A novel manganese(III)-peroxo complex bearing a proline-derived pentadentate aminobenzimidazole ligand

Article abstract of DOI:10.1016/j.cclet.2018.04.036

Manganese(III)-peroxo complexes are invoked as key intermediates in the enzymatic cycles of Mn-containing enzymes, and the synthesis of reactive manganese(III)-peroxo complexes with rationally designed ligand has been of great interest in the communities of bioinorganic and biomimetic chemistry. Herein, we designed a novel pentadentate aminobenzimidazole ligand and obtained its manganese(II) complex, which was successfully applied in the synthesis of a reactive manganese(III)-peroxo complex by treatment with hydrogen peroxide in the presence of triethylamine. The manganese(III)-peroxo complex was well characterized with various spectroscopic techniques, including ultraviolet–visible (UV-vis) spectrophotometry, coldspray ionization time-of-flight mass spectrometry (CSI-TOF MS), and continuous wave electron paramagnetic resonance (CW-EPR) spectroscopy. Besides, its reactivity in aldehyde deformylation was investigated, demonstrating second-order kinetics in the reaction with 2-phenylpropionaldehyde and affording acetophenone as the sole product.

Full text of DOI:10.1016/j.cclet.2018.04.036

Accepted Manuscript
Title: A novel manganese(III)-peroxo complex bearing a
proline-derived pentadentate aminobenzimidazole ligand
Authors: Junyi Du, Chengxia Miao, Chungu Xia, Wei Sun
PII:
DOI:
Reference:
S1001-8417(18)30187-6
https://doi.org/10.1016/j.cclet.2018.04.036
CCLET 4532
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Chinese Chemical Letters
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29-3-2018
24-4-2018
28-4-2018
Please cite this article as: Junyi Du, Chengxia Miao, Chungu Xia,
Wei Sun,
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Communication
A novel manganese(III)-peroxo complex bearing a proline-derived pentadentate
aminobenzimidazole ligand
Junyi Dua,b, Chengxia Miaoa,Chungu Xiaa, Wei Suna,*
aState Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese
Academy of Sciences, Lanzhou 730000, China
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
Corresponding author.
E-mail address: wsun@licp.cas.cn (W. Sun).
Graphical asbtract
A novel nonheme manganese(III)-peroxo complex bearing a proline-derived pentadentate aminobenzimidazole ligand was
synthesized and spectroscopically characterized, and its reactivity in aldehyde deformylation was investigated.
ARTICLE INFO
Article history:
Received 30 March 2018
Received in revised form 24 April 2018
Accepted 26 April 2018
Available online
ABSTRACT
Manganese(III)-peroxo complexes are invoked as key intermediates in the enzymatic cycles of Mn-containing enzymes, and the
synthesis of reactive manganese(III)-peroxo complexes with rationally designed ligand has been of great interest in the communities of
bioinorganic and biomimetic chemistry. Herein, we designed a novel pentadentate aminobenzimidazole ligand and obtained its
manganese(II) complex, which was successfully applied in the synthesis of a reactive manganese(III)-peroxo complex by treatment
with hydrogen peroxide in the presence of triethylamine. The manganese(III)-peroxo complex was well characterized with various
spectroscopic techniques, including ultraviolet–visible (UV-vis) spectrophotometry, coldspray ionization time-of-flight mass
spectrometry (CSI-TOF MS), and continuous wave electron paramagnetic resonance (CW-EPR) spectroscopy. Besides, its reactivity in
aldehyde deformylation was investigated, demonstrating second-order kinetics in the reaction with 2-phenylpropionaldehyde and
affording acetophenone as the sole product.

