Embrace interlocking of dipyrazinylpyridine complexes involving N?π interactions

Article abstract of DOI:10.1016/j.inoche.2010.03.004

Fe(II) and Ni(II) complexes of 4-p-tolyl-2,6-di(2-pyrazinyl)pyridine (tdpzpy) have been synthesized and characterized. The N?π interaction involving the uncoordinated pyrazinyl N atom, which is stronger than the corresponding C-H?π hydrogen bonding in a s

Full text of DOI:10.1016/j.inoche.2010.03.004

Inorganic Chemistry Communications 13 (2010) 625–629
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Inorganic Chemistry Communications
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Embrace interlocking of dipyrazinylpyridine complexes involving N⋯π interactions
⁎
Jing-Wei Dai, Ben-Zhen Li, Ying-Lin Chen, Guang Huang, Bin Cai, Ying Yu, Jian-Zhong Wu
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Fe(II) and Ni(II) complexes of 4-p-tolyl-2,6-di(2-pyrazinyl)pyridine (tdpzpy) have been synthesized and
characterized. The N⋯π interaction involving the uncoordinated pyrazinyl N atom, which is stronger than the
Received 21 December 2009
Accepted 2 March 2010
Available online 9 March 2010
corresponding C–H⋯π hydrogen bonding in
a similar 4-p-tolyl-2,2′:6′,2″-terpyridine complex, was
distinguished as a supramolecular motif. It acts as an edge-to-face force in assembling cations of [Fe
(tdpzpy)2]2+ or [Ni(tdpzpy)2]2+ into chains via embrace interlocking. C–H⋯π hydrogen bonds further link
the chains into higher dimension net. The electronic absorption and emission properties of [Fe(tdpzpy)2]2+
and [Ni(tdpzpy)2]2+ are largely ligand-centered. [Fe(tdpzpy)2]2+ displays an MLCT absorption at 567 nm.
The fluorescence of [Ni(tdpzpy)2]2+ is different markedly in methanol and acetonitrile. The ligand-based
fluorescence is quenched to some extent by coordination to the d-orbital unfulfilled Fe(II) or Ni(II) atom.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Crystal structure
N⋯π interaction
C–H⋯π hydrogen bond
Electronic absorption
Fluorescence
2,2′:6′,2″-Terpyridine (terpy) and its substituted analogues are
among the most studied tridentate ligands for a wide range of
transition metal ions.[1] One of the studying areas of terpy type
ligands is their application in supramolecular assembly. Dance and his
coworkers have identified a crystal supramolecular motif which was
termed “terpy embrace” in a number of octahedral meridional [M
(terpy)2]n+ complexes.[2] The terpy embrace involves one offset-
face-to-face (OFF) and two edge-to-face (EF) interactions among the
outer pyridyl rings of the ligand between neighbored complex cations.
This phenomenon has been found not only in complexes of terpy
ligands, but also in some other terimine tridentate ligands, and
depending on the ligand's structure, some other motifs may also exist
in the crystal packing structures.[3] For example, the pendant phenyl,
[3] pyridine [3] or tolyl [3] groups bring additional π⋯π or hydrogen
bonding interactions.
It is some surprising that 2,5-di(2-pyrazinyl)pyridine (dpzpy,
Scheme 1), the isoelectronic species of terpy, and other dipyrazinyl-
pyridines have received much scarce attention. Up to now only five
papers appeared concerning this family of compounds.[4] The ligand
4-p-tolyl-2,6-di(2-pyrazinyl)pyridine (tdpzpy, Scheme 1) was first
obtained in 2001 in a short communication, but the synthetic details
and characterization data (except crystal analysis) were not given.[4]
The ligation investigation of tdpzpy was limited to [Ru(tdpzpy)2]
(PF6)2·2CH3CN [4] and the essential coordinate data of this complex
were unavailable either in that paper or in the CSD database (Refcode
EFUREC). In order to further investigate the coordination behaviour of
the dipyrazinylpyridine ligands, here we present the synthesis [5] of
tdpzpy and two of its complexes [Fe(tdpzpy)2]Cl2·3.5H2O (1) and [Ni
(tdpzpy)2](NO3)2·4H2O (2). Interestingly, the crystal analysis [6]
reveals that an N⋯π interaction between the pyrazinyl moieties can be
distinguished which has never been reported before.
Comparing to its isoelectronic species 4-p-tolyl-2,2′:6′,2″-
terpyridine (ttpy, Scheme 1) (m.p. 168 °C),[4,7] the melting point
of tdpzpy is 90 °C higher, reflecting enhancement of intermolecular
interaction by replacement of the two peripherical pyridyl rings with
pyrazinyls. The iron complex 1 was obtained hydrothermally.
Although FeCl3 was used as the metal source, in the product the
iron atom is divalent and in low-spin state, clearly proved by that 1
H
NMR signals of 1 are sharp and well-resolved in regular range
(chemical shifts 0–10 ppm), indicative of diamagnetic property.
Reduction of Fe(III) to Fe(II) in the presence of an aromatic N ligand
was ever reported in another case.[8] In contrast, the 1H NMR
spectrum of the nickel complex 2 demonstrates it is paramagnetic,
similar to that found for Ni(II)-terpy type complexes.[9]
1 and 2 are virtually isostructural, with the same symmetry
(triclinic P-1) and similar cells. [6] They are analogous in the
coordination cation structure and the inter-cationic interaction
dominates the whole molecular packing as will be described in detail
below. The counter anions (chloride and nitrate in 1 and 2,
respectively) and water molecules possess the void spaces and have
no remarkable effects on the crystal packing, similar to a number of [M
(terpy)2]n+ complexes with small counter anions.[2] Each Fe(II) or Ni
(II) atom is hexa-coordinated by two orthogonal tridentate tdpzpy
ligands. The outer M–Npz bonds (averaged 1.966 and 2.104 Å for Fe
and Ni–Npz, respectively) are longer than the central M–Npy bonds
(averaged 1.87 and 1.973 Å for Fe and Ni–Npy, respectively). Due to
the low-spin state of the Fe(II) atom in 1, the Fe–N bonds in 1 are
markedly shorter than the corresponding Ni–N bonds in 2. The
twisting of the pendant tolyl moieties with respect to the connected
central pyridine rings is very similar for 1 and 2 (twisting angles 19.2
⁎
Corresponding author.
E-mail address: wujzh@scnu.edu.cn (J.-Z. Wu).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.03.004

