Selective and effective stabilization of MoVI=O Bonds by NH...S hydrogen bonds via trans influence

Article abstract of DOI:10.1021/ic301597d

A monooxomolybdenum(IV) complex containing two intramolecular NH...S hydrogen bonds, (NEt₄)₂[Mo^IVO(1,2-S ₂-3-t-BuNHCOC₆H₃)₂], was synthesized. The trans isomer was crystallized as the major product, and the molecular structure was determined by X-ray analysis. The trans isomer was isomerized by heating in solution to give a 1:1 mixture of trans and cis isomers. Oxidation of these isomers by Me₃NO afforded (NEt ₄)₂[Mo^VIO₂(1,2-S₂-3-t- BuNHCOC₆H₃)₂]. ¹H NMR analysis revealed that the dioxomolybdenum(VI) complex existed as a single isomer where both oxo ligands were trans to each of the two hydrogen-bonded thiolate ligands. The Mo^VI=O bond was effectively stabilized by the NH...S hydrogen bond via trans influence, which was determined using resonance Raman spectroscopy. These results were supported by preliminary density functional theory calculations.

Full text of DOI:10.1021/ic301597d

Article
pubs.acs.org/IC
Selective and Effective Stabilization of MoVIO Bonds by NH···S
Hydrogen Bonds via Trans Influence
Taka-aki Okamura,* Miki Tatsumi, Yui Omi, Hitoshi Yamamoto, and Kiyotaka Onitsuka
Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
S
* Supporting Information
ABSTRACT: A monooxomolybdenum(IV) complex containing
two intramolecular NH···S hydrogen bonds, (NEt4)2[MoIVO-
(1,2-S2-3-t-BuNHCOC6H3)2], was synthesized. The trans isomer
was crystallized as the major product, and the molecular structure
was determined by X-ray analysis. The trans isomer was
isomerized by heating in solution to give a 1:1 mixture of trans
and cis isomers. Oxidation of these isomers by Me3NO afforded
(NEt4)2[MoVIO2(1,2-S2-3-t-BuNHCOC6H3)2]. 1H NMR analysis
revealed that the dioxomolybdenum(VI) complex existed as a
single isomer where both oxo ligands were trans to each of the
two hydrogen-bonded thiolate ligands. The MoVIO bond was
effectively stabilized by the NH···S hydrogen bond via trans influence, which was determined using resonance Raman
spectroscopy. These results were supported by preliminary density functional theory calculations.
donation of the oxo ligand via trans influence; in addition, the
SC bond was shortened, suggesting a degree of thioketone
(SC) character,11,12 which is closely related to the non-
innocent behavior found in [MoVI(bdt)3].13 In contrast, the
MoVIO bond was stabilized by the weak MoS bond at the
trans position.
INTRODUCTION
■
Molybdenum enzymes are widely distributed in both
eukaryotes and prokaryotes, and contain one or two unique
pyranopterin dithiolene moieties called molybdopterin cofac-
tors, with the exception of nitrogenase and few other proteins
with unknown function.1,2 Molybdopterin cofactors have been
found exclusively in molybdenum and tungsten enzymes,3,4
strongly suggesting that the dithiolene ligand is essential for the
activity of these enzymes. The enzymes catalyze the
oxidoreduction of various substrates in the redox cycle using
MoIV and MoVI oxidation states. Molybdoenzymes are classified
into three major families according to the number and kinds of
ligands they contain.3 The members of dimethyl sulfoxide
(DMSO) reductase family possess two molybdopterin cofactors
and an oxo ligand in the oxidized state. The reduced
molybdenum(IV) center of DMSO reductase or trimethyl-
amine N-oxide reductase reductively eliminates an oxygen atom
from the substrate, DMSO or Me3NO, respectively, resulting in
the formation of a MoVIO bond. The oxygen-atom-transfer
reactions are thought to be the appropriate model reactions,
and hence, numerous model complexes have been reported.5−8
In 1990, we found that the monooxomolybdenum(IV)
benzenedithiolate complex, (NEt4)2[MoIVO(bdt)2] (bdt =1,2-
benzenedithiolato), originally reported by Garner et al.,9
catalyzed the biomimetic oxygen-atom-transfer reaction
between amine N-oxide and benzoin to give amine, benzil,
and water.1 0 Moreover, the oxidized complex
(NEt4)2[MoVIO2(bdt)2] was isolated, and its molecular
structure was successfully determined by X-ray analysis as the
first such example of a dioxomolybdenum(VI) bis(dithiolate)
complex.11 The structure revealed that the MoS bond trans
to the MoO bond was elongated due to the strong electron
In our previous report, we demonstrated that the
introduction of four NH···S hydrogen bonds into [MoIVO-
(bdt)2]2− significantly accelerated the reduction of Me3NO.14
Unfortunately, the oxidized dioxomolybdenum(VI) complexes
proved to be too unstable to be isolated. However, the
corresponding dioxotungsten(VI) derivatives are fairly stable,
and the analysis of isolated complexes indicated that the NH···S
hydrogen bond stabilizes the WVIO bond at the trans
position.15 The crystal structure and IR spectra of
(NEt4)2[WVIO2{1,2-S2-3,6-(CH3CONH)2C6H2}2] demonstra-
ted that the NH···S hydrogen bond at the trans position to the
oxo ligand is significantly stronger than that at cis position.
These facts suggest that the hydrogen bond at the cis position
does not contribute to the stabilization of MO but, on the
contrary, destabilizes the oxidized state. This suggestive result
inspired us to propose the presence of an intramolecular
NH···S hydrogen bond in the pterin cofactor (Figure 1a).14,15
In this Article, we describe the introduction of an NH···S
hydrogen bond unsymmetrically into a bdt ligand (Figure 1b)
and the use of this ligand to synthesize monooxomolybdenum-
(IV) and dioxomolybdenum(VI) complexes with two intra-
molecular NH···S hydrogen bonds. Model studies using
unsymmetrical ligands are limited due to synthetic difficulties;
Received: July 21, 2012
Published: October 17, 2012
© 2012 American Chemical Society
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Article
(NEt4)2[MoIVO(1,2-S2-3-t-BuNHCOC6H3)2] (1). The complex was
synthesized by a ligand exchange reaction using the dithiolate ligand or
thioester.
