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Stability and reactivity of low-spin ferric hydroperoxo and alkylperoxo complexes with bipyridine and phenantroline ligands
Journal of Molecular Catalysis A: Chemical 159 Ž2000. 233–245 www.elsevier.comrlocatermolcata
Stability and reactivity of low-spin ferric hydroperoxo and alkylperoxo complexes with bipyridine and phenantroline ligands A.P. Sobolev a , D.E. Babushkin b, E.P. Talsi b,) a b
Department of Natural Sciences, NoÕosibirsk State UniÕersity, NoÕosibirsk 630090, Russia BoreskoÕ Institute of Catalysis, Pr. Akademika LaÕrentieÕa 5, NoÕosibirsk 630090, Russia
Received 1 May 1999; received in revised form 14 February 2000; accepted 28 February 2000
Abstract In this work the first-order rate constants of self-decomposition of hydroperoxo and alkylperoxo complexes wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 Ž2a-Py., wFeŽphen. 2 ŽOOH.PyxŽNO 3 . 2 Ž2b-Py. and wFeŽbpy. 2 ŽOOt Bu.CH 3 CNxŽNO 3 . 2 Ž3aCH 3 CN. were determined in the presence of various substrates and at various temperatures. It was observed, that the alkylperoxo species are far less stable than corresponding hydroperoxo intermediates, k s 1.2 = 10y2 sy1 Ž3a-CH 3 CN in CH 3 CN at y108C. and k s 2 = 10y4 sy1 Ž2a-Py in CH 3 CN at y108C.. The sixth ligand ŽPy in 2a-Py and 2b-Py; CH 3 CN in 3a-CH 3 CN. can be replaced by other donor molecules B in appropriate solvent systems. Using d 9-t BuOOH, 2 D NMR signals of t BuOO moieties of complexes 3a-CH 3 CN, 3a-CH 3 OH and 3a-H 2 O were observed. The rate of decomposition of hydroperoxo complexes wFeŽbpy. 2 ŽOOH.BxŽNO 3 . 2 Ž2a-B., where B are derivatives of Py Ž3-Br-Py, 3-Me-Py, 4-Me-Py and 4-Me 2 N-Py. increases with the growth of basisity of B Žpush effect.. Such effect is markedly smaller for alkylperoxo species wFeŽbpy. 2 ŽOOt Bu.BxŽNO 3 . 2 Ž3a-B.. The addition of organic substrates Žcyclohexane, cyclohexene, methyl phenyl sulfide. in concentrations up to 3 M at y108C to q208C does not noticeably change the rate of self-decomposition of 2a-B, wFeŽphen. 2 ŽOOH.BxŽNO 3 . 2 Ž2b-B. and 3a-B. Thus the intermediates concerned do not directly react with organic substrates. The reactivity patterns of 2a-B, 2b-B and 3a-B were characteristic for free radical oxidation. OH P and HO P2 radicals were trapped in solution containing 2a-Py, and t BuOO P free radicals were detected in solution in the presence of 3a-B. The determined rates of self-decomposition of complexes 2a-B, 2b-B and 3a-B can be used for evaluation of the upper limit for their reactivity towards organic substrates. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Ferric hydroperoxo complexes; Ferric alkylperoxo complexes; EPR; NMR spectroscopy; Reactivity studies
1. Introduction There have been several efforts aimed at modelling of the alkane functionalization chem)
Corresponding author. Tel.: q7-383-234-1877; fax: q7-383234-3056. E-mail address: [email protected] ŽE.P. Talsi..
istry of nonheme iron enzymes; most prominent of these are the catalysts known as the ‘‘Gif’’ systems w1–3x, wFe 2 OŽ bpy. 4 x 4qrt BuOOH w4x and wFeŽTPA. Clx 2qrt BuOOH w5,6x combinations ŽTPA — trisŽ2-pyridylmethyl. amine. . Two alternative mechanisms of alkane oxidation by these catalytic systems are supposed now in the literature. According to one of them,
1381-1169r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 1 - 1 1 6 9 Ž 0 0 . 0 0 1 9 7 - 7
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metal based oxidant, either a metal-peroxide intermediate or a high-valent iron-oxo species derived therefrom, must participate in the alkane functionalization reactions w1–5x. According to the other, alkylperoxy or alkoxy radicals are supposed to be reactive intermediates w7–10x. Since high-valent iron-oxo intermediates have never been observed spectroscopically for nonheme catalytic systems, the assumption that ferric-hydroperoxide could be itself a highly reactive species is rather spread in the literature Žsee Refs. w11–13x.. Nevertheless, this possibility requires much more experimental and theoretical studies. During the last 5 years, molecular composition of several low-spin alkylperoxo and hydroperoxo iron intermediates wFeŽ BLM. ŽOOH.x Žactivated bleomycin. w14–16x, wFe IIIŽ bpy . 2 Ž OO t Bu . HOR x 2q w17 x, wFe III Ž TPA . ŽOOt Bu.ŽHOR.x 2q w18x, wFe III ŽN4Py.Ž OOH.x 2q Žwhere N4Py is N-ŽbisŽ2-pyridyl. -methyl.-N, NbisŽ 2-pyridylmethyl. amine. w19x, wFe III Ž Py5. ŽOOH.x 2q Žwhere Py5 is 2,6-bis-ŽbisŽ 2-pyridyl. methoxymethane.pyridine. w20x and wFe III Ž TPA. ŽOOH.x 2q w21,22x has been reliably determined using EPR, resonance Raman and electrospray ionization mass spectroscopies. Some intermediates wFe III Ž TPP.Ž OOH.Ž OH.x w23x, wFe III Ž cyclyd .Ž OOH .x w24 x, wFe III Ž F20TPP .Ž OOC Ž O . Ar.x w25x, wFe III ŽPMA.ŽOOH.x and wFe III Ž PMA. Ž O O t B u .x w 26,27 x , w Fe Ž bpy . 2 Ž O O H . Py x ŽNO 3 . 2 and wFeŽphen. 2 ŽOOH.PyxŽNO 3 . 2 w28x have been characterized by EPR spectroscopy based on their highly characteristic anisotropy of g-tensor. However in all cases, except activated bleomycin FeŽ BLM.Ž OOH. w14,15x, the reactivity data were insufficient to evaluate the impact of these intermediates to the oxidation of organic substrates. In this work we describe our detailed studies of the stability and reactivity of ferric hydroperoxo and alkylperoxo complexes wFeŽbpy. 2Ž OOH. PyxŽ NO 3 . 2 Ž 2a-Py. , wFeŽ phen. 2 Ž OOH. PyxŽ NO 3 . 2 Ž 2b-Py. and wFeŽ bpy. 2 Ž OOt Bu. CH 3 CNxŽNO 3 . 2 Ž 3a-CH 3 CN.. These complexes
were previously found in the reaction of HOOH with wFe 2 OŽ bpy. 4 P 2H 2 OxŽ NO 3 . 4 Ž 1a. and wFe 2 OŽphen. 4 P 2H 2 OxŽNO 3 . 4 Ž1b. in PyrAcOH mixture as a solvent w28x, and in the reaction of t BuOOH with 1a in ROHrCH 3 CN mixture w17x.
2. Experimental part 2.1. Materials Cyclohexane, methyl phenyl sulfide, pyridine, 3-Me–, 4-Me–, 3-Br–, 2-Me–, 4-dimethylamino pyridines, glacial acetic acid, acetonitrile, methanol and corresponding deuterated solvents were purchased from Aldrich and used without additional purification. Purchased cyclohexene was purified by double fractional distillation just before using. Hydrogen peroxide 30% was concentrated in vacuum up to 90–95% and titrated with KMnO4 just before experiments. Tert-butyl hydroperoxide and deuterated tert-butyl hydroperoxide Žd 9-t BuOOH. were obtained from tert-butyl alcohol and deuterated tert-butyl alcohol Ž d 9-t BuOH. according to procedure described in Refs. w29,30x. Complexes 1a and 1b were prepared as described in Refs. w31,32x, respectively. 2.2. EPR monitoring of hydroperoxo complex 2a-Py in the catalytic system 1a r HOOHr AcOH r Py To start the reaction, 0.2 ml of 0.01 M solution of complex 1a in 2:1 PyrAcOH molar mixture or in this mixture containing cyclohexane was added to Žfrozen in liquid nitrogen. 0.02 ml of 90% HOOH Ž370 equiv.. directly in an EPR tube. Then the sample was immersed in a constant-temperature bath with water and solutions of 1a and HOOH were mixed. To stop the reaction, the EPR tube was immersed in liquid nitrogen followed by registration of EPR
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spectrum at y1968C. Thus, dependencies of 2a-Py concentration versus time were obtained. 2.3. Preparation of the solutions of 2a-Py, 2b-Py and 3a-CH3 CN with reduced concentration of peroxides A mixture of 0.6 ml of 1:1 Pyr90% HOOH volume was added dropwise into 1 ml of 0.01 M solution of complex 1a or 1b in 2:1 Pyr AcOH molar mixture at y208C Žcatalyst:oxidant ratio s 1:1100 equiv.. . The sample was warmed up to 08C and kept 5–10 min at this temperature to rich 10y3 M concentration of 2a-Py or 2b-Py. Then the solution was mixed with 20 ml of diethyl ether at y608C and kept 30–40 min at this temperature to obtain precipitate, containing 2a-Py or 2b-Py as an admixture. After that, the liquid part was decanted and the precipitate was again washed with 20 ml of diethyl ether at y608C. The washing procedure was repeated three times and then residual diethyl ether was removed under vacuum. The residue was dissolved in appropriate solvent system at low temperature to give solutions of w F e Ž b p y . 2 Ž O O H . B xŽ N O 3 . 2 Ž 2 a -B . o r wFeŽphen. 2 ŽOOH.BxŽNO 3 . 2 Ž2b-B. with the reduced concentration of HOOH. Concentration of residual HOOH Ž monitored by iodometric titration. was less than 10y2 M and that of 2a-B or 2b-B Žmonitored by EPR. was about 2 = 10y4 M. To study the interaction with organic substrates, the solution containing 2a-Py or 2b-Py was sampled into several EPR tubes at low temperature. Some of these tubes contained the appropriate amount of a substrate. The reaction was started by immersing all tubes into a constant-temperature bath with ethyl alcohol. The solutions in EPR tubes were thoroughly mixed. The reaction was stopped by simultaneously freezing all the samples in liquid nitrogen. Then EPR spectra of hydroperoxo intermediates were recorded at y1968C. The reaction was started again by immersing all the tubes in the bath. Thus obtained kinetic curves for complexes 2aPy or 2b-Py decomposition in the presence and
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in the absence of various substrates can be reliably compared. The procedure used for preparation of 3aCH 3 CN samples with reduced concentration of t BuOOH was similar to that described above. In this case, the interaction of 1a Žw1ax s 0.01 M. with 500 equiv. of t BuOOH was performed in 1:4 MeOHrCH 3 CN molar mixture at y408C. The characteristic green color indicated formation of 3a-CH 3 CN. Solid samples containing 3a-CH 3 CN were isolated by the same procedure as in the case of 2a-Py. 2.4. EPR measurements EPR spectra Žy1968C. were recorded at 9.2–9.3 GHz on a Bruker ER-200D spectrometer in glass cylindrical tubes Ž d s 5 mm. . They were simulated using an extended version of the EPR 1 program, described in Ref. w33x. Measurements were made in a quartz Dewar with liquid nitrogen. The dual EPR cavity furnished with the spectrometer was used. Periclase crystal ŽMgO. , with impurities of Mn2q and Cr 3q, served as a side reference, was placed into the center of the second compartment of the dual cavity. EPR signals were quantified by double integration with copper chloride as a standard at y1968C. A flat quartz cell was used for registration of EPR spectra of liquid samples at room temperature. 2.5. NMR measurements 2
D NMR spectra were recorded at 61.40 MHz in 10-mm glass sample tubes, using pulsed FT-NMR technique, with a Bruker MSL-400 NMR spectrometer. The typical operating conditions used for 2 D NMR measurements were as follows: sweep width 3 kHz; spectrum accumulation frequency 2.5 Hz; number of transients 5000–10 000; 458 pulse at 10 ms. 1 H NMR spectra were monitored on the Bruker MSL-400 NMR spectrometer at 400.13 MHz and a Bruker DPX-250 NMR instrument at 250.10 MHz as well.
