Dinuclear Mn(ii,ii) complexes: Magnetic properties and microwave assisted oxidation of alcohols

Article abstract of DOI:10.1039/c3dt52774c

A series of six new mixed-ligand dinuclear Mn(ii,ii) complexes of three different hydrazone Schiff bases (H₃L¹, H ₃L² and H₃L³), derived from condensation of the aromatic acid hydrazides benzohydrazide, 2-aminobenzohydrazide or 2-hydroxybenzohydrazide, with 2,3-dihydroxy benzaldehyde, respectively, is reported. Reactions of Mn(NO₃) ₂·4H₂O with the H₃L^1-3 compounds, in the presence of pyridine (1:1:1 mole ratio), in methanol at room temperature, yield [Mn(H₂L¹)(py)(H₂O)] ₂(NO₃)₂·2H₂O (1·2H₂O), [Mn(H₂L²)(py)(CH ₃OH)]₂(NO₃)₂·4H₂O (2·4H₂O) and [Mn(H₂L³)(py)(H ₂O)]₂(NO₃)₂ (3) respectively, whereas the use of excess pyridine yields complexes with two axially coordinated pyridine molecules at each Mn(ii) centre, viz. [Mn(H₂L ¹)(py)₂]₂(NO₃)₂· H₂O (4·H₂O), [Mn(H₂L²)(py) ₂]₂(NO₃)₂·2H₂O (5·2H₂O) and [Mn(H₂L³)(py) ₂]₂(NO₃)₂·2CH₃OH (6·2CH₃OH), respectively. In all the complexes, the (H ₂L^1-3)^- ligand coordinates in the keto form. Complexes 1·2H₂O, 2·4H₂O, 4·H ₂O, 5·2H₂O and 6·2CH₃OH are characterized by single crystal X-ray diffraction analysis. The complexes 1, 2 and 6, having different coordination environments, have been selected for variable temperature magnetic susceptibility measurements to examine the nature of magnetic interaction between magnetically coupled Mn(ii) centres and also for exploration of the catalytic activity towards microwave assisted oxidation of alcohols. A yield of 81% (acetophenone) is obtained using a maximum of 0.4% molar ratio of catalyst relative to the substrate in the presence of TEMPO and in aqueous basic solution, under mild conditions.

Full text of DOI:10.1039/c3dt52774c

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Dinuclear Mn(II,II) complexes: magnetic properties
and microwave assisted oxidation of alcohols†
Cite this: DOI: 10.1039/c3dt52774c
a
a,b
a
Manas Sutradhar, Luísa M. D. R. S. Martins, M. Fátima C. Guedes da Silva,
a,b
c
a
Elisabete C. B. A. Alegria, Cai-Ming Liu and Armando J. L. Pombeiro*
A series of six new mixed-ligand dinuclear Mn(II,II) complexes of three different hydrazone Schiff bases
1
2
3
(H
3
L , H
3 3
L and H L ), derived from condensation of the aromatic acid hydrazides benzohydrazide, 2-amino-
benzohydrazide or 2-hydroxybenzohydrazide, with 2,3-dihydroxy benzaldehyde, respectively, is reported.
1–3
Reactions of Mn(NO
3
)
2
·4H
2
O with the H
3
L
compounds, in the presence of pyridine (1 : 1 : 1 mole ratio),
1
2
in methanol at room temperature, yield [Mn(H
2
L )(py)(H (NO
2
O)]
2
3 2 2 2 2
) ·2H O (1·2H O), [Mn(H L )(py)-
3
(CH
3
OH)]
2
(NO
3
)
2
2
·4H O (2·4H
2
O) and [Mn(H
2
L )(py)(H
2
O)] (NO ) (3) respectively, whereas the use of
2
3 2
excess pyridine yields complexes with two axially coordinated pyridine molecules at each Mn(II) centre,
1
2
3
viz. [Mn(H
py) (NO
the keto form. Complexes 1·2H
2
L )(py)
2
]
2
(NO
3
)
2
·H
2
O
(4·H
OH), respectively. In all the complexes, the (H
O, 2·4H O, 4·H O, 5·2H O and 6·2CH OH are characterized by single
2
O), [Mn(H
2
L )(py)
2
]
2
(NO
3
)
2
·2H
2
O
(5·2H
2
O) and [Mn(H
–3 −
2
L )-
1
(
2
]
2
3
)
2
·2CH OH (6·2CH
3
3
2
L
) ligand coordinates in
2
2
2
2
3
crystal X-ray diffraction analysis. The complexes 1, 2 and 6, having different coordination environments,
have been selected for variable temperature magnetic susceptibility measurements to examine the nature
of magnetic interaction between magnetically coupled Mn(II) centres and also for exploration of the cata-
lytic activity towards microwave assisted oxidation of alcohols. A yield of 81% (acetophenone) is obtained
using a maximum of 0.4% molar ratio of catalyst relative to the substrate in the presence of TEMPO and in
aqueous basic solution, under mild conditions.
Received 4th October 2013,
Accepted 19th December 2013
DOI: 10.1039/c3dt52774c
www.rsc.org/dalton
are efficient catalysts for the oxygenation of heteroatom con-
taining organic compounds and also behave as excellent cata-
Introduction
6
Manganese plays an important and diverse role in the bio- lysts for the highly enantioselective asymmetric epoxidation of
7
chemistry of many microorganisms, plants and animals. olefins. Nowadays, microwave-assisted oxidation of alcohols
Dinuclear manganese containing active sites are found in a has attracted special attention because it can be a faster,
1
large variety of enzymes that catalyze biological processes, e.g. simpler, and more energy efficient technique compared to the
1
c,g–j,2
3
Mn catalases
and arginase, and have received significant conventional methods. Manganese oxides and some simple
8
attention in bioinorganic chemistry. Besides the biological salts showed a significant activity as catalysts for this process,
importance, the relevance in catalysis, under homogeneous but, to the best of our knowledge, manganese complexes have
and heterogeneous conditions, is also well recognized. Dinuc- not yet been explored as catalysts under these conditions.