Method 1. A mixture of (NEt4)[MoVO(SPh)4] (99 mg, 0.15 mmol)
and N-tert-butyl-2,3-di(pivaloylthio)benzamide (120 mg, 0.29 mmol)
in 1,2-dimethoxyethane (DME) (5 mL) was stirred at 60 °C for 6
days. The precipitate was collected by centrifugal separation and
washed with DME and diethyl ether. When the precipitate contains
molybdenum(V) species, a gray precipitate was deposited; however,
the reduction by NEt4BH4 in acetonitrile was effective to give a clear
yellow precipitate. The yellow precipitate was recrystallized from
acetonitrile/diethyl ether to afford orange blocks. Yield 5 mg (4%).
The spectral data were identical with those of the product synthesized
by the following method.
Figure 1. Proposed intramolecular NH···S hydrogen bond in a
molybdopterin (a) and the model ligand (b).
Method 2. A solution of N-tert-butyl-2,3-dimercaptobenzamide
(49.2 mg, 0.204 mmol) in DME (1.5 mL) was added dropwise to a
solution of (NEt4)2[MoIVO(S-4-ClC6H4)4] (91.2 mg, 0.096 mmol) in
acetonitorile (1 mL) to give a green solution. The mixture was stirred
for 3 h to give a reddish brown solution, and concentrated to dryness
under reduced pressure. The residue was washed with DME several
times to give a yellow powder. Recrystallization from acetonitorile gave
orange microcrystals, which were washed with diethyl ether (1 mL ×
2) and dried under reduced pressure. Yield 28.8 mg (35.3%). 1H NMR
(303 K, CD3CN): δ 9.14 (s, 1H, NH), 7.68 (d, J = 6.2 Hz, 1H, Ar),
7.48 (d, J = 7.3 Hz, 1H, Ar), 6.83 (t, J = 7.4 Hz, 1H, Ar), 2.97 (q, J =
7.3 Hz, 8H, CH2(NEt4)), 1.54 (s, 9H, t-Bu), 1.06 (tt, J = 7.3, 1.9 Hz,
12H, CH3(NEt4)). Anal. Calcd for C38H66N4O3S4Mo·H2O: C, 52.51;
H, 7.89; N, 6.45. Found: C, 52.58; H, 7.77, N, 6.47.
however, both early16,17 and recent18,19 work using unsym-
metrically heterocyclic substituted dithiolenes clearly indicate
the contribution of the resonance form over dithiolene and the
adjacent aromatic ring with unsymmetrical thione−thiolate
electronic structure. Here, we discuss the formation of an
unsymmetrical dithiolato ligand with an NH···S hydrogen
bond, and the contributions of this hydrogen bond to the
unsymmetrical electronic structures of model complexes using
X-ray analysis, spectral measurements, and theoretical calcu-
lations.
EXPERIMENTAL SECTION
■
(NEt4)2[MoVIO2(1,2-S2-3-t-BuNHCOC6H3)2] (2). To a DMF
solution (0.51 mL) of 1 (1.67 mg, 1.9 μmol) was added 50 μL of
DMF solution containing 0.29 mg (3.9 μmol) of trimethylamine N-
oxide at room temperature. The pale yellow solution turned
immediately red. The solution was concentrated to dryness under
reduced pressure. The product was used for the spectral measurements
without further purification. 1H NMR spectra revealed that the
oxidation proceeded quantitatively to afford pure 2 as described in the
Results section.
All procedures were performed under argon atmosphere by the
Schlenk technique. All solvents were dried and distilled under argon
before use.
Materials. 2,3-Dimercapto-1-benzoic acid,20 (NEt4)[MoVO-
(SPh)4],21 and (NEt4)2[MoIVO(S-4-ClC6H4)4],22 were prepared by
the reported methods.
N-tert-Butyl-2,3-di(pivaloylthio)benzamide. 2,3-Dimercapto-1-
benzoic acid (2.87 g, 15 mmol) was dissolved in THF (160 mL). To
the solution was added dropwise triethylamine (8.66 mL, 62 mmol)
and pivaloyl chloride (6.26 mL, 51 mmol) cooling in an ice-salt bath.
The obtained yellow suspension was allowed to warm up to room
temperature, and stirring was continued overnight. The volatile
materials were removed under reduced pressure, and THF (60 mL)
was added to the residue. To the mixture was added dropwise t-
butylamine (1.63 mL, 15 mmol), and the mixture was stirred
overnight. The yellow suspension turned to blue. The blue suspension
was concentrated in vacuo. To the residue was added water and ethyl
acetate. The organic layer was separated, washed successively with 2%
HCl aq, sat. NaCl aq, 4% NaHCO3 aq, and sat. NaCl aq, and then
dried over Na2SO4. The solution was concentrated under reduced
pressure to give a green residual oil, which was recrystallized from hot
Physical Measurements. UV−vis absorption spectra were
recorded using a SHIMADZU UV-3100PC spectrometer. Infrared
(IR) spectroscopic measurements in the solid state were done on a
Jasco FT/IR-8300 spectrometer. Samples were prepared as KBr disks
and CH2Cl2 solution (2 mM). 1H NMR spectra were obtained with a
Jeol LA-500 spectrometer in dimethylsulfoxide-d6, dichloromethane-
d2, or acetonitrile-d3 at 30 °C. Nuclear Overhauser effect (NOE)
correlated spectroscopy (NOESY), heteronuclear single quantum
coherence (HSQC), gradient enhanced NOE spectroscopy (GOESY),
and heteronuclear multiple bond connectivity (HMBC) spectra were
recorded on a Varian UNITYplus 600 MHz at −10 °C. Electrospray
ionization mass spectroscopy (ESI-MS) measurements were per-
formed on a Finnigan MAT LCQ ion trap mass spectrometer in a
acetonitrile solution. The measurements of cyclic voltamograms in
DMF solusion were carried out on a BAS 100B/W instrument with a
three-electrode system: glassy carbon working electrode, a Pt-wire
auxiliary electrode, and reference electrode. The scan rate was 100
mV/s. Concentration of sample was 2 mM, containing 0.1 M of n-
Bu4NClO4 as a supporting electrolyte. Potentials were determined at
room temperature versus saturated calomel electrode (SCE) or Ag/
AgNO3 (10 mM in acetonitrile) as a reference. In the case of SCE,
values uncorrected for junction potential were obtained. In the same
condition, ferrocene/ferrocenium redox couple was observed at +0.48
V versus SCE. Although the junction potential presumably depends on
the conditions of the interface, the reproducibility within 0.01 V was
observed. The data were consistent with the values versus Ag/AgNO3.
