New sterically encumbered arylimido hexamolybdates for organic oxidation reactions

Article abstract of DOI:10.1039/c5nj02330k

Facile synthesis of mono- and di-functionalised arylimido hexamolybdates, (ⁿBu₄N)₂[Mo₆O₁₈(L1)] (1), (ⁿBu₄N)₂[Mo₆O₁₇(L1)₂] (2) and (ⁿBu₄N)₂[Mo₆O₁₈(L2)] (3) (L1 = 4-bromo-2,6-diisopropylaniline, L2 = 2,2′,6,6′-tetraisopropylbenzidine), has been achieved through the reaction of parent (ⁿBu₄N)₂[Mo₆O₁₉] (4) with a series of aniline derivatives. The arylimido derivatives 1-3 have been characterised by both analytical and spectroscopic studies (¹H NMR, IR, UV-Vis and mass spectral studies), apart from structural elucidation of all the three compounds by single crystal X-ray diffraction studies. Steric hindrance at the ortho-positions of the arylamine seems to strengthen and effectively protect the newly formed MoN bonds. The surface modification of the hexamolybdate by aryl amines results in the transformation of otherwise inactive parent compound 4 into useful catalysts 1-3, which effectively catalyse the oxidation of cyclohexene to cyclohexene epoxide and benzyl alcohol to benzaldehyde and benzoic acid.

Full text of DOI:10.1039/c5nj02330k

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ARTICLE TYPE
New sterically encumbered arylimido hexamolybdates for organic oxidation
reactions
Ritambhara Jangir,a Rajendran Antony,a and Ramaswamy Murugavel*a
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
5
DOI: 10.1039/b000000x
Facile synthesis of monoꢀ and diꢀfunctionalised arylimido hexamolybdates, (nBu4N)2[Mo6O18(L1)] (1), (nBu4N)2[Mo6O17(L1)2] (2) and
(nBu4N)2[Mo6O18(L2)] (3) (L1=4ꢀbromoꢀ2,6ꢀdiisopropylaniline, L2=2,2',6,6'ꢀtetraisopropylbenzidine), has been achieved through the
reaction of parent (nBu4N)2[Mo6O19] (4) with a series of aniline derivatives. The arylimido derivatives 1ꢀ3 have been characterised by
both analytical and spectroscopic studies (1H NMR, IR, UVꢀVis and mass spectral studies), apart from structural elucidation of all the
10 three compounds by single crystal Xꢀray diffraction studies. Steric hindrance at the orthoꢀpositions of the arylamine seems to strengthen
and effectively protect the newly formed Mo≡N bonds. The surface modification of the hexamolybdate by aryl amines results in the
transformation of otherwise inactive parent compound 4 into useful catalysts 1ꢀ3, which effectively catalyse the oxidation of cyclohexene
to cyclohexene epoxide and benzyl alcohol to benzaldehyde and benzoic acid.
45
Interesting optical and electronic properties, desirable
chemical reactivity, high stability towards oxidising conditions,
and their ability to accept multiple electrons have made nanoꢀ
sized POMs as promising candidates in several oxidative
chemical transformations in the presence of suitable oxidants.43
Introduction
15
Although polyoxomolybdates (POMs) have been
known for several decades, newer anions with different size and
structures continue to be discovered even in recent times.1 The
field of hybrid POMs, on the other hand, is relatively new and has
tremendously grown in the last two decades.2ꢀ5 Owing to the
50 Hybrid POMs assemblies, because of their facile synthesis and
combinatorial properties, have proven as multifunctional catalysts
in oxidation reactions. Selective oxidation of cyclohexene to its
epoxide (cyclohexene oxide) is an everꢀgreen reaction owing to
numerous applications of epoxide in synthetic organic
55 chemistry.48 Similarly, oxidation of benzyl alcohol to
benzaldehyde is also of interest since benzaldehyde plays a key
role in pharmaceutical and dye industries.49,50
20 structural diversity and varied applications, several research
groups have been carrying out investigations on different aspects
in order to improve their utility as useful materials both in solid
and solution state.6ꢀ8 Polyoxomolybdates usually comprise
octahedrally coordinated discrete polyanionic metal–oxide
25 nanoclusters of Mo(VI) with adjacent polyhedra shared at corners
and edges.9ꢀ13 The Lindqvist hexamolybdate cluster, comprising
[Mo6O19]2−anion, represents one of the ideal building blocks to
construct new organic–inorganic hybrid assemblies.14ꢀ19 The
functionalisation can be performed by substitution of the counter
30 cations through an electrostatically driven process22,23 or
insertion of heteroꢀgroups in the metalꢀoxide subꢀunits of POMs
under acidic conditions20,21 or substitution of the terminal M=O
groups covalently with conjugated organic species.12,24ꢀ29
Although large number of POM based catalysts have
been reported for oxidation of substrates such as cyclohexene and
60 benzyl alcohol, newer POMs are being explored to augment
maximum selectivity in these reactions.51ꢀ53 Further
functionalisation of POMs has received attention in order to
attain highest selectivity in such reactions.54 Thus, the objective
of the present work is to develop suitable organicꢀhybrid POMs
65 for increasing both conversion and selectivity in oxidative
chemical transformations. In order to achieve this, we have
prudently introduced bulky isopropyl groups on the ortho
position of a series of anilines (including a highly substituted
benzidine derivative) and used them as starting materials for
70 introducing arylimido ligands on one or more molybdenum
centres of the hexamolybdate anion. The details are presented in
this present article.
