Battlement-shaped 1D coordination polymer based on a bis(N-methylimidazole- 2-yl)butadiyne ligand

Article abstract of DOI:10.1039/c3ce41652f

Bis(N-methylimidazole-2-yl)butadiyne (bmib) has been prepared starting from N-methylimidazole following two different pathways. Bmib fluoresces and is capable of forming 1D coordination polymers by clamping together metal units. The molecular structure of a zinc coordination polymer based on bmib and zinc acetate resembles a battlement of a fortress.

Full text of DOI:10.1039/c3ce41652f

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Volume 15 | Number 47 | 21 December 2013 | Pages 10145–10406
COVER ARTICLE
Guldi, Burzlaff et al.
Battlement-shaped 1D coordination polymer based on a
bis(N-methylimidazole-2-yl)butadiyne ligand

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Battlement-shaped 1D coordination polymer based on
a bis(N-methylimidazole-2-yl)butadiyne ligand†
Cite this: CrystEngComm, 2013, 15,
10157
Thomas Waidmann,a Nico Fritsch,a Johannes Tucher,a Marc Rudolf,b Felix Glaser,a
b
a
*
*
Dirk M. Guldi and Nicolai Burzlaff
Received 12th July 2013,
Accepted 21st August 2013
DOI: 10.1039/c3ce41652f
www.rsc.org/crystengcomm
Bis(N-methylimidazole-2-yl)butadiyne (bmib) has been prepared
starting from N-methylimidazole following two different
pathways. Bmib fluoresces and is capable of forming 1D
coordination polymers by clamping together metal units. The
molecular structure of a zinc coordination polymer based on
bmib and zinc acetate resembles a battlement of a fortress.
square-grid architectures.6 Recently, we reported on
trans-1,2-bis(N-methylimidazole-2-yl)ethylene(trans-bie) which
turned out to be a versatile building block for 2D and espe-
cially 1D coordination polymers.7 Inspired by the results
gained, we decided to aim for the new linear
bis(N-methylimidazole-2-yl)butadiyne (bmib) (7).
The first approach to get hands on the bmib ligand
started with 1-methylimidazole-2-carbaldehyde (2) that is acces-
sible from 1-methylimidazole (1) (Scheme 1).8 A Wittig reaction
of 2 and chloromethylene-(triphenyl)phosphine resulted in
2-(2′-chlorovinyl)-N-methylimidazole that was obtained as a 2 : 1
mixture of E/Z isomers 3a/b. Treating 2-(2′-chlorovinyl)-
N-methylimidazole (3a/b) with potassium tert-butoxide yielded
the 2-ethynyl-N-methylimidazole (6) suitable for a Glaser cou-
pling. Due to a rather cumbersome purification of 3a/b, mostly
caused by unreacted Wittig reagent, we decided to follow another
route to 2-ethynyl-N-methylimidazole (6). This second approach
involved 2-iodo-1-methylimidazole (4) which is gained by a reac-
tion of 1-methylimidazole (1) with n-butyl lithium and I2.9 The
resulting imidazole 4 was treated under Sonogashira coupling
conditions to yield 2-trimethylsilyethynyl-1-methylimidazole (5).10
After desilylation with KF, bis(N-methylimidazole-2-yl)butadiyne
(bmib) (7) was obtained by a homocoupling Glaser reaction of 6
catalyzed by CuCl in pyridine (Scheme 1).
Unsaturated organic ligands in combination with transition
metals can be used as precursors for inorganic–organic
materials. These materials, based on their architecture and
characteristics, are of great interest regarding catalysis,
nonlinear optics, or properties that may be useful in
electronic applications or as molecular sensors.1 Molecular
self-assembly is at the heart of crystal engineering and is
based on complementary and explicit interactions between
the components in order to generate the final product.2
Two new branches excelled in this still emerging field,
namely metal–organic coordination networks (MOCNs) or
metal–organic frameworks (MOFs) and supramolecular coor-
dination complexes (SSCs).3 Synthesis of highly ordered
coordination polymers can lead to molecular networks with
different dimensions, e.g. linear polymers or “zigzag” chains
and 2D planar or 3D polymer networks.1–4 Our special interest
focuses on the synthesis of new appropriate organic N,N-ligands
suitable for 1D coordination polymers.2a,b,5 In recent years,
polyyne-bridged heterocycles, such as bis(4-pyridyl)butadiyne,
have been proven to be powerful ligands for chain and
Bmib (7) crystallizes in the space group P21/c, and the
