Synthesis and molecular structure of a zinc complex of the vitamin K 3 analogue phthiocol

Article abstract of DOI:10.1016/j.molstruc.2013.05.057

The complex [Zn(phthiocol)₂(H₂O)₂]; 1, where phthiocol is 2-hydroxy-3-methyl-1,4-naphthoquinone, has been synthesized and characterized by elemental analysis, FT-IR, ¹H NMR, UV-vis spectroscopy, thermogravimetric (TG) analysis, electrochemical and single crystal X-ray diffraction studies. The m_C=O stretch shifts to lower frequencies upon complexation of phthiocol to Zn²⁺. ¹H NMR spectra show an upfield shift of the benzenoid ring protons in 1. There is a bathochromic shift of the LMCT band in the UV-vis spectra of 1. Single crystal X-ray structure of 1 show distorted octahedral geometry around Zn²⁺. Two phthiocol ligands are in plane with the metal, while water molecules are trans to this plane. Coordination of deprotonated phthiocol ligands is 'trans, trans' to Zn²⁺. Intra as well as intermolecular interactions are observed in 1. Molecules of 1 show three dimensional network through C-H...O and O-H...O interactions. Additional anodic peaks are observed in cyclic voltammogram of phthiocol ligand due to oxidation of reduced species formed during reduction. One-electron reduction of 1 is shown to be reversible and DFT studies define this redox event as ligand-centered.

Full text of DOI:10.1016/j.molstruc.2013.05.057

Journal of Molecular Structure 1048 (2013) 223–229
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and molecular structure of a zinc complex of the vitamin K3
analogue phthiocol
Laxmi Kathawate a, Stephen Sproules b, Omkar Pawar a, Ganesh Markad a, Santosh Haram a,
Vedavati Puranik c, Sunita Salunke-Gawali a,
⇑
a Department of Chemistry, University of Pune, Pune 411 007, India
b WestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK
c Center for Material Characterization, National Chemical Laboratory, Pune 411 007, India
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
Zinc complex of the vitamin K3 analogue phthiocol is synthesized and characterized.
Molecules of 1 show three dimensional network through CAHꢁ ꢁ ꢁO, OAHꢁ ꢁ ꢁO interactions.
Additional anodic peaks are observed in cyclic voltammogram of phthiocol ligand.
DFT studies define this redox event as ligand-centered.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 April 2013
Received in revised form 20 May 2013
Accepted 20 May 2013
Available online 31 May 2013
The complex [Zn(phthiocol)2(H2O)2]; 1, where phthiocol is 2-hydroxy-3-methyl-1,4-naphthoquinone,
has been synthesized and characterized by elemental analysis, FT-IR, 1H NMR, UV–vis spectroscopy, ther-
mogravimetric (TG) analysis, electrochemical and single crystal X-ray diffraction studies. The mC@O
stretch shifts to lower frequencies upon complexation of phthiocol to Zn2+ 1H NMR spectra show an
.
upfield shift of the benzenoid ring protons in 1. There is a bathochromic shift of the LMCT band in the
UV–vis spectra of 1. Single crystal X-ray structure of 1 show distorted octahedral geometry around
Zn2+. Two phthiocol ligands are in plane with the metal, while water molecules are trans to this plane.
Coordination of deprotonated phthiocol ligands is ‘trans, trans’ to Zn2+. Intra as well as intermolecular
interactions are observed in 1. Molecules of 1 show three dimensional network through CAHꢁ ꢁ ꢁO and
OAHꢁ ꢁ ꢁO interactions. Additional anodic peaks are observed in cyclic voltammogram of phthiocol ligand
due to oxidation of reduced species formed during reduction. One-electron reduction of 1 is shown to be
reversible and DFT studies define this redox event as ligand-centered.
Keywords:
Vitamin K3
Naphthoquinone
Phthiocol
Zinc complex
Trans coordination
Hydrogen bonding
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
Menadione (2-methyl-1,4-naphthoquinone) is artificial form of
vitamin K3 known for its anticancer activities [9]. Phthiocol (2-hy-
Chemistry and coordination chemistry of redox active ligands
[1] is of growing importance in recent years because of their mul-
tifunctional applications [2], especially to catalytic applications [3].
