Complexes of cis-dioxomolybdenum(VI) with a chiral tetradentate tripodal-like ligand system: Syntheses, structures and catalytic activities

Article abstract of DOI:10.1016/j.poly.2016.08.025

Racemic complexes with the general formula cis-[MoO₂(bzacLⁿ)] (1–4) (H₂bzacLⁿ?=?2-((4/5-R-2-hydroxyphenylamino)(pyridin-2-yl)methyl)-1-phenylbutane-1,3-dione, where n?=?1–4 for R?=?H, 5-Me, 5-Cl and 4-Me, respectively and 2Hs represent the dissociable phenolic proton and the active tertiary CH proton) have been synthesized in 75–82% yields by reacting [MoO₂(bzac)₂] (Hbzac?=?benzoylacetone) with the potentially N₂O-donor 5,5-membered fused chelate rings forming Schiff bases 4/5-R-2-(2-pyridylaldimine)phenols (HLⁿ; n?=?1–4 for R?=?H, 4-Me, 4-Cl and 5-Me, respectively) in hot methanol. The chiral ligand system (bzacLⁿ)^2?in 1–4 is formed via metal assisted Mannich-type addition of benzoylacetonate methine to the azomethine fragment of HLⁿ. All four complexes have been characterized by elemental (CHN) analysis, solution conductivity, magnetic susceptibility, spectroscopic (IR, UV–Vis and NMR) and electrochemical measurements. The molecular structures of 1–3 have been established by single crystal X-ray crystallography. In each complex, the chiral (bzacLⁿ)^2?acts as a tetradentate, N₂O₂-donor, tripodal-like ligand system and along with the two mutually cis oxo groups forms a distorted octahedral N₂O₄coordination environment around the molybdenum(VI) center. All four complexes are diamagnetic and non-electrolytic. The infrared spectra are generally consistent with the structural formulas of 1–4. The electronic spectra of 1–4 in dimethylformamide display two strong absorption bands in the range 245–300?nm. The cyclic voltammograms of 1–4 in dimethylformamide exhibit a metal centered one-electron reduction response within ?0.64 to ?0.74?V. All these complexes (1–4) and the analogous cis-[MoO₂(acacL^1–4)] (5–8) synthesized from [MoO₂(acac)₂] (Hacac?=?acetylacetone) and HL^1–4have been evaluated for their bromoperoxidase activities.

Full text of DOI:10.1016/j.poly.2016.08.025

Polyhedron xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Polyhedron
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Complexes of cis-dioxomolybdenum(VI) with a chiral tetradentate
tripodal-like ligand system: Syntheses, structures and catalytic activities
⇑
Sabari Ghosh, Sathish Kumar Kurapati, Samudranil Pal
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e i n f o
a b s t r a c t
Racemic complexes with the general formula cis-[MoO2(bzacLn)] (1–4) (H2bzacLn = 2-((4/5-R-2-hydrox-
yphenylamino)(pyridin-2-yl)methyl)-1-phenylbutane-1,3-dione, where n = 1–4 for R = H, 5-Me, 5-Cl
and 4-Me, respectively and 2Hs represent the dissociable phenolic proton and the active tertiary CH pro-
ton) have been synthesized in 75–82% yields by reacting [MoO2(bzac)2] (Hbzac = benzoylacetone) with
the potentially N2O-donor 5,5-membered fused chelate rings forming Schiff bases 4/5-R-2-(2-pyridy-
laldimine)phenols (HLn; n = 1–4 for R = H, 4-Me, 4-Cl and 5-Me, respectively) in hot methanol. The chiral
ligand system (bzacLn)2À in 1–4 is formed via metal assisted Mannich-type addition of benzoylacetonate
methine to the azomethine fragment of HLn. All four complexes have been characterized by elemental
(CHN) analysis, solution conductivity, magnetic susceptibility, spectroscopic (IR, UV–Vis and NMR) and
electrochemical measurements. The molecular structures of 1–3 have been established by single crystal
X-ray crystallography. In each complex, the chiral (bzacLn)2À acts as a tetradentate, N2O2-donor, tripodal-
like ligand system and along with the two mutually cis oxo groups forms a distorted octahedral N2O4
coordination environment around the molybdenum(VI) center. All four complexes are diamagnetic and
non-electrolytic. The infrared spectra are generally consistent with the structural formulas of 1–4. The
electronic spectra of 1–4 in dimethylformamide display two strong absorption bands in the range
245–300 nm. The cyclic voltammograms of 1–4 in dimethylformamide exhibit a metal centered one-elec-
tron reduction response within À0.64 to À0.74 V. All these complexes (1–4) and the analogous cis-
[MoO2(acacL1–4)] (5–8) synthesized from [MoO2(acac)2] (Hacac = acetylacetone) and HL1–4 have been
evaluated for their bromoperoxidase activities.
