Oxygen atom transfer between DMSO and benzoin catalyzed by: Cis -dioxidomolybdenum(vi) complexes of tetradentate Mannich bases

Article abstract of DOI:10.1039/c7nj03551a

The reaction of dibasic tetradentate ONNO donor Mannich bases derived from ethylenediamine and 2,4-di-tert-butylphenol (H₂L¹I), 2,4-di-methylphenol (H₂L²II), 2-tert-butyl-4-methylphenol (H₂L³

Full text of DOI:10.1039/c7nj03551a

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Oxygen atom transfer between DMSO and benzoin catalyzed by cis–
dioxidomolybdenum(VI) complexes of tetradentate Mannich Bases†
Mannar R. Maurya, *a Bekele Mengesha,a Bhawna Uprety,a Nancy Jangra,a Reshu Tomara and
Fernando Avecillab
aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee–247667, India. E–mail:
rkmanfcy@iitr.ac.in; Fax: +91 1332 273560; Tel: +91 1332 285327
https://www.iitr.ac.in/CY/rkmanfcy
bGrupo Xenomar, Centro de Investigacións Científicas Avanzadas (CICA), Departamento de
Química, Facultade de Ciencias, Universidade da Coruña, Campus de A Coruña, 15071 A
Coruña, Spain
† Electronic supplementary information (ESI) available: Figs. S1–S4 contain crystal packing of
complexes 1, 2, 3 and 5. Figs. S5 – S12 contain 1H NMR spectra of ligands and complexes. Fig.
S13 and Table S1present catalytic results for 4ꢀchlorobenzene. CCDC 1573125 (for 1), 1573126
(for 2), 1573127 (for 3) and 1573128 (for 5) contain the supplementary crystallographic data. For
ESI and crystallographic data in CIF or other electronic format see DOI: xxxxxxxx
<<Abstract>>
The reaction of dibasic tetradentate ONNO donor Mannich bases derived from ethylenediamine
and 2,4–di–tert–butylphenol (H2L1), 2,4–di–methylphenol (H2L2), 2–tert–butyl–4–methylphenol
(H2L3), 2,4–di–chlorophenol (H2L4) and 2ꢀnaphthol (H2L5) with [MoVIO2(acac)2] (Hacac =
acetylacetone) in a 1:1 molar ratio in refluxing MeOH gave the corresponding cisꢀ
dioxidomolybdenum(VI) complexes [MoVIO2(L1)], [MoVIO2(L2)], [MoVIO2(L3)], [MoVIO2(L4)],
and [MoVIO2(L5)], respectively. All complexes were characterized by elemental analysis, various
spectroscopic (FTꢀIR, UV/Vis, 1H and13C NMR) techniques and singleꢀcrystal Xꢀray analysis (of
1, 2, 3 and 5). These complexes adopt a distorted sixꢀcoordinated octahedral geometry where
ligands act as tetradentate, coordinating through the two Ophenolate and two Namine atoms in a cisꢀα
type binding mode involving coordination of one of the Namine atom in the apical position and one
Ooxido terminal oxygen atom in the equatorial position. These complexes catalyze oxygen atom
transfer between benzoin and dimethyl sulfoxide (DMSO) in acetonitrile at 80 °C. The formation

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of benzil could easily be monitored by HPLC. The electronic effect caused by the para
substituent and the steric effect caused by the ortho substituent influence the formation of benzil
and therefore conversion varied between 68 ꢀ 99% in 18 h of reaction time under optimized
conditions; most active catalyst being [MoVIO2(L5)]. Almost similar trend has also been obtained
with 4ꢀchlorobenzoin. The pseudo first order rate constant for [MoVIO2(L5)] was found to be
0.0998 h–1. During catalytic reaction, the formation of the binuclear intermediate and its fast
decay into the initial dioxidomolybdenum(VI) complex was established by time dependent
UV/Vis studies.
Introduction
The most prominent role of second row transition element, molybdenum, in chemistry and
biology is that of an important bio–catalyst, since a wide variety of chemical transformations in
biology are catalyzed by molybdenum enzymes.1 Molybdenum enzymes have long enjoyed the
status of being vital eukaryotic enzymes catalyzing reactions involved in carbon, nitrogen and
sulfur bio geochemical cycles in the atmosphere.2 Apart from nitrogenases (in which the metal is
incorporated into a unique [MoFe7S9] cluster) all molybdenum enzymes are quite uniform in
their active site structure and can be divided into three large and non–overlapping families, as
exemplified by the enzymes xanthine oxidase, sulfite oxidase and DMSO reductase.3,4 In all of
these families, the molybdenum cofactor (commonly abbreviated as Moco), consists of a
molybdenum ion coordinated to either one or two bidentate pterindithiolene ligands ([2–amino–
4(1H)–pteridinone]).3,5 The enzymes from the DMSO reductase family (the most diverse of all)
have only been found in bacteria and archea whilst enzymes from the other two families are
found in all forms of life.6,7
The DMSO reductase family is the largest family of molybdopterin enzymes, and is
widely studied for its reactivity and mechanism. They oxidize or reduce suitable substrates (for
e.g. DMSO to DMS), and thus catalyze oxygen atom transfer (OAT) reactions according to
equation 1.2b The process is accompanied by two electron reduction, and usually the oxygen
removed from the substrate (SO) coordinates the metal in the oxidized state. Thus, the presence
of Mo=O moiety become an essential functional requirement for model studies for this class of
enzymes.6,8,9

