Catalysis of aerobic olefin oxidation by a ruthenium perhaloporphyrin complex

ELSEVIER

lnorganica Chimica Acta 270 (1998) 433439

Catalysis of aerobic olefin oxidation by a ruthenium perhaloporphyrin complex 1 Eva R. Birnbaum, Jay A. Labinger *, John E. Bercaw, Harry B. Gray Arthur Amos Noyes Laboratory. CaliJornia Institute of Technology, Pasadena. CA 91125, USA

Received 16 April 1997;accepted 2 September 1997

Abstract

The perhalogenated porphyrin ruthenium complex (TFPPCls)Ru(CO) (TFPPC18 = octachlorotetrakis(pentafluorophenyl)porphyrin) catalyzes aerobic oxidation of olefins at room temperature. Cyclohexene is oxidized primarily at the allylic position, and styrene primarily to benzaldehyde, indicating a radical autoxidation mechanism. Reactions are enhanced by visible light. Reaction with m-chloroperbenzoic acid converts the ruthenium complex to (TFPPCIs)Ru(O)2, but such oxo complexes do not appear to participate in catalytic aerobic oxidation. © 1998 Elsevier Science S.A. Kevwords: Olefin oxidation; Ruthenium complexes; Perhaloporphyrincomplexes

1. Introduction

The search for biomimetic analogs of the remarkably facile and selective hydrocarbon oxidations catalyzed by cytochrome P-450 [ 1 ] has been vigorously pursued for at least two decades [ 2 ]. In the majority of cases studied, the catalysts are unable to mimic the biological utilization of molecular oxygen, instead requiring more expensive O-atom donors as oxidants. Halogenated metalloporphyrins have attracted attention in recent years, for reasons including increased stability to porphyrin degradation, greater steric bulk, and modification of electronic properties. A number of examples of oxidations utilizing 02 are catalyzed by this class of porphyrins [3,4]. The aerobic oxidation of isobutane to t-butanol, catalyzed by iron complexes of tetrakis(pentafluorophenyl)porphyrin (TFPP) and its octahalo derivatives (TFPPCI8 and TFPPBr~) sparked particular interest, as they appeared to represent long-sought examples of the generation of oxidizing M = O active sites from 02 without the need for a co-reductant; it was proposed that the active oxidant is (porph)FeWO, made sufficiently reactive by the electronwithdrawing halogen substituents (the active oxidant in P450 systems is one oxidation state higher) [3]. However, mechanistic studies revealed that these reactions actually proceed via radical-chain autoxidation paths [5,6]. Their appli* Correspondingauthor. Tel.: + 1-626-3956520; fax: + 1-626-4494159; e-mail: [email protected] Dedicated to ProfessorJack Halpern. 0020-1693/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved PHSO020- 1693 (97)06000-3

cability hence is limited to compatible transformations, such as the above hydroxylation at a tertiary position; alkenes undergo primarily allylic oxidation rather than the more desirable epoxidation [7]. A variety of ruthenium porphyrin complexes have been found to effect oxidations of hydrocarbons [ 8-12] 2, and in some cases there is good evidence for M = O species as the active oxidant. Ruthenium(VI) diexo porphyrin complexes of the form (porph) RuO2, where porph = octaethylporphyrin (OEP), tetraphenylporphyrin (TPP) or tetramesitylporphyrin (TMP), oxidize alkenes stoichiometrically; the TMP complex also catalyzes the aerobic oxidation of olefins, thioethers, steroids and esters under mild conditions. The mechanism for the catalytic reaction is shown in Scheme 1; key steps are the disproportionation of (TMP)Ru~VO to the active Ru vt species and solvated ( T M P ) R u n, with the latter reacting with 02 to complete the catalytic cycle. The system does not catalyze alkane oxidation, though, and even with alkenes deactivates after a moderate number of turnovers [ 8 ]. From these results and our investigations of oxygenation reactions catalyzed by iron complexes ofperhalogenated porphyrins [5-7,13 ], we hoped that ruthenium complexes of the latter might exhibit the additional reactivity and stability needed for alkane oxidation within a non-radical chain, metal 2 Ru complexesof halogenated, sterically hindered porphyrinsare active catalysts for aerobic oxidation of neat olelinic and saturated hydrocarbons, at ~ 90°C and only at low ( ~ 10- 5 M ) catalyst concentration, by a radical mechanism [ 12b].

