Alkene epoxidation catalyzed by manganese complexes with electronically modified 1,10-phenanthroline ligands

Article abstract of DOI:10.1016/j.ica.2011.12.029

The syntheses and alkene epoxidation activities of four manganese(II) complexes with electronically modified 1,10-phenanthroline ligands are reported. Each phenanthroline has a different substituent in the 5-position; the placement ensures that the modifications influence the reactivity through largely electronic effects. All four manganese complexes catalyze the conversion of alkenes to epoxides by commercially available peracetic acid. With each of the four alkenes investigated, the yield of the epoxide product is greater when the 5-substituent on the catalyst is strongly electron-donating, contrary to what has been observed with reactions catalyzed by manganese-salen complexes.

Full text of DOI:10.1016/j.ica.2011.12.029

Inorganica Chimica Acta 384 (2012) 340–344
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Inorganica Chimica Acta
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Alkene epoxidation catalyzed by manganese complexes with electronically
modified 1,10-phenanthroline ligands
⇑
Christian R. Goldsmith , Wenchan Jiang
Department of Chemistry and Biochemistry, Auburn University, Auburn, AL 36849, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The syntheses and alkene epoxidation activities of four manganese(II) complexes with electronically
modified 1,10-phenanthroline ligands are reported. Each phenanthroline has a different substituent in
the 5-position; the placement ensures that the modifications influence the reactivity through largely
electronic effects. All four manganese complexes catalyze the conversion of alkenes to epoxides by com-
mercially available peracetic acid. With each of the four alkenes investigated, the yield of the epoxide
product is greater when the 5-substituent on the catalyst is strongly electron-donating, contrary to what
has been observed with reactions catalyzed by manganese–salen complexes.
Received 8 September 2011
Received in revised form 14 December 2011
Accepted 15 December 2011
Available online 30 December 2011
Keywords:
Manganese
Alkene epoxidation
Electronic effects
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
5-position of the phen ligands in order to minimize the influence
of steric effects on the alkene epoxidation. Scheme 1 shows
The epoxidation of terminal and electron-deficient olefins re-
mains a challenging task in synthetic chemistry. Although many al-
kenes react directly with peracids, these uncatalyzed reactions
often require extended reactions times and/or elevated tempera-
tures, both of which can reduce selectivity. The use of transition
metal catalysts can enhance the selectivity, reduce reaction times,
and allow reactivity at ambient temperatures [1–10]. Many homo-
geneous and heterogeneous catalysts for epoxidation reactions
employ manganese as the active metal [1–7].
the molecular structure of the phen derivatives and a sketch of
the anticipated coordination around the Mn(II) ion [15]. Each of
the [Mn(R-phen)2Cl2] compounds (R = NH2, CH3, H, NO2) can cata-
lyze the oxygenation of alkenes by peracetic acid (Scheme 2). The
yields of the reactions are correlated with the electron-donating
capacity of the substituent on the 5-position, with electron-with-
drawing groups impeding, not hastening, the alkene epoxidation.
2. Experimental
Electron-rich olefins are generally more reactive, to the extent
that the development of catalysts capable of activating more elec-
tron-deficient olefins, such as terminal alkenes, remains an intense
area of research [1–3,10]. The terminal oxidant and the acidity of
the reaction also strongly influence the activity [1,2,7,11]. System-
atic studies on the influence of the electronic nature of the catalyst,
as opposed to that of the substrate [12], on alkene epoxidation are
comparatively rare [13,14]. In studies involving manganese–salen
catalysts, the addition of electron-donating substituents onto the
catalyst’s ligand were found to slow the rate and improve the stere-
oselectivity of olefin oxidation [5,13]. This has been attributed to
the ability of the electron donors to stabilize the Mn@O bond in
the Mn(V) oxo species that is believed to be the active oxidant [14].
