Hydrogen-Atom Transfer Oxidation with H2O2 Catalyzed by [FeII(1,2-bis(2,2′-bipyridyl-6-yl)ethane(H2O)2]2+: Likely Involvement of a (μ-Hydroxo)(μ-1,2-peroxo)diiron(III) Intermediate

Article abstract of DOI:10.1002/ijch.201700059

The iron(II) triflate complex (1) of 1,2-bis(2,2′-bipyridyl-6-yl)ethane, with two bipyridine moieties connected by an ethane bridge, was prepared. Addition of aqueous 30 % H₂O₂ to an acetonitrile solution of 1 yielded 2, a green compound with λ_max=710 nm. Moessbauer measurements on 2 showed a doublet with an isomer shift (δ) of 0.35 mm/s and a quadrupole splitting (ΔE_Q) of 0.86 mm/s, indicative of an antiferromagnetically coupled diferric complex. Resonance Raman spectra showed peaks at 883, 556 and 451 cm^?1 that downshifted to 832, 540 and 441 cm^?1 when 1 was treated with H₂ ¹⁸O₂. All the spectroscopic data support the initial formation of a (μ-hydroxo)(μ-1,2-peroxo)diiron(III) complex that oxidizes carbon-hydrogen bonds. At 0 °C 2 reacted with cyclohexene to yield allylic oxidation products but not epoxide. Weak benzylic C?H bonds of alkylarenes were also oxidized. A plot of the logarithms of the second order rate constants versus the bond dissociation energies of the cleaved C?H bond showed an excellent linear correlation. Along with the observation that oxidation of the probe substrate 2,2-dimethyl-1-phenylpropan-1-ol yielded the corresponding ketone but no benzaldehyde, and the kinetic isotope effect, k_H/k_D, of 2.8 found for the oxidation of xanthene, the results support the hypothesis for a metal-based H-atom abstraction mechanism. Complex 2 is a rare example of a (μ-hydroxo)(μ-1,2-peroxo)diiron(III) complex that can elicit the oxidation of carbon-hydrogen bonds.

Full text of DOI:10.1002/ijch.201700059

Full Paper
DOI: 10.1002/ijch.201700059
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Hydrogen-Atom Transfer Oxidation with H2O2 Catalyzed by
[FeII(1,2-bis(2,2’-bipyridyl-6-yl)ethane(H2O)2]2+: Likely
Involvement of a (m-Hydroxo)(m-1,2-peroxo)diiron(III)
Intermediate
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Alexander M. Khenkin,[a] Madhu Vedichi,[a] Linda J. W. Shimon,[b] Matthew A. Cranswick+,[c]
Johannes E. M. N. Klein,[c] Lawrence Que, Jr.,*[c] and Ronny Neumann*[a]
Abstract: The iron(II) triflate complex (1) of 1,2-bis(2,2’- cyclohexene to yield allylic oxidation products but not
bipyridyl-6-yl)ethane, with two bipyridine moieties connected epoxide. Weak benzylic CÀH bonds of alkylarenes were also
by an ethane bridge, was prepared. Addition of aqueous 30% oxidized. A plot of the logarithms of the second order rate
H2O2 to an acetonitrile solution of 1 yielded 2, a green constants versus the bond dissociation energies of the
compound with lmax =710 nm. Moessbauer measurements cleaved CÀH bond showed an excellent linear correlation.
on 2 showed a doublet with an isomer shift (d) of 0.35 mm/s Along with the observation that oxidation of the probe
and a quadrupole splitting (DEQ) of 0.86 mm/s, indicative of substrate 2,2-dimethyl-1-phenylpropan-1-ol yielded the corre-
an antiferromagnetically coupled diferric complex. Resonance sponding ketone but no benzaldehyde, and the kinetic
Raman spectra showed peaks at 883, 556 and 451 cmÀ1 that isotope effect, kH/kD, of 2.8 found for the oxidation of
downshifted to 832, 540 and 441 cmÀ1 when 1 was treated xanthene, the results support the hypothesis for a metal-
with H218O2. All the spectroscopic data support the initial based H-atom abstraction mechanism. Complex 2 is a rare
formation of a (m-hydroxo)(m-1,2-peroxo)diiron(III) complex example of a (m-hydroxo)(m-1,2-peroxo)diiron(III) complex
that oxidizes carbon-hydrogen bonds. At 08C 2 reacted with that can elicit the oxidation of carbon-hydrogen bonds.
Keywords: OÀO bond activation · Bridging ligands · peroxo · CÀH bond activation · hydrogen-atom abstraction
32 1. Introduction
droanthracene) or oxygenated products (e.g. xanthone from
xanthene). Evidence from UV-vis, resonance Raman, and
Moessbauer spectroscopy along with titrations by 2,6-lutidine
supports the formation of a (m-hydroxo)(m-1,2-peroxo)diiron
(III) complex as the active species. Furthermore, rate
correlation experiments, a substrate probe and isotope labeling
33
34 The activation of carbon-hydrogen bonds is a central and
35 general topic of broad scope.[1] In this context, selective
36 hydrocarbon oxidation involving carbon-hydrogen bond acti-
37 vation, especially using environmentally friendly oxidants
38 such as molecular oxygen and hydrogen peroxide is a long-
39 standing research goal. Some of these efforts have been
40 inspired by the many iron enzymes that have been charac-
41 terized in the past quarter century and found to have nonheme
42 active sites consisting of histidine and carboxylate ligands.[2]
43 The biomimetic approach has played an important role mostly
44 from a basic mechanistic perspective that also has broad
45 implications from a practical point of view.[3] A wealth of
46 information concerning the nature and reactivity of the active
47 species involved in the action of various metalloenzymes with
48 mononuclear and dinuclear iron active sites has been uncov-
49 ered from synthetic complexes.[4,5] Bioinspired iron catalysts
50 have also been developed that use H2O2 as oxidant and are
51 capable of selective oxidation of aliphatic CÀH bonds.[6]
[a] A. M. Khenkin, M. Vedichi, R. Neumann
Department of Organic Chemistry, Weizmann Institute of Science
Rehovot, Israel 76100
phone: +972-8-9343354
E-mail: Ronny.Neumann@weizmann.ac.il
[b] L. J. W. Shimon
Department of Chemical Research Support, Weizmann Institute of
Science
Rehovot, Israel 76100
[c] M. A. Cranswick,+ J. E. M. N. Klein, L. Que, Jr.
