Square-planar metal dithiolene complexes (M = Ni, Au) exhibiting cis and trans forms based on structurally flexible o-xylylene backbone

Article abstract of DOI:10.1016/j.poly.2012.08.026

The square-planar metal dithiolene complexes of o-xylenediyldithioethylene- 1,2-dithiolate (oxddt) ligand, (Et₄P)[Ni(oxddt)₂] (1), (Ph₄P)[Ni(oxddt)₂] (2) and (Ph₄P)[Au(oxddt) ₂] (3), were prepared and characterized. Among them, the Ni complexes 1 and 2 showed cis-anti-form structures, whose two benzene rings of the Ni(oxddt)₂ molecule are closely located to the dithiolene rings but located on the opposite side with respect to the dithiolene ring. In contrast to these Ni complexes, the Au complex 3 has a trans-anti form, suggesting that a Ni/Au substitution in the [M(oxddt)₂] modifies those molecular structures in the crystals. An intramolecular π orbital overlap at the α-LUMO + 5 level between benzene and dithiolene rings was observed in the [Ni(oxddt)₂] complex with the cis-anti-form. Redox behavior of 1 and 3 were investigated by cyclic voltammetry measurements.

Full text of DOI:10.1016/j.poly.2012.08.026

Polyhedron 47 (2012) 9–15
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Polyhedron
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Square-planar metal dithiolene complexes (M = Ni, Au) exhibiting cis and trans
forms based on structurally flexible o-xylylene backbone
⇑
Mitsushiro Nomura , Kazuto Harada, Toshinori Suzuki, Masatsugu Kajitani, Toru Sugiyama
Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1, Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The square-planar metal dithiolene complexes of o-xylenediyldithioethylene-1,2-dithiolate (oxddt)
ligand, (Et4P)[Ni(oxddt)2] (1), (Ph4P)[Ni(oxddt)2] (2) and (Ph4P)[Au(oxddt)2] (3), were prepared and char-
acterized. Among them, the Ni complexes 1 and 2 showed cis-anti-form structures, whose two benzene
rings of the Ni(oxddt)2 molecule are closely located to the dithiolene rings but located on the opposite
side with respect to the dithiolene ring. In contrast to these Ni complexes, the Au complex 3 has a
trans-anti form, suggesting that a Ni/Au substitution in the [M(oxddt)2] modifies those molecular struc-
Received 19 July 2012
Accepted 8 August 2012
Available online 17 August 2012
Keywords:
Metal dithiolene complex
Cis-trans forms
Molecular structure
Orbital overlap
Eight-membered ring
tures in the crystals. An intramolecular
p orbital overlap at the a-LUMO + 5 level between benzene and
dithiolene rings was observed in the [Ni(oxddt)2] complex with the cis-anti-form. Redox behavior of 1 and
3 were investigated by cyclic voltammetry measurements.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
[7,8], because of the p
-delocalized radical anion [M(dithiolene)2]ꢀÅ
state. In contrast to the introduction of a rigid system to the dithio-
lene ring, introductions of flexible alkyl chains to the square-planar
[M(dithiolene)2] complexes are efficient ways toward thermo-
tropic metal complexes, allowing for the formations of smectic
and nematic [9] as well as discotic mesophases [10,11]. Which
mesophase we can obtain is due to the length and shape of the
flexible alkyl chain.
Metal dithiolene complexes involve delocalized p-electron sys-
tems with planar five-membered metallacycles (metalladithio-
lene). In monodithiolene complexes [CpM(dithiolene)] (M = Co,
Rh, Ir; Cp =
ized 6 -electrons system suggesting that the metalladithiolene
g
5-cyclopentadienyl), the metallacycle has a delocal-
p
ring is pseudo-aromatic by the Hückel’s rule [1]. In addition to
the pseudo-aromaticity, unsaturation coexists in the metallacycle.
