Nickel(II) complexes bearing a pincer ligand containing thioamide units: Comparison between SNS- And SCS-pincer ligands

Article abstract of DOI:10.1016/j.ica.2010.04.012

Nickel(II) complexes bearing a K³SNS pincer ligand, 2,5-bis(benzylaminothiocarbonyl)pyrrolyl (L1) and a K³SCS-pincer ligand, 2,6-bis(benzylaminothiocarbonyl)phenyl (L2), were synthesized, and their structures and electrochemical properties were elucidated. The crystal staictures of [Ni(SNS)Br] (2) and [Ni(SCS)Br] (5) were determined by X-ray crystallography. The electrochemical and crystallographic data obtained from the complexes revealed that the K³SCS ligand has a stronger electron-donating ability than the K³SNS ligand.

Full text of DOI:10.1016/j.ica.2010.04.012

Inorganica Chimica Acta 363 (2010) 2474–2480
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Nickel(II) complexes bearing a pincer ligand containing thioamide units:
Comparison between SNS- and SCS-pincer ligands
Take-aki Koizumi a, , Takuya Teratani a,b, Ken Okamoto a,b, Takakazu Yamamoto a, Yukihiro Shimoi c,d
,
*
Takaki Kanbara b,e,
**
a Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
b Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan
c Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
d National Institute for Nanotechnology, National Research Council of Canada, 11421 Saskatchewan Drive, Edmonton, Albert, Canada T6G 2M9
e Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Nickel(II) complexes bearing a
j
tures and electrochemical properties were elucidated. The crystal structures of [Ni(SNS)Br] (2) and
[Ni(SCS)Br] (5) were determined by X-ray crystallography. The electrochemical and crystallographic data
j
3SNS pincer ligand, 2,5-bis(benzylaminothiocarbonyl)pyrrolyl (L1) and a
3SCS-pincer ligand, 2,6-bis(benzylaminothiocarbonyl)phenyl (L2), were synthesized, and their struc-
Received 20 November 2009
Received in revised form 31 March 2010
Accepted 6 April 2010
Available online 13 April 2010
obtained from the complexes revealed that the
the
3SNS ligand.
j
3SCS ligand has a stronger electron-donating ability than
j
Keywords:
Nickel
Ó 2010 Elsevier B.V. All rights reserved.
Pincer complex
X-ray crystal structures
Electron-donating ability
1. Introduction
role unit as a central aromatic ring are considered to be different
from those of benzene-centered pincer complexes. However, there
have been no reports on the electrochemical properties of pyrrole-
centered pincer complexes to the best of our knowledge. In this pa-
per, we compared the SNS- and SCS-pincer ligand frameworks and
elucidated the structure and electronic properties of their pincer
nickel complexes.
In recent years, nickel complexes bearing pincer ligands [1–6]
have been widely investigated because of their interesting chemical
properties. Many reports have been published on the electrochem-
ical behavior of Ni–pincer complexes [2]. The strong electron-
donating ability of the pincer ligand leads to a negative shift of
the Ni(III)/Ni(II) redox potential [3]. However, most of the reported
Ni–pincer complexes have a benzene unit as a central aromatic ring
2. Experimental
of the pincer ligand. Recently, we have synthesized (j
3SNS)-pincer
complexes constructed of a pyrrole unit as a central aromatic ring
and thioamide units as side arms as shown in Scheme 1 [4].
These complexes are stable in air, and the results of X-ray stud-
ies suggest that delocalization of the electrons occur through the
central pyrrole ring and the thioamide group. Such a replacement
of the central benzene unit by other aromatic or non-aromatic
units modulates the chemical and electronic properties of the pin-
cer complexes, and revealing the effect of modulation will be
intriguing [3b,d,5]. From the chemical and electrochemical view-
points, the electronic properties of pincer complexes having a pyr-
2.1. General, measurement, and materials
1H NMR spectra were recorded on a JEOL Lambda-300 NMR
spectrometer. 13C{1H} NMR spectra were recorded on a JEOL JNM
EX-400 NMR and Bruker Avance III 400 MHz NMR spectrometer.
