Vanadium(v) complexes of a tripodal ligand, their characterisation and biological implications

Article abstract of DOI:10.1039/c5dt02716k

The reaction of the tripodal tetradentate dibasic ligand 6,6′-(2-(pyridin-2-yl)ethylazanediyl)bis(methylene)bis(2,4-di-tert-butylphenol), H₂L¹I, with [V^IVO(acac)₂] in CH₃CN gives the V^VO-complex, [V^VO(acac)(L¹)] 1. Crystallisation of 1 in CH₃CN at ～0 °C gives dark blue crystals of 1, while at room temperature it affords dark green crystals of [{V^VO(L¹)}₂μ-O] 3. Upon prolonged treatment of 1 in MeOH, [V^VO(OMe)(MeOH)(L¹)] 2 is obtained. All three complexes were analysed by single-crystal X-ray diffraction, depicting a distorted octahedral geometry around vanadium. In the reaction of H₂L¹ with V^IVOSO₄ partial hydrolysis of the tripodal ligand results in the elimination of the pyridyl fragment of L¹ and the formation of H[V^VO₂(L²)] 4 containing the ONO tridentate ligand 6,6′-azanediylbis(methylene)bis(2,4-di-tert-butylphenol), H₂L²II. Compound 4, which was not fully characterised, undergoes dimerization in acetone yielding the hydroxido-bridged [{V^VO(L²)}₂μ-(OH)₂] 5 having a distorted octahedral geometry around each vanadium. In contrast, from a solution of 4 in acetonitrile, the dinuclear compound [{V^VO(L²)}₂μ-O] 6 is obtained, with a trigonal bipyramidal geometry around each vanadium. The methoxido complex 2 is successfully employed as a functional catechol-oxidase mimic in the oxidation of catechol to o-quinone under air. The process was confirmed to follow a Michaelis-Menten type kinetics with respect to catechol, the V_max and K_M values obtained being 7.66 × 10^-6 M min^-1 and 0.0557 M, respectively, and the turnover frequency is 0.0541 min^-1. A similar reaction with the bulkier 3,5-di-tert-butylcatechol proceeded at a much slower rate. Complex 2 was also used as a catalyst precursor for the oxidative bromination of thymol in aqueous medium. The selectivity shows quite interesting trends, namely when not using excess of the primary oxidizing agent, H₂O₂, the para mono-brominated product corresponds to ～93% of the products and no dibromo derivative is formed.

Full text of DOI:10.1039/c5dt02716k

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Table of Contents Synopsis
Vanadium (V) complexes of a tripodal ligand, their characterisation and biological
implications
M.R. Maurya, B. Uprety, F. Avecilla, P. Adão, and J. Costa Pessoa
Different vanadium(V) complexes with 6,6'–(2–(pyridin–2 yl)ethylazanediyl)bis(methylene)
bis(2,4–di–tert–butylphenol) (H2L1) are isolated, characterized and one of them,
[VVO(OMe)(MeOH)(L1)], is used as catalyst precursor as functional mimic of catechol–
oxidase and for the oxidative bromination of thymol.
1

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DOI: 10.1039/C5DT02716K
Vanadium (V) complexes of a tripodal ligand, their characterisation and biological
implications†
Mannar R. Maurya,*a Bhawna Uprety,a Fernando Avecilla,b Pedro Adão,c and J. Costa Pessoa*c
a Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee–247667, India. E–
mail: rkmanfcy@iitr.ernet.in; Fax: +91 1332 273560; Tel: +91 1332 285327
b
Departamento de Química Fundamental, Universidade da Coruña, Campus de A Zapateira,
15071 A Coruña, Spain
c Centro de Química Estrutural, Instituto Superior Técnico, Universidade Lisboa, 1049–001
Lisboa, Portugal. E–mail: joao.pessoa@ist.utl.pt; Fax: +351 21 8464455; Tel: +351 21
8419268
Abstract
The reaction of the tripodal tetradentate dibasic ligand 6,6'–(2–(pyridin–2–
yl)ethylazanediyl)bis(methylene)bis(2,4–di–tert–butylphenol), H2L1 I, with [VIVO(acac)2] in
CH3CN gives the VVO–complex, [VVO(acac)(L1)] 1. Crystallisation of 1 in CH3CN at ~0 ºC,
gives dark blue crystals of 1, while at room temperature it affords dark green crystals of
[{VVO(L1)}2µ–O] 3. Upon prolonged treatment of 1 in MeOH [VVO(OMe)(MeOH)(L1)] 2 is
obtained. All three complexes are analysed by single–crystal X–ray diffraction, depicting
distorted octahedral geometry around vanadium. In the reaction of H2L1 with VIVOSO4 partial
hydrolysis of the tripodal ligand results in elimination of the pyridyl fragment of L1 and the
formation of H[VVO2(L2)] 4, containing the ONO tridentate ligand 6,6'–
azanediylbis(methylene)bis(2,4–di–tert–butylphenol), H2L2 II. Compound 4, which was not
fully characterised, undergoes dimerization in acetone yielding the hydroxido–bridged
[{VVO(L2)}2µ–(HO)2] 5, having distorted octahedral geometry around each vanadium. In
contrast, from a solution of 4 in acetonitrile, the dinuclear compound [{VVO(L2)}2µ–O] 6 is
obtained, with trigonal bipyramidal geometry around each vanadium. The methoxido
complex 2 is successfully employed as a functional catechol–oxidase mimic in the oxidation
of catechol to o–quinone under air. The process is confirmed to follow a Michaelis–Menten
type kinetics with respect to catechol, the Vmax and KM values obtained being 7.66×10–6
M
min1 and 0.0557 M, respectively, and the turnover frequency is 0.0541 min–1. The similar
reaction with the bulkier 3,5–di–tert–butylcatechol proceeds at much slower rate. Complex 2
is also used as a catalyst precursor for the oxidative bromination of thymol in aqueous
medium. The selectivity shows quite interesting trends, namely when not using excess of
2

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primary oxidizing agent, H2O2: the para mono–brominated product corresponds to ~93 % of
the products and no dibromo derivative is formed.
__________________________________
† Electronic supplementary information (ESI) available: Details of 51V NMR and other
spectral studies. CCDC 1413240-1413244. For ESI and crystallographic data in CIF or other
electronic format see DOI: $$$$$.
Introduction
The understanding of the role of vanadium in biological systems and its possible use in
therapeutics has gathered increasing attention over the recent years. One of the most explored
areas is the role of vanadium in decreasing blood glucose levels, where vanadium compounds
are known to mimic most of the actions of insulin by inhibiting protein tyrosine phosphatases
(PTPs).1 Additionally, several vanadium compounds have also been successfully tested for
their antitrypanosomal, antileishmanial, antiamoebic and DNA binding activities.1(a), 2
On the other hand, a similarly relevant field of interest in vanadium chemistry is
based on the catalytic potential of vanadium compounds, namely for oxidation, oxidative
halogenations and sulfoxidations of a variety of organic substrates.3 Haloperoxidases are
peroxidases that are able to catalyse the oxidation of halides by hydrogen peroxide, thereby
achieving the halogenation of organic substrates.1(f),4 Several structural and functional models
of vanadium haloperoxidases (VHPOs) have been prepared and tested, and most of these
compounds contain VIVO/VVO and/or VVO2 units coordinated to O,N,O/S donor ligands.
