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pected, the plot follows a S=15/2 Brillouin function (Figure 3,
inset).
Physical measurements
Elemental analyses were carried out with a Perkin–Elmer 240 ele-
mental analyzer. 1H NMR spectra were recorded in CDCl3 with
a Bruker 300 MHz NMR spectrophotometer using tetramethylsilane
(d=0 ppm) as an internal standard. Electronic spectra were record-
ed with an Agilent-8453 diode array UV/Vis spectrophotometer. In-
frared spectra (400–4000 cmÀ1) were recorded from KBr pellets
with a Nicolet Magna IR 750 series-II FTIR spectrophotometer. Mag-
netic susceptibility measurements were carried out on polycrystal-
line samples using a Quantum Design MPMS-5 SQUID susceptome-
ter working in the range 2–300 K under magnetic fields of 0.3 T
(300–30 K) and 0.03 T (30–2 K) to avoid saturation effects. Diamag-
netic corrections were estimated from Pascal tables.
Although, mixed oxo or hydroxo/azide bridges have been
widely studied[55] and usually they promote ferromagnetic cou-
pling, the corresponding alkoxo/azide bridgeshave been
scarcely reported to date.[56] Compound 1 shows an unprece-
dented phenoxo/azido bridge between MnII centers, and the
ferromagnetic interaction between the spin carriers is consis-
tent with the Mn-Ophenoxo-Mn and Mn-Nazido-Mn bond angles
that lie in the 99.4(5)–103.4(4)8 range, which is usually associat-
ed with ferromagnetic coupling.[55]
Catalytic studies
Conclusion
The catalytic epoxidation reactions were carried out in the liquid
phase in a batch reactor at approximately 558C. During our stud-
ies, a 50 mL two-necked, round-bottomed flask equipped with
a water condenser that contained alkene (65 mmol in the case of
styrene and 50 mmol in that of cyclooctene) in acetonitrile (MeCN;
10 mL) solvent and catalyst (0.03 mmol) was kept in a preheated
oil bath. TBHP (2 mL) was then added to the mixture and it was
magnetically stirred continuously for 24 h. TBHP (0.5 mL) was
added intermittently at a time interval of 60 min. The products of
the epoxidation reactions were collected at different time intervals
and were identified and quantified by means of gas chromatogra-
phy.
A dodecadentate N8O2 donor ligand, H2L, was generated in
situ, which, upon reacting with manganese(II) acetate in meth-
anol in the presence of azide ion, gives a linear trinuclear man-
ganese(II) complex [Mn3(L)(N3)4]·2H2O (1) as a brown rod-
shaped crystal. The structure determination by means of
single-crystal X-ray crystallography reveals that it is a linear tri-
nuclear MnII complex in which all the MnII centers are in
pseudo-octahedral geometry. The central manganese atom
(Mn2) is connected to two terminal manganese (Mn1 and
Mn3) atoms by two bridging phenolate-O atoms and two
bridging azide-N atoms with Mn···Mn internuclear distances in
the range of 3.37–3.88 . Compound 1 is the first example of
a linear trinuclear model that showed pronounced epoxidation
of olefins by tert-butylhydroperoxide with a TON>950. Al-
though epoxidation of olefins by m-chloroperbenzoic acid (m-
CPBA) was found to be complete within 20 minutes of mixing,
the conversion was not more than 40%, and corresponding al-
cohols were found to be major products, whereas with H2O2 as
oxidant there was no visible catalytic epoxidation of olefins.
HRMS analysis revealed that the catalytic reactions proceed
through the formation of [(Mn3L)(N3)4(m-CPBA)] intermediate
with an expansion of coordination number. However, we failed
to identify such an intermediate for TBHP as oxidant. Further-
more, it was characterized by temperature-dependent magnet-
ic susceptibility measurements, and a total spin ground state
of St =5/2 was determined, which was found to be involved in
ferromagnetic interactions with the neighboring MnII centers.
Synthesis of proligand 2-[N,N-bis(2-methylpyridyl)amino-
methyl]-6-carbaldehyde-4-methylphenol (HL)
2-Chloromethyl-6-carbaldehyde-4-methylphenol
(1.515 g,
8.2 mmol) was dissolved in dry THF (15 mL) in a round-bottomed
flask. Bis-picolylamine (1.630 g, 8.2 mmol) and triethylamine (Et3N)
(1.659 g, 16.4 mmol) were dissolved in dry THF (10 mL), and this
mixture was added dropwise to the 2-chloromethyl-6-carbalde-
hyde-4-methylphenol solution. Instant precipitation of Et3NHCl was
observed, and the solution turned bright yellow (Scheme 2). After
24 h of stirring, the precipitate Et3NHCl was removed by filtration.
Solvent (THF) was removed under reduced pressure to obtain an
oily product, which, upon storage in the refrigerator for a couple
of days, yielded a light-yellow crystalline solid. The solid product
1
was then filtered and washed with cold ether. Yield: 78%. H NMR
(CDCl3, 300 MHz): d=2.27 (s, 3H; Me), 3.80, 3.85 (s, 2H; Ar-CH2-N),
3.94 (s, 4H; N-CH2-py); 7.16–7.68 (8H; pyridine H), 7.40 and 7.37
(2H; aromatic H), 10.39 (s, 1H; OH), 11.48 ppm (s, 1H; CHO).
Synthesis of complex [Mn3(L)(N3)4]·2H2O (1)
Experimental Section
2-[N,N-Bis(2-methylpyridyl)aminomethyl]-6-carbaldehyde-4-methyl-
phenol (0.347 g, 1.00 mmol) and ethylenediamine (0.03 g,
0.50 mmol) in methanol (30 mL) were heated to reflux together for
40 min. After cooling to room temperature, manganese(II) acetate
tetrahydrate (0.368 g, 1.50 mmol) was added and heated to reflux
for 1 h. Sodium azide (0.260 g, 4 mmol) was then added, and the
mixture was stirred for another 24 h. The yellow solution turned
brown (Scheme 3). It was filtered and kept in a rack. Slow evapora-
tion of methanol gave rod-shaped brown crystals suitable for X-ray
studies. Elemental analysis calcd (%) for C44H46Mn3N20O4 (1083.82):
C 48.72, H 4.15, N 25.83; found: C 48.55, H 4.18, N 25.70.
Reagents
2-(Chloromethyl)-6-carbaldehyde-4-methylphenol and 2-[N,N-bis(2-
methylpyridyl)aminomethyl]-6-carbaldehyde-4-methylphenol were
prepared by the reported method.[57] Mn(OAc)2·4H2O (Aldrich), eth-
ylenediamine (Merck, India), benzyl amine (Merck, India), triethyl-
amine (Merck, India), bis-picolylamine (Aldrich), and sodium azide
(Merck, India) were of reagent grade and used as received. Sol-
vents such as MeCN (Merck India), methanol, ethanol, and other
solvents were of reagent grade and were dried using standard
methods before use.
ChemPlusChem 2015, 80, 1440 – 1447
1445
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Dutta, Arpan