IR spectra were recorded on a JASCO IR-810 spectrophotometer.
UV–Vis spectra were measured with a Shimadzu UV-2550 UV–
Vis spectrophotometer. Elemental analysis was carried out by the
Center for Advanced Materials Analysis (Suzukakedai) in Tokyo
Institute of Technology. Thermal analysis was performed with a
Yanagimoto Seisakusho Micro Melting Point Apparatus. Electro-
chemical measurements were performed with a Hokuto Denko
HSV-100 automatic polarization system. A conventional three-
electrode configuration was used, with glassy carbon working
(BAS electrode) and platinum wire auxiliary electrodes (Tokuriki,
special order) and an Ag/0.1 M AgNO3 reference (BAS RE-5). Cyclic
* Corresponding author. Tel.: +81 45 924 5222; fax: +81 45 924 5976.
** Corresponding author at: Tsukuba Research Center for Interdisciplinary Mate-
rials Science (TIMS), University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8573,
Japan.
E-mail address: tkoizumi@res.titech.ac.jp (T.-a. Koizumi).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.04.012

T.-a. Koizumi et al. / Inorganica Chimica Acta 363 (2010) 2474–2480
2475
M.p.: 146–148 °C (dec.). Anal. Calc. for C22H19BrN2NiS2: C, 51.39;
H, 3.72; N, 5.45. Found: C, 50.98; H, 4.18; N, 6.07%. 1H NMR
(300 MHz, acetone-d6): d 10.18 (2H, s), 7.67 (2H, d, J = 7.9 Hz),
7.50 (4H, d, J = 6.6 Hz), 7.36 (7H, m), 5.08 (4H, d, J = 6.1 Hz).
13C{1H} NMR (100 MHz, acetone-d6):
d
198.816, 147.506,
136.290, 129.114, 128.325, 128.203, 125.763, 122.188, 50.083.
FT-IR (KBr, cmꢀ1): 3362, 3225, 1651, 1553, 1495, 1454, 1290,
1184, 1024, 928, 797, 754, 700.
2.2.5. [2,6-Bis(benzylaminothiocarbonyl)-j2S,S0-phenyl-
j
Scheme 1. Molecular structure of the M-(j
3SNS) complex.
C1]iodonickel (6)
The procedure for
4 was modified using NiI2 (31.25 mg,
voltammograms were recorded at a scan rate of 50 mV sꢀ1. Fc+/
Fc = +115 mV versus 0.10 M AgNO3/Ag, and +425 mV versus SCE.
N,N0-dibenzyl-1H-pyrrole-2,5-dicarbothioamide (L1H), N,N0-diben-
zyl-benzene-1,3-dicarbothioamide (L2H), and [2,5-bis(benzylami-
nothiocarbonyl)pyrrolyl-j3N1,S,S0]chloronickel (1) were prepared
according to the literature methods [4].
0.1 mmol) to afford a red precipitate of 6 (42 mg, 77% yield).
M.p.: 178–185 °C (dec.). Anal. Calc. for C22H19IN2NiS2: C, 47.09; H,
3.41; N, 4.99. Found: C, 46.61; H, 3.44; N, 4.91%. 1H NMR
(300 MHz, acetone-d6): d 10.23 (2H, s), 7.74 (2H, d, J = 8.0 Hz),
7.49 (2H, d, J = 7.9 Hz), 7.36 (8H, m), 7.11 (1H, t, J = 8.0 Hz), 5.08
(4H, d, J = 6.1 Hz). 13C{1H} NMR (100 MHz, acetone-d6):
d
202.061, 147.244, 135.659, 128.704, 128.281, 127.963, 124.231,
121.929, 50.227. FT-IR (KBr, cmꢀ1): 3362, 3264, 1553, 1495,
1455, 1420, 1343, 1291, 1181, 1022, 922, 795, 739, 718, 698.