Department of Chemistry and Center for Metals in Biocatalysis
University of Minnesota, 207 Pleasant Street SE, Minneapolis,
Minnesota 55455, USA
52
In this paper we describe our findings on a newly
E-mail: larryque@umn.edu
53 synthesized complex, {[FeII(1,2-bis(2,2’-bipyridyl-6-yl)etha-
54 ne](OH2)2}2+, that in the presence of hydrogen peroxide
55 oxidizes alkylarenes by CÀH bond activation resulting in
56 oxydehydrogenation products (e.g. anthracene from dihy-
[+] Present address: Department of Chemistry, Colorado State Uni-
versity – Pueblo, 2200 Bonforte Blvd Pueblo, CO 81001
Supporting information for this article is available on the WWW
under https://doi.org/10.1002/ijch.201700059
Isr. J. Chem. 2017, 57, 1–10
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support the possibility that oxidation occurs via a rate
determining hydrogen atom transfer (HAT) step.
2. Results and Discussion
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The synthesis of the catalyst precursor, Scheme 1, involved the
preparation of 1,2-bis(2,2’-bipyridyl-6-yl)ethyne by reacting 6-
bromo-2,2-bipyridine with sodium acetylide through consec-
utive Suzuki and Sonogashira coupling reactions followed by
hydrogenation over Pd/C to yield the tetradentate ligand, 1,2-
bis(2,2’-bipyridyl-6-yl)ethane (L).[7] Metalation of L with FeII
(CF3SO3)2 ·4 MeCN yielded an orange crystalline compound
[FeII(L)](H2O)2](CF3SO3)2 (1), Figure 1. The crystal structure
obtained is similar to an analogous [FeII(L)](SCN)2 compound
recently published.[8] Thus, 1 has a distorted octahedral
structure where the tetradentate L coordinates FeII in the
equatorial plane in a twisted planar configuration with FeÀN
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˚
bond lengths of 2.213, 2.212, 2.216 and 2.199 A (ravg
=
II
˚
2.21 A). The aqua ligands are bonded to Fe in the axial
Scheme 1. Synthetic pathway for the preparation of [FeII(L)](H2
O)2](CF3SO3)2, 1.
˚
positions with FeÀO bond lengths of 2.109 and 2.096 A. These
bond lengths are indicative of a high-spin iron(II) center.
The reaction of 1 (UV-vis lmax =435 nm, e=580 MÀ1 cmÀ1,
Figure 2) with 30% H2O2 generates a green solution with a
lmax =710 nm at À408C as shown in Figure 2. This species, 2,
decays upon standing to yield a brown solution, with no peaks in
the visible region (Figure 2) with t1/2 values of 240 and 18 min at
À40 and 08C, respectively (Figure S1). Given the nature of the
reaction components and the thermal instability of 2, the
chromophore formed is likely an iron(III)-peroxo complex. Its
further characterization is described below.
The zero-field Moessbauer spectrum of 2 formed by the
reaction of 10 equiv. 30% H2O2 with 57Fe-labeled 1 in
acetonitrile at À358C shows a doublet with an isomer shift (d)
of 0.35 mmsÀ1 and
a quadrupole splitting (DEQ) of
0.86 mmsÀ1 with linewidths of ~0.7 mmsÀ1 (Figure 3). For
comparison, values of d=1.08 mmsÀ1 and DEQ =3.70 mmsÀ1
are found for 1, which are typical for high-spin iron(II)
centers. On the other hand, the isomer shift found for 2 falls at
the low end of the range associated with high-spin ferric
centers.[9]
Figure 1. ORTEP representation (50% probability) of [FeII
(L)(OH2)2](CF3SO3)2 (1). Carbon-gray; Nitrogen-blue; Oxygen-red;
Iron-orange; Hydrogen-white. The triflate anions and hydrogen
atoms bound to carbon atoms are not shown.
Figure 2. UV-visible spectra of 0.5 mmsolution of [FeII(L)(OH2)2](CF3
Figure 3. The zero-field Moessbauer spectrum at 110 K of a frozen
green solution obtained by the addition of a 10-fold excess of 30%
H2O2 to a 10 mmsolution of [57FeII(L)(H2O)2](CF3SO3)2 in acetonitrile
at À358C.
SO3)2 in acetonitrile at À408C with lmax =435 nm (e=580 MÀ1 cmÀ1
)
(black), after addition of 10 eq. of 30% H2O2 (green), and after
warming to 08C (red).
