Thiohydrazide complexes of molybdenum and their relevance to reduction of dinitrogen to ammonia

Article abstract of DOI:10.1039/c4dt01945h

Aqueous solution of sodium molybdate reacts with aromatic thiohydrazides like thiobenzhydrazide, 2-hydroxythiobenzhydrazide, furan-2-thiohydrazide and thiophen-2-thiohydrazide to form green, neutral diamagnetic 13 chelates. They were characterized by elemental analysis and spectroscopic methods. Tris(2-hydroxythiobenzhydrazido)molybdenum(vi) was crystallized from benzene and the crystal structure shows that molybdenum(vi) is hexacoordinated to three sulfur and three nitrogen atoms from three identical ligands in facile trigonal prismatic geometry. The OH group is involved in intramolecular and intermolecular hydrogen bonding. The bound ligands of tris(thiohydrazido)molybdenum(vi) undergo a redox reaction differently depending on the solvents. Two complexes of molybdenum bound to those oxidized ligands were isolated and characterized and their structures were also solved. Of them a binuclear complex, containing two MoO₂²⁺ ions, of the ligand N-2-hydroxybenzoyl-N′-2-hydroxythiobenzoylhydrazine showed some capability to catalyze the reduction of dinitrogen to ammonia by sodium borohydride. This journal is

Full text of DOI:10.1039/c4dt01945h

Dalton
Transactions
View Article Online
View Journal
PAPER
Thiohydrazide complexes of molybdenum and
their relevance to reduction of dinitrogen to
ammonia†
Cite this: DOI: 10.1039/c4dt01945h
Dolan Sengupta,a,b Snigdha Gangopadhyay,*a Michael G. B. Drewc and
Pijush Kanti Gangopadhyaya
Aqueous solution of sodium molybdate reacts with aromatic thiohydrazides like thiobenzhydrazide,
2-hydroxythiobenzhydrazide, furan-2-thiohydrazide and thiophen-2-thiohydrazide to form green, neutral
diamagnetic 1 : 3 chelates. They were characterized by elemental analysis and spectroscopic methods.
Tris(2-hydroxythiobenzhydrazido)molybdenum(VI) was crystallized from benzene and the crystal structure
shows that molybdenum(VI) is hexacoordinated to three sulfur and three nitrogen atoms from three identi-
cal ligands in facile trigonal prismatic geometry. The OH group is involved in intramolecular and inter-
molecular hydrogen bonding. The bound ligands of tris(thiohydrazido)molybdenum(VI) undergo a redox
reaction differently depending on the solvents. Two complexes of molybdenum bound to those oxidized
ligands were isolated and characterized and their structures were also solved. Of them a binuclear
Received 27th June 2014,
Accepted 5th November 2014
2+
complex, containing two MoO2 ions, of the ligand N-2-hydroxybenzoyl-N’-2-hydroxythiobenzoyl-
DOI: 10.1039/c4dt01945h
hydrazine showed some capability to catalyze the reduction of dinitrogen to ammonia by sodium
borohydride.
www.rsc.org/dalton
bound to a bimetallic molybdenum center.4 Since the model
compounds mimicking the Fe–Mo cofactor of nitrogenase
Introduction
Coordination compounds of high-valent molybdenum(V and VI) have been shown2 to contain the cluster core of Fe7MoS9 or
with bidentate ligands having nitrogen and sulfur as their Fe6Mo2S9, the study of the nature of molybdenum complexes
donor sites, are of interest because of their relevance to redox of bidentate ligands with nitrogen and sulfur as donor sites,
active molybdo-enzymes.1 Of the various enzymes containing particularly having –S–Mo–N–N– linkage, may be of interest to
molybdenum, nitrogenase probably received a greater atten- explore the role of the metal in nitrogen fixation. The pioneer-
tion.2 Despite intensive investigations, the mechanism of con- ing work in this field has revealed some interesting structural
version of nitrogen to ammonia by nitrogenase, especially the types of non-oxomolybdenum5 complexes with trigonal prism
role of molybdenum in the Fe–Mo cofactor, has not yet been geometry. Dioxomolybdenum(VI) complexes of similar ligands,6
completely understood. However, recent studies3 have shown however, behave in a predictable manner. Some hydrazido-
that dinitrogen can bind to a sterically protected single molyb- bridged binuclear molybdenum complexes of ligands with
denum center forming an end-on monometallic complex and additional S-donor atoms have also been reported.7 In the
subsequently be reduced to ammonia. Alternatively, dinitrogen present paper, synthesis of tris(thiohydrazido)molybdenum(VI)
can be reduced by a reducing agent at an end-on hydrazine complexes of some aromatic thiohydrazide ligands, namely,
thiobenzhydrazide, 2-hydroxythiobenzhydrazide, furan-2-thio-
hydrazide and thiophen-2-thiohydrazide has been described.
aDepartment of Chemistry, Presidency University (formerly Presidency College),
The general scheme for the synthesis of the molybdenum com-
Kolkata 700073, India. E-mail: snigdhagangopadhyay@yahoo.com;
plexes are shown in Scheme 1. The spectral properties of all
Tel: +91 33 22413893
bDepartment of Chemistry, Dumdum Motijheel College, Kolkata 700074, India.
E-mail: dolansengupta@yahoo.co.in; Tel: +91 33 255125 85, +91 33 22413893
cDepartment of Chemistry, University of Reading, Berkshire, RG6 6AH, UK.
E-mail: m.g.b.drew@reading.ac.uk; Fax: +44 (0)118 378 8454;
the compounds have been discussed. The behaviour of tris(2-
hydroxythiobenzhydrazido)molybdenum(VI) in acetone and
DMSO has also been studied. The catalytic properties of the
isolated compounds in relation to reduction of dinitrogen to
ammonia by a reducing agent, sodium borohydride, have been
examined and reported. The structure of the compounds has
also been described when single crystals could be isolated.