Naphthoquinones are redox active ligands, as compared to other
non-innocent ligands, the coordination chemistry and catalytic
applications of these ligands are least explored. However their bio-
logical applications for example antiproliferative [4], antifungal [5],
antiviral [6], antimalarial [7], etc. are well established. Wide vari-
ety of applications of naphthoquinone in biology is due to their re-
dox activity and radical forming abilities [8].
droxy-3-methyl-1,4-naphthoquinone) is synthesized from mena-
dione and is an hydroxy derivative of vitamin K3 family members
also known for its antifungal [10] and antionconogenic properties
[11]. Single crystal X-ray structures show that hydroxy naphtho-
quinones and their metal complexes are polymeric in nature
formed by three dimensional hydrogen bonding network through
OAHꢁ ꢁ ꢁO and CAHꢁ ꢁ ꢁO interactions [12]. Molecular associations
of naphthoquinones through hydrogen bonding help them to inter-
act with biomolecules and stabilization of reduced forms com-
monly known as naphthosemiquinone radicals. Besides biological
applications of hydroxy naphthoquinones, they can act as a bi or
monodentate ligands and could provide redox active functional
materials [13], thus synthesis of metal complexes with naphtho-
quinone ligands are highly encouraged.
⇑
Corresponding author. Tel.: +91 2025601397/25601395x531; fax: +91
2025693981/2025691728.
E-mail address: sunitas@chem.unipune.ac.in (S. Salunke-Gawali).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.05.057

224
L. Kathawate et al. / Journal of Molecular Structure 1048 (2013) 223–229
Metal complexes with crystal structures with phthiocol ligands
used for structure solution and full matrix least squares refinement
on F2. Hydrogen atoms were included in the refinement as per the
riding model but the hydrogens of the water molecule were ob-
tained by difference Fourier and were refined during the refine-
ment. Data collection and refinement parameters are listed in
Table 1. X-ray analysis revealed the conformation of the molecule
it is found that half of the complex molecule is related to another
half by symmetry.
are rare although their synthesis is known [14] only single crystal
X-ray structure of Mn complex [15] is known so far, since the
structure of phthiocol in 1965 [16]. In present investigation syn-
thesis, characterization, single crystal X-ray diffraction studies
and electrochemical properties of zinc metal complex of vitamin
K3 analogue; phthiocol viz. [Zn(phthiocol)2(H2O)2]; 1 are discussed.
Intraligand bond lengths of the one- and two-electron reduced
forms are computed by density functional theory (DFT). Phthiocol
ligand can potentially bind to metal ions in three different oxida-
tion states, viz. naphthoquinone, naphthosemiquinone and catech-
olate as shown in (Scheme 1).
2.2.4. DFT studies
All DFT calculations were performed with the ORCA program
[21]. Geometry optimized structures of 1, [1]1ꢃ and [1]2ꢃ were
achieved using the B3LYP method [22,23]. The conductor-like
screening model (COSMO) was applied using water as the solvent
[24]. The all electron basis sets of triple-f quality were those devel-
oped by the Ahlrichs group [25]. Auxiliary basis sets were used to
expand the electron density in the calculations were chosen the
match the orbital basis. The geometry search was carried out in
redundant internal coordinates without imposing geometry con-
straints, and stationary points were characterized via numerical
calculations. Spin density plots were obtained using Molekel [26].
2. Experimental
2.1. Material and methods
All the chemicals used in the synthesis were analytical grade
and used without further purification. Menadione (2-methyl-1,4-
napthoquinone) have been obtained from Sigma–Aldrich. Zinc ace-
tate was obtained from Qualigens Chemicals, India. Milli-Q water is
used in estimation of Zn2+ volumetrically [17]. Anhydrous metha-
nol was purified by the literature method [18].
2.2.5. Cyclic voltammetric studies
The electrochemical measurements were performed with the
help of Metrohm Potentiostat/Galvanostat (Model Autolab PGSTAT
100). A commercial Pt disk electrode (CHI Instruments, USA, 2 mm
diameter), Ag wire, and Pt wire loop were used as working, qua-
sireference and counter electrodes respectively. The voltammetric
measurements were carried out in an indigenously developed vac-
uum electrochemical cell having a special provision to transfer the
analyte with minimum exposure to the laboratory atmosphere
[27]. After fixing the electrodes to the cell 0.239 g of tetra butyl
ammonium perchlorate (typically 100 mM in 7 mL solution) was
transferred and vacuum dried in situ at 80 °C for an hour. The cell
was cooled down to room temperature and brought to atmo-
spheric pressure by relieving the vacuum through high purity ar-
gon gas. Predried 5 mL solvent was injected into the cell through
silicon septum under argon atmosphere. The blank or controlled
voltammograms were acquired in tetra butyl ammonium perchlo-
rate–DMSO mixture prior to the measurements. Sample dispersed
in small amount of solvent injected in the cell for further measure-
ment (analyte concentration 1 mg mLꢃ1). At the end of each set of
experiments the potentials were calibrated with respect to the nor-
mal hydrogen electrode (NHE) using ferrocene as an internal
standard.