Among the many organic hybrids of POMs,
35 organoimido derivatives are of great interest because the organic
πꢀelectrons may extend their conjugation to the inorganic
framework, thus resulting in strong d–π interactions. Such
hybrids exhibit widespread applications in areas as diverse as
electrochemistry, catalysis, medicine, nanomaterials, sensor
40 design, magnetism, biochemistry and photochemistry.8,26ꢀ46 The
six terminal oxygen atoms in the hexamolybdate anion can be
partially or completely replaced by organoimido ligands,
depending on (a) stoichiometry of the starting materials, (b)
reaction temperature, and (c) duration of the reaction.17,36
Results and Discussion
75
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duration still produces only the monoꢀsubstituted product 1.
Extending the duration of the reaction to a total of 18 h produces
the biꢀsubstituted product 2 with no trace of 1 in the isolated
reaction product. The formation of 3, on the other hand, does not
Synthesis and Characterization
When a mixture containing (Bu4N)2[Mo6O19] (4), an
aromatic amine and dicyclohexyl carbodiimide (DCC) is boiled
under reflux in anhydrous acetonitrile under nitrogen, the
5
corresponding organoimido derivatives (nBu4N)2[Mo6O18(L1)] 30 depend on the initial amount of 2,2',6,6'ꢀtetraisopropylbenzidine
(1), (nBu4N)2[Mo6O17(L1)2] (2) and (nBu4N)2[Mo6O18(L2)] (3)
(L1 4ꢀbromoꢀ2,6ꢀdiisopropylaniline, L2 2,2',6,6'ꢀ
(L2) added to the reaction mixture (up to 6 equivalents have been
added).
=
=
tetraisopropylꢀbenzidine) are formed. Completion of the reaction
takes place in about 12ꢀ18 hours. The reaction has been found not
Compounds 1ꢀ3 have been characterised by analytical
and spectroscopic methods. The single crystals obtained from the
10 to proceed in solvents other than acetonitrile. DCC is used as 35 reaction mixture were found to be pure and the elemental analysis
water scavenger and its quantity in the reaction mixture has to be
maintained at around the same amount as water produced in the
reaction mixture (a small excess amount of DCC is favoured).
Use of excess DCC usually results in unexpected side reactions
15 and reduces the yield of the desired products.36
results obtained for all the three compounds have been found to
be consistent with the proposed molecular formula. The 1H NMR
spectra of compounds 1ꢀ3 are included in the ESI. All the H
NMR spectra exhibit clearly resolved signals, which can be
40 unambiguously assigned. The integrated intensities of individual
resonances are consistent with the proposed structure in each
case. In all the three complexes, the resonances due to the ≡NR
group have downfield shifted compared to the corresponding
resonances in the parent ligands as a result of MoꢀN bond
1
The extent of substitution (either one or two of the
terminal Mo=O linkages by arylimido groups Mo≡NR, leading to
the formation of functionalised hybrid POMs), depends on the
initial stoichiometry of the starting materials as well as the
20 duration of the reaction under reflux conditions. For example, 45 formation. Compounds 1 and 2 exhibit a single resonance at 7.32
compound
1
is isolated as exclusive product when
and 7.27 ppm, respectively, for aromatic CꢀH protons. The four
m,mˈꢀaromatic protons on coordinated L2 in 3 however exhibit
two different signals at 7.39 and 7.34 ppm, owing to the nonꢀ
equivalence created by binding of POM fragment on only one of
(nBu4N)2[Mo6O19] (4) and 4ꢀbromoꢀ2,6ꢀdiisopropylaniline (L1)
were taken in a 1:1 stoichiometry and the reaction was carried out
for only 12 h. Change of initial stoichiometry of the starting
25 materials to 1:2 and carrying out the reaction for the same 50 the two available –NH2 terminals (figure S1ꢀS3).
Scheme 1. Synthesis of 1ꢀ3 (DCC=dicyclohexyl carbodiimide, DTMAB=dodecyltrimethylammonium bromide).
55
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ESIꢀMS studies further support the molecular formula
respectively, which are comparable to similar vibrations in the
assigned to these compounds, also indicating their structural
integrity in solution under mass spectral ionisation conditions.