We report three novel monomeric manganese complexes with
electronically modified derivatives of 1,10-phenanthroline (phen)
that are related to two highly active catalysts reported by Murphy
and Stack [2]. The electronic perturbations are installed on the
2.1. Materials and methods
Acetonitrile (MeCN), manganese(II) chloride (MnCl2),
1,10-phenanthroline (phen), 5-nitro-1,10-phenanthroline (NO2-
phen), 5-chloro-1,10-phenanthroline (Cl-phen), cyclohexene,
cyclohexene oxide, 3-cyclohexenol, 3-cyclohexenone, cyclooctene,
1-octene, 2,4-hexadienoic acid (sorbic acid), peracetic acid (32% in
acetic acid, PAA), and 1,2-dichlorobenzene were purchased from
Sigma–Aldrich and used as received. 5-Amino-1,10-phenanthro-
line (NH2-phen) and 5-methyl-1,10-phenanthroline (Me-phen)
were acquired from Acros. Anhydrous ethanol (EtOH) and anhy-
drous diethyl ether (ether) were bought from Pharmco-Aaper
and Fisher Scientific, respectively. Deuterated acetonitrile (CD3CN)
was purchased from Cambridge Isotopes.
2.2. Instrumentation
⇑
All 1H magnetic resonance (NMR) spectra were acquired on a
400 MHz AV Bruker NMR spectrometer at 294 K. All NMR
Corresponding author. Tel.: +1 334 844 6463; fax: +1 334 844 6959.
E-mail address: crgoldsmith@auburn.edu (C.R. Goldsmith).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.029

C.R. Goldsmith, W. Jiang / Inorganica Chimica Acta 384 (2012) 340–344
341
mental Anal. Calc. for C26H20Cl2MnN4°H2O: C, 58.66; H, 4.17; N,
10.53. Found: C, 58.83; H, 3.83; N, 10.52%. FT-IR (KBr, cmÀ1):
3043 (w), 2951 (s), 2922 (s), 2852 (s), 2725 (w), 2673 (w), 1462
(m), 1377 (m).
2.3.4. Dichloro(bis(5-amino-1,10-phenanthroline))manganese(II),
[Mn(NH2-phen)2Cl2]
NH2-phen (0.103 g, 0.51 mmol) was dissolved in 10 mL of EtOH.
The resultant solution was added to 0.051 g of MnCl2 (0.25 mmol)
and heated to dissolve as much of the residual manganese salt as
possible. The hot solution was filtered, then cooled to deposit the
product as a yellow powder (0.112 g, 87%). Solid-state magnetic
Scheme 1.
susceptibility (294 K):
leff = 5.7 lB. Elemental Anal. Calc. for
C
24H18Cl2MnN6: C, 55.83; H, 3.51; N, 16.28. Found: C, 55.24; H,
3.60; N, 15.97%. FT-IR (KBr, cmÀ1): 3385 (w), 2922 (s), 2854 (s),
2725 (w), 2671 (w), 1529 (m), 1514 (m), 1462 (m), 1377 (m),
1350 (m).
Scheme 2.
resonances were referenced to internal standards. A Shimadzu IR
Prestige-21 FT-IR spectrophotometer was used to record infrared
spectra on samples that were prepared as KBr pellets. All samples
that were submitted for elemental analysis were powdered and
dried before shipment; Atlantic Microlabs (Norcross, GA) per-
formed all elemental analysis. The magnetic susceptibilities of so-
lid samples were measured on a Johnson Matthey balance (model
MK I#7967). The diamagnetic contribution to each magnetic sus-
ceptibility was estimated using Pascal’s constants [16]. Electron
paramagnetic resonance (EPR) spectra were collected on a Bruker
EMX-6/1 X-band EPR spectrometer operated in the perpendicular
mode. Each EPR sample was run as a frozen 3:2 MeCN/CHCl3 solu-
tion in a quartz tube, and the program EasySpin was used to ana-
lyze the acquired EPR data. Gas chromatography (GC) was
obtained on a ThermoScientific Trace GC Ultra spectrometer with
a flame ionization detector (FID).
2.4. Reactivity studies
All reactions were run in MeCN at 0 °C under N2. The initial con-
centrations of the catalyst, olefin substrate, and terminal oxidant
(PAA) were 0.010, 1.00, and 2.00 mM, respectively. The reaction
mixtures were stirred for 1 h, at which point excess ether was
added to precipitate the manganese compounds. With all sub-
strates except sorbic acid, the quenched reaction mixtures were
sequentially filtered through plugs of neutral alumina, basic alu-
mina, and silica gel to remove the remaining PAA and metal salts.