In fact, we have observed diverse addition reactions on the M–S
bond [2]. Bisdithiolene complexes [M(dithiolene)2] with group 10
In this work, we focused on an introduction of a dialkylene-aro-
matics group, which can be a mixture of rigid benzene and flexible
dialkyl chains, to the dithiolene ring. The metal complex of the o-
xylenediyldithioethylene-1,2-dithiolate (oxddt) ligand is just the
case. In the oxddt ligand, there is an eight-membered ring between
dithiolene and benzene rings (Chart 1). According to some previous
works, one could observe cis and trans structural isomers based on
the folding angle of the eight-membered ring such as 1,5-cycloocta-
diene compounds [12]. Baik et al. first reported the molecular struc-
ture of a bis(cyclopentadienyl) metal complex of the oxddt ligand,
which is formulated as [Cp2Mo(oxddt)] [13]. The oxddt ligand
showed the cis-form (closed-form) as show in Chart 1. We previ-
ously reported that syntheses and molecular structures of the
mono(cyclopentadienyl) [CpM(oxddt)] (M = Co, Ni) complexes and
their derivatives [14,15]. It was different that [CpM(oxddt)] com-
plexes showed the trans-form (opened-form). It was notable that
the alkylidene adduct of the [CpCo(oxddt)] showed the cis-form.
In addition to the [CpCo(oxddt)] derivatives, the oxone derivative
of oxddt, [O@C(oxddt)], has the trans-form (Chart 1) [14], although
the corresponding thione [S@C(oxddt)] has the cis-form [16].
Thus, these oxddt compounds can be structurally flexible based
on the eight-membered ring moiety and the folding angle in the
and 11 metals (M = Ni, Pd, Pt, Au) have relatively larger p-delocal-
ized systems compared with the monodithiolene complexes be-
cause of the square-planar geometry [3]. Trisdithiolene
complexes (M = Mo, W) with a trigonal prismatic geometry show
three-dimensional p-electron delocalization [4]. Among these me-
tal complexes noted above, the square-planar [M(dithiolene)2]
complexes have received many interests for various molecular
materials based on the particular electronic structures.
We can reduce a HOMO–LUMO gap of the [M(dithiolene)2]
complex, when the dithiolene ring is directly connected with other
rigid
p-conjugated systems to extend the effective
p-conjugation
length [5]. Many [M(dithiolene)2] complexes with
p-extended
structures show near-IR electronic absorption by the low energy
HOMO–LUMO gap [6]. Magnetic and conducting properties based
on the [M(dithiolene)2] complex has been intensively investigated
⇑
Corresponding author. Current address: Condensed Molecular Materials Labo-
ratory, RIKEN, 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan.
E-mail address: mitsushiro@riken.jp (M. Nomura).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.08.026

10
M. Nomura et al. / Polyhedron 47 (2012) 9–15
oxidation, and then the addition of Ph4PBr gave the target (Ph4-
P)[Au(oxddt)2] (3) salt (Scheme 1). The single crystals of 1–3 were
obtained by recrystallizations using vapor diffusion of Et2O into
these acetone solutions. 1–3 crystallized in the monoclinic system
with space group C2/c, respectively (Table 2). The three complexes
are perfectly symmetric and each central metal atom as well as
phosphorus atom lie on a crystallographic inversion center. There-
fore, one half of each anion (or cation) is crystallographically un-
ique (Fig. 1). The Et4P+ cation in 1 is disordered with two
possible orientations (Fig. 2).
2.2. Structural determinations of [M(oxddt)2] complexes
The ORTEP drawings of the [M(oxddt)2]ꢀ anion moieties in 1
and 3 as well as definition of dihedral angles (h1–h4) of them are
displayed in Fig. 1. The selected bond lengths, bond angles and
dihedral angles are summarized in Table 1. All the bond lengths
and angles of 1–3 in these metallacycles are in the range of normal
[Ni(dithiolene)2]ꢀ and [Au(dithiolene)2]ꢀ complexes [3]. Two ben-
zene rings of the Ni(oxddt)2 molecule in 1 are closely located to-
ward the dithiolene rings by folding of the eight-membered ring
(closed-form) but each benzene ring is located on the opposite side
with respect to the dithiolene ring, forming the cis-anti-form struc-
ture (Fig. 1(a)). In addition to 1, the corresponding Ph4P+ salt 2 also
showed the cis-anti-form based on the [Ni(oxddt)2]ꢀ radical anion
molecule. However, the structure of [Ni(oxddt)2]ꢀ radical anion
moiety was slightly modified by the cation effect. The dihedral an-
gle h1 (dithiolene folding angle) of 1 or 2 was 7.23° or 2.73°, respec-
tively. The other dihedral angles h2, h3 and h4 were also modified by
the cation effect (Table 1). There are short intramolecular non-
chemical bondings of C1ꢁ ꢁ ꢁC5 and C2ꢁ ꢁ ꢁC6 between dithiolene
and benzene rings (Fig. 1(a)). The bonding distances in 2 (2.932
and 2.952 Å) are obviously shorter than those in 1 (3.038 and
3.083 Å). This result is reflected by degree of the dihedral angle
h4 (61.98° for 1 and 50.06° for 2).
