Hydrogen bond and steric effect directed preparation and structures of dioxovanadium(V) complexes with tridentate Schiff bases

Article abstract of DOI:10.1080/00958972.2013.780205

Three new dioxovanadium(V) complexes, [VO₂L¹} ₂ (1), [VO₂L²] (2) and [VO₂L ³] (3) (HL¹=2-[(2-ethylaminoethylimino)methyl]-4- methylphenol, HL² = 2-[(2-isopropylaminoethylimino)methyl]-6- methylphenol, HL³ = 5-methoxy-2-[(pyridin-2-ylmethylimino)methyl] phenol), have been prepared and structurally characterized by physico-chemical methods and single crystal X-ray diffraction. Complex 1 is a centrosymmetric dimeric dioxovanadium(V) complex with V V distance of 3.187(2)A. V in 1 is octahedral. Complexes 2 and 3 are mononuclear dioxovanadium(V) complexes. V in 2 is in a distorted square-pyramidal coordination and that in 3 is intermediate between square-pyramidal and trigonal bipyramidal. During the synthesis and crystallization of the complexes from oxovanadium(IV) starting material, aerial oxygen probably acts as the oxidizing agent. The final structures of the complexes are directed by hydrogen bonds and steric effects among the Schiff base ligands. The existence of amino groups and small steric effects such as ethyl group can result in the formation of dimeric structures.

Full text of DOI:10.1080/00958972.2013.780205

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Hydrogen bond and steric effect
directed preparation and structures
of dioxovanadium(V) complexes with
tridentate Schiff bases
Shao-Song Qian a , Zhong-Lu You b , Ying-Nan Li b , Xue Hu b , Mei
Zhang b & Hai-Liang Zhu a
a School of Life Sciences, Shandong University of Technology, ZiBo,
P.R. China
b Department of Chemistry and Chemical Engineering, Liaoning
Normal University, Dalian, P.R. China
Accepted author version posted online: 28 Feb 2013.Version of
record first published: 05 Apr 2013.
To cite this article: Shao-Song Qian , Zhong-Lu You , Ying-Nan Li , Xue Hu , Mei Zhang & Hai-
Liang Zhu (2013): Hydrogen bond and steric effect directed preparation and structures of
dioxovanadium(V) complexes with tridentate Schiff bases, Journal of Coordination Chemistry, 66:8,
1311-1319
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Journal of Coordination Chemistry, 2013
http://dx.doi.org/10.1080/00958972.2013.780205
Hydrogen bond and steric effect directed preparation and
structures of dioxovanadium(V) complexes with tridentate
Schiff bases
SHAO-SONG QIAN†, ZHONG-LU YOU*‡, YING-NAN LI‡, XUE HU‡, MEI ZHANG‡
and HAI-LIANG ZHU*†
†School of Life Sciences, Shandong University of Technology, ZiBo, P.R. China;
‡Department of Chemistry and Chemical Engineering, Liaoning Normal University,
Dalian, P.R. China
(Received 16 July 2012; in final form 21 December 2012)
Three new dioxovanadium(V) complexes, [VO2L1]2 (1), [VO2L2] (2) and [VO2L3] (3) (HL1 = 2-
[(2-ethylaminoethylimino)methyl]-4-methylphenol, HL2 = 2-[(2-isopropylaminoethylimino)methyl]-
6-methylphenol, HL3 = 5-methoxy-2-[(pyridin-2-ylmethylimino)methyl]phenol), have been prepared
and structurally characterized by physico-chemical methods and single crystal X-ray diffrac-
tion. Complex 1 is a centrosymmetric dimeric dioxovanadium(V) complex with VÁ Á ÁV distance
of 3.187(2) Å. V in 1 is octahedral. Complexes 2 and 3 are mononuclear dioxovanadium(V)
complexes. V in 2 is in a distorted square-pyramidal coordination and that in 3 is intermediate
between square-pyramidal and trigonal bipyramidal. During the synthesis and crystallization of the
complexes from oxovanadium(IV) starting material, aerial oxygen probably acts as the oxidizing
agent. The final structures of the complexes are directed by hydrogen bonds and steric effects
among the Schiff base ligands. The existence of amino groups and small steric effects such as ethyl
group can result in the formation of dimeric structures.