The negative ion mode ESIꢀMS of 1 contains a signal due to the
parent ion [Mo6O18(L1)]ꢀ2 at m/z 559.6452, exhibiting the
expected calculated isotopic pattern for this structural unit.
Similarly, compounds 2 and 3 also exhibit in their ESIꢀMS the
parent ions at m/z 678.6162 ([Mo6O17(L1)2]ꢀ2) and 607.7228
([Mo6O18(L2)]2ꢀ), respectively (Figure 1).
previously reported hexamolybdates.55 The ν(Mo–N) bands
appear at 971ꢀ975 cmꢀ1 as a shoulder of the strong ν(Mo–Oterminal
)
25 band, indicating the formation Mo≡N bond (Figure 2).
5
10
1
ʋ (Mo≡N)
Figure 2. IR spectra of 1ꢀ4 (as KBr disc, 2000ꢀ500 cmꢀ1).
2
Figure 3. UVꢀVis (1.0×10ꢀ5M) [λmax, 254(ε, 0.67 × 105 Mꢀ1 cmꢀ1), λmax
30 398 (ε, 0.69×105 Mꢀ1 cmꢀ1) for 1; λmax, 258(ε, 0.68 × 105 Mꢀ1 cmꢀ1), λmax
,
,
,
3
416(ε, 0.70×105 Mꢀ1 cmꢀ1) for 2; λmax, 260(ε, 0.81 × 105 Mꢀ1 cmꢀ1), λmax
425 (ε, 0.83×105 Mꢀ1 cmꢀ1)for 3; λmax, 260(ε, 0.86 × 105 Mꢀ1 cmꢀ1), λmax
325 (ε, 0.35×105 Mꢀ1 cmꢀ1)for 4] spectra in DMSO.
,
35
The lowest energy electronic transition in the UVꢀVis
spectrum of compounds 1ꢀ3 exhibits a bathochromic shift (1: 398
nm, 2: 416 nm, and 3: 425 nm) compared to that of the parent
[Mo6O19]2ꢀ ion (325 nm)36,55 (Figure 3). This band is assigned to
the electronic transition from the terminal oxygen πꢀtype nonꢀ
40 bonding HOMO to the molybdenum πꢀtype LUMO in
[Mo6O19]2.47,57 This energy gap is presumably reduced in the case
of arylimido derivatives 1ꢀ3 as a result of strong dꢀπ interaction
between the conjugated organic ligand and the hexamolybdate
framework, indicating strong electronic interaction between the
45 metalꢀoxygen cluster and the organic ≡NR segment.
15
Figure 1. Experimental and calculated ESIꢀMS spectrum of 1 (top), 2
(middle) and 3 (bottom). Spectra recorded in CH3CN in negative ion
mode.
Two strong absorption bands in the IR spectra of 1ꢀ3 in
20 the range 795ꢀ798 cmꢀ1 and 951ꢀ954 cmꢀ1 are due to ν(Mo–
Thermogravimetric analysis (TGA) was performed for
the new POMs in the range of 30–800 °C at the heating rate of
Obridged–Mo) and ν(Mo–Oterminal
)
stretching vibrations,
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10 °C min−1 under flow of dinitrogen (see ESI, Figure S4). An 35 comparable to those found in other orthoꢀsubstituted arylimido
o
initial weight loss of about 5% occurring up to 250 C is due to
the loss of adsorbed or occluded solvent molecules. In the
hexamolybdates (Table 1).55 The Mo≡N formulation is also
consistent with nearly linear Mo–N–C linkages (171.3(1)o for 1
and 169.8(6)o for 3). The bond length between imidoꢀbearing
molybdenum atom and central oxo atom (Mo(2)ꢀO(1)=2.214 Ǻ,
o
temperature range 250ꢀ600 C, the weight loss occurring in two
stages can be assigned to the loss of organic cation and the
5
arylamido ligands to result in the final ceramic material MoO3 40 1; Mo(2)ꢀO(1)=2.193 Ǻ, 3) is slightly shorter than that of the
(40% of the initial mass) at 800 oC (Figure S4).
other remaining Mo–O distances (2.350 Ǻ, 1; 2.363 Ǻ, 3) due to
“trans influence” by the aryl imide moiety.
Molecular structure of 1 and 3
10
In order to ascertain the structural changes resulting
from the substitution of the terminal oxo groups by arylimido
functionalities, the molecular structures of 1ꢀ3 have been
determined by single crystal Xꢀray diffraction studies. The ball
and stick models of the molecular structures of mono substituted
15 derivatives 1 and 3 are depicted in Figures 4 and 5, respectively.
The terminal oxo group of the hexamolybdate cluster is replaced
by sterically hindered arylimido ligands (L1 and L2). The anionic
cluster is made up of six distorted octahedral molybdenum
centres which themselves are arranged in the form a metal
20 octahedron with a ꢀ6ꢀO2ꢀ occupying the centre of the polyhedron.