Chart 1. trans-Form (opened-form) and cis-form (closed-form) structures of oxddt
derivatives.
solid state is controllable by chemical modifications. However, we
have found that there are a few examples of square-planar dithio-
lene complexes of the oxddt ligand. Underhill et al. previously re-
ported preparations of the four-coordinate [M(oxddt)2]ꢀ (M = Ni,
Cu, Au) anion salts with an ammonium cation and their oxidized
species, but these products were not structurally characterized
[17]. No structure of a bisdithiolene complex with the oxddt ligand
has been known so far. Here we report three structurally character-
ized [M(oxddt)2]ꢀ (M = Ni, Au) anion salts with a phosphonium
cation. Electrochemical measurements and DFT calculations on
the [M(oxddt)2]ꢀ anion salts were also carried out to discuss their
electronic structures.
2. Results and discussion
2.1. Preparations of [M(oxddt)2] complexes (M = Ni, Au)
In contrast to the Ni complexes with cis-anti-forms, each oxddt
ligand of the Au complex 3 has a trans-form (opened-form). Since
two benzene rings in 3 are located on the opposite side toward
the dithiolene ring, this is a trans-anti form (Fig. 1(c)). The h2 and
h3 dihedral angles in 3 (107.40° and 118.97°) are remarkably differ-
ent from those of [CpCo(oxddt)] with the trans-form (116.683° and
108.892°) [14]. The packing diagrams of 1 and 3 show intermolec-
ular interactions between benzene rings with the plane-to-plane
distances of 3.627 and 3.786 Å, respectively (Figs. 2 and 3). How-
ever, no interaction was realized between dithiolene rings in 1
and 3 as well as 2, whereas in the neutral [CpM(oxddt)] complexes,
there were the intermolecular face-to-face interactions between
The oxo derivative of oxddt, [O@C(oxddt)], was reacted with 2
equiv of sodium methoxide in methanol solution to generate air-
sensitive oxddt2ꢀ. A successive addition of NiIICl2ꢁ6H2O into the re-
sulted solution under Ar atmosphere gave the nickel complex with
bis-oxddt ligand, [Ni(oxddt)2]2ꢀ (Scheme 1). An aerial oxidation of
the [Ni(oxddt)2]2ꢀ and then an addition of phosphonium cation
(Et4P+ or Ph4P+) gave the corresponding radical anion salt (Et4-
P)[Ni(oxddt)2] (1) or (Ph4P)[Ni(oxddt)2] (2), respectively. The reac-
tion of the in situ generated oxddt2ꢀ with NaAuIIICl4ꢁ2H2O directly
afforded the air-stable monoanion [Au(oxddt)2]ꢀ without aerial
Scheme 1. Preparations of [M(oxddt)2] salts (M = Ni, Au).

M. Nomura et al. / Polyhedron 47 (2012) 9–15
11
Table 1
Selected bond lengths (Å), bond angles (°) and dihedral angles (°) in metalladithiolene ring.