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o II

UV-Vis spectra were recorded on an Hewlett Packard 8452 diode array spectrophotometer or on an Oils-modified Cary-14 spectrophotometer interfaced to an IBM 386 computer. Infrared spectra were recorded as solutions in carbon tetrachloride or benzene on a Perkin-Elmer model 1600 FTIR spectrophotometer. NMR spectra were obtained on a Brtiker AM-500 in CDCI3 or deuterated acetone. Oxidation products were characterized and quantitated using a Hewlett Packard gas chromatograph with a SD 1 column. Samples were identified by matching retention times to authentic samples. An internal standard (toluene) was added to each aliquot before injection in the GC and used to determine the concentration of each product.

I ]

2.2. Catalytic oxidations

1/202

d:5 = Ruthenium teu'amesitylporphyrin

Scheme 1. oxo-based mechanism. In fact (TFPPCIs)Ru(CO), which we had previously synthesized and characterized [ 14], does function as a catalyst precursor for the oxidation of alkenes by molecular oxygen as well as iodosobenzene (PhIO), but apparently not via Ru=O species. We report here on our observations for this system, and their implications concerning the possibility of an aerobic oxidation catalyst based on Ru=O. While this work was in progress, some related hydrocarbon oxidations catalyzed by (TFPP)Ru(CO) were described [ 15 ].

2-3 mg of porphyrin (approximately 2 Ixmol) along with oxidant (PhlO, mCPBA, or TBHP) when used, were added to the reaction vessel (a Kjeldahl flask modified with a stopcock on the side for sampling), which was then flushed with O2 (or the appropriate alternate atmosphere), allowing gas to escape through the open stopcock. 15 ml freshly distilled methylene chloride (saturated with Ar o r 02) were added by syringe into the reaction vessel, followed by 1 ml of freshly distilled substrate. From the solubility of oxygen in methylene chloride and the volume of the flask, the dioxygen reactions were calculated to have approximately 1240 i~mol of Oz, or ~600 equiv, based on ruthenium. 'Ambient light' experiments were conducted with a 60 watt light bulb approximately 3 feet above the reaction flask. For 'dark' reactions the flask was wrapped in AI foil, while for 'photolyzed' reactions a standard 150 watt light bulb was positioned approximately one foot from the reaction vessel, which was placed in a water bath to maintain temperature. The solutions were stirred for 24~18 h, and aliquots taken by syringe every few hours for analysis of oxidation products. 2.3. Titration o f (TFPPCI~)Ru( CO)

2. E x p e r i m e n t a l

2.1. Materials" and methods

Cyclohexene, cyclooctene (Aldrich) and methylene chloride (EM Science) were distilled under argon before use. Cyclohexene oxide, 2-cyclohexen- 1-ol, 2-cyclohexen- l-one, m-cbloroperoxybenzoic acid (mCPBA), triphenylphosphine, pyridine, tert-butylhydroperoxide and styrene were purchased from Aldrich. Hydrogen peroxide, acetone (EM Science), iodosobenzene and cyclohexene-d~o (TCI) were used as received. Carbon monoxide, ethylene and dioxygen lecture bottles were purchased from Matheson. (TFPPCIs)Ru(CO) was obtained by metalation of the free ligand, H2TFPPCI8, with Ru3(CO)t2, followed by purification by column chromatography and HPLC [ 1411.