Keywords: Schiff base; Oxovanadium complex; Crystal structure; Hydrogen bond; Steric effects
1. Introduction
Considerable interest has focused on Schiff bases and their metal complexes in coordina-
tion and biological chemistry [1–3]. Vanadium complexes have interesting biological activ-
ities such as normalizing high blood glucose levels and acting as models of
haloperoxidases [4–6]. Construction of specific vanadium complexes seems important to
explore effective biological agents. Factors that influence the final structures of metal
complexes include metal and ligand ratio, solvents, reaction temperature, and weak forces,
such as hydrogen bonds and πÁ Á Áπ stacking interactions [7, 8]. Mokry and co-workers
reported that the steric bulk of the substituent on benzene can be used to modify the
coordination geometry and/or number of vanadiums by preventing dimerization [9]. To fur-
ther explore synthesis of such complexes, three new dioxovanadium(V) complexes,
[VO2L1]2 (1), [VO2L2] (2) and [VO2L3] (3) (HL1 = 2-[(2-ethylaminoethylimino)methyl]
*Corresponding authors. Email: youzhonglu@yahoo.com.cn (Z.-L. You); hailiang_zhu@163.com (H.-L. Zhu)
Ó 2013 Taylor & Francis

1312
S.-S. Qian et al.
-4-methylphenol, HL2 = 2-[(2-isopropylaminoethylimino)methyl]-6-methylphenol, HL3 = 5-
methoxy-2-[(pyridin-2-ylmethylimino)methyl]phenol), respectively, were synthesized and
characterized (scheme 1).
2. Experimental
2.1. General remarks and physical measurements
Reagents and solvents were purchased from commercial suppliers and used without purifi-
cation. Elemental analyses were performed on a Perkin-Elmer 240C elemental analyzer. IR
spectra were recorded on a Jasco FT/IR-4000 spectrometer as KBr pellets from 4000–
200 cm–1. Molar conductances of the complexes at 10À3 M in absolute methanol were
measured with a Shanghai DDS–11A conductometer. The conductometer was calibrated
before use. First, the temperature compensation was set to 25 °C. Second, the measure
switch was set to ‘adjust’. Then, by adjusting the constant correction knob, we can make
the instrument display the actual coefficient of the conductivity cell. X-ray diffraction was
1
carried out on a Bruker SMART 1000 CCD area diffractometer. H NMR spectra were
recorded on a Varian INOVA300 pulse Fourier-transform NMR spectrometer.
