Complexes of cis-dioxomolybdenum(VI) with unsymmetrical tripodal NO3-donor ligands: Synthesis, characterization and catalytic applications

Article abstract of DOI:10.1016/j.ica.2015.02.022

A series of cis-dioxomolybdenum(VI) complexes of general formula cis-[MoO₂(HLⁿ)] (1-4) have been synthesized in 80-85% yields by reacting equimolar amounts of [MoO₂(acac)₂] (acac^- = acetylacetonate) with 2,2′-(2-hydroxy-3,5-R₁,R₂-benzylazanediyl)diethanols (H₃Lⁿ, n = 1-4) in methanol. Characterization of the complexes has been performed by elemental analysis, spectroscopic (IR, UV-Vis, ¹H and ¹³C NMR) and electrochemical measurements. The molecular structures of all four complexes have been determined by single-crystal X-ray diffraction studies. In each of these analogous complexes, the metal centre is in a distorted octahedral NO₅ coordination sphere assembled by the single edge shared 5,5,6-membered chelate rings forming NO₃-donor (HLⁿ)^2- and two cis oriented oxo groups. Crystal structures of the complexes reveal formation of discrete centrosymmetric dimeric species via a pair of reciprocal intermolecular O-HO hydrogen bonding interactions. Spectroscopic data of all the complexes are consistent with their molecular structures. In the cyclic voltammograms, the redox-active complexes display a quasi-reversible to irreversible metal centred reduction with the cathodic peak potential in the range -0.92 to -1.12 V (versus Ag/AgCl). All the complexes have been evaluated for their catalytic activities in oxidative bromination reactions of styrene and salicylaldehyde and in benzoin oxidation reaction.

Full text of DOI:10.1016/j.ica.2015.02.022

Accepted Manuscript
Complexes of cis-dioxomolybdenum(VI) with unsymmetrical tripodal NO3-do-
nor ligands: Synthesis, characterization and catalytic applications
Sathish Kumar Kurapati, Swamy Maloth, Samudranil Pal
PII:
S0020-1693(15)00115-2
http://dx.doi.org/10.1016/j.ica.2015.02.022
ICA 16440
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
9 January 2015
20 February 2015
21 February 2015
Please cite this article as: S.K. Kurapati, S. Maloth, S. Pal, Complexes of cis-dioxomolybdenum(VI) with
unsymmetrical tripodal NO3-donor ligands: Synthesis, characterization and catalytic applications, Inorganica
Chimica Acta (2015), doi: http://dx.doi.org/10.1016/j.ica.2015.02.022
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Complexes of cis-dioxomolybdenum(VI) with unsymmetrical tripodal
NO3-donor ligands: Synthesis, characterization and catalytic applications
Sathish Kumar Kurapati, Swamy Maloth, Samudranil Pal*
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
ABSTRACT
A series of cis-dioxomolybdenum(VI) complexes of general formula cis-[MoO2(HLn)] (1−4)
have been synthesized in 80−85% yields by reacting equimolar amounts of [MoO2(acac)2] (acac−
= acetylacetonate) with 2,2'-(2-hydroxy-3,5-R1,R2-benzylazanediyl)diethanols (H3Ln, n = 1−4) in
methanol. Characterization of the complexes has been performed by elemental analysis,
1
spectroscopic (IR, UV-Vis, H- and 13C-NMR) and electrochemical measurements. The
molecular structures of all four complexes have been determined by single-crystal X-ray
diffraction studies. In each of these analogous complexes, the metal centre is in a distorted
octahedral NO5 coordination sphere assembled by the single edge shared 5,5,6-membered
chelate rings forming NO3-donor (HLn)2− and two cis oriented oxo groups. Crystal structures of
the complexes reveal formation of discrete centrosymmetric dimeric species via a pair of
reciprocal intermolecular O−H···O hydrogen bonding interactions. Spectroscopic data of all the
complexes are consistent with their molecular structures. In the cyclic voltammograms, the
redox-active complexes display a quasi-reversible to irreversible metal centred reduction with
the cathodic peak potential in the range −0.92 to −1.12 V (vs. Ag/AgCl). All the complexes have
been evaluated for their catalytic activities in oxidative bromination reactions of styrene and
salicylaldehyde and in benzoin oxidation reaction.
Keywords:
cis-dioxomolybdenum(VI) complexes, Tetradentate tripodal ligands; Crystal
structures; Physicochemical properties; Catalytic activities
1. Introduction
Complexes of cis-dioxomolybdenum(VI) are of considerable contemporary interest due to
their potential applications as catalysts in a variety of organic oxidation reactions [1–7] and as
models for mimicking the function of several molybdenum containing oxotransferase enzymes
[8−10]. As a result, a vast literature on the complexes containing the cis-{MoO2}2+ unit with a
variety of ligands is now available [5−7,11,12]. Majority of these complexes are with bi- and
tridentate ligands, while reports on its complexes with tetradentate tripodal ligands are rather
* Corresponding author.
E-mail address: spal@uohyd.ac.in (S. Pal)
1

limited [13−23]. Tripodal ligands are structurally and electronically easily tunable and they
create a pocket to hold the metal ion leaving one side open for ancillary ligands. Compared to
bi- or tridentate ligands tripodal ligands are expected to bind metal ions more strongly and
provide more stable complexes due to their increased denticity and hence greater chelate effect.
The structural uniqueness and stability of the complexes with such ligands have led to their
utilization in a wide variety of research areas such as activation of small molecules [24,25],
stabilization and scrutiny of reactive intermediates [26,27], biomodeling and biomimicking [24,
25,28−30], catalysis in organic synthesis [30−32] and sensors [33,34].
In the present work, we have explored the coordination chemistry of unsymmetrical
tripodal tetradentate NO3-donor 2,2'-(2-hydroxy-3,5-R1,R2-benzylazanediyl)diethanols (H3Ln, n =
1−4) with {cis-MoO2}2+ and synthesized a series of complexes having the formula cis-
[MoO2(HLn)] (1−4) (Chart 1). The catalytic abilities of the isolated complexes in oxidative
bromination reactions of styrene and salicylaldehyde and benzoin oxidation reaction have been
also assessed. The reasons to study these particular reactions using 1−4 as catalysts are as
follows.
The bromination of organic compounds via haloperoxidase reactions is an
environmentally benign approach compared to the traditional bromination reactions with Br2.
Oxovanadium(V) complexes have been extensively studied as haloperoxidase mimics [35−37],
whereas very few oxomolybdenum(VI) complexes have been employed for this purpose [38–40].
α-Dicarbonyls belong to an important class of organic compounds in the context of their
applications in synthetic organic chemistry. Hence, selective catalytic oxidation of benzoin to
benzil is an important key reaction [41−43]. However, not many reports on benzoin oxidation
reaction catalyzed by oxomolybdenum(VI) species are available in literature [38,40,44,45].
Herein, we describe the details of synthesis, X-ray structures, physicochemical properties and
catalytic behaviours of cis-[MoO2(HLn)] (1−4).
2. Experimental
2.1. Materials
Bis(acetylacetonato)dioxomolybdenum(VI), [MoO2(acac)2], was prepared according to a
literature procedure [46]. 2,2'-(2-hydroxy-3,5-R1,R2-benzylazanediyl)diethanols (H3Ln) were
synthesized using a procedure very similar to that reported earlier [47,48]. A general synthetic
2

