X-ray structure analysis, electronic and vibrational circular dichroism of chiral-at-metal dioxidovanadium(V) complexes with amino acids derived Schiff base ligands

Article abstract of DOI:10.1016/j.poly.2014.06.052

Five new chiral dioxidovanadium Schiff base complexes derived from l-valine, l-leucine, l-histidine, l-threonine and d-threonine were synthesized and characterized by elemental analysis, ¹H and ⁵¹V NMR and IR spectra. The absolute co

Full text of DOI:10.1016/j.poly.2014.06.052

Accepted Manuscript
X-ray structure analysis, electronic and vibrational circular dichroism of chiral-
at-metal dioxidovanadium(V) complexes with amino acids derived Schiff base
ligands
Lukáš Krivosudský, Peter Schwendt, Róbert Gyepes, Zdirad Žák
PII:
S0277-5387(14)00462-8
DOI:
http://dx.doi.org/10.1016/j.poly.2014.06.052
POLY 10850
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
27 May 2014
26 June 2014
Please cite this article as: L. Krivosudský, P. Schwendt, R. Gyepes, Z. Ž ák, X-ray structure analysis, electronic and
vibrational circular dichroism of chiral-at-metal dioxidovanadium(V) complexes with amino acids derived Schiff
base ligands, Polyhedron (2014), doi: http://dx.doi.org/10.1016/j.poly.2014.06.052
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X-ray structure analysis, electronic and vibrational circular dichroism of chiral-at-metal
dioxidovanadium(V) complexes with amino acids derived Schiff base ligands.
a
a,*
b
c, d
Lukáš Krivosudský , Peter Schwendt , Róbert Gyepes , Zdirad Žák
a
Department of Inorganic Chemistry, Comenius University, Faculty of Natural Sciences, Mlynská dolina,
42 15 Bratislava, Slovakia
Faculty of Education, J. Selye University, Bratislavská cesta 3322, 945 01 Komárno, Slovakia
8
b
c
Department of Inorganic Chemistry, Faculty of Natural Sciences, Masaryk University, Kotlářská 2, 611 37
Brno, Czech Republic
d
CEITEC – Central European Institute of Technology, Masaryk University, Kamenice 5, 625 00 Brno,
Czech Republic
*
Corresponding author. Tel.: +421 0260296273
E-mail address: schwendt@fns.uniba.sk (P. Schwendt).
Keywords: vanadium complexes; absolute configuration; circular dichroism.
Abstract
Five new chiral dioxidovanadium Schiff base complexes derived from L-valine, L-leucine, L-histidine, L-
1
51
threonine and D-threonine were synthesized and characterized by elemental analysis, H and V NMR and
IR spectra. The absolute configurations of the complexes were determined by X-ray single crystal analysis
3
in solid state and by electronic and vibrational circular dichroism in CH CN solution. The L-threoninato
complex, which contains two stereogenic centres in the amino acid site, does not show epimerization of the
amino acid, in contrast with the analogical isoleucine-derived complexes reported recently. Conglomerate
crystallization was observed in racemic DL-valine- and DL-threonine-derived complexes.
1
. Introduction
Vanadium Schiff base complexes have shown catalytic activity in several types of asymmetric oxidations of
organic substrates [1‒13]. Therefore chiral complexes of vanadium(IV) and (V) are of a great interest and
during recent years some diffraction and spectroscopic studies focused on the chirality-at-metal were
published. It is assumed that the formation of tetragonally pyramidal and octahedral chiral Schiff base
complexes of vanadium is diastereospecific meaning that if there is one stereogenic centre in the Schiff base
and one stereogenic centre is generated on the vanadium atom upon complexation only one stereoisomer
instead of two diastereomers is observed in the solid state structure. This assumption is proven by many
crystal structures of tetragonally pyramidal [14‒24] and octahedral [14, 25‒34] complexes of vanadium(V)
with ONO Schiff base donor set where enantiomerically pure Schiff bases were used. If a racemic Schiff
base is used in the synthesis racemic complexes are isolated from the reaction system and the complex
formation occurs diastereospecifically again [14, 35] although obviously not many syntheses starting from
racemic ligands have been performed. This situation clearly illustrates the compound [VO{N-(2-
oxidobenzylidene)phenylalaninato}(OMe)(HOMe)]: one diastereomer was isolated when L-phenylalanine
derivative was used in the synthesis (instead of two possible) [36] while two enantiomers with defined
chirality mode on the vanadium atom were present in the crystal structure if DL-phenylalanine was
introduced in the synthesis (although four stereoisomers are theoretically possible) [37]. Finally, if a Schiff
base without a stereogenic centre is applied in the synthesis both possible stereoisomers (enantiomers
defined by the stereogenic centre at the metal site) are observed, either tetragonally pyramidal [14, 38‒43] or
octahedral [14, 29, 38, 42, 44‒52].