Tel: +44 (0) 118 378 8454I
†Electronic supplementary information (ESI) available. CCDC 903770–903772.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4dt01945h
This journal is © The Royal Society of Chemistry 2014
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Tris(thiobenzhydrazido)molybdenum(VI) (I). Yield 72%.
Anal. Calc. for [(C6H5CSNNH)3Mo]: C, 46.15; H, 3.30; N, 15.38;
S, 17.58. Found: C, 46.30; H, 3.12; N, 15.31; S, 17.82%. Elec-
tronic spectral absorption maxima in benzene solution:
15 710 cm−1 (6205 M−1 cm−1), 25 880 cm−1 (10 050 M−1 cm−1).
IR (cm−1) 3230w (νN–H), 1600s (aromatic), 1565m (νC–N),
1470s (aromatic), 730m (νC–S), 490w (νMo–N), 390w (νMo–S).
1H NMR: δ (ppm) 6.98–7.26 (m, 2H, 3J 8.6 Hz), 7.45 (t, 1H,
3J 9.2 Hz), 7.54 (d, 2H, 3J 7.7 Hz) 10.22 br. (1H, N–H).
Tris(2-hydroxythiobenzhydrazido)molybdenum(VI) (II). Yield
94%. Anal. Calc. for [(C6H4(OH)CSNNH)3Mo] C, 42.42; H, 3.03;
N, 14.14; S, 16.16. Found: C, 42.57; H, 3.10; N, 14.05; S,
16.38%. Electronic spectral absorption maxima in benzene:
14 460 cm−1 (11 714 M−1 cm−1), 24 700 cm−1 (18 941 M−1
cm−1).
Scheme 1 General scheme for synthesis for molybdenum(VI) com-
plexes of thiohydrazides.
IR (cm−1) 3400–3050 br. (νO–H and νN–H), 1600s (aro-
matic), 1585m (νC–N), 1480s (aromatic), 735m (νC–S), 485w
(νMo–N), 385w (νMo–S). 1H NMR δ (ppm) 6.96–7.06 (m, 1H,
Experimental
Materials and methods
3
3
3J 7.4 Hz), 7.12–7.26 (m, 1H, J 6.8 Hz), 7.55 (d, 1H, J 7.2 Hz),
7.97 (d, 1H, 3J 7.6 Hz), 10.41br. (N–H), 10.92br. (O–H).
Tris(furan-2-thiohydrazido)molybdenum(VI) (III). Yield 68%.
Anal. Calc. for [(C4H3O.CSNNH)3Mo] C, 34.88; H, 2.33; N,
16.28; S, 18.60. Found: C, 34.95; H, 2.12; N, 16.31; S, 18.80%.
Electronic spectral absorption maxima in benzene solution:
14 440 cm−1 (10 987 M−1 cm−1), 24 680 cm−1 (15 678 M−1
cm−1).
Na2MoO4 (>98%, Sigma-Aldrich) was used to prepare the
molybdenum complexes. The ligands were synthesized as
described earlier.8 All other chemicals were of reagent grade
and used without further purification; solvents were purified
and dried according to the standard procedures. Electronic
absorption and IR spectra were recorded on a Hitachi (Model
U-3210) spectrophotometer and Unicam SP300S (KBr disks
IR (cm−1) 3290w (νN–H), 1590s (aromatic), 1570m (νC–N),
1
1
and Nujol mulls) respectively. H NMR spectra were recorded
820m (νC–S), 495w (νMo–N), 390w (νMo–S). H NMR: δ (ppm)
6.41–6.66 (m, 1H, 3J 3.6 Hz), 6.81 (d, 1H, 3J 4.2 Hz), 7.36 (d,
1H, 3J 5.4), 13.57 (N–H).
on a Bruker 500 MHz instrument in CDCl3 (99.9%, Cambridge
Isotopes Laboratories Inc.) solutions. Continuous wave EPR
spectra were recorded on a Bruker X-band EPR instrument in
dichloromethane–toluene mixture solution (v/v 1 : 1) (Sigma-
Aldrich) at 70 K temperature using liquid helium. The solu-
tions were degassed by argon flow for 15 min prior to freezing.
Magnetic susceptibility was measured with a MAGWAY MFG
Mk 1 (Sherwood Scientific Ltd, UK). Cyclic voltammetric
measurements were carried out with an EG&G Potentiostat
(model 263A) instrument using a platinum working electrode,
a platinum wire auxiliary electrode and an Ag/AgCl (saturated
KCl) as the reference electrode. All cyclic voltammetric experi-
ments were performed under a nitrogen atmosphere in
Tris(thiophen-2-thiohydrazido)molybdenum(VI) (IV). Yield
74%. Anal. Calc. for [(C4H3S.CSNNH)3Mo] C, 31.91; H, 2.13; N,
14.89; S, 34.04. Found: C, 31.71; H, 2.10; N, 14.82; S, 34.25%.
Electronic spectral absorption maxima in benzene solution:
14 510 cm−1 (11 539 M−1 cm−1), 24 780 cm−1 (16 330 M−1
cm−1).
IR (cm−1) 3290w (νN–H), 1607s (aromatic), 1590m (νC–N),
1
710m (νC–S), 495w (νMo–N), 390w (νMo–S). H NMR: δ (ppm)
7.03–7.18 (m, 1H, 3J 4.6 Hz), 7.41 (d, 1H, 3J 5.0 Hz), 7.67 (d,
1H, 3J 5.6 Hz), 13.32 (N–H).
distilled dichloromethane solution (0.1
M TBAP). Cyclic
X-ray crystallography
voltammetric data were collected at 298 K and are uncorrected
for junction potential.