A solution of (TFPPC18)Ru(CO) in methylene chloride was prepared such that the absorbance (at either the Soret or Q band) was close to 1, and the exact concentration determined from the extinction coefficient. The porphyrin solution (2.5 ml) was titrated with 10 to 20 Ixl aliquots of a standard solution of mCPBA, while monitoring changes in the UVvisible spectrum. Following completion, the resulting solution of (TFPPCIx)Ru (O)2 was titrated with PPh3, or allowed to react with excess olefin, again under monitoring by UVvisible spectroscopy. 2.4. Photochemical studies

A solution of RuTFPPC18(CO) in methylene chloride in a quartz laser cuvette was degassed by three cycles of the freeze-pump-thaw method on a high vacuum line, and backfilled with the desired gas (argon, carbon monoxide, oxygen

E.R. Birnbaum et al. / lnorganica Chimica Acta 270 (1998) 433-439

435

Table 1 Oxidation of olefins catalyzed by TFPPCI 8) Ru (CO):comparison of 02 and PhIO as oxidants Substrate

Oxidant

No. of turnovers after 24 h

Selectivity to epoxide

Cyclohexene

O2 Ph[O 02 02 PhiO

300 10 42 2.5 " 26 "

15 42 100

Cyclooctene Slyrene

Selectivity to alcohol + ketone 85 58 t, 100 ~ 19 c

81

" After 3 h. ~' Several minor products detected by GC but not identified. Benzaldehyde sole product (other than epoxide) detected. Table 2 Oxidation of cyclohexene catalyzed by (TFPPCIs)Ru (CO):effect of reaction conditions Run

Conditions

No. of turnovers after 8 h

Selectivity to epoxide

Selectivity to cyclohexenol

Selectivity to cyclohexenone

1 2 3 4 5 6 7

Standard " Dark Photolyzed Intermittent photolysis PhIO, no 02 CO/O2 02 + 5 equiv, mCPBA

78 l0 270 280 l0 h 83 20

18 16 17 13 42 7 :35

59 59 61 63 56 55 52

21 25 22 24 2 38 13

02 atmosphere, ambient light (see text for details). b Alter 24 h.

or ethylene) to 1 atm pressure. The excitation source for the time-resolved absorption experiments was the third harmonic of a Quanta-Ray Nd-YAG laser with a 20 ns pulse width; for prolonged photolysis a PTi arc lamp was used. Details of data acquisition and analysis are given elsewhere [ 16].

3. R e s u l t s

3. I. Oxidation of alkenes (TFPPCI8) Ru(CO) catalyzes the oxidation of alkenes by dioxygen (Tables 1 and 2). Reactions were initiated by addition of 1 ml substrate to a 0.1 mM solution of catalyst in methylene chloride under an 02 atmosphere. Significant activity is observed, with up to 300 turnovers (based on Ru) of cyclohexene in 24 h. The porphyrin appears quite stable under the conditions used for catalysis (room temperature, 1 aunosphere 02), as over 90% of the catalyst could be recovered at the end of a catalytic run. Cyclohexene oxidation continued until all oxygen was consumed and resumed if more oxygen was added. Aerobic oxidation of cyclooctene and styrene was slower, with 42 and 3 turnovers in 24 h, respectively. For both cyclohexene and styrene, little epoxidation was observed. Benzaldehyde is the sole product of styrene oxidation, while cyclohexene gives primarily the allylic oxidation products 2-cyclohexen-l-ol (58%) and 2-cyclohexen- 1-one (27%) along with cyclohexene oxide (15%).

Cyclooctene, in contrast, gave mostly epoxide, along with a few (not identified) minor products. Catalytic aerobic oxidation is light dependent (Table 2, Fig. 1). Long induction periods were observed upon complete exclusion of light, with significant oxidation occurring only after 10 h. Continuous irradiation with a low intensity tungsten lamp resulted in 3.5 times as many turnovers in 8 h relative to one in ambient light (Table 2, runs I and 3). Such irradiation has no significant effect on product distribution. These oxidations appear to be photo-initiated rather than • • * •

300 250

ambient light no light visible photolysis with addition of mCPBA

200 150 t100 50 •

0

,'

0

'?'

2

•

•

'"~'',"P',', 4 6

|

',' 8

l' 10

Time (h) Fig, 1. Effects of light and of addition of mCPBA on aerobic oxidation of cyclohexene catalyzed by (TFPPCIx) Ru(CO).