2.2. Synthesis of the Schiff bases
To a methanolic solution (50 cm3) of substituted salicylaldehyde (2.0 mmol) a methanolic
solution (30 cm3) of primary amine (2.0 mmol) was added with continuous stirring. The
mixture was stirred for 30 min at room temperature to give yellow solution. The solvent
was evaporated to give yellow paste of the Schiff base, which was washed with methanol
and dried in air. Yields: 80–90%. Anal. Calcd for C12H18N2O (HL1): C, 69.9; H, 8.8; N,
13.6%. Found: C, 69.7; H, 8.8; N, 13.7%. IR data: 3372 (br, w) ν(O–H), 3211 (w)
1
ν(N–H), 1645 (s) ν(C=N), 1207 (s) ν(C–O). H NMR (DMSO-d6, ppm): δ 1.07 (t, 3H),
2.25 (s, 3H), 2.64 (q, 2H), 2.91 (s, 2H), 3.69 (s, 2H), 6.83 (d, 1H), 7.00 (s, 1H), 7.07 (d,
1H), 7.25 (s, 1H), 8.30 (s, 1H), 9.81 (s, 1H). Anal. Calcd for C13H20N2O (HL2): C, 70.9;
H, 9.2; N, 12.7%. Found: C, 70.6; H, 9.1; N, 12.8%. IR data: 3365 (br, w) ν(O–H), 3205
1
(w) ν(N–H), 1645 (s) ν(C=N), 1205 (s). H NMR (DMSO-d6, ppm): δ 1.08 (d, 6H), 2.23
(s, 3H), 2.81 (m, 1H), 2.91 (m, 2H), 3.70 (t, 2H), 6.78 (t, 1H), 7.08 (d, 1H), 7.17 (d, 1H),
7.25 (s, 1H), 8.36 (s, 1H), 13.5 (s, 1H). Anal. Calcd for C14H14N2O2 (HL3): C, 69.4; H,
5.8; N, 11.6%. Found: C, 69.5; H, 5.9; N, 11.4%. IR data: 3327 (br, w) ν(O–H), 1627 (s)
1
ν(C=N), 1193 (s) ν(C–O). H NMR (DMSO-d6, ppm): δ 3.78 (s, 3H), 4.86 (s, 1H), 6.49
(m, 2H), 7.19 (m, 3H), 7.40 (m, 1H), 7.65 (m, 1H), 8.37 (s, 1H), 8.54 (d, 1H).
H
N
H
N
N
N
N
N
OH
MeO
OH
OH
HL2
HL1
Scheme 1. The Schiff bases.
H2L3

Dioxovanadium(V) complexes
2.3. Synthesis of the complexes
1313
A methanolic solution (20 cm3) of the Schiff base (0.5 mmol) was added with stirring to a
methanolic solution (20 cm3) of VO(acac)2 (0.5 mmol, 0.133 g). The mixtures were stirred
at room temperature for 30 min to give yellow solutions. X-ray quality single crystals were
formed by slow evaporation of the solutions in air after a few days.
2.3.1. bis(μ2-Oxo)bis(2-[(2-ethylaminoethylimino)methyl]-4-methylphenolato)dioxo
divanadium(V) (1). Yellow single crystals. Yield: 76.4 mg (53%). Anal. Calcd for
C24H34N4O6V2: C, 50.0; H, 5.9; N, 9.7%. Found: C, 49.8; H, 6.1; N, 9.6%. IR data: 3228
(w) ν(N–H), 1636 (s) ν(C=N), 1292 (s) ν(C–O), 933 (m) ν(V=O), 849 (s) ν(V=O), 562
1
(w) ν(V–N), 445 (w) ν(V–O). H NMR (DMSO-d6, ppm): δ 1.30 (t, 3H), 2.25 (m, 3H),
2.59 (d, 1H), 2.82 (m, 1H), 3.30 (s, 2H), 3.91 (s, 1H), 4.09 (m, 1H), 5.57 (s, 1H), 6.70
(d, 1H), 7.30 (m, 2H), 8.84 (s, 1H). ΛM=12 Ω–10 cm2 mol–1.
2.3.2. (2-[(2-Isopropylaminoethylimino)methyl]-6-methylphenolato)dioxovanadium
(V) (2). Yellow single crystals. Yield: 107.2 mg (71%). Anal. Calcd for C13H19N2O3V:
C, 51.7; H, 6.3; N, 9.3%. Found: C, 51.5; H, 6.4; N, 9.1%. IR data: 3187 (w) ν(N–H),
1643 (s) ν(C=N), 1292 (s) ν(C–O), 927 (s) ν(V=O), 848 (s) ν(V=O), 589 (w) ν(V–N), 458
1
(w) ν(V–O). H NMR (DMSO-d6, ppm): δ 1.22 (d, 3H), 1.42 (d, 3H), 2.11 (s, 3H), 2.59
(d, 1H), 3.07 (s, 1H), 3.34 (s, 1H), 3.97 (m, 2H), 5.54 (m, 1H), 6.71 (t, 1H), 7.38 (t, 2H),
8.87 (s, 1H). ΛM = 30 Ω–1 cm2 mol–1.