2.2. Synthesis and characterization
2.2.1. [2,5-Bis(benzylaminothiocarbonyl)pyrrolyl-j
3N1,
S,S0]bromonickel (2)
2.3. Crystal structure determination
NiBr2 (43.7 mg, 0.20 mmol) and L1H (75.2 mg, 0.20 mmol) were
dissolved in EtOH (5 mL) and stirred at room temperature for 24 h.
The generated red precipitate was collected by filtration, washed
with hexane, EtOH, and water, and dried in vacuo. (77.5 mg, 77%
yield). M.p.: 148–150 °C (dec.). Anal. Calc. for C20H18BrN3NiS2: C,
47.75; H, 3.61; N, 8.35. Found: C, 47.45; H, 3.82; N, 8.13%. 1H
NMR (300 MHz, acetone-d6): d 10.14 (2H, s), 7.37 (10H, m), 6.57
(2H, s), 4.93 (4H, d, J = 6.7 Hz). 13C{1H} NMR (100 MHz, acetone-
d6): d 187.732, 144.997, 135.761, 128.747, 128.353, 128.050,
112.359, 50.391. FT-IR (KBr, cmꢀ1): 3280, 3086, 2957, 1558,
1496, 1390, 1344, 1261, 1180, 752, 418.
Crystals of 2 and 5 for X-ray analysis were obtained as described
in the preparations. The crystal of 2 suitable for X-ray diffraction
study was obtained by recrystallization from MeCN/EtOH and that
of 5 was obtained from DMF. The suitable crystal was mounted on
a glass fiber. Data collection for 2 and 5 was performed at ꢀ160 °C
on a Rigaku/MSC Saturn CCD diffractometer with graphite mono-
chromated Mo K
to a maximum 2h value of 55°. A total of 720 oscillation images
were collected. A sweep of data was done using scans from
ꢀ110° to 70° in 0.5° steps, at = 45.0° and u = 0.0°. The structures
a radiation (k = 0.7107 Å). The data were collected
x
v
were solved by using the CRYSTALSTRUCTURE software package [7].
Atom scattering factors were obtained from the literature. Refine-
ments were performed anisotropically for all non-hydrogen atoms
by the full-matrix least-square method. Hydrogen atoms except for
H1 and H2 were placed at the calculated positions and were in-
cluded in the structure calculation without further refinement of
the parameters. H1 and H2 of 2 and 5 were determined by differ-
ence Fourier map and refined isotropically. The residual electron
densities were of no chemical significance. Crystal data and pro-
cessing parameters are summarized in Table 6.
2.2.2. [2,5-Bis(benzylaminothiocarbonyl)pyrrolyl-j3N1,S,S0]iodonickel
(3)
The procedure for 2 was adopted using NiI2 (31.3 mg, 0.1 mmol)
to afford a red precipitate of 3 (42.0 mg, 77% yield). M.p.: 128–
130 °C (dec.). Anal. Calc. for C20H18IN3NiS2: C, 43.67; H, 3.3; N,
7.64. Found: C, 42.81; H, 3.67; N, 7.03%. 1H NMR (300 MHz, ace-
tone-d6): d 10.15 (2H, s), 7.36 (10H, m), 6.67 (2H, d, J = 8.1 Hz),
4.93 (4H, d, J = 6.7 Hz). 13C{1H} NMR (100 MHz, acetone-d6): d
188.529, 144.616, 135.690, 128.764, 128.312, 128.076, 112.252,
50.513. FT-IR (KBr, cmꢀ1): 3289, 3248, 1653, 1387, 1348, 1263,
1240, 1055, 1028, 887, 744, 696.
2.4. Computational details
2.2.3. [2,6-Bis(benzylaminothiocarbonyl)-j2S,S0-phenyl-
j
C1]chloronickel (4)
All the TD-DFT calculations and natural population analysis
(NPA) reported in this study were carried out using the GAUSSIAN
03 suite of programs [8]. We employed the B3PW91 functional
with LANL2DZ basis set implemented in GAUSSIAN 03 programs suits.