In the present condition, ferrocene/ferrocenium redox couple was
observed at +0.28 V versus Ag/AgNO3. Raman spectra were measured
at 298 K on a Jasco NR-1800 with liq N2 cooled CCD detector.
Exciting radiation was provided by Ar+ ion (514.5 nm).
1
ethyl acetate to give off-white powder. Yield 2.69 g (43%). H NMR
(dimethylsulfoxide-d6): δ 1.23 (s, 9H), 1.24 (s, 9H), 1.32 (s, 9H), 7.42
(d, 1 H), 7.54 (t, 1H), 7.58 (d, 1H), 7.79 (s, 1H). Anal. Calcd for
C21H31NO3S2: C, 61.58; H, 7.63; N, 3.42. Found: C, 61.49; H, 7.66;
N, 3.42.
N-tert-Butyl-2,3-dimercaptobenzamide. To a solution of N-
tert-butyl-2,3-di(pivaloylthio)benzamide (312 mg, 0.76 mmol) in
methanol (10 mL) was added dropwise 2 M NaOH aq (2.5 mL, 5.0
mmol) at 0 °C. The reaction mixture was stirred overnight at 25 °C to
give a yellow solution. The solution was concentrated under reduced
pressure to dryness. To the residue was added water and acidified with
2% HCl aq. The separated product was extracted with ethyl acetate.
The organic layers were combined and washed with water and sat.
NaCl aq, and then dried over Na2SO4. Removal of the solvent under
reduced pressure gave off-white solid, which was recrystallized from
diethyl ether to give colorless needles. Yield 63.5 mg (35%). 1H NMR
(CDCl3): δ 7.39 (dd, J = 7.8, 1.4 Hz, 1H, Ar), 7.18 (dd, J = 7.6, 1.4 Hz,
1H, Ar), 7.02 (t, J = 7.7 Hz, 1H, Ar), 5.71 (br, 1H, NH), 5.40 (s, 1H,
SH), 3.77 (s, 1H, SH), 1.47 (s, 9H, t-Bu). Anal. Calcd for
C11H15NOS2: C, 54.74; H, 6.26; N, 5.80. Found: C, 54.28; H, 6.17;
N, 5.74.
Kinetic Measurements. Reaction systems containing the
monooxomolybdenum(IV) complex and Me3NO were monitored
spectorometrically in the region 250−700 nm. A typical measurement
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Scheme 1. Synthetic Route for Monooxomolybdenum(IV) Complex
by 1H NMR spectra. When the crude product was
contaminated with a molybdenum(V) species, reduction
using a small amount of NEt4BH4 was effective for obtaining
a pure product. An alternative to the reported procedure30 is
here. Dithiol was prepared by hydrolysis of the thioester and
allowed to react with (NEt4)2[MoIVO(S-4-ClC6H4)4]22 to give
1 in a reasonable yield. 1H NMR spectra of the microcrystalline
product revealed predominantly the trans isomer with a trans/
cis ratio of about 4:1, which suggests that the formation of the
trans isomer is kinetically favorable. The structural assignments
are described in the following section. A similar preferential
formation or crystallization of one isomer over the other was
found previously in a similar case using unsymmetrical
ligands.17
The monooxomolybdenum(IV) complex 1 readily reacted
with Me3NO to afford the dioxomolybdenum(VI) complex,
(NEt4)2[MoVIO2(1,2-S2-3-t-BuNHCOC6H3)2] (2), which was
relatively stable in comparison with (NEt4)2[MoVIO2(bdt)2]
( 6 ) 1 1 , 1 2 a n d ( N E t 4 ) 2 [ M o V I O 2 { 1 , 2 - S 2 - 3 , 6 -
(CH3CONH)2C6H2}2].14 Compound 2 was isolated as a
solid and exhibited sufficient stability for spectral analysis. The
solid was dissolved in acetonitrile and analyzed using ESI-MS.
In a negative mode, anionic [(NEt4)[MoVIO2(1,2-S2-3-t-
BuNHCOC6H3)2]]− (calcd m/z 738.1) and [MoVIO2(1,2-S2-
3-t-BuNHCOC6H3)2]2− (calcd m/z 304.0) were detected, and
the characteristic isotope patterns were in agreement with
simulations. Some intense fragment peaks were observed,
which were probably caused by the decay of the parent ions in
the ionization process. All major peaks were assignable to
reasonable fragment ions (Figure S2). The molybdenum(VI)
center was sometimes electrically reduced to molybdenum(V)
in the ion source.
was carried out using a 1-mm UV cell containing a solution of
monooxomolybdenum(IV) complex 1 (1 mM, 0.3 mL) at 27 °C. After
thermal equilibrium, a Me3NO solution (60 mM, 10 μL, also at 27 °C)
was injected through a silicone rubber cap, and the cell contents were
quickly mixed by shaking. The time dependence of the absorbance for
dioxomolybdenum(VI) was measured. All calculations for the data
analysis were performed using the absorbance at 506 nm (O and S →
MoVI charge transfer band).
X-ray Structure and Determination. Each single crystal of 1 and
N-tert-butyl-2,3-di(pivaloylthio)benzamide was selected carefully and
mounted in a loop with Nujol, which was frozen immediately in a
stream of cold nitrogen at 200 K. Data collection was made on a
Rigaku RAXIS-RAPID Imaging Plate diffractometer with graphite
monochromated Mo Kα radiation (0.710 75 Å). The structures were
solved by SIR9223 and expanded by Fourier technique using SHELXL-
97.24 All non-hydrogen atoms were refined anisotropically. All
hydrogen atoms were placed at the calculated positions using the
riding model.