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2.6. The analysis of the oxidation products The yield of the reaction products Žcyclohexanone, cyclohexanol, cyclohexenone and cyclohexenol. was measured using GC technique Žchromatograph ‘‘Tsvet 500’’, FID, Ar, a steel column Ž2 m = 3 mm. packed with 15% Carbowax 20M on Chromaton N-AW-HMDS. . Probes Ž 0.5 ml. of the reaction solutions were sampled, followed by the extraction of the reaction products with diethyl ether Ž 3 = 1 ml.. The diethyl ether layers were combined; styrene was added as an internal standard. Diethyl ether was evaporated and the mixture was analyzed by GC. Products were identified and quantitated by comparison with authentic samples.
3. Results and discussion 3.1. Characterization of hydroperoxo complexes 2a-B and 2b-B The intensities of the EPR signals of complexes wFeŽbpy. 2 Ž OOH.PyxŽNO 3 . 2 Ž2a-Py. and wFeŽphen. 2 ŽOOH.PyxŽNO 3 . 2 Ž2b-Py. ŽFig. 1a and b. correspond to their yield as much as 20% based on starting complexes 1a or 1b. The g-values of complexes 2a-Py Ž2.14, 2.11, 1.97. and 2b-Py Ž2.13, 2.12, 1.97. were rather close to those for recently well characterized hydroperoxo complexes wFe III Ž N4Py.Ž OOH.x 2q Ž2.17, 2.12, 1.98. w19x, wFe III Ž Py5.Ž OOH.x 2q Ž2.15, 2.13, 1.98. w20x and wFe III ŽTPA.ŽOOH.x 2q Ž2.19, 2.14, 1.97. w21x. The typical g-values for stable low-spin ferric complexes with O,N-donor ligands containing no OOH or OOR moieties are in the range g 1 s 2.4–2.6, g 2 s 2.14–2.2, g 3 s 1.85–1.95 w34,35x. The observed low anisotropy of g-factors for the complexes 2a-Py and 2b-Py is a highly characteristic property of low-spin ferric hydroperoxo or alkylperoxo species. The attempts to characterize 2a-Py and 2b-Py by electrospray ionization mass spectrometry were unsuccessful. However, it is unlikely that the supposed molecular composition
Fig. 1. X-band EPR spectra of complexes 2a-Py Ža. and 2b-Py Žb. at y1968C in 2:1 pyridinerAcOH molar mixture. Spectrometer settings: microwave power 20 mW, modulation frequency 100 kHz, modulation amplitude 5 G.
of the complexes 2a-Py and 2b-Py will be essentially refined. Indeed, their monomeric structure is evident from EPR spectra typical of a mononuclear low-spin ironŽIII. center in an octahedral environment. The presence of at least two bipyridine or phenantroline ligands is necessary to obtain low-spin ferric species, the presence of the hydroperoxo moiety is evident from the characteristic anisotropy of g-tensor. Low-spin ferric complexes must be six-coordinated. This sixth ligand is pyridine. A sample of 2a-Py obtained after extraction of HOOH from the reaction mixture and removal of the major part of PyrAcOH Ž the residual concentration of HOOH was ca. 10y2 M and that of AcOH or Py was also not more than 10y2 M, see Section 2. was dissolved in CH 3 CN. The EPR spectrum of this new sample only slightly differs from that of 2a-Py. Thus, the replacement of Py in 2a-Py by CH 3 CN does not occur, otherwise it should lead to noticeable changes in EPR spectral parameters as in the case of related complexes wFeŽbpy. 2OOt Bu.BxŽNO 3 . 2 3a-B. Their EPR spectra are very sensitive to the nature of the sixth ligand ŽCH 3 CN or Py, see Table 1. . The g-values of 2a-Py change insignificantly when Py is replaced by its derivatives B with various p K a
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Table 1 EPR spectroscopic data for low-spin ferric peroxo and alkylperoxo complexes Complex
Solvent
g1
g2
g3
Ref.
wFeŽN4Py.ŽOOH.x2q wFeŽPy5.ŽOOH.x 2q wFeŽTPA.ŽOOH.x2q wFeŽbpy. 2 ŽOOH.Pyx 2q, 2a-Py wFeŽbpy. 2 ŽOOH.Pyx 2q, 2a-Py wFeŽphen. 2 ŽOOH.Pyx 2q, 2b-Py wFeŽ4,4X-Me 2-bpy. 2 ŽOOH.Pyx 2q wFeŽbpy. 2 ŽOOt Bu.CH 3 CNx 2q, 3a-CH 3 CN wFeŽbpy. 2 ŽOOt Bu.MeOHx 2q, 3a-MeOH wFeŽbpy. 2 ŽOOt Bu.H 2 Ox 2q, 3a-H 2 O wFeŽbpy. 2 ŽOOt Bu.Pyx 2q, 3a-Py wFeŽbpy. 2 ŽOOt Bu.Ž3-Br-Py.x 2q, 3a-3-Br-Py wFeŽphen. 2 ŽOOt Bu.CH 3 CNx 2q, 3b-CH 3 CN
CH 3 CN CH 3 CN CH 3 CN PyrAcOH CH 3 CNrPy PyrAcOH CH 3 CNrPy CH 3 CN MeOH CH 3 CN CH 3 CNrPy CH 3 CNr3-Br-Py CH 3 CN
2.17 2.15 2.19 2.14 2.14 2.13 2.14 2.18 2.18 2.18 2.18 2.18 2.16
2.12 2.13 2.14 2.11 2.11 2.12 2.11 2.12 2.16 2.16 2.13 2.13 2.10
1.97 1.98 1.97 1.97 1.97 1.97 1.97 1.98 1.98 1.98 1.975 1.975 1.97
w19xa w20x b w21x c w28x this work w28x this work this work this work this work this work this work this work
a
N4Pys N-ŽbisŽ2-pyridyl.-methyl.-N, N-bisŽ2-pyridylmethyl.amine. Py5 s 2,6-bis-ŽbisŽ2-pyridyl.methoxymethane.pyridine. c TPA s tris-Ž2-pyridylmethyl.amine. b
Ž3-Br-Py, 3-Me-Py, 4-Me-Py or 4-Me 2 N-Py. . This replacement can be readily performed by the addition of B at a concentration of 1 M to a sample of 2a-Py in CH 3 CN Žresidual concentration of Py was less than 10y2 M. . 3.2. Characterization of alkylperoxo complexes 3a-B Recently, alkylperoxo complexes with supposed structure wFeŽbpy. 2 Ž OOt Bu.ROHxŽClO4 . 2 Ž 3a-ROH. , where ROH s benzyl alcohol, methanol and phenol, were detected by EPR in the reaction of 1a with t BuOOH in CH 3 CNr ROH mixtures. The presence of OOt Bu moiety in 3a-ROH was confirmed by resonance Raman spectroscopy w17x. However, the EPR signal Ž g s 2.18, 2.12, 1.98. attributed in Ref. w17x to 3a-ROH belongs in fact to complex 3a-CH 3 CN with CH 3 CN molecule on the sixth coordination site ŽFig. 2c.. Complex 3a-MeOH Ž g s 2.18, 2.16, 1.98. can be obtained in neat methanol ŽFig. 2a., while in methanolracetonitrile mixtures the complexes 3a-MeOH and 3a-CH 3 CN both are present ŽFig. 2b.. Complexes 3a-B ŽB is a donor molecule. can be prepared by dissolution of a sample, containing 3a-CH 3 CN in CH 3 CNrB solvent system at y408C. EPR pa-
rameters of complexes 3a-B are presented in Table 1. It is seen that g-factors for complexes 3a-MeOH and 3a-H 2 O are close. Further 2 D NMR spectroscopic studies provide new insight into the structures of the 3a-B intermediates, formed in the reaction of 1a with t BuOOH in CH 3 CNrMeOH mixtures. When deuterated d 9-t BuOOH was added to the solution of 1a in 1:3 MeOHrCH 3 CN molar mixture at y358C, two weak resonances at y3.1 and at y3.7 ppm along with an intense signal of d 9-
Fig. 2. X-band EPR spectra of complexes 3a-MeOH and 3aCH 3 CN formed in the reaction of 1a Ž5=10y3 M. with 200 equiv. of t BuOOH at y358C in various MeOHrCH 3 CN molar mixtures: Ža. in neat MeOH; Žb. 1:1 MeOHrCH 3 CN; Žc. 1:40 MeOHrCH 3 CN. The spectra were recorded at y1968C. The signal denoted by asterisks belongs to t BuOO P radical.
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t BuOOH and d 9-t BuOH at 1.22 ppm were observed in the 2 D NMR spectrum ŽFig. 3a.. The intensities of these resonances correlated with the intensities of the EPR signals for the complexes 3a-MeOH and 3a-CH 3 CN detected in the same sample. The signals at y3.1 and y3.7 ppm can be ascribed to the alkylperoxo moieties of 3a-MeOH and 3a-CH 3 CN respectively. The ratio of their intensities in 2 D NMR spectra was proportional to that of MeOH and CH 3 CN concentrations. Addition of water to the reaction mixture afforded a new signal at y2.5 ppm attributed to the t BuOO group of 3a-H 2 O ŽFig. 3b.. The intensity of the particular resonance was found to be proportional to the concentration of the corresponding ligand Ž MeOH, CH 3 CN or H 2 O. in solution. It implies the occurrence of equilibria between the complexes 3a-MeOH, 3a-CH 3 CN and 3a-H 2 O, as shown in Eqs. Ž1. – Ž 3. 3a-MeOHq CH 3 CN
° 3a-CH CN q MeOH K1
3
Ž1.
° 3a-H O q MeOH 3a-CH CN q H O ° 3a-H O q CH CN 3a-MeOHq H 2 O
K2
2
K3
Ž2.
Fig. 3. 2 D NMR spectra recorded in the course of the reaction of 1a Žw1ax s 0.01 M. with 100 equiv. of d 9-t BuOOH at y358C Ža. in 1:3 MeOHrCH 3 CN molar mixture; Žb. in 1:1.5 MeOHr CH 3 CN molar mixture, where H 2 O ŽwH 2 Ox s1 M. was added.
tively w36x. The 1 H NMR resonances of bipyridine ligands of complexes 3a-B were either too broad to be observed or masked by signals of the initial complex 1a and by those of undeuterated admixtures in the solvent and peroxide used.
Ž3.
