Supramolecular structures of mononuclear and dinuclear dioxomolybdenum(VI) complexes via hydrogen bonds and π-π Stacking, thermal studies and electrochemical measurements

Article abstract of DOI:10.1016/j.poly.2013.08.059

Two dioxomolybdenum(VI) complexes, [Mo₂O₄(HBAP) ₂L] (1) and [MoO₂(HMBI)] (2) (HBAP = ((E)-2-(2- hydroxybenzylidene)amino)phenol, HMBI = ((E)-N′-(2-hydroxy-3- ethoxybenzylidene)isonicotinohydrazide) and L = (4,4′

Full text of DOI:10.1016/j.poly.2013.08.059

Polyhedron 67 (2014) 11–18
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Supramolecular structures of mononuclear and dinuclear
dioxomolybdenum(VI) complexes via hydrogen bonds and p–p
stacking, thermal studies and electrochemical measurements
b
Samir Alghool a, , Carla Slebodnick
⇑
a Department of Chemistry, Faculty of Science, Port Said University, Port Said 42521, Egypt
b Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Two dioxomolybdenum(VI) complexes, [Mo2O4(HBAP)2L] (1) and [MoO2(HMBI)] (2) (HBAP = ((E)-2-(2-
hydroxybenzylidene)amino)phenol, HMBI = ((E)-N0-(2-hydroxy-3-ethoxybenzylidene)isonicotinohydraz-
ide) and L = (4,40-dipyridyl)), have been synthesized and structurally characterized by elemental analysis,
1H NMR spectroscopy and X-ray single crystal diffraction analysis. In complex (1), a 1D-chain is formed
through Cꢀ ꢀ ꢀHꢀ ꢀ ꢀO hydrogen-bonding interactions. In complex (2), a dimer is formed through Oꢀ ꢀ ꢀHꢀ ꢀ ꢀN
hydrogen-bonding interactions. This dimer is extended into a 1D chain through secondary Cꢀ ꢀ ꢀHꢀ ꢀ ꢀO
Received 11 June 2013
Accepted 6 August 2013
Available online 5 September 2013
Keywords:
Supramolecular
Dioxomolybdenum
Hydrogen bond
p–p Stacking
Electrochemical
hydrogen-bonding interactions, in addition to p–p stacking interactions. Electrochemical measurements
showed that both complexes undergo an irreversible reduction in DMF solution. The thermogravimetric
analyses clearly indicate that the complexes have high thermal stability.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
The field of supramolecular frameworks has developed rapidly
because of its structural diversity and potential applications [20–
The chemistry of molybdenum has become a prospective area
for research. This is due to molybdenum’s ability to form com-
pounds with most inorganic and organic ligands, in addition to
forming bi- and polynuclear compounds. These compounds have
the ability to form molybdenum–molybdenum bonds as well as
being complexes containing bridging ligands [1–5]. Molybdenum
compounds have many and varied potential applications. Recently,
molybdenum complexes have been paid great interest because of
the presence of molybdenum in metalloenzymes and its significant
role in hydroxylase and oxotransferase enzymes [6–12].
24]. There are large numbers of 1D, 2D and 3D supramolecular
frameworks that have been studied, with various types of hydro-
gen bonds and
p–p stacking interactions [25–27]. The selection
and design of suitable ligands as a building blocks are playing
key roles in the construction of supramolecular frameworks. Addi-
tionally, the coordination geometry of the metal and the orienta-
tion of the binding sites are key factors in the supramolecular
design. Despite the great importance of molybdenum complexes,
few articles present molybdenum complexes as supramolecular
structures [28–32].
An exceptional feature of molybdenum is that it has versatile
oxidation states, from (ꢁ2) to (6), and coordination numbers rang-
ing from 4 to 8 [13]. It is well known that catalytic reactions of
molybdoenzymes involve molybdenum in its (+6) and (+4) oxida-
tion states [14]. Therefore, molybdenum complexes are considered
to be very effective catalysts for oxidation reactions [15]. Molybde-
num Schiff base complexes have continued to play the role of one
of the most important models of molybdoenzymes due to their
preparative accessibility [16,17]. Schiff bases derived from salicyl-
aldehyde and amino alcohols have been widely used for the prep-
aration of oxomolybdenum complexes containing the MoO22þ core
[18,19].