Isr. J. Chem. 2017, 57, 1–10
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Table 1. UV-vis and Resonance Raman properties of synthetic peroxodiferric complexes.
lmax, nm
n(OÀO),
n(FeÀO2ÀFe), n(FeÀOÀFe),
d (DEQ),
Ref.
(e, MÀ1 cmÀ1
)
cmÀ1
(D18O2) cmÀ1 (D18OH2) cmÀ1 mmsÀ1
4
2
710 (1300)
883 (À51)
451 (À11),
556 (À13)
461, 521
0.35 (0.86)
This work
This work
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2+base
520 (1000)
645 (600)
840
[FeIII2(m-O)(m-1,2-O2)(6-HPA)]2+
490 (1130) 670 (1060) 826 (À51)
0.351 (1.635) 10
[FeIII2(m-OH)(m-1,2-O2)(BnBQA)2]3+
730 (2400)
925 (À53)
468 (À6),
550 (À17)
424 (À11)
523 (À16)
0.57 (À1.35) 11a
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0.56 (À0.96)
[FeIII2(m-O)(m-1,2-O2)(BnBQA)2]2+ (5)
[FeIII2(m-OH)(m-1,2-O2)(6-Me2BPP)2]2+
505 (1250)
650 (1300)
644 (3000)
854 (À47)) 460 (À13),
511 (À19)
0.55 (1.43)
0.50 (1.31)
0.50 (1.46)
0.54 (1.68)
–
11a
11b
11b
11c
11c
11d
908 (À47)
847 (À33)
847 (À44)
844 (À44)
874 (À38)
460 (À13),
548 (À18)
465 (À19)
[FeIII2(m-O)(m-1,2-O2)(6-Me2BPP)2]2+ (8) 462 (1100)
577 (1500)
[FeIII2(m-O)(m-1,2-O2)(6-Me3TPA)2]2+ (4) 494 (1100)
648 (1200)
463 (À21),
533 (À25)
464 (À17),
523 (À20)
458 (À13)
511 (À12)
[FeIII2(m-O)(m-1,2-O2)(BQPA)2]2+ (6)
480 (1000)
620 (1000)
690(1500)
[FeIII2(m-O)(m-1,2-O2)(IndH)2 (10)
–
Ligand abbreviations: 6-HPA=1,2-bis[2-{bis(2-pyridylmethyl)-aminomethyl}pyridin-6-yl]ethane; BnBQA=N-benzyl-N,N-di(quinolin-2-ylmethyl)
amine; 6-Me3TPA=tris(6-methyl-2-pyridylmethyl)amine; BQPA=1-(isoquinolin-1-yl)-N-(isoquinolin-1-ylmethyl)-N-(pyridin-2-ylmethyl)metha-
namine; 6-Me2BPP=bis(6-methyl-2-pyridylmethyl)-3-aminopropionate; IndH=1,3-bis(2-pyridylimino)isoindoline; pb=(À)4,5-pinene deriva-
tive of 2,2’-bipyridine.
The appearance of a single quadrupole doublet for 2
28 indicates that it is not a mononuclear iron(III) complex, as
29 such species typically give rise to broadened six-line spectra at
30 liquid He temperatures due to magnetic hyperfine interactions
31 of the S=5/2 center.[9] The observed quadrupole doublet is
32 instead consistent with an antiferromagnetically coupled
33 diferric complex with very similar coordination environments
34 for the two Fe atoms, which would have an S=0 ground state.
35 Given the reaction conditions, a peroxide very likely serves as
36 a bridge between the two iron centers to mediate the
37 antiferromagnetic interaction. Compared to other diferric-
38 peroxo complexes listed in Table 1, 2 exhibits an isomer shift
39 that is 0.2 mmsÀ1 lower than most of the others but is
40 essentially identical to the one reported for [FeIII2(m-O)(m-1,2-
41 O2)(6-HPA)]2+ (see footnote in Table 1 for ligand abbrevia-
42 tions).[10] However, the smaller quadrupole splitting of 2
43 relative to that of the 6-HPA complex suggests that the oxo
44 bridge is protonated in 2.
Figure 4. UV-visible spectra of a 0.5-mmsolution of [FeII(L)](CF3SO3)2
in acetonitrile at À408C after addition of 5 eq. of 30% H2O2 and
consecutive additions of 4 aliquots of 0.5 equivalents of 2,6-lutidine.
45
Indeed the visible spectrum of 2 resembles those found for
polydentate ligands, 3 is much more stable than its conjugate
acid 2. The formation of 3 is reversible as can be shown by the
addition of two equivalents of trifluoroacetic acid.
46 (m-1,2-peroxo)diiron(III) complexes that do not have an addi-
47 tional oxo bridge (Table 1).[10] The chromophore reaches its
48 maximum intensity upon addition of one equivalent 2,6-
49 lutidine (Figure 4) and has an estimated extinction coefficient
50 (e) of 1300 MÀ1 cmÀ1. Introduction of a second equivalent of
51 2,6-lutidine elicits a hypsochromic shift in the visible spectrum
52 and the appearance of new peaks at lmax =520 and 645 nm
53 with respective e values of 1000 and 800 MÀ1 cmÀ1, corre-
54 sponding to its conjugate base 3. These features strongly
55 suggest the formation of a (m-oxo)(m-1,2-peroxo)diiron(III)
56 complex (Table 1). Like such complexes supported by other
The structures proposed for 2 and 3 in Scheme 2 have been
tested by applying resonance Raman (rR) spectroscopy on
these samples, as the observed visible features likely arise
from peroxo-to-iron(III) charge transfer transitions. Intermedi-
ate 2 was prepared by reacting a CH3CN solution of 1 with
H216O2, and its rR spectrum was obtained with 647.1-nm
excitation. This spectrum reveals peaks at 883, 556 and
451 cmÀ1 (Figure 5, top), all three of which respectively
downshift to 832, 543 and 440 cmÀ1 for the sample prepared
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completely consistent with the initial formation of a (m-
hydroxo)(m-1,2-peroxo)diiron(III) complex (2) and the subse-
quent deprotonation of the hydroxo bridge upon addition of
2,6-lutidine to form a (m-oxo)(m-1,2-peroxo)diiron(III) com-
plex (3) (Scheme 2).
