Simultaneous formation of non-oxidovanadium(iv) and oxidovanadium(v) complexes incorporating phenol-based hydrazone ligands in aerobic conditions

Article abstract of DOI:10.1039/c9nj06114b

A new family of non-oxidovanadium(iv) [V^IV(L)₂] (1-4) complexes along with methoxy bonded oxidovanadium(v) [V^VO(L)(OCH₃)] (5 and 6) and [V^VO(L)(OCH₃)]₂ (7 and 8) complexes have been synthesized by refluxing [V^IVO(aa)₂] [aa^- being the deprotonated form of acetylacetone (Haa)] with a family of hydrazone ligands (H₂L^1-4, general abbreviation H₂L, obtained by the condensation of 2-aminobenzoylhydrazide with 2-hydroxyacetophenone and its 5-substituted derivatives) using laboratory grade methanol as a solvent in the presence of air in ～30% yield. The DFT calculated changes in Gibbs free energy (ΔG), enthalpy (ΔH) and internal energy (ΔE) for the reaction to form complexes 1-4 in methanol: 2H₂L + [V^IVO(aa)₂] + ?O₂ + CH₃OH → [V^IV(L)₂] + 2Haa + CH₃OH + ?O₂ are on average ～10, ～5 and ～14 kcal mol^-1 respectively, while these values for the reaction in methanol to form complexes 5-8: 2H₂L + [V^IVO(aa)₂] + ?O₂ + CH₃OH → [V^VO(L)(OCH₃)] + H₂L + 2Haa + ?H₂O are on average ～-16, ～-20 and ～-12 kcal mol^-1 respectively, suggesting that the formation of 1-4 is not thermodynamically feasible through this method. In practice, however, these complexes are also formed probably due to their very low solubility in methanol along with complexes 5-8. The complexes were characterized by analytical and spectral methods. The structures of the H₂L² and H₂L³ ligands and complexes 3 and 5-8 were determined by X-ray diffraction. Complexes 1-4 display quasi-reversible one-electron oxidation and reduction peaks in the potential windows of 0.66-0.70 and -0.22 to -0.39 V respectively while complexes 5-8 exhibit quasi-reversible one-electron reduction peaks in the 0.15-0.22 V region. The EPR spectral parameters indicate that the odd electron in complexes 1-4 is present in the d_xy orbital, a suggestion which is also supported by a DFT study. The substituents in the aryloxy ring of the hydrazone ligands exhibited a significant effect on λ_max for the LMCT transition, A_‖, E_1/2, E_SOMO/E_HOMO and E_LUMO of the complexes. Complexes 1, 3, and 5-7 showed a wide range of toxicity in a dose dependent manner against lung cancer cells and they kill the cells through apoptosis. The binding ability of the complexes with CT DNA has been determined by a fluorescence emission study and the binding constant value obtained by this method was supported by a molecular docking study.

Full text of DOI:10.1039/c9nj06114b

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Simultaneous formation of non-oxidovanadium(IV) and oxidovanadium(V)
DOI: 10.1039/C9NJ06114B
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complexes incorporating phenol-based hydrazone ligands in aerobic
condition
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Nirmalendu Biswas,a Sachinath Bera,b Nayim Sepay,b Amrita Pal,c Tanmoy Halder,d Sudipta
Ray,d Swarnali Acharyya,e Anup Kumar Biswas,f Michael G. B. Drew,g and Tapas Ghosh*,a
aPost Graduate Department of Chemistry, Ramakrishna Mission Vivekananda Centenary
College, Rahara, Kolkata-700118, India
bDepartment of Chemistry, Jadavpur University, Kolkata 700032, India
cDepartment of Mechanical Engineering, National University of Singapore, 9 Engineering
Drive 1. Singapore 117576
dDepartment of Botany, University of Calcutta, 35, Ballygunge Circular Road, Kolkata -
700019, India
eDepartment of Pathology and Cell Biology, Columbia University, New York, NY, United
States
fHerbert Irving Comprehensive Cancer Centre, Columbia University, 1130 St. Nicholas
Avenue, New York, NY 10032, USA
gDepartment of Chemistry, The University of Reading, PO Box 224, Whiteknights, Reading,
RG6, 6AD, UK
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DOI: 10.1039/C9NJ06114B
Abstract
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A new family of non-oxidovanadium(IV) [VIV(L)2] (1-4) complexes along with methoxy
bonded oxidovanadium(V) [VVO(L)(OCH3)] (5,6) and [VVO(L)(OCH3)]2 (7, 8) complexes
have been synthesized by refluxing [VIVO(aa)2] [aa- being the deprotonated form of
acetylacetone(Haa)] with a family of hydrazone ligands (H2L1-4, general abbreviation H2L,
obtained by the condensation of 2-aminobenzoylhydrazide with 2-hydroxyacetophenone and
its 5-substituted derivatives) using laboratory grade methanol as solvent in presence of air in
~30% yield. DFT calculated changes in Gibbs free energy (∆G), enthalpy (∆H) and internal
energy (∆E) for the reaction to form complexes 1-4 in methanol: 2H2L + [VIVO(aa)2] + ¼O2
+ CH3OH → [VIV(L)2] + 2Haa + CH3OH + ¼O2 are on average ~10, ~5 and ~14 kcal mol-1
respectively while these values for the reaction in methanol to form complexes 5-8: 2H2L +
[VIVO(aa)2] + ¼O2 + CH3OH → [VVO(L)(OCH3)] + H2L + 2Haa + ½H2O are on average ~-
16, ~-20 and ~-12 kcal mol-1 respectively suggesting that the formation of 1-4 is not
thermodynamically feasible through this method. In practice however, these complexes are
also formed probably due to their very low solubility in methanol along with 5-8 complexes.
Complexes were characterized by analytical and spectral methods. The structures of H2L2 and
H2L3 ligands and complexes 3, 5-8 were determined by X-ray diffraction. Complexes 1-4
display quasi-reversible one-electron oxidation and reduction peaks in the potential windows
of 0.66-0.70 and -0.22 to -0.39 V respectively while complexes 5-8 exhibit quasi-reversible
one-electron reduction peak in the 0.15-0.22 V region. The EPR spectral parameters indicate
that the odd electron in complexes 1-4 is present in the dxy orbital, a suggestion which is also
supported by DFT study. Substituents in the aryloxy ring of hydrazone ligands exhibited
significant effect on λmax for LMCT transition, A׀׀, E1/2, ESOMO/EHOMO and ELUMO of
complexes. Complexes 1, 3, 5-7 showed wide range of toxicity in a dose dependent manner
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against lung cancer cells and they kill the cells through apoptosis. The bindinDgOIa: 1b0i.1l0it3y9/Co9fNJ06114B
complexes with CT DNA has been determined by fluorescence emission study and the
binding constant value obtained by this method was supported by molecular docking study.
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1. Introduction
Vanadium is recognized as a bio-element and its biological function is well established and
documented1-3. Vanadium complexes have the ability to suppress the growth of existing
tumours by inhibiting tumour cell proliferation, including apoptosis and limiting the invasion
and metastatic potential of neoplastic cells4. Cisplatin is widely used in various types of
cancer chemotherapy including lung cancer. However, it has several undesirable side effects5
(viz., high nephrotoxicity, neurotoxicity, hair loss, vomiting tendency and drug resistance of
metastatic cancer). In this context, a suitably designed vanadium complex containing
biologically relevant ligand having lesser side effects may be a promising candidate in lung
cancer chemotherapy.