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One of the most explored enzymes of DMSO reductase family is the name enzyme,
DMSO reductases. It catalyses the reduction of DMSO to DMS, and the molybdenum center
shuttles between Mo(VI) and Mo(IV) oxidation states during the catalytic cycle. A number of
structurally diverse dioxidomolybdneum(VI) complexes have been developed and evaluated for
their oxygen atom transfer abilities for biomimetic modelling as well as industry relevant
processes.10ꢀ13 Most of these model studies are based on oxygen atom transfer involving tertiary
phosphines and/or DMSO. Tertiary phosphines are not the biological substrate for DMSO
reductases, however, they are often a substrate of choice for OAT modelling studies, since their
properties can be desirably tuned and the reactions can be easily monitored by 31P NMR
studies.13ꢀ20
In our recent reports, we have evaluated the OAT ability of cis–dioxidomolybdenum(VI)
complexes built on aminobisphenoltripodal ligands,21 between DMSO and benzoin in
o
acetonitrile at 80 C in 24 h of reaction time. Similar reactions have also been reported by
Sillanpää et al. and Ng et al. in DMSO at ambient temperature.22ꢀ24 A detailed mechanistic study
had revealed an overall steric control in our reactions.21 We have further extended the study to a
new series of cis–[MoO2]2+ complexes derived from Mannich bases of ethylenediamine and 2,4–
disubstituted phenols. The present systems employed for catalytic OAT reactions were made
sterically less hindered by using dipodal ligands, as against tripodal ligands reported earlier by
us. We have achieved almost 99 % conversion in only 18 h of reaction time. We have also
derived the second order rate equation using the most active catalyst, [MoO2L5].
HO
R
R
OH
OH
H
HO
H
N
N
H
H
N
N
R'
R'
R
R’
Ligand
H2L5 V
tert–butyl tert–butyl H2L1 I
Methyl
tert–butyl Methyl
Chloro Chloro
Methyl
H2L2 II
H2L3 III
H2L4 IV
Scheme 1 Structure of ligands used in this study.

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Experimental Section
Materials and general methods
Ammonium molybdate, acetylacetone (SRL, India), 2,4–di–tert–butylphenol, 2,4–di–
methylphenol, 2–tert–butyl–4–methylphenol, 2,4–di–chlorophenol, ethylenediammine (Himedia,
India), benzoin (S.D. fine chemicals, India), formaldehyde and DMSO (Rankem, India) were
used as obtained. Other chemicals and solvents were of AR grade. The precursor complex
[MoVIO2(acac)2] was prepared following a literature method.25
All measurements were made after drying the metal complexes at 100 ˚C. Elemental
analyses of the complexes were carried out on an Elementar model Vario–EI–III after drying the
samples at 100 °C. IR spectra were recorded as KBr pellets on a Nicolet 1100 FT–IR
spectrometer. Electronic spectra of the complexes were recorded in acetonitrile on a Shimadzu
1
13
1601 UV–Vis spectrophotometer. H and C NMR spectra were recorded in CDCl3/DMSOꢀd6
on a JEOL ECX 400 MHz spectrometer. GC–MS was run on a Perkin–Elmer Clarus 500
instrument. HPLC was performed on a Shimadzu LC–2010A HT instrument using 250 × 4.6 mm
Cꢀ18 column in low pressure gradient mode with a flow rate of 0.5 mL min–1 and injection
volume of 40 µL. The mobile phase used was MeCN : H2O : trifluoroacetic acid, 60 : 40 : 0.02.
Preparation of ligands
H2L1 I. A solution of ethylenediammine (0.300 g, 5 mmol), 39% formaldehyde (0.770 g, 10
mmol,) and 2,4−di−tert−butylphenol (2.063 g, 10 mmol) in MeOH (20 mL ) was refluxed for ca.
18 h. During this period, a white solid slowly separated; the solid was collected by filtration,
washed well with cold MeOH and then dried under vacuum. Yield: 1.90 g (76%). (Found: C
76.69; H, 10.35; N, 5.70%. Calcd for C32H52N2O2 (496.0 g molꢀ1): C, 77.37; H, 10.55; N,
1
5.64%). H NMR (400 MHz, CDCl3): δ 10.73 (br, 2H, OH), 7.25 (s, 2H), 6.85 (s, 2H)
(aromatic), 3.92 (s, 4H, ꢀCH2ꢀ), 3.54 (s, 2H, ꢀNH), 3.03 (s, 4H, ꢀCH2CH2ꢀ), 1.44 (s, 18H), 1.30
13
(s, 18H, ꢀCH3). C NMR (400 MHz, CDCl3): δ 150.83, 142.17, 136.68, 121.87, 118.89, 82.23,
51.35, 49.69, 34.89, 34.27, 31.58, 29.61.
Ligands H2L2 II, H2L3 III, H2L4 IV and H2L5
V were prepared by the procedure outlined for
H2L1 using 10 mmol each of 2,4ꢀdiꢀmethylphenol, 2–tert–butyl–4–methylphenol, 2,4–di–
chlorophenol and 2ꢀnaphtol, respectively, in place of 2ꢀtertꢀbutylꢀ4ꢀmethylphenol.

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H2L2 II. Yield: 1.22 g (74%). (Found: C, 73.46; H, 8.42; N, 8.81%. Calcd for C20H28N2O2
(328.0g molꢀ1): C, 73.14; H, 8.59; N, 8.53%. 1H NMR (400 MHz, CDCl3): δ 10.42 (br, 2H, OH),
6.89 (s, 2H), 6.66 (s, 2H) (aromatic), 3.85 (s, 4H, ꢀCH2ꢀ), 3.54 (s, 2H, ꢀNH), 2.97 (s, 4H, ꢀ
CH2CH2ꢀ), 2.22 (s, 12H, ꢀCH3). 13C NMR (400 MHz, CDCl3): δ 153.13, 131.24, 127.86, 126.29,
124.88, 120.61, 58.36, 51.77, 20.46, 15.86.
H2L3 III. Yield: 1.04 g (50%). (Found: C, 75.17; H, 9.54; N, 6.90%. Calcd for C26H40N2O2
(412.0 g molꢀ1): C, 75.68; H, 9.77; N, 6.79%. 1H NMR (400 MHz, CDCl3): δ 10.58 (br, 2H, OH),
6.98 (s, 2H), 6.66 (s, 2H) (aromatic), 3.85 (s, 4H, ꢀCH2ꢀ), 3.44 (s, 2H, ꢀNH), 2.99 (s, 4H, ꢀ
13
CH2CH2ꢀ), 2.22 (s, 6H), 1.41 (s, 18H) (CH3). C NMR (400 MHz, CDCl3): δ 154.21, 136.07,
127.47, 126.83, 121.78, 100.03, 58.05, 50.89, 34.48, 29.89, 20.64.
H2L4 IV. Yield: 1.28 g (62%). (Found: C, 48.44; H, 4.06; N, 6.92%. Calcd for C16H16N2O2Cl4
(410.12 g molꢀ1): C, 46.86; H, 3.93; N, 6.83%. 1H NMR (400 MHz, CDCl3): δ 7.34 (s, 2H), 7.07
13
(s, 2H) (aromatic), 3.97 (s, 4H, ꢀCH2ꢀ), 4.96 (s, 2H, ꢀNH), 2.79 (s, 4H, ꢀCH2CH2ꢀ). C NMR
(400 MHz, DMSOꢀd6): δ 154.26, 127.92, 127.33, 126.84, 121.33, 121.18, 50.71, 46.48.
H2L5 V. Yield: 1.02 g (55%). (Found: C, 73.86; H, 6.61; N, 7.73%. Calcd for C24H24N2O2
(372.46 g molꢀ1): C, 77.39; H, 6.49; N, 7.52%).1H NMR (400 MHz, d6ꢀ[DMSO]): δ 6.12 (d, 2H),
7.12 (d, 2H), 7.26 (dd, 2H), 7.68 (dd, 2H), 7.71 (d, 2H), 7.98 (d, 2H), (aromatic), 4.74 (s, 4H ꢀ
CH2ꢀ), 5.00 (s, 2H, ꢀNH), 4.07 (s, 4H ꢀCH2CH2ꢀ).
Preparations of complexes
[MoO2L1] 1. To a suspension of H2L1 (0.496 g, 1.0 mmol) in MeOH (10 mL) was added a
filtered solution of metal precursor [MoVIO2(acac)2] (0.326 g, 1.0 mmol) in MeOH (10 mL) with
magnetic stirring where a clear orange solution was obtained within a few minutes. The resulting
reaction mixture was then heated under reflux for 4 h on a water bath. After cooling to room
temperature, the precipitated orange solid was collected by filtration, washed with cold MeOH
and dried under vacuum. Yield 0.461 g (74%). (Found: C, 61.36; H, 8.24; N, 4.58%. Calcd for
1
C32H50N2O4Mo (622.0 g mol−1): C, 61.72; H, 8.09; N, 4.50%). H NMR (400 MHz, CDCl3): δ
7.26 (s, 2H), 6.86 (s, 2H) (aromatic), 5.17ꢀ5.18 (d, 1H), 5.20ꢀ5.21 (d, 1H), 3.93ꢀ3.94 (d, 1H),

