4,5-Dicyano-3,6-diethylbenzo-1,2-diselenete, a highly Stable 1,2-diselenete: Its preparation, structural characterization, calculated molecular orbitals, and complexation with tetrakis(triphenylphosphine)palladium

Article abstract of DOI:10.1021/ic5000765

The first isolable benzo-1,2-diselenete, 4,5-dicyano-3,6-diethylbenzo-1,2- diselenete (4), was prepared by the reaction of 4,5-(o-xylylenediseleno)-3,6- diethylphthalonitrile (3) with aluminum chloride in toluene. X-ray crystallographic analysis demonstrated that 4 contains a trapezoidal diselenide ring rather than a benzo-1,2-diselenone structure. In crystal form, 4 undergoes self-assembly and generates structures based on layered molecular sheets since the unit cell contains only one molecule. While the cyclic voltammogram of 4 exhibited only one irreversible peak (E_p = 1.59 V) during oxidation and two quasireversible couples during reduction, three peaks were observed in the differential pulse voltammogram of the reduction couples (E_1/2 = -1.19, -0.75, and -0.69 V). Although a THF solution of 4 in the presence of sodium metal was EPR silent, various signals were readily observed in its ¹H, ¹³C, and ⁷⁷Se NMR spectra. Molecular orbital calculations for 4 demonstrated that the HOMO orbital is primarily localized at the two selenium atoms and four of the benzene carbon atoms while the LUMO orbital is situated solely on the diselenete ring. It appears that the HOMO and LUMO orbitals of 4 receive significant stabilization from the nitrile groups compared to the level of stabilization in the unsubstituted benzo-1,2-diselenete (BDS). The reaction of 4 with tetrakis(triphenylphosphine) palladium in benzene was found to produce a dinuclear palladium complex (8), and the structure of this complex was determined by X-ray crystallographic analysis. The central four membered ring of 8 consists of the Pd1, Se2, Pd2, and Se3 atoms and is not planar but rather adopts a folded arrangement.

Full text of DOI:10.1021/ic5000765

Article
pubs.acs.org/IC
4,5-Dicyano-3,6-diethylbenzo-1,2-diselenete, a Highly Stable 1,2-
Diselenete: Its Preparation, Structural Characterization, Calculated
Molecular Orbitals, and Complexation with
Tetrakis(triphenylphosphine)palladium
Takeshi Kimura,*,† Tsukasa Nakahodo,‡ Hisashi Fujihara,‡ and Eiichi Suzuki§
†Center for Instrumental Analysis, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan
‡Department of Applied Chemistry, Kinki University, Higashi-Osaka 577-8502, Japan
§Department of Chemistry and Bioengineering, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan
S
* Supporting Information
ABSTRACT: The first isolable benzo-1,2-diselenete, 4,5-dicyano-3,6-
diethylbenzo-1,2-diselenete (4), was prepared by the reaction of 4,5-(o-
xylylenediseleno)-3,6-diethylphthalonitrile (3) with aluminum chloride in
toluene. X-ray crystallographic analysis demonstrated that 4 contains a
trapezoidal diselenide ring rather than a benzo-1,2-diselenone structure.
In crystal form, 4 undergoes self-assembly and generates structures based
on layered molecular sheets since the unit cell contains only one
molecule. While the cyclic voltammogram of 4 exhibited only one irreversible peak (Ep = 1.59 V) during oxidation and two
quasireversible couples during reduction, three peaks were observed in the differential pulse voltammogram of the reduction
couples (E1/2 = −1.19, −0.75, and −0.69 V). Although a THF solution of 4 in the presence of sodium metal was EPR silent,
various signals were readily observed in its 1H, 13C, and 77Se NMR spectra. Molecular orbital calculations for 4 demonstrated that
the HOMO orbital is primarily localized at the two selenium atoms and four of the benzene carbon atoms while the LUMO
orbital is situated solely on the diselenete ring. It appears that the HOMO and LUMO orbitals of 4 receive significant
stabilization from the nitrile groups compared to the level of stabilization in the unsubstituted benzo-1,2-diselenete (BDS). The
reaction of 4 with tetrakis(triphenylphosphine)palladium in benzene was found to produce a dinuclear palladium complex (8),
and the structure of this complex was determined by X-ray crystallographic analysis. The central four membered ring of 8 consists
of the Pd1, Se2, Pd2, and Se3 atoms and is not planar but rather adopts a folded arrangement.
there exist several reports of four membered selenium
derivatives, the 1,2-diselenetes and benzo-1,2-diselenetes.7
However, these studies have involved either unstable or
theoretical derivatives. The detection of 1,2-diselenete, for
example, required photolysis of 1,4-diselenin derivatives at low
temperature (−196 °C) using an argon matrix.7a A possible
exception is 3,4-bis(trifluoromethyl)-1,2-diselenete, which was
found to be stable at low temperatures but to slowly dimerize at
0 °C to form the corresponding 3,4,7,8-tetrakis-
(trifluoromethyl)-1,2,5,6-tetraselenocin.7b In these reports,
1,2-dithiete and 1,2-diselenete are considered to be in
equilibrium with their ring-opening valence isomers, 1,2-
dithione and 1,2-diselenone, respectively. In addition, benzo-
1,2-diselenete is expected to be extremely unstable because of
the steric strain caused by fusing with the benzene ring.