Although these conclusions are quite straightforward, little is known about the composition of the reaction
mixtures, the mother liquors and the solutions of dissolved stereoisomers in respect to chirality-at-metal. It
has been shown that despite the crystal structure of [VO{N-(2-oxido-1-naphthylmethylene)-L-
S
S
alaninato}OBu (HOBu )] carries four centres of chirality (the vanadium atom, the alanine moiety,
secondary carbons of coordinated butanol and butanolate), it contains only two stereoisomers (ASRR and

5
1
CSSS), and V NMR spectrum of the solution of the compound exhibits three signals corresponding to three
diastereomers out of eight possible stereoisomers. The situation may be even more complicated due to
5
1
overlapping signals in V NMR spectra, dissociation of ligands and disfavourable formation of specific
diastereomers [53].
Our primary goal was to prepare a new family of chiral dioxido complexes of vanadium(V) derived from
amino acids as a low-cost source of chirality. As a continuation of our work on understanding the
stereochemistry of chiral vanadium(V) complexes [54‒59] we took a closer look at the chirality of the
compounds prepared. Recently we have reported on the vanadium catalyzed epimerization of isoleucine and
a controlled crystallization of stereoisomers of [N(C
with chiroptical characterization of isolated stereoisomers. The synthetic procedure was utilized to prepare
five new chiral dioxido complexes of vanadium(V): [N(C ][VO (N-salicylidene-L-valinato)] (VO2L-
val), [N(C ][VO (N-salicylidene-L-leucinato)] (VO2L-leu), [N(C ][VO (N-salicylidene-L-
histidinato)]·2H (VO2L-his), [N(C ][VO (N-salicylidene-L-threoninato)] (VO2L-thr) and
N(C ][VO (N-salicylidene-D-threoninato)] (VO2D-thr) (Fig. 1). The absolute configuration of the
4
H
9
)
4
][VO
2
(N-salicylidene-isoleucinato)] [60] together
4
H
9
)
4
2
4
H
9
)
4
2
4
9
H )
4
2
2
O
4
H
9
)
4
2
[
4
H
9
)
4
2
complexes prepared was confirmed not only by X-ray structure analysis, but also by electronic and
vibrational circular dichroism which provide comprehensive information on the chirality. Moreover,
crystallizations of racemic mixtures of VO2DL-val and VO2DL-thr were examined as well bringing
surprising proof of rarely occurring conglomerate crystallization of enantiomers.
Fig. 1. Structural formulae of vanadium(V) complexes.
2
2
. Experimental
.1 General information and materials
All reagents and solvents were obtained from commercial sources and used without further purification.
V
2
O
5
4 3
was prepared by thermal decomposition of NH VO . Elemental analyses (C, H, N) were performed on
a Vario MIKRO cube (Elementar). Vanadium(V) was determined volumetrically by titration with FeSO
4
using diphenylamine as indicator. Solid state IR spectra were recorded on Thermo Scientific Nicolet 6700
1
FT IR spectrometer in nujol mulls and in KBr pellets. The H NMR spectra were recorded with a Varian
1
Mercury plus instrument (300 MHz for H). Chemical shifts (δ) are given in ppm relative to
5
1
tetramethylsilane. The V NMR spectra of solutions were registered at 278 K on a Varian Mercury Plus 600
5
1
MHz spectrometer operating at 157.88 MHz ( V) in 5 mm tubes. Chemical shifts (δ) are given in ppm
relative to VOCl as external standard (δ = 0 ppm). Solution IR and VCD spectra were recorded on a Bruker
Tensor 27 FTIR spectrometer with 4 cm resolution equipped with the Bruker PMA 50 VCD sidebench
module, using a data collection time of 6 h. CD CN solution samples were placed in a 100 µm path length
BaF cell. All spectra were corrected for background effects by solvent subtraction. ECD and UV-VIS
spectra were recorded on a JASCO J-815 CD spectrometer in CH CN solutions with a 1 cm cell.
3
−1
3
2
3
2
.2 X-ray diffraction
Diffraction data were collected using a Kappa Apex II (Bruker) diffractometer equipped with a
Cryostream Cooler (Oxford Cryosystems). Data were collected using graphite-monochromated Mo K
α

radiation (λ = 0.71073 Å) and were corrected for radiation absorption by methods incorporated in the
diffractometer software. The phase problem was solved by direct methods (SHELXS97 [61]) and refined by
2
full-matrix least-squares based on F (SHELXL97 [61]). All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms were included in idealized positions and refined as
riding atoms. Geometric data were obtained with a recent version of the PLATON [62] program. Graphics
were obtained with DIAMOND [63].
2
.3 Syntheses
The vanadium(V) complexes reported herein as well as [N(C
VO2L-ile) applied as a reference for designation of absolute configurations were prepared by a modified
procedure described previously [60].