For these substrates, the preceding workup did not remove organic
starting materials or products from the reaction mixture. For reac-
tions using sorbic acid as the substrate, the reaction mixtures were
not run through the basic alumina plug; the workup was otherwise
identical to that previously described. All products were identified
by both GC and NMR. The yields of the products were quantified
relative to both an internal standard of 1,2-dichlorobenzene, which
does not react under these conditions, and the remaining alkene
starting material. All reactions were run at least six times to ensure
reproducibility; the yields were averaged to provide the values
listed on Table 1.
2.3. Syntheses
2.3.1. Dichloro(bis(5-nitro-1,10-phenanthroline))manganese(II),
[Mn(NO2-phen)2Cl2]
A solution of NO2-phen (0.105 g, 0.47 mmol) in 10 mL of EtOH
was added to 0.046 g of MnCl2 (0.23 mmol). The resultant slurry
was heated until the residual manganese salt had dissolved. The
solution was filtered and cooled to precipitate the product as a yel-
low powder (0.113 g, 85%). Solid-state magnetic susceptibility
3. Results
3.1. Syntheses and characterization of the manganese complexes
(294 K):
leff = 5.7 lB. Elemental Anal. Calc. for C24H14Cl2MnN6O4:
All four organic ligands are commercially available (Scheme 1),
and the syntheses of the associated Mn(II) complexes are straight-
forward. The complex [Mn(phen)2Cl2] had been reported previ-
ously [17]; the other three Mn(II) compounds can prepared
through the same fundamental procedure. The yields for the metal
complex syntheses range from 70% to 83%, with no correlation
between the yield and the identity of the substituent on the 5
-position of the phen. Spectroscopically, the four compounds are
typical Mn(II) complexes. Each is pale yellow, with no
C, 50.02; H, 2.45; N, 14.58. Found: C, 49.80; H, 2.51; N, 14.54%.
FT-IR (KBr, cmÀ1): 3422 (m), 3325 (m), 3226 (m), 3051 (m), 2951
(m), 2922 (m), 2852 (m), 2729 (w), 2694 (w), 1634 (s), 1628 (s),
1501 (s), 1480 (s), 1378 (m).
2.3.2. Dichloro(bis(1,10-phenanthroline))manganese(II),
[Mn(phen)2Cl2]
The synthesis of this compound has been reported previously
[17]. Solid-state magnetic susceptibility (294 K):
leff = 5.8 lB. Ele-
mental Anal. Calc. for C24H16Cl2MnN4: C, 59.28; H, 3.32; N, 11.52.
Found: C, 58.41; H, 3.25; N, 11.35.FT-IR% (KBr, cmÀ1): 2951 (w),
2922 (w), 2852 (w), 2727 (w), 2673 (w), 1635 (m), 1544 (m),
1494 (m), 1300 (m).
Table 1
Yields of epoxides for Mn(II)-catalyzed olefin oxidation by PAA.
Substrate
R = NH2
R = Me
R = H
R = NO2
Controla
Cyclohexene
1-Octene
Cyclooctene
Sorbic acid
41 ( 7)
39 ( 4)
87 ( 6)
30 ( 3)
40 ( 7)
36 ( 6)
78 ( 7)
25 ( 3)
36 ( 7)
29 ( 7)
76 ( 7)
23 ( 5)
24 ( 5)
23 ( 7)
69 ( 9)
16 ( 5)
5 ( 3)
0b
2.3.3. Dichloro(bis(5-methyl-1,10-phenanthroline))manganese(II),
[Mn(Me-phen)2Cl2]
20 ( 3)
0b
Me-phen (0.100 g, 0.52 mmol) and MnCl2 (0.051 g, 0.26 mmol)
were added to 10 mL of EtOH. The resultant slurry was refluxed
until the solids had completely dissolved. The solution was filtered
and cooled to precipitate the product as a yellow powder (0.106 g,
Initial concentrations in MeCN: [substrate] = 1.00 mM, [PA] = 2.00 mM,
[Mn(R-phen)2Cl2] = 0.010 mM. Reactions ran at 0 °C for 1 h under N2.
a
Experiments were run in the absence of Mn(II) with identical concentrations of
substrate and PA.
b
No epoxide products were observed above the limit of detection.