Article history:
Received 5 July 2016
Accepted 18 August 2016
Available online xxxx
Keywords:
cis-Dioxomolybdenum(VI) complexes
Chiral tripodal-like ligands
Crystal structures
Physicochemical properties
Catalytic applications
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
nation site trans to one of the two strong trans-labilizing oxo
groups for a monodentate ligand in their complexes. During the
The growing interest in the coordination complexes of cis-diox-
omolybdenum(VI) is primarily due to their potential abilities to act
as catalysts in various organic oxidation reactions [1–7] and as
structural and functional models of a number of molybdenum con-
taining enzymes [8–12]. Consequently, there are continuous efforts
in synthesizing new cis-dioxomolybdenum(VI) complexes which
will perform as better catalysts or will serve as better enzyme
models. The general approach for realizing such complexes is to
modify the denticity as well as the electronic and the steric charac-
teristics of the ligands being used. Tridentate meridionally span-
ning Schiff bases are very attractive due to the following reasons:
they are easy to prepare, their electronic and steric characteristics
can be readily tuned by varying the substituents on the amine or
on the carbonyl or on both and the availability of the sixth coordi-
past few years, we have been working on cis-dioxomolybdenum
(VI) complexes with some tri- and tetradentate ligands and their
applications as catalysts in oxidative bromination and benzoin
and sulfide oxidation reactions [13–16]. Recently in our attempts
to prepare complexes of cis-dioxomolybdenum(VI) with a poten-
tially meridional N2O-donor 5,5-membered fused chelate rings
forming Schiff base system (HLn) derived by condensing 2-pyridy-
laldehyde with 2-aminoethanol and/or 2-amino-4/5-R-phenol
using [MoO2(acac)2] (acacÀ = acetylacetonate) as the starting
material, we have encountered a metal assisted ligand transforma-
tion reaction and isolated a series of racemic complexes of formula
cis-[MoO2(acacLn)], where (acacLn)2À represents the transformed
ligand system [15]. The N2O2-donor chiral tripodal-like (acacLn)2À
is the product of the Mannich-type addition of the methine group
(@CHA) of acacÀ to the azomethine group (ACH@NA) of HLn. Very
recently the same ligand transformation reaction assisted by cis-
dioxomolybdenum(VI) and the racemic complexes with the
⇑
Corresponding author.
E-mail address: spal@uohyd.ac.in (S. Pal).
http://dx.doi.org/10.1016/j.poly.2016.08.025
0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: S. Ghosh et al., Polyhedron (2016), http://dx.doi.org/10.1016/j.poly.2016.08.025

2
S. Ghosh et al. / Polyhedron xxx (2016) xxx–xxx
transformed ligands which are analogous to (acacLn)2À have been
reported by another research group [17]. In the present work, we
have examined the generality of this ligand transformation reac-
tion using [MoO2(bzac)2] (bzacÀ = benzoylacetonate) and 4/5-R-
2-(2-pyridylaldimine)phenols (HLn; n = 1–4 for R = H, 4-Me, 4-Cl
and 5-Me, respectively). Here also the Mannich-type reaction
involving bzacÀ and HLn occurs and a series of analogous racemic
complexes with the formula cis-[MoO2(bzacLn)] (1–4) is formed
(Scheme 1). The identities of these complexes have been estab-
lished by various physical measurements and X-ray crystallogra-
phy. In addition, the catalytic abilities of 1–4 as well as those of
our previously reported cis-[MoO2(acacLn)] (5–8) [15] in oxidative
bromination of various organic substrates have been also investi-
gated. In the following account, we have described the syntheses,
structures and physical properties of 1–4 and the bromoperoxidase
activities of 1–8.
was used to measure the solution electrical conductivities. The
infrared spectra were recorded on a Thermo Scientific Nicolet
380 FT-IR spectrophotometer. A Shimadzu UV-3600 UV–Vis–NIR
spectrophotometer was used to collect the electronic spectra. The
1H (400 MHz) and the 13C (100 MHz) NMR spectra were recorded
with the help of a Bruker NMR spectrometer. Cyclic voltammo-
grams were collected on a CH Instruments model 620A electro-
chemical analyzer.
2.3. Syntheses of [MoO2(bzacLn)] (1–4)
A general procedure was used for the syntheses of all four com-
plexes. The details are as follows: solid [MoO2(bzac)2] (0.2 mmol)
was added to a solution of the corresponding Schiff base HLn
(0.2 mmol) in 30 ml of methanol and the resulting mixture was
heated on a steam bath for 45 min. After cooling to room temper-
ature the reaction mixture was filtered to remove any solid pre-
sent. The filtrate was allowed to evaporate slowly at room
temperature till the volume reduces to about 2–3 ml. The complex
separated as a yellow to light brown crystalline material was col-
lected by filtration, washed with ice-cold methanol and dichloro-
methane and finally dried in air. The yield was in the range of
75–82%.
2. Experimental
2.1. Materials
Bis(benzoylacetonato)dioxomolybdenum(VI),
[MoO2(bzac)2],
was prepared using a literature method [18]. The Schiff bases
(HLn) were prepared from equimolar amounts of 2-pyridylalde-
hyde and the corresponding amino phenols according to literature
procedures [19,20]. All other chemicals and solvents used in this
work were of analytical grade and were used as supplied without
further purification.