436

E.R. Birnbaum et al. / lnorganica Chimica Acta 270 (1998) 433-439

• • ,~

320

1.5

ambientfight I visiblephotolysis | intermittentphotolysis1

280

/.,

240 200

e.

,11,

5X

A

160 120

'e'

.<

A

80 •

A

40 0

t

/h

i

0

[

2

i

A

i

I , .

~

4

I

t , ,

6

I,

8

F

v

I

,

10

Time (h) Fig. 2. Comparison of effects of continuous and intermittent photolysis on aerobic oxidation of cyclohexene catalyzed by (TFPPCls) Ru (CO),

1200

• •

lO00

0 2 atmosphere I CO/O2 mixt'Rl'e

800

480

higher selectivity for epoxidation - - 81% styrene oxide and 42% cyclohexene oxide - - than those utilizing 02.

,..1 400~

3.2. Generation and reactivity of (TFPPCIs)Ru(O)2

I'D

240 400 160 200

80 IPT~Lq~'I

0

10

....

I ....

20 30 T i m e 0a)

I ....

40

650

Fig. 4. UV-Vis spectral changes during titration of (TFPPCI8) Ru (CO) with mCPBA.

6OO

0

350 400 450 500 550 600 Wavelength (nm)

560

320 o

300

0

50

Fig. 3. Effect of CO on aerobic oxidation of cyclohexene catalyzed by (TFPPCIs) Ru (CO).

photo-catalytic, in that a reaction irradiated for 3 h and then placed in the dark continues unabated; also a reaction kept in the dark for 30 min, followed by alternating 2 h periods of photolysis and darkness, shows the same activity as under continuous photolysis (Table 2, runs 3 and 4; Fig. 2). Under a mixed O2/CO atmosphere (ambient light), reactivity was little changed from that under pure 02 (Table 2, runs 1 and 6; Fig. 3), except for some decrease in epoxide formation. The activities are virtually identical, as seen in Fig. 3, until the O2/CO reaction cuts off as Oz becomes virtually exhausted. ( TFPPCls ) Ru (CO) catalyzes the oxidation of alkenes by PhIO, although at significantly lower levels of activity than with molecular oxygen. These oxidations also show much

Titration of ('ITPPCIs)Ru(CO) with m C P B A results in complete transformation to a new species, as monitored by changes in the UV-Vis spectrum, after addition of 2 equiv. of peracid. Isosbestic points are seen at 390, 417, 525 and 585 nm. The final spectrum (Fig. 4) shows a red-shifted Soret band and peaks (Am~x= 420, 514,552 nm) very similar to those reported for (TFPPBr8) Ru (O) 2 [ 15 ]. The complete disappearance of the carbonyl stretch in the IR indicates removal of the CO ligand, so the solution must contain the dioxo complex (TFPPCI8)Ru(O)2. (While no strong peaks were observed where expected for v ( R u = O ) ( = 830 cm - 1 ) , this region was significantly obscured by strong solvent signals.) All attempts to concentrate the solution and/or obtain the complex as a solid led to decomposition Titration of (TFPPCI~)Ru(O) z with PPh:~, up to 2 equiv., results in a decrease in the 514 nm band and the 600 nm shoulder associated with the dioxo complex, and new Q bands at 542 and 553 nm, with clearly resolved isosbestic points (Fig. 5). Further addition of PPh3, up to a total of 4 equiv., resulted in a blue shift of the Q bands to 517 and 540 nm, again with clean isosbestic behavior. The 3~p N M R spectrum of the final solution shows a resonance at 29.9 ppm for Ph3PO as well as a second signal at 40 ppm. The latter signal is consistent with coordination of triphenylphosphine to Ru H, but a complex was not isolated. No resonance for free triphenylphosphine was observed. Addition of dioxygen and cyclohexene to a 0.1 mM solution of (TFPPCIs) Ru (O) 2 gave only 20 turnovers of product, with much higher selectivity to the epoxide (Table 2, run 7) ;

E.R. Birnbaum et al. / lnorganica Chimica Acta 270 (1998) 433-439

(a)

3.3. Photochemical experiments

1.61 ÷ 2 eq PPh 3

1.4 1.2

~

t

"~

0:i

<

1).6

c

0.4 0.2 ,Jl,,

0 500

I , , , I J , , l l l l [ l l l l l l

520

540

?