In this article, we report the synthesis and crystal structure
determination of supramolecular mono and dinuclear dioxomolyb-
denum(VI) complexes based on Schiff base ligands. The electro-
chemistry of the dioxomolybdenum(VI) complexes was studied
as well as the thermal degradation of the complexes.
2. Experimental
2.1. Materials
MoO2(acac)2, salicaldehyde, 4,40-dipyridyl, O-ominophenol,
O-vanillin and isonicotinic acid hydrazide were purchased from Al-
drich. Reagent grade solvents were dried and distilled prior to use.
All the starting chemicals were analytically pure and were used
⇑
Corresponding author. Tel.: +20 010 96200949; fax: +20 066 3657601.
E-mail address: Alghools@yahoo.com (S. Alghool).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.08.059

12
S. Alghool, C. Slebodnick / Polyhedron 67 (2014) 11–18
OCH3
OH
O
HO
OH
N
N
H
N
N
(E)-2-((2-hydroxybenzylidene) amino) phenol (HBAP)
(E)-N'-(2-hydroxy-3-methoxybenzylidene) isonicotinohydrazide (HMBI)
Fig. 1. Structure of the ligands.
Table 1
Crystal data and structural refinements for complexes (1) and (2).
Compound
Complex (1)
36H26Mo2N4O8
834.49
100.3
Complex (2)
Empirical formula
Formula weight
T (K)
C
C15H15MoN3O6
429.24
100(1)
k (Å)
Crystal system
Space group
0.71073
monoclinic
P121/c
0.7107
monoclinic
P121/n
Unit cell dimensions
a (Å)
b (Å)
c (Å)
10.8386(13)
6.5964(6)
22.722(2)
6.75499(13).
30.2838(5)
7.89920(13)
a
(°)
90
90
b (°)
91.349(9)
93.3546(16)
c
(°)
90
1624.1(3)
90
V (Å3)
1613.14(5)
Z
2
4
Dcalc (Mg/m3)
1.706
0.834
836
1.767
0.852
864
Absorption coefficient (mmꢁ1
F(000)
)
Crystal size (mm)
Theta range for data, collection (°)
Index ranges
0.2361 ꢂ 0.0385 ꢂ 0.0263
0.3955 ꢂ 0.2024 ꢂ 0.0552
3.57–32.51
3.63–32.05
ꢁ16 6 h 6 15, ꢁ9 6 k 6 9, ꢁ34 6 l 6 33
13610
ꢁ10 6 h 6 9, ꢁ44 6 k 6 44, ꢁ11 6 l 6 11
Reflections collected
20275
Independent reflections
Completeness to theta = 30.00°
Absorption correction
5347 [Rint = 0.0913]
99.90%
5279 [Rint = 0.0312]
99.20%
Maximum and minimum transmission
Refinement method
0.988 and 0.936
0.956 and 0.794
full-matrix least-squares on F2
5347/36/245
full-matrix least-squares on F2
5279/3/231
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
1.049
1.11
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0668, wR2 = 0.0853
R1 = 0.1296, wR2 = 0.1033
0.757 and ꢁ0.866
R1 = 0.0277, wR2 = 0.0573
R1 = 0.0345, wR2 = 0.0601
0.543 and ꢁ0.442
Largest difference in peak and hole (e Åꢁ3
)
without further purification. The Schiff bases were synthesized
using the procedure reported in the literature [33].
internal standard. Measurements were made at a rate of 100 mV/
s. The supporting electrolyte was 0.1 M nBu4NPF6 in spectral grade
DMF. Solutions were deoxygenated by bubbling with argon, and
the working electrode was manually cleaned prior to analysis.
Thermogravimetric analyses (TG/DTG) were carried out at 25–
800 °C under a steam of nitrogen using a TGA Q500 thermal ana-
lyzer. The experimental conditions were platinum crucible, nitro-
gen atmosphere with a 40 ml/min flow rate and a heating rate
10 °C/min.