3.3. Stability of hydroperoxo complexes 2a-B and 2b-B
According to EPR data the time of establishing of these equilibria was 5–10 min at y358C. The equilibrium constants K 1 –K 3 determined from 2 D NMR spectra Žat y358C. have the following values: K 1 s 0.5 " 0.1; K 2 s 8 " 1; K 3 s 16 " 2. It is noteworthy that the substitution of MeOH by H 2 O molecule on the sixth coordination site of 3a-MeOH gives rise to noticeable change in the chemical shift of the alkylperoxo moiety while the difference between EPR parameters of 3a-MeOH and 3aH 2 O is negligible. The NMR spectra of Fig. 3 is the first example of the NMR detection of the alkylperoxo moiety for low-spin ferric alkylperoxo species. The reported chemical shifts for a and b deuterium atoms of OOCD 2 CD 3 group in the high-spin alkylperoxo complex FeŽ TPP. ŽOOCD 2 CD 3 . were 180 and 4 ppm, respec-
First of all, let us consider the results of EPR monitoring of the hydroperoxo intermediate 2aPy in the catalytic system 1arHOOHrPyr AcOH. Fig. 4a shows the time-dependence of the 2a-Py concentration in the course of the reaction of 1a with HOOH at 208C in 2:1 PyrAcOH mixture Žw1ax s 0.01 M, wHOOHx s 3 M.. It is seen that the concentration of complex 2a-Py reached maximum during 3 h, was in steady state for 1 h, and then decreased by a pseudo-first-order kinetics with the rate constant Ž3 " 1. = 10y4 sy1. The steady-state concentration of 2a-Py dropped 5 " 0.5 times if C 6 H 12 ŽwC 6 H 12 x s 1 M. had been previously added to a sample Ž Fig. 4b.. The value of this drop depended on the cyclohexane concentration and was 3 " 0.6 at
3
2
2
3
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Fig. 4. Concentration of 2a-Py as a function of time in the reaction of 1a Žw1ax s 0.01 M. with 90% HOOH ŽwHOOHx o s 3 M. at 208C in 2:1 PyrAcOH Ža. and in this mixture, containing C 6 H 12 Ž1 M. Žb.. The sample in Ža., if C 6 H 12 ŽwC 6 H 12 x s1 M., was added 4 h after the reaction beginning Žc.. The difference between curves Ža. and Žc. before addition of C 6 H 12 shows the level of reproducibility of the samples.
wC 6 H 12 x s 0.5 M and 2 " 0.4 at wC 6 H 12 x s 0.25 M. The addition of cyclohexane ŽwC 6 H 12 x s 1 M. to the sample of Fig. 4a at a moment when the steady-state concentration of 2a-Py had already been established, gave rise to the rapid decrease of the concentration of 2a-Py down to the value corresponding to the presence of cyclohexane in solution initially Ž Fig. 4c. . This conversion followed a first-order kinetics with k s Ž1.5 " 0.5. = 10y3 sy1. The concentration of 2a-Py is determined by the competition of the rates of its formation Ž W1 . and decay Ž W2 .. The effect of cyclohexane ŽFig. 4c. could result either from decrease of W1 or increase of W2 . To distinguish between these two possibilities, it is necessary to measure the rate of self-decomposition of 2a-Py and determine the influence of organic substrates on this rate. The reasonable way to measure self-decay kinetics of 2a-Py is to dramatically diminish the concentration of HOOH in the reaction solution at low temperature and thus sharply decrease the rate of 2a-Py formation. The subsequent monitoring of the decrease of the concentration of 2a-Py at higher temperatures allows determination of the rate of its self-decomposition. This procedure will give the real value of the rate of
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2a-Py self-decomposition, if the loss of 2a-Py concentration during the removal of HOOH at low temperature is markedly smaller than the decrease of the concentration of HOOH. The extraction of peroxide from reaction mixture by diethyl ether at low temperature Žsee Section 2. permits fulfillment of these requirements. The loss of the 2a-Py concentration during extraction of HOOH Žfrom 2 = 10y3 to 2 = 10y4 M. was markedly smaller than the decrease of HOOH concentration Ž from 3 to 10y2 M. . The similar procedure was used for preparation of complexes 2b-Py and 3a-CH 3 CN. Further, if not especially noted, the data for samples with the reduced concentration of peroxides will be present. The procedure described above allows us to obtain 2a-Py, 2b-Py and 3a-CH 3 CN only as small admixtures Ž 1–3%. to other ferric complexes. According to 1 H NMR data, in the case of 2a-Py and 2b-Py, the major part of iron species represents trinuclear basic ironŽIII. acetate wFe 3Ž m 3-O.ŽCH 3 CO 2 . 6 Py3 xŽNO 3 . formed upon dissolution of the Ž m-oxo. diironŽ III. complexes wFe 2 OŽ bpy. 4 P 2H 2 OxŽ NO 3 . 4 and wFe 2 OŽphen. 4 P 2H 2 OxŽNO 3 . 4 in pyridineracetic acid mixture. Its identity was confirmed by 1 H NMR, using wFe 3Ž m 3-O.ŽCH 3 CO 2 . 6 Py3 xŽClO4 . independently prepared as described in Ref. w37x. Complex 3a-CH 3 CN can be obtained as an admixture to the initial complex 1a wFe 2 OŽbpy. 4 P 2H 2 OxŽNO 3 . 4 . Fortunately, all iron species except alkylperoxo and hydroperoxo intermediates display no signals near g s 2 and thus the concentration of these intermediates can be readily monitored by EPR. The obtained kinetic data for self-decay of 2a-B, 2b-Py and 3a-B under various conditions are collected in Table 2. Complex 2a-Py dissolved in Py or PyrAcOH mixture was stable at y608C and decayed with a first-order kinetics at higher temperatures Ž Fig. 5.. The rate constants of this decay determined at various temperatures are the same in Py and in 2:1 PyrAcOH mixture and have the following values: k s 5 = 10y4 sy1 Žy278C.; 1 = 10y3 sy1 Ž y208C. ; 2.8 = 10y3 sy1
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Table 2 First order rate constants of low-spin ferric hydroperoxo and alkylperoxo complexes self-decay a Complex wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.Ž3-Br-Py.xŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.Ž3-Me-Py.xŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.Ž4-Me-Py.xŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.Ž4-Me 2 NPy.xŽNO 3 . 2 wFeŽ4,4X-Me 2-bpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽphen. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.CH 3 CNxŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.CH 3 CNxŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.CH 3 CNxŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.MeOHxŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.Ž3-Br-Py.xŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.Ž4-Me 2 N-PyxŽNO 3 . 2
Temperature Ž8C. y27 y20 y10 5 5 20 20 20 20 20 20 20 y37 y30 y20 y35 y30 y30 y30
Solvent system b
PyrAcOH PyrAcOH b PyrAcOH b PyrCH 3 CN c PyrH 2 O c 3-Br-PyrCH 3 CN d PyrCH 3 CN c 3-Me-PyrCH 3 CN e 4-Me-PyrCH 3 CN f 4-Me 2 N-PyrCH 3 CN g PyrCH 3 CN c PyrCH 3 CN c CH 3 CN CH 3 CN CH 3 CN MeOH PyrCH 3 CN c 3-Br-PyrCH 3 CN d 4-Me 2 N-PyrCH 3 CN g
k = 10 3 Žsy1 . 0.5 1.0 2.8 0.5 2.0 0.4 2.0 2.0 2.0 6 6 8 1.0 2.5 5.8 2.4 2.3 2.0 3.0
a
All of the constants were determined with error less than 20%. 3:1 PyrAcOH volume mixture. c wPyx s 1 M. d w3-Br-Pyx s 1 M. e w3-Me-Pyx s 1 M. f w4-Me-Pyx s 1 M. g w4-Me 2 N-Pyx s 1 M. b
Žy108C., Ea s 11.6 " 3 kcalrmol Ž the initial concentration of 2a-Py is 10y4 to 5 = 10y4 M..
Fig. 5. Ža. Low-field part of the EPR signal of complex 2a-Py in the course of its self-decay in 2:1 PyrAcOH molar mixture at y238C. The interval between the spectra is 10 min. Inset Žb.: Plot of the kinetic data from Ža. after applying the equation lnŽ Cr C0 . sy kt.
The nature of the solvent markedly perturbs the stability of 2a-Py. Compare k s 5 = 10y4 sy1 Ž58C. in CH 3 CNrPy ŽwPyx s 1 M., k s 7 = 10y3 sy1 Ž58C. in Py and k s 2 = 10y3 sy1 Ž58C. in H 2 OrPy ŽwPyx s 1 M. . Note that the rate constant of self-decay of activated bleomycin k s 5.8 = 10y3 sy1 Ž48C. in H 2 O w14x is rather close to that for the decay of 2a-Py Ž k s 2 = 10y3 sy1 . at 58C in H 2 OrPy ŽwPyx s 1 M.. To elucidate the influence of the basicity of the sixth ligand B on the rate of the decay of 2a-B, the following derivatives of Py with various p K a values shown in brackets were used w38x: 3-Br-Py Ž2.84. , Py Ž 5.23. , 3-Me-Py Ž 5.68. , 4-Me-Py Ž6.02. and 4-Me 2 N-Py Ž9.58.. Py and its derivatives were added into the aliquots of the same solution of 2a-Py in CH 3 CN Žto make wBx s 1 M.. The following rate constants were obtained at q208C: 4 = 10y4 sy1 Ž3-Br-Py.; 2 = 10y3 sy1 ŽPy.; 2 = 10y3 sy1 Ž3-Me-Py.; 2
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= 10y3 sy1 Ž4-Me-Py.; 6 = 10y3 sy1 Ž4-Me 2N-Py.. It is seen that with the increase of basicity of B, the rate constant of 2a-B self-decay noticeably grows. The similar ‘‘push effect’’ of B was previously observed for heterolytic decay of acylperoxo ironŽIII. porphyrin complexes in CH 2 Cl 2 , that affords oxoferryl cation radical complexes ŽPor qP.Fe IV s O w39x. The replacement of 2,2X-bipyridine ligands in 2a-Py by 4,4X-dimethyl-2,2X-bipyridine ligands results in threefold increase of the rate constant of its decay at 208C in CH 3 CNrPy ŽwPyx s 1 M.. This observation is in agreement with the reported data for acylperoxoironŽ III. porphyrin complexes, where introduction of electron donating substituents at the meso position of the porphyrin ring accelerates the rate of their selfdecay w39x. Unfortunately, in contrast to the iron porphyrin systems, we have no data on the structure of intermediates of the hydroperoxo complexes decomposition. The hydroperoxo complex 2b-Py with phenantroline ligands is less stable than the hydroperoxo complex 2a-Py with bipyridine ligands. The rate constant of the 2b-Py decomposition in CH 3 CNrPy ŽwPyx s 1 M. at 208C is 8 = 10y3 sy1 versus 2 = 10y3 sy1 for 2a-Py in the same solvent and at the same temperature. 3.4. ReactiÕity of the hydroperoxo complexes 2a-B and 2b-B To study the reactivity of the hydroperoxo complexes 2a-Py and 2b-Py, a substrate Žcyclohexane, cyclohexene, methyl phenyl sulfide. was added to their solutions in Py or CH 3 CNrPy ŽwPyx s 1 M. at y108C to q208C. The initial concentration of 2a-Py or 2b-Py was 2 = 10y4 to 5 = 10y4 M. It was found that the presence of listed substrates in solution Ž concentrations were up to 3 M. did not noticeably perturb the rate of self-decay of 2a-Py and 2b-Py. This rather unexpected result is consistent with the previous data obtained for activated bleomycin, where DNA also does not affect the rate of activated bleomycin self-decay
241