Among the wide range of oxidation states (-III to +V) of vanadium, the +III, +IV, and
+V states are biologically important. In the two higher (+IV and +V) oxidation states,
vanadium is oxophilic and forms varieties of oxido species viz., VO2+, VO3+, (OV-µ-O-
VO)3+/4+ [i.e., (V2O3)3+/4+] and VO2 that involve extremely strong and inert V≡O bonds as is
+
evident from the unusually high V≡O bond energy6,7 (atomization energy = 140 kcal/mol)
with T1/2 for O-exchange with bulk water is ca. 400 min. Though, in the biological domain,
different types of non-oxido complexes are abundant [e.g., the presence of non-oxido V(IV)
compound amavadin in fly agaric8, the presence of non-oxido V(V) compound in the co-
factor of an alternative form of nitrogenase, known as VNase, which catalyses the biological
conversion of atmospheric dinitrogen to bio-available ammonia9,10 but, in the abiological
domain, the formation of non-oxido or “bare” vanadium(IV) and vanadium(V) complexes is
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not common because the strong and inert V≡O oxido bond must be broken andDtOhIe: 10o.1x03y9g/Ce9nNJ06114B
atom removed from the vanadium as a water molecule. In fact, until now, such compounds
are limited in number6,11-25. It should be noted that almost all of these compounds have
vanadium centres in six-coordinated O/N donor environments6,11-18 containing at least one
phenolic-O or alcoholic-O moiety, though a few complexes containing bidentate
dithiolenate19, acetylacetonate20 and benzoylacetonate20 have also been reported. We have
very recently reported25 two non-oxidovanadium(IV) complexes incorporating hydrazone
ligands having tridentate enolic-O, imine-N and amide-O moieties. Some eight-coordinated
non-oxido V(IV) complexes with VN4O4 coordination have been reported along with the
structural characterization of one compound24. A few examples of four- and five-coordinated
non-oxido V(IV) complexes with VO422 and VNO423 coordination are also to be found in the
literature. Some six-coordinated non-oxido V(IV) complexes with VO2N2S2 coordination
have also been reported very recently6.
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The synthesis of non-oxidovanadium(IV) complexes utilizing suitable multidentate
ligands starting from an oxido-precursor in the presence of air using laboratory grade solvents
without using a base is an important area of research to inorganic chemists as most of the
reported non-oxidovanadium(IV) complexes were synthesised in the absence of air, using dry
solvent and base. Moreover, it should be noted that during the formation of non-
oxidovanadium(IV) complexes from an oxido-V(IV) precursor under aerobic conditions, the
formation of oxido complexes is also thermodynamically feasible (vide infra). Unfortunately,
until now, almost all reports have focused only on the formation of non-oxidovanadium(IV)
species apart from three reports19i,25,26. In this context, multidentate hydrazone ligands are
relevant as hydrazone moieties are important pharmaceutical cores of several anti-cancer,
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anti-inflamatory and anti-platelet drugs27. Multidentate hydrazone-based ligands have wide
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As a part of our programme on the synthesis and characterization of non-
oxidovanadium(IV) complexes under aerobic conditions using multidentate hydrazone based
O, N donor ligands25, we have used four hydrazone based ligands (H2L1-4, general
abbreviation H2L, Scheme 1) which are synthesized by the condensation of 2-
aminobenzoylhydrazide (hereafter ah) with 2-hydoxyacetophenone and its 5-substituted
derivatives. Different substituents at the 5-position have been used to study the electronic
effect of these substituents on the electronic properties of vanadium in the resulting
complexes. These ligands contain phenolic-O, imine-N, amine-N and amide-O moieties. The
motive behind the choice of phenolic rather than enolic moieties is to examine effects on the
electronic properties of vanadium in the resulting complexes. In this paper, we report a new
family of non-oxidovanadium(IV) [VIV(L1-4)2] (1-4) complexes along with methoxy bonded
oxidovanadium(V) [VVO(L1-2)(OCH3)] (5, 6) and [VVO(L3-4)(OCH3)]2 (7, 8) complexes
which were formed simultaneously by the reaction of [VIVO(aa)2] with the respective H2L
ligand in laboratory grade methanol in aerobic conditions. We also report the results obtained
on performing the reactions stated above in dry methanol and in the absence of air. These
complexes were fully characterized by various physicochemical methods namely: infrared
(IR) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy (for non-oxido
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V(IV) complexes), electrospray ionization (ESI) mass spectrometry, H NMR and V NMR
(for oxidovanadium(V) complexes) spectroscopies, electronic spectroscopy along with
magnetic susceptibility and electrochemical measurements. The structure of H2L2 and H2L3
ligands and some of these complexes (3 and 5-8) were determined by single-crystal X-ray
diffraction. Density functional theory (DFT) was used extensively to predict the formation of
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probable complexes, calculate the bond lengths and bond angles as well as cDoOmI: p10u.1t0e39/tCh9eNJ06114B
absorption maxima of all the complexes reported here. The cytotoxic activity of these
complexes was also examined.
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Scheme 1. Hydrazone ligands used in this study.
2. Experimental and computational section
2. 1 Materials and methods
2-aminobenzoylhydrazide, 2-hydroxyacetophenone, 2-hydroxy-5-methylacetophenone, 2-
hydroxy-5-methoxyacetophenone and 5-chloro-2-hydroxyacetophenone were obtained from
Aldrich. Acetylacetone, vanadyl sulphate pentahydrate were obtained from Loba chemical
company (India). [VIVO(aa)2] was prepared by the reported method30. All other reagents were
of A. R. grade obtained from commercial sources and used without further purification.
2.2 Synthesis of the ligands (H2L1-4). These ligands were synthesized in good yield (70-
76%) by refluxing 2-aminobenzoylhydrazide (5 mmol) with the equimolar amount of
respective 2-hydroxyacetophenone (5 mmol) in methanol (40 mL) for 2 h. The reaction
mixture was then kept at room temperature. After 3 days, the yellow crystalline compound
(suitable for structure determination by X-ray crystallography) formed was collected by
filtration followed by washing with methanol and dried over fused CaCl2. Elemental
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analyses, H NMR, IR and ESI-MS data confirmed the identity of all four ligands. The
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analytical data of H2L1 ligand are not described here as this ligand has already been reported
in the literature31.
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2.2.1 H2L2. Yield: 1.02 g (72%). Anal. calcd for C16H17N3O2: C, 67.84%; H, 6.01%; N,
14.84%. Found: C, 67.64%; H, 6.10%; N, 14.78%. FT-IR bands (KBr pellet, cm-1): 2918 (N-
H), 3444 (O-H), 983 (N-N), 1254 (C-Oenolic), 1373 (C-Ophenolic), 1620 (C=N), 1668 (C=O).
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UV-Vis (CH2Cl2) [λmax/nm (ε, M-1 cm-1)]: 352 (17610), 294 (15410). H NMR (400 MHz,
DMSO-d6, 298 K, δ/ppm; J/Hz): 6.75- 6.80 (m, 2H, H-3, H-13), 7.09 (dd, 1H, H-4, J = 8.0,
1.2), 7.41 (d, 1H, H-6, J = 1.2), 7.22 (dt, 1H, H-12, J = 8.0, 1.2), 6.59 (dt, 1H, J = 7.6, 1.2),
7.61 (dd, 1H, H-14, J = 7.6, 1.2), 2.49 (s, 3H, H-15), 2.26 (s, 3H, H-16), 13.15 (s, 1H, H(2-
OH)). ESI-MS (positive) in CH3CN: 283.13 (M), 284.14 (M+H+).