2.3.3. bis(5-Methoxy-2-[(pyridin-2-ylmethylimino)methyl]phenolato)dioxovanadium(V)
(3). Yellow single crystals. Yield: 90.7 mg (56%). Anal. Calcd for C14H13N2O4V: C,
51.9; H, 4.0; N, 8.6%. Found: C, 51.6; H, 4.1; N, 8.7%. IR data: 1611 (s) ν(C=N), 1231
(s) ν(C–O), 925 (s) ν(V=O), 563 (w) ν(V–N), 438 (w) ν(V–O). 1H NMR (DMSO-d6,
ppm): δ 4.02 (s, 3H), 5.03 (m, 1H), 6.57 (m, 2H), 6.63 (t, 1H), 7.36 (m, 3H), 7.57
(m, 2H), 8.86 (s, 1H). ΛM = 25 Ω–1 cm2 molÀ1
.
2.4. X-ray crystallography
Diffraction intensities for the complexes were collected at 298(2) K using a Bruker
SMART 1000 CCD area diffractometer with MoKα radiation (λ = 0.71073 Å). The
collected data were reduced using SAINT [10] and multi-scan absorption correction was
performed using SADABS [11]. The structures were solved by direct methods and
refined against F2 by full-matrix least-squares using the SHELXTL package [12]. All
non-hydrogen atoms were refined anisotropically. The amino hydrogens in 1 and 2 were
located from difference Fourier maps and refined isotropically, with NÀH distances
restrained to 0.90(1) Å, and with Uiso(H) set to 0.08 Å2. The remaining hydrogens in the
complexes were placed in calculated positions and constrained to ride on their parent.
The crystallographic data for the complexes are summarized in table 1. Selected bond
lengths and angles are listed in table 2.

1314
S.-S. Qian et al.
Table 1. Crystallographic and experimental data for 1–3.
Complex
1
2
3
Formula
Formula weight
T (K)
C24H34N4O6V2
576.4
298(2)
C13H19N2O3V
302.2
298(2)
C14H13N2O4V
324.2
298(2)
Crystal shape/color
Crystal size (mm3)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
Block/yellow
0.17 Â 0.15 Â 0.15
Triclinic
P-1
7.6441(6)
8.5500(7)
11.3268(9)
78.838(1)
78.566(1)
67.047(1)
662.58(9)
1
Block/yellow
0.20 Â 0.17 Â 0.15
Monoclinic
P21/n
12.724(2)
6.779(1)
17,082(3)
90
104.261(3)
90
1427.9(4)
4
1.406
Block/yellow
0.17 Â 0.15 Â 0.15
Monoclinic
P21/n
11.594(2)
10.380(3)
12.014(3)
90
109.679(2)
90
1361.4(5)
4
1.582
b (°)
γ (°)
V (Å3)
Z
Dc (g cm–3
)
1.445
0.750
300
μ (Mo-Kα) (mmÀ1
F(000)
)
0.700
632
0.746
664
Data collected
Unique data
3902
2822
6481
2345
6326
2520
Observed data (I P 2σ(I))
Parameters
2499
169
1672
178
1884
191
Restraints
1
1
0
Min. and max. transmission
Goodness-of-fit on F2
R1, wR2 [I P 2σ(I)]a
R1, wR2 (all data)a
0.883 and 0.896
1.055
0.0325, 0.0790
0.0389, 0.0833
0.347 and À0.233
0.873 and 0.902
1.039
0.0453, 0.1057
0.0683, 0.1196
0.377 and À0.249
0.884 and 0.896
1.037
0.0384, 0.0874
0.0574, 0.0974
0.279 and À0.251
Large diff. peak and hole (eÅÀ3
)
aR1 = Fo À Fc/Fo, wR2 À [Σw(Fo À Fc2)/Σw(Fo2)2]1/2
.