Crystallographic data were collected with MoKα (λ = 0.71 073)
radiation at 150(2) K using the Oxford Diffraction X-Calibur
CCD system. The crystal was positioned at 50 mm from the
Synthesis of tris(thiohydrazido) complexes of molybdenum
Sodium molybdate (20.6 mg, 0.1 mmol) was dissolved in water CCD. Frames of 321 were measured with a counting time of 10 s.
and diluted to about 100 ml. Acidity of the solution was Data analysis was carried out with the ChrysAlis program.9
adjusted between 0.5 and 1 M with respect to acetic acid and it The structures were solved using direct methods with
was heated nearly to boiling. A solution of the ligand SHELXS97.10 The non-hydrogen atoms were refined with ani-
(0.5 mmol) in ethanol was added with constant stirring and sotropic thermal parameters. The hydrogen atoms bonded to
the mixture was allowed to stand at 80 °C for about 2 h. A deep carbon, nitrogen and oxygen were included in geometric posi-
green precipitate, thus formed, was separated by filtration tions and given thermal parameters equivalent to 1.2 times
under suction, washed thoroughly with boiling water and those of the atom to which they were attached. An absorption
dried at 110 °C. Yield of each compound was recorded on the correction was carried out using the ABSPACK program.11 The
basis of molybdenum.
structure was refined on F2 using SHELXL97.10
Dalton Trans.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Reduction of dinitrogen to ammonia
are dominated by two peaks in the visible region, one in the
region of 14 500 cm−1 and the other at around 24 700 cm−1
,
A weighed amount of green thiohydrazide complex of molyb-
denum was dissolved in 5 ml of DMSO and allowed to stand
for 2 hours. The solution was then quantitatively transferred to
a 3-necked flask fitted to a pressure equalizer dropping funnel,
an inlet and an outlet for nitrogen. The outlet leads to an
Erlenmeyer flask containing a measured volume of 0.01 M
hydrochloric acid to absorb expelled ammonia. About 2 g of
sodium borohydride was added and then ethanol was added
through the dropping funnel. Nitrogen gas was bubbled
through for 2 h. The amount of ammonia that could be
expelled was determined either by the spectrophotometric
method with Nessler’s reagent or by titration of excess acid
with the standard sodium hydroxide. The results were
expressed as the percentage of ammonia on the basis of nitro-
gen present in the compound and were calculated using the
the energy of separation being about 10 200 cm−1. Compound
I under similar conditions, however, showed two peaks at
slightly higher energy with almost the same difference in
energy. The molar absorbance of the compounds at the peaks
is of the order of 104 dm3 mole−1 cm−1 and hence bands may
be considered as LMCT bands. Comparing the electronic spec-
tral features of these compounds with those of tris-cis-stilbene-
dithiolatomolybdenum(VI) the lower energy band may be
assigned as A1 → E and the higher energy band as A1 → E,
1A2. These bands are characteristics of six coordinate molyb-
denum(VI) complexes with the trigonal prismatic structure.12 It
has been mentioned earlier that compound II in acetone solu-
tion gradually turns brown on standing. On following the elec-
tronic spectrum with time it was found that the two peaks at
14 460 cm−1 and 24 700 cm−1 lost their intensities gradually
and a new peak appeared at 21 460 cm−1 with three isosbestic
1
1
1
1
n  100
n  100
equations
for compounds I–IV and
for com-
6 Â m
2m
pound IIc where n = number of moles of ammonia purged and points at 17 860 cm−1, 22 730 cm−1 and 26 310 cm−1 (ESI
m = moles of the compound used. The compounds I–IV Fig. S1†). Compound IIb isolated from a concentrated solution
contain six moles of nitrogen per mole of the complex while of II in acetone solution showed only one peak in the visible
compound IIc contains two moles of nitrogen per mole of the region at 20 830 cm−1. Compounds isolated from the acetone
complex. The experiment was repeated using different passage solution of I, III and IV showed peaks at 22 025 cm−1
,
times of nitrogen.
20 800 cm−1 and 20 877 cm−1, respectively, in the visible
region of the electronic spectra. Compound IIc isolated from
the DMSO solution of II also showed only one peak in the
visible region at 20 330 cm−1
.
Results and discussion
The infrared spectral data relevant for elucidation of the
structure of thiohydrazido complexes are discussed here. None
of the tris(thiohydrazido)molybdenum(VI) complexes showed
any band in the region of 880–950 cm−1. This suggests the
absence of MovO in the complexes. The characteristic ν(C–S)
in the complex suffers a change of ∼25 cm−1 with respect to
that of the corresponding ligand and it may be due to the for-
mation of complex through the S-centre. The ν(C–N) of the
complexes are marginally higher than those of ligands
showing the increase of double bond character of the C–N
bond. The weak bands near 485–495 cm−1 and at
385–390 cm−1 that are not observed in the IR spectra of the
ligands may be assigned to the Mo–N and Mo–S stretching fre-
quencies, respectively. The IR spectrum of compound IIb
showed a medium intensity band at 1695 cm−1 in addition to
all other characteristic peaks of compound II. This peak is
assigned to CvO stretching of the acetone molecule linked to
the compound through the H-bond. The IR spectrum of com-
pound IIc showed characteristic absorption at 921 and
894 cm−1 due to νas and νs of the cis-MoO2 group.13 The νSvO
band of the coordinated DMSO molecule in the compound is
found at 1003 cm−1 (νSvO in free DMSO-1058 cm−1). This indi-
cates coordination of the DMSO molecule through oxygen to
the Mo-centre.14
Behavior of the compounds in different solvents
The green compounds (I–IV) retain their color in benzene,
toluene, chloroform and dichloromethane solution. While
green color persists in benzene and toluene solutions for more
than a week, solutions in chloroform and dichloromethane
turn red slowly after 24 hours. It is found that in solvents con-
taining oxygen and nitrogen the color of the compounds
turned brown quickly. The solvents include acetone, aceto-
nitrile, ethanol, methanol, DMSO and DMF.
