Synthesis, structure, and luminescence properties of arylpyridine- substituted terpyridine Zn(II) and Cd(II) complexes

Article abstract of DOI:10.1016/j.poly.2012.08.054

Two terpyridine derivatives, 4′-(4-(pyridin-4-yl)phenyl)-[2,2′: 6′,2″]terpyridine (L1) and 4′-(4′-(pyridin-4-yl)-[1, 1′-biphenyl]-4-yl)-[2,2′:6′,2″]terpyridine (L2) have been synthesized by palladium-catalyzed cross-coupling reactions, and their metal com

Full text of DOI:10.1016/j.poly.2012.08.054

Polyhedron 52 (2013) 435–441
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, structure, and luminescence properties of arylpyridine-substituted
terpyridine Zn(II) and Cd(II) complexes
Emi Kubota a, Young Hoon Lee a, Akira Fuyuhiro b, Satoshi Kawata c, Jack M. Harrowfield d, Yang Kim e,
Shinya Hayami a,f,
⇑
a Department of Chemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
b Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
c Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan-Ku, Fukuoka 814-0180, Japan
d Laboratoire de Chimie Supramoléculaire, Institut de Science et d’Ingénierie Supramoléculaires, Université de Strasbourg, 8, allée Gaspard Monge, 67083 Strasbourg, France
e Department of Chemistry & Advanced Materials, Kosin University, 194, Wachi-Ro, Yeongdo-gu,Busan 606-701, South Korea
f JST, CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan
a r t i c l e i n f o
a b s t r a c t
Two terpyridine derivatives, 40-(4-(pyridin-4-yl)phenyl)-[2,20:60,200]terpyridine (L1) and 40-(40-(pyridin-
4-yl)-[1,10-biphenyl]-4-yl)-[2,20:60,200]terpyridine (L2) have been synthesized by palladium-catalyzed
cross-coupling reactions, and their metal complexes [Zn(L1)2](ClO4)2ꢀ2MeOHꢀCHCl3 (1) and [Cd(L2)2]
(ClO4)2ꢀDMFꢀ2.8H2O (2) characterized by single crystal X-ray crystallography and spectroscopic methods
including electronic absorption and emission measurements. In the crystalline materials, both metal ions
have the expected pseudo-octahedral N6 coordination environment and the lattice structures appear to
be influenced by aromatic–aromatic interactions involving the heteroaryl tails and the pyridyl rings of
the terpyridine units. In solution and in the solid state, the complexes display blue emission attributed
to an intramolecular charge transfer transition.
Article history:
Available online 30 August 2012
Dedicated to the memory of Alfred Werner
on the 100th Anniversary of his Nobel Prize
in Chemistry in 1913.
Keywords:
Crystal structure
Luminescence
Metal complex
Heteroarylterpyridine
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
tives have been used, however, not only in this way as building
blocks to construct, as further examples, luminescent coordination
Of the extraordinarily numerous derivatives of 2,20:60,200-terpyr-
idine synthesized to endow both the ligand and its metal com-
plexes with novel properties, those involving heteroaromatic
substituents, notably (poly)pyridyl, have been prominent [1–3].
Most commonly, substituents have been introduced on the 40-posi-
tion of terpyridine, so that a pyridyl unit introduced there can pro-
vide a metal ion coordination site which is divergent from that
defined by the N-donor atoms of the original terpyridine chelate
site, thus facilitating the formation of various heterometallic, olig-
omeric complexes, some of which have been shown to have excep-
tional electronic, magnetic and electrochemical properties [4]. In
our earlier work on polymeric Co(II) complexes of 40-(4-pyridyl)-
2,20:60,200-terpyridine, for example, an abrupt spin transition was
found to be inducible by the mere change of lattice solvent [5].