2.2. Characterization studies
2.2.1. FT-IR, elemental analysis, 1H NMR
Infrared spectra were recorded on a SHIMADZU FT 8400 spec-
trometer as KBr discs. Elemental analyses were conducted with a
Thermo Finnigan EA 1112 Flash series Elemental Analyzer. 1H
NMR spectra were collected on a Varian Mercury 300 MHz in
DMSO-d6 and calibrated to tetramethylsilane.
2.2.2. Thermogravimetric (TG) studies and LC–MS
Thermogravimetric measurements were carried out with a
home built instrument previously detailed [19]. Liquid chromato-
graph mass spectra have been recorded on Shimadzu, LCMS-
2010EV.
2.2.3. X-ray crystallography
Single crystals of 1 were grown by slow evaporation of the solu-
tion in methanol. Red colored needle type crystal of approximate
size 0.17 ꢂ 0.05 ꢂ 0.02 mm3, was used for data collection on Bruker
SMART APEX CCD diffractometer using Mo K
a radiation with fine
focus tube with 50 kV and 30 mA. Crystal to detector distance
6.05 cm, 512 ꢂ 512 pixels/frame, hemisphere data acquisition.
Total frames = 1271, Oscillation/frame ꢃ0.3°, exposure/frame
= 10.0 s/frame, maximum detector swing angle = ꢃ30.0°, beam cen-
ter = (260.2, 252.5), in plane spot width = 1.24, SAINT integration, h
range = 2.09–24.99°, completeness to h of 24.99° is 99.9%. SADABS
correction applied. All the data were corrected for Lorentzian,
polarization and absorption effects. SHELX-97 (ShelxTL) [20] was
2.3. Synthesis
2.3.1. Synthesis of 2-hydroxy-3-methyl-1,4-naphthoquinone
(phthiocol)
The preparation followed a slight modification of the published
procedure [28]. Menadione (1.0 g; 5.8 mmol) was dissolved in
Scheme 1. Oxidation levels of the phthiocol ligand (I: Naphthoquinone, II: Naphthoqsemiquinone, III: Catechol).

L. Kathawate et al. / Journal of Molecular Structure 1048 (2013) 223–229
225
Table 1
3. Result and discussion
Crystal structure data and structure refinement for 1.
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C22H18O8Zn
475.73
296(2) K
0.71073 Å
Monoclinic
P21/n
a = 5.015(1) Å, a = 90°
b = 12.447(3) Å, b = 98.280(3)°
c = 15.785(4) Å,
975.1(2) Å3
2
Coordination of hydroxy naphthoquinone ligands to metal ions
will take place by deprotonation of hydroxyl group and it needs
basic conditions, this can be achieved by addition of pyridine
[29], sodium acetate [30], ammonia [31], 4,40-bipyridine [32],
triethylamine [33], etc. Based on the synthetic conditions the coor-
dination of hydroxynaphthoquinone ligands to metal ions may be
either ‘cis, cis’ or ‘trans, trans’ for example, the coordination of
2-hydroxy-1,4-naphthoquinone (lawsone) to Zn2+ is ‘cis, cis’ when
1 mmol of triethylamine is used while, ‘trans, trans’ coordination
is achieved with 2 mmol of triethylamine [29]. Coordination of
hydroxynaphthoquinone with metal ions is always ‘anti, anti’,
when acetate ions are used in reaction mixture [30,12b]. Reaction
of zinc acetate with phthiocol at room temperature forms red
precipitate of 1: [Zn(phthiocol)2(H2O)2].
Various analytical tools are used to determine the oxidation
states of redox active ligands viz. frequency of mC@O by FT-IR spec-
troscopy [1b,e] and CAO bond length by single crystal X-ray dif-
fraction studies, etc., however sometimes they may lead to
misleading results, for antiferromagnetic interaction between the
semiquinone radicals [34]. Oxidation states of hydroxy naphtho-
quinone ligands could also be determined by thermogravimetric
analysis [19]. To reveal the oxidation states of phthiocol ligands
in 1 characterization is performed by FT-IR, thermogravimetric
analysis, single crystal X-ray diffraction and electrochemical stud-
ies. The molecular and electronic structures of the one- and two-
electron reduced forms are evaluated by DFT studies.