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3.97ꢀ3.98 (d, 1H) (ꢀCH2ꢀ), 2.94 (s, 2H, ꢀNH), 2.68ꢀ2.73 (t, 2H), 2.84ꢀ2.87 (t, 2H) (ꢀCH2CH2ꢀ),
1.43 (s, 18H), 1.28 (s, 18H) (ꢀCH3). 13C NMR (400 MHz, CDCl3): δ 154.23, 140.84, 135.83,
123.43, 123.20, 120.97, 59.34, 51.87, 35.01, 34.28, 31.80, 29.75.
Complexes
2–5
were prepared similarly using [MoVIO2(acac)2] (0.326 g, 1.0 mmol) and
respective ligands (1.0 mmol).
[MoO2L2] 2. Yield 0.363 g (80%). (Found: C, 75.17; H, 9.54; N 6.90%. Calcd for
1
C20H26N2O4Mo (454.0 g mol−1): C, 52.86; H, 5.77; N 6.17%). H NMR (400 MHz, CDCl3): δ
6.68 (s, 2H), 6.89 (s, 2H) (aromatic), 5.21ꢀ5.24 (d, 2H), 3.94ꢀ3.96 (d, 2H) (ꢀCH2ꢀ), 2.70 (s, 2H, ꢀ
13
NH), 3.05 (s, 2H), 2.91 (s, 2H) (ꢀCH2CH2ꢀ), 2.23 (s, 6H), 2.19 (s, 6H) (CH3). C NMR (400
MHz, CDCl3): δ 156.36, 137.86, 130.45, 129.47, 119.56, 99.81, 53.45, 45.49, 20.52, 17.03.
[MoO2L3] 3. Yield 0.480 g (89 %). (Found: C, 57.47; H, 7.23; N, 5.41%. Calcd for
1
C26H38N2O4Mo (538.0 g mol−1): C, 57.98; H, 7.11; N, 5.20%). H NMR (400 MHz, CDCl3): δ
6.89 (s, 2H), 6.68 (s, 2H) (aromatic), 5.21 (d, 1H), 5.24 (d, 2H), 3.95 (d, 1H), 3.97 (d, 1H) (ꢀCH2ꢀ
), 2.91 (s, 2H, ꢀNH), 3.05ꢀ3.08 (t, 2H), 2.70ꢀ2.73 (t, 2H) (ꢀCH2CH2ꢀ), 2.27 (s, 6H), 1.58 (s, 18H)
(CH3). 13C NMR (400 MHz, CDCl3): δ 156.71, 138.5, 129.18, 126.71, 127.77, 121.23, 53.20,
45.71, 34.58, 29.36, 20.60.
[MoO2L4] 4. Yield 0.487 g (91%). (Found: C, 35.59; H, 2.54; N, 5.29%. Calcd for
1
C16H14Cl4N2O4Mo (536.0 g mol−1): C, 35.85; H, 2.63; N 5.23%). H NMR (400 MHz, DMSO):
δ 7.20 (s, 2H), 7.40 (s, 2H) (aromatic), 4.72 (dd, 2H), 3.95 (dd, 2H) (ꢀCH2ꢀ), 5.40 (s, 2H, ꢀNH),
13
2.17 (t, 2H), 2.80ꢀ2.76 (t, 2H) (ꢀCH2CH2ꢀ). C NMR (400 MHz, DMSOꢀd6): δ 154.43, 128.17,
127.53, 125.42, 123.36), 123.04, 52.31, 45.72.
[MoO2L5] 5. Yield 0.417 g (83%). (Found: C, 58.23; H, 4.54; N, 5.47%. Calcd for
1
C24H22N2O4Mo (498.0 g mol−1): C, 57.84; H, 4.45; N 5.62%). H NMR (400 MHz, CDCl3): δ
7.00 (d, 2H), 7.40 (d, 2H), 7.62 (dd, 2H), 7.75 (dd, 2H), 7.86 (d, 2H), 7.98 (d, 2H), (aromatic),
3.09 (dd, 2H), 4.40 (dd, 2H) (ꢀCH2ꢀ), 5.00 (s, 2H, ꢀNH), 1.27 (t, 2H), 1.75 (t, 2H) (ꢀCH2CH2ꢀ).