In addition to the central oxygen, the molybdenum centres are
further held together by twelve µ2ꢀoxygen centres to form the
metal cage. While in the parent hexamolybdate anion each of the
six Mo centres contains a terminal oxide (Mo=O) ligand, in
25 clusters 1 and 3, one of the six Mo centres feature a terminal
imido linkage (Figures 4 and 5). Thus five of the total six Mo
Figure 4. Molecular structure of the anion of 1. Hydrogen atoms omitted
45 for clarity.
In 3, the imidoꢀnitrogen to aryl carbon bond distance is
shorter (N(1)ꢀC(1A)=1.370(9) Ǻ) than the other ArꢀN bonds
(N(2)ꢀC(0EA)=1.410(2) Ǻ), suggesting a complete delocalisation
50 of the nitrogen lone pair over CꢀNꢀMoꢀO segment. As a result,
the central O(1) atom is shifted slightly toward the imidoꢀbearing
molybdenum atom Mo(2). The bonds between imidoꢀbearing
molybdenum atom and ꢀ2ꢀoxygen atom are longer than all other
Mo–O distance, because the imidoꢀbearing molybdenum atom
55 demonstrates weaker electropositivity than other molybdenum
atoms as a consequence of coordination with better electron
donating alkyl imido ligands (Figure 4 and 5).
centres feature
a distorted MoO6 octahedral coordination
environment (one µ6ꢀoxygen, one terminal oxo and four µ2ꢀ
oxygen), the sixth molybdenum reveals a distorted MoO5N
30 octahedral coordination environment (one µ6 oxygen, four µ2ꢀ
oxygen and one terminal arylimido group).
The observed MoꢀN distances (Mo(2)ꢀN(1) 1.735(8) Å
for 1 and Mo(2)ꢀN(1) 1.748(6) Å in 3) are suggestive of a formal
Mo≡N bond in these systems. Further, these distances are also
60 Table 1. Structural comparison of 1ꢀ3 with a few selected alkyl and aryl imido hexamolybdates (Mo≡NꢀR).
Mo−µ6O (Ǻ)a
(trans to Mo≡N)
2.261(4)
2.229(2)
2.229(4)
2.229(4)
2.230(4)
2.252(7)
2.223(3)
Mo−µ6Ob (Ǻ)
(all other)
2.351(4)
2.335(2)
2.342(4)
2.345(4)
2.338(4)
2.333(7)
2.358(3)
2.337(6)
2.312(10)
2.345(3)
2.349 (4)
2.375(2)
2.353(4)
C−N−Mo (o)
S. No.
ꢀR
Mo≡N (Ǻ)
Ref.
1
2
3
4
4
Et
1.705(11)
1.704(4)
1.713(7)
1.720(7)
1.718(7)
1.750(1)
1.709(4)
1.723(7)
1.739(15)
1.731(4)
1.733(6)
1.745(2)
1.729(5)
178.1(10)
177.9(5)
175.0(7)
178.4(8)
178.3(8)
177.5(8)
174.6(7)
178.4(7)
176.3(15)
176.3(4)
172.8(6)
171.17 (3)
172.5(5)
55
55
55
55
55
55
55
36
13
56
36
57
36
nPr
iPr
nBu
tBu
Cy
Hex
5
6
7
2,6ꢀdimethylphenyl
2,6ꢀdiisopropylphenyl
2.230(6)
2.254(10)
2.211(3)
2.222(4)
8
9
4ꢀbromoꢀ2,6ꢀdimethylphenyl
4ꢀiodoꢀ2,6ꢀdimethylphenyl
4ꢀiodoꢀ2,6ꢀdiisopropylphenyl
4ꢀethynylꢀ2,6ꢀdimethylphenyl
10
11
2.225(2)
2.232(4)
12
13
4ꢀbromoꢀ2,6ꢀdiisopropylphenyl (1) 1.735(8)
2.214(1)
2.345(1)
171.3(1)
this work
this work
4ꢀbromoꢀ2,6ꢀdiisopropylphenyl (2) 1.738(5),
2.225(5),
2.227(4)
2.193(4)
2.375(5),
2.374(4)
2.363(5)
172.9(5),
174.4(6)
169.8(6)
1.742(5)
14
3,3',5,5'ꢀtetraisopropylꢀ[1,1'ꢀ
1.748 (6)
this work
biphenyl]ꢀ4ꢀamine (3)
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Within the crystal lattice, the POM hybrid ions of 1 are
organised into weakly held dimers. As shown in Figure 6, the
“dimerisation” is clearly driven by π–π stacking of the aryl rings
(3.97 Å).
5
Figure 5. Molecular structure of the anion part of 3. Hydrogen atoms
omitted for clarity.
