UV-vis absorption spectra of (a) 1, (b) 2, and (c) 3 in 0.1 M ical means, as both the [ R u i v ( 0 ) ( H 2 0 ) ]and [Ruiv(OH),] comCF3S03H. plexes should have the same pH dependence of E I 1 2values. Figure 4 shows the ORTEP plot of the trans-[RuIV(L)coordinated geometry about the Ru atom is distorted octahedral (0)(H2O)l2+ (2) cation with atomic numbering scheme. The with the oxo and H20 ligands trans to each other. The Ru atom is essentially located at the center of the macrocycle with just (1 1 ) (a) Moyer, B. A.; Meyer, T. J. J . Am. Chem. SOC.1978,100, 3601. 0.057-A deviation from the mean plane defined by the N(1), N(2), (b) Moyer, B. A,; Meyer, T. J. Inorg. Chem. 1981,20,436. (c) Roecker, L.; O( l ) , and O(2) atoms. The configuration adopted by the macKutner, W.; Gilbert, J. A.; Simmons, M.; Murray, R. W.; Meyer, T. J. Inorg. rocycle is similar to the nonmethylated N 2 0 2ligand (N202repChem. 1985, 24, 3784. resents 3,4:9,1 O-dibenzo- 1,12-diaza-5,8-dioxacyclopentadecane) (12) For oxometal systems, the d orbitals are in the order, d, < d,,,d I < dxiy2 < dzz, after the works of Winkler Gray (Inorg. Chem. 1986, 25, $46). in [Ni(N,0,)C12],5Cin which the chelate rings incorporating the
-
-
-
-
-
*s
o'811
I; I
I
9052 J . Am. Chem. SOC..Vol. 11I, No. 25, 1989
n
m
Che et al.
a
V
0 li
a Figure 5. Stereoscopic diagram to show the packing of the trans[RU'~(L)(O)(H,O)] (C104), molecules.
200
300
400
600
500
700
h / nm
b
I I
I
P)
uE
m
P
0
m
c1 .4
112
400
500
600
700
X / nm Figure 3. UV-vis absorption spectra of (a) 1 and (b) 2 at pH 1.81 and 12.0. C 141
C
Figure 4. ORTEP plot of the tran~-[Ru'~(L)(O)(H,0)]~+ cation with atomic numbering scheme.
benzene rings are bent almost equally (40 and 46') to the same side of the N202plane and the lone pairs of nitrogen atoms are oriented syn to each other. It is probable that this represents the most stable form of this type of macrocycles. The N-CH3 and R u 4 groups are on the same side of the macrocycle. This leads to a nonbonded repulsive interaction and hence deviation of the
O-Ru-N(l) (94.6 (2)') and 0-Ru-N(2) (92.9 (2)') angles from 90'. The coordination geometry about the nitrogen atoms is tetrahedral with the C-N-C angles (av 107.8 ( 5 ) ' ) expected for tertiary amines.2cJ3 An important structural feature of the molecule is the measured Ru=O distance of 1.739 (2) A, which is the shortest one ever reported for the oxoruthenium(1V) system. It is considerably shorter than those values in t r a n ~ - [ R u ~ ~ ( T M C ) ( 0 ) ( X(1.765 )]"+ (1.86 (8) A)I4 and even (5) A)2c and trans-[R~'~(py)~(O)(Cl)]+ comparable to the Ru=O bonds in some ruthenium(V1)-oxo complexes such as [ R U ~ ' ( ~ ~ ~ ) ( O ) ~ ( C H ~ C [ ROUO( O) ~) ~] -, (HI06)2]6-,and Ba[RuV'O3(0H),] where the Ru=O distances are 1.726 (l),Ib 1.732 @),Id and 1.755 A,15respectively. Previous work has shown that the nature of axial ligands in the trans[RutV(TMC)(0)(X)]"+system has no effect on the Ru=O distances.2c Thus the difference in the d(Ru=O) values in 2 and t r u n s - [ R ~ ' ~ ( T M C ) ( 0 ) ( X )is] "likely + due to the effect of the equatorial macrocycle. Since L is a weaker donor than TMC,5a-8 the ruthenium ion in 2 is more electropositive, leading to an enhanced p,(O)-d,(Ru) r-bonding interaction. The measured Ru-O(1) and Ru-O(2) distances of 2.115 (2) and 2.153 (2) A are similar to the Ru-OH2 