Unit cell dimensions
c = 90°
Volume
Z
q
1.620 g/cc
1.310 mmꢃ1
488
Absorption coefficient
F(000)
Crystal size
Theta range for data collection 2.09–24.99°
Index ranges
Reflections collected
Independent reflections
Completeness to theta = 24.99° 99.9%
Absorption correction
0.17 ꢂ 0.05 ꢂ 0.02 mm3
ꢃ5 <= h <= 5, ꢃ14 <= k <= 14, ꢃ18 <= l <= 12
4803
1698 [R(int) = 0.0335]
Semi-empirical from equivalents
Max. and min. transmission
Refinement method
0.9806 and 0.8052
Full-matrix least-squares on F2
1698/0/151
Data/restraints/parameters
Goodness-of-fit on F2
1.168
Final R indices [I > 2sigma(I)]
R indices (all data)
Largest diff. peak and hole
R1 = 0.0510, wR2 = 0.1038
R1 = 0.0686, wR2 = 0.1092
0.508 and ꢃ0.249 eÅꢃ3
10 mL methanol and chilled on an ice bath. A solution of 0.2 g of
anhydrous Na2CO3 and 1 mL of 30% H2O2 in 5 mL of water was then
added and the reaction mixture maintained at 0 °C. Addition of
100 mL of chilled water led to the precipitation of 2-methyl-1,4-
naphthoquinoneoxide as colorless crystals which were collected
by filtration and dried in air. The solid was treated with 5 mL con-
centrated H2SO4 and allowed to stand for 10 min. The addition of
20 mL water afforded a yellow precipitate which was recrystallized
from methanol containing a few drops of concentrated H2SO4. The
crude product was column chromatographed using 5% methanol in
toluene.
3.1. Characterization of 1
The FT-IR spectra of phthiocol and complex 1 are overlaid in
Fig. 1. The peak at ꢄ3290 cmꢃ1 in the spectrum of 1 is characteris-
tic of coordinated water as opposed to water trapped in the KBr
matrix that manifests as
a considerably sharper peak at
3400 cmꢃ1 in the spectrum of phthiocol. The considerable width
of this peak obscures the mCAH stretches of the aromatic protons
>3000 cmꢃ1 in 1. These are clearly visible in the spectrum of free
phthiocol together with the corresponding vibrations of the
C(3)ACH3 substituent ꢄ2900 cmꢃ1
.
The mC@O vibration at
2.3.1.1. Characterization of phthiocol. Yellow solid, Yield: 0.84 g
(84%). FT-IR; (KBr, cmꢃ1): 3371 s, 1660 s, 1591 s, 1392 s, 1276
s, 1211 s, 833 s, 727 s, 684 s, 634 sh. 1H NMR; (DMSO-d6,
300 MHz): d 1.956 (s, 3H, Ar), 7.969 (d, J = 6.3, 2H, Ar), 7.821 (t,
J = 6.15, 2H, Ar), 7.778 (t, J = 6.3, 2H, Ar), 7.989 (d, J = 5.4, 2H,
Ar), d 10.900 (s, ArAOH). UV–vis; (methanol, kmax, nm): 287,
330, 388, 479. LC-MS (m/z): 188.
ꢄ1635 cmꢃ1 of phthiocol bound to Zn2+ in 1 is shift ꢄ25 cmꢃ1 to
lower energy than the free entity. The mC@C vibrations
(ꢄ1587 cmꢃ1) also shift to slightly lower energy upon complexa-
tion of phthiocol. A new band observed at ꢄ1550 cmꢃ1 is of delo-
calization of charge between mC@C and mC@O after coordination of
the free phthiocol ligand [34]. The p-naphthoquinone vibration in
1 is observed ꢄ1290 cmꢃ1, an increase of ꢄ10 cmꢃ1 compared to
phthiocol ligand (ꢄ1280 cmꢃ1). Similarly the mC(2)AO vibration is
shifted 21 cmꢃ1 to higher energy.
1H NMR spectrum of phthiocol show two singlet due to AOH
and C(3)ACH3, two doublets due to C(5)H and C(8)H, and two trip-
lets due to C(6)H and C(7)H (Fig. 2). The singlet peak due to AOH
disappears upon coordination of a Zn2+ ion forming 1. There is up-
field shift of 0.11–0.23 ppm observed for the benzenoid ring pro-
tons in 1 compared to free phthiocol.