.
2.2.2 H2L3. Yield: 1.05 g (70%). Anal. calcd for C16H17N3O3: C, 64.21%; H, 5.68%; N,
14.04%. Found: C, 64.34%; H, 5.56%; N, 14.02%. FT-IR bands (KBr pellet, cm-1): 2993 (N-
H), 3472 (O-H), 987 (N-N), 1248 (C-Oenolic), 1377 (C-OPhenolic), 1620 (C=N), 1662 (C=O).
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UV-Vis (CH2Cl2) [λmax/nm (ε, M-1 cm-1)]: 361 (15850), 286 (14440). H NMR (400 MHz,
DMSO-d6, 298 K, δ/ppm; J/Hz): 6.79 (d, 1H, H-3, J = 8.5), 6.93 (1H, H-4, J = 8.5, 3.0), 7.12
(d, 1H, H-6, J = 3.0), 6.85 (d, 1H, H-11, J = 8.5), 7.24 (t, 1H, H-12, J = 8.5), 6.61 (t, 1H, H-
13, J = 8.0), 7.63 (d, 1H, H-14, J = 8.0), 2.46 (s, 3H, H-15), 3.76 (s, 1H, H-16), 12.87 (s, 1H,
H(2-OH)). ESI-MS (positive) in CH3CN: 300.13 (M).
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2.2.3 H2L4. Yield: 1.15 g (76%). Anal. calcd for C15H14N3O2Cl: C, 59.21%; H, 4.60%; N,
13.81%. Found: C, 59.30%; H, 4.56%; N, 13.78%. FT-IR bands (KBr pellet, cm-1): 3059 (N-
H), 3468 (O-H), 989 (N-N), 1250 (C-Oenolic), 1385 (C-OPhenolic), 1651 (C=N), 1666 (C=O).
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UV-Vis (CH2Cl2) [λmax/nm (ε, M-1 cm-1)]: 353 (21220), 295 (18320). H NMR (400 MHz,
DMSO-d6, 298 K δ/ppm; J/Hz): 6.77 (d, 1H, H-3, J = 8.0), 7.31 (dd, 1H, H-4, J = 8.0, 2.4).
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7.62 (d, 1H, H-6, J = 2.4), 6.92 (d, 1H, J = 8.8), 7.23 (dd, 1H, H-12, J = 8.8, 0.8), 7.61Vi(edwdAr,ticle Online
DOI: 10.1039/C9NJ06114B
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1H, H-14, J = 8.0, 0.8), 2.45 (s, 3H, H-15), 13.52 (s, 1H, H(2-OH)). ESI-MS (positive) in
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CH3CN: 304.09 (M).
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2.3 Synthesis of complexes [VIV(L)2] (1-4)
2.3.1 [VIV(L1)2] (1). To a methanolic solution (20 mL) of [VIVO(aa)2] (0.27 g, 1 mmol) was
added a warm methanolic solution (20 mL) of H2L1 (0.27 g, 1 mmol) drop-wise with stirring.
The reaction mixture was then heated under reflux for 2 h. After cooling the reaction mixture
to room temperature, a brownish-black compound was obtained which was filtered and
washed with methanol and dried over CaCl2 (fused). Yield: 0.19 g (32% based on
[VIVO(aa)2]). However, on using two equivalents of H2L1, the yield of 1 was increased to
70% (based on [VIVO(aa)2]). Anal. calcd for C30H26N6O4V: C, 61.54%; H, 4.44%; N,
14.36%. Found: C, 61.49%; H, 4.33%; N, 14.29%. FT-IR bands (KBr pellet, cm-1): 3066 (N-
H), 1615, 1589 (C=Nazomethine), 1362 (C-Ophenolic), 1258 (C-Oenolic), 1084 (N-N). UV-Vis
(CH2Cl2) [λmax/nm (ε, M-1 cm-1)]: 575 (6280), 492 (9610), 380 (23250). ESI-MS (positive) in
CH3CN: 586.15 (M+H+), 608.13 (M+Na+), 624.11 (M+K+).
2.3.2 [VIV(L2)2] (2). This compound was prepared following an identical method as described
for compound 1 using the ligand H2L2 instead of H2L1. Yield: 0.19 g (31%). However, the
yield of 2 was increased to 72% on using two equivalents of H2L2. Anal. calcd for
C32H30N6O4V: C, 62.64%; H, 4.89%; N, 13.70%. Found: C, 62.59%; H, 4.85%; N, 13.68%.
FT-IR bands (KBr pellet, cm-1): 2924 (N-H), 1616, 1539 (C=Nazomethine), 1363 (C-Ophenolic),
1255 (C-Oenolic), 1031 (N-N). UV-Vis (CH2Cl2) [λmax/nm (ε, M-1 cm-1)]: 560 (1840), 486
(3100), 376 (10080). ESI-MS (positive) in CH3CN: 614 (M+H+), 635.86 (M+Na+), 651.83
(M+K+).
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2.3.3 [VIV(L3)2] (3). This compound was obtained as crystalline compound (DsuOIi:t1a0b.1l0e39/fCo9rNJ06114B
structure determination by X-ray diffraction) following an identical method as described for
compound 1 using the ligand H2L3 instead of H2L1. Yield: 0.20 g (31%). Using two
equivalents of H2L3, the compound 3 was obtained in 74% yield. Anal. calcd for
C32H30N6O6V: C, 59.53%; H, 4.65%; N, 13.02%. Found: C, 59.51%; H, 4.62%; N, 13.01%.
FT-IR bands (KBr pellet, cm-1): 3051 (N-H), 1612, 1590 (C=Nazomethine), 1362 (C-Ophenolic),
1260 (C-Oenolic), 1041 (N-N). UV-Vis (CH2Cl2) [λmax/nm (ε, M-1 cm-1)]: 557 (1830), 484
(2620), 377 (7980). ESI-MS (positive) in CH3CN: 646.20 (M+H+), 668.17 (M+Na+), 684.07
(M+K+).
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2.3.4 [VIV(L4)2] (4). This compound was also prepared following an identical method as
described for compound 1 using the ligand H2L4 instead of H2L1. Yield: 0.22 g (33%), while
use of two equivalents of H2L4 the yield of compound 4 was increased to 76%. Anal. calcd
for C30H24N6O4Cl2V: C, 55.04%; H, 3.67%; N, 12.86%. Found: C, 55.03%; H, 3.63%; N,
12.80%. FT-IR bands (KBr pellet, cm-1): 2924 (N-H), 1614, 1591 (C=Nazomethine), 1362 (C-
Ophenolic), 1257 (C-Oenolic), 1027 (N-N), UV-Vis (CH2Cl2) [λmax/nm (ε, M-1 cm-1)]: 595 (4190),
502 (5600), 370 (15220). ESI-MS (positive) in CH3CN: 653.82 (M+H+), 675.93 (M+Na+),
691.77 (M+K+).
