Divalent transition metal(II) complexes of two heterocyclic ligands with pendant pyridinyl groups: Synthesis, characterization and electrochemistry

Article abstract of DOI:10.1016/j.poly.2013.11.032

Treatment of the pentadentate macrocyclic ligand 4-(pyridin-2-ylmethyl)-1, 7-dithia-4,10-diazacyclododecane, L, and the hexadentate macrocyclic ligand 4,10-bis(pyridin-2-ylmethyl)-1,7-dithia-4,10-diazacyclododecane, L′, with one equivalent of a M²⁺ metal salt (M = Co, Ni, Cu) in methanol yielded six mononuclear complexes, [CoL(MeCN)](ClO₄)₂ (1), [NiL(H₂O)]Cl₂ (2), [CuL(MeCN)](ClO₄) ₂ (3) and [ML′](ClO₄)₂ (4-6). Their solid state crystal structures were determined using X-ray single crystal diffraction analysis. In complexes 1-3, the metal centres exhibit a slightly distorted octahedral geometry, resulting from the coordination of three nitrogen and two sulfur donating atoms in the ligand L and one solvent molecule (MeCN for complexes 1 and 3, water for complex 2) whereas in complexes 4-6, the coordination around the metal centres was satisfied by four nitrogen and two sulfur atoms from the ligand L′. The electronic spectra and electrochemistry of these complexes are reported

Full text of DOI:10.1016/j.poly.2013.11.032

Polyhedron 69 (2014) 181–187
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Divalent transition metal(II) complexes of two heterocyclic ligands with
pendant pyridinyl groups: Synthesis, characterization
and electrochemistry
Zhenhong Wei a, Yan Peng a, David L. Hughes c, Jia Zhao a, Lingman Huang a, Xiaoming Liu a,b,
⇑
a Department of Chemistry, Nanchang University, Nanchang 330031, China
b College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing 314001, China
c School of Chemistry, University of East Anglia, Norwich NR4 7TJ, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
Treatment of the pentadentate macrocyclic ligand 4-(pyridin-2-ylmethyl)-1,7-dithia-4,10-diazacyclod-
odecane, L, and the hexadentate macrocyclic ligand 4,10-bis(pyridin-2-ylmethyl)-1,7-dithia-4,10-diaza-
cyclododecane, L0, with one equivalent of a M2+ metal salt (M = Co, Ni, Cu) in methanol yielded six
mononuclear complexes, [CoL(MeCN)](ClO4)2 (1), [NiL(H2O)]Cl2 (2), [CuL(MeCN)](ClO4)2 (3) and
[ML0](ClO4)2 (4–6). Their solid state crystal structures were determined using X-ray single crystal diffrac-
tion analysis. In complexes 1–3, the metal centres exhibit a slightly distorted octahedral geometry, result-
ing from the coordination of three nitrogen and two sulfur donating atoms in the ligand L and one solvent
molecule (MeCN for complexes 1 and 3, water for complex 2) whereas in complexes 4–6, the coordina-
tion around the metal centres was satisfied by four nitrogen and two sulfur atoms from the ligand L0. The
electronic spectra and electrochemistry of these complexes are reported.
Received 4 September 2013
Accepted 18 November 2013
Available online 1 December 2013
Keywords:
Macrocyclic ligands
Transition metal ions
Crystal structures
Pyridinyl derivatives
Electrochemistry
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
ing capability can stabilise a transition metal ion of a high oxida-
tion state. The pyridinyl group is both a
r-donor and p-acceptor,
Busch was the first to show that thioether macrocyclic ligands
effectively bind transition metal ions [1]. Since then, the develop-
ment of macrocyclic ligand assemblies continues to being stimu-
lated in that such ligands form transition metal complexes with
tunable physicochemical and functional properties, and thus these
complexes have diverse application in pharmaceuticals [2], solar
cells as photosensitizers [3], organic transformations as catalysts
[4], molecular devices [5], mimics of metalloenzymes [6] and cat-
alytic hydrolysis [7].
Macrocyclic ligands with pendant substituents on the rings
have become important since the pendant arms render manoeu-
vrability to further tune the properties of their transition metal
complexes to achieve particular functionalities [8–12]. Although
the macrocycle ring size largely controls the coordination chemis-
try, the pendant substituents on the ring also make contributions
[13–16]. For example, macrocycles armed with substituents of a
with moderate strength in the spectrochemical series [17]. It
shows strong binding ability towards transition metal ions and is
effective in stabilising low oxidation states of metal ions.