INTRODUCTION
■
Recently, considerable attention has been focused on three or
four membered dichalcogenide ring systems, due to their
strained structures and high reactivities.1 The smallest cyclic
disulfide, 1,2-dithiiran, was first isolated and reported by Ishii et
al. in 1994.2 Since then, many related compounds have been
prepared, and studies of their structures and reactivities have
accumulated. In addition, both saturated and unsaturated four
membered cyclic disulfides, the 1,2-dithietanes and 1,2-
dithietes, have been investigated by many organic chemists.1
On the basis of these studies, it is known that these ring
systems are quite unstable and that 1,2-dithiete incorporates
more steric strain than does 1,2-dithietane. Although the cyclic
disulfides exhibit high reactivities, several 1,2-dithietanes3 and
1,2-dithietes,4 along with their oxidized derivatives,4b,5 can be
isolated in stable forms. There have also been many reports of
unstable benzo-annelated 1,2-dithietes, the benzo-1,2-dithietes.1
To date, several examples of stable benzo-1,2-dithietes have
been described,6 and it has been demonstrated that these
compounds are stabilized either by steric protection or by
fusion with the 1,2-benzoquinone skeleton. Although there are
no actual examples of three membered diselenide ring systems,
Previously, we have reported the preparation, structural
determination, and optical and electrochemical properties of
several phthalocyanines and tetraazaporphyrins with peripheral
sulfur- or selenium-based functional groups.8 In that prior work,
Received: January 11, 2014
Published: April 11, 2014
© 2014 American Chemical Society
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4,5-(o-Xylylenediseleno)-3,6-diethylphthalonitrile. Compound 2
(6.72 g, 12 mmol) and CuCN (4.33 g, 48 mmol) were placed in a glass
reactor, DMF (50 mL) was added under Ar, and the solution was
stirred at 140 °C for 5.5 h. After cooling the reactor, aqueous FeCl3·
6H2O solution was added, and the solution was stirred at 70 °C for 15
min. After filtration, the residue was dissolved with CHCl3. The
solution was dried, and the solvent was evaporated. The residue was
purified by column chromatography (Wakogel C-300HG, n-hexane/
we used 5,6-dibromo-4,7-diethylbenzotrithioles and the corre-
sponding selenium derivatives as precursors to phthalocyanines.
In related work with phthalocyanines having four diselenadi-
thiafulvalene units,9 we observed the formation of a reddish-
purple solid when 5,6-dicyano-4,7-diethylbenzo[1,3]diselenole-
2-one (5) was prepared by the sequential treatment of 4,5-(o-
xylylenediseleno)-3,6-diethylphthalonitrile (3) with aluminum
chloride in toluene followed by carbonyl diimidazole.
Unexpectedly, the reddish-purple product was determined to
be a benzo-1,2-diselenete, specifically 4,5-dicyano-3,6-dieth-
ylbenzo-1,2-diselenete (4). Compound 4 is an unprecedented
example of a stable benzo-1,2-diselenete and represents the first
four membered diselenide ring system examined by X-ray
crystallography. This Article reports the preparation, structural
determination, and electrochemical properties of 4, along with
molecular orbital calculations based on density functional
theory (DFT). In addition, the complexation reaction of 4 with
tetrakis(triphenylphosphine)palladium and the structural anal-
ysis of the resulting complex are also reported.
1
CHCl3 = 1:1) to produce 3 in 12% yield (659 mg). H NMR (500
MHz, CDCl3): δ = 1.19 (t, J = 7.5 Hz, 6H), 3.10 (q, J = 7.5 Hz, 4H),
4.46 (s, 4H), 6.94 (s, 4H). 13C NMR (101 MHz, CDCl3): δ = 14.8,
30.9, 31.5, 114.8, 115.1, 127.8, 129.6, 135.0, 146.7, 151.8. 77Se NMR
(95 MHz, CDCl3): δ = 406.9. HRMS calcd for C20H18N280Se2
445.9800; found m/z = 445.9797 (M+).