2.4 Synthesis of [VVO(L1-2)(OCH3)] (5,6) and [VVO(L3-4)(OCH3)]2 (7,8) complexes
2.4.1 [VVO(L1)(OCH3)] (5). This compound was isolated from the filtrate (after separation
of compound 1 as described earlier) on slow evaporation of the solvent methanol at room
temperature for 4 days. The shiny brownish black crystals (suitable for structure
determination by X-ray crystallography) of 5 were collected by filtration, washed with
methanol and dried over CaCl2 (fused). Yield: 0.12 g (33% based on [VIVO(aa)2]). However,
on using two equivalents of H2L1, the yield of 5 (after separation of compound 1 as described
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earlier) was 25%. Anal. calcd for C16H16N3O4V: C, 52.60%; H, 4.38%; N, 11.5D0O%I: .10F.10o3u9/nCd9N:J06114B
C, 52.51%; H, 4.34%; N, 11.46%. FT-IR bands (KBr pellet, cm-1): 3057 (N-H), 1614, 1577
(C=Nazomethine), 1367 (C-Ophenolic), 1254 (C-Oenolic), 1049 (N-N), 991 (V=O). UV-Vis (CH2Cl2)
[λmax/nm (ε, M-1 cm-1)]: 408 (5520), 326 (7650). 1H NMR (400 MHz, CDCl3, 298 K, δ/ppm;
J/Hz): 7.00-7.03 (m, 1H, H-3), 7.36-7.38 (m, 2H, H-4, H-12), 7.30-7.32 (m, 2H, H-5, H-13),
7.68-7.72 (m, 2H, H-6, H-14), 6.88-6.92 (m, 1H, H-11), 2.94 (s, 3H, 3 × H-15), 3.49 (s, 3H, 3
× H-16). 51V NMR (400 MHz, CDCl3, 300 K, δ/ppm): -536, -542. ESI-MS (positive) in
CH3OH: 366.23 (M+H+).
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2.4.2 [VVO(L2)(OCH3)] (6). This compound was prepared following an identical method as
described for compound 5 using the ligand H2L2 instead of H2L1. Yield: 0.12 g (32%).
However, on using two equivalents of H2L2, the yield of 6 (after separation of compound 2 as
described earlier) was 24%. Anal. calcd for C17H18N3O4V: C, 53.83%; H, 4.75%; N, 11.08%.
Found: C, 53.81%; H, 4.78%; N, 11.04%. FT-IR bands (KBr pellet, cm-1): 3064 (N-H), 1616,
1573 (C=Nazomethine), 1363 (C-Ophenolic), 1256 (C-Oenolic), 1032 (N-N), 997 (V=O). UV-Vis
(CH2Cl2) λmax/nm (ε/dm3 mol-1 cm-1): 402 (8660), 326 (12930) .1H NMR (400 MHz, CDCl3,
298 K, δ/ppm; J/Hz): 6.50 (s, 1H, H-3), 7.54-7.57 (m, 3H, H-4, H-12, H-14), 6.70 (s, 1H, H-
6), 7.48-7.52 (m, 2H, H-11, H-13), 2.46 (s, 3H, 3 × H-15), 3.49 (s, 3H, 3 × H-16), 2.84 (s,
51
3H, 3 × H-17). V NMR (400 MHz, CDCl3, 300 K, δ/ppm): -530, -535. ESI-MS (positive)
in CH3OH: 380.42 (M+H+).
2.4.3 [VVO(L3)(OCH3)]2 (7). This compound was prepared following an identical method as
described for compound 5 using the ligand H2L3 instead of H2L1. Yield: 0.13 g (32%).
However, on using two equivalents of H2L3, the yield of 7 (after separation of compound 3 as
described earlier) was 22%. Anal. calcd for C34H36N6O10V2: C, 51.64%; H, 4.55%; N,
10.63%. Found: C, 51.60%; H, 4.50%; N, 10.60%. FT-IR bands (KBr pellet, cm-1): 3311 (N-
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H), 1616, 1585 (C=Nazomethine), 1369 (C-Ophenolic), 1264 (C-Oenolic), 1031 (N-N), 993 (VV=ieOw A)r.ticle Online
DOI: 10.1039/C9NJ06114B
5
6
1
UV-Vis (CH2Cl2) [λmax/nm (ε, M-1 cm-1)]: 400 (6210), 324 (10440). H NMR (400 MHz,
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8
9
CDCl3, 298 K, δ/ppm; J/Hz): 6.56 (d, 1H, H-3, J = 8.0), 6.75 (d, 1H, H-4, J = 8.0), 7.29 –7.31
(m, 1H, H-6), 7.13-7.21 (m, 2H, H-11, H-13), 7.29-7.31 (m, 1H, H-12), 7.37 (d, 1H, H-14, J
= 8.0), 2.90 (s, 3H, 3 × H-15), 3.49 (s, 3H, 3 × H-16), 3.89 (s, 3H, 3 × H-17). 51V NMR (400
MHz, CDCl3, 300 K, δ/ppm): -525, -529. ESI-MS (positive) in CH3OH: 396.00 (M+H+).
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2.4.4 [VVO(L4)(OCH3)]2 (8). This compound was prepared following an identical method as
described for compound 5 using the ligand H2L4 instead of H2L1. Yield: 0.12 g (30%).
However, on using two equivalents of H2L4, the yield of 8 (after separation of compound 4 as
described earlier) was 21%. Anal. calcd for C32H30Cl2N6O8V2: C, 48.12%; H, 3.75%; N,
10.52%. Found: C, 48.10%; H, 3.70%; N, 10.50%. FT-IR bands (KBr pellet, cm-1): 3279 (N-
H), 1616, 1577 (C=Nazomethine), 1363 (C-Ophenolic), 1252 (C-Oenolic), 1075 (N-N), 977 (V=O).
1
UV-Vis (CH2Cl2) [λmax/nm (ε, M-1 cm-1)]: 412 (5800), 324 (9040). H NMR (400 MHz,
CDCl3, 298 K, δ/ppm; J/Hz): 6.97-7.00 (m, 2H, H-3, H-11), 8.18 (d, 1H, H-4, J = 6.8), 7.54
(s, 1H, H-6), 7.67-7.70 (m, 1H, H-12, H-14), 7.33-7.35 (m, 1H, H-13), 2.93 (s, 3H, 3 × H-
51
15), 3.49 (s, 3H, 3 × H-16). V NMR (400 MHz, CDCl3, 300 K, δ/ppm): -537, -541. ESI-
MS (positive) in CH3OH: 400.00 (M+H+).
2.5 Physical Measurements
Elemental analyses were performed on a Perkin-Elmer 2400 CHN analyzer. IR spectra (as
KBr pellets) were recorded on a Shimadzu FT-IR spectrophotometer, IR Affinity-1. Magnetic
susceptibilities were measured at 298 K using a PAR model 155 vibrating sample
magnetometer fitted with Walker Scientific L75 FBAL magnet using Hg[Co(SCN)4] as the
corr
calibrant. Diamagnetic corrections were made using Pascal’s constants to obtain χM and
the resulting µeff value is reported. The ESI-MS in positive ion mode were measured on a
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Micromass QTOF YA 263 mass spectrometer. The X-band EPR spectra of complexes 1-4
5
6
7
8
were recorded on a Magnettech GmbH MiniScope MS400 spectrometer (equipped with
temperature controller TC H03), where the microwave frequency was measured with an
FC400 frequency counter. Tetracyanoethene (tcne) (g = 2.0027) was used to calibrate the
EPR spectra. The EPR spectra of complexes 1-4 were simulated using Easy Spin32 software.