Solution of II in benzene on slow evaporation formed beau-
tiful green crystals of the molecular formula [(C6H4(OH)
CSNNH)3Mo].3C6H6 (IIa) (Anal. Calc. C, 56.53; H, 4.35; N,
10.14; S, 11.59%. Found: C, 56.54; H, 4.32; N, 10.15; S,
11.57%). The compound is diamagnetic.
A black needle shaped crystal obtained from the acetone
solution of II is found to be paramagnetic (μ = 1.58 BM) and
has the molecular formula [C24H21N6O3S3]Mo·(CH3)2CO(IIb)
(Anal. Calc. C, 46.89; H, 3.91; N, 12.16; S, 13.89; Mo, 13.88.
Found: C, 46.88; H, 3.92; N, 12.17; S, 13.87; Mo, 13.90%). A
brown crystalline diamagnetic compound of the molecular
formula C18H20Mo2N2O9S3 (IIc) (Anal. Calc. C, 31.03; H, 2.87;
N, 4.02; S, 13.79; Mo, 27.57. Found: C, 31.01; H, 2.88; N, 4.04;
S, 13.78; Mo, 27.59%) was isolated from the DMSO solution
of II.
1
The H NMR spectrum of compound I showed two distinct
sets of peaks in the region of 6.92–8.16 ppm and at 10.22 ppm.
The signals in the region 6.92–8.16 are due to aromatic
Spectral data
Electronic spectra of three tris(thiohydrazido) complexes of protons revealing the presence of three sets of aromatic
molybdenum(VI), (II, III and IV) in benzene or toluene solution protons and the N–H peak is found at 10.22 ppm. Compound
This journal is © The Royal Society of Chemistry 2014
Dalton Trans.

View Article Online
Paper
Dalton Transactions
II showed four peaks in the region for aromatic protons Crystal structure of the complexes
(6.96–8.3 ppm) and two broad peaks at 10.41 and 10.92 ppm.
The crystal data, together with the data collection and struc-
ture refinement parameters of compounds IIa, IIb and IIc are
presented in ESI (Table S1†).
The perspective view of the structure of compound IIa is
shown in Fig. 1.
The peaks at 10.41 and 10.92 ppm are due to N–H and O–H,
respectively. Compound III showed peaks in two distinct
regions of 6.41–7.36 ppm and 13.57 ppm. The first set of three
peaks is for the furan ring region and the peak at 13.57 is due
to N–H. In compound IV three peaks for the thiophene ring
protons were found in a narrow region of 7.05–7.67 ppm while
the peak due to the N–H proton was observed at 13.32 ppm.
The large downfield shifts of N–H protons in thiohydrazido
complexes may be attributed to coordination of nitrogen to
molybdenum in the high oxidation state. In the NMR spec-
trum of compound IIb several peaks were found in the aro-
matic region of 6.85–8.30 ppm and three sharp peaks at 2.06,
2.44 and 2.64 ppm are due to the methyl protons. The peak
at 2.06 ppm is for acetone and the other two peaks are
assigned to two non-equivalent methyl groups of the hydra-
zone. The OH peaks for the three non-equivalent ligands are
noticed at 9.15, 11.37 and 13.40 ppm. It may be mentioned
that the aromatic protons and the protons in the heterocyclic
rings have been identified by comparing them with those
found in NMR spectra of the free ligands. The X-band EPR
spectrum of compound IIb in frozen dichloromethane–
toluene (1 : 1) solution is characteristic of molybdenum(V)
with S = 1/2 and showed g values of 2.0037, 1.9999 and
1.9987. No hyperfine splitting due to 14N (S = 1) could be
observed because of the large H-strain (anisotropic line
broadening).Hx, Hy and Hz values are ∼50 MHz, 50 MHz and
50 MHz (ESI Fig. S2†). Fine splitting due to 95Mo (I = 5/2)
could not be observed. No EPR signal could be observed with
any of the tris(thiohydrazido) complexes. Absence of the EPR
signal and diamagnetism of tris(thiohydrazido) complexes of
molybdenum are consistent with the formal oxidation state
of +6 (d0) of the metal. Compound IIc also did not exhibit
any EPR signal.
The selected bond lengths and angles of the compound are
listed in Table 1. The crystal structure of compound IIa
[(C6H4(OH)CSNNH)3Mo].3C6H6 contains discrete [(C6H4(OH)-
CSNNH)3Mo] entities and three C6H6 molecules. The com-
pound consists of monomeric molecules and each Mo(VI) is
hexacoordinated to three nitrogen and three sulfur atoms from
three identical ligands forming a facial structure. The structure
shows Mo(VI) in a slightly distorted trigonal prismatic environ-
ment being bonded to three bidentate ligands through N and S.
One triangular face containing three sulfur atoms is an equi-
lateral triangle of side length 3.295 Å while the other face con-
taining three nitrogen atoms is also another equilateral
triangle of side length 2.954 Å. The difference in the side
length of two triangular faces arises from the difference in van
der Waals radii of S and N and it accounts for the distortion in
the trigonal prismatic facial structure. The Mo(1)–N(11) and
Mo(1)–S(20) bond lengths are 1.9943(17) and 2.4191(5) Å,
respectively. These bond-lengths are in agreement with
those in the tris(thiobenzoyldiazene)molybdenum complex.5
The C(13)–S(20) bond length of 1.722(2) Å is larger than the
Electrochemistry
Two cathodic peak potentials at −0.193 V and −0.821 V were
observed in the cyclic voltammogram (Ag/AgCl as the reference
electrode) of compound II. The corresponding anodic peaks
were found at +0.065 V and −0.520 V. The large separation of
Ep and Ep values (>250 mV) suggests that the electron transfer
c
a
processes are irreversible or quasi-reversible. The first Ep value
c
corresponds to one electron reduction of Mo(VI) to Mo(V) and
the second one to Mo(V)/Mo(IV).5 Compound I also showed
Fig. 1 Molecular structure of compound IIa with the numbering
scheme adopted. Thermal ellipsoids are shown at the 50% probability
level.