distance of 2.136 (4) A in [ ( b ~ y ) ~ l6 but significantly longer than those values found (H20)Ru11']204+ in the structures of [Ru(H20)6]3' (2.016 (4)-2.037 (5) A)]' and tr~ns-[Ru~*'(bpy),(OH)(OH~)]~+ (2.007 (3) A). The unusual long Ru-O(3) distance of 2.199 (3) A is probably a consequence of the trans effect of the oxo group. The Ru-N distances of 2.094 (2) and 2.103 (3) 8, are comparable to those values found in the Ru-TMC system.2a,2c Figure 5 shows the drawing for the crystal lattice of the Ru(1V) complex. An important structural feature of the crystal-packing diagram is that the Ru=O moieties of the truns-[Rdv(L)(0)(H2O)l2+cations orient toward one another. Despite the fact that the close approach of two trans-[R~~(L)(O)(H,0)]~+ cations would lead to unfavorable coulombic re ulsion, the intermolecular separation of the Ru=O units is 2.85 , indicating the existence of [Ru=O--O=Ru] interaction, though small. Such interaction of Ru=O moieties is undoubtedly involved in the coupling reactions of ruthenium-xo complexes and has long been suggested to be crucial for the water oxidation mediated by binuclear ruthenium complexes via
8:
(13) Che, C. M.; Cheng, W. K.; Lai, T. F.; Poon, C. K.; Mak, T. C. W. Inorg. Chem. 1987, 26, 1678. (14) Yukawa, Y . ; Aoyagi, K.; Kurihara, M.; Shinai, K.; Shimiza, K.; Mukaida, M.; Takeuchi, T.; Kakihana, H. Chem. Left. 1985, 283. (15) Nowogrocki, G.; Abraham, F.; Trehoux, J.; Thomas, D. Acta Crystallogr., Sect. B 1976, 832, 2413. (16) Gilbert, J. A.; Eggleston, D. S.; Murphy, W. R. Jr.; Gaselowitz, D. A.; Gersten, S. W.; Hodgson, D. J.; Meyer, T. J. J . Am. Chem. SOC.1985, 107, 3855.
(17) Dong, V.; Keller, H. J.; Endress, H.; Moroni, W.; Nothe, D. Acta Crystallogr. Sect. B. 1977, 833, 2428. (18) Durham, B.; Wilson, S. R.; Hcdgson, D. J.; Meyer, T. J. J . Am. Chem. SOC.1980, 102, 600. (19) Rotzinger, F. R.; Munavalli, S.; Comte, P.; Hurst, J. K.; gratzel, M.; Pern, F. J.; Frank, A. J. J . Am. Chem. SOC.1987, 109, 6619.
Novel Ru-Oxo Complexes of Saturated Macrocycles
J. Am. Chem. Soc., Vol. 111, No. 25, 1989 9053 III
We believe that, in the highly oxidizing cationic ruthenium-oxo complexes such as 2, the oxo groups are electrophilic and possess "oxene" character,20 which may be responsible for the observed [Ru=O-.O=Ru] interaction. Electrochemistry. The electrochemistry of trans-[RuV1(L)(0)2]2+(3) is similar to its bpyZbor T M C analog~e.'~In aqueous solutions, the cyclic voltammograms of 1-3 are the same except the potentials of zero current are at different values. Three reversible/quasi-reversiblecouples 1-111 a t the respective E , values of 0.26,0.62, and 0.92 V, corresponding to the electrode reactions
#
0.0
1
I
a
2
0.4
-0.I
0.1
0.6
1.D
0.8
I 2
Y V I SCE
-
trans-[Ru111(L)(OH)(H20)]Z+ + H + + e-
trans- [Ru"(L) (H20),] (I)
trans-[R~~~(L)(O)(H~0)]~+ + H + + etr~ns-[Ru~~~(L)(OH)(H~0)]~ (11)
-
+
