Preparation, characterization, and crystal structures of dioxomolybdenum(VI) complexes with tridentate schiff base ligands

Article abstract of DOI:10.1080/15533174.2012.753622

Two new mononuclear dioxomolybdenum(VI) complexes, [MoO₂L ¹(CH₃OH)] (1) and [MoO₂L²(CH ₃OH)] (2), have been prepared by the reaction of MoO ₂(acac)₂ with the tridentate Schiff bases 5-diethylamino-2-[(2-hydroxypropylimino)methyl]phenol (H₂L ¹) and 2-[(2-hydroxypropylimino)methyl]-5-methoxyphenol (H ₂L²), respectively, in methanol. The complexes were characterized by elemental analyses, infrared spectra, and single-crystal X-ray diffraction. Complex (1) crystallizes in monoclinic space group P₂₁/c with unit cell dimensions a = 16.464(2), b = 8.319(1), c = 14.056(2) A, β = 114.272(2)°, V = 1755.0(4) A³, Z = 4, R ₁ = 0.0287, and wR₂ = 0.0718. Complex (2) crystallizes in triclinic space group with unit cell dimensions a = 6.774(2), b = 8.481(2), c = 13.198(3) A, α = 76.183(2), β = 78.963(2), γ = 87.515(2)°, V = 722.7(3) A³, Z = 2, R₁ = 0.0239, and wR₂ = 0.0573. Both complexes consist of cis-MoO₂ core coordinated by Schiff base ligand through two deprotonated hydroxyl groups and one imino nitrogen atom. The octahedral coordination around the molybdenum atom is completed by the neutral methanol ligand. Supplemental materials are available for this article. Go to the publisher's online edition of Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry to view the supplemental file.