4
H
9
)
4
][VO (N-salicylidene-L-isoleucinato)]
2
(
2
4 9 4 2
.3.1 [N(C H ) ][VO (N-salicylidene-L-valinato)] (VO2L-val)
Salicylaldehyde (0.531 mL, 5 mmol) in tert-butanol (3 mL) was added to a solution of L-valine (0.580 g, 5
mmol) dissolved in acetone (3 mL) and tetrabutylammonium hydroxide (3.6 mL of 40 % w/w solution, 5
mmol) and the solution was stirred for 1 hour. After V O (0.45 g, 2,5 mmol) was dissolved in
2 5
tetrabutylammonium hydroxide (3.6 mL of 40 % w/w solution, 5 mmol) at room temperature the resulting
colourless solution was added to the solution of a Schiff base and the solution was stirred for 15 minutes. To
the orange solution 3 mL of distilled water were added. Pale yellow crystals appear on the following day.
49 2 5
Yield: 52%. Anal. Calc. for C28H N O V: C, 61.75, H, 9.07, N, 5.14, V, 9.35. Found: C, 61.55; H, 9.18, N,
−
1
5
1
2
6
3
.19, V, 9.16%. IR (cm ): 458, 514, 558, 588, 616, 740, 771, 807, 848, 887, 921, 935, 980, 1005, 1030,
039, 1059, 1125, 1137, 1148, 1137, 1204, 1307, 1341, 1402, 1544, 1599, 1620, 1670. UV-VIS [λmax, nm]:
27, 260, 370. H NMR (300 MHz, CDCl
.74 (t, 1H), 4.11 (d, 1H), 3.49 (broad t, 8H), 2.37 (m, 1H), 1.74 (broad m, 8H), 1.5 (broad m, 8H), 1.13 (d,
H), 1.08 (d, 3H), 1.00 (t, 12H). V NMR (CH
1
3
, ppm) δ 8.32 (s, 1H), 7.36 (dt, 1H) 7.31 (dd, 1H), 6.98 (d, 1H),
51
3
CN, ppm): −534.
2
.3.2 [N(C ][VO (N-salicylidene-L-leucinato)] (VO2L-leu)
4
H
9
)
4
2
The synthesis of VO2L-leu was similar to the procedure described for VO2L-val except that L-valine was
replaced by L-leucine (0.65 g, 5 mmol). Yield: 32%. Anal. Calc. for C29 V: C, 62.35, H, 9.20, N,
51 2 5
H N O
−1
5
7
1
.01, V, 9.12. Found: C, 62.45; H, 9.17, N, 4.94, V, 9.21%. IR (cm ): 455, 476, 540, 557, 574, 615, 687,
40, 771, 811, 873, 895, 916, 938, 958, 1005, 1029, 1079, 1107, 1134, 1147, 1169, 1205, 1224, 1292, 1311,
340, 1380, 1403, 1426, 1451, 1546, 1599, 1619, 1670. UV-VIS [λmax, nm]: 227, 260, 370. H NMR (300
1
MHz, CDCl
3
, ppm) δ 8.31 (s, 1H), 7.37 (dt, 1H) 7.31 (dd, 1H), 6.99 (d, 1H), 6.74 (t, 1H), 4.34 (t, 1H), 3.50
(
1
broad t, 8H), 2.03‒1.83 (broad m, 1H + 2H), 1.76 (broad m, 8H), 1.5 (broad m, 8H), 1.03 (d, 6H), 0.99 (t,
5
1
2H). V NMR (CH
3
CN, ppm): −535.
2
.3.3 [N(C ][VO
4
H
9
)
4
2
(N-salicylidene-L-histidinato)]·2H O (VO2L-his)
2
The synthesis of VO2L-his was similar to the procedure described for VO2L-val except that L-valine was
replaced by L-histidine (0.768 g, 5 mmol). In the final step 5 mL of water and L-histidine (0.768 g, 5 mmol)
were added to the solution to precipitate the compound. Yield: 35%. Anal. Calc. for C29
H, 8.31, N, 9.06, V, 8.23. Found: C, 56.42; H, 8.39, N, 9.10, V, 8.15%. IR (cm ): 419, 449, 459, 501, 530,
H
51
N
4
O
7
V: C, 56.30,
−1
5
1
54, 564, 618, 629, 655, 707, 746, 774, 808, 821, 858, 882, 919, 934, 968, 985, 1030, 1072, 1087, 1108,
126, 1149, 1176, 1210, 1236, 1276, 1307, 1341, 1361, 1400, 1485, 1538, 1571, 1622, 1653. UV-VIS [λmax
,
1
nm]: 227, 260, 370. . H NMR (300 MHz, CDCl
3
, ppm) δ 7.77 (s, 1H), 7.54 (s, 1H), 7.32 (d, 1H) 7.05 (d,
1
8
H), 6.86 (d, 1H), 6.67 (t, 1H), 4.54 (broad t, 1H), 3.37 (broad t, 8H), 3.12 (broad dd, 2H), 1.68 (broad m,
5
1
H), 1.44 (broad m, 8H), 0.97 (t, 12H). V NMR (CH
3
CN, ppm): −535.
