Design, syntheses, and characterization of dioxo-molybdenum(vi) complexes with thiolate ligands: Effects of intraligand NH...S hydrogen bonding

Article abstract of DOI:10.1039/b714386a

Presence of the hydrogen bonding near a metal center can influence the properties of the complex. Here, we describe changes in redox and spectral properties in discrete dioxo-molybdenum centers coordinated by a single thiolato ligand that can support an intra-ligand hydrogen bond. We have utilized thiophenolato ligands that can harbor hydrogen bonding between the thiophenolato sulfur with an amide functionality creating either a five- or a six-membered ring. Methylation of the amide functionality removes the NH...S hydrogen bonding thus providing a basis for understanding the effect of hydrogen bonding. These thiophenolato ligands have been used in synthesizing dioxo-Mo^VI complexes of type Tp*MoO₂(S-o-RC₆H₄), where R = CONHMe (11), CONMe₂ (12), NHCOMe (13), and N(Me)COMe (14). The complexes have been characterized by NMR, infrared, and UV-visible spectroscopy. Spectroscopic data clearly indicate the presence of hydrogen bonding in both 11 and 13, and stronger in 13, where hydrogen bonding stabilizes a five-membered ring. All complexes exhibit a Mo^VI/Mo^V redox couple and redox potentials are modulated by the nature of H-bonding. Compound 14 with the electron-releasing N(Me)COMe group has the highest reduction potential and is more difficult to reduce. The Royal Society of Chemistry.

Full text of DOI:10.1039/b714386a

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Design, syntheses, and characterization of dioxo-molybdenum(VI) complexes
with thiolate ligands: effects of intraligand NH · · · S hydrogen bonding
Raghvendra S. Sengar, Jonathan J. Miller and Partha Basu*
Received 18th September 2007, Accepted 28th February 2008
First published as an Advance Article on the web 27th March 2008
DOI: 10.1039/b714386a
Presence of the hydrogen bonding near a metal center can influence the properties of the complex. Here,
we describe changes in redox and spectral properties in discrete dioxo-molybdenum centers coordinated
by a single thiolato ligand that can support an intra-ligand hydrogen bond. We have utilized
thiophenolato ligands that can harbor hydrogen bonding between the thiophenolato sulfur with an
amide functionality creating either a five- or a six-membered ring. Methylation of the amide
functionality removes the NH · · · S hydrogen bonding thus providing a basis for understanding the
effect of hydrogen bonding. These thiophenolato ligands have been used in synthesizing dioxo-MoVI
complexes of type Tp*MoO2(S-o-RC6H4), where R = CONHMe (11), CONMe2 (12), NHCOMe (13),
and N(Me)COMe (14). The complexes have been characterized by NMR, infrared, and UV-visible
spectroscopy. Spectroscopic data clearly indicate the presence of hydrogen bonding in both 11 and 13,
and stronger in 13, where hydrogen bonding stabilizes a five-membered ring. All complexes exhibit a
MoVI/MoV redox couple and redox potentials are modulated by the nature of H-bonding. Compound
14 with the electron-releasing N(Me)COMe group has the highest reduction potential and is more
difficult to reduce.
for molybdenum enzymes. We are primarily interested in the
Introduction
former approach, where a hydrogen-bonding unit is placed by
The presence of hydrogen bonding is extensive in biological
design. Severalsyntheticmodels for theoxo-molybdenumcenter of
systems.1 It plays important roles in the functioning of biological
molybdoenzymes have been synthesized, where the ligand provides
molecules such as structural organization, control of redox
one or more hydrogen-bonded unit(s) in close proximity to a metal
potential, substrate entry and product release. Hydrogen bonding
center. This proximity strongly influences the properties of the
is ubiquitous in metalloenzymes including mononuclear molybde-
metal center. Much emphasis has been given to the 5-membered,
num enzymes which carry out many essential life processes.2 We are
2-(acylamino)benzene thiolate ligands (type B, Fig. 2), which ex-
interested in understanding how hydrogen bonding can influence
hibited a large positive shift in redox potential in alkylammonium
salts of [MoVO(S-o-RꢀCONHC6H4)4
and [MoIVO{1,2-S2−3,6-
11,12
the redox properties of oxo-molybdenum centers, which are the
(RꢀCONH)2C6H2}2] (Rꢀ = CH3, CF3, t-Bu, Ph3C).13,14 The effect
of intramolecular hydrogen bonding was evaluated by comparing
the corresponding N-methylated complexes where such hydrogen
bonding is not present. The sensitivity of the redox potential to
similar hydrogen bonding has been reported for the molybdenum
nitrosyl compound, Tp*Mo(NO)(SCH2CONHCH3)2.15 Several
other metals such as tungsten,16 iron,17 zinc,18 cadmium,18d,19
mercury,20 cobalt,21 copper,22 platinum and palladium23 have also
been reported to form a complex with 2-(acylamino)benzene
thiolate-type ligands. In these complexes, various thiolato ligands
have an influence on the properties of the metal center. The
presence of a NH · · · S hydrogen bond not only contributes to
the positive shifts in the redox potentials in complexes, but also
affects their spectral and structural properties by modulating the
metal–sulfur interaction.