Figure 7. Molecular structure of the anion part of 2. Hydrogen atoms
30 omitted for clarity.
Arylimido POMs 1ꢀ3 as oxidation catalysts
Hybrid polyoxometallates have been extensively
studied as oxidation catalysts, including our recent study on D4R
35 zincꢀphosphate based octaꢀmolybdates [Mo8O26]4ꢀ, phosphaꢀ
molybdates [PMo11VO40]4ꢀ, and silaꢀtungstates [SiMo12O40]4ꢀ for
olefin expoxidation.47 Although there are many alkyl/arylimido
POMs known in the literature, their catalytic activity in organic
oxidation reactions have not been explored. To test the utility of
40 this class of compounds, cyclohexene and benzyl alcohol
oxidation reactions were carried out using 1ꢀ3 as catalysts
(Scheme 2).
10 Figure 6. Dimeric structure of cluster anions of 1 formed through πꢀπ
interactions (3.97 Å).
Molecular structure of 2
Compound 2 crystallises in the triclinic space group P ꢀ
15 1. The asymmetric part of the unit cell contains two
crystallographycally independent molecules. Unlike in 1 and 3,
two terminal oxo groups of the hexamolybdate cluster have been
replaced by L1 in 3, where the arylimido moieties are cis to each
other on the Mo6 octahedron. The bond distances between Mo(1)ꢀ
20 N(1) and Mo(2)ꢀN(2) are 1.738(5) and 1.742(5) Ǻ, respectively,
once again indicating triple bond character (Table 1). The bond
angles C(1)ꢀN(1)ꢀMo(1) (172.9(5)o) and C(13)ꢀN(2)ꢀMo(2)
(174.4(6)o) are close to linear. The bond lengths between imidoꢀ
bearing molybdenum atom and central oxo atom (Mo(1)ꢀO(1)=
25 2.227(4), Mo(2)ꢀO(1)= 2.225(4) Ǻ) are shorter than other MoꢀO
bonds (Mo(4)ꢀO(1)= 2.374(4), Mo(6)ꢀO(1)= 2.375(4) Ǻ) (Figure
7).
Scheme 2. Schematic representation of cyclohexene and benzyl alcohol
45 oxidations catalysed by POM catalysts.
Acetonitrile and toluene have often been reported as
effective solvents in homogeneous cyclohexene and benzyl
alcohol oxidation reactions catalysed by POMs.58ꢀ61 Therefore, in
50 the present study, we have performed cyclohexene oxidation
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using both acetonitrile and toluene as solvent with 1 as catalyst to
optimise the solvent for these reactions. Toluene was found to be
superior to acetonitrile since 1 showed much better catalytic
activity with almost 100% selectivity for the epoxide (Table 2).
On the other hand, 1 in acetonitrile, under otherwise identical
conditions, exhibited lower activity and poorer selectivity (49%
conversion; 2% epoxide, 51% cyclohexenꢀ1ꢀol and 47%
cyclohexenꢀ1ꢀone). Similarly the catalytic reactions were found
to be ineffective at room temperature in toluene. Hence the
5
10 temperature was raised to 80 ºC at which the highest conversion
was observed.
The results of oxidation reactions run in toluene using
cumene peroxide as the oxidant and 1ꢀ3 as catalysts (0.05%) at
o
80 C in toluene are summarised in Table 2. Irrespective of the
15 catalyst employed for the oxidation of cyclohexene, its epoxide
was obtained as the major product with a selectivity of ~95%.
Cyclohexeneꢀ1ꢀol was formed as the only minor product (~5%).
Unlike cyclohexene oxidation, the oxidation of benzyl alcohol
under similar reaction conditions produced two major products,
20 viz. benzaldehyde and benzoic acid with nearly equal selectivity
(cf. Table 2), which is primarily due to further oxidation of
benzaldehyde formed to benzoic acid.
Figure 8. Effect of time on product selectivity and % conversion in
45 cyclohexene oxidation by catalyst 1.
It was further noted that the selectivity in case of
cyclohexene oxidation is almost invariant as a function of
reaction time and conversion rate as can be seen from Figure 8. In
50 case of benzyl alcohol oxidation, the selectivity scenario is
somewhat different as depicted in Figure 9. It appears that in the
early stages of the oxidation reaction, the benzyl alcohol gets
predominately converted into the aldehyde. After ~5 hours of the
catalytic run, it appears that a second catalytic process involving
55 the conversion of initially formed benzaldehyde to benzoic acid
sets in. As a result, longer reaction runs tend to produce more of
benzoic acid and hence reduce the selectivity for benzaldehyde.