Coordination complexes of pyridyl-substituted terpyridine deriva-
polymers [6], heteronuclear photocatalysts [7] and organized mon-
olayers [8], but also as antennae to promote lanthanide lumines-
cence [9], and as reagents for the photocleavage of biomolecules
[10]. Of further interest with a pyridyl substituent is the fact that
it expands the heteroaromatic ligand core and thus enhances its
capacity for involvement in weak interactions that may modulate
the reactivity of the system [11]. Further still, it is a nucleophilic
site that can be alkylated to give complexes of the cationic ligand
which can be used as luminescent anion sensors [12] and constit-
uents of electrochromic materials [13], while the reaction of alkyl-
ation itself can be used to form metallamacrocycles [14] and
mechanically interlocked molecules [15].
In extension of our studies of pyridyl-appended terpyridines,
we have now investigated the consequences of introducing
unsubstituted aromatic substituents between the terpyridine and
pyridine units by synthesizing the ligands 40-(4-(pyridin-4-yl)
phenyl)-[2,20:60,200]terpyridine (L1; previously known [9]) and 40-
(40-(pyridin-4-yl)-[1,10-biphenyl]-4-yl)-[2,20:60,200]terpyridine (L2)
and conducting a preliminary structural and spectroscopic charac-
terization of their complexes with Zn(II) and Cd(II), two metal ions
expected to give luminescent species.
⇑
Corresponding author at: Department of Chemistry, Graduate School of Science
and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555,
Japan.
E-mail address: hayami@sci.kumamoto-u.ac.jp (S. Hayami).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.08.054

436
E. Kubota et al. / Polyhedron 52 (2013) 435–441
were obtained by vapor diffusion of methanol into a DMF solution
2. Experimental
2.1. General
of the product. Anal. Calc. for C67H56.6CdCl2N9O11.8: C, 59.17; H,
4.20; N, 9.27. Found: C, 59.00; H, 4.21; N, 9.24%. ESI–MS: m/
z = 519.33 [Cd(L2)2]2+. UV spectrum in CH3CN: kmax
(
emax (ꢁ 104
Mꢂ1 cmꢂ1)) = 236 nm (sh, 4.62), 287 nm (6.97), 332 nm (8.35). IR
spectrum (KBr disc, cmꢂ1), 3066 (Ar C-H), 1089 (ClO4ꢂ).
Elemental analyses for C, H and N were carried out at the Instru-
mental Analysis Center of Kumamoto University. 1H NMR spectra
were measured on a JEOL (500-ECX) instrument (500 MHz) using
deuterated solvents with TMS as internal reference. UV–vis and
fluorescence spectra were recorded with a Shimadzu UV-3600
spectrophotometer and a Perkin-Elmer LS55 spectrofluorimeter,
respectively. Infrared spectra were recorded on a Shimadzu FT-IR
8700 instrument. Mass spectral data were collected on a JEOL
JMS-BU-20GC-mate spectrometer operating in positive ion fast
atom bombardment mode (FAB+) with an NBA matrix.
2.3. Crystal structure determinations
X-ray structural analyses for 1 and 2 were carried out using RIG-
AKU Saturn CCD diffractometer with a confocal mirror using
graphite-monochromated Mo
Ka radiation (k = 0.71075 Å) at
100 K. The structures were solved by direct methods (SHELXL-97
[17]) for 1 and by heavy-atom Patterson methods (DIRDIF99
PATTY) for 2, and refined by full-matrix least-squares method on
F2
(SHELXL-97).
2.2. Synthesis
All chemicals and solvents were used as received without
further purification. 40-(40-(Pyridin-4-yl)-phenyl)-[2,20:60,200]ter-
pyridine (L1) [9] and 40-(40-bromo-biphenyl-4-yl)-[2,20:60,200]ter-
pyridine (Brbptpy) [16] were prepared according to literature
procedures.
