Towards molecular construction platforms: Synthesis of a metallotricyclic spirane based on bis(2,2′:6′,2"-terpyridine)RuII connectivity

Article abstract of DOI:10.1002/chem.201403840

The design and construction of the first multicomponent stepwise assembly of a II-tpy>-based (tpy=terpyridine), three-dimensional, propeller-shaped trismacrocycle, 8, are reported. Key steps in the synthesis involve the preparation of a

Full text of DOI:10.1002/chem.201403840

DOI: 10.1002/chem.201403840
Communication
&
Metallomacrocycles
Towards Molecular Construction Platforms: Synthesis of
a Metallotricyclic Spirane Based on Bis(2,2’:6’,2“-Terpyridine)RuII
Connectivity
Ting-Zheng Xie,[a] Kai Guo,[a] Mingjun Huang,[a] Xiaocun Lu,[a] Sheng-Yun Liao,[b]
Rajarshi Sarkar,[a] Charles N. Moorefield,[a] Stephen Z. D. Cheng,[a] Chrys Wesdemiotis,*[a] and
George R. Newkome*[a]
Abstract: The design and construction of the first multi-
component stepwise assembly of a <tpy-RuII-tpy>-based
(tpy=terpyridine), three-dimensional, propeller-shaped
trismacrocycle, 8, are reported. Key steps in the synthesis
involve the preparation of a hexaterpyridinyl triptycene
and its reaction with dimeric, 608-directional, bisterpyri-
dine-RuII building blocks. Characterization includes ESI-
1
and ESI-TWIM-MS and TEM, along with 1D and 2D H NMR
spectroscopy.
Inspiration arising from fascinating natural systems has long
prodded synthetic chemists to mimic the shapes and proper-
ties of biomolecular scaffolds[1] from which a wide variety of
supramolecular structures has been investigated and devel-
oped.[2] Many such constructs are predicated on molecular self-
Scheme 1. Syntheses of the hexakisterpryidinyl triptycene core 4 and the
monoRuII capping units 6.
assembly, which is driven by precisely designed complementa-
ry components and, in many cases, employs noncovalent inter-
actions.[3] These protocols serve as the foundation for stepwise
coordination-driven assembly, which can overcome entropic
disadvantages and provide excellent avenues to nano- and
macroscopic structures.[4] In many cases, novel complex hetero-
leptic materials have been prepared.[5,6]
py core (6 and 4, respectively, Scheme 1). The critical hexakis-
terpyridinyl ligand employed for the core of the nanotricycle
was constructed on a tryptycene moiety that provided the
requisite directionality exemplified in pseudorotaxanes.[8] Previ-
ously reported <tpy-RuII-tpy>-constructed architectures pos-
sessing the triangular motif include triangle-based fibers,[9] but-
terfly and bowtie spirane motifs,[10] along with rhombic,[11] and
spoked-wheel metallocyclic structures.[12] Construction of the
metallospirane is thus predicated on this ubiquitous, stable,
topological element.
Herein, we report the coordination-driven assembly of
a
nanoscopic, three-bladed, propeller-shaped spirane (8,
Scheme 2) comprised of nine linear terpyridine-RuII-terpyridine
(<tpy-RuII-tpy>) coordination linkages with architectures remi-
niscent of Meijere’s[7] and Kozhushkov’s cyclopropane-based
triangulanes. The metallotricycle 8 was obtained by the combi-
nation of two extended tpy ligands; a 608-based <bistpy-RuII-
bistpy> capping component and a triptycene-based hexakist-
Each dimeric capping unit possesses a central benzene ring
connector that imposes a 608 angle between two attached ter-
pyridines that is required for triangle formation.[13] Preparation
of the metallotricyclic spirane began with the synthesis of the
tryptycene-based core 4 (Scheme 1) that was accessed by
a six-fold Suzuki-coupling reaction using the hexakisbromotrip-
tycene 2 (prepared by a literature procedure[14]) and 4-terpyri-
dine-phenylboronic acid 1 (obtained from reaction of commer-
cially available 4-formylphenylboronic acid with 2-acetyl pyri-
dine and base, followed by NH4OAc).[12a] Isolation followed by
column chromatography and recrystallization gave the desired
[a] Dr. T.-Z. Xie, K. Guo, M. Huang, Dr. X. Lu, R. Sarkar, Dr. C. N. Moorefield,
Prof. S. Z. D. Cheng, Prof. C. Wesdemiotis, Prof. G. R. Newkome
Departments of Polymer Science and Chemistry and
The Maurice Morton Institute for Polymer Science
The University of Akron, Akron, OH 44325 (USA)
E-mail: wesdemiotis@uakron.edu
newkome@uakron.edu
[b] S.-Y. Liao
Department of Chemistry, Nankai University, Tianjin, 300071 (P.R. China)
1
hexadentate product 4 (60% yield). The H NMR spectrum of 4
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403840.
shows only one set of peaks for the tpy units, indicating
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Communication
a single type of hexaterpyridinyl ligand as well as confirming
its symmetric motif.
a saturated aqueous NH4PF6 solution to give the MeCN soluble
À
complex [8(PF6 )18].