2.3.1. cis-[MoO2(bzacL1)] (1)
Yield 78%. Anal. Calc. for C22H18MoN2O5 (486.32): C, 54.33;
H,3.73; N,5.76. Found C, 54.23; H, 3.79; N, 5.68. Selected IR bands
(cmÀ1): 3129 (NAH), 1616 (C@O), 926 and 882 (cis-MoO2). UV–Vis
data (kmax (nm) (e
(103 MÀ1 cmÀ1))): 246 (12.6) and 289 (8.9). 1H
NMR data (d (ppm) (J (Hz))): 8.90 (4) (d, 1H, H1), 8.89 (s, 1H,
NH), 7.98 (8, 8) (dd, 1H, H3), 7.82 (8) (d, 1H, H4), 7.62–7.54 (m,
3H, H2, H18, H22), 7.47–7.39 (m, 3H, H19–21), 7.23 (8) (d, 1H, H12),
7.09 (8, 8) (dd, 1H, H10), 6.87 (8, 8) (dd, 1H, H11), 6.66 (8) (d, 1H,
H9), 5.55 (s, 1H, H6), 2.09 (s, 3H, Hs on C13). 13C NMR data (d
(ppm)): 207.0 (C16), 195.9 (C14), 172.0 (C8), 161.9 (C5), 157.3 (C3),
148.7 (C1), 141.6 (C17), 141.2 (C20), 133.2 (C10), 132.4 (C7), 129.7
(C4), 129.1 (C19), 128.9 (C21), 126.3 (C18), 125.6 (C22), 122.6 (C2),
121.2 (C11), 117.4 (C12), 114.4 (C9), 65.1 (C15), 31.2 (C6), 24.0
2.2. Physical measurements
Elemental (CHN) analysis data were obtained with the help of a
Thermo Finnigan Flash EA1112 series elemental analyzer. A Shi-
madzu LCMS 2010 liquid chromatograph mass spectrometer was
used for the purity verification of the Schiff bases. Solid state mag-
netic susceptibility measurements were performed with a Sher-
wood Scientific balance. A Digisun DI-909 conductivity meter
(C13). E1/2 (V) (
D
Ep (mV)): À0.69 (320).
2.3.2. cis-[MoO2(bzacL2)] (2)
Yield 80%. Anal. Calc. for C23H20MoN2O5 (500.35): C, 55.21;
H,4.03; N,5.60. Found C, 55.17; H, 4.12; N, 5.56. Selected IR bands
(cmÀ1): 3114 (NAH), 1622 (C@O), 931 and 896 (cis-MoO2). UV–Vis
data (kmax (nm) (e H
(103 MÀ1 cmÀ1))): 251 (15.1) and 289 (11.1). 1
NMR data (d (ppm) (J (Hz))): 8.88 (4) (d, 1H, H1), 8.77 (s, 1H, NH),
7.96 (6, 6) (dd, 1H, H3), 7.80 (6) (d, 1H, H4), 7.60 (6, 6) (dd, 1H, H2),
7.58–7.39 (m, 5H, H18–22), 7.20 (s, 1H, H12), 6.88 (4) (d, 1H, H10),
6.53 (8) (d, 1H, H9), 5.54 (s, 1H, H6), 3.15 (s, 3H, 5-Me), 2.17 (s,
3H, Hs on C13). 13C NMR data (d (ppm)): 207.0 (C16), 196.0 (C14),
172.0 (C8), 159.7 (C5), 157.4 (C3), 148.6 (C1), 141.6 (C17), 141.3
(C20), 132.9 (C11), 132.4 (C12), 130.4 (C10), 130.2 (C7), 129.1 (C19),
128.9 (C21), 126.3 (C18), 125.5 (C22), 122.7 (C4), 116.9 (C2), 114.5
(C9), 65.0 (C15), 31.2 (C6), 24.0 (5-Me), 20.5 (C13). E1/2 (V) (
(mV)): À0.68 (350).
DEp
2.3.3. cis-[MoO2(bzacL3)] (3)
Yield 75%. Anal. Calc. for C22H17ClMoN2O5 (520.77): C,50.74;
H,3.29; N,5.38. Found C, 50.65; H, 3.24; N, 5.31. Selected IR bands
(cmÀ1): 3100 (NAH), 1625 (C@O), 935 and 890 (cis-MoO2). UV–Vis
data (kmax (nm) (e H
(103 MÀ1 cmÀ1))): 245 (21.5) and 295 (15.4). 1
NMR data (d (ppm) (J (Hz))): 8.95 (s, 1H, NH), 8.89 (4) (d,1H, H1),
8.01 (8, 8) (dd, 1H, H3), 7.83 (8) (d, 1H, H4), 7.63–7.43 (m, 6H, H2,
H18–22), 7.24 (s, H12), 7.14 (8) (d, 1H, H10), 6.69 (8) (d, 1H, H9),
5.61 (s, 1H, H6), 2.08 (s, 3H, Hs on C13). 13C NMR data (d (ppm)):
207.0 (C16), 196.0 (C14), 171.0 (C8), 161.0 (C5), 157.1 (C3), 148.7
Scheme 1. Synthesis of cis-[MoO2(bzacLn)] and molecular diagrams of 4/5-R-2-(2-
pyridylaldimine)phenol (HLn), benzoylacetonate (bzacÀ), 2-((4/5-R-2-hydrox-
yphenylamino)-(pyridin-2-yl)methyl)-1-phenylbutane-1,3-dione (H2bzacLn) and
cis-[MoO2(bzacLn)].
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S. Ghosh et al. / Polyhedron xxx (2016) xxx–xxx
3
(C1), 141.7 (C17), 141.0 (C20), 134.2 (C10), 132.6 (C7), 129.7 (C4),
Table 1
129.1 (C19), 129.1 (C21), 126.5 (C18), 125.7 (C22), 123.9 (C11),
Selected crystal data and structure refinement summary.
