Synthesis and characterization of a redox-active bis(thiophenolato)amide ligand, [SNS]3-, and the homoleptic tungsten complexes, W[SNS] 2 and W[ONO]2

Article abstract of DOI:10.1021/ic302506e

A new tridentate redox-active ligand platform, derived from bis(2-mercapto-p-tolyl)amine, [SNS^cat]H₃, has been prepared in high yields by a four-step procedure starting from commericially available bis(p-tolyl)amine. The redox-active pincer-type ligand has been coordinated to tungsten to afford the six-coordinate, homoleptic complex W[SNS]₂. To benchmark the redox behavior of the [SNS] ligand, the analogous tungsten complex of the well-known redox-active bis(3,5-di-tert- butylphenolato)amide ligand, W[ONO]₂, also has been prepared. Both complexes show two reversible reductions and two partially reversible oxidations. Structural, spectroscopic, and electrochemical data all indicate that W[ONO]₂ is best described as a tungsten(VI) metal center coordinated to two [ONO^cat]^3- ligands. In contrast, experimental data suggests a higher degree of S→W π donation, giving the W[SNS]₂ complex non-innocent electronic character that can be described as a tungsten(IV) metal center coordinated to two [SNS ^sq]^2- ligands.

Full text of DOI:10.1021/ic302506e

Article
pubs.acs.org/IC
Synthesis and Characterization of a Redox-Active
Bis(thiophenolato)amide Ligand, [SNS]3−, and the Homoleptic
Tungsten Complexes, W[SNS]2 and W[ONO]2
David W. Shaffer, Geza Szigethy, Joseph W. Ziller, and Alan F. Heyduk*
́
Department of Chemistry, University of California, Irvine, California, 92697, United States
S
* Supporting Information
ABSTRACT: A new tridentate redox-active ligand platform,
derived from bis(2-mercapto-p-tolyl)amine, [SNScat]H3, has
been prepared in high yields by a four-step procedure starting
from commericially available bis(p-tolyl)amine. The redox-
active pincer-type ligand has been coordinated to tungsten to
afford the six-coordinate, homoleptic complex W[SNS]2. To
benchmark the redox behavior of the [SNS] ligand, the
analogous tungsten complex of the well-known redox-active
bis(3,5-di-tert-butylphenolato)amide ligand, W[ONO]2, also
has been prepared. Both complexes show two reversible
reductions and two partially reversible oxidations. Structural, spectroscopic, and electrochemical data all indicate that W[ONO]2
is best described as a tungsten(VI) metal center coordinated to two [ONOcat]3− ligands. In contrast, experimental data suggests a
higher degree of S→W π donation, giving the W[SNS]2 complex non-innocent electronic character that can be described as a
tungsten(IV) metal center coordinated to two [SNSsq]2− ligands.
Chart 1
INTRODUCTION
■
Metal complexes containing redox-active ligands with a
tridentate, pincer-type architecture have received increased
attention recently owing to their intriguing electronic proper-
ties and rich reactivity.1−4 Pincer-type ligands offer a robust,
meridional coordination environment for the binding of
transition metal ions and can include both hard donor atoms
such as oxygen and nitrogen and soft donor atoms such as
sulfur and phosphorus. With regard to redox-active ligands, the
pincer motif is advantageous because the tridentate coordina-
tion prevents ligand dissociation during oxidation state changes
to the ligand; furthermore, the meridional coordination mode
helps to maintain a planar geometry allowing redox-changes to
be delocalized over the entire ligand framework. The
delocalized nature of the redox-active ligand orbitals may also
help to avoid the kinetic barriers associated with geometric
rearrangements that occur for localized changes to metal
oxidation states. Examples of redox-active, pincer-like ligands
include bis(imino)pyridines,3 bis(phosphino)pyridines and
amides, bis(anilido)amides,1 and various other tridentate
ligands.5−7 Metal complexes supported by such ligands have
shown promise in promoting multielectron reactions, and
warrant the development of new redox-active pincer-type ligand
platforms.
and the monoanionic iminoquinonate, [ONOq]−.1 While
homoleptic complexes with the formulation M[ONO]2 were
originally synthesized by the aerobic condensation of NH3 with
3,5-di-tert-butylcatechol in the presence of divalent metal
salts,11 more recently, the synthesis, characterization, and
reactivity of metal complexes containing a single [ONO]
ligand have been elucidated by way of isolated [ONOcat]H3 or
[ONOq]K.16,17
Another attractive pincer-type ligand platform would be one
that preserves the redox-activity of the [ONO] ligand while
incorporating softer donor atoms for metal binding. Given the
importance of sulfur-based ligands in both synthetic and
biological coordination chemistry, and the long-known non-
innocent behavior of transition metal dithiolene com-
plexes,18−23 it is surprising that sulfur ligands derived from
One of the best studied redox-active pincer ligands is the
[ONO] platform, derived from bis(3,5-di-tert-butylphenol)-
amine [ONOcat]H3.8−15 The [ONO] ligand has been
characterized in three oxidation states when coordinated to
metal ions (Chart 1): the fully reduced catecholate trianion
[ONOcat]3−, the dianionic, radical semiquinonate, [ONOsq●]2−,
Received: November 15, 2012
Published: February 6, 2013
© 2013 American Chemical Society
2110
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Article
Electronic absorption spectra were recorded with Perkin-Elmer
Lambda 800 and 900 UV−vis spectrophotometers in 1-cm quartz
cells using dry, degassed CH2Cl2. Atmospheric-pressure chemical
ionization mass spectrometry (APCI-MS) and electrospray ionization
mass spectrometry (ESI-MS) data were collected on a Waters LCT
Premier mass spectrometer.
Bis(2-bromo-p-tolyl)amine. This compound was synthesized
according to a literature preparation, but the reaction proceeded in
lower yield, and the final product required an added recrystallization
step.34 An effective recrystallization method was found to be
dissolution of the solids in a minimum amount of hot EtOH (until
no second phase of oil is observed). With continued rapid stirring, the
recrystallization flask was transferred to an ice water bath and stirred as
it cooled, which seized the solution with a fine white solid. This
material was collected by filtration, washed with cold EtOH, and dried
under vacuum to yield product usable for the next synthetic step, albeit
with significant loss of total yield to about 60−75%.