active centers of mononuclear molybdenum enzymes. Extended
hydrogen bonding constitutes catalytic pockets often creating
a positively charged channel conducive to binding of anionic
product or substrate. For example, in the active site of sulfite
2−
oxidase (SO), a network of H-bonded water molecules from SO3
to the oxomolybdenum center has been observed,3–6 which raises
questions about the roles of hydrogen bonding in modulating
properties of the metal center. Sulfite oxidase is considered to be
a prototypical oxotransferase, where water serves as the ultimate
source of oxygen atom.7 A simplified view of the reductive half
cycle of sulfite oxidase is shown in Fig. 1, where a water molecule
is hydrogen bonded to the substrate (sulfite). In this model, the
product (sulfate) is generated via electron- and proton-transfer
processes.
Synthetic models of molybdenum enzymes generally use two
main approaches for introducing hydrogen bonding—one that
harbors H-bonding within the ligand, and one where H-bonding
involves an anion, cation or a solvent molecule.8–10 Both ap-
proaches have been successfully utilized in developing models
2-(Alkylcarbamoyl)benzene thiolate ligands (type A, Fig. 2)
constitute a different class of ligands, which can potentially
=
exhibit intramolecular hydrogen bonding such as SH · · · O C or
S · · · HN in protonated thiol forms or in deprotonated thiolato
forms respectively. While such units have not been fully explored
in oxo-molybdenum chemistry, their mercury, platinum, palla-
dium, iron and gallium complexes have been reported.23–25 These
Department of Chemistry and Biochemistry, Duquesne University, Pitts-
burgh, PA, 15282, USA. E-mail: basu@duq.edu
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Fig. 1 Transfer of an oxygen atom from the H-bonded remote second-coordination-sphere water molecule to oxidizing sulfite to sulfate.7
1
Room temperature H and 13C NMR spectra were recorded
using a Bruker ACP-300 spectrometer at 300.133 MHz and
75.469 MHz frequencies respectively. Additional 1H NMR spectra
were collected on a Bruker 400 MHz spectrometer. Deuterated
solvents were obtained from Cambridge Isotope laboratories and
were used without further purification. Solid-state infrared spectra
were recorded on a Perkin-Elmer FT-IR 1760X spectrometer
in KBr pellets. Solution IR spectra were recorded using a flow
through CaF2 cell with 0.1 mm solvent path length. Mass spectra
were collected on Micromass ZMD mass spectrometer using
both negative and positive ionization modes. Acetonitrile (or
methanolic) solutions of the samples were injected via a syringe
pump with a flow rate of 0.1–0.2 ml min−1. Electronic spectra of
complexes were collected on a Cary 3 spectrophotometer. Cyclic
voltammograms (CVs) of complexes were recorded on a Bioan-
alytical systems (BAS) model CV-50W using a standard three-
electrode system, consisting of Pt-disk working and reference
electrodes and a Pt-wire auxiliary electrode. Measurements were
performed in dry and degassed acetonitrile using 0.2M NBu4ClO4
of supporting electrolyte, and internally referenced with an Fc+/Fc
couple, and the potentials are expressed with respect to the Fc+/Fc
couple.
Fig. 2 Two modes of intra-ligand NH · · · S hydrogen bonding used in this
investigation.
complexes, e.g. [Pt(bpy)(S-o-MeNHCOC6H4)2] and (NEt4)2[Hg(S-
o-MeNHCOC6H4)4], possess weak NH · · · S intra-ligand (type A)
hydrogen bonding.23,24 In the alkyl analogue of type A thiolates,
based on infrared studies Zuppiroli et al. have proposed extended
structures of the neutral HSC2H4CONHCH3 ligand.26 This was
also found in Tp*Mo(NO)(S(CH2)2CONHCH3) 15, where no six-
membered cyclic structure containing S · · · HN was found in its
crystal structure. Instead, an interligand O · · · HN was observed.
Interestingly, complexes like Tp*Mo(NO)(S(CH2)2CONHCH3)
and Tp*Mo(NO)(S(CH2)2CONH(CH2)2S) 27 do not possess an
intra-ligand H-bond but their redox potentials are shifted towards
higher values compared to the N-methylated complexes.
Herein we report the syntheses and characterization of dioxo-
molybdenum complexes with ligands harboring type A and type B
hydrogen-bonding units. In addition, we also report the syntheses
and characterization of complexes of the corresponding N-
methylated ligands that cannot exhibit similar hydrogen bonding.
We demonstrate that in both types (type A and type B) H-bonding
can influence the spectral as well as the electrochemical properties
of the complexes.
