Synthesis of symmetric N,O-donor ligands derived from pyridoxal (vitamin B6): DFT studies and structural features of their binuclear chelate complexes with the oxofilic uranyl and vanadyl(V) cations

Article abstract of DOI:10.1016/j.ica.2013.12.008

The synthesis and the structural characterization of symmetric dimers containing uranium and vanadium atoms provide an outstanding opportunity for the study of hydrogen bonding in supramolecular architectures and unusual interactions. On the search of lig

Full text of DOI:10.1016/j.ica.2013.12.008

Inorganica Chimica Acta 412 (2014) 6–14
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis of symmetric N,O-donor ligands derived from pyridoxal
(
vitamin B6): DFT studies and structural features of their binuclear
chelate complexes with the oxofilic uranyl and vanadyl(V) cations
a,
⇑
a,
⇑
a
a
Davi Fernando Back , Gelson Manzoni de Oliveira , Daiane Roman , Marco Aurélio Ballin ,
a
b
Roger Kober , Paulo Cesar Piquini
a
Departamento de Química, Laboratório de Materiais Inorgânicos, Universidade Federal de Santa Maria, UFSM, 97115-900 Santa Maria, RS, Brazil
Departamento de Física, Universidade Federal de Santa Maria, UFSM, 97115-900 Santa Maria, RS, Brazil
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 August 2013
Received in revised form 27 November 2013
Accepted 6 December 2013
Available online 14 December 2013
The synthesis and the structural characterization of symmetric dimers containing uranium and vanadium
atoms provide an outstanding opportunity for the study of hydrogen bonding in supramolecular
architectures and unusual interactions. On the search of ligands able to coordinate itself to two metal ions
simultaneously, we have synthesized the Schiff bases bis((3-hydroxy-5-(hydroxymethyl)-2-methylpyri-
din-4-yl)methylene) oxalohydrazide (H
pyridin-4-yl)methylene) succinohydrazide (H10pyr
6
Pyr
2
oxdihyd) and bis((3-hydroxy-5-(hydroxymethyl)-2-methyl-
sucdihyd), efficient symmetric ligands with an
2
Keywords:
inversion center, obtained through the reaction of pyridoxine/pyridoxal hydrochloride with oxalyl dihy-
drazide and succinic dihydrazide. Their reactions and the products obtained with the oxofilic uranyl(VI)
and vanadyl(V) cations were discussed, as well as computational methods were used as complementary
tools in the study of intra and intermolecular bonds.
2+
and VO3+
Chelates of UO
Schiff base ligands
Ligands derived from pyridoxal
2
Ó 2013 Elsevier B.V. All rights reserved.
1
. Introduction
A large amount of the enzymatic reactions of amino acids cata-
In the search of Schiff bases presenting coordination sites with
different charges and hard/soft character for the synthesis of metal
complexes with N,O-donor ligands, we have further prepared the
Schiff base {3-hydroxyl-5-(hydroxymethyl)-2-methylpyridine-4-
yl-methylene} benzohydrazidehydrochloride monohydrated, or,
lyzed by pyridoxal phosphate-containing enzymes reported earlier
by Metzler et al. [1] could be reproduced by non-enzymatic reac-
tions, in which pyridoxal [2,3] or other appropriate aldehydes [4]
and a suitable metal salt act as catalysts [5,6]. Therefore, these
reactions also demonstrated that the catalytic potentialities of
pyridoxal phosphate metalloenzymes are basically those of their
prosthetic group (the pyridoxal metal complex), and that the
non-enzymatic and enzymatic reactions proceed by similar mech-
anisms. These finds increased significantly the interest for the
coordination chemistry of compounds derived from the vitamin
B6 complex.