=
3,6-Diethyl-4,5-dicyanobenzo-1,2-diselenete (4). To a mixture of
3 (60.9 mg, 0.14 mmol) and AlCl3 (136 mg, 1.02 mmol) was added
toluene (18 mL) under Ar, and the solution was stirred at room
temperature for 2 h. To the solution was added H2O. The product was
extracted with toluene, and the solvent was evaporated. The product
was purified with column chromatography (Wakogel C-300HG,
CHCl3) to produce 4 in 63% yield (29.3 mg): red-purple crystal;
1
mp 192−195 °C. H NMR (500 MHz, CDCl3): δ = 1.29 (t, J = 7.7
Hz, 6H, CH3), 2.62 (q, J = 7.7 Hz, 4H, CH2). 13C NMR (126 MHz,
CDCl3): δ = 13.2, 24.3, 111.6, 114.9, 137.8, 143.6. 77Se NMR (95
EXPERIMENTAL SECTION
■
MHz, CDCl3): δ = 685.3. IR (KBr): ν
̃
= 2219.7 cm−1 (CN). UV−vis
General. NMR spectra were obtained with a Bruker AVANCE 500
III spectrometer. Mass spectra were acquired using a JEOL JMS-700
mass spectrometer based on fast atom bombardment mass
spectrometry (FAB MS) and electron ionization high-resolution
mass spectrometry. UV−vis spectra were recorded with a JASCO
Ubest V-570 spectrophotometer, and IR spectra were obtained
employing a JASCO FT/IR-4200 spectrometer. A Hokuto Denko
Co. model HAB-151 apparatus and a Cypress Systems CS-1200
galvanostat/potentiostat apparatus were used when measuring
oxidation and reduction potentials, and elemental analysis was
performed using a Yanako MT-6 analyzer.
(CHCl3): λmax (ε) = 346.5 (10 900), 421.5 nm (183). HRMS calcd for
C12H10N280Se2 = 341.9174; found m/z = 341.9168 (M+). X-ray data:
red platelet crystal, triclinic P1 (No. 1); a = 5.205(3) Å, b = 6.870(4)
Å, c = 8.803(5) Å, α = 106.190(7)°, β = 91.612(7)°, γ = 103.098(6)°,
V = 293.1(3) Å3, Z = 1, Dcalc = 1.927 g/cm3, μ(Mo Kα) = 62.807
cm−1. Temperature for data collection = −172.8 °C. Number of
reflections measured = total, 2253; unique, 1737 (Rint = 0.0464);
reflection/parameter ratio = 11.90; R1 [I > 2.00σ(I)] = 0.0291; wR2
(all reflections) = 0.0732; GOF = 1.062; maximum peak in the final
difference Fourier map = 0.69 e/Å3.
3,6-Diethyl-4,5-dicyano-1,2-benzenedithiol. To a mixture of 4,5-
(o-xylylenedithio)-3,6-diethylphthalonitrile (178.1 mg, 0.5 mmol) and
AlCl3 (400 mg, 1.02 mmol) was added toluene (18 mL) under Ar, and
the solution was stirred at room temperature for 2 h. To the solution
was added H2O. The product was extracted with toluene, and the
solvent was evaporated. The product was purified with column
chromatography (Wakogel C-300HG, CHCl3) to produce the
corresponding dithiol in 74% yield (93.6 mg), as a light-yellow solid.
1H NMR (500 MHz, CDCl3): δ = 1.29 (t, J = 7.6 Hz, 6H, CH3), 3.09
X-ray Crystallographic Analysis. X-ray crystallographic data
were acquired with a Rigaku Saturn724 diffractometer using graphite
monochromated Mo Kα radiation. Diffraction patterns were collected
at −172 1 °C to a maximum 2θ value of 55.0°. Data were collected
and processed using the CrystalClear software package (Rigaku), and
the structures were solved by a direct method and expanded using
Fourier techniques. The non-hydrogen atoms were refined anisotropi-
cally while the hydrogen atoms were refined using the riding model.
All calculations were performed using the CrystalStructure crystallo-
graphic software package, except in the case of refinement, which was
performed using SHELXL-97.
Oxidation−Reduction Potentials. All oxidation−reduction
measurements were performed by cyclic voltammetry using Ag/
AgNO3 (0.01 mol dm−3) as a reference electrode, glassy carbon as a
working electrode, and Pt as a counter electrode (scan rate: 200 mV
s−1). A solution of n-Bu4NClO4 in CH3CN (0.1 mol dm−3) was used
as the electrolyte. The oxidation potential of ferrocene was found to be
E1/2 = 0.09 V using this apparatus without applying any correction.
5,6-Dibromo-4,7-diethylbenzotriselenole (1). Compound 1 was
prepared by the procedure described previously.8a
(q, J = 7.6 Hz, 4H, CH2), 4.36 (s, 2H, SH). 13C NMR (126 MHz,
CDCl3): δ = 13.0, 29.0, 113.1, 114.9, 139.2, 145.8. HRMS calcd for
C12H12N2S2 = 248.0442; found m/z = 248.0441 (M+).