Synthesis
Tp*MoO2Cl and Tp*MoO2(SPh) were prepared following the
published methods.28 Similarly, N-[2-(2-acetylaminophenyldisul-
fanyl)phenyl]acetamide, 7, and N-{2-[2-(acetylmethylamino)-
phenyldisulfanyl]phenyl}-N-methylacetamide, 8, were synthesized
according to the literature procedure.29
2,2ꢀ-Disulfanediyldibenzoic acid, 1. To
a solution of 2-
Experimental
mercapto benzoic acid (1.5 g, 9.74 mmoles) in 95% EtOH (100 ml)
was added a saturated solution of I2 in 95% EtOH in an open
atmosphere until a light-yellow color of the reaction mixture
persisted. Precipitation of a white solid occurred after ∼30 min
of stirring, which was collected and washed with ethanol. Solid
was further dried under vacuum at 50 ◦C. Yield, 60% (0.9 g,
2.94 mmoles). NMR (1H, d/ppm) in (CD3)2SO: 7.33 (t, 2H, J =
7.3 Hz,); 7.55 (t, 2H, J = 7.3 Hz); 7.62 (d, 2H, J = 8.1 Hz); 8.02
(d, 2H, J = 7.5 Hz). NMR (13C, d/ppm) in (CD3)2SO: 125.04;
125.96; 128.12; 131.55; 133.21; 138.93; 167.58. IR (KBr, cm−1):
2920, 2661, 1683, 1587, 1561, 1461, 1416, 1310, 1292, 1260, 1151,
1056, 1038, 898, 809, 738, 695, 651. ESI−-MS (CH3OH): m/z:
153.03 [1/2(M − 2H)]2−, (M = C14H10O4S2, 306.36).
Unless otherwise specified, all the reactions were carried out in
oven-dried glassware in dry atmospheres of argon using standard
Schlenk techniques. All work-up and chromatographic purifica-
tions of complexes were conducted in air using distilled solvents.
Solvents were purified as follows: dichloromethane (CH2Cl2) from
CaH2; toluene, hexane and tetrahydrofuran (THF) from Na-
benzophenone; ethanol (EtOH) by refluxing with freshly prepared
sodium ethoxide. Triethylamine and pyridine were purified by
distilling over KOH pellets. For adsorption chromatography, silica
˚
gel (60 A, 63–200 lm) from Sorbent Technologies was used. Thin-
layer chromatography (TLC) was performed on silica-gel-coated
plastic plates also purchased from Sorbent Technologies.
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2-Chlorocarbonyl-phenyl disulfanyl benzyl chloride, 2. To a
mixture of 1 (3 g, 9.8 mmoles) and N,N-dimethylformamide
(1 ml) in THF (25 ml) was slowly added oxalyl chloride (8.5 ml,
97.4 mmoles) at 0 ◦C. The reaction mixture was gradually warmed
to room temperature and further stirred for 3 h. Evaporation
of solvents to dryness under reduced pressure afforded a yellow
solid in quantitative yield which was characterized without further
purifications. NMR (1H, d/ppm) in (CD3)2SO: d = 7.39 (t, 2H, J =
7.2 Hz); 7.55 (t, 2H, J = 7.6 Hz); 7.76 (d, 2H, J = 9.1 Hz); 8.38 (d,
2H, J = 7.8 Hz). NMR (13C, d/ppm) in (CD3)2SO: 126.18; 126.44;
130.46; 135.12; 135.60; 141.55; 167.50. IR (KBr, cm−1): 1718, 1583,
1555, 1446, 1435, 1199, 1133, 1039, 898, 879, 764, 721, 695, 655.
EtOAc yielded a yellow liquid, which was purified by adsorption
chromatography on silica gel using a 1 : 3 mixture of acetonitrile
and toluene to yield 90% (1.49 g, 3.58 mmoles) of the final product.
NMR (1H, d/ppm) in CD3CN: 2.84 (d, 3H, J = 5.8 Hz); 5.00 (s,
1H); 6.85 (m, 1H); 7.17 (t, 1H, J = 7.2 Hz); 7.28 (t, 1H, J = 7.3 Hz);
7.35 (d, 1H, J = 7.2 Hz); 7.45 (d, 1H, J = 7.3 Hz). NMR (13C,
d/ppm) in CD3CN: 26.69; 126.02; 129.05; 131.50; 134.10; 169.82.
IR (KBr, cm−1): 3315, 2936, 2538, 1621, 1588, 1544, 1407, 1319,
1269, 1173, 1042, 1007, 745, 714. ESI−-MS (CH3CN): m/z: 166
[M − H]−, (M = C8H9NOS, 167).
2-Mercapto N,N-dimethyl benzamide, 6. This compound was
synthesized by reducing 4 (2.0 g, 5.55 mmoles) with NaBH4 (0.85 g,
22.37 mmoles) in THF–EtOH (12 ml and 12 ml) as described for
the synthesis of 5. Yield, 87.5% (1.75 g, 9.71 mmoles). NMR
(1H, d/ppm) in CD3CN: 2.83 (s, 3H); 3.09 (s, 3H); 3.74 (s, 1H);
7.15–7.20 (m, 3H); 7.29 (d, 1H, J = 7.4 Hz). NMR (13C, d/ppm)
in CD3CN: 34.57, 38.27; 125.78; 126.71; 128.15; 129.09; 130.69;
136.47; 169.90. IR (KBr, cm−1): 2929, 2515, 1619, 1394. ESI−-MS
(CH3OH): m/z: 180.2 [M − H]−, (M = C9H11NOS, 181.25).