w14x. Thus, 2a-Py and 2b-Py do not directly react with organic substrates and the observed decrease of the quasi steady-state concentration of 2a-Py after the addition of cyclohexane into the catalytic system 1arHOOHrPyrAcOH ŽFig. 4. is not caused by the increase of the rate of complex 2a-Py decay ŽW2 ., but by the decrease of the rate of its formation Ž W1 .. It is still not entirely clear how cyclohexane can perturb W1. If it is granted that free radicals ŽOH P, HO 2P . participate in formation of 2a-Py, the possible way of reducing W1 is the decrease of the concentration of free radicals in solution via their reaction with cyclohexane. As a rule, alkylperoxo and hydroperoxo ironŽIII. intermediates were detected only at high excesses of oxidants Ž ROOH, HOOH. , when a number of unidentified substrate-sensitive reactions could determine their concentrations. Thus, the attempts to derive data on the reactivity of LFeŽOOH. or LFeŽOOR. species from the drop of their quasi steady-state concentrations after the addition of a substrate to the reaction mixture w25,27,28x were inadequate. The observed pseudo-first-order decrease of the concentration of wFe III ŽTPA.Ž OOt Bu. ŽHOR.x 2q w18x can reflect the kinetics of the decomposition of excessive t BuOOH rather than the kinetics of the decay of the alkylperoxo intermediate, and thus the effect of cyclohexanol on this kinetics can not be connected with the reactivity of the alkylperoxo intermediate. The only informative reactivity study of an hydroperoxo intermediate is the study of the activated bleomycin Ž Fe III Ž BLM.Ž OOH.. , which is believed to be responsible for the oxidative damage of DNA under aerobic conditions by Fe II-BLM w14,15x. In contrast to the other mentioned intermediates, detected only at the excess of oxidants, activated bleomycin can be prepared via stoichiometric oxidation of Fe II-BLM by dioxygen and thus kinetics of its self-decay can be reliably measured. As was mentioned, DNA did not affect the rate of activated bleomycin self-decay, however the rate of accumulation of DNA degradation products coin-
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cided kinetically with the decay of activated bleomycin. The yield of oxidation products achieved 50% of the concentration of activated bleomycin decomposed w14x. This evidences in favour of the rate determining conversion of Fe III ŽBLM.ŽOOH. to another active species prior to the reaction with substrate. The results of this part of our study are the determination of the rates for self-decomposition processes of the complexes 2a-Py, 2b-Py and demonstration of the fact that organic substrates do not perturb these rates. These data allow evaluation of the upper limit for the rates of the reaction of 2a-B or 2b-B with organic substrates. The maximal values of these rates are not more than those of the rates of complexes 2a-B or 2b-B self-decomposition. In the opposite case, the addition of substrates would noticeably perturb the decay of these species. Secondly, the obtained data show that organic substrates can influence the steady-state concentration of the hydroperoxo and alkylperoxo ironŽIII. intermediates diminishing the rate of their formation rather than increasing the rate of their decay. This possibility should be taken into account in reactivity studies of peroxo iron species. The main products of cyclohexane oxidation by the catalytic system 1arHOOHrPyrAcOH Žthe sample of Fig. 4. are cyclohexanone and cyclohexanol in 1:1 ratio. Catalyst:substrate ratio was 1:100 equiv., ketone and alcohol were formed in 4% yield each based on substrate. The oxidation of cyclohexene by this system affords products of allylic oxidation Ž cyclohexenone and cyclohexenol, their yields were 4% and 3%, respectively. . This reactivity pattern is typical for oxidation by free radicals. The same oxidation products are observed for samples with reduced concentration of HOOH. It should be noted that the residual concentration of HOOH Ž10y2 M. was much larger than the concentration of 2a-Py Ž10y4 M.. The overall concentration of cyclohexane oxidation products in these samples was 3- to 10-fold larger than that of 2a-Py. Thus, we cannot exclude that the
Fenton decomposition of residual HOOH affords the major part of the products. We have applied a spin-trapping technique for the characterization of transient free radicals in a solution containing complex 2a-Py in CH 3 CN. The 3,3,5,5-tetramethyl-1-pyrroline N-oxide ŽTMPO. purchased from Sigma was used as a spin trap. The nitrone spin traps DMPO and TMPO are widely used to provide evidence for the involvement of free radicals in many reactions in chemical and biological systems w40–42x. They are particularly useful for identifying oxygen-centered radicals, e.g. superoxide radical anion, peroxyl, alkoxyl and hydroxyl radicals. Similarly to all nitrones, TMPO captures short-living free radicals to give more stable nitroxyl radicals Žspin adducts.. The bhydrogen hyperfine splitting constants Ž a H . of spin adducts vary markedly with the nature of the radical trapped. Thus radicals can be recognized w40,41x. EPR spectrum recorded 3 min after the addition of TMPO Ž to make its concentration 0.1 M. to a sample containing 2a-Py in CH 3 CN at 208C ŽFig. 6a. is a superposition of the spectra of two adducts: aN s a H s 14 G and a N s 13 G, aH s 6.5 G with relative weights 1:2. The relative weight of the spectrum with a N s 13 G, a H s 6.5 G grows with time Ž Fig. 6b and c.. According to hyperfine splitting constants the signal with aN s a H s 14 G belongs
Fig. 6. X-band EPR spectra of TMPO spin adducts in a sample with reduced content of HOOH Žca. 10y2 M. containing 2P10y4 M 2a-Py in CH 3 CN at various moments of time after addition of TMPO Žto make its concentration 0.1 M. at 208C: Ža. 3 min, Žb. 11 min, Žc. 27 min.
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to spin adduct TMPOr POH. Formation of this adduct was observed for the catalytic system HOOHq CoŽacac. 2 in CH 3 CN w43x. In this system Fenton decomposition of hydrogen peroxide takes place. The EPR spectrum with a N s 13 G, a H s 6.5 G is characteristic for the adduct TMPOrHOP2 w41x. Thus POH and HO P2 free radicals can be trapped in the solution of 2a-Py P P in CH 3 CN. Recently, Oy 2 rHO 2 free radicals were trapped in a water solution of activated bleomycin w35x. Note, that the attempts to trap free radicals directly in the catalytic system 1arHOOHrPyrAcOH were unsuccessful due to rapid decomposition of TMPO. Free t BuOO P radicals can be directly observed in EPR spectra in the course of the reaction of 1a with t BuOOH in CH 3 CN. Their EPR spectrum in a frozen solution represents a sharp signal with axial anisotropy of g-factor, gIs 2.04, g Hs 2.008 ŽFig. 2c, the signal denoted by asterisks.. 3.5. Stability and reactiÕity of alkylperoxo complexes 3a-B The alkylperoxo complex 3a-CH 3 CN obtained after extraction of t BuOOH from a reaction mixture rapidly disappears even at low temperatures with the following rate constants: k s 10y3 sy1 Ž y378C. ; 2.5 = 10y3 sy1 Ž y308C.; 5.8 = 10y3 sy1 Žy208C.; E a s 14 " 3 kcalrmol. The rate constant of 3a-MeOH self-decay in MeOH was found to be 2.4 = 10y3 sy1 at y358C. The addition of Py or its derivatives ŽwBx s 1 M. to a solution of 3a-CH 3 CN in CH 3 CN gives rise to its conversion to 3a-B monitored by EPR. The rate constants for complexes 3a-B decay in CH 3 CNrB ŽwBx s 1 M. at y308C were as follows: 2.0 = 10y3 sy1 Ž3-Br-Py.; 2.3 = 10y3 sy1 ŽPy.; 3 = 10y3 sy1 Ž4-Me 2 N-Py.. It is seen that the basicity of the sixth ligand B only slightly perturbs the rates of decomposition of alkylperoxo species 3a-B in contrast to hydroperoxo species 2a-B Žsee Section 3.3.. The similar small push effect was reported for homolytic decomposition of
243
acylperoxoironŽ III. porphyrin in toluene that affords oxoferryl complex Ž Por.Fe IV s O w39x. The alkylperoxo species are far less stable than corresponding hydroperoxo intermediates, k s 1.2 = 10y2 sy1 Ž3a-CH 3 CN in CH 3 CN at y108C. and 2 = 10y4 sy1 Ž2a-Py in CH 3 CN at y108C.. The different effect of axial ligation on the rate of self-decomposition of hydroperoxo Ž2a-B. and alkylperoxo Ž3a-B. species could provide evidence in favor of different mechanisms of their decay. However, we have still not obtained any additional support for this assumption. The addition of cyclohexane, cyclohexene or methyl phenyl sulfide in concentrations up to 3 M to the reaction mixture does not perturb the rate of the complex 3a-CH 3 CN decomposition at y258C. It indicates that 3a-CH 3 CN does not directly react with these organic substrates similarly to the hydroperoxo complexes 2a-Py and 2b-Py. GC analysis reveals mainly cyclohexanone and cyclohexanol in 1:1 ratio as the products of cyclohexane oxidation with solution of 3a-CH 3 CN in CH 3 CN Žobtained after extraction of t BuOOH. at y208C. As the overall concentration of the products was not more than the initial concentration of 3a-CH 3 CN, its decomposition could produce an intermediate responsible for oxidation of the substrate. Based on 1:1 ratio of ketone to alcohol typical of free radical oxidation, this intermediate could be t BuO P radicals. The catalytic system 1ar t BuOOHrCH 3 CN also oxidizes cyclohexane mainly into cyclohexanol and cyclohexanone in 1:1 ratio at 08C. Probably, the reactive species of this oxidation are t BuO PŽ t BuO P2 . radicals formed in the course of t BuOOH decomposition. t BuO P2 radicals at a concentration of ca. 10y5 M were observed in the catalytic system 1artBuOOHrCH 3 CN ŽFig. 2c..
4. Conclusions The first-order rate constants of self-decomposition of hydroperoxo and alkylperoxo com-
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plexes wFeŽ bpy. 2 Ž OOH. PyxŽ NO 3 . 2 Ž 2a-Py. , wFe Ž phen . 2 Ž OOH . Py xŽ NO 3 . 2 Ž 2b-Py . and wFeŽbpy. 2 ŽOOt Bu.CH 3 CNxŽNO 3 . 2 Ž3a-CH 3 CN. were determined in the presence of various substrates and at various temperatures. The rate of decomposition of hydroperoxo complexes 2a-B, where B are derivatives of Py Ž 3-Br-Py, 3-Me-Py, 4-Me-Py and 4-Me 2 N-Py. increases with the growth of basisity of B Ž push effect. . Such an effect is markedly smaller for alkylperoxo species 3a-B. 2 D NMR signals of t BuOO moieties of low-spin ferric alkylperoxo complexes 3a-CH 3 CN, 3a-CH 3 OH and 3a-H 2 O were detected for the first time. The addition of organic substrates Žcyclohexane, cyclohexene, methyl phenyl sulfide. at a concentration up to 3 M at y108C to q208C does not noticeably change the rate of self-decomposition of 2a-B, 2b-Py and 3a-B. Thus the intermediates concerned do not directly react with organic substrates. The reactivity patterns of 2a-B, 2b-Py and 3a-B were characteristic for free radical oxidation. The determined rates of self-decomposition of complexes 2a-B, 2b-B and 3a-B can be used for evaluation of the upper limit for their reactivity towards organic substrates. To date there is an example w21x, when a low-spin ferric hydroperoxo intermediate wFeŽ TPA .Ž OOH .x 2q detected in wFeŽ TPA . ŽCH 3 CN. 2 xŽClO4 . 2rHOOHrCH 3 CN system is believed to be responsible for epoxidation of 1-hexene that is typical for non radical oxidation. Further it would be interesting to study the influence of 1-hexene on the rate of wFeŽTPA.ŽOOH.x 2q self-decay and to correlate the rate of this decay with the rate of epoxide formation to elucidate whether this hydroperoxo complex directly reacts with olefin or via formation of another reactive species.
Acknowledgements This work was supported by Russian Fund of Basic Research, grant no.97-03-32495a and by Haldor Topse ArS.