4
2 2
lear manganese complexes of functionalized N O Schiff Recently our group reported that Cu(II) complexes are efficient
9
bases, e.g., [Mn(2-OHsalpn)] (2-OHsalpn = 1,3-bis(salicylidene- catalysts in the microwave-assisted oxidation of alcohols.
2
amino)-2-propanol), are found to be efficient peroxide dispro- Therefore, the exploration of manganese complexes with suit-
5
portionation catalysts in acetonitrile. Mn(III)-salen complexes able N, O donor ligands as catalysts for this reaction would be
timely and constitutes an aim of the current work.
On the other hand, manganese complexes are well recog-
nized as magnetic materials, and the polynuclear manganese
a
Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade
de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal.
E-mail: pombeiro@tecnico.ulisboa.pt
10
cluster [Mn O (CH COO) (H O) ]·2CH COOH·4H O was
1
2
12
3
16
2
4
3
2
1
1
b
Chemical Engineering Department, ISEL, R. Conselheiro Emídio Navarro,
reported to be the first single molecule magnet (SMM).
1
959-007 Lisboa, Portugal
SMMs have been synthesized with many different nuclearities,
c
Beijing National Laboratory for Molecular Sciences, Center for Molecular Science,
12
13
ranging from dinuclear and trinuclear metal complexes to
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
14
others as large as the Mn 84 wheel.
†
CCDC 964300–964304 for 1·2H
respectively. For crystallographic data in CIF or other electronic format see DOI:
0.1039/c3dt52774c
2 2 2 2 3
O, 2·4H O, 4·H O, 5·2H O and 6·2CH OH
Therefore, manganese complexes in general and the dinuc-
lear Mn(II,II) compounds in particular with N/O donor ligands
1
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1–3
have received considerable attention in recent years. In addition, attempts of reactions of Mn(NO
3
)
2
·6H
2
O with H
3
L
in metha-
hydrazone Schiff bases form highly stable complexes and can be nol in the absence of pyridine did not lead to complex for-
15
tuned easily by changing different carbonyl derivatives. In this mation, and the presence of an excess of this base did not
context, herein we report a series of six dinuclear Mn(II,II) com- promote enolization during the desired complexation process.
plexes (1–6) using three hydrazone Schiff base ligands derived Therefore, pyridine (base/co-ligand) allowed Mn(II) to form
from condensation of three different aromatic acid hydrazides stable complexes and promoted exclusively the keto binding
with 2,3-dihydroxy benzaldehyde. This work also focuses on vari- mode of coordination of the hydrazino ligands. Such a binding
able temperature magnetic (VTM) susceptibility measurements of mode was also observed in one Mn(II) complex with a similar
17
three dinuclear Mn(II,II) complexes (1, 2 and 6) to examine the type of ligand. We selected the three different types of dinuc-
nature of their magnetic behaviour and on their application as lear Mn(II,II) complexes 1, 2 and 6 to examine the nature of mag-
catalysts in microwave assisted oxidation of alcohols.
netic interactions and also to study their catalytic properties
towards microwave assisted oxidation of alcohols.
Results and discussion
General description of the crystal structures
1
2
3
3 3 3
Three hydrazone Schiff bases (H L , H L and H L ), derived X-ray quality crystals of the compounds were obtained upon
from the reaction of 2,3-dihydroxy benzaldehyde separately
with benzohydrazide, 2-aminobenzohydrazide or 2-hydroxy-
slow evaporation of their methanolic solutions, at room
temperature. The crystallographic data and processing
benzohydrazide, respectively, have been used in this study. parameters are summarized in Table 1, representative plots are
Hydrazone Schiff bases exhibit keto–enol tautomerism in solu-
tion and in the majority of cases the enol form undergoes
1
6
complexation in the presence of a metal ion. Here a series of
mixed-ligand complexes of Mn(II) is reported with these Schiff
bases where, in contrast with the above, coordination takes
place via the keto form (Scheme 1).
1
–3
Reactions of Mn(NO ) ·4H O with the H L
compounds,
3
2
2
3
in the presence of pyridine (1 : 1 : 1 mole ratio), in methanol
and at room temperature, yield the dinuclear manganese(II)
1
complexes [Mn(H L )(py)(H O)] (NO ) ·2H O (1·2H O), [Mn-
2
2
2
3 2
2
2
2
3
(H
H
2
L )(py)(CH
3
OH)]
2
(NO
3
)
2
2 2 2
·4H O (2·4H O) and [Mn(H L )(py)-
(
2
O)] (NO ) (3), respectively. The use of excess pyridine in
2
3 2
the reaction medium yields dinuclear manganese(II) complexes
with two axially coordinated pyridine molecules at each Mn(II)
1
2
centre, viz. [Mn(H
2
L )(py)
2 2 3 2 2 2 2
] (NO ) ·H O (4·H O), [Mn(H L )-
3
(
py) ] (NO ) ·2H O (5·2H O) and [Mn(H L )(py) ] (NO ) ·
1–3 −
2
2
3 2
2
2
2
2 2
3 2
Scheme 1 Mode of coordination of the ligands (H2L
) in the dinuc-
2
CH OH (6·2CH
3 3
OH), respectively. It is noteworthy that lear Mn(II,II) complexes.
