Sterically congested, hexameric tetrakispyridinyl-PdII/Cd II-metallomacrocycles: Self-assembly and structural characterization

Article abstract of DOI:10.1039/c1cc10649j

Hexagonal Pd^II- or Cd^II-tetrakispyridinyl-based macrocycles are quantitatively self-assembled from 4′-(3-pyridinyl)-4, 4′′-di(tert-butyl)-2,2′:6′,2″-terpyridine and structurally confirmed by NMR and TWIM-MS. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc10649j

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COMMUNICATION
Sterically congested, hexameric tetrakispyridinyl-PdII/CdII-
metallomacrocycles: self-assembly and structural characterizationw
Sujith Perera,a Xiaopeng Li,b Mingming Guo,a Chrys Wesdemiotis,*ab Charles N. Moorefielda
and George R. Newkome*ab
Received 1st February 2011, Accepted 23rd February 2011
DOI: 10.1039/c1cc10649j
Hexagonal PdII- or CdII-tetrakispyridinyl-based macrocycles
are quantitatively self-assembled from 40-(3-pyridinyl)-4,400-
di(tert-butyl)-2,20 : 60,20 0-terpyridine and structurally confirmed
by NMR and TWIM-MS.
Herein, we report the self-assembly of hexagonal PdII- or
CdII-polypyridinyl-based macrocycles using both mono- and
tridentate coordination sites. Hexagonal macrocyclic forma-
1
tion from a one-pot reaction is confirmed from H and 13C
NMR, 2D COSY NMR, and TWIM-MS methods. Moreover,
hydrodynamic radii obtained from DOSY NMR are in accord
with radii calculated from preliminary molecular modeling
and the drift time trends observed TWIM-MS data.
Self-assembly procedures have been elegantly used to construct
diverse 2D and 3D macromolecular architectures.1 Among
these self-assembly processes, inorganic metal coordination
has played a pivotal role2 and instilled important utilitarian
electronic, catalytic, photophysical, and magnetic properties.3
Tridentate terpyridine-metal coordination has been successfully
used to assemble diverse metallomacrocycles generating boxes,
helixes, and other polygonal shapes.4 Similarly, bisterpyridinyl
ligands possessing 60–1201 angles between the two ligating
termini, have been shown to self-assemble into various metallo-
macromolecules.5 Designing tridentate and monodentate
coordination sites within the same ligand along with tunable
directionality provides an opportunity to utilize stronger and
weaker coordination sites for the assembly of new polygons.
Under mild ionization conditions, ESI-MS has been used in
the identification and characterization of supramolecules.6–9
Unfortunately, the peaks from different charge states are
known to superimpose; even with the high resolving power
of Fourier Transform Mass Spectrometry (FTMS), only
a few isotope patterns of different charge states have been
deconvoluted.7 Recently, traveling wave ion mobility mass
spectrometry (TWIM-MS)10 and traditional IM-MS11 have been
employed in the detection and characterization of complex
metallomacrocycles.12 This technique alleviates isomer super-
position and deconvolutes the isotope patterns of differently
charged ions, which significantly facilitates structure elucidation.
Combining molecular modeling and 2D diffusion ordered NMR
spectroscopy (DOSY) experiments, with TWIM-MS data affords
complementary and reliable structural information.
In order to enhance the solubility and ascertain the issues
associated with potential steric hindrance, the synthesis of
40-(3-pyridinyl)-4,400-di(tert-butyl)-2,20 :60,200-terpyridine (1) was
undertaken (Scheme 1). Acetylation of 4-tert-butylpyridine13
in the 2-position generated the previously prepared14,15 precursor
4-tert-butyl-2-acetylpyridine; however, our one-pot acetyl-
ation of 4-tert-butylpyridine leads to a higher yield and
enhanced purity. The desired ligand 1 was prepared using a
modified procedure for 40-(3-pyridinyl)-2,20 : 60,200-terpyridine16
based on sequential addition of 4-tert-butyl-2-acetylpyridine
(see Scheme S1w). The 1H NMR spectra of 1 showed0 t0h0 e
presence of singlet peaks at d 9.14 (s, Ha), 8.79 (s, PyH3 ,3 ),
0
00
8.71 (s, PyH3 ,5 ) (see Fig. 1) and 1.48 [s, C(CH3)]; the proper
peak assignments were confirmed using two-dimensional 2D
COSY NMR. 13C NMR and ESI-MS spectra also supported
the structure of 1: (m/z) = 423.2 [M+H]+ (calcd m/z = 423.2).
a Department of Polymer Science, The University of Akron, Akron,
OH 44325-4717, USA. E-mail: newkome@uakron.edu,
wesdemiotis@uakron.edu; Fax: +1 330-972-2413;
Tel: +1 330-972-6458
b Department of Chemistry, The University of Akron, Akron,
OH 44325-3601, USA
w Electronic supplementary information (ESI) available: Detailed experi-
mental procedures with analytical data. See DOI: 10.1039/c1cc10649j