2.3.1. Crystal data for 1
C55H45Cl5N8O10Zn, M = 1276.09, triclinic, space group P1, a =
12.5578(6), b = 14.4664(5), c = 14.7705(7) Å, = 89.702(2), b =
79.220(2),
= 88.108(2)°, V = 2634.5 Å3, Z = 2, Dcalc = 1.609 g cmꢂ3
(Mo K
) = 7.912 cmꢂ1, T = 100 K, 26051 reflections collected.
Refinement of 12071 reflections (712 parameters) with I > 2 (I)
ꢀ
a
c
,
l
a
r
2.2.1. Synthesis of 40-(40-pyridin-4-yl-biphenyl-4-yl)-
[2,20:60,200]terpyridine (L2)
converged to a final R1 = 0.0797, wR2 = 0.2349. Goodness-of-fit
(GOF) = 1.080.
An oven-dried three-neck round-bottom flask was charged with
Brbptpy (0.20 g, 0.43 mmol), 4-pyridinyl-boronic acid (0.15 g,
1.2 mmol), and Pd(PPh3)4 (0.1 g, 0.086 mmol), and then filled with
argon gas. After successive additions of DME (30 mL) and degassed
aqueous Na2CO3 (2 M, 15 mL), the mixture was heated at reflux for
24 h under argon. The solvent was evaporated off under reduced
pressure and the residue dissolved in dichloromethane. A black so-
lid and Na2CO3 salt were filtered out through Celite and the filter
bed washed with dichloromethane. The filtrate was taken to dry-
ness under vacuum and the residue purified by column chromatog-
raphy on silica gel with ethyl acetate/hexane (1:1). Yield: 0.18 g
(91 %). MS (FAB+): m/z = 463.21 [M+H]. 1H NMR (500 MHz, CDCl3):
2.3.2. Crystal data for 2
ꢀ
C67H56.6CdCl2N9O11.8, M = 1359.95, triclinic, space group P1, a =
14.896(2), b = 15.260(1), c = 15.358(2) Å,
(2),
= 83.140(2)°, V = 3101.7(4) Å3, Z = 2, Dcalc = 1.456 g cmꢂ3
(Mo K
) = 5.098 cmꢂ1, T = 100 K, 22436 reflections collected.
Refinement of 12703 reflections (804 parameters) with I > 2 (I)
a = 80.168(2), b = 64.542
c
,
l
a
r
converged to a final R1 = 0.1093, wR2 = 0.2894. Goodness-of-fit
(GOF) = 1.054.
3. Results and discussion
0
0
00
d = 8.74 (s, 2H, PyH3 ,5 ), 8.69 (d, 2H, PyH6,6 ), 8.64–8.62 (m, 4H,
00
00 000,6000
000,5000
3.1. Synthesis and structure
PyH2,6 ,2 ), 7.98 (d, 2H, PyH3 ), 7.85 (t, 2H, PyH5,3 ), 7.75–7.69
00
(m, 6H, ArH), 7.54 (d, 2H, ArH3,5), 7.32 (t, 2H, PyH4,4 ), UV spectrum
Both heteroaryl-substituted terpyridine ligands L1 (previously
known [9]) and L2, possessing phenylpyridine and biphenylpyri-
dine moieties, respectively, were obtained in good yield through
Pd-catalyzed Suzuki cross-coupling reactions [9], as depicted in
Scheme 1. The intermediate Brbptpy was prepared by using (see
Section 2) our previously reported synthetic procedures [16]. L2
was obtained as a white solid by coupling Brbptpy with 4-pyr-
idinylboronic acid. The bis(ligand) complexes of Zn(II) with L1
(1) and of Cd(II) with L2 (2) were obtained by reacting stoichiom-
etric quantities of the ligands and the metal perchlorates in mixed
solvents.
in CH3CN: kmax
296 nm (3.57).