In recent years, we have been interested in synthesizing ligands
bearing pyridinyl groups, as well as their transition metal com-
plexes. Of the reported ligands of this type were macrocyclic ligands
containing an ‘‘NS’’ donor-set [18–22]. In previous work, novel li-
gands were designed, synthesized and reacted with transition metal
ions; some were found to have potential applications in recognition
towards Cu2+ and Hg2+ ions [23–25]. In this work, as part of our con-
tinued efforts in investigating the role of donor atoms in the stability
of complexes, we have prepared a macrocyclic ligand possessing an
{N3S2}-donor set, with one pendant pyridinyl arm, 4-(pyridin-2-yl-
methyl)-1,7-dithia-4,10-diazacyclododecane (L), [23] and a macro-
cyclic ligand possessing an {N4S2}-donor set with two pendant
pyridinyl arms, 4,10-bis(pyridin-2-ylmethyl)-1,7-dithia-4,10-diaz-
acyclododecane (L0); their Co2+, Ni2+ and Cu2+ complexes were also
synthesized. The structures of these complexes were established
using both X-ray crystallographic analysis and microanalysis. The
electronic properties of the complexes were investigated using
UV–Vis spectroscopic techniques and cyclic voltammetry. Our re-
sults revealed that the complexes coordinated with L0 exhibited im-
proved reversibility compared to the complexes of the ligand L. This
p-accepting nature can enhance the stability of the low oxidation
state of a transition metal ion, whereas a moiety of only
r-donat-
⇑
Corresponding author at: Department of Chemistry, Nanchang University,
Nanchang 330031, China. Tel./fax: +86 0573 83643937.
E-mail addresses: xiaoming.liu@mail.zjxu.edu.cn, xiaoming.liu@ncu.edu.cn (X.
Liu).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.11.032

182
Z. Wei et al. / Polyhedron 69 (2014) 181–187
Table 1
Crystallographic data for complexes 1–2 and 4–6.
Complexes
1
2
4
5
6
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
C
16H26CoN4S2,2(ClO4)
C14H25N3NiOS2,2Cl,5(H2O)
C20H28CoN4S2,2(ClO4)
646.41
monoclinic
C2/c
14.030(7)
17.103(8)
11.264(5)
90
C20H28N4NiS2,2(ClO4)
646.19
monoclinic
C2/c
13.732(4)
16.939(5)
11.466(4)
90
C20H28CuN4S2,2(ClO4)
651.02
monoclinic
C2/c
13.9755(14)
17.0085(17)
11.2534(11)
90
596.37
535.18
monoclinic
C2/m
21.229(10)
12.032(6)
9.549(5)
90
orthorhombic
P212121
10.7201(5)
12.1620(5)
18.3304(8)
90
b (Å)
c (Å)
a
(°)
b (°)
90
90
100.539(8)
90
2398(2)
4
106.250(10)
90
2595(2)
4
103.495(4)
90
2593.4(14)
4
104.836(2)
90
2585.8(4)
4
c
(°)
V (Å3)
Z
2389.88(18)
4
F(000)
1228
1.166
1070
1.185
1332
1.081
1336
1.168
1340
1.266
l
(Mo K
a
, mmꢁ1
)
Total No. of reflns.
hmax
18129
28.26
6679
27.62
5910
24.00
10644
26.83
7195
26.25
No. of unique reflns (Rint
No. of observed refns
No. of variables
R1, wR2 (obsd data)
R1, wR2 (all data)
GOF, S
)
5768 (0.0534)
3050
299
0.0589, 0.1345
0.0936, 0.1400
1.053
2626 (0.0299)
1927
155
0.0483, 0.1434
0.0707, 0.1562
1.044
2041 (0.0352)
1519
168
0.0451, 0.1128
0.0668, 0.1212
1.067
2786 (0.0552)
2341
163
0.0611, 0.1750
0.0719, 0.1892
1.037
2601 (0.0262)
1829
190
0.0359, 0.0923
0.0571, 0.0979
1.077
improvement is attributed to the increase in the number of pendant
pyridinyl groups.
(Heraeus CHN-O-Rapid). The macrocycle L and its complex (3)
were synthesised as reported in our previous work [23].
Caution! Perchlorate salts are potentially explosive, only small
quantities should be used and they should be handled with great
care.