The molecular and crystal structures of all Ni–pincer complexes
derivative have been determined by X-ray diffraction were used
for the NPA calculations. Geometry optimization for TD-DFT calcu-
lation was carried out under the constraint of Cs symmetry.
NiCl2ꢁ6H2O (120 mg, 0.5 mmol) and L2H (188 mg, 0.5 mmol)
were dissolved in toluene (5 mL) and stirred at 110 °C for 72 h.
The precipitation were collected by filtration, washed with hexane
and small amount of EtOH, and dried in vacuo to afford 4 as a red
powder. (163 mg, 70% yield). M.p.: 120–125 °C (dec.). Anal. Calc.
for C22H19ClN2NiS2: C, 56.26; H, 4.08; N, 5.96. Found: C, 55.90; H,
3.94; N, 5.71%. 1H NMR (300 MHz, acetone-d6): d 10.13 (2H, s),
7.59 (2H, d, J = 7.7 Hz), 7.45 (4H, d, J = 7.1 Hz), 7.34 (6H, m), 7.03
(1H, t, J = 7.7 Hz), 5.04 (4H, d, J = 6.2 Hz). 13C{1H} NMR (100 MHz,
3. Results and discussion
DMSO-d6):
d 197.9, 171.56, 146.71, 135.56, 128.34, 127.58,
127.42, 125.06, 121.42, 49.43 FT-IR (KBr, cmꢀ1): 3360, 3231,
1558, 1454, 1419, 1344, 1288, 1228, 1180, 1022, 924, 754, 733, 700.
3.1. Synthesis of Ni complexes
The pincer ligand L1H [4] reacted with NiCl2 in ethanol at room
2.2.4. [2,6-Bis(benzylaminothiocarbonyl)-j2S,S0-phenyl-
j
temperature to give the nickel–j
3SNS-pincer complex 1 with a 68%
C1]bromonickel (5)
yield as shown in Eq. (1). Similarly, Br-complex 2 and I-complex 3
were obtained from the reaction of L1H with NiBr2 and NiI2, both
with a 77% yield.
The procedure for
4 was modified using NiBr2 (43.7 mg,
0.2 mmol) to afford a red precipitate of 5 (54.5 mg, 53% yield).

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ð1Þ
SCS-pincer Ni complexes 4–6 were prepared by the reaction of
NiX2 (X = Cl, Br, I) with L2H in toluene as shown in Eq. (2).
1.859(4) Å) [2a,9], and shorter than those in benzene-centered
PCP pincer complexes (1.879(2)–1.931(2) Å) [3b,3d,10]. The
ð2Þ
All complexes, 1–6, were characterized by IR and 1H NMR spec-
troscopy. In the 1H NMR spectra of 1, 2, and 3, a broadened N–H
resonance of the thioamide group appeared at d 10.09, 10.14, and
10.15, respectively. The peak was shifted to a lower magnetic field
from that of L1H (d 8.98) by 1.11–1.17 ppm. The resonance of the
pyrrole N–H proton in L1H (d 10.62) reasonably disappeared for
1–3. On the other hand, in the 1H NMR spectra of 4–6, the reso-
nance based on the N–H in the thioamide group appeared at d
10.13, 10.18, and 10.23, respectively. They were also shifted to a
lower magnetic field from that of the N–H (thioamide) signal of
L2H.
3.2. Molecular structures of Ni 2 and 5
Fig. 2. ORTEP drawing of 5ꢁDMF with 50% ellipsoidal level. Hydrogen atoms except
Figs. 1 and 2 depict the molecular structures of 2 and 5, respec-
tively, determined by X-ray crystallography, and selected bond
lengths and angles of the complexes are summarized in Tables 1
and 2, respectively. Complexes 2 and 5 have an almost planar
structure; the sum of the bond angles around the Ni center of 2
and 5 are 359.98° and 360.00°, respectively. The Ni1–C1 bond
length in 5 is 1.8630(11) Å, which is similar or longer than those
in benzene-centered Ni(II)–NCN pincer complexes (1.814(2)–
for H1 and H2 and a solvated DMF molecule are omitted for clarity.