procedure and some characterization (LC-MS and NMR) data for H3Ln have been provided in
the supplementary material. All other chemicals were of analytical grade available commercially
and used as received. Solvents were purified by standard procedures [49].
2.2. Physical measurements
A Thermo Finnigan Flash EA1112 series elemental analyzer was used to obtain the
elemental (C, H, N) analysis data. A Shimadzu LCMS 2010 liquid chromatograph mass
spectrometer was used for the purity verification of H3Ln. Room temperature magnetic
susceptibility measurements were performed with a Sherwood Scientific balance. Solution
electrical conductivities were measured using a Digisun DI-909 conductivity meter. The infrared
spectra were obtained with the help of a Thermo Scientific Nicolet 380 FT-IR
spectrophotometer. The electronic spectra were collected with the help of a Shimadzu UV-3600
1
UV-Vis-NIR spectrophotometer. The H (400 MHz) and 13C (100 MHz) NMR spectra were
recorded using tetramethylsilane as internal standard on a Bruker NMR spectrometer. The cyclic
voltammetric measurements were performed on a CH Instruments model 620A electrochemical
analyzer. Gas chromatographic measurements were performed with the help of Shimadzu GC-
2010 Plus equipped with FID and Shimadzu GCMS-QP2010 instruments.
2.3. Synthesis of complexes cis-[MoO2(HL1−4)](1−4)
All the complexes were synthesized by using the following general procedure: 65 mg (0.2
mmol) of solid [MoO2(acac)2] was added to a hot methanol solution (30 ml) of 0.2 mmol of the
corresponding H3Ln (n = 1−4) The resulting mixture was boiled at reflux for 1 h. Then the
reaction mixture was cooled to room temperature, filtered to remove suspended particles and
allowed to evaporate slowly in air. After about a day the yellow microcrystalline material
separated was collected by filtration, washed first with cold methanol and then with
dichloromethane and hexane (1:1) mixture and finally dried in air.
2.3.1. cis-[MoO2(HL1)] (1)
Yield 81%. Anal. calcd for C12H17MoNO5: C, 41.04; H, 4.88; N, 3.99. Found C, 41.28;
H, 4.56; N, 4.12 %. Selected IR data (cm−1): 2947 (νC−H of CH3), 2837−2668 (νC−H of –CH2− ),
908 and 881 (cis-{MoO2}2+). UV-Vis data (λmax (nm) (10−3 x ε (M−1 cm−1)): 320sh (4.0), 285sh
3

1
(6.7). H NMR (δ (ppm) (J (Hz))): 9.03 (s, 1H, Mo-OH), 6.92 (8) (d, 1H, H3), 6.89 (s, 1H, H5),
6.56 (8) (d, 1H, H2), 4.77 (15) (d, 1H, H7a), 4.56 (m, 1H, H10a), 4.40 (m, 1H, H10b), 4.00 (15) (d,
1H, H7b), 3.66 (m, 1H, H8a), 3.58 (m, 1H, H8b), 3.26 (m, 1H, H11a), 3.19 (m, 1H, H11b), 2.76 (m,
13
1H, H9a), 2.55 (m, 1H, H9b), 2.20 (s, 3H, CH3 of 4-Me). C NMR (δ (ppm)): 160.50 (1C, C1),
132.18 (1C, C4), 131.30 (1C, C6), 131.03 (1C, C2), 124.66 (1C, C5), 120.62 (1C, C3), 74.52 (1C,
C7), 64.16 (1C, C10), 62.32 (1C, C8), 59.47 (1C, C11), 57.83 (1C, C9) and 22.75 (1C, 4-Me). E1/2
(V) (∆Ep (mV)): −0.74 (360).
2.3.2. cis-[MoO2(HL2)] (2)
Yield 80%. Anal. calcd for C13H19MoNO5: C, 42.75; H, 5.24; N, 3.83. Found C, 42.68;
H, 4.91; N, 3.95 %. Selected IR data (cm−1): 2964 (νC−H of CH3), 2854−2662 (νC−H of –CH2− ),
914 and 892 (cis-{MoO2}2+. UV-Vis data (λmax (nm) (10−3 x ε (M−1 cm−1)): 321sh (2.6), 282sh
1
(4.5). H NMR (δ (ppm) (J (Hz))): 9.03 (s, 1H, Mo-OH), 6.95 (8) (d, 1H, H3), 6.91 (s, 1H, H5),
6.59 (8) (d, 1H, H2), 4.82 (15) (d, 1H, H7a), 4.57 (m, 1H, H10a), 4.40 (m, 1H, H10b), 4.03 (15) (d,
1H, H7b), 3.67 (m, 1H, H8a), 3.58 (m, 1H, H8b), 3.26 (m, 1H, H11a), 3.19 (m, 1H, H11b), 2.77 (m,
1H, H9a), 2.56 (m, 1H, H9b), 2.49 (2H, CH2 of 4-Et), 1.13 (8) (t, 3H, CH3 of 4-ethyl ). 13C NMR
(δ (ppm)): 158.47 (1C, C1), 135.38 (1C, C4), 128.73 (1C, C6), 127.85 (1C, C2), 122.44 (1C, C5),
118.43 (1C, C3), 72.34 (1C, C7), 62.04 (1C, C10), 60.10 (1C, C8), 57.22 (1C, C11), 57.65 (1C, C9),
27.74 (1C, CH2 of 4-Et) and 16.28 (1C, CH3 of 4-Et). E1/2 (V) (∆Ep (mV)): −0.77 (440).
2.3.3. cis-[MoO2(HL3)] (3)
Yield 83%. Anal. calcd for C15H23MoNO5: C, 45.81; H, 5.89; N, 3.56. Found C, 45.86;
H, 5.68; N, 3.51 %. Selected IR data (cm−1): 2958 (νC−H of CH3), 2859−2670 (νC−H of –CH2− ),
925 and 886 (cis-{MoO2}2+). UV-Vis data (λmax (nm) (10−3 x ε (M−1 cm−1)): 320sh (2.7), 275sh
1
(5.0). H NMR (δ (ppm) (J (Hz))): 9.05 (s, 1H, Mo-OH), 7.14 (8) (d, 1H, H3), 7.08 (s, 1H, H5),
6.60 (8) (d, 1H, H2), 4.80 (15) (d, 1H, H7a), 4.59 (m, 1H, H10a), 4.40 (m, 1H, H10b), 4.07 (15) (d,
1H, H7b), 3.68 (m, 1H, H8a), 3.60 (m, 1H, H8b), 3.27 (m, 1H, H11a), 3.20 (m, 1H, H11b), 2.77 (m,
13
1H, H9a), 2.55 (m, 1H, H9b), 1.23 (s, 9H, CH3 of 4-t-Bu). C NMR (δ (ppm)): 158.19 (1C, C1),
142.37 (1C, C4), 126.01 (1C, C6), 125.38 (1C, C2), 121.93 (1C, C5), 118.11 (1C, C3), 72.40 (1C,
C7), 62.24 (1C, C10), 60.01 (1C, C8), 57.17 (1C, C11), 55.70 (1C, C9), 34.19 (1C, 4-t-Bu) and
31.82 (3C, 4-t-Bu). Epc (V): −0.96.
4