The turnover numbers displayed in Table 1 suggest that the
POMs 1ꢀ3 are better catalysts for cyclohexene oxidation
60 compared to benzyl alcohol oxidation, probably owing to the
competing reactions in the latter. The integrity of the catalysts
after the catalytic run have been examined by ESI mass spectral
studies on 3 and it has been found that the spectrum obtained for
the recovered catalyst is identical to that of the original
65 compound 3 (see ESI, Figure S9).
Table 2. Product yield and selectivity % of cyclohexene and benzyl
25 alcohol oxidations catalyzed by POM catalysts
Cyclohexene oxidation
Benzyl alcohol
oxidation
Selectivity
(%)
Selectivity
(%)
1
2
3
82
81
76
95
94
95
5
6
5
1892
1869
1753
39
45
51
59
57
58
41
43
42
900
1038
1176
Conditions: Catalyst (13 µmol); substrate (30 mmol); oxidant (6 mmol);
toluene (5 ml); temperature (80 ºC) and time (48 h)
30
Although all three catalysts displayed similar catalytic
performance for cyclohexene epoxidation, as can be seen from
Table 2, catalysts 1 and 2 marginally perform better than catalyst
3. For oxidation of benzyl alcohol, however, catalyst 3 has been
found to be a better catalyst (51% conversion) than 1 (39%
35 conversion) and 2 (45% conversion), when reactions were carried
out for same amount of time (48 h). The origin of this reversed
conversion efficiency between the two different substrates is not
clearly understood although it is reasonable to assume that the
free –NH2 group has a definite role to play through hydrogen
40 bonding interactions with the benzyl alcohol substrate in the early
part of the catalytic cycle.
Figure 9. Effect of time on product selectivity and % conversion for
benzyl alcohol oxidation by catalyst 3.
70
Conclusions
6
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DOI: 10.1039/C5NJ02330K
It has been demonstrated that it is possible to modify
the hexamolybdate anion to the corresponding arylimido
derivatives by their reaction with substituted anilines. The use of
Anal. Calcd. for C44H88BrMo6N3O18: C, 32.97; H, 5.53; N, 2.62.
Found: C, 33.04; H, 5.32; N, 2.93. FTꢀIR (KBr disc, cmꢀ1); 2962
(s), 2873 (s), 1463 (s), 1381 (s), 975 (s), 953 (s), 798 (s). H
1
4ꢀbromoꢀ2,6ꢀdiisopropylaniline in one or two equivalents results 60 NMR (400 MHz, DMSOꢀd6): δ (ppm) 7.32 (s, 2 H, ArH), 3.51
5
in the isolation of either monoꢀ or diꢀsubstituted arylimido
hexamolybdates 1 and 2. The free ArꢀBr linkages in 1 and 2 are
highly amenable for further organic reactions (viz. cross coupling
CꢀC bond formation reactions) and hence can be useful synthons
for polymeric hybrid materials incorporating polyoxomolybdates.
(spt, 4 H, CH), 1.19 (d, 24 H, 3J=2.16 Hz, ArCH3) 3.03 (m, 16 H,
NCH2), 1.58 (m, 16 H, CH2), 1.28 (m, 16H, CH2), 0.94 (t, 24 H,
CH3). ESIꢀMS (ꢀve mode): m/z calc. for dianion
C12H16BrMo6NO18 558.9565; found 558.6462 [M]ꢀ2.
65
Synthesis of (nBu4N)2[Mo6O17(L1)2] (2): A mixture of
10 Interestingly, the reaction of tetraisopropylbenzidine with
hexamolybdates neither proceeds beyond monoꢀsubstitution nor
captures the hexamolybdates at both the –NH2 ends, thus leaving
a highly functionalised hybrid polyoxomolybdate 3 with free –
4ꢀbromoꢀ2,6ꢀdiisopropylaniline (L1) (0.61 g, 2.40 mmol),
dicyclohexyl carbodiimide (0.49
g
2.40 mmol) and
(nBu4N)2[Mo6O19] (1.62 g 1.20 mmol) was boiled under reflux in
acetonitrile (20 mL) under nitrogen for 18 hours. After being
NH2 group. All the three new polyoxomolybdates have been 70 cooled to room temperature, the resulting dark red solution was
15 found to be efficient and selective catalysts for olefin
epoxidation, while 3 was found to be more efficient for benzyl
alcohol oxidation, indicating the possible role of –NH2 group in
the catalysis.
filtered to remove the white precipitate of N,N'ꢀdicyclohexylurea.
After 12 h, yellow crystals of unreacted starting material was
formed, which were discarded and the red solution was further
kept undisturbed for crystallisation. After 2 days, dark red
75 coloured crystals of 2 were formed, which were found suitable
o
for single crystal Xꢀray diffraction. M.p: >250 C. Anal. Calcd.