2
.3.4 [N(C ][VO (N-salicylidene-L-threoninato)] (VO2L-thr)
4
H
9
)
4
2

The synthesis of VO2L-thr was similar to the procedure described for VO2L-val except that L-valine was
replaced by L-threonine (0.589 g, 5 mmol). In the final step 5 mL of water and L-threonine (0.589 g, 5
mmol) were added to the solution to precipitate the compound. Yield: 29%. Anal. Calc. for C27
C, 59.33, H, 8.67, N, 5.12, V, 9.32. Found: C, 59.02; H, 8.55, N, 5.20, V, 9.08%. IR (cm ): 421, 444, 460,
H
47
N
2
O
6
V:
−1
5
1
39, 551, 566, 617, 723, 743, 775, 816, 858, 881, 910, 925, 971, 983, 1009, 1031, 1062, 1084, 1098, 1124,
135, 1148, 1206, 1224, 1251, 1299, 1330, 1342, 1378, 1451, 1524, 1546, 1602, 1628, 1684. UV-VIS [λmax
,
1
nm]: 227, 260, 370. H NMR (300 MHz, CDCl
3
, ppm) δ 8.44 (s, 1H), 7.39 (dt, 1H) 7.33 (dd, 1H), 6.97 (d,
1
8
H), 6.76 (t, 1H), 4.40 (d, 1H), 4.25 (broad m, 1H) 3.47 (broad t, 8H), 1.74 (broad m, 8H), 1.49 (broad m,
5
1
H), 1.33 (d, 3H), 1.00 (t, 12H). V NMR (CH
3
CN, ppm): −534.
2
.3.5 [N(C ][VO (N-salicylidene-D-threoninato)] (VO2D-thr)
4
H
9
)
4
2
The synthesis of VO2D-thr was similar to the procedure described for VO2L-thr except that L-threonine was
replaced by D-threonine (0.589 g, 5 mmol). Yield: 25%. Anal. Calc. for C27 V: C, 59.33, H, 8.67, N,
47 2 6
H N O
−1
5
7
1
3
4
1
.12, V, 9.32. Found: C, 59.35; H, 8.54, N, 5.18, V, 9.42%. IR (cm ): 421, 444, 460, 539, 551, 566, 617,
23, 743, 775, 816, 858, 881, 910, 925, 971, 983, 1009, 1031, 1062, 1084, 1098, 1124, 1135, 1148, 1206,
224, 1251, 1299, 1330, 1342, 1378, 1451, 1524, 1546, 1602, 1628, 1684. UV-VIS [λmax, nm]: 227, 260,
1
70. H NMR (300 MHz, CDCl
3
, ppm) δ 8.45 (s, 1H), 7.39 (dt, 1H) 7.34 (dd, 1H), 6.96 (d, 1H), 6.76 (t, 1H),
.39 (d, 1H), 4.17 (broad m, 1H) 3.46 (broad t, 8H), 1.74 (broad m, 8H), 1.48 (broad m, 8H), 1.33 (d, 3H),
5
1
.00 (t, 12H). V NMR (CH
CN, ppm): −534.
3
3
3
Results and discussion
.1 Syntheses
The branched-chain amino acids derived complexes VO2L-val and VO2L-leu were synthesized following
the procedure described for VO2L-ile. VO2L-his, VO2L-thr and VO2D-thr showed higher solubility and
therefore some excess of corresponding amino acids were necessary to initialize crystallizations. Threonine
and isoleucine are the only naturally occurring amino acids carrying two stereogenic centres, α- and β-
carbons of the amino acids. While in solutions of VO2L-ile we observed surprising vanadium-catalyzed
1
epimerization at the α-carbon position of isoleucine [60], H NMR studies of solutions of VO2L-thr and
VO2D-thr showed no such reaction. Furthermore, some decomposition reactions were detected which may
originate in higher reactivity of the hydroxyl group attached to β-carbon in threonine in comparison to the
ethyl group in isoleucine at the same position. The syntheses performed with DL-valine and DL-threonine
resulted in conglomerate crystallizations of the stereoisomers and the corresponding crystal structures are
further discussed. DL-leucine provided racemate as well, but the crystals were of a bad quality and therefore
it was not possible to distinguish between conglomerate crystallization and racemic compound.