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Preparation, Characterization, and Crystal Structures
of Dioxomolybdenum(VI) Complexes With Tridentate
Schiff Base Ligands
Hong-Ge Zhang a
a Department of Chemistry and Chemical Engineering , Baoji University of Arts and
Sciences , Baoji , P. R. China
Accepted author version posted online: 18 Mar 2013.Published online: 02 May 2013.
To cite this article: Hong-Ge Zhang (2013): Preparation, Characterization, and Crystal Structures of Dioxomolybdenum(VI)
Complexes With Tridentate Schiff Base Ligands, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal
Chemistry, 43:8, 1029-1033
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:1029–1033, 2013
Copyright ꢀ Taylor & Francis Group, LLC
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ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2012.753622
Preparation, Characterization, and Crystal Structures
of Dioxomolybdenum(VI) Complexes With Tridentate
Schiff Base Ligands
Hong-Ge Zhang
Department of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji,
P. R. China
plexes might be interesting model systems. In recent years, a
number of dioxomolybdenum complexes have been synthe-
sized and characterized to mimic the biological systems.[3–5]
Schiff bases are a kind of important ligands in coordination
chemistry. However, the number of dioxomolybdenum com-
plexes with Schiff bases is small. We report here the synthesis
and characterization of two new dioxomolybdenum(VI) com-
plexes, [MoO2L1(CH3OH)] (1) and [MoO2L2(CH3OH)] (2),
derived from tridentate Schiff base ligands 5-diethylamino-
2-[(2-hydroxypropylimino)methyl]phenol (H2L1) and 2-[(2-
hydroxypropylimino)methyl]-5-methoxyphenol (H2L2).
Two new mononuclear dioxomolybdenum(VI) complexes,
[MoO2L1(CH3OH)] (1) and [MoO2L2(CH3OH)] (2), have been
prepared by the reaction of MoO2(acac)2 with the tridentate Schiff
bases 5-diethylamino-2-[(2-hydroxypropylimino)methyl]phenol
(H2L1) and 2-[(2-hydroxypropylimino)methyl]-5-methoxyphenol
(H2L2), respectively, in methanol. The complexes were character-
ized by elemental analyses, infrared spectra, and single-crystal
X-ray diffraction. Complex (1) crystallizes in monoclinic space
group P21/c with unit cell dimensions a = 16.464(2), b = 8.319(1),
c = 14.056(2) Å, β = 114.272(2)◦, V = 1755.0(4) Å3, Z = 4, R1 =
0.0287, and wR2 = 0.0718. Complex (2) crystallizes in triclinic
¯
space group P1 with unit cell dimensions a = 6.774(2), b =
8.481(2), c = 13.198(3) Å, α = 76.183(2), β = 78.963(2), γ =
87.515(2)◦, V = 722.7(3) Å3, Z = 2, R1 = 0.0239, and wR2 = 0.0573.
Both complexes consist of cis-MoO2 core coordinated by Schiff
base ligand through two deprotonated hydroxyl groups and one
imino nitrogen atom. The octahedral coordination around the
molybdenum atom is completed by the neutral methanol ligand.
EXPERIMENTAL
Materials and Physical Measurements
All chemicals and solvents (AR grade) were obtained from
Fluka (Beijing, China), and used without further purification.
[MoO2(acac)2] was prepared as described in literature.[6] Ele-
mental analyses of the Schiff bases and the complexes were car-
ried out using a Vario EL III CHNS analyzer (College of Chem-
istry and Chemical Engineering, Lanzhou University, China).
Infrared spectra were recorded on a JASCO FT/IR-4100 type A
spectrometer (Baoji University of Arts and Sciences, China) in
the range 4000–400 cm−1 using KBr discs.
Supplemental materials are available for this article. Go to the
publisher’s online edition of Synthesis and Reactivity in Inorganic,
Metal-Organic, and Nano-Metal Chemistry to view the supplemen-
tal file.
Keywords crystal structure, hydrogen bonding, molybdenum, Schiff
bases, X-ray diffraction
INTRODUCTION
Synthesis of the Schiff Bases
For H2L1, to a stirred methanolic solution (50 mL) of 4-
diethylaminosalicylaldehyde (10 mmol, 1.93 g) was added drop-
wise a methanolic solution (30 mL) of 1-aminopropan-2-ol
(10 mmol, 0.75 g). The mixture was then stirred for 1 h at
reflux. Afterwards the mixture was cooled to room temperature,
and the yellow products formed were collected by filtration.
Yield, 73%. Anal. Calcd. for C14H22N2O2: C, 67.2; H, 8.9; N,
11.2. Found (%): C, 67.0; H, 8.8; N, 11.1.
Molybdenum has been found to be a very important element
in biological systems. Apart from the nitrogenases in where
a molybdenum atom can be a component of the multinuclear
active sites,[1] there are a number of mononuclear molybde-
num enzymes that are often referred to as oxomolybdenum en-
zymes.[2] Therefore, the mononuclear dioxomolybdenum com-
H2L1 was prepared by a similar method as described for
H2L1, but with 4-diethylaminosalicylaldehyde replaced by 4-
methoxysalicylaldehyde (10 mmol, 1.52 g). Yield, 79%. Anal.
Calcd. for C11H15NO3: C, 63.1; H, 7.2; N, 6.7. Found (%): C,
63.2; H, 7.3; N, 6.6.
Received 6 June 2012; accepted 21 November 2012.
This research was supported by the Scientific Research Projects of
the Education Department of Shaanxi Province (Grant No. 12JK0630).
Address correspondence to Hong-Ge Zhang, Department of Chem-
istry and Chemical Engineering, Baoji University of Arts and Sciences,
Baoji 721007, P. R. China. E-mail: zhang hongge@126.com
1029