result of an X-ray single crystal structure determination
shows the presence of intermolecular π-stacking (Fig. 1). The
average distance between the centres of the stacking imidaz-
oles is approximately 3.6–3.7 Å. Bond distances of the acety-
lene chain show prevalent values with a triple bond distance
of 1.202 Å.6a,11
a Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg,
Egerlandstrasse 1, 91058 Erlangen, Germany. E-mail: nicolai.burzlaff@fau.de;
Fax: +49 9131 85 27387; Tel: +49 9131 85 28976
b Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg,
Egerlandstrasse 3, 91058 Erlangen, Germany. E-mail: dirk.guldi@fau.de; Fax:
+49 9131 85 27341; Tel: +49 9131 85 28307
Similar to related compounds,12 the absorption spectrum
(Fig. 2) of bmib (7) gives rise to vibronic transitions in the
280 to 350 nm range with maxima at 292, 308, 328, and 350 nm
with an energetic spacing of 2000 cm−1. These are assigned to
the triple bond centered π–π* excited state. Following excitation
at a wavelength of 300 nm, an emission spectrum that gives
† Electronic supplementary information (ESI) available: Experimental details
for synthesis and characterization details, crystallographic data for 7 & 8.
CCDC 949213–949214. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3ce41652f
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Scheme 1 Synthesis of bmib via two different pathways. (a) n-BuLi, DMF, THF.
(b) n-BuLi, (chloromethyl)triphenylphosphonium chloride, THF. (c) KOtBu, NH4Cl,
THF. (d) n-BuLi, I2, THF. (e) Pd(PPh3)2Cl2, CuI, Et3N, DMF, TMS-acetylene. (f) KF,
MeOH. (g) CuCl, O2, pyridine.
Fig. 1 The molecular structure of bmib (7); thermal ellipsoids are drawn at the
30% probability level, and hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (°): C3–C2, 1.366(19); C2–C1, 1.202(19); C1–C11,
1.417(18); C14–N12, 1.454(17); C2–C1–C11, 179.05(14); C3–C2–C1, 178.35(14);
N11–C11–C21–N21, 178.04(14).
Fig. 2 The absorption (solid line) and emission (dotted line) spectra for bmib (7) in
CHCl3 at 300 nm excitation wavelength.
Fig. 3 (a) Differential absorption spectra (visible and near-infrared) obtained upon
femtosecond pump probe experiments (258 nm) of bmib (7) in argon-saturated THF
with several time delays between 0 and 150 ps at room temperature. (b) Time–
absorption profiles of the spectra shown in (a) at 555 and 939 nm, monitoring the
intersystem crossing. (c) Differential absorption spectra (visible and near-infrared)
obtained upon nanosecond flash photolysis (266 nm) of bmib (7) in argon-saturated
THF at 50 ns after excitation at room temperature. (d) Time–absorption profile of
the spectrum shown in (c) at 450 nm, monitoring the excited state decay.
rise to a 115 nm red-shifted maximum at 415 nm evolves.
Emission quantum yields were as high as 1.5 × 10−3 and
1.1 × 10−3 in CHCl3 and THF, respectively.
Further insights into the excited state deactivation of bmib
(7) came from transient absorption measurements following
femtosecond and nanosecond excitation in THF (Fig. 3, top).
Upon 258 nm excitation, we note intense transient absorp-
tion features throughout the visible and near-infrared region,
which maximize at 490, 555, and 939 nm. We interpret these
differential absorption changes as the population of the
singlet excited state of bmib (7). On a rather short timescale,
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Fig. 4 Battlement arrangement of coordination polymer 8.
that is, 25 5 ps, these transient absorption features decay.
At the end of this decay, we note a broad transient absorption
throughout the visible and near-infrared region with maxima
at 450 and around 1100 nm. The latter are stable and are still
discernible at the end of the experimental timescale of
7.5 ns. Tentatively, we ascribe these transients to the triplet
excited state of bmib (7). To explore the triplet excited state
deactivation, nanosecond transient absorption measurements
with 266 nm laser pulses were performed (Fig. 3, bottom). The
nanosecond transient absorption spectrum is in perfect agree-
ment with what was observed at the end of our femtosecond
transient absorption measurements. In particular, transient
absorption features with maxima at 450 and around 1100 nm