2
3. Results and discussion
The Schiff bases were synthesized by reaction of equimolar quantities of substituted
salicylaldehydes with primary amines in methanol. All complexes were synthesized by
reaction of methanol solution of the Schiff bases with VO(acac)2. The compounds were
characterized by elemental analysis and IR spectra. Structures of the complexes were
further confirmed by X-ray crystallography. The molar conductance values of 1, 2, and
3 measured in methanol at 10–3 M are 12, 30, and 25 Ω–1 cm2 mol–1, indicating the
non-electrolytic nature of the complexes [13]. Vanadium in the complexes is + 5 oxida-
tion state and therefore EPR silent. During the synthesis and crystallization of the
complexes from oxovanadium(IV) starting material, aerial oxygen must act as the
oxidizing agent.
The three Schiff bases coordinate to V through phenolate O, imino N, and amino/pyr-
idine N. The hydroxy groups deprotonate upon coordination. In dimeric 1, there are four
NÀHÁ Á ÁO hydrogen bonds (table 3) between the two [VO2L1] units, while in mononu-
clear 2, there are two NÀHÁ Á ÁO hydrogen bonds. The difference might be caused by
the steric effects of the terminal groups, ethyl for 1, and isopropyl for 2. As for 3, there
are no hydrogen bonds among the molecules, leading to formation of mononuclear
complex.

Dioxovanadium(V) complexes
Table 2. Selected bond lengths (Å) and angles (°) for 1–3.
1315
1
V1–O1
V1–O3
V1–N2
1.909(1)
1.664(1)
2.152(2)
V1–O2
V1–N1
V1–O3A
1.610(2)
2.156(2)
2.396(1)
O2–V1–O3
O3–V1–O1
O3–V1–N2
O2–V1–N1
O1–V1–N1
O2–V1–O3A
O1–V1–O3A
N1–V1–O3A
2
107.6(1)
O2–V1–O1
O2–V1–N2
O1–V1–N2
O3–V1–N1
N2–V1–N1
O3–V1–O3A
N2–V1–O3A
102.0(1)
98.8(1)
92.0(1)
98.5(1)
84.8(1)
169.7(1)
85.3(1)
74.6(1)
92.5(1)
158.3(1)
152.1(1)
77.0(1)
78.1(1)
78.6(1)
V1–O1
V1–O3
V1–N2
1.896(2)
1.703(2)
2.130(3)
V1–O2
V1–N1
1.598(2)
2.125(3)
O2–V1–O3
O3–V1-O1
O3–V1–N1
O2–V1–N2
O1–V1–N2
3
109.6(1)
O2–V1–O1
O2–V1–N1
O1–V1–N1
O3–V1–N2
N1–V1–N2
109.0(1)
93.1(1)
144.1(1)
101.9(1)
146.4(1)
105.1(1)
83.7(1)
88.6(1)
75.7(1)
V1–O1
V1–O4
V1–N2
1.911(2)
1.618(2)
2.115(2)
V1–O3
V1–N1
1.617(2)
2.125(2)
O3–V1–O4
O4–V1–O1
O4–V1–N2
O3–V1–N1
O1–V1–N1
V1–O5
109.1(1)
O3–V1–O1
O3–V1–N2
O1–V1–N2
O4–V1–N1
N2–V1–N1
V1–O6
101.8(1)
98.1(1)
90.7(1)
120.2(1)
84.2(1)
1.578(2)
2.094(2)
109.2(1)
99.7(1)
88.0(1)
99.9(1)
83.0(1)
93.5(1)
158.6(1)
129.0(1)
75.1(1)
1.744(2)
V1–N1
O5–V1–O6
O6–V1–O1
O6–V1–O3
O5–V1–N1
O1–V1–N1
O5–V1–O1
O5–V1–O3
O1–V1–O3
O6–V1–N1
O3–V1–N1
105.4(1)
102.5(1)
146.6(1)
148.7(1)
74.4(1)
Table 3. Hydrogen-bond geometry (Å, °) of 1 and 2.