20 Experimental section
for C56H104Br2Mo6N4O17: C, 36.53; H, 5.69; N, 3.04. Found: C,
36.54; H, 5.90; N, 3.76. FTꢀIR (KBr disc, cmꢀ1); 2965 (s), 2873
Methods, materials and instruments. Acetonitrile
was purified using P2O5 and distilled over molecular sieves as
described in the literature.62 All the reactions were carried out
under N2 atmosphere using Schlenk line techniques. 2,6ꢀ
25 Diisopropylaniline (Alfa Aesar), sodium molybdate (Sigma
Aldrich), and dicyclohexyl carbodiimide (Merck) were procured
from commercial sources and used either as received or purified
where necessary. 4ꢀBromoꢀ2,6ꢀdiisopropylaniline (L1) and
2,2',6,6'ꢀtetraisopropylbenzidine (L2) were synthesized using
1
(s), 1463 (s), 1385 (s), 971 (s), 954 (s), 795 (s). H NMR (400
80 MHz, DMSOꢀd6): δ (ppm) 7.27 (s, 4 H, ArH), 3.78 (spt, 4 H,
3
CH), 2.08 (d, 48 H, J=2.16 Hz, ArCH3) 3.06 (m, 16 H, NCH2),
1.56 (m, 16 H, CH2), 1.31 (m, 16 H, CH2), 0.93 (t, 24H, CH3).
ESIꢀMS (ꢀve mode): m/z calc. for dianion C24H32Br2Mo6N2O17
678.0425; found 678.6162 [M]ꢀ2.
85
Synthesis of (nBu4N)2[Mo6O18(L2)] (3): A mixture of
30 reported procedure in the literature.63,64
Synthesis of
2,2',6,6'ꢀtetraisopropylbenzidine (L2) (0.49 g, 1.42 mmol),
(nBu4N)2[Mo6O19] was accomplished by using a previously
published report.65
dicyclohexyl carbodiimide (0.29
g
1.42 mmol) and
(nBu4N)2[Mo6O19] (1.93 g 1.42 mmol) was heated under reflux
conditions in acetonitrile (20 mL) under nitrogen flow for 72
The melting points of the new products were measured
in glass capillaries and are reported uncorrected. Elemental
35 analyses were performed on a Thermo Finnigan (FLASH EA
1112) microanalyzer. FTꢀIR spectra were recorded on a Perkin
Elmer Spectrum One Infrared Spectrometer as KBr diluted discs.
The absorption spectra were recorded on a Varian Cary Bio 100
90 hours. After being cooled to room temperature, the resulting dark
red solution was filtered to remove the white precipitate of N,N'ꢀ
dicyclohexylurea. After 24 h, yellow crystals of unreacted
starting material were obtained, which was discarded by
filtration. From the remaining dark red solution, dark red crystals
o
1
95 of 3 were obtained after 4 days. M.p: >250 C. Anal. Calcd. for
UVꢀVis spectrophotometer. H NMR spectra were recorded on a
C56H106Mo6N4O18: C, 39.59; H, 6.29; N, 3.30 Found: C, 39.06; H,
5.86; N, 3.31. FTꢀIR (KBr disc, cmꢀ1); 3441 (b), 2961 (s), 2934
(s), 2873 (s), 1480 (s), 1469 (s), 973 (s), 951 (s), 924 (s), 796 (s).
1H NMR (400 MHz, Acetoneꢀd6): δ (ppm) 7.39 (s, 2 H, ArH),
40 Bruker 400 and Varian 400 MHz NMR spectrometers. ESIꢀMS
were recorded on a Bruker Maxis Impact or a Waters QꢀTOF
micro mass spectrometer. Thermogravimertic analyses were
performed on Perkin Elmer Pyris Diamond TGA instrument.
3
100 7.34 (s, 2 H, ArH), 4.03 (spt, 4 H, CH), 2.04 (d, 24 H, J=2.16
Hz, ArCH3) 3.05 (m, 16 H, NCH2), 1.76 (m, 16H, CH2), 1.42 (m,
16 H, CH2), 0.96 (t, 24 H, CH3). ESIꢀMS (ꢀve mode): m/z calc.
for dianion C24H34Mo6N2O18 607.1420; found 607.7228 [M]ꢀ2.
45
Synthesis of (nBu4N)2[Mo6O18(L1)] (1): A mixture of
4ꢀbromoꢀ2,6ꢀdiisopropylaniline (L1) (0.30 g, 1.20 mmol),
dicyclohexyl carbodiimide (0.25
g
1.20 mmol) and
(nBu4N)2[Mo6O19] (1.62 g 1.20 mmol) was refluxed in 20 mL of
acetonitrile under flow of nitrogen gas for 12 hours. After being
105
Catalytic oxidation studies. Prior to the catalytic
oxidation of cyclohexene or benzyl alcohol, 25 ml two necked
reaction flask was equipped with a magnetic stir bar. To this
flask, toluene, cumene hydroperoxide, cyclohexene or benzyl
alcohol and catalyst were added. The obtained catalytic mixture
50 cooled to room temperature, the resulting dark red solution was
filtered to remove the white precipitate of N,N'ꢀdicyclohexylurea
formed. After 24 h, yellow crystals of unreacted starting material
were obtained. Hence the solution was decanted and kept
undisturbed for a further crystallisation. After 4 days dark red
55 block shaped crystals of 1 formed. These crystals were found to
be suitable for single crystal Xꢀray diffraction. M.p: >250 oC.