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Stability and reactivity of low-spin ferric hydroperoxo and alkylperoxo complexes with bipyridine and phenantroline ligands A.P. Sobolev a , D.E. Babushkin b, E.P. Talsi b,) a b
Department of Natural Sciences, NoÕosibirsk State UniÕersity, NoÕosibirsk 630090, Russia BoreskoÕ Institute of Catalysis, Pr. Akademika LaÕrentieÕa 5, NoÕosibirsk 630090, Russia
Received 1 May 1999; received in revised form 14 February 2000; accepted 28 February 2000
Abstract In this work the first-order rate constants of self-decomposition of hydroperoxo and alkylperoxo complexes wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 Ž2a-Py., wFeŽphen. 2 ŽOOH.PyxŽNO 3 . 2 Ž2b-Py. and wFeŽbpy. 2 ŽOOt Bu.CH 3 CNxŽNO 3 . 2 Ž3aCH 3 CN. were determined in the presence of various substrates and at various temperatures. It was observed, that the alkylperoxo species are far less stable than corresponding hydroperoxo intermediates, k s 1.2 = 10y2 sy1 Ž3a-CH 3 CN in CH 3 CN at y108C. and k s 2 = 10y4 sy1 Ž2a-Py in CH 3 CN at y108C.. The sixth ligand ŽPy in 2a-Py and 2b-Py; CH 3 CN in 3a-CH 3 CN. can be replaced by other donor molecules B in appropriate solvent systems. Using d 9-t BuOOH, 2 D NMR signals of t BuOO moieties of complexes 3a-CH 3 CN, 3a-CH 3 OH and 3a-H 2 O were observed. The rate of decomposition of hydroperoxo complexes wFeŽbpy. 2 ŽOOH.BxŽNO 3 . 2 Ž2a-B., where B are derivatives of Py Ž3-Br-Py, 3-Me-Py, 4-Me-Py and 4-Me 2 N-Py. increases with the growth of basisity of B Žpush effect.. Such effect is markedly smaller for alkylperoxo species wFeŽbpy. 2 ŽOOt Bu.BxŽNO 3 . 2 Ž3a-B.. The addition of organic substrates Žcyclohexane, cyclohexene, methyl phenyl sulfide. in concentrations up to 3 M at y108C to q208C does not noticeably change the rate of self-decomposition of 2a-B, wFeŽphen. 2 ŽOOH.BxŽNO 3 . 2 Ž2b-B. and 3a-B. Thus the intermediates concerned do not directly react with organic substrates. The reactivity patterns of 2a-B, 2b-B and 3a-B were characteristic for free radical oxidation. OH P and HO P2 radicals were trapped in solution containing 2a-Py, and t BuOO P free radicals were detected in solution in the presence of 3a-B. The determined rates of self-decomposition of complexes 2a-B, 2b-B and 3a-B can be used for evaluation of the upper limit for their reactivity towards organic substrates. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Ferric hydroperoxo complexes; Ferric alkylperoxo complexes; EPR; NMR spectroscopy; Reactivity studies
1. Introduction There have been several efforts aimed at modelling of the alkane functionalization chem)
Corresponding author. Tel.: q7-383-234-1877; fax: q7-383234-3056. E-mail address: [email protected] ŽE.P. Talsi..
istry of nonheme iron enzymes; most prominent of these are the catalysts known as the ‘‘Gif’’ systems w1–3x, wFe 2 OŽ bpy. 4 x 4qrt BuOOH w4x and wFeŽTPA. Clx 2qrt BuOOH w5,6x combinations ŽTPA — trisŽ2-pyridylmethyl. amine. . Two alternative mechanisms of alkane oxidation by these catalytic systems are supposed now in the literature. According to one of them,
1381-1169r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 1 - 1 1 6 9 Ž 0 0 . 0 0 1 9 7 - 7
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metal based oxidant, either a metal-peroxide intermediate or a high-valent iron-oxo species derived therefrom, must participate in the alkane functionalization reactions w1–5x. According to the other, alkylperoxy or alkoxy radicals are supposed to be reactive intermediates w7–10x. Since high-valent iron-oxo intermediates have never been observed spectroscopically for nonheme catalytic systems, the assumption that ferric-hydroperoxide could be itself a highly reactive species is rather spread in the literature Žsee Refs. w11–13x.. Nevertheless, this possibility requires much more experimental and theoretical studies. During the last 5 years, molecular composition of several low-spin alkylperoxo and hydroperoxo iron intermediates wFeŽ BLM. ŽOOH.x Žactivated bleomycin. w14–16x, wFe IIIŽ bpy . 2 Ž OO t Bu . HOR x 2q w17 x, wFe III Ž TPA . ŽOOt Bu.ŽHOR.x 2q w18x, wFe III ŽN4Py.Ž OOH.x 2q Žwhere N4Py is N-ŽbisŽ2-pyridyl. -methyl.-N, NbisŽ 2-pyridylmethyl. amine. w19x, wFe III Ž Py5. ŽOOH.x 2q Žwhere Py5 is 2,6-bis-ŽbisŽ 2-pyridyl. methoxymethane.pyridine. w20x and wFe III Ž TPA. ŽOOH.x 2q w21,22x has been reliably determined using EPR, resonance Raman and electrospray ionization mass spectroscopies. Some intermediates wFe III Ž TPP.Ž OOH.Ž OH.x w23x, wFe III Ž cyclyd .Ž OOH .x w24 x, wFe III Ž F20TPP .Ž OOC Ž O . Ar.x w25x, wFe III ŽPMA.ŽOOH.x and wFe III Ž PMA. Ž O O t B u .x w 26,27 x , w Fe Ž bpy . 2 Ž O O H . Py x ŽNO 3 . 2 and wFeŽphen. 2 ŽOOH.PyxŽNO 3 . 2 w28x have been characterized by EPR spectroscopy based on their highly characteristic anisotropy of g-tensor. However in all cases, except activated bleomycin FeŽ BLM.Ž OOH. w14,15x, the reactivity data were insufficient to evaluate the impact of these intermediates to the oxidation of organic substrates. In this work we describe our detailed studies of the stability and reactivity of ferric hydroperoxo and alkylperoxo complexes wFeŽbpy. 2Ž OOH. PyxŽ NO 3 . 2 Ž 2a-Py. , wFeŽ phen. 2 Ž OOH. PyxŽ NO 3 . 2 Ž 2b-Py. and wFeŽ bpy. 2 Ž OOt Bu. CH 3 CNxŽNO 3 . 2 Ž 3a-CH 3 CN.. These complexes
were previously found in the reaction of HOOH with wFe 2 OŽ bpy. 4 P 2H 2 OxŽ NO 3 . 4 Ž 1a. and wFe 2 OŽphen. 4 P 2H 2 OxŽNO 3 . 4 Ž1b. in PyrAcOH mixture as a solvent w28x, and in the reaction of t BuOOH with 1a in ROHrCH 3 CN mixture w17x.
2. Experimental part 2.1. Materials Cyclohexane, methyl phenyl sulfide, pyridine, 3-Me–, 4-Me–, 3-Br–, 2-Me–, 4-dimethylamino pyridines, glacial acetic acid, acetonitrile, methanol and corresponding deuterated solvents were purchased from Aldrich and used without additional purification. Purchased cyclohexene was purified by double fractional distillation just before using. Hydrogen peroxide 30% was concentrated in vacuum up to 90–95% and titrated with KMnO4 just before experiments. Tert-butyl hydroperoxide and deuterated tert-butyl hydroperoxide Žd 9-t BuOOH. were obtained from tert-butyl alcohol and deuterated tert-butyl alcohol Ž d 9-t BuOH. according to procedure described in Refs. w29,30x. Complexes 1a and 1b were prepared as described in Refs. w31,32x, respectively. 2.2. EPR monitoring of hydroperoxo complex 2a-Py in the catalytic system 1a r HOOHr AcOH r Py To start the reaction, 0.2 ml of 0.01 M solution of complex 1a in 2:1 PyrAcOH molar mixture or in this mixture containing cyclohexane was added to Žfrozen in liquid nitrogen. 0.02 ml of 90% HOOH Ž370 equiv.. directly in an EPR tube. Then the sample was immersed in a constant-temperature bath with water and solutions of 1a and HOOH were mixed. To stop the reaction, the EPR tube was immersed in liquid nitrogen followed by registration of EPR
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spectrum at y1968C. Thus, dependencies of 2a-Py concentration versus time were obtained. 2.3. Preparation of the solutions of 2a-Py, 2b-Py and 3a-CH3 CN with reduced concentration of peroxides A mixture of 0.6 ml of 1:1 Pyr90% HOOH volume was added dropwise into 1 ml of 0.01 M solution of complex 1a or 1b in 2:1 Pyr AcOH molar mixture at y208C Žcatalyst:oxidant ratio s 1:1100 equiv.. . The sample was warmed up to 08C and kept 5–10 min at this temperature to rich 10y3 M concentration of 2a-Py or 2b-Py. Then the solution was mixed with 20 ml of diethyl ether at y608C and kept 30–40 min at this temperature to obtain precipitate, containing 2a-Py or 2b-Py as an admixture. After that, the liquid part was decanted and the precipitate was again washed with 20 ml of diethyl ether at y608C. The washing procedure was repeated three times and then residual diethyl ether was removed under vacuum. The residue was dissolved in appropriate solvent system at low temperature to give solutions of w F e Ž b p y . 2 Ž O O H . B xŽ N O 3 . 2 Ž 2 a -B . o r wFeŽphen. 2 ŽOOH.BxŽNO 3 . 2 Ž2b-B. with the reduced concentration of HOOH. Concentration of residual HOOH Ž monitored by iodometric titration. was less than 10y2 M and that of 2a-B or 2b-B Žmonitored by EPR. was about 2 = 10y4 M. To study the interaction with organic substrates, the solution containing 2a-Py or 2b-Py was sampled into several EPR tubes at low temperature. Some of these tubes contained the appropriate amount of a substrate. The reaction was started by immersing all tubes into a constant-temperature bath with ethyl alcohol. The solutions in EPR tubes were thoroughly mixed. The reaction was stopped by simultaneously freezing all the samples in liquid nitrogen. Then EPR spectra of hydroperoxo intermediates were recorded at y1968C. The reaction was started again by immersing all the tubes in the bath. Thus obtained kinetic curves for complexes 2aPy or 2b-Py decomposition in the presence and
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in the absence of various substrates can be reliably compared. The procedure used for preparation of 3aCH 3 CN samples with reduced concentration of t BuOOH was similar to that described above. In this case, the interaction of 1a Žw1ax s 0.01 M. with 500 equiv. of t BuOOH was performed in 1:4 MeOHrCH 3 CN molar mixture at y408C. The characteristic green color indicated formation of 3a-CH 3 CN. Solid samples containing 3a-CH 3 CN were isolated by the same procedure as in the case of 2a-Py. 2.4. EPR measurements EPR spectra Žy1968C. were recorded at 9.2–9.3 GHz on a Bruker ER-200D spectrometer in glass cylindrical tubes Ž d s 5 mm. . They were simulated using an extended version of the EPR 1 program, described in Ref. w33x. Measurements were made in a quartz Dewar with liquid nitrogen. The dual EPR cavity furnished with the spectrometer was used. Periclase crystal ŽMgO. , with impurities of Mn2q and Cr 3q, served as a side reference, was placed into the center of the second compartment of the dual cavity. EPR signals were quantified by double integration with copper chloride as a standard at y1968C. A flat quartz cell was used for registration of EPR spectra of liquid samples at room temperature. 2.5. NMR measurements 2
D NMR spectra were recorded at 61.40 MHz in 10-mm glass sample tubes, using pulsed FT-NMR technique, with a Bruker MSL-400 NMR spectrometer. The typical operating conditions used for 2 D NMR measurements were as follows: sweep width 3 kHz; spectrum accumulation frequency 2.5 Hz; number of transients 5000–10 000; 458 pulse at 10 ms. 1 H NMR spectra were monitored on the Bruker MSL-400 NMR spectrometer at 400.13 MHz and a Bruker DPX-250 NMR instrument at 250.10 MHz as well.