Theoretical Calculations. Geometry optimizations and natural
bond order (NBO) analysis were performed at the density functional
theory (DFT) level using Becke’s three parameter hybrid functionals
(B3LYP) in Gaussian 0325 program package. The basis set used for
Mo was MINI-126 supplemented by the 5p AO with the same
exponent as that for the 5s AO.27 For the other atoms (H, C, N, O, F,
S), 6-31G** basis set was employed. The coordinates of the crystal
structures, (NEt4 )2 [MoO(S2 -3-t-BuNHCOC6H3 )2] (1),
(NEt4 )2 [MoO{1,2-S2 -3,6-(CH3 CONH)2 C6 H2 }2 ],14 and
(NEt4)2[WO2{1,2-S2-3,6-(CH3CONH)2C6H2}2]15 were used for the
initial structures with some modifications.
RESULTS
■
Synthesis. A precursor of the dithiolato ligand, N-tert-butyl-
2,3-di(pivaloylthio)benzamide, was synthesized using a tradi-
tional mixed anhydride method,28 as shown in Scheme 1. The
1
thioester was fully characterized by H NMR, X-ray analysis,
Molecular Structure in the Crystal. The crystal structure
of 1 is shown in Figure 2. Complex 1 has essentially square-
pyramidal geometry, and thus both trans and cis configurations
are possible, as illustrated in Chart 1. Only one isomer, trans-1,
was crystallized selectively. Selected geometrical parameters are
listed in Table 1 with the related compounds, (NEt4)2[MoIVO-
(bdt)2] (3), which lacks hydrogen bonds, and (NEt4)2[MoIVO-
{1,2-S2-3,6-(CH3CONH)2C6H2}2] (4), which contains four
intramolecular NH···S hydrogen bonds. Amide NH groups of 1
are directed toward the neighboring thiolate sulfur atom,
potentially forming intramolecular NH···S hydrogen bonds,
where N1···S12 and N2···S22 are 3.016(7) and 3.006(7) Å,
and elemental analysis. The results of X-ray analysis are given in
the Supporting Information. The two thioester moieties and an
amide group were approximately perpendicular to the benzene
ring and formed intermolecular NH···OC hydrogen bonds in
a dimeric structure (Figure S1). The thioester reacted slowly
with (NEt4)[MoVO(SPh)4] in 1,2-dimethoxyethane (DME)
accompanying the autoreduction of Mo(V) to Mo(IV) to
afford (NEt4)2[MoIVO(1,2-S2-3-t-BuNHCOC6H3)2] (1) in a
low yield. The reaction most likely proceeded via a thioester
exchange reaction following autoreduction. Preliminary experi-
ments using (NEt4)2[NiII(SPh)4]29 revealed that thioester
exchange occurred, and the resulting t-BuCOSPh was detected
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of the Mo−S bond is shorter than that of 3. The MoS12 and
MoS22 bonds are significantly longer than the others in 1,
suggesting that the NH···S hydrogen bond electrostatically or
covalently stabilizes the thiolate anion, causing the decrease in
electron-donation from S to Mo.
Solution Structure of 1. The configuration of trans-1 was
sufficiently stable in solution at room temperature for
spectroscopic analysis. When crystals of 1 were dissolved in
acetonitrile-d3, only one isomer, assumed to be trans-1, was
1
observed in H NMR spectra (Figure 3a). Next the solution
Figure 2. Molecular structure of the anion part of (NEt4)2[MoIVO-
(1,2-S2-3-t-BuNHCOC6H3)2] (1).
Chart 1. Two Isomers of 1
Table 1. Selected Bond Distances (Å) for the Anions of
(NEt4)2[MoIVO(1,2-S2-3-t-BuNHCOC6H3)2] (1),
(NEt4)2[MoIVO(bdt)2] (3), and (NEt4)2[MoIVO{1,2-S2-3,6-
(CH3CONH)2C6H2}2] (4)
Figure 3. 1H NMR spectra of (NEt4)2[MoIVO(1,2-S2-3-t-BuNH-
COC6H3)2] (1) in acetonitrile-d6 at 30 °C: (a) trans-1 isolated as
crystals, (b) heated at 50 °C for 1 h, and (c) heated at 50 °C for 3 h.
The trans isomer was isomerized gradually by heating to cis-1, and
trans/cis ratio converged to 1:1. The set of chemical shifts for the
trans/cis isomers are 9.17/9.39 (NH), 7.69 (4-H), 7.49/7.53 (6-H),
6.84/6.85 (5-H), and 1.541/1.538 (t-Bu).
a
b
1
3
4
MoO
1.698(6)
2.369(17)
2.348(2)
2.385(2)
0.037
1.699(6)
2.388(2)
2.391(2)
2.384(2)
0.007
1.672(4)
2.386(6)
2.383(2)
2.377(2)
0.006
MoS (mean)
MoS11
MoS12
was heated to 50 °C, and the conversion was monitored by 1H
NMR at 30 °C. Upon heating, trans-1 gradually isomerized to
cis-1, which was detected as distinct peaks. After 1 h, the trans/
cis ratio was 6:1 (Figure 3b) and converged to 1:1 within 3 h
(Figure 3c). These results demonstrated that the thermody-
namic stabilities of the two isomers were identical.
c
Δ(MoS)
MoS21
MoS22
2.356(2)
2.385(2)
0.029
2.391(2)
2.384(2)
0.007
2.393(2)
2.389(2)
0.004
c
Δ(MoS)
a
Two anions I and II are present, and the dimensions are not
significantly different from each other. Here is shown the data for
1H NMR spectra of trans-1 in dichloromethane-d2 exhibited
an NH peak at 8.98 ppm at 30 °C. In contrast, the NH peak of
N-tert-butyl-2,3-di(pivaloylthio)benzamide without intramolec-
ular NH···S hydrogen bonds appeared at 5.68 ppm under the
same conditions. The downfield shift of ∼3.3 ppm strongly
suggests the presence of tight NH···S hydrogen bonds in 1 that
maintain their conformation in the crystal although the effects
caused by the coordination of a molybdenum ion should be
considered.
b
c
anion I (ref 9). Reference 14. Δ(Mo−S) = |(Mo−S11) − (Mo−
S12)| or |(Mo−S21) − (Mo−S22)|.
respectively. Estimated geometry using fixed NH distance
(0.88 Å) indicated preferable distances and angles, H1···S12
(2.36 Å), H2···S22 (2.22 Å), N1H1···S12 (132°), and N2
H2···S22 (149°), for the formation of NH···S hydrogen bonds.