123.0 (C2), 118.7 (C12), 114.4 (C9), 65.0 (C15), 31.2 (C6), 23.2 (C13).
Complex
1
2ÁMeOH
3ÁMeOH
E1/2 (V) (
D
Ep (mV)): À0.64 (380).
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
C
22H18MoN2O5 C24H24MoN2O6 C23H21MoN2O6
486.32
orthorhombic
Pbca
12.152(3)
15.959(4)
20.241(5)
90
532.39
monoclinic
P21/n
11.8850(13)
15.8012(16)
12.1851(13)
90
91.786(2)
90
2287.2(4)
4
1.546
0.617
18504
3997
3168
552.81
monoclinic
P21/n
11.916(3)
15.769(4)
12.205(3)
90
91.530(4)
90
2292.7(9)
4
1.602
0.731
21716
4043
3666
2.3.4. cis-[MoO2(bzacL4)] (4)
Yield 82%. Anal. Calc. for C23H20MoN2O5 (500.33): C,55.21;
H,4.03; N,5.60. Found C, 55.28; H, 4.09; N, 5.71. Selected IR bands
(cmÀ1): 3117 (NAH), 1638 (C@O), 937 and 888 (cis-MoO2). UV–
b (Å)
c (Å)
e
(103 MÀ1 cmÀ1))): 254 (21.5) and 300sh
a
(°)
Vis data (kmax (nm) (
b (°)
90
90
(12.8). 1H NMR data (d (ppm) (J (Hz))): 8.89 (4) (d,1H, H1), 8.79
(s, 1H, NH), 7.98 (8, 8) (dd, 1H, H3), 7.81 (8) (d, 1H, H4), 7.64–
7.53 (m, 4H, H2, H18, H20, H22), 7.45 (8, 8) (dd, 1H, H19, H21), 7.26
(8) (d, 1H, H12), 6.67 (8) (d, 1H, H11), 6.47 (s, 1H, H9), 5.51 (s, 1H,
H6), 2.14 (s, 3H, 4-Me), 2.09 (s, 3H, Hs on C13). 13CNMR data (d
(ppm)): 207.0 (C16), 196.0 (C14), 172.0 (C8), 161.7 (C5), 157.4 (C3),
148.7 (C1), 141.6 (C10), 141.3 (C17), 139.4 (C20), 132.4 (C7), 130.6
(C4), 129.1(C19), 128.9 (C21), 125.6 (C18), 125.6 (C22), 122.6 (C2),
122.0 (C11), 117.6 (C12), 114.4 (C9), 65.0 (C15), 31.2 (C6), 24.0 (4-
c
(°)
V (Å3)
3925.4(16)
8
1.646
0.706
34693
3454
Z
q
l
(g cmÀ3
(mmÀ1
)
)
Reflections collected
Reflections unique
Reflections [I P 2
Parameters
R1, wR2 [I P 2
R1, wR2 [all data]
Goodness-of-fit (GOF)
on F2
r
(I)]
2518
272
302
301
r
(I)]
0.0563, 0.1179 0.0437, 0.1018 0.0335, 0.0884
0.0813, 0.1272 0.0578, 0.1083 0.0371, 0.0909
1.023
Me), 21.2 (C13). E1/2 (V) (
D
Ep (mV)): À0.74 (400).
1.040
1.060
Maximum/minimum
0.719/À0.438
0.767/À0.264
0.755/À0.498
2.4. General procedure for catalytic bromoperoxidation reactions
Dq )
(e ÅÀ3
cis-[MoO2(bzacLn)] or cis-[MoO2(acacLn)] (0.01 mmol), substrate
(5 mmol), KBr (1.2 g, 10 mmol), H2O2 (1.05 ml of 30% w/w aqueous
solution, 10 mmol) and HClO4 (1.1 ml of 60% aqueous solution,
10 mmol) were added to 6 ml of acetonitrile–water (1:1) mixed
solvent in a 10 ml round bottom flask. The mixture was stirred
for the desired time period at room temperature. After stirring it
was diluted with 25 ml of water and then extracted with dichlor-
omethane (2 Â 7 ml). The dichloromethane extracts were com-
bined, dried over anhydrous sodium sulfate and then subjected
to GC–MS analysis for identification of the products and calculation
of the yields from the areas under the respective peaks.
HLn in hot methanol (Scheme 1). The elemental (CHN) analyses
data are in good agreement with their general molecular formula
as [MoO2(bzacLn)] where (bzacLn)2À denotes the chiral tetraden-
tate tripodal-like transformed ligand system. The diamagnetic
characters established by magnetic susceptibility measurements
clearly indicate the +6 oxidation state of the metal centers in 1–
4. The yellow to brown complexes are freely soluble in dimethyl-
formamide and dimethylsulfoxide, reasonably soluble in methanol
and acetonitrile and sparingly soluble in chloroform and dichloro-
methane. In solution, the electrically non-conducting behavior of
all four complexes is consistent with their general formulation as
neutral molecules.