as acronym, (hhmmbH)ClꢀH
2
O [12]. This ligand is derived from
the vitamin B6 (pyridoxine) and presents a hard/medium basic
character. In contrast with other Schiff base ligands obtained start-
ing from the vitamin B6, (hhmmbH)ClꢀH
2
O was not prepared by
linkage of two pyridoxal rings through a polyamine like (for exam-
ple, en or dien), being rather obtained through the reaction of pyr-
idoxine hydrochloride with benzoic acid hydrazide, after treatment
with manganese dioxide and concentrated sulfuric acid. With this
ligand we obtained the chelate complexes of vanadium and ura-
We reported some reactions [7–10] of vitamin B6 derivatives
with metal ions like uranium, thorium and lanthanides, and these
derivatives are almost all symmetric parents of the Schiff base
nium [VO
[UO Cl(hhmmb)(CH
In this work we describe the synthesis of the ligands H
oxdihyd (bis((3-hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-
yl)methylene) oxalohydrazide) (Chart A) and H10pyr sucdihyd
2
(hhmmb)]ꢀPy, [UO
2
Cl(hhmmb)(H
2
O)]NO
3
ꢀ2H
2
O
and
2
3
OH)]NO
3
3
ꢀCH OH [13].
6
pyr
2
0
N,N -bis(pyridoxylideneiminato)ethylene, derived from the con-
densation of pyridoxal with ethylenediamine, which has been
developed to investigate substitution reactions in compounds of
2
(bis((3-hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-yl)methy-
lene) succinohydrazide) (Chart B), as well as the synthesis, the
0
vanadium with the Schiff base {N,N -ethylenediaminebis(salicy-
lideneiminato)}, sal
2
en [11].
structural features and DFT studies of the reaction products
+
[
(UO
2
)
2
(H
6
pyr
2
oxdihydꢁ4H )(DMSO)
4 2 2
] (1) and [(VO) (H10pyr suc
] (2). The binuclear chelate complexes of the ura-
+
dihydꢁ4H )(MeO)
2
⇑
Corresponding authors. Tel.: +55 55 3220 8757; fax: +55 55 3220 8031.
E-mail addresses: daviback@gmail.com (D.F. Back), manzonideo@smail.ufsm.br
G. Manzoni de Oliveira).
nyl(VI) and vanadyl (V) cations were obtained as result of the ability
of the ligands to ‘‘grasp’’ the metal ions in both its extremities.
(
0
020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.12.008

D.F. Back et al. / Inorganica Chimica Acta 412 (2014) 6–14
7
Pyridoxine hydrochloride (0.191 g, 1 mmol) was dissolved in
00 mL of water. In this solution 0.88 g of 85% MnO was sus-
1
2
pended and 1 mL of concentrated sulfuric acid was added very
slowly. Thereafter the mixture was heated in an oil-bath at 60–
7
0 °C for ꢂ4 h until whole dissolution of the manganese dioxide.
The solution becomes brown however translucent due to the
whole oxidation of pyridoxine to pyridoxal. The ligand H pyr2-
6
dihyd precipitated after the addition of 0.118 g of oxalic acid
hydrazide. The mixture was heated in an oil-bath at 40 °C for about
1
0 min and the ligand was collected on a filter, washed with dis-
Chart A.
tilled water and dried. Yield: 0.167 g (86%).
Melting point: 220 °C. Anal. Calc: C, 51.92; H, 4.80; N, 20.19.
Found: C, 51.04; H, 4.84; N, 20.02%.
IR (KBr pellets; s, strong; m, middle; w, weak): 3195 [m, m(NH)];
ꢁ
1
3
021 [w,
m
(CH)ar]; 1718 [s,
m
(C@O)]; 1529 [s, m(C@N)]; 1256 cm
[
s,
m(CꢁO)phenol.].
+
2 2
2.2.2. [(UO ) (H
6
pyr
2
oxdihydꢁ4H )(DMSO)
4
] (1)
The ligand H
0 mL of anhydrous methanol and stirred under argon atmosphere
(NO O (0.05 g, 0.1 mmol),
ꢀ6H
.5 mL of triethylamine were added dropwise and the mixture
was refluxed for 2 h and then cooled to room temperature. After
days an orange precipitate was isolated by filtration and its
6
pyr
2
dihyd (0.034 g, 0.05 mmol) was dissolved in
Chart B.
1
for 15 min. After addition of UO
2
3
)
2
2
0
2
. Experimental and computational details
2
recrystallization from DMSO yielded, after 3 days, orange crystals
suitable for X-ray analysis. Yield: 0.047 g (74%).
2.1. General
40 6 14 4 2
Melting point: 212–213 °C. Anal. Calc. for C26H N O S U : C,
4.69; H, 3.19; N, 6.64; S, 10.14. Found: C, 24.85; H, 3.22; N,
.74; S, 10.05%.