Table 1 Crystal data and structure refinement details of complexes 1·2H
·2H 2·4H
2
O, 2·4H
2
O, 4·H
4·H
2
O, 5·2H
2
O and 6·2CH
5·2H
3
OH
1
2
O
2
O
2
O
2
O
3
6·2CH OH
Empirical formula
Formula weight
Temp./K
C
38
H
40Mn
N O
2 8 16
C
40
H
42Mn
N O
2 10 18
C
48
H
44Mn
N O
2 10 13
C
48
H
48Mn
N O
2 12 14
50 2 10 16
C H50Mn N O
1156.88
150
974.66
150
1068.78
296
1078.81
296
1126.86
150
Crystal system
Space group
Orthorhombic
Pbca
Monoclinic
C2/c
Monoclinic
C2/c
Triclinic
Pˉ1
Monoclinic
P2 /c
1
a/Å
b/Å
c/Å
α/°
β/°
γ/°
11.1784(7)
18.6890(9)
19.5725(9)
90
90
90
4089.0(4)
4
1.583
0.702
18 229/4153/2382
0.0929
27.7167 (12)
10.8872 (5)
19.6740 (10)
90
126.3820(10)
90
4779.6 (4)
4
1.474
0.612
26 060/4103/2809
0.0735
0.0530, 0.1469
1.044
12.8357 (4)
17.3807 (5)
23.5331 (6)
90
100.2330 (10)
90
5166.6 (3)
4
1.387
0.560
17 872/3953 /3062
0.0322
0.0568, 0.1456
1.022
10.5455 (8)
10.5706 (9)
12.8509 (6)
18.0597 (9)
12.2873 (6)
90
108.603 (2)
90
2702.7 (2)
2
1.422
0.545
20 228/5524/3729
0.0504
0.0442, 0.1061
1.027
11.9344 (10)
95.750 (5)
106.408 (5)
103.690 (6)
1219.84 (17)
1
1.534
0.599
9698/4154/2075
0.1010
0.0868, 0.2086
1.023
3
V (Å )
Z
D
calc (g cm−
3
)
−
1
μ(Mo Kα) (mm
Rfls. collected/unique/observed
)
R
int
a
1
b
Final R
Goodness-of-fit on F
, wR
2
(I ≥ 2σ)
0.0519, 0.1018
0.973
2
a
b
2
2
2 2
4 1/2
o
R = ∑||F | − |F
c
||/∑|F
o
|. wR(F ) = [∑w(|F
o
| − |F
c
| ) /∑w|F
o
| ]
.
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i
displayed in Fig. 1, and selected dimensions are presented in imposed inversion centres in the middle of the Mn1–Mn1
Table 2. Relevant hydrogen bond interactions are shown in bonds, and therefore in the heart of the respective Mn O2
2
Table 3.
planes. The Schiff bases act as monoanionic and tetradentate
X-ray single-crystal analyses of the dinuclear complexes ONOO equatorial ligands, coordinating to one of the metal
1
·2H O, 2·4H O, 4·H O, 5·2H O and 6·2CH OH show that their centres via the keto oxygen, the hydrazino nitrogen and the
2
2
2
2
3
asymmetric units contain half of the molecules, viz. one deprotonated 2-hydroxyl oxygen, and to the other metal cation
manganese(II) cation with one coordinated ligand, one pyri- through the phenoxy group and the 3-hydroxyl moiety. Thus,
dine and one water (or one methanol) molecule (1·2H O or each structure exhibits two µ-O bridges which link the two
2
2
·4H
pyridine moieties (4·H
metric units also contain one (1·2H O, 4·H O and 5·2H O) or geometries. The axial positions are occupied by the pyridine
2
O, respectively), or one manganese(II) cation with two manganese(II) centres which, by themselves, display hepta
2
O, 5·2H O and 6·2CH OH). The asym- coordination and present distorted pentagonal bipyramidal
2
3
2
2
2
two (2·4H
2
O) water molecules or a methanol molecule nitrogen atoms and the water or the methanol oxygen atoms,
(
6·2CH OH). All the structures contain crystallographically as appropriate.
3
Fig. 1 Thermal ellipsoid plots (drawn at 30% probability level) of complexes (a) 1·2H
the partial atom labeling scheme. Nitrate anions, water and methanol molecules are omitted for clarity. Symmetry codes to generate equivalent
atoms: (i) −x,−y, 1 − z (1·2H O), 1 − x, 1 − y, 2 − z (2·4H O), 1.5 − x, 1/2 − y, 1 − z (4·H O), 1 − x, 1 − y, 2 − z (5·2H O), −x, −y, −z (6·2CH OH).
2 2 2 2 3
O, (b) 2·4H O, (c) 4·H O, (d) 5·2H O and (e) 6·2CH OH with
2
2
2
2
3
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a
Table 2 Selected bond distances (Å) and angles (°) in complexes 1·2H
2
O, 2·4H
2
O, 4·H
2
O, 5·2H
2
O and 6·2CH
3
OH
Parameter
2
1·2H O
2
2·4H O
2
4·H O
2
5·2H O
3
6·2CH OH
Mn environment
N–N
Mn–Nhyd
Mn–NPyr
N
2
O
5
N
2
O
5
N
3
O
4
N
3
O
4
3 4
N O
1.382(4)
2.340(3)
2.254(3)
1.391(4)
2.350(3)
2.284(4)
1.451(4)
2.412(3)
2.310(3)
1.385(9)
2.345(6)
2.282(7)
2.295(7)
2.146(5)
2.290(5)
2.276(5)
2.291(6)
—
175.9(3)
—
74.4(2)
105.63(19)
4.04
1.375(3)
2.336(2)
2.267(2)
2.282(2)
2.1802(16)
2.2672(16)
2.2925(17)
2.3291(18)
—
173.86(8)
—
74.47(7)
105.53(7)
3.02
2.320(3)
Mn–Ocore
2.140(3)
2.160(2)
2.269(2)
2.282(2)
2.340(2)
2.200(2)
—
172.83(11)
75.24(9)
104.76(9)
—
2.216(2)
2.375(2)
2.329(3)
2.412(3)
—
173.49(12)
—
73.39(11)
106.61(10)
8.87
2.288(2)
Mn–Ool
Mn–OKeto
Mn–OH2O(MeOH)
2.297(3)
2.335(3)
2.163(3)
—
166.89(13)
74.30(9)
105.69(9)
—
∠
∠
∠
∠
∠
∠
N
N
O
pyr–Mn–Npyr
pyr–Mn–OH2O(MeOH)
core–Mn–Ocore
Mn–Ocore–Mn
P
pyr/Ppyr
Pcore/Pphenyl
9.27
3.5306(8)
30.05
3.5088(10)
10.11
3.6829(7)
35.28
3.5356(18)
22.04
3.5413(5)
Mn⋯Mn
a
Nhyd = coordinated N-hydrazino atom; Npyr = coordinated N-pyridine atom. Ool, Oketo and OH2O(MeOH) refer to the deprotonated, keto and water
(
or methanol) O-atoms, respectively. Pcore, Ppyr and Pphenyl represent the mean planes of the Mn
2
(μ-O)
2
cores, the pyridine rings and the non-
coordinated phenyl ring, in this order.