D–HÁ Á ÁA
d(D–H) (Å)
d(HÁ Á ÁA) (Å)
d(DÁ Á ÁA) (Å)
(D–HÁ Á ÁA) (°)
1
N2–H2Á Á ÁO3i
N2–H2Á Á ÁO1i
2
0.90(1)
0.90(1)
2.43(2)
2.32(2)
2.886(2)
3.037(2)
112(2)
138(2)
N2–H2Á Á ÁO3ii
0.90(1)
2.00(1)
2.890(4)
170(4)
Symmetry codes: (i) Àx, 1 À y, Àz; (ii) 1/2 À x, À1/2 + y, 3/2 À z.
3.1. Structure description of 1
The molecular structure of 1 is shown in figure 1. X-ray crystallography reveals that the
complex is a centrosymmetric dimeric oxovanadium(V) compound, with inversion center
located at the midpoint between the two vanadiums. The VÁ Á ÁV distance is 3.187(2) Å.
Each V in the complex is six-coordinate through three bonds to oxo groups and three

1316
S.-S. Qian et al.
Figure 1. A perspective view of the molecular structure of 1 with the atom labeling scheme. The thermal
ellipsoids are drawn at the 30% probability level. Hydrogen bonds are shown as dashed lines. Unlabeled atoms
are related to the symmetry position –x, 1–y, –z.
bonds to the tridentate Schiff base, forming an octahedral geometry. The distance between
V1 and O2 is 1.609(2) Å, indicating it is a typical V=O bond [14, 15]. O3 in the complex
is a bridging group, linking two different but symmetry related vanadiums. The V1–O3
distance of 1.664(1) Å is longer than V1–O2, in accord with those observed in similar
structures [14–18]. The V–O and V–N bonds (table 2) agree well with those observed in
other Schiff base oxovanadium complexes [14–18]. The V1–O1 bond length of 1.909(1) Å
confirms the Schiff base ligand is a monoanion [15, 16]. Distortion of octahedral coordina-
tion can be observed by coordinate bond angles, ranging from 74.6(1) to 107.6(1)° for the
perpendicular angles and from 152.1(1) to 169.7(1)° for diagonal angles. Four NÀHcdotsO
hydrogen bonds form between the two [VO2L1] units. Examination of the complex and
dimeric oxovanadium(V) complexes reported previously show most exist with intramolecu-
lar hydrogen bonds [14–18]. The molecules are further stacked along the a axis (figure 2).
3.2. Structure description of 2 and 3
Molecular structures of 2 and 3 are shown in figures 3 and 4, respectively. The two Schiff
base ligands for 2 and 3 are coordinated in a similar manner. L2 in 2 coordinates to V
Figure 2. Molecular packing of 1 viewed along the a axis. Hydrogen bonds are shown as dashed lines.

Dioxovanadium(V) complexes
1317
through phenolate O, imine N, and amine N. L3 in 3 coordinates to V through phenolate
O, imine N, and pyridine N. In each complex V is coordinated by the Schiff base ligand
and two oxo. V in 2 is distorted square-pyramidal coordination, while 3 is intermediate
between square-pyramidal and trigonal bipyramidal. The extent of distortion of a square-
pyramidal geometry toward a trigonal-bipyramidal geometry can be measured by the value
of τ which is defined as (β – α)/60°, where α and β are the two largest coordinate bond
angles [19]. For an ideal square-pyramidal geometry τ is 0 and for an ideal trigonal-bipyra-
midal geometry τ is 1. The values of τ are 0.039 for 2 and 0.493 for 3. The V1–O1 and
V1–N1 lengths are comparable to each other and also to those observed in similar oxova-
nadium complexes [20, 21]. V1–N2 in 2 is longer than in 3, which might be influenced by
formation of N2–H2cdotsO3 hydrogen bonds in 2, and also might be influenced by the dif-
ferent geometries of N2. It is not unusual that M–Namino bonds are longer than M–Npyridine
bonds [9, 16, 17, 22]. Molecules in 2 are linked through intermolecular NÀH Á Á Á O
hydrogen bonds, forming zigzag chains running along the b axis, as shown in figure 5.