Table 1
Selected bond lengths (Å) and angles (°) of 2ꢁMeCN.
Complex 2
Ni1–Br1
Ni1–S2
S1–C5
N2–C5
2.321(2)
2.241(4)
1.717(12)
1.348(7)
1.473(18)
177.6(3)
93.90(12)
88.0(3)
122.7(8)
115.1(11)
115.9(9)
98.3(4)
Ni1–S1
Ni1–N1
S2–C13
N3–C13
2.229(3)
1.817(12)
1.703(15)
1.311(6)
1.43(2)
94.38(11)
171.72(15)
83.7(3)
130.0(9)
110.1(12)
119.2(10)
97.0(5)
C1–C5
C4–C13
Br1–Ni1–N1
Br1–Ni1–S2
S1–Ni1–N1
Ni1–N1–C1
N1–C1–C5
S1–C5–C1
Ni1–S1–C1
Br1–Ni1–S1
S1–Ni1–S2
S2–Ni1–N1
Ni1–N1–C4
N1–C4–C13
S2–C13–C4
Ni1–S2–C13
Ni1–Br1 bond length in 2 is 2.321(2) Å, which is shorter than that
in 5 (2.3686(2) Å); this is due to the trans influence of the benzene
unit is more significant than that of the pyrrole unit.
Fig. 1. ORTEP drawing of 2ꢁMeCN with 50% ellipsoidal level. Hydrogen atoms
except for H1 and H2 and a solvated MeCN molecule are omitted for clarity.

T.-a. Koizumi et al. / Inorganica Chimica Acta 363 (2010) 2474–2480
2477
Table 2
The X-ray crystallographic data of 2 reveal that it contains
solvated acetonitrile in the crystal state. The two N–H protons in
2 undergo on intermolecular interaction with other molecules.
One N–H proton in 2 is positioned near the Br atom in the neigh-
boring Ni complex (N–Hꢁ ꢁ ꢁBr, 2.67(11) Å), and the other N–H pro-
ton interacts with the N atom in solvated acetonitrile (N–Hꢁ ꢁ ꢁN,
2.09(15) Å). Fig. 3a shows the packing structure of 2ꢁMeCN. As
shown in Fig. 3a, 2ꢁMeCN forms an alternating network by hydro-
gen bonding. The N–H hydrogens of 5ꢁDMF also undergo intermo-
lecular interactions. As depicted in Fig. 3b, one of the N–H
hydrogen atoms interacts with the Br atom in the neighboring mol-
ecule with an Ni–Brꢁ ꢁ ꢁH–N distance of 2.750(15) Å. The other N–H
hydrogen interacts with solvated DMF with a C@Oꢁ ꢁ ꢁH–N distance
of 1.909(13) Å. 2ꢁMeCN and 5ꢁDMF have different packing struc-
tures form each other. The packing structure of 2ꢁMeCN is similar
to that of the previously reported 1ꢁMeCN [4], and 5ꢁDMF has a
packing structure similar to that of a DMF-solvated SCS–Ni–Cl
complex [6b].
Selected bond lengths (Å) and angles (°) of 5ꢁDMF.
Complex 5
Ni1–Br1
Ni1–S2
S1–C7
N1–C7
2.3686(2)
2.1757(3)
1.7163(12)
1.3144(15)
1.4626(15)
175.82(3)
90.797(10)
87.73(3)
121.46(8)
114.35(10)
115.83(8)
100.52(3)
Ni1–S1
Ni1–C1
S2–C15
N2–C15
2.1735(3)
1.8630(11)
1.7034(12)
1.3203(14)
1.4614(16)
94.007(10)
175.195(14)
87.47(3)
121.56(8)
114.58(10)
115.80(8)
100.41(4)
C6–C7
C2–C15
Br1–Ni1–C1
Br1–Ni1–S2
S1–Ni1–C1
Ni1–C1–C2
C1–C6–C7
S1–C7–C6
Ni1–S1–C7
Br1–Ni1–S1
S1–Ni1–S2
S2–Ni1–C1
Ni1–C1–C6
C1–C2–C15
S2–C15–C2
Ni1–S2–C15
In the IR spectra, the
m
(N–H) band of 1–3 was observed as a
(N–
shoulder peak at about 3300 cmꢀ1. On the other hand, the
m
H) peaks of 4–6 were clearly observed at 3360–3362 cmꢀ1. As
mentioned above, L1 in 2 has an electron-delocalized resonance
structure as shown in Scheme 2 [4]. The electronic structure ap-
The electronic spectra of 2 and 5 are shown in Fig. 4, and the
spectral data of 1–6 are summarized in Table 3. Complexes 1–3 ex-
hibit intense absorption bands at kmax = 286–292 and 369–372 nm,
pears to give a lower shift of the wavenumber of the m(N–H) band
in complexes 1–3.