1
2
3
[CpCo(oxddt)]
[CpCo(CH2)(oxddt)]
Structure type
Bond length
M1–S1
M1–S2
S1–C1
cis-anti
cis-anti
trans-anti
trans
cis
2.1429(8)
2.1378(4)
2.3137(8)
2.118(3)
2.111(3)
1.751(10)
1.711(12)
1.361(13)
2.1858(7)
2.2042(7)
1.767(2)
1.720(2)
1.352(3)
2.1496(9)
1.737(3)
1.726(3)
1.353(5)
2.1486(4)
1.7300(17)
1.7259(17)
1.362(2)
2.3195(9)
1.751(3)
1.745(3)
1.340(4)
S2–C2
C1–C2
Bond angles
S1–M1–S2
M1–S1–C1
M1–S2–C2
S1–C1–C2
S2–C2–C1
Non-chemical bonding
C1ꢁ ꢁ ꢁC5
91.38(3)
104.43(11)
104.44(11)
119.5(2)
91.313(15)
104.79(6)
104.72(6)
119.58(13)
119.46(12)
89.12(3)
101.94(10)
102.56(11)
123.7(2)
91.14(12)
104.5(3)
106.6(3)
119.2(8)
118.0(8)
91.19(2)
103.77(8)
102.93(8)
118.30(15)
123.10(15)
120.0(2)
122.5(2)
b
b
b
b
3.038
3.083
2.932
2.952
2.908
2.959
C2ꢁ ꢁ ꢁC6
Dihedral anglesa
h1
h2
h3
7.23
120.10
121.82
61.98
2.73
112.52
117.51
50.06
3.08
107.40
118.97
11.57
8.026
116.683
108.892
7.791
7.896
115.327
114.484
50.232
[14]
h4
References
This work
This work
This work
[14]
a
Dihedral angles (h1–h4) are defined in Fig. 1.
Not relevant in the trans structure.
b
Table 2
Crystallographic data.
Compound
Formula
1
2
3
C
28H36NiPS8
C44H36NiPS8
910.89
C44H36AuPS8
1049.19
Formula weight (g molꢀ1
)
718.73
Crystal color
Crystal shape
Crystal size (mm)
Crystal system
Space group
Unit cell dimensions
T (K)
Brown
Block
0.50 ꢂ 0.40 ꢂ 0.35
Monoclinic
C2/c (No. 15)
Brown
Block
0.50 ꢂ 0.40 ꢂ 0.30
Monoclinic
C2/c (No. 15)
Yellow
Platelet
0.15 ꢂ 0.13 ꢂ 0.07
Monoclinic
C2/c (No. 15)
293(2)
293(2)
293(2)
a (Å)
b (Å)
c (Å)
25.606(5)
9.6765(17)
16.812(3)
126.5960(10)
3344.4(11)
4
25.8325(16)
8.5894(2)
22.7112(11)
123.2290(10)
4215.3(3)
4
23.674(7)
11.481(4)
17.025(5)
90.1674(15)
4627(2)
4
b (°)
V (Å3)
Z
Dcalc (g cmꢀ3
)
1.427
1.146
1.435
0.927
1.506
3.615
l
(mmꢀ1
)
Total reflections
11920
15589
17425
Unique reflections (Rint
)
3756(0.0200)
3355
0.0559
0.1733
1.117
4763(0.0172)
4438
0.0324
0.0873
1.046
14069(0.030)
5269
0.0495
0.1130
1.038
Unique reflections (I > 2
R1 (I > 2 (I))
wR2 (I > 2 (I))
Goodness-of-fit (GOF)
r(I))
r
r
w(Fo ꢀ Fc2)2/
R
wFo
]
.
2
4 1/2
R1
=
R
||Fo| ꢀ |Fc||/
R
|Fo|; wR2 = [
R
dithioethylene-1,2-dithiolate moieties [14,15]. One reason why we
did not observe such interaction in the [M(oxddt)2]ꢀ anion salt
may be due to Coulomb repulsion of the anionic molecule.
tal structures (e.g. intermolecular arrangement) except some iso-
structural couples [20–22]. However, the molecular structure
itself has not been changed by a substitution of the central metal.
In the [M(oxddt)2] complexes, however, it is quite rare that the Ni/
Au substitution modifies the molecular structure. One reason is
probably due to a structural flexibility of the eight-membered ring
moiety of the oxddt ligand. We think that our result is just similar
case to cis/trans isomers of eight-membered 1,5-cyclooctadiene
derivatives [12].