Bis(4,5-dicyano-3,6-diethylbenzodiselenolato)bis-
(triphenylphosphine)dipalladium (8). To a mixture of 4 (42.7 mg,
0.13 mmol) and tetrakis(triphenylphosphine)palladium (148 mg, 0.13
mmol) was added dry benzene (15 mL) under Ar, and the solution
was stirred at room temperature for 16 h. After the solvent was
evaporated, the residue was purified with column chromatography
(Wakogel C-300HG, CHCl3) to produce 8 in 84% yield (75 mg): dark
brown crystal; mp 273−275 °C (decomp). 1H NMR (500 MHz,
CDCl3): δ 0.94 (t, J = 7.5 Hz, 6H, CH3), 1.11 (t, J = 7.5 Hz, 6H,
CH3), 1.80 (dq, J = 13.9, 7.5 Hz, 2H, CH2), 2.29 (dq, J = 13.9, 7.5 Hz,
2H, CH2), 2.72 (dq, J = 13.7, 7.5 Hz, 2H, CH2) 2.78 (dq, J = 13.7, 7.5
Hz, 2H, CH2), 7.28−7.35 (m, 12H, ArH), 7.39−7.45 (m, 6H, ArH),
7.52−7.60 (m, 12H, ArH). 31P NMR (202 MHz, CDCl3): δ = 27.1 (s,
JP−Se = 120, 103, 37 Hz as satellite coupling constants). 77Se NMR (95
MHz, CDCl3): δ = 349.7 (JSe−P = 120, 103 Hz), 604.952, 604.956
(JSe−P = 37 Hz). FABMS m/z = 1424 (M+). Anal. Calcd for
C60H54N4P2Pd2Se4: C, 50.69; H, 3.83; N, 3.94%. Found: C, 50.55; H,
1,2-(o-Xylylenediseleno)-4,5-dibromo-3,6-diethylbenzene (2). To
a solution of 1 (263.9 mg, 0.5 mmol) and K2CO3 (70 mg, 0.5 mmol)
in 30 mL of THF and 10 mL of MeOH was added NaBH4 (80 mg, 2.1
mmol) slowly, and the solution was stirred for 10 min.
Xylylenedichloride (106 mg, 0.6 mmol) was added, and the solution
was stirred at room temperature for 24 h. After addition of aqueous
HCl and evaporation of the solvent, the product was extracted with
CHCl3 and the solvent was dried and evaporated. The residue was
purified by column chromatography (Wakogel C-300HG, n-hexane/
1
CHCl3) to produce 2 in 73% yield (203.1 mg). H NMR (500 MHz,
3.52; N, 3.61%. X-ray data: red platelet crystal, triclinic P1 (No. 2); a =
̅
CDCl3): δ = 1.10 (t, J = 7.4 Hz, 6H), 3.19 (q, J = 7.4 Hz, 4H), 4.37 (s,
4H), 6.89−6,96 (m, 4H). 13C NMR (101 MHz, CDCl3): δ = 13.5,
30.3, 35.6, 127.2, 128.1, 129.6, 135.4, 139.0, 148.0. 77Se NMR (95
14.3131(17) Å, b = 15.8611(15) Å, c = 16.521(2) Å, α = 70.492(4)°, β
= 86.635(5)°, γ = 66.389(4)°, V = 3227.0(6) Å3, Z = 2, Dcalc = 1.705
g/cm3, μ(Mo Kα) = 31.54 cm−1, temperature for data collection =
−172.8 °C, number of reflections measured = total, 21 165; unique,
10 563 (Rint = 0.0299); reflection/parameter ratio = 13.46, R1 [I >
MHz, CDCl3): δ = 412.7. HRMS calcd for C18H1879Br280Se2
551.8106; found m/z = 551.8104 (M+).
=
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The 1H NMR spectrum of 4 in chloroform-d exhibits signals
attributed to the ethyl groups at 1.29 and 2.62 ppm, while the
methylene signal is situated at a higher magnetic field than the
corresponding signals of 5,6-dicyano-4,7-diethylbenzo[1,2,3]-
triselenole (7) (Table 1).8a In addition, the 13C NMR spectrum
2.00σ(I)] = 0.0365, wR2 (all reflections) = 0.0919, GOF = 1.074,
maximum peak in the final difference Fourier map = 1.42 e/Å3.