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XꢀRay crystal structure determination
Threeꢀdimensional Xꢀray data were collected on a Bruker Kappa Apex CCD diffractometer at
low temperature for and at room temperature for and , by the scan method.
Reflections were measured from a hemisphere of data collected from frames, each of them
covering 0.3º in . A total of 78032 for , 47390 for , 64461 for and 36101 for reflections
measured were corrected for Lorentz and polarization effects and for absorption by multiꢀscan
methods based on symmetryꢀequivalent and repeated reflections. A total of 5937 for , 4529 for
, 3886 for and 3813 for , independent reflections exceeded the significance level
F F) > 4.0, respectively. After data collection, in each case an empirical absorption
correction (SADABS)26 was applied, and the structures were solved by direct methods and
refined by full matrix lea stꢀsquares on F2 data using SHELX suite of programs.27 In
, hydrogen
atoms were included in calculated position and refined in the riding mode, except for N(1) and
N(2), which were located in difference Fourier map and fixed to the nitrogen atoms. In and
hydrogen atoms were located in difference Fourier map and left to refine freely, except for C(2),
C(10), C(11), C(19) and C(20) in , and C(8), C(9), C(10), C(11), C(23), C(24), C(25) and C(26)
in , which were included in calculated position and refined in the riding mode, respectively. In
, hydrogen atoms were included in calculated position and refined in the riding mode, except for
2
1
,
3
5
φꢀω
ω
1
2
3
5
1
2
3
5
(
/σ
1
2
3,
2
3
5
C(8), N(1) and N(2), which were located in difference Fourier map and left to refine freely.
Refinements were done with allowance for thermal anisotropy of all nonꢀhydrogen atoms. A
final difference Fourier map showed no residual density outside, except for compound
disordered water molecules which were not refined: 1.369 and ꢀ0.524 e.Å–3 for
, 0.411 and ꢀ
. A weighting scheme w =
1, due to a
1
0.318 for
2
and 0.605 and ꢀ0.588 e.Å–3 for
3
and 0.569 and ꢀ0.735 for 5
2
2
1/[σ2(Fo ) + (0.108000 P)2 + 2.394000 P] for
1
, w = 1/[σ2(Fo ) + (0.024300 P)2 + 1.332900 P] for
2, , w = 1/[σ2(Fo ) + (0.054500 P)2 + 0.00000 P] for
3
and w = 1/[σ2(Fo ) + (0.066700 P)2 +
2
2
0.00000 P] for
important disorder on
two atomic sites have been observed and refined with the anisotropic atomic displacement
parameters for two ꢀbutyl groups. More specifically this disorder was refined using 243
5
, where P = (|Fo|2 + 2|Fc|2)/3, were used in the latter stages of refinement. An
t
ꢀbutyl groups appear in the crystal of . This disorder has been refined and
1
t
restraints (ISOR, SADI, SIMU and DELU restraints were used). The site occupancy factors were
0.43339 for C(15A), C(16A) and C(17A) and 0.45583 for C26(A), C(27A) and C(28A). Further
details of the crystal structure determination are given in Table 1.

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Table 1 Crystal data and structure refinement for [MoO2(L1)]
1
, [MoO2L2]
2
, [MoO2(L3)]
3 and
[MoO2(L5)]
5
. . CCDC: 1573125ꢀ1573128.
1
2
3
5
Formula
C32 H50 Mo N2 O4
622.68
C20 H26 Mo N2 O4
454.37
C26 H38 Mo N2 O4
538.52
C24 H22 Mo N2 O4
498.38
Formula weight
T, K
293(2)
99(2)
293(2)
296(2)
Wavelength, Å
Crystal system
Space group
a/Å
0.71073
Trigonal
P3
0.71073
Monoclinic
P21/c
0.71073
Orthorhombic
Pcab
0.71073
Monoclinic
P21/c
19.0694(6)
12.1153(11)
10.9427(10)
15.7160(14)
90
12.9405(4)
14.0380(4)
28.3268(10)
90
15.164(2)
12.8563(18)
10.6927(14)
90
b/Å
c/Å
17.1332(6)
90
α/º
β/º
90
112.296(4)
90
90
90.963(7)
90
γ/º
120
90
V/Å3
5395.6(3)
6
1927.8(3)
4
5145.8(3)
8
2084.2(5)
4
Z
F000
1980
936
2256
1016
D
calc/g cmꢀ3
/mmꢀ1
/ (º)
1.150
0.397
1.19 to 28.28
0.0453
1.566
1.390
1.588
0.709
0.543
0.664
ꢀ
1.82 to 28.32
0.0216
1.44 to 28.40
0.0973
1.34 to 28.37
0.0644
θ
Rint
Crystal size/ mm3
Goodnessꢀofꢀfit on F2
R1[I>2σ(I)]a
0.30 × 0.23 × 0.07
1.031
0.50 × 0.40 × 0.35
1.077
0.31 × 0.19 × 0.15
0.937
0.25 × 0.13 × 0.05
1.093
0.0521
0.0196
0.0367
0.0352
wR2 (all data)b
0.1827
0.0524
0.1059
0.1228
Largest differences peak
and hole (eÅꢀ3)
1.369 and ꢀ0.524
0.411 and ꢀ0.318
0.605 and ꢀ0.588
0.569 and ꢀ0.735
2 2
aR1 = ΣFo ꢀ Fc/ΣFo. bwR2 = {Σ[w(Fo2 ꢀFc2)2]/Σ[w(Fo ) ]}1/2