~

520

540

560 580 600 Wavelength (nm)

I i r i]

620

640

660

(b) 1.6 1.4

~ + 4 eq pph3 (total)

1.2 1

0.8

<

437

0.6 0.4 0.2 i 0

Transient absorption laser spectroscopy was used to probe the behavior of the catalyst in an excited state. Excitation of (TFPPC18)Ru(CO) at 355 or 480 nm generates a transient species with a positive absorption peak at 620 nm and negative net absorption in the Soret region. The excited species undergoes biexponential decay, with rate constants (under Ar atmosphere) of 7.1 × 106 S-t and 7.6× 105 s -1. Under CO or O~ atmospheres the transient spectrum is essentially the same at both 5 and 50 I~s, corresponding to before and after the first (faster) decay process is complete. The rate constants do show some changes; the faster one is 1.25 × 107 s L under 02 and 3.9 × 106 s - z under CO. After prolonged photolysis at the excitation wavelengths, under carbon monoxide, dioxygen, argon or ethylene atmosphere, UV-Vis spectroscopy reveals no new products, although there is significant degradation of the starting complex under CO or 02. To test the possibility that the photochemical catalysis mechanism might involve energy transfer from an excitedstate porphyrin triplet to O2 generating singlet oxygen, photocatalytic experiments were performed using (TFPPCIs) Zn and Rose Bengal. In both cases, lhe level of cyclohexene oxidation was no higher than background levels (i.e., no catalyst, ambient light).

' 500

560 580 600 Wavelength (tam)

620

640

660

Fig. 5. UV-Vis spectral changes during titration of (TFPPCIs) Ru(O) 2 with PPh 3.

but the latter can be ascribed in part to stoichiometric epoxidation by excess mCPBA. Furthermore, unlike reactions employing (TFPPC18)Ru(CO) as catalyst precursor, which maintain activity until all oxygen is consumed, the reaction with (TFPPCIs)Ru(O) 2 ceased after 2 h, when less than 3% of the oxygen had reacted. At lower concentrations ( < 10 ~M), reaction of (TFPPCIs)Ru(O)z with cyclohexene or styrene can be observed by UV-Vis spectroscopy; there is a large decrease in the intensity of both the Soret and Q band absorptions. (Concentrations were too low to allow accurate determination of organic products.) The new spectrum is similar to that reported for (OEP)RuW(O) [9] and (TPP)RuW(O) [17], with a slight blue shift in the Sorer band relative to the dioxo complex, and broad absorptions in the Q band region. The Soret intensity recovers slowly on allowing the solution to stand for 24 h, while addition of carbon monoxide gas regenerates the (TFPPCIs)Ru(CO) spectrum with over 85% of the original intensity. Similarly, addition of 10 equiv, of t-butylhydroperoxide (TBHP) or triethylamine oxide to (TFPPC18)Ru(CO) produces little or no increase in the rate of catalytic oxidation of cyclohexene by O~. Some catalyzed decomposition of TBHP is observed, but the rate ( ~ 4.5 TBHP decomposed per Ru in 4 h) is relatively slow, compared to the much higher activity found for (TFPPXs)FeCI [3,13e].

