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Letter V
EPR Spectroscopic Detection of the Elusive Fe =O Intermediates in Selective Catalytic Oxofunctionalizations of Hydrocarbons Mediated by Biomimetic Ferric Complexes Oleg Y. Lyakin, Alexandra M. Zima, Denis G. Samsonenko, Konstantin P. Bryliakov, and Evgenii P. Talsi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b00169 • Publication Date (Web): 23 Mar 2015 Downloaded from http://pubs.acs.org on March 25, 2015
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ACS Catalysis
EPR Spectroscopic Detection of the Elusive FeV=O Intermediates in Selective Catalytic Oxofunctionalizations of Hydrocarbons Mediated by Biomimetic Ferric Complexes Oleg Y. Lyakin,a,b Alexandra M. Zima,a,b Denis G. Samsonenko,b,c Konstantin P. Bryliakov,a,b and Evgenii P. Talsi*,a,b a
Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russian Federation Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russian Federation c Nikolaev Institute of Inorganic Chemistry, Pr. Lavrentieva 3, Novosibirsk 630090, Russian Federation b
ABSTRACT: Herein, we report the EPR spectroscopic study of the bioinspired catalyst systems for selective hydrocarbon oxofunctionalizations based on dinuclear ferric complexes with TPA* and PDP* aminopyridine ligands , hydrogen peroxide and acetic acid (TPA* = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine, PDP* = bis(3,5-dimethyl-4-methoxypyridylmethyl)(S,S)-2,2′-bipyrrolidine). Using very low temperatures −75 to −85 °C, the extremely unstable and reactive iron-oxygen intermediates, directly reacting with olefins even at −85 °C have been detected for the first time. Their EPR parameters (g1 = 2.070–2.071, g2 = 2.005–2.008, g3 = 1.956–1.960) were very similar to those of the known oxoiron(V) complex [(TMC)FeV=O(NC(O)CH3)]+ (g1 = 2.053, g2 = 2.010, g3 = 1.971, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). On the basis of EPR and reactivity data, the detected intermediates were assigned to the FeV=O active oxidizing species of the catalyst systems studied. KEYWORDS: bioinspired catalysis, enantioselective epoxidation, EPR, iron, mechanism
The selective oxidation of organic molecules is fundamentally important to life.1a While in Nature, metalloenzyme-catalyzed oxidations often exhibit exquisite substrate specificity as well as regioselectivity and/or stereoselectivity, man-designed synthetic bioinspired, or biomimetic, systems may have broader substrate scope and tunable selectivity, which makes them challenging protagonists of environmentally friendly catalytic chemistry of the near future.1 The detection and characterization of the active species of biomimetic catalyst systems, which can also provide important insights into biological pathways, is a hot topic of oxidation chemistry. Catalyst systems based on iron complexes with tetradentate N4-donor ligands, H2O2, and carboxylic acid as an additive have attracted particular attention in the last decade due to high efficiency and selectivity in the preparative oxidations of C–H and C=C groups of various organic molecules.1–3 The systems 1–3/H2O2/CH3COOH (Chart 1) are among the most extensively studied ones.2,3 It is generally agreed that species of the type [(L)FeV=O(OC(O)CH3)]2+ are active intermediates of these catalyst systems (L is the corresponding tetradentate ligand).1e,2b,3c,4 Direct spectroscopic evidence for the putative (L)FeV(O)(OH) oxidant was obtained by Costas, Cronin and coworkers from variable temperature electrospray mass spectrometric experiments in studies of the catalyst system 4/H2O2 (Chart 1).4 However, the active species of the catalyst systems 1–3/H2O2/CH3COOH have not been unambiguously identified so far.