Computational Methods. All calculations were performed using
the Gaussian 09 program. Optimized structures, HOMO and LUMO
energies and NMR shielding constants for 4, 7, benzo-1,2-diselenete
(BDS), 3,6-diethylbenzo-1,2-diselenete (BDSET) and 4,5-dicyano-
benzo-1,2-diselenete (BDSCN) were calculated using density func-
tional theory (DFT) at the B3LYP/6-311++G (d, p) level.10
Table 1. Partial Summary of Observed and Calculated NMR
a
Data for 4 and 7
RESULTS AND DISCUSSION
■
4 δ (ppm)
7 δ (ppm)
As a precursor to compound 4, 4,5-(o-xylylenediseleno)-3,6-
diethylphthalonitrile (3) was prepared from 5,6-dibromo-4,7-
diethylbenzotriselenole (1)8a via 1,2-(o-xylylenediseleno)-4,5-
dibromo-3,6-diethylbenzene (2) (Scheme 1). In the process,
obsd
calcd
obsd
calcd
1H
13C
77Se
1.29, 2.62
13.2, 24.3
685.3
1.26, 2.58
16.1, 29.1
1.29, 3.01
14.0, 33.2
1.30, 2.97
16.1, 38.9
463.5, 563.5
a
Calculated chemical shifts were obtained by the DFT method (vide
Scheme 1. Preparation of 4 by the Reaction of 3 with
Aluminum Chloride in Toluene
infra).
of 4 shows signals due to the ethyl carbons at 13.2 and 24.3
ppm, while these signals in the spectrum of 7 appear at 14.0
and 33.2 ppm. Finally, the 77Se NMR spectrum of 4 exhibits a
singlet peak at 685.3 ppm, at a lower magnetic field than the
two selenium signals of 7. The IR spectrum of 4 has a nitrile
peak at 2219.7 cm−1, slightly below the position of the
corresponding peak in the spectrum of 7 (2222.6 cm−1). The
UV−vis spectrum of 4 in THF showed λmax values at 346.5 nm
(ε = 10 900) and 421.5 nm (ε = 183).
It was found that 4,5-bis(benzylseleno)-3,6-diethylphthaloni-
trile (6), a precursor to 2,3,9,10,16,17,23,24-octakis-
(benzylseleno)phthalocyanine,8a gradually decomposed both
in the solid state and in solution to give 4. On monitoring the
the cyanation reaction of 2 unfortunately produced a low yield
compared to the results obtained with sulfur derivatives, likely
due to the lower bond energy of the Se−C bond compared
with that of the S−C bond. As reported in a previous paper, the
1,3-diselenole-2-one derivative 5 (Chart 1) was obtained when
1
decomposition reaction of 6 via H NMR in chloroform-d at
room temperature, the signals associated with 6 disappeared
while the signals attributed to the ethyl group of 4 appeared
and increased in intensity, together with new signals due to
benzaldehyde. Although the reaction mechanism is not clear, 4
and benzaldehyde may be produced via the oxidation of
selenide 6 with oxygen, followed by a Pummerer-type reaction.
The structure of 4 was determined by X-ray crystallography,
which indicated unexpected variations in the molecular
arrangements within the crystals. Compound 4 therefore
appears to exhibit crystal polymorphism.13 Since 4 is
crystallizing in a triclinic form with the space group P1 (No.
1) and Z = 1, the crystal contains a unique molecular structure
in the unit cell. The ORTEP drawing indicates that the 4,5-
dicyanobenzo-1,2-diselenete skeleton is planar in the crystal
(Figure 1A). In addition, the two ethyl groups of 4 are situated
such that they extend in opposite directions. The bond lengths
of the Se1−Se2, Se1−C1, Se2−C2, and C1−C2 bonds are
2.3736(13), 1.915(5), 1.905(6), and 1.376(7) Å, respectively
[see Supporting Information (SI)]. The Se−Se bond of 4 is
longer than those of either the diaryl diselenide or 7 while the
Se−C bonds of 4 and the diaryl diselenide are similar in
length.14 The Se−Se and Se−C bond lengths of bis-
(pentafluorophenyl) diselenide are 2.319(4) and 1.910 Å (a
mean value), respectively, and those of 7 are 2.319 and 1.94 Å
(mean values). The bond angles of 4 at the Se2−Se1−C1,
Se1−Se2−C2, Se1−C1−C2, and Se2−C2−C1 junctions are
74.52(15)°, 75.21(13)°, 105.3(4)°, and 104.9(4)°, respectively.
X-ray crystallographic analysis demonstrated that 4 contains a
trapezoidal diselenide ring rather than a benzo-1,2-diselenone
structure in the crystal.
Chart 1
3 was treated with aluminum chloride in toluene followed by
carbonyl diimidazole.9 In contrast, treatment of 3 with
aluminum chloride in toluene without the application of
carbonyl diimidazole gave product 4 in 63% yield. This process,
which is a modification of Decurtins’ procedure, is the Friedel−
Crafts-like deprotection reaction of the xylylene group from
3.11,12 Compound 4 appears to be highly stable at room
temperature. The similar treatment of 4,5-(o-xylylenedithio)-
3,6-diethylphthalonitrile with aluminum chloride in toluene
gave the corresponding 1,2-benzenedithiol in 74% yield. These
results suggest that the benzo-1,2-diselenete ring was formed by
the oxidation of the corresponding 1,2-benzenediselenolate
after elimination of the xylylene group.