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Results and Discussion
A controlled Mannich condensation between ethylenediamine, formaldehyde and 2,4–
disubstituted phenols or 2−naphthol (1 : 2 : 2 molar ratio) in MeOH yields the corresponding
secondary amine Mannich base ligands, H2L1–5 in moderate to excellent yields. The reaction of
the dibasic tetradentate Mannich base ligands, H2L1−5 in 1:1 molar ratio with [MoVIO2(acac)2] in
MeOH, results in the formation of the corresponding dioxidomolybdenum(VI) complexes,
[MoVIO2(L1−5)] (1–5) in good yields. All complexes are yellow/orange air stable solids, soluble
in common organic solvents such as MeOH, CHCl3, MeCN, DMSO, DMF etc.
Solid state characterizations
The coordination style of the ligands and the resulting geometry around the metal centre was
confirmed by SCꢀXRD studies. ORTEP diagrams for the compounds [MoO2(L1)]
[MoO2(L3)] and [MoO2(L5)]
are presented in Figs. 1, 2, 3, and 4, respectively. Selected bond
distances and angles are given in Table 2.
The structure of the complexes 1, 2, 3 and 5 adopt a distorted sixꢀcoordinated octahedral
1 2,
, [MoO2L2]
3
5
geometry with two Ooxido terminal oxygen atoms and the ligands act as tetradentate, coordinating
through the two Ophenolate and two Namine atoms in a cisꢀα type binding mode involving
coordination of one of the Namine atom in the apical position and one Ooxido terminal oxygen atom
in the equatorial position.28 The axial sites are occupied by the oxido atoms, O(1), and by the
nitrogen atoms, N(1), of the ligand. The equatorial plane is formed for the three atoms of the
ligand [N(2), O(3) and O(4)] and one of the terminal oxygen atoms, O(2), and it is distorted with
respect to the planarity, [mean deviation from the plane for N(2), O(2), O(3), O(4), 0.0041(2) Å
in
displaced toward the apical oxido ligand, O(1), from the equatorial plane defined by N(2), O(2),
O(3) and O(4), 0.3025(3) Å in , 0.3185(2) Å in , 0.2712(12) Å in and 0.2751(2) Å in . The
MoꢀOoxido bond lengths [Mo(1)ꢀO(1) and Mo(1)ꢀO(2) (see Table 2)] and bond angles [O=Mo=O
angle are: 108.08(11)º in , 108.93(6)º in ,106.67(11)º in and 107.51(14)º in ] are within the
ranges for the bond distances typically observed in these type of compounds.29
1, 0.0208(6) Å in 2, 0.0298(11) Å in 3 and 0.0088(13) Å in 5]. The molybdenum atoms are
1
2
3
5
1
2
3
5

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πꢀ
π
Interactions were not observed in the crystal packing of the four complexes. Intermolecular
hydrogen bonds were observed in compounds and (see Table 3). Complex forms selfꢀ
assembled dimers in the solid state packing (Fig. 5), which are stabilized by CꢀH··· non
1
,
2
3
2
π
covalent interactions in an antiparallel architecture. The distance between hydrogen atoms of the
ethyl groups and centroids of the phenyl rings are 2.578 Å.30 Interestingly, crystal packing of
three complexes are entirely different while they belong to very similar ligand system (Figs. S1ꢀ
S4).
Table 2 Bond lengths [Å] and angles [°] for [MoO2(L1)]
[MoO2(L5)]
1
, [MoO2L2]
2
, [MoO2(L3)]
3 and
5
.
Bond lengths
1
2
3
5
Mo(1)ꢀO(1)
Mo(1)ꢀO(2)
Mo(1)ꢀO(3)
Mo(1)ꢀO(4)
Mo(1)ꢀN(1)
Mo(1)ꢀN(2)
1.710(2)
1.707(2)
1.939(2)
1.936(2)
2.351(3)
2.356(3)
1.7055(10)
1.7248(10)
1.9460(10)
1.9546(9)
1.707(2)
1.701(2)
1.9364(18)
1.954(2)
2.345(2)
2.357(3)
1.708(2)
1.717(2)
1.934(2)
1.965(2)
2.314(3)
2.316(3)
2.3501(11)
2.3077(11)
Angles
1
2
3
5
O(1)ꢀMo(1)ꢀO(2)
O(1)ꢀMo(1)ꢀO(4)
O(2)ꢀMo(1)ꢀO(4)
O(1)ꢀMo(1)ꢀO(3)
O(2)ꢀMo(1)ꢀO(3)
O(4)ꢀMo(1)ꢀO(3)
O(1)ꢀMo(1)ꢀN(1)
O(2)ꢀMo(1)ꢀN(1)
O(4)ꢀMo(1)ꢀN(1)
O(3)ꢀMo(1)ꢀN(1)
108.23(12)
96.42(11)
97.42(11)
99.67(12)
93.78(11)
156.43(10)
160.52(11)
90.69(12)
75.99(10)
83.23(11)
108.83(5)
96.13(4)
98.34(4)
98.76(5)
96.92(4)
153.94(4)
164.15(4)
86.63(4)
77.70(4)
82.26(4)
106.67(11)
98.98(9)
95.58(8)
93.18(9)
99.61(8)
156.94(8)
163.31(10)
89.50(10)
83.15(8)
79.76(8)
107.40(13)
97.21(10)
91.96(10)
96.02(11)
102.04(11)
156.88(10)
162.22(11)
90.37(12)
82.16(10)
79.47(10)

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O(1)ꢀMo(1)ꢀN(2)
O(2)ꢀMo(1)ꢀN(2)
O(4)ꢀMo(1)ꢀN(2)
O(3)ꢀMo(1)ꢀN(2)
N(1)ꢀMo(1)ꢀN(2)
89.33(11)
161.92(11)
84.47(10)
78.64(10)
72.25(11)
90.98(5)
160.13(4)
80.45(4)
78.04(4)
73.69(4)
90.94(10)
162.35(10)
80.18(8)
89.18(11)
160.48(12)
75.55(10)
85.77(10)
73.31(10)
80.11(8)
73.03(9)
Table 3 Hydrogen bonds for [MoO2(L1)] , [MoO2L2] and [MoO2(L3)]
1 2 3
Compound DꢀH...A
d(DꢀH)
0.95
d(H...A)
2.31
d(D...A)
3.067(4)
<(DHA)
137.0
N(2)ꢀH(2N)...O(1)#1
1
2
3
N(2)ꢀH(2N)...O(2)#2
N(1)ꢀH(1N)...O(1)#3
0.85(2)
0.96(3)
2.06(2)
2.59(3)
2.8663(14) 157.5(18)
3.194(3)
121(2)
Symmetry transformations used to generate equivalent atoms: #1 xꢀy,x,ꢀz+1
z+1/2 #3 x,yꢀ1/2,ꢀz+1/2
#2 ꢀx+1,yꢀ1/2,ꢀ
Fig. 1 ORTEP plot of complex [MoO2(L1)]
1
. All the nonꢀhydrogen atoms are presented by their
50% probability ellipsoids.

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Fig. 2 ORTEP plot of complex [MoO2L2]
2. All the nonꢀhydrogen atoms are presented by their
50% probability ellipsoids.
Fig. 3 ORTEP plot f complex [MoO2(L3)]
3
. All the nonꢀhydrogen atoms are presented by their
50% probability ellipsoids.