N-(2-Mercaptophenyl)acetamide, 9. This compound was syn-
thesized by reducing the corresponding disulfide, 7 (1.1 g,
3.31 mmoles), following the procedure described for the synthesis
of 5 with NaBH4 (0.51 g, 13.4 mmoles) in THF–EtOH (16 ml and
16 ml). Yield, 83% (0.92 g, 5.5 mmoles). NMR (1H, d/ppm) in
CD3CN: 2.53 (s, 3H); 7.11 (t, 1H, J = 7.3 Hz); 7.22 (t, 1H, J =
7.3 Hz); 7.63 (d, 1H, J = 7.4 Hz); 7.72 (d, 1H, J = 7.4 Hz). NMR
(13C, d/ppm) in CD3CN: 22.13; 122.45; 122.91; 125.53; 126.73;
136.56; 154.23; 168.09. IR (KBr, cm−1): 3235, 2939, 2546, 1640,
1577, 1528, 1437, 1437, 1298, 1263, 1077, 752. ESI−-MS (CH3CN):
m/z: 166 [M − H]−, (M = C8H9NOS, 167).
N-(2-Mercaptophenyl) N-methyl acetamide, 10. It was syn-
thesized by reducing the corresponding disulfide, 8 (1.5 g,
4.16 mmoles), by following the procedure described for the
synthesis of 5 using NaBH4 (0.64 g, 16.84 mmoles) in a THF–
EtOH (20 ml and 20 ml) medium. Yield, 95% (1.43 g, 7.90 mmoles).
NMR (1H, d/ppm) in D2O–(CD3)2SO: 3.05 (s, 3H); 4.19 (s, 3H);
7.67 (t, 1H, J = 7.0 Hz); 7.77 (t, 1H, J = 7.2 Hz); 7.97 (d, 1H, J =
7.4 Hz); 8.08 (d, 1H, J = 7.4 Hz). NMR (13C, d/ppm) in D2O–
(CD3)2SO: 18.07; 37.31; 117.79; 125.16; 129.99; 131.14; 143.07;
177.55. IR (KBr, cm−1): 2415, 1662, 1512, 1462, 1443, 760. ESI+-
MS (CH3CN): m/z: 363.29 [2M + H]+, (M = C9H11NOS, 181.06).
2,2ꢀ-Disulfanediylbis(N,N-dimethylbenzamide), 3. A solution
of freshly prepared 2 (3.36 g, 9.8 mmoles) in CH2Cl2 (30 ml) was
slowly added to a mixture of MeNH2·HCl (2.0 g, 29.6 mmoles)
◦
and Et3N (8.2 ml, 58.9 mmoles) in CH2Cl2 (15 ml) at 0 C. The
reaction mixture was gradually warmed to room temperature and
stirred overnight. The reaction was quenched with water and a
white precipitate was filtered, washed with water and dried in a
desiccator. Yield, 75% (2.44 g, 7.35mmoles). NMR (1H, d/ppm) in
(CD3)2SO: 2.73 (d, 6H, J = 4.3 Hz); 7.30 (t, 2H, J = 7.4 Hz); 7.45
(t, 2H, J = 7.3 Hz); 7.63 (m, 4H); 8.58 (m, 2H). This compound
was sparingly soluble in CHCl3. NMR (1H, d/ppm) in CDCl3 2.99
(d, 6H, J = 4.8 Hz), 6.16 (s, 2H), 7.25 (t, J = 7.3 Hz), 7.37 (t, 2H,
J = 8.0 Hz), 7.48 (d, 2H, J = 7.7 Hz), 7.76 (d, 2H, J = 7.8 Hz).
NMR (13C, d/ppm) in (CD3)2SO: 26.18; 125.85; 127.80; 131.04;
133.71; 136.64; 167.24. IR (KBr, cm−1): 3284, 3084, 1632, 1551,
1406, 1327, 745, 695. ESI+-MS (CH3CN): m/z: 333.31 [M + H]+,
(M = C16H16N2O2S2, 332.07).
2-(Dimethylcarbamoyl-phenyldisulfanyl)-N,N-dimethyl benza-
mide, 4. This compound was synthesized by following the pro-
cedure described for 3 using compound 2 (4.48 g, 13.06 mmoles),
Me2NH·HCl (3.26 g, 40 mmoles), and Et3N (11.0 ml, 79.5 mmoles)
in CH2Cl2 (50 ml). After the addition of water, the organic layer
was separated, dried over anhydrous MgSO4 and evaporated to
yield a light-pink solid, which was further purified by a silica
column using a 5–30% gradient of acetonitrile in toluene. Yield,
80% (3.77 g, 10.45 mmoles). NMR (1H, d/ppm) in CD3CN: 2.74 (s,
6H); 3.05 (s, 6H); 7.24 (d, 2H, J = 7.4 Hz); 7.34 (t, 2H, J = 7.3 Hz);
7.41 (t, 2H, J = 7.8 Hz), 7.73 (d, 2H, J = 7.7 Hz). NMR (13C,
d/ppm) in CD3CN: 22.06; 35.98; 127.19; 128.40; 128.67; 129.41;
135.26; 141.65; 165.8. NMR (1H, d/ppm) in CDCl3: 2.84 (s, 6H);
3.12 (s, 6H); 7.20 (d, 2H, J = 7.0 Hz); 7.26 (t, 2H, J = 7.2 Hz);
7.33 (t, 2H, J = 7.4 Hz); 7.68 (d, 2H, J = 7.2 Hz). NMR (13C,
d/ppm) in CDCl3: 35.00; 38.90; 127.01; 127.64; 128.83; 130.03;
134.24; 136.75; 169.39. IR (KBr, cm−1): 3049, 2926, 1648, 1620,
1586, 1508, 1392. ESI+-MS (CH3CN): m/z: 360.1 [M + H]+, (M =
C18H20N2O2S2, 360.49).