2.5. X-ray crystallography
Single crystals of [MoO2(bzacL1À3)] (1–3) were collected
directly from the products isolated from their synthesis reaction
mixtures in methanol. Complex 1 crystallizes as it is without any
solvent molecule, while the other two complexes crystallize as
2ÁMeOH and 3ÁMeOH. Determination of the unit cell dimensions
and the intensity data collection for all three crystals were carried
out on a Bruker-Nonius SMART APEX CCD single crystal X-ray
3.2. X-ray molecular structures
The structure of 1 has been solved in the orthorhombic Pbca
space group, while the structures of both 2ÁMeOH and 3ÁMeOH
have been solved in the monoclinic P21/n space group (Table 1).
The perspective views of all three structures are shown in Fig. 1.
The bond lengths and bond angles associated with the metal cen-
ters are listed in Table 2. In each complex, the pyridine-N, the sec-
ondary amine-N, the enolate-O and the phenolate-O donor
(bzacLn)2À and two mutually cis oriented oxo groups assemble a
distorted octahedral N2O4 coordination environment around the
molybdenum atom. The chiral tripodal-like (pivotal atom C6)
(bzacLn)2À has three different coordinating arms. These are the
N-donor 2-pyridyl arm, the enolate-O donor 1-phenyl-1-one-2-
butenyl-3-olate arm, and the bidentate secondary amine-N and
the phenolate-O donor aminophenolate arm. The (bzacLn)2À caps
one face of the octahedron using the nitrogen atoms from the 2-
pyridyl and the aminophenolate arms (N1 and N2, respectively)
and the oxygen atom from the 1-phenyl-1-one-2-butenyl-3-olate
arm (O4), while the second coordinating atom of the aminopheno-
late arm the phenolate-O (O3) and the two oxo groups (O1 and O2)
occupy the opposite triangular face. Interestingly, the 1-phenyl-1-
one-2-butenyl-3-olate arm of (bzacLn)2À coordinates the metal
center through the enolate-O that is at the methyl end rather than
at the phenyl end (Scheme 1 and Fig. 1). This preference for the
oxygen atom of MeC(OÀ)@ is most likely due to its more basic
character compared to that of the oxygen atom of PhC(OÀ)@. In
diffractometer using graphite monochromated Mo K
a radiation
(k = 0.71073 Å) at 298 K. Data acquisition and data extraction were
performed using the SMART and the SAINT-Plus packages [21], respec-
tively. The SADABS program [22] was used for absorption correction.
Each of the three structures was solved by direct methods and
refined on F2 by full-matrix least squares procedures. All non-
hydrogen atoms were refined anisotropically. All hydrogen atoms
were included in the structure factor calculations at geometrically
idealized positions by using a riding model. Structure solution and
refinement were performed using SHELX-97 programs [23] available
in the WinGX suite [24]. Molecular graphics were done with the
help of the Mercury package [25]. Selected crystal data and struc-
ture refinement summaries for all three structures are listed in
Table 1.
3. Results and discussions
3.1. Synthesis and characterization
The complexes 1–4 were synthesized in 75–82% yields by react-
ing equimolar amounts of [MoO2(bzac)2] and the corresponding
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4
S. Ghosh et al. / Polyhedron xxx (2016) xxx–xxx
Table 2
Selected bond lengths (Å) and angles (°) for 1, 2ÁMeOH and 3ÁMeOH.
Complex
1
2ÁMeOH
3ÁMeOH
Mo–O(1)
Mo–O(2)
Mo–O(3)
Mo–O(4)
Mo–N(1)
Mo–N(2)
1.693(4)
1.718(3)
1.939(4)
1.941(3)
2.353(5)
2.307(4)
1.713(3)
1.683(3)
1.946(2)
1.948(2)
2.339(3)
2.327(3)
107.80(14)
99.12(12)
95.94(12)
161.23(12)
92.24(11)
96.78(12)
99.13(12)
90.84(12)
159.77(12)
153.57(10)
80.32(10)
76.82(9)
1.711(2)
1.686(2)
1.953(2)
1.944(2)
2.334(2)
2.327(2)
107.67(12)
98.53(10)
96.13(10)
161.56(10)
92.14(9)
96.55(10)
100.02(10)
90.72(10)
159.88(10)
153.36(8)
80.20(8)
76.54(7)
78.88(9)
80.81(7)
69.60(8)
O(1)–Mo–O(2)
O(1)–Mo–O(3)
O(1)–Mo–O(4)
O(1)–Mo–N(1)
O(1)–Mo–N(2)
O(2)–Mo–O(3)
O(2)–Mo–O(4)
O(2)–Mo–N(1)
O(2)–Mo–N(2)
O(3)–Mo–O(4)
O(3)–Mo–N(1)
O(3)–Mo–N(2)
O(4)–Mo–N(1)
O(4)–Mo–N(2)
N(1)–Mo–N(2)
107.13(18)
98.24(18)
96.63(17)
164.06(17)
93.47(16)
96.54(16)
101.36(16)
88.81(17)
159.16(16)
152.18(15)
79.69(16)
76.94(14)
79.58(15)
78.81(14)
70.63(14)
78.48(10)
81.02(9)
69.28(10)
Table 3
Hydrogen bonding parameters.