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DFT calculations were carried out to assess the structures
proposed for 2 and 3. One very important consequence of the
assignment of (m-hydroxo)(m-1,2-peroxo) and (m-oxo)(m-1,2-
peroxo) cores in 2 and 3, respectively, is a change of the
coordination environment when compared to the X-ray
structure of the iron(II) precursor 1 shown in Figure 1. In 1 the
1,2-bis(2,2’-bipyridyl-6-yl)ethane ligand occupies all equatori-
al positions, hence adopting a meridional coordination mode.
As a result, the two aqua ligands occupy the axial positions
and are trans to each other. Such a coordination environment
does not allow for the formation of the proposed cores in 2
and 3, which require the labile ligands to be placed cis to each
other. In order to probe the flexibility of the ligand we carried
out Kohn-Sham density functional theory (DFT) calculations
using the M06-L functional,[12] which we recently found to
perform well for the determination of spin ground states of
iron complexes.[13] Geometries were computed using the def2-
SVP(Fe:def2-TZVP) basis set combination and single point
energies were obtained using the def2-TZVPP basis set.[14]
Solvation effects of acetonitrile were mimicked using the
COSMO solvation model.[15] Full computational details are
provided in the experimental section.
We have first probed the conformational flexibility of the
tetradentate ligand by optimizing monomeric model com-
plexes bearing the 1,2-bis(2,2’-bipyridyl-6-yl)ethane ligand for
the +III oxidation state with two hydroxide ligands in either a
cis or trans arrangement. We find the S=5/2 spin state to be
energetically more favored for both complexes. Interestingly,
we find that placing the two hydroxide ligands cis to each
other is energetically favored by 3.7 kcalmolÀ1 over the trans
arrangement and that the two CH2 groups of the ethylene
bridge of the ligand are in a staggered conformation relative to
each other. These results clearly indicate that in the +III
oxidation state (i) the ligand possesses sufficient flexibility to
accommodate various coordination modes and (ii) a coordina-
tion environment providing two cis labile sites is energetically
favored.
Scheme 2. Proposed structures for 2 and 3.
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Figure 5. Resonance Raman spectra (lexc =647.1 nm) at 77 K of a
frozen green solution obtained from the reaction of a 3 mmsolution
of 1 with a 10-fold excess of H216O2 (top, CH3CN) or H218O2 (middle,
CD3CN) at À408C and a similar spectrum after addition of two
equiv. 2,6-lutidine to the solution of the intermediate from 1 with a
10-fold excess of H216O2 (bottom, CD3CN).
40 with H218O2 (Figure 5 middle). The fact that all three peaks are
41 sensitive to the 16O/18O substitution of the added H2O2
42 indicates that they all derive from a bound peroxo moiety.
43 Such features and their 18O isotopic shifts resemble those
44 found for a number of (m-1,2-peroxo)diiron(III) complexes
45 (Table 1) and can be assigned respectively to the n(OÀO), nas
46 (FeÀO2ÀFe), and ns(FeÀO2ÀFe) modes, corroborating the
47 presence of an FeÀOÀOÀFe unit.
Merging two of these monomeric Fe(III) fragments via a
m-(hydr)oxo and a m-1,2-peroxo bridge generates the proposed
(m-hydroxo)(m-1,2-peroxo)diferric and (m-oxo)(m-1,2-peroxo)
diferric cores in 2 and 3, respectively. We tested a few
representative conformations, with a representative example
for 2 shown in Figure 6. Note that the complex shown in
Figure 6 is computed for a S=0 spin state. This electronic
configuration was obtained from an anti-ferromagnetically
coupled pair of S=5/2 Fe(III) centers, as illustrated by the
spin density plot in Figure 6. The corresponding ferromagenti-
cally coupled S=5 complexes were also computed and found
to be energetically disfavored by 6.–6.7 and 11.9–13.3 kcam
molÀ1 for complexes 2 and 3, respectively. These can be found
in the supporting material.