a
Table 3 Hydrogen bond geometry (Å, °) in the manganese compounds 1·2H
2
O, 2·4H
2
O, 4·H
2
2 3
O, 5·2H O and 6·2CH OH
Compound
D–H⋯A
d(H⋯A)
d(D⋯A)
∠(D–H⋯A)
Symmetry operation
1
·2H
2
O
O
N2–H2⋯O20
2.17(3)
1.794(19)
1.984(19)
1.87(2)
2.11(3)
2.18
1.75
1.94
2.00
2.22(8)
2.14
1.90(5)
2.05(8)
1.81
2.110
2.030
2.13(8)
1.99(4)
2.08(4)
1.69
2.01
1.98
1.71
1.74(3)
2.982(4)
2.675(4)
2.851(4)
2.779(5)
2.855(4)
2.907(4)
2.651(4)
2.769(9)
2.770(5)
2.732(4)
2.913(5)
2.819(9)
2.884(7)
2.728(5)
2.735(9)
2.880(10)
2.876(10)
2.876(13)
2.920(10)
2.594(9)
2.646(3)
2.822(3)
2.628(2)
2.692(3)
168(4)
173(3)
175(3)
176(3)
142(3)
142.6
161.2
148.0
138.6
116(6)
149.8
—
O2–H2O⋯O12
O10–H10A⋯O11
O10–H10B⋯O13
O20–H20B⋯O13
N12–H12A⋯O13
O3–H3⋯O1W
1/2 − x, 1/2 + y, z
−1/2 + x, 1/2 − y, −z
−1 + x, y, z
−1/2 + x, y, 1/2 − z
1/2 − x, −1/2 + y, 1/2 − z
1/2 + x, 1/2 − y, −1/2 + z
1/2 + x, 1/2 + y, z
1/2 + x, 1/2 + y, z
Intra
2·4H
2
O20–H20⋯O3W
O20–H20⋯O2W
N2–H2B⋯O2
N12–H12A⋯O20W
O20W–H20A⋯O41
O20W–H20A⋯O40
O3–H3A⋯O42
N3–H3A⋯O2
N3–H3B⋯O11
N12–H12A⋯O12
O10–H10B⋯O11
O10–H10A⋯N3
O3–H3⋯O10
N12–H12A⋯O4
O20–H20⋯O13
O3–H3O⋯O20
O4–H4O⋯O12
4·H
2
O
—
—
—
154(10)
141(10)
170.2
x, −y, −1/2 + z
Intra
5·2H
2
O
128.0
163.7(5)
155(9)
158(7)
153(6)
157.8
128.6
174.6
1 − x, 2 − y, 1 − z
1 + x, y, z
1 − x, 1 − y, 1 − z
1 − x, 1 − y, 1 − z
x, y, −1 + z
Intra
6·2CH
3
OH
—
174.53(12)
173(3)
1 + x, y, 1 + z
—
a
Only the strongest classical hydrogen bond interactions are shown (D⋯A distances lower than 3 Å).
Although run at low (1, 5 and 6) and at room temperatures Npyr–Mn–Npyr groups are nearly linear (minimum of 173.49
2 and 4), and considering the similarities between the ligands (12)° for 4·H O) and exceed the Npyr–Mn–OH2O(MeOH) values of
and the obtained structures, some general comparisons can 166.95(12)° (1·2H O) and 172.80(11)° (2·4H O). The latter
tentatively be made (Table 2). The bond distances of 1.380(4)– values are intimately related with the H-bond interactions in
.392(4) Å for the N–N groups suggest their single bond hydra- which the water and methanol ligands are involved (see
zino character. The Mn–Nhydrazino bond distances [average below).
(
2
2
2
1
2
2
.357(3) Å] are longer than the Mn–Npyridine bonds [average
.287(4) Å] and, for those complexes with two pyridine ligands 104.79(9) to 106.61(10)° and the O–Mn–O angle assumes values
O, 5·2H O and 6·2CH OH), one of the of 73.39(11)–75.21(9)° (Table 2); expectedly, the widening of the
bonds slightly exceeds the other (by 0.010–0.016 Å). The former leads to a tapering of the latter. Comparing the various
In the Mn (μ-O) cores the Mn–O–Mn angles range from
2
2
per metal cation (4·H
2
2
3
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Mn–O bond distances, it is observed that those involving the ultimately interact with it; in contrast, the OH group in 6 is
keto oxygen are larger, the only exception being 5·2H O where engaged with the H-atom of the hydrazine set.
2
this parameter is similar to the one of the Mn–Ocore
.