N-benzyl-bis(2-bromo-p-tolyl)amine. A solution of bis(2-
bromo-p-tolyl)amine (5.84 g, 16.5 mmol, 1 equiv) in 50 mL of
THF was frozen, and KH (691 mg, 17.2 mmol, 1.05 equiv) was added
upon thawing. The mixture was allowed to warm to room temperature
and stirred until gas evolution ceased, giving a yellow, homogeneous
solution. The solution was again frozen and immediately upon
thawing, benzyl bromide was added (1.96 mL, 16.5 mmol, 1 equiv).
This mixture was allowed to warm to room temperature and was then
stirred for 2 days to afford a pale yellow suspension. Solvent was
removed under vacuum, the residue was coevaporated once with Et2O,
and the solid residue was transferred to a Soxhlet extractor and
extracted into 125 mL of Et2O. Solvent was removed from the yellow
extract to yield N-benzyl-bis(2-bromo-p-tolyl)amine as a white solid
(7.13 g, 97%). Analytically pure samples were recrystallized from Et2O
at −4 °C as fine white needles. Anal. Calcd. (Found) for C21H19NBr2
(%): C, 56.66 (56.86); H, 4.30 (4.30); N, 3.15 (3.14). 1H NMR
(C6D6) δ/ppm: 1.82 (s, 6H, CH3), 4.65 (s, 2H, CH2), 6.62 (d, J = 8.5
Hz, 2H, aryl−H), 6.77 (d, J = 8.0 Hz, 2H, aryl−H), 6.95 (t, J = 7.5 Hz,
1H, aryl−H), 7.10 (t, J = 7.5 Hz, 2H, aryl−H), 7.27 (s, 2H, aryl−H),
7.57 (d, J = 8.0 Hz, 2H, aryl−H). 13C{1H} NMR (C6D6) δ/ppm:
20.17 (CH3), 57.12 (CH2), 121.61 (aryl−C), 125.28 (aryl−C), 127.22
(aryl−C), 127.93 (aryl−C), 128.62 (aryl−C), 134.94 (aryl−C), 135.12
(aryl−C), 138.43 (aryl−C), 145.11 (aryl−C). MS (ESI+) m/z: 465.8
(28%, MNa+), 467.8 (39%, MNa+).
bis(thiophenol)amines, analogous to the [ONO] platform, are
not known in the literature. Coordination complexes of
bidentate dithiolates and aminothiolates have been well-studied,
and a variety of metal complexes containing tridentate sulfur-
containing ligands have been described including such motifs as
(ONS),24 (ONSR),25,26 (NNS),27 (NNRS),28 (NSN),29,30 and
(RSNSR).31 Notably, an (SNMeS) platform has been
synthesized,32 but methylation of the central nitrogen donor
prevents conjugation, thus reducing the ligand’s redox-activity
compared to the [ONO] platform.
Described herein is the synthesis of a new redox-active,
pincer-type ligand derived from bis(2-mercapto-p-tolyl)amine,
[SNScat]H3. Preparation of the ligand precursor relies on a
dimethyldisulfide oxidation of a benzyl-protected, dilithium salt
of di-p-tolylamine, that should be applicable to the preparation
of a variety of [SNS] derivatives with different backbone
substitution patterns. To benchmark the metal-binding and
redox-active properties of the [SNS] platform, the homoleptic
W[SNS]2 complex was prepared and characterized by NMR,
UV−vis−NIR, and IR absorption spectroscopies, APCI mass
spectrometry, cyclic voltammetry, and X-ray crystallography.
For comparison, the previously uncharacterized tungsten
complex, W[ONO]2, was also examined. Both tungsten
complexes are neutral, diamagnetic species: W[ONO]2 is well
described as a WVI complex containing two [ONOcat]3− ligands,
while W[SNS]2 has non-innocent character suggestive of a WIV
center with two [SNSsq]2− ligands.
EXPERIMENTAL SECTION
■
General Considerations. Some of the complexes described below
are air and moisture sensitive, necessitating that manipulations be
carried out under an inert atmosphere of argon or nitrogen gas using
standard Schlenk, vacuum-line, and glovebox techniques unless
otherwise noted. Hydrocarbon solvents were sparged with nitrogen
and then deoxygenated and dried by passage through Q5 and activated
alumina columns, respectively. Ethereal and halogenated solvents were
sparged with nitrogen and then dried by passage through two activated
alumina columns. To test for effective oxygen and water removal,
nonchlorinated solvents were treated with a few drops of a purple
solution of sodium benzophenone ketyl in tetrahydrofuran (THF).
Electrochemical Methods. Electrochemical experiments were
performed on a Gamry Series G 300 Potentiostat/Galvanostat/ZRA
(Gamry Instruments, Warminster, PA, U.S.A.) using a 3.0 mm glassy
carbon working electrode, a platinum wire auxiliary electrode, and a
silver wire reference electrode. Electrochemical experiments were
performed at room temperature in a glovebox, under an atmosphere of
nitrogen. Electrochemical samples were 1.0 mM analyte solutions in
THF or CH2Cl2 containing 0.10 M [n-Bu4N][PF6] as the supporting
electrolyte. All potentials were referenced to the [Cp2Fe]+/0 couple
using ferrocene or decamethylferrocene as an internal standard.33
Ferrocene and decamethylferrocene (Acros) were purified by
sublimation under reduced pressure and tetra-n-butylammonium
hexafluorophosphate (Acros) was recrystallized from ethanol three
times and dried under vacuum. To verify that electrode processes were
diffusion-controlled, forward peak currents were plotted with respect
to the square root of scan rates in the range of 50 to 1600 mV/s and
found to be linear.
N-benzyl-bis(2-methanethio-p-tolyl)amine. A homogeneous
solution of N-benzyl-bis(2-bromo-p-tolyl)amine (7.11 g, 16.0 mmol,
1 equiv) in 180 mL of Et2O was frozen in a liquid-nitrogen cold well.