similar peaks only at a slightly shifted position (Ep = −0.36 V
c
and −1.12 V; Ep = −0.12 V and −0.74 V). The Ep value of −1.12 V
a
c
is close to a cathodic peak at −0.98V in the cyclic voltammo-
gram of the free ligand and hence it may be due to the ligand
based irreversible reduction process. Compounds III and IV
Table 1 Selected bond lengths (Å) and angles (°) of compound IIa
had reduction peaks (Ep ) at −0.388 V and −0.459 V, respect-
c
Mo(1)–N(11)
1.9943(17)
1.722(2)
1.343(2)
76.18(5)
98.40(7)
119.05(17)
Mo(1)–S(20)
C(13)–N(12)
C(15)–O(150)
N(12)–N(11)–Mo(1)
C(13)–N(12)–(N11)
2.4191(5)
1.329(2)
1.362(2)
128.22(12)
114.69(17)
ively, with their corresponding Ep values at −0.268 V and C(13)–S(20)
a
N(11)–N(12)
N(11)–Mo(1)–S(20)
C(13)–S(20)–Mo(1)
N(12)–C(13)–S(20)
−0.305 V. All these redox processes are irreversible with high
ΔEp (240–305 mV) and it may be due to the ligand based elec-
tron transfer process.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
CvS bond length of 1.61–1.63 Å15 while the C(12)–N(13) bond
length of 1.329(2) Å is shorter than the C–N single bond length
of 1.47 Å. From these data it is obvious that the thioenol tauto-
mer of the ligand participates in complex formation.16 In com-
pound IIa the N(11)–N(12) bond length of 1.343(2) Å is shorter
than the N–N distance of 1.4187(16) Å as was determined by
X-ray crystallography of the free ligand 2-hydroxythiobenzhydra-
zide monohydrate.17 The C–N bond length in compound IIa
remains the same as that of the ligand, while the C–S
bond length increases by 0.047 Å from that of the ligand.
The shorter N–N distance and constancy in the C–N bond
length in the complex in comparison with the ligand may
suggest a diazenide formulation but fails to explain the
d0 electronic configuration of molybdenum(VI). Therefore, it is
reasonable to assume that extensive delocalization of π-elec-
trons is responsible for shortening of the N–N bond and con-
stancy of the C–N bond length. This will also explain the
diamagnetic nature of compound IIa and the absence of any
signal in its EPR spectrum. Furthermore, there is an intra-
molecular hydrogen bond between O(150)–H and N(12). O(150)
is also involved in an intermolecular hydrogen bond with N(11)H
of another molecule of [(C6H4(OH)CSNNH)3Mo]. These hydro-
gen bonds may play a vital role in crystal formation. Moreover,
the distance between S(20) and H(21) amounting to 3.215 Å is
greater than the sum of their van der Waals radii (3.00 Å) and
therefore, it may be assumed that there is no direct short
contact between benzene molecules of crystallization and the
tris(2-hydroxythiobenzhydrazido)molybdenum(VI) entity. Com-
pound IIa is related to compound II in that it contains three
molecules of benzene of crystallization. Benzene molecules
may have some role in crystal packing in such a way that the
green color is retained.
Table 2 Selected bond lengths (Å) and angles (°) of compound IIb
Mo(1)–N(34)
Mo(1)–N(54)
Mo(1)–S(11)
C(32)–S(31)
C(52)–S(51)
C(12)–N(13)
N(33)–N(34)
N(53)–N(54)
1.783(5)
2.199(5)
2.4682(16)
1.734(7)
1.766(6)
1.325(8)
1.313(7)
1.416(8)
1.359(8)
73.57(16)
76.57(15)
129.4(4)
97.1(2)
Mo(1)–N(14)
Mo(1)–S(31)
Mo(1)–S(51)
C(12)–S(11)
C(32)–N(33)
C(52)–N(53)
N(13)–N(14)
C(40)–O(41)
2.000(5)
2.4693(17)
2.4697(17)
1.738(7)
1.342(8)
1.294(8)
1.356(7)
1.379(9)
1.356(8)
76.16(15)
141.9(4)
118.7(4)
98.3(2)
C(20)–O(21)
C(60)–O(61)
N(34)–Mo(1)–S(31)
N(54)–Mo(1)–S(51)
N(13)–N(14)–Mo(1)
C(32)–S(31)–Mo(1)
C(52)–S(51)–Mo(1)
C(12)–N(13)–N(14)
N(33)–C(32)–S(31)
N(53)–C(52)–S(51)
N(14)–Mo(1)–S(11)
N(33)–N(34)–Mo(1)
N(53)–N(54)–Mo(1)
C(12)–S(11)–Mo(1)
C(32)–N(33)–N(34)
C(52)–N(53)–N(54)
N(13)–C(12)–S(11)
96.6(2)
141.9(4)
116.6(5)
119.5(5)
115.5(5)
117.0(5)
123.1(5)
Fig. 2 represents the view of the structure of compound IIb
and selected bond lengths and angles are shown in Table 2.
The crystal structure of compound IIb reveals that the Mo(V)
is bonded to three distinct, nonidentical ligand units
and one acetone of crystallization. The compound consists of
Chart 1 Bonding scheme of three non-equivalent ligands in compound IIb.
monomeric units containing Mo(V) ions hexacoordinated to N
and S atoms of three different ligands in a distorted trigonal
prismatic structure. The bonding scheme in compound IIb is
presented in Chart 1.
The compound may be represented as [Mo(2-OHC6H4CSN2)-
(2-OHC6H4C:SN:N)(2-OHC6H4CSNNCMe2)]·(CH3)2CO. The
internal bond angles of N(14)Mo(1)S(11) [76.1°], N(34)Mo(1)S(31)
[73.5°] and N(54)Mo(1)S(51) [76.5°] in the compound are close
to those in compound IIa which has a trigonal prismatic struc-
ture. The only difference is that none of the triangular faces
are equilateral.
