Bimetallic cis-dioxomolybdenum(VI) complex containing hydrazone ligand: Syntheses, crystal structure and catalytic studies

Article abstract of DOI:10.1016/j.inoche.2017.09.018

Bimetallic molybdenum(VI) complex [(MoO₂)₂(slsch)(H₂O)₂] containing dihydrazone ligand was synthesized by reaction of ligand with MoO₂(acac)₂ in 1:2 M ratio in methanol. The bimetallic complex obtained was characterized by various spectroscopic studies. The structure of complex was assigned using Single Crystal X-ray Crystallography and DFT method. We have also explored the catalytic behavior of complex for oxidation of primary benzylic, aliphatic, allylic, and propargylic alcohols.

Full text of DOI:10.1016/j.inoche.2017.09.018

Inorganic Chemistry Communications 86 (2017) 39–43
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Inorganic Chemistry Communications
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Short communication
Bimetallic cis-dioxomolybdenum(VI) complex containing hydrazone
ligand: Syntheses, crystal structure and catalytic studies
Sunshine Dominic Kurbah a, Arvind Kumar b, Ibanphylla Syiemlieh a, M. Asthana c, Ram A. Lal a,
⁎
a
Centre for Advanced Studies, Department of Chemistry, North-Eastern Hill University, Shillong 793022, India
Department of Chemistry, Faculty of Science and Technology, The University of West-Indies, St. Augustine, Trinidad and Tobago
b
c
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 August 2017
Received in revised form 1 September 2017
Accepted 14 September 2017
Available online 19 September 2017
Bimetallic molybdenum(VI) complex [(MoO2)2(slsch)(H2O)2] containing dihydrazone ligand was synthesized
by reaction of ligand with MoO2(acac)2 in 1:2 M ratio in methanol. The bimetallic complex obtained was charac-
terized by various spectroscopic studies. The structure of complex was assigned using Single Crystal X-ray Crys-
tallography and DFT method. We have also explored the catalytic behavior of complex for oxidation of primary
benzylic, aliphatic, allylic, and propargylic alcohols.
© 2017 Elsevier B.V. All rights reserved.
Keywords:
DFT
Bimetallic
Molybdenum(VI) complex
Single crystal
1. Introduction
evaporation of water/acetonitrile solution (3:2, v/v), at room tempera-
ture. The crystal structure of the complex and optimized geometry are
Molybdenum complexes are of considerable interest, especially
those featuring high valent oxidation states are used as catalysts for ox-
idation reactions in recent years [1–4]. In biological systems, molybde-
num has been found in many enzymes such as oxotransferases [5],
sulfite oxidase [6], aldehyde oxidase [7], nitrate reductase [8], and xan-
thine oxidase/dehydrogenase [9]. Molybdenum ions when coordinate
to protein usually exist in +4, +5 and +6 oxidation states [10,11].
Oxotransferases are a broad class of enzymes which are involved in ox-
ygen atom transfer processes and are often referred as oxomolybdenum
enzymes [12]. The active site in these enzymes usually consists of a cis-
dioxo molybdenum(VI) cation with one oxo group and one or two
molybdopterin ligands [13,14]. In order to mimic the activity of these
enzymes, the search for new model compounds containing high valent
[MoO2]2+ core have been synthesized [15,16].
So, in this communication we have described the synthesis of bime-
tallic molybdenum complex containing multi-dentate hydrazone ligand
[17]. The structure of [MoO2)2(slsch)(H2O)2] was established with the
help of single X-ray crystallography. We also report the catalytic oxida-
tion of alcohols to their corresponding aldehyde using H2O2 as the ter-
minal oxidant.
shown in Fig. 1. Complex crystallized in monoclinic P21/n space group.
Crystal data and structure refinement for complex are given in Table
1, selected bond length and bond angles are presented in Table S1.
The ligand coordinate to the metal ions in tridentate fashion with O1,
O2, N1 and O4 at the equatorial position, whereas the axial position
are occupied by O3 and O5 forming six coordinate octahedral geometry
around molybdenum(VI) ions.