2. Experimental
2.1. Instrument and materials
2.2. Synthesis
The reactions were performed under an argon atmosphere
using standard Schlenk techniques when necessary. Solvents were
freshly distilled by using appropriate drying agents prior to use. 1H
and 13C NMR spectra were recorded on an AVANCE DRX 400
(Bruker) spectrometer in CDCl3. UV–Vis spectra were recorded
with a Cary 50 COCN (Varian) spectrometer at room temperature.
Electrochemical measurements of the complexes were performed
on an Echo Chemie Autolab PGESTAT 30 in dry acetonitrile with
[NBu4][BF4] as the supporting electrolyte; platinum and Ag/AgCl
were used as the working and reference electrodes, respectively.
The microanalysis service was provided by Nanjing University
2.2.1. 2-(Chloromethyl)pyridine
Pyridin-2-ylmethanol (3.0 g, 27.5 mmol) in dry CH2Cl2 (25 mL)
was cooled in an ice bath before thionyl chloride (5.0 g, 37.6 mmol)
was dropwise added under rapid stirring. After the addition, the
reaction mixture was warmed to room temperature and left
standing overnight under continuously stirring. The solvent was
removed under vacuum to give an off-white solid which was
re-dissolved by adding a solution of sodium hydroxide (10%,
50 mL). The aqueous layer was extracted with CH2Cl2
(3 ꢀ 50 mL). The organic layers were separated and combined
Scheme 1. The synthetic route to the ligand L0 and complexes 1–6.

Z. Wei et al. / Polyhedron 69 (2014) 181–187
183
2.2.2. 4,10-Bis(pyridin-2-ylmethyl)-1,7-dithia-4,10-
diazacyclododecane, L0
A mixture of L (1.5 g, 5 mmol) and 2-(chloromethyl)pyridine
(0.64 g, 5 mmol) in dry toluene (50 mL) was refluxed under an Ar
atmosphere for 48 h in the presence of K2CO3 (1.40 g, 10.14 mmol)
and KI (0.84 g, 5.05 mmol). Insoluble inorganic solids were filtered
off and the solvent was removed under reduced pressure to pro-
duce a pale yellow solid. This crude product was further purified
by chromatography using ethyl acetate as the eluant to give a pale
yellow solid (1.5 g, 79%). 1H NMR (CDCl3): 8.455 (d, J = 4.52 Hz, Py–
H), 7.604 (t, J = 15.08 Hz, Py–H), 7.475 (d, J = 7.79 Hz, Py–H), 7.097
(t, J = 12.18 Hz, Py–H), 3.739 (s, Py–CH2), 2.802 (t, J = 13.08 Hz,
NCH2CH2), 2.729 (t, J = 13.02 Hz, NCH2CH2). 13C NMR (CDCl3):
159.26, 149.06, 136.50, 123.05, 122.12, 61.35, 52.54, 26.66.
2.2.3. Complexes 1 and 2
A solution of ligand L (0.51 g, 1.7 mmol) in methanol (2 mL) was
added dropwise to a solution of Co(ClO4)2ꢂ6H2O (0.62 g, 1.7 mmol)
in methanol (2 mL). The reaction mixture was stirred at room tem-
perature for 4 h to give a pink precipitate. The solid (complex 1,
0.81 g, 80%) was collected by filtration, washed with methanol
and dried under vacuum. The product was recrystallized by diffu-
sion of diethyl ether into its acetonitrile solution to give crystals
suitable for X-ray crystallography. Anal. Calc. for complex 1 (C14-
H22Cl2CoN3O8S2ꢂ2H2O, FW = 590.3): C, 28.48; H, 4.44; N, 7.12.
Found: C, 27.94; H, 4.09; N, 6.60%.
Complex 2 was prepared similarly using NiCl2ꢂ6H2O (0.40 g,
1.7 mmol). The reaction was stirred at room temperature for 4 h
to give a clear solution before being concentrated. Any insoluble
solids were filtered and the filtrate was left in a refrigerator for a
few days to produce green crystals (0.75 g, 83%). Anal. Calc. for
complex 2 (C14H25N3OS2Cl2Niꢂ5H2O, FW = 535.1): C, 31.40; H,
6.54; N, 7.85. Found: C, 31.93; H, 6.51; N, 7.99%.