2.3.4. cis-[MoO2(HL4)] (4)
Yield 85%. Anal. calcd for C19H31MoNO5: C, 50.78; H, 6.95; N, 3.12. Found C, 50.72;
H, 6.71; N, 3.32 %. Selected IR data (cm−1): 2953 (νC−H of CH3), 2848−2660 (νC−H of –CH2− ),
903 and 881 (cis-{MoO2}2+). UV-Vis data (λmax (nm) (10−3 x ε (M−1 cm−1)): 332 (3.6), 278sh
(5.7). 1H NMR δ ((ppm) (J (Hz))): 8.81 (s, 1H, Mo-OH), 7.12 (s, 1H, H5), 6.95 (s, 1H, H3), 4.75
(14) (d, 1H, H7a), 4.54 (m, 1H, H10a), 4.40 (m, 1H, H10b), 4.04 (14) (d, 1H, H7b), 3.65 (m, 1H,
H8a), 3.47 (m, 1H, H8b), 3.31 (m, 1H, H11a), 3.17 (m, 1H, H11b), 2.81 (m, 1H, H9a), 2.58 (m, 1H,
13
H9b), 1.31 (s, 9H, CH3 of 2-t-Bu) 1.23 (s, 9H, CH3 of 4-t-Bu). C NMR (δ (ppm)): 159.42 (1C,
C1), 143.51 (1C, C4), 139.34 (1C, C2), 126.34 (1C, C6), 124.56 (1C, C5), 124.28 (1C, C3), 73.77
(1C, C7), 64.91 (1C, C10), 62.62 (1C, C8), 59.59 (1C, C11), 57.62 (1C, C9), 36.55 (1C, 2-t-Bu),
32.32 (3C, 2-t-Bu), 37.42 (1C, 4-t-Bu) and 34.12 (3C, 4-t-Bu). E1/2 (V) (∆Ep (mV)): −0.71 (830).
2.4. Procedure for oxidative bromination of styrene
A 10 ml round bottom flask containing 4 ml of 1:1 acetonitrile and water mixture was
charged with 0.01 mmol of cis-[MoO2(HLn)], 520 µl (5 mmol) of styrene, 1.2 ml of 30% w/w
aqueous solution (10 mmol) of H2O2, 1.2 g (10 mmol) of KBr and 1.2 ml of 60% aqueous (10
mmol) HClO4. The mixture was stirred at 60° C for 4 h and then cooled to room temperature.
After cooling it was diluted with 15 ml of water and extracted with dichloromethane (2 x 4 ml).
The combined dichloromethane extracts were dried over anhydrous sodium sulphate and then
subjected to GC analysis for calculation of yields from the areas under the peaks. The products
were identified by GC-MS.
2.5. Procedure for oxidative bromination of salicylaldehyde:
0.01 mmol of the cis-[MoO2(HLn)], 530 µl (5 mmol) of salicylaldehyde, 1.2 ml of 30%
w/w aqueous solution (10 mmol) of H2O2, 1.2 g (10 mmol) of KBr and 1.2 ml of 60% aqueous
(10 mmol) HClO4 were added to 4 ml of 1:1 acetonitrile and water mixture in a 10 ml round
bottom flask. The mixture was stirred at 60°C for 1 h. After cooling to room temperature the
mixture was diluted with 15 ml of water and then extracted with dichloromethane (2 x 4 ml).
The dichloromethane extracts were combined, dried over anhydrous sodium sulphate and then
subjected to GC-MS analysis for identification of the products and calculation of yields from the
areas under the peaks.
5

2.6. Procedure for benzoin oxidation:
0.01 mmol of the cis-[MoO2(HLn)] and 1.2 ml of 30% w/w aqueous solution (10 mmol)
of H2O2 were added to 1.07 g (5 mmol) of benzoin taken in 3 ml of methanol. The resulting
mixture was stirred at 60° C for 4 h. The reaction progress was monitored with TLC. After
completion of the reaction the mixture was diluted with 15 ml of water and then extracted with
dichloromethane (2 x 4 ml). After drying over anhydrous sodium sulphate the combined
dichloromethane extracts were analyzed by GC-MS to identify the products and calculate the
yields from the areas under the peaks.
2.7. X−ray crystallography
Single crystals of 1 and 3 were obtained by recrystallization of the corresponding
complexes from hot acetonitrile and that of 2 and 4 were collected from the crystalline materials
obtained during their synthesis. The unit cell parameters and the intensity data at 298 K for 1, 3
and 4 were obtained with the help of an Oxford Diffraction Xcalibur Gemini single crystal X-ray
diffractometer equipped with a graphite monochromator and a Mo Kα fine-focus sealed tube (λ
= 0.71073 Å). The CrysAlisPro software [50] was used for data collection, reduction and
absorption correction. Some residual absorption effects in the dataset of 4 were treated with
XABS2 program [51]. Determination of the unit cell parameters and the collection of the
intensity data at 298 K for 2 were performed on a Bruker-Nonius SMART APEX CCD single
crystal diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The
SMART and the SAINT-Plus [52] programs were used for data acquisition and data extraction,
respectively. An empirical absorption correction was applied using the SADABS program [53].
The structure of each of 1−4 was solved by direct method and refined on F2 by full−matrix least-
squares procedures. All non-hydrogen atoms were refined using anisotropic thermal parameters.
All hydrogen atoms were included in the structure factor calculations at idealized positions by
using a riding model. The SHELX-97 programs [54] provided in the WinGX package [55] were
used for structure solution and refinement. The Platon [56] and the Mercury [57] packages were
used for molecular graphics. Selected crystallographic data for 1−4 are listed in Table 1.
6