Immediately upon thawing, a solution of nBuLi in hexanes (2.47 M,
12.9 mL, 31.9 mmol, 2 equiv) was added, and the resultant
heterogeneous mixture was allowed to warm with stirring until it
became homogeneous (ca. 20 min), at which point MeSSMe (2.83
mL, 31.9 mmol, 2 equiv) was added. This mixture became
heterogeneous immediately and was stirred overnight at room
temperature. The reaction was quenched by the addition of 100 mL
of water and 200 mL of CH2Cl2. The layers were separated, and the
aqueous layer was extracted twice with CH2Cl2 (2 × 50 mL). The
combined organic layers were washed with water (50 mL) and
saturated brine (25 mL) and dried over Na2SO4 before the solvent was
removed under vacuum. The resultant solid was stirred in 10 mL of
Et2O, filtered, and washed twice with Et2O (2 × 10 mL) to yield N-
benzyl-bis(2-methanethio-p-tolyl)amine as a white, microcrystalline
solid (5.13 g, 81%). Analytically pure samples were recrystallized from
Et2O at −4 °C as fine white needles. Anal. Calcd. (Found) for
1
C23H25NS2 (%): C, 72.78 (72.42); H, 6.64 (6.61); N, 3.69 (3.65). H
Physical Methods. NMR spectra were collected on a Bruker
Avance 500 MHz spectrometer in dry, degassed C6D6. 1H NMR
spectra were referenced to TMS using the residual proteo impurities of
the solvent at δ = 7.16 ppm; 13C NMR spectra were referenced to
TMS using the natural abundance 13C impurities of C6D6 at δ = 128.4
ppm. All chemical shifts are reported using the standard δ notation in
parts per million; positive chemical shifts are to a higher frequency
from the given reference. Infrared spectra were recorded as KBr pellets
with a Perkin-Elmer Spectrum One FTIR spectrophotometer.
NMR (C6D6) δ/ppm: 1.94 (s, 6H, CH3), 2.06 (s, 6H, SCH3), 4.83 (s,
2H, CH2), 6.67 (d, J = 8.0 Hz, 2H, aryl−H), 6.78 (s, 2H, aryl−H),
6.95−6.96 (m, 3H, aryl−H), 7.13 (d, J = 8.0 Hz, 2H, aryl−H), 7.71 (d,
J = 7.5 Hz, 2H, aryl−H). 13C{1H} NMR (C6D6) δ/ppm: 14.28
(SCH3), 20.03 (CH3), 56.56 (CH2), 123.69 (aryl−C), 125.28 (aryl−
C), 125.76 (aryl−C), 126.96 (aryl−C), 127.96 (aryl−C), 128.50
(aryl−C), 133.74 (aryl−C), 136.45 (aryl−C), 139.45 (aryl−C), 143.93
(aryl−C). MS (ESI+): m/z = 380.1 (100%, MH+).
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Bis(2-mercapto-p-tolyl)amine, [SNScat]H3. A suspension of N-
benzyl-bis(2-methanethio-p-tolyl)amine (5.03 g, 13.3 mmol) in 60 mL
of condensed anhydrous ammonia was stirred in a Schlenk flask and
maintained between −70 and −60 °C by cooling in a dry ice/
isopropanol bath. Solid sodium metal was added in small pieces,
allowing the deep blue color associated with the solvated electron to
quench before the addition of the next piece. Once the blue color
persisted for 15 min (between 6 and 9 equiv of sodium), the mixture
was quenched by the addition of excess NH4Cl. The liquid ammonia
was evaporated under a stream of nitrogen. The residue was dissolved
in water, cooled in an ice water bath and treated dropwise with
concentrated HCl until pH < 1. The resulting yellow suspension was
extracted with Et2O (2 × 75 mL). The combined organics were
washed with water and saturated brine, dried with Na2SO4, and the
solvent was removed under vacuum. The residual oil was passed
through a glass wool filter to remove a small amount of solids, using
petroleum ether to collect the last fractions of oil. Residual solvent was
removed under vacuum to yield the product as an air-stable, viscous,
yellow oil (3.20 g, 92%). 1H NMR (C6D6) δ/ppm: 1.99 (s, 6H, CH3),
2.95 (s, 2H, SH), 6.19 (s, 1H, NH), 6.70 (d, J = 8.0 Hz, 2H, aryl−H),
6.84 (d, J = 8.0 Hz, 2H, aryl−H), 7.09 (s, 2H, aryl−H). 13C{1H} NMR
(C6D6) δ/ppm: δ 20.42 (CH3), 119.11 (aryl C), 120.50 (aryl−C),
128.86 (aryl−C), 131.64 (aryl−C), 133.87 (aryl−C), 140.58 (aryl C).
and refined on F2 by full-matrix least-squares techniques. Analytical
scattering factors for neutral atoms were used throughout the
analyses.35 Hydrogen atoms were included using a standard riding
model. ORTEP diagrams were generated using ORTEP-3 for
Windows.36 Diffraction data for W[SNS]2 and W[ONO]2 are given
in Table 1. Least-squared planes, trigonal twist angles, and prismatic
compression values given in Table 3 were calculated using Mercury
3.0.
Table 1. X-ray Diffraction Data Collection and Refinement
Parameters for W[SNS]2 and W[ONO]2
W[SNS]2·C7H8
W[ONO]2
empirical formula
formula weight
crystal system
space group
a/Å
C35H32N2S4W
792.72
C56H80N2O4W
1029.07
monoclinic
P21/c
triclinic
P1
̅
19.7206(6)
10.6756(3)
15.2817(5)
90
11.6164(7)
12.0666(7)
20.5721(12)
82.7281(6)
73.9437(6)
76.1399(6)
2684.8(3)
2
b/Å
c/Å
α/deg
β/deg
107.2028(3)
90
IR (KBr) υmax/cm−1: 3340 (N−H), 2525 (S−H). MS (ESI+) m/z:
̅
γ/deg
V/Å3
262.0 (MH+, 42%).