The presence of the unsymmetrical ligand resulted in an
unsymmetrical coordination environment. The two MoS
bonds are distinguishable from each other, and the mean length
IR and Raman Spectra. The presence and the strength of
the NH···S hydrogen bonds were established and evaluated
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using IR spectra. The data relating to the amide NH and CO
stretching bands of 1 and 2 are listed in Table 2. Evaluation of
is probably associated with distortion of the MoOS4 local
structure.34
The resonance Raman spectrum of 2 in the solid state is
shown in Figure S3. Characteristic symmetric and asymmetric
vibration modes of the MoO2 moiety were observed as
described in a previous report.12,35 The νs(MoO) and
νas(MoO) values are listed with the related compounds,
(NEt4)2[MoVIO2(bdt)2] (6),12 which does not have substitu-
ents, and (NEt4)2[MoVIO2(bdtCl2)2],36 which contains elec-
tron-withdrawing groups (Table 4). The values of 2 are higher
Table 2. IR Data of (NEt4)2[MoIVO(1,2-S2-3-t-
BuNHCOC6H3)2] (1) and (NEt4)2[MoVIO2(1,2-S2-3-t-
BuNHCOC6H3)2] (2)
a
d
a
ν(NH)
Δ ν(NH)
ν(CO)
Δ ν(CO)
b
1
2
solid state
in CH2Cl2
3231
3247
−198
−182
1643
1634
1641
−16
−25
−18
c
solid state
b
3220
c
−209
a
d
In cm−1. In KBr disk. In Nujol mull. Difference from the values for
Table 4. Raman Bands (cm−1) of (NEt4)2[MoVIO2(1,2-S2-3-t-
BuNHCOC6H3)2] (2) and the Related Complexes
the free amide group of (S-2-t-BuNHCOC6H4)2 in CH2Cl2 (ref 31).
νs(MoO)
νas(MoO)
hydrogen bonds was performed utilizing a reported method
using the values of the so-called free amide group without a
hydrogen bond.31 The values of Δν(NH) and Δν(CO) are
the differences from the free ν(NH) and ν(CO) of (S-2-t-
BuNHCOC6H4)2, which lacks hydrogen bonds in CH2Cl2. The
large negative values of Δν(NH) clearly indicate the presence
of intramolecular NH···S hydrogen bonds in both the solid
state and in solution. These results agree with the observations
made using 1H NMR spectra discussed above. The lower
wavenumber of ν(NH) of 2 suggests the presence of a stronger
NH···S hydrogen bond in comparison with that of 1. The
relatively small Δν(CO) was caused by an inductive effect of
the NH···S hydrogen bond and indicated the absence of an
NH···OC hydrogen bond, which was found in the other
complexes.31−33
2
871
840
a
(NEt4)2[MoVIO2(bdt)2] (6)
858
829
b
(NEt4)2[MoVIO2(bdtCl2)2]
867 (868)
(839)
a
b
Data from ref 12. Reference 36. IR data are shown in the
parentheses.
than those of the other compound, suggesting that the NH···S
hydrogen bonds efficiently stabilized the MoO bonds as
reported for analogous dioxotungsten(VI) complexes.15
Electrochemical Properties. The cyclic voltammogram of
1 exhibited pseudoreversible Mo(V)/Mo(IV) redox couple at
E1/2 = −0.26 V versus SCE, as shown in Figure S4. The redox
potential is listed in Table 3 along with those of 3−5. The value
of 1 with two NH···S hydrogen bonds is more positive by 0.12
V than that of 3, which lacks hydrogen bonds. Conversely, the
measured redox potential for 4 with four NH···S hydrogen
bonds was −0.13 V, which is +0.25 V (approximately twice as
large as 0.12 V) in comparison with the redox potential of 3. A
similar simple correlation between the number of NH···S bonds
and a positive shift in redox potential was identified previously
for another series of complexes.32,33,37 Direct electron-with-
drawing groups can effectively induce a positive shift in redox
potential, as observed for 5.36
Absorption Spectra. The UV−vis spectrum of 1 is shown
as an initial curve in Figure 4, and absorption maxima of 1 and
3−5 are summarized in Table 3. The spectra of 3−5 resemble
each other, but 1 shows an intense peak at 358 nm. This
maximum peak persisted during the oxidation to 2, and thus
most likely arises from absorption of the dithiolato ligand.
Because the absorption peak at ∼300 nm observed for 3−5 is
The data regarding MoO stretching of 1 in IR and
resonance Raman spectra are listed in Table 3 along with values
Table 3. Spectroscopic Data and Redox Potentials of
(NEt4)2[MoIVO(1,2-S2-3-t-BuNHCOC6H3)2] (1),
(NEt4)2[MoIVO(bdt)2] (3), (NEt4)2[MoIVO{1,2-S2-3,6-
(CH3CONH)2C6H2}2] (4), and (NEt4)2[MoIVO(bdtCl2)2]
(5)
ν(MoO)
d
/cm−1
E1/2 /V
vs
SCE
vs Ag/
e
λmax nm (ε, M−1 cm−1
)
Raman
IR
AgNO3
1
3
4
5
281 (35 000), 358 (10 800),
440 (890)
918
902
921
918 −0.26
905 −0.38
922 −0.13
−0.44
a
b
c
328 (12 000), 385 (980),
452 (420)
−0.58
333 (5100), 380 (650),
452 (420)
330 (9500), 394 (950)
910 −0.10
a
b
c
d
e
Referenes 9 and 30. Reference 14. Reference 36. In DMF. In
acetonitrile.
for the related compounds. Complex 1 exhibited higher
ν(MoO) than (NEt4)2[MoIVO(bdt)2] (3), which lacks
NH···S hydrogen bonds, and was slightly lower than that of
(NEt4)2[MoIVO{1,2-S2-3,6-(CH3CONH)2C6H2}2] (4), which
contains four intramolecular NH···S hydrogen bonds. These
results suggest that the NH···S hydrogen bond decreases the
electron donation from S to Mo and electron density on Mo,
resulting in increased donation from O to Mo. A similar
tendency was found in (NEt4)2[MoIVO(bdtCl2)2] (5) with
electron-withdrawing substituent groups (Cl). The trend is
reasonable, but the relationship to the number and strength of
hydrogen bonds could not be strictly described. This ambiguity
Figure 4. UV−vis spectral change (every 4 min) in the stoichiometric
reduction of Me3NO by 1 in DMF ([MoIV] = 1 mM, [Me3NO] = 2
mM) at 27 °C. The spectra show spectral change from 1 to 2. Inset
shows difference of the absorption between (NEt4)2[MoVIO2(1,2-S2-3-
t-BuNHCOC6H3)2] (2) and 1.