It appears that 4 has small intermolecular distances between
neighboring molecules in the crystal. For example, the two
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Figure 1. (A) ORTEP drawing of 4 viewed along the b axis. The thermal ellipsoids are drawn at the 50% probability level. (B) Short contact
distances of 4 viewed along the a axis: Se1−N2, 3.124(6) Å; Se2−N2, 3.049(6) Å.
selenium atoms, Se1 and Se2, are close to nitrogen atom N2 of
the nitrile group on the adjacent molecule, as shown in Figure
1B. The intermolecular distances between the Se1−N2 and
Se2−N2 atoms are 3.124(6) and 3.049(6) Å, respectively,
which are less than the sum of the van der Waals radii of
selenium and nitrogen atoms (3.45 Å).15 It is expected that
these interactions between selenium and nitrogen atoms
strongly affect the molecular arrangement of 4 in the crystal.
As a result, the unit cell contains only one molecule
(Supporting Information Figure S2), and the molecules of 4
can undergo self-assembly and form molecular sheets
(Supporting Information Figures S3 and S4).
The oxidation−reduction potentials of 4 were measured by
cyclic voltammetry using Ag/AgNO3 as a reference electrode
(solvent, CH3CN; scan rate, 200 mV s−1). The voltammogram
showed one irreversible peak potential (Ep = 1.59 V) during
oxidation and two quasireversible peaks during reduction
(Figure 2). Since the shapes of the two reduction couples were
It was anticipated that, while it exhibits low reactivity with an
oxidizing reagent, 4 would be readily reduced with a reducing
reagent. On treatment of 4 with sodium metal in a THF
solution, the compound took on a reddish-brown coloration
and the UV−vis absorption spectrum of the resulting solution
exhibited λmax values of 347.0 nm (ε = 9120) and 424.0 nm (ε
= 3970). In solution, the absorption of 4 is slightly shifted to
lower energies while the molar absorption coefficient at 421.5 is
increased from 183 to 3970 (424.0 nm). Although this solution
was also analyzed using EPR at room temperature, no signals
1
were observed in the resulting spectrum. When the H NMR
spectrum of 4 in THF-d8 was acquired following treatment with
sodium metal, the spectrum of the reddish-brown solution
contained a sharp triplet and a broadened quartet at δ = 1.12
and 2.67 ppm, respectively, such that the quartet was at a
slightly lower magnetic field position than the equivalent peaks
in the spectrum of unreacted 4. Since the quasireversible first
reduction couple in the cyclic voltammogram is caused by a
two-electron transfer, the species in the NMR tube must have
been generated by the reduction of 4 with the two electrons.
Under similar conditions, the 13C NMR spectrum showed six
signals at δ = 13.8, 30.5, 111.5, 115.9, 142.8, and 145.2 ppm.
Although no signals were observed in the 77Se NMR spectrum
acquired at room temperature, a singlet peak was present at
622.5 ppm in the spectrum obtained at −40 °C, suggesting that
there may be an equilibrium between the ring-opening and the
ring-closing forms. This peak appeared at a higher magnetic
field position than the equivalent peak in the spectrum of
unreacted 4. After quenching the THF solution with hydro-
chloric acid, compound 4 was recovered. These results suggest
that 4 produced an anionic rather than a radical anionic species
in the THF solution.
The optimized structures, HOMO and LUMO energies, and
NMR shielding constants for 4 and for the related compounds,
benzo-1,2-diselenete (BDS), 3,6-diethylbenzo-1,2-diselenete
(BDSET), 4,5-dicyanobenzo-1,2-diselenete (BDSCN), and 7,
were calculated using the Gaussian 09 program (Chart 1).10
The structures of these compounds were optimized using the
DFT method at the B3LYP/6-311++G (d, p) level.
Figure 2. Oxidation−reduction potentials of 4 as measured by cyclic
voltammetry using Ag/AgNO3 as a reference electrode. Inset:
differential pulse voltammetry data.