Fourmigué et al. reported that diverse molecular structures
including trans-syn or trans-anti form derived from bis(2,20-diflu-
oropropylenedithio)tetrathiafulvalene or non-fluorinated bis(pro-
pylenedithio)tetrathiafulvalene radical cation salt with several
different anions [18,19]. The molecular structure of the tetra-
thiafulvalene (TTF) derivative can be controlled by a halogen bond-
ing (or hydrogen bonding) interaction between the terminal
propylene group of TTF cation and halogen atom of the selected an-
ion (Fig. 4). On the basis of the structural determinations of Ni/
Au(oxddt)2 complexes, we observed that the Ni/Au substitution
causes the cis/trans structural change. The Ni/Au substitution in
the normal [M(dithiolene)2] complexes often modifies those crys-
2.3. Molecular orbitals
Molecular orbital distributions of [Ni(oxddt)2]ꢀ radical anion
and [Au(oxddt)2]ꢀ anion were obtained by density functional the-
ory (DFT) methods. These frontier orbital distributions (HOMO,

12
M. Nomura et al. / Polyhedron 47 (2012) 9–15
Fig. 1. ORTEP drawings of the anion moieties of (a) 1 and (c) 3. Thermal ellipsoids are drawn at 40% probability level. Dotted lines in 1 show short intramolecular non-
chemical bondings between dithiolene and benzene rings. Definition of dihedral angles for the anion moieties of (b) 1 and (d) 3: h1 (dithiolene folding angle) = M1S1S2/
S1S2C1C2S3S4, h2 = S1S2C1C2S3S4/S3S4C3C4, h3 = S3S4C3C4/benzene, h4 = S1S2C1C2S3S4/benzene.
Fig. 2. Projection view along the b-axis of 1 demonstrating a
pꢀp stacking through
the benzene ring. The Et4P+ cation is disordered with two possible orientations.
Fig. 4. (a) The trans-syn form of bis(2,20-difluoropropylenedithio)tetrathiafulvalene
radical cation salt with ICl2 anion and (b) the trans-anti form of bis(propylenedi-
thio)tetrathiafulvalene radical cation salt with ICl2 anion (Adapted from Ref. [18]).
Dotted lines show Hꢁ ꢁ ꢁCl halogen (or hydrogen) bonding interactions.
dithiolene-
p orbital for the a
-SOMO in the [Ni(oxddt)2]ꢀ radical
anion (Fig. 5(b)) and for the HOMO in the [Au(oxddt)2]ꢀ anion
(Fig. 5(e)), but small contributions of d orbital for them. These
SOMO and HOMO are mostly distributed to the five-membered
metallacycle. In addition, the dithiolene-
p orbital largely contrib-
uted for the
a
-LUMO of [Ni(oxddt)2]ꢀ radical anion (Fig. 5(a)). It
is different that the LUMO of [Au(oxddt)2]ꢀ anion is due to the anti-
bonding combination of the metal dxy and the dithiolene ligand
orbitals (Fig. 5(d)). The overlap between these two orbitals is favor-
able and provides an efficient pathway for ligand-to-metal
r elec-
tron donation, which is similar to the LUMO of [Au(bdt)2]
(bdt = benzene-1,2-dithiolate) as previously obtained by Wieg-
hardt et al. [23]. Furthermore, the
Fig. 3. Projection view along the b-axis of 3 demonstrating a
the benzene ring.
pꢀp stacking through
a
-LUMO + 5 of [Ni(oxddt)2]ꢀ
radical anion is distributed in the whole molecule including the
terminal benzene rings. Among them, it is notable that there is
SOMO and LUMO) are similar to those of typical Ni/Au(dithiolene)2
species [23,24]. As shown in Fig. 5, there are large contributions of
an intramolecular overlap of
p orbital between benzene and C@C

M. Nomura et al. / Polyhedron 47 (2012) 9–15
13
Fig. 5. (a)
isovalue for the orbital plots is 0.02.
a-LUMO, (b) a-SOMO, (c) a
-LUMO + 5 of [Ni(oxddt)2]ꢀ radical anion moiety, (d) LUMO and (e) HOMO of [Au(oxddt)2]ꢀ anion moiety obtained by DFT methods. The
bond moiety in the metallacycle (Fig. 5(c)). The short intramolecu-
lar non-chemical bondings of C1ꢁ ꢁ ꢁC5 and C2ꢁ ꢁ ꢁC6 obtained by X-
ray analyses (Fig. 1(a)) and obtained by the optimized structure
(2.966 and 3.016 Å) support this
p orbital overlap. Namely, one
interesting thing of the cis-form is an intramolecular approach of
these aromatic rings.