Chart 1. Structures of Aminopyridine Iron Complexes R2
R2
N
N N R1 R2
OTf
II
N
Fe
N
OTf N II Fe N OTf N
OTf
N R1 R2
2
OTf
II
N OTf
N
N
II
Fe
N
OTf OTf
N
R1
R2 = OCH3
N
N III
Fe N
OH OH
III
Fe
5
N
N N
N N
N
N
OCH3
OCH3
OCH3
H3CO
OCH3
OCH3
N
4
3: R1 = R2 = H 3*: R1 = CH3,
R2 = OCH3
H3CO
Fe
R2
1: R1 = R2 = H 1*: R1 = CH3,
N
N N
R1
R1
R1
R1
R1
R1
R1
OCH3 (OTf)4
Fe
III
OH
III
Fe OH
N N
N
N
(OTf)4 OCH3
H3CO
6
Previously, we have observed by EPR very unstable and reactive intermediates 1a–3a in the catalyst systems 1– 3/H2O2/CH3COOH at −70 to −80 °C and assigned them to the
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Table 1. EPR Spectroscopic Data for S = 1/2 Iron Species Formed in the Catalyst Systems Based on Iron Complexes with N4-donor Tetradentate Ligands no
compound V
g1
+
g2
g3
ref
1
[(TMC)Fe =O(NC(O)CH3)]
2.053
2.010
1.971
6a
2
[(TAML)FeV=O]−
1.99
1.97
1.74
6b
3
[(TPA)FeV=O(OC(O)CH3)]2+ (1a)
2.71
2.42
1.53
5
4
[(BPMEN)FeV=O(OC(O)CH3)]2+ (2a)
2.69
2.42
1.70
5
V
2+
5
[((S,S)-PDP)Fe =O(OC(O)CH3)] (3a)
2.66
2.42
1.71
3c
6
[(TPA*)FeV=O(OC(O)CH3)]2+ (5a)
2.070
2.005
1.956
this work
7
[((S,S)-PDP*)FeV=O(OC(O)CH3)]2+ (6a)
2.071
2.008
1.960
this work
8
[(TPA*)FeIII(κ2-OC(O)CH3)]2+ (X5)
2.58
2.38
1.72
this work
2.54
2.41
1.79
this work
9
III
2
2+
[((S,S)-PDP*)Fe (κ -OC(O)CH3)] (X6)
TMC – tetramethylcyclam, TAML – macrocyclic tetraamide ligand.
[(L)FeV=O(OC(O)CH3)]2+ species.3c,5 It should be noted that the EPR parameters of 1a–3a (g1 = 2.7, g2 = 2.4, g3 = 1.5–1.7, see entries 3–5 of Table 1) drastically differed from those of known FeV=O species (Table 1, entries 1, 2),6 and were close to those of low-spin ferric complexes. Relying on these data and on the results of DFT calculations, Wang et al. hypothesized that 1a–3a were ferric-peracetate complexes FeIII– OOC(O)CH3 rather than iron(V)-oxo complexes.7 In 2014, Oloo et al. reported the detection of the intermediate X (g1 = 2.58, g2 = 2.38 and g3 = 1.72) in the catalyst system 1*/H2O2/CH3COOH in acetonitrile at −40 °C.8 In contrast to complex 1 bearing unsubstituted TPA ligand, the TPA* ligand of 1* contains donor substituents at the pyridine rings (Chart 1). By a combination of EPR, Mössbauer, resonance Raman and ESI-MS methods, X was identified as a low-spin acylperoxoiron(III) species [(TPA*)FeIII(κ2-OOC(O)CH3)]2+.8 The rate of decay of complex X at −40 °C was unaffected by the presence of cyclooctene, 1-octene or 2-heptene in the reaction solution, ruling out the assignment of X to the actual oxygen transferring species. So, two important problems remain unresolved: (1) detection of the true oxidizing species of the catalyst system 1*/H2O2/CH3COOH and (2) identification of the intermediates 1a–3a. In this contribution, we report EPR spectroscopic studies of the catalyst systems 5/H2O2/CH3COOH and 6/H2O2/CH3COOH (Chart 1). The dinuclear ferric complexes 5 and 69 were used as the precursors to the active sites, instead of mononuclear ferrous complexes 1* and 3*, since 1* and 3* immediately convert into 5 and 6 just after the addition of H2O2 (see below). Using low temperatures (−75 to −85 °C), the extremely unstable and reactive iron-oxygen intermediates 5a and 6a have been detected by EPR spectroscopy in the systems 5,6/H2O2/CH3COOH and assigned to the oxoiron(V) intermediates on the basis of their reactivity towards electrondonating alkenes and close similarity of their EPR spectra to those of the known oxoiron(V) species. The starting dinuclear complex 5 is EPR silent. Just after the addition of 50 equiv of CH3COOH to the 0.05 M solution of 5 in CH2Cl2/CH3CN = 1.2:1 (by volume) mixture at room temperature and freezing the sample at −196 °C, the EPR signal of a new ferric complex with proposed structure [(TPA*)FeIII(κ2OC(O)CH3)]2+ (X5) is observed (Figure 1A). This complex
displays resonances at g1 = 2.58, g2 = 2.38 and g3 = 1.72. We note that related complexes [(TPA)FeIII(acac)]2+ and [(5Me3TPA)FeIII(acac)]2+ (Hacac = acetylacetone) show very similar EPR spectra (g1 = 2.57, g2 = 2.35 and g3 = 1.71).10a The EPR spectrum of X5 disappears with the half-life time of τ1/2 = 2 min at room temperature. Besides the resonances of X5, the EPR spectrum of Figure 1A exhibits the intense resonances at g = 5.6 and 4.3 from stable unidentified high-spin (S = 5/2) ferric species. 2.58 2.38 5.6
X5
X5
4.3 1.72
A)
X5
2.070
5a X5
5a 2.005
*
B)
X5 2.201 2.163
5a 1.956
1000
2000
3000
4000
5000 H / G
Figure 1. EPR spectra (−196 °C) of the sample (A) 5/CH3COOH ([CH3COOH]/[5] = 50, [5] = 0.05 M) frozen 1 min after mixing the reagents at room temperature in a 1.2:1 CH2Cl2/CH3CN mixture, and the sample (B) 5/H2O2/CH3COOH ([5]:[H2O2]:[CH3COOH] = 1:6:20, [5] = 0.02 M) frozen 1.5 min after mixing the reagents at −80 °C in a 1.8:1 CH2Cl2/CH3CN mixture.