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Keywords:
Enzyme models
Manganese
Manganese(III)-peroxo complex
Aldehyde deformylation
Kinetics
Manganese(III)-peroxo complexes have been suggested as key intermediates in the enzymatic cycles of Mn-containing enzymes,
such as manganese superoxide dismutase (MnSOD) [1], catalase [2], and the oxygen-evolving complex (OEC) of photosystem II [3].
In biomimetic studies, a variety of rationally designed heme and nonheme ligands have been applied in the synthesis of reactive
manganese(III)-peroxo complexes [4-9]. A notable example of the supporting ligands is the heme ligand TPP (TPP =
tetraphenylporphyrin dianion), with which Valentine and co-workers obtained the first X-ray crystal structure of the manganese(III)-
peroxo complexes [10]. For nonheme systems, based on the facially-coordinating trispyrazolyl ligand TpiPr2 (TpiPr2 = hydridotris(3,5-
diisopropylpyrazolyl)borate anion), two more manganese(III)-peroxo complexes were synthesized and structurally characterized by
Moro-oka and co-workers[11,12]. The nonheme macrocyclic TMC (TMC = N-tetramethylated cyclam) ligand series is versatile in the
chemistry of metal-oxygen adducts, and several structurally characterized and highly reactive manganese(III)-peroxo complexes of
TMC have been developed by Nam and co-workers [13-15].
The nonheme amino N-heteroarene ligands, such as aminopyridine, aminoquinoline and aminoimidazole ligand, have also been
extensively investigated in the synthesis of manganese(III)-peroxo complexes (see Fig. 1 for selected examples of aminopyridine
2
ligand N4Py, aminoquinoline ligand L7q2, and aminoimidazole ligand imL5 ) [16-25]. On the other hand, the aminobenzimidazole
ligand series, another member of the nonheme amino N-heteroarene ligand family, has been frequently used in the metal complex-
catalyzed oxidation of organic substrates [26-35]. Recently, our group has succeeded in highly efficient (enantioselective) oxidation of
hydrocarbons, olefins and alcohols with aqueous H2O2 as a terminal oxidant under mild conditions, in which the proline- derived
aminobenzimidazole ligands play key roles in modulating the catalytic reactivity of the manganese complexes [26,36-39].
On the basis of these above work, we are interested in the investigation of the activity of manganese(III)-peroxo complex bearing an
aminobenzimidazole supporting ligand. Herein, we report the synthesis and characterization of a novel manganese(III)-peroxo complex
supported by a proline-derived aminobenzimidazole ligand Pro3Bzim (Fig. 1). The corresponding manganese(III)-peroxo complex is
characterized by means of various spectroscopic methods, and its reactivity in aldehyde deformylation is also investigated.
We initiated our study by preparation of the precursor MnII complex using an aminobenzimidazole ligand Pro3Bzim (Pro3Bzim =
(S)-N,N’,N’-tris(N-methylbenzimidazol-2-ylmethyl)-2-pyrrolidinemethanamine), which was readily synthesized from proline-derived
diamine skeleton (S)-2-pyrrolidinemethanamine and 2-chloromethyl-N-methylbenzimidazole by following a modified procedure for
the synthesis of its analogue Pro3Py (Pro3Py = (S)-N,N’,N’-tris(2-picolinyl)-2-pyrrolidinemethanamine) in our previous report [40]
1
(see the Supporting information for H and 13C NMR spectra of ligand Pro3Bzim). Reacting Pro3Bzim with equimolar amounts of
Mn(OTf)2 in CH3CN at ambient temperature afforded the desired [MnII(Pro3Bzim)(OTf)](OTf) complex (1-OTf) (Scheme 1).
Single crystals of 1-OTf were obtained by diffusing ether into the CH3CN solution. X-ray crystallographic analysis reveals that the
MnII center resides in a distorted octahedral geometry and the Pro3Bzim ligand is coordinated to MnII ion with five N donor atoms, two
from the diamine backbone and three from the benzimidazole moieties. A triflate ligand is bound to the manganese center in the
capacious apical position, indicating a labile site for the binding of peroxo ligand (See also Table S1 and S2 in the Supporting
information for crystallographic data of 1-OTf).
With the MnII starting material in hand, we set out to synthesize the corresponding MnIII-peroxo complex by following a
conventional protocol [4]. Treating the colorless solution of 1 in CH3CN with H2O2 (40 equiv.) in the presence of triethylamine (5
equiv.) at 20 °C for about 2 min resulted in the completely formation of a pink species (2) with a characteristic ultraviolet–visible (UV-
vis) absorption band at 540 nm (ε = ~140 L mol-1 cm-1, Fig. 3a), which is in agreement with the reported manganese(III)-peroxo
complexes [23]. It should be noted that 2 is metastable with a half-life about 0.5 h at 20 °C (see Fig. S3 in the Supporting information
for the time profile of the natural decay of 2), and the metastable property has often been regarded as a characteristic feature of the
reactive metal-oxygen adducts [41-44]. In order to identify the structure of 2, it was further characterized with coldspray ionization
time-of-flight mass spectrometry (CSI-TOF MS) and continuous wave electron paramagnetic resonance (CW-EPR). The CSI-TOF-MS
of 2 in positive mode exhibited a dominant ion peak at a mass-to-charge (m/z) ratio of 619.2, whose mass and isotope distribution
pattern correspond to [MnIII(Pro3Bzim)(O2)]+ (calculated m/z 619.2) (Fig. 3 b). When 2 was prepared with isotopically labeled H218O2
in the presence of triethylamine, a dominant ion peak corresponding to [MnIII(Pro3Bzim)(18O2)]+ appeared at m/z 623.2 (calculated m/z
623.2) (Fig. 3b, inset). The mass shift of 4 Daltons confirms that species 2 contains an O2 moiety in the coordination sphere. The X-
band CW-EPR spectrum of a frozen solution of 2 appears silent, as expected for the MnIII oxidation state, which is in accordance with
the manganese(III)-peroxo complexes in literature [13,14]. Based on the spectroscopic characterizations above, the new species 2
proves to be a manganese(III)-peroxo complex, assigned as [MnIII(Pro3Bzim)(O2)]+. Given that the structure of ligand Pro3Bzim is
similar to Pro3Py, taken together, it is likely that 2 possesses a similar coordination geometry to [MnIII(Pro3Py)(O2)]+ and the peroxo
moiety is bound side-on as deduced for the latter and some other manganese(III)-peroxo complexes [6,40].