3
.2 Crystal structures
Crystal data and structure refinement of compounds VO2L-val, VO2D-val, VO2L-leu, VO2L-his, VO2L-thr
and VO2D-thr are summarized in Table 1. All compounds crystallize in orthorhombic lattices, space group
P2
1
2
1
2
1
, except VO2L-his, which crystallizes in a monoclinic lattice, space group P2
1
. Each asymmetric unit
+
consists of one tetrabutylammonium cation, [N(C
4
H
9
)
4
] , and one chiral dioxidovanadium anion, [VO (N-
2
−
salicylidene-amino acidato)] (Fig. 2 and Fig. 3). The asymmetric unit of VO2L-his contains two additional
crystallization water molecules. In all complex anions, the penta-coordinated vanadium atom adopts a
distorted tetragonal pyramidal environment, filled by the oxido ligand (O1) in the apical position and the
oxido ligand (O2) and ONO donor set of the Schiff base (O3N1O4) in the equatorial positions. The
calculated τ parameters refer to a clear square pyramid geometry with an obvious tendency to
pyramidalization giving the same displacement of the vanadium atom from the tetragonal pseudoplane
towards the oxido ligand (O1) in all complexes (Table 2). The shorter bond length V1‒O1 than V1‒O2 is in
accordance with the general tendency in square pyramidal geometry [64] except for VO2L-thr and VO2D-
thr. All the other bond lengths and angles are consistent for all complexes.

Table 1 Crystal data and structures refinement.
Compound
Empirical formula
Formula weight
T (K)
VO
2
L-val
VO
2
D
-val
VO
2
L
-leu
VO
2
L
-his
VO
2
L
-thr
2
VO D-thr
C
28
H
49
N
2
O
5
V
C
28
H
49
N
2
O
5
V
C
29
H
51
N
2
O
5
V
C
29
H
51
N
4
O
7
V
C
27
H
47
N
2
O
6
V
27 47 2 6
C H N O V
546.61
150
544.63
150
544.63
150
558.66
150
618.68
150
546.61
200
λ (Å)
0.71073
orthorhombic
P2
0.71073
0.71073
orthorhombic
P2
0.71073
monoclinic
P2
0.71073
0.71073
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
orthorhombic
P2
orthorhombic
P2
orthorhombic
P2
1
2
1
2
1
1
2
1
2
1
1
2
1
2
1
1
1
2
1
2
1
1 1 1
2 2
12.3286(3)
13.6471(3)
17.7314(5)
90.0
12.3293(2)
13.6538(2)
17.7242(3)
90.0
11.7875(2)
13.7137(2)
18.6580(4)
90.0
9.6610(3)
13.4854(4)
13.3814(3)
90.0
13.2899(3)
13.6162(3)
16.2361(4)
90.0
13.0433(12)
13.2202(13)
16.4850(16)
90.0
α (°)
β (°)
γ (°)
V (Å )
90.0
90.0
2983.30(13)
4
1.213
90.0
90.0
2983.72(8)
4
1.212
90.0
90.0
3016.07(9)
4
1.230
108.2460(10)
90.0
1655.71(8)
2
1.241
0.347
90.0
90.0
2938.05(12)
4
1.236
90.0
90.0
2842.6(5)
4
1.275
3
Z
D
−3
calc (g·cm )
−1
Absorption coefficient (mm ) 0.369
0.369
0.367
0.378
0.390
F(000)
1176
1176
1208
664
1176
1172
Crystal size (mm)
0.45 × 0.40 × 0.16 0.74 × 0.31 × 0.24 0.42 × 0.28 × 0.16 0.49 × 0.39 × 0.13 0.56 × 0.28 × 0.12 0.25 × 0.20 × 0.15
θ Range for data collection (°) 2.23 to 27.46
1.88 to 27.49
− 13 ≤ h ≤ 16
− 17 ≤ k ≤ 17
− 23 ≤ l ≤ 18
26017/6825
1.84 to 26.01
− 14 ≤ h ≤ 12
− 16 ≤ k ≤ 16
− 23 ≤ l ≤ 22
18094/5914
1.60 to 27.05
− 12 ≤ h ≤ 11
− 17 ≤ k ≤ 17
− 17 ≤ l ≤ 17
18032/7238
1.95 to 26.01
− 15 ≤ h ≤ 16
− 16 ≤ k ≤ 15
− 16 ≤ l ≤ 20
16033/5775
2.47 to 27.50
− 16 ≤ h ≤ 13
− 13 ≤ k ≤ 17
− 18 ≤ l ≤ 21
13062/6395
Limiting indices
− 15 ≤ h ≤ 16
−
−
17 ≤ k ≤ 17
23 ≤ l ≤ 23