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1
51
1
The H and V NMR spectra of the complexes in CDCl3 and the H NMR spectra of H2L2
ligand in DMSO-d6 were recorded at 293 K using a Bruker AM 300L 400 MHz
superconducting FT NMR spectrophotometer. The 51V NMR spectra were referenced to
VOCl3 (0 ppm) as an external reference. UV-vis spectra were recorded on a Shimadzu UV-
1700 Spectrophotometer. Electrochemical measurements were performed at 298 K in CH2Cl2
solution for ca. 1×10-3 mol dm-3 using tetrabutylammonium hexafluorophosphate (TBAHP)
as the supporting electrolyte under a dry N2 atmosphere on a BASi Epsilon-EC
electroanalytical instrument. The BASi platinum working electrode, platinum auxiliary
electrode and Ag/AgCl electrode were used for measurements. The redox potential data are
referenced to a ferricenium/ferrocene, Fc+/Fc couple. Emission intensity measurements were
carried out using an RF-5301 PC Shimadzu spectroflourimeter.
2.6 Crystallographic data collection and refinement
X-ray quality single crystals of two ligands (H2L2 and H2L3) and five complexes (3, 5-8) were
obtained from the slow evaporation of the solvent CH3OH of the reaction mixtures at room
temperature. X-ray data were measured with MoKα radiation (λ = 0.71069 Å) for H2L2,
H2L3, 5, 7 and 8 at 150 K and for 3 and 6 at 295 K using the Oxford Diffraction X-calibur
CCD system. Data analyses were carried out with the CrysAlis program33. The structures
were solved using direct methods with the Shelxs97 program34. The non-hydrogen atoms
were refined with anisotropic thermal parameters. The hydrogen atoms bonded to carbon
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were included in geometric positions and given thermal parameters equivalent to 1.2 tVimieweAsrticle Online
DOI: 10.1039/C9NJ06114B
5
6
7
8
those of the atom to which they were attached. Hydrogen atoms bonded to nitrogen and
oxygen atoms were located in difference Fourier maps and refined with distance constraints.
Absorption corrections were carried out using the ABSPACK program35. All seven structures
were refined on F2 using Shelxl16-636. Crystallographic data are collected in Table 1 and
Table 2. CCDC 1851470-1851476 contain the supplementary crystallographic data for H2L2,
H2L3 and complexes 3, 5-8.
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Table 1. Summary of X-ray crystallographic data for ligands H2L2, H2L3 and complex 3.
H2L2
H2L3
3
Empirical formula
C16H17N3O2
283.32
C16H17N3O3
299.32
C32H30N6O6V
645.56
Formula weight
Crystal system
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
I2/a
Space group
a/Å
b/Å
c/Å
/
8.4314(10)
7.1284(10)
23.591(3)
91.638(12)
16.7983(14)
7.3360(8)
12.7009(13)
111.721(11)
11.0164(9)
9.5390(9)
27.584(2)
92.035(8)
e/Å3
1417.3(3)
4
1450.0(3)
4
2896.9(4)
4
Z
ρcalc/g cm-3
1.328
1.367
1.480
/mm-1
0.090
0.097
0.400
F(000)
600
632
1340
Data/restraints/parameters
2482/0/188
4065/0/216
2413/8/2012
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1.060
DOI: 10.1039/C9NJ06114B
Goodness-of-fit on F2
1.251
1.038
5
6
7
8
R indices, [I>2σ(I)] R1, wR2
R indices (all data): R1, wR2
Residual electron density (e/Å3)
0.1044, 0.2240
0.1167, 0.2305
0.265, -0.261
0.0805, 0.1570
0.1407, 0.1827
0.244, -0.236
0.1015, 0.1211
0.1716, 0.1430
0.359, -0.355
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Table 2. Summary of X-ray crystallographic data for complexes 5, 6, 7 and 8.
5
6
7
8
Empirical formula
C16H16N3O4V
365.26
C17 H18N3O4V
379.28
Monoclinic
P21/c
C34H36N6O10V2 C32H30Cl2N6O8V2
Formula weight
790.56
Monoclinic
P21/c
799.40
Crystal system
Monoclinic
P21/c
Triclinic
P-1
Space group
a/Å
b/Å
c/Å
α/o
9.7308(4)
13.0374(5)
12.7577(6)
(90)
8.4289(7)
14.4775(9)
14.0191(14)
(90)
8.9923(2)
12.1935(3)
15.8556(5)
(90)
6.8992(8)
10.7180(12)
11.0208(12)
103.797(9)
92.767(9)
91.011(9)
790.15(15)
1
105.085(5)
(90)
101.407(9)
(90)
104.014(3)
(90)
/
γ/o
V/Å3
1686.80(8)
4
1676.9(2)
4
1562.72(11)
2
Z
ρcalc/g cm-3
1.552
1.502
1.557
1.680
 /mm-1
0.661
0.619
0.623
0.825
F(000)
752
784
816
408
Data/restraints/parameters
Goodness-of-fit on F2
4398/0/226
0.933
3549/0/235
0.929
4740/0/244
0.975
4414/0/228
1.104
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R indices, [I>2σ(I)] R1, wR2
0.0400, 0.0919
0.0633, 0.0967
0.0608, 0.1378 0.0342, 0.0891
DOI: 10.10309/.C096N7J066,1104.B1902
R indices (all data): R1, wR2
0.0865, 0.1537 0.0469, 0.0915 0.0913, 0.2049
6
7
8
9
Residual electron density 0.571, -0.379
(e/Å3)
0.391, -0.700
0.373, -0.398
0.734, -0.667
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2.7 DFT Calculations
The geometries of complexes 1-8 were optimized in the gas phase with the Gaussian 09
(revision B. 01) software37 using the 6-31+G(d,p) basis set38 and the PBEPBE functional39.
Starting models were obtained from the crystal structures where possible or derived using
such data. The structures were optimized in the gas phase and also in methanol solution using
the IEF-PCM solvation model40 with default convergence criteria as implemented in
Gaussian 09 program. Vibrational frequency analyses were obtained to verify the stationary
points to be the real minima and also to obtain thermodynamic energy corrections. TD-DFT
calculations were done using PBEPBE functional and 6-31+G(d,p) basis set with the
Gaussian 09 package.
2.8 Cytotoxicity Assay
Cytotoxicity of the complexes was determined by Promega CellTiter 96® AQueous One
Solution Cell Proliferation Assay kit (Cat # G3580) containing tetrazolium complex
following manufacturer’s instructions. Briefly, 10,000 HARA cells (human squamous cell
lung carcinoma cell line), kindly provided by Dr. Haruo Iguchi41, Clinical Research Institute,
National Hospital Organization Shikoku Cancer Center, Matsuyama, Japan were plated in
100 µl RPMI growth media (supplemented with 10% foetal bovine serum and penicillin-
streptomycin)/well in 96 well plates. After overnight incubation at 37 ºC in CO2 incubator
with 5% CO2, the complexes were added to cells at various concentrations with 200 µl
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growth medium/well. Each concentration was used in duplicate wells. All theDOcIo: 1m0.1p0l3e9x/Ce9sNJ06114B
were dissolved in 100% DMSO and diluted in growth media to obtain the various
concentrations used. After 72 h incubation, medium was carefully aspirated from the wells
and 100 µl growth medium without phenol red was added to each well. Then, 20 μl of
CellTiter 96 AQueous One Solution Reagent was added into each well of the 96-well assay
plate. After 1-4 h incubation at 37 ºC in CO2 incubator, the amount of soluble formazan was
determined by absorbance at 450 nm. NADPH or NADH produced by dehydrogenase
enzymes in metabolically active cells reduce MTS tetrazolium compound into a coloured
formazan product that is soluble in tissue culture medium42,43 . Thus, the amount of formazan
produced is directly proportional to the number of living cells in culture. Results are
expressed as percentages of untreated control, which was arbitrarily assigned 100% viability,
and represented as the standard error of the mean (SEM) of the duplicate wells.