All manipulations were conducted by use of standard argon
2
6
atmosphere. Elemental analyses for C, H and N were performed
at a Shimadzu EA 112 microanalysis instrument. IR spectra were
recorded on a Tensor 27-Bruker spectrometer with KBr pellets in
IR (KBr pellets): 3125 [m,
m
(OꢁH)]; 1597 [s,
m
(C@N)]; 1406 [m,
ꢁ1
d(SꢁCH3)]; 1277 [s,
m
(CꢁO)phenol.]; 1013 [m, d(CꢁS@O)]; 957 cm
ꢁ1
the 4000 to 400 cm region.
[
s, m(O@U@O)as].
2
2
.2. Preparations
2
.2.3. Bis((3-hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-
.2.1. Bis((3-hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-yl)meth-
yl)methylene) succinohydrazide (H10pyr
2
sucdihyd)
6 2
ylene) oxalohydrazide (H Pyr oxdihyd)

8
D.F. Back et al. / Inorganica Chimica Acta 412 (2014) 6–14
Pyridoxal hydrocloride (0.406 g, 2 mmol), dissolved in 10 ml of
anhydrous methanol, was mixed with 0.146 g (1 mmol) succinic
dihydrazide. The white solution was stirred by 1 h under Ar atmo-
sphere. The light yellow precipitate was removed by filtration. The
slow evaporation of the mother solution yielded light pale yellow
crystals. Yield: 82%.
least-squares on F2 with anisotropic displacement parameters for
all non-hydrogen atoms. Hydrogen atoms were included in the
refinement in calculated positions, but those hydrogen atoms that
realize special bonds were located in the Fourier map. Crystal data
and more details of the data collection and refinements of the com-
plexes 1 and 2 are contained in Table 1.
Properties: light yellow crystals. Melting point: 235–238 °C.
Anal. Calc.: C, 45.82; H, 5.19; N, 15.93. Found: C, 46.24; H, 5.33;
N, 16.19%.
2
.4. Computational details
IR (KBr pellets cmꢁ1): 3561 [m,
m(NꢁH)]; 3107,2932 [w,
The interactions stabilizing the resulting molecular packing in
m
(OꢁH)];
the molecular crystals have been studied through a methodology
that treats differently the parts of the molecules directly involved
in the intermolecular interactions from those that are secondary
to these stabilizing interactions. Each of the molecules in the
molecular crystal is divided in two layers (ONIOM approach)
1
694 [s,
m
(C@O)]; 1623 [m,
m
(C@N)]; 1544 [s,
m
(PyꢁH)]; 1142 [s,
m
(CꢁO)alcohols].
+
2
2.2.4. [(VO) (H10pyr
2
sucdihydꢁ4H )(MeO)
2
] (2)
[
15]. A first principles density functional theory (DFT) is used to
The ligand H10pyr
ml of methanol and mixed with 0.2 mmol (0.052 mg) of 98% van-
adyl(IV) acetylacetonate and 200 L of Et N. The mixture was stir-
red for 3.5 h at 60 °C. The few precipitate was removed by filtration
and after 3 days the solvent evaporation (under normal conditions
of temperature and pressure) yielded deep red crystals. Yield: 94%.
Properties: deep red crystals. Melting point (decomp.): 147 °C.
2
sucdihyd (0.2 mmol, 0.104 g) was dissolved in
treat the layer that is most directly involved in the interaction be-
tween the molecules, while the remaining atoms in the comple-
mentary layer are studied through the semi-empirical PM6
methodology [16]. The DFT approach uses the hybrid CAM-B3LYP
functional to describe the exchange and correlation interactions
[17], and the 6-31+G(d, p) basis set to represent the molecular
orbitals. Our calculations are performed on a model system con-
taining three molecules, according to Fig. 6, which contains all
the relevant structural informations that are necessary to study
the intermolecular interactions in the molecular crystal. This
‘‘three molecules’’ model is obtained directly from the molecular
crystal, keeping the structure of each molecule and their relative
configurations as in the bulk. Fig. 6 shows the atomic layers: a
small one that is treated with the density functional theory (repre-
sented by balls), and a larger one for which the semi-empirical ap-
proach is used (represented by sticks). Total energy calculations for
this model system are performed according to the scheme illus-
trated in Fig. 7. The total energy is calculated using the PM6 ap-
proach for the whole molecule plus the total energy of the
smaller layer using the DFT approach, minus the total energy of
this same smaller layer calculated through the PM6 methodology.