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Aldehyde deformylation is frequently used as a probe reaction to evaluate the reactivity of metal-peroxo complexes [4,40,45-50].
Therefore, we were inspired to examine the kinetic reactivity of 2 by using 2-phenylpropionaldehyde (2-PPA) as a probe substrate with
UV-vis spectrophotometry. Upon addition of 2-PPA to 2 in CH3CN at 20 °C, the absorption band at 540 nm, the typical absorption of
manganese(III)-peroxo 2, decreased with a pseudo-first-order kinetics profile (Fig. 4a). GC-MS analysis of the resulting solution
revealed the formation of acetophenone as the sole product, as observed in the deformylation of 2-PPA by other metal-peroxo
complexes [15,40,45-49]. The first-order rate constant (kobs), determined by pseudo-first-order fitting of the kinetic data for the decay
of 2 monitored at 540 nm, increased linearly with increasing concentration of 2-PPA, thereby giving a second-order rate constant (k2)
of 0.10 L mol-1 s-1 at 20 °C (Fig. 4b). This reaction rate is comparable with the reported manganese(III) -peroxo complexes [15,40,50].
Thus, we demonstrate that 2 is capable of performing oxidative deformylation reaction of aldehyde with second-order kinetics.
In summary, we have employed a proline-derived pentadentate aminobenzimidazole ligand (Pro3Bzim) to synthesize the N5-MnII
complex (1), which was crystallographically characterized and further used in the preparation of a novel manganese(III)-peroxo
complex (2) by a conventional protocol with H2O2 in the presence of triethylamine. The structure of 2 was well characterized with
various spectroscopic techniques, including UV-vis, CSI-TOF MS and CW-EPR, while the reactivity of 2 was investigated in oxidative
deformylation of aldehyde, demonstrating second-order kinetics in the reaction with 2-phenylpropionaldehyde and affording
acetophenone as the sole product. The aminobenzimidazole ligands have proven to be excellent candidates in the development of
enzyme models toward to catalytic oxidation reactions. The present work provides valuable example for this type ligands in the study
of synthetic metal-oxygen adducts. Further investigations focusing on the development of more effective and versatile amino N-
heteroarene ligands are underway in our lab.
Acknowledgments
This work is financially supported by the National Natural Science Foundation of China (No. 21473226) and Natural Science
Foundation of Jiangsu Province (No. BK20161261). We acknowledge Prof. Wonwoo Nam (Ewha Womans University, Korea) for the
measurements of CSI-TOF MS and CW-EPR.

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Fig. 1. Selected examples of nonheme amino N-heteroarene ligands for manganese(III)-peroxo complexes.
Fig. 2. ORTEP plot for [MnII(Pro3Bzim)(OTf)]+ (1; CCDC-1832630) with 50% probability thermal ellipsoids and H atoms omitted for clarity. Atom colors:
Mn, violet; C, gray; N, blue; O, red; F, green; S, yellow.
(c)
(a)
(b)
540 nm
0.1
0.0
0.2
0.1
0.0
619.2
619.2
623.2
at 540 nm
100
50
0
0
100 200 300
Time, s
619 621 623 625
400
600
800
1000
1000
2000
3000
Gauss
4000
5000
400
600
800
1000
Wavelength, nm
m/z
Fig. 3. Spectroscopic characterizations of 2: (a) The changes of UV-vis absorption spectra showing the formation of 2 (red line) observed in the oxidation of 1 (1
mmol/L, blue line) by H2O2 (40 mM) in the presence of triethylamine (5 mmol/L) in CH3CN at 20 °C. Inset shows the time profile monitored at 540 nm for the
formation of 2. (b) CSI-TOF-MS spectrum of 2. Inset shows the isotope distribution patterns of 2-16O (blue line) and 2-18O (red line). (c) X-band CW-EPR
spectrum of 2 recorded at 5 K.
(b)
1.5
1.0
0.5
0.0
(a)
k2 = 0.10 M-1 s-1
540 nm
0.1
0.0
0.2
0.1
0.0
at 540 nm
0
200 400 600
Time, s
0
30
60
90
120
150
400
600
800
1000
[2-PPA], mM
Wavelength, nm
Fig. 4. Kinetics of the reaction of 2 with 2-PPA: (a) UV-vis absorption spectral changes observed in the reaction of 2 (1 mmol/L, red line) with 2-PPA (90
mmol/L) in CH3CN at 20 °C. Inset shows the time profile of 2 monitored at 540 nm obeying pseudo-first-order kinetics. (b) Plot of the first-order rate constant
(kobs) against the concentration of 2-PPA to determine the second-order rate constant (k2) in the reaction of 2-PPA with 2 (1 mmol/L) in CH3CN at 20 °C.

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Scheme 1. Syntheses of precursor complex [MnII(Pro3Bzim)(OTf)]+ (1) and manganese(III)-peroxo complex [MnIII(Pro3Bzim)(O2)]+ (2).