3. Results and discussion
3.1. Synthesis and some properties
The tripodal 2,2'-(2-hydroxy-3,5-R1,R2-benzylazanediyl)diethanols (H3Ln) (Chart 1) were
synthesized in 75−85% yields by Mannich condensation reactions of equimolar amounts of
bis(2-hydroxyethyl)amine, corresponding substituted phenols and formaldehyde in methanol
under reflux condition. The complexes cis-[MoO2(HLn)] (1−4) were synthesized by reacting the
appropriate H3Ln and [MoO2(acac)2] in hot refluxing methanol. All four complexes were
isolated as yellow microcrystalline materials in very good yields (80−85%). The elemental
analysis data are in good agreement with those calculated for 1−4. Magnetic susceptibility
measurements indicate the diamagnetic character of all the complexes. The diamagnetic
behaviour is consistent with the +6 oxidation state of the metal centre in each of the four
complexes. All the complexes are highly soluble in dimethylformamide and dimethylsulfoxide,
moderately soluble in methanol, ethanol, acetonitrile, chloroform and dichloromethane and
insoluble in diethylether, hexane and toluene. In solution, each of the four complexes is
electrically non-conducting.
3.2. X-ray structural studies
Complexes 1, 2 and 4 crystallize in the monoclinic P21/c space group without any lattice
solvent molecule, while 3 crystallizes in the triclinic P space group with acetonitrile molecules
1
(Table 1). Asymmetric unit of each of 1, 2 and 4 contains one complex molecule and that of 3
contains one complex and two acetonitrile molecules. The molecular structures of 1 and 2 are
illustrated in Fig. 1 and those for 3 and 4 are provided in Fig. S1 (Supplementary material). The
bond lengths and angles related to the metal centres are listed in Table 2. Overall molecular
structures of all the complexes are very similar. In each complex, the dianionic tripodal ligand
(HLn)2− coordinates the metal centre through the amine-N, the ethanol-O, the ethanolate-O and
the phenolate-O atoms. The intraligand bond parameters are unexceptional. The phenolate-O,
the amine-N and the ethanolate-O atoms span meridionally, while the ethanol-O and the two
mutually cis oxo groups occupy the other meridian and complete a distorted octahedral NO5
coordination sphere around the metal centre. The Mo(1)−O(3) (2.287(6)−2.292(4) Å) and the
Mo(1)−N(1) (2.320(8)−2.364(4) Å) bonds, which are trans to the two oxo groups (O(4) and
7

O(5)), are significantly longer compared to the Mo(1)−O(1) (1.922(6)−1.941(4) Å) and
Mo(1)−O(2) (1.958(6)−1.967(3) Å). It is very likely that the trans lengthening of the
Mo(1)−O(3) facilitates the proton to be on O(3) rather than on the more strongly metal bound
O(1) or O(2), where it will be more acidic and hence easily dissociable. There is no effect of the
single substituent at para to the phenolate-O (O(1)) on the Mo−O(1) bond lengths in 1−3.
However, the Mo−O(1) bond length is shorter by ~0.02 Å in 4 compared to the corresponding
bond lengths in 1−3 (Table 2). This shortening indicates that the Mo−O(1) bond is relatively
stronger in 4 than in 1−3. This is very likely due to the combined positive inductive effect of the
two t-butyl groups at both ortho and para positions with respect to the phenolate-O (O(1)) in 4.
Overall, the bond lengths and the bond angles associated with the metal centres in all four
complexes are within the ranges reported for cis-dioxomolybdenum(VI) complexes with ligands
bearing similar coordinating atoms [13−23,40,58−66].
Considering the presence of one protonated ethanol arm of (HLn)2− and several hydrogen
bond acceptors in cis-[MoO2(HLn)] (1−4), we have searched for the conventional intermolecular
hydrogen bonding interaction and the subsequent self-assembled supramolecular structure for all
four complexes. Indeed the metal coordinated ethanol-OH (O(3)−H) participates in a strong
O−H···O interaction involving the metal coordinated ethanolate-O (O(2)) of another molecule.
The O(3)···O(2) distances vary within the range 2.586(4)−2.606(2) Å, while the O(3)−H···O(2)
angles are ~175° for this interaction in 1−4 (Table 3). In the crystal lattice, the molecules of each
of the four complexes dimerise via two such reciprocal O(3)−H···O(2) interactions [66]. The
dimers of 1 and 2 are represented in Fig. 2 and those of 3 and 4 are shown in Fig. S2
(Supplementary material).
We have also collected the powder X-ray diffraction patterns using the crystalline
samples of all the complexes and compared them with the corresponding simulated diffraction
patterns generated from the single crystal X-ray diffraction data (Fig. S3, Supplementary
material). The satisfactory agreement between the simulated and the experimental diffraction
patterns indicates the bulk purity of sample and phase for each of 1, 2 and 4. However, the
experimental curve is somewhat different than the simulated curve for 3⋅2CH3CN (Fig. S3). In
8

all probability, this difference is due to the loss of some of the lattice acetonitrile molecules
during grinding of the sample.
3.3. Spectroscopic properties
Infrared spectra of H3Ln and the corresponding complexes (1−4) were recorded in KBr
pellets. The spectra show a large number of bands of various intensities in the range 4000−400
cm−1. However, except for the following selected few we have not attempted to assign the
remaining bands. The spectra of H3Ln show a strong and broad band around 3350 cm−1
corresponding to its three hydroxyl functionalities. In contrast, the complexes display a very
weak broad band centred at ~3420 cm−1. The disappearance of the strong broad band is
consistent with deprotonation of two out of three hydroxyl groups of the ligand due to
complexation. The weak band at ~3420 cm−1 is perhaps due to the lone hydrogen bonded metal
coordinated OH group in the complexes. The bands corresponding to the symmetric and
asymmetric stretches of the cis-{MoO2}2+ core in 1−4, are observed in the ranges 925−903 and
892−881 cm−1, respectively [13−15,18,21,23,59−66].
The electronic spectra of 1–4 were recorded in dimethylsulfoxide solutions. The spectral
profiles are very similar. The spectrum of 2 is shown in Fig. 3, while the spectra of the
remaining complexes are depicted in Fig. S4. All the complexes display two strong absorptions
in the ranges 332−317 and 285−280 nm. Comparable absorptions observed for complexes of
cis-dioxomolybdenum(VI) with ligands having similar coordinating atoms have been assigned to
ligand-to-metal charge transfer and ligand centred transitions, respectively [40,59−66].
13
13
1H-, C- and C-DEPT NMR spectroscopic measurements with DMSO-d6 solutions of
1–4 have been performed. All the spectra are illustrated in Figs. S5−S7 (Supplementary
material). The spectral data of all four complexes are consistent with the corresponding
molecular structures as established by X-ray crystallography. The 1H-NMR spectra (Fig. S5) of
1–3 show two doublets in the ranges δ 6.56−6.60 and 6.92−7.14 ppm for the protons at C2 and
C3, respectively (Chart 1). The singlet observed at δ 7.12 ppm in the spectrum of 4 is assigned to
the proton at C3. The C5 proton of 1−4 resonates as a singlet within δ 6.89−7.08 ppm. Each
signal corresponding to the protons at C3 and C5 show a small splitting due to the spatial
interaction with protons of the alkyl substituent. The methyl group (at C4) protons of 1 are
9