2.3.2. Synthesis of 1
A solution of zinc acetate (0.22 g; 1.0 mmol) in 10 mL of anhy-
drous methanol was added drop-wise to phthiocol (0.38 g;
2.0 mmol) dissolved in 25 mL anhydrous methanol over 30 min.
with constant stirring. The precipitate of dark red 1 was collected
by filtration, washed with methanol, then diethyl ether and dried
under vacuum.
UV–vis spectra for phthiocol and 1 were recorded between 200
and 800 nm in methanol, with both exhibiting three bands
(Fig. S1). Two excitation in the UV region at 292 nm and 329 nm
2.3.2.1. Characterization of 1. Red solid, Yield: 0.33 g (70%). FT-IR;
(KBr; cmꢃ1): 3354 br, 1635 s, 1585 s, 1381 s, 1290 s, 1232 s, 844
s, 734 s, 682 s, 665 s. 1H NMR (DMSO-d6, 300 MHz): d 1.839 (s,
Ar), 7.508 (t, J = 7.8, 2H, Ar), 7.665 (t, J = 6.6, 2H, Ar), 7.758 (d,
for 1 are assigned as p ?
pꢅ transitions of quinonoid and benze-
noid rings, respectively. The latter shows a modest hyposochromic
shift in 1 compared with free phthiocol. The single peak in the vis-
ible region at 479 nm for phthiocol and shifted to 491 nm in 1 has
been previously described as the n ? pꢅ charge transfer (CT) tran-
sition [35]. There is a bathochromic shift observed to the band
ꢄ491 nm in 1 due to ligand-to-metal charge transfer (LMCT).
J = 6.6, 2H, Ar), 7.858 (t, J = 7.2, 2H, Ar). UV–vis; (methanol, kmax
,
nm): 292, 329, 491. Anal. Data Calc. for [C22H18O8Zn] (475.76 g):
C, 55.54; H, 3.81, Zn, 13.74%. Found: C, 54.95; H, 3.71, Zn, 12.77%.

226
L. Kathawate et al. / Journal of Molecular Structure 1048 (2013) 223–229
Fig. 1. FT-IR spectra of phthiocol and 1.
3.2. X-ray crystallography
1 crystallizes in monoclinic space group P21/n. Fig. 4 shows
ORTEP diagram of 1 and details of crystallographic data is shown
in Table 1. The Zn2+ ion is octahedrally coordinated through O(1)
and O(2) of two phthiocol ligands, the fifth (O(4)) and sixth(O(5))
coordination site is occupied by two water molecules (Fig. 2). Coor-
dination of phthiocol monoanion are ‘trans, trans’ in 1 [33]. The
water molecules occupy axial position and are perpendicular to
plane of phthiocol ligands. The ZnAO distances varies with O(1),
O(2) and O(4), ZnAO(1) is longest distance of 2.136(3) Å (Table 2).
The carbonyl bond distances C(1)AO(1) is 1.227 Å and C(2)AO(2)
1.291 Å, this distances fall under oxidized form of phthiocol ligand
and are similar to Cu, Zn complexes of hydroxynaphthoquinone
ligands [12b,c]. Most affected distance is carbonyl C(4)AO(3),
observed to be longer 1.246 Å, this distance is fall in naphthosem-
iquinone form (form II, Scheme 1) of the ligand or it is results of
delocalization of charge density [34]. Bond angle of O(4)AZnAO(2)
and O(4)AZnAO(1) is more than 90°.
Fig. 2. Representation of 1 with ligand numbering scheme.
Nonisothermal TG curve of phthiocol and 1 in air are shown in
Fig. 3 and thermodynamic data is given in Table S1. Bottei et al.
have been reported data for Zn, Co, Ni and Cu complexes with qui-
none after correcting for adsorbed water [36]. The decomposition
of phthiocol ligand occurs in two stages with a decomposition ratio
of 71:29 over the temperature range 52–557 °C. For 1, the first step
at 100 °C releases adsorbed water, whereas the second step ac-
counts for the loss of coordinated H2O molecules below 200 °C.
Water molecules are unavoidably adsorbed during sample prepa-
ration despite the rigorous drying of the samples under vacuum
beforehand. The decomposition of phthiocol ligands in final step
in 1 occurs between 287 and 565 °C. The plateau beyond 565 °C
stems from the constant mass of residual ZnO. Decomposition of
the phthiocol ligands occurs gradually and in single step (step III
in Fig. 3), hence the coordination both phthiocol ligands are in
same oxidation level, namely the fully oxidized form (form I,
Scheme 1) of phthiocol, as reduced ligands often shows abrupt
mass losses in non-isothermal thermogravimetric studies [30,31].
Thus, a thermogravimetric study reveals the coordination of
phthiocol ligands to Zn2+ are in the same oxidation level.
The fascinating fact of hydroxynaphthoquinone ligands and the
metal complexes is their hydrogen bonding network. The oxygens
of quinonoid ring viz. O(2) and O(3), often forms bi or multifurcat-
ed OAHꢁ ꢁ ꢁO hydrogen bonding network. In few cases the hydro-
gen’s of benzenoid ring forms CAHꢁ ꢁ ꢁO type of hydrogen
bonding, moreover the benzenoid and quinonoid rings can also
forms
p–p stacked chains. Three dimensional hydrogen bonding
network of hydroxynaphthoquinone ligands holds the molecules
together and these molecules can have opportunity to interact
with one another.
Both intra and intermolecular hydrogen bonding interactions
are observed in 1 (Table 3). Interestingly C(11)AH(11B) group is in-
volved in CAHꢁ ꢁ ꢁO type intramolecular hydrogen bonding interac-
tion with oxygen of C(4)AO(3). Bifurcated hydrogen bonding is
observed to oxygens O(2) and O(3) of phthiocol anion in 1. Each
complex unit is linked to ten other monomeric units through inter-
molecular OAHꢁ ꢁ ꢁO, CAHꢁ ꢁ ꢁO hydrogen bondings. The acceptor
oxygens are O(2) and O(3) and hydrogen’s of water molecules
O(4)AH(1), O(4)AH(2), C(5)H and C(8)H are hydrogen donors.
I
II
0
Phthiocol
1
20
40
A
III
3.2.1. Hydrogen bonding interaction through O(2)
60
80
1 forms polymeric chain via OAHꢁ ꢁ ꢁO hydrogen bonding be-
tween the coordinated water molecules and quinonoid oxygen
O(2) of phthiocol ligand (Fig. 5a). The monomeric units are hydro-
gen bonded through O(4)AH(1)ꢁ ꢁ ꢁO(2) leads to one dimensional
polymeric chain. This polymeric chain is stabilized by intermolec-
B
100
0
200
400
600
Temperature / 0C
ular CAHꢁ ꢁ ꢁ interaction [C(11)H(11)ꢁ ꢁ ꢁC(1), 3.62 Å, \149.61°], be-
p
tween the neighboring phthiocol anions on the same side of the
Fig. 3. Thermogram of phthiocol and 1.
chain. The intra chain Znꢁ ꢁ ꢁZn distance was found to be 5.015 Å,

L. Kathawate et al. / Journal of Molecular Structure 1048 (2013) 223–229
227
Fig. 4. ORTEP diagram of 1, ellipsoids are drawn at 50% probability.
of 10.631 Å. This distance is on higher side as compare to Cu and Fe
complexes of 2-hydroxy-1,4-naphthoquinone [12b,37].
Table 2
Selected bond distances for geometry optimized structures.a
[1]0
[1]1ꢃ
[1]2ꢃ
3.3. Electrochemical studies
Bond
Exptl
Calcd
Calcd
Calcd
Cyclic voltammogram of phthiocol studied in DMSO with tetra-
butyl ammonium hexafluorophosphate as a supporting electrolyte
by Gonzalez [38]. Reduction of phthiocol takes place by electron
transfer followed by protonation reaction, thus the reduction take
place via ECE-DISP1-DISP2 mechanism [39]. Reduction studied
with tetrabutyl ammonium perchlorate as a electrolyte (Fig. 6
(top)) shows two additional anodic peaks IIIa and IVa at +0.45 V
and 1.00 V vs NHE (Table 4) respectively. These peaks are due to
reoxidation of reduced phthiocol.
A quasireversible cathodic peak is observed for 1 at ꢃ0.52 V vs
NHE, this peak is assigned to one electron reduction of phthiocol
anion. Peak positions of Ic and Ia in 1 are shifted to more negative
potentials while IIc and IIa shifted to positive potentials compared
to free phthiocol ligand. More than a single process occurred dur-
ing reduction, anodic peaks (IIIa and IVa) are the result of oxidation
of electrochemical products formed during reduction. Biradicals in
zinc quinone complexes are stable and show characteristic triplet
signal in X-band EPR spectra [40]. Peak current is reduced in 1 as
compared to phthiocol ligand although same concentration is used
for the experiments.