This may be due to the non-equivalent nature of the
ligands. The hydrazido ligand unit bears a charge of 3− due to
loss of all the three protons attached to the two nitrogen
atoms and the double bonded nature of the Mo–N(34) bond is
in conformity with the very short bond distance of 1.783(5)
Å.18 That the ligand is in the thiol form can be predicted from
the longer C(32)–S(31) bond length of 1.734(7) Å. Another
ligand molecule is modified to a uninegative diazenido unit
Fig. 2 Molecular structure of compound IIb with the numbering
scheme adopted. Thermal ellipsoids are drawn at the 50% probability
level.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Table 3 Hydrogen bond lengths (Å) and angles (°)
Table 4 Selected bond lengths (Å) and angles (°) of compound IIc
Compound
Donor (D) Acceptor (A) H⋯A D–H⋯A D⋯A
Mo(1)–O(11)
9415(19) Mo(1)–O(19)
2.4027(16) Mo(1)–N(20)
2.076(4)
2.248(2)
1.713(2)
1.334(5)
1.400(5)
81.93(8)
97.35(10)
168.23(9)
Mo(1)–S(19)i
Mo(1)–O(1)
1.701(2)
1.318(4)
1.736(3)
1.359(3)
Mo(1)–O(3)
C(18)–O(19)
N(20)–N(20)*
O(11)–Mo(1)–N(20)
Compound IIa O(150)H
N(12)
O(150#)
1.813 152.02
2.181 140.24
2.585
2.895
C(18)–N(20)
N(11)H
C(18)–S(19)i
C(12)–O(11)
Compound IIb O(21)H
O(41)H
N(13)
N(33)
N(53)
1.866 150.32
1.891 147.66
1.853 154.27
2.636
2.651
2.645
O(1)–Mo(1)–O(3)
O(3)–Mo(1)–O(11)
O(3)–Mo(1)–O(21)
105.90(11) O(1)–Mo(1)–O(11)
106.26(9) O(1)–Mo(1)–O(21)
85.86(10) O(11)–Mo(1)– O(19)* 143.66(14)
O(61)H
O(11)–Mo(1)–S(19)i* 155.61(7)
where molybdenum is singly bonded to the N-14 atom as found
from the relatively longer Mo–N(14) bond length [2.000(5) Å].
The third ligand in its thiol form is converted to a hydrazone of
acetone and it binds with Mo(V) through its N(54) and S(51).
The hydrazone ligand thus behaves as a uninegative bidentate
ligand. The thiol form is confirmed by the long C(52)–S(51)
bond distance of 1.766(6) Å and the comparatively shorter
C(52)–N(53) bond distance of 1.294(8) Å. The unusually long
Mo(1)–N(54) bond length of 2.199(5) Å is due to either the bulky
hydrazone group or due to the coordinative nature of bonding.
The structure shows that O(21)H, O(41)H and O(61)H are hydro-
gen bonded to N(13), N(33) and N(53), respectively. This H-
bonding may be responsible for the more symmetric orthor-
hombic unit cell of compound IIb. The hydrogen bonding para-
meters in compounds IIa and IIb are presented in Table 3.
The distance between N(14) (diazenido nitrogen) and O(91)
(acetone oxygen) of another molecule is 2.865 Å which is less
than the sum of the van der Waals radii of O and N (3.07 Å).
Therefore some short term electrostatic interaction may exist
between the acetone and the N(14) moiety of the complex.
The crystal structure of compound IIc represented in Fig. 3
shows a centrosymmetric dimeric structure which can be
represented as [(MoO2)2(C6H4(O)–C(vO)– N–N–C(vS)(O)C6H4)-
(DMSO)2].
Selected bond lengths and bond angles are shown in
Table 4. Each Mo atom is in a distorted octahedral environ-
ment being hexacoordinated to two mutually cis terminal
oxygen atoms, nitrogen, sulfur/oxygen, oxygen atoms of the
deprotonated OH group of the ligand N-2-hydroxybenzoylN′-2-
hydroxythiobenzoylhydrazine and the oxygen atom of one
DMSO molecule. The structure is disordered with regard to
O(19) and S(19)i atoms which were refined with 50% occupancy.
The crystallographic results do not distinguish between the
occurrence of asymmetric molecules containing O and S.
The structure is disordered over the centre of symmetry or
symmetric dimer, 50% containing O(19) and 50% containing
S(19)i as shown in Fig. 3.
The ligand thus has two tridentate donor sites each of
2+
which coordinates with a MoO2 unit in a mer configuration
forming a six- and a five-membered ring with bite angles
O(11)Mo(1)N(20) and N(20)Mo(1)S(19)i/N(20)Mo(1)O(19) of
81.93°(8), 80.80°(7)/66.33°(14), respectively. It is clear from the
structure that the benzoylated N atom and the sulfur atom of
the thiobenzoyl group S(19)i coordinate with one Mo-centre
whereas the thiobenzoylated N atom (N′) and the oxygen atom
of the benzoyl group(O19) coordinate with the other Mo-
centre. The ligand is significantly distorted from planarity due
to the stereochemical requirement of accommodating two cis
oxygen atoms in the coordination sphere. Two thiohydrazido
ligands in compound IIa are oxidized to a hydrazine derivative
on exposure to DMSO. It cannot be predicted for certain
whether the oxidation is incurred through DMSO oxygen or
aerial oxygen. However, smell of dimethylsulfide in the course
of the reaction supports the prediction that DMSO is respon-
sible for oxygen transfer. The Mo-centers are also oxygenated
in this environment probably through DMSO, which has been
previously reported.19. Finally the structural requirement to
accommodate the newly formed diazene ligand and two cis
dioxo groups around each Mo-centre forced the third ligand
out of the coordination sphere. The short Mo(1)–O(1) and
Mo(1)–O(3) bond distance [1.701(2) Å and 1.713(2) Å] proves the
double bond character of the MovO bonds. The electrostatic
interaction between O(1) and S(19)i of another molecule of
compound IIc is predicted from the shorter distance of separ-
ation between the atoms (3.24 Å) than the sum of their van der
Waals radii (3.32 Å). The N–N covalent bond distances in com-
pound IIc is 1.400(5) Å which is close to the N–N single bond
distance. The shortening of the bond length may be explained
by extensive delocalization of π-electrons over the entire
Fig. 3 Molecular structure of compound IIc with the numbering
scheme adopted. Thermal ellipsoids are shown at the 30% probability
level.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
hydrazine moiety. The delocalization is also reflected in the
lengthening of the CvN bond length [1.418(4) Å] from the
usual CvN bond distances in hydrazones [C(52)–N(53) 1.294(8)
Å] in compound IIb.