observed as a singlet at δ 2.19 ppm, while the t-butyl group (at C4) protons of each of 3 and 4
appear as singlet at δ 1.23 ppm. In the case of 2, the methyl protons of the ethyl group (at C4)
resonate as a triplet at δ 1.13 ppm, while the signal corresponding to the methylene protons of
the same ethyl substituent coincides with the residual DMSO signal at 2.49 ppm. The tertiary
butyl group (at C2) protons of 4 resonate as a singlet at δ 1.31 ppm. A relatively broad signal
observed within δ 8.81–9.05 ppm for 1–4 is attributed to the metal coordinated hydroxyl group
proton. Because of diastereotopicity, the five methylene group protons of each complex show
ten signals in the range δ 2.5−5.0 ppm. The methylene group (ortho to the phenolate) protons (at
C7) show two clear doublets at δ ~4.79 and ~4.04 ppm. The protons at C10, the methylene group
(bonded to N) of the protonated arm of (HLn)2− appear as two multiplets at δ ~4.57 and ~4.40
ppm. On the other hand, the corresponding methylene group (bonded to N) protons at C8 of the
deprotonated arm of (HLn)2− are observed as two multiplets at δ ~3.67 and ~3.53 ppm. The
methylene group (bonded to coordinated OH) protons at C11 show two multiplets at δ ~3.29 and
3.19 ppm. In contrast, the multiplets corresponding to the methylene group (bonded to the
ethanolate-O) protons at C9 are observed at δ ~2.79 and ~2.57 ppm.
The 13C-NMR spectra (Fig. S6) of 1–4 display the resonances corresponding to the
aromatic carbons C1–C6 in the ranges δ 158.2–160.5, 125.4–139.3, 118.1–124.4, 132.2–143.5,
121.9–124.7 and 126.0–131.3 ppm, respectively. The methylene carbon (C7) attached to the
phenolate ring resonates within δ 72.3–74.5 ppm. Two signals observed in the ranges δ 60.0–
62.6 and 55.7–57.8 ppm are attributed to the methylene carbon atoms C8 and C9, respectively of
the ethanolate arm of (HLn)2–. Two more signals appeared within δ 62.0–64.9 and 55.7−59.6
ppm are assigned to the methylene carbon atoms C10 and C11 of the ethanol arm of (HLn)2–. The
carbon atom of the methyl substituent in 1 resonates at δ 22.75 ppm, while the primary and the
secondary carbon atoms of the ethyl substituent in 2 are observed at δ 16.28 and 27.74 ppm,
respectively. The quaternary carbon atom of the t-butyl group at para position of the phenolate
ring appears at δ 34.19 and 37.42 ppm for 3 and 4, respectively. The primary carbon atoms of
the same t-butyl substituent in 3 and 4 are observed at δ 31.82 and 34.12 ppm, respectively. The
resonances for the quaternary and the primary carbon atoms of the t-butyl group at ortho position
10

of the pehenolate ring in 4 are observed at δ 36.55 and 32.32 ppm, respectively. The 13C-DEPT
NMR spectra (Fig. S7) of 1−4 also substantiates the spectral assignments described above.
3.4. Electron-transfer properties
Electron-transfer properties of 1−4 have been investigated by cyclic voltammetry using
their ~10−3 M solutions in acetonitrile-dimethylformamide (9:1) containing tetrabutylammonium
perchlorate as the supporting electrolyte. The measurements were carried out under nitrogen
atmosphere at 298 K with the three electrode setup comprised of a Pt-disk working electrode, a
Pt-wire auxiliary electrode and an Ag/AgCl reference electrode. The ferrocenium/ferrocene
(Fc+/Fc) couple appeared at E1/2 = 0.63 V under identical condition. The cyclic voltammogram
of 2 is illustrated in Fig. 4 and the cyclic voltammograms of 1, 3 and 4 are provided in Fig. S8
(Supplementary material). All the complexes display a quasi-reversible to irreversible reduction
response on the cathodic side of the Ag/AgCl reference electrode. The Epc values of this
reduction span the range −0.92 to −1.12 V. The reduction is irreversible for 3 while the Epa
values for the remaining three complexes (1, 2 and 4) are within −0.29 to −0.56 V. The ratio of
peak currents (ipc/ipa) for the quasi-reversible responses varies from 2 to 4. The cathodic peak
currents are comparable with that of Fc+/Fc couple and other known one electron-transfer
processes [63,67]. Similar electron-transfer responses observed for cis-dioxomolybdenum(VI)
complexes with ligands containing comparable donor atoms have been assigned to Mo(VI) →
Mo(V) process [14,15,40,63−65].
3.5. Catalytic studies
The catalytic behaviours of 1–4 in oxidative bromination reactions of styrene and
salicylaldehyde and in benzoin oxidation reaction have been investigated. Various solvents,
different temperatures and catalyst loadings have been screened using 1 as the catalyst for
optimization of all the three reaction conditions. In each reaction, irrespective of the types of the
products and their selectivities, only complete or nearly complete conversion of the substrate has
been considered during the optimization. These optimized reaction conditions have been used
for scrutiny of the remaining three complexes (2–4) as catalysts (Scheme 1). The details of the
yields, selectivity and TOF values are summarized in Table 4. Generally in the present catalytic
11

studies less number of significant products has been detected when compared with earlier studies
on the same reactions in presence of cis-dioxomolybdenum(VI) complexes with various
tridentate ligands as catalysts [38−40]. However, for all the three reactions the TOF values
obtained with 1–4 as catalysts are lower than the corresponding values reported earlier.
3.5.1. Oxidative bromination of styrene
The catalyst cis-[MoO2(HLn)], styrene, H2O2, KBr and HClO4 in the mole ratio of
0.01:5:10:10:10 in acetonitrile-water (1:1) mixture at 60 °C have been found as the best
condition for maximum conversion of the substrate in about 4 h. Under this condition, three
products 2-bromo-1-phenylethane-1-ol (a), 2-bromo-1-phenylethane-1-one (b) and styrene oxide
(c) have been identified (Scheme 1) for each of the four catalysts (1–4). The yield of b is
significantly higher than that of the other two products (a and c). It is important to note that the
formation of b has occurred most likely through the oxidation of a. The change of the catalyst
from 1 to 4 does not affect too significantly the percentage substrate conversion and product
selectivity. Under specified condition as mentioned above the control experiment without
catalyst has been carried out. The control experiment shows only 10% conversion of the
substrate to 2-bromo-1-phenylethane-1-ol (a) as the major product. The product selectivities
observed here are slightly different than that reported for the bromination reaction of the same
substrate catalyzed by cis-dioxomolybdenum(VI) complexes [38−40]. In the previous studies,
with a and c formation of 1,2-diol and 1,2-dibromo derivatives instead of b have been observed.
We could detect a trace amount (< 1%) of the dibromo product but no diol.
3.5.2. Oxidative bromination of salicylaldehyde
The optimized reaction condition for oxidative bromination of salicylaldehyde using cis-
[MoO2(HLn)] as catalyst is same as described above for oxidative bromination of styrene. But
here the maximum conversion of the substrate has been realized within 1 h. The catalytic
behaviours of 1–4 in this reaction are comparable (Table 4). In all the cases, 5-
bromosalicylaldehyde (d), 3,5-dibromosalicylaldehyde (e) and 3-bromosalicylaldehyde (f) have
been identified as the products (Scheme 1). The 5-bromosalicylaldehyde (d) has been produced
more selectively than the other two products with essentially complete conversion of the
12