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J.-W. Dai et al. / Inorganic Chemistry Communications 13 (2010) 625–629
in which the pyrazine π system acts as either a π Lewis acid or a π Lewis
base. This is quite similar to the OFF stacking between pyridine rings in
[Fe(ttpy)2]2+. As regards the EF interaction, the difference is much
remarkable. In [Fe(ttpy)2]2+ and other bis(terpy) type complexes, the
EF interaction between two nearly perpendicular pyridine rings
involves (sp2)C–H⋯π hydrogen bonds. But in [Fe(tdpzpy)2]2+ or [Ni
(tdpzpy)2]2+, the EF interaction between two pyrazine rings utilizes the
uncoordinated N atom in a pyrazine ring. The N⋯pyrazine distances in
[Fe(tdpzpy)2]2+ are comparable to the H⋯pyridine distances in [Fe
(tdpzpy)2]2+, as shown in Fig. 1a and b.
Scheme 1. Structures of relevant ligands, with ring and atom labelling in tdpzpy used
for NMR characterisation.
(2) and 48.0(2)° in 1, 19.2(1) and 46.6(1)° in 2, Fig. 1b and c). The
dihedral angle between the two tolyl moieties is not large, 14.5(2) and
16.3(1)° for 1 and 2 respectively.
In order to quantitatively compare the novel N⋯π interaction with
the C–H⋯π interaction, preliminary theoretical calculations based on
crystal structural data of [Fe(tdpzpy)2]2+ and [Fe(ttpy)2]2+ were
carried out, taking the embracing pyrazine–pyrazine or pyridine–
pyridine moieties as the simplified models, respectively. The DFT
method in Guaussian 03 program with B3LYP/6-31G* basis set and 50%
basis set superposition error (BSSE) correction was used for the
calculation.[10] Since the anions have little influence on the embrace
interlocking as mentioned above, they were ignored in the calculation.
The interaction energies for the actual N⋯π or H⋯π distances are
compiled in Table 1. It is clear that the interaction energies of N⋯π
Like the bis(terpy) type complexes, the “terpy embrace”, or more
generally embrace interlocking, is the dominant interaction between
adjacent [Fe(tdpzpy)2]2+ or [Ni(tdpzpy)2]2+ cations, as illustrated in
Fig. 1b and c, respectively. The embrace of [Fe(ttpy)2]2+ cations in [Fe
(ttpy)2][PF6]2·2.2MeCN,[3] drawn based on the CIF file (Refcode
KOGLAU) is shown in Fig. 1a for comparison, where one of the two
sets of disordered positions adopted. There are obvious OFF stacking
between adjacent pyrazine rings in [Fe(tdpzpy)2]2+ or [Ni(tdpzpy)2]2+
,
Fig. 1. Assembling of the cations into a chain by linkage of alternating [Fe(ttpy)2]2+ (a), [Fe(tdpzpy)2]2+ (b), or [Ni(tdpzpy)2]2+ (c) via OFF (green bold lines) and EF (red bold lines)
interactions. For clarity, most of the H atoms are omitted. The values are the corresponding centroid···centroid, N⋯centroid or H⋯centroid distances in angstrom.