The M\\O bond lengths of the complex are 1.909(7) Å (Mo\\O1),
1.967(7) Å (Mo\\O2), 1.698(7) Å (Mo\\O3), 1.708(6) Å (Mo\\O4)
and 2.272(8) Å (Mo\\O5), whereas Mo1\\N1 bond distance is
2.271(9) Å. The equatorial and axial bond angles are 151.6(3)0,
155.6(3)0 and 171.3(3)0, respectively. Crystal packing diagram of the
complex (Fig. S1) shows O\\H…O hydrogen bond interaction between
O5\\H(A)…O3 and O5\\H(B)…O4 with geometrical hydrogen bond
length of 2.714(13) Å and 2.698(13) Å, whereas the hydrogen bond an-
gles are 1220 and 1590, respectively. The metal…metal distance is 9.566
Å while the packing efficiency is 68.37% (Table 2).
The gas phase optimized geometry of the complex were performed
using density functional theory (DFT) with hybrid functional B3LYP
with 6-31+G(d,p) basis set for carbon, hydrogen, oxygen, nitrogen
and bromine atoms and LANL2DZ [18] for molybdenum ions imple-
mented in Gaussian 09 program [19]. The optimized geometrical bond
lengths and bond angles of the complex are comparable with experi-
mental solid state structure, slight difference being observed between
these two because geometrical optimization are carried out in gas
Complex was synthesized following the standard reported literature
[10]. The single crystal of the complex were grown by a slow
⁎
Corresponding author.
E-mail address: ralal@rediffmail.com (R.A. Lal).
https://doi.org/10.1016/j.inoche.2017.09.018
1387-7003/© 2017 Elsevier B.V. All rights reserved.

40
S.D. Kurbah et al. / Inorganic Chemistry Communications 86 (2017) 39–43
Fig. 1. (a) Molecular structure of bimetallic complex and (b) optimized geometrical structure.
The use of molybdenum complexes as catalysts for the oxida-
Table 1
tion reaction has become a topical research area in recent years.
Thus, having synthesized homobimetallic molybdenum(VI) com-
plex, we investigated the catalytic activity in the oxidation reac-
tion of alcohols using hydrogen peroxide as a mild oxidant. In
order to achieve the best catalytic performance, we carried out
the oxidation of benzyl alcohol as a standard substrate [20]
(Scheme 1).
The catalytic studies were carried out using different amount of
catalyst and found that 0.01 mmol gave the best results at a temper-
ature of 70 °C (Figs. 3 & 4) with 49% of the oxidized product. Differ-
ent solvent were tested under similar condition and was found that
acetonitrile (Fig. 5) gave best yield. In order to improve the catalytic
studies and selectivity of the product, we also introduced some base
in the reaction medium and it seems that addition of a base decrease
the reaction rate. The catalytic oxidation of benzyl alcohol were also
tested using different metal sources such as, MoO3, Mo2(OAC)4 and
MoO2(acac)2 but it was found that the catalytic activity of complex
is superior giving high yield. Additionally, the reaction was also car-
ried out under dinitrogen atmosphere but only a trace amount of the
product was observed, hence the catalytic oxidation reaction pro-
ceed solely with air.
Crystal data and structure refinement for complex.
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
b/Å
c/Å
C9H9MoN2O5
321.12
294.11(10)
Monoclinic
P21/n
8.7716(11)
11.5108(13)
11.4795(17)
90
109.170(16)
90
1094.8(3)
4
1.210
3503
2240/0/155
1.195
α/°
β/°
γ/°
Volume/Å3
Z
μ/mm−1
Reflections collected
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I N =2σ (I)]
Final R indexes [all data]
R
R
1 = 0.0751, wR2 = 0.2098
1 = 0.0829, wR2 = 0.2147
Table 2
Represents hydrogen bond parameters of the complex.
