Stable, trinuclear Zn(ii)- and Cd(ii)-metallocycles: TWIM-MS, photophysical properties, and nanofiber formation

Article abstract of DOI:10.1039/c2dt31813j

A series of trimeric, Zn(ii)- and Cd(ii)-metallocycles is reported. Structural characterization of the highly stable triangles was supported by traveling-wave ion mobility-mass spectrometry (TWIM-MS) and gradient tandem mass spectrometry (gMS²). Their unique photophysical properties and self-assembly to form nanofibers are also described.

Full text of DOI:10.1039/c2dt31813j

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Stable, trinuclear Zn(II)- and Cd(II)-metallocycles: TWIM-MS, photophysical
properties, and nanofiber formation†
Anthony Schultz,a Yan Cao,b Mingjun Huang,b Stephen Z. D. Cheng,b Xiaopeng Li,a Charles N. Moorefield,c
Chrys Wesdemiotis*a,b and George R. Newkome*a,b,c
Received 18th April 2012, Accepted 16th August 2012
DOI: 10.1039/c2dt31813j
A series of trimeric, Zn(II)- and Cd(II)-metallocycles is
reported. Structural characterization of the highly stable tri-
angles was supported by traveling-wave ion mobility-mass
spectrometry (TWIM-MS) and gradient tandem mass spec-
trometry (gMS2). Their unique photophysical properties and
self-assembly to form nanofibers are also described.
Self-assembly of predesigned architectures has attracted much
attention in recent years.1–6 Crystal engineering,7–9 as practiced
with non-covalent structural control, provides insight into the
use of supramolecular synthons for the generation of highly
ordered structures. With few exceptions, ionic interactions
provide a superb pathway to ordered, stable assemblies, and net-
works. Whereas, complex polyionic interactions have played
critical roles in the development of novel materials that include
rotaxanes,10 porphyrins,11 and dendrimers;12 with one excep-
tion,13 there has been limited reports in the area of nanofiber for-
mation. Nanofibers prepared by traditional methods have found
application in gene delivery,14 photonics and electronics,15 and
offer new avenues to functional materials. We herein report the
creation and characterization of nanoscale fibers, based on the
polyionic-mediated assembly of metallomacrocycles, constructed
Scheme 1 Metallocycle synthesis.
from bis(2,2′:6′,2′′-terpyridinyl) ligands possessing
a 60°
orientation.
Three ligands (5–7; Scheme 1) possessing 60° directionality
were synthesized by Suzuki coupling16 with known aryl halides
1–3 and 4-terpyridinylphenylboronic acid (4) by refluxing in a
3 : 3 : 1 toluene : water : t-butanol mixture for 48 hours. The pre-
viously unsynthesized ligands were readily purified by either
column chromatography or simple recrystallization from the
reaction mixture to give the bisterpyridines in good yields. These
ligands can be identified by their easily assignable 1H NMR
spectra (Fig. 1), as well as by ESI-MS (ESI†). When these
aDepartment of Chemistry, The University of Akron, Akron,
OH 44325-4717, USA
Fig. 1 Stacked 1H NMR spectra for terpyridine ligand 5, with
cadmium and zinc complexes 8 and 11.
bDepartment of Polymer Science, The University of Akron, Akron,
OH 44325-4717, USA
cThe Maurice Morton Institute for Polymer Science, The University of
Akron, Akron, OH 44325-4717, USA. E-mail: newkome@uakron.edu,
wesdemiotis@uakron.edu; Fax: +1 330-972-2368;
Tel: +1 330-972-8595
†Electronic supplementary information (ESI) available: All experimen-
tal data and spectroscopic data can be found in the Supplementary Infor-
mation. See DOI: 10.1039/c2dt31813j
ligands were mixed stoichiometrically with Zn(BF4)2 as well as
Cd(NO3)2 followed by precipitation with NH4PF6, a series of six
triangular metallomacrocycles (8–13) was generated in 80–92%
yield.
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The triangles exhibited 1H NMR spectra indicative of their
equilateral symmetry that facilitates easy structure elucidation.
Characteristic upfield shifts (Fig. 1) for the terpyridinyl 6,6′′
protons from ∼8.7 ppm for the free ligand to ∼8.1 and
∼7.8 ppm for the cadmium and zinc triangles, respectively, are
indicative of <M2+> (where, < = terpyridine) octahedral complex
formation. The 13C–1H and 1H–1H COSY NMR spectra (see
ESI†) of the products are also consistent with the triangular
structures.
10 was examined using gradient tandem mass spectrometry17
(gMS2, Fig. 2C) by selecting the 4+ charge state (m/z = 722) and
subjecting it to collisionally activated dissociation prior to ion
mobility separation at collision energies ranging from 6 to 32
V. At an ion trap voltage 32 V, the 4+ complex completely dis-
appears, corresponding to a center-of-mass collision energy
(Ecm) of 1.75 eV.
All triangles 8–13 displayed similar spectra (see ESI†). In
contrast, previous studies on 4+ ions of hexacadmium macro-