Reduction of dinitrogen to ammonia
The percentage of ammonia that could be expelled is shown in
Table 5. It is observed that of all the compounds only com-
pound II could liberate more than 120% ammonia in 2 h on
the basis of total nitrogen present in the compound. On the
other hand compounds I, III and IV could generate only 6 to
23% of ammonia in 6 h. Admittedly, the total amount of
ammonia that was formed cannot be purged simply by bub-
bling nitrogen. In spite of that formation of more than 100%
of ammonia clearly indicates that there is some other source
of nitrogen that was reduced to ammonia in the presence of
compound II. Compound II differs from the other compounds
in this series in that it contains a free –OH group in the 2-
postion of the phenyl ring. This –OH group is capable of pro-
viding an additional donor site.
Scheme 2 Probable scheme for oxidation of 2-hydroxy thiobenzhydra-
zide by DMSO.
It was proposed earlier4 that an important factor that can
strongly influence the catalytic activity of molybdenum in the
reduction of dinitrogen to ammonia is the formation of a binuc-
end. The formation of a hydrazone of acetone in solution of
compound II in that the solvent is a clear indication of lability
of the thiohydrazido complex of the molybdenum(VI) complex
at the nitrogen bound end.
lear complex of the type
having a common
The first part of the reaction scheme shown in a box of
Scheme 3 suggests that Mo(HO·C6H4·CS·N·NH)3 (compound II)
may serve as a model for the active site of DMSO reductase.19
Oxygen of DMSO is not only responsible for the generation of
MoO22+, but also for the oxidation of coordinated ligand 2-
hydroxy thiobenzhydrazide to N-2-hyroxybenzoyl-N′-2-hyroxy-
thiobenzoyl hydrazine [(B) of Scheme 2]. From the reaction
scheme it may be observed that N-N′-di(2-hydroxythiobenzoyl)-
hydrazine [(A) of Scheme 2] is produced. Possibility of its invol-
vement to generate N-2-hyroxybenzoyl-N′-2-hyroxythiobenzoyl
hydrazine [(B) of Scheme 2] cannot be ruled out. Since the
oxidation state of molybdenum in the binuclear complex is
+6 (d0) a strong reducing agent like sodium borohydride in the
presence of ethanol is necessary to reduce molybdenum(VI) to
a lower oxidation state to provide a source of electron in the
vicinity of the coordinated nitrogen to produce ammonia.
Therefore, as a mechanism for reduction of dinitrogen to
ammonia, it is proposed that molybdenum reduced by sodium
borohydride acts as a source electron for the bridged nitrogen
atoms of hydrazine in IIc to ammonia, opening up the link
between two molybdenum atoms. After that dinitrogen is
inserted in that position leading to the formation of a bridged
dinitrogen complex which is successively reduced to azenide
and hydrazide and the reaction continues. The proposed reac-
tions in the presence of sodium borohydride are shown in
Scheme 3 (outside the box).
ligand binding both metal atoms and dinitrogen closing the
cycle at coordination. Compound II can fulfil this requirement
in the presence of DMSO. In order to explain the role of DMSO
the following reaction scheme involving the free ligand shown
in Scheme 2, in the absence of a reducing agent, may be
suggestive.
Both dimethylsulfide and sulfur-dioxide could be detected
when compound II was dissolved in DMSO. However, when
the free ligand is dissolved in DMSO neither dimethylsulfide
nor sulfur-dioxide could be detected and hence it is stable in
DMSO solution. The presence of a metal ion is necessary to
catalyze these reactions. Earlier it was observed17 that rhodium(III)
could catalyze such oxidation by oxo-transfer reaction of
DMSO. In the case of the molybdenum complex the co-
ordinated ligands are oxidized, suggesting that the compound
is labile and undergoes dissociation at the nitrogen bound
Table 5 Percentage of ammonia produced on the basis of nitrogen
present in the compound
Time for
passing N2
% NH3
Compound
formeda
I
2 h
6 h
2 h
6 h
1 h
2 h
6 h
2 h
6 h
2 h
6 h
16
18
II
IIc
120
125
180
200
210
6
6
20
23
We could not isolate all the intermediates that we proposed.
It is definite that compound IIc was isolated and fully charac-
terized. In addition compound II could produce more than
100% ammonia on the basis of nitrogen present in the com-
pound. The above facts are sufficient indication in favour of
our proposed mechanism.
III
IV
a Percentages have been presented as the nearest integer.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Scheme 3 Probable scheme representing the formation of compound IIc from compound II and the catalytic activity of IIc in the presence of
sodium borohydride.