substrate. In the control experiment without any catalyst, only 30% of substrate conversion with
100% selectivity for 5-bromosalicylaldehyde (d) has been observed in 4 h of reaction time. In
earlier studies on bromination of salicylaldehyde using cis-dioxomolybdenum(VI) complexes as
catalysts [38,40], d was the major product as observed here. However, unlike the present
observation 2,4,6-tribromo phenol was also reported with e and f as a minor product.
3.5.3. Benzoin oxidation
The cis-[MoO2(HLn)] catalyzed benzoin oxidation reactions have been carried out in
methanol employing H2O2 as the oxidant. The mole ratio of catalyst, substrate and oxidant used
for the reaction is 0.01:5:10. It may be noted that comparable results were obtained when
acetonitrile or ethanol was used instead of methanol as the reaction solvent. In all the cases,
benzoic acid (g), benzil (h) and α,α-dimethoxytoluene (i) have been detected as the products with
complete conversion of benzoin (Scheme 1, Table 4). Benzoic acid (g) is preferentially formed
as the major product followed by benzil (h) and then α,α-dimethoxytoluene (i). A trace amount
(< 1%) of methylbenzoate was also detected. As observed in the preceding two catalytic
reactions, here also the catalytic efficiencies of 1–4 do not vary in a significant way. Previous
studies on benzoin oxidation reactions employing cis-dioxomolybdenum(VI) complexes as
catalysts have demonstrated benzoic acid (g) to be the major product as found here, but along
with (h) and (i) a comparable amount of methylbenzoate has been also reported as an additional
minor product [38,40].
4. Conclusions
The N-capped unsymmetrical tripodal tetradentate 2,2'-(2-hydroxy-3,5-R1,R2-
benzylazanediyl)diethanols (H3Ln) afford complexes of general molecular formula cis-
[MoO2(HLn)] (1−4) in very good yields. X-ray crystallographic studies reveal the same gross
molecular structure for all the complexes. In each complex, the ligand (HLn)2− acts as 5,5,6-
membered chelate rings forming NO3-donor towards the cis-{MoO2}2+ unit. In the crystal
lattice, the molecules of each complex form discrete dimers via two reciprocal intermolecular
O−H···O interactions involving the ethanol-OH and the ethanolate-O both being metal
coordinated. The spectroscopic (IR, UV-Vis and NMR) features of 1−4 complement their
13

molecular structures very well. The redox-active complexes are also catalytically active in
oxidative bromination reactions of styrene and salicylaldehyde and in benzoin oxidation reaction.
The overall catalytic performances of 1−4 in these reactions are generally invariant to the type
and number of substituents on the phenolate moiety of the ligands.
Appendix A. Supplementary material
CCDC 1042110−1042113 contain the supplementary crystallographic data for 1−4,
respectively. These data can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Supplementary materials (synthetic
procedure and the characterization data for H3Ln and Figs. S1−S8) associated with this article
can be found in the online version.
Acknowledgements
S. K. Kurapati S. Maloth thank the Council of Scientific and Industrial Research (CSIR),
New Delhi for research fellowships. We are grateful to the Department of Science and
Technology (DST), New Delhi and the University Grants Commission (UGC), New Delhi for
the facilities provided under the FIST and the CAS programs, respectively.
References
[1] T. Punniyamurthy, S. Velusamy, J. Iqbal, Chem. Rev. 105 (2005) 2329−2363.
[2] F.E. Kühn, A.M. Santos, M. Abrantes, Chem. Rev. 106 (2006) 2455−2475.
[3] R. Sanz, M.R. Pedrosa, Curr. Org. Synth. 6 (2009) 239−263.
[4] M.R. Maddani, K.R. Prabhu, J. Indian Inst. Sci. 90 (2010) 287−297.
[5] J.A. Brito, B. Royo, M. Gómez, Catal. Sci. Technol. 1 (2011) 1109−1118.
[6] R.G. de Noronha, A.C. Fernandes, Curr. Org. Chem. 16 (2012) 33−64.
[7] A. Syamal, M.R. Maurya, Coord. Chem. Rev. 95 (1989) 183−238.
[8] R. Hille, Chem. Rev. 96 (1996) 2757−2816.
[9] J.H. Enemark, J.J.A. Cooney, Chem. Rev. 104 (2004) 1175−1200.
[10] A. Majumdar, S. Sarkar, Coord. Chem. Rev. 255 (2011) 1039−1054.
[11] M.J. Morris, Coord. Chem. Rev. 172 (1998) 181−245.
[12] R.D. Chakravarthy, D.K. Chand, J. Chem. Sci. 123 (2011) 187−199.
14

[13] C.J. Hinshaw, G. Peng, R. Singh, J.T. Spence, J.H. Enemark, M. Bruck, J.
Kristofzski, S.L. Merbs, R.B. Ortega, P.A. Wexler, Inorg. Chem. 28 (1989)
4483−4491.
[14] Y.--L. Wong, Y. Yan, E.S.H. Chan, Q. Yang, T.C.W. Mak, D.K.P. Ng, J. Chem.
Soc., Dalton Trans. (1998) 3057--3064.
[15] A. Lehtonen, M. Wasberg, R. Sillanpää, Polyhedron 25 (2006) 767−775.
[16] S. Khatua, H. Stoeckli• Evans, T. Harada, R. Kuroda, M. Bhattacharjee, Inorg.
Chem. 45 (2006) 9619−9621.
[17] S. Khatua, T. Harada, R. Kuroda, M. Bhattacharjee, Chem. Commun. (2007)
3927−3929.
[18] Y.L. Wong, L.H. Tong, J.R. Dilworth, D.K.P. Ng, H. K. Lee, Dalton Trans. 39 (2010)
4602−4611.
[19] L. H. Tong, Y. L. Wong, H. K. Lee, J. R. Dilworth, Inorg. Chim. Acta. 383 (2012) 91−97.
[20] F. Madeira, S. Barroso, S. Namorado, P.M. Reis, B. Royo, A.M. Martins, Inorg. Chim.
Acta. 383 (2012) 152−156.
[21] X. Lei, N. Chelamalla, Polyhedron 49 (2013) 244−251.
[22] A. Riisioe, A. Lehtonen, M.M. Hanninen, R. Sillanpää, Eur. J. Inorg. Chem. (2013)
1499−1508.
[23] T. Heikkila, R. Sillanpää, A. Lehtonen, J. Coord. Chem. 67 (2014)1863−1872.
[24] X. Hu, K. Meyer, J. Organomet. Chem. 690 (2005) 5474--5484.
[25] L.M. Berreau, Eur. J. Inorg. Chem. (2006) 273--283.
[26] J.G. Verkade, Acc. Chem. Res. 26 (1993) 483−489.
[27] R.A. Schrock, Acc. Chem. Res. 30 (1997) 9−16.
[28] A.S. Borovik, Acc. Chem. Res. 38 (2005) 54−61.
[29] A.G. Blackman, Polyhedron 24 (2005) 1--39.
[30] M. Palaniandavar, M. Velusamy, R. Mayilmurugan, J. Chem. Sci. 118 (2006)
601−610.
[31] Yamaguchi, H. Masuda, Sci. Technol. Adv. Mater. 6 (2005) 34--47.
[32] A.G. Blackman, Eur. J. Inorg. Chem. (2008) 2633--2647.
15