48
The addition of two equivalents of base to the solution of 2
49 gives rise to an rR spectrum in which the three peaks of 2 shift
50 to 840, 520 and 461 cmÀ1, respectively. The significant 43-
51 cmÀ1 downshift for the n(OÀO) mode follows a pattern that
52 has been previously observed for two other acid/base pairs of
53 complexes, namely [FeIII2(m-OH/O)(m-1,2-O2)(BnBQA)2]3+
54 and [FeIII2(m-OH/O)(m-1,2-O2)(6-Me2BPP)2]+ (Table 1),[10,11]
55 reflecting a decrease in the Fe···Fe distance due to the shorter
56 FeÀmÀO bonds. The Raman results for 2 and 3 are thus
Isr. J. Chem. 2017, 57, 1–10
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attempts to oxidize cyclohexene at temperatures ranging from
0 to À408C showed no epoxidation but instead afforded allylic
oxidation products. Other substrates with a weak allylic or
benzylic CÀH bonds such as xanthene, cyclohexadiene, 9,10-
dihydroanthracene, 1,2-dihydronapthalene, fluorene, indene,
diphenylmethane, and triphenylmethane were oxidized at
À408C, mostly to xanthone, benzene, anthracene, 5,6-dihy-
dronaphthelone, fluorenone, indenone, benzophenone and 9-
phenylfluorene, respectively. As one can see from Table 2, the
yields versus iron complex are not high due to decomposition
of the peroxo complex even at À408C. The dienes such as
1,3-cyclohexadiene and 9,10-dihydroanthracene were oxidized
to their aromatic derivatives – benzene and anthracene,
respectively, while other alkylaromatic compounds were oxy-
genated to ketones. Intriguing is the observation of 9-phenyl-
fluorene as the major product in the oxidation of triphenyl-
methane. As previously reported,[16] this result suggests the
formation of an intermediate that leads to activation of the
ortho-aryl positions for dimerization. Oxidation of 1-phenyl-
2,2-dimethyl-1-propanol yielded phenyl tert-butyl ketone as
the only product observed by GC-MS and GC. It was shown
that this substrate could be used as a mechanistic probe;
hydrogen atom transfer leads to a ketone, while electron
transfer gives rise to a radical cation that decomposes to
benzaldehyde, Scheme 3.[17] The observation of the ketone
implicates a hydrogen atom transfer mechanism. The green
compound was decomposed during the reaction, Figure S1.
This brown solution contained no active species and therefore
conversions of the arylalkanes were limited, which could also
not be improved by addition of more 30% H2O2.
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Figure 6. Structural depiction (A) and spin density plot (B, isosur-
face=0.005) of 2 at the M06-L/def2-SVP(Fe:def2-TZVP)/COSMO
(MeCN) level of theory. Hydrogen atoms of the 1,2-bis(2,2’-bipyridyl-
6-yl)ethane ligand are removed for clarity.
Table 2. Oxidation of Substrates with H2O2 in the presence of 1
Substrate
Product
Yield (%)
The computed Fe···Fe distances are in the range of 3.51–
xanthene
fluorene
xanthone
9-fluorenone
16
2
˚
˚
36 3.56 A and 3.15–3.22 A for 2 and 3 respectively, compara-
37 ble to distances found experimentally for corresponding
38 BnBQA (BnBQA=N-benzyl-bis(quinolyl-2-methylamine)
39 complexes.[11a] In addition, we find that the OÀO bond
9,10-dihydroanthracene
1,2-dihydronapthalene
4-methoxytoluene
1H-indene
diphenylmethane
triphenylmethane
anthracene
5
7
naphthalen-1(2H)-one
4-methoxybezaldehyde
1H-inden-1-one
benzophenone
benzophenone
9-phenylfluorene
cyclohex-2-en-1-one
cyclohex-2-en-1-ol
benzene
0.6
2.5
2.2
2.1
18.9
7.2
5.8
35
˚
40 distances for 2 are shorter (1.34 A) than for 3 (1.37–
˚
41 1.38 A). This trend is consistent with observations by
42 resonance Raman spectroscopy where the OÀO stretching
43 frequency changes from 883 cmÀ1 for 2 to 840 cmÀ1 for 3. A
44 similar change has also been found for the corresponding
45 BnBQA complexes.[11a] Overall, we find our computed
46 structures to further support the proposed structures of 2
47 and 3.
cyclohexene
1,3-cyclohexadiene
Reaction conditions: 1=10 mM, H2O2 =20 mM, substrate=50 mM
(yield in %mol products/mol 1)in acetonitrile, 08C.
48
The green intermediate, 2, decays more rapidly upon
49 addition of several substrates (see below); in contrast 3 was
50 unreactive. As the formation of such green chromophores is
51 often a hallmark for the formation of reactive iron species, the
52 reactivity of 1 in the presence of 30% H2O2 was surveyed. For
53 example, in an acetonitrile solution of 0.1 mM thioanisole,
54 0.02 mM 1 and 0.2 mM of H2O2 at À208C, thioanisole was
55 nearly quantitatively converted to the corresponding sulfoxide
56 with some sulfone formation (~5%) within 5 min. Parallel
Scheme 3. Oxidation of 1-phenyl-2,2-dimethyl-1-propanol as a reac-
tion probe.