79%). Solid-state magnetic susceptibility (294 K): leff = 5.6 lB. Ele-

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C.R. Goldsmith, W. Jiang / Inorganica Chimica Acta 384 (2012) 340–344
ligand-to-metal charge transfer bands past 300 nm. The magnetic
moments of the compounds range from 5.6 to 5.8 lB and are most
consistent with high-spin d5 metal centers. The electronic modifi-
cations to the ligand are not great enough to force a change in the
spin-state.
3.2. Reactivity studies with manganese compounds
The abilities of the four manganese compounds to act as cata-
lysts for olefin epoxidation were tested with four olefin substrates:
cyclohexene, cyclooctene, 1-octene, and sorbic acid (Scheme 3,
Table 1). The latter two are relatively electron-deficient and resis-
tant to epoxidation. In the absence of a metal catalyst, no epoxida-
tion above the detection limit of the GC is observed over the course
of 1 h for sorbic acid and 1-octene. The more electron-rich cyclo-
hexene and cyclooctene, conversely, will react directly with PAA,
with non-negligible amounts of products observable at 1 h. With
all four olefin substrates, the yield of the manganese-catalyzed
epoxidation dwarfs that of the uncatalyzed reaction. As shown in
Fig. 1, the epoxidation reaction is mostly finished at 1 h. The yield
continues to increase past this point, but this additional activity
can be attributed to the uncatalyzed reaction between the residual
olefin and the peracid.
Fig. 1. Epoxidation of cyclooctene by peracetic acid catalyzed by [Mn(NO2-
phen)2Cl2] in MeCN at 0 °C under N2. The results from four independent kinetic
runs are superimposed.
Only epoxides and unreacted alkene starting materials are ob-
served in the product mixtures. Cyclohexene is often oxidized at
the allylic position to 3-cyclohexenol and 3-cyclohexenone
[6,18], but neither of these side-products is observed. Diols, which
have been observed as products in some transition metal-catalyzed
epoxidation reactions [9,19], are likewise not seen above the limit
of detection. The diene sorbic acid is oxygenated at the more elec-
tron-rich alkene farther from the carboxylic acid functional group,
yielding 4,5-sorbic acid oxide as the sole organic product.
3.3. EPR spectroscopy of [Mn(NO2-phen)2Cl2]
The reaction between [Mn(NO2-phen)2Cl2] and PAA was fol-
lowed by EPR (Fig. 2). The addition of PAA to a solution of the
Mn(II) compound in 3:2 MeCN/CHCl3 does not generate Mn(IV)
species above the limit of detection, as had been observed by
Ottenbacher et al. in a recent study of manganese complexes with
neutral N-donor ligands [7]. The spectra of the solutions with 0, 2,
4, and 8 equiv of PAA are nearly identical, with integrated signal
intensities that are within error of each other. Although the solvent
and the concentrations of PAA in these samples are different than
those used in the catalytic epoxidation reactions, they were suffi-
cient to generate Mn(IV) species in the aforementioned studies of
Mn(II) complexes with aminopyridyl ligands [7]. When 50 equiv
of PAA are added, the integrated signal intensity decreases by
15%. This is accompanied by a noticeable browning of the solution,
which is typical of oxidation to either Mn(III) or Mn(IV) [20]. Mon-
itoring the reaction by optical spectroscopy reveals that the brown
color is due to an absorption band at 365 nm.
Fig. 2. X-band EPR spectra of a solution of 1.1 mM [Mn(NO2-phen)2Cl2] in 3:2
MeCN/CHCl3 (solid). To this solution was added 2, 4, 8, and 50 equiv of peracetic
acid. The following signal intensities were calculated through the double integra-
tion of the first-derivative spectra with subsequent normalization to the intensity of
the 0 Equiv spectrum: 1.00 (0 Equiv), 0.99 (2 Equiv), 0.97 (4 Equiv), 1.01 (8 Equiv),
0.85 (50 Equiv). The temperature of all samples was 77 K.
the current work, Mn(II) complexes with electronically modified
phenanthroline (phen) ligands are prepared and used to catalyze
the epoxidation of alkenes by peracetic acid (PAA). The electronic
perturbations are installed on the 5-position of the phen heterocy-
cle (Scheme 1); this placement minimizes the ability of the added
functional groups to modulate the chemistry through steric effects.