which are assigned to the
p
p* transitions in the SNS pincer
ꢀ
Scheme 2. Delocalization of electrons in the Ni(SNS) complex.
Fig. 3. Packing diagrams of (a) 2ꢁMeCN and (b) 5ꢁDMF.

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T.-a. Koizumi et al. / Inorganica Chimica Acta 363 (2010) 2474–2480
Fig. 4. Electronic spectra of (a) 2 and (b) 5 in MeCN.
Fig. 6. Cyclic voltammograms of (a) 1 and (b) 4 in MeCN; 0.10 M [N(n-Bu)4]PF6,
scan rate 50 mV sꢀ1, glassy carbon working electrode.
Table 3
UV–Vis spectral data for the complexes in MeCN.
Table 5
kmax (nm) (e )
/L molꢀ1 cmꢀ1
Electrochemical data for complexes 1–6.
E/V (versus Fc+/Fc)
1
2
3
4
5
6
292 (43 100), 372 (16 700)
287 (42 700), 369 (9800)
246 (24 900), 286 (43 300), 369 (20 200)
248 (64 400), 452 (9000)
248 (57 400), 444 (8800)
246 (69 100), 440 (9250)
[Ni]+/[Ni]0
[Ni]2+/[Ni]+
[Ni]3+/[Ni]2+
1
2
3
4
5
6
Epa = 0.397
Epc = 0.274
Epa = 0.768 (irr)
Epa = 0.464
Epc = 0.223
Epa = 0.847 (irr)
Epa = 0.457
Epc =0.369
Table 4
Epa = 0.102
Epc =0.008
Epa = 0.566 (irr)
Epa = 0.362 (irr)
Epa = 0.362 (irr)
Epa = 0.807 (irr)
Epa = 0.787 (irr)
Selected calculated singlet excited-state transitions for complexes for 2 and 5.
Complex Wavelength Oscillator
Assignment (% of major transition
contributing to the band)
Epa = 0.092
Epc = ꢀ0.012
Epa = 0.139
Epc = ꢀ0.098
(nm)
strength
2
5
506
437
401
0.0368
0.0724
0.0654
HOMO ? LUMO (89%)
HOMOꢀ2 ? LUMO (46%)
HOMOꢀ4 ? LUMO (80%)
Condition: Sample: 1 mM/MeCN; electrolyte: [N(n-Bu)4]PF6 0.1 M; working elec-
trode: glassy carbon; counter electrode: Pt wire. Sweep rate: 50 mV sꢀ1
.
ligand. Weak absorption is observed in the region of 390–420 nm,
which can be assigned to a metal-to-ligand charge transfer (MLCT)
band [7,11]. In 4–6, a significant MLCT band is observed at
kmax = 440–452 nm, whose intensities are stronger than those of
theory (TD-DFT) calculations (Table 4) on isolated complexes.
Three-dimensional plots of the HOMOs and LUMOs of 2 and 5,
and their molecular orbitals (MO) are depicted in Fig. 5 using
GaussView 4.1 [12]. From the results of the TD-DFT calculation, 2
1–3. The assignments of the
pꢀ
p
* transition and the MLCT absorp-
tion bands were examined by time-dependent density functional
Fig. 5. Molecular orbitals (MO) and their levels of (a) 2 and (b) 5 calculated at the B3PW91/LANL2-DZ level, respectively.