Recently vanadium compounds, predominantly with tripodal ligands, have also been
employed in the catalysis of olefin polymerization. These vanadium catalysts display
excellent thermal stability and generate high molecular mass poly–olefins with narrow range
of molecular mass distribution.5 Such types of VV–complexes with tetradentate tripodal
ligands exhibit interesting structural and solution phase properties.6 They have also been
applied for several catalytic oxidations.7,8 In contrast, reports on vanadium based catechol
oxidase mimics have seldom appeared.9
Catechol oxidase is an enzyme that catalyses the oxidation of phenols such as
catechol. It is a copper–containing enzyme whose activity is similar to that of tyrosinase, a
related class of copper oxidases. The enzyme contains the so–called type–3 Cu binding site,
where dopa (3-hydroxy-L-tyrosine) is oxidized to dopa quinone coupled with 2e/2H+
reduction of O2 to H2O. The generated dopa quinone is further oxidized, in a four electron
3

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process, to indolquinone. Indolquinone undergoes auto–polymerization producing a brown
polyphenolic pigment, i.e., melanin, a process which is considered to protect damaged tissues
against pathogens or insects.10 The active site contains the {Cu(His)3}2-O core, where two
of the CuII–centers are coordinated to three histidine N–atoms and bridged by an external
hydroxide moiety in a trigonal pyramidal geometry.11-13 Several Cu based models have been
prepared and studied for the establishment of structure/activity relationships.14
Notwithstanding, there are numerous reports in literature where functional models with
Co(II/III), Mn(III/II), Fe(III), Ni(II), Zn(II) centers, instead of Cu(II), have been used to
achieve moderate to excellent conversion of catechol to o–quinone.15
Ramadan et al. reported a series of VIVO–picolyl hydrazones as functional catechol
oxidase models in the presence of a base at room temperature.9 Similar results, but at elevated
temperature, were reported by Hu et al. by using a V–polyoxidometalate.16 In another report
GdIII/VIVO cryptate was used to mimic catechol oxidase action.17 However, to the best of our
knowledge, there have been no reports of VV–complexes acting as catechol oxidase
functional mimics. Recently some of us reported a polymer supported vanadium based
polyphenol oxidase mimicking system for the oxidation of pyrogallol.18 Herein, we report the
synthesis and characterization of oxido/oxidomethoxido VV–compounds, based on a tripodal
ligand derived from aminoethylpyridine, formaldehyde and 2,4–di–tert–butylphenol using a
modified Mannich condensation. Our attempts towards isolation of VIV–complexes led to the
hydrolysis of the tripodal amine, leaving an O,N,O donor tridentate amine coordinated to a
VV–centre. Some of these complexes mimic the action of catechol oxidase at room
temperature, oxidising catechol to its quinone, without the use of base.
Experimental Section
Materials and general methods
V2O5 (Loba Chem, India), 2–aminoethylpyridine (Sigma Aldrich, USA), acetyl acetone
(SRL, India), formaldehyde (Rankem, India), 2,4–di–tert–butylphenol (Himedia, India), 30%
aqueous H2O2 (Qualigens, India), catechol (SDFCL, India), thymol (SDFCL, India) were
used as obtained. Other chemicals and solvents were of AR grade. The precursor complex
[VIVO(acac)2] was prepared following a reported method.19
Elemental analyses of the complexes were carried out on an Elementar model Vario–EI–III
after drying the samples at 100 ˚C. IR spectra were recorded as KBr pellets on a Nicolet 1100
FT–IR spectrometer. Electronic spectra of complexes were recorded in chloroform or
4

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1
dichloromethane on a Shimadzu 1601 UV–Vis spectrophotometer. H NMR and 13C NMR
51
spectra were recorded in CDCl3 on a Bruker Avance 500 MHz spectrometer. The V NMR
spectra were obtained either on Bruker Avance+ 400 MHz or 300 MHz Spectrometers. The
51V chemical shifts (δV) are expressed in ppm relative to neat VVOCl3, used as an external
reference sample. High resolution mass spectra (HRMS) in positive ion electrospray
ionization mass spectrometry mode were obtained on a Bruker micrOTOF–Q II mass
spectrometer in CH3CN. The electron paramagnetic resonance (EPR) spectra were measured
with a Bruker ESP 300E X-band spectrometer, in frozen samples at 77 K, using DPPH radical
as reference. The measured spectra (1st derivative X-band EPR) were simulated with the
computer program developed by Rockenbauer and Korecz.20
X–Ray crystal structure determinations
Three–dimensional X–ray data were collected on a Bruker Kappa Apex CCD diffractometer
at low temperature for 1, 2, 5 and 6 and at room temperature for 3, by the – scan method.
Reflections were measured from a hemisphere of data collected from frames, each of them
covering 0.3º in . 132448 for 1, 96907 for 2, 76068 for 3, 48486 for 5 and 22722 for 6
reflections measured were corrected for Lorentz and polarization effects and for absorption
by multi–scan methods based on symmetry–equivalent and repeated reflections. Of them,
6289 for 1, 7353 for 2, 6272 for 3, 3839 for 5 and 3408 for 6, independent reflections
exceeded the significance level (F/F) > 4.0, respectively. After data collection, in
each case an empirical absorption correction (SADABS) was applied, and the structures were
solved by direct methods and refined by full matrix least–squares on F2 data using SHELX
suite of programs.21,22 In 1 and in 3, hydrogen atoms were included in calculated position and
refined in the riding mode. In 2, hydrogen atoms were included in calculated positions and
refined in the riding mode, except for O(2M), O(3M), C(3), C(5), C(10), C(12), C(18), C(19),
C(20) and C(21), which were located in the difference Fourier map and left to refine freely.
In 5, hydrogen atoms were included in calculated positions and refined in the riding mode,
except for N(1), which was located in the difference Fourier map and left to refine freely, and
for O(1), which was located in the difference Fourier map and then fixed to the oxygen atom.
In 6, H–atoms were included in calculated positions and refined in the riding mode, except
for N(1), C(3), C(5), C(18) and C(20), which were located in the difference Fourier map and
left to refine freely. Refinements were done with allowance for thermal anisotropy of all non–
hydrogen atoms. Final difference Fourier maps showed no residual density outside: 2.200
5

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and –0.537 e.Å–3 for 1, 0.567 and –0.732 for 2, 0.498 and -0.461 for 3, 0.539 and –0.674 for 5
2
and 0.913 and –0.769 e.Å–3 for 6. A weighting scheme w = 1/[σ2(Fo ) + (0.082900 P)2 +
2
2
50.612396 P] for 1, w = 1/[σ2(Fo ) + (0.094900 P)2 + 0.702300 P] for 2, w = 1/[σ2(Fo ) +
2
(0.126300 P)2 + 0.000000 P] for 3, w = 1/[σ2(Fo ) + (0.065900 P)2 + 1.096700 P] for 5 and
2
w = 1/[σ2(Fo ) + (0.124900 P)2 + 0.000000 P] for 6, where P = (|Fo|2 + 2|Fc|2)/3, were used in
the latter stages of refinement. The crystal of 1 presents a high residual electron density next
to O(1), which has not been possible to refine. An important disorder on t–butyl groups
appear in the crystal of 3. These disorders were refined and two atomic sites have been
observed and refined with the anisotropic atomic displacement parameters for three t–butyl
groups. More specifically these disorders were refined using 274 restraints (SADI, SIMU,
DELU and DFIX restraints were used). The site occupancy factors were 0.56667 for C(27A)–
C(28A)–C(29A), 0.44182 for C(31A)–C(32A)–C(33A) and 0.83216 for C(60A)–C(61A)–
C(62A). Other disorders appear around C(6) and C(43) atoms in the ligand, which were
refined and two atomic sites were observed and refined with the anisotropic displacement
parameters for the two atoms. These disorders were refined using 82 restraints (SADI, SIMU
and DELU restraints were used). The site occupancy factors were 0.84836 for C(6A) and
0.74491 for C(43A). A disordered acetone molecule is present in the crystal packing of 5.
More specifically this disorder was refined using 79 restraints (SADI, SIMU and DELU
restraints were used). The site occupancy factor was 0.41198 for C(1SA)–C(2SA)–O(1SA).
Further details of the crystal structure determinations are given in Table 1.
6

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Table 1 Crystal Data and Structure Refinement for [VVO(acac)(L1)]·0.5CH3CN 1, for
[VVO(OMe)(MeOH)(L1)]·MeOH 2, [{VVO(L1)}2µ–O] 3, [{V(IV)O(L2)}2µ(OH)2]·2(CH3)2CO
5 and [{V(V)O(L2)}2µ–O] 6.