(
emax (ꢁ 104 Mꢂ1 cmꢂ1)) = 258 nm (sh, 1.91),
2.2.2. Synthesis of [Zn(L1)2](ClO4)2ꢀ2MeOHꢀ CHCl3 (1)
To a solution of L1 (0.03 g, 0.078 mmol) in MeOH/CHCl3 (10 mL,
v/v = 1:1), Zn(ClO4)2ꢀ6H2O (0.014 g, 0.039 mmol) in MeOH (10 mL)
was added. The mixture was stirred for 2 h at room temperature
and then the solvent volume was reduced under vacuum to
ꢃ5 mL. The white precipitate formed was collected by filtration,
washed with methanol and dried in vacuo (Yield: 0.04 g). Crystals
suitable for a structure determination were obtained by liquid dif-
fusion of methanolic Zn(ClO4)2ꢀ6H2O into a chloroform solution of
L1. Anal. Calc. for C55H45Cl5N8O8Zn: C, 54.12; H, 3.72; N, 9.18.
Found: C, 54.42; H, 3.74; N, 9.27 %. ESI–MS: m/z = 418.58
N
[Zn(L1)2]2+
.
UV spectrum in CH3CN: kmax
(
emax (ꢁ 104 Mꢂ1
-
Br
cmꢂ1)) = 236 nm (4.44), 269 nm (sh, 4.95), 287 nm (6.60), 315 nm
(6.17). IR spectrum (KBr disc, cmꢂ1) 3076 (Ar C–H), 1089 (ClO4ꢂ).
B(OH)2
n
n
, Pd(PPh3)4
N
2.2.3. Synthesis of [Cd(L2)2](ClO4)2ꢀDMFꢀ2.8H2O (2)
O
aq. Na2CO3,
O
To a solution of L2 (0.030 g, 0.065 mmol) in DMF (15 mL),
Cd(ClO4)2ꢀ6H2O (0.014 g, 0.033 mmol) in DMF (5 mL) was added.
The mixture was stirred for 2 h at 100 °C. The solvent was removed
under reduced pressure. The solid residue was washed with MeOH
and the white product was collected by filtration and dried in va-
cuo. Yield: 0.03 g. Crystals suitable for a structure determination
N
N
N
N
N
N
n = 1, L1; n = 2, L2
Scheme 1. Ligand syntheses via Suzuki cross-coupling reactions.

E. Kubota et al. / Polyhedron 52 (2013) 435–441
437
Crystals of 1 suitable for a structure determination were ob-
tained by slow diffusion over several weeks after layering a meth-
anol solution of Zn(ClO4)2ꢀ6H2O on a CHCl3 solution of L1 in a glass
tube. A single-crystal X-ray structural determination was success-
fully carried out at 100 K, the space group being found to be tri-
gives rise to the possibility of other forms of such interactions [2].
As for other complexes of terpyridines with linear polyaromatic
substituents [20], lattice planes can be identified in which the cat-
ions form sheets containing these substituents. In 1, these sheets
lie parallel to the (10ꢂ1) plane and, from the apparent proximity
of substituents of adjacent cations, chained polymeric structures
can be identified (Fig. 2). The Zn. . .Zn separations of centrosym-
metric pairs within these rods alternate between 19.99(2) and
20.07(2) Å, and for the longer of these two, the intervening pyridyl-
phenyl substituents (involving N7) are arrayed so that the pyridyl
of one projects (offset) on the phenyl of the other, with the ring
planes being close to parallel and the centroid. . .centroid separa-
tion 3.659(3) Å, with the shortest interatomic contact being C. . .N
of 3.366(8) Å. These parameters are indicative of a significant offset
face-to-face (OFF) interaction [21,23]. For the centrosymmetric
pair with the slightly shorter Zn. . .Zn separation of 19.99(2) Å,