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2479
Table 6
stronger transitions in the MLCT band originating from HOMOꢀ2
to LUMO and HOMOꢀ4 to LUMO. These occupied levels possess a
certain amount of weight at the d orbitals of the nickel atom and
Crystal data and details of the structure refinement of 2ꢁMeCN and 5ꢁDMF.
2ꢁMeCN
22H21BrN4NiS2
5ꢁDMF
C25H26BrN3NiS2O
587.22
monoclinic
P21/c (no. 14)
9.1750(15)
18.545(3)
the
p orbitals of the ligand, respectively, and the LUMOs possess
Formula
C
a strong ligand character.
Molecular weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
544.16
monoclinic
P21/c (no. 14)
9.084(10)
16.121(17)
15.850(16)
3.3. Electrochemical and optical data of the Ni complexes
Cyclic voltammograms (CVs) of 1–6 were recorded in a DMF
solution containing 0.10 M [N(n-Bu)4]PF6 as the supporting elec-
trolyte using a glassy carbon working electrode. The CVs of 1 and
4 in the region of ꢀ0.3 to +0.5 V are displayed in Fig. 6, and the
electrochemical data of complexes 1–6 are summarized in Table 5.
In the region as shown in Fig. 6, 1 exhibits a quasi-reversible wave
based on the Ni(III)/Ni(II) redox couple [13] at E1/2 = +0.336 V (ver-
sus Fc+/Fc). The E1/2 value of the Ni(III)/Ni(II) redox couple of 4 is
observed at +0.055 V (versus Fc+/Fc). The occurrence of a more fac-
ile oxidation of 4 than that of 1 suggests a higher electron-donating
ability of L2 to Ni than that of L1. A similar tendency is also ob-
served for other couples of complexes (2 versus 5 and 3 versus 6,
Table 5). These results are consistent with the X-ray crystallo-
graphic and electronic spectral data. In almost all cases, many of
the Ni–pincer complexes exhibited an irreversible oxidative wave
in a more positive region, suggesting the formation of higher oxi-
dized Ni species or oxidation of the thioamide ligand, however,
the formed species have not been identified.
14.721(2)
a
b (°)
(°)
103.260(5)
96.0577(9)
c
(°)
V (Å3)
2259(4)
4
28.265
1016.00
1.479
5083
5083
271
0.0808
0.1165
0.1878c
2490.8(7)
4
25.790
1200.00
1.566
5482
5482
402
0.0253
0.0315
0.0350d
Z
l
(cmꢀ1
)
F(0 0 0)
Dcalc (g cmꢀ3
)
No. of unique reflections
No. of reflections used
No. of variables
R1
R
a
b
Rw
P
P
a
b
c
R1
Rw
=
¼
||Fo| ꢀ |Fc||/ |Fo| for I > 2.0
r
:
(I) data.
P
P
2
2
1=2
½wðF2o ꢀ F2c Þ = wðFo2Þ ꢂ
Wei0067hting Scheme 1/[0:0159F2o þ 1:0000
r
ðF2oÞ].
Weighting Scheme 1/[0:0005F2o þ 1:0000
r
ðF2oÞ].
d
has a relatively weak absorption at 506 nm that is dominated by a
transition from HOMO to LUMO. On the other hand, 5 has two
The DFT calculation also suggests the higher electron-donating
ability of L2 than that of L1. The contour plots of the HOMOs of 2
Fig. 7. Natural charge of (a) 2 and (b) 5 calculated by NPA.