3073.32(16)
4
Synthesis of W[SNS]2. A dark blue suspension of WCl6 (162 mg,
408 μmol, 1 equiv) in 10 mL of toluene was treated with a yellow
solution of [SNScat]H3 (219 mg, 837 μmol, 2.05 equiv) in 16 mL of
toluene. The resulting dark maroon solution was heated to reflux for
8.5 h. After solvent removal under reduced pressure, the crude product
was redissolved in 2 mL of toluene, and 10 mL of pentane was added
to induce precipitation of the product. The solid was removed by
filtration, washed with 3 × 2 mL of pentane, and dried under reduced
pressure to provide the product as a purplish-brown solid in 65% yield
(186.1 mg). 1H NMR (C6D6) δ/ppm: 2.14 (s, 12H, −Me), 6.59 (ddd,
3JHH = 8.7 Hz, 4JHH = 1.8 Hz, 4JHH = 0.5 Hz, 4H, aryl−H), 7.07 (br s,
Z
refl. collected
indep. refl.
36891
31079
7748
12287
a
R1 (I > 2σ)
0.0151
0.0203
b
wR2 (all data)
0.0351
0.0511
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = {∑[w(Fo − Fc )2]/
2
∑[w(Fo )2]}1/2
.
RESULTS
3
4H, aryl−H), 7.43 (d, JHH = 8.6 Hz, 4H, aryl−H). 13C{1H} NMR
■
Synthesis of [SNScat]H3, W[SNS]2, and W[ONO]2. The
(C6D6) δ/ppm: 20.3 (−Me), 121.3 (aryl−C), 126.5 (aryl−C), 129.3
(aryl−C), 137.3 (aryl−C), 152.0 (aryl−C), 154.2 (aryl−C). APCI-MS
(toluene) m/z: 697.9 ([M]+), 698.9 ([M+H]+). UV−vis (CH2Cl2)
λmax/nm (ε/M−1 cm−1): 461 (19,300), 766 (3,620).
tridentate ligand precursor bis(2-mercapto-p-tolyl)amine,
[SNScat]H3, was prepared in four scalable synthetic steps
from commercially available di(p-tolyl)amine (Scheme 1).
Synthesis of W[ONO]2. A dark blue suspension of WCl6 (111 mg,
279 μmol, 1 equiv) in 10 mL of toluene was treated with a colorless
solution of [ONOcat]H3 (249 mg, 586 μmol, 2.1 equiv) in 12 mL of
toluene. The resulting dark orange solution was stirred for 7 h at room
temperature then heated to reflux for 16 h. The solvent was removed
under reduced pressure. The crude product was redissolved in 5 mL of
toluene and 10 mL of MeCN was added to effect precipitation of the
product. The solid was removed by filtration, washed with MeCN (2 ×
2 mL) and cold Et2O (2 × 2 mL), and dried to provide the product as
an orange-brown solid in 74% yield (211.8 mg). Anal. Calcd. (Found)
for C56H80N2O4W (%): C, 65.36 (65.15); H, 7.84 (7.80); N, 2.72
Scheme 1
1
(2.58). H NMR (C6D6) δ/ppm: 1.28 (s, 18H, −tBu), 1.40 (s, 18H,
4
4
−tBu), 6.92 (d, JHH = 1.7 Hz, 4H, aryl−H), 7.41 (d, JHH = 1.8 Hz,
4H, aryl−H). 13C{1H} NMR (C6D6, 125.8 MHz) δ/ppm: 30.1
(−tBu), 32.1 (−tBu), 35.2 (−tBu), 35.4 (−tBu), 121.6 (aryl−C),
113.8 (aryl−C), 136.9 (aryl−C), 139.3 (aryl−C), 146.2 (aryl−C),
162.9 (aryl−C). APCI-MS (toluene) m/z: 1026.6 ([M]+), 1027.6 ([M
+H]+). UV−vis (CH2Cl2) λmax/nm (ε/M−1 cm−1): 286 (38,600), 378
(25,200), 420 (29,700).
Crystallographic Methods. X-ray diffraction data were collected
on single crystals of W[SNS]2 and W[ONO]2 mounted on glass fibers
using a Bruker CCD platform diffractometer equipped with a CCD
detector. Measurements were carried out at 163 K using Mo Kα (λ =
0.71073 Å) radiation, which was wavelength selected with a single-
crystal graphite monochromator. The SMART program package was
used to determine unit-cell parameters and to collect data. The raw
frame data were processed using SAINT and SADABS to yield the
reflection data files. Subsequent calculations were carried out using the
SHELXTL program suite. Structures were solved by direct methods
Bromination of the diarylamine derivative was achieved
according to published procedures;34 however, the product,
as-isolated, required an additional purification step to be carried
through the rest of the synthesis. Benzyl protection of the
central nitrogen allowed the dibromide to be lithiated with
nBuLi and subsequently oxidized with dimethyldisulfide to
install the sulfur donors as methyl sulfide groups in N-benzyl-
bis(2-methanethio-p-tolyl)amine. Simultaneous deprotection of
both the benzylated nitrogen and the methylated sulfur atoms
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Inorganic Chemistry
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was achieved using Birch conditions37 to afford the [SNScat]H3
product in excellent yield as a yellow, viscous oil.
Metalation of the [SNScat]H3 ligand was readily achieved
using tungsten hexachloride (eq 1). The addition of yellow
[SNScat]H3 to a dark blue suspension of WCl6 in toluene
resulted in an immediate color change to dark maroon. To
drive the reaction to completion via elimination of 6 equiv of
HCl, the reaction mixture was heated to reflux for several hours.
Shorter reaction times or reactions carried out at room
temperature resulted in the sustained presence of protonated
ligand resonances in the 1H NMR spectrum indicative of
incomplete product formation. The desired product, W[SNS]2,
was isolated as a purplish-brown solid from a mixture of toluene
and pentane in 65% yield.
Figure 1. ORTEP diagram of W[SNS]2. Ellipsoids are shown at 50%
probability. Hydrogen atoms and a toluene molecule have been
omitted for clarity. Inset: Core structure of W[SNS]2 viewed along
N(1)−W(1)−N(2).