The calculated structure of 4 is completely planar, which is
similar to the results obtained from X-ray crystallography. The
bond lengths calculated for 4 were 2.4291 (Se1−Se2), 1.9226
(Se1−C1 and Se2−C2), and 1.3958 (C1−C2) Å; these values
are slightly longer than those determined by X-ray crystallog-
raphy. The Se−C bond of 4 is longer than that calculated for
benzohexaselenone,7d suggesting that the four membered
diselenide ring is more favorable than the benzo-1,2-diselenone
structure.16 The calculated bond angles at the Se2−Se1−C1
(Se1−Se2−C2) and Se1−C1−C2 (Se2−C2−C1) junctions of
4 are 74.41° and 105.59°, respectively, and these angles are in
not clear, we measured the reduction potentials using
differential pulse voltammetry. As shown in the inset of Figure
2, we observed one overlaid peak for the first reduction couple
(E1/2 = −0.75 and −0.69 V) and one peak for the second
reduction couple (E1/2 = −1.19 V), suggesting that the first
reduction couple in the cyclic voltammogram contains two one-
electron-transfer processes. While the first reduction potential
of 4 is similar to that of 7 (E1/2 = −0.68 V), the oxidation
potential is higher than that of 7 (E1/2 = 0.87 V).
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Figure 3. Molecular orbitals of 4: (A) HOMO and (B) LUMO.
good agreement with the X-ray crystallography data. The
calculated HOMO and LUMO orbitals of 4 are shown in
Figure 3. While the HOMO orbital is primarily localized at the
two selenium atoms and four of the benzene carbon atoms, the
LUMO orbital is situated solely on the diselenete ring. The
HOMO orbital exhibits antibonding character between the
Se1−Se2 and C4−C5 bonds, while the LUMO orbital shows
antibonding character between the Se1−Se2 and C1−C2
bonds. On the basis of this result, it is expected that anionic
species generated from 4 in the THF/Na solution may have a
ring-opening structure.
The molecular orbital energies of 4 and the related
compounds demonstrate that the HOMO and LUMO orbitals
in these molecules are highly stabilized by the nitrile groups and
slightly destabilized by the ethyl groups compared with the
degree of stabilization in the unsubstituted benzo-1,2-diselenete
(BDS), as shown in Table 2 and Figure 4. This finding suggests
Table 2. Molecular Orbital Energies of 4 and Related
a
Compounds (eV)
BDS
4
BDSET
BDSCN
7
Figure 4. Molecular orbital energies of 4, BDS, BDSET, BDSCN, and
7.
LUMO − 2
LUMO − 1
LUMO
−0.87
−1.12
−2.90
−6.03
−6.45
−7.81
−1.84
−2.48
−3.54
−6.74
−7.20
−8.16
−0.63
−0.96
−2.75
−5.84
−6.30
−7.23
−2.24
−2.71
−3.79
−7.00
−7.44
−8.82
−2.28
−2.81
−3.22
−6.41
−7.11
−7.77
tetrakis(triphenylphosphine)palladium in benzene at room
temperature did in fact proceed, to give the palladium complex
8 in 84% yield (Scheme 2). This complex was stable under air
HOMO
HOMO − 1
HOMO − 2
Scheme 2. Reaction of 4 and
Tetrakis(triphenylphosphine)palladium
a
MO calculations were performed using Gaussian 09 with DFT,
B3LYP6-311++G(d,p).
that the stability of the 1,2-diselenete rings of 4 and BDSCN is
strongly affected by the two nitrile groups. The shielding
constants of 4 were subsequently calculated using the same
1
basis sets, and the results showed that the calculated H and
13C NMR chemical shifts of the ethyl groups of 4 are at higher
magnetic field positions than those of 7 (Table 1). In addition,
1
while the calculated H NMR chemical shifts of 4 are at a
slightly higher magnetic field than the measured values, the
calculated 13C NMR chemical shifts are at a slightly lower
magnetic field than those obtained experimentally.
It is well-known that a zerovalent palladium atom can
oxidatively add to the organo-disulfide, diselenide, and,
ditelluride bonds to produce the corresponding Pd com-
plexes.17,18 It was therefore anticipated that 4 would react in the
same manner with zerovalent palladium to produce the
corresponding palladium complex. The reaction of 4 with
and soluble in chloroform. We observed two ethyl signals as
double quartet patterns as well as complex aromatic signals in
the 1H NMR spectrum of the product, suggesting that the ethyl
and phenyl groups are sterically congested and that 8 has an
unsymmetrical structure on the NMR time scale. In the 31P
NMR spectrum of this compound, a singlet signal at δ = 21.7
ppm was found together with satellite peaks due to coupling
with the selenium atoms (see Supporting Information). It is
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expected that one phosphorus atom can affect four selenium
atoms in terms of spin−spin coupling because of the
unsymmetrical structure of 8 caused by steric hindrance, and
the 77Se NMR spectrum showed split signals at δ = 349.7 and
605.0 ppm at a higher magnetic field compared to the signal in
the spectrum of 4. However, since we were unable to observe
spin−spin coupling between selenium atoms even after 20 000
times data acquisition, the corresponding coupling constants via
the palladium atom must be extremely small. In contrast, the
selenium atoms of 8 do exhibit spin−spin coupling with the
phosphorus atom. In the case of the 31P−77Se coupling of 8, the
selenium peaks at δ = 349.7 ppm which are assigned to bridged
selenium show two coupling constants: JSe−P = 120 and 103 Hz
(see Supporting Information). The peaks at δ = 605.0 ppm
would be due to the outer selenium atom, and were observed as
a doublet signal (JSe−P = 37 Hz). Therefore, the 31P NMR
spectrum of 8 is expected to show one singlet peak together
with satellite peaks based on interactions with the selenium
atoms (JP−Se = 120, 103, 37 Hz). A singlet-like signal was
observed at δ = 605.0 ppm between the doublet signals in the
77Se NMR spectrum, although the signal was not clear since it
was obscured by the major doublet signals. This peak would be
assigned to the outer selenium atom of 8 and may be split due
to spin−spin coupling with another phosphorus atom (4JSe−P).