2.4. Electrochemical behavior
In order to study electrochemical behavior, the cyclic voltam-
mograms of 1 and 3 were obtained in dichloromethane solution
containing tetra-n-butylammonium perchlorate (TBAP). 1 showed
a reversible reduction wave at E1/2(red) = ꢀ1.13 V (versus Fc/Fc+),
a reversible oxidation wave at E1/2(ox) = ꢀ0.29 V and an irrevers-
ible oxidation wave at Ep(ox) = +0.72 V (Fig. 6(a)). This CV result
suggests that there are stable dianionic [Ni(oxddt)2]2ꢀ and neutral
[Ni(oxddt)2]0 states on the time scale of this CV measurement but
very unstable cationic [Ni(oxddt)2]+ state. However, the multiple
potential scan of the first oxidation of 1 demonstrated decrease
of the redox current with increasing the number of potential scan,
and then irreversibility of the first oxidation wave (Fig. 6(b)). We
assume that robust precipitation of the neutral [Ni(oxddt)2]0 spe-
cies on the electrode occurred during the multiple potential scan.
In fact, after the multiple scan, a potential scan to ꢀ1.6 V showed
a large cathodic current (Fig. 6(c)), indicating the electrochemical
reduction and following desorption of the precipitated
[Ni(oxddt)2]. On the other hand, 3 showed a reversible reduction
wave at E1/2(red) = ꢀ2.02 V and an irreversible oxidation wave at
Ep(ox) = +0.05 V. Furthermore, there were several oxidation waves
at more than +0.5 V (Fig. 6(d)). Thus, the CV of the Au complex 3
showed negative shift of a reduction potential and positive shift
of an oxidation potential, compared with each of the Ni complex
1. Namely, it seems that [Ni(oxddt)2]ꢀ radical anion has a small
HOMO–LUMO gap to explain its near-IR absorption at kmax = 934 -
Fig. 6. (a) Cyclic voltammograms of 1, (b) multiple scan of the first oxidation wave
of 1, (c) reduction wave after the multiple scan of 1 and (d) CV of 3: all materials
were measured in dichloromethane containing tetra-n-butylammonium perchlo-
rate (TBAP).
absorption energy of [Ni(oxddt)2]ꢀ is higher than those of
[Ni(dddt)2]ꢀ (1175 nm), [Ni(pddt)2]ꢀ (938 nm) and [Ni(dmit)2]ꢀ
(1137 nm), where dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate,
nm (e
= 9400 Mꢀ1 cmꢀ1) in dichloromethane solution, although the

14
M. Nomura et al. / Polyhedron 47 (2012) 9–15
pddt = 6,7-dihydro-5H-1,4-dithiepin-2,3-dithiolate and dmit = 1,3-
dithiol-2-thione-4,5-dithiolate [25]. Furthermore, the irreversible
in 50% yield by using NaAuCl4ꢁ2H2O instead of NiCl2ꢁ6H2O. After
the addition of NaAuCl4ꢁ2H2O, no aerial oxidation is necessary.
Spectroscopic and analytical data of 3 are as follows: 1H NMR
(CDCl3, versus TMS, 500 MHz) d 7.08 (br-s, 4H, benzene), 4.14
(br-s, 4H, CH2). UV–Vis (CH2Cl2) kmax/nm
(16500). Elemental Anal. Calc. for C44H36AuPS8: C, 50.37; H, 3.46.
oxidation wave of
3 suggested that the neutral radical
[Au(oxddt)2]0 is very unstable and then immediately undergoes a
chemical reaction. This result is usual because a neutral [Au(dithio-
lene)2]0 complex is normally unstable unless there is a bulky group
or a long alkyl chain on the dithiolene ligand [26].