1030
H.-G. ZHANG
For [MoO2L2(CH3OH)] (2), this complex was prepared by
the similar method as described for (1), but with H2L1 replaced
by H2L2. Anal. Calcd. for C12H17MoNO6: C, 39.2; H, 4.7; N,
3.8. Found (%): C, 39.3; H, 4.7; N, 3.9.
TABLE 1
Crystallographic and experimental data for complexes (1)
and (2)
Complex
(1)
(2)
Empirical formula
Formula weight
Crystal color, shape
Crystal size (mm)
C15H24MoN2O5 C12H17MoNO6
X-Ray Structure Determination
408.3
367.2
Intensity data for complexes (1) and (2) were collected on a
Bruker APEX II CCD area diffractometer (College of Chem-
istry and Chemical Engineering, Lanzhou University, China)
equipped with graphite-monochromatized Mo Kα radiation
(λ = 0.71073 Å) and ω scan technique. Crystal data, exper-
imental conditions, details of structure determination, and fi-
nal refinement parameters are summarized in Table 1. Selected
bond distances and angles are listed in Table 2. Both com-
plexes were solved by direct method and Fourier synthesis using
SHELXS97[7] and refined by full-matrix least-squares method
assuming anisotropic temperature factors for all non-hydrogen
atoms using SHELXL97.[8] The final Fourier maps revealed po-
sitions of all H atoms, but some with poor geometry. Therefore
all hydrogen atoms attached to carbon atoms were generated on
geometrical grounds. The hydrogen atoms bound to the oxygen
atoms of coordinated methanol molecules were located from
difference Fourier maps and refined isotropically.
Yellow, block
0.18 × 0.17 ×
0.15
Yellow, block
0.17 × 0.13 ×
0.13
Crystal system
Space group
a (Å)
Monoclinic
P21/c
Triclinic
P1
¯
16.464(2)
8.319(1)
14.056(2)
90
114.272(2)
90
6.774(2)
8.481(2)
13.198(3)
76.183(2)
78.963(2)
87.515(2)
722.7(3)
2
b (Å)
c (Å)
α (◦)
β (◦)
γ (◦)
V (Å3)
Z
1755.0(4)
4
ρcalc (g cm−3
F(000)
)
1.545
1.688
840
372
μ(MoKα) (mm−1
θ range (◦)
)
0.772
0.931
2.7–27.9
9921
2.5–28.2
5499
No. of reflections
measured
Unique reflections
Observed reflections
[I > 2σ(I)]
3791
3117
3052
2749
TABLE 2
Selected bond lengths (Å) and angles (◦) for complexes (1)
and (2)
Parameters
215
187
Restraints
Rint
1
1
(1)
(2)
0.0213
0.0174
Goodness-of-fit on F2
Max. and min.
Transmission
Final R indices [I >
2σ(I)]
1.033
0.8735, 0.8929
1.073
0.8578, 0.8886
Mo1-O1
Mo1-O2
Mo1-O3
Mo1-O4
Mo1-O5
Mo1-N1
1.956(2)
1.958(2)
2.395(2)
1.707(2)
1.692(2)
2.235(2)
80.5(1)
152.2(1)
79.9(1)
81.2(1)
75.1(1)
101.5(1)
96.1(1)
157.3(1)
82.6(1)
95.95(1)
99.7(1)
171.1(1)
106.1(1)
96.1(1)
75.5(1)
1.951(2)
1.952(2)
2.375(2)
1.709(2)
1.700(2)
2.259(2)
80.7(1)
152.3(1)
80.5(1)
80.8(1)
75.1(1)
100.6(1)
96.9(1)
157.8(1)
82.0(1)
97.5(1)
98.3(1)
172.2(1)
105.8(1)
96.0(1)
76.3(1)
R1 = 0.0287,
wR2 = 0.0718
R1 = 0.0372,
wR2 = 0.0771
0.664, –0.341
R1 = 0.0239,
wR2 = 0.0573
R1 = 0.0289,
wR2 = 0.0602
0.668, –0.565
R indices
O1-Mo1-N1
O1-Mo1-O2
O1-Mo1-O3
O2-Mo1-O3
O2-Mo1-N1
O4-Mo1-O1
O4-Mo1-O2
O4-Mo1-N1
O4-Mo1-O3
O5-Mo1-O1
O5-Mo1-O2
O5-Mo1-O3
O5-Mo1-O4
O5-Mo1-N1
N1-Mo1-O3
Largest difference in
peak and hole (e Å−3
)
Synthesis of the Complexes
For [MoO2L1(CH3OH)] (1), to a methanolic solution (10 mL)
of H2L1 (1.0 mmol, 0.25 g) was added a methanolic solution
(10 mL) of MoO2(acac)2 (1.0 mmol, 0.33 g) at room tempera-
ture. The mixture was stirred for 1 h and the solvent was removed
under reduced pressure to give a quantitative yield of the prod-
uct. Analytically pure samples and crystals for X-ray structural
determination were recrystallized from methanol solvent. Anal.
Calcd. for C15H24MoN2O5: C, 44.1; H, 5.9; N, 6.9. Found (%):
C, 44.3; H, 6.0; N, 6.8.