8
l
3
Anal. Calc. for C22
Found: C, 40.02; H, 4.33; N, 12.64%. IR (KBr, cm ): 3431 [sh,
m(OAH)alcohols]; 2924 [w, (CAH)]; 1604 [s, (C@O)]; 1510 [m,
(CAN)]; 1219 [m, (CAO)alcohols] 984 [m,
26 6 10 2
H N O V (636): C, 41.37; H, 4.38; N, 13.16.
ꢁ1
m
m
m(C@N)]; 1303 [m,
m
m
m(VO)sym].
2.3. X-ray crystallography
Data were collected with a Bruker APEX II CCD area-detector
diffractometer and graphite-monochromatized Mo K radiation.
a
The structure was solved by direct methods using SHELXS [14]. Sub-
sequent Fourier-difference map analyses yielded the positions of
the non-hydrogen atoms. Refinements were carried out with the
SHELXL package [14]. All refinements were made by full-matrix

D.F. Back et al. / Inorganica Chimica Acta 412 (2014) 6–14
9
Table 1
Crystal data and structure refinement for 1 and 2.
Complex
1
2
Empirical formula
Formula weight
T (°K)
Radiation, k (Å)
Crystal system, space group
Unit cell dimensions
a (Å)
C
26
H
40
N
6
O
14
S
4
U
2
22 26 6 10 2
C H N O V
1264.94
273(2)
0.71073
636.37
293(2)
0.71073
monoclinic, P2
1
/c
1
monoclinic, P2 /n
6.2867(10)
31.201(4)
10.0480(12)
90
102.906(6)
90
1921.1(4)
2, 2.187
8.706
8.5426(6)
14.9955(13)
9.5856(7)
90
94.895(5)
90
1223.44(16)
2, 1.727
0.836
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
3
V (Å )
Z, dcalc (g cm 3
Absorption coefficient
ꢁ
)
ꢁ
1
(
mm
)
F (000)
1196
652
Crystal size (mm)
h range (°)
Index ranges
0.265 ꢃ 0.094 ꢃ 0.051 0.18 ꢃ 0.10 ꢃ 0.09
2.18–28.30
1.72–28.43
ꢁ10 6 h 6 10,
ꢁ17 6 k 6 19,
ꢁ12 6 l 6 12
16431
ꢁ8 6 h 6 3,
ꢁ
ꢁ
41 6 k 6 30,
13 6 l 6 13
+
Fig. 1. Molecular structure of the binuclear chelate [(UO
DMSO) ] (1).
)
2 2
(H
6
pyr
2
oxdihydꢁ4H )(-
Reflections collected
Reflections unique
Completeness to theta max.
17016
4
4633 [Rint = 0.0754]
98.6
2598 [Rint = 0.0925]
98.3
(
%)
Table 2
2 2 2 2 4
Selected bond lengths (Å) and angles (°) for [(UO ) (H pyr oxdihyd)(DMSO) ] (1) and
Absorption correction
Maximum and minimum
transmissions
GAUSSIAN
GAUSSIAN
0.738 and 0.6197
0.9286 and 0.8641
[(VO)
2 6 2 2
(H pyr sucdihyd)(MeO) ] (2).