2.2. Physical measurements
Elemental analyses (C, H and N) were performed using Desert
Analytics (Tucson, AZ). UV–Vis spectral measurements were car-
ried out in DMSO solution at a concentration of 1.0 ꢂ 10ꢁ3 M using
a Jenway 6405 spectrophotometer with a 1 cm quartz cell, in the
range 200–800 nm. 1H NMR spectra were recorded on a Varian
Unity 400 MHz NMR spectrometer and referenced to the residual
solvent peaks. FAB-mass spectra were recorded on a JEOL SX
102/DA-6000 mass spectrometer/data system using argon/xenon
(6 kV, 10 A) as the FAB gas. IR spectra (4000–400 cmꢁ1) were re-
corded as KBr pellets on a Bruker FT-IR spectrophotometer. Elec-
trochemistry, cyclic voltammetry and Osteryoung squarewave
voltammetry were performed with an Epsilon potentiostat from
Bioanalytical Systems, Inc. in one compartment, three electrode
cells using a carbon working electrode, platinum wire auxiliary
electrode and a Ag wire pseudo reference electrode calibrated
against the ferrocene/ferrocenium couple, FeCp2/FeCp2+, as an
2.3. X-ray crystallography
An orange-brown needle (0.026 ꢂ 0.038 ꢂ 0.24 mm3) of com-
plex (1) and an orange block (0.06 ꢂ 0.20 ꢂ 0.40 mm3) of complex
(2) were centered on the goniometer of an Agilent Gemini E Ultra
diffractometer operating with Mo Ka radiation. The data collection
routine, unit cell refinement and data processing were carried out
with the program CrysAlisPro [34]. The Laue symmetry and sys-
tematic absences were consistent with the monoclinic space group
P21/c for complex (1) and P21/n for complex (2). The structures
were solved using SHELXS-97 [35] and refined using SHELXS-97 [35]

S. Alghool, C. Slebodnick / Polyhedron 67 (2014) 11–18
13
Fig. 2. (a) Molecular diagram for complex (1) showing the atom-labelling scheme (non-hydrogen) (50% thermal ellipsoids), (b) Molecular diagram of the asymmetric unit for
complex (1) showing the atom-labelling scheme, hydrogen atoms are omitted for clarity (50% thermal ellipsoids).
Table 2
concentrated and cooled to room temperature, then the orange
precipitate that formed was filtered, and washed with 2 ml meth-
Selected bond lengths (Å) and angles (°) for complex (1).
Bond lengths
Bond angles
anol and 5 ml diethyl ether, dried in vacuo to obtain HBAP, Fig. 1
(yield 87%, 1.86 g). Anal. Calc. for C13H11NO2: C, 73.33; H, 5.20; N,
6.57. Found: C, 73.64; H, 5.17; N, 6.81%. 1H NMR (400 MHz,
DMSO-d6) d: 8.53 (s, 1H, imine), 7.56–6.71 (m, 2 ꢂ 4H, Ph), 4.71
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(4)
Mo(1)–N(1A)
Mo(1)–N(1B)
Mo(1)–N(2)
O(3)–C(1)
O(4)–C(13)
N(1A)–C(2)
N(1A)–C(7A)
N(1B)–C(7B)
N(1B)–C(8)
N(2)–C(14)
N(2)–C(18)
1.707(3)
1.705(3)
1.956(3)
1.930(3)
2.275(4)
2.27(3)
2.424(3)
1.352(5)
1.344(5)
1.442(6)
1.290(8)
1.30(4)
O(1)–Mo(1)–O(3)
O(1)–Mo(1)–O(4)
O(1)–Mo(1)–N(1A)
O(1)–Mo(1)–N(1B)
O(1)–Mo(1)–N(2)
O(2)–Mo(1)–O(1)
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–O(4)
O(2)–Mo(1)–N(1A)
O(2)–Mo(1)–N(1B)
O(2)–Mo(1)–N(2)
O(3)–Mo(1)–N(1A)
O(3)–Mo(1)–N(1B)
O(3)–Mo(1)–N(2)
O(4)–Mo(1)–O(3)
O(4)–Mo(1)–N(1A)
O(4)–Mo(1)–N(1B)
O(4)–Mo(1)–N(2)
N(1A)–Mo(1)–N(2)
N(1B)–Mo(1)–N(2)
C(1)–O(3)–Mo(1)
96.46(14)
100.57(14)
161.82(15)
161.9(6)
84.65(13)
106.77(14)
99.82(13)
98.95(13)
90.13(14)
86.9(6)
168.57(13)
73.55(16)
92.7(7)
78.13(12)
149.74(13)
82.91(16)
64.9(7)
(s, 2 ꢂ 1H, Ph–OH). IR (KBr, in cmꢁ1):
m(OH) 3324, m(C–N) 1629.