These results suggest that the dinuclear complex may have an
asymmetric character at the central four membered ring. The
structure of 8 was further supported by FAB MS data and by
elemental analysis.
coordinate to both the palladium atoms, and the lengths of the
associated Pd−Se bonds are 2.4227(8) Å for Pd1−Se2,
2.4856(6) Å for Pd2−Se2, 2.5016(7) Å for Pd1−Se3, and
2.4271(6) Å for Pd2−Se3. The interatomic distance between
the two Pd atoms is 3.0751(3) Å, and the lengths of the Pd2−
Se2 and Pd1−Se3 bonds are longer than those of the Pd1−Se2
and Pd2−Se3 bonds. The bond angles at the Pd1−Se2−Pd2,
Pd1−Se3−Pd2, Se2−Pd1−Se3, and Se2−Pd2−Se3 junctions
are 77.58(2)°, 77.189(18)°, 77.28(2)°, and 77.505(17)°,
respectively. As a result, the Pd2 atom deviates from the
Se2−Pd1−Se3 plane, forming not a planar structure but rather
a folded arrangement.
The bond angles at the Se1−Pd1−Se3, Se2−Pd2−Se4, Se2−
Pd1−P1, and Se3−Pd2−P2 junctions are 164.55(3)°,
166.16(2)°, 173.29(4)°, and 177.14(5)°, respectively. There
have been several reports concerning similar complexes with a
four membered Pd−Se−Pd−Se ring,17 and it appears that the
structure of the central four membered Pd−Se−Pd−Se ring of
8 is similar to that of the dinuclear Pd complex prepared by the
reaction of cycloalkeno-1,2,3-selenadiazole or 1,4-diselenine
derivatives with tetrakis(triphenylphosphine)palladium.17c
However, while the lengths of the Pd−Se bonds within the
central four membered ring of 8 are slightly longer than the
equivalent bonds in Morley’s complex, the bond angles in the
ring structure of 8 are slightly smaller than those found in the
Morley complex.
CONCLUSION
■
The structure of 8 was also examined by X-ray crystallog-
raphy. The results showed that 8 is a dinuclear complex and
that both Pd atoms are a four-coordinated structure which has
an almost square planar geometry (Figure 5). The lengths of
the Pd1−Se1 and Pd2−Se4 bonds are 2.3841(7) and 2.3796(6)
Å (see Supporting Information). The Se2 and Se3 atoms
The highly stable benzo-1,2-diselenete derivative 4 was
obtained by the reaction of 3 with aluminum chloride in
toluene. The spontaneous and gradual decomposition of 6 also
gave 4. X-ray crystallographic analysis showed that 4 contains a
trapezoidal four membered diselenide ring. In crystal form, 4
undergoes self-assembly to form layered molecular sheets since
the unit cell contains only one molecule. Molecular orbital
calculations for 4 demonstrated that the HOMO and LUMO
energies are stabilized by the electron-withdrawing nitrile
groups. The reaction of 4 with tetrakis(triphenylphosphine)-
palladium resulted in the dinuclear palladium complex 8, the
structure of which was determined by X-ray crystallographic
analysis. Since the Pd2 atom deviates from the Se2−Pd1−Se3
plane, 8 appears to adopt a folded arrangement at the central
four membered Pd−Se−Pd−Se ring rather than a planar form.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data and NMR spectra of 4 and 8.19 This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kimura@iwate-u.ac.jp.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript. These authors contributed equally.
Figure 5. (A) ORTEP drawing of 8. Two chloroform molecules and a
water molecule are removed for clarity. The thermal ellipsoids are
drawn at the 50% probability level. (B) Partial molecular structure
showing the folded arrangement of the central Pd1−Se2−Pd2−Se3
ring.
Notes
The authors declare no competing financial interest.
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Article
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ACKNOWLEDGMENTS
■
This work was supported by a Special Grant from Iwate
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