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1
assignable to the ligand-to-metal charge-transfer absorption
(LMCT) band,14 the LMCT band of 1 presumably overlapped
in the shoulder of the intense peak in this region.
The reaction was also followed by H NMR. When an
equimolar mixture of trans/cis isomers was used, the two
isomers reacted with Me3NO to the same extent; that is, the 1:1
ratio was maintained during the reaction, as shown in Figure
S5. The reaction was clean and afforded pure 2 as the only
structural isomer accompanying the conversion of Me3NO into
Me3N. These results suggest that the reactivities of the trans-
and cis-isomers of 1 are identical.
Reduction of Trimethylamine N-Oxide. The O atom
transfer reduction of Me3NO by 1 was monitored by UV−vis
spectroscopy. A series of spectra taken every 4 min is shown in
Figure 4, where [Mo] = 1 mM and [Me3NO] = 2 mM in DMF
at 27 °C. During the reaction, absorption at 281 nm decreased,
and the absorption maximum at 506 nm increased slowly,
showing isosbestic points at 302 and 358 nm. The reaction was
virtually completed within ∼40 min. To exclude the intense
absorption of the ligand, the difference of the absorption
between 2 and 1 is shown in the inset of Figure 4. The maxima
at 390 and 500 nm are assignable to LMCT bands, which are
close to the reported values of 389 and 531 nm for
(NEt4)2[MoVIO2{1,2-S2-3,6-(CH3CONH)2C6H2}2].14
Structural Characterization of Dioxomolybdenum(VI)
Complex. The purity of isolated 2 was confirmed by 1H NMR,
where neither unreacted Me3NO nor product Me3N could be
found. The 1H NMR spectrum of 2 in acetonitrile-d3 for
aromatic and t-Bu regions is shown in Figure 5 in comparison
The reaction was monitored at 506 nm and analyzed using
pseudo-first-order kinetics at the initial stage. The observed rate
constant was kobs = (9.6
1.0) × 10−4 s−1 for 2 equiv of
Me3NO. When 10 equiv of Me3NO ([Me3NO] = 10 mM) was
used, kobs = (37 6) × 10−4 s−1. The reaction obeyed second-
order kinetics, and the obtained rate constants were k2 = (6.1
0.8) × 10−1 M−1s−1 for 2 equiv of Me3NO and k2 = (4.4 0.5)
× 10−1 M−1 s−1 for 10 equiv. Under similar conditions, the
observed rate constants for 3 and 4 with 2 equiv of Me3NO
were found to be kobs = (5.4 0.1) × 10−4 s−1 and kobs = (49
1) × 10−4 s−1, respectively.14 Compound 1, containing two
NH···S hydrogen bonds, exhibited a rate constant approx-
imately twice that of 3. While the presence of NH···S hydrogen
bonds probably accelerated the reduction of Me3NO by a
similar mechanism as that described previously for 4, which
contains four NH···S hydrogen bonds,14 the contribution per
hydrogen bond is small. The difference is presumably caused by
the difference in the two substituent groups, carbamoyl
(RNHCO) and acylamino (RCONH). The former prefers to
form six-membered NH···S hydrogen bonds but retains
flexibility,31 and the latter forms five-membered rings in rigid
or fixed conformations. In the solid state or in less polar
solvents, e.g., CH2Cl2, 1 formed strong NH···S hydrogen bonds
Figure 5. 1H NMR spectra of (a) trans-(NEt4)[MoIVO(1,2-S2-3-t-
BuNHCOC6H3)2] (trans-1), and (b) (NEt4)2[MoVIO2(1,2-S2-3-t-
BuNHCOC6H3)2] (2) in acetonitrile-d3 at 30 °C. The aromatic
region was enlarged along vertical axis.
with 1. As described in detail below, the peaks were fully
assigned by HMBC, HSQC, GOESY, and NOESY spectra, and
the assignments are shown with labeled chemical structures.
1
as established by IR and H NMR. However, in DMF, the
1
The H−13C HSQC correlation spectrum indicates directly
amide groups were solvated, and NH groups interacted with
carbonyls of DMF, resulting in weak hydrogen bonds and
insufficient contributions to the reaction. Because of the
solubility limits of Me3NO and instability of the product 2 in
CH2Cl2, the reaction could not be carried out in less polar
solvents.
1
1
bound H−13C (Figure S6b). The H−13C HMBC spectrum
revealed multiple bond interactions, including single bonds
(Figure S6a). To reduce the thermal motion and increase the
sharpness of the signals, two-dimensional spectra were recorded
at −10 °C. Consequently, we were able to make a reasonable
assignment, as shown in Figure 5. A single set of proton signals
for the ligand indicated the presence of a single symmetric
T h e s t o i c h i o m e t r i c r e a c t i o n b e t w e e n
a
monooxomolybdenum(IV) complex and Me3NO is following
essentially second-order kinetics. In the case of complex 4,
saturation kinetics were shown in the range 6 mM ≤ [Me3NO]
≤ 10 mM.14 Similar kinetic behavior was reported for
[MoIVO(1,2-S2-3-Ph3SiC6H3)2]2− with bulky substituents.30 In
these cases, the trans−cis rearrangement of the intermediate is
the rate-determining step. The sufficient bulkiness and/or
NH···S hydrogen bonds stabilized the intermediates and
retarded the rearrangements. In the present case, such a
significant saturation behavior was not found. The hydrogen
bond stabilized the intermediate moderately, and the
insufficient bulkiness allowed a facile rearrangement. To reveal
the relationship among the strength of the hydrogen bond, the
bulkiness of the substituent group, the rate constant, and the
reaction mechanism, further systematical investigations are
required, which will be reported in the near future.