Donor – H…Acceptor → D - H⋯⋯H…A⋯⋯D…A D - H…A
O(5) –H(5A)…O(4)#1⋯⋯⋯0.86⋯⋯1.88⋯⋯2.698(13)⋯⋯159
O(5) –H(5B)…O(3)#2⋯⋯⋯0.86⋯⋯2.16 → 2.714(13)⋯⋯122
#1: -x, 1 − y,-z; #2: 1/2 − x, -1/2 + y,1/2 − z
phase in which intermolecular interactions are absent and moreover its
structure is more extended compared to solid state structure, in which
intermolecular interactions become prominent (Fig. 2).
Scheme 1. Oxidation of benzyl alcohol using synthesized complex.
Fig. 2. Overlay structure of space fill model with zigzag one dimensional hydrogen bonding.

S.D. Kurbah et al. / Inorganic Chemistry Communications 86 (2017) 39–43
41
corresponding aldehydes and ketones in good to excellent yields (Table
3, Entry 1–4, 6–12). Primary benzyl alcohols bearing electron donating
groups such as, p-methoxy benzyl alcohol gave 90% isolated yield
of p-methoxy benzaldehyde (GC: 94%), p-methyl benzyl alcohol
yields 88% of p-methyl benzaldehyde, whereas 3,4,5-trimethoxy
benzyl alcohol gave 90% of isolated yield within 3 h. Electron donat-
ing group present at the ortho and para position of the benzene ring
such as, o-methyl benzyl alcohol gave 79% of o-methyl benzaldehyde
and m-methyl benzyl alcohol gave 78% of m-methyl benzaldehyde.
Electron withdrawing groups such as p-nitro benzyl alcohol and m-
chloro benzyl alcohol gave 81% (Table 3, Entry 3) and 78.4% (Table
3, Entry 8) isolated yield lower to that of electron donating groups
present in para position of the benzene ring. Benzyl alcohol contain-
ing double bond functional groups, such as cinnamyl alcohol was
also oxidized to cinnamaldehyde with selectively 87% conversion
without oxidation of double bond. 2-hydroxy-1,2-diphenylethan-1-
one was also oxidized to benzil with 85% yield within 2 h. Secondary
benzyl alcohols such as phenyl ethanol (entry 6), phenyl propanol
(entry 7), cyclohexanol (entry 5) and cycloheptanol (entry 13)
were either not react or only poorly oxidized to their corresponding
ketone 0.7%, not reactive, not reactive and 0.5% yield, respectively.
On the other hand, hetero-aryl alcohols like furan-2-yl-methanol,
pyridine-2-yl methanol and thiophene methanol (entries 14, 15,
16) gave 88%, 89% and 77.9% of their corresponding aldehydes. In
case of aliphatic alcohols such as n-propanol (entry 20), n-butanol
(entry 18), and n-octanol (entry 21) were also oxidized for a period
of 48 h, and gave moderate to excellent yields of 69%, 78%, and 35.3%,
respectively. Propargylic alcohols such as 2-hexyne-1-ol, were also
oxidized to 2-hexyne-1-al with lower yield 17% (entry 19)
(Scheme 2).
Fig. 3. Effect of concentration of catalyst on oxidation of benzyl alcohol.
The result above shows that homobinuclear molybdenum(VI)
complex containing hydrazone ligand can be used as a model cata-
lyst for oxidation of wide range of benzylic alcohol, hetero-aryl alco-
hols, aliphatic alcohols and propargylic alcohols using H2O2 as
terminal oxidant. The catalytic behavior of the binuclear
molybdenum(VI) complexes are comparable to that of the reported
complexes containing molybdenum ions [21]. The process is efficient
and the catalyst can be recovered without any appreciable loss in the
catalytic process (Fig. S11).
Fig. 4. Effect of temperature on oxidation of benzyl alcohol.
The catalytic study of complex was applied to diverse substrates by
oxidizing various benzylic, allylic and propargylic alcohols (Table 3)
using the optimization method to assess the reaction scope and limita-
tion. Both primary and secondary benzyl alcohols were reacted to afford
In conclusion, binuclear complex [MoO2)2(slsch)(H2O)2] has been
prepared and well characterized. Crystal structure shows that ligand co-
ordinates to metal ion in bis-(tridentates) fashion in tetra-deprotonated
form. The crystal structure of complex (water coordinated) confirmed
the distorted octahedral geometry on each metal ion. The complex can
be used as a model catalyst for oxidation of primary benzylic alcohols,
aliphatic, allylic, and propargylic alcohols in the present of 15% H2O2.