Refinement method
full-matrix least-
squares on F
4633/0/212
1.081
full-matrix least-
squares on F2
2598/1/176
1.017
1
2
2
Data/restraints/parameters
Goodness-of-fit (GOF) on F
Bond lengths
UAO4
UAO3
UAO1
UAO5
UAO7
UAO6
UAN2
S2AO7
Bond lengths
VAO5
VAO4
VAO1
VAO3
2
1.766(7)
1.778(7)
2.230(7)
2.323(7)
2.387(7)
2.391(7)
2.569(8)
1.516(8)
1.607(6)
1.7249(12)
1.857(5)
1.930(5)
2.102(5)
Final R indices [I > 2
R indices (all data)
Largest difference in peak
r
(I)]
R
1
= 0.0588,
wR = 0.1035
= 0.0969,
wR = 0.1174
2.694 and ꢁ2.376
R
1
= 0.0662,
wR = 0.1531
= 0.1247,
wR = 0.1841
0.832 and ꢁ0.835
2
2
R
1
R
1
VAN2
2
2
Bond angles
O5AVAO4
O5AVAO1
O4AVAO1
O5AVAO3
O4AVAO3
O1AVAO3
O5AVAN2
O1AVAN2
O4AVAN2
O3VAN2
C2AO1AV
ꢁ
3
and hole (e Å
)
103.4(2)
102.6(3)
101.38(15)
100.0(3)
93.00(15)
149.5(2)
92.4(3)
83.3(2)
161.95(17)
75.5(2)
Bond angles
O4AUAO3
O4AUAO1
O3AUAO1
O4AUAO5
O3AUAO5
O1UAO5
O4AUAO7
O3AUAO7
O1AUAO7
O5AUAO7
O3UAO6
O1AUAO6
O5AUAO6
O7AUAO6
O3AUAN2
O1AUAN2
O5AUAN2
O7AUAN2
O6AUAN2
O4AUAO6
S2AO7AU
S1AO6AU
179.2(3)
All calculations have been done using the GAUSSIAN 09 simulation
package [18]. The ball and sticks figures are made using the VMD
code [19], while the 2D graphs are obtained through the use of
the Grace code [20].
87.3(3)
92.6(3)
88.5(3)
91.0(3)
131.1(2)
90.9(3)
89.6(3)
153.4(2)
75.3(2)
86.2(3)
78.6(3)
150.3(3)
75.1(3)
81.9(3)
69.1(3)
63.1(2)
137.3(3)
144.9(3)
94.6(3)
142.8(4)
143.0(5)
133.1(4)
3
. Results and discussion
3
.1. Crystal structure
The molecular structure of the chelate, binuclear complex
+
[
(UO
2
)
2
(H
6
pyr
2
oxdihydꢁ4H )(DMSO)
4
]
(1) is represented in
Fig. 1. Selected bond lengths and angles of complex 1 are resumed
in Table 2.
In the molecule of 1 two uranyl cations are chelated at the
extremities of the ligand H Pyr dihyd, which undergoes deproto-
6 2
nation in the two NH groups, as well as in the phenolic OH group
of both rings. Two molecules of DMSO complete the coordination
number seven and the characteristic distorted pentagonal bipyra-
midal geometry of both the uranyl cations. The double (covalent)
bonds UAO3 (1.778) and UAO4 (1.766 Å) are shorter than the
coordinative ones UAO1 (2.230), UAO5 (2.323), UAO6 (2.391),
UAO7 (2.387) and UAN2 (2.569 Å).
The molecule of [(UO
well as the ligand H Pyr
Intermolecular interactions of the type Oꢀ ꢀ ꢀH, as well as CAHꢀ ꢀ ꢀ
p
contacts have not been detected in compound 1.
The occurrence of binuclear complexes of the uranyl cation is
not uncommon, since chelate complexes of UO
2
+
2
with oligoden-
+
2
)
2
(H
6
pyr
2
oxdihydꢁ4H )(DMSO)
4
] (1), as
tate ligands have been already reported [23–26]. Along the crystal-
lographic direction 010 could be observed the existence of a classic
hydrogen bond, as shown in Fig. 2, between the atoms
N3(A)ꢀ ꢀ ꢀH2#ꢁO2#(D) (A = acceptor; D = donator), in order of
0.850 Å in length (DAH); 2.008(9) Å (Hꢀ ꢀ ꢀA), and 2.789(1) Å
6
2
oxdihyd, present an inversion center be-
0
tween the atoms C10 and C10 . The attainment of NꢁH intermolec-
ular [21,22] interactions allowed the visualization of
supramolecular assemblies along the two-dimensional plane bc.

1
0
D.F. Back et al. / Inorganica Chimica Acta 412 (2014) 6–14
Fig. 2. Molecular assembly and hydrogen interaction in complex 1. Symmetry operations used to generate equivalent atoms: (#) ꢁx, 0.5 + y, 2.5 ꢁ z.