Complex
D–HÁ Á ÁA
DÁ Á ÁA (Å)
2.932(5)
\D–HÁ Á ÁA (°)
162.7
1
2
N(2)–H(2A)Á Á ÁO(2)i
N(2)–H(2A)Á Á ÁO(1)ii
O(6)–H(6A)Á Á ÁO(5)
N(2)–H(2A)Á Á ÁO(1)ii
O(6)–H(6A)Á Á ÁO(5)
2.922(4)
2.688(5)
174.5
165.5
3
2.893(3)
2.702(4)
175.3
163.4
Symmetry transformations used: (i) x À 1/2, y, Àz + 1/2. (ii) Àx + 1, Ày, Àz + 1.
N,N-chelation of HLn; nucleophilic attachment of the methine-C
(C15) of one of the two ( 1-bzac)À to the polarized azomethine
g
C-atom (C6); 1,3-proton shift from C15 to C6 and finally elimination
of the second spectator (
1-bzac)À as Hbzac by coordination and
g
deprotonation of the OH of the aminophenol arm leading to the
formation of cis-[MoO2(bzacLn)] (Scheme 1).
Although all three structures are solved in the centrosymmetric
space groups, but the corresponding molecules are chiral due to
the presence of two chiral centers, the pivotal carbon atom (C6)
and the metal coordinated secondary amine nitrogen atom (N2)
in (bzacLn)2À. In each of 1–3, the coordinated NAH group is
involved in an intermolecular hydrogen bonding with a metal
bound oxo group (Table 3). However, this NAHÁ Á ÁO hydrogen bond
assisted self-assembly patterns are not identical. In the case of
unsolvated 1, the enantiomers form parallel inversion symmetry
related one-dimensional chain-like structures via this hydrogen
bonding (Fig. 2). On the other hand, for each of the two methanol
solvates of 2 and 3, the enantiomers are involved in a pair of recip-
rocal NAHÁ Á ÁO interactions to form discrete centrosymmetric
dimers (Fig. 3). Thus, not surprisingly, all three complexes prefer
to crystallize in the centrosymmetric space groups.
Fig. 1. ORTEPs of cis-[MoO2(bzacL1)] (1) (top), cis-[MoO2(bzacL2)]ÁMeOH (2ÁMeOH)
(middle) and cis-[MoO2(bzacL3)]ÁMeOH (3ÁMeOH) (bottom). For all three structures
non-hydrogen atoms are drawn at their 40% probability thermal ellipsoids. For
clarity except for the chiral carbon atom no other carbon atom has been labeled.
both methanol solvates 2ÁMeOH and 3ÁMeOH, the methanol mole-
cules are strongly hydrogen bonded to the corresponding free ben-
zoyl carbonyl oxygen atoms (O5) of (bzacLn)2À (Fig. 1 and Table 3).
The very similar molecular structures and bond parameters of 1–3
indicate that the overall structure is independent of the nature of
the substituent on (bzacLn)2À. The metal centered bond lengths
in 1–3 are comparable to the corresponding bond lengths observed
for cis-dioxomolybdenum(VI) complexes containing analogous
coordinating atoms [13–17,25–28]. All three structures confirm
the formation of the transformed ligand (bzacLn)2À due to the Man-
nich-type addition of the methine (@CHA) group of (bzac)À across
the azomethine (ACH@NA) group of (Ln)À (Scheme 1 and Fig. 1). A
plausible mechanism for the formation of the transformed ligand
system (bzacLn)2À as proposed in our earlier report [15] is as fol-
lows: cleavage of the two metal to benzoyl-O bonds which are
trans to the two cis oriented oxo-groups in [MoO2(bzac)2];
3.3. Spectroscopic features
The infrared spectra of 1–4 were recorded in the range 4000–
400 cmÀ1 using KBr pellets. All the spectra display a rich array of
bands with various intensities. Except for the following few impor-
tant ones we have not tried to assign the remaining bands. The N–
H stretch appears in the region 3129–3100 cmÀ1 as a broad band. A
medium intensity band observed within 1638–1616 cmÀ1 is
attributed to the benzoyl C@O group of the dangling part of the
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S. Ghosh et al. / Polyhedron xxx (2016) xxx–xxx
5
Fig. 2. Intermolecular NAHÁ Á ÁO hydrogen bond assisted parallel one-dimensional chain-like assemblies of the enantiomers of cis-[MoO2(bzacL1)] (1).
1-phenyl-1-one-2-butenyl-3-olate arm of the ligand. Two charac-
teristic strong bands a sharp one and a relatively broad one associ-
ated with the cis-{MoO2}2+ unit appear in the ranges 937–926 and
895–882 cmÀ1, respectively [13–17,26–29].
The electronic spectral profiles of 1–4 recorded in dimethylfor-
mamide are quite similar with those of the analogous cis-[MoO2
(acacLn)] (5–8) [15]. Each of 1–4 displays two strong absorption
bands in the wavelength ranges 300–289 and 254–245 nm. The
low energy band has a broad tail extending up to 350–400 nm.
Similar absorptions for complexes of cis-dioxomolybdenum(VI)
with N,O donor ligands have been attributed to ligand to metal
charge transfer and ligand centered transitions, respectively
[13–17,26–29].