The process went on smoothly and moreover the complexes can be eas-
ily recovered without any decrease in percentage yield in the next
cycles.
Acknowledgements
Authors are thankful to, SAIF and DST-PURSE SCXRD, North-Eastern
Hill University, Shillong-793022, India for providing NMR and X-ray
analysis. Sunshine D. Kurbah would like to thank UGC, New Delhi for
NFST fellowship. We also extend our thanks to computer center of the
university, for providing high computing performance facilities.
Appendix A. Supplementary material
The crystallographic data (CCDC number: 1566197), can be obtained
via https://summary.ccdc.cam.ac.uk/structure-summary-form, from
Cambridge Crystallographic Data Centre. Supplementary data associat-
ed with this article can be found online at. https://doi.org/10.1016/j.
inoche.2017.09.018.
Fig. 5. Effect of solvent on oxidation of benzyl alcohol.

42
S.D. Kurbah et al. / Inorganic Chemistry Communications 86 (2017) 39–43
Table 3
Oxidation of alcohols catalyzed by [(MoO2)2slsch(H2O)2] under mild conditiona.
Sl. no
1
Alcohol
Product
Time (h)
3
Yield (%) (isolated)b R-CHO
89
Yield (%) (GC %)c R-CHO
90
2
3
3
3
90
81
94
85.8
4
5
6
3
3
5
87
90
–
0.7
No reaction
No reaction
7
8
5
4
No reaction
78.4
No reaction
80.6
9
4
3
88
78
89
10
82.8
11
12
3
2
79
85
87.7
89.7
13
14
15
3
3
3
No reaction
0.5
90
91
88
89
16
17
2
3
77.9
90
83
91
18
19
48
14
78
–
81.7
17
20
21
48
48
69
32
75.7
35
a
Reaction condition: catalyst (0.010 mmol); 15% H2O2 (4.85 mmol); alcohol (4.84 mmol).
Yield.
GC yield.
b
c
[3] (a) M. Schubert, J. Leppin, K. Wehming, D. Schollmeyer, K. Heinze, S.R. Waldvogel,
Angew. Chem. Int. Ed. 53 (2014) 2494;
(b) R. Sanz, M. Pedrosa, Curr. Org. Synth. 6 (2009) 239.
[4] M. Amini, M.M. Haghdoost, M. Bagherzadeh, Coord. Chem. Rev. 257 (2013) 1093.
[5] (a) R. Hille, Dalton Trans. 42 (2013) 3029;
(b) M. Bagherzadeh, M. Amini, H. Parastar, M. Jalali-Heravi, A. Ellern, L.K. Woo,
Inorg. Chem. Commun. 20 (2012) 86.
[6] G. Schwarz, R.R. Mendel, Annu. Rev. Plant Biol. 57 (2006) 623.
[7] C. Coelho, M. Mahro, J. Trincão, A.T.P. Carvalho, M.J. Ramos, M. Terao, E. Garattini, S.
Leimkühler, M.J. Romao, J. Biol. Chem. 287 (2012) 40690.
[8] M.J. Barber, L.P. Solomonson, Polyhedron 5 (1986) 577.
[9] C. Enroth, B.T. Eger, K. Okamoto, T. Nishino, E.F. Pai, Proc. Natl. Acad. Sci. U. S. A. 97
(2000) 10723.
[10] S.D. Kurbah, A. Kumar, S. Shangpung, I. Syiemlieh, I. Khongjoh, R.A. Lal, Z. Anorg.
Allg. Chem. 643 (2017) 794.
[11] S.S. Paul, M.D. Selim, A. Sahab, K.K. Mukherjea, Dalton Trans. 43 (2014) 2835.
[12] R. Hille, Chem. Rev. 96 (1996) 2757.
[13] R. Hille, Biochim. Biophys. Acta 1184 (1994) 143.
[14] R. Hille, Chem. Rev. 96 (1996) 2757.