Δ
[SNScat]H3 + WCl6 ⎯⎯⎯⎯⎯⎯→ W[SNS]2 + 6HCl
(1)
toluene
The new tungsten complex was identified to be the desired
homoleptic W[SNS]2 complex by a battery of analytical
techniques. The APCI mass spectrum of W[SNS]2 displayed
overlapping peaks corresponding to [M]+ and [M+H]+ at 697.9
and 698.9 amu, respectively, with each signal displaying the
expected isotopic distributions. 1H and 13C NMR spectra
indicated D2d symmetry for the complex in solution. A single
Table 2. Selected Bond Distances (Å) and Angles (deg) for
W[ENE]2 (E = S, O)
W[SNS]2
W[ONO]2
Bond Distances/Å
2.3942(4)
2.3718(4)
2.3296(4)
2.3732(4)
2.0592(14)
2.0843(13)
1.7310(17)
1.7545(17)
1.7463(17)
1.7385(18)
1.396(2)
W(1)−E(1)
W(1)−E(2)
W(1)−E(3)
W(1)−E(4)
W(1)−N(1)
W(1)−N(2)
E(1)−C(2)
E(2)−C(16/9)
E(3)−C(30/16)
E(4)−C(44/23)
N(1)−C(1)
N(1)−C(15/8)
N(2)−C(29/15)
N(2)−C(43/22)
C(1)−C(2)
1.9057(13)
1.9061(13)
1.9123(13)
1.9130(13)
2.0449(15)
2.0380(15)
1.363(2)
1.365(2)
1.363(2)
1.365(2)
1.416(2)
1.415(3)
1.413(2)
1.413(2)
1.400(3)
1.402(3)
1.401(3)
1.401(2)
1
methyl resonance in the H NMR spectrum at 2.14 ppm was
accompanied by three aryl proton resonances. The 13C NMR
spectrum showed the methyl carbon and six unique aryl carbon
resonances in the 120−155 ppm region. In square-planar
complexes of platinum(II) and palladium(II) containing the
semiquinonate form of aminothiophenol ligands, M(1,2-
C6H6SNR)2, 13C NMR resonances were shifted to higher
frequency, in the region 166−170 ppm.38
Similar to the preparation of W[SNS]2, the bis(phenolate)
derivative, W[ONO]2, was prepared from tungsten hexa-
chloride and [ONOcat]H3 (eq 2). The bis[ONO] tungsten
complex was isolated as an orange-brown solid in 74% yield
from toluene upon addition of acetonitrile. The APCI mass
spectrum of W[ONO]2 displayed [M]+ and [M+H]+ peaks at
1026.7 and 1027.7 amu, respectively. Similar to W[SNS]2, the
1.418(2)
1.415(2)
1.410(2)
1.410(2)
C(15/8)−C(16/9)
C(29/15)−C(30/16)
C(43/22)−C(44/23)
1.404(2)
1
1.396(2)
W[ONO]2 derivative displayed D2d symmetry in both the H
and 13C NMR spectra. Notably, the C−O carbon of W[ONO]2
resonated at 162.9 ppm in the 13C NMR spectrum, consistent
with the resonance for that carbon in complexes of the fully
reduced [ONOcat]3− ligand.39−41
1.405(2)
Bond Angles/deg
78.76(4)
N(1)−W(1)−E(1)
N(1)−W(1)−E(2)
N(2)−W(1)−E(3)
N(2)−W(1)−E(4)
N(1)−W(1)−N(2)
E(1)−W(1)−E(2)
E(3)−W(1)−E(4)
76.55(6)
76.54(6)
76.37(6)
76.53(6)
175.81(6)
152.82(6)
152.87(6)
79.41(4)
80.09(4)
Δ
[ONOcat]H3 + WCl6 ⎯⎯⎯⎯⎯⎯→ W[ONO]2 + 6HCl
77.82(4)
(2)
toluene
155.27(5)
145.170(16)
152.646(15)
Structures of W[SNS]2 and W[ONO]2. Single-crystal X-ray
diffraction was used to establish the molecular structure of
W[SNS]2. X-ray quality single crystals of W[SNS]2 were grown
from a saturated toluene/acetonitrile solution of the complex
chilled to −35 °C. The complex crystallized in the monoclinic
spacegroup P21/c, and the asymmetric unit contained one
cocrystallized toluene molecule. Figure 1 shows the molecular
structure of W[SNS]2 and Table 2 contains selected metrical
parameters for the structure.
Neither [SNS] ligand in six-coordinate W[SNS]2 can be
described as planar. The nitrogen atom in each [SNS] ligand
displays planar, sp2 hybridization (359° sum of angles around
N); however, the ligand aryl rings are not coplanar. Defining
the aryl rings of the [SNS] ligands as Πn according to the
attached sulfur atom, S(n) (see Figure 2a), the intraligand
angles between the aryl rings are ∠(Π1−N(1)−Π2) = 46° and
Figure 2. Diagrams of metrical parameters calculated for W[SNS]2 (E
= S) and W[ONO]2 (E = O).
∠(Π3−N(2)−Π4) = 38°. As shown in the inset of Figure 1,
these distortions from planarity within the [SNS] ligand
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manifest a distorted geometry for the tungsten center. The
N(1)−W−N(2) bond angle of 155° is significantly smaller than
that expected for an octahedral complex. Intraligand S−W−S
angles are 145° and 153° while interligand S−W−S angles vary
from 80° to 105°. It should be noted that while interligand S−S
bond formation has been observed in complexes with ONS
chelating ligands,24 in the case of W[SNS]2, all S···S distances
are greater than 3.05 Å.
The overall tungsten geometry in W[SNS]2 is in between
trigonal prismatic and trigonal antiprismatic (pseudo-octahe-
dral).42 The primary measure of prismatic versus antiprismatic
geometry is the twist angle (θ) between opposite trigonal faces
(Figure 2b). In W[SNS]2, the two trigonal planes can be
defined by the nitrogen and one sulfur atom from one [SNS]
ligand and a sulfur atom from the other ligand (N(1)−S(1)−
S(3) and N(2)−S(2)−S(4)), and the twist angle is then
defined as the torsion angle between the vertices of the trigonal
planes and the centroids of those planes. In an ideal trigonal
prism, θ = 0° whereas in an antiprism, θ = 60°.42 A second
metrical parameter used to differentiate trigonal prismatic and
trigonal antiprismatic geometries is the prismatic compression,
defined as the ratio of the prism height, h, to the (average)
length of the trigonal face, s. For an ideal trigonal prism s/h =
1.00, whereas in an ideal octahedron s/h = 1.22.42 Table 3
the aryl rings in each W[SNS]2 molecule are in close contact
(<3.7 Å) with nearby aryl rings on adjacent molecules, setting
up π−π stacking interactions.43,44 The Π1 and Π4 rings stack at
a distance of 3.64 Å from adjacent Π4 and Π1 rings, respectively,
whereas the Π3 ring stacks at a distance of 3.53 Å from an
adjacent Π3 ring. Only the Π2 ring lacks intermolecular π-
stacking interactions.