2.2.4. Complexes 4–6
A solution of ligand L0 (63 mg, 0.17 mmol) in methanol (2 mL)
was added dropwise to a solution of Co(ClO4)2ꢂ6H2O (0.62 g,
0.17 mol) in methanol (2 mL). The reaction was stirred at room
temperature for 4 h to give a precipitate which was collected by fil-
tration, washed with methanol and dried under vacuum. Recrystal-
lisation by diffusion of diethyl ether into an acetonitrile solution
produced crystals of complex 4 suitable for X-ray crystallography
Fig. 1. Molecular structures of the cations of complex 1 (top) and complex 2
(bottom). In all the crystallographic diagrams, hydrogen atoms have been omitted
for clarity and thermal ellipsoids are drawn at the 30% probability level.
before being dried with MgSO4. Removal of the solvent yielded a
colourless liquid (3.1 g, 89%). The product was used for the next
reaction without further purification.
(0.82 g, 75%). Anal. Calc. for complex
4 (C20H28N4O8S2Cl2Co,
Table 2
Selected bond lengths (Å) and angles (°) for complexes 1–3.
1
2
3 [23]
Co(1)–N(2) = 2.142(4)
Ni(1)–N(2) = 2.117(4)
Cu(1)–N(2) = 2.260(3)
Co(1)–N(1) = 2.057(4)
Ni(1)–N(1) = 2.063(4)
Cu(1)–N(1) = 1.992(3)
Co(1)–N(3) = 2.081(4)
Ni(1)–N(3) = 2.098(4)
Cu(1)–N(3) = 2.007(3)
Co(1)–S(1) = 2.479(2)
Ni(1)–S(1) = 2.4293(15)
Cu(1)–S(1) = 2.421(1)
Co(1)–S(2) = 2.481(2)
Ni(1)–O(1) = 2.088(4)
Cu(1)–S(2) = 2.447(1)
Co(1)–N(4) = 2.031(4)
N(1)–Ni(1)–O(1) = 90.68(17)
N(1)–Ni(1)–N(3) = 178.59(16)
O(1)–Ni(1)–N(3) = 90.73(17)
N(1)–Ni(1)–N(2) = 81.54(16)
O(1)–Ni(1)–N(2) = 172.22(16)
N(3)–Ni(1)–N(2) = 97.06(17)
N(1)–Ni(1)–S(1) = 95.78(3)
O(1)–Ni(1)–S(1) = 93.35(3)
N(3)–Ni(1)–S(1) = 84.14(3)
N(2)–Ni(1)–S(1) = 87.47(3)
S(1)a–Ni(1)–S(1) = 166.57(5)
Cu(1)–N(4) = 2.369(4)
N(4)–Co(1)–N(1) = 92.17(17)
N(1)–Co(1)–N(2) = 82.15(15)
N(3)–Co(1)–N(2) = 96.19(16)
N(4)–Co(1)–S(2) = 94.48(19)
N(1)–Co(1)–S(2) = 92.82(16)
N(3)–Co(1)–S(2) = 83.97(15)
N(2)–Co(1)–S(2) = 86.45(17)
N(4)–Co(1)–S(1) = 94.50(19)
N(1)–Co(1)–S(1) = 99.59(16)
N(3)–Co(1)–S(1) = 83.34(14)
N(2)–Co(1)–S(1) = 85.89(17)
N(4)–Co(1)–N(3) = 89.52(17)
N(1)–Cu(1)–N(4) = 93.9(1)
N(1)–Cu(1)–N(2) = 80.5(1)
N(3)–Cu(1)–N(2) = 98.3(1)
N(4)–Cu(1)–S(2) = 93.7(1)
N(1)–Cu(1)–S(2) = 96.88(8)
N(3)–Cu(1)–S(2) = 85.18(8)
N(2)–Cu(1)–S(2) = 85.24(8)
N(4)–Cu(1)–S(1) = 95.5(1)
N(1)–Cu(1)–S(1) = 92.59(8)
N(3)–Cu(1)–S(1) = 85.15(8)
N(2)–Cu(1)–S(1) = 86.52(8)
N(3)–Cu(1)–N(4) = 87.3(1)
a
Symmetry code: x, 1 – y, z.