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Fig. 4 ORTEP plot of complex [MoO2(L5)]
5. All the nonꢀhydrogen atoms are presented by their
50% probability ellipsoids.
Fig. 5 Xꢀray fragment of [MoO2L2]
and they are drawn in dashed black lines.
2. CꢀH···π interactions in 2 are present in the crystal packing

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Spectral Studies
The coordination modes of the ligands towards metal centre was further confirmed by
spectroscopic analyses of all the ligands and the metal complexes. The IR spectra of all
complexes exhibits two characteristics IR bands around 883ꢀ902 and 902ꢀ955 cm–1 (Table 4),
due to the respective symmetric and asymmetric stretching of cisꢀ[MoO2] core.31 Additionally,
the broad peak around ca. 3400 cm–1 present in the spectra of all the ligands due to phenolic –OH
is absent in the corresponding complexes indicating the coordination of phenolic oxygen
following deprotonation. Coordination of nitrogen functionalities could not be ascertained
unequivocally by IR spectral study because the spectra of the ligands as well as complexes both
exhibit ν(NH) bands around 3050–3100 cmꢀ1. However, this coordination is well supported by
single crystal Xꢀray study (vide supra). Other characteristic bands present in the spectra of the
ligands as well as the complexes include the stretching frequencies around 2800–3000 cm–1 due
to different alkyl substitution at the phenolic rings.
Table 4 Selected IR data (in cm–1) for the ligands and complexes with tentative assignments.
Entry
Compounds
H2L1,
ν(OH)
ν(NꢀH)
ν
ꢀ
asym(O=Mo=O) νsym(O=Mo=O)
ꢀ
1
I
3270(b)
3000
2
[MoVIO2L1],
H2L2, II
[MoVIO2L2],
H2L3, III
[MoVIO2L3],
H2L4, IV
[MoVIO2L4],
1
2
3
4
5
2960, 3260
3130(b)
3182, 3295
3050
927
883
3
3440(b)
3425(b)
3440(b)
3450(b)
4
927
ꢀ
884
ꢀ
5
6
3300
904
884
7
2990, 3128(b)
3220
8
914
ꢀ
899
ꢀ
9
H2L5,
[MoVIO2L5],
V
3055
10
3055, 3245
955
902

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Table 5 presents UV/Vis spectral data of ligands and their cis–[MoO2]2+ complexes
recorded in MeCN and Fig. 6 presents spectra of complexes. The UVꢀVis spectra of the ligands
display two spectral bands around 224–280 and 286–336 nm which can be attributed to π→π
*
and n→π* transitions, respectively (See Table 5 for exact values). These transitions undergo a
hypsochromic shift in the metal complexes, and thus the corresponding UVꢀVis spectra of the
complexes display only the n→π* band around 260–336 nm. This is possibly due to
rearrangement of the ligand structure after coordination with metal ion. Additionally, all
complexes also display an intense but broad band in the 330–380 nm due to ligand to metal
charge transfer transition.
Fig. 6 UV/Vis spectra of complexes [MoVIO2L1–5].

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Table 5 UV/Vis spectral data of ligands and complexes.
λ[nm] (ɛ
, litre mole–1cm–1)
Compound
H2L1,
[MoVIO2L1],
H2L2, II
[MoVIO2L2],
H2L3, III
[MoVIO2L3],
H2L4, IV
[MoVIO2L4],
H2L5,
[MoVIO2L5],
I
234(2.48×103), 294(1.22×103)
280(4.2×103), 345(2.39×103)
224(1.46×103), 286 (4.73×103)
284(3.25×103), 324(2.04×103)
226(1.75×103), 286(1.02×103)
285(3.46×103), 330(2.19×103)
237(1.43×103), 295(6.66×102)
277(4.90×103), 342(3.09×103)
280(2.23×103), 336(1.24×103)
260(4.82×103), 336 (2.17×103), 380 (6×103)
1
2
3
4
5
V
The coordinating modes of ligands to the metal complexes were further ascertained by
1
recording their H NMR spectra in CDCl3. Experimental section presents spectral data, Fig. 7
1
provides the representative spectra (ligand II and complex
2
) and Figs. S5 – S12 provide H
NMR spectra of other ligands and complexes for the series. All ligands’ spectra show a signal at
δ= 10.25–10.73 ppm due to two phenolic –OH protons which are equivalent in nature. The
disappearance of this signal in the spectra of complexes indicates the coordination of the
phenolic oxygen after their deprotonation. A relatively broad signal appearing at δ =3.45–3.64
ppm due to –NH protons in ligands shows slight shift in their position in the corresponding
complexes, indicating their coordination to the metal center and the resulting change in electron
density. Signals due to methylene groups, connecting to –NH and aromatic ring, appears as small
singlet at δ =3.77ꢀ3.92 ppm except ligand H2L5 which exhibits two singlets at δ = 4.80 (s, 2H)
and 4.3 (s, 2H) ppm.This signal become diastereotopic following coordination and splits into two
doublets (in
2
,
4
and
5
) with two protons each or four doublets (in
1 and 3) with one proton each
and appear at relatively higher
δ
values. This trend is more common in tripodal ligand containing
complexes.32 Similarly, the ethylene protons attached to two dipodal –NH appears as singlet at δ
=2.57ꢀ3.03 ppm in ligands and this splits into two triplets and appears at slightly lower positions.
Signals due to aromatic protons, methyl and tertꢀbutyl groups in ligands as well as in complexes
appear in the expected region with slight variations.