The bond distances for W[SNS]2 are consistent with a
partially oxidized [SNS] ligand. The average W−S distance of
2.37 Å is similar to those observed in W(pdt)3 and [W(pdt)3]−
(pdt2− = 1,2-diphenyl-1,2-dithiolate), which were recently
described as WV and WIV complexes, respectively, with partially
oxidized dithiolene ligands.45 The average C−N and C−S
distances in W[SNS]2 are 1.41 and 1.74 Å, respectively. In
bidentate ortho-aminothiophenolate ligands, the catecholate
oxidation state is characterized by C−N and C−S distances of
1.41 and 1.76 Å, respectively, whereas the semiquinonate
oxidation state is characterized by C−N and C−S distances of
1.36 and 1.72 Å, respectively.24,38,46,47 Oxidized ortho-amino-
thiophenolate ligands also display partial localization of double-
bond character within the aryl rings with differences between
the longest and shortest C−C bonds, (ΔC−C)max, of about
0.06 Å. In W[SNS]2, partial double bond localization is evident
from (ΔC−C)max values ranging from 0.015 to 0.04 Å for the
four aryl rings.
In contrast to the structure of W[SNS]2, the single-crystal X-
ray diffraction analysis of W[ONO]2 revealed a pseudo-
octahedral environment around the tungsten center with nearly
planar [ONO] ligands. The molecular structure of W[ONO]2
is shown in Figure 4, and the corresponding metrical
Table 3. Additional Measured and Calculated Metrical
Parameters for W[SNS]2 and W[ONO]2, As Defined in
a
Figure 2
W[SNS]2·C7H8
W[ONO]2
∠ Π1−N(1)−Π2
∠ Π3−N(2)−Π4
θE(1), E(4)
46.5
38.0
10.2
27.4
33.4
1.11
6.0
10.6
35.3
47.1
45.2
1.23
θN(1),E(2)
θE(3), N(2)
s/h
a
The plane defined by the six carbon atoms of each (thio)phenolate
moiety are indicated by Πn, where “n” is the number of the attached
atom E (E = S, O).
summarizes the twist angles and prismatic compression values
for W[SNS]2 as determined from the single crystal X-ray data.
Both parameters fall midway between the expected values for a
trigonal prism and a trigonal antiprism.
Examination of the extended solid state structure of
W[SNS]2 revealed intermolecular contacts between the aryl
groups of the [SNS] ligand, as illustrated in Figure 3. Three of
Figure 4. ORTEP diagram of W[ONO]2. Ellipsoids are shown at 50%
probability. Hydrogen atoms have been omitted for clarity. Inset: Core
structure of W[ONO]2 viewed along N(1)−W(1)−N(2).
parameters for W[ONO]2 are provided in Tables 2 and 3.
Single crystals of W[ONO]2 were grown by evaporation of
solvent from a saturated CH2Cl2 solution of the complex. The
geometry around the tungsten center of W[ONO]2 is
consistent with a trigonal antiprism with the two [ONO]
ligands perpendicular to one another given an average
interligand O−W−O bond angle of 93.15° and an N(1)−
W−N(2) angle of 176°. The meridional binding of the [ONO]
ligands give rise to nearly coplanar intraligand aryl rings. As
Figure 3. Left: Diagram highlighting the π−π interactions in the solid
state structure of W[SNS]2. Right: The unit cell of W[SNS]2.
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shown in Table 3, the Π1−N(1)−Π2 and Π3−N(2)−Π4 angles
are only 6° and 11°, respectively. Trigonal twist angles for
W[ONO]2 are larger than those measured for W[SNS]2 (Table
3); however, the limited bite angle of the [ONO] ligand
platform still keeps these values shy of the ideal value for a
trigonal antiprism. Nevertheless, a prismatic compression value
of 1.23 is consistent with an antiprismatic structure.
Ligand oxidation states for W[ONO]2 are readily assigned
from the high-resolution X-ray data. The average tungsten−
oxygen distance of 1.91 Å is comparable to those in octahedral
W(O-p-tolyl)6 (W−Oave = 1.90 Å) and W(O2C2Me4)3 (W−
O
ave = 1.91 Å).48,49 Within the [ONO] ligands, C−N and C−O
distances of 1.41 and 1.36 Å, respectively, are entirely
consistent with the C−N and C−O distances observed in
[ONOcat]TaVCl2(OEt2) and [ONOcat]ZrIVCl(THF)2 com-
plexes.1,50 The aryl rings of the ligand show no signs of
localized double-bonds, which would be expected for an
oxidized ligand.
Spectroscopic and Electrochemical properties of
W[SNS]2 and W[ONO]2. The electronic absorption spectra
of W[SNS]2 and W[ONO]2, shown in Figure 5, are dominated
Figure 6. Cyclic voltammograms of W[SNS]2 and W[ONO]2 in (a)
THF and (b) CH2Cl2. Measurements were made under N2 using a
scan rate of 200 mV s−1 on 1.0 mM analyte solutions containing 0.10
M [Bu4N][PF6] electrolyte. Potentials were referenced to [Cp2Fe]+/0
using an internal standard.
tion. The electrochemical behavior of the complexes was similar
in THF, MeCN, and CH2Cl2. While the oxidations of both
W[SNS]2 and W[ONO]2 were best observed in CH2Cl2, the
second reduction of W[ONO]2 was too negative to be
observed in the CH2Cl2 solvent window. Thus, the cathodic
portion of the cyclic voltammograms of Figure 6 were acquired
in THF while the anodic portion of the cyclic voltammograms
of Figure 6 were acquired in CH2Cl2. The measured potentials
for W[SNS]2 and W[ONO]2, referenced to [Cp2Fe]+/0 using
an internal standard, are listed in Table 4. The W[SNS]2
a
Table 4. Electrochemical Data for W[SNS]2 and W[ONO]2
E1°′
E2°′
E3,pa°′
E4,pa°′
c
b
b
c
([M]1−/2−
)
([M]0/1−
)
([M]1+/0
)
([M]2+/1+
)
W[SNS]2
−1.55
−2.77
−0.61
−1.38
0.70
0.63
1.05
1.16
W[ONO]2
Figure 5. UV−vis absorption spectra for W[SNS]2 and W[ONO]2 in
CH2Cl2.
a
Potentials were referenced to [Cp2Fe]+/0 using an internal standard.