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2.9 Method of nuclear staining for apoptosis assay
We used microscopic methods to detect apoptotic nucleus using a nucleic acid staining dye 4,
6-diamidino-2-phenylindole (DAPI). We followed the procedure as previously described19l in
which HARA cells were plated on glass chamber slides (Lab-TekII Chamber slides, Nunc)
and grown for 24 hours before being treated with the complexes or left untreated (non-
apoptotic control). After 48 hours of treatment cells were fixed with 3.7% formaldehyde for
15 minutes. Subsequently cells were washed three times with 1X phosphate buffered saline
(PBS), permeabilized with 0.1% Triton X-100 and stained with 1 µg/ml DAPI (prepared in
1X PBS) for 5 minutes at 37 ºC. After the DAPI solution was aspirated, cells were washed
three times with 1X PBS, mounted with mounting media; cover slips are applied in place and
examined by fluorescence microscopy using Leica DM IL LED microscope at 40X objective.
2.10 Fluorometric study
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The binding constant of these complexes with CT DNA has been determined by fDluOoI: 1r0o.1m03e9t/Cri9cNJ06114B
method. In the fluorometric experiment, 50µL of a CT DNA (10-4 M) solution in Tris-HCl–
NaCl buffer (5 × 10-2 M and 5 × 10-2 M NaCl, pH = 7.2) was added to 2 mL of ethidium
bromide (1.5 × 10-4 M) in the same buffer medium to get the maximum fluorescence
intensity. Aliquots of stock solution (0.01 M) of the complex in DMSO were added to the
ethidium bromide bound CT DNA solution and the fluorescence was measured for each test
solution after 2 hours of incubation at 37 ºC. The solutions were excited at 515 nm (with
excitation and emission slit 5 nm) and emission spectra were recorded from 520 nm to 700
nm. The solutions containing an equivalent amount of a CT DNA solution, each of
complexes 1-8 and buffer when excited at 515 nm did not exhibit any fluorescence emission.
The fluorescence quenching data were further analyzed using the Stern-Volmer equation44.
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2.11 Method for molecular docking study
The DFT optimized geometries of complexes 1-8 were used in docking. For docking studies
the structural coordinates of CT DNA were obtained from the protein data bank (PDB ID:
1BNA). Molecular docking for these two complexes was carried out using AutoDock 4.245.
The conformation of the DNA was fixed and the conformations of the complexes were
allowed to change during molecular docking. The data obtained from docking was examined
by PyMOL (http://pymol.sourceforget.net/) software46. Discovery Studio 4.1 Client and
Chimera 1.10.1rc were used for visualization effects.
3. Results and Discussion
3.1 Theoretical predictions for the formation of different type of complexes
With a view to obtain ideas about the types of complex that may form in the reaction between
[VIVO(aa)2] [aa- representing the monoanionic form of acetylacetone(Haa)] with H2L1-4
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ligands (general abbreviation H2L) in the laboratory grade (LG) methanol (contains 0.1%
5
6
7
8
H2O) in the presence of air, we carried out a computational study using the 6-31+G(d,p) basis
set and PBEPBE functional set to evaluate the formation of two possible complexes [VIV(L)2]
and [VVO(L)(OCH3)] with all the four ligands through the calculation of the difference in
enthalpy (∆H), Gibbs free energy (∆G) and internal energy (∆E) between the probable
products and respective reactants in methanol solution37-40 using the equations as described in
Scheme 2. It has been found that ∆H, ∆G and ∆E values (Table 3) are on average ~10, ~5 and
~14 kcal mol-1 respectively for the formation of the non-oxidovanadium(IV) complexes 1-4
but for the formation the oxidovanadium(V) complexes 5-8 these are on average ~-16, ~-20
and ~-12 kcal mol-1 respectively suggesting the non-feasibility of the formation of complexes
1-4 by this method. However, it should be noted that for these two types of complexes [viz.,
oxidovanadium(V) and non-oxidovanadium(IV)], the value of these parameters decreases
with the increase of electron donating property of the substituent in the aryloxy ring of the
hydrazone ligands.
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22
23
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25
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 3. ∆G, ∆H and ∆E values (in kcal/mol) calculated using the PBEPBE functional and 6-
31+G (d,p) basis set in methanol solution utilizing the IEF-PCM solvation model) for the
formation of [VIV(L)2] and [VVO(L)(OCH3)] complexes using scheme 2.
1
5
2
6
3
7
4
8
∆Hgas 7.7
∆Ggas 2.1
∆Hsolv 10.5
∆Gsolv 4.6
-18.0
-21.1
-16.4
-19.3
-12.0
6.8
1.6
9.3
3.4
13.2
-18.1 5.1
-20.8 0.9
-16.8 7.7
-19.6 2.3
-12.6 12.2
-18.5 7.6
-20.9 2.8
-17.0 10.8
-19.8 5.5
-12.4 14.4
-17.5
-20.1
-15.9
-18.9
-11.3
∆E
13.8
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Scheme 2. Schematic representation for the formation of probable tetravalent and pentavalent
complexes on reaction of [VIVO(aa)2] with two equivalents of H2L in LG (Laboratory Grade)
CH3OH in air.
3.2 Experimental verification of theoretical predictions
To verify these theoretical predictions, we carried out several experiments utilizing the
tetravalent precursor [VIVO(aa)2] with varying amounts of respective H2L. On refluxing
[VIVO(aa)2] with equivalent amounts of H2L in laboratory grade (LG) CH3OH under aerobic
conditions both non-oxidovanadium(IV) and oxidovanadium(V) complexes were obtained in
~30% yield (based on [VIVO(aa)2]) though by theoretical calculations the formation of non-
oxidovanadium(IV) complexes is not indicated. However, on reaction of [VIVO(aa)2] with
two equivalents of respective H2L, the % yield of non-oxidovanadium(IV) complexes was
increased to ~70% (based on [VIVO(aa)2]) while that of oxidovanadium(V) complexes
decreased to ~25%. The formation of these non-oxidovanadium(IV) complexes, not favoured
theoretically, may be due to their low solubility in CH3OH (Scheme 2). However, on
performing the above-mentioned reaction in 1:1 ratio in dry methanol in the absence of air, as
expected, only the non-oxidovanadium(IV) complexes were obtained in ~45% yield
suggesting that aerial dioxygen is the oxidizing agent. It is to be noted that for the formation
of non-oxidovanadium(IV) complexes from the oxido precursor [VIVO(aa)2], the required
loss of oxido moiety from V=O group as water molecule most likely occurs via the formation
of intermediate hydroxy (V—OH) group through protonation by the ligands’ acidic group(s)
associated with the coordination of respective ligand replacing aa- species (which is removed
as Haa) followed by further protonation of OH moiety to form water.
