Z. Lv et al.
Molecular Catalysis 448 (2018) 46–52
sodium
fluoromethanesulfonate (Mg(OTf)
Sc(OTf) ), ytterbium trifluoromethanesulfonate (Yb(OTf)
trifluoromethanesulfonate (Y(OTf) ) came from Accela ChemBio Co., Ltd
or Alfa Aesar. Other trifluoromethanesulfonates including Ca(OTf) , Ba
OTf) , Zn(OTf) and Al(OTf) were purchased from Aladdin or Shanghai
trifluoromethanesulfonate
), scandium trifluoromethanesulfonate
) and yttrium
(NaOTf),
magnesium
tri-
2
(
3
3
3
2
(
2
2
3
Dibai Chemical Company. The alkenes, such as cyclooctene, cyclohexene,
norbornene, styrene, 1-hexene and 1-dodecene, and their corresponding
epoxides were obtained from Aldrich, Alfa Aesar or TCI (Shanghai)
Development Co., Ltd. Ammonium hexafluorophosphate was purchased
IV
Scheme 1. Chemical structure of [Mn
2
3 3 2 6 2
(μ-O) (Me tacn) ](PF ) catalyst.
Addressing the functional roles of non-redox metal ions is highly related
to the mechanism of both metalloenzymes in nature and multimetallic
catalysts in chemical oxidation, and their roles have attracted con-
siderable attentions in both the biological and chemical communities
36–76].
In our previous works, the effective strategy of introducing non-redox
metal ions as Lewis acids was widely applied to modulate the reactivity of
transition metal catalysis including manganese, iron, palladium, vana-
dium, ruthenium and osmium complexes in versatile homogeneous reac-
tions, such as hydroxylation, N-dealkylation, CeC coupling, dehy-
drogenation, isomerization and epoxidation [61–76]. Recently, we have
communicated the first example that the addition of non-redox metal ions
can greatly improve the oxygen atom transfer efficiency in catalytic
from Alfa Aesar. Common solvents, H
inorganic manganese salt, MnCl •4H O, came from Sinopharm Chemical
Reagent Co., Ltd. The ligand, 1,4,7-trimethyl-1,4,7-triazacyclononane
Me tacn), and its corresponding dinuclear manganese(IV) complex,
2 2
O (30% aqueous solution) and
2
2
(
[
3
[
IV
Mn
literature procedures [82–84], and the chemical structure of [Mn
O) (Me tacn) ](PF is displayed in Scheme 1.
2
(μ-O)
3 3 2 6 2
(Me tacn) ](PF ) , were synthesized according to the previous
IV
2
(μ-
3
3
2
6 2
)
Characterization equipment
Gas chromatography-mass spectrometry (GC–MS) analysis was
conducted on an Agilent 7890A/5975C spectrometer. FT-IR spectra
were collected on a Bruker VERTEX70. UV–vis spectra were obtained
on an Analytik Jena Specord 205 UV–vis spectrometer. Electron para-
magnetic resonance (EPR) experiments were performed at 130 K on a
Bruker A200 instrument, with a center field of 3352.488 G, frequency
of 9.395 GHz, power of 19.44 mW, modulation amplitude of 2.00 G and
III
IV
epoxidation by dissociating the dinuclear Mn -(μ-O)
2
-Mn complex
which is very sluggish for olefin epoxidation [74–76]. We further com-
pared the promotion effect of Lewis acid with that of Brønsted acid and
found that they showed very similar reaction pathway and promotion
effect. However, the organic oxidant, PhI(OAc)
while all other oxidants including H were not efficient, giving sluggish
performance. For economic and environmental reasons, catalytic olefin
oxidations based on H are preferred over traditional stoichiometric
2
, was used in these cases,
3
receiver gain of 1.00 × 10 . The content of Mn ion was determined by
2 2
O
atomic absorption spectroscopy (AAS) analysis with an Analyst 300
Perkin Elmer. Mass spectra (MS) were measured in negative mode in
the range m/z 50–3000 by a Bruker SolariX 7.0T spectrometer (ESI-
MS). Elemental analysis (EA) was performed on a Vario Micro cube.