4. D i s c u s s i o n

The underlying goal of this project was to achieve aerobic alkane oxidation via a non-radical chain mechanism, such as the one in Scheme 1, by increasing the reactivity of the RuVt(O)z state by means of electronegative substitution on the porphyrin ring. The target species, (TFPPCI~)Ru(O)2, can be readily generated by oxidation of the carbonyl with mCPBA, as shown by the close spectral agreement with the Br analog, prepared similarly [ 15], as well as the stoichiometric oxidation of 2 equiv, of PPh3 to the phosphine oxide. However, solutions so prepared are significantly less catalytically active than the carbonyl-bound precursor. The lower activity could be accounted for in either of two ways: (TFPPCIs)Ru(O)2 might be a much less powerful oxidant than anticipated, or it might not be possible to regenerate it from 02 once it has delivered an oxygen atom (by the mechanism of Scheme 1 or a related process). In fact, both of these statements appear to be true. (TFPPC18) Ru(O ) z does oxidize PPh3 (not surprisingly), and undergoes reduction in the presence of olefins (as detected by UV-Vis spectral changes), but Groves et al. have shown that the closely related (TFPP) Ru (O) z is not capable of hydroxylating even such a reactive alkane as adamantane [ 15]. Regeneration may be prevented by a lower tendency to disproportionate, as suggested by the relative longevity of the partially reduced (TFPPCIs) Ru (O) species in UV-Vis experiments, and / or a reduced affinity for Ru H to bind oxygen. The fact that preoxidation to the RuW(O)2 state slows overall activity for oxi-

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E.R. Birnbaum et al. / lnorganica Chirnica Acta 270 (1998) 433-439

dation by 02 strongly suggests that it is not a viable intermediate in a catalytic cycle, even if it does have the ability to effect substrate oxidations. It is of interest to compare the high activities for hydrocarbon oxidation (both alkane hydroxylation and olefin epoxidation) observed by Groves et ai. for the (TFPP)Ru(O)2/ 2,6-dichloropyridine-N-oxide system [15]. Mechanistic studies revealed that the active oxidant is a RuV(O) species, and the catalytic cycle involves the RunI/Ru v couple. A key feature is that the pyridine-N-oxide is not able to generate the RuV~(O) 2 state in that system; preformation of the latter was shown to slow activity, as we have found. None of our experiments gives clean access to the Rum/Ru v manifold, as attempted oxidative decarbonylation (other than with mCPBA, which goes to Ru w) led to porphyrin degradation; while redox potentials [ 14a] indicate that Ru In will not be accessible without loss of CO. The product distributions observed here clearly indicate a radical pathway. With cyclohexene, the relative proportions of allylic oxidation products and epoxide are virtually identical to those obtained with the analogous iron complex as catalyst [7]; the latter reactions are known to follow the radical-chain autoxidation mechanism [5,6]. (The higher selectivity for epoxide with PhIO as oxidant may indicate some contribution of a non-radical chain mechanism involving Ru=O.) The high selectivity to cyclooctene epoxide is not inconsistent with this mechanism, as the rate for ROO. addition to olefin relative to that for abstraction of an allylic hydrogen is considerably higher for cyclooctene than for cyclohexene [ 18 ]. Despite some similarities, the functions of (TFPPC18)Ru(CO) and (TFPPXs)FeCI are clearly not completely parallel. The Ru catalyst does not effect alkane hydroxylation; it is not a particularly good catalyst for TBHP decomposition; and exposure to light is required for greatest activity. This is perhaps not surprising, as the key property of the iron catalyst that accounts for its activity is the redox potential of the Fe 3+/2+ couple, which allows Eq. ( 1 ) to proceed at a rate sufficient to sustain long chains in both peroxide decomposition and alkane oxidation [5,6]. The Ru catalyst, strongly stabilized in the R u lI oxidation state by the 7r-acceptor CO ligand, has no comparable redox couple [ 14a]. ROOH + M 3+ ---)ROO. + H + + M 2+

( 1)

R O O H + M 2+ ~ R O . + O H -

(2)