however, the rings of the overlapping tails do not lie parallel, the
centroid. . .centroid separation is long at 4.054(3) Å and no inter-
atomic separation is shorter than 3.5 Å. Here, it must be noted that
a cation (at Zn. . .Zn 11.20(1) Å) in an adjacent rod is positioned
such that the terminal pyridyl groups of the tails are close to par-
allel to peripheral pyridyl groups of the terpyridine head groups,
with a centroid. . .centroid separation of 3.672(3) Å and two inter-
atomic contacts <3.5 Å. Further, there is another neighbor (Zn. . .Zn
11.30(1) Å) for which one of the phenyl groups apparently involved
in interactions in the 20.07(2) Å rod-pair lies close to parallel to a
peripheral pyridyl of its neighbor’s head group, with a cen-
troid. . .centroid separation of 3.604(3) Å and 4 interatomic con-
tacts at <3.5 Å. In all these situations, there are CH. . .C contacts
ꢀ
clinic P1. Fig. 1 shows the cation present in the lattice of 1 with
a partial atomic numbering scheme. Selected bond distances and
angles are given in Table 1. The central metal ion is coordinated
by six nitrogen donor atoms of two terpyridine units. As usual with
terpyridine complexes [2,18], the Zn–N bonds to the central pyri-
dine-N donors (2.067(4)–2.084(4) Å) are significantly shorter than
those to the peripheral-ring donors (2.169(5)–2.202(5) Å), giving
the ZnN6 unit a distorted octahedral geometry. The complex cation
has very close to a linear, rod-like form (N7. . .Zn1. . .N8 177.8°;
N7. . .N8 26.90(3) Å) with, again as usual (see, for example,
[6,11,13,14,19,20], the phenyl and pyridyl rings slightly twisted
away from coplanarity with one another and the terpyridine head
group. This twisting is markedly different for the N8 tail to that of
the N7, so that the two tails are clearly inequivalent.
Concerning the lattice of 1, it is apparent that grafting of a pyr-
idylphenyl substituent to terpyridine must be added to the list of
factors known to disrupt the ‘‘terpyridine embrace’’ [21], ascribed
to aromatic–aromatic interactions, found in the lattices of a large
number of complexes of unsubstituted terpyridine (and indeed in
some with small substituents such as hydroxyl [22] and even a sin-
gle pyridyl ring [13]). Obviously, the presence of an aromatic tail,
and particularly one containing a polarized aromatic unit (pyridyl),
<3 Å which may be indicative of edge-to-face CH. . .
p interactions
[23–25]. Thus, any analysis of aromatic–aromatic interactions
within the lattice of 1 is exceedingly complicated and is rendered
even more so by recognizing that there are, of course, other inter-
actions to be considered. Most obvious and probably dominant
[21], are cation–anion attractions (which may of course pass
through an anion. . .p pathway [26]) but in the case of the
Fig. 1. Molecular structure of the [Zn(L1)2]2+ cation in 1 with thermal ellipsoids
plotted at the 50 % probability level. For clarity, hydrogen atoms are omitted.
Table 1
Selected bond lengths (Å) and bond angles (°) for 1 and 2.