(e ) 345
/Mꢀ1 cmꢀ1
Found: C, 50.44; H, 3.51.
4.3. CV measurements
3. Conclusion
All electrochemical measurements were performed under an
argon atmosphere. Solvents for electrochemical measurements
were dried by molecular sieve 4A before use. A platinum wire
served as a counter electrode, and the reference electrode Ag/AgCl
was corrected for junction potentials by being referenced inter-
nally to the ferrocene/ferrocenium (Fc/Fc+) couple. A stationary
platinum disk (1.6 mm in diameter) was used as a working elec-
trode. The Model CV-50W instrument from BAS Co. was used for
cyclic voltaꢀm3metry (CV) measurements. CVs were measured in
1 mmol dm dichloromethane solutions of complexes containing
0.1 molꢁdmꢀ3 tetra-n-butylammonium perchlorate (TBAP) at 25 °C.
In this work, we studied on the square-planar metal dithiolene
complexes of the o-xylenediyldithioethylene-1,2-dithiolate
(oxddt) ligand exhibiting cis and trans structures. Previously, it
has been known that the molecular structure of the mono(dithio-
lene) complex, [CpCo(oxddt)], can be modified by addition of an
alkylidene group on the Co–S bond (Chart 1). However, we ob-
served that
a metal substitution of bis(dithiolene) complex,
[M(oxddt)2], also modifies the molecular structure. Such diverse
structural change is probably due to the structurally flexible oxddt
ligand with an eight-membered ring. It may be suggesting that we
can observe diverse molecular structures by introducing a large-
membered ring (e.g. more than eight-member) to the dithiolene li-
gand. In addition to such attractive structural changes, we also
4.4. Density functional theory (DFT) calculation
found intramolecular
metallacycle, based on DFT calculations. It is just rare that the
orbital of organic aromatics overlapped with that of aromatic
metallacycle.
p orbital overlap between benzene and
Geometries of [Ni(oxddt)2]ꢀ radical anion and [Au(oxddt)2]ꢀ an-
ion were optimized with no constraint using the GAUSSIAN 03 package
[28] and the hybrid functional B3LYP for closed-shell molecules or
UB3LYP for opened-shell molecules [29]. The standard 3-21Gꢃ basis
set [30] was used for H, C and S together with the LanL2DZ for Co. En-
ergy minima were confirmed by frequencies analysis.
p
4. Experimental
4.1. Materials and instrumentation
4.5. X-ray diffraction study
All reactions were carried out under argon atmosphere by
means of standard Schlenk techniques. MeOH was obtained from
Wako Pure Chemical Industries, Ltd. and then distilled by CaH2 be-
fore use. MeONa was prepared by dry MeOH with sodium metal.
NiCl2ꢁ6H2O, NaAuCl4ꢁ2H2O, Ph4PBr and Et4PBr were obtained from
Wako Pure Chemical Industries, Ltd. o-Xylenediyldithioethylene-
1,3-dithiol-2-one, O@C(oxddt),[27] was prepared by the reaction
of S@C(oxddt) with mercuric acetate. Elemental analyses were
determined by using a Shimadzu PE2400-II instrument. NMR spec-
tra were measured with a JEOL LA500 spectrometer. UV–Vis spec-
tra were recorded on a Hitachi model UV-2500PC.
Single crystals of 1–3 were obtained by recrystallization using
vapor diffusion of Et2O into the acetone solution. A single crystal
was mounted on the top of a thin glass fiber. The measurement
was made on a Rigaku Mercury diffractometer with graphite-
monochromated MoKa radiation. The data were corrected for Lor-
entz and polarization effects. The structure was solved by direct
methods and expanded using Fourier techniques [31]. The non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were refined using the riding model. All the calculations were car-
ried out using the CRYSTAL STRUCTURE CRYSTALLOGRAPHIC software package
[32] for 3 and WINGX software package for 1 and 2 [33]. Crystallo-
graphic data are summarized in Table 2.
4.2. Preparation of [M(oxddt)2] salts
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Sodium methoxide in MeOH (2.8 ml, 2.8 mmol) was added to a
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