Synthesis, redox properties, and X-ray diffraction structure of 2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide

Article abstract of DOI:10.1007/s10870-004-7652-1

Refluxing 2,3-dichloromaleic anhydride with p-anisidine in benzene with water removal gives the condensation product 2,3-dichloro-N-(p-MeOC ₆H₄)maleimide (1) 75% yield. This new maleimide compound reacts with added ethanethiol in the

Full text of DOI:10.1007/s10870-004-7652-1

Journal of Chemical Crystallography, Vol. 34, No. 11, November 2004 (ꢀC 2004)
Synthesis, redox properties, and X-ray diffraction structure
of 2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide
William H. Watson,(1)∗ Tao Chen,(2) and Michael G. Richmond(2)∗
Received August 4, 2003
Refluxing 2,3-dichloromaleic anhydride with p-anisidine in benzene with water removal
gives the condensation product 2,3-dichloro-N-(p-MeOC6H4)maleimide (1) 75% yield.
This new maleimide compound reacts with added ethanethiol in the presence of Et3N or
DBU to furnish the bidentate sulfide ligand 2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide
(2) in 85% yield. Each product has been characterized in solution by IR, NMR,
and UV-vis spectroscopies, and the solid-state structure of 2,3-bis(ethylthio)-N-(p-
MeOC6H4)maleimide was unequivocally established by single-crystal X-ray diffraction
analysis. 2,3-bis(Ethylthio)-N-(p-MeOC6H4)maleimide crystallizes in the monoclinic
◦
˚
˚
˚
space group C2/c, a = 20.035(3) A, b = 9.188(1) A, c = 16.887(2) A, β = 93.696(2) ,
V = 3102.3(8) A , Z = 8, and Dcalcd = 1.385 mg/m3; R = 0.0268, Rw = 0.0676 for
3
˚
2025 reflections with I > 2σ(I). The nature of the LUMO in 2,3-bis(ethylthio)-N-(p-
MeOC6H4)maleimide (2) has been determined by extended Hu¨ckel molecular orbital cal-
culations, and these data are discussed relative to the cyclic voltammetry results and other
structurally relevant compounds prepared in our labs.
KEY WORDS: Maleimide heterocycle; bidentate sulfide; cyclic voltammetry; extended Hu¨ckel MO calculations.
due to the availability of a low-lying LUMO,2,3
Introduction
whose nodal properties approximate that of ψ4 of
The compounds 2,3-dichloromaleic anhy-
common 6π electron systems such as maleic an-
hydride and hexatriene,4 coupled with facile P C
bond cleavage sequences that are often triggered
upon the coordination of these ligands to a variety
of mono- and polynuclear metal complexes.5
Recently, our groups have embarked on the
synthesis of disulfide compounds derived from
2,3-dichloromaleic anhydride and 4,5-dichloro-
4-cyclopenten-1,3-dione.6,7 Three such bidentate
disulfide compounds prepared by our groups are
shown subsequently, and one of the more inter-
esting aspects that has emerged from our studies
of these disulfides is the quasi-reversible one-
electron reduction found for 4,5-bis(p-tolylthio)-
4-cyclopenten-1,3-dione. The quasi-reversible
dride and 4,5-dichloro-4-cyclopenten-1,3-dione
serve as starting materials for the redox-
active
diphosphine
complexes
2,3-bis-
(diphenylphosphino)maleic anhydride (bma)
and 4,5-bis(diphenylphosphino)-4-cyclopenten-
1,3-dione (bpcd).1 The interest and continued
investigation of these diphosphine ligands derive
from their ability to function as electron reservoirs
(1) Department of Chemistry, Texas Christian University, Fort Worth,
Texas 76129.
(2) Department of Chemistry, University of North Texas, Denton,
Texas 76203.
∗
To whom correspondence should be addressed at; e-mails:
w.watson@tcu.edu; cobalt@unt.edu.
765
ꢀ
C
1074-1542/04/1100-0765/0 2004 Springer Science+Business Media, Inc.