1
2
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–N(3)
Zn(1)–N(4)
Zn(1)–N(5)
Zn(1)–N(6)
2.178(4)
2.067(4)
2.181(4)
2.169(5)
2.084(4)
2.202(5)
Cd(1)–N(1)
Cd(1)–N(2)
Cd(1)–N(3)
Cd(1)–N(4)
Cd(1)–N(5)
Cd(1)–N(6)
2.319(7)
2.308(9)
2.368(8)
2.345(8)
2.297(9)
2.353(7)
N(1)–Zn(1)–N(2)
N(1)–Zn(1)–N(3)
N(1)–Zn(1)–N(4)
N(1)–Zn(1)–N(5)
N(1)–Zn(1)–N(6)
N(2)–Zn(1)–N(3)
N(2)–Zn(1)–N(4)
N(2)–Zn(1)–N(5)
N(2)–Zn(1)–N(6)
N(3)–Zn(1)–N(4)
N(3)–Zn(1)–N(5)
N(3)–Zn(1)–N(6)
N(4)–Zn(1)–N(5)
N(4)–Zn(1)–N(6)
N(5)–Zn(1)–N(6)
75.84(15)
151.93(14)
95.81(16)
106.39(15)
90.74(15)
76.26(15)
103.98(15)
177.76(15)
104.54(16)
87.86(16)
101.52(15)
99.23(15)
76.14(15)
151.48(14)
75.38(16)
N(1)–Cd(1)–N(2)
N(1)–Cd(1)–N(3)
N(1)–Cd(1)–N(4)
N(1)–Cd(1)–N(5)
N(1)–Cd(1)–N(6)
N(2)–Cd(1)–N(3)
N(2)–Cd1)–N(4)
N(2)–Cd(1)–N(5)
N(2)–Cd(1)–N(6)
N(3)–Cd(1)–N(4)
N(3)–Cd(1)–N(5)
N(3)–Cd(1)–N(6)
N(4)–Cd(1)–N(5)
N(4)–Cd(1)–N(6)
N(5)–Cd(1)–N(6)
70.3(3)
139.5(3)
97.5(3)
126.7(3)
100.9(3)
69.4(3)
117.5(3)
161.4(3)
101.5(3)
97.0(3)
93.7(3)
91.1(3)
70.9(3)
140.6(3)
70.2(3)
Fig. 2. A partial view (stick representation) perpendicular to the (01ꢂ1) plane of
one of the sheets of cations in 1 where the tails lie within the sheet. One of the
apparent chains of rod-like cation units is shown in black. For clarity, anions,
solvent molecules and hydrogen atoms are not shown (gray = C, blue = N,
yellow = Zn).

438
E. Kubota et al. / Polyhedron 52 (2013) 435–441
11.30(2) Å cation pair, for example, they can also be considered to
be linked through chains involving H-bonding between terminal-
pyridine-N, methanol (O(10)ꢀꢀꢀN(7) 2.784(6) Å) and chloroform
3.768(5) Å) but still relatively long and no C. . .C contact shorter
than 3.50 Å is evident. The 22.11(1) Å pair may be bridged by
CH. . .N interactions (C15. . .N8 3.18(2), H. . .N 2.41 Å) and, if (aro-
matic)C. . ..O(perchlorate) contacts 63.40 Å are taken as indicative
of interactions, then both pairs of cations are bridged by anions,
so no attraction between the aromatic tails need necessarily be
postulated. Nonetheless, the two cation pairs just discussed are
far from the closest within the lattice and for the closest pair (at
8.98(1) Å), there is even evidence for some degree of retention of
the terpyridine embrace in that flanking pyridyl rings of the terpyr-
idine heads have a centroid. . .centroid separation of 3.619(6) Å and
three reciprocal C. . .C separations well below 3.50 Å. A very similar
situation applies for the next-closest pair (at 9.06(1) Å), where the
centroid. . .centroid separation is 3.638(4) Å and two C. . .C contacts
are (just) below 3.50 Å. In fact these two pairs can be considered to
form a one-dimensional array along a line parallel to the (111)
plane which, especially when viewed in the absence of the tail
atoms, resembles very closely a portion of the two-dimensional
terpyridine embrace found in simpler complexes (Figs. 5 and S2).
Further, for the again centrosymmetric pair found 10.593(7) Å
apart, the shortest centroid. . .centroid separation of all
(3.586(4) Å) is found between the first phenyl group of a tail and
a peripheral terpyridine pyridyl ring of its neighbor and is associ-
ated with one C. . .C contact as short as 3.29(1) Å. Irrespective of
the near pairs considered, all are bridged by anion contacts with
C. . .O 63.40 Å, although any detailed analysis of these interactions
is hampered by disorder of the oxygen atom locations. Note that
contacts between the cations and anions are concentrated on the
bound terpyridine head groups, so that their extension through
(C55. . .O10 3.37(1) Å) along with Cl. . .
p interactions [27]
(Cl3. . .C46 3.331(7) Å). In addition, this involves but one of the
two methanol molecules of the stoichiometric unit; the other is
also involved in H-bonding to pyridine-N (O(9)ꢀꢀꢀN(8) 2.899(6) Å)
but along with O. . .HC(aromatic) interactions and without any H-
bonding to chloroform (see Fig. 3).X-ray quality, pale violet crystals
of 2 were obtained by slowly diffusing methanol vapor into a DMF
ꢀ
solution of the complex, the space group again proving to be P1.