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2.6. The analysis of the oxidation products The yield of the reaction products Žcyclohexanone, cyclohexanol, cyclohexenone and cyclohexenol. was measured using GC technique Žchromatograph ‘‘Tsvet 500’’, FID, Ar, a steel column Ž2 m = 3 mm. packed with 15% Carbowax 20M on Chromaton N-AW-HMDS. . Probes Ž 0.5 ml. of the reaction solutions were sampled, followed by the extraction of the reaction products with diethyl ether Ž 3 = 1 ml.. The diethyl ether layers were combined; styrene was added as an internal standard. Diethyl ether was evaporated and the mixture was analyzed by GC. Products were identified and quantitated by comparison with authentic samples.
3. Results and discussion 3.1. Characterization of hydroperoxo complexes 2a-B and 2b-B The intensities of the EPR signals of complexes wFeŽbpy. 2 Ž OOH.PyxŽNO 3 . 2 Ž2a-Py. and wFeŽphen. 2 ŽOOH.PyxŽNO 3 . 2 Ž2b-Py. ŽFig. 1a and b. correspond to their yield as much as 20% based on starting complexes 1a or 1b. The g-values of complexes 2a-Py Ž2.14, 2.11, 1.97. and 2b-Py Ž2.13, 2.12, 1.97. were rather close to those for recently well characterized hydroperoxo complexes wFe III Ž N4Py.Ž OOH.x 2q Ž2.17, 2.12, 1.98. w19x, wFe III Ž Py5.Ž OOH.x 2q Ž2.15, 2.13, 1.98. w20x and wFe III ŽTPA.ŽOOH.x 2q Ž2.19, 2.14, 1.97. w21x. The typical g-values for stable low-spin ferric complexes with O,N-donor ligands containing no OOH or OOR moieties are in the range g 1 s 2.4–2.6, g 2 s 2.14–2.2, g 3 s 1.85–1.95 w34,35x. The observed low anisotropy of g-factors for the complexes 2a-Py and 2b-Py is a highly characteristic property of low-spin ferric hydroperoxo or alkylperoxo species. The attempts to characterize 2a-Py and 2b-Py by electrospray ionization mass spectrometry were unsuccessful. However, it is unlikely that the supposed molecular composition
Fig. 1. X-band EPR spectra of complexes 2a-Py Ža. and 2b-Py Žb. at y1968C in 2:1 pyridinerAcOH molar mixture. Spectrometer settings: microwave power 20 mW, modulation frequency 100 kHz, modulation amplitude 5 G.
of the complexes 2a-Py and 2b-Py will be essentially refined. Indeed, their monomeric structure is evident from EPR spectra typical of a mononuclear low-spin ironŽIII. center in an octahedral environment. The presence of at least two bipyridine or phenantroline ligands is necessary to obtain low-spin ferric species, the presence of the hydroperoxo moiety is evident from the characteristic anisotropy of g-tensor. Low-spin ferric complexes must be six-coordinated. This sixth ligand is pyridine. A sample of 2a-Py obtained after extraction of HOOH from the reaction mixture and removal of the major part of PyrAcOH Ž the residual concentration of HOOH was ca. 10y2 M and that of AcOH or Py was also not more than 10y2 M, see Section 2. was dissolved in CH 3 CN. The EPR spectrum of this new sample only slightly differs from that of 2a-Py. Thus, the replacement of Py in 2a-Py by CH 3 CN does not occur, otherwise it should lead to noticeable changes in EPR spectral parameters as in the case of related complexes wFeŽbpy. 2OOt Bu.BxŽNO 3 . 2 3a-B. Their EPR spectra are very sensitive to the nature of the sixth ligand ŽCH 3 CN or Py, see Table 1. . The g-values of 2a-Py change insignificantly when Py is replaced by its derivatives B with various p K a
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Table 1 EPR spectroscopic data for low-spin ferric peroxo and alkylperoxo complexes Complex
Solvent
g1
g2
g3
Ref.
wFeŽN4Py.ŽOOH.x2q wFeŽPy5.ŽOOH.x 2q wFeŽTPA.ŽOOH.x2q wFeŽbpy. 2 ŽOOH.Pyx 2q, 2a-Py wFeŽbpy. 2 ŽOOH.Pyx 2q, 2a-Py wFeŽphen. 2 ŽOOH.Pyx 2q, 2b-Py wFeŽ4,4X-Me 2-bpy. 2 ŽOOH.Pyx 2q wFeŽbpy. 2 ŽOOt Bu.CH 3 CNx 2q, 3a-CH 3 CN wFeŽbpy. 2 ŽOOt Bu.MeOHx 2q, 3a-MeOH wFeŽbpy. 2 ŽOOt Bu.H 2 Ox 2q, 3a-H 2 O wFeŽbpy. 2 ŽOOt Bu.Pyx 2q, 3a-Py wFeŽbpy. 2 ŽOOt Bu.Ž3-Br-Py.x 2q, 3a-3-Br-Py wFeŽphen. 2 ŽOOt Bu.CH 3 CNx 2q, 3b-CH 3 CN
CH 3 CN CH 3 CN CH 3 CN PyrAcOH CH 3 CNrPy PyrAcOH CH 3 CNrPy CH 3 CN MeOH CH 3 CN CH 3 CNrPy CH 3 CNr3-Br-Py CH 3 CN
2.17 2.15 2.19 2.14 2.14 2.13 2.14 2.18 2.18 2.18 2.18 2.18 2.16
2.12 2.13 2.14 2.11 2.11 2.12 2.11 2.12 2.16 2.16 2.13 2.13 2.10
1.97 1.98 1.97 1.97 1.97 1.97 1.97 1.98 1.98 1.98 1.975 1.975 1.97
w19xa w20x b w21x c w28x this work w28x this work this work this work this work this work this work this work
a
N4Pys N-ŽbisŽ2-pyridyl.-methyl.-N, N-bisŽ2-pyridylmethyl.amine. Py5 s 2,6-bis-ŽbisŽ2-pyridyl.methoxymethane.pyridine. c TPA s tris-Ž2-pyridylmethyl.amine. b
Ž3-Br-Py, 3-Me-Py, 4-Me-Py or 4-Me 2 N-Py. . This replacement can be readily performed by the addition of B at a concentration of 1 M to a sample of 2a-Py in CH 3 CN Žresidual concentration of Py was less than 10y2 M. . 3.2. Characterization of alkylperoxo complexes 3a-B Recently, alkylperoxo complexes with supposed structure wFeŽbpy. 2 Ž OOt Bu.ROHxŽClO4 . 2 Ž 3a-ROH. , where ROH s benzyl alcohol, methanol and phenol, were detected by EPR in the reaction of 1a with t BuOOH in CH 3 CNr ROH mixtures. The presence of OOt Bu moiety in 3a-ROH was confirmed by resonance Raman spectroscopy w17x. However, the EPR signal Ž g s 2.18, 2.12, 1.98. attributed in Ref. w17x to 3a-ROH belongs in fact to complex 3a-CH 3 CN with CH 3 CN molecule on the sixth coordination site ŽFig. 2c.. Complex 3a-MeOH Ž g s 2.18, 2.16, 1.98. can be obtained in neat methanol ŽFig. 2a., while in methanolracetonitrile mixtures the complexes 3a-MeOH and 3a-CH 3 CN both are present ŽFig. 2b.. Complexes 3a-B ŽB is a donor molecule. can be prepared by dissolution of a sample, containing 3a-CH 3 CN in CH 3 CNrB solvent system at y408C. EPR pa-
rameters of complexes 3a-B are presented in Table 1. It is seen that g-factors for complexes 3a-MeOH and 3a-H 2 O are close. Further 2 D NMR spectroscopic studies provide new insight into the structures of the 3a-B intermediates, formed in the reaction of 1a with t BuOOH in CH 3 CNrMeOH mixtures. When deuterated d 9-t BuOOH was added to the solution of 1a in 1:3 MeOHrCH 3 CN molar mixture at y358C, two weak resonances at y3.1 and at y3.7 ppm along with an intense signal of d 9-
Fig. 2. X-band EPR spectra of complexes 3a-MeOH and 3aCH 3 CN formed in the reaction of 1a Ž5=10y3 M. with 200 equiv. of t BuOOH at y358C in various MeOHrCH 3 CN molar mixtures: Ža. in neat MeOH; Žb. 1:1 MeOHrCH 3 CN; Žc. 1:40 MeOHrCH 3 CN. The spectra were recorded at y1968C. The signal denoted by asterisks belongs to t BuOO P radical.
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t BuOOH and d 9-t BuOH at 1.22 ppm were observed in the 2 D NMR spectrum ŽFig. 3a.. The intensities of these resonances correlated with the intensities of the EPR signals for the complexes 3a-MeOH and 3a-CH 3 CN detected in the same sample. The signals at y3.1 and y3.7 ppm can be ascribed to the alkylperoxo moieties of 3a-MeOH and 3a-CH 3 CN respectively. The ratio of their intensities in 2 D NMR spectra was proportional to that of MeOH and CH 3 CN concentrations. Addition of water to the reaction mixture afforded a new signal at y2.5 ppm attributed to the t BuOO group of 3a-H 2 O ŽFig. 3b.. The intensity of the particular resonance was found to be proportional to the concentration of the corresponding ligand Ž MeOH, CH 3 CN or H 2 O. in solution. It implies the occurrence of equilibria between the complexes 3a-MeOH, 3a-CH 3 CN and 3a-H 2 O, as shown in Eqs. Ž1. – Ž 3. 3a-MeOHq CH 3 CN
° 3a-CH CN q MeOH K1
3
Ž1.
° 3a-H O q MeOH 3a-CH CN q H O ° 3a-H O q CH CN 3a-MeOHq H 2 O
K2
2
K3
Ž2.
Fig. 3. 2 D NMR spectra recorded in the course of the reaction of 1a Žw1ax s 0.01 M. with 100 equiv. of d 9-t BuOOH at y358C Ža. in 1:3 MeOHrCH 3 CN molar mixture; Žb. in 1:1.5 MeOHr CH 3 CN molar mixture, where H 2 O ŽwH 2 Ox s1 M. was added.
tively w36x. The 1 H NMR resonances of bipyridine ligands of complexes 3a-B were either too broad to be observed or masked by signals of the initial complex 1a and by those of undeuterated admixtures in the solvent and peroxide used.
Ž3.