Fig. 3. Hydrogen interactions in complex 1 (dashed lines). Symmetry operations used to generate equivalent atoms: (#) 1 + x, y, z.
(
Dꢀ ꢀ ꢀA), with an angle of 158.84(51)° (DAHꢀ ꢀ ꢀA). Symmetry trans-
formations used to generate equivalent atoms: (#) ꢁx, ꢁ0.5 + y,
.5 ꢁ z.
However, when the ac crystallographic plane of complex 1 is
Table 3
Selected bond lengths (Å) and angles (°) for complex 1.
2
(
DAHꢀ ꢀ ꢀA)
DAH (Å)
Hꢀ ꢀ ꢀA (Å)
2.423(7)
2.631(7)
Dꢀ ꢀ ꢀA (Å)
3.204(14)
3.568(15)
DAHꢀ ꢀ ꢀA(°)
138.24(75)
165.37(74)
O3#ꢀ ꢀ ꢀH11aAC11
O3#ꢀ ꢀ ꢀH14aAC14
0.960(11)
0.959(13)
observed, also bifurcated hydrogen bonds of the type ‘‘receptor
forked’’, with two donors atoms, are identified [27,28]. The O3
atom of the uranyl cation acts as acceptor for two hydrogen bonds
from the hydrogens of the methyl groups of the neighboring di-
methyl sulfoxide groups. These type of atomic interactions allow
us to link this structure with species of three-dimensional stairs
dences of growth of the supramolecular assembly of complex 1
along the crystallographic plane ac (see Fig. 3).
Similarly to what happens with complex 1, the ligand H10pyr2-
sucdihyd contains a ‘‘spacer’’ (succinohydrazide), which allows the
complexation of two (VO) cations at two ligand places, as repre-
sented in Fig. 4. Each ‘‘side’’ of the hexadentate ligand occupies
(Fig. 3, symmetry transformations used to generate equivalent
+
3
atoms: (#) 1 + x, y, z) [29]. The intermolecular bonds and distances
showed in Table 3 for the atoms CAHꢀ ꢀ ꢀO3(#) support the evi-

D.F. Back et al. / Inorganica Chimica Acta 412 (2014) 6–14
11
+
Fig. 4. Molecular structure of the binuclear chelate [(VO)
2
(H10pyr
2
sucdihydꢁ4H )(MeO)
2
] (2).
three coordination positions at each oxidocation (VO)+3: one oxy-
gen atom from the deprotonated phenolic group, a bond from
the substituent C@N (imino) and the third bond is formed by the
negative charged oxygen of the group.
Table 5
Secondary interactions: lengths (Å) and angles (°) for complex 2.
(
DAHꢀ ꢀ ꢀA)
DAH (Å)
Hꢀ ꢀ ꢀA (Å)
Dꢀ ꢀ ꢀA (Å)
DAHꢀ ꢀ ꢀA (°)
165.64(16)
O2AH2ꢀ ꢀ ꢀO3#
0.987(1)
1.860(6)
2.827(7)
C
N
O
Table 6
Selected bond lengths (Å) for complex 2.
Atom
Dꢀ ꢀ ꢀA (Å)
The two cations (VO)+3 achieve a distorted quadratic pyramidal
geometry through further coordination to two deprotonated mole-
cules of methanol.
The deprotonation of the hydroxilic methanol groups can be
easily confirmed by comparison of the VAO (methanol) distances
listed en Table 4, for both protonated and not protonated hydroxyl
methanolic groups, which are considerably lower than in proton-
ated cases [28,30–32].
As in the complex of uranium (1), in compound 2 the hydrogen
of the fragment of the primary alcohol from pyridoxal performs
connections between the nitrogen atoms of the neighboring pyri-
dines (Table 5).
O2ꢀ ꢀ ꢀO3#
O2ꢀ ꢀ ꢀN2#
O2ꢀ ꢀ ꢀO1#
O2ꢀ ꢀ ꢀO4#
2.774(6)
2.942(7)
2.915(4)
3.032(2)
occurs between the atoms O2AO4# (3.032 (2) Å), being also in
agreement with the sum of the Van der Waals radii listed in Table 6.