766
Watson, Chen, and Richmond
nature of the 0/1− redox couple in 4,5-bis(p-
tolylthio)-4-cyclopenten-1,3-dione was totally
unexpected, as the site of electron accession in this
and related complexes is associated with the π∗
of the dione moiety, which has typically afforded
stable reduction products. However, the kinetic in-
stability of the radical anion relative to the known
diphosphine and other disulfide derivatives is due
to the cleavage of the C(dione) S(p-tolyl) bond
and release of the stable p-tolylthiolate anion.
(CH2Cl2) and NMR (CDCl3) solvents used in
these studies were distilled from P2O5. The
above solvents were distilled under argon using
standard inert atmosphere techniques and were
stored in Schlenk storage vessels under argon.8
All other solvents were of reagent grade and
used as received. The tetra-n-butylammonium
perchlorate (TBAP) was purchased from Johnson
Matthey Electronics and recrystallized from ethyl
acetate/hexane, followed by drying under vacuum
for at least 48 h before use. The reported melting
points were recorded on a Meltemp apparatus
and are uncorrected.
Routine infrared spectra were recorded on a
Nicolet 20 SXB FT-IR spectrometer in 0.1 mm
NaCl cells, using PC control and OMNIC soft-
ware, while the 1H NMR spectrum was recorded
at 200 MHz on a Varian Gemini-200 spectrometer.
The UV-vis spectrum was collected on a Hewlett-
Packard 8425A diode array spectrophotometer in
a 1.0 cm quartz cell.
Herein, we report our data on the syn-
thesis and solution characterization of the new
bidentate sulfide ligand 2,3-bis(ethylthio)-N-
(p-MeOC6H4)maleimide (2). The solid-state
structure was established by X-ray crystallogra-
phy and the results of extended Hu¨ckel molecular
orbital calculations reveal that the LUMO in 2,3-
bis(ethylthio)-N-(p-MeOC6H4)maleimide (2) is
essentially identical to that found in other such 6π
electron systems. The electrochemical properties
of 2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide
(2) have been studied by cyclic voltammetric
and double-potential step chronocoulometric
experiments.
Synthesis of 2,3-bis(ethylthio)-N-(p-MeOC6 H4)
maleimide
To 2.00 g (16.0 mmol) of p-anisidine in
60 mL of toluene was added 2.70 g (16.0 mmol) of
2,3-dichloromaleic anhydride, after which 50 mg
of the acid catalyst p-toluenesulfonic acid (PTSA)
was added. The reaction was refluxed at 110 ◦C
overnight, while the water was removed through
theuseofaDean-Starktrap. Afterthesolutionwas
allowed to cool to room temperature, the solvent
was removed and the initial product, 2,3-dichloro-
N-(p-MeOC6H4)maleimide (1), was obtained by
column chromatography over silica gel using
CH2Cl2/petroleum ether (60:30 mixture) as the
eluent. 2,3-Dichloro-N-(p-MeOC6H4)maleimide
(1): yield 3.20 g (75%). mp: 212–213 ◦C. IR
(CH2C12): ν(CO) 1739 (antisymm, maleimide)
Experimental section
General
1
The compounds p-anisidine, ethanethiol,
cm−1. H NMR (CDC13): δ 7.09 (aromatic AB
2,3-dichloromaleic
anhydride,
and
1,8-
quartet, JH–H = 9.0 Hz), 3.82 (MeO).