Extensive hydrogen bond interaction can be found in all
ligands to the the compounds of this work (Fig. 2, 3 and Table 3). Such inter-
Mn(II) cation in the complexes of this study is also worth dis- actions result from the presence of coordinated (in 1·2H O) or
1
–3 −
The mode of coordination of the (H
2
L
)
2
2 2 2 2
cussing, in what concerns their planarity here evidenced by free (in 1·2H O, 2·4H O, 4·H O and 5·2H O) water molecules
the twistings of the phenyl rings (denoted as Pphenyl, Table 2) which act both as proton donors (usually to nitrate anions;
relative to the plane of the Mn (μ-O) core (symbolized by Pcore, eventually to the N-aniline moiety in 2·4H O and 5·2H O) and
2
2
2
2
Table 2). While for complexes 1·2H
meter assumes values of 9.27 and 10.11°, respectively, for com- HN-aniline protons in 4·H
plexes 2·4H O and 5·2H O, which possess NH substituents in in 2·4H O and 5·2H O). These interactions are usually strong
2
O and 4·H
2
O, that para- as proton acceptors (from coordinated methanol in 2·4H
2
O,
2
O or the protonated phenol group
2
2
2
2
2
those phenyl groups, it adopts values as high as 30.05 and [d(D⋯A) distances in the 2.594(9)–2.982(4) Å range, and ∠(D–
5.28°, in this order; complex 6, with an OH substituent, pre- H⋯A) between 116 and 176°], the most intense one [d(D⋯A)
3
sents an intermediate value (22.04°). These twistings are cer- 2.594(9) Å; ∠(D–H⋯A) 157.8°] being found in 5·2H O and con-
2
tainly related to hydrogen bond interactions (Table 3) cerning the coordinated OH phenol group which acts as a
involving such substituents which, in the relevant cases donor to a free water molecule. It is compound 2·4H
2
O, which
(
2·4H O, 5·2H O and 6·2CH OH), act as donor groups towards presents four crystallization water molecules per unit cell, that
2
2
3
2 2
nitrate. Moreover, the NH phenyl moieties in 2·4H O and discloses the most intricate network, as expressed by the
5
·2H O are directed towards the coordinated keto atom and higher number of H-bonds detected (Table 3).
2
2 2 2 2 3
Fig. 2 Hydrogen bond interactions (in dashed light blue line) for complexes (a) 1·2H O, (b) 2·4H O, (c) 4·H O, (d) 5·2H O and (e) 6·2CH OH.
Symmetry operations are given in Table 3.
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Fig. 3 Packing diagram of complexes (a) 1·2H
2
O, (b) 2·4H
2
O, (c) 4·H
2
O, (d) 5·2H
2
O and (e) 6·2CH
3
OH highlighting the hydrogen bond interactions
(
in dashed light blue line).
Magnetism
The values of χ T almost remain constant above 50 K on
M
cooling and then deeply decrease reaching a minimum of
3
−1
−3
−1
−3
The temperature dependence of the magnetic susceptibilities 0.88 cm K mol for 1, 0.75 cm K mol for 2 and 0.61 cm
of 1, 2 and 6 has been investigated. The thermal variation of χ K mol− for 6 at 2 K, suggesting an overall antiferromagnetic
1
2
+
and χT for three complexes in the temperature range 2–300 K interaction between Mn ions in these complexes. The binuc-
T values of lear Mn(II) model utilized for [Mn
.43 cm K mol for 1, 9.04 cm K mol for 2 and 9.38 cm K (ClO ) ·2py was adopted to simulate the magnetic suscepti-
mol for 6 are a little larger than the value expected for two bility of 1. Eqn (1) is derived from the Hamiltonian H = −JS S .
1 2
non-coupled Mn(II) ions (8.655 cm K mol , assuming g = 2.0). Here, N, k and β constants have their usual meaning.
1
7
II
is shown in Fig. 4. The room temperature χ
M
2
(HL)
2 3 2 2
(CH OH) (py) ]-
3
−1
3
−1
3
9
4
2
−
1
3
−1
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oxidation of 1-phenylethanol). Selected results are summar-
ized in Table 4.
Under typical conditions, yields up to 30% are obtained,
but the presence of 2,2,6,6-tetramethylpiperidyl-1-oxyl
(
(
TEMPO) (see below) can lead to the yield increase up to 81%
entry 20, Table 4). Compound 6 appears to be a more active
catalyst precursor than 1 or 2. This eventually can be
accounted for by the presence of the basic amino-substituent
at the Schiff base ligand. Amino groups can have a favourable
1
8
effect on oxidation catalysis, as was observed for various cata-
lytic systems in peroxidative alkane oxidations with catalysts
bearing aminoalcohol ligands.
The influence of the amount of catalyst 6 was studied
(
entries 6–10, Table 4, Fig. 5). Its increase results in a yield
enhancement, e.g., from 6% to 32% upon changing the
amount of catalyst from 5 μmol (0.1 mol% vs. substrate) to
M
Fig. 4 Temperature dependence of the χ T product for complexes 1, 2
100 μmol (2 mol% vs. substrate). As expected, the increase of
and 6 in the temperature range of 2–300 K.
the catalyst amount results in a corresponding TON (moles of
product per mole of catalyst) lowering from 51 to 12. Blank
tests (in the absence of any catalyst) were performed under
common reaction conditions and no significant conversion
was observed.
2
2
χ ¼ 2Ng β A=kTB
ð1Þ
where
9
,19
It is known
that MW can provide a more efficient syn-
A ¼ expðJ=kTÞ þ 5expð3J=kTÞ þ 14expð6J=kTÞ
þ 30expð10J=kTÞ þ 55expð15J=kTÞ
thetic method than conventional heating, allowing us to
achieve the same yield in a shorter time and/or to improve the
yield and/or selectivity. A favourable effect of MW is also
observed in this study even at the low power of 10 W, as
B ¼ 1 þ 3expðJ=kTÞ þ 5expð3J=kTÞ þ 7expð6J=kTÞ
þ 9expð10J=kTÞ þ 11expð15J=kTÞ
9
reported for other systems. Hence, for example, the product
yield increases from 18% (entry 23) to 81% (entry 20), in the
−
1
2 3
The best fitting gives J = −1.45 cm , g = 2.06, R = 2.65 × presence of TEMPO/K CO (see below), upon MW irradiation,
0− for 1, J = −2.05 cm , g = 2.02, R = 7.87 × 10 for 2, and the other conditions being the same, although in the absence
5
−1
−5
1
J = −1.69 cm 1, g = 2.06, R = 5.35 × 10−5 for 6. These J values of such additives a lower yield enhancement was observed
−
II
are
(
a
little more negative than that of [Mn
2
(HL)
2
-
(entry 22 vs. 3).