Fig. 5 shows the above related interactions between the atoms (red
dashed lines).
These interactions are considered strong to moderate, since the
distances between the atoms are in the range of 0.987 Å (O2AH2)
and 1.860(6) Å (O3ꢀ ꢀ ꢀH2#). The bond angle is 165°. These values
are in agreement with the models proposed by T. Steiner and
G.A. Jeffrey [41,42].
Although this hydrogen bond can be considered strongꢁmoder-
ate due to a preferential spatial arrangement, the oxygen O2 from
the alcohol function makes interactions between the atoms O3#
3.2. Theoretical statements
In complex 2 there are two main interactions responsible for
the crystalline stabilization: (i) the one between the O2 atom in
molecule A and the V-centered group of atoms in molecule B
(see Fig. 6), and (ii) the hydrogen bond interaction between the
H2 atom in molecule A and the N1# atom in molecule C. In order
to compare the relative importance of these interactions on the
stabilization of the crystal structure, total energy calculations for
different O2AV and H2AN1# interatomic separations have been
performed.
(
ketone function), N2# (imine function) O1# (phenol function)
and O1# (deprotonated methanol oxygen). The major interaction
For the O2AV interaction between molecules A and B, we dis-
place molecules A and C relative to each other as a rigid block along
the vector joining the O2 and V atoms. No geometry optimization
is performed. The total energy curve obtained for each O2AV inter-
atomic separation is shown in Fig. 8.
The calculated equilibrium distance of 2.58 Å is in close agree-
ment with the crystallographic data (2.592 Å). In order to under-
stand which atoms are most affected by this interaction, the
natural atomic charges are calculated and their variation relatively
to their values at a O2AV distance of 15 Å is displayed in Fig. 9 (the
charge variation for O2AV distances from 4 to 15 Å follow a mono-
tonic path, not shown in Fig. 9). As can be seen from this Figure, the
greater variations are observed at the V atom that turns more and
Table 4
Bond lengths (Å) of selected vanadium compounds described in the literature.
Reference VAO distance (deprotonated
VAO distance (protonated
methanol)
methanol)
[33]
[34]
[35]
[36]
[36]
[37]
[38]
[39]
[40]
1.775
1.768
1.767
1.775
1.761
1.769
1.793
1.781
1.776
2.398
2.346
2.398
2.333
2.340
2.327
2.311
2.236
2.295

1
2
D.F. Back et al. / Inorganica Chimica Acta 412 (2014) 6–14
Fig. 5. Multiple interactions in complex 2. Symmetry operations used to generate equivalent atoms: (#) ꢁ0.5 + x, 1.5 ꢁ y, 0.5 + z and (2#) 0.5 + x, 1.5 ꢁ y, ꢁ0.5 + z.
Fig. 6. The three molecules model used to calculate the total energy and Mulliken charges. Letters A, B, and C are used to label the different molecules while the atomic labels
distinguish the atoms relevant to the analysis in the text.
Fig. 7. Scheme used to calculate the total energy of the model system. The left side shows the layers treated by the DFT (balls) and PM6 (sticks) approaches. The total energy
is obtained by calculating the total energy of the two layers using the PM6 approach (first term on the right side), plus the total energy of the smaller layer using the DFT
approach (second term on the right side), minus the total energy of the smaller layer using the PM6 approach (third term on the right side).

D.F. Back et al. / Inorganica Chimica Acta 412 (2014) 6–14
13
Fig. 8. Calculated potential energy profiles for both the hydrogen bonding between
atoms H2 and N1# atoms (black curve) and the electrostatic interaction between
the O2 and V atoms (red (deeper) curve). (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. Calculated Natural charge variation on selected atoms (see Fig. 1), as a
function of the H2AN1# interatomic separation, relatively to their values at a
H2AN1# distance of 10 Å.
are shown in Fig. 10. From these Natural charges one can see that
the most pronounced charge variation appears for the N1# and O2
atoms. As shorter the H2AN1# distance more negatively charged
turns the N1# atom, until the H2AN1# distance is shorter than
2
.0 Å, when its charge remains the same, independently of the
H2AN1# bond distance. Further, the charge variations on H2 and
V atoms show opposite behaviors, with the H2 and V turning pos-
itively and negatively charged, respectively, as the H2AN1# dis-
tance turns shorter. These tendencies are reverted for H2AN1#
distances lower than 2.0 Å. A very similar behavior is observed
for the Natural charges of the O3# e N2# atoms, as can be seen
in Fig. 10.