Cyclic voltammetry (CV) were conducted with a CS CorrTest electro-
chemical workstation equipped with glassy carbon as both working and
counter electrodes and saturated calomel as the reference electrode.
Electrochemical data were collected in dry acetonitrile with 0.1 M tet-
rabutylammonium perchlorate as the supporting electrolyte.
2 2
O
oxidations because of its low cost, high atom efficiency and en-
vironmentally benign by-product (water) [77–80].
In the previous reports by other groups, it was found that H
sharply decomposed by the classic dinuclear manganese complex,
(Me tacn) ](PF ) , in the acetonitrile solution [10,23,81].
3 3 2 6 2
Although the unwanted manganese-catalysed self-destruction of H
could be suppressed by the application of acetone as the reaction sol-
vent, this proposal was not acceptable for large-scale synthesis because
of the risk of generation of explosive cyclic peroxides. These findings
provide a challenge for us to explore whether the wasteful catalase-like
decomposition can be suppressed by the introduction of non-redox
metal ions serving as Lewis acids with acetonitrile as the solvent.
3
Encouraged by the classic Mn-Me tacn complexes and the effective
strategy of introducing non-redox metal ions, herein, we have in-
vestigated the oxygen atom transfer reaction of alkene catalysed by the
dinuclear manganese(IV) complex [Mn
2 2
O was
IV
[
Mn
2
(μ-O)
2 2
O
Reaction procedure
General procedure for non-redox metal-ions-accelerated alkene epoxidation
by the dinuclear manganese(IV) complex
The solution of 5 mL of acetonitrile containing 0.1 M alkene, 1 mM
dinuclear manganese(IV) complex and 2 mM non-redox metal salt as
Lewis acid was cooled in an ice water bath (273 K). Then 1.5 mmol
H
2
O
2
(30% aqueous solution) was added to the mixture solution to
initialize the reaction. The reaction mixture was magnetically stirred at
73 K in the ice water bath for 2 h (set intervals for kinetics study). The
IV
2
3 3 2 6 2
(μ-O) (Me tacn) ](PF ) with
2 2
H O as the solely terminal oxidant in the promotion of non-redox
2
metal ions. Compared with the effect of carboxylic acids in abundant
reports, the introduction of non-redox metal ions as Lewis acid can also
induce the remarkably promotional effect by dissociating the dinuclear
yield of epoxide and the conversion of alkene were quantitatively
analyzed by GC using the internal standard method. Control experi-
ments using the dinuclear manganese (IV) complex or different non-
redox metal salts alone as the catalyst were performed in parallel under
identical conditions. All the reactions were conducted at least in tri-
plicate and the average data were applied in the results and discussion
section.
3
Mn-(μ-O) -Mn core in the epoxidation reaction and better efficiency is
observed in the case of Lewis acids with higher positive charge. The
primary mechanism in catalytic process was further explored and the
open-loop dinuclear manganese complex was proposed as the key ac-
tive species to be capable of the alkene epoxidation process. This work
demonstrates a novel strategy to improve the catalytic reactivity of
some μ-oxo-bridged complexes and inspires us to explore novel cata-
lysts for current challenges and societal demands.
Results and discussion
Non-redox metal-ions-accelerated effect in alkene epoxidation
Experimental section
The promotional effect with non-redox metals on epoxidation was
initially conducted with cyclooctene as the typical substrate and
IV
Chemicals
[Mn
2
3 3 2 6 2 2 2
(μ-O) (Me tacn) ](PF ) as the catalyst, during which H O was
applied as the solely terminal oxidant. The catalytic epoxidation ex-
periments in the promotion of trivalent metals like Al3+ and Sc were
investigated at different reaction temperatures as shown in Table S1.
3+
All chemicals were commercially available and used without further
purification unless otherwise indicated. The non-redox metal salts, such as
47