3.3. Stability of hydroperoxo complexes 2a-B and 2b-B
According to EPR data the time of establishing of these equilibria was 5–10 min at y358C. The equilibrium constants K 1 –K 3 determined from 2 D NMR spectra Žat y358C. have the following values: K 1 s 0.5 " 0.1; K 2 s 8 " 1; K 3 s 16 " 2. It is noteworthy that the substitution of MeOH by H 2 O molecule on the sixth coordination site of 3a-MeOH gives rise to noticeable change in the chemical shift of the alkylperoxo moiety while the difference between EPR parameters of 3a-MeOH and 3aH 2 O is negligible. The NMR spectra of Fig. 3 is the first example of the NMR detection of the alkylperoxo moiety for low-spin ferric alkylperoxo species. The reported chemical shifts for a and b deuterium atoms of OOCD 2 CD 3 group in the high-spin alkylperoxo complex FeŽ TPP. ŽOOCD 2 CD 3 . were 180 and 4 ppm, respec-
First of all, let us consider the results of EPR monitoring of the hydroperoxo intermediate 2aPy in the catalytic system 1arHOOHrPyr AcOH. Fig. 4a shows the time-dependence of the 2a-Py concentration in the course of the reaction of 1a with HOOH at 208C in 2:1 PyrAcOH mixture Žw1ax s 0.01 M, wHOOHx s 3 M.. It is seen that the concentration of complex 2a-Py reached maximum during 3 h, was in steady state for 1 h, and then decreased by a pseudo-first-order kinetics with the rate constant Ž3 " 1. = 10y4 sy1. The steady-state concentration of 2a-Py dropped 5 " 0.5 times if C 6 H 12 ŽwC 6 H 12 x s 1 M. had been previously added to a sample Ž Fig. 4b.. The value of this drop depended on the cyclohexane concentration and was 3 " 0.6 at
3
2
2
3
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Fig. 4. Concentration of 2a-Py as a function of time in the reaction of 1a Žw1ax s 0.01 M. with 90% HOOH ŽwHOOHx o s 3 M. at 208C in 2:1 PyrAcOH Ža. and in this mixture, containing C 6 H 12 Ž1 M. Žb.. The sample in Ža., if C 6 H 12 ŽwC 6 H 12 x s1 M., was added 4 h after the reaction beginning Žc.. The difference between curves Ža. and Žc. before addition of C 6 H 12 shows the level of reproducibility of the samples.
wC 6 H 12 x s 0.5 M and 2 " 0.4 at wC 6 H 12 x s 0.25 M. The addition of cyclohexane ŽwC 6 H 12 x s 1 M. to the sample of Fig. 4a at a moment when the steady-state concentration of 2a-Py had already been established, gave rise to the rapid decrease of the concentration of 2a-Py down to the value corresponding to the presence of cyclohexane in solution initially Ž Fig. 4c. . This conversion followed a first-order kinetics with k s Ž1.5 " 0.5. = 10y3 sy1. The concentration of 2a-Py is determined by the competition of the rates of its formation Ž W1 . and decay Ž W2 .. The effect of cyclohexane ŽFig. 4c. could result either from decrease of W1 or increase of W2 . To distinguish between these two possibilities, it is necessary to measure the rate of self-decomposition of 2a-Py and determine the influence of organic substrates on this rate. The reasonable way to measure self-decay kinetics of 2a-Py is to dramatically diminish the concentration of HOOH in the reaction solution at low temperature and thus sharply decrease the rate of 2a-Py formation. The subsequent monitoring of the decrease of the concentration of 2a-Py at higher temperatures allows determination of the rate of its self-decomposition. This procedure will give the real value of the rate of
239
2a-Py self-decomposition, if the loss of 2a-Py concentration during the removal of HOOH at low temperature is markedly smaller than the decrease of the concentration of HOOH. The extraction of peroxide from reaction mixture by diethyl ether at low temperature Žsee Section 2. permits fulfillment of these requirements. The loss of the 2a-Py concentration during extraction of HOOH Žfrom 2 = 10y3 to 2 = 10y4 M. was markedly smaller than the decrease of HOOH concentration Ž from 3 to 10y2 M. . The similar procedure was used for preparation of complexes 2b-Py and 3a-CH 3 CN. Further, if not especially noted, the data for samples with the reduced concentration of peroxides will be present. The procedure described above allows us to obtain 2a-Py, 2b-Py and 3a-CH 3 CN only as small admixtures Ž 1–3%. to other ferric complexes. According to 1 H NMR data, in the case of 2a-Py and 2b-Py, the major part of iron species represents trinuclear basic ironŽIII. acetate wFe 3Ž m 3-O.ŽCH 3 CO 2 . 6 Py3 xŽNO 3 . formed upon dissolution of the Ž m-oxo. diironŽ III. complexes wFe 2 OŽ bpy. 4 P 2H 2 OxŽ NO 3 . 4 and wFe 2 OŽphen. 4 P 2H 2 OxŽNO 3 . 4 in pyridineracetic acid mixture. Its identity was confirmed by 1 H NMR, using wFe 3Ž m 3-O.ŽCH 3 CO 2 . 6 Py3 xŽClO4 . independently prepared as described in Ref. w37x. Complex 3a-CH 3 CN can be obtained as an admixture to the initial complex 1a wFe 2 OŽbpy. 4 P 2H 2 OxŽNO 3 . 4 . Fortunately, all iron species except alkylperoxo and hydroperoxo intermediates display no signals near g s 2 and thus the concentration of these intermediates can be readily monitored by EPR. The obtained kinetic data for self-decay of 2a-B, 2b-Py and 3a-B under various conditions are collected in Table 2. Complex 2a-Py dissolved in Py or PyrAcOH mixture was stable at y608C and decayed with a first-order kinetics at higher temperatures Ž Fig. 5.. The rate constants of this decay determined at various temperatures are the same in Py and in 2:1 PyrAcOH mixture and have the following values: k s 5 = 10y4 sy1 Žy278C.; 1 = 10y3 sy1 Ž y208C. ; 2.8 = 10y3 sy1
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Table 2 First order rate constants of low-spin ferric hydroperoxo and alkylperoxo complexes self-decay a Complex wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.Ž3-Br-Py.xŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.Ž3-Me-Py.xŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.Ž4-Me-Py.xŽNO 3 . 2 wFeŽbpy. 2 ŽOOH.Ž4-Me 2 NPy.xŽNO 3 . 2 wFeŽ4,4X-Me 2-bpy. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽphen. 2 ŽOOH.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.CH 3 CNxŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.CH 3 CNxŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.CH 3 CNxŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.MeOHxŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.PyxŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.Ž3-Br-Py.xŽNO 3 . 2 wFeŽbpy. 2 ŽOOt Bu.Ž4-Me 2 N-PyxŽNO 3 . 2
Temperature Ž8C. y27 y20 y10 5 5 20 20 20 20 20 20 20 y37 y30 y20 y35 y30 y30 y30
Solvent system b
PyrAcOH PyrAcOH b PyrAcOH b PyrCH 3 CN c PyrH 2 O c 3-Br-PyrCH 3 CN d PyrCH 3 CN c 3-Me-PyrCH 3 CN e 4-Me-PyrCH 3 CN f 4-Me 2 N-PyrCH 3 CN g PyrCH 3 CN c PyrCH 3 CN c CH 3 CN CH 3 CN CH 3 CN MeOH PyrCH 3 CN c 3-Br-PyrCH 3 CN d 4-Me 2 N-PyrCH 3 CN g
k = 10 3 Žsy1 . 0.5 1.0 2.8 0.5 2.0 0.4 2.0 2.0 2.0 6 6 8 1.0 2.5 5.8 2.4 2.3 2.0 3.0
a
All of the constants were determined with error less than 20%. 3:1 PyrAcOH volume mixture. c wPyx s 1 M. d w3-Br-Pyx s 1 M. e w3-Me-Pyx s 1 M. f w4-Me-Pyx s 1 M. g w4-Me 2 N-Pyx s 1 M. b
Žy108C., Ea s 11.6 " 3 kcalrmol Ž the initial concentration of 2a-Py is 10y4 to 5 = 10y4 M..
Fig. 5. Ža. Low-field part of the EPR signal of complex 2a-Py in the course of its self-decay in 2:1 PyrAcOH molar mixture at y238C. The interval between the spectra is 10 min. Inset Žb.: Plot of the kinetic data from Ža. after applying the equation lnŽ Cr C0 . sy kt.
The nature of the solvent markedly perturbs the stability of 2a-Py. Compare k s 5 = 10y4 sy1 Ž58C. in CH 3 CNrPy ŽwPyx s 1 M., k s 7 = 10y3 sy1 Ž58C. in Py and k s 2 = 10y3 sy1 Ž58C. in H 2 OrPy ŽwPyx s 1 M. . Note that the rate constant of self-decay of activated bleomycin k s 5.8 = 10y3 sy1 Ž48C. in H 2 O w14x is rather close to that for the decay of 2a-Py Ž k s 2 = 10y3 sy1 . at 58C in H 2 OrPy ŽwPyx s 1 M.. To elucidate the influence of the basicity of the sixth ligand B on the rate of the decay of 2a-B, the following derivatives of Py with various p K a values shown in brackets were used w38x: 3-Br-Py Ž2.84. , Py Ž 5.23. , 3-Me-Py Ž 5.68. , 4-Me-Py Ž6.02. and 4-Me 2 N-Py Ž9.58.. Py and its derivatives were added into the aliquots of the same solution of 2a-Py in CH 3 CN Žto make wBx s 1 M.. The following rate constants were obtained at q208C: 4 = 10y4 sy1 Ž3-Br-Py.; 2 = 10y3 sy1 ŽPy.; 2 = 10y3 sy1 Ž3-Me-Py.; 2
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= 10y3 sy1 Ž4-Me-Py.; 6 = 10y3 sy1 Ž4-Me 2N-Py.. It is seen that with the increase of basicity of B, the rate constant of 2a-B self-decay noticeably grows. The similar ‘‘push effect’’ of B was previously observed for heterolytic decay of acylperoxo ironŽIII. porphyrin complexes in CH 2 Cl 2 , that affords oxoferryl cation radical complexes ŽPor qP.Fe IV s O w39x. The replacement of 2,2X-bipyridine ligands in 2a-Py by 4,4X-dimethyl-2,2X-bipyridine ligands results in threefold increase of the rate constant of its decay at 208C in CH 3 CNrPy ŽwPyx s 1 M.. This observation is in agreement with the reported data for acylperoxoironŽ III. porphyrin complexes, where introduction of electron donating substituents at the meso position of the porphyrin ring accelerates the rate of their selfdecay w39x. Unfortunately, in contrast to the iron porphyrin systems, we have no data on the structure of intermediates of the hydroperoxo complexes decomposition. The hydroperoxo complex 2b-Py with phenantroline ligands is less stable than the hydroperoxo complex 2a-Py with bipyridine ligands. The rate constant of the 2b-Py decomposition in CH 3 CNrPy ŽwPyx s 1 M. at 208C is 8 = 10y3 sy1 versus 2 = 10y3 sy1 for 2a-Py in the same solvent and at the same temperature. 3.4. ReactiÕity of the hydroperoxo complexes 2a-B and 2b-B To study the reactivity of the hydroperoxo complexes 2a-Py and 2b-Py, a substrate Žcyclohexane, cyclohexene, methyl phenyl sulfide. was added to their solutions in Py or CH 3 CNrPy ŽwPyx s 1 M. at y108C to q208C. The initial concentration of 2a-Py or 2b-Py was 2 = 10y4 to 5 = 10y4 M. It was found that the presence of listed substrates in solution Ž concentrations were up to 3 M. did not noticeably perturb the rate of self-decay of 2a-Py and 2b-Py. This rather unexpected result is consistent with the previous data obtained for activated bleomycin, where DNA also does not affect the rate of activated bleomycin self-decay
241