The relative influences of the O2AV and H2AN1# interactions
for the molecular crystal stabilization can be estimated by compar-
ing the potential energy profiles in Fig. 8. It should be stressed that
the absolute values for the binding energies obtained from calcula-
tions using the ONIOM approach are far from being conclusive,
since we are not taken into account the basis set superposition er-
ror [43]. We will then focus on the differences between the poten-
tial energy profiles.
Fig. 9. The calculated Natural charge variation on selected atoms as a function of
the O2AV interatomic separation. The values are taken relatively to the ones at a
O2AV distance of 15 Å.
The O2AV interaction presents a sharper potential energy pro-
file, while the hydrogen bond shows a broader one. It indicates that
the H2AN1# hydrogen bond is softer (lower vibrational frequen-
cies) than the O2AV electrostatic interaction.
From this analysis it could be suggested that the H2AN1#
hydrogen bond interaction drives the initial steps of the crystalli-
zation process but, when the molecules turn closer to each other,
the O2AV electrostatic interaction would effectively stick the mol-
ecules together, stabilizing the crystalline packing.
more negatively charged until the O2AV (equilibrium) distance of
2
.58 Å where this tendency is stopped and slightly reverted. An
opposite behavior is observed for the N2# and O2 atoms, which
turn more and more positively charged (relatively to its value at
long distances) as the O2 atom at molecule A approaches the V
center at molecule B. The O2 atom revert this behavior for O2AV
distances lower than the one corresponding to the lowest total
energy.
These results show that when the O2 atom of molecule A ap-
proaches the V center in molecule B, a charge transfer occurs from
atoms N2# to the V atom within molecule B that drives the electro-
static interaction between molecules A and B. This mechanism is
effective until an O2AV distance around 2.58 Å. For shorter
O2AV distances this interaction turns repulsive, with the charge
on atoms O2 and V remaining approximately constant or even
changing the sign of their variation.
Total energy calculations for the hydrogen bond interaction be-
tween molecules A and C have been performed for different
H2AN1# interatomic distances by displacing molecules A and C
relative to each other, as rigid blocks, along the vector joining the
H2 and N1# atoms. Again no geometry optimization is performed.
The total energies for each considered H2AN1# separation are
shown in Fig. 8. An equilibrium distance of 2.16 Å is obtained,
which is relatively larger (16%) than the one obtained from the
crystallographic data (1.86 Å). The variation of the atomic natural
charges relatively to their values at a H2AN1# separation of 10 Å
4. Conclusions
The reactions reported in this work, together with already pub-
lished results [7–10], make evident that pyridoxal and pyridoxine
2
+
derivatives present a remarkable ability to react with [UO
2
]
and
+
3
[VO] attaining the chelation of two metal atoms pro ligand
molecule.
6 2 2
The ligands H Pyr dihyd and H10pyr sucdihyd, like other pyri-
doxal- or pyridoxine-containing Schiff base ligands, combine the
metabolic potential of the vitamin B6 family with its chemical abil-
ity to form stable chelate complexes. Because of that, and in theory,
the synthesis and the structure elucidation of the title complexes 1
and 2 should also represent a previous qualitative contribution to
the research on models of chelation of oxofilic elements.
We have also observed that molecules containing pyridoxal
have been shown a differentiated behavior regarding the formation

1
4
D.F. Back et al. / Inorganica Chimica Acta 412 (2014) 6–14
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of supramolecular assemblies, since this molecule (pyridoxal) has
more potential sites for hydrogen bonding and specific interac-
tions. The theoretical (computational) data presented are useful
tools to qualify and quantify the intra and intermolecular interac-
tions involved in the title compounds.
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Acknowledgment
Brazilian Research Councils: CNPq – Edital No. 14/2011; FA-
PERGS – Fundação de Amparo à Pesquisa do Estado do Rio Grande
do Sul – Edital No 002/2011 – PqG; FIPE ARD CCNE – 2011.
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