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Table 2. Oxidation of Cyclohexene with H2O2 Catalyzed by Mononuclear and Dinuclear Non-Heme Iron Complexes a entry
catalyst
1
1*
2 3 4
5
CH3COOH (equiv)b
cyclohexene oxide c
2-cyclohexen1-one c
2-cyclohexen1-ol c
selectivity (%)d
–
27
59
52
20
10
61
38
31
47
100
64
23
19
60
–
23
37
43
22
5
10
62
24
22
57
6
100
67
13
11
74
7
500
72
5
8
85
–
78
36
42
50
10
93
23
26
65
8
3*
9 10 11
6
100
91
20
22
68
–
31
25
31
36
12
10
96
13
18
76
13
100
96
12
12
80
14
500
95
4
5
91
a
H2O2 was delivered by syringe pump over 25 min at room temperature, and 5 extra minutes of stirring were allowed before subjecting the solution to GC analysis. [Fe]:[H2O2]:[C6H10] = 1:10:300. [Fe] = 2.7 × 10−3 M. Observed yields of 1,2-cyclohexanediol were 100%, as well as 2-cyclohexen-1-ol/2-cyclohexen-1-one ratio of ca. 1 were found to be unaffected by the addition of triphenylphosphine after the completion of the reaction.13 Apparently, the 2cyclohexen-1-peroxyl radicals (formed via trapping O2 by the initially generated 2-cyclohexen-1-yl radicals) rapidly convert into the corresponding ketone and alcohol in the reaction mixture before subjecting to GC. The amounts of 2-cyclohexen-1ol and 2-cyclohexen-1-one formed in the course of oxidation dropped upon increasing the amount of acetic acid, thus demonstrating that the latter promotes the major (metal-oxomediated) pathway to the epoxide. Notably, much higher epoxide selectivity (up to 99%) was achieved with 10 times larger amount of H2O2 ([Fe]:[H2O2]:[C6H10] = 1:102:100; Table S1, Supporting Information) with respect to those observed under conditions where cyclohexene was in a large excess ([Fe]:[H2O2]:[C6H10] = 1:10:300; Table 2). Since the amount of O2 in the reaction mixture is independent of the catalytic conditions applied, the obtained result corroborates that the allylic oxidation products come from oxygen via a radical mechanism, while the epoxide arises from H2O2 in the course of a metal-oxo mediated reaction. To evaluate the reactivity of 6a towards alkenes, the sample (6/H2O2/CH3COOH = 1:6:20, [6] = 0.02 M) containing preliminary generated 6a was prepared. An alkene was added to this sample at −90 °C within 1 min, and decay of 6a was monitored at −85 °C. While in the absence of alkene or in the presence of 1-acetyl-1-cyclohexene and cyclohexene-1carbonitrile, 6a decays with τ1/2 = 4 min at −85 °C, the EPR resonances of 6a disappeared within 99% selectivity. They exhibit rhombic EPR spectra with g-tensor values very similar to those of known S = 1/2 oxoferryl complexes. Studies of the catalyst systems based on aminopyridine iron complexes with various electron-donating and electron-withdrawing substituents in the pyridine rings are planned to collect a broader library of EPR data on iron-oxygen species active towards the oxidation of olefins and alkanes.
ASSOCIATED CONTENT Experimental details, data on the catalytic alkene oxidations, EPR and 1H NMR spectra, X-ray data for 6. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC 1020311 (complex 6) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from
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the Cambridge Crystallographic Data Centre www.ccdc.cam.ac.uk/data_request/cif and from the authors.
via
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(17) Tse, C.-W.; Chow, T. W.-S.; Guo, Z.; Lee, H. K.; Huang, J.S.; Che, C.-M. Angew. Chem. Int. Ed. 2014, 53, 798–803.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT Financial support from the Russian Scientific Foundation (Grant 14-13-00158) is gratefully acknowledged. We thank Mr. M. V. Shahkov for the GC-MS measurements.
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