184
Z. Wei et al. / Polyhedron 69 (2014) 181–187
FW = 646.4): C, 37.15; H, 4.37; N, 8.67. Found: C, 37.10; H, 4.32; N,
8.53%.
dangled from this N atom, and trans to this amine N was a loosely
bound acetonitrile (1 and 3) or water (2) molecule. It is interesting
to note that there is a crystallographic plane passing through the
Ni1, O1, N2 and N3 atoms and the pyridinyl ring in the structure
of the cation of complex 2. Although analogous planes are found
in the structures of complexes 1 and 3, they are not as perfectly
symmetrical as that in complex 2. The M–N bond lengths of the
three complexes are as expected, as shown in Table 2. When the
The corresponding Ni2+ and Cu2+ complexes were prepared and
crystallised in the same manner (complex 5, 0.89 g, 78%; complex
6, 0.77 g, 70%). Anal. Calc. for complex 5 (C20H28N4O8S2Cl2Niꢂ5H2O,
FW = 736.2): C, 32.61; H, 5.16; N, 7.60. Found: C, 32.84; H, 4.94; N,
7.16%. Anal. Calc. for complex 6 (C20H28N4O8S2Cl2Cu, FW = 651.0):
C, 36.87; H, 4.30; N, 8.60. Found: C, 36.90; H, 4.33; N, 8.48%.
bond possesses
p-back bonding (the pyridinyl group or acetoni-
trile), the bond length is shorter than that to the
r
-only bond.
2.2.5. Single-crystal X-ray diffraction analysis
One exception is Cu–NMeCN (2.369 Å) [23], where the lengthening
is attributed to the Jahn Teller effect. The Ni1–O(1) bond length
of 2.088(4) Å in complex 2 is similar to the corresponding Ni–Owater
In the crystallographic data collection of complexes 1–6, stan-
dard procedures were used for mounting the crystals on a Bruker
Apex-II area-detector diffractometer at ca 293 K. The crystals were
routinely coated with paraffin oil before being mounted. Intensity
0
data were recorded using MoKa radiation (k = 0.71073 ÅA) using the
u
- and x-scan modes. The structures were solved by the direct
method routines in the SHELXS program and refined on F2 in SHELXL
[26].
In complex 2, two Clꢁ ions were disordered and have been split
in two fragments, Cl1, Cl2/O5, O6, which were constrained to be
identical with occupancy factors of 0.5/0.5. In complex 5, the per-
chlorate anion exhibited disorder over two positions and the oxy-
gen atoms bonded to the Cl atom were split into two fragments
O(1)–O(4)/O(1A)–O(4A) with occupancy factors of 0.69/0.31. In
complex 6, the O72–O74 atoms in the perchlorate anion and the
C3 and C4 atoms in the ethylene group were each disordered over
two positions with occupancy factors O(72)–O(74)/O(75)–O(77) of
0.877/0.123 and C2, C3/C2x, C3x of 0.774/0.226. All non-hydrogen
atoms were modelled anisotropically. Except for water hydrogen
atoms, which were not added, all other hydrogen atoms were posi-
tioned geometrically and treaded as riding on their parent 0atoms
with C–H distances of 0.97 (ethyl), 0.96 (methyl) and 0.93 ÅA (aro-
matic rings), and with Uiso(H) = 1.2Ueq (C) (ethyl and aromatic
rings), and Uiso(H) = 1.5Ueq (C) (methyl). Details of the crystallo-
graphic and refinement data are given in Table 1.
3. Results and discussion
3.1. Synthesis and characterization
The ligand L in its copper(II) complex (3) was reported previ-
ously [23]. As shown in Scheme 1, the ligand L0 was obtained by
further reaction of ligand L with 2-(chloromethyl)pyridine in the
presence of K2CO3 and KI under reflux in toluene. The ligand L0
was isolated as a yellow solid in a high yield. 1H and 13C NMR con-
firmed unambiguously its identity. Subsequently, treatment of the
ligands L and L0 with the perchlorate salts of cobalt(II), nickel(II)
and copper(II) in MeOH led to complexes 1 and 3–6. These com-
plexes could be easily crystallised in MeCN-Et2O systems to give
single crystals suitable for X-ray diffraction. Crystals of complex
2 were obtained by the direct reaction of the ligand L with NiCl2
in methanol solution. Complexes 1–6 are air and moisture stable
in their solid state, and soluble in polar solvents such as MeCN,
DMSO and DMF.
3.2. Structural analysis of complexes 1–6
The crystal structure of complex 3 was reported in our previous
work [23]. The structures of complexes 1 and 2 are shown in Fig. 1.