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δ ppm
Fig. 7 1H NMR spectra of H2L2 and
2 in CDCl3.
The binding modes of the ligands were further supported by the study of the
13
coordination–induced C NMR chemical shifts. The spectra were recorded in CDCl3/DMSOꢀd6
and the relevant data are presented in Table 6. All carbons resonate well within the expected
regions; however, significant shifts, ꢁ
of the carbon atoms in the vicinity of the coordinating atoms. Thus, the carbons bearing the
phenolic oxygens undergo a shift, ꢁ between ~3.4 to ꢀ0.68 ppm (see Table 6). Similarly, the
effect of coordination is also well pronounced on the ethylene protons, and they resonate with an
average shift of ꢁ between ꢀ11.48 to 7.99 ppm. A complete list of all peaks is included in the
experimental section (see Fig. 8 as a representative).
δ = [δ(complex) – δ(ligand)], were observed in the signals
δ
δ

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Table 6. 13C NMR spectral data of ligands and complexes.
Compound a
H2L1
[MoVIO2L1]
C1/C1′
150.83
154.23
(3.4)
C4/C4′
142.17
140.84
(2.03)
131.24
130.58
(ꢀ0.66)
136.07
138.5
C7/C7′
49.69
51.87
(2.18)
58.36
53.59
(ꢀ4.77)
58.05
53.20
(ꢀ4.85)
52.31
50.20
(ꢀ2.11)
C8/C8′
51.35
59.34
(7.99)
51.77
45.63
(ꢀ11.48)
50.89
45.71
(ꢀ5.18)
45.72
45.97
(0.25)
(ꢁδ)
H2L2
[MoVIO2L2]
153.13
156.49
(3.36)
154.21
156.71
(2.5)
(ꢁδ)
H2L3
[MoVIO2L3]
(ꢁδ)
H2L4
[MoVIO2L4]
(2.43)
128.17
127.40
(ꢀ0.77)
154.43
153.75
(ꢀ0.68)
(ꢁδ)
a ꢁδ = [δ (complex) ꢀ δ (free ligand)].

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δ ppm
Fig. 8 13C NMR spectra [chemical shifts (δ) in ppm] of H2L3 and
3 in CDCl3.
Oxygen atom transfer between benzoin and DMSO
Cis–dioxidomolybdenum(VI) complexes are known to catalyze oxygen atom transfer (OAT) in
the presence of DMSO in a fashion similar to DMSO reductases. Most of these reactions
reported in literature, are based on oxygen atom transfer between phosphines and/ or DMSO;[13ꢀ
20]
and only a few reports are available where OAT has been explored with a suitable organic
substrate.[21ꢀ24] In our recent studies, we have reported the oxygen atom transfer ability of cis
–
o
dioxidomolybdenum(VI) complexes between DMSO and benzoin in acetonitrile at 80 C. We
have also proposed an overall steric control over the reaction mechanism, and identified suitable
reaction intermediates.[21] The present study further explores the mechanistic details involving
the oxygen atom transfer between DMSO and benzoin, catalyzed by cis
–
dioxidomolybdenum(VI) complexes. As the cis–dioxidomolybdenum (VI) complexes reported
here are built on aminobisphenols, we have evaluated their electronic environment effect on
OAT reactions between DMSO and benzoin in acetonitrile at 80 oC. The reaction was monitored
over a period of 24 h, and the maximum conversion of 99 % to benzil selectively was achieved
within 18 h at 80 oC using [MoO2L5]
5 as a representative catalyst. The reaction was followed by

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measuring the decrease in the concentration of benzoin and formation of benzil via periodic
HPLC analysis of the reaction mixture, eluted with waterꢀacetonitrileꢀtrifluoroacetic acid mixture
(60:40:02) (Fig. 9).
Fig. 9 The progress of the reaction as monitored by HPLC analysis of the reaction mixture at
different time intervals using [MoO2(L5)]
5 as the catalyst. Mobile phase: MeCN : H2O : TFA 60
: 40 : 0.02 (for details, see text).
The reaction was initiated by taking benzoin (1.00 g, 5 mmol), DMSO (1 mL, 14 mmol)
and the complex as catalyst (3.00 mg, 6.02 µmol) in 10 mL of acetonitrile in a two neck 50 mL
5
round bottom flask. DMSO was always taken in excess with respect to benzoin as well as the
catalyst in all the reactions. The temperature of the reaction mixture was maintained at 80 oC for
a period of 24 h, and the progress of the reaction was monitored by withdrawing small aliquots
of the reaction mixture periodically and analyzing the same by HPLC analysis. A maximum of
99% conversion was achieved in 18 h and after that no further conversion was noted. Under the
same reaction conditions, the other four complexes were also tested and the obtained results are
collected in Table 7 and Fig. 10. Thus, the best conversion of 99 % was achieved by using 3.00
mg (6.02 µmol) of 5.

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Fig. 10 Results for oxygen atom transfer reaction between DMSO and benzoin catalyzed by cis
–
[MoO2]2+complexes (for details, see text).
Table 7 Dioxidomolybdenum(VI) complexes catalyzed oxygen atom transfer between benzoin
and DMSO in acetonitrile at 80 oC in 18 h of reaction time; conversion, TOF and selectivity data.
Entry Catalyst
Catalyst [mg, µmol] Conv.[%]
Selectivity [%] TOF [h–1]
1
2
3
4
5
[MoVIO2(L1)] 1
3.76, 6.02
2.73, 6.02
3.24, 6.02
3.22, 6.02
3.00, 6.02
70
79
68
66
99
100
100
100
100
100
33
37
31
31
46
[MoVIO2(L2)] 2
[MoVIO2(L3)] 3
[MoVIO2(L4)] 4
[MoVIO2(L5)] 5
The results of the catalytic OAT reaction between DMSO and benzoin using different
catalysts, show a stark variation while changing the substituent on the phenol rings. Therefore,
the complex with naphthyl rings, i.e. [MoO2(L5)]
renders an excellent conversion of 99 %. This
5
conversion drops to 70 – 80 % with alkyl substitution while the chloro substitution further slows
down the reaction, and the overall conversion drops to 66 % in 18 h of reaction time for
[MoVIO2(L4)] 4. These results can directly be correlated with the electronic environment of the
cis–[MoO2]2+ core of the catalyst during the catalytic process and its consequences on the
reaction mechanism. Most OAT reactions are thought to proceed via an associative mechanism;

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the reaction being initiated by the nucleophilic attack of the substrate (DMS in case of DMSO
reductase) onto one of the more labile oxido groups of cis–[MoO2]2+moiety.[17,33,34] Thus, the
substitutions on the ligands can affect the OAT reactions in two ways: (i) the electronic effect
caused by the para substituent, and (ii) the steric effect caused by the ortho substituent. A bulky
ortho substitution would hinder the approach of the incoming nucleophile, thus inhibiting the
reaction. On the other hand, an electron donating para substitution would facilitate the
nucleophilic attack of the substrate by compensating the electron density on the metal center
because of the +I effect. Thus, when the ortho and para positions are substituted by chloro
group, the reaction conversion drops to lowest, since the electron withdrawing chloro group
exerts an –I effect, thus destabilizing the reaction intermediate. Consequently, complex
4
delivers the lowest conversion of 66 %. However, when the phenolic rings are replaced with
naphthyl rings, the steric hindrance is reduced to minimal at the ortho positions, enabling an
effective and strong interaction of the nucleophile with the metal center. Further, they also
contribute towards stabilization of the reaction intermediate due to enhanced conjugation. Thus,
the maximum conversion of 99 % is achieved with complex 5. The results obtained with the
other three catalysts are also in line with the ortho effects of alkyl substitution.[21] The Hammet
plot also shows a good correlation for all the four electronic para substitutions (R2= 0.98 and ρ =
–1.78) (Fig. 11). This further substantiates our hypothesis of an electron deficient transition state
during the reaction mechanism.
Fig. 11 Hammet plot for OAT reaction between DMSO and benzoin catalyzed by cis–[MoO2]2+
complexes.