were noted. A kinetic analysis revealed two lifetimes—a short-
lived component with 1.7 0.2 and a long-lived component
with 7.9 0.5 μs. The two components are likely to arise from
triplet–triplet annihilation and ground-state quenching. Sup-
port for the assignment that the transient features correlate
with the triplet excited state of bmib (7) came from analogous
experiments in oxygen-saturated solutions. In the latter, the
triplet excited lifetime is reduced to 80 5 ns.
Reaction of bmib with Zn(OAc)2 yielded a 1D polymeric
compound. The X-ray structure analysis revealed coordination
polymer [Zn5(OAc)10(bmib)2]n consisting of alternating dinu-
clear [Zn2(O2CCH3)4] paddle-wheel units and trinuclear
[Zn3(O2CCH3)6] units “clamped” together by bmib ligands. This
rather unusual coordination mode of the bmib ligand causes a
one-dimensional sequence that resembles a battlement of a
fortress (Fig. 4). Zinc acetate is capable of forming coordination
polymers with N,N-donor ligands by either having a paddle-
wheel or trinuclear inorganic unit.13–16,18 To the best of our
knowledge, such alternating paddle-wheel and trinuclear units
have been reported only once in literature before.19
tetrahedrally by three oxygen atoms of the acetate groups and
by the nitrogen atom of the bmib ligand. Two different types
of carboxylate coordination were found in this unit. Four ace-
tate ligands show κ2-coordination, forming syn–syn bridges
between central and terminal zinc ions. The other two ace-
tate ligands are κ1-coordinated and function as Zn–O–Zn
μ-bridges with one uncoordinated oxygen atom slightly
involved in
a weak interaction with the zinc atom
(d(Zn3⋯O32) = 2.645(4) Å).17 The paddle-wheel unit is
constructed by two symmetrically equivalent Zn atoms, which
are μ-bridged by four acetate ligands. Each Zn atom in this
unit is in a distorted square pyramidal environment, with
four oxygen atoms from acetates, to form the equatorial
plane, and one nitrogen atom of the bmib ligand. In the
paddle-wheel unit, the Zn–O bond distances range from
2.035(3) to 2.077(4) Å and the Zn–N bond distance amounts
to 2.034(4), which compare well to values of related com-
pounds.13,18 In the trinuclear unit, the Zn–O bond distances
of the κ2-OAc range from 1.938(4) to 2.118(3) Å. The κ1-OAc
shows a bond distance of 2.129(3) for Zn1–O31 and 1.980(3)
for Zn3–O31. The Zn–N bond distance of 2.002(4) Å is slightly
shorter than in the dinuclear unit. The Zn–Zn distances of
dinuclear and trinuclear units are 2.9489(10) and 3.3287(6) Å,
which are similar to literature values.20 The bond distances
of the bmib ligand do not vary significantly from those of the
free ligand. It is worthwhile mentioning that obviously bmib
(7) acts as some sort of molecular clip for metal ions with
almost fixed angles ∠(M–N11–N21) close to 90° and torsion
angles ∠(M–N11–N21–M′) close to 0°. A similar coordination
property has recently been reported for the slightly shorter
1,4-bis(N-methylimidazole-2-yl)benzene ligand.20
Conclusion
The trinuclear Zn(II) acetate unit is constructed by six
bridging acetate ligands and similar to previously described
trinuclear zinc carboxylates with axial N- or O-donor
ligands.13–16 There is an inversion centre on the central Zn
atom. Out of three Zn atoms, the central Zn is all O-donor
hexacoordinated from six acetate groups to form a distorted
octahedron. The two terminal Zn centres are coordinated
The ligand 1,4-bis(N-methylimidazole-2-yl)butadiyne (bmib) (7)
shows fluorescent properties in solution and intermolecular
π-stacking in the crystal structure. Bmib (7) possesses remark-
able coordination properties, as is indicated by the formation
of a coordination polymer [Zn5(OAc)10(bmib)2]n (8). The X-ray
diffraction revealed an interesting battlement-type structure
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with alternating trinuclear and dinuclear paddle-wheel zinc
acetate units clamped together by bmib (7). The metal–bmib–
metal units resemble the geometry of retaining clips.
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Acknowledgements
This work was supported by “Solar Technologies go Hybrid”—an
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