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3.3 Description of crystal structures
3.3.1 Description of crystal structure of H2L2 and H2L3
8
9
The molecular structures of the hydrazone ligands H2L2 (Fig. 1) and H2L3 (Fig. S1), the
condensed products of 2-aminobenzoylhydrazine with respectively 2-hydroxy-5-
methylacetophenone and 2-hydroxy-5-methoxyacetophenone reveal that they exist in the
enamine form in the solid state. However, in the presence of vanadium, they undergo
enolisation to bind the metal (vide infra). Relevant dimensions for H2L2 and H2L3 are
collected in Table 4. The C(8)-N(2) and N(1)-N(2) bond lengths suggest single bond
character while C(8)-O(2) and C(7)-N(1) bond lengths show double bond nature. The
molecules are slightly twisted at the N(1)-N(2) bond so that the C(7)-N(1)-N(2)-C(8) torsion
angles are 174.7(2), 170.1(2)o respectively. In addition, it is noteworthy that the N(2)-O(8)-
C(9)-C(10) torsion angles are 9.3(2), 27.3(2)o in the two structures, showing a remarkable
twist in the structure of H2L3. Two intramolecular hydrogen bonds, one involving the O(1)-
H(1) with N(1) and the other involving N(3)-H(3A) with O(2) are also observed. Dimensions
for the hydrogen bonds are given in Table S1.
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56
57
58
59
60
Fig.1. Molecular structure of the H2L2 ligand with ellipsoids at 30% probability. Hydrogen
bonds are shown as dotted bonds.
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Table 4. Selected bond lengths (Å) in ligands H2L2 and H2L3.
5
6
7
8
H2L2
H2L3
1.362(2)
C2 -O1
C8 -O2
C7- N1
C8- N2
N1-N2
1.361(5)
1.235(5)
1.289(5)
1.376(5)
1.372(5)
9
1.226(2)
1.290(2)
1.368(3)
1.378(2)
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3.3.2 Description of crystal structure of 3
The structure of complex 3 which has crystallographic C2 symmetry is shown in Fig 2. The
metal atom has a distorted trigonal prismatic environment and is bonded to two ligands with
bond lengths V-O1 1.877(4), V-O2 1.905(4), V-N1 2.085(5) Å (Table 5). In the trigonal
prism, the two triangular faces are made up of O1, N1, O2(i) and O1(i), N1(i) and O2. [(i) =
symmetry element 3/2-x, y, 1-z]. In addition, there is an intramolecular H-bond between N3-
H and N2, dimensions are given in Table 6.
Fig. 2. Molecular structure of 3 with ellipsoids at 30% probability. Hydrogen bonds are
shown as dotted bonds.
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Table 5. Selected bond lengths (Å) and angles (°) in complex 3.
5
6
7
8
9
V-O1
V-O2
1.877(4)
1.905(4)
2.085(5)
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34
35
36
37
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46
47
48
49
50
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52
53
54
55
56
57
58
59
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V-N1
Bond angles, deg
O1-V-O1(i)
O1-V-O2(i)
85.2(3)
86.2(2)
125.8(2)
138.7(3)
82.7(2)
O1-V-O2
O2-V-O2(i)
O1-V-N1
O1-V-N1(i)
O2-V-N1(i)
O2-V-N1
144.4(2)
86.6(2)
74.4(2)
N1-V-N1(i)
124.5(3)
(i) = symmetry element 3/2-x, y, 1-z
Table 6. Dimensions of hydrogen bonds (distances, Å, angles (°)) in the metal complexes.
Complex
D-H…A
d(D-H)
d(H…A)
2.09(6)
2.09(2)
2.08(4)
2.50(4)
2.11(2)
d(D…A)
2.719(9)
2.728(2)
2.730(4)
3.206(4)
2.779(2)
<(DHA)
128(6)
134(2)
130(3)
142(4)
139(2)
symmetry element
3
5
6
N3-H3A…N2 0.87(2)
N3-H3A…N2 0.83(2)
N3-H3A…N2 0.89(4)
N3-H3B…O4 0.84(4)
N3-H3A…N2 0.82(2)
2-x, -1/2+y, ½-z
7
8
N3-H3A…N2 0.85(2)
2.06(4)
2.730(5)
135(5)
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3.3.3 Description of crystal structures of 5-8.
5
6
7
8
9
The dimensions of the four structures which have a common numbering system are compared
in Table 7. The structures of 5 and 6 are very similar and consist of a [VVO(L)(OCH3)] unit
[L2- = (L1)2- and (L2)2-] as shown in Fig. 3 and Fig. 4 respectively. The metal atom is five-
coordinate in a square pyramidal geometry with one terminal oxygen atom O3 in an axial
position. The three equatorial positions are occupied by the three donor atoms O1, O2 and N1
of the planar tridentate L2- ligand while the fourth equatorial position is occupied by O4 of an
OCH3 group. There are only small differences between the geometry of the coordination
spheres in 5 and 6. Thus the V-O3 (terminal) distances are 1.590(1), 1.583(2) Å. The V-O2
distances [1.917(1), 1.905(2) Å] are longer than the V-O1 distances [1.821(1), 1.822(1) Å].
The V-N1 distances are 2.0972(2), 2.097(2) Å. The V-O4 distances [1.768(1), 1.777(2) Å] as
expected, are shorter than both V-O1 and V-O2 distances. Least squares planes calculations
show that the four atoms in the equatorial planes show r.m.s. deviations of 0.122, 0.131 Å
with the metal atom 0.486(1), 0.508(1) Å from the plane in the direction towards the terminal
oxygen O3.
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Fig. 3. Molecular structure of 5 with ellipsoids at 30% probability. Hydrogen bonds are
shown as dotted bonds.
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5
6
7
8
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Fig. 4. Molecular structure of 6 with ellipsoids at 30% probability. Hydrogen bonds are
shown as dotted bonds.
Complexes 7 and 8 have similar structures (Fig. 5 and Fig. 6 respectively) but the
molecules form centrosymmetric dimers as the metal forms weak bonds with the N3(ii) [(ii) =
symmetry element 1-x, 1-y, 2-z in 7; -x, 1-y, -z in 8) in axial positions at 2.577(1) and
2.874(4) Å respectively. Despite this weak interaction in the axial position thus forming a
distorted octahedron, the other five bond lengths around the metal are very similar to those
found in 5 and 6. However it is interesting to note that because of the presence of N3(ii) in an
axial site, the deviation of the metal atom from the equatorial plane of the four donor atoms is
significantly less than in 5 and 6 being 0.325(1) and 0.382(1) Å in 7 and 8 with the r.m.s.
deviation 0.049 and 0.003 Å respectively. There is no obvious reason why the structures of 7
and 8 are different from the monomeric 5 and 6 in which there are no atoms approaching the
vacant axial position. In all four structures 5-8 there is an intramolecular hydrogen bond
between N3-H and N2, dimensions are given in Table 6. The second proton on N3
participates in an intermolecular hydrogen bond in complex 6 (Table 6) and while there are
weaker interactions in the other structures, the second proton is free of hydrogen bonds.
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The DFT optimized geometrical parameters of 3, 5, 6, 7 and 8 are inDOvIe: 1r0y.10g39o/Co9dNJ06114B
agreement with the values obtained from the X-ray crystallographic data. Some selected
optimized bond lengths and bond angles of complexes are given in Table S2 and Table S3.
However, the dimerisation in complexes 7 and 8 is also supported by the DFT study as their
dimers are more stable than their respective monomer by 2.08 and 3.00 kcal mol-1
respectively.