cycles exhibited a Ecm value of 0.75 eV.22 Notably, a MALDI
mass spectrum was also obtained for the triangle 8 (Fig. S17†);
this technique is typically useful for the characterization of the
more stable bisterpyridine–Ru2+ complexes.23 Terpyridine-based
Cd2+ and Zn2+ complexes have been observed to be much
weaker than the Ru2+ counterparts resulting in inconclusive
spectra.24 Note that for the first time intact terpyridine-based
supramolecular macrocycles with Cd2+ were observed in
MALDI. This suggests that the triangular rigid geometry could
be playing an important role in self-assembly and stability.
The UV-visible and photoluminescence (PL) spectra of the
free ligands 5–7 and the corresponding Cd- and Zn-based tri-
angles were recorded. A bathochromic shift is observed in the
UV absorptions (Fig. 3A) of the ligands 5–7 that is attributed to
the increasing electron-donating capability of the peripheral
groups (i.e., H-, Me-, and MeO-) of the corner benzene rings.
While, UV absorption in the metallocycles 8–13 shows little
difference, due presumably to the d10 metals inhibition of the
MLCT bands.
Electrospray ionization mass spectrometry (ESI-MS) coupled
with
traveling-wave
ion
mobility-mass
spectrometry
(TWIM-MS),17–19
a
variant of ion mobility mass spec-
trometry,20,21 was employed to aid in the characterization of
these triangular architectures. A typical ESI mass spectrum for
the triangles is shown in Fig. 2A, which exhibits the charge state
distribution observed for the methoxy-substituted, Cd-based tri-
angle 10, ranging from Z = 2+ to 6+, as well as the theoretical
and experimental isotope patterns for the 4+ charge state. Analy-
sis by ESI-TWIM-MS ensured that there were no superimposed
isomers or conformers within the sample as corroborated by the
single, regular step-like spectrum (Fig. 2B) and only minor frag-
ments at low drift times; hence, the intact triangle was the
primary species observed. Subsequently, the triangle stability for
However, the PL spectra of the H- and Me-modified trigonal
complexes 8, 9, 11, and 12 exhibit a significant bathochromic
shift from 467 to 499 nm (Cd-based) and 474 to 510 nm (Zn-
based), respectively. For both triangles 10 and 13 possessing a
methoxy substituent, nearly complete quenching of the photo-
luminescence is observed.
Attempts to investigate the utility of the macrocycles to self-
assemble into ordered arrays resulted in the formation of compo-
site nanofibers. Pairing the hexavalent positively charged
Fig. 3 UV-vis (A) and photoluminescence (B) spectra of the H-,
methyl-, and methoxy-substituted bisterpyridines 5–7 and triangles 8–13
were obtained using spectrophotometric grade DMF, along with a photo-
graph (C) of the luminescence properties of 11–13. Excitation wave-
length was 285 nm for ligands was 315 nm for complexes. Data was
collected at a concentration of 5.0 × 10−5 M for ligands and 5.0 × 10−6
M for complexes, respectively.
Fig. 2 ESI mass spectrum of triangle 10 (A), TWIM-MS plot of
10 (B), and gMS2 plot of m/z 721.5 (4+ charge state) from 10.
11574 | Dalton Trans., 2012, 41, 11573–11575
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from this model, and further studies are on-going to determine
the exact location of the hexacarboxylate. A tabulated summary
of the SAXD for the microcrystals (see ESI†) gives all unit cell
and theoretical data.
In conclusion, new bisterpyridine ligands possessing a 60°
interior angle with respect to the ligating moieties have been syn-
thesized and their facile self-assembly with cadmium and zinc
afforded high yields of the corresponding metallotriangles.
TWIM-MS and gMS2 show the triangular motif to be unexpect-
edly stable and the PL properties can be engineered to generate
good candidates for electronic applications, such as in LEDs.
The potential to self-assemble offers avenues to higher ordered
supramacromolecular materials.
Fig. 4 TEM images of fibers formed from triangles 13 (left), and 12
(right) with benzenehexacarboxylate (inset: SAXD diffraction pattern
from trimer 12).
Acknowledgements
The authors acknowledge support from the National Science
Foundation (DMR-0705015 and CHE-1151991
– G.R.N.;
DMR-0821313 and CHE-1012636 – C.W.) and the Ohio Board
of Regents.
Notes and references
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Powder XRD (X-ray diffraction) data obtained from micro-
crystalline regions within the fibers supports the SAXD data and
provides insight into the long range order in the fibers; ortho-
rhombic unit cell dimensions of 39.82, 8.38, and 49.14 Å for a,
b, and c, respectively, were found. Based on these data, a
packing model for the triangles in the fiber was derived from
molecular modeling (Fig. 5). The counterions have been omitted
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 11573–11575 | 11575