[33] B. Kuswandi, Nuriman, W. Verboom, D.N. Reinhoudt, Sensors 6 (2006) 978−1017.
[34] Z. Dai, J.W. Canary, New J. Chem. 31 (2007) 1708--1718.
[35] V. Contea, A. Coletti, B. Floris, G. Licini, C. Zonta, Coord. Chem. Rev. 255 (2011)
2165--2177.
[36] M.R. Maurya, J. Chem. Sci. 123 (2011) 215−228.
[37] R. Wever, M.A.V. Horst, Dalton Trans. 42 (2013) 11778--11786.
[38] M.R. Maurya, S. Dhaka, F. Avecilla, Polyhedron 67 (2014) 145−159.
[39] M.R. Maurya, N. Kumar, F. Avecilla, J. Mol. Catal. A. Chem. 392 (2014) 50--60.
[40] S. Pasayat, S.P. Dash, S. Roy, R. Dinda, S. Dhaka, M.R. Maurya, W. Kaminsky,
Y.P. Patil, M. Nethaji, Polyhedron 67 (2014) 1--10.
[41] M. Kirihara, Y. Ochiai, S. Takizawa, H. Takahata, H. Nemoto, Chem. Commun.
(1999) 1387--1388.
[42] K.A. Al-Sou'od, B.F. Ali, R. Abu-El-Halawa, A.-A.-H.H. Abu-Nawas, Int. J. Chem.
Kinet. 37 (2005) 444--449.
[43] C. Joo, S. Kang, S.M. Kim, H.Han, J.W. Yang, Tetrahedron Lett. 51 (2010) 6006--
6007.
[44] N. Ueyama, N. Yoshinaga, A. Nakamura, J. Chem. Soc. Dalton. Trans. (1990) 387--
394.
[45] R.K. Bhatia, G.N. Rao, J. Mol. Catal. A. Chem. 121 (1997) 171--178.
[46] G.J.J. Chen, J.W. McDonald, W.E. Newton, Inorg. Chem. 15 (1976) 2612−2615.
[47] P.–P. Yang, H.–B. Song, X.–F. Gao, L.–C. Li, D.–Z. Liao, Cryst. Grow. Des. 9 (2009)
4064−4069.
[48] A. Joohuan, M. Yanyan, R. Chris, C. Rodolphe, B. Colette, Aust. J. Chem. 62 (2009)
1124−1129.
[49] D.D. Perrin, W.L.F. Armarego, D.P. Perrin, Purification of Laboratory Chemicals,
2nd ed., Pergamon, Oxford, 1983.
[50] CrysAlisPro version 1.171.36, Agilent Technologies, Yarnton, Oxfordshire, UK,
2013.
[51] S. Parkin, B. Moezzi, H. Hope, J. Appl. Cryst. 28 (1995) 53−56.
16

[52] SMART version 5.630 and SAINT• plus version 6.45, Bruker• Nonius Analytical
X• ray Systems Inc., Madison, WI, USA, 2003.
[53] G.M. Sheldrick, SADABS, Program for Area Detector Absorption Correction,
University of Göttingen, Göttingen, Germany, 1997.
[54] G.M. Sheldrick, Acta Crystallogr., Sect. A, 64 (2008) 112--122.
[55] L.J. Farrugia, J. Appl. Crystallogr. 45 (2012) 849• 854.
[56] A.L. Spek, Platon, A Multipurpose Crystallographic Tool, Utrecht University,
Utrecht, The Netherlands, 2002.
[57] C.F. Macrae, I.J. Bruno, J.A. Chisholm, P.R. Edgington, P. McCabe, E. Pidcock, L.
Rodriguez-Monge, R. Taylor, J. van de Streek, P.A. Wood, J. Appl. Cryst. 41 (2008)
466• 470.
[58] S.K. Kurapati, Acta Crystallogr. Sect. E 69 (2013) m460−m461.
[59] A. Lymberopoulou-Karaliota, D. Hatzipanayioti, M. Kamariotaki, M. Potamianou,
C. Litos, V. Aletras, Inorg. Chim. Acta 358 (2005) 2975--2995.
[60] Y. Zhu, W. Yan, Y. Zhang, J. Coord. Chem. 64 (2011) 1104−1112.
[61] S.K. Kurapati, U. Ugandhar, S. Maloth, S. Pal, Polyhedron 42 (2012) 161−167.
[62] X. Lei, N. Chelamalla, Polyhedron 49 (2013) 244--251.
[63] S.K. Kurapati, S. Pal, Dalton Trans. 44 (2015) 2401−2408.
[64] S. Gupta, S. Pal, A.K. Barik, S. Roy, A. Hazra, T.N. Mandal, R.J. Butcher, S.K.
Kar, Polyhedron 28 (2009) 711−720.
[65] N.K. Ngan, K.M. Lo, C.S.R. Wong, Polyhedron 30 (2011) 2922–2932.
[66] M. Bagherzadeh, M.M. Haghdoost, A. Ghanbarpour, M. Amini, H.R. Khavasi, E. Payab,
A. Ellern, L.K. Woo, Inorg. Chim. Acta 411 (2014) 61–66.
[67] S. Pal, R.N. Mukherjee, M. Tomas, L.R. Falvello, A. Chakravorty, Inorg. Chem. 25 (1986)
200−207.
17