2480
T.-a. Koizumi et al. / Inorganica Chimica Acta 363 (2010) 2474–2480
[3] (a) D.M. Grove, G. van Koten, P. Mul, R. Zoet, J.G.M. Van der Linden, J. Legters,
J.E.J. Schmitz, N.W. Murrall, A.J. Welch, Inorg. Chem. 27 (1988) 2466;
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Clarkson, M. Lutz, A.L. Spek, G.P.M. van Klink, G. van Koten, Organometallics 26
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25 (2006) 602;
and 5 are shown in Fig. 6. The HOMO level of 2 (ꢀ0.208 eV) is
slightly deeper than that of 5 (ꢀ0.197 eV). We have also calculated
the electron densities of 2 and 5 using a DFT method. The atomic
charges were estimated by natural population analysis (NPA)
[14], which is well known to be less dependent on the basis set
than Mulliken population analysis. The NPA charges of 2 and 5
are shown in Fig. 7. These results indicate that the natural charge
of the nickel atom of 2 is more positive than that of 5, which is con-
sistent with the results of the X-ray crystallographic study and cyc-
lic voltammetry.
(c) L.-C. Liang, P.-S. Chien, J.-M. Lin, M.-H. Huang, Y.-L. Huang, J.-H. Liao,
Organometallics 25 (2006) 1399;
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D.J. Mindiola, Inorg. Chem. 46 (2007) 6271;
4. Conclusion
(g) H. Meguro, T. Koizumi, T. Yamamoto, T. Kanbara, J. Orgnomet. Chem. 693
(2008) 1109;
(h) G.W. Bates, P.A. Gale, M.E. Light, M.I. Ogden, C.N. Warriner, Dalton Trans.
(2008) 4106.
We have elucidated the synthesis, structures, and electrochem-
ical properties of Ni–SNS and Ni–SCS-pincer complexes. The pincer
ligands smoothly coordinate to Ni in a tridentate fashion via a
cyclometalation process. The Ni(III)/Ni(II) redox potentials of com-
plexes 1–6 are significantly influenced by the electronic effects of
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(b) K. Okamoto, Ph.D. Thesis, Tokyo Institute of Technology, 2008.
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Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin,
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G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
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[9] (a) D.M. Grove, G. van Koten, H.J.C. Ubbels, R. Zoet, Organometalics 3 (1984)
1003;
the N- and C-
r-bonded unit in the pincer ligand. The X-ray crys-
tallography of 2 and 5 indicated that the trans influence of L2 is lar-
ger than that of L1. These results reveal that the centered benzene
unit of L2 has a higher electron-donating capability to the metal
center than centered pyrrole unit of L1. These findings are ex-
pected to contribute to the modulation of chemical properties of
the pincer complexes.
Acknowledgement
Y.S. acknowledges partial financial support from the Grant-in-
Aid for Science Research in a Priority Area ‘‘Super-Hierarchical
Structures” (17067018) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan.
(b) D.M. Grove, G. van Koten, W.P. Mul, A.A.H. van der Zeijden, J. Terheijden,
Organometallics 5 (1986) 322;
Appendix A. Supplementary material
(c) J.A.M. van Beek, G. van Koten, M.J. Ramp, N.C. Coenjaarts, D.M. Grove, K.
Goubitz, M.C. Zoutberg, C.H. Stam, W.J.J. Smeets, A.L. Spek, Inorg. Chem. 30
(1991) 3059.
CCDC 707890 and 707891 contain the supplementary crystallo-
graphic data for 2 and 5. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via http://
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2010.04.012.
[10] (a) M.E. van der Boom, S.-Y. Liou, L.J.W. Shimon, Y. Ben-David, D. Milstein,
Inorg. Chim. Acta 357 (2004) 4015;
(b) J. Capora, P. Palma, D. del Rio, E. Alvarez, Organometallics 23 (2004) 1652;
(c) J. Capora, P. Palma, D. del Rio, M.M. Conejo, E. Alvarez, Organometallics 23
(2004) 5653;
(d) D. Benito-Garagorri, V. Bocokie, K. Mereiter, K. Kirchner, Organometallics
25 (2006) 3817.
[11] The MLCT absorption band of Ni–pincer complexes has been discussed
previously: L.A. van de Kuil, D.M. Grove, J.W. Zwikker, L.W. Jenneskens, W.
Drenth, G. van Koten, Chem. Mater. 6 (1994) 1675.
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