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5
6
7
8
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Fig. 5. Structure of the centrosymmetric dimer in 7 with ellipsoids at 30% probability.
Hydrogen bonds are shown as dotted bonds.
Fig. 6. Structure of the centrosymmetric dimer in 8 with ellipsoids at 30% probability.
Hydrogen bonds are shown as dotted bonds.
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Table 7. Selected bond lengths (Å) and angles (°) in complexes 5-8.
5
6
5
6
7
8
7
8
9
Bond lengths, Å
V-O1
1.821(1)
1.917(1)
1.590(1)
1.768(1)
2.078(2)
1.822(2)
1.905(2)
1.583(2)
1.777(2)
2.097(3)
1.842(1)
1.948(1)
1.585(1)
1.782(1)
2.130(1)
2.577(1)
1.824(3)
1.922(3)
1.591(3)
1.765(3)
2.094(3)
2.846(4)
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60
V-O2
V-O3
V-O4
V-N1
V-N3(ii)
Bond angles, deg
O3-V-O4
O3-V-O1
O4-V-O1
O3-V-O2
O4-V-O2
O1-V-O2
O3-V-N1
O4-V-N1
O1-V-N1
O2-V-N1
O3-V1-N3(ii)
O4-V1- N3(ii)
O1-V1-N3(ii)
O2-V1-N3(ii)
N1-V1- N3(ii)
105.61(7)
106.69(7)
100.78(6)
110.29(6)
86.35(6)
138.78(6)
96.80(7)
154.72(6)
83.43(6)
74.85(5)
105.40(11)
107.97(12)
98.77(11)
111.25(13)
87.66(10)
136.86(11)
97.57(11)
155.06(10)
82.96(10)
74.87(10)
102.19(6)
100.94(6)
102.51(5)
100.69(6)
93.06(5)
149.90(5)
94.23(6)
161.09(6)
83.25(5)
74.53(5)
176.29(5)
81.50(5)
78.41(5)
78.64(5)
82.08(4)
106.18(17)
102.31(15)
102.07(14)
100.30(15)
88.59(14)
151.07(13)
96.54(14)
154.32(15)
84.19(13)
75.50(12)
174.52(14)
79.04(14)
74.65(12)
81.26(12)
78.72(12)
(ii) = symmetry element 1-x, 1-y, 2-z in 7 and -x, 1-y, -z in 8.
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3.4 IR Spectroscopy
5
6
7
8
9
In the IR spectra of the ligands, the C=O and C=N stretching vibrations are observed in the
1628-1668 cm-1 and 1610-1651 cm-1 regions respectively. Two bands in the region 3444-
3477 and 2918-3059 cm-1 are assigned to phenolic O-H and N-H bonds respectively. The N-
N band is observed in the 914-921 cm-1 region. Disappearance of characteristic C=O and N-H
bands of the ligands and the appearance of a new band in the 1254-1264 cm-1 region is
attributed to ν(C-Oenolic) indicating the enolisation. The strong band in the 977-997 cm-1
region is assigned to V≡O bond for 5-8 complexes whereas no such stretching vibration in
the 900-1000 cm-1 region is observed for 1-4 complexes suggesting the presence of non-
oxido form of vanadium in these complexes. The N-N band of the complexes is found in the
1027-1084 cm-1 region indicating a 109-122 cm-1 shift towards higher wave number upon
complexation due to diminished repulsion between the lone pairs of adjacent nitrogen atoms.
A sharp band, observed in the 3085-3445 cm-1 region is likely to be associated with the
intramolecularly H-bonded N-H group.
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3.5 UV-vis spectroscopy
Electronic spectra of the complexes 1-8 were recorded in CH2Cl2 solution. The spectral data
are listed in Table 8 and the representative spectra (along with deconvoluted) of 1 and 5 are
displayed in Fig. 7 and Fig. 8 respectively while the electronic spectra of 2-4 are displayed in
Fig. S2- Fig. S4 and of 6-8 complexes in CH2Cl2 solution are displayed in Fig. S5 - Fig. S7
respectively. The electronic spectra of the complexes 1-4 are closely similar and showed a
broad absorption band in the visible region with a main band in the 575-595 nm and a broad
peak in the 484-502 nm regions. Both of these two bands, falling in the visible region, are
expected to be due to d-d transitions but their high intensity and analogy with other non-
oxido vanadium(IV) complexes19,23,25 suggest that they are most reasonably assigned to
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ligand-to-metal-charge-transfer (LMCT) transitions. The expected low intensity Viedw-Adrticle Online
DOI: 10.1039/C9NJ06114B
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8
transitions are masked by the highly intense LMCT transitions. The other bands in the higher
energy region (370-380 nm) are likely to be due to intra-ligand π→π* transitions. The
expected LMCT transition for the pentavalent 5-8 complexes is observed in the 400-412 nm
region while the ligand-centred π→π* transition is observed in the 324-326 nm region.
9
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60
Fig. 7. Electronic spectra of 1 in CH2Cl2. (black experimental; red, green, blue and sky
deconvoluted)
Fig. 8. Electronic spectra of 5 in CH2Cl2. (black experimental; red, green and blue
deconvoluted)
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An increase in λmax values (for the LMCT transition) for complexes 1-4 and 5-8DiOsI:o10b.1s0e3r9v/Ce9dNJ06114B
with the increase in electron withdrawing property of the substituent while the reverse is true
for electron donating substituent(s) which is quite expected from the ligands’ basicity point of
view.
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11
12
13
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16
17
18
19
20
21
22
23
24
25
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Table 8. Electronic spectral and electrochemical data at 298K of the complexes 1-8 in CH2Cl2
solution.
Complex
λmax, nm (ε, M-1 cm-1)
E1/2 (V)
(∆E) (mV)
[VIV(L1)2] (1)
[VIV(L2)2] (2)
575(6280), 492(9610), 380(23250) -0.32(200), 0.68(190)
560(1840), 486(3100), 376(10080) -0.37(260), 0.67(160)
[VIV(L3)2] (3)
557(1830), 484(2620), 377(7980)
-0.39(250), 0.66(130)
[VIV(L4)2] (4)
595(4190), 502(5600), 370(15220) -0.22(150), 0.70(80)
[VVO(L1)(OCH3)] (5)
[VVO(L2)(OCH3)] (6)
[VVO(L3)(OCH3)]2 (7)
[VVO(L4)(OCH3)]2 (8)
408(5520), 326(7650)
402(8660), 326(12930)
400(6210), 324(10440)
412(5800), 324(9040)
0.19(170)
0.17(170)
0.15(190)
0.22(150)
All potentials are vs Ag/AgCl reference. E1/2 = 0.5 (Epc + Epa), calculated from cyclic
voltametric data. ∆E = Epc - Epa. Scan rate = 100 mVs-1. Epa and Epc representing anodic and
cathodic peak potential value respectively.
3.6 1H NMR Spectroscopy
1
The H NMR spectra of the ligands have been recorded in DMSO-d6 solvent while those of
1
complexes 5-8 were recorded in CDCl3 solvent. The H NMR spectral data are given in the
experimental section. The phenolic O-H group of the acetophenone moiety of the free ligands
exhibits resonance in the region δ12.63-13.23 ppm. However, this signal is not observed in
the complexes 5-8 suggesting that the phenolic OH group of the acetophenone moiety is
involved in bonding with vanadium through deprotonation. All the aromatic protons of the
ligands in the complexes are observed in the δ6.68-8.16 ppm region.
29