Table 1
Selected crystal data and structure refinement summary
Complex
1
2
3⋅2CH3CN
4
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
C12H17MoNO5
351.21
C13H19MoNO5
365.23
C19H29MoN3O5
475.39
C19H31MoNO5
449.39
Monoclinic
P21/c
Monoclinic
P21/c
Triclinic
Monoclinic
P21/c
P
1
6.582(4)
32.2225(12)
7.0812(9)
90
13.9819(9)
6.9127(4)
15.0916(10)
90
7.0365(5)
11.8658(8)
14.0224(10)
70.158(6)
77.337(6)
86.791(5)
1074.27(13)
2
22.232(3)
7.3594(14)
12.894(2)
90
b (Å)
c (Å)
α (°)
β (o)
119.653(6)
90
94.6510(10)
90
90.071(17)
90
γ (°)
V (Å3)
1340.09(19)
4
1453.84(16)
4
2109.6(6)
4
Z
ρ (g cm−3)
µ (mm−1)
1.741
1.669
1.47
1.415
0.994
0.920
0.644
0.648
Reflections collected
Reflections unique
Reflections [I ≥ 2σ(I)]
Parameters
4886
13382
6862
5203
2349
2567
3780
3251
2217
2342
3134
2121
172
182
255
241
R1, wR2 [I ≥ 2σ(I)]
R1, wR2 [all data]
GOF on F2
0.0613, 0.1064
0.0663, 0.1081
1.449
0.0242, 0.0650
0.0269, 0.0663
1.083
0.0504, 0.1171
0.0640, 0.1281
1.042
0.0832, 0.1585
0.1263, 0.1778
1.056
Largest diff. peak / hole (e Å−3)
0.663 / −1.107
0.360 / −0.313
0.913 / −0.886
1.662 / −1.174
18

Table 2
Selected bond lengths (Å) and angles (o) for 1, 2, 3⋅2CH3CN and 4
Complex
−O(1)
1
2
3.2CH3CN
4
Mo
1.941(4)
1.960(4)
2.292(4)
1.693(4)
1.704(5)
2.361(5)
153.01(19)
80.29(17)
94.5(2)
1.9406(17)
1.9639(16)
2.2932(17)
1.6938(18)
1.7009(17)
2.3422(18)
153.26(7)
80.66(7)
1.941(3)
1.922(6)
1.958(6)
2.287(6)
1.686(6)
1.724(6)
2.320(8)
152.8(3)
80.6(2)
96.0(3)
104.1(3)
78.6(3)
85.1(3)
93.7(3)
96.9(3)
75.0(3)
169.1(3)
83.1(3)
71.4(2)
107.8(3)
97.8(3)
153.7(3)
Mo−O(2)
Mo−O(3)
Mo−O(4)
1.967(3)
2.288(3)
1.693(3)
−O(5)
Mo
1.698(4)
Mo−N(1)
2.364(4)
−Mo−O(2)
O(1)
152.45(15)
80.20(11)
94.94(14)
104.35(14)
78.45(12)
85.39(11)
94.58(14)
97.11(15)
74.77(13)
168.35(16)
83.45(14)
71.69(12)
108.08(18)
97.02(15)
154.26(14)
O(1)−Mo−O(3)
−Mo−O(4)
O(1)
O(1)−Mo−O(5)
−Mo−N(1)
94.05(9)
103.3(2)
78.69(18)
84.94(16)
95.1(2)
104.07(9)
78.73(6)
O(1)
O(2)−Mo−O(3)
−Mo−O(4)
85.69(7)
O(2)
O(2)−Mo−O(5)
−Mo−N(1)
95.03(9)
87.4(2)
96.82(8)
O(2)
O(3)−Mo−O(4)
−Mo−O(5)
75.24(2)
167.1(2)
84.12(19)
71.55(17)
108.6(2)
96.0(2)
75.17(6)
168.51(8)
82.89(8)
O(3)
O(3)−Mo−N(1)
−Mo−O(5)
71.53(6)
O(4)
O(4)−Mo−N(1)
−Mo−N(1)
108.36(9)
97.53(8)
O(5)
155.0(2)
153.57(8)
19

Tabel 3
Hydrogen bonding parameters
Complex
D−H···A
H···A (Å)
D···A (Å)
D−H···A (o)
1
2
3
4
O(3)−H(3A)···O(2)i
1.77
1.79
1.77
1.78
2.586(6)
2.606(2)
2.586(4)
2.594(9)
175
174
174
176
−H(3A)···O(2)ii
O(3)
−H(3A)···O(2)iii
O(3)
O(3)−H(3A)···O(2)iv
Symmetry transformations used:
(i) −x + 2, −y, −z + 2; (ii) −x, −y + 2, −z + 2; (iii) −x, −y + 2, −z + 1; (iv) −x + 1, −y, −z + 1.
20

Table 4
Catalysis data
Reaction and catalyst
Conversion TOF
Product selectivity
(%)
(%)
(h−1)
Bromination of styrene
(a)
(b)
73
68
73
68
(e)
19
26
28
34
(h)
29
28
30
33
(c)
1
99
98
96
95
124
123
120
119
15
23
10
15
(d)
74
70
67
62
(g)
59
57
53
60
11
7
2
3
11
10
(f)
7
4
Bromination of salicyladehyde
1
99.5
99.8
99.6
99.7
497
499
498
499
2
4
3
5
4
4
Benzoin oxidation
(i)
12
15
17
7
1
2
3
4
99.9
99.9
99.9
99.9
125
125
125
125
21

Figure Captions
Chart 1. Tripodal 2,2'-(2-hydroxy-3,5-R1,R2-benzylazanediyl)diethanols (H3Ln) and the
corresponding complexes cis-[MoO2(HLn)].
Fig. 1. X-ray molecular structures of cis-[MoO2(HL1)] (1) (top) and cis-[MoO2(HL2)] (2)
(bottom). For both structures all non-hydrogen atoms are shown in their 30% probability
thermal ellipsoids.
Fig. 2. Hydrogen-bonded dimers of cis-[MoO2(HL1)] (1) (top) and cis-[MoO2(HL2)] (2)
(bottom).
Fig. 3. Electronic spectrum of cis-[MoO2(HL2)] (2) in (CH3)2SO.
Fig. 4. Cyclic voltammogram (scan rate of 100 mVs−1) of cis-[MoO2(HL2)] (2) in CH3CN-
(CH3)2NC(O)H (9:1) employing the experimental condition described in the text.
Scheme 1. Substrates and the products obtained using cis-[MoO2(HL1−4)] (1−4) as catalysts.
22

Chart 1
23

Fig. 1
24

Fig. 2
25

Fig. 3
26

Fig. 4
27

Scheme 1
28

Graphical Abstract: Pictogram
Graphical Abstract: Synopsis
Four new complexes of cis-{MoO2}2+ with tripodal NO3-donor 2,2'-(2-hydroxy-3,5-R1,R2-
benzylazanediyl)diethanols (H3Ln) are reported. Molecular structures of the diamagnetic and redox
active complexes of general formula cis-[MoO2(HLn)] are established by spectroscopic and X-ray
crystallographic measurements. The catalytic activities of all the complexes in some organic
oxidation reactions are described.
29