Tp*MoO2(S-o-MeNHCOC6H4), 11. A mixture of thiol 5
(0.39 g, 2.34 mmoles) and Et3N (1 ml, 7.2 mmoles) in CH2Cl2
(15 ml) was added to the suspension of Tp*MoO2Cl (1.10 g,
2.38 mmoles) in CH2Cl2 (25 ml) at room temperature and the
reaction mixture was stirred for 6 h. Solvent was removed under
vacuum and the brown solid residue was chromatographed on
silica gel using a mixture of MeCN–toluene (1 : 4). The first brown
band was collected and the solvent was evaporated. Yield, 25%
(0.35 g, 0.59 mmoles). NMR (1H, d/ppm) in CDCl3: 2.36 (s, 3H);
2.41 (s, 6H); 2.61 (s, 3H); 2.63 (s, 6H); 2.90 (d, 3H, J = 4.3 Hz);
5.86 (s, 3H); 6.62 (m, 1H); 7.11 (t, 1H, J = 7.3 Hz); 7.44 (t, 1H, J =
7.5); 7.73 (d, 1H, J = 7.2); 8.02 (d, 1H, J = 8.4 Hz). NMR (13C,
d/ppm) in (CD)3SO: 12.52; 12.80; 15.09; 15.25, 26.70; 107.34;
107.53; 125.39; 129.23; 130.15; 136.57; 136.95; 140.54; 144.79;
147.2; 153.26; 154.44; 168.95. ESI+-MS (CH3CN + 0.05%TFA):
m/z: 616 [M + Na]+, (M = C23H30O3N7SBMo, 593).
2-Mercapto N-methyl benzamide, 5. To a solution of 3 (1.65 g,
1.99 mmoles) in EtOH (20 ml) and THF (20 ml) was added
NaBH4 (0.30 g, 7.96 mmoles) in small portions at 0 ◦C and
reaction mixture was warmed to room temperature and stirred for
4 h. Solvents were removed under vacuum, the resulting yellow
solid was dissolved in distilled water (10 ml) and dilute AcOH
was added to acidify the solution to pH ∼ 4. A yellowish, oily
material separated out from the solution, which was extracted with
EtOAc (60 ml) and dried over anhydrous MgSO4. Evaporation of
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Tp*MoO2(S-o-Me2NCOC6H4), 12. This complex was synthe-
sized using a procedure similar to that described for the synthesis
of 11 using Tp*MoO2Cl (1.27 g, 2.74 mmoles), thiol 6 (0.51 g,
2.81 mmoles) and Et3N (1.16 ml, 8.4 mmoles) in CH2Cl2 (45 ml).
The reaction mixture was stirred for 18 h. The brown liquid was
evaporated to dryness and redissolved in a 1 : 1 mixture of MeCN
and toluene and loaded in the silica column. A gradient of MeCN
and toluene starting from 10% (MeCN in toluene) was used for
purification. Yield, 28% (0.47 g, 0.77 mmoles). NMR (1H, d/ppm)
in CDCl3: 2.36 (s, 6H); 2.39 (s, 3H); 2.65 (s, 9H); 2.83 (s, 3H);
3.03 (s, 3H); 5.86 (m, 3H); 7.15–7.26 (m); 7.42 (m, 1H); 8.1 (d,
1H, J = 7.5). NMR (13C, d/ppm) in (CD)3SO: 12.95; 13.02; 16.04;
34.80, 38.40; 51.20; 107.44; 124.89; 126.88; 128.75; 129.36; 133.04;
140.50; 169.50. ESI+-MS (CH3CN + 0.05%TFA): m/z: 608 [M +
H]+, (M = C24H32O3N7SBMo, 607).
literature procedure,31 which included first reacting 1 with oxalyl
chloride to prepare the acid chloride, 2, followed by its reaction
with MeNH2 or Me2NH. Compound 2 was found to be sensitive to
moisture and readily hydrolyzed to 1 upon contact with moisture.
Disulfides 3 and 4 were obtained in good yields and were stable
in air. Disulfides were reduced to thiol ligands using NaBH4 in
THF–ethanol media. Thiols were found to be moderately sensitive
towards air and methyl sulfoxide.
Ligands were characterized by solution-phase 1H and 13C NMR
spectroscopy and infrared spectroscopy. Due to the low solubility
of thiols in chloroform, the NMR spectra were recorded in
CD3CN with the exception of compound 10 which was recorded
in (CD3)2SO. The aromatic signals of thiols 5, 6, 9 and 10 show
downfield shifts compared to the corresponding disulfides 3, 4,
8 and 9 respectively; however, methyl protons show an opposite
upfield shift in thiols compared to the disulfides. The thiols also
exhibit a characteristic strong infrared stretch around 2530 cm−1
for the S–H group.