TWO DIXOMOLYBDENUM(VI) COMPLEXES
1031
FIG. 1. ORTEP plot of (1) showing the labeling system of non-hydrogen atoms. Displacement ellipsoids are plotted at 30% probability level.
region 900–940 cm−1 characteristic of the cis-[MoO2].[9,10] The
middle stretching frequencies in the region 1120–1140 cm−1
were attributed to the coordinated C-O groups.
RESULTS AND DISCUSSION
Synthesis
The Schiff base ligands were synthesized in-situ from 4-
diethylaminosalicylaldehyde or 4-methoxysalicylaldehyde with
1-aminopropan-2-ol in methanol. The Schiff bases reacted with
bis(acetylacetonato)dioxomolybdenum(VI) in methanol to form
the methanol-coordinated dioxomolybdenum(VI) complexes.
The complexes are yellow solids and could be crystallized from
methanol.
Structure Description of the Complexes
Crystal structures of complexes (1) and (2) are built up of
mononuclear complex molecules with cis-MoO2 core (Figures 1
and 2). In both complexes, the Schiff base ligands act in a di-
anionic tridentate manner. The coordination geometry around
the Mo center in these molybdenum complexes can be best de-
scribed as a distorted octahedron in which the ONO tridentate
ligand occupies meridional position with two anionic oxygens
Infrared Spectra
In the infrared spectra of the complexes the intense bands at mutually trans and are cis to the oxygens of cis-dioxo group.
1600 cm−1 for (1) and (2) are assigned to the azomethine groups, The octahedral geometry of molybdenum center is additionally
ν(C N).[9] The weak and broad bands at about 3450 cm−1 are completed by coordinated methanol solvent molecule. In (1) and
assigned to the O-H vibrations of the methanol ligands. Each (2), the Mo atoms deviate from the corresponding least-squares
spectrum of the complexes exhibits two strong bands in the planes defined by the equatorial donor atoms by 0.330(1) and
FIG. 2. ORTEP plot of (2) showing the labeling system of non-hydrogen atoms. Displacement ellipsoids are plotted at 30% probability level.

1032
H.-G. ZHANG
FIG. 3. Molecular packing diagram of (1), viewed along the c-axis. Intermolecular O−H···O hydrogen bonds are shown as dashed lines.
observed in the similar structures with coordinated methanol
molecules.[11–13]
0.328(1) Å, respectively, toward O5 axial donor atoms. Both
Mo O distances, ranging from 1.692(2) to 1.709(2) Å, and
the O Mo O angles, from 105.8(1) to 106.1(1)◦, are in the
In the crystal structures of (1) and (2), two neighboring
usual range for cis-MoO2 complexes.[11–13] In the structures of molecules are linked through intermolecular O−H···O hydro-
both complexes, the tridentate Schiff base ligands form one six- gen bonds [for (1), O−H = 0.85(1) Å, H···O = 1.86(1) Å, O···O
and one five-membered chelate rings, with Mo-O bond lengths = 2.695(3) Å, O−H···O = 175(4)◦; for (2), O−H = 0.85(1) Å,
varying from 1.951(2) to 1.958(2) Å, and Mo-N distances vary- H···O = 1.88(1) Å, O···O = 2.701(2) Å, O−H···O = 168(4)◦], to
ing from 2.235(2) to 2.259(2) Å. The longest Mo-O bonds are form centrosymmetric dimeric structures, as shown in Figures 3
Mo-Omethanol, ranging from 2.375(2) to 2.395(2) Å, as already and 4, respectively.

TWO DIXOMOLYBDENUM(VI) COMPLEXES
1033
FIG. 4. Molecular packing diagram of (2), viewed along the a-axis. Intermolecular O−H···O hydrogen bonds are shown as dashed lines.
SUPPLEMENTARY MATERIALS
6. Chen, G.J.; McDonald, J.W.; Newton, W.E. Synthesis of Mo(IV) and Mo(V)
complexes using oxo abstraction by phosphines - mechanistic implications.
Inorg. Chem. 1976, 15, 2612–2615.
7. Sheldrick, G.M. SHELXS-97, Program for the Solution of Crystal Struc-
tures; University of Go¨ttingen, Germany, 1997.
8. Sheldrick, G.M. SHELXL-97, Program for the Refinement of Crystal Struc-
tures; University of Go¨ttingen, Germany, 1997.
9. Zhang, C.; Rheinwald, G.; Lozan, V.; Wu, B.; Lassahn, P.-G.; Lang, H.;
Janiak, C. Structural study and solution integrity of dioxomolybdenum(VI)
complexes with tridentate Schiff base and azole ligands. Z. Anorg. Allg.
Chem. 2002, 628, 1259–1268.
Crystallographic data for the structures reported in this pa-
per have been deposited at the Cambridge Crystallographic
Data Centre, CCDC Nos. 885121 (1) and 885122 (2). Copies
of this information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: +44-1223-336-033; e-mail: deposit@ccdc.cam.ac.uk;
www: http://www.ccdc.cam.ac.uk).
10. Cindric, M.; Strukan, N.; Vrdoljak, V.; Kajfez, T.; Kamenar, B. A series of
molybdenum(VI) complexes with tridentate Schiff base ligands. Z. Anorg.
Allg. Chem. 2002, 628, 2113–2117.
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