Reflections collected/unique
38189/6828
int = 0.031
R
Rint = 0.018
R
int = 0.025
Rint = 0.024
R
int = 0.017
Rint = 0.109
Completeness to 2θ (%)
100
100
99.7
99.5
99.8
99.5
Data/restraints/parameters
Goodness-of-fit (GOF) on F
Final R indices [I > 2σ(I)]
6828/6/331
1.05
6825/6/332
1.03
5914/12/ 340
1.03
7238/6/381
1.03
5775/2/360
1.02
6395/0/ 331
1.03
2
R
1
= 0.0287
wR
= 0.0690
= 0.0336
wR
= 0.0713
− 0.009(13)
R
1
= 0.0256
wR
= 0.0671
= 0.0281
wR
= 0.0686
0.0(14)
R
1
= 0.0335
wR
= 0.0801
= 0.0393
wR
= 0.0828
0.005(19)
R
1
= 0.0402
wR
= 0.1016
= 0.0478
wR
= 0.1059
R
1
= 0.0333
wR
= 0.0868
= 0.0379
wR
= 0.0906
R
1
= 0.0713
wR
= 0.1600
= 0.0800
wR
= 0.1726
-0.04(3)
2
2
2
2
2
2
R indices (all data)
R
1
R
1
R
1
R
1
R
1
R
1
2
2
2
2
2
2
Flack parameter
− 0.003(18)
− 0.019(18)
−
3
−3
−3
−3
−3
−3
Largest difference in peak
− 0.284 e Å and
− 0.275 e Å
0.317 e Å and
0.562 e Å and
1.067 e Å and
0.483 e Å and
1.2 e Å and
−
3
−3
−3
−3
−3
−3
−3
and hole (e Å )
− 0.177 e Å
− 0.498 e Å
− 0.389 e Å
− 0.529 e Å
− 0.71 e Å
Fig. 2 The molecular structures of VO
2
L
-val, VO
2
D
-val, VO
2
L
-thr and VO -thr complex anions with
2
D
displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of arbitrary
radius.
Fig. 3 The molecular structures of VO
2
L
-leu and VO -his complex anions with displacement ellipsoids
2
D
drawn at the 50% probability level. H atoms are shown as spheres of arbitrary radius.

Table 2 Selected bond lengths (Å), angles (°) and structural parameters.
VO2L-val
VO2D-val
VO2L-leu
VO2L-ile [60]
CSS
VO2L-his
VO2L-thr
VO2D-thr
Absolute
configuration
CS
AR
CS
CS
ASR
ARS
[
a]
V1‒O1
V1‒O2
V1‒O3
V1‒O4
V1‒N1
1.616(6)
1.631(1)
1.994(1)
1.934(1)
2.158(1)
110.40(7)
117.32(6)
131.95(6)
0.40
1.618(1)
1.632(1)
1.992(1)
1.933(1)
2.160(1)
110.44(6)
117.28(6)
131.94(5)
0.40
1.614(2)
1.634(2)
1.995(2)
1.933(2)
2.154(2)
110.8(1)
114.44(9)
134.31(8)
0.35
1.619(1)
1.631(1)
1.997(1)
1.935(1)
2.157(1)
110.15(6)
119.06(5)
130.42(5)
0.44
1.618(2)
1.636(2)
1.992(1)
1.918(2)
2.148(2)
109.52(9)
116.51(8)
133.41(8)
0.39
1.624(2)
1.624(2)
1.972(2)
1.929(1)
2.154(2)
109.93(9)
113.93(7)
135.66(8)
0.30
1.631(3)
1.621(3)
1.977(3)
1.932(2)
2.154(3)
110.2(1)
115.3(1)
134.2(1)
0.34
O1‒V1‒O2
N1‒V1‒O1
N1‒V1‒O2
[
b]
τ
[c]
Average θ
105.96
105.96
105.77
106.02
105.50
106.26
106.2
Displacement of
V atom from the
0.4741(3) Å 0.4732(2) Å 0.4673(4) Å 0.4799(2) Å
0.4704(4) Å 0.4885(3) Å 0.4764(6)
tetragonal pseudoplane
towards the oxido ligand (O1)
[
a]
The absolute configuration is composed of the stereodescriptor A/C for the vanadium atom and the stereodescriptor R/S for the α-carbon (and β-carbon in
isoleucine and threonine) of the amino acid moiety.
For coordination number 5 the τ parameter is calculated as follows: τ = (β − α)/60; α = N1‒V1‒O2, β = O3‒V1‒O4. The geometry of the polyhedron adopts the
[
b]
structure of trigonal bipyramid for τ = 1 or square-base pyramid for τ = 0 [64].
[
c]
Pyramidalization in square-planar geometries is quantified by means of the value of the angle θ between the apical and equatorial ligands (O1‒V1‒O2,
O1‒V1‒O3, O1‒V1‒O4, O1‒V1‒N1) [64].