The monoRuII terpyridinyl dimer 6 was synthesized from the
dibenzyloxybisterpyridinylbenzene 5, which was prepared by
a two-fold Suzuki-coupling reaction utilizing dibromide 3 and
4-terpyridinylphenylboronic acid (1).[10] Dimer 6 was isolated
(65% yield) and used with chloride counterions to form the
bisRuIII adduct 7. Support for the formation of dimer 6 includ-
The H NMR spectrum of this metallotricyclic spirocomplex 8
1
exhibited a set of peaks that were consistent with the forma-
tion of a discrete, symmetrical product. The D3h symmetry of
trispirane 8 arises from the arrangement of the three simple
triangles that give rise to broadened absorptions owing to the
three chemically non-equivalent terpyridinyl units exhibiting
1
1
ed the H NMR spectrum, which revealed two singlets assigned
close chemical shifts. The H NMR peaks assigned to the cap-
to the protons of the benzyloxy methylene moieties at 5.30
and 5.29 ppm, indicating different diastereotopic environments
after the formation of the Ru dimer. Two singlets observed at
9.37 and 8.73 ppm were assigned to the 3’,5’ protons from co-
ordinated and non-coordinated terpyridine moieties, respec-
tively. The peak at m/z 955.28 in the mass spectrum of 6 fur-
ther supports its structure.
ping terpyridines nearly overlap and appear as a single broad
peak (Figure 1). Where resolution permits, the terpyridinyl sig-
The paramagnetic bisRuIII adduct 7 of dimer 6 was synthe-
sized (Scheme 2) by treatment with
3 equivalents of
RuCl3·nH2O in EtOH heated at reflux, affording the capping
Figure 1. Stacked 1H NMR spectra of the hexakisterpyridinyl core 4 (top), the
metallospirane complex 8 (middle), and the bisRuII bisterpyridine dimer 7
(bottom). Arrows depict assigned resonance shifts that occur upon complex
formation.
nals appear as two sets of peaks in the ratio of 1:2, identifying
the core and capping components, respectively. Thus, the 3’,5’
capping terpyridinyl protons appear together at 9.05 ppm and
those attributed to the inner core terpyridines appear at
9.10 ppm. The protons attached to the tertiary carbon on the
central triptycene unit are identified by a single peak at
6.36 ppm, further supporting the symmetry of this spirocom-
plex. A noticeable 0.51 ppm downfield shift of the triptycene
moieties’ tertiary CÀH, compared with that of the metal-free
core ligand 4, was observed owing to a strong electropositive
environment created by the proximity of nine RuII centers. Also
notable, the benzyloxy methylene protons appear as a singlet
and move slightly downfield to 5.39 ppm from approximately
5.30 ppm in the precursor. Assignment of all terpyridinyl and
aromatic peaks was accomplished with NOESY and COSY ex-
periments. For example, the 1D NMR broad signals arising
from Htpy3,3“, Htpy4,4”, Htpy5,5“, and Htpy6,6” in 8 were assigned to
Scheme 2. Synthesis of the metallotrispirane nonacomplex 8.
component for the polyspirane (98% yield). Subsequent treat-
ment of the bisRuIII adduct 7 with the hexadentate triptycene
4 in a 3:1 ratio in the presence of N-ethylmorpholine in a mix-
ture of MeOH and CHCl3 (v/v 2:1) afforded a deep red solution
upon heating at reflux for 24 h. Following column chromatog-
raphy using silica, the pure product was isolated (30% yield) as
À
the NO3 salt. The counterions were exchanged by employing
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8.69, 7.89, 7.17, 7.45 ppm, respectively. The small singlet at
8.11 ppm was assigned to the Hc(interior) protons and the upfield
shifted Hc(outer) protons assigned to 7.40 ppm.
The computer-generated, energy-minimized structure of
complex 8 also supports a rigid, shape-persistent, and highly
symmetric spirane-shaped architecture with D3h symmetry
(Scheme 2). The distance between two Ru ions in an individual
triangle is approximately 1.2 nm, whereas the distance be-
tween two Ru ions in adjacent triangles at the outer rim is ap-
proximately 4 nm. Same-side, corner-to-corner distances mea-
sure approximately 5.3 nm.