t r a n s - [ R ~ ~ ~ ( L ) ( O+) ~2H+ ] ~ + 2etrans-[ R U ~ ~ ( L ) ( O ) ( H , O2+) ] (111) are found at pH 1.1 as shown in Figure 6a. Controlled-potential coulometry of 1-3 in 0.1 M C F 3 S 0 3 Hwith potentials held at 0.2, 0.5, and 0.7 V vs S C E established n = 1 for couples I and I1 and n = 2 for couple I11 in accordance with their formulation. For couple I, the peak to peak separation (Up 60 mV) and current ratio ( i / i p c = 1) show little dependence on scan rates and electrod)easurfaces. For couples I1 and 111, because the electrode reactions are accompanied with a slow proton-transfer step, their reversibilities are strongly affected by scan rates and the nature of electrode surfaces.3a Quasi-reversible couples are observed only with edge-plane pyrolytic graphite electrode and at scan rates of 20-100 mV s-l. With glassy-carbon or platinum electrode, the couples become less reversible or irreversible. As pH is increased, couple I11 begins to split into two quasi-reversible one-electron couples, IV and V, appearing at 0.70 and 0.49 V, respectively, a t pH 8.0 (Figure 6b). These two couples are assigned to Ru(VI)/Ru(V) and Ru(V)/Ru(IV) couples and the corresponding electrode reactions are
-
-
+ t r a n s - [ R ~ ~ ( L ) ( O )+ ~ ]H + + + e-
t r a n ~ - [ R u ~ ~ ( L ) ( O ) ~e] ~ + t r a n ~ - [ R u ~ ( L ) ( O ) ~ ] + (IV)
-
trans- [RuIV(L)(O)(OH)]+ (V) When the pH of the medium is raised, the El values of couples 1-111 and V (Table IV) will shift cathodically while that for couple IV remains independent of pH as expected. The plots of E l l 2 versus pH for all these couples at pH 1-8 are shown in Figure 7. Linear plots for couples 111 and V with slopes of -58 mV/pH unit are found in accordance with their formulations as two-proton two-electron and one-proton one-electron transfer processes, respectively. For couple 11, the plot with a slope of -58 mV/pH unit is linear over the pH range of 1-8 indicative of a one-proton one-electron transfer reaction throughout this pH range. The result is not unexpected since the pKa values for trans-[Ru"'(L)(OH)( H 2 0 ) I 2 +and trans- [ R U ~ ~ ( L ) ( O ) ( H , O ) ]are ~ + similar. In the case of couple I, there are two straight line portions with different slopes. At 1 < pH < 5.4,the slope is -58 mV/pH unit (20) The term "oxene" is used by Sawyer and co-workers to formulate the valency of oxygen in metal-oxo compounds. For example, see: Sawyer, T.
J. Comments Inorg. Chem. 1987, 6, 103.
1 -0.5
-0.3
0.3
0.1
0.5
0.9
1.IYrr5CT
Figure 6. Cyclic voltammograms of rruns-[R~~~~(L)(OH)(H,0)]~+ at (a) pH 1.0 and (b) pH 8.0: working electrode, pyrolytic graphite; scan rate, 50 mV
s-l.
Table IV. Ell* Values for Various Redox Couples of c ~ ~ ~ ~ - I R u * ~ ~ ( L ) ( H , Oat~ 25 ( O"C H)~Z+ E 1 p V vs SCE pH III/II IV/III V/IV VI/IV 1.10 0.26 0.62 0.92 1.81 0.24 0.58 0.87 2.21 0.22 0.55 0.85 2.56 0.20 0.53 0.82 3.29 0.16 0.47 0.78 3.78 0.14 0.45 0.75 0.11 0.43 0.73 4.10 4.50 0.085 0.39 0.7 1 5.02 0.065 0.37 0.70 5.73 0.025 0.34 0.67 6.37 -0.035 0.30 0.60 -0.07 0.28 0.56 6.80 -0.11 0.25 0.53 7.24 7.54 -0.14 0.22 0.51 8.00 -0.19 0.20 0.49
VI/V
0.70 0.70 0.70 0.70 0.70
whereas at pH > 5.4, it changes to -1 17 mV/pH unit. On the basis of this finding, the electrode reactions are for 1 < pH < 5.4
tr~ns-[Ru~~~(L)(OH)(H~0)]~+ + e- + H +
-
trans- [ R U ~ ~ ( L ) ( H ~ O ) ~ ] * + and for 5.4 < p H < 8 t r a n s - [ R ~ ~ ~ ~ ( L ) ( O H )e-~ ] + 2H+
+ +
-
trans- [Ru"( L) (H20)2]2+
since, a t pH > 5.4, the electrode reaction is a two-proton oneelectron transfer process; a slope of -1 17 mV/pH unit is expected. The break point of the plot for couple 111occurs a t pH 5.4, which is logically assigned to the pK, value of trans-[RulI1(L)( O H ) ( H 2 0 ) ] 2 + . This value is close to the 5.3 0.2 value determined by spectrophotometric method as described in the earlier section.