-t-M 3+

The catalytic species in the Ru system, then, must be much less active than the Fe complex; it is able to catalyze autoxidation of cyclohexene but not the more difficult alkane hydroxylation. The 'relative oxidizability' of cyclohexene compared to isobutane is about 20:1 for general autoxidation mechanisms, as determined by the relative rates of propagation (ROO. + R H - - * R O O H + R . ) and termination (2ROO---* non-radical species + O2) [19]. The iron catalysts are able to effect isobutane oxidation under mild

conditions precisely because the decomposition of ROOH is so efficient that steady-state levels of [ROO. ] are lowered to the point that the termination step shown above becomes nearly negligible: [5 ]. While we cannot identify the precise species responsible for catalyzing aerobic oxidation here, it is obviously not the 'precatalyst' (TFPPCIs) Ru(CO) itself, but rather a complex produced therefrom in low concentration, with its formation being much enhanced by light irradiation. The most straightforward explanation would be that photodecarbonylation affords a small, steady-state concentration of (TFPPCI~)Ru which is the active catalyst, but that is inconsistent with several observations. First, reactivity is unaffected by CO in the atmosphere, which would certainly reduce the steady-state concentration of (TFPPCIs) Ru; also the fact that intermittent irradiation is just as effective as continuous irradiation argues against that hypothesis. Lastly, the photochemical behavior in flash photolysis studies are most consistent with a triplet excited state rather than photodecarbonylation, as a CO atmosphere does not not increase the rate of decay of the excited state. A second explanation could involve energy transfer from the excited-state triplet to 02, yielding singlet oxygen, which would certainly be a strong enough oxidant to initiate radical chains. Again, though, this does not seem compatible with the results under intermittent irradiation. Furthermore, known singlet oxygen sensitizers were not found to catalyze cyclohexene oxidation under these conditions. We are left with the conclusion that the active catalyst is irreversibly formed from (TFPPCIs)Ru(CO), probably by a combination of photolysis and oxidation. Related behavior was observed in the Ru-catalyzed oxidations by pyridine-Noxide, where the initial Run(CO) complex is converted lirst to a porphyrin radical cation and then to a Ru m species, which participates in the actual catalytic cycle [ 15]. It is tempting to propose that a similar species, (TFPPCIs)RuInx2 (where X = solvent or some other axial ligand), might be responsible for the aerobic radical-chain oxidation observed here. Failure to catalyze alkane hydroxylation would not be surprising in view of the fact that the second-row metal complex is almost certainly not high-spin, in contrast to the iron analog. We have found in studies on related Fe-salen complexes that a spin-state change has a major effect on catalytic activity, probably because low- or intermediate-spin complexes undergo ligand exchange reactions much less readily, decreasing the rate of (inner-sphere) electron-transfer processes [20]. It should also be noted that, just as in Groves' et al. work [ 15], overoxidation (as for example with mCPBA) destroys the active species and substantially slows catalytic activity. In conclusion, while (TFPPCIs)Ru(CO) does catalyze aerobic oxidations, they are relatively slow, and require highly reactive substrates such as cyclohexene with its allylic C-H bonds. Most probably the electron-withdrawing nature of the halogenated porphyrin, intended to increase the reactivity of Ru=O intermediates, has the opposite effect of

E.R. Birnbaum et al. / lnorganica Chimica Acta 270 (1998) 433-439

d e c r e a s i n g the r e a c t i v i t y o f R u n t o w a r d s 0 2 (as was also o b s e r v e d in the F e s y s t e m [ 6 ] ) , so that R u = O species are n e v e r g e n e r a t e d f r o m 02. F u r t h e r m o r e , o n the basis o f b o t h this a n d p r e v i o u s w o r k [ 15], it a p p e a r s that a n y i n c r e a s e in R u = O r e a c t i v i t y is m o d e s t at best. A g e n e r a l strategy for n o n radical c h a i n a e r o b i c h y d r o c a r b o n r e m a i n s an e l u s i v e target.

oxidation

catalysis

Acknowledgements W e t h a n k D r H a n s N i k o l a n d D r Jay W i n k l e r for helpful d i s c u s s i o n s . T h i s w o r k w a s s u p p o r t e d b y the N a t i o n a l S c i e n c e F o u n d a t i o n , the D e p a r t m e n t o f E n e r g y , the G a s R e s e a r c h Institute, a n d S u n C o m p a n y , Inc. F e l l o w s h i p support f r o m the P a r s o n s F o u n d a t i o n ( E . R . B . ) is a c k n o w l e d g e d .

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