Scheme 2 represents the synthetic route to dioxo-MoVI com-
plexes possessing ortho-substituted thiophenol ligands, which
follows the ligand substitution reaction similar to Tp*MoO2(SPh).
Upon addition of a thiol–Et3N solution into a suspension of
Tp*MoO2Cl in CH2Cl2, the reaction mixture changes color from
yellow to brown. The progress of the reactions was monitored
by thin-layer chromatography (TLC). In general, however, the
reactions with substituted ligands took a longer time for com-
pletion. Syntheses of complexes 11 and 13 took about 6–7 h, while
their methylated partners 12 and 14 required overnight for the
reaction to be completed. In all four cases, multiple spots on
the TLC plate were observed, indicating side reactions and/or
decomposition. Complexes 11–14 were successfully purified by
column chromatography over silica gel using a solvent gradient of
MeCN in toluene. Complexes 12 and 14, were further purified by
crystallization using toluene and hexane.
Tp*MoO2(S-o-MeCONHC6H4), 13. This complex was syn-
thesized using a procedure similar to that described for the
synthesis of 11 using Tp*MoO2Cl (0.80 g, 1.73 mmoles), thiol
7 (0.36 g, 1.80 mmoles), and Et3N (0.75 ml, 5.4 mmoles) in
CH2Cl2 (35 ml). The reaction mixture was stirred for 7 h.
Solvent was removed at a reduced pressure and the residue
was chromatographed on silica gel using a mixture of MeCN–
toluene (1 : 4). The first brown band was collected and the solvent
was evaporated to give a solid compound. Yield, 30% (0.31 g,
0.52 mmoles). NMR (1H, d/ppm) in CDCl3: 2.16 (s, 3H); 2.36 (s,
3H); 2.42 (s, 6H); 2.56 (s, 3H); 2.69 (s, 6H); 5.86 (s, 1H); 5.90(s,
2H); 7.08 (t, 1H, J = 7.3 Hz); 7.18 (t, 1H, J = 7.3); 7.83 (d,
1H, J = 7.3); 8.20 (s, 1H); 8.30 (d, 1H, J = 7.4 Hz). NMR (13C,
d/ppm) in CDCl3: 12.50; 12.76; 15.19; 24.87; 30.82; 107.33; 107.62;
120.01; 123.56; 127.26; 131.56; 134.83; 137.66; 144.96; 147.16;
153.23; 154.26; 168.22. ESI+-MS (CH3CN + 0.05%TFA): m/z: 594
[M + H]+, (M = C23H30O3N7SBMo, 593).
The molybdenum complexes were found to be sensitive in
solution and to degrade spontaneously releasing the thiophenolato
ligand. The sensitivity poses a challenge towards chromatographic
separation and contributes to the lower yields. We were unable
to obtain suitable quality crystals for structural studies. In
fact in most cases we obtained dimeric molybdenum complexes
without any thiolato coordination even when we started with
spectroscopically (NMR) pure solutions of the complexes. This
problem has also resulted in less than optimal elemental analysis
of the isolated product. Solutions of the isolated product showed
multiple spots on thin-layer chromatography and additional peaks
appeared in the NMR spectra.
Tp*MoO2(S-o-MeCONMeC6H4), 14. This complex was syn-
thesized using a procedure similar to that described for the
synthesis of 12 using Tp*MoO2Cl (1.0 g, 2.16 mmoles), thiol 10
(0.35 g, 1.93 mmoles), and Et3N (0.80 ml, 5.8 mmoles) in CH2Cl2
(40 ml). The reaction mixture was stirred overnight. Solvent was
removed at reduced pressure and the residue was chromatographed
on silica gel using a mixture of MeCN–toluene (1 : 4). The first
brown band was collected and the solvent was evaporated resulting
in a solid brown compound. Yield, 18% (0.24 g, 0.39 mmoles).
NMR (1H, d/ppm) in: d = 1.83 (s, 3H); 2.36 (s, 3H); 2.40 (s, 6H);
2.60 (s, 6H); 2.62 (s, 3H); 3.18 (s, 3H); 5.86–5.92 (m, 3H); 7.22
(d, 1H, J = 7.0 Hz); 7.35 (t, 1H, J = 7.5); 7.45 (t, 1H, J = 7.5);
7.62 (d, 1H, J = 7.6 Hz). NMR (13C, d/ppm) in (CD)3SO: 7.52;
18.60; 22.05; 58.70, 115.20; 116.10, 140.15; 142.50, 143.22; 150.64;
163.20; 163.92; 164.45; 171.80; 172.50.