1.0 mM analyte in THF containing 0.1 M [Bu4N][PF6]. 1.0 mM
b
c
analyte in CH2Cl2 containing 0.1 M [Bu4N][PF6].
by intense transitions in the UV−visible region of the spectrum.
The dominant absorption of W[SNS]2 is centered at 461 nm (ε
= 19,300 M−1 cm−1) with shoulders on both sides of the
maximum. A lower intensity transition can be seen at 766 nm
(ε = 3,620 M−1 cm−1). These absorption energies and
intensities are similar to those observed for tris(dithiolene)
complexes of tungsten such as W[bdt]3 and W[tbbdt]3 (bdt2−
= benzene-1,2-dithiolate; tbbdt2− = 3,5-di-tert-butylbenzene-
1,2-dithiolate).45,51 The W[ONO]2 derivative has better
resolved optical features with a sharp, high-energy absorption
at 286 nm (ε = 45,100 M−1 cm−1) and two overlapping
absorptions at 378 and 420 nm (ε = 25,200 and 29,700 M−1
cm−1, respectively). The electronic absorption spectrum of
W[ONO]2 is quite different than the spectra reported for other
homoleptic, transition-metal complexes, M[ONO]2 (M = Ti,
Fe, Mn, Co, Ni, Cu), which typically contain one or more
oxidized [ONO] ligands and display intense absorptions in the
700−1000 nm wavelength region.9,11,12
complex displays two reductive and two oxidative processes.
Based on the relative intensities for these signals, all four
processes appear to be single electron in nature. While the
reductive processes for W[SNS]2 appear to be reversible (ipa/ipc
≃ 1.0), the oxidative processes do not display significant
reversible character. The two reductions of W[SNS]2 are
separated by 0.94 V.
The electrochemical data for W[ONO]2 is significantly
different from that recorded for the sulfur homologue.
W[ONO]2 displays two reversible reductions that are shifted
cathodically relative to the reductions in W[SNS]2; moreover,
the two reductions are separated by almost 1.4 V. In the anodic
portion of the voltammogram, W[ONO]2 displays two
oxidative processes that are at least partially reversible. It is
noteworthy that comparison of the peak intensities of the
reductive and oxidative processes suggest that the oxidations
may be two-electron in nature. This proposal is qualitatively
supported by the plateaued cathodic portion of the first
oxidation of W[ONO]2, which shows two features at slow scan
rates, and may be indicative of partial resolution of two one-
electron processes.
Cyclic voltammetry studies of W[SNS]2 and W[ONO]2
revealed multiple one-electron reductive and oxidative
processes. Figure 6 shows cyclic voltammogram traces for
W[SNS]2 and W[ONO]2; the scan rate dependencies of the
cyclic voltammetry data are provided as Supporting Informa-
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chemistry of W[ONO]2 shows two one-electron reductions at
very negative potentials and with a large peak-to-peak
separation. These data are consistent with sequential reduction
of a tungsten(VI) center first to tungsten(V) and then to
tungsten(IV).59,60 Similarly negative first and second reduction
potentials, with large peak-to-peak separations, have been
observed for homoleptic aryloxides and catecholate complexes
of tungsten. On the oxidative side, integration of the oxidative
process at +0.63 V suggests that it is a two-electron process.
Given a WVI[ONOcat]2 electron configuration and an
orthogonal arrangement of the [ONO] ligands, a two-electron
oxidation could arise from coincident one-electron oxidations
of each [ONO] ligand. It is worth noting that such an
DISCUSSION
■
The new redox-active pincer-type ligand precursor, [SNS]H3, is
readily accessible from commercially available diphenylamine
derivatives. Installation of the sulfur donor atoms relied on an
initial arene oxidation ortho to the amine functionality followed
by lithiation and subsequent oxidation with dimethyldisulfide.
While the synthetic route required four steps, including
protection of the amine and deprotection of the amino and
thiol groups, the steps proceeed in high yields making the
synthetic procedure viable for the preparation of the ligand in
gram quantities. Furthermore, the procedure should work for
other diphenylamine derivatives with substituents in the 4 and
4′ position to guard against para bromination in the first
reaction step.
+
assignment implies that the cation W[ONO]2 would be a
Robin−Day Class I mixed-valence complex,56,61 but more
detailed electrochemical and spectroscopic analyses are
required to confirm such a characterization.
The homoleptic W[SNS]2 and W[ONO]2 complexes were
readily accessed by the reaction of appropriate ligand with
WCl6 to extrude HCl. H NMR experiments indicated that
1
Structural and electrochemical data for W[SNS]2 suggest a
subtly different electron distribution from that of W[ONO]2. In
W[SNS]2, better spatial and energetic overlap between the
tungsten d orbitals and sulfur p lone-pair orbitals would lead to
a more pronounced π interaction between the metal and [SNS]
ligand. In the limit of purely covalent π bonding between the
tungsten center and the [SNS] ligand, the experimental metal
and ligand oxidation states would be best described as
WIV[SNSsq]2. It is important to note that this oxidation state
assignment does not require any radical (or diradical) character
in W[SNS]2. Instead, it is only meant to imply equal sharing
between the tungsten center and the two [SNS] ligands of two
electron pairs in two perpendicular π orbitals. This electronic
structure assignment is consistent with studies of tungsten
tris(dithiolene) complexes, which have been assigned as WIV or
WV species coordinated by a mixture of dianionic and
monoanionic dithiolenes.23,45,62−65 While the structural differ-
ences observed for W[SNS]2 are too small for an unequivocal
assignment of metal and ligand oxidation states, especially given
the irregular solid-state geometry of the complex, the W−S and
S−C bond distances are consistent with a partially oxidized
[SNS] ligand. Similarly, while the electrochemistry of W[SNS]2
is superficially similar to that of W[ONO]2, subtle differences
suggest a different frontier electronic configuration for the
sulfur derivative. Two reversible, one-electron reductions are
observed for W[SNS]2, but they occur at significantly milder
potentials than the reductions observed for W[ONO]2, and
they are separated by only 0.94 V. This result is surprising since
simple electronegativity arguments would suggest that the
oxygen-containing, [ONO] derivative would be easier to reduce
than the sulfur-containing, [SNS] derivative. Within the model
of a WIV[SNSsq]2 electron configuration, each reduction would
add an electron to a molecular orbital delocalized over the
tungsten metal and [SNS] ligand; therefore, electron−electron
repulsion would be smaller owing to the delocalized nature of
these orbitals leading to less negative reduction potentials. For
the oxidation of W[SNS]2, the electrochemical processes are
less well-resolved, and less reversible than for W[ONO]2, but
there is no indication that they are two-electron processes.