1
isomer in the H NMR time scale. When octahedral geometry
is assumed and enantiomers are neglected, three isomers 2A−
2C are possible (Figure 6). In the case of the catecholate
derivative of dioxomolybdenum(VI), cis-(NMe4)2[MoVIO2(2,3-
dhb)2] (2,3-H2dhb = 2,3-dihydroxybenzoic acid),38 which
contains two intramolecular OH···O hydrogen bonds and
similar octahedral geometry to 2, two distinct isomers in a
roughly 2:1 ratio were detected by 1H NMR. The major isomer
had symmetric coordination as ascertained by the single set of
proton resonances of the ligand observed, where both
hydrogen-bonded O− ligands were trans to the oxo ligand
like isomer 2A. The minor isomer, like isomer 2C, had
unsymmetrical coordination as ascertained by the two sets of
proton resonance with the same integral intensity observed. For
complex 2, either isomer A or B (Figure 6) was possible
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basis of the X-ray structures with some modifications. The
optimized structures are shown in Figure S9, and selected
parameters are summarized in Table S2. In the table, Wiberg
bond index and the atom−atom overlap-weighted natural
atomic orbital (NAO) bond orders are shown in order to
estimate the covalency of the bond. The hydrogen-bonded
sulfur is labeled as S1 bound to C1, and the second sulfur is
labeled as S2. In 3, S1 and S2 are identical. The formation of
the NH···S hydrogen bond was confirmed by the attractive
interaction between the NH proton and S, the bond order of
which was calculated as a positive value. On the basis of the
bond order, the strength of the NH···S hydrogen bonds in
trans-1 and cis-1 are approximately the same. Introduction of an
amide group or an NH···S hydrogen bond in 3 shortened the
MoS bonds. These results are consistent with the X-ray
structure of 1. The relatively longer length of the MoS1 bond
compared with MoS2 also agreed with the crystal structure.
Slightly shorter distances and a larger bond order of the Mo
O bond in trans-1 and cis-1 than in 3 are consistent with the
observed ν(MoO) values in the Raman spectra.
Contributions of the NH···S hydrogen bonds to the MS
bonds have been investigated systematically. In the case of
tetrakis(arenethiolato)metal complexes or monooxomolybde-
num tetrakis(arenethiolate), the hydrogen bonds shorten MS
bonds. This phenomenon was explained by antibonding MS
interactions in the HOMO being diminished by NH···S
hydrogen bonds, resulting in the increase of MS
covalency.32,34,39 For porphyrin complexes, the situation is
different. Such antibonding MS character is not involved in
the HOMO, and thus, the hydrogen bond simply decreases the
electron density on sulfur; that is, it lowers the basicity of the
thiolate ligand. Consequently, MS bonds were elon-
gated.33,40,41
For the optimized structure of trans-1, the HOMO is
represented in Figure S10. Because an essentially nonbonding
interaction between Mo and S was found, the donation of S to
Mo should be reduced by NH···S hydrogen bonds. The bond
order was significantly decreased by the hydrogen bond, while
the calculated MoS1 bond length of 1 was apparently not
elongated in the comparison with 3. In contrast, the bond order
of MoS2 of 1 was higher than that of 3. This indirect
contribution of the carbamoyl group was most likely acting
through the dithiolene moiety or aromatic ring via π-
conjugation. Such influences propagating through the delocal-
ization system suggest to us the importance of the dithiolene
moiety and the unsymmetrically connected pterin ring to the
stabilization and electronic fine-tuning of the metal center in
molybdenum enzymes.
The unsymmetrical electronic structure of 1 probably does
not affect directly the binding of Me3NO to the molybdenum
center. The NH···S hydrogen bonds delocalize the electron on
the molybdenum center through the thiolate ligand, resulting in
facile acceptance of electrons from Me3NO. The unsymmetrical
structure of the ligand provides enough space to rotate the
ligand, which is required by the trans−cis rearrangement.
In comparison with 3, the presence of the NH···S hydrogen
bonds augments the natural charge of Mo atoms, although the
coordinated atoms lose their negative charge. These results
indicate that the negative charge is delocalized on the ligand. In
particular, the carbamoyl group and the hydrogen bond support
the charge electrostatically. The more positive charge on Mo
agrees with the positive shift found in the cyclic voltammo-
grams.
Figure 6. Structures of isomers 2A−2C (2C′) of [MoVIO2(1,2-S2-3-t-
BuNHCOC6H3)2]2− obtained by DFT geometrical optimization. 2C
and 2C′ are identical.
1
because of the observation of the single set of signals. The H
NMR signals of 2 were found upfield from those of 1 (Figure
5). The proton signal of the t-Bu group was observed at 1.16
ppm, which is upfield from that of 1 by 0.29 ppm. The NH
signal of 2 was found at 8.63 ppm, which is upfield from that of
1 by 0.45 ppm, although IR spectra revealed that the NH···S
hydrogen bond of 2 is stronger than or of comparable strength
to that of 1. These results suggest that the upfield shifts of these
protons are attributable to shielding effects induced by the
aromatic rings, that is, the t-Bu group is located over the
neighboring benzene ring of the other ligand in isomer 2-A. It
seems reasonable to conclude that isomer 2-A is the
predominant structure in solution, although conformational
thermal motion must be present.
1H NMR spectra of 2 at −10 °C revealed sharp peaks
because of the restricted thermal motion. To estimate the
spatial distances, GOESY spectra were measured, which
indicated the presence of t-Bu groups in close proximity to
three aromatic protons (Figure S7). The NOE correlation
between t-Bu and 4-H is most likely due to an intraligand
interaction with 4−5 Å distance (Figure S8). Because the 5-H
and 6-H protons are ≥7 Å apart from the t-Bu group, the NOE
must arise from an interligand interaction within the 2-A
isomer. The two ligands of 2-B are directly opposite each other,
making it impossible to assay the NOE-observable distance
between protons. For the thioester N-tert-butyl-2,3-di-
(pivaloylthio)benzamide, no NOE correlation was observed
between the t-Bu group and 5,6-H protons under similar
conditions. One of the ligands of 2-A was found capable of
approaching another ligand within ∼4 Å by thermal motion.