1H NMR spectroscopy. The 1H NMR spectra of molybdenum
complexes have a 2 : 1 ratio for the three-pyrazole methyl groups
indicating the Cs local symmetry in solution.28,30 The amide NH
signal in 11 appeared at 6.68 ppm in chloroform-d, which is about
0.52 ppm downfield compared to the NH signal in corresponding
disulfide 3 (6.16 ppm in chloroform). Similarly, the amide NH
signal in 13 appeared at 8.20 ppm, i.e. shifted about 0.30 ppm
downfield compared to the corresponding disulfide, 7.11 These
shifts are consistent with the presence of a H-bond in 11 and 13
as described for other metal complexes.20–25 Additional support
Results and discussion
Synthesis and characterization
The functionalized thiol ligands used in this investigation were
synthesized by reducing preformed disulfides as outlined in
Scheme 1. 2-Mercaptobenzoic acid was oxidized to 1 using I2
in ethanol.30 Disulfides 3 and 4 were synthesized by modifying a
1
comes from the H-NMR spectra of the complexes containing
amide –NHCO– and N-methylated –NMeCO– moieties. Fig. 3
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Scheme 1
Scheme 2
shows 1H NMR chemical shifts of aromatic protons of thiopheno-
lato ligand in the molybdenum complexes. The doublet resonances
(represented as grey bars [red online]) are due to the protons next
to the sulfur and the amide substituent, which show upfield shifts
upon N-methylation, e.g. two doublets (a1 and d1) of thiophenol
ligand in 11 move from 7.73 and 8.02 ppm (peak separation
∼0.29 ppm) to 7.39 ppm (c1) and 7.67 ppm (b1) (peak separation
∼0.28 ppm) in 12. Similar shifts in doublets (a2 and d2) are also
observed from 13 to 14. The NMR chemical shifts are primarily
due to electronic effects of substituents and the presence of H-
bonding. The substituents containing amide functionality and
the corresponding N-methylated species have similar electronic
coefficients32 and thus are unlikely to give such a large shift of the
NMR signals. Thus, we propose the presence of intramolecular
H-bonding in 11 and 13, which is consistent with solution IR
spectra (vide infra). The formation of an NH · · · S hydrogen bond
can decrease the electron density on the sulfur atom, which results
in a downfield shift of ring protons compared to N-methylated
complexes where there is no H-bonding present. A larger shift in
the pairs of doublets and triplets in 13 to 14 indicates stronger H-
bonding compared to 11, which is again consistent with solution
IR-spectra (vide infra).
IR spectroscopy. The infrared spectra of the dioxo-Mo(VI)
complexes have, due to the cis-MoO2 unit, two strong bands
around 924 and 893 cm−1 which have been assigned as symmetric
and asymmetric vibrational modes respectively. In addition, a
sharp B–H stretch is present at 2545 cm−1 in all the molybdenum
=
complexes (Table 1). IR bands for amide m(N–H) and m(C O)
for 11 and 13 are summarized in Table 2 which also lists
similar bands for the corresponding thiols and disulfides. Fig. 4
=
represents the amide region m(N–H) and m(C O) in the solid-
state IR spectra of complex 11 and the corresponding thiol (5)
=
and disulfide (3). Complex 11 exhibits broad N–H and C O
bands indicating poor crystal packing. Thiol 5 exhibits a slightly
larger shift in NH band compared to the disulfide 3, which is
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Fig. 4 IR spectra of (top) thiol 5 (on NaCl plate) , (middle) disulfide 3
(in KBr pellet), and (bottom) complex 11 (in KBr pellet).
Fig. 3 1H NMR chemical shifts of the thiophenol phenyl ring in Mo–
complexes (black lines [blue online] represent triplet resonances whereas
grey lines [red online] represent doublets). All spectra were collected in
chloroform-d at room temperature in freshly prepared solutions.
described previously. It should be noted that thiol 9 is likely to
form intramolecular NH · · · S hydrogen bonding.23
The presence of intramolecular H-bonding in Mo-complexes
was examined in solutions of chloroform-d, and the extent of
dissociation into m(N–Hbound) and m(N–Hfree) is determined by
integrating the intensities of the m(N–H) peaks of 11 and 13.
Table 2 also summarizes the vibrational frequencies of m(N–H)
consistent with 2-tBuNHCOC6H4SH (m(N–H) ∼ 3303 cm−1) and
(S-2-tBuNHCOC6H4)2 (m(N–H) ∼ 3294 cm−1).24 The thiol ligands
and m(C O) in the Mo-complexes in solid and in chloroform-d.
=
Both 11 and 13 exhibit two m(N–H) amide stretching bands in
solution compared to a single band in the solid state, indicating
the presence of hydrogen-bonded m(N–Hbound) and non-hydrogen-
bonded m(N–Hfree) groups as shown in Fig. 5. In the solution state,
11 exhibits two distinct N–H bands at 3325 and 3452 cm−1 while
in the solid state only one broad N–H band at 3313 cm−1 appears.