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The initial rates of substrate oxidation were measured by
UV-vis methods at À408C by following the decrease of the
optical density of 2 at 710 nm. These reactions are first order
in substrate and in 2. Pseudo-first order fitting of the kinetic
data allowed us to determine kobs values and to calculate the
second order rate constant k taking into account background
decomposition of 2. Then log k values were plotted versus the
gas phase ionization potentials[18] and the CÀH bond dissocia-
tion energies[19] of the substrates. The results clearly show that
Raman spectra support the formation of a (m-hydroxo)(m-1,2-
peroxo)diiron(III) complex. Reactivity studies show that 2 is
an active species for carbon-hydrogen bond activation and
subsequent oxygenation or aromatization. Although 2 is
insufficiently stable to be a practical oxidant, mechanistic
studies showed that oxidation occurs through a hydrogen-atom
abstraction mechanism. Results that support this conclusion
are the linear correlation of rate constants with bond
dissociation energies, the use of 2,2-dimethyl-1-phenylpropan-
1-ol as a probe substrate that showed formation of 2,2-
dimethyl-1-phenylpropan-1-one and not benzaldehyde, and a
kinetic isotope effect, kH/kD =2.8, for the oxidation of
xanthene. Intermediate 2 is a rare example of a (m-hydroxo)(m-
1,2-peroxo)diiron(III) complex that exhibits the ability to
cleave CÀH bonds.[10c,11d] Further work on these species should
shed light on how such chemistry may be carried out at the
diferric-peroxo centers formed in the active sites of aliphatic
CÀH bond cleaving diiron enzymes such as methane mono-
10 there is a good correlation of log k with CÀH bond
11 dissociation energy (r2 =0.93), Figure 7, but there was no
12 correlation with the ionization potential of the substrate,
13 Figure S2. The observation that the rate constants decrease
14 with the increase of the CÀH BDE of the substrate supports an
15 H-atom transfer as the rate determining step for the
16 oxidation.[5] This conclusion is consistent with the observation
17 of a kinetic isotope effect kH/kD =2.8 for the competitive
18 oxidation of xanthene and xanthene-d2 at À408C. These
19 observations strongly suggest that 2 may either react directly
oxygenase[2e] and deoxyhypusine hydroxylase,
a human
20 with the substrate or be in equilibrium with a high-valent
enzyme that hydroxylates a deoxyhypusine residue of the
eukaryotic translation initiation factor 5A (eIF5A), a modifica-
tion that is essential for eIF5A to promote peptide synthesis at
the ribosome.[20] Complex 2 is particularly germane for the
latter enzyme, as spectroscopic studies have demonstrated that
its dioxygen adduct is in fact a (m-hydroxo)(m-1,2-peroxo)
diiron(III) complex.[21]
21 derivative that actually cleaves the CÀH bond. The reactivity
22 presented in Figure 7 is not common among peroxodiferric
23 complexes and has in fact only been reported for one other
24 family of complexes supported by dinucleating octadentate
25 ligands that were developed by Kodera and coworkers.[10] It is
26 clear that there is a very interesting parallel between the
27 reactivities of 2 and the peroxodiferric complexes described
28 by Kodera, which will be investigated in future efforts.
29
30
31
32
33
34
35
36
37
38
39
40
41
42
Experimental Part
Synthesis of 1,2-bis(2,2’-bipyridyl-6-yl)ethyne. Sodium ace-
tylide (0.22 mL, 18 wt % slurry in xylene/light mineral oil,
95%) was suspended in a 10 mL of dry THF in a 50 mL round
bottom flask under a dry argon atmosphere. To this mixture
was added B(OMe)3 (0.23 mL) and immediately the reaction
mixture became clear. Subsequently, 6-bromo-2,2’-bipyridine
(0.200 g, 0.86 mmol) and then Pd(dppf)Cl2 (0.040 g,
0.049 mmol) were added. This reaction mixture was heated to
reflux at 85–908C for 24 h. The resulting dark colored
reaction mixture was cooled to RT and the solvent was
removed by evaporation under vacuum. The resulting brown
solid was titurated with CH2Cl2 and H2O. The organic layer
was dried over MgSO4 and L was purified by silica column
chromatography with hexane: EtOAc (2:1) eluant to give a
50 mg of 1,2-bis(2,2’-bipyridyl-6-yl)ethyne as a white solid
(17.4% Yield). IR (KBr pellet): (n/cmÀ1)=2006, 1581, 1559,
1463, 1430, 1261, 1162, 1096, 1079, 989, 774, 742, 642, 623,
43
44
Figure 7. The initial rate of the disappearance of 2 in the presence of
arylalkanes as a function of the CÀH bond disassociation energy,
45
46
47
48
49
50
51
BDE. (X – xanthene, DHA – dihydroanthracene, BA – benzyl alcohol,
I – indene, TPM – triphenylmethane, F – fluorene, DHN – 1,2-
dihydronaphthalene, DPM – diphenylmethane, MA – 4-methylani-
sole). Reaction conditions: 20 mmol substrate, 10 mmol 30% H2O2
2 mmol 1 in 2 mL of acetonitrile at À408C.
1
575. H NMR (300 MHz, d6-DMSO): dH 8.78 (qq, 2H), 8.48
(m, 4H), 8.09 (t, 2H), 8.03 (dt, 2H), 7.89 (dd, 2H), 7.55(m,
2H); 13C NMR (400 MHz, d6-DMSO): dC 155.85, 154.15,
149.28, 140.96, 138.13, 137.38, 127.91, 124.55, 120.72,
120.64, 87.46. Anal. Calcd for C22H14N4: C, 79.02; H, 4.22; N,
16.76. Found: C, 78.79; H, 3.99; N, 16.75 ESI-MS (CHCl3),
m/z=334.56, 335.52 (M+1).
52 3. Conclusion
53
54 Addition of aqueous 30% H2O2 to an acetonitrile solution of
55 [FeII(1,2-bis(2,2’-bipyridyl-6-yl)ethane(H2O)2]triflate2
(1)
56 yields a green complex, (2) whose Moessbauer and resonance
Alternative two-step method. Step 1: Synthesis of 6-
ethynyl-2,2’-bipyridine. Sodium acetylide (0.5 mL, 18 wt %
Isr. J. Chem. 2017, 57, 1–10
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slurry in xylene/light mineral oil, 95%) was suspended in a
10 mL of dry THF in a 50 mL round bottom flask under a dry
argon atmosphere. To this mixture was added B(OMe)3
(0.4 mL) and immediately the reaction mixture became clear.