Previously, it was found that Mn(II) complexes with 2-derivatized
phen ligands displayed severely curtailed oxidative catalysis [2].
The functional groups present on the 2-position either block the
access of substrate and/or terminal oxidant to the metal center
or weaken the MnAN bonds enough to promote the dissociation
of the ligands from the manganous ions [2]. Either of these events
would account for the diminished reactivity.
4. Discussion
The reactivity was tested using commercially available PAA as
the terminal oxidant. Commercially available PAA is not as
Installing electronic modifications on organic ligands has been
shown to impact the reactivity of transition metal catalysts. In
Scheme 3.

C.R. Goldsmith, W. Jiang / Inorganica Chimica Acta 384 (2012) 340–344
343
effective as a previously reported, less basic grade of PAA [21]; con-
sequently, the yields (Table 1) and reaction times (Fig. 1) of the re-
ported olefin epoxidations compare poorly to previously reported
epoxidation catalyzed by Mn(II)-phen species [2]. The greatest
number of catalytic turnovers is 87, corresponding to the epoxida-
tion of cyclooctene to cyclooctene oxide by [Mn(NH2-phen)2Cl2],
and 20 of those turnovers can be accounted for by the background
reactivity (Table 1). The oxidative efficiency, defined as the yield
with respect to the terminal oxidant, is also inferior, reaching a
maximum of 44% in the aforementioned cyclooctene reaction.
The selectivities of the catalyzed reactions for the epoxide, how-
ever, are retained [2], and no products corresponding to allylic
oxidation or dihydroxylation are observed.
The less extensive reactivity offers an advantage in that it allows
us to differentiate the oxygenating activities of the four manganese
catalysts. Murphy and Stack had previously compared the reactivity
of manganese complexes with 1,10-phenanthroline and 5-chloro-
1,10-phenanthroline. With a 5 min reaction time, they did not ob-
serve a meaningful difference in the epoxidation activity using
either commercial or custom-made PAA [2]. Using a less efficient
terminal oxidant and extending the reaction times to 1 h reveal
the impact of such electronic modifications. When an electron-
withdrawing substituent (NO2) is used, the yields of all four alkene
epoxidation reactions decrease (Fig. 3). Conversely, electron-donat-
ing substituents (NH2, Me) increase the yields of the reactions mea-
sured at 1 h. The decrease in the yield going from R = NH2 to NO2 is
approximately 15% for each substrate, with a strong linear correla-
these already electron-deficient metal centers and provide a greater
thermodynamic impetus for the reduction of the manganese oxi-
dant, with concomitant oxidation of the alkene [13,14]. That we
see the opposite effect may indicate either a different rate-deter-
mining step or a fundamentally different mechanism.
The greater negative charge on the salens used in the aforemen-
tioned work would be anticipated to better stabilize more posi-
tively charged metal centers (e.g. Mn(V)) relative to neutral
ligands. The EPR spectra of [Mn(NO2-phen)2Cl2] and the [Mn
(NO2-phen)2Cl2]/PAA mixtures differ only slightly, and we observe
no unambiguous evidence for Mn(IV) even with a 50-fold excess of
terminal oxidant. Parallel experiments monitored by NMR did not
reveal any paramagnetic resonances that would be indicative of
Mn(III). The signal intensity of the Mn(II), however, does decrease
15% when a 50-fold excess of PAA is added. This loss of signal
intensity and the concurrent browning of the solution under these
circumstances suggest that the manganese is being oxidized. The
data suggest that [Mn(R-phen)2Cl2] complexes may be more diffi-
cult to oxidize than related salen and aminopyridyl manganese
complexes but certainly cannot preclude higher-valent oxidants
in the catalytic cycle. One mechanistic possibility is that the reac-
tivity proceeds through a Mn(IV) oxo species but with the rate-lim-
iting step being the oxidation of the Mn(II) rather than the oxygen
atom transfer from the oxidized manganese species to the alkene.