The cation present in the lattice of 2 is shown in Fig. 4, and selected
bond distances and angles are given in Table 1. Again, the Cd(II)
metal ion has a distorted octahedral CdN6 coordination geometry
with, as also in the Zn(II) species, a significant deviation (ꢃ8°) of
the mean planes of the terpyridine units from orthogonality. With
its longer substituent chains, giving N7. . .N8 35.79(3) Å, the overall
form of the cation is more obviously bent, the angle N7–Cd1–N8
being 166.9°, but the lattice can still be divided into sheets of cat-
ions with the tails lying within the sheets, here parallel to the
(10ꢂ1) plane. Undulating linear arrays, similar to the more strictly
linear arrays in the Zn complex lattice, involving overlap in projec-
tion of the tail units can be discerned within these sheets (Fig. S1),
the Cd. . .Cd separations alternating between 21.69(2) and
22.11(1) Å. Appearances here are particularly deceptive, however,
as for the longer 22.11(1) Å separation, all centroid. . .centroid dis-
tances for the apparently approximately parallel and overlapping
rings are >4.5 Å (viz. very long), while for the cations 21.69(2) Å
apart, the centroid. . .centroid distances are shorter (3.941(5),
Fig. 3. Stick representations of various cation pairs which can be discerned within the lattice of 1: (a) the pair with Zn. . .Zn 19.99(2) Å; (b) the pair with Zn. . .Zn 20.07(2) Å; (c)
the pair with Zn. . .Zn 11.20(1) Å; (d) the pair with Zn. . .Zn 11.30(1) Å. Black spheres define the centroids of aromatic rings in a stacked array, except for (a), where the rings are
not parallel. For (a), dashed lines indicate CH. . .N contacts, while for (d), the dashed lines define links between the cations involving H-bonding and Cl. . .
p interactions
(red = O, green = Cl).

E. Kubota et al. / Polyhedron 52 (2013) 435–441
439
Fig. 4. Molecular structure of [Cd(L2)2]2+ cation in 2 with thermal ellipsoids plotted at the 50% probability level. For clarity, hydrogen atoms are omitted.
Fig. 5. (a) Views of the apparent stacking arrays for (left) the closest cation pairs (8.98(1) Å apart) and (right) the next closest pairs (at 9.06(1) Å) present in the lattice of 2.
Black spheres define the centroids of the stacked rings and C. . .C contacts <3.5 Å are shown as dashed lines, the separations/Å being indicated. (b) Orthogonal partial views of
one of the strands formed by cations in which the Cd. . .Cd separations alternate between 8.98(1) and 9.06(1) Å. The upper view is that down the perpendicular to the (111)
plane. (c) The stacking contacts involving the shortest centroid. . .centroid distance (3.586(4) Å) of any intercationic aromatic ring pair, occurring for cations 10.593(7) Å apart
in the lattice of 2 (violet = Cd).
the lattice may be influenced by differences in the size of Zn(II) and
Cd(II) and thus explain some of the differences between the two
present structures (other than the obvious difference due to the
nature of the tails). If as a measure of the size of the complex core
the distances between the C atoms to which the tails are attached
(9.68(1) Å for Zn(II), 10.04(2) Å for Cd(II)) and between the two sets
of C atoms para to the N-donor atoms of the peripheral rings
(9.45(1), 9.46(1) Å for Zn(II); 9.47(2), 9.51(2) Å for Cd(II)) are taken,
at least the longitudinal difference can be regarded as significant in
terms of a change in any interaction distance but the lateral differ-
ences, probably more relevant in relation to anion bridging, are
quite small.