w14x. Thus, 2a-Py and 2b-Py do not directly react with organic substrates and the observed decrease of the quasi steady-state concentration of 2a-Py after the addition of cyclohexane into the catalytic system 1arHOOHrPyrAcOH ŽFig. 4. is not caused by the increase of the rate of complex 2a-Py decay ŽW2 ., but by the decrease of the rate of its formation Ž W1 .. It is still not entirely clear how cyclohexane can perturb W1. If it is granted that free radicals ŽOH P, HO 2P . participate in formation of 2a-Py, the possible way of reducing W1 is the decrease of the concentration of free radicals in solution via their reaction with cyclohexane. As a rule, alkylperoxo and hydroperoxo ironŽIII. intermediates were detected only at high excesses of oxidants Ž ROOH, HOOH. , when a number of unidentified substrate-sensitive reactions could determine their concentrations. Thus, the attempts to derive data on the reactivity of LFeŽOOH. or LFeŽOOR. species from the drop of their quasi steady-state concentrations after the addition of a substrate to the reaction mixture w25,27,28x were inadequate. The observed pseudo-first-order decrease of the concentration of wFe III ŽTPA.Ž OOt Bu. ŽHOR.x 2q w18x can reflect the kinetics of the decomposition of excessive t BuOOH rather than the kinetics of the decay of the alkylperoxo intermediate, and thus the effect of cyclohexanol on this kinetics can not be connected with the reactivity of the alkylperoxo intermediate. The only informative reactivity study of an hydroperoxo intermediate is the study of the activated bleomycin Ž Fe III Ž BLM.Ž OOH.. , which is believed to be responsible for the oxidative damage of DNA under aerobic conditions by Fe II-BLM w14,15x. In contrast to the other mentioned intermediates, detected only at the excess of oxidants, activated bleomycin can be prepared via stoichiometric oxidation of Fe II-BLM by dioxygen and thus kinetics of its self-decay can be reliably measured. As was mentioned, DNA did not affect the rate of activated bleomycin self-decay, however the rate of accumulation of DNA degradation products coin-
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cided kinetically with the decay of activated bleomycin. The yield of oxidation products achieved 50% of the concentration of activated bleomycin decomposed w14x. This evidences in favour of the rate determining conversion of Fe III ŽBLM.ŽOOH. to another active species prior to the reaction with substrate. The results of this part of our study are the determination of the rates for self-decomposition processes of the complexes 2a-Py, 2b-Py and demonstration of the fact that organic substrates do not perturb these rates. These data allow evaluation of the upper limit for the rates of the reaction of 2a-B or 2b-B with organic substrates. The maximal values of these rates are not more than those of the rates of complexes 2a-B or 2b-B self-decomposition. In the opposite case, the addition of substrates would noticeably perturb the decay of these species. Secondly, the obtained data show that organic substrates can influence the steady-state concentration of the hydroperoxo and alkylperoxo ironŽIII. intermediates diminishing the rate of their formation rather than increasing the rate of their decay. This possibility should be taken into account in reactivity studies of peroxo iron species. The main products of cyclohexane oxidation by the catalytic system 1arHOOHrPyrAcOH Žthe sample of Fig. 4. are cyclohexanone and cyclohexanol in 1:1 ratio. Catalyst:substrate ratio was 1:100 equiv., ketone and alcohol were formed in 4% yield each based on substrate. The oxidation of cyclohexene by this system affords products of allylic oxidation Ž cyclohexenone and cyclohexenol, their yields were 4% and 3%, respectively. . This reactivity pattern is typical for oxidation by free radicals. The same oxidation products are observed for samples with reduced concentration of HOOH. It should be noted that the residual concentration of HOOH Ž10y2 M. was much larger than the concentration of 2a-Py Ž10y4 M.. The overall concentration of cyclohexane oxidation products in these samples was 3- to 10-fold larger than that of 2a-Py. Thus, we cannot exclude that the
Fenton decomposition of residual HOOH affords the major part of the products. We have applied a spin-trapping technique for the characterization of transient free radicals in a solution containing complex 2a-Py in CH 3 CN. The 3,3,5,5-tetramethyl-1-pyrroline N-oxide ŽTMPO. purchased from Sigma was used as a spin trap. The nitrone spin traps DMPO and TMPO are widely used to provide evidence for the involvement of free radicals in many reactions in chemical and biological systems w40–42x. They are particularly useful for identifying oxygen-centered radicals, e.g. superoxide radical anion, peroxyl, alkoxyl and hydroxyl radicals. Similarly to all nitrones, TMPO captures short-living free radicals to give more stable nitroxyl radicals Žspin adducts.. The bhydrogen hyperfine splitting constants Ž a H . of spin adducts vary markedly with the nature of the radical trapped. Thus radicals can be recognized w40,41x. EPR spectrum recorded 3 min after the addition of TMPO Ž to make its concentration 0.1 M. to a sample containing 2a-Py in CH 3 CN at 208C ŽFig. 6a. is a superposition of the spectra of two adducts: aN s a H s 14 G and a N s 13 G, aH s 6.5 G with relative weights 1:2. The relative weight of the spectrum with a N s 13 G, a H s 6.5 G grows with time Ž Fig. 6b and c.. According to hyperfine splitting constants the signal with aN s a H s 14 G belongs
Fig. 6. X-band EPR spectra of TMPO spin adducts in a sample with reduced content of HOOH Žca. 10y2 M. containing 2P10y4 M 2a-Py in CH 3 CN at various moments of time after addition of TMPO Žto make its concentration 0.1 M. at 208C: Ža. 3 min, Žb. 11 min, Žc. 27 min.
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to spin adduct TMPOr POH. Formation of this adduct was observed for the catalytic system HOOHq CoŽacac. 2 in CH 3 CN w43x. In this system Fenton decomposition of hydrogen peroxide takes place. The EPR spectrum with a N s 13 G, a H s 6.5 G is characteristic for the adduct TMPOrHOP2 w41x. Thus POH and HO P2 free radicals can be trapped in the solution of 2a-Py P P in CH 3 CN. Recently, Oy 2 rHO 2 free radicals were trapped in a water solution of activated bleomycin w35x. Note, that the attempts to trap free radicals directly in the catalytic system 1arHOOHrPyrAcOH were unsuccessful due to rapid decomposition of TMPO. Free t BuOO P radicals can be directly observed in EPR spectra in the course of the reaction of 1a with t BuOOH in CH 3 CN. Their EPR spectrum in a frozen solution represents a sharp signal with axial anisotropy of g-factor, gIs 2.04, g Hs 2.008 ŽFig. 2c, the signal denoted by asterisks.. 3.5. Stability and reactiÕity of alkylperoxo complexes 3a-B The alkylperoxo complex 3a-CH 3 CN obtained after extraction of t BuOOH from a reaction mixture rapidly disappears even at low temperatures with the following rate constants: k s 10y3 sy1 Ž y378C. ; 2.5 = 10y3 sy1 Ž y308C.; 5.8 = 10y3 sy1 Žy208C.; E a s 14 " 3 kcalrmol. The rate constant of 3a-MeOH self-decay in MeOH was found to be 2.4 = 10y3 sy1 at y358C. The addition of Py or its derivatives ŽwBx s 1 M. to a solution of 3a-CH 3 CN in CH 3 CN gives rise to its conversion to 3a-B monitored by EPR. The rate constants for complexes 3a-B decay in CH 3 CNrB ŽwBx s 1 M. at y308C were as follows: 2.0 = 10y3 sy1 Ž3-Br-Py.; 2.3 = 10y3 sy1 ŽPy.; 3 = 10y3 sy1 Ž4-Me 2 N-Py.. It is seen that the basicity of the sixth ligand B only slightly perturbs the rates of decomposition of alkylperoxo species 3a-B in contrast to hydroperoxo species 2a-B Žsee Section 3.3.. The similar small push effect was reported for homolytic decomposition of
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acylperoxoironŽ III. porphyrin in toluene that affords oxoferryl complex Ž Por.Fe IV s O w39x. The alkylperoxo species are far less stable than corresponding hydroperoxo intermediates, k s 1.2 = 10y2 sy1 Ž3a-CH 3 CN in CH 3 CN at y108C. and 2 = 10y4 sy1 Ž2a-Py in CH 3 CN at y108C.. The different effect of axial ligation on the rate of self-decomposition of hydroperoxo Ž2a-B. and alkylperoxo Ž3a-B. species could provide evidence in favor of different mechanisms of their decay. However, we have still not obtained any additional support for this assumption. The addition of cyclohexane, cyclohexene or methyl phenyl sulfide in concentrations up to 3 M to the reaction mixture does not perturb the rate of the complex 3a-CH 3 CN decomposition at y258C. It indicates that 3a-CH 3 CN does not directly react with these organic substrates similarly to the hydroperoxo complexes 2a-Py and 2b-Py. GC analysis reveals mainly cyclohexanone and cyclohexanol in 1:1 ratio as the products of cyclohexane oxidation with solution of 3a-CH 3 CN in CH 3 CN Žobtained after extraction of t BuOOH. at y208C. As the overall concentration of the products was not more than the initial concentration of 3a-CH 3 CN, its decomposition could produce an intermediate responsible for oxidation of the substrate. Based on 1:1 ratio of ketone to alcohol typical of free radical oxidation, this intermediate could be t BuO P radicals. The catalytic system 1ar t BuOOHrCH 3 CN also oxidizes cyclohexane mainly into cyclohexanol and cyclohexanone in 1:1 ratio at 08C. Probably, the reactive species of this oxidation are t BuO PŽ t BuO P2 . radicals formed in the course of t BuOOH decomposition. t BuO P2 radicals at a concentration of ca. 10y5 M were observed in the catalytic system 1artBuOOHrCH 3 CN ŽFig. 2c..
4. Conclusions The first-order rate constants of self-decomposition of hydroperoxo and alkylperoxo com-
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plexes wFeŽ bpy. 2 Ž OOH. PyxŽ NO 3 . 2 Ž 2a-Py. , wFe Ž phen . 2 Ž OOH . Py xŽ NO 3 . 2 Ž 2b-Py . and wFeŽbpy. 2 ŽOOt Bu.CH 3 CNxŽNO 3 . 2 Ž3a-CH 3 CN. were determined in the presence of various substrates and at various temperatures. The rate of decomposition of hydroperoxo complexes 2a-B, where B are derivatives of Py Ž 3-Br-Py, 3-Me-Py, 4-Me-Py and 4-Me 2 N-Py. increases with the growth of basisity of B Ž push effect. . Such an effect is markedly smaller for alkylperoxo species 3a-B. 2 D NMR signals of t BuOO moieties of low-spin ferric alkylperoxo complexes 3a-CH 3 CN, 3a-CH 3 OH and 3a-H 2 O were detected for the first time. The addition of organic substrates Žcyclohexane, cyclohexene, methyl phenyl sulfide. at a concentration up to 3 M at y108C to q208C does not noticeably change the rate of self-decomposition of 2a-B, 2b-Py and 3a-B. Thus the intermediates concerned do not directly react with organic substrates. The reactivity patterns of 2a-B, 2b-Py and 3a-B were characteristic for free radical oxidation. The determined rates of self-decomposition of complexes 2a-B, 2b-B and 3a-B can be used for evaluation of the upper limit for their reactivity towards organic substrates. To date there is an example w21x, when a low-spin ferric hydroperoxo intermediate wFeŽ TPA .Ž OOH .x 2q detected in wFeŽ TPA . ŽCH 3 CN. 2 xŽClO4 . 2rHOOHrCH 3 CN system is believed to be responsible for epoxidation of 1-hexene that is typical for non radical oxidation. Further it would be interesting to study the influence of 1-hexene on the rate of wFeŽTPA.ŽOOH.x 2q self-decay and to correlate the rate of this decay with the rate of epoxide formation to elucidate whether this hydroperoxo complex directly reacts with olefin or via formation of another reactive species.
Acknowledgements This work was supported by Russian Fund of Basic Research, grant no.97-03-32495a and by Haldor Topse ArS.
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