The mass spectrum gave a molecular peak at 213 m/z.
HMBI was synthesized and purified by a similar method, using
isonicotinic acid hydrazide instead of O-aminophenol, and O-vanil-
lin instead of salicylaldehyde, Fig. 1 (yield 70%, 1.9 g). Anal. Calc. for
C
14H13N3O3: C, 61.99; H, 4.83; N, 15.49. Found: C, 62.11; H, 4.54; N,
15.87%. 1H NMR (400 MHz, DMSO-d6) d: 12.1 (s, 1H, imine), 8.82–
1.70(3)
1.331(5)
1.327(5)
7.80 (m, 4H, pyridine ring), 7.56–6.79 (m, 3H, Ph), 7.45 (s, 1H, ben-
zylidene), 3.8 (s, 3H, OCH3). IR (KBr, in cmꢁ1):
m(OH) 3332,
m(NH)
3255,
m(C–N) 1639, m(C@O) 1678. The mass spectrum gave a
molecular peak at 271 m/z.
78.77(12)
78.49(13)
82.0(6)
2.5. Synthesis of the dioxomolybdenum(VI) complexes
123.8(3)
2.5.1. Complex (1)
via OLEX2 [36]. A 2-position disorder model was used for the Schiff-
base ligand, with relative occupancies that refined to 0.844(8) and
0.156(8). The final refinement model involved anisotropic dis-
placement parameters for non-hydrogen atoms and a riding model
for all hydrogen atoms. The anisotropic displacement parameters
of the atoms in the disordered region of the Schiff base were re-
strained with the SIMU command. The hydrogen atom position
of the CH3OH alcohol group was located from the residual electron
density map and the position refined with distance restraints.
A solution of the Schiff base ligand HBAP (0.1 mmol, 0.213 g) in
15 ml methanol was added to a stirred solution of MoO2(acac)2
(0.1 mmol, 0.331 g) in 5 ml methanol and 2 ml dichloromethane.
The reaction mixture was heated under reflux for 1 h with contin-
uous stirring, while the color changed from yellow to orange, the
linker (4,40-dipyridyl (0.05 mmol, 0.08 g) in 5 ml methanol was
added dropwise over a period of 10 min. The resulting reaction
solution was heated under reflux with stirring for an addition
1 h. The solvent was removed in vacuo and the residue was dis-
solved in 10 ml dichloromethane. A very fine colorless precipitate
was filtered over celite. The orange filtrate was recrystallized from
dichloromethane and methanol. Upon standing at ꢁ5 °C and after
4 days, orange solid crystals appeared (yield 43%, 0.360 g). Anal.
Calc. for C36H26Mo2N4O8 (834.49): C, 51.81; H, 3.14; N, 6.71.
Found: C, 52.23; H, 3.57; N, 6.54%.
2.4. Synthesis of the Schiff base ligands
O-aminophenol (1.09 g, 10 mmol) in methanol (30 ml) was
added to salicylaldehyde (1.221 g, 10 mmol) in methanol (10 ml).
The resulted deep orange solution was heated at 80 °C for 2 h,

14
S. Alghool, C. Slebodnick / Polyhedron 67 (2014) 11–18
Fig. 3. (a) Diagram for complex (1) showing the hydrogen-bonding between O2 and H7A (the hydrogen bond is represented as a black dashed line), non-bonding hydrogen
atoms are omitted for clarity, (b) Packing diagram for complex (1) around the b axis, hydrogen atoms are omitted for clarity.
(0.1 mmol, 0. 331 g) in 5 ml methanol and 2 ml dichloromethane.