It has been observed that compounds I, III and IV could libe- tion. In a polar solvent like acetone molybdenum(VI) is even
rate only a small amount of ammonia due to the reduction of reduced to molybdenum(V) at the expense of one ligand being
the hydrazido part of the ligand in the complexes by sodium oxidized to diazenide. The most interesting feature of this
borohydride and ethanol. In those compounds the additional work is that the complex of the ligand 2-hydroxythiobenz-
coordination sites, like the –OH group in compound II are not hydrazide is involved in oxo-transfer with DMSO to form a
available. As a result none of them could be converted to a binuclear complex of O2Mo2+ of a newly generated ligand N2-
binuclear complex having similarity with compound IIc. There- hydroxybenzoylN′2-hydroxythiobenzoylhydrazine and the com-
fore the reduction process could not be sustained.
pound has the capability to catalyze the reduction of dinitro-
gen to ammonia by strong reducing agent sodium
a
borohydride at room temperature.
Conclusions
The present study shows that molybdenum(VI) in a weakly
acidic medium forms green, diamagnetic, 1 : 3 inner metallic
chelates with thiohydrazide ligands where the charge on the
Acknowledgements
ligands is 2 – due to the loss of two protons. The compounds The authors thank EPSRC (UK), and the University of Reading
are stable in the solid state and also in non-polar solvent solu- for funds for the diffractometer. Financial assistance received
Dalton Trans.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
from the University Grants Commission, New Delhi and the
Higher Education Department, Government of West Bengal is
gratefully acknowledged.
(c) A. Shyamal and M. R. Maurya, Coord. Chem. Rev., 1989,
95, 183–238.
7 R. Mattes and H. Scholand, Angew. Chem., Int. Ed. Engl.,
1984, 23, 382–383.
8 P. Basak, S. Gangopadhyay, S. De, M. G. B. Drew and
P. K. Gangopadhyay, Inorg. Chim. Acta, 2010, 363, 1495–
1499.
Notes and references
1 (a) J. T. Spence, Coord. Chem. Rev., 1969, 4, 475–498;
9 CrysAlis, Oxford Diffraction Ltd, Abingdon, UK, 2006.
(b) J. T. Spence, Coord. Chem. Rev., 1983, 48, 59–82; 10 (a) G. M. Sheldrick, SHELXS-97, Program for solution of
(c) M. Hidai and Y. Mizobe, Chem. Rev., 1995, 95, 1115–
1133; (d) E. I. Steifel, J. Chem. Soc., Dalton Trans., 1997,
3915–3924; (e) A. Majumdar and S. Sarkar, Coord. Chem.
Rev., 2011, 255, 1039–1054; (f) R. Hille, J. Hall and P. Basu,
Chem. Rev., 2014, 114, 3963–4038.
crystal structures, University of Göttingen, Germany, 1997;
(b) G. M. Sheldrick, SHELXL-97, Program for refinement of
crystal structures, University of Göttingen, Germany, 1997;
(c) G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystal-
logr., 2008, 64, 112–122.
2 (a) M. K. Chan, J. Kim and D. C. Rees, Science, 1993, 260, 11 ABSPACK, Oxford Diffraction Ltd, Oxford, UK, 2005.
792–794; (b) B. A. MacKey and M. D. Fryzuk, Chem. Rev., 12 E. I. Stiefel, R. Eisenberg, R. C. Rosenberg and H. B. Gray,
2004, 104, 385–401; (c) M. L. Hlavinka, T. Miyaji,
R. J. Staples and R. H. Holm, Inorg. Chem., 2007, 46, 9192– 13 S. K. Dutta, D. B. Mclonville, W. J. Youngs and
9200; (d) I. Dance, Dalton Trans., 2010, 39, 2972–2983.
M. Chaudhury, Inorg. Chem., 1997, 36, 2517–2522.
3 (a) G. J. Leigh, Acc. Chem. Res., 1992, 25, 177–181; 14 H. J. Kim and B. K. Koo, Bull. Korean Chem. Soc., 1994, 15,
(b) R. R. Schrock, Acc. Chem. Res., 2005, 38, 955–962; 766–771.
(c) R. R. Schrock, Angew. Chem., Int. Ed. Engl., 2008, 47, 15 P. Arjunan, V. Ramamurthy and K. Venkatesan, Acta
J. Am. Chem. Soc., 1966, 88, 2956–2966.
5512–5522; (d) J. M. Chin, R. R. Schrock and P. Müller,
Inorg. Chem., 2010, 49, 7904–7916.
Crystallogr., Sect. C: Cryst. Struct. Commun., 1984, 40, 556–
558.
4 (a) A. E. Shilov, Pure Appl. Chem., 1992, 64, 1409–1420; 16 M. X. Li, J. Zhou, Z. L. Wang and J. P. Wang, Z. Naturforsch.,
(b) A. E. Shilov, Russ. Bull. Chem. Int. Ed. Engl., 2003, 52,
2555–2562.
5 (a) J. R. Dilworth, J. Hyde, P. Lyford, P. Vella,
B: Chem. Sci., 2008, 63, 1–5.
17 S. Gangopadhyay, P. Basak, M. Drew and P. K. Gangopadhyay,
Chem. Commun., 2010, 46, 7436–7438.
K. Venkatasubramaman and J. A. Zubieta, Inorg. Chem., 18 C. Bustos, L. Alvarez-Thon, A. Ibanez and C. Sanchez, Acta
1979, 18, 268–274; (b) P. Dahlstrom, M. Kustyn, J. Zubieta
and J. R. Dilworth, Transition Met. Chem., 1979, 4, 396–397.
Crystallogr., Sect. E: Struct. Rep. Online, 2011, 67, m675–
m676.
6 (a) M. D. FitzRoy, J. M. Frederiksen, K. S. Murray and 19 (a) C. J. Whiteoak, G. J. P. Britovsek, V. C. Gibson and
M. R. Snow, Inorg. Chem., 1985, 24, 3265–3270;
(b) S. Purohit, A. P. Koley, L. S. Prasad, P. T. Manoharan
and S. Ghosh, Inorg. Chem., 1989, 28, 3735–3742;
A. J. P. White, Dalton Trans., 2009, 2337–2344; (b) B. S. Lim,
J. P. Donahue and R. H. Holm, Inorg. Chem., 2000, 39,
263–273.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans.