Necessity of fine tuning in Mo(iv) bis(dithiolene) complexes to warrant nitrate reduction

Article abstract of DOI:10.1039/b815436h

Four desoxo Mo(iv) bis(dithiolene) complexes, [Et₄N][Mo ^IV(PPh₃)(SC₆H₄-p-Me)(mnt) ₂] (1) (mnt = maleonitrile dithiolate), [PNP][Mo ^IV(PPh₃)(SC₆H₄

Full text of DOI:10.1039/b815436h

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Necessity of fine tuning in Mo(IV) bis(dithiolene) complexes to warrant
nitrate reduction†
Amit Majumdar, Kuntal Pal and Sabyasachi Sarkar*
Received 4th September 2008, Accepted 9th December 2008
First published as an Advance Article on the web 28th January 2009
DOI: 10.1039/b815436h
Four desoxo Mo(IV) bis(dithiolene) complexes, [Et4N][MoIV(PPh3)(SC6H4-p-Me)(mnt)2] (1) (mnt =
i
maleonitrile dithiolate), [PNP][MoIV(PPh3)(SC6H4-o-COOH)(mnt)2]·CH3CN· PrOH (2) (PNP is
[Ph3PNPPh3]+), [Et4N][MoIV(PPh3)(NCS)(mnt)2] (3) and [Et4N]2[MoIV(NCS)2(mnt)2] (4) are synthesized
and characterized structurally. Complexes 1 and 2 are found to release the pentacoordinated species,
[MoIV(SR)(mnt)2]1- (R = –C6H4-o-COOH, –C6H4-p-Me) due to the dissociation of coordinated PPh3 in
solution and this pentacoordinated species can then reduce nitrate to nitrite. But unlike the model
complex of nitrate reductase, [Et4N][MoIV(PPh3)(SPh)(mnt)2] (5) (A. Majumdar, K. Pal and S. Sarkar,
J. Am. Chem. Soc. 2006, 128, 4196), these reactions remain incomplete. Such inefficiency of complexes 1
and 2 to mediate a complete reduction of nitrate is attributed to the steric bulk exerted by the
para-methyl and ortho-carboxylic acid functionalities thereby retarding the formation of the nitrate
bound Mo(IV) complex necessary for nitrate reduction. Complex 3 does not release a pentacoordinated
species in solution and hence is unable to reduce nitrate. Although 4 dissociates one coordinated NCS
1-
ligand in solution to release a penta-coordinated species {[MoIV(NCS)(mnt)2]} , which remains inactive
towards nitrate reduction indicating the necessity of thiolate coordination and hence the necessity of
fine electronic tuning to effect nitrate reduction. Complexes 1, 2 and 5 can be converted to 3 or 4
depending on the amount of thiocyanate employed. Complexes 3 and 4 undergo a facile interconversion
among themselves. Substitution of essential thiolate coordination rather than replacing the dissociable
PPh3 in 1 and 2 by thiocyanate resulted in inactivation (formation of 3) towards nitrate reduction which
is somewhat similar to the dead-end type inhibition often encountered in native systems. These results
followed by theoretical calculations at DFT level establish the necessity of fine stereoelectronic tuning at
the axial position of desosxo molybdenum bis(dithiolene) complexes to warrant nitrate reduction.
Introduction
Nitrate reductases1–5 belong to the DMSO reductase family of
enzymes and catalyze the reduction of nitrate to nitrite via oxygen
atom transfer reaction according to eqn (1).
-
-
NO3 + 2H+ + 2e- ꢀ NO2 + H2O (E0¢ = +420 mV, pH 7) (1)
Nitrate was proposed to bound to the molybdenum center of
the reduced cofactor with the formation of an enzyme–substrate
complex, followed by the essential oxotransfer reaction and the
elimination of the nitrite ion (Scheme 1). Thiocyanate, a potential
ligand to Mo in most of its oxidation states, has been shown
to be a competitive inhibitor of nitrate in native protein.6–9 It
was postulated that the thiocyanate ligand may bind to the active
site via the sulfur atom9 with the second MGD ligand (Chart 1)
being dissociated from the Mo center. This proposed mode
Scheme 1 Proposed reaction pathway for nitrate reduction.3 Also shown
is the pyranopterindithiolate cofactor (R is absent or a nucleotide).
Department of Chemistry, Indian Institute of Technology, Kanpur, India.
E-mail: abya@iitk.ac.in; Fax: +91-512-2597265; Tel: +91-512-2597265
of thiocyanate coordination was quite surprising and a critical
analysis of the literature5–16 strongly suggests that isothiocyanate
mode of ligation (binding through nitrogen atom) to molybdenum
in the inhibited enzyme would have been more plausible.
Nitrate reduction by Mo(IV) complexes without any
dithiolene type of coordination is long known.17 Various
penta-coordinated molybdenum(IV) bis(dithiolene) monothiolate
† Electronic supplementary information (ESI) available: Gas phase geom-
etry optimized structures, corresponding coordinates, selected molecular
orbitals for {[MoIV(NCS)(mnt)2]1-} without geometry optimization, first
few frequencies for the penta-coordinated species released in solution by
1, 2 and 4, cyclic voltammetric traces and differential pulse polarographs
for 1, 2 and 3]. CCDC reference numbers CCDC 691999 & 692000 for 1
and 2; 673220 & 673221 for 3 and 4. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b815436h
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Two more complexes, [Et4N][MoIV(PPh3)(NCS)(mnt)2] (3) and
[Et4N]2[MoIV(NCS)2(mnt)2] (4) are synthesized to clarify the
importance of axial thiolate ligation further. Thiocyanate as a
potential inhibitor6–9 of nitrate reductase system1–5 is investigated
and it is shown that unlike a competitive inhibition as observed in
native systems9 it acts like a dead end inhibitor in the present model
study. Complexes 1, 2 and 5 are converted to 3 upon addition of
one equivalent of thiocyanate while two equivalents of thiocyanate
converted them to 4. Although structurally similar to the model
active site of NR (5), 3 does not dissociate triphenylphosphine to
release the pentaccordinated species in solution accounting for its
inability to reduce nitrate. On the other hand, 4 is shown to release
one coordinated isothiocyanate group in solution, but the penta-
Chart 1 Designations and abbreviations.
complexes have are reported with structural resemblance to
the active site of NR (Chart 1).18–21 The hexacoordinated
complex, [MoIV(CO)(SPh)(S2C2Me2)2]1- was irradiated to release
[MoIV(SPh)(S2C2Me2)2]1- as the active model cofactor along
with the removal of CO. Unfortunately this attempt resulted in
decomposition18 forming [MoV(S2C2Me2)3]1-. The penta-coordi-
nated tungsten(IV) complex, [WIV(SC6H2-2,4,6-Pri3)(S2C2Me2)2]1-
is reported to reduce nitrate following a second order kinet-
ics. The analogous molybdenum complex, [MoIV(SC6H2-2,4,6-
Pri3)(S2C2Me2)2]1- (6)18 would have been the best candidate to
mediate a clean reduction of nitrate as it closely resembles the
active site of nitrate reductase3 from a structural point of view. It
was quite surprising that this compound appeared to be inactive
towards nitrate reduction21 as it underwent decomposition to form
[MoIV(S2C2Me2)3]1- at rates faster than oxo transfer. No reason
for the unexpected failure of this compound to mediate nitrate
reduction has been invoked so far.
1-
coordinated species, {[MoIV(NCS)(mnt)2]} thus made available
could not reduce nitrate. These studies show that, (i) thiolate
ligation is extremely essential in the penta-coordinated Mo(IV)
bis(dithiolene) complexes to mediate nitrate reduction, (ii) fine
steric tuning at the axial thiolate ligation is rather more important
to warrant a clean and complete reduction of nitrate and
(iii) thiocyanate acts as a dead end inhibitor in the present model
study. These experimental findings are successfully correlated with
theoretical calculations at DFT level.
Result and discussion
Synthesis and reactivity
The chemical reactions22–26 of [MoIVO(mnt)2]2- 27,28 suggest that
=
Recently we have demonstrated a clean reduction of ni-
trate to nitrite (80% conversion) by the model complex,
[Et4N][MoIV(PPh3)(SPh)(mnt)2] (5) following Michaelis–Menten
saturation kinetics.22 Preference of nitrate over other potent
biological oxo transfer agents like trimethylamine-N-oxide and
dimethylsulfoxide have also been demonstrated.23 We were further
interested to examine the chemical differences between 522 and
618 as both should have been able to mediate the reduction of
nitrate. This difference in reactivity towards nitrate reduction is
attributed to the steric crowding of triisopropyl benzenethiolate
group which was deliberately used to isolate the penta-coordinated
species. To support this possibility we have synthesized
two new complexes, [Et4N][MoIV(PPh3)(SC6H4-p-Me)(mnt)2] (1),
and [PNP][MoIV(PPh3)(SC6H4-o-COOH)(mnt)2]·CH3CN.iPrOH
(2) which are structurally analogous to the model complex 5
reported earlier by us.22 These two complexes release the important
penta-coordinated species, {[MoIV(SR)(mnt)2]1-} (R = C6H4-p-
Me, C6H4-o-COOH) in solution as a result of reversible dissoci-
ation of triphenylphosphine and are shown to reduce nitrate like
5. But unlike 5, the reactions remain incomplete. Inefficiency of 1
and 2 towards nitrate reduction is attributed to the steric crowding
exerted by the fanning movement of p-Me-benzenethiolate and o-
COOH-benzenethiolate groups thereby preventing the approach
of nitrate to come and bind at the Mo(IV) center of the released
penta-coordinated species. Obviously, this steric crowding reaches
its extreme point in 6 (containing triisopropyl benzenethiolate
group), leading to the inability of 6 to mediate any nitrate
reduction at all. Thus it becomes evident that penta-coordinated
monothiolate Mo(IV) bis(dithiolene) moiety can not warrant
nitrate reduction unless proper tuning at the thiolate ligand is
met.
the {Mo O} double bond can easily be protonated with the
addition of an acid (HX) to yield {MoIV(OH)(X)} group, and
when the counter anion, X, is a non-coordinating anion like
methanesulfonate, the species may well respond to hydrolysis
finally yielding the stable tris-chelated species, [MoIV(mnt)3]2-29–34
in 67% yield based on the molybdenum present. Its formation
can be made quantitative upon addition of one equivalent of free
mnt35–38 anion into the reaction medium.39 Direct formation of
the bluish green tris product was prevented by acidification of
[MoIVO(mnt)2]2- in the presence of triphenylphosphine (PPh3),
where the initial brownish green color of the solution rapidly
changed to red presumably due to the formation of the probable
reaction intermediate, {[MoIV(PPh3)(OH)(mnt)2]1-}. Addition of
paramethylthiophenol, 2-mercaptobenzoic acid and ammonium
thiocyanate into this red solution yielded the complexes 1, 2 and
3 in 90% yield (Scheme 2). Presence of excess PPh3 at this stage
is crucial in isolating these complexes in excellent yield, as excess
PPh3 prevents the back reaction in forming the hypothetical lone
hydroxo species which rapidly changed to the tris species.29–34
Acidification of the starting material, [MoIVO(mnt)2]2-27–28 in
presence of ammonium thiocyanate alone yielded 4 (Scheme 2).
Complexes 1–3 are isostructural to 522 and other related
complexes23 reported earlier in the context of clean and efficient re-
duction of nitrate. Complexes 1 and 2 exhibited the dissociation of
coordinated triphenylphosphine in solution resulting in the release
of an important penta-coordinated species, {[MoIV(SR)(mnt)2]1-}
(R = –C6H4-p-Me, –C6H4-o-COOH). This was confirmed by 31P
NMR spectroscopic measurements, wherein upon repetitive scan
of a solution of 1 and 2 in dichloromethane (CH2Cl2) showed
gradual development of a peak at d -8.18 ppm due to dissociated
PPh3 (compared with commercially available PPh3 in CH2Cl2) with
1928 | Dalton Trans., 2009, 1927–1938
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Fig. 2 Conversion of 2 to [MoIV(mnt)3)]2- in dichloromethane. Concen-
tration of 2 = 2 ¥ 10-4 M. (a) First stage of the conversion: Total time =
3 min, scan rate = 10 s/scan. (b) Second stage of the conversion: Total
time = 3 min, scan rate = 10 s/scan.
Scheme 2 Synthesis of 1–5. Also shown are the interconversion of the
synthesized complexes and catalytic nitrate reduction by 5.22.
shown in Fig. 3. Complex 3, unlike the isostructural complexes
1, 2 and 522 is extremely stable in both solid and solution
state. 31P NMR spectroscopic monitoring of a dichloromethane
solution of 3 showed no dissociation of coordinated PPh3 and the
absorption spectroscopic signature also remained unchanged up
to 4 h. Complex 4 on the other hand is unstable in solution and
the absorption spectroscopic monitoring of a dichloromethane
solution of 4 showed an interesting feature (Fig. 4). The parent
concomitant decrease in intensity of the peak due to Mo(IV) bound
PPh3. Dissociation of triphenylphosphine (PPh3) from these type
of complexes has earlier been established by us22,23 and is shown for
1 in the ESI†. Reversibility of the PPh3 dissociation was checked
by addition of PPh3 into the solution which restored the peak due
to Mo(IV) bound PPh3 to almost its original intensity.
Complexes 1 and 2 are highly unstable in solution towards disso-
ciation of PPh3 and can be stabilized in solution only in presence
of excess PPh3 added externally. These two complexes are con-
verted to the thermodynamically stable tris-chelated species,29–34
[MoIV(mnt)3]2- in both non-polar solvents (dichloromethane) and
polar solvents (acetonitrile, dimethylformamide). Rapid conver-
sions of 1 and 2 to the tris-chelated species in dichloromethane
solution are monitored by absorption spectroscopic measurements
and are shown in Fig. 1 and 2. It can be clearly seen that conversion
to the tris-chelated species is not complete in case of 1 (Fig. 1)
whereas in case of 2 a complete conversion is effected in essentially
two stages (Fig. 2).
Fig. 3 Conversion of 2 to [MoIVO(mnt)2)]2- in dimethylformamide.
Concentration of 2 = 2 ¥ 10-4 M, total time = 4.5 min, scan rate =
10 s/scan. Inset: (a) 2 in dichloromethane and (b) 2 in dimethylformamide.
Fig. 1 Conversion of 1 to [MoIV(mnt)3)]2- in dichloromethane. Concen-
tration of 1 = 2 ¥ 10-4 M, total time = 5 min, scan rate = 10 s/scan.
Unlike 1, 2 exhibits a different absorption spectroscopic sig-
nature in dimethylformamide (lmax (eM): 555 (2830), 515 (sh,
2580) nm) than that observed in dichloromethane solution (lmax
(eM): 500 (3610), 550 (3220) nm). This may be due to the
deprotonation of the carboxylic acid group in dimethylformamide
thereby changing the chromophore. Conversion of the desoxo
complex, 2, back to the brownish green colored starting material,
[MoIV(O)(mnt)2]2- is monitored by absorption spectroscopy and is
Fig.
4
Conversion of [Et4N]2[MoIV(NCS)2(mnt)2] (4) to the hy-
pothetical penta-coordinated complex {[Et4N][MoIV(NCS)(mnt)2]} in
dichloromethane. Concentration of 4 = 1 ¥ 10-4 M. Total time = 30 min;
scan rate = 2 min/scan.
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peak of 4 at 520 nm gradually shifted to 504 nm. Another peak
at 377 nm decreased its intensity along with the concomitant
development of a new peak at 710 nm with tight isosbestic
points at 406 and 590 nm indicating a clean conversion. After
the formation of this intermediate species (stable for 30 min),
the newly developed peaks gradually decreased with time (not
shown here) with concomitant development of a new peak at
665 nm characteristic of tris-chelated species,29–34 [MoIV(mnt)3]2-.
This indicated that in solution one isothiocyante may be disso-
ciated from the Mo(IV) center of 4 leading to the generation of
an unstable penta-coordinated species, {[MoIV(NCS)(mnt)2]1-},
which slowly changed to the thermodynamically stable tris
species (completion of the reaction took 11 h). All attempts
to isolate the penta-coordinated species resulted in a mixture
which on work up finally yielded a mixture of 4 and tris-chelated
species.29–34
as a classical ene-1,2-dithiolate, as observed in other reported
dithiolene ligated molybdenum complexes.18–28
Crystal structure of 2 showed that two molecules of an-
ion, [MoIV(PPh3)(SC6H4-o-COOH)(mnt)2]1-, are attached together
with the aid of hydrogen bonding between two carboxylic acid
˚
groups. This feature is shown in Fig. 6. Mo–P bond in 3 (2.538 A)
˚
˚
is close to that in case of 1 (2.578 A), 2 (2.586 A) and that
˚
reported (2.540–2.590 A) for earlier isostructural desoxo Mo(IV)
complexes.22,23 Despite quite similar Mo–P bond distances in 1, 2, 3
and 5,22 PPh3 dissociation from Mo(IV) center does not take place
only in case of 3. This can be correlated with the steric crowding
between the phenyl rings of PPh3 and thiolate ligands which forces
the PPh3 to dissociate from Mo(IV) center of 1, 2 and 5 in solution.
Mo–N–C angles (167.82◦ in case of 3; 165.45◦ and 167.74◦ in case
of 4) are quite higher bearing an isothiocyanate ligated to Mo(V)
center.11 However these angles fall in the range of 160–180◦ as
reported in the literature.12–16 Mo–N distances in 4 are 2.384 and
˚
2.419 A.
X-Ray structure description
All the four synthesized complexes are characterized by X-ray
structure determinations. Structures (ORTEP view) are shown in
Fig. 5 and the metric parameters are collected in Table 1. For the
most part, leading structural parameters are in good agreement
with the other related molybdenum bis(dithiolene) complexes.22–28
All the complexes represent a trigonal prismatic geometry. Mean
C–C and S–C (dithiolene) bond distances are in the ranges 1.36–
Interconversion of 3 and 4
Complexes 3 and 4 undergo a facile interconversion process
(Scheme 2). Addition of thiocyanate into a dichloromethane
solution of 3 leads to the formation of 4, while addition of PPh3
into a dichloromethane solution of 4 is sufficient to revert it back to
3. These interconversions are monitored by UV-Vis spectroscopy
and are shown in Fig. 7 (conversion of3 to 4) and Fig. 8 (conversion
of 4 to 3). Appearance of a tight isosbestic point at 465 nm in both
the conversions indicates a clean reaction. This interconversion
˚
˚
1.37 A and 1.73–1.74 A, respectively, which are sufficiently close
2
2
˚
=
to the typical bond lengths (sp )C C(sp ) = 1.331(9) A and
2
35–38
˚
S–C(sp ) = 1.75(2) A and hence establishes the ligand ‘mnt’
Fig. 5 Structure (ORTEP view) of anions of 1 (a), 2 (b), 3 (c) and 4 (d) showing 50% probability thermal ellipsoids with partial atom labeling scheme.
Hydrogen atoms have been omitted for clarity.
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Table 1 Crystallographic dataa for complexes 1–4
Complexes
1
2
3
4
Formula
C42H44Cl2MoN5PS5
976.98
C71H53MoN6O2P3S5
1371.39
C35H35MoN6P S5
826.95
Orthorhombic
Pna21
100
C26H40MoN8S6
753.02
Monoclinic
P21/c
100
Formula weight
Crystal system
Space group
T/K
Triclinic
Triclinic
¯
¯
P1
P1
100
100
Z
a/A
b/A
2
2
8
4
˚
˚
˚
11.086(5)
11.646(5)
18.305(5)
100.126(5)
101.684(5)
97.983(5)
2240.7(15)
1.427
0.716
2.32–28.30
1.147
0.0751(0.1509)
0.1154(0.2109)
13.671(5)
14.423(5)
17.921(5)
85.404(5)
87.751(5)
80.342(5)
3471(2)
1.312
0.456
2.14–28.33
1.199
0.0702(0.1723)
0.1027(0.2487)
25.720(5)
19.829(5)
15.048(5)
90
90
90
7675(4)
1.431
0.689
2.05–28.28
1.069
0.0475(0.0871)
0.0630(0.0954)
11.857(5)
15.812(5)
22.444(5)
90
121.891(13)
90
3573(2)
1.400
0.747
2.02–34.49
0.961
0.0923(0.2085)
0.1470(0.2443)
c/A
a/◦
b/◦
g /◦
3
˚
V/A
dc/g cm-3
m/mm-1
q range/◦
GOF (F2)
b
c
R1 (wR2 )
Rint (wR2(all data))
ꢀ
ꢀ
ꢀ
ꢀ
a Mo Ka radiation. b R1 = ꢀF0| - |Fcꢀ/ |F0|. c wR2 = { [w(F0 - Fc2)2]/ [w(F02)2]}1/2; Flack parameter = 0.0(4) for 3.
2
Fig. 6 Hydrogen bonding in 2.
Fig. 8 Conversion of 4 to 3 in dichloromethane. Concentration of 4 =
1 ¥ 10-4 M; concentration of PPh3 = 2 ¥ 10-4 M. Total time = 12 min; scan
rate = 10 s/scan. Inset: Absorption spectra of 3 and 4 in dichloromethane.
Fig. 7 Conversion of 3 to 4 in dichloromethane. Concentration of 3 =
1 ¥ 10-4 M; concentration of [PPh4][SCN] = 2 ¥ 10-4 M. Total time =
1 min; scan rate = 2 s/scan. Inset: Absorption spectra of 3 and 4 in
dichloromethane.
involving facile ligand exchange reactions establishes the lability
of ligands in desoxo bis(dithiolene) Mo(IV) complexes.
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Electrochemical investigation for the existence of
{[MoIV(NCS)(mnt)2]1-} species released in solution by 4
The electrochemical properties of 1–3 have been investigated by
cyclic voltammetry and by differential pulse polarography and
are presented in the ESI†. Complex 4 exhibited one irreversible
reduction with Epc = -0.552 V (-0.480 V, DPP) attributable
to the reduction of Mo(IV) to Mo(III) as shown in Fig. 9.
In the oxidative scan the electrochemical behavior of 4 is
complicated due to the presence of dissociated free thiocyanate
ion along with the formation of an unstable penta-coordinated
species, {[MoIV(NCS)(mnt)2]1-} in solution. Therefore species
present in the solution of 4 under electrochemical study may
be [MoIV(NCS)2(mnt)2]2-, {[MoIV(NCS)(mnt)2]1-} and free SCN-.
Complex 4 exhibited four consecutive irreversible oxidations at
Epa = 0.530 V, 0.710 V, 0.980 V and another around 1.128 V
as shown in Fig. 9. The irreversible oxidation at Epa = 1.128 V
may be due to the oxidation of coordinated “mnt” ligand. Since,
[PPh4][SCN] showed (Fig. 10, (c)) one irreversible oxidation
at Epa = 0.710 V, this oxidation can be correlated with the
oxidation of free thiocyanate ion released by the dissociation of
one coordinated isothiocyanate from 4 in solution. The irreversible
oxidation at Epa = 0.530 V can be attributed to the oxidation
of Mo(IV) to Mo(V) in 4. This explanation is supported by the
electrochemical response of 4 in the presence of [PPh4][SCN]
Fig. 10 Cyclic voltammograms (scan rate = 100 mV s-1) for (a) 4,
(b) 4 with [PPh4][SCN] (externally added) and (c) [PPh4][SCN] itself in
acetonitrile.
suggests that in solution 4 upon dissociation released the penta-
coordinated species, {[MoIV(NCS)(mnt)2]1-}, which showed an
irreversible oxidation of its Mo(IV) at Epa = 0.980 V.
Nitrate reduction by the synthesized complexes
Complexes 1 and 2 are found to reduce nitrate as shown in
eqn (2). The oxidized species reacts further with the dissociated
PPh3 to give back the active penta-coordinated MoIV species with
concomitant formation of triphenylphosphine oxide (Ph3PO) as
shown in eqn (3).
(added externally) where the current intensity of the peak at Epa
=
0.530 V is increased suggesting the rise in the concentration of
undissociated 4 ((b), Fig. 10) along with an increase in current
height of the peak at Epa = 0.710 V (due to free thiocyanate ion) in
comparison to that in the case of 4 alone ((a), Fig. 10) in solution.
Interestingly a shoulder around 0.980 V which appeared along
with the Epa = 1.128 V (Fig. 10) remained absent under oxidative
scan in the presence of [PPh4][SCN] (Fig. 10, (b)). This observation
-
[MoIV(PPh3)(SR)(mnt)2]- + NO3 → [MoVI(O)(SR)(mnt)2]- +
-
NO2 + PPh3; R = –C6H6-p-Me, –C6H4-o-COOH
(2)
[MoVI(O)(SR)(mnt)2]- + PPh3 → [MoIV(SR)(mnt)2]- +
Ph3PO; R = –C6H6-p-Me, –C6H4-o-COOH
(3)
Reduction of nitrate to nitrite by 1 and 2 in solution is confirmed
by Griess’s reagent test40 and checked with a control system
(blank).22,23 Reduction of nitrate by 1 and 2 is monitored by
absorption spectroscopy and are shown in Fig. 11 and 12 respec-
tively. The reduction of nitrate by 1 and 2 remains incomplete,
which is evident from the absorption spectroscopic monitoring
of nitrate reduction by 1 and 2 as shown in Fig. 11 and 12. The
parent absorption bands of 1 at 516 nm and 580 nm decrease with
Fig. 9 (a) Cyclic voltammogram (solid line, scan rate = 100 mV s-1) and
differential pulse polarograph (dotted line, scan rate = 20 mV s-1, pulse
width = 50 ms, pulse period = 200 ms, pulse amplitude = 50 mV) for 4 in
acetonitrile. (b) to (d) show scanning of individual processes.
Fig. 11 Absorption spectroscopi◦c monitoring for the reaction of 1 with
nitrate in dichloromethane at 25 C. Concentration of 1 = 2 ¥ 10-4 M,
concentration of [Bu4N][NO3] = 10 ¥ 10-4 M, total time = 2 min, scan
rate = 2 s/scan. Inset: Expanded view of selected region.
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center via nitrogen and not via the sulfur atom. Thus, instead
of competing for the substrate (nitrate) binding site, thiocyanate
replaces the essential thiolate coordination in 1, 2 and 5, leading
to the formation of ‘thiocyanate modified’ model system, 3, which
is unable to reduce nitrate (tested with Griess reagent40). No
appreciable PPh3 dissociation was observed (up to 4 h) as followed
by 31P NMR spectroscopy of a dichloromethane solution of 3.
This indicates that in the presence of nitrate its binding site is
not available as it is blocked by PPh3 coordination leading to the
inactivity of 3 towards nitrate reduction. Such type of binding of
a molecule or ion to the active site thereby preventing its normal
function (here, capability to reduce nitrate) is known as ‘dead-end’
inhibition.
Curiously 4 releases the plausible penta-coordinated species
{[MoIV(NCS)(mnt)2]1-} in solution by dissociation of one thio-
cyanate ion, yet it was found to be unable to reduce nitrate
(tested with Griess reagent40). These observations establish the
indispensable role of thiolate coordination in 1, 2, 522 and in other
related model complexes23 to mediate the essential oxotransfer
reaction with nitrate. This ‘thiocyanate modified’ model system, 3
is different from the postulated thiocyanate inhibited active site of
nitrate reductase9 where thiocyanate binds to the Mo(IV) center via
sulfur atom at the substrate binding site arising from competitive
inhibition.
Based on the facile interconversion of 3 and 4, (Fig. 7 and 8)
the conversion of 1, 2 and 5 to 3 and 4 are analyzed critically.
Complexes 1, 2 and 5 are rapidly converted to 3 upon addition
of a stoichiometric amount of thiocyanate in dichloromethane
solution while addition of 2 equivalents of thiocyanate into a
dichloromethane solution of 1, 2 and 5 lead to the formation of
4. Both these conversions are very fast (~100% conversion within
15 s) as monitored by UV-Vis spectroscopy. The formation of 4
upon addition of excess thiocyanate into a solution of 1, 2 and
5 can always be prevented by prior addition of PPh3. It was also
found that 3 and 4 cannot be reverted back to 1, 2 and 5, which
indicates that once displaced by an external ligand like thiocyanate
the thiolate ligand can not displace it. This facile thiolate to –NCS
exchange phenomenon in 1, 2 and 522 thus establishes the easy
lability of the thiophenolate ligand in presence of external agents
thereby forming a dead end type complex.
Fig. 12 Absorption spectroscopi◦c monitoring for the reaction of 2 with
nitrate in dichloromethane at 25 C. Concentration of 2 = 2 ¥ 10-4 M,
concentration of [Bu4N][NO3] = 10 ¥ 10-4 M, total time = 2 min, scan
rate = 2 s/scan. Inset: Expanded view of selected region.
time (Fig. 11). The weak development of the absorption band at
780 nm (characteristic of the oxidized species) in case of 1 (Fig. 11)
indicates an incomplete reaction with nitrate. Same situation is
encountered while monitoring the reaction of 2 with [Bu4N][NO3]
(Fig. 12). Parent absorption bands at 495 and 545 nm decreases
with time along with the weak development of a new absorption
band at 640 nm (characteristic of the oxidized species). Complexes
1 and 2 with [Bu4N][NO3] in 1 : 5 ratio in dichloromethane solution
were investigated with 31P NMR spectroscopy. According to eqn
(3), the amount of Ph3PO produced should roughly equal the
amount of nitrite produced. The intensity of the peaks due to
in situ generated Ph3PO in case of 1 and 2 were compared with that
of commercial Ph3PO in dichloromethane solution. This investiga-
tion exhibited about 20% conversions which agreed quite well with
colorimetrically determined amount of nitrite. Complex 3 despite
of being isostructural to 1, 2 and 5 is unable to show any reductive
activity towards nitrate. The coordinated triphenylphosphine in 3
does not dissociate in solution to provide a vacant site for nitrate
to bind at Mo(IV) center, thereby eliminating the possibility of
3 to take part in an oxo transfer reaction with nitrate. Complex
4 on the other hand, dissociates one coordinated isothiocyanate
ligand in solution thereby leading to the generation of a penta-
coordinated species, {[MoIV(NCS)(mnt)2]1-}. Formation of this
species is indicated by absorption spectroscopic monitoring of
a solution of 4 in dichloromethane (Fig. 4) and established by
electrochemical investigation (Fig. 9, 10). In spite of this, 4 is
found to be unable to reduce nitrate which clearly indicates
the importance of thiolate coordination in penta-coordinated
molybdenum(IV) bis(dithiolene) complexes to participate in an
oxygen atom transfer reaction with nitrate.
Rationalization of the differing reactivity towards nitrate
The differing reactivity of complexes reported here (1, 2, 4)
and earlier (522, 618) can be summarized as shown in Table 2.
Complexes 1, 2, 5 and 6 fall in the same category, where the thiolate
coordination is provided by a benzenethiolate moiety (in case of
5) with different substitutions (in case of 1, 2 and 6). Complex 4
is quite different from this series as this contains a isothiocyanate
coordination to the molybdenum center.
Thiocyanate inhibition
To address the different pattern of reactivity of these com-
plexes towards nitrate, a gas phase population analysis has been
performed with the gas phase geometry optimized structures of
the penta-coordinated species released in solution by 1, 2, 4,
and that of 6. The selected molecular orbitals of the complexes
are shown in Fig. 13. The molecular orbital participating in
oxygen atom transfer reaction with nitrate should be the HOMO
of the concerned active complex. From Fig. 13 it is evident
that all the thiolate coordinated complexes (Fig. 13 (a), (b) and
To address the possible mode of thiocyanate binding at the Mo(IV)
center, 1, 2 and 5 were treated with [NH4][SCN] in presence of PPh3
in dichloromethane. Compounds thus isolated was found to be 3
(identical UV-Vis, IR and 31P NMR spectral data) synthesized
directly from [Et4N]2[MoIV(O)(mnt)2]. A competitive inhibitor by
definition should bind to the substrate binding site. But in this
reaction, instead of replacing the dissociable PPh3, thiocyanate
replaces the essential thiolate coordination and binds to Mo(IV)
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Table 2 Changes in reactivity towards nitrate upon changing the substitution at the axial thiolate ligand
Complexesa
R group attached to the S atom of thiolate ligand (–SR)
Ability of reducing nitrate to nitrite
522
Reduces nitrate to nitrite obeying Michaelis–Menten kinetics
1
2
Incomplete reduction of nitrate
Incomplete reduction of nitrate
618
Unable to reduce nitrate at all
a In case of 1, 2 and 522, it is the penta-coordinated species released in solution that actually reduces nitrate to nitrite.
been correlated with the efficiency of nitrate reduction by 5
compared with the inactivity of isostructural halogen coordinated
complexes.41 Therefore all these complexes (1, 2, 5, and 6) seem to
be in a thermodynamically favourable situation to mediate oxygen
atom transfer reaction with nitrate. However, this is not the case
and in reality, it is only 5 that efficiently reduces nitrate obeying
Michaelis–Menten saturation kinetics similar to the native enzyme
system.22 Complexes 1 and 2 can only mediate an inefficient
reduction of nitrate whereas 6 is unable to reduce nitrate at all.
Therefore the reason for this differing reactivity towards nitrate is
not thermodynamic; rather it may be a kinetic problem involving
steric factors.
Complex 4 dissociates a penta-coordinated species, {[MoIV-
(NCS)(mnt)2]1-}, in solution which with the virtue of bearing a
vacant site might have reduced nitrate. But from Fig. 13, it can
be seen that the corresponding HOMO is predominantly ligand
centered (metal = 5.26%), thereby accounting for its inactivity
towards nitrate. The geometry of isothiocyanate coordinated
species is different in crystal structure than that obtained by
geometry optimization of the hypothetical penta-coordinated
species in a sense that the isothiocyanate moiety is bent
(Mo–N–C angle = 165.45◦ and 167.74◦) in the structure of 4
obtained from crystal structure, whereas in geometry optimized
structure it is linear (Mo–N–C angle = 180◦). One might inquire
that the geometry of the species concerned can alter the population
of molecular orbital. To address this aspect, a single point energy
calculation is also performed with the penta-coordinated species
without optimizing the geometry, which essentially gives almost
the same population of molecular orbitals (metal = 7.62%, see ESI
for a pictorial representation†) thereby eliminating the possibility
of severe alteration of population in the molecular orbitals due to
change in geometry.
Fig. 13 Selected molecular orbitals for the penta-coordinated species,
{[MoIV(SR)(mnt)2]1-} ((a), R = p-Me-Ph; (b) R = o-COOH-Ph) re-
leased in solution by reversible dissociation of triphenylphosphine
from 1 and 2, (c) [MoIV(SC6H2-2,4,6-Pri3)(S2C2Me2)2]1- (6)18 and (d)
{[MoIV(NCS)(mnt)2]1-} released from 4. Color codes: pink, Mo; yellow, S;
red, O; blue, N; grey, C; green, H. Isosurface cut off value = 0.04.
The diminished efficiency of 1 and 2 towards nitrate reduction
compared with 5 emerges out to be a result of steric factor exerted
(c)) possess a predominantly metal centered HOMO (mainly
metal dxy). Presence of a metal centered HOMO has recently
1934 | Dalton Trans., 2009, 1927–1938
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by the substituents. The space filling models of these complexes
are shown in Fig. 14 to show the increasing steric bulk at the
nitrate binding site upon traveling from 5 to 1, 2 to 6. When there
is a methyl substitution at the para position of the coordinated
benzenethiolate ligand (1), the complex loses its efficiency towards
nitrate reduction. The same situation is encountered when there
is a carboxylic acid substituent in the ortho position of the
benzene thiolate group (2). This diminishing efficiency towards
nitrate reduction can be attributed to the steric bulk exerted by
the free rotation (shown in Fig. 14 (a)) of substituted (para-
methyl and ortho-carboxylic acid) benzenethiolate group at the
axial position of corresponding penta-coordinated bis(dithiolene)
Mo(IV) complexes thereby retarding the nitrate moiety to bind at
Mo(IV) center of 1 and 2. Hence, 1 and 2 remain inefficient towards
a complete reduction of nitrate. This situation reaches its extreme
point in the case of 6,18 where the three isopropyl substituents
(Table 2) exert an enormous amount of steric crowding and, as a
result of this, 6 is found to be completely inactive towards nitrate
reduction. In the case of 6 the use of sterically demanding 2,4,6-
triisopropylbenzenethiolate ligand essentially made the isolation
of the penta-coordinated species possible.18 However, it now
becomes clear that the same steric reason rendered this complex
inactive towards reduction of nitrate.
using [Ph4P][Br] and [PNP][Cl] instead of [Et4N][Br]. [PPh4][SCN]
was prepared by cation metathesis reaction with [Ph4P][Br] and
[NH4][SCN]. X-Ray data were collected on a Bruker-AXS Smart
APEX CCD diffractometer. Infrared spectra were recorded on a
Bruker Vertex 70, FT-IR Spectrophotometer as pressed KBr disks.
Elemental analyses for carbon, hydrogen, nitrogen and sulfur
were carried out with Perkin-Elmer 2400 microanalyser. Electronic
spectra were recorded using a USB 2000 (Ocean Optics Inc.) UV-
Visible spectrophotometer equipped with fiber optics. 31P NMR
spectroscopic measurements in dichloromethane (CH2Cl2) were
recorded with JEOL JNM-LA 400, FT-NMR. Cyclic voltammet-
ric measurements were made with BASi Epsilon–EC Bioanalytical
Systems Inc. Cyclic voltammograms of 10-3 M solution of the
compounds (1, 2, 3 and 4) were recorded (scan rate = 100 mV s-1)
using glassy carbon electrode as working electrode, 0.2 M
[Bu4N][ClO4] as supporting electrolyte, Ag/AgCl electrode as
reference electrode and platinum wire as an auxiliary electrode.
Sample solutions were prepared in acetonitrile. Differential pulse
polarography was performed with scan rate = 20 mV s-1, pulse
width = 50 ms, pulse period = 200 ms, pulse amplitude =
50 mV. All electrochemical experiments were done under argon
atmosphere at 298 K. Potentials are referenced against internal
ferrocene (Fc) and are reported relative to the Ag/AgCl electrode
(E1/2(Fc+/Fc) = 0.459 V vs. Ag/AgCl electrode).
Synthesis
[Et4N][MoIV(PPh3)(p-Me-SPh)(mnt)2] (1). To a solution of
2.62 gm (4 mmol) of [Et4N]2[MoIVO(mnt)2] and 10.49 gm
(40 mmol) of PPh3 in 100 ml dichloromethane was slowly added
◦
1 ml of methanesulfonic acid with constant stirring at 0 C. The
solution became bright red after 10 min and 3 ml (24 mmol)
of para-methyl thiophenol was added to this solution. Stirring
was continued for another 15 min wh◦en the solution became
◦
dark violet. Petroleum ether (60 C–80 C) was added to it and
kept overnight at 4 ◦C to obtain bright violet coloured needle
shaped diffraction quality crystals. Yield: 3.52 gm (90%). Required
for MoPS5C42H44N5Cl2: C 51.62, H 4.54, N 7.16, S 16.40%;
found C 51.68, H 4.58, N 7.20, S 16.48%. Absorption spectrum
(dichloromethane) lmax (eM): 380 (10250), 515 (5200), 575 (4360)
Fig. 14 Space filling model for the gas phase geometry optimized
structure of the penta-coordinated species released in solution from 521
(a), 1 (b), 2 (c) and 6 (d) indicating the increasing steric bulk on going
from (a) to (d). In case of 6, structure was taken from the reported crystal
structure itself.18 Arrows indicate the free movements of axial thiolate
ligands thereby preventing the approach of nitrate towards Mo(IV) center.
Color codes: pink, Mo; yellow, S; red, O; blue, N; grey, C; green, H.
=
nm. IR (KBr pellet): n 1480 (C C), 2201 (CN), 3053 (aromatic
CH stretching), 745 (aromatic CH bending) cm-1. 31P NMR: d
49.68 ppm.
[Ph3PNPPh3][MoIV(PPh3)(o-COOH SPh)(mnt)2]. CH3CN.
iPrOH (2). To
a solution of 2.62 gm (4 mmol) of
[Et4N]2[MoIVO(mnt)2] and 10.49 gm (40 mmol) of PPh3 in 100 ml
dichloromethane was slowly added 1 ml of methanesulfonic
acid with constant stirring at 0 ◦C. The solution became bright
red after 10 min and 3.70 gm (24 mmol) of 2-mercaptobenzoic
acid was added to this solution. Stirring was continued for
another 15 min when the solution became dark violet. Petroleum
ether (60 ◦C–80 ◦C) was added to it to yield a reddish pink
coloured solid. This was dissolved in acetonitrile followed by
addition of isopropanol saturated with 2-mercaptobenzoic acid.
Finally diethyl ether was added to this reaction mixture and kept
overnight at 4 ◦C to obtain reddish pink coloured needle shaped
diffraction quality crystals. Yield: 5.15 gm (90%). Required
for C71H53MoN6O2P3S5: C 62.18, H 3.89, N 6.12, S 11.69%;
found C 62.12, H 4.34, N 5.95, S 11.28%. Absorption spectrum
Experimental section
Materials and physical methods
All reactions and manipulations were performed under a pure
argon atmosphere using modified Schlenk technique. [NH4][SCN]
and CH3SO3H was obtained from S. D. Fine Chemicals Ltd.
India. [Ph4P][Br], [PNP][Cl] and Para-methyl thiophenol was ob-
tained from Aldrich Chemicals and 2-mercaptobenzoic acid was
obtained from ACROS ORGANICS. Solvents were distilled and
dried by standard procedure before used. [Ph4P]2[MoIVO(mnt)2]
and [PNP]2[MoIVO(mnt)2] were prepared following the similar
procedure for the preparation of [Et4N]2[MoIVO(mnt)2]27,28 by
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(dichloromethane) lmax (eM): 364 (10000), 390 (sh, 6150), 500
(3600), 550 (3200) nm. IR (KBr pellet): n 3400 (broad, carboxylic
described by Blessing.46 The space group of the compounds was
determined based on the lack of systematic absence and intensity
statistics. Full-matrix least squares/difference Fourier cycles were
performed which located the remaining non-hydrogen atoms. All
non-hydrogen atoms were refined with anisotropic displacement
parameters. Complex 1 contains one anion, one cation and one
dichloromethane molecule in its asymmetric unit. Complex 2
contain one anion, one cation and one acetonitrile molecule in
its asymmetric unit. The structure of 2 suffered from disorder of
unidentified solvent (CH2Cl2/isopropanol) in the lattice, which
was not included in the refinement but was taken care of by the
=
OH), 3053 (aromatic CH stretching), 2203 (CN), 1692 (C O),
-1
1480 (C C), 745 (aromatic CH bending) cm . 31P NMR: d
=
49.70 ppm.
[Et4N][MoIV(PPh3)(NCS)(mnt)2] (3). To a solution of 2.62 g
(4 mmol) of [Et4N]2[MoIVO(mnt)2] and 10.49 g (40 mmol) of PPh3
in 30 mL dichloromethane in cold was added 1 mL methanesul-
fonic acid dropwise with constant stirring. The solution became
bright red after 10 min and 1.82 g (24mmol) of NH4SCN was
added to this solution. Stirring was continued for another 30 min.
Petroleum ether (60 ◦C–80 ◦C) was added to it and this was
allowed to stand overnight at 4 ◦C when dark red colored crystals
embedded in an oily mass were formed. The whole mass was
filtered, washing with isopropyl alcohol followed by diethyl ether
removed the oily mass to separate out the crystalline solid which
was dried in vacuum to get needle shaped dark red coloured
diffraction quality crystals. Yield: 2.98 g (90%). Required for
C35H35MoN6PS5: C, 50.83; H, 4.26; N, 10.16; S, 19.39%; found
C, 50.87; H, 4.30; N, 10.20; S 19.43%. Absorption spectrum
(dichloromethane) lmax (€M): 378 (7350), 415 (5776), 454 (sh,
3645), 538 (5015) nm. IR (KBr pellet): n 2207 (CN), 2050 (NCS),
3050 (aromatic CH stretching), 740 (aromatic CH bending) cm-1.
31P NMR: d 49.82 ppm, s.
SQUEEZE procedure (from PLATON). The volume occupied by
3
˚
the solvent was 172 A , and the number of electrons per unit cell
deduced by SQUEEZE was 35. Asymmetric units contain two
formula weights of 3 and one formula weight and a tetraethy-
lammonium cation of 4. Complex [MoIV(NCS)(PPh3)(mnt)2]1- (3)
occurred as a racemic twinned crystal, as determined by the
Twin RotMax routine in PLATON. The data were detwinned by
incorporation of TWIN instruction in SHELXL This complex
contained one major component, as indicated by the batch scale
factors (BASF) of 0.47. Some of the carbon atoms of ethyl group of
the tetraethylammonium cation of complex [MoIV(NCS)2(mnt)2]2-
(4) were disordered over two positions and were refined with free
part instruction using SHELXL-97.44
[Et4N]2[MoIV(NCS)2(mnt)2] (4). To a mixture of 2.62 g
(4 mmol) of [Et4N]2[MoIVO(mnt)2] and 3.04 g (40 mmol) of
NH4SCN in 30 mL dichloromethane in cold 1 mL methanesulfonic
acid was very slowly added with constant stirring. The mixture
was stirred for 30 min to give a violet solution. This was filtered
and petroleum ether (60 ◦C–80 ◦C) was added to it in excess
to precipitate out a violet microcrystalline solid immediately.
The microcrystalline solid mass was washed with isopropanol
followed by diethyl ether and dried in vacuum. Block shaped
diffraction qualit◦y crystals were obtained upon adding petroleum
and on standing overnight at 0 ◦C. Yield: 2.56 g (85%). Required
for C26H40MoN8S6: C, 41.47; H, 5.35; N, 14.88; S, 25.55%; found
C, 41.50; H, 5.39; N, 14.91; S 25.58%. Absorption spectrum
(dichloromethane) lmax (€M): 377 (9580), 520 (5550) nm. IR (KBr
pellet): n 2207 (CN), 2050 (NCS) cm-1.