Similarly, oxidations of tris(dithiolene) complexes of tungsten
are also irreversible, which has been attributed to unprotected
sulfur atoms with significant radical character.51 Within the
WIV[SNSsq]2 model, the oxidations would again come from
molecular orbitals delocalized over the tungsten metal and
[SNS] ligands, resulting in significant radical character on the
sulfur atoms.
relatively long reaction times (hours) and relatively high
temperatures (refluxing toluene) were required to drive the
reaction to completion. Under milder conditions, unidentified
tungsten products were observed including species with a
nitrogen-protonated [SNHS] ligand.
While W[ONO]2 displayed a regular trigonal antiprismatic
structure in the solid state, the structure of W[SNS]2 proved to
be an irregular six-coordinate species that is intermediate
between trigonal prismatic and trigonal antiprismatic geo-
metries. The reason for the distorted geometry in W[SNS]2 is
unclear. It could be attributed to packing effects stemming from
the π−π stacking interactions observed in the solid-state
structure. Alternatively, it is well-known that homoleptic
tungsten thiolate complexes show a range of six-coordinate
geometries.23 Thus, the observed geometry of W[SNS]2 may
be a product of competing electronic factors that favor a
trigonal prismatic structure23 and steric limitations of a planar
[SNS] ligand that favor an antiprismatic structure. While it
might be expected that a partially oxidized [SNS] ligand should
favor a planar ligand geometry, previous studies have shown
that oxidation of thiolate ligands occurs primarily at the sulfur
atom, with decreased delocalization of the resultant hole
through the aromatic ring system.52 Another important caveat
to rationalizing the solid-state structure is that on the NMR
time scale there is no evidence for the unsymmetrical structure
of W[SNS]2 in solution. This result could result from either a
more symmetrical and regular structure in solution (i.e., loss of
π−π stacking interactions), or a fluxional coordination
environment that averages inequivalent chemical positions.
Of particular interest here is a comparison of the electronic
properties of the [SNS] and [ONO] ligand platforms,
particularly their redox activity. In this respect, the two
homoleptic complexes W[SNS]2 and W[ONO]2 provide a
case study of the two ligand platforms.
The homoleptic W[ONO]2 complex is best described as a
WVI[ONOcat]2 complex. The single crystal X-ray diffraction
data for W[ONO]2 shows W−O, O−C, and N−C bond
distances that are consistent with a tungsten(VI) metal center
coordinated to [ONO] ligands in the fully reduced, catecholate
oxidation state. Furthermore, C−C bond distances within the
[ONO] ligand show no evidence for the cyclohexadiene
character manifested with ligand oxidation. In contrast, other
homoleptic M[ONO]2 complexes reported in the literature
have been assigned to contain some combination of
[ONOsq● 2− and [ONOq]− ligands.8−10,12,15,53−58 The electro-
]
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(12) Larsen, S. K.; Pierpont, C. G. J. Am. Chem. Soc. 1988, 110,
1827−1832.
SUMMARY
■
The new amidedithiolate ligand, [SNScat]3−, offers a redox-
active ligand platform for the preparation of coordination
complexes. Structural and electrochemical studies suggest that
the [SNS] platform maintains similar redox properties to the
well-established [ONO] ligand, but whereas the hard oxygen
and nitrogen donors of [ONO] favor a WVI[ONOcat]2
electronic structure, the softer sulfur donor atoms of the
[SNS] ligand seem to increase metal−ligand covalency in the π-
bonding manifold, increasing the WIV[SNSsq]2 character of the
complex. Because the [SNS] ligand is readily prepared in good
yields, and since metalation is straightforward, the [SNS] ligand
should receive considerable attention for the preparation of
new coordination complexes in which ligand-based redox-
activity, meridional binding, and softer donor atoms are
desirable. Given the importance of sulfur-based ligands in
metalloenzymes, the tridentate [SNS] ligand offers a new
platform for pursuing reactivity models for various hydro-
genase, reductase, and oxygen-atom transfer enzymes.
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ASSOCIATED CONTENT
■
Chem. 2009, 48, 3783−3791.
S
* Supporting Information
(25) Hubner, R.; Weber, S.; Strobel, S.; Sarkar, B.; Zal
́
is,
̌
S.; Kaim, W.
̈
X-ray crystallographic data for W[SNS]2·C6H5Me and W-
Organometallics 2011, 30, 1414−1418.
[ONO]2 (CIF format); H and 13C NMR data for W[SNS]2
1
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and W[ONO]2, and full cyclic voltammograms of W[SNS]2
and W[ONO]2 in both THF and CH2Cl2 (PDF format). This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
(30) Xiao, J.; Deng, L. Organometallics 2011, 31, 428−434.
(31) Harkins, S. B.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 2885−
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Corresponding Author
*E-mail: aheyduk@uci.edu.
Notes
(32) Hahn, R.; Nakamura, A.; Tanaka, K.; Nakayama, Y. Inorg. Chem.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The authors acknowledge the National Science foundation for
suppoting this work (CHE-1152543). A.F.H. is a Camille
Dreyfus Teacher-Scholar.
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