ZnAO(1)
ZnAO(2)
ZnAO(4)
ZnAO(5)
2.136(3)
2.136(3)
2.096(3)
2.096(3)
1.227(5
1.292(4)
1.509(5)
1.472(5)
1.363(5)
1.449(5)
1.497(5)
1.494(6)
1.245(5)
1.370(6)
1.389(5)
1.380(6)
1.381(6)
1.385(6)
1.397(6)
2.175
1.969
2.332
2.299
1.237
1.295
1.500
1.466
1.375
1.457
1.498
1.506
1.227
1.392
1.389
1.393
1.388
1.396
1.401
2.145
2.008
2.374
2.212
1.237
1.273
1.510
1.396
1.455
1.475
1.484
1.496
1.219
1.391
1.388
1.394
1.386
1.398
1.408
2.102
2.091
2.198
2.223
1.238
1.254
1.517
1.446
1.425
1.486
1.464
1.491
1.214
1.393
1.387
1.401
1.384
1.404
1.419
O(1)AC(1)
O(2)AC(2)
C(1)AC(2)
C(1)AC(9)
C(2)AC(3)
C(3)AC(4)
C(3)AC(11)
C(4)AC(10)
C(4)AO(3)
C(5)AC(6)
C(5)AC(10)
C(6)AC(7)
C(7)AC(8)
C(8)AC(9)
C(9)AC(10)
a
O(4) and O(5) are water ligands; C(11) the 3-methyl group.
which is similar to [Cu(Lw)2(H2O)2]n [12b]. Each chain is further
intermolecularly hydrogen bonded to four neighboring chains
(Fig. S2) through OAHꢁ ꢁ ꢁO interaction of two trans water mole-
cules and the oxygens O(3) of the two trans phthiocol anions and
through CAHꢁ ꢁ ꢁO type of hydrogen boning [C(5)AH(5)ꢁ ꢁ ꢁO(2) and
C(8)AH(8)ꢁ ꢁ ꢁO(2)]. This forms three dimensional polymeric net-
work of 1 molecules.
3.4. DFT studiesss
The observed quasireversible redox processes for 1 have been
investigated by density function theoretical (DFT) calculations.
The ground state geometry of 1 was geometry-optimized using
the spin-unrestricted Kohn–Sham density functional B3LYP in the
gas phase. The salient ZnAO and intraligand bond distances are
in very good agreement with the experimentally determined met-
rics as listed in Table 3. They depict two co-planar phthiocol
monoanions bound to an octahedral central Zn2+ ion as shown by
the short C(4)AO(3) distance of 1.227 Å typical of a ketone moiety.
The C(1)AO(1) bond of 1.237 Å is slightly longer as a consequence
of metal coordination, whereas the C(2)AO(2) distance is the lon-
gest of the at 1.295 Å as was original a hydroxyl substituent prior
to complexation, and hence more closely held to the Zn2+ ion.
The two ZnAOH2 bond distances are ꢄ0.24 Å longer in the opti-
mized structure (Fig. S3), which highlights the intricate array of
intermolecular interactions found in the crystal structure excluded
in the optimization.
3.2.2. Hydrogen bonding interaction through O(3)
The OAHꢁ ꢁ ꢁO hydrogen bonding through carbonyl (C(4)AO(3))
of the quinonoid moiety and the coordinated water molecules form
a square shaped molecular assembly (Fig. 5b). Each of molecules 1
in a square is almost perpendicular to other with Znꢁ ꢁ ꢁZn distance
Table 3
Analysis of potential hydrogen bonds of 1.
Sr.
no.
DAHꢁ ꢁ ꢁA
DAH
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
DAHꢁ ꢁ ꢁA
(°)
0
0
0
(AÅ)
(AÅ)
(AÅ)
1
2
3
4
5
O(4) AH(1)ꢁ ꢁ ꢁO(2)a
O(4) AH(2)ꢁ ꢁ ꢁO(3)b
C(8) AH(8)ꢁ ꢁ ꢁO(3)c
C(5) AH(5)ꢁ ꢁ ꢁO(2)d
C(11)
0.72(5)
0.75(6)
0.93(1)
0.93(1)
0.96(1)
2.01(5)
2.05(6)
2.720(5) 172(4)
2.782(4) 168(6)
2.616(1) 3.545(4) 176(7)
2.647(2) 3.445(2) 144(3)
2.389(3) 2.792(5) 105(3)
The one-electron reduction of 1 to the monoanionic form [1]1ꢃ
was deemed quasireversible by electrochemistry. The geometry
about the Zn2+ ion remains octahedral however the phthiocol li-
gands are now pitched with a dihedral angle ꢄ25° relative to each
other (Fig. S4). There is a modest contraction of the ZnAO(1) bond
whereas the ZnAO(2) bond lengthens as a resonance mechanism
scatters charge throughout the quinonoid ring as portrayed in
AH(11B)ꢁ ꢁ ꢁO(3)intra
a
b
c
1 ꢃ x, ꢃy, ꢃz.