The reaction mixture was heated under reflux for 2 h with contin-
uous stirring, while the color changed from yellow to orange, then
the solvent was removed in vacuo. The residue was dissolved in
10 ml dichloromethane and 2 ml MeOH and was left for 2 days at
ꢁ5 °C. Orange solid crystals suitable for X-ray single crystal analy-
sis appeared (yield 45%, 0.196 g). Anal. Calc. for C15H15MoN3O6
(429.24): C, 41.97; H, 3.52; N, 9.79. Found: C, 42.13; H, 3.66; N,
9.58%.
3. Results and discussion
Two dioxomolybdenum(VI) compounds were synthesized from
the reaction of the tridentate ligands HBAP and HMBI with bis-
(acetylacetonate)dioxomolybdenum(VI), in the presence of 4,40-
Fig. 4. Molecular diagram for complex (2) showing the atom-labelling scheme (50%
thermal ellipsoids), hydrogen atoms are omitted for clarity.
dipyridyl as a linker in the case of complex (1). In complex (2),
no linker was used because the terminal nitrogen of the pyridine
ring was expected to act as a linker. X-ray single crystal diffraction
analysis reveals that the terminal nitrogen atom did not coordinate
to the metal ion as expected, but it was involved in a hydrogen
2.5.2. Complex (2)
A solution of the Schiff base ligand HMBI (0.1 mmol, 0. 271 g) in
15 ml methanol was added to a stirred solution of MoO2(acac)2

S. Alghool, C. Slebodnick / Polyhedron 67 (2014) 11–18
15
Table 3
and soluble in DMF and DMSO. X-ray quality crystals were
obtained by keeping the solutions in a mixture of dichloromethane
and methanol at ꢁ5 °C for 4 days.
Selected bond lengths (Å) and angles (°) for complex (2).
Bond lengths
Bond angles
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(4)
Mo(1)–O(6)
Mo(1)–N(1)
O(3)–C(1)
O(4)–C(8)
O(5)–C(7)
O(5)–C(14)
O(6)–C(15)
N(1)–N(2)
N(1)–C(2)
1.7127(12)
1.7045(12)
2.0237(12)
1.9375(12)
2.3470(12)
2.2315(14)
1.330(2)
1.3479(19)
1.363(2)
1.436(2)
1.437(2)
1.3983(19)
1.296(2)
1.298(2)
1.344(2)
O(1)–Mo(1)–O(3)
O(1)–Mo(1)–O(4)
O(1)–Mo(1)–O(6)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–O(1)
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–O(4)
O(2)–Mo(1)–O(6)
O(2)–Mo(1)–N(1)
O(3)–Mo(1)–O(6)
O(3)–Mo(1)–N(1)
O(4)–Mo(1)–O(3)
O(4)–Mo(1)–O(6)
O(4)–Mo(1)–N(1)
N(1)–Mo(1)–O(6)
C(1)–O(3)–Mo(1)
C(8)–O(4)–Mo(1)
C(7)–O(5)–C(14)
C(15)–O(6)-Mo(1)
N(2)–N(1)–Mo(1)
C(2)–N(1)–Mo(1)
97.41(5)
102.13(5)
83.75(5)
157.43(5)
105.34(6)
96.41(6)
98.68(6)
170.72(5)
95.85(5)
80.32(5)
72.15(5)
151.09(5)
80.86(5)
81.87(5)
3.1. The structure of complex (1)
Complex (1) crystallized in the monoclinic space group P21/c,
Table 1. The molecular structure is shown in Fig. 2a, with relevant
bond distances and angles collected in Table 2. The coordination
geometry around the Mo(VI) centre is distorted octahedral. The
asymmetric unit of the compound consists of a [MoO2(HBAP)]
moiety and half of a 4,40-dipyridyl molecule, Fig. 2b. The Schiff base
ligand is bonded to the Mo(VI)O22þ ion in the xy-plane through two
phenoxide oxygen donor atoms, an imine nitrogen atom and a
nitrogen atom of 4,40-bipyridine. Two[MoO2(HBAP)] moieties are
linked together via the 4,40-dipyridyl ligand to form a dinuclear
dioxomolybdenum complex. The [MoO2(HBAP)] moiety occupies
the equatorial plane. The terminal nitrogen of 4,40-dipyridyl and
the second oxo ligand (O) occupy the axial positions. The ligand
coordinates to the Mo ion, forming five and six member chelate
rings with O(3)–Mo(1)–N(1A) and O(4)–Mo(1)–N(1A) bite angles
of 73.55(16) and 82.91(16)°, respectively. The non-bonding inter-
atomic Moꢀ ꢀ ꢀMo separation within the dimeric cation is 11.96 Å.