[15] A. Dupe, M.E. Judmaier, F. Belaj, K. Zanggerb, N.C. Mosch-Zanetti, Dalton Trans. 44
(2015) 20514.
[16] (a) C. Schulzke, Eur. J. Inorg. Chem. (2011) 1189;
Scheme 2. Bimetallic complex as precatalyst for the catalytic oxidation alcohol (only one
unit of complex is shown).
(b) M.R. Maurya, L. Rana, N. Jangra, F. Avecilla, Chem. Select. 2 (2017) 6767.
[17] (a) H.H. Monfared, N.A. Lalami, A. Pazio, K. Wozniak, Christoph Janiak, Inorg. Chim.
Acta 406 (2013) 241;
References
(b) N.A. Lalami, H.H. Monfared, H. Noei, P. Mayer, Transit. Met. Chem. 36 (2011) 669.
(c) [MoO2)2(slsch)(H2O)2]. To a solution of molybdenum acetylacetonate (0.326 g,
1mmol) in methanol (20 mL), H4slsch (0.175 g, 0.5 mmol) was added with
[1] M.R. Maurya, L. Ranaa, F. Avecilla, New J. Chem. 41 (2017) 724.
[2] A. Dupe, M.E. Judmaier, F. Belaj, K. Zanggerb, N.C. Mosch-Zanetti, Dalton Trans. 44
(2015) 20514.

S.D. Kurbah et al. / Inorganic Chemistry Communications 86 (2017) 39–43
43
constant stirring and reflux for 20 minutes. The reaction mixture was cool to
room temperature and evaporated under vacuum to obtain the desired com-
plex. The single light yellow crystals suitable for X-ray analysis were obtained
by a slow evaporation of their CH3CN/water solutions (2:1, v/v), in air at room
temperature. Anal (%): Calcd for C24 H28 N6 O10 Mo2: C, 38.31; H, 3.75; N,
[20] Benzyl alcohol (0.05 g, 0.5 mmol), was added to a mixture of 15% H2O2 (0.3 g,
8.83 mmol), complex [MoO2)2(slsch)(H2O)2] (0.01 g, 0.017 mmol) in acetonitrile
solution in 50 mL round bottom flask. The mixture was first stirred at ambient tem-
perature for 20 mins and then the temperature was raised to 70 °C for 2 h. The
crude product was separated by a separating funnel and dried over anhydrous sodi-
um sulfate. The oxidized product was purified by column chromatography to afford
benzaldehyde. The isolated product was ascertained by comparison with the au-
thentic sample using 1H and 13C NMR spectroscopies.
11.17; Found: C, 38.72; H, 3.71; N, 11.60. IR data (cm−1 KBr): 3400 (sbr)
,
ν(OH), 1621 (s) ν(C_N), 1558 (s) Amide II + (C-O) (Phenyl), 1545 (vs)
ν(NCO-), 1341 (m), 1270 (m) ν(C-O), 1017 (w) ν(N-N), 925 (s), 907 (s),
ν(M_O), 578 (w) (Mo-O) (phenolic), 466 (w) (M-O)(carbonyl). 1H NMR
(400 MHz, DMSO-d6, Me4Si): δ (ppm): 7.94 (s, 1H, H-C_O), 6.88–7.94 (m,
8H, Ar-H), 8.28–8.76 (s, 2H, C(H)_N), 2.5 (s, 4H, (−CH2)). Electronic spectrum
CH3CN Solution, λmax: 310, 384.
[21] (a) S.K. Maiti, K.M. Abdul Malik, R. Bhattacharyya, Inorg. Chem. Commun. 7 (2004)
823;
(b) M.R. Maurya, L. Rana, F. Avecilla, New J. Chem. 41 (2017) 724;
(d) M.R. Maurya, N. Saini, F. Avecilla, Inorg. Chim. Acta 438 (2015) 168.
[18] L. Fernandez, F.F. Perez-Pla, I. Tunon, E. Llopis, J. Phys. Chem. A 120 (2016) 9636.
[19] M.J. Frisch, et al., GAUSSIAN 09, Revision C.01, Gaussian Inc., Walling-ford, CT, 2009.