DISCUSSION
■
Contribution of the NH···S Hydrogen Bond to MoS
Bond Length. To evaluate the influence of NH···S hydrogen
bonds, DFT calculations were carried out using B3LYP. The
initial structures of trans-1, cis-1, and 3 were constructed on the
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The oxidation potential is correlated to the relative energy
level of HOMO in DFT calculations. The calculated energy
levels of HOMOs are listed in Table S2. One-electron oxidation
from Mo(IV) to Mo(V) is removal of one electron on HOMO,
which is formed primarily from orbitals of Mo(dxy) and
dithiolene moieties (Figure S10). The energy levels of trans-
and cis-isomers of 1 are approximately equal. The difference of
the calculated energy levels between trans-1 and 3 is 0.033 au
(0.91 eV). That is to say, the electron on HOMO of 3 is more
easily removable than that of 1 by 0.9 V. The observed redox
potential of 3 is more negative (i.e., easily oxiziable) than that
of 1 by 0.12 V. Although contributions of solvating medium,
used basis sets, theoretical method, and electronic configuration
should be considered, the tendency agreed with the
observation.
natural charge clearly indicates the lopsided dithiolate ligand,
where negative charge is localized on S1 in 2-A. Such an
unsymmetrical structure is usually unstable; however, the
effective and selective NH···S hydrogen bonding stabilizes the
localized negative charge and lowers the total energy.
Contribution of the NH···S hydrogen bond to the trans
influence is illustrated in Figure 7. As described above, strong
Thermodynamic Stability of Isomers of 1 and 2. The
major isomer in the crude product of 1 is the trans isomer,
indicating that the trans form is kinetically favorable in the
ligand-exchange reaction. When trans-1 was heated for several
hours, it was converted to a 1:1 mixture of trans-1 and cis-1,
indicating that these isomers possess essentially equal
thermodynamic stability. DFT calculations clearly indicated
their identical stability as well. The calculated total energy is
given in Table S2. The difference was only ∼0.2 kcal mol−1,
which is a very small value even if it was underestimated.
As described in the discussion regarding 1H NMR spectra, 2-
A is a unique isomer in solution. Unfortunately, 2 did not
crystallize, and therefore the molecular structure could not be
determined by X-ray analysis. Its thermodynamic stability was
evaluated by theoretical calculations. The total energy of 2-A
was calculated to be less than that of 2-B by ∼6.0 kcal mol−1
(Table S3). This difference is not as large but clearly
demonstrates that 2-A is the more stable isomer.
Figure 7. Schematic drawing of trans influence (a) without and (b)
with NH···S hydrogen bond. The arrows represent the direction and
degree of electron donation.
donation of S to Mo restricts the donation of O by trans
influence, resulting in a weak MoO bond (Figure 7a). When
an NH···S hydrogen bond is formed to the coordinated sulfur,
electrons on the sulfur atom are used for the hydrogen bond,
and thus, the donation from S to Mo is diminished.
Consequently, the electron-deficient Mo requires electrons
from the oxo ligand, which results in the strengthening of the
MoO bond.
Stabilization of the MoVIO Bond by NH···S Hydro-
gen Bonds via Trans Influence. The first example of a
d i o x o m o l y b d e n u m ( V I ) b i s ( d i t h i o l a t e ) ,
(NEt4)2[MoVIO2(bdt)2] (6), exhibited an extraordinarily long
MoS (thiolate) bond (∼2.6 Å) trans to the oxo ligand,11,12
which is comparable to the MoS (thioether)42 or MoS
(thioketone) bond.43 This long MoS bond is caused by
strong donation of the oxo ligand at the trans position via trans
influence,12 and is stabilized by the adjacent dithiolene or
aromatic ring to form a dithioketone-like contributing structure,
closely related to the noninnocent behavior of the ligand.
Conversely, a weak MoS bond stabilizes the MO bond at
the trans position.
Contributions of NH···S hydrogen bonds were evaluated by
DFT calculations in both isomers 2-A and 2-B (Figure S9,
Table S3). The bond order of NH···S revealed the presence of
a hydrogen bond with covalent character, as well as the greater
strength of the hydrogen bond of 2-A over that of 2-B. The
MoO bond of 2-A was significantly shorter than that of 6,
which is consistent with the Raman spectra. In addition, the
MoS bond trans to MoO in 2-A is longer than that of 6.
On the contrary, the hydrogen bond at the cis position of 2-B
elongated the MoO bond and shortened the MoS1 bond.
These results demonstrate that the NH···S hydrogen bond
stabilized the long MS bond and strengthened the MoO
bond at the trans position.
CONCLUSIONS
■
Monooxomolybdenum(IV) and dioxomolybdenum(VI) com-
plexes with two intramolecular NH···S hydrogen bonds were
successfully synthesized. Unsymmetrical introduction of an
NH···S hydrogen bond into the benzenedithiolato ligand
created an unsymmetrical coordination environment surround-
ing the molybdenum core. Two distinct MoS lengths were
observed. The hydrogen bond stabilized and maintained the
long MoS bond by moving the excessive electrons on the
basic thiolate anion to the acidic NH. The dithiolene moiety
most likely stabilized the local negative charge on the sulfur
through the conjugated system that extended over the
dithiolene and metal ion, which is known as noninnocent
behavior. The dioxomolybdenum(VI) existed as a single isomer
where MoO was trans to the hydrogen-bonded thiolato
ligand, even though at least three isomers were possible. This
selectivity indicates that the MoVIO bond was effectively
stabilized by the NH···S hydrogen bond via trans influence.
These results suggest that the unsymmetrical coordination
environment and conjugation system induced by the pterin
cofactor are essential to regulating the activity of the
molybdoenzymes, and that the NH···S hydrogen bonds most
likely play a key role in the fine control of the reactivity of the
active site.
Furthermore, the hydrogen bond reduced the strained
thioketone structure. The calculated distances of the SC
bonds are listed in Table S3. Relatively mild shortening of S1
C1 was found in 2-A in comparison with 6. Distribution of the
In the present Article, unsymmetrical structure within one
ligand was described; however, the two ligands are identical in a
complex except for the reaction intermediates. In the active
sites of DMSO reductase and trimethylamine N-oxide
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Article
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ASSOCIATED CONTENT
* Supporting Information
■
S
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E.01); Gaussian, Inc.: Pittsburgh PA, 2004.
1
voltammograms of 1 (Figre S4); H NMR spectra of 1 and 2
(Figure S5); HMBC, HSQC, GOESY, NOESY spectra of 2
(Figures S6−S8); optimized structures of model complexes
(Figure S9); and geometrical parameters (Tables S2, S3),
HOMO of trans-1 (Figure S10). This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tokamura@chem.sci.osaka-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI Grant 23655049.
■
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