CH OH) (py) ](ClO ) ·2py (−0.98 cm− ), which could be
1
The use of hydrogen peroxide (30% aqueous solution)
ascribed to the larger Mn–O–Mn angles of 1 (105.68°), 2 instead of TBHP results in a marked yield lowering from 14%
3
2
2
4 2
II
(
104.81°) and 6 (105.54°) in comparison with [Mn
2
(HL)
2
-
to 2% (entries 8 and 12, respectively, Table 4), in accord with
(
CH OH) (py) ](ClO4) ·2py (mean value of 103.83°).
the expected decomposition of H under the applied reac-
2 2
O
3
2
2
2
tion conditions (80 °C). The use of more oxidant does not lead
to a better conversion (entries 8 and 11, Table 4).
Performing the reaction in the presence of acetonitrile does
not change significantly the yield, e.g. for the 1-phenylethanol
oxidation in the presence of 6 in acetonitrile, the yield
decreases from 14%, under added solvent-free conditions
Catalysis
The manganese complexes 1, 2 and 6 have been tested as cata-
lysts (or catalyst precursors) for the oxidation of common sec-
ondary alcohols (usually 1-phenylethanol) to the respective
t
ketones using tert-butylhydroperoxide (Bu OOH, TBHP) (2 eq.)
(
entry 8, Table 4), to 11% in NCMe (entry 15, Table 4), whereas
as an oxidizing agent, under typical conditions of 80 °C, low
power (10 W) microwave irradiation (MW), 3 h reaction time
and in the absence of any added solvent (Scheme 2 for the
the use of water as a solvent results in a more significant yield
reduction from 14%, without any added solvent, to 1% in
water (entry 16, Table 4), under the same reaction conditions.
2 3
In contrast, the use of a basic solution of 1 M K CO (entry 17,
Table 4) results in a significant increase of the conversion of
the alcohol to the ketone compared to simple water (from 1%
only in water to 32% in basic solution) or even to the added
solvent-free system (14% yield, entry 3, Table 4). The role of
basic additives, which facilitate the deprotonation of the
2
0,21
alcohol, has already been observed in other cases.
The relevance of the ligands to the catalytic activity of the
metal complexes is shown by testing also the catalytic
Scheme 2 Added solvent-free oxidation of 1-phenylethanol to
acetophenone.
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a
Table 4 Oxidation of selected secondary alcohols using 1, 2 and 6 as catalyst precursors
b
c
Entry
Catalyst
Substrate
Reaction time [h]
Catalyst amount [μmol]
TON
Yield [%]
1
2
3
4
5
6
7
8
9
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
—
6
6
6
2
2
2
2
1
1
1
1
1
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
Cyclohexanol
0.5
1
3
6
15
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
15
3
3
6
15
3
3
20
20
20
20
20
5
23
27
36
50
75
51
36
34
18
12
31
7
6
7
14
20
31
6
11
14
17
32
14
2
10
20
40
100
20
20
20
20
20
20
20
20
20
20
100
20
20
—
20
20
20
10
20
20
20
20
20
20
20
50
40
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
d
e
f
1
2
g
—
23
2
—
11
1
32
37
36
81
73
9
18
4
28
22
5
h
i
j
80
59
49
203
36
27
34
9
21
74
37
38
26
29
77
37
39
58
106
15
15
k
l
m
k
n
m,n
o
2-Octanol
3-Octanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
8
11
12
31
9
16
23
42
30
12
m
m
m,p
Mn(NO
3 2 2
) ·4H O
3
a
Reaction conditions unless stated otherwise: 5 mmol of substrate, 1–100 μmol of catalyst (0.02–1.4 mol% vs. substrate), 10 mmol of TBHP
b
c
(
2 eq.), 80 °C, 3 h reaction time, microwave irradiation (10 W). Turnover number = number of moles of product per mole of catalyst. Moles of
d
e
f
ketone product per mole of alcohol. 20 mmol of TBHP (4 eq.).
H O
2 2
30% aqueous solution instead of TBHP. In the presence of Ph
2
NH.
g
h
i
j
k
l
In the presence of CBrCl
3
.
In NCMe. In H
2
O. In K
2 3
CO aqueous solution (1 M). In the presence of TEMPO (0.125 mmol). In the presence
m
n
of TEMPO (0.250 mmol). In the presence of TEMPO (0.125 mmol) and in 1 M K
2 3
CO aqueous solution. Without microwave irradiation.
o
p
TEMPO as a catalyst. Included for comparative purposes.
performance of Mn(NO ) ·4H O in the oxidation of 1-phenyl-
3
2
2
ethanol and comparing it, for the same reaction conditions,
with those of 1, 2 and 6. Hence, in the presence of the TEMPO/
K CO combination, the use of that salt leads to a 12% yield of
2
3
acetophenone (entry 37, Table 4), which is much lower than
those achieved in the presence of 1, 2 and 6 (42, 31 and 81%;
entries 35, 31 and 20, Table 4) under the same experimental
conditions.
The addition to the reaction mixture of Ph
well known oxygen- or carbon radical traps, respectively,
2 3
NH or CBrCl ,
2
2
results in an important yield drop, compared to the reaction
carried out under the same conditions (20 μmol, 80 °C, MW,
3
h) but in the absence of a radical trap. This suggests the
generation of oxygen and carbon radicals in the reaction,
which are trapped by those radical scavengers. The mechanism
may involve coordination of 1-phenylethanol followed by
metal-centred dehydrogenation and oxidation of the alcohol
Fig. 5 Effect of the amount of catalyst 6 on the yield (dotted line) and
turnover number (TON) (solid line) for the oxidation of 1-phenylethanol
to acetophenone.
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through hydrogen abstraction or one-electron oxidation pro-
cesses.