diazabicyclo[5.4.0]undec-7-ene (DBU) were
purchased from Aldrich Chem. Co. and were
used as received. The IR and cyclic voltammetry
Compound 1 was next employed in the
synthesis of 2,3-bis(ethylthio)-N-(p-MeOC6H4)-
maleimide (2). Here, 0.5 g (1.80 mmol) of

2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide
767
Proserpio,13 using weighted Hꢁijs. The input
Z matrix for 2,3-bis(thio)-maleimide was con-
structed upon a model possessing C2ν symmetry,
using bond distances and angles from the X-ray
diffraction coordinates for 2,3-bis(ethylthio)-N-
(p-MeOC6H4)maleimide. Here, the p-MeOC6H4
and the ethyl groups were replaced by hydrogen
atoms in order to simplify the calculations, with
2,3-dichloro-N-(p-MeOC6H4)maleimide
in
20 mL of CH2C12 was treated with 0.4 mL
(5.40 mmol) of ethanethiol, followed by the drop-
wise addition of 0.67 mL of DBU. The reaction
was stirred for 2 h at room temperature, at which
time TLC analysis confirmed the presence of the
desired disulfide product. 2,3-bis(ethylthio)-N-
(p-MeOC6H4)maleimide was isolated by column
chromatography using CH2Cl2/petroleum ether
(60:30 mixture) as the eluent. 2,3-bis(Ethylthio)-
N-(p-MeOC6H4)maleimide (2): yield 0.5 g
(85%). mp: 56–58 ◦C. IR (CH2Cl2): ν(CO)
1772 (w, symm, maleimide), 1715 (s, antisymm,
maleimide) cm−1. 1H NMR (CDCl3): δ 7.07
(aromatic AB quartet, JH–H = 8.5 Hz), 3.80 (s,
MeO), 3.34 (q, methylene), 1.35 (t, methyl).
UV-vis (CH2Cl2): λmax 421 (ꢀ = 2480), 309
(ꢀ = 2790), 250 (ꢀ = 3360).
the N H and S H bond lengths assigned to val-
14
˚
˚
ues of 1.05 A and 1.35 A, respectively.
Cyclic voltammetry data
The cyclic voltammetric and double-
potential step chronocoulometric studies were
recorded on a PAR Model 273 potentio-
stat/galvanostat, equipped with positive feedback
circuitry to compensate for iR drop. The air-
tight CV cell used was based on a three-electrode
design, and a platinum disk was employed as
the working and auxiliary electrodes. The refer-
ence electrode utilized a silver wire as a quasi-
reference electrode. All potential data reported
have been referenced to the formal potential of the
Cp2Fe/Cp2Fe+ (internally added) redox couple,
taken to have E1/2 = 0.307 V.15
X-ray diffraction structure of 2,3-bis(ethylthio)-
N-(p-MeOC6 H4)maleimide
Single crystals of 2,3-bis(ethylthio)-N-
(p-MeOC6H4)maleimide suitable for X-ray
crystallography were obtained from a CH2Cl2
solution containing 2,3-bis(ethylthio)-N-(p-
MeOC6H4)maleimide that had been layered with
hexane. X-ray data were collected on a Bruker
SMART 1000 CCD-based diffractometer at
213 K. The frames were integrated with the
available SAINT software package using a
narrow-frame algorithm,9 and the structure
was solved and refined using the SHELXTL
program package.10 The molecular structure was
checked by using PLATON.11 All nonhydrogen
atoms were refined anisotropically. Refinement
converged at R = 0.0268 and Rw = 0.0676 for
2025 independent reflections with I > 2σ(I).
Discussion
Synthesis, spectroscopic data, and
molecular structure
The p-toluenesulfonic acid-catalyzed re-
action between 2,3-dichloromaleic anhydride
and p-anisidine proceeds in toluene under
reflux, coupled with removal of water, to
furnish the corresponding condensation prod-
uct
2,3-dichloro-N-(p-MeOC6H4)maleimide.
2,3-Dichloro-N-(p-MeOC6H4)maleimide was
isolated as the sole isolable product by column
chromatography and was characterized in
solution. The IR spectrum of 2,3-dichloro-N-
(p-MeOC6H4)maleimide in CH2Cl2 exhibits a
strong IR stretch at 1739 cm−1, readily assignable
to antisymmetric carbonyl stretch belonging to
Extended Hu¨ckel MO calculations
The extended Hu¨ckel calculations on 2,3-
bis(ethylthio)-N-(p-MeOC6H4)maleimide were
conducted with the original program developed
by Hoffmann,12 as modified by Mealli and

768
Watson, Chen, and Richmond
the maleimide moiety.16 The expected higher
frequency symmetric counterpart of this latter
ν(CO) stretch is extremely weak and was not
observed.16a The 1H NMR spectrum of 2,3-
Table 1. X-ray Crystallographic Data and Processing Parameters for
2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide (2)
CCDC entry no.