3.3. IR and NMR spectra
IR spectra of HL1, HL2, 1 and 2 exhibit sharp bands at 3150–3250 cm–1, assigned to
ν(N–H). Weak and broad absorptions at 3300–3400 cm–1 in spectra of the free Schiff bases
Figure 3. A perspective view of the molecular structure of 2 with the atom labeling scheme. The thermal
ellipsoids are drawn at the 30% probability level.
Figure 4. A perspective view of the molecular structure of 3 with the atom labeling scheme. The thermal
ellipsoids are drawn at the 30% probability level.

1318
S.-S. Qian et al.
Figure 5. Molecular packing of 2 viewed along the b axis. Hydrogen bonds are shown as dashed lines.
are absent in the complexes, indicating that the three Schiff bases coordinate to V through
deprotonated phenolate. This is also supported by the shift of ν(C–O) of free Schiff bases
and corresponding complexes. Strong absorptions at 1636 cm–1 in 1, 1643 cm–1 in 2, and
1611 cm–1 in 3 are attributed to ν(C=N) [23], which are shifted to lower wavenumbers
when compared to those of the free Schiff bases. This is caused by coordination of imino
N. Bands at 933 and 849 cm–1 in 1, 927 and 848 cm–1 in 2, and 925 cm–1 in 3 are assigned
to ν(V=O) [16]. There are two distinct bands attributed to V=O stretches in spectra of 1
and 2. However, there is only one band attributed to V=O stretch in 3. This can be
explained by V=O bond distances. The V=O bonds in 1 and 2 are 1.610(1) and 1.664(1)
Å, and 1.598(2) and 1.703(2) Å, respectively. However, in 3, the two V=O bonds are very
similar, viz. 1.617(2) and 1.618(2) Å.
The DMSO-d6 solutions of the Schiff bases and their complexes were used to record
proton NMR spectra. The aromatic protons appear as multiplets at δ 6.7–7.7 ppm. For HL1
and HL2 and their complexes, methyl groups attached to aromatic rings resonate as a singlet
near δ 2.2 ppm, while those of the ethyl or isopropyl groups are δ 1.1 ppm for HL1 and
HL2, and δ 1.30 ppm for 1 and δ 1.22 ppm for 2. The singlet observed at δ 3.78 ppm for
HL3 and δ 4.02 ppm for 3 are attributed to methyl of the methoxy substituents on the salicy-
lidene fragments. The –NH– protons of HL1 and HL2 are broad singlets at δ 9.8 ppm and δ
13.5 ppm, respectively. The azomethine (–CH=N–) protons in the Schiff bases are singlets
at δ 8.3–8.6 ppm, while in the complexes they are observed at δ 8.8–8.9 ppm. These chemi-
cal shifts are not unusual for complexes containing the oxovanadium(V) units [21, 24, 25].
4. Conclusion
This paper reports the synthesis and structures of three new dioxovanadium(V) complexes
with Schiff bases. The hydrogen bonds and steric effects of the Schiff bases influence the

Dioxovanadium(V) complexes
1319
final structures of the complexes. The amino group, which readily forms intermolecular
hydrogen bonds with adjacent ligand, and the presence of small steric effects of ethyl can
result in formation of dimeric structures. Besides the amino group, sizes of the groups
attached to the amino groups are very sensitive. Comparing 1 and 2 the small adjustment
from ethyl to isopropyl, only mononuclear complex can be obtained.
Supplementary material
CCDC 891283 for 1, 891287 for 2, and 891285 for 3 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic
Data Center via http://www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was financially supported by the Distinguished Young Scholars Program of
Higher Education of Liaoning Province (Grant No. LJQ2011114).
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