J.-W. Dai et al. / Inorganic Chemistry Communications 13 (2010) 625–629
627
Table 1
Comparison of the interaction energies of N⋯π and C–H⋯π interactions.
Complex
Interaction type
N (or H)⋯π/Å
Energy/kcal mol−1
[Fe(tdpzpy)2]2+
Npz⋯πpz
3.120
3.067
2.872
2.992
3.103
3.124
1.537
1.753
1.006
0.948
0.563
0.694
[Fe(ttpy)2]2+
CpyH⋯πpy
interactions are stronger than those of C–H⋯π hydrogen bonds,
although the values are almost in the same magnitude. As C–H⋯π
hydrogen bonds have remarkable effects in the supramolecular
assembly,[11] the N⋯π interaction would likely play some important
role, as demonstrated in this paper. 1 and 2 represent two rare
examples involving the N⋯π interaction. Searching the CSD database
gives several organic or coordination compounds (Refcodes GALTES,
VUDMEM, SIBDAJ, YIDTAH, TISWOI and TISWOI01) with analogous
aromatic N⋯π contacts, which were not definitely indicated by the
authors.[12] In [Ni(tdpzpy)2]2+, the EF and OFF interactions are some
weaker than those in [Fe(tdpzpy)2]2+ as judged from the N⋯π and π⋯π
related structural data (Fig. 1), consistent with the larger coordination
sphere of [Ni(tdpzpy)2]2+ as indicated above.
Fig. 3. Electronic absorption spectra of tdpzpy and its complexes in methanol
(concentration 10 μM) at room temperature.
almost colinear, with the M⋯M⋯M angle being 167.2(1) and 168.9(1)°
for [Fe(tdpzpy)2]2+ and [Ni(tdpzpy)2]2+ respectively, close to 174.5(1)°
for [Fe(ttpy)2]2+. The substituent tolyl groups are aligned orthogonal to
the metal line pointing towards two opposite directions. These tolyl
groups also have some structural contribution for the supramolecular
assembling. As shown in Fig. 2, the intermolecular sp3–C–H⋯π interac-
tions involving the methyl C–H and the pyrazine ring in [Fe(tdpzpy)2]2+
link the chains into two-dimensional layers. Other sp2–C–H⋯π interac-
tions involving the phenyl and pyrazine rings further connect the layers
into a three-dimensional network.
For the prototype [M(terpy)2]n+ complexes, normally each complex
cation forms eight EF and four OFF interactions with its four neighbours
and hence there is a 4-fold two-dimensional net of these embraces.[2]
But when the terpy is replaced by a terimine ligand having pendant
substituents the situation may become different. [Fe(ttpy)2]2+, for
example, forms four EF and two OFF interactions with two neighbours.
[3] The similar embrace pattern in [Fe(tdpzpy)2]2+ and [Ni(tdpzpy)2]2+
leads the cations align alternately to form one-dimensional
chains, as illustrated in Fig. 1. The metal atoms inside a chain are
It is some regretful that the X-ray crystallographic data for 1 and 2
were not good enough to assure the exact positioning of the water
proton atoms. So it is not very safe to describe accurately the water
involved hydrogen bonds. But the water molecules and the anions
Fig. 2. Chains of [Fe(tdpzpy)2]2+ expand to a layer by methyl involved C–H⋯π hydrogen bonds. Only relevant H atoms are shown. Green and cyan lines represent OFF and C–H⋯π
interactions, respectively.