440
E. Kubota et al. / Polyhedron 52 (2013) 435–441
3.2. Absorption and emission spectra
and 7. The ligand emission bands, found in the near-UV region in
both cases, are strongly red-shifted by metal ion coordination. At
the concentrations used, it is possible that the complexes were dis-
sociated to some extent and that this may have affected the spec-
tra, although there is close similarity to what is seen in the solid
state. Emission from the Cd(II) complex occurs at a considerably
lower energy than that from the Zn(II) complex and it is assumed
that this is primarily a consequence of the extension of the ligand
structure rather than the change in metal ion. Although the pres-
ence of the pyridyl substituent in the tails must reduce the overall
polarity of the ligand, the characteristics of the emission spectra re-
main consistent with the emitting excited state being that of an
intramolecular charge transfer (ICT) transition [3,28].
The absorption spectra of the ligands and the complexes in CH3-
CN are shown Fig. S3. Peak wavelengths and their typically high
absorption coefficients are given in Table 2. Comparison of the
two ligand spectra shows that the lower-energy absorption peak
is slightly red-shifted by the extension of the length of the 40 sub-
stituent. As expected [19], there is a red-shift induced by coordina-
tion of the metal ions.
The emission spectra of the free ligands and their metal com-
plexes in CH3CN solution and the solid state are shown in Figs. 6
Table 2
Absorption data in CH3CN (molar absorption coefficient) for the ligands and the
complexes.
4. Conclusions
The grafting of pyridylphenyl and pyridylbiphenyl tails onto the
40 position of terpyridine has the effect on the structure of the
Zn(II) and Cd(II) complexes of these ligands of eliminating almost
entirely the ‘‘terpyridine embrace’’ typifying the lattices of com-
plexes of terpyridine itself. While evidence for various other forms
of aromatic–aromatic interactions is apparent within the lattices,
the patterns are quite different in the two cases studied, indicating
that these interactions are subsidiary to others, in particular the
cation–anion interactions. This is consistent with recent theoreti-
abs
Molecules
k
nm (emax (ꢁ 104 Mꢂ1 cmꢂ1))
max
L1
L2
1
258 (sh, 1.31)
258 (sh, 1.91)
236 (4.44), 269 (sh, 4.95)
236 (sh, 4.62)
285 (2.22)
296 (3.57)
287 (6.60)
287 (6.97)
315 (6.17)
332 (8.35)
2
cal evidence [29] that
p-stacking only becomes a force of greater
significance than dispersion interactions when the contact surfaces
are quite large, well beyond the confrontation of single aromatic
rings.
Acknowledgements
This work was supported by Innovative Areas ‘‘Coordination
Programming’’ (area 2107) from the MEXT,Japan. Y.H. Lee was also
supported by the JSPS program of Postdoctoral Fellowships for For-
eign Researchers (No. 2200341).
Appendix A. Supplementary data
CCDC 879317 and 879318 contain the supplementary crystallo-
graphic data for the complexes 1 and 2. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk.
Fig. 6. Emission spectra of L1 (solid line) and 1 (dotted line) in solution (5 lM in
CH3CN) and the solid state (inset) as a powder. The excitation wavelengths were
289 (L1) and 291 nm (1).
Fig. S1 – a view, perpendicular to the (10ꢂ1) plane of the cation
array present in sheets within which the ligands tails lie. One cat-
ion chain is identified in black.
Fig. S2 – a view of the same cation array as shown in the upper
part of Fig. 5(b) but without the ligand tail atoms being shown.
This is compared with the two-dimensional terpyridine embrace
array found, for example, in [Co(terpyridine)2]I2ꢀH2O.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.08.054.
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