Space group
21269
Monoclinic, C2/c
20.035(3)
9.188(1)
16.887(2)
93.696(2)
3102.3(8)
C15H17NO3S2
323.42
˚
a (A)
dichloro-N-(p-MeOC6H4)maleimide
exhibits
˚
b (A)
a methoxy singlet at δ 3.82 and an aromatic
resonance that appears as an AB quartet centered
at δ 7.09 (JH–H = 9.0 Hz).
˚
c (A)
β (◦)
3
˚
V (A )
Molecular formula
fw
Treatment of 2,3-dichloro-N-(p-MeOC6H4)
maleimide (1) with a slight excess of ethanethiol
in the presence of DBU or Et3N, which
were employed to scavenge the liberated HC1,
gives the desired compound 2,3-bis(ethylthio)-
N-(p-MeOC6H4)maleimide in good yield. TLC
analysis of the crude reaction solution us-
ing a 6:4 mixture of CH2Cl2/petroleum ether
revealed the presence of a new spot (Rf =
0.30), which was subsequently isolated by
column chromatography over silica gel us-
ing the same solvent system as the eluent.
The IR spectrum of the 2,3-bis(ethylthio)-N-(p-
MeOC6H4)maleimide revealed the presence of a
pair of ν(CO) stretches at 1772 (w) and 1715 (vs)
that belong to the kinematically coupled carbonyl
Formula units per cell (Z)
Dcalcd (Mg/m3)
8
1.385
˚
λ (MoKα) (A)
0.71073
0.352
Absorption coefficient (mm−1
)
Rmerge
0.0400
Absolute correlation factor
Total reflections
0.2747/0.2127
6178
Independent reflections
2025
Data/res/parameters
R
2025/0/194
0.0268
Rw
0.0676
GOF
Weights
1.052
[0.04F2 + (σ F)2]−1
The thermal ellipsoid drawing of 2,3-
bis(ethylthio)-N-(p-MeOC6H4)maleimide (2)
shown in Fig. 1 confirms the functionalization of
the C(2) and C(3) ring atoms of the maleimide
1
groups of the maleimide ring.16 The H NMR
spectrum of compound 2,3-bis(ethylthio)-N-(p-
MeOC6H4)maleimide (2) exhibits an AB quar-
◦
˚
Table 2. Selected Bond Distances (A) and Angles ( ) in for 2,3-
bis(erthylthio)-N-(p-MeOC6H4)maleimide (2)a
tet for the aromatic protons at δ 7.07 (JH–H
=
8.5 Hz), with additional high-field resonances at
δ 3.80 (s), 3.34 (q), and 1.35 (t), belonging
to the p-OMe group, and the ethylthio methy-
lene and methyl moieties, respectively. Finally,
the UV-vis spectrum of 2,3-bis(ethylthio)-N-(p-
MeOC6H4)maleimide (2) in CH2C12 displayed
moderately intense absorbances at 421, 309, and
250 nm.
Bond distances
S(1) C(2)
S(2) C(3)
O(1) C(1)
O(3) C(8)
N(1) C(1)
N(1) C(5)
C(2) C(3)
1.732(2) S(1) C(12)
1.737(2) S(2) C(14)
1.208(2) O(2) C(4)
1.366(2) O(3) C(11)
1.385(2) N(1) C(4)
1.431(2) C(1) C(2)
1.351(2) C(3) C(4)
1.806(2)
1.816(2)
1.204(2)
1.427(2)
1.402(2)
1.499(3)
1.501(3)
Bond angles
The
molecular
structure
of
2,3-
C(2) S(1) C(12)
C(8) O(3) C(11)
C(1) N(1) C(5)
O(1) C(1) N(1)
N(1) C(1) C(2)
C(3) C(2) S(1)
C(2) C(3) C(4)
C(4) C(3) S(2)
O(2) C(4) C(3)
104.83(9) C(3) S(2) C(14) 104.66(8)
118.1(1) C(1) N(1) C(4) 109.9(1)
124.8(1) C(4) N(1) C(5) 125.3(1)
125.2(2) O(1) C(1) C(2) 127.6(2)
107.2(1) C(3) C(2) C(1) 108.2(2)
136.4(1) C(1) C(2) S(1) 115.4(1)
107.9(2) C(2) C(3) S(2) 128.1(1)
123.0(1) O(2) C(4) N(1) 124.7(2)
128.5(2) N(1) C(4) C(3) 106.8(1)
bis(ethylthio)-N-(p-MeOC6H4)maleimide (2)
was next established by X-ray diffraction
analysis. Single crystals of 2,3-bis(ethylthio)-N-
(p-MeOC6H4)maleimide (2) were found to exist
as discrete molecules in the unit cell with no
unusually short inter- or intramolecular contacts.