The Mo–O, Mo–N and Mo@O bonds are within the normal ranges
and are comparable to those observed in similar dioxomolybde-
num(VI) complexes [37,38].
N(2)–C(1)
N(3)–C(11)
N(3)–C(12)
74.89(5)
1.345(2)
118.46(10)
136.03(11)
116.56(14)
122.52(11)
115.78(10)
129.10(12)
bonding interaction instead. The complexes were isolated as
orange crystals. They are insoluble in weak polar solvents, only
sparingly soluble in strong polar solvents (acetonitrile, CH2Cl2)
Fig. 5. (a) Molecular diagram illustrating the formation of the dimer as result of hydrogen bonding between H6 and N3, (b) Diagram illustrating the formation of the infinite
one dimensional hydrogen bonded chain in complex (2). Hydrogen bonding between two molecules to form a dimer is represented by a dashed blue line (N3ꢀ ꢀ ꢀH6), the
extended hydrogen bond is represented by a dashed black line (H2ꢀ ꢀ ꢀO2), non-bonding hydrogen atoms are omitted for clarity.

16
S. Alghool, C. Slebodnick / Polyhedron 67 (2014) 11–18
Fig. 6. (a) Crystal packing of complex (2) around the a axis. (b) Structure diagram of complex (2) showing the intermolecular p–p stacking interactions between two pyridine
rings, hydrogen atoms are omitted for clarity.
Fig. 7. TGA curves of the Mo complexes (1) and (2).
A hydrogen bond was observed in the packing structure, Fig. 3a,
between the oxo ligand of molybdenum (O2) and H7A (a hydrogen
atom of a methylene group) with bond lengths O2ꢀ ꢀ ꢀH7A 2.35 Å,
O2ꢀ ꢀ ꢀC7A 3.14 Å and angle O2ꢀ ꢀ ꢀH7Aꢀ ꢀ ꢀC7A 140.6°, forming a 1D
chain (ladder structure), Fig. 3b. The hydrogen bond distance and
angle values are in good agreement with previously reported val-
ues [5].
the isonicotinic moiety and an imine nitrogen atom, with one
methanol molecule completing the distorted octahedral geometry.
The methanol ligand and the second oxo ligand occupy the axial
positions. The complex crystallizes in the spacegroup P121/n1, Ta-
ble 1. Selected bond lengths and angles are given in Table 3. The
Mo ion forms five and six member chelate rings with bite angles
of 72.15(5) and 81.87(5)°, respectively. The ligand is coordinated
in the enol form, which is evident from the O(3)–C(1) and N(2)–
C(1) bond lengths, with values of 1.330(2) and 1.298(2) Å, respec-
tively. In the crystal packing structure of complex (2) it is obvious
that adjacent molecules are linked through Oꢀ ꢀ ꢀH bonds to form a
dimer. The hydrogen bond exists between the hydroxyl group of a
coordinated methanol molecule and the nitrogen atom of a
pyridine ring to form an intermolecular O6ꢀ ꢀ ꢀH6ꢀ ꢀ ꢀN3 hydrogen
bonding interaction, as shown in Fig. 5a, with bond lengths
3.2. The structure of complex (2)
The coordination geometry around the Mo ion in the structure
of complex (2) can be described as slightly distorted octahedral.
The molecular structure of complex (2) is shown in Fig. 4. The
Schiff base ligand is bonded to the Mo(VI)O22þ ion in the xy-plane
through one phenoxide oxygen donor atom, the ketone oxygen of

S. Alghool, C. Slebodnick / Polyhedron 67 (2014) 11–18
17
Fig. 8. Electrochemical measurements of the Mo complexes (1) and (2).