Subsequently, 6-bromo-2,2’-bipyridine (0.235 g, 1 mmol) and
then Pd(dppf)Cl2 (0.060 g, 0.073 mmol) were added. This
reaction mixture was heated to reflux at 85–908C for 24 h.
The resulting dark colored reaction mixture was cooled to RT
and the solvent was removed by evaporation under vacuum.
10 mL of dry acetonitrile and Fe(CF3SO3)2·4CH3CN (70 mg,
0.151 mmol) was added. The reaction mixture was stirred for
3 h at room temperature under an argon atmosphere. The
reaction mixture was filtered and filtrate was diffused by ether.
Orange crystals of [FeII(L)](H2O)2](CF3SO3)2 formed which
were filtered out, washed with diethyl ether and dried under
vacuum. Yield - 95 mg (90% based on Fe). IR (KBr pellet):
(n/cmÀ1)=1656, 1601, 1568, 1492, 1456, 1429, 1220, 1159,
1109, 1029, 828, 780, 760, 634, 582, 514. Anal. Calcd for C24
H18F6FeN4O6S2: C, 41.63; H, 2.62; N, 8.09. Found: C, 41.56;
H, 2.41; N, 8.45.
10 The resulting brown solid was titurated with CH2Cl2 and H2O.
11 The organic layer was dried over MgSO4 and 1,2-bis(2,2’-
12 bipyridyl-6-yl)ethyne was purified by silica column chroma-
13 tography with hexane: EtOAc (2:1) eluant to give two white
14 solids: 15 mg of 1,2-bis(2,2’-bipyridyl-6-yl)ethyne (5.2%
15 Yield) and 80 mg of 6-ethynyl-2,2’-bipyridine (44.4% Yield).
16 IR (KBr pellet): (n/cmÀ1)=2101, 1580, 1558, 1476, 1450,
17 1426, 1259, 1150, 1094, 1079, 995, 986, 822, 772, 742, 639,
18 622, 581. 1H NMR (300 MHz, CDCl3): dH 8.65 (dd, 1H), 8.46
19 (m, 3H), 7.81 (m, 3H), 7.49 (dd, 1H), 7.31 (m, 1H), 3.18(s,
20 1H); 13C NMR (500 MHz, CDCl3): dC 156.66, 155.36, 149.20,
21 141.79, 137.27, 137.13, 127.63, 124.23, 121.66, 121.10, 83.19.
22 Anal. Calcd for C12H8N2: C, 79.98; H, 4.47; N, 15.54. Found:
23 C, 79.03; H, 3.89; N, 14.98. ESI-MS (CHCl3), m/z=180.44,
24 181.61 (M+1).
Synthesis of Deuterated Compounds. Xanthene-9-d2 was
prepared as previously described by reduction of xanthen-9-
one with AlCl3-LiAlD4.[22]
Resonance Raman experiments. Resonance Raman spec-
tra were collected using Spectra-Physics model 2060 Kr+ and
2030–15 Ar+ lasers and an Acton AM-506 monochromator
equipped with a Princeton LN/CCD data collection system.
Low-temperature spectra in CH3CN or CD3CN were obtained
at 77 K using a 1358 backscattering geometry. Samples were
frozen onto a gold-plated copper coldfinger in thermal contact
with a dewar flask containing liquid nitrogen. Raman
frequencies were calibrated to indene prior to data collection.
Rayleigh scattering was attenuated using a holographic notch
filter (Kaiser Optical Systems) for each excitation wavelength.
The monochromator slit width was set for a band pass of
4 cmÀ1 for all spectra. The plotted spectra are averages of 32
scans with collection times of 30 s. All spectra were intensity
corrected to the 710 and 773 cmÀ1 solvent peak of CD3CN and
CH3CN, respectively.
Computational Details. All calculations were carried out
using Turbomole v. 7.0.1.[23] The M06-L functional was used
in combination with the def2-TZVP basis set for Fe and def2-
SVP for all other elements for geometry optimizations.[14]
Single point energy calculations were performed using the
larger def2-TZVPP basis set for all atoms.[14] Solvation effects
of MeCN were accounted for using the COSMO solvation
model[15] and electronic energies include an outlying charge
correction.[24] Calculations were accelerated using the MARIÀJ
approach[25] in combination with suitable fitting basis sets.[26]
Numerical second derivatives were computed to validate that a
local minimum was reached in the geometry optimizations. A
differentiation increment of 0.02 was used, unless stated
otherwise. The multiple grid m5 was used in all calculations.
Structural depictions and spin density plots were made using
IboView.[27]
25
Step 2: 6-ethynyl-2,2’-bipyridine (0.180 g, 1 mmol), 6-
26 bromo-2,2’-bipyridine (0.235 g, 1 mmol) and then Pd(PPh3)4
27 (0.116 g, 0.1 mmol) were suspended in a 10 mL of dry THF in
28 a 50 mL round bottom flask under a dry argon atmosphere. To
29 this mixture was added 3 mL of triethylamine and then CuI
30 (1.5 mg, 0.008 mmol). The mixture was allowed to stir at
31 808C temperature for 18 hr. The solvent was removed under
32 vacuum, and the resulting mixture was subjected to silica gel
33 chromatography using hexane: EtOAc (2:1) as an eluent to
34 yield 280 mg (84%) of 1,2-bis(2,2’-bipyridyl-6-yl)ethyne as a
35 white solid.