The X-ray data collection and processing param-
eters are listed in Table 1, whereas Table 2 gives
selected bond distances and angles.
aNumbers in parentheses are estimated standard deviations in the
least significant digits.

2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide
769
Fig. 1. Thermal ellipsoid plot of 2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide (2)
showing the thermal ellipsoids at the 50% probability level.
residue by an ethylthio group. The C(1) C(2)
¨
Cyclic voltammetry data and extended Huckel
molecular orbital calculations
˚
˚
[1.499(3) A] and C(3) C(4) [1.501(3) A] C C
single-bond distances, along with the C(2) C(3)
˚
The redox chemistry of 2,3-bis(ethylthio)-
N-(p-MeOC6H4)maleimide (2) was initially
studied by cyclic voltammetry at a platinum elec-
trode in CH2Cl2 solvent containing 0.2 M TBAP
at room temperature. Scanning the sample at a
rate of 200 mV/s over a potential range of 1.0 V
to −1.7 V revealed the presence of single redox
response at E1/2 = −1.22 V, with Fig. 2 showing
the cathodic scan CV over the region of −0.6 V
out to the switching potential of −1.7 V and back
to −0.6 V. Normal electrochemical criteria indi-
cate that this 0/1− redox couple corresponds to
a reversible one-electron reduction,15,17 a feature
that is consistent with the CV behavior reported
for the diphosphine ligands bma and bpcd.3e,18
Of interest to us is the fact that 2,3-
bis(ethylthio)-N-(p-MeOC6H4)maleimide (2)
forms a relatively stable radical anion upon
reduction with no evidence for cleavage of
either the S C(dione) or the S Et bond on
the CV time scale. The kinetic stability of 2,
3-bis(ethylthio)-N-(p-MeOC6H4)maleimide (2)
during one-electron reduction was validated
[1.351(2) A] bond length for the C C π bond
exhibit acceptable values for C C and C C
bond lengths.6,14 The three different C N
˚
distances range from 1.385(2) A [N(1) C(1)] to
˚
1.431(2) A [N(1) C(5)], with an average length
of 1.406 A.6,14 The five- and six-membered rings
˚
are planar and the interplanar angle between the
two rings reveals a dihedral angle of 59.4(1)◦.
The torsion angles of 27.1(2)◦ and −146.2(2)◦
for the atoms C(3) C(2) S(1) C(12) and
C(2) C(3) S(2) C(14), respectively, indicate
that the ancillary ethyl groups are tipped out
of the plane defined by the maleimide ring.
There are three intermolecular nonclassical
hydrogen-bond like interactions in the structure:
˚
C(9) H(9)· · ·O(3) with distances of 0.94 A,
◦
˚
˚
2.50 A, 3.371(2) A and an angle of 154 ;
˚
C(10) H(10)· · ·O(2) with distances of 0.94 A,
◦
˚
˚
2.51 A, 3.421(2) A and an angle of 161 ;
˚
C(11) H(11c)· · ·O(1) with distances of 0.97 A,
◦
˚
˚
2.59 A, 3.320(2) A and an angle of 132 . The
remaining bond distances and angles in 2 are
unexceptional and do not require comment.