In tetragonal pyramidal complexes the stereodescriptors C/A are used to determine the absolute
configuration of the central atom [65]. The polyhedron is oriented so that the viewer looks along the formal
4
C axis, from the axial ligand (O1) towards the central atom (V1). The priority numbers of the ligating atoms
in the perpendicular plane are then considered. The procedure for assigning priorities in mononuclear
coordination systems is based on the standard sequence rules developed for chiral carbon compounds by
Cahn, Ingold and Prelog (CIP rules) [66, 67]. The structure is assigned the symbol C or A according to
whether the clockwise (C) or anticlockwise (A) sequence of priority numbers is lower at the first point of
difference. In the complexes reported herein the following priority numbers were assigned: 1 O2, 2 O3, 3
O4, 4 N1. The symbol C or A was chosen depending on the sequence of atoms O2, O3. It is clear that the
orientation of the cis-dioxidovanadium group attached to the vanadium atom determines the absolute
configuration at the metal centre (Fig. 2).
The configuration on the vanadium atom in VO2L-val, VO2L-leu, VO2L-his is C (Fig. 2 and Fig. 3), as has
been found in the analogical VO2L-ile complex [60]. Crystallization of a racemic mixture VO2DL-val
provided well developed crystals suitable for X-ray structure analysis, which has shown that the racemate
crystallizes as a conglomerate incorporating each enantiomer in a separate crystal. The crystal containing D-
valine derivative was picked from the mixture of crystals by chance (the rod-like shape of the crystals did
not allow analysis of the crystal shapes) and the crystal structure of VO2D-val has been solved in that
manner. The configuration on the vanadium atom in VO2D-val is A as expected. However, the threonine
derivatives VO2L-thr and VO2D-thr (Fig. 2) showed surprisingly the opposite configuration on the
vanadium atom in respect to the configuration on the alpha carbon (C2) of the amino acid site in comparison
to other complexes. VO2L-thr, derived from L-threonine (2S,3R), has the configuration on the vanadium
atom A, and its enantiomer, VO2D-thr, derived from D-threonine (2R,3S), has the configuration on the
vanadium atom C.
3
.2 Electronic and vibrational circular dichroism
Thanks to the long-term stability of prepared dioxidocomplexes in CH
3
CN solution (over 48 h) it was
possible to collect reliable electronic and vibrational circular dichroism spectra (ECD and VCD). It is
5
1
important to check the stability of complexes by V NMR prior to measurement of these spectra, because
even partial decomposition may significantly influence the final spectrum measured because of the different
5
1
molar extinction coefficients of the free (dissociated) and coordinated Schiff base. The V NMR chemical
shifts of all compounds lie in the region typical for dioxido vanadium(V) complexes and are very similar

being −534 ppm for VO2L-val, VO2L-thr and VO2D-thr, and −535 ppm for VO2L-leu, VO2L-his,
compared to −539 for VO2L-ile [60].
The previously prepared and reported VO2L-ile complex was taken as a reference. We have shown that the
bands in ECD and VCD spectra of this complex anion originate mainly in the chelate ring around the
vanadium atom and the benzene ring of the N-salicylidene moiety [60] (in the observable regions). The
3
electronic spectra of VO2L-ile, VO2L-val, VO2L-leu, VO2L-his, VO2L-thr and VO2D-thr in CH CN
solution show ligand bands in the region 200‒420 nm (Fig. 4). The broad band with maximum at 370 nm
belongs to HOMO→LUMO transition (predominantly π→π* character) and has practically no origin at the
metal site. The bands at lower wavelengths (227, 260 nm) correspond to n→π* and π→π* transitions with a
small contribution of LMCT transitions [60].
−3
3
Fig. 4 UV-visible absorption spectra of the prepared compounds in CH CN. Concentrations (mol·dm ):
−
5
−5
−5
−5
VO2L-val 2.1666 × 10 , VO2L-leu 2.2196 × 10 , VO2L-thr 2.1953 × 10 , VO2D-thr 2.1588 × 10 ,
−
5
−5
VO2DL-thr 2.3783 × 10 , VO2L-his 2.4245 × 10 .
The sign of the Cotton effect in electronic CD spectra has been empirically found to reflect the configuration
of the five-membered chelate ring of a Schiff base complex [68]. Thus, S configuration on the alpha carbon
of the amino acid induces the same shape of ECD spectra (Fig. 5) of L-amino acid derived complexes
although the configuration on the vanadium atom in VO2L-thr is different from the other complexes.
Significantly lower ∆ε values for VO2L-thr and VO2D-thr (+/− 5 vs. − 6.5 for HOMO→LUMO transition)
and for VO2L-his may be attributed to small geometric variations of the complexes in solution. However,
formation of diastereomers of the complexes indistinguishable by NMR in solution may also contribute to
the final intensity of measured ECD spectrum. Although the chemical shift difference for diastereomers
where one of the centers of chirality is vanadium is commonly 5 to 15 ppm [53], we have shown that
−
chemical shifts of the diastereomers of [VO
2
(N-salicylidene-isoleucinato)] differ in 1 ppm [60]. VO2DL-
thr showed no signals in ECD spectrum confirming that the compound behaves as a racemate.