1/2 + x, 1/2 ꢃ y, 1/2 + z.
x, 1/2 + y, 1/2 ꢃ z.
d
1/2 ꢃ x, 1/2 + y, 1/2 ꢃ z.

228
L. Kathawate et al. / Journal of Molecular Structure 1048 (2013) 223–229
Fig. 5. (a) Polymeric chain formed by quinonoid oxygen O(2) and coordinated H2O molecules. (b) OAHꢁ ꢁ ꢁO hydrogen bonding network in 1.
the unit is reduced. The benzenoid ring remains largely unaffected
by the increase in charge on the phthiocol ligands as foretold by
resonance structures (Scheme 1). The distribution of the additional
spin is quantified by a Mülliken spin population analysis shown in
Fig. 7 consequence of their co-planar arrangement, spin is spread
across both ligands with the majority found at C(3). This is in
agreement with EPR and theoretical data obtained on one-electron
reduced lawsone, an analogue of phthiocol [41].
The second one-electron reduction generates a dianionic spe-
cies, [1]2ꢃ, which possesses the same octahedral geometry seen
in the other two members of this electron transfer series. Here,
the dihedral angle between reduced phthiocol ligands decreases
to ꢄ10° (Fig. S5). This nearly co-planar arrangement provides
strong overlap of the oxygen p orbitals with the filled zinc d orbi-
tals facilitating an efficient super exchange pathway. Here, phthio-
col radical dianions are strongly coupled via this superexchange
mechanism leading to a well-isolated diamagnetic ground state
for [1]2ꢃ. The ZnAO(1) and ZnAO(2) bonds lengthen and shorten,
respectively, in accord with reduction of the second phthiocol li-
gand, and allows the water molecules to bind more closely to the
Zn2+ ion. Additionally, both C(3)AC(11) and C(4)AO(3) contract
while C(1)AC(2) and C(3)AC(4) elongate. Interestingly, C(2)AC(3)
becomes shorter which may stem from more co-planar ligands
here than in [1]1ꢃ. Again, the aromatic CAC distances in the benze-
noid ring remains unchanged.
Fig. 6. Cyclic voltammograms of phthiocol (top) and 1 (bottom) at indicated scan
rates.
Table 4
Electrochemical data (in V) for phthiocol and 1 at a scan rate of 100 mV sꢃ1
.
Ic (V)
Ia (V)
D
Ep
IIc
IIa
D
Ep
IIIa
IVa
Phthiocol
1
ꢃ0.40
ꢃ0.52
ꢃ0.29
ꢃ0.39
0.11
0.13
ꢃ1.27
ꢃ1.18
ꢃ0.94
ꢃ1.04
0.32
0.10
0.45
0.50
1.00
1.04
Scheme 1. This is reflected in a shortening of the O(2)AC(2),
C(3)AC(11) and C(4)AO(3) bonds with a concomitant elongation
of C(1)AC(2), C(2)AC(3) and C(3)AC(4) bonds of the quinonoid ring
as each gains and loses some bonding character, respectively, when
Fig. 7. Mülliken spin population analysis for [1]1ꢃ from B3LYP-DFT calculations.

L. Kathawate et al. / Journal of Molecular Structure 1048 (2013) 223–229
229
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4. Conclusions
Zn2+ complex of phthiocol, a vitamin K3 analogue [Zn(phthio-
col)2(H2O)2]; 1 has been structurally and spectroscopically charac-
terized. Coordination of anionic phthiocol ligands to Zn2+ is in
oxidized form (naphthoquinone form) as revealed by FT-IR, ther-
mogravimetric analysis and X-ray diffraction studies. UV–vis spec-
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X-ray diffraction studies of 1 show, distorted octahedral geom-
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additional anodic peaks at +0.45 V and 1.00 V vs NHE. These peaks
are due to the oxidation of reduced phthiocol anion. A quasirevers-
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assigned to one electron reduction of phthiocol anion in 1.
The observed quasireversible redox processes for 1 have been
investigated by density function theoretical (DFT) calculations.
The ground state geometry of 1 was geometry-optimized using
the spin-unrestricted Kohn–Sham density functional B3LYP in the
gas phase. The salient ZnAO and intraligand bond distances in very
good agreement with the experimentally determined metrics.
Bond distances, geometry around Zn2+ and phthiocol anion co-pla-
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