The NMR (1H and 13C) spectra of 1–4 were recorded using their
DMSO-d6 solutions. The chemical shifts and the tentative assign-
ments are given in the experimental section under the respective
complexes. In the 1H NMR spectra, the NH proton appears as a sin-
glet in the range of d 8.77–8.95 ppm. The proton (H6) present at the
chiral pivotal carbon center (C6) of (bzacLn)2À (Scheme 1) resonates
as a singlet within d 5.51–5.61 ppm. A three-proton singlet
observed in the range d 2.08–2.17 ppm is attributed to the methyl
group of the 2-butenyl-3-olate arm of the ligand. The protons of
the methyl substituent on the aminophenolate arm are observed
as a three-proton singlet at d 3.15 and 2.14 ppm for 2 and 4, respec-
tively. Among the aromatic protons of (bzacLn)2À, the chemical
shifts of the metal coordinated pyridine ring protons are generally
more downfield than the others. The ortho proton (H1) is most
deshielded and resonates at d 8.88–8.90 (d) ppm. The following
two signals observed in the ranges d 7.96–8.01 (dd) and 7.79–
7.83 (d) ppm are ascribed to the pyridine ring ortho (H3) and one
of the two meta (H4) protons, respectively. The remaining aromatic
ring protons appear within d 6.47–7.63 ppm with the expected
spin coupling patterns.
In general, the 13C NMR spectra of 1–4 are in good agreement
with the structure of the metal coordinated tripodal-like
(bzacLn)2À (Scheme 1). The carbonyl carbon (C16) of the benzoyl
fragment has the most downfield chemical shift (d 207.0 ppm).
The following two resonances at around d 196 and 172 ppm are
Fig. 3. Dimerization of cis-[MoO2(bzacL2)]ÁMeOH (2ÁMeOH) (top) and cis-[MoO2
(bzacL3)]ÁMeOH (3ÁMeOH) (bottom) via reciprocal NAHÁ Á ÁO hydrogen bonds.
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6
S. Ghosh et al. / Polyhedron xxx (2016) xxx–xxx
assigned to the carbon atoms bonded to the enolate-O (C14) and
the phenolate-O (C8), respectively. The aromatic ring carbons
(C1À4, C7,9À12 and C17À22) appear in the chemical shift range d
114.4–161.9 ppm. Among the remaining carbon atoms, the carbon
atom of the methyl group of the 2-butenyl-3-olate arm appears at
the most upfield chemical shift (d 20.5–24.0 ppm). The preceding
two resonances at d 31.2 and 65.0 ppm are attributed to the chiral
pivotal tertiary carbon (C6) and the enolate carbon C15 atoms,
respectively. The carbon of the methyl substituent on the
aminophenolate arm of 2 is observed at d 24.1 ppm, while that of
4 appears at d 24.0 ppm.
3.4. Redox behavior
Cyclic voltammetric experiments with 1–4 were performed at
298 K using their ꢀ10À3 M dimethylformamide solutions contain-
ing tetrabutylammonium perchlorate as the supporting electrolyte
under nitrogen atmosphere with a Pt-disk working electrode, a Pt-
wire auxiliary electrode and an Ag/AgCl reference electrode. Under
identical condition the Fc+/Fc (Fc = ferrocene) couple was observed
at E1/2 = 0.62 V. All the complexes display a quasi-reversible reduc-
tion response in the potential range À0.64 to À0.74 V. The current
heights of this response are comparable with known one-electron
transfer processes including the Fc+/Fc process [30,31]. A represen-
tative cyclic voltammogram is illustrated in Fig. 4. Similar reduc-
tion responses observed for cis-dioxomolybdenum(VI) complexes
having comparable coordinating atoms are generally assigned to
molybdenum(VI) to molybdenum(V) reduction process [26,28,29].
3.5. Bromoperoxidase activities
Scheme 2. Substrates and the products obtained using cis-[MoO2(bzacL1–4)] (1–4)
and cis-[MoO2(acacL1–4)] (5–8) as catalysts.
Complexes of oxovanadium(V) have been extensively studied
for bromoperoxidase activities due to the presence of this oxo-
metal ion in most naturally occurring bromoperoxidases [32–35].
Despite the well-known catalytic abilities of cis-dioxomolybde-
num(VI) complexes in various organic oxidation reactions [1–7]
there are very few reports on their bromoperoxidase activities
[14,16,28,36–40]. Thus we have explored the catalytic activities
of not only cis-[MoO2(bzacL1À4)] (1–4), but also our previously
reported cis-[MoO2(acacL1À4)] (5–8) [15] as well in bromoperoxi-
dation reactions of some aromatic and olefinic substrates. Opti-
mization of the reaction conditions by screening solvents,
catalyst loadings and different reaction temperatures was done
using cis-[MoO2(acacL1)] (5) as the model catalyst for each of the
five substrates aniline, salicylaldehyde, styrene, cyclohexene and
1-octene. The maximum conversion of the substrate occurred in
15–45 min when the reaction was conducted by using the catalyst,
substrate, KBr, H2O2 and HClO4 in 0.01:5:10:10:10 mol ratio in
MeCN–H2O (1:1) solvent mixture at room temperature (298 K).
The reaction time required for aniline and salicylaldehyde was
the shortest (15 min), while for cyclohexene and 1-octene it was
the longest (45 min). Styrene required the intermediate time of
30 min. Under these optimized conditions the remaining seven
complexes 1–4 and 6–8 were used as catalysts for oxidative bromi-
nation of all five substrates. The various brominated products
detected are shown in Scheme 2 and the substrate conversion per-
centages, product selectivities, yields and the TOF values are listed
in Table 4.