Che et al.
9054 J . Am. Chem. SOC..Vol. 111, No. 25, 1989
0.9
0.7
w u ro y1
ly
e' 2
0.5
> >
s
h
c
0.3
0.1
-0.1 0 250
300
60-0
500
hOO
700
h /nm
-0.3 I
I
I
I
I
I
I
1
2
3
4
5
6
7
I
8
,
9
Figure 8. UV-vis spectral changes for the oxidation of styrene by trans-[R~~'(L)(O)~]*+ in CH$N at 298 K: scan interval, - 3 min.
PH
I
Figure 7. Plots of versus pH for all the redox couples of trans[ Ru"'( L)(OH)(Hz0)l2+.
styrene
With references to previous works,21*22 the direct two-electron reduction of 3 to 2 in acidic solutions proceeds through a Ru(V) intermediate, which is thermodynamically unstable with respect to disproportionation into Ru(V1) and Ru(1V). The disproportionation equilibrium constant, Kdisp, for the reaction
+
2trans-[RuV(L)(0),]+ 2H+ trans- [ RuV1(L) (O),] 2+
-+
0.4
c y=looctene
trans- [Ru"'(L)(O) (OH,)] 0.3
can be calculated by the equations E1/2disp = El/2(V/1V) - El/2(V1/V) E1/2(V/IV) = 2E,/,(VI/IV) - El/,(VI/V) where El/2(V/IV), E l ,(VI/IV), and E,/,(VI/V) refer to the half-wave potentials ofl the respective half-cell reactions trans-[Ruv(L)(0),]+
/
0.5
+ + -
+ 2H+ + e-
3
I
\ Y) 111
g
0.2
X
-
0.1
trans- [ R U ~ ~ ( L ) ( O ) ( H , O ) ] ~ +
+
t r a n s - [ R ~ ~ ' ( L ) ( O ) ~ ] ~2H+ + trans- [ RuV1(L)(O),]2+
2etrans- [ R U ' ~ ( L ) ( O ) ( H ~ O ) ] ~ +
e-
trans- [RuV(L)(O),]+
is 0.44V at pH 1.1 and hence For the Ru(L)(O) system, is 2.77 x 10' mol-' dm3 at 25 OC. Similar calculation on the Ru(TMC)(O) system gives a 0.86 V and 3.18 X 10" mol-' dm3 for the respective and &is values. The much higher redox potential of t r a n s - [ R ~ ~ ' ( L ) ( O ) ~than ] ~ + that for trans[RuV1(TMC)(O),l2+ for Ru(VI)/Ru(IV) couple is 0.66 V vs S C E at pH 1.O] is attributed to the weaker donor strength of L. This would lead to the destabilization of the dioxoruthenium(V1) complex and hence a decrease in the &sp value. Stoichiometric and Catalytic Oxidation of Organic Substrates. We previously reported that t r a n s - [ R ~ ~ ' ( T M C ) ( 0 ) is ~ ]a~mild + oxidant, and its reactions with toluene and olefins are very slow even a t 50 0C.2c In this work, the ruthenium-oxo complexes of L demonstrate enhanced oxidative reactivities. From electrochemical experiments, complexes 2 and 3 are 0.22 and 0.26 V more
&p
(21) Che, C. M.; Wong, K.Y . J. Chem.Soc., Chem. Commun. 1986,229. (22) Lau, K. Ph.D. Thesis, University of Hong Kong, 1988.