110 was stirred at 80 ºC under atmospheric pressure conditions. The
progress of the catalysis was monitored by analysing aliquots of
reaction mixture drawn at different time intervals. By following
the similar procedure, two blank reactions (one without catalyst
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DOI: 10.1039/C5NJ02330K
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cyclohexene and benzyl alcohol oxidation. PerkinꢀElmer gas
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60
65
70
75
80
85
90
95
5
Xꢀray crystallography of 1ꢀ3. The Xꢀray diffraction
intensity data for 1 and 2 were collected on a Rigaku Saturn 724+
CCD diffractometer fitted with a MoKα radiation source (λ =
10 0.71075 Å) at 130 K and 293 K, respectively. Intensity data
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CuKα radiation source (λ =1.54190 Å), under continuous flow of
cooled nitrogen gas. Rigaku CrystalClear software was used for
15 data integration and indexing of reflections for compound 1 and
2, apart from application of numerical method to correct for
absorption effects. The data integration and indexing were
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employed to correct for absorption for 3. All calculations were
20 carried out using the programs in WinGX module66 and the
structures were solved by direct methods implemented in SIRꢀ
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compounds.
All
nonꢀhydrogen
atoms
were
refined
25 anisotropically. The hydrogen atoms were refined isotropically as
rigid atoms in their idealised locations. The aryl rings of
arylimido group in 2 and 3 were found to be highly disordered
and hence it has been refined using partial occupancies for
various ring atoms. Similarly, the disorder of bromine atoms in 2
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Acknowledgement
The authors thank DST / SERB, New Delhi (No:
SB/S1/ICꢀ48/2013) for financial support. R. M. thanks
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35 Department of Atomic Energy, Mumbai for
Outstanding Investigator Award (No: 2010/21/04ꢀBRNS) and
DST New Delhi for a J.C. Bose Fellowship. R. J. thanks CSIRꢀ
New Delhi for a Research Fellowship. R. A. thanks IIT Bombay 100
for an Institute Postdoctoral Fellowship. The authors thank the
40 crystallography referee for useful suggestions on the quality of Xꢀ
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a DAEꢀSRC
compound 2.
105
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Notes and references
a
Department of Chemistry, Indian Institute of Technology Bombay,
45 Powai, Mumbai 400 076, India. Fax: +91 22 2576 7152; Tel: +91 22
110
2576 7163; Eꢀmail: rmv@chem.iitb.ac.in
† Electronic Supplementary Information (ESI) available: [1H NMR
spectra, TGA and selectivity and % conversion cures]. Crystallographic
data of 1ꢀ3. CCDC 1420505ꢀ1420507. For ESI and crystallographic data
50 in CIF or other electronic format see DOI: 10.1039/b000000x/
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Table 3. Summary of crystallographic data for compounds 1ꢀ3.
Compounds
1
2
3
Formula
F.W
C44H88BrMo6N3O18
C56H104Br2Mo6N4O17
C56 H105.54 Mo6 N4 O18
1698.62
1602.72
triclinic
P ꢀ1
1840.89
triclinic
P ꢀ1
Cryst. syst.
monoclinic
Space group
P21/c
Size/mm3
0.30 × 0.29× 0.13
11.843(8)
0.12 × 0.12× 0.08
16.201(4)
0.25 x 0.23 x 0.20
23.8986(5)
a/Å
b/Å
c/Å
12.310(9)
19.611(5)
22.481(6)
15.7862(2)
20.1555(5)
20.833(14)
α (º)
99.327(14)
99.865(12)
98.01(5)
96.77(4)
90.00
β (º)
112.883(3)
γ (º)
95.802(12)
2927(4)
2
90.14(6)
7022.4(3)
4
90.00
7005.6(2)
4
V/A˚ 3
Z
ρc/Mg mꢀ3
1.819
1.740
1.611
Refl. collected
Unique refl.
(Rint.)
22141
10207
52661
24481
51470
12339
0.0296
0.0391
0.0225
0.0529
0.0669
0.0546
Final R indices
[I>2sigma(I)]
wR2
0.0889
1.091
0.1533
1.088
0.1372
0.967
GOF
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TOC
Surface modification of the parent hexamolybdate by aryl amines results in useful catalysts for the oxidation of cyclohexene to
cyclohexene epoxide and benzyl alcohol to benzaldehyde and benzoic acid.