=
of type A have been proposed to form a SH · · · O C H-bond,
whereas the thiolato form of the ligand harbors a NH · · · S H-
bonding.24 In the case of thiol 9, the m(N–H) vibration appears
at 3202 cm−1 and for the corresponding disulfide 7 the m(N–
H) band appears at ∼3238 cm−1, opposite to the type A motif
Table 1 Spectroscopic properties of molybdenum complexes
Selected IR bands in
KBr pellet, m/cm−1
Complex
m(MoO2)
m(B–H)
Electronic spectra in MeCN, k/nm (e/M−1 cm−1
)
Redox potential,a E1/2/V (DEp/V)
11
893, 924
890, 924
894, 921
896, 925
895, 923
2549
2553
2538
2548
2549
258 (12980); 403 (2825); 517 (sh, 465)
252 (3580); 407 (970); 520 (sh, 125)
255 (13645); 398 (2350); 519 (sh, 397)
254 (15945); 396 (2785); 504 (sh, 825)
262 (4140); 409 (1225); 526 (sh, 120)
−1.04 (0.23)
−1.14 (0.24)
−1.11 (0.12)
−1.11 (0.12)
−1.30 (0.15)
12
Tp*MoO2(SPh)
13
14
a In MeCN, room temperature. Scan rate: 0.1 V s−1. Pt-disk working and reference electrodes and a Pt-wire auxiliary electrode; 0.2 M NBu4ClO4 of
supporting electrolyte.
Table 2 IR bands of amine and carbonyl stretches
Disulfide
3
Thiol
5
Molybdenum complexes
7
9
11
13
KBra
KBra
NaClb
NaClb
KBra
CDCl3
KBra
CDCl3
c
c
m(N–H)
m(C O)
3284
1634
3238
1662
3315
1620
3202
1662
3313
1650
3325, 3452
1650
3375
1700
3370, 3611
1686
=
a KBr pellet. b NaCl plate. c Chloroform-d solution.
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Fig. 5 Comparison of IR spectra in solid and liquid phases: (a) 11 in KBr pellet, (b) 11 in chloroform-d, (c) 13 in KBr pellet, and (d) 13 in chloroform-d.
Liquid spectra were recorded using a CaF2 flow-through cell. (Solution path length = 0.1 mm and concentration ∼ 20 mM.)
Thus, in solution the two vibrations are separated by 127 cm−1.
Similarly, for 13, the N–H vibration at 3375 cm−1 in the solid
state, appears as two bands at 3370 and 3611 cm−1 in solution, a
separation of 241 cm−1. The N–H · · · S hydrogen bonds in 11 and
low intensity around 515 nm is found to be more sensitive to
electronic effects. From 13 to 14 the band position shifts from
504 to 526 nm indicating the involvement of intramolecular H-
bonding in 13 as discussed earlier. From 11 to 14 the band showed
a positive shift of 13 nm. A strong charge-transfer transition
around 260 nm can be assigned to the ligand-to-ligand charge
transfer. In addition, the intensity of the low-energy transition
(i.e. ∼500 nm) for complexes with H-bonding (11 and 13), is
higher than that in the N-methylated complexes (12 and 14). These
observations indicate that NHCOMe induces stronger H-bonding
than CONHMe.
13 are ∼26% and ∼11% dissociated with respect to the m(N–Hfree
forms. Complex 13 shows a larger separation between m(N–Hbound
)
)
and m(N–Hfree) compared to 11, by 114 cm−1, indicating stronger
N–H · · · S hydrogen bonding in 13. This is consistent with the 1H
NMR data described above.
UV-Visible spectroscopy. UV-Visible spectra of dioxo-
complexes were recorded in MeCN at room temperature (Fig. 6)
and the characteristic band positions are listed in Table 1. A band
around 400 nm is due to the S → Mo charge-transfer transition.33
Interestingly, from 13 to 14, a red shift of 13 nm was observed due
to the absence of intramolecular H-bonding from thiophenolato
ligand. Similar electronic effects have also been observed for
complexes, 11, 12, and 14, where a blue shift was observed
from the electron-donating ligand (NMeCOMe, 409 nm) to the
electron-withdrawing group (CONHMe, 403 nm). A shoulder of
Electrochemistry
The electrochemical behavior of the Mo-complexes, 11–14, was
studied by cyclic voltammetry along with Tp*MoO2(SPh), and
the redox potentials are summarized in Table 1. Cyclic voltammo-
grams for all dioxo complexes are shown in Fig. 7, which show one-
electron quasi-reversible redox couple, assigned to the MoV/MoVI
Fig.
temperature.
6
Electronic spectra of Mo-complexes in MeCN at room
Fig. 7 Cyclic voltammograms of (a) 11, (b) 12, (c) 13, and (d) 14 in MeCN
at room temperature. Scan rate: 0.1 V s−1
.
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process.30,34 No other reductive response was observed within the
solvent window. Consistent with the diffusion-controlled process,
the peak currents follow a linear relation with the square root of
the scan rate while the half-wave potentials remain independent.
Interestingly, the ratios of the peak currents are significantly
different from unity, indicating the irreversible nature of the
redox couple. Consequently, no attempt was made to generate
the corresponding Mo(V) complexes.
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The unsubstituted dioxo-Mo(VI) complex, Tp*MoO2(SPh), also
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Dioxo-MoVI complexes containing ortho-substituted thiopheno-
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Acknowledgements
Financial support from the National Institutes of Health
(GM61555) and an equipment grant from the National Science
Foundation (CHE 0614785) are gratefully acknowledged. We
acknowledge Ms Tina Bhatnagar for preliminary experiments
and Ms Eranda Perera for valuable experimental assistance and
discussion.
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