H6ꢀ ꢀ ꢀN3 1.84 Å, N3ꢀ ꢀ ꢀO6 2.701 Å and angle N3ꢀ ꢀ ꢀH6ꢀ ꢀ ꢀO6 165°. This
4. Conclusions
dimer is extended into a 1D chain through hydrogen bonding,
Fig. 5b, between the oxo ligand of molybdenum (O2) and a hydro-
gen atom of a methylene group, O2ꢀ ꢀ ꢀH2, with bond lengths
O2ꢀ ꢀ ꢀH2 2.303 Å, O2ꢀ ꢀ ꢀC2 3.20 Å and angle O2ꢀ ꢀ ꢀH2ꢀ ꢀ ꢀC2 167°. In
addition to the hydrogen bond, the crystal packing structure of
Supramolecular mononuclear and dinuclear dioxomolybde-
num(VI) complexes have been synthesized and characterized. A
hydrogen bond was observed in the packing structure of complex
(1) between an oxo ligand of molybdenum (O2) and H7A (a hydro-
gen atom of a methylene group) to form a 1D chain. It is obvious
that complex (2) forms a dimer via hydrogen bonding, and this di-
mer is extended into a 1D chain via a second hydrogen bond and
complex (2), Fig. 6a, shows further stabilisation via
p–p stacking
interactions, centroid–centroid distance 3.52 Å, face-to-face, with
a dihedral angle of 0°, Fig. 6b, and this value is similar to the values
reported for other p–p interactions [21,39].
p
–
p
stacking interactions. The presence of
tions in complex (2) is due to the existence of an electron-with-
drawing nitrogen atom, which reduces the -electron density in
the ring and consequently the -electron repulsion. These com-
p–p stacking interac-
p
3.3. Thermal studies
p
plexes show thermal stability up to 252 and 306 °C for complexes
(1) and (2), respectively. The electrochemical measurements of the
two complexes show two irreversible reduction processes (ꢁ1 to
ꢁ2 V) and no oxidation peaks were observed.
The thermal behaviors of the complexes have been studied. TGA
experiments were performed under a nitrogen atmosphere with a
heating rate of 10 °C minꢁ1 in the temperature range 25–800 °C.
The thermal curves are exhibited in Fig. 7. The TGA curve of com-
plex (1) reveals that the complex is stable up to 252 °C. Decompo-
sition of the organic part is within the range 252–370 °C. The final
product, consisting of two Mo atoms, was formed above 400 °C.
Complex (2) is thermal stable up to 306 °C. The weight loss in
the first stage is due to the release of the methanol molecule, in
the range 120–170 °C. Decomposition of the organic part is within
the range 306–700 °C. The final product was formed above 700 °C,
consisting of MoO2 and one carbon residue, with an estimated
mass loss of 33.34% (calculated mass loss 33.07%). As expected,
the complexes have high thermal stability due to the formation
of the hydrogen bonds.
Acknowledgment
We thank Prof. Brian E. Hanson, Professor of Inorganic Chemis-
try, Virginia polytechnic institute of technology and state univer-
sity, for his support and his advice. Thanks go to Dr. Jessica Knoll,
Virginia polytechnic institute of technology and state university,
for helping with electrochemical measurements.
Appendix A. Supplementary data
CCDC cs1753 and cs1943 contains the supplementary crystallo-
graphic data for complex (1) and complex (2). These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk.
3.4. Electrochemical measurements
The electrochemical behavior of the two dioxomolybdnum(IV)
complexes has also been studied. The reduction process has been
observed in DMF in the range +1.0 to ꢁ2.5 V. The electron transfer
behavior of the two complexes shows two irreversible reduction
processes (ꢁ1 to ꢁ2 V), which may be attributed to the elimination
of oxo ligands as OHꢁ or as H2O in the case of complex (1), and the
elimination of an oxo ligand and the methanol ligand in the case of
complex (2), which depends on the existence of protons. On the
other hand, no oxidation peak was observed. For complex (1) the
reduction peaks are at ꢁ1.15 and ꢁ1.86 V, whilst for complex (2)
the reduction peaks are at ꢁ1.04 and ꢁ1.56 V, Fig. 8.
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