Selected bond lengths and angles are summarized in Table 2. The
structures of these complexes are essentially superimposable. An
octahedral coordination geometry was adopted by the central me-
tal ions. In these structures, five ligating atoms from the ligand L
coordinate to M2+ in a nearly perfect square-pyramidal geometry,
with one of the amine N atoms as the vertex. The pyridinyl group
Fig. 2. The molecular structures of the cations of complexes 4 (top), 5 (middle) and
6 (bottom).

Z. Wei et al. / Polyhedron 69 (2014) 181–187
185
Table 3
Selected bond lengths (Å) and angles (°) for complexes 4–6.
4
5
6
Co–N(22) = 2.113(3)
Ni(1)–N(1) = 2.066(3)
Cu–N(12)= 2.003(2)
Co–N(1) = 2.240(3)
Ni(1)–N(2) = 2.156(3)
Cu–N(1) = 2.128(2)
Co–S(4) = 2.491(2)
Ni(1)–S(1) = 2.440(1)
Cu–S(4) = 2.6669(8)
N(22)–Co–N(1) = 76.5(1)
N(1)a–Co–N(1) = 120.3(2)
N(22)a–Co–S(4) = 88.22(9)
N(22)–Co–S(4) = 116.83(9)
N(1)a–Co–S(4) = 82.41(9)
N(1)–Co–S(4) = 80.28(9)
N(22)a–Co–N(22) = 91.9(2)
N(1)–Ni(1)–N(2) = 81.0(1)
N(2)b–Ni(1)–N(2) = 105.2(2)
N(1)b–Ni(1)–S(1) = 83.6(1)
N(1)–Ni(1)–S(1) = 107.4(1)
N(2)b–Ni(1)–S(1) = 86.91(9)
N(2)–Ni(1)–S(1) = 83.23(9)
N(1)b–Ni(1)–N(1) = 96.2(2)
N(12)–Cu–N(1) = 82.02(9)
N(1)–Cu–N(1)a = 106.0(1)
N(12)–Cu–S(4) = 84.22(6)
N(12)a–Cu–S(4) = 112.78(6)
N(1)–Cu–S(4) = 84.66(6)
N(1)a–Cu–S(4) = 80.67(6)
N(12)–Cu–N(12)a = 95.0(1)
a
Symmetry codes: a: 1 – x, y, 0.5 – z.
Symmetry codes: 1 ꢁ x, y, 1.5 ꢁ z.
b
bond lengths in other similar compounds: [Ni(Batp)(H2O)](CF3-
SO3)2 (2.079 Å) [21] and [Ni(H2O)2(PyNS)](BF4)2 (2.082(2) Å) [27].
The crystals of complexes 4–6 are essentially isostructural and
their structures are shown in Fig. 2. Selected bond lengths and an-
gles are listed in Table 3. The structures of the three cations adopt
an octahedral geometry about each metal centre, but the geometry
is severely distorted, due to the presence of the additional pyridi-
nyl group, compared to those of complexes 1–3. The coordination
in each structure involves the two pyridinyl nitrogen atoms, two
amino nitrogen atoms and two thioether sulfur atoms of the hexa-
dentate ligand L0. In all these structures, the pairs of aromatic nitro-
gen atoms are cis to each other, as are the pairs of amino N atoms,
whereas the two thioether sulfur atoms are in the opposing (trans)
positions. These structures possess a crystallographic two fold
symmetry (C2) axis that passes through the metal centre. In com-
plexes 4–6, the M–Namino bond lengths are longer than the M–
Npy bond lengths, reflecting the existence of the back donating
bond between the metal and the pyridinyl group.
p
All the complexes are structurally highly similar, adopting an
octahedral geometry. In general, the bond lengths of the complexes
bearing two pyridinyl groups (4–6) are slightly longer than the
analogous bond lengths of the other complexes (1–3) derived from
ligand L. The lengthening is certainly attributed to the steric hin-
drance caused by the hexa-coordination of the hexadentate ligand
L0. However, the length (2.6669 Å) of the Cu–S bond of complex 6 is
about 0.2 Å longer than the other M–S bonds (ca 2.4 Å). The cause
of this lengthening is probably due to the Jahn–Teller effect.