36
Synthesis of 1,2-bis(2,2’-bipyridyl-6-yl)ethane, L. 10%
37 Pd/C (25 mg) was added to a solution of 1,2-bis(2,2’-
38 bipyridyl-6-yl)ethyne (50 mg, 0.149 mmol) in 7.5 mL 2:1
39 THF:EtOH in Fischer-Porter pressure tube. The suspension
40 was stirred at room temperature for 18 h under an atmosphere
41 of H2 (140 psi). TLC showed that no starting material
42 remained. Dichloromethane (10 mL) was added to the reaction
43 mixture, the catalyst was removed by filtration through Celite,
44 and the filtrate was concentrated in vacuum, to afford 1,2-bis
45 (2,2’-bipyridyl-6-yl)ethane (45 mg, 90% yield) as a crystalline
46 white solid. IR (KBr pellet): (n/cmÀ1)=1581, 1475, 1456,
47 1431, 1260, 1153, 1093, 1044, 993, 897, 828, 781, 747, 646,
48 620, 556. 1H NMR (300 MHz, CDCl3): dH 8.64 (dd, 2H), 8.43
49 (dd, 2H), 8.17 (dd, 2H), 7.75 (td, 2H), 7.64 (t, 2H), 7.24(m,
50 2H), 7.14(dd, 2H), 7.24(m, 4H). 13C NMR (300 MHz, CDCl3):
51 dC 160.84, 156.68, 155.63, 149.21, 136.96, 137.20, 123.66,
52 123.16, 121.39, 118.50, 37.80. Anal. Calcd for C22H18N4: C,
53 78.08; H, 5.36; N, 15.56. Found: C, 77.25; H, 5.76; N, 16.14.
54 ESI-MS (CHCl3), m/z=338.13, 339.47 (M+1).
General Procedure for Catalytic Runs. Oxidations of
alkylarenes were performed at 08C in 2 mL vials sealed with
Teflon-faced silicon septa under magnetic stirring. The
reactions were run by adding at once 20 mmol of H2O2
(oxidant) to a solution of 10 mmol [FeII(L)](H2O)2]-(CF3SO3)2
and 2 mL of cyclooctane (internal standard) in 1 mL
acetonitrile under argon. Immediately after the solution
became deep green, 50 mmol substrate was added, and the
color of the solution slowly turned brown. The quantities of
standard, substrate and product(s) in the reaction mixtures
55
Synthesis of [FeII(L)](H2O)2](CF3SO3)2. 1,2-bis(2,2’-bi-
56 pyridyl-6-yl)ethane, L, (50 mg, 0.147 mmol) was suspended in
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were analyzed by GC and GC/MS. Each reaction was
performed at least three times and the reported data represent
the average of these reactions. Control reactions in the absence
of the catalyst that were carried out under the same conditions
as the catalytic runs show in all cases no conversion.
UV-vis Spectroscopy. UV-visible spectra were recorded
on Agilent 89090A spectrophotometer equipped with Unisoku
cooling system using 1 mmsolution of the complex in
acetonitrile at À408C. Complex 2 formed upon addition of
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10 30% H2O2 (5 eq). Then 10 eq. of substrate was added.
11
Moessbauer Spectroscopy. Mossbauer spectra were
12 recorded at 110 K and 298 K using 57Co point source. An
13 iron foil was used for the calibration of Doppler velocity.
14 The Moessbauer spectrum of 57[FeII(L)](H2O)2](CF3SO3)2,
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18 acetonitrile at À408C and then was frozen.
19
Resonance Raman Spectroscopy. Resonance Raman
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25 Dewar flask containing liquid nitrogen.
26
X-ray Crystallography. Single crystal X-ray data for FeII
27 (L)(H2O2(](CF3SO3)2 formed by recrystallizationwas collected
28 on a Rigaku XtaLab PRO equipped with PILATUS 200
29 diffractometer with Mo Ka (l=0.71073 nm) radiation and
30 graphite monochromator. Measurement were performed at
31 100 K under liquid N2 to achieve better quality data. The data
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34 Full-matrix least-squares refinement was based on F2 with
35 SHELXL-2016. Crystal data collection and refinement param-
36 eters are given in the crystallographic CIF files also available
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42 This research was supported by the United States-Israel
43 Binational Science Foundation (grant #2004270 to R. N. and
44 L. Q.) and the US National Institutes of Health (grant
45 GM38767 to L. Q. and postdoctoral fellowship ES017390 to
46 M. A. C.). J.E.M.N.K. thanks the Alexander von Humboldt
47 Foundation for a Feodor Lynen Research Fellowship. We also
48 acknowledge Prof. Christopher J. Cramer and the Minnesota
49 Supercomputing Institute (MSI) at the University of Minnesota
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Received: June 7, 2017
Accepted: August 24, 2017
Published online on && &&, 0000
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© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.ijc.wiley-vch.de
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A. M. Khenkin, M. Vedichi, L. J. W.
Shimon, M. A. Cranswick, J. E. M. N.
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Hydrogen-Atom Transfer
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Oxidation with H2O2 Catalyzed by
[FeII(1,2-bis(2,2’-bipyridyl-6-yl)
ethane(H2O)2]2+: Likely Involve-
ment of a (m-Hydroxo)(m-1,2-
peroxo)diiron(III) Intermediate
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