In the case of aniline, mono-, di- and also tribrominated
products À 4-bromoaniline, 2,4-dibromoaniline and 2,4,6-tribromo-
aniline have been obtained. The yield of the 2,4-dibromoaniline is
significantly higher than that of the other two products. In the con-
trol experiment without catalyst only 15% conversion of the sub-
strate occurs and 2,4-dibromoaniline has been obtained as the
major product. The change of the catalyst from 1 to 8 does not
affect too significantly the product selectivity and the percentage
substrate conversion. In contrast, salicylaldehyde produces only
the mono- and dibrominated derivatives 5-bromosalicylaldehyde
and 3,5-dibromosalicylaldehyde, respectively. Although the ratio
of the yields of the two products spans a somewhat wide range
due to variation of the catalyst, but generally the yield of the
monobrominated derivative is higher than the yield of the
dibrominated derivative. The controlled reaction without catalyst
Fig. 4. Cyclic voltammogram (scan rate of 100 mV sÀ1) of cis-[MoO2(bzacL2)] (2) in
dimethylformamide employing the experimental conditions described in the text.
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S. Ghosh et al. / Polyhedron xxx (2016) xxx–xxx
7
Table 4
Catalysis data.a
Reaction and catalyst
Conversion (%)
TOF (hÀ1
)
Product selectivity (%)
Bromination of aniline
(a)
23
28
25
24
24
22
14
25
(b)
68
67
70
71
71
72
80
69
(c)
8
4
4
4
4
5
5
5
1
2
3
4
5
6
7
8
99
99
99
99
99
99
99
99
1980
1980
1980
1980
1980
1980
1980
1980
Bromination of salicylaldehyde
(d)
86
65
64
83
52
62
56
88
(e)
14
34
36
16
47
37
44
12
1
2
3
4
5
6
7
8
>99
99
>99
99
99
99
1998
1980
1998
1980
1980
1980
1998
1998
>99
>99
Bromination of styrene
(f)
71
69
77
71
77
68
73
69
(g)
15
9
8
15
9
11
5
14
(h)
11
19
13
11
12
17
17
15
1
2
3
4
5
6
7
8
97
97
98
97
98
96
95
98
970
970
980
970
980
960
950
980
Bromination of cyclohexene
(i)
(j)
1
2
3
4
5
6
7
8
99
>99
99
>99
99
>99
>99
>99
660
666
660
666
660
666
666
666
51
54
48
49
51
49
49
49
48
46
51
51
48
51
51
51
Bromination of 1-octene
(k)
7
8
16
8
8
9
8
7
(l)
(m)
33
34
38
34
33
33
31
33
1
2
3
4
5
6
7
8
99
98
99
99
>99
>99
>99
99
660
653
660
660
666
666
666
660
59
56
45
57
59
57
61
59
a
Reaction conditions: MeCN–H2O (1:1) (6 ml), catalyst (0.01 mmol), substrate (5 mmol), KBr (10 mmol), H2O2 (10 mmol), HClO4 (10 mmol), temperature 298 K.
has produced 5-bromosalicylaldehyde as the only product but in a
much longer reaction time (2 h) and with 65% conversion of the
substrate. On oxidative bromination, the vinyl fragment of styrene
gets mono- as well as dibrominated. However, styrene epoxide has
been also detected as the third product. The monobrominated 2-
bromo-1-phenylethanol is the dominant product, while the yields
of the dibrominated (1,2-dibromoethyl)benzene and styrene epox-
ide are relatively much lower. The change of catalyst does not
make any significant difference in the product selectivities as well
as the percentages of the substrate conversion. Only 10% substrate
conversion has been observed in the controlled reaction without
catalyst. The results with 1-octene are quite similar to those with
styrene. Three products 2-bromooctan-1-ol, 1,2-dibromooctane
and octane-1,2-diol have been obtained. The dibromo derivative
has been the major product followed by octane-1,2-diol and 2-bro-
mooctan-1-ol. The product selectivity and the percentage of sub-
strate conversion remain invariant with the change of the
catalyst. In the controlled reaction without catalyst, 48% conver-
sion of the substrate occurs in 2 h and 1,2-dibromooctane has been
obtained as the major product. Unlike styrene and 1-octene, oxida-
tive bromination of cyclohexene has not produced either epoxide
or diol. Only the mono- and the dibrominated products À 2-bro-
mocyclohexanol and 1,2-dibromocyclohexane have been obtained
in almost equal amounts. Catalyst variation has no significant
effect on the product selectivities and the substrate conversion
percentages. The controlled reaction without catalyst has also pro-
duced the same products but in a much longer reaction time of 4 h
and with only 40% substrate conversion.
4. Conclusion
Racemic complexes having the general formula cis-[MoO2
(bzacL1À4)] (1–4) synthesized from [MoO2(bzac)2] and 4/5-R-2-
(2-pyridylaldimine)phenols (HL1À4
) have been reported. The
transformed chiral N2O2-donor tripodal-like ligand system
(bzacLn)2À has been formed via Mannich-type nucleophilic addi-
tion of the methine (@CHA) group of the benzoylacetonate (bzac)A
across the polarized azomethine (ACH@NA) group of the Schiff
base system HLn. X-ray crystal structures of the complexes 1–3
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8
S. Ghosh et al. / Polyhedron xxx (2016) xxx–xxx
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