3.3. Electronic absorption spectra
The electronic spectra of these complexes and the ligands, in
acetonitrile, are shown in Fig. 3. Since the ligand L0 possesses only
one more pyridinyl group than ligand L, the two ligands have very
similar electronic spectra. The two sets of complexes, 1–3 and 4–6,
show three types of transitions, viz ligand-based, ligand-to-metal
and d-d transitions. The absorption band of L and L0 at about
Fig. 3. UV–Vis absorption spectra of the ligand L and its complexes 1–3 (top) and
the ligand L0 and its complexes 4–6 (bottom) in MeCN (C = 1.8 ꢀ 10ꢁ4 mol Lꢁ1
(Inset: d–d transitions of complexes 1–6).
)
272 nm is assigned to the p–
p⁄ transition of the pyridinyl moiety.
3.4. Electrochemical investigation
This ligand-based transition is essentially intact in the spectra of
the complexes. For the complexes of the ligand L (1–3), additional
absorption band(s) shift gradually towards longer wavelengths, at
about 290 nm for the Ni2+ complex, at 315 nm (a shoulder) for the
Co2+ complex and at 373 nm (a strong peak) for Cu2+, as shown in
Fig. 3 (top). Similar absorption bands with moderate intensity and
analogous variation trends were also observed for complexes 4–6,
Fig. 3 (bottom). These transition bands are assigned to ligand-
to-metal charge-transfer (LMCT) absorption bands. In the long
wavelength range, weak absorption bands are observed for all
the complexes which are assigned to d–d transitions, Fig. 3 (inset).
The cyclic voltammograms of the complexes are shown in Fig. 4.
The potentials of the redox processes and their assignments are
tabulated in Table 4. The copper complexes (3 and 6) exhibit two
reduction processes, assigned to Cu2+/Cu+ and Cu+/Cu0, whereas
the complexes of Co2+ and Ni2+ show both one reduction and one
oxidation process. The two redox processes (for Co and Ni) are as-
signed to M2+/M+ and M3+/M2+. The assignment for the copper
complexes is supported by the observation of the sharp stripping
peak at about ꢁ0.14 V for both complexes during reverse scanning,
Fig.
4 (bottom). The electrochemistry of these complexes

186
Z. Wei et al. / Polyhedron 69 (2014) 181–187
demonstrated well how the additional pyridinyl group, both a
r-
donor and -acceptor, stabilises the reduced species. The improve-
p
ment in stability is particularly significant for both the cobalt and
nickel ions. For the copper ion, the improvement is exhibited by
the cleaner electrochemistry after the first reduction. Although
the influence of the additional pyridinyl group on the oxidation
process is not as obvious as that on the reduction, improvement
in the reversibility of the oxidation of the cobalt complex is still ob-
servable, Fig. 4 (top).
4. Conclusions
In summary, the ligand L0 was synthesised by the further func-
tionalisation of the ligand L, which was reported in our previous
work [23]. Compared to the ligand L, the ligand L0 possesses an ex-
tra pyridinyl group. Reaction of the ligands with three transition
metal ions formed complexes [MLSol]2+ (1–3, Sol = MeCN (1 and
3) or H2O (2)) and [ML0]2+ (4–6) (M = Co2+, Ni2+, Cu2+), respectively.
These complexes adopt an octahedral geometry and the additional
pyridinyl ligand in the ligand L0 led to severely distorted octahedral
structures. Their UV–Vis spectra suggest that the electronic struc-
tures are similar and the MLCT bands of complexes 4–6 shift to-
wards long wavelengths, probably due to the additional pyridinyl
group lowering the energy of the LUMOs of the complexes. The
electrochemistry of these complexes reveals that the presence of
an additional pyridinyl group in the ligand L0 improves the revers-
ibility of the redox processes by stabilising the redox products
through the
r-donor and p-acceptor properties of this ligand.
The improvement is particularly significant in the reduction of
these complexes.
Acknowledgements
The authors thank NSF of China (Grant No.: 20871064,
21171073), the Government of Zhejiang Province (Qianjiang Pro-
fessorship, XL) for financial support, and the Education Department
of Jiangxi Province (Grant No.: GJJ13106).
Appendix A. Supplementary data
CCDC 918653–918657 contains the supplementary crystallo-
graphic data for complexes 1–2 and 4–6. These data can be ob-
tained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk.
Fig. 4. The CVs of complexes 1 and 4 (top), 2 and 5 (middle), 3–6 (bottom, the
coloured lines are CVs without going through the second reduction) in 0.1 mol Lꢁ1
MeCN solution at a scanning rate of 0.1 V sꢁ1 (298 K).
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