−3
3
Fig. 5 ECD spectra of the prepared compounds in CH CN. Concentrations (mol·dm ): VO2L-ile 2.0585 ×
−
5
−5
−5
−5
−5
1
0 , VO2L-val 2.1666 × 10 , VO2L-leu 2.2196 × 10 , VO2L-thr 2.1953 × 10 , VO2D-thr 2.1588 × 10 ,
−5
−5
VO2DL-thr 2.3783 × 10 , VO2L-his 2.4245 × 10 .
Vibrational circular dichroism (VCD) is one of the most reliable methods for determination of absolute
configuration in solution [69, 70]. During recent years the absolute configurations of several metal
complexes has been determined by VCD [71‒81]. VCD spectra of complexes VO2L-ile, VO2L-val and
VO2L-leu derived from branched-chain amino acids exhibit the same bands in principle (Fig. 6). The bands
present in the observable region correspond mainly to coupled C‒H bending vibrations (CH groups on
α‒carbon and in the benzene ring) with a contribution of C‒O stretching vibration. VO2L-thr and VO2D-thr
−1
exhibit an additional band at 1330 cm which may be attributed to the C‒H bending vibrations (CH group
on β-carbon) [60]. Interestingly, the sign of VCD spectrum of the VO2L-thr enantiomer is in an agreement
with other complexes derived from L-amino acids as has been observed in ECD spectra. It is obvious that
the electronic transitions as well as vibrations observable in the infrared spectra in the region 1200‒1600
−1
cm originate mainly in the organic ligand and thus the configuration on the vanadium atom, despite of

being incorporated in the chelate ring, has practically no influence on the shape of ECD and VCD spectra.
The VO -his complex showed no VCD signal above the background noise level.
2
L
−
3
Fig. 6 VCD spectra of the prepared compounds in CD
3
CN. Concentrations (mol·dm ): VO -his 8.08 ×
2
L
−2
−2
−2
−2
−2
1
8
0 , VO
.32 × 10 .
2
L
-ile 6.98 × 10 , VO
2
L-val 7.16 × 10 , VO
2
L
-leu 6.80 × 10 , VO
2
L
-thr 8.32 × 10 , VO
2
D-thr
−2
4
Conclusions
Five new chiral dioxidovanadium(V) complexes with Schiff base ligands derived from
L-valine, L-leucine,
L
-histidine, -threonine and -threonine and salicylaldehyde with tetrabutylammonium cation have been
L
D
1
51
prepared. The complexes have been characterized by IR, UV–VIS, H and V NMR, X-ray structure
analysis as well as electronic and vibrational circular dichroism. Moreover, crystallizations from systems
containing DL-valine and DL-threonine provided evidences of conglomerate crystallizations and the crystal
1
structure of
D
-valine-derived complex has been solved. H NMR studies of the solutions of threonine-
derived complexes pointed to partial decomposition of the ligand and no epimerization of the amino acid,
although in previously described isoleucine-derived complexes possessing two stereogenic centres at the
same positions as threonine, vanadium catalyzed epimerization of the amino acid was observed in a
significant rate.
The chirality of the prepared compounds has been discussed deeply. It seems that in general the chirality
mode on the vanadium atom is not induced by the amino acid. While the complex anions VO
val, VO -leu, VO -his possess C configuration on the vanadium atom and S configuration on the α-
carbon of the amino acid, VO -thr possess A configuration on the vanadium atom although the
configuration on the α-carbon of the amino acid is S as well. The absolute configurations of the complexes
2
L-ile, VO
2
L-
2
L
2
L
2
L

were determined by electronic and vibrational circular dichroism in solution as well. These methods were
found to be less informative regarding the absolute configuration on vanadium atom, since all complexes
possessing S configuration on the α-carbon showed the same sign pattern. Although we observed decreased
magnitudes in ECD and VCD spectra of VO2L-thr and VO2D-thr, this might be attributed not only to
opposite configuration on the vanadium atom, but more probably to the influence of slightly different
geometries of the complexes. This is supported by the significantly lower intensities of ECD spectrum and
no VCD signal of VO2L-his complex, which possesses the same configuration on chiral atoms as VO2L-ile,
VO2L-val and VO2L-leu.
Acknowledgements
This work was supported by the Grant Agency of the Ministry of Education of the Slovak Republic and
Slovak Academy of Sciences VEGA project no. 1/0336/13.
Appendix A. Supplementary data
CCDC 1005224-1005229 contain the supplementary crystallographic data for VO2L-val, VO2D-val, VO2L-
leu, VO2L-his, VO2L-thr and VO2D-thr. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2, IEZ, UK; fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk.
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Graphical abstract