This would prevent the accumulation of Mn(IV) above the detec-
tion threshold of EPR and would explain the decreased activity
with more electron-withdrawing ligands.
tion between the yield at 1 h and the
r
p parameter for the substitu-
In their analysis of non-porphyrin manganese catalysts, Murphy
and Stack proposed that the acid in many epoxidation reactions
quenches the reactivity by protonating the phenanthroline, which
facilitates their removal from the manganese ions [1]. In their con-
trol studies, these unbound manganese ions were found to be unpro-
ductive as catalysts for epoxidation. The more electron-deficient
phen derivatives are anticipated to bind to the metal ions less avidly
and [Mn(R-phen)2Cl2] complexes with these ligands may be more
susceptible to acid-induced deactivation. Although we do not
observe any of the 1H NMR features associated with dissociated
phenanthroline ligands in mixtures of the [Mn(R-phen)2Cl2] species
with either acetic acid or peracetic acid, this remains a plausible
alternative explanation for the reduced activity of [Mn(NO2-
phen)2Cl2].
Another possibility consistent with the EPR data is that the ac-
tive oxidant is a Mn(II)–PAA adduct and that higher-valent manga-
nese species are not relevant to the hydrocarbon oxidation. Similar
low-valent metal species have been mentioned as potential oxi-
dants in other transition metal-based epoxidations [7,24]. Further-
more, complexes with metals that generally do not exhibit much
redox activity (e.g. Ti(IV), Al(III), Zn(II)) have been found to act as
competent catalysts for alkene epoxidation [18,25]. If the metal
ion were merely acting as a Lewis acid, however, more strongly
electron-withdrawing ligands would be anticipated to amplify
the reactivity, which is contrary to what we observe.
ent on the 5-position of the phen. With cyclohexene, 1-octene, and
sorbic acid, the reactivity is approximately halved upon switching
the ligand from NH2-phen to NO2-phen. The R values for the fits
in Fig. 3 range from 0.95 (cyclohexene) to 0.98 (cyclooctene). These
results differ from the manganese–salen chemistry developed by
Jacobsen, in which electron-withdrawing substituents were found
to increase the rate of epoxidation [5,13,14]. As with other reported
systems, more electron-rich olefins (cyclohexene, cyclooctene) are
oxygenated to a greater extent than more electron-deficient ones
(sorbic acid, 1-octene) [1,7,13,18,22], and the diene sorbic acid is
oxygenated at the more electron-rich 4,5-alkene.
There is compelling evidence that Jacobsen’s system proceeds
through a Mn(V) oxo species [5,23], which installs the oxygen atom
onto the alkene through either a single two-electron or two one-
electron steps [12]. A study of Mn(II) complexes with aminopyridyl
ligands also implicates higher-valent oxidants in epoxidation, spe-
cifically Mn(IV) oxo adducts with the terminal oxidant [7]. The pres-
ence of electron-withdrawing groups would further destabilize
5. Conclusions
A series of Mn(II) complexes with electronically modified phe-
nanthroline ligands has been prepared. The three novel compounds
catalyze the epoxidation of alkenes by commercially available per-
acetic acid. Unexpectedly, the more electron-rich ligands lead to
faster reactivity with all alkenes investigated, as assessed by the
yields of the olefin oxidations measured at 1 h. The results may sug-
gest that [Mn(phen)2Cl2] and related complexes may oxidize al-
kenes to epoxides through a fundamentally different free energy
pathway than Mn(III)–salen catalysts. Further experimental and
theoretical work is needed to fully elucidate these differences.
Fig. 3. Plot of the reaction yields (as assessed at 1 h) for the oxygenation of each
alkene as
a function of the rp value of the 5-substituent on the catalyst’s
phenanthroline ligands.

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[d] F.G. Gelalcha, G. Anilkumar, M.K. Tse, A. Brückner, M. Beller, Chem. Eur. J. 14
Acknowledgments
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[e] M. Bühl, R. Schurhammer, P. Imhof, J. Am. Chem. Soc. 126 (2004) 3310.
[9] [a] K. Chen, M. Costas, J. Kim, A.K. Tipton, L. Que Jr., J. Am. Chem. Soc. 124
(2002) 3026;
This work was funded by Auburn University and a Doctoral
New Investigator Grant from the American Chemical Society-
Petroleum Research Fund. The authors are also grateful to Prof.
Evert Duin for his assistance acquiring and interpreting the EPR
data.
[b] R. Mas-Ballesté, L. Que Jr., J. Am. Chem. Soc. 129 (2007) 15964.
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