Determination of nitrite
Nitrite generated in the system containing [Bu4N][NO3] and 1 or
2 was measured quantitatively following a colorimetric method40
and was compared with control system (blank test). The control
system containing [Bu4N][NO3] and triphenyl phosphine (PPh3)
in dichloromethane did neither respond to Griess reagent40 test
nor did it show the formation of any triphenylphosphineoxide
(Ph3PO). Immediately after completion of the oxo transfer re-
action, the solvent in the reaction mixture was removed under
vacuum and the residue was extracted with water. Griess reagent40
(1 : 1 mixture of 0.1% N-1-napthylethylenediamine dihydrochlo-
ride in water and 1% sulfanilamide in 5% phosphoric acid) was
added to the extract to give a purple solution. Absorbance at
550 nm was measured and the concentration was calculated with
the help of a standard curve made with pre-known concentration
of NaNO2. Nitrite generated in these systems corresponded
to about 20% of the theoretical conversion for a complete
reaction.
◦
ether (60 C–80 C) to a dichloromethane solution of this solid
X-Ray crystallography
Suitable diffraction quality single crystals were obtained from the
crystallization procedures described in each synthesis. The crystals
used in the analyses were glued to glass fibers and mounted on
BRUKER SMART APEX diffractometer. The instrument was
equipped with CCD area detector and data were collected using
Computational details
All calculations were performed using the Gaussian 03 (Revision
B.04) package47 on an IBM PC platform. Molecular orbitals were
visualized using ‘Gauss View’ and geometry optimized molecular
structures were shown with Diamond 3.1e. Gas phase geometry
optimization and population analysis of the molecular orbitals
were carried out at DFT level. The method used was Becke’s three-
parameter hybrid-exchange functional,48 the non-local correlation
provided by the Lee, Yang, and Parr expression,49 and Vosko, Wilk,
and Nuair 1980 local correlation functional (III) (B3LYP).48,49 6-
31G*+ basis set50 was used for S, P, C, N, and H atoms. The
LANL2DZ basis set51 and LANL2 pseudopotentials of Hay and
Wadt52,53 were used for the Mo atom. The optimized minima
˚
graphite-monochromated Mo Ka radiation (l = 0.71069 A) at
low temperature (100 K). Cell constants were obtained from the
least-squares refinement of three-dimensional centroids through
the use of CCD recording of narrow w rotation frames, completing
almost all-reciprocal space in the stated q range. All data
were collected with SMART 5.628 (BRUKER, 2003), and were
integrated with the BRUKER SAINT42 program. The structure
was solved using SIR9743 and refined using SHELXL-97.44 Crystal
structures were viewed using ORTEP45 and Diamond 3.1e. The
empirical absorption correction was applied using SADABS, as
1936 | Dalton Trans., 2009, 1927–1938
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were characterized by harmonic-vibration-frequency calculation
with the same method and basis set in which the minimum
has no imaginary frequency. For geometry optimization of all
the complexes, initial geometries were taken from corresponding
crystal structures. In case of 618 geometry was not optimized and
the geometry from reported crystal structure is used for population
analysis. The assignment of the type of each MO was made
on the basis of its composition and by visual inspection of its
localized orbital. The coordinate frame for both the compounds
7 S. J. Elliott, K. R. Hoke, K. Heffron, M. Palak, R. A. Roth-
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2
were assigned by the visual inspection of the dz and dxy orbital.
13 T. Shibahara, H. Kuroya, K. Matsumoto and S. Ooi, Bull. Chem. Soc.
Jpn., 1983, 56, 2945.
14 N. W. Murrall and J. Welch, J. Organomet. Chem., 1986, 301, 109.
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Conclusions
Four new desoxo Mo(IV) bis(dithiolene) complexes (1–4) are syn-
thesized and characterized structurally as well as by spectroscopic,
electrochemical and theoretical study in relevance to the model
reactions of nitrate reductase. The complexes 1 and 2 (isostructural
to the model complex of nitrate reductase active site (5)22) are
found to mediate incomplete reactions with nitrate. It is well
known that a thiolate coordination at the axial position of penta-
coordinated Mo(IV) bis(dithiolene) complexes is essential to me-
diate a clean reduction of nitrate. A comparative theoretical study
along with experimental results presented here unambiguously
establishes the important fact that a fine tuning at the axial thiolate
position is rather more necessary to warrant nitrate reduction. It
can be concluded that use of sterically demanding ligands is not a
permanent solution in modeling the active sites of oxidoreductase
class of molybdenum enzymes. Sometimes the steric solution can
be helpful to isolate the desired model complexes18 with structural
resemblance to the native active site, whereas the same steric reason
can restrict the substrate binding thereby leading to the inactivity
of the model complex towards biological substrates.21
23 A. Majumdar, K. Pal and S. Sarkar, Inorg. Chem., 2008, 47, 3393.
24 A. Majumdar, K. Pal, K. Nagarajan and S. Sarkar, Inorg. Chem., 2007,
46, 6136.
25 A. Majumdar, J. Mitra, K. Pal and S. Sarkar, Inorg. Chem., 2008, 47,
5360.
26 K. Nagarajan, H. K. Joshi, P. K. Chaudhuri, K. Pal, J. A. Cooney, J. H.
Enemark and S. Sarkar, Inorg. Chem., 2004, 43, 4532.
27 S. K. Das, P. K. Chaudhury, D. Biswas and S. Sarkar, J. Am. Chem.
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28 R. Maity, K. Nagarajan and S. Sarkar, J. Mol. Struct., 2003, 656, 169.
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30 H.-Wu. Xu, Z.-N. Chen and J.-G. Wu, Acta Crystallogr., Sect E: Struct.
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31 D. V. Formitchev, B. S. Lim and R. H. Holm, Inorg. Chem., 2001, 40,
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Complexes 3 and 4 are investigated to address the possible mode
of thiocyanate inhibition in model complexes. It is found that
instead of a competitive inhibition observed in native systems,
thiocyanate acts as a dead end inhibitor in the present model
study. Complex 4 releases a penta-coordinated species in solution
bearing a vacant site for possible binding of nitrate. However it
failed to react with nitrate thereby once again establishing the
indispensable role of thiolate ligation to effect nitrate reduction.
32 K. Wang, J. M. McConnachie and E. I. Stiefel, Inorg. Chem., 1999, 38,
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33 G. Matsubayashi, K. Douki, H. Tamura, M. Nakano and W. Mori,
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35 G. Bahr and B. Schleltzer, Chem. Ber., 1955, 88, 1771.
36 G. Bahr, Angew. Chem., 1956, 68, 525.
37 E. I. Stiefel, L. E. Bennet, Z. Dori, T. H. Crawford, C. Simo and H. B.
Gray, Inorg. Chem., 1970, 9, 281.
38 A. Davidson and R. H. Holm, Inorg. Synth., 1967, 10, 8.
39 S. K. Das, D. Biswas, R. Maiti and S. Sarkar, J. Am. Chem. Soc., 1996,
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Acknowledgements
40 P. Griess, Ber. Dtsch. Chem. Ges.,1879, 12, 427, as cited in F. Feigl, Spot
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