1·0.5CH3CN
2·MeOH
3
5·2(CH3)2CO
6
C43H60.5 N2.5 O5 V
Formula
C40 H63 N2 O6 V
C74 H104 N4 O7 V2
C66 H104 N2 O10 V2
C60 H90 N2 O7 V2
743.38
Formula weight
718.86
1263.49
293(2)
1187.39
100(2)
1053.22
100(2)
T, K
100(2)
100(2)
0.71073
Wavelength, Å
0.71073
Triclinic
P1
0.71073
Triclinic
P1
0.71073
Monoclinic
P21/c
0.71073
Monoclinic
P21/n
Crystal system
Orthorhombic
Pbcn
Space group
15.7992(10)
13.8716(8)
19.3649(12)
a/Å
b/Å
c/Å
/º
β/º
7.9065(5)
14.5215(9)
19.1809(12)
67.981(3)
82.406(4)
86.889(3)
2023.7(2)
2
10.6149(6)
15.8957(9)
23.0172(12)
80.123(3)
85.922(3)
75.312(3)
3699.6(4)
2
9.7921(19)
28.523(6)
11.769(3)
24.9968(16)
11.8900(8)
19.5013(15)
91.760(2)
93.467(15)
γ/º
V/Å3
Z
3280.9(12)
2
5796.0(7)
4
4242.0(4)
4
1596
F000
776
1356
1280
2264
calc/g cm–3
1.180
1.134
1.202
1.207
1.164
D
/mm–1
0.290
0.304
0.341
0.374
0.278
1.69 to 26.46
0.1098
/ (º)
1.15 to 27.81
0.0672
0.90 to 28.06
0.1012
2.08 to 29.25
0.1841
1.63 to 25.51
0.1019
Rint
Crystal size/ mm3
Goodness–of–fit on F2
R1[I>2(I)] a
wR2 (all data) b
0.47× 0.36 × 0.32
1.041
0.29 × 0.21 × 0.16 0.25 × 0.13 × 0.05
0.35 × 0.21 × 0.19
1.055
0.30 × 0.23 × 0.07
1.156
0.943
1.025
0.1032
0.0771
0.0753
0.0502
0.0851
0.3247
0.2632
0.2032
0.1573
0.2319
2.200 and -0.537
0.498 and -0.461
0.539 and -0.674
Largest differences peak and
hole (eÅ–3)
0.567 and –0.732
0.913 and –0.769
2 2
aR1 = Fo – Fc/Fo. bwR2 = [w(Fo2 –Fc2)2]/[w(Fo ) ]1/2
7

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Synthetic procedures
6,6'–(2–(pyridin–2–yl)ethylazanediyl)bis(methylene)bis(2,4–di–tert–butylphenol), H2L1
I. The compound was prepared following a literature method with slight modifications.23
A
solution of 2–aminoethylpyridine (2.440 g, 20.0 mmol), formaldehyde 30% (3.076 g, 40.0
mmol) and 2,4–di–tert–butylphenol (8.254 g, 40.0 mmol) was refluxed in MeOH (30.0 mL)
for 12 h. Upon cooling to room temperature, a white solid precipitated out, which was
filtered, washed with MeOH and dried in vacuum. Yield: 5.281 g (47%). (Found: C, 79.93;
H, 9.47; N, 5.03; C37H54N2O2 (558.84 g mol−1) requires, C, 79.52; H, 9.74; N, 5.01).
[VVO(acac)(L1)] 1. A solution of [VIVO(acac)2] (0.265 g, 1.0 mmol) in acetonitrile (10.0
mL) was added to a solution of I (0.559 g, 1.0 mmol) in hot acetonitrile (30.0 mL). The
resulting solution was refluxed in a water bath for 8 h, affording a dark blue solution. After
reducing the solvent volume to ca. 15 mL and cooling to room temperature, a black
crystalline product slowly separated. It was filtered, washed with cold MeCN and dried in a
vacuum desiccator over silica gel. Crystals of 1·0.5CH3CN suitable for X–ray diffraction
(XRD) analysis were grown by slow evaporation of an acetonitrile solution at 4 ˚C in a
refrigerator. Yield: 0.585 g (81%). (Found: C, 69.50; H, 7.94; N, 3.58; C42H59N2O5V (722.87
g mol−1) requires, C, 69.78; H, 8.23; N, 3.88). IR υmax/ cm–1: 2957, 2906, 2866, 1584, 1525,
1
1470, 1436, 1360, 1246, 1170, 959 (V=O), 853, 759, 580, 481. H NMR (CDCl3)/ : 8.44–
8.42 (d, 1H, Py), 7.51–7.47 (m, 1H, Ar), 7.31–7.30 (d, 2H, Ar), 7.06–7.05 (m, 3H, Ar), 6.87–
6.85 (d, 1H, Ar), 5.60 (s, 1H, –CH=), 3.44–3.41 (d, 2H, NCH2Ar), 2.92 (s, 2H, NCH2Ar), 2–
2.05 (d, 2H, NCH2CH2), 2.25(s, 2H, NCH2CH2), 1.93 (s, 6H, –CH3), 1.31(s, 18H, C(CH3)3)
1.57(s, 18H, –C(CH3)3).
[VVO(OMe)(MeOH)(L1)] 2. A solution of [VVO(acac)(L1)] 1 (0.723 g, 1.0 mmol) in MeOH
(40.0 mL) was heated under reflux for 3 days with stirring. The resulting solution was cooled
and kept in air for slow precipitation of the black crystalline compound. It was filtered,
washed with cold MeOH and dried in a vacuum desiccator over silica gel. Crystals of
2·MeOH suitable for X–ray diffraction analysis were grown by slow evaporation of its
MeOH solution. Yield: 0.515 g (75%). (Found: C, 68.50; H, 8.85; N, 4.15; C39H59N2O5V
(686.84 g mol−1) requires, C, 68.20; H, 8.66; N, 4.08). IR υmax/ cm–1: 3412, 2954, 2903, 2867,
1474, 1442, 1361, 1236, 1204, 1169, 1049, 1024 (V=O), 1008, 843, 756, 621, 598, 559.
[{VVO(L1)}2µ–O] 3. Method A: A solution of [VVO(OMe)(MeOH)(L1)] 2 (0.687 g, 1.0
mmol) in acetone (40.0 mL) or MeCN was heated under reflux for 24 h. The resulting
solution was cooled at 4 ºC for ca. 2 days from where the black complex 3 slowly
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precipitated out. It was filtered, washed with cold acetone and dried in vacuum. Yield: 0.482
g (74%). (Found: C, 69.88; H, 8.0; N, 4.43; C74H104N4O7V2 (1263.52 g mol−1) requires, C,
70.34; H, 8.30; N, 4.43). IR υmax/ cm–1: 2954, 2903, 2866 (tert-butyl), 1467, 1441, 1361,
1
1279, 1238, 1170, 936 (V=O), 855 (V–µ–O–V) 758, 689, 668, 569, 476. H NMR (CDCl3)/
: 9.83 (s, 2H Py), 7.36–7.33 (t, 2H, Ar), 7.02–6.99 (t, 2H, Ar), 6.96 (s 4H, Ar), 6.86 (s, 4H,
Ar) 6.70–6.68 (d, 2H, Ar), 6.03–6.00 (d, 4H, NCH2Ar), 3.24–3.21 (d, 4H, NCH2Ar), 2.80 (s,
4H, NCH2CH2), 2.51 (s, 4H, NCH2CH2), 0.86 (s, 36H, –C(CH3)3), 1.24 (s, 36H, –C(CH3)3).
Method B: Slow evaporation of a filtered acetonitrile solution of [VVO(acac)(L1)] 1 at
room temperature produced dark green crystals of 3 in poor yields in ca. 3 days.
H[VVO2(L2)] 4. A solution of H2L1 (0.558 g, 1.0 mmol) in 20.0 mL ethanol was added to a
solution of VOSO4·5H2O (0.253 g 1.0 mmol) in aqueous–ethanol (1:4, 5.0 mL). The resulting
solution was refluxed for 8 h to obtain a dark brown solution. After reducing the solution
volume to ca. 10 mL and keeping at room temperature, a dark brown solid slowly
precipitated. It was filtered, washed with cold ethanol and dried in vacuum. Attempts to
obtain adequate crystals of this compound were not successful. Yield: 0.274 g (51%). (Found:
C, 67.24; H, 8.14; N, 2.60; C30H44NO4V (535.63 g mol−1) requires, C, 67.27; H, 8.66; N,
2.61). No satisfactory 1H or 51V NMR could be obtained for this compound. The samples had
a very weak EPR signal, due to the presence of a small amount of a VIV-species, probably an
impurity (see below). IR υmax/ cm–1 (see Fig. S1): 3448 (OH), 3181 (NH), 2955, 2913, 2867
(tert-butyl), 1467, 1440, 1357, 1239, 1168, 1130, 1047, 994, 969 (VO2), 872, 854, 759, 645,
596.
[{VVO(L2)}2µ(OH)2] 5. Method A: A solution of 4 (0.200 g, 0.37 mmol) in acetone (40.0
mL) was heated under reflux for 3 h. The resulting solution was cooled at 4 ˚C for 2 days,
where black crystals of 5·2(CH3)2CO were slowly formed. The solid was filtered, washed
with cold acetone, and dried in a vacuum desiccator over silica gel. Yield: 0.187 g (89%).
(Found: C, 67.06; H, 8.63; N, 2.43; C60H92N2O8V2 (1071.26 g mol−1) requires, C, 67.27; H,
8.66; N, 2.61). For X–ray diffraction single crystals of 5·2(CH3)2CO were used as such. IR
υ
max/ cm–1 (see Fig. S2): 3568, 3433, 3259, 2955, 2904, 2866, 1458, 1362, 1239, 1203, 1171,
957 (V=O), 870 (V–µ–O–V), 777, 756, 603, 586. 1H NMR (CDCl3)/ : 7.31 (s, 1H, Ar), 7.27
(s, 2H, Ar), 7.05 (s, 4H, Ar), 7.00 (s, 1H, Ar), 4.01–3.98 (d, 3H, NCH2Ar), 3.47–3.42 (t, 3H,
NCH2Ar), 3.23 (s, 1H, NCH2Ar), 3.06 (s, 1H, NCH2Ar), 1.41–1.42 (d, 36 H, –C(CH3)3),
1.28–1.31 (d, 36H, –C(CH3)3), 4.20 (br s, 1H, NH), 4.13 (br s, 1H, NH).
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Method B: A solution of H2L1 (0.558 g, 1.0 mmol) in acetonitrile (20.0 mL) was added to a
solution of VIVOSO45H2O (0.253 g, 1.0 mmol) in aqueous:acetonitrile (1:4, 5.0 mL). The
resulting solution was refluxed for 8 h to obtain a dark coloured solution. It was neutralized
by drop wise addition of 1 equivalent of sodium hydroxide, whereupon a black solid
precipitated. It was filtered, washed with cold acetone and dried in a vacuum desiccator over
silica gel. Yield: 63%.
[{VVO(L2)}2µ–O] 6. A solution of 4 (0.100 g, 0.19 mmol) in MeCN (40.0 mL) was heated
under reflux for 18 h. The resulting solution was maintained at 4 ˚C for ca. 2 days, from
where black crystals were obtained. The product was filtered, washed with cold acetonitrile
and dried in a vacuum desiccator over silica gel. Yield: 0.075 g (74%). (Found: C, 67.50; H,
8.18; N, 2.84; C60H90N2O7V2 (1053.56 gmol−1), including ½ H2O requires, 67.84; H, 8.63; N,
2.64). For X–ray diffraction single crystals of 6 were used as such. IR υmax/ cm–1 (see Fig.
S3): 3261, 2959, 2905, 2866, 1465, 1437, 1362, 1235, 1170, 1127, 1002 (V=O), 839 (V–µ–
O–V), 764, 723, 702, 601, 572, 540. 1H NMR (CDCl3)/ : 7.28 (s, 4H, Ar), 7.05 (s, 2H, Ar),
7.00 (s, 2H, Ar), 4.00–3.98 (d, 4H, NCH2Ar), 3.47–3.42 (m, 2H, NCH2Ar), 3.21(s, 1H,
NCH2Ar), 2.97 (s, 1H, NCH2Ar), 1.41–1.43 (d, 36H, –C(CH3)3), 1.31–1.32 (d, 36H, –
C(CH3)3), 4.58 (br s, 1H, NH), 4.19 (br s, 1 H, NH).
Catechol Oxidase Mimetic Activity
The oxidomethoxido complex 2 was chosen as representative to study the catalytic oxidation
of catechol. The reaction was carried out in normal methanol and was studied by monitoring
the increase in absorbance at 383 nm due to o–benzoquinone formation using a UV–visible
spectrophotometer for a period of 1 h. Two sets of reactions were carried out: one in
atmospheric air and the other in an atmosphere of molecular oxygen. The reaction was
initiated with the addition of a 5.0 mL solution of [VVO(OMe)(MeOH)(L1)] (3.9834×10–4 M)
in methanol to 10.0 mL of a solution of catechol (1.00×10–1 M). The steady state kinetic
experiments were carried out by varying the catechol concentration (0.0250–0.1750 M) in the
presence of [VVO(OMe)(MeOH)(L1)] (3.9834×10–4 M). The reaction was studied by mixing
1.0 mL of the solution containing the catalyst in methanol to 2.0 mL of catechol solution in
methanol, and monitoring the increase of absorbance at 383 nm (ε = 1900 M–1 cm–1 for 3,5–
di–tert–butyl quinone)11 as a function of time on a UV–Vis spectrophotometer after 3
minutes of preparation. The initial rates were calculated from the slope of the plot of
absorbance versus time, over a period of 3 minutes. The initial reaction rates were then fitted
to the Michaelis–Menten equation and Lineweaver Burk plot to calculate the parameters:
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maximal velocity (Vmax), Michaelis constant (KM) and catalytic constant or turnover number
(kcat) from nonlinear curve fit using Origin 8.0 software.
Vanadium haloperoxidase activity: catalytic oxidative bromination of thymol
The oxidomethoxido complex 2 was used to catalyse the oxidative bromination of thymol in
aqueous medium. In a typical reaction, thymol (1.500 g, 10.0 mmol), KBr (2.380 g, 20.0
mmol), H2O2 (2.267 g, 20.0 mmol) and water (20.0 mL) were taken in a 50 mL reaction
flask. The catalyst (0.0010 g) and HClO4 (0.717 g, 0.50 mmol) were added and the mixture
was stirred at room temperature. An additional 1.50 mmol of HClO4 was added in three equal
portions at intervals of fifteen minutes. After 2 h the reaction mixture was extracted with
dichloromethane, washed with brine (NaCl) solution and dried over anhydrous Na2SO4. The
reaction products were separated using a silica gel column. The products were analysed and
identified by GC, GC–MS and 1H NMR.
Results and Discussion
The reaction of the ligand H2L1 I with one equivalent of [VIVO(acac)2] in acetonitrile under
ambient conditions gave dark blue crystals of [VVO(acac)(L1)] 1 where the ligand acts as
ONO tridentate coordinating through N–imine and O–phenolate atoms in a cis fashion (vide
infra). The VIV–center is oxidised to VV during the process. A hot MeCN solution of 1, upon
cooling to 0 ˚C, yielded dark blue crystals of 1, while a suspension of 1 in MeCN at room
temperature slowly turned dark green in ca. 3 days and resulted in green crystals of an oxido
bridged dinuclear complex [{VVO(L1)}2µ–O] 3. Upon prolonged treatment of 1 with MeOH
the coordinated acac is removed and the corresponding methoxido complex,
[VVO(OMe)(MeOH)(L1)] 2, was isolated. This methoxido compound yielded 3 upon
refluxing in MeCN or acetone. Attempts made to isolate the VIV–complex of H2L1 by the
reaction of ligand with VIVOSO4 in ethanol resulted in the formation of H[VVO2(L2)] 4
(where H2L2 II is the tridentate ligand obtained upon loss of the ethylpyridine fragment of I).
The proton may either be protonating an O-phenolate, or the N-amine atom (or even the O-
oxido atom). The elemental analysis of 4 does not differ much from those of 5 and 6, but the
IR spectrum depicts some significant differences (Figs. S1 to S3), namely the absence of the
strong sharp bands at 702 and 723 cm1, present in 6. In 5 strong sharp bands are detected at
958 and 870 cm1, while in 4 sharp bands are observed at 994, 969 and 872 cm1 (medium
intensity), as well as a sharp band at 854 cm1. What we designate by 4 probably is not a pure
compound and thus we cannot consider it as fully characterized.
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Compound 4 in acetone gave black crystals of [{VVO(L2)}2µ–(OH)2]·2(CH3)2CO 5.
The effect of solvent is yet again noticeable: while the reaction of I with VIVOSO4.5H2O in
ethanol gave 4 as the only product, the same reaction in MeCN followed by base
neutralization directly yielded the dinuclear species 5 as the major product. Moreover, a
solution of 4 in acetonitrile resulted in the formation of the oxido bridged dinuclear
compound [{VVO(L2)}2µ–O] 6.
The structural formulae of the compounds prepared are summarized in Scheme 1. All
complexes are soluble in common organic solvents such as MeOH, EtOH, CH2Cl2, CHCl3,
acetone and acetonitrile. The presence of tert–butyl groups also allows solubility of these
compounds in hydrocarbon–type solvents such as hexane and heptane.
Scheme 1 Structural formulae and numbering of the compounds along with their synthetic
routes. Conditions: (i) [VIVO(acac)2]/ MeCN, ∆; (ii) MeCN/ RT; (iii) MeOH, ∆; (iv) MeCN,
∆; (v) VIVOSO45H2O/ EtOH, ∆; (vi) MeCN; (vii) Acetone; (viii) VIVOSO45H2O/ MeCN, ∆.
L2 corresponds to the ONO tridentate ligand obtained upon elimination of the ethylpyridine
moiety of H2L1.
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Description of structure of the compounds
Compound [VVO(acac)(L1)]·0.5CH3CN 1 crystallized from acetonitrile as black crystals and
Fig. 1 depicts an ORTEP representation. The asymmetric unit of 1 contains one mononuclear
complex and half acetonitrile molecule. Compound [VVO(OMe)(MeOH)(L1)]·MeOH 2
crystallized from methanolic solution as black crystals and Fig. 2 presents an ORTEP
representation. In 2 the asymmetric unit contains one mononuclear complex and one
methanol molecule. Compound 3, [{VVO(L1)}2µ–O], crystallized from acetonitrile solution
and the asymmetric unit contains two half’s of two dinuclear complexes (see Fig. 3). In the
dinuclear compound 5, [{VVO(L2)}2µ–(OH)2]·2(CH3)2CO, the ligand that binds to each
vanadium, L2, corresponds to L1 after the elimination of the aminoethylpyridine group; it
crystallized from acetone as black crystals. Fig. 4 shows the dinuclear compound, and the
asymmetric unit contains half dinuclear complex and one acetone molecule. In compound
[{VVO(L2)}2µ–O] 6 the ligand is the same as in 5 and the asymmetric unit only contains half
complex; Fig. 5 depicts a representation of this compound. Table 2 provides selected bond
lengths and bond angles for compounds 1, 2, 3, 5 and 6.
Table 2
Bond Lengths [Å] and Angles [°] for [VVO(acac)(L1)]·0.5CH3CN 1,
[VVO(OMe)(MeOH)(L1)]·MeOH 2, [{VVO(L1)}2µ–O] 3, [{VVO(L2)}2µ(OH)2]·2(CH3)2CO 5
and [{VVO(L2)}2µ–O] 6.
Bond lengths
1
2
3
5
6
V(1)-O(1)
V(1)-O(2)
V(1)-O(3)
V(1)-O(4)
V(1)-O(5)
V(1)-O(1)#1
V(1)-O(1M)
V(1)-O(2M)
V(1)-N(1)
V(1)-N(2)
V(2)-O(5)
V(2)-O(6)
V(2)-O(7)
V(2)-O(8)
V(2)-N(3)
V(2)-N(4)
1.594(4)
1.839(4)
1.817(4)
1.989(4)
2.008(5)
1.5884(14)
1.8455(14)
1.8548(14)
1.843(3)
1.856(3)
1.606(3)
1.7932(8)
1.965(3)
1.836(3)
1.600(3)
1.844(3)
1.832(3)
1.859(3)
1.583(3)
1.7896(9)
1.991(3)
2.350(4)
1.8187(14)
2.2834(15)
2.3472(16)
2.452(5)
2.395(3)
2.328(4)
1.851(3)
1.843(3)
1.603(3)
1.7941(7)
2.386(3)
2.337(4)
2.149(4)
Angles
1
2
3
5
6
O(1)–V(1)–O(3)
98.5(2)
100.28(7)
96.34(14)
97.13(13)
109.46(15)
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O(1)–V(1)–O(2)
O(3)–V(1)–O(2)
O(1)–V(1)–O(4)
O(3)–V(1)–O(4)
O(2)–V(1)–O(4)
O(1)–V(1)–O(5)
O(3)–V(1)–O(5)
O(2)–V(1)–O(5)
O(4)–V(1)–O(5)
O(1)–V(1)–N(1)
O(3)–V(1)–N(1)
O(2)–V(1)–N(1)
O(4)–V(1)–N(1)
O(5)–V(1)–N(1)
O(3)–V(1)–O(1)#1
O(2)–V(1)–O(1)#1
O(4)–V(1)–O(1)#1
O(1)–V(1)–O(1)#1
V(1)–O(4)–V(1)#2
O(1)#1–V(1)–N(1)
V(1)–O(1)–V(1)#1
O(1)–V(1)–O(1M)
O(1M)–V(1)–O(2)
O(1M)–V(1)–O(3)
O(1)–V(1)–O(2M)
O(1M)–V(1)–O(2M)
O(2)–V(1)–O(2M)
O(3)–V(1)–O(2M)
O(1M)–V(1)–N(1)
O(2M)–V(1)–N(1)
O(3)–V(1)–N(2)
O(4)–V(1)–N(2)
O(1)–V(1)–N(2)
O(2)–V(1)–N(2)
N(2)–V(1)–N(1)
O(7)–V(2)–O(8)
O(7)–V(2)–O(6)
O(8)–V(2)–O(6)
O(7)–V(2)–O(5)
O(8)–V(2)–O(5)
O(6)–V(2)–O(5)
O(7)–V(2)–N(4)
O(8)–V(2)–N(4)
O(6)–V(2)–N(4)
O(5)–V(2)–N(4)
O(7)–V(2)–N(3)
O(8)–V(2)–N(3)
O(6)–V(2)–N(3)
O(5)–V(2)–N(3)
N(4)–V(2)–N(3)
100.6(2)
99.2(2)
96.3(2)
88.66(18)
160.01(19)
94.2(2)
165.91(19)
84.54(19)
83.73(19)
176.2(2)
82.05(18)
82.96(18)
79.93(17)
84.94(17)
100.04(7)
154.98(6)
161.36(13)
96.48(14)
95.59(10)
100.97(12)
95.19(11)
93.85(12)
97.47(13)
157.28(12)
98.65(13)
100.17(13)
140.34(13)
105.25(15)
92.66(10)
107.06(16)
95.03(14)
90.73(6)
82.56(6)
82.73(6)
82.24(12)
170.83(15)
82.94(12)
88.19(9)
81.45(12)
178.38(14)
81.88(13)
82.94(12)
81.31(14)
93.84(15)
77.77(13)
159.06(15)
98.20(13)
160.62(12)
88.68(12)
72.94(13)
82.14(12)
107.06(13)
100.75(7)
95.42(6)
94.98(6)
174.62(7)
84.63(6)
79.53(6)
78.87(6)
168.51(6)
83.89(6)
86.78(15)
172.26(10)
83.52(14)
83.75(15)
84.06(13)
101.94(11)
96.37(13)
96.73(9)
95.53(14)
95.01(10)
161.18(13)
86.39(15)
171.58(11)
83.38(14)
82.87(14)
169.87(13)
88.18(8)
82.35(12)
83.37(13)
83.49(13)
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Symmetry transformations used to generate equivalent atoms:
#1 –x,–y,–z+1
#2 –x+2,y,–z+1/2
The molecular structure of 1 consists on a six–coordinated mononuclear complex with
a distorted octahedral geometry. The acac ligand normally binds strongly and the pyridine
moiety does not participate in complex formation at least partly due to the oxophilicity of the
VV.24 The axial sites of the octahedron are occupied by the oxido O–atom and by the
bridgehead N–amine atom [V1–O1, 1.594(4) Å, V1–N1, 2.452(5) Å]. The remaining
coordinating atoms approximately form a plane [O2–O3–O4–O5, mean deviation from plane,
0.0371(21) Å] and the V–atom is displaced toward the apical oxido ligand by 0.2502 Å. The
two O–atoms from the acac ligand occupy two positions cis to V=O, a type of binding not
commonly found in literature.25,26 As expected the bond length to the amine N–atom trans to
the oxido atom (V1–N1) is quite elongated, as a consequence of its trans influence.27 The V–
OArO bonds, which are cis to each other, have similar V–O distances (average 1.828(4) Å).
The carbon–carbon and carbon–oxygen distances of the acac ligand do not allow to
differentiate the ketonic oxygen from the enolic one, [O4–C39, 1.272(8) Å, O5–C41,
1.270(8) Å, C39–C40, 1.376(10) Å, C40–C41, 1.391(10) Å]. 25,26
Fig. 1 ORTEP plot of the mononuclear VV–complex in [VVO(acac)(L1)]·0.5CH3CN 1. All
the non–hydrogen atoms are presented by their 50% probability ellipsoids. Hydrogen atoms
are omitted for clarity.
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The structure of 2 corresponds to a mononuclear complex with a six–coordinated
distorted octahedral geometry. The axial sites of the octahedron are occupied by the oxido O–
atom and by one methanol molecule [V1–O1, 1.5884(14) Å, V1–O2M, 2.2834(15) Å]. The
other coordinated atoms form a plane [O2–O3–N1–O1M, mean deviation from plane,
0.0689(7) Å] with the vanadium atom displaced toward the apical oxido ligand by 0.2741 Å.
The distances V1–OArO [V1–O2, 1.8455(14) Å and V1–O3, 1.8548(14) Å] are similar to V1–
O
methoxido [V1–O1M, 1.8187(14) Å]. As expected the bond length to the function trans to the
oxido group (V1–O2M) is somewhat elongated, as a consequence of the trans influence,27
but is shorter than in 1. The methoxido and methanol molecules can easily occupy different
positions and in 2 the V–OArO bonds are trans to each other. Intra and intermolecular
hydrogen bonds exist between coordinated methoxido and coordinated and non–coordinated
methanol molecules (see Table 3).
Fig.
2
ORTEP plot of the mononuclear VV–complex in compound
[VVO(OMe)(MeOH)(L1)]·MeOH 2. All the non–hydrogen atoms are presented by their 50%
probability ellipsoids. Hydrogen atoms are omitted for clarity.
The molecular structure of 3 consists of a dinuclear complex corresponding to a
[V2O3]4+ core with anti–linear configuration,28 each VV being bound to a tetradentate ONNO
ligand (L1) [Fig. 3 (a and b)]. The geometry around each vanadium centre is distorted
octahedral with L1 bound through the pyridine–N atoms [V1–N2, 2.328(4) Å and V2–N4,
2.338(4) Å], the bridgehead amine–N atoms (Namine) [V1–N1, 2.328(4) Å and V2–N3, V2-
N3, 2.386(3) Å], two phenolate–O atoms (OArOe) [V1–O1, 1.843(3) Å and V1–O2, 1.856(3)
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Å, as well as V2–O5, 1.851(3) Å and V2–O6, 1.843(3) Å], one terminal O–atom [V1-O3,
1.606(3) Å and V2-O7, 1.603(3) Å] and one oxido bridge in each complex [V1-O4,
1.7932(8) Å and V2-O8, 1.7941(7) Å]. The V–atoms in each unit are displaced toward the
apical oxido ligand from the equatorial plane defined by the two OArO, one Namine and –oxo
atoms by 0.1751 Å (in respect to the plane O1–O2–O3–N1, mean deviation from plane
0.0213(16) Å), and by 0.1909 Å (in respect to the plane O5–O6–O7–N3, mean deviation
from plane 0.0214(14) Å). The two V=O bonds are very similar [average: 1.6015(30) Å] and
characteristic of oxido–type O–atoms with strong  bonding. The V––oxo bonds also depict
some –bonding character and have an average length of 1.7937(8) Å. The V–OArO bonds,
which are trans to each other, are significantly shorter (average 1.848(3) Å) than the N–atoms
that are trans to –oxo atoms or to V=O bond (average 2.362(3) Å) in a cis– type
structure.29,30 The V–V separation is 3.587 Å in the two dinuclear units.31
(a)
(b)
Fig. 3 ORTEP plot of [{VVO(L1)}2µ–O] 3. (a) Asymmetric unit showing two independent half
molecules of 3, and (b) the full dinuclear structure of 3. All the non–hydrogen atoms are
presented by their 30% probability ellipsoids. Hydrogen atoms are omitted for clarity.
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Table 3 Hydrogen bonds for [VVO(OMe)(MeOH)(L1)]·MeOH 2 and [{VVO(L2)}2µ–
(OH)2]·2(CH3)2CO 5.
D–H...A (compound)
O(2M)–H(2M)...O(3M)#1 2
O(3M)–H(3M)...O(1M) 2
N(1)-H(1N)...O(3)#2 (5)
d(D–H)
0.90(3)
0.89(3)
0.74(4)
d(H...A)
1.76(3)
1.89(3)
2.48(4)
d(D...A)
2.650(2)
2.741(2)
3.118(5)
<(DHA)
169(3)
160(3)
145(4)
Symmetry transformations used to generate equivalent atoms:
#1 –x+1,–y+1,–z+1 #2 –x,–y,–z+1
In the molecular structure of 5 and 6 the complexes consist in dinuclear species
containing two VV–centers with ligand L2, the difference being in the bridge between the VV–
atoms. In 5, the bridge is formed by two OH groups, in an anti–orthogonal configuration. In
6, the bridge is formed only by one O–oxido moiety in a twist quasi–linear configuration
[V(1)–O(4)–V(1)#1, 166.6(3)º]. The V–centres adopt a distorted six–coordinated octahedral
geometry in 5. In 6, the geometric parameter τ = (β − α)/60, where β and α are the two largest
L–M–L angles, describes a coordination environment of five coordinated atoms; it has a
value of 1 if the geometry is trigonal bipyramidal and 0 if it is square pyramidal.32 For 6
(O1–V1–O2 = 140.34º, βO4–V1–N1 = 159.06º) the value of τ is 0.31, indicating a distorted square
pyramidal geometry.32,33 The ligand L2 bound through the bridgehead amine–N atoms [V1–
N1, 2.350(3) Å in 5 and 2.149(4) Å in 6], two phenolic–O atoms [V1–O2, 1.836(3) Å in 5
and 1.859(3) Å in 6, and V1–O4, 1.844(3) Å in 5 and 1.7896(9) Å in 6] and terminal O–
atoms [V1–O3, 1.600(3) Å in 5 and 1.583(3) Å in 6]. In 5, the vanadium atoms in each core
are displaced toward the apical oxido ligand from the equatorial plane defined by the two
OArO atoms and two –hydroxide groups by 0.2600 Å (in respect to the plane O1–O2–O4–
O1A, mean deviation from plane, 0.0022 Å). The V–V separation is 3.182 Å. In 6, the axial
position is occupied by an oxido–type O–atom with strong  bonding [V1–O3, 1.583(3) Å].
The basal positions are occupied by two OArO atoms, one Namine atom and one oxido bridge,
[V1–O1, V1–O2, V1–N1, V1–O4]. The torsion angle between the planes containing V1–O3
and V1A–O3A bonds (N1, O3, V1, O4 and N1A, O3A, V1A, O4) is 75.2º. The V––oxido
bonds also depict some –bonding character [V1–O4, 1.7896(9) Å] and the V–V separation
is 3.554 Å. Intermolecular hydrogen bonds exist between protonated Namine and oxido O–
atoms of different complexes in 5. (see Table 3).
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Fig. 4 ORTEP plot of [{VVO(L2)}2µ–(OH)2]·2(CH3)2CO 5. All the non–hydrogen atoms are
presented by their 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.
Fig. 5 ORTEP plot of [{VVO(L2)}2µ–O] 6. All the non–hydrogen atoms are presented by
their 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.
IR Spectral Studies
A partial list of IR spectral data of the compounds is presented in Table 4 (For IR spectra of
4-6 see Figs. S1–S3). All VV–complexes exhibit strong ν(V=O) bands in the range 936–1024
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cm–1. Dinuclear complexes 3, 5 and 6 show one strong peak at 936–1002 cm–1 due to
ν(V=O) and another one arising at 853–880 cm–1, probably due to the ν(V–O–V) stretch.34 In
the spectrum of 5 a sharp peak appearing at 3567 cm–1 is probably due to the bridging OH
group; the presence of single peak in the 936–1002 cm–1 region indicates the equivalence of
the two bridged halves. The spectrum of complex 1 also displays the signature of carbonyl
stretch at 1525 cm–1 due to the coordinated acac moiety. The hydrolysis of the N–atom
(acting like the tripodal) and the consequent presence of free NH is reflected by the sharp
peak due to ν(NH) stretch in complexes 4, 5 and 6 at 3180–3261 cm–1. The presence of two
pairs of peaks at 969, 854 cm–1 and 992, 872 cm–1 in the spectrum of 4, may be attributed to
ν
asymm(O=V=O) and νsymm(O=V=O) stretches in two distinct environments (e.g. two isomeric
forms); some of these bands may also not be V=O stretches.
Table 4 Selected IR data for the compounds obtained with tentative assignments.
O

V
V
S.No. Compounds
ν(–OH) ν(V=O)
ν(N–H)
1. [VVO(acac)(L1)] 1
959
[VVO(OMe)(MeOH)(L1)] 2
1024 a
3400
2.
3. [{VVO(L1)}2µ–O] 3
4. H[VVO2(L2)] 4
936
855
969/994
872/854
3180
5. [{VVO(L2)}2µ–(OH)2] 5
6. [{VVO(L2)}2µ–O] 6
3400
957
870
838
3259
3261
1002
a There are a few weak IR bands in the region 930-915 cm¯1, and three medium/strong bands
at 1008, 1049 and1024 cm¯1 (order of increasing intensity). In 2002, for C-[VIVO(sal-L-
Ala)(H2O)], from a frequency calculation with the ADF-2000 program, the V=O stretching
was calculated at 1028.7 cm¯1, while the experimental ν(V=O) was found at 996 cm¯1 (ref.
35), thus here we tentatively assign the ν(V=O) to the and 1024 cm¯1 band.
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Electronic Spectral Studies
The electronic spectra of all compounds were recorded in either chloroform or
dichloromethane as solvent and their λmax values along with the extinction coefficients are
presented in Table 5. The UV spectrum of I displays three absorption bands at 285 nm, 268
nm and 228 nm. These can be assigned to n  π*, π  π* and φ  φ* transitions. These
bands are also present in all complexes, but at slightly shifted max values. In addition, all the
complexes also display a medium intensity shoulder band around 300–350 nm resulting from
charge transfer (CT) transitions. The low energy broad bands appearing around 500–670 nm
in all complexes are assigned to CT bands of phenolate to vanadium (dπ), being also
characteristic of VVO–aminobisphenol complexes.8
Table 5 UV–Vis spectral data for the VV–compounds obtained in this work.
S.No. Compound
Solvent
CHCl3
λmax/nm (ε M–1cm–1)
1. [VVO(acac)(L1)] 1
667 (3423), 508 (1973), 384 (1222), 333
(4345), 283 (15580), 262 (18159)
2. [VVO(OMe)(MeOH)(L1)] 2 CHCl3
670 (1466), 328 (2643), 280 (9825), 266
(10875)
3. [{VVO(L1)}2µ–O] 3
4. H[VVO2(L2)] 4
CHCl3
DCM
DCM
DCM
576 (2451), 364 (11394), 308 (20034), 279
(26428), 267 (28956),241 (31785)
609 (1240), 522 (1237), 344 (22011), 269
(17085), 229 (29568)
5. [{VVO(L2)}2µ–(OH)2] 5
6. [{VVO(L2)}2µ–O] 6
628 (4074), 518 (3622), 310 (14605), 278
(16628), 227 (24952)
620 (2382), 518 (2214), 347 (18175), 273
(23138), 230 (32081)
1H NMR Studies
1
The modes of coordination in the VV–complexes were also confirmed by H NMR spectral
1
studies. H NMR spectra were recorded in CDCl3 and the data are presented in Table S1. A
broad signal appearing at δ 9.6 ppm for the two phenolic –OH in the spectrum of I is not
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detected in the VV–complexes, indicating deprotonation and coordination of the two phenolic
O–atoms. The spectrum of the methoxido compound 2 is complex due to the presence of the
two isomeric forms (see 51V NMR description for more details), and thus the number of
protons could not be integrated.
The methylenic protons connecting the tripodal N–atom with the phenyl rings
becomes diastereotopic upon coordination. Thus, the singlet corresponding to the four
methylene protons in the spectrum of the ligand at 3.8 ppm, appears at 2.9 ppm and 3.41–
3.44 ppm in 1. The spectrum of 1 displays additional signals at 5.6 ppm (1H) and a singlet at
1.9 ppm (6H) corresponding to the coordinated acetylacetonato moiety. Signals for aromatic
protons for ligand and complexes resonate well within the expected region, but those of the
complexes are significantly up-shielded, probably due to anisotropy of the phenol rings.8 The
proton adjacent to the pyridyl N–atom has an up-field shift of –0.364 ppm {Δδ = [δ(complex)
– δ(ligand)]} in 1. However, the same proton appears to be remarkably down-shielded in 3
with ∆δ = 1.035 ppm, probably due to the coordination of pyridyl-N atom to vanadium. Also,
the pair of diastereotopic methylene protons in 3, in special proximity with the coordinated
pyridyl–N atom, appears significantly deshielded and resonates as a doublet at an unusually
high δ value of 6.00–6.03 ppm, while the other set resonates similarly to that of 1 (δ = 3.21–
3.24 ppm). Complexes 5 and 6 bearing ligand L2 also exhibit similar resonances, with
additional peaks around δ = 4.58 and 4.19 ppm corresponding to the secondary amine
protons. The spectrum of 5 exhibits two broad signals at 3.06 ppm and 3.23 ppm assigned to
the bridging hydroxide groups.
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1
Fig. 6 H NMR spectra of H2L1 I, [VVO(acac)(L1)] 1 and [{VVO(L1)}2µ–O] 3 recorded in
CDCl3.
13C NMR Studies
The binding modes of the ligands were further supported by the study of the coordination–
induced 13C NMR chemical shifts. The spectra were recorded in CDCl3 and the relevant data
are presented in Table 6. Significant downfield shifts, Δ = [(complex) – (ligand)], were
observed in the signals of the carbon atoms in the vicinity of the coordinating atoms. Thus the
phenolic carbons become diastereotopic following coordination and appear as two distinct
signals with Δ values varying between 15.93 to 10.09 ppm. The carbons of the pyridine ring
in 1 appear similar to those of the ligand, however, the coordination of pyridine N–atom in 3
leads to stark differences in geometry, and this is reflected in the greater Δ values observed
for 3. The carbons adjacent to the pyridine-N atom suffer significant downfield shifts (Δ =
14.29 ppm for C5 and 7.68 ppm for C1), due to its coordination. The signals due to the
pyridyl ring are not present in compounds 5 and 6 because of the elimination of the
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ethylpyridine fragment. Also the carbons C18/C19 become enantiotopic owing to symmetry
and thus appear as a singlet around 51.30–59.30 ppm.
Table 6 13C NMR data for some of the compounds prepared.
Compound C1
C5
C6/C12
148.97
C9/C15/C11/C17 C18/C19
136.21 57.01, 54.09
C20/C21
tert–butyl
I
153.11
159.08
35.04, 34.81 31.81, 9.74
1 (Δ) a
153.08 (- 160.49
159.06 (10.09), 137.54, 136.39,
149.35 136.18, 135.96
56.98 (–0.03), 35.52, 35.01 30.37, 9.70
54.05 (–0.04)
0.03)
(1.41)
160.79
(7.68)
173.37
(14.29) 148.95
153.60 (4.63), 136.72, 134.40
65.72 (8.71), 34.64, 34.32 29.67, 1.76
55.09 (1.00)
3 (Δ)
5 (Δ)
6 (Δ)
–
–
–
–
164.88 (15.91), 135.60, 128.75
145.6
59.30 (2.29)
51.30 (-5.71)
–
–
29.93, 1.67,
34.68, 5.40
164.83 (15.93), 135.55, 134.33
145.55
29.86, 31.58,
31.62, 35.32
a Δ = [(complex) – (ligand)]
51V NMR Studies
The 51V NMR resonances of the VV–compounds were measured in CD2Cl2 solution and the
spectra obtained are included in the Supporting Information section (Figs. S4–S8). The 51V
NMR spectrum obtained for compound 2 is presented in Figure 7 as an example. The
respective experimental chemical shifts are listed in Table 7.
Table 7 Experimental 51V NMR chemical shifts for 1, 2, 3 and 6 in CD2Cl2 solution.
Compound V (ppm)
1
2
3
6
– 455.4 (major), –470
– 445 , –455 , –470 (major), – 500, –517.1 (2nd major)
–454, –470.1 (major), –514
–421.6 (3rd major), –514, –548, -560.5 (2nd major), –579.1 (major)
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Fig. 7. 51V NMR spectrum of 2 in CD2Cl2. There are two major resonances at V = -470 and -
517 ppm and several other minor resonances, some of them specified in Table 7.
Compound 1 exhibits two resonances at –455 and –470 ppm which are comparable to
those reported for similar VVO complexes.8,36 These signals may be due to the likely VVO
conformers present in CD2Cl2 solution bearing the following donor atom sets: i)
VVO[OArO,Namine,OArO,Oacac]eq[Oacac axial
VVO[OArO,Namine,Oacac,Oacac]eq[OArO axial
might correspond to e.g. VVO[OArO,OArO,OMeOH,OMeOH][Namine axial
]
ii) VVO[OArO,OArO,Oacac,Oacac]eq[Namine axial
. For the resonance at – 470 ppm, it is plausible that it
. In fact some small
]
and iii)
]
]
amount of solvent might be present, and a similar band is the major one in the 51V NMR
spectra of 2 and 3.
In the case of 2, where the acac ligand is replaced by the monodentate methoxido and
methanol ligands, the possibility of additional conformational arrangements is favored.
Therefore, in addition to the signals at –454 and –470 ppm, two other signals at –445 and –
517 ppm are present. These four signals may be due to the following VVO species present in
solution:
VVO[OArO,Namine,OArO,OMeOH]eq[OMeO axial
iv)
VVO[OArO,OMeOH,OArO,OMeO]eq[Namine axial
VVO[OArO,Npy,OArO,OMeO]eq[Namine axial
N-pyridine donor atom displaces the methanol ligand may also be considered.
i)
VVO[OArO,Namine,OArO,OMeO]eq[OMeOH axial
; iii) VVO[OArO,Namine,OMeOH,OMeO]eq[OArO axial
In addition, species such
]
;
ii)
and
as
]
]
]
.
]
and VVO[OArO,Namine,OArO,OMeO]eq[Npy]axial, where the
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The spectrum of the –oxido compound 3 resembles that obtained for 2. In this case,
possible binding sets for species present in solution are: i) VVO[OArO,Namine,OArO,O–
O]eq[Npy]axial; ii) VVO[OArO,Npy,OArO,O–O]eq[Namine axial
O]eq[OArO axial
]
and iii) VVO[Npy,Namine,OArO,O–
]
.
The spectrum obtained for 6 differs significantly from those of 1, 2 and 3. In fact,
complex 6 involves a distinct ligand, which acts as tridentate, allowing the binding of
additional monodentate ligands such as –oxido and water. Indeed, two major signals are
shifted up-field (–560 and –579 ppm) compared to the single major species obtained for L1, a
tetradentate ligand. This is possibly due to the more electronegative character of O–donor
atoms relative to the N–donors of pyridines 37, but this may be a rather simplified view, as the
complexes differ significantly, and other factors may also be relevant. In the case of 6
plausible
VVO[OArO,Npy,OArO,Owater]eq[O–O axial
VVO[Namine,OArO,OV=O,O–O]eq[OArO axial
binding
sets
are:
VVO[OArO,Namine,OArO,O–O]eq[Owater axial
V=O···V=O dimer such as iii)
]
;
ii)
]
and
a
]
.
Reactivity with H2O2
Reactivity of complex 1 towards H2O2 was monitored by UV–vis spectroscopy upon
progressive additions of a H2O2 solution (2.22 ×10–2 M, 10.0 mL MeOH) to 25.0 mL of a ca.
1.66×10–4 M solution of [VVO(acac)(L1)] 1 in CHCl3. Two sets of spectral changes were
observed. As shown in Fig. 8, after addition of ca. 500 µL of the above H2O2 solution, the
bands at 674 and 525 nm gradually decrease in intensity and finally become weak shoulders,
while the band at 326 nm decreases in intensity; the changes observed result in the presence
of two isosbestic points at 457 nm and 345 nm (Fig. 8a). These spectral changes may be
attributed to formation of the corresponding VVO–(O2) species through [VVO2]+ species.
Further additions of H2O2, ca. 500 µL, result in the shifts of 326 nm band to 335 nm and the
generation of a band at 405 nm along with the formation of an isosbestic point at 476 nm
(Fig. 8b). The 51V NMR spectrum recorded in a 3:2 CH3OH/CD3OD mixture indicated that
weak peaks at –521, –543 and –546 ppm formed upon dissolution of 1 (0.61 mM) in
methanol, and the solution acquired a brown coloration. Upon addition of 1 equivalent of
H2O2 the coloration changed from brown to light yellow and the respective 51V NMR
spectrum showed a new species with V at –645 ppm (Fig S7). This chemical shift is
characteristic of inorganic [VVO–(O2)]–type species38, thus it is probably due either to
HVO3(O2)2¯ or to a peroxido complex derived from 1.
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(a)
(b)
Fig. 8 Two sequential sets (a) and (b) of spectral changes obtained by progressive additions
of a H2O2 solution ((2.22 ×10–2 M) of 30% H2O2 dissolved in 10.0 mL of MeOH) to 25.0 mL
of a ca. 1.66×10–4 M solution of [VVO(acac)(L1)] 1 in CHCl3.
We also monitored the spectral changes (in the visible range if any) during the
conversion of [VVO(acac)(L1)] 1 into the dinuclear [{VVO(L1)}2µ–O] 3 in acetonitrile (see
experimental section) over a period of 7 days. The resulting spectral changes do not confer
much information, since no new distinct band was generated in the spectra during this period.
Notwithstanding, the intensity of bands at 674 and 525 nm initially increased (during the first
7 h), and then decreased only slightly with time; Fig 9. The existence of an isosbestic point at
ca. 490 nm suggests the existence of the two complexes (1 and 3) in equilibrium in solution.
Upon standing the solution at ca. 0 ˚C, the major product 1 crystallizes out from the solution,
while at room temperature the equilibrium slowly moves towards 3.
Fig. 9 Spectral changes observed in the electronic spectrum of [VVO(acac)(L1)] 1 in MeCN
with time.
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The addition of a H2O2 solution (2.22 ×10–2 M, 10.0 mL MeCN) to a solution of 1
(25.0 mL of a ca. 1.16×10–4 M solution in acetonitrile) also showed a similar trend, i.e. an
initial increase in the intensity of the 674, 525 nm and 326 nm bands. This trend continued till
the addition of ca. 2.1 mL of the H2O2 solution; Fig. 10a. These changes also resulted in the
observation of two isosbestic points at 434 nm and at 389 nm. Further additions of H2O2
yielded only a small decrease in intensity for all three bands in the spectra. After addition of
ca. 500 µL of H2O2 solution, the system stabilized, and showed no further H2O2 effect [Fig.
10b]. Thus, the two bands at 674 and 525 nm, which almost disappeared in MeOH/CHCl3
solution, are still present when a similar experiment was done in acetonitrile. In contrast to
what was observed in methanol, there was no observable change either in sample coloration
(dark blue) or in the respective 51V NMR spectrum of 1 in 3:2 CH3CN/CD3CN mixture (0.62
mM), even after the addition of 8 equivalents of H2O2 (see Fig. S8). This also indicates that 1
is much more resistant to dissociation in the presence of H2O2 when in polar aprotic medium.
Spectral changes observed by sequential addition of a solution of H2O2 in MeCN to a
solution of [VVO(OMe)(MeOH)(L1)] 2 in MeCN are shown in Fig. 11. The intensity of the
670 nm band decreases gradually and finally becomes flat. Simultaneously a new clearly
distinct band generates at 460 nm. Since no other bidentate ligand (like in 1) is present in this
complex, we may conclude that 2 forms the peroxido complex in solution. In fact, the
VVO(O2) complex was reported to have a band with max = 455 nm in aqueous acidic
medium.39,40 Unlike what was observed for the 51V NMR spectrum of 1 in 3:2
CH3CN/CD3CN, there was a significant change either in sample coloration (dark blue) and in
the respective 51V NMR spectrum of 2 in 3:2 CH3CN/CD3CN mixture (2.3 mM) after the
addition of 8 equivalents of H2O2 (see Fig. 12). A weak signal appears at –542 ppm after the
addition of 1 equivalent of H2O2, but the VVO(O2)–L2–type species formed (resonances at –
585, –595 and 603 ppm) and the ligand free inorganic VVO(O2)2–type species (resonances at
–698 and –707 ppm) are only detected after the addition of 8 equivalents of H2O2.38,41 The
presence of signals beyond –600 ppm, typical of VVO(O2)–type species, indicates that 2,
when in polar aprotic medium, is not as resistant to dissociation as compound 1 in the
presence of H2O2. All 51V NMR signals disappear ~72 h after the addition of oxidant.
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(a)
(b)
Fig. 10 Two sequential sets (a) and (b) of spectral changes obtained by progressive additions
of a H2O2 solution (2.22 ×10–2 M, of 30 % H2O2 dissolved in 10.0 mL of MeCN) to 25.0 mL
of a ca. 1.16×10–4 M solution of [VVO(acac)(L1)] 1 in MeCN.
Fig. 11 Spectral changes obtained by progressive additions of a H2O2 solution (2.22 ×10–2 M)
to 15.0 mL of a ca. 1.23×10–4 M solution of [VVO(OMe)(MeOH)(L1)] 2 in MeCN.
29