In an attempt to increase the activity of 1, 2 and 6 in
Experimental section
General materials and procedures
2
1a,d
solvent-free MW-assisted peroxidative oxidation of 1-phenyl- All synthetic work was performed in air. The reagents and sol-
ethanol, we have investigated the influence of 2,2,6,6-tetra- vents were obtained from commercial sources and used as
methylpiperidyl-1-oxyl (TEMPO), a nitroxyl radical that is a received, i.e., without further purification or drying. Mn(NO ) ·
3
2
9
b,21a,c,d,23
known
promoter of oxidation catalysis of alcohols. In 4H O has been used as a precursor for the synthesis of com-
2
accord, a significant yield increase was observed for the plexes 1–6. C, H, and N elemental analyses were carried out by
1
-phenylethanol oxidation in the presence of 6 (from 14% in the Microanalytical Service of the Instituto Superior Técnico.
the absence of TEMPO to ca. 37% in its presence; entries 8, 18 Infrared spectra (4000–400 cm− ) were recorded on a BIO-RAD
1
and 19, Table 4, respectively).
FTS 3000MX or a Jasco FT/IR-430 instrument in KBr pellets;
Gas chromatographic (GC)
The reactions performed in the presence of TEMPO wavenumbers are in cm−1
.
(0.125 mmol) and in 1 M K
2
CO
3
aqueous solution (entries 35, measurements were carried out using a FISONS Instruments
3
1 and 20, Table 4) give the highest yields of 42, 31 and 81% GC 8000 series gas chromatograph with a FID detector and a
for 1, 2 and 6, respectively (Fig. 6).
capillary column (DB-WAX, column length: 30 m; internal dia-
Recently, our group reported some efficient systems invol- meter: 0.32 mm). The temperature of injection was 240 °C.
9
a,b
ving copper(II) triazapentadienate,
bis- and tris-pyridyl The initial temperature of the column was maintained at
9
c
−1
amino and imino thioether Cu and Fe complexes for the 120 °C for 1 min, and then raised at 10 °C min up to 200 °C,
MW-assisted oxidation of secondary alcohols to the corres- and held at this temperature for 1 min. Helium was used as
ponding ketones, the in situ generated copper(II)-diimine com- the carrier gas. The magnetic susceptibility measurements
plexes towards the TEMPO-mediated oxidation of benzylic were carried out on polycrystalline samples with a Quantum
2
1a
alcohols in aqueous media
and copper(II) complexes con- Design MPMS-XL5 SQUID magnetometer in the temperature
taining arylhydrazones from methylene-active nitriles towards range of 2–300 K at an applied field of 2000 Oe. Diamagnetic
the selective oxidation of primary and secondary alcohols to corrections were estimated from Pascal’s constants for all con-
9
b
24
the corresponding carbonyl compounds. The above catalytic stituent atoms. The catalytic investigations under microwave
systems lead to higher yields and/or TONs than those of this irradiation (10 W) were performed in a focused microwave
work. Other manganese-based systems were reported for alco- CEM Discover reactor (10 mL, 13 mm diameter) fitted with a
hols oxidation, namely the silica-supported manganese rotational system and an IR detector of temperature.
1
3
dioxide (MnO ) used in the oxidation of benzyl alcohol under
3 3
Synthesis of the pro-ligands H L –H L . The Schiff base
2
8
b
1
solvent-free conditions, which yielded acetophenone in 88%, pro-ligands (2,3-dihydroxybenzylidene)benzohydrazide (H L ),
relative to 2-amino-(2,3-dihydroxybenzylidene)benzohydrazide (H L ) and
the substrate (5 : 1 molar ratio), while in our system we (2,3-dihydroxybenzylidene)-2-hydroxybenzohydrazide
3
2
under MW for 20 s, but required an excess of MnO
2
3
3
(H L )
3
2
5
achieved 81% yield using a maximum of 0.4% molar ratio of (Scheme 1) were prepared by a reported method upon con-
the catalyst relative to the substrate.
densation of the corresponding aromatic acid hydrazides and
2,3-dihydroxybenzaldehyde.
Synthesis of dinuclear Mn(II,II) complexes (1–6)
1
[
Mn(H L )(py)(H O)] (NO ) ·2H O (1·2H O). To
a
15 mL
0.256 g (1.00 mmol), 0.256 g
O was added and the reaction
2
2
2
3 2
1
2
2
methanol solution of H
1.02 mmol) of Mn(NO
3
L
(
3
)
2
·4H
2
mixture was stirred for 15 min at room temperature. There-
after, 0.08 g (1.01 mmol) of pyridine was added and stirring
was continued for 1 h. The resulting yellow solution was fil-
tered and the filtrate was kept in open air at room temperature
for crystallization. After 3 days, single crystals suitable for X-ray
diffraction were isolated.
Yield 74%. Anal. calcd for C H N O Mn (1): C, 48.63; H,
3
8
36
8
14
2
3
.87; N, 11.94. Found: C, 48.56; H, 3.84; N, 11.86. IR (KBr;
−
1
cm ): 3386, 3224 ν(OH), 3058 ν(NH), 1638, 1613 ν(CvN),
1
384 ν(NO3−), and 1071 ν(N–N).
Complexes 2 and 3 have been synthesized using the same
procedure.
2
[
Mn(H
2 3 2 3 2 2 2
L )(py)(CH OH)] (NO ) ·4H O (2·4H O). Yield 68%.
Anal. calcd for C H N O Mn (2): C, 48.20; H, 4.25; N,
4
0
42 10 14
2
−
1
1
(
4.05. Found: C, 48.12; H, 4.21; N, 13.97. IR (KBr; cm ): IR
KBr; cm− ): 3364, 3242 ν(OH), 2988 ν(NH), 1632, 1608
1
Fig. 6 Influence of different solvents and additives on the yield of
acetophenone from oxidation of 1-phenylethanol.
−
ν(CvN), 1384 ν(NO3 ), and 1066 ν(N–N).
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3
[
Mn(H
2
L )(py)(H
2
O)]
2
(NO
3
)
2
(3). Yield 78%. Anal. calcd for in the microwave reactor and the system was left under stirring
C H N O Mn (3): C, 47.02; H, 3.74; N, 11.54. Found: C, and under irradiation (10 W) for 0.5–15 h at 80 °C.