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Dioxidomolybdenum(VI) complexes catalyzed oxygen atom transfer between 4ꢀ
o
chlorobenzoin and DMSO have also been carried out in acetonitrile at 80 C in 18 h of reaction
time (Table S1 and Fig. S13). While 89ꢀ100% conversion was achieved with these complexes,
the electron withdrawing chloro group again exerts an –I effect, destabilizing the reaction
intermediate and consequently complex
4 gives the lowest conversion of 89 %.
We have performed a detailed kinetic analysis using the most active complex,
representative catalyst and deduced the rate equation for oxygen atom transfer between DMSO
and benzoin. For this, a solution of (3.00 mg, 6.02 µmol), benzoin (5 mmol) and DMSO (1
5 as the
5
mL) in 10 mL MeCN was maintained at 80 oC. The reaction was performed for a period of 18 h
and reaction mixture was analyzed as mentioned earlier.
The integrated rate equation for the OAT reaction between catalyst, benzoin and DMSO
can be represented by equation 2. Since, the effective concentration of the catalyst does not
change during the reaction, the reaction may be assumed to be following pseudo first order
reaction kinetics. This was confirmed by the pseudo first order straight line plot obtained
between log of decrease in concentration of benzoin vs time (Fig. 12). Equation 3 represents the
resulting pseudo first order rate equation. The pseudo first order rate constant for
be 0.0998 h–1.
5 was found to

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Fig. 12 Variation of [Benzoin] with time for the cis–[MoVIO2(L5)]
5
catalysed oxygen atom
transfer reaction in the presence of DMSO. Reaction conditions: [Benzoin]0 (5 mmol), DMSO
(1.00 mL), catalyst (3.00 mg) and MeCN (10 mL).
The order of the reaction with respect to catalyst concentration was determined by
solving equation 5, using the data inferred from Fig. 12. Thus, the rate of decrease in benzoin
o
concentration after 18 h of reaction time at 80 C in acetonitrile was monitored using different
concentrations of the catalyst, [MoVIO2(L5)]. The relevant plot (lnk1 vs ln[catalyst]) is shown in
Fig. 13 and the observed order with respect to catalyst is found to be 0.94 and kobs is 0.038 M–1h–
1. Therefore, the overall rate equation may be represented as:
Rate = kobs[Benzoin][Catalyst]
kobs = 0.038 Mꢀ1 hꢀ1
(6)

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Fig. 13 Plot of lnk1 vs ln[catalyst] for oxido transfer reaction between benzoin and DMSO
catalysed by [MoVIO2(L5)] at 80 oC for 18 h reaction time.
5
Most oxygen transfer reactions are thought to proceed via the formation of a Mo(V)
dinuclear intermediate [LMoV–µ–O–MoVL] (equations 7–8).[21] However, its formation is fast
and reversible, and irreversible binuclear formation is often related to catalyst poisoning. During
our studies, the formation of the binuclear intermediate and its fast decay into the initial
dioxidomolybdenum(VI) complex was established by time dependent UV–Visible studies (Fig.
14). The reversible nature of the process is also authenticated by the fact that the order of the
reaction in terms of catalyst was found to be unity.

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Fig. 14 Timeꢀdependent spectral changes observed in the OAT reaction between 4, benzoin and
DMSO in acetonitrile after 8 h of reaction time. The spectra were recorded every 10 s from 80 oC
to room temperature.
[MoVIO2(Ln)] + DMS
[MoIVO(Ln)] + DMSO (7)
O
O
MoVLn]
[LnMoV
O
[MoVIO2(Ln)] + [MoIVO(Ln)]
(8)
Conclusions
Five dibasic tetradentate ONNO donor Mannich bases H2L1–5 derived from ethylenediamine and
2,4–di–tert–butylphenol, 2,4–di–methylphenol, 2–tert–butyl–4–methylphenol, 2,4–di–
chlorophenol and 2ꢀnaphthol and their corresponding cisꢀMoO2 complexes, [MoVIO2(L1–5)] have
been isolated and characterized. The isolated complexes have successfully been used as catalysts
for oxygen atom transfer between benzoin or 4ꢀchlorobenzoin and dimethyl sulfoxide (DMSO)
in acetonitrile at 80 °C, similar to DMSO reductase. As high as 100% conversion, monitored by
HPLC was achieved with complex
5
. The electron withdrawing chloro group exerts an –I effect,
gives the lowest conversion
destabilizing the reaction intermediate and consequently complex
4

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of 66% for benzoin and 89% for 4ꢀchlorobenzoin. The naphthyl ring reduces the steric hindrance
at the ortho position, stabilizing the reaction intermediate due to enhanced conjugation and
consequently complex
5 gives highest conversion. Other substituents have intermediate effect.
The Hammet plot also presents a good correlation for all the four para substitutions (R2= 0.98
and ρ = –1.78). A detail kinetic study supports the pseudo first order reaction; the obtained
pseudo first order rate constant for
5
is 0.0998 h–1. UV/Vis spectroscopy suggested the plausible
formation of the binuclear intermediate [LMoV–ꢂꢀO–MoVL] during catalytic action and its fast
decay to the initially taken monomeric complex.
Conflicts of Interest
There are no conflicts of interest to declare.
Acknowledgments
M. R. M. thanks the Science and Engineering Research Board (SERB), Department of Science
and Technology, Government of India, New Delhi for financial support of the work (grant
number EMR/2014/000529). B. M. is thankful to the Indian Institute of Technology Roorkee for
an MHRD fellowship.
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