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J.-W. Dai et al. / Inorganic Chemistry Communications 13 (2010) 625–629
have little influence on the crystal packing comparing to the cationic
embrace interlocking as described above. Thermogravity analysis
implys that the water involved hydrogen bonding is not strong
enough to markedly increase the temperature for complete water
removing (below 130 °C).
The electronic absorption spectra of complexes 1 and 2, as well as the
ligand in methanol are shown in Fig. 3. Below 310 nm there are several
strong ligand-centered absorption bands for the complexes. The
coordination induced new absorption was observed around 335 nm
for both complexes. The Fe(II) complex 1 displays a moderately intense
metal-to-ligand charge-transfer (MLCT) band at 567 nm and is
responsible for the dark purple colour of this complex, supporting the
supposed low-spin state of Fe(II), analogous to the Fe(II)–terpyridine
type complexes.[13] The absorption spectra of the compounds in
acetonitrile are essentially the same as those in methanol. Excitation of
the MLCT transition of complex 1 could not trigger off photoemission.
But when excited with UV light, both 1 and 2 in methanol exhibited
Fig. 5. Luminescence spectra of tdpzpy and its complexes in solid state (excited at
285 nm) at room temperature.
fluorescence around 400 nm and weaker fluoroscence around 320 nm,
attributable to the ligand-based transition by comparison with the
ligand, as illustrated in Fig. 4. In another solvent acetonitrile the weak
bands disappeared, while the strong bands decreased to some extent.
The emission of the Ni(II) complex 2 is more likely to be affected by the
solvent, asthe410 nmemission inmethanol dramaticallyshifted blue to
390 nm in acetonitrile, accompanied by a remarkable intensity decrease
(36%). Comparing the fluorescence intensities in the same solvent
(Fig. 4) suggests that coordination to Fe(II) or Ni(II) leads to slight
quenching of the emission from tdpzpy, implying some charge-transfer
from the ligand to the d-orbital unfulfilled metal atom. Such quenching
becomes more drastic for the solid state fluorescence (Fig. 5).
In summary, the Fe(II) and Ni(II) complexes with a rarely utilized
dipyrazinylpyridine ligand have been synthesized. The supramolec-
ular embrace interlocking between the coordination cations dom-
inates the whole crystal packing. The rarely seen N⋯π interaction,
stronger than C–H⋯π hydrogen bonding, has been found participating
the embrace motif. The electronic absorption and emission properties
of the complexes have been preliminarily investigated.
Acknowledgement
Financial support from the National Natural Science Foundation of
China (grant no. 20975041), the Guangdong Natural Science Foun-
dation (grant no. 5005935) and the SRFROCS program, State
Education Ministry of China, are gratefully acknowledged.
Appendix A. Supplementary data
CCDC 745109 and 745110 contains the supplementary crystallo-
graphic data for this paper. 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 associated with
this article can be found, in the online version, at doi:10.1016/j.
inoche.2010.03.004.
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8.76–8.75 (m, 4 H, HA2/A3), 8.67 (d, 2 H, HB1), 7.78 (d, 2 H, HC1), 7.34 (d, 2 H, HC2),
2.44 (s, 3 H, CH3). ESI-MS: m/z=326.51 ([tdpzpy+H]+). Synthesis of 1: tdpzpy
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,
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