770
Watson, Chen, and Richmond
The nature of the LUMO in 2,3-
bis(ethylthio)-N-(p-MeOC6H4)maleimide (2)
was explored by extended Hu¨ckel MO cal-
culations, in order to establish the site of
electron accession in the corresponding radical
anion. The bottom portion of Fig. 2 shows
the three-dimensional CACAO drawing13 for
the a2-based LUMO in the model compound
(HS)C C(SH)C(O)NHC(O). The calculated
LUMO energy level (−10.05 eV) and nodal
pattern found in (HS)C C(SH)C(O)NHC(O)
are similar to the π∗ LUMO (ψ4) reported
for the 6π electron systems maleic anhy-
dride and 1,3,5-hexatriene,4,21 the diphosphine
ligands bma and bpcd,1–3,22and the mono-
substituted cyclopenten-1,3-dione compound
(Cl)C C(NMe2)C(O)CH2C(O).23 Minor out-
of-phase π∗ contributions to the LUMO from
a lone-electron pair associated with each thiol
group are also observed. Mulliken population
analysis confirms that the π∗ thiol interactions
contribute less than 5% to the LUMO. This
latter antibonding interaction between the thiol
groups and the carbocyclic ring system pro-
vides the basis by which thiolate expulsion is
achieved during electrochemical reduction. We
have also replaced the maleimide hydrogen
in (HS)C C(SH)C(O)NHC(O) with a phenyl
group and have studied the effect that the
ancillary phenyl group has on the LUMO in
(HS)C C(SH)C(O)NPhC(O).(3) The MO calcu-
lations on (HS)C C(SH)C(O)NPhC(O) revealed
that the LUMO (not shown) possessed a π∗-based
orbital consistent with that of ψ4 of other 6π
electron systems and exhibited a calculated en-
ergy of −10.05 eV. Clearly, the substitution of the
N Ph versus a N H group does not dramatically
alter the nature and energy level of the LUMO
Fig. 2. Cathodic scan cyclic voltammogram for 2,3-
bis(ethylthio)-N-(p-MeOC6H4)maleimide (2) ca. 10−3 M in
CH2C12 containing 0.25 M TBAP at 200 mV/s at room tem-
perature (top), and the CACAO drawing for the LUMO of
2,3-bis(thio)-maleimide (bottom).
by double-potential step chronocoulometric
(DPSC) studies.19 The DPSC data revealed
that the stability of 2,3-bis(ethylthio)-N-(p-
MeOC6H4)maleimide (2−•) was on the order
of 3–5 s at room temperature, and these results
stand in sharp contrast to the stability of the
radical anion derived from 4,5-bis(p-tolylthio)-
4-cyclopenten-1,3-dione,7 whose half-life may
be estimated at ca. <0.5 s by both DPSC and
dimensionless CV current ratio measurements
at room temperature.20 The latter radical anion
decomposes according to the pathway depicted
in Eq. (1) with the expulsion of the weak
p-tolylthiolate anion.
(3)In the original calculations on (HS)C C(SH)C(O)NPhC(O),
the thiol linkages were constrained to the plane defined by the
maleimide ring and were set coincident with the closest carbonyl
group (torsion angle = 0◦). Setting the rotation of the phenyl group
about the N C(aryl) bond as a free variable in the calculations
showed only a small variation in the energy level of the LUMO.
No major change was observed in the gross composition of the
LUMO.

2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide
771
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Conclusions
The synthesis of the maleimide com-
pound 2,3-dichloro-N-(p-MeOC6H4)maleimide
(1), followed by its conversion into the
disulfide compound 2,3-bis(ethylthio)-N-(p-
MeOC6H4)maleimide (2) has been described.
Both new compounds have been fully character-
ized in solution by IR and NMR spectroscopies,
and the X-ray diffraction structure of the
latter compound has been determined. The
one-electron reduction product observed by
cyclic voltammetry for 2,3-bis(ethylthio)-N-
(p-MeOC6H4)maleimide (2) derives from the
occupation of a π∗ orbital on the maleimide
ring in keeping with this genre of redox-active
compounds. The site of the electron accession
in 2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide
(2) was verified by extended Hu¨ckel MO
calculations.
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Acknowledgment
Financial support from the Robert A. Welch
Foundation (Grants P-0074-WHW and B-1093-
MGR) is appreciated.
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