3
8
36
8
16
2
4
2
6.96; H, 3.69; N, 11.47. IR (KBr; cm−1): 3394, 3224 ν(OH),
After cooling to room temperature, 300 μL of benzaldehyde
−
956 ν(NH), 1642, 1614 ν(CvN), 1383 ν(NO
3
), and 1064 (internal standard) and 5 mL of NCMe (to extract the substrate
ν(N–N).
and the organic products from the reaction mixture) were
Complexes 4–6 have been synthesized using the same pro- added. The obtained mixture was stirred for 10 min and then
cedure as that mentioned above but only the amount of pyri- a sample (1 μL) was taken from the organic phase and ana-
dine was taken in excess (0.032 g, 4.04 mmol).
lysed by GC using the internal standard method.
1
[
Mn(H L )(py) ] (NO ) ·H O (4·H O). Yield 78%. Anal. calcd
2 2 2 3 2 2 2
for C48
H
42
N
10
O
2
12Mn (4): C, 54.35; H, 3.99; N, 13.20. Found: C,
−1
5
2
4.29; H, 3.95; N, 13.12. IR (KBr; cm ): 3346, 3216 ν(OH), Conclusion
984 ν(NH), 1646, 1618 ν(CvN), 1384 ν(NO3−), and 1064
Depending on the molar ratio of pyridine used in the reaction
ν(N–N).
Mn(H
calcd for C H N O Mn (5): C, 52.85; H, 4.07; N, 15.41.
2
medium, the reactions of Mn(NO ) ·4H O with the Schiff
3
2
2
[
2
L )(py)
2
]
2
(NO
3
)
2
·2H
2
O
2
(5·2H O). Yield 78%. Anal.
1
–3
bases H3L
in methanol at room temperature yield the dinuc-
4
8
44 12 12
2
−
1
lear manganese(II) complexes 1–6. In all the structures, the
Found: C, 52.79; H, 4.02; N, 15.34. IR (KBr; cm ): 3399, 3236
ν(OH), 2924 ν(NH), 1632, 1614 ν(CvN), 1384 ν(NO
1
–3 −
−
ligand (H2L
) coordinates via the keto form and two ligand
3
), and
molecules occupy the equatorial planes. In complexes 1–3
(where pyridine was used in a 1 : 1 molar ratio), the axial posi-
tions are occupied by one pyridine molecule and one solvent
molecule (water or methanol), but in 4–6 (where excess pyri-
dine was used) each of the manganese(II) centres is axially co-
ordinated by two pyridine molecules.
1
068 ν(N–N).
3
[
Mn(H L )(py) ] (NO ) ·2CH OH (6·2CH OH). Yield 76%.
2
2 2
3 2
3
3
Anal. calcd for C48
1
3
H
42
N
10
O
14Mn
2
(6): C, 52.76; H, 3.87; N,
−
1
2.82. Found: C, 52.68; H, 3.84; N, 12.77. IR (KBr; cm ): 3380,
214 ν(OH), 2986 ν(NH), 1618 ν(CvN), 1383 ν(NO3 ), and 1065
−
ν(N–N).
Among the six dinuclear Mn(II,II) complexes, three of them
(1, 2 and 6) having different coordination environments have
been examined for variable temperature magnetic suscepti-
bility measurements and have also been tested for their cataly-
tic activity towards microwave assisted oxidation of alcohols.
Magnetic analysis of these three complexes reveals that anti-
ferromagnetic interaction is propagated between the Mn(II)
centres. A yield of 81% has been found using 0.4% molar ratio
of the catalyst relative to the substrate in the presence of
TEMPO and in 1 M K CO aqueous solution under mild
X-ray structure determinations
Crystals of 1·2H O, 5·2H O and 6·2CH OH were immersed in
2
2
3
cryo-oil, mounted in a nylon loop and measured at a tempera-
ture of 150 K; crystals of 2·4H O and 4·H O were determined at
2
2
room temperature. Intensity data were collected using a Bruker
AXS-KAPPA APEX II diffractometer with graphite monochro-
matic Mo-Kα (λ 0.71073) radiation. Data were collected using
omega scans of 0.5° per frame and a full sphere of data were
obtained. Cell parameters were retrieved using Bruker SMART
2
3
2
6
conditions.
software and refined using Bruker SAINT on all the observed
reflections. Absorption corrections were applied using
Hence, this work has successfully extended the use of
dinuclear manganese complexes as catalyst precursors to the
microwave assisted oxidation of alcohols. Tuning the ligand
system or modifying the structures of the dinuclear Mn(II,II)
complexes using, e.g., different secondary ligands will be
attempted to achieve a higher activity.
2
6
SADABS. Structures were solved by direct methods by using
2
7
27
the SHELXS-97 package and refined using SHELXL-97. Cal-
culations were performed using the WinGX System-Version
2
8
1
.80.03. All hydrogen atoms were inserted in calculated posi-
tions. There were disordered molecules present in the struc-
tures of complex 6. Since no obvious major site occupations
were found for those molecules, it was not possible to model
2
9
Acknowledgements
them. PLATON/SQUEEZE was used to correct the data and a
3
2
potential volume of 219.7 (4·H O) Å was found with 56 elec-
The authors are grateful to the Foundation for Science and
Technology (FCT) (projects PTDC/QUI-QUI/102150/2008,
PTDC/EQU-EQU/122025/2010 and PEst-OE/QUI/UI0100/2013),
Portugal, for financial support. M.S. acknowledges the FCT,
Portugal for a postdoctoral fellowship (SFRH/BPD/86067/2012).
trons per unit cell worth of scattering. The electron count
roughly suggests the presence of ca. one molecule of water for
each of the unit cells. Least square refinements with anisotro-
pic thermal motion parameters for all the non-hydrogen atoms
and isotropic ones for the remaining atoms were employed.
Catalytic studies
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