Oa0
0.05
0.10
0.15
0.20
0.25
substrate / M
Figure 9. Plots of the experimental pseudo-first-order rate constants versus concentration of the organic substrates for the oxidative reactions of 3 in acetonitrile at 298 K.
oxidizing than trans- [ R U ~ ~ ( T M C ) ( O ) ( O H , ) ]and ~ + trans[RuV1(TMC)(O),l2+. This increase in redox potential leads 3 to be a better oxidant. Addition of organic substrate such as alcohol, aldehyde, or olefin to an acetonitrile (or acetone) solution of 3 a t room temperature results in the rapid reduction of Ru(V1); typical UV-vis spectral changes for the reaction with styrene a t shown in Figure 8. The immediate ruthenium product in each case is Ru(1V) since its optical spectrum is similar to that of 2 in acetonitrile (or acetone) and it does not further react with the organic substrate unless with benzyl alcohol at high temperature (50 "C). Kinetic studies showed a second-order kinetics with rate = k[Ru(VI)] [organic substrate] for the oxidation of organic substrates by 3. Figure 9 shows the plots of the experimental pseudo-first-order rate constants (versus concentration of the
Novel Ru-Oxo Complexes of Saturated Macrocycles Table V. Stoichiometric Oxidation of Organic Substrates by rrans-IRuV1(L)(0),l2+in C H X N at Room Temoerature substrate product yield: % benzyl alcohol benzaldehyde isopropyl alcohol acetone cyclohexene cyclohexenone cyclooctene cyclooctene oxide styrene benzaldehyde norborylene norborylene oxide cis-stilbene benzaldehyde trans-stilbene benzaldehyde toluene benzaldehyde adamantane 1-adamantanol 2-adamantanol acetophenone ethylbenzene sec-hphenylethyl alcohol phenylisopropyl alcohol cumene y-butyrolactone tetrahydrofuran cyclobutanone cyclobutanol cvclohexanone cvclohexanol Reaction time, 4 h.
J . Am. Chem. SOC..Vol. 1 1 1. No. 25, 1989 9055
~~
Table VI. Oxidation of Organic Substrates by PhIO Catalyzed by trans-IRu'V(L)(0)(H,0)12+ under Degassed Conditions substrate product yield: % norbor ylene norborylene oxide 50 styrene benzaldehyde 30 cyclohexene cyclohexene oxide c3 cyclohexen-1-01 20 cyclohexenone 40 cyclooctene cyclooctene oxide 35 Based on the amount of PhI produced. The reaction usually took 4-5 h when all PhIO solids were completely dissolved.
organic substrates) for the oxidation reactions of 3 in C H 3 C N a t 298 K. Linear plots are obtained from which the respective second-order rate constants of 0.045,0.21, and 0.140 mol-' dm3 s-I for isopropyl alcohol, styrene, and cyclooctene are deduced. The measured rate constant of 0.21 mol-' dm3 s-l for the styrene oxidation is even higher than that found for oxochromium(V) salen complex [0.014-0.113 mol-' dm3 s-' a t 298 K under similar condition^].^^ Table V summarizes the results on the stoichiometric oxidation of organic substrate. It oxidizes alcohols to aldehydes/ketones and epoxidizes olefins. Since cyclobutanol is oxidized to cyclobutanone in quantitative yield, 3 acts as a twoelectron oxidant. With styrene, oxidative cleavage of the C=C double bond is the major reaction pathway. The oxidation of cyclohexene to give cyclohexenone exclusively but with trace amount of cyclohexane oxide is similar to that found for trans-[R~~'(TMC)(0)~]~+.~~ This suggests that 3 has a high affinity toward allylic C-H bond oxidation. It is likely that the oxo groups in trans-dioxoruthenium(V1) complexes of L or T M C are very electrophilic and preferentially undergo hydrogen atom/hydride abstraction rather than a concerted oxo-transfer reaction. An important finding, which suggests the potential of trans-dioxoruthenium(V1) in C-H bond oxidation, is that 3 can oxidize tetrahydrofuran, aromatic hydrocarbons, and adamantane to the corresponding oxidized products in high yields. Since oxidation of adamantane gives 1-adamantan01 exclusively, 3 preferentially attacks a 3' C-H bond in the presence of a 2O C-H bond. Attempts have been made to develop catalytic oxidation by reoxidation of the Ru(1V) to Ru(V1) utilizing an oxygen atom donor. The results on the oxidation of organic substrates by iodosylbenzene (PhIO) in acetone in the presence of a catalytic amount of 2 are given in Table VI. For a given substrate such as styrene, the same distribution of organic products was found in both the stoichiometric and catalytic oxidations. This suggests that the catalytic oxidation proceeded with 3 as the possible reactive intermediate. Evidence to support this suggestion comes (23) Samsel, E. G.; Srinivasan, K.; Kochi, J. K. J . Am. Chem. SOC.1985, 107, 7606.
or
"' . i
n