À
The ESI-MS spectrum of the PF6 salt of the spirocomplex 8
with 18+ charges further confirms its structure (Figure 2a). A
series of peaks ranging from m/z 591.59–1696.43 corresponds
Further proof-of-structure was provided by determining the
collision cross-section (CCS) of 8 from the drift times measured
in the TWIM-MS experiments and comparing it with the theo-
retical CCS for 100 candidate structures obtained by molecular
modeling. The CCSs measured for nanospirane ions in various
charge states are listed in Table 1. Only slight fluctuations were
Table 1. Drift times and collision cross sections for the spirane.[a]
Charge
Drift time [ms]
Collision cross section [ꢁ2]
15+
14+
13+
12+
11+
10+
9+
2.71
2.44
2.89
3.34
3.97
4.87
6.05
1433
1455
1514
1530
1554
1586
1606
[a] All drift times were collected at a traveling-wave velocity of 350 msÀ1
and a traveling-wave height of 7.5 V.
observed as the charge ranged from 15+ to 9+ (1519Æ86 ꢁ2),
indicating rigidity and shape-persistence for the 3D-shaped ar-
chitecture. Compared with the spoke-wheel metalloarchitec-
ture,[12] this 3D metallotricyclic spirane is only slightly smaller.
The theoretical CCS calculated by the trajectory (TJ) method[17]
for the energy-minimized structure of 8 (no counterions) is
1580 ꢁ2; this value agrees well with the average experimental
CCS for charge states 9+ to 15+ (1519 ꢁ2).
Notably, the use of ZnII or CdII ions, as in the synthesis of our
isomeric supramolecular bowtie and butterfly structures[10]
with ester substituted RuII-based capping dimers and tetrater-
pyridinyl cores, afforded a substantial amount the correspond-
ing cyclic tetramer[13] as a byproduct (as determined by
1H NMR).
Figure 2. a) ESI-MS spectrum of complex 8. Monoisotopic m/z values and
charge states are marked on top of the corresponding peaks; the inset
shows an expanded view of the measured and calculated isotope pattern
for the 14+ ion. b) 2D ESI-TWIM-MS plot (m/z vs. drift time) of complex 8.
The charge states of intact assemblies are marked.
to the spirane cations possessing charge states from 15+ to
6+, respectively. In contrast to a related bowtie-shaped spi-
rane,[10] complex 8 exhibited fewer fragments in both the 1D
and 2D ESI mass spectra, indicative of greater stability.
ESI-MS coupled with traveling-wave ion mobility (TWIM)
spectrometry, a variant of ion mobility spectrometry,[15] was ap-
plied to further validate the structure and to determine wheth-
er there are other conformers, based on differing drift times.
The TWIM-MS spectrum of complex 8 (Figure 2b) reveals a set
of narrow, symmetric peaks, confirming that there is only
a single species existing in the solution under the conditions
of ESI-MS. This is consistent with the NMR results. Compared
with the multiple metal ion (ꢀ6) macrocycles,[16] this nanospir-
ane exhibits the enhanced stability of the trigonal motif;[9] gra-
dient tandem MS (gMS2)[15a] was performed on the 12+ charge
state (m/z 775.7) by subjecting it to collisionally activated dis-
sociation prior to ion mobility separation at collision energies
ranging from 10 to 40 eV. The 12+ complex ions dissociated
completely when the trap voltage reached 35 V, which corre-
sponds to a center-of-mass collision energy (Ecm) of 1.80 eV.
This compares very well with the stability observed for other
triangular constructs.[9,11]
Transmission electron microscopy (TEM) experiments have
facilitated visualization of spirane 8 (Figure 3). The TEM images
revealed direct correlation of both size and shape of individual
molecules upon deposition of a dilute (~10À7 m) MeCN solu-
À
tion of complex 8 with PF6 counterions on carbon-coated
grids (Cu, 400 mesh). The outline of single molecules located
on the film with three triangles vertically erected can be ob-
served; a slight tilt of the molecule also shows the structure’s
concave side. The average distance (5.3 nm) between the two
edges perfectly fits the size obtained from the optimized mo-
lecular model.
In conclusion, we have developed a novel all-RuII polyspir-
ane-shaped metallomacrocycle through programmed coordi-
nation-driven assembly based on the logical combination of
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The authors gratefully acknowledge funding from the National
Science Foundation [CHE-1151991 (GRN) and CHE-1012636 and
CHE-1308307 (CW)].
Keywords: metallomacrocycles · ruthenium · self-assembly ·
spiranes · terpyridine
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Received: June 6, 2014
Published online on July 23, 2014
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