Scheme 1 Self-assembly of the macrocyclic palladium (2) and cadmium
(3) hexamers: (a) MeCN, Pd(MeCN)4(BF4)2, 24 h, 60 1C; (b) CDCl3/
CD3OD (3 : 2), Cd(NO3)2ꢀ4H2O, 5 min., 25 1C.
c
4658 Chem. Commun., 2011, 47, 4658–4660
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differently charged species with their linear fragments generated
during ESI; therefore, TWIM-MS was conducted on 2 and 3 to
prove the existence of the cyclic hexameric structure.
The ions possible at m/z 1320 from 2 are [2L+2Pd+3BF4]+,
, . Note that
[4L+4Pd+6BF4]2+ and [6L+6Pd+9BF4]3+
[6L+6Pd+9BF4]3+ could be linear or cyclic. After ion mobility
separation, four species are detected at 4.50, 2.89, 1.99, and
1.71 ms, respectively (Fig. 2). The composition of each peak
with unique drift time was determined from the corresponding
isotope distribution and spacing.12a–c For instance, due to
their isotope spacing of 0.33 amu, both the signals at 1.71
and 1.99 ms were assigned to [6L+6Pd+9BF4]3+. In the mobility
separation, compact ions drift faster, while more extended ions
drift more slowly. Thus, the peaks at 1.71 and 1.99 ms correspond
to cyclic and linear ions of [6L+6Pd+9BF4]3+, respectively. The
linear [6L+6Pd+9BF4]3+ ions could be generated by ring-
opening of cyclic structure 2, either in the ESI source or during
IM separation, and further fragmentation could lead to the signals
at 2.89 and 4.50 ms, corresponding to [4L+4Pd+6BF4]2+ and
[2L+2Pd+3BF4]+, respectively. Losses of neutral BF3 were
observed from each charge state (see Table S3w for resulting m/z
values from hexamer 2). The pattern of BF3 losses further
confirms the charge state and composition assigned to each signal.
The ions at m/z 2023 from 2 were also selected for ion
mobility separation (Fig. S5w). Analysis of the corresponding
isotope patterns reveals the presence of three charge states at
m/z 2023, having spacing of Dm = 1 (8.39 ms), 0.5 (4.78 ms),
Fig. 1 The 1H NMR of ligand 1 and complexes 2 and 3 showing
aromatic regions (1 in CDCl3/CD3OD, 2 in CD3CN, 3 in CDCl3/
CD3OD).
The polydentate ligand 1 possessing two unique, 1201
angularly juxtaposed coordination sites was treated with
one equivalent of either [Pd(MeCN)4(BF4)2] in MeCN or
Cd(NO3)2ꢀ4H2O in CDCl3 :CD3OD (3 : 2) to give the hexametallic
macromolecules [Pd6(1)6(BF4)12] (2) and [Cd6(1)6(NO3)12] (3),
respectively, in quantitative yields (Scheme 1). 1H NMR
data established the formation of a single type of tridentate
II
terpyridine coordination with either PdII or Cd with absorptions
0
0
00
at d 8.65Pd (s, tpyH3 ,5 ) or 8.59Cd (s, tpyH3,3 ) and d 7.81Pd or
and 0.33 (3.52 ms) amu, corresponding to [3L+3Pd+5BF4]1+
,
00
7.74Cd (d, tpyH5,5 ). The downfield shifts of Ha singlet signal
[6L+6Pd+10BF4]2+, and [9L+9Pd+15BF4]3+, respectively.
The only doubly charged species is assigned to the cyclic
[6L+6Pd+10BF4]2+ hexamer.12a,c No linear component is
deteꢁcted for the 2+ ions, presumably because the additional
BF4 counterion (as compared to the 3+ ions) enhances the
stability of the macrocyclic structure, thus increasing the energy
needed for ring-opening. The ions of [9L+9Pd+15BF4]3+
[from d 9.14 to 9.26Pd, (Dd = 0.12) or d 9.33Cd (Dd = 0.19)]
and the Hb doublet signal [from d 8.69 to 8.86Pd, (Dd = 0.17)
or d 8.83Cd (Dd = 0.14)] support the coordination of mono-
dentate pyridine unit to the metal center (see Fig. 1). Sharp
singlets assigned to the C(CH3)3 moieties (dPd/Cd = 1.47) for 2
and 3, along with a single type of metal-coordinated terpyridine
unit in each macrocycle are indicative of the formation of
symmetric, cyclic structures. The hexameric sizes were validated
by ESI-MS: macrocycle 2 gave rise to signals at m/z 2023
[6L+6Pd+10BF4]2+, 1987 [6L+6Pd+9BF4+1F]2+ (ꢁ1 BF3),
1320 [6L+6Pd+9BF4]3+, and 1297 [6L+6Pd+8BF4+1F]3+
(ꢁ1 BF3); whereas, macrocycle 3 showed peaks at m/z 1915
[6L+6Cd+10NO3]2+ and 1256 [6L+6Cd+9NO3]3+. Notably,
the symmetry observed in the 1H and 13C NMR spectra further
corroborated the formation of cyclic structures; 2D COSY NMR
1
confirmed each of the H NMR peak assignments.
Full mass spectra of both PdII and CdII metallomacrocycles
are given in Fig. S3 and S4,w respectively. Corresponding
spectra consist of different charge states of the hexameric
metallomacrocycle and its linear fragments. The fragmentation
of metal–ligand complexes in ESI is well-documented.9,12a,c
All possible combinations of fragments for complexes 2 and 3
with different m/z values, due to the loss of counterions
are given in Tables S1 and S2.w Table S3 shows different m/z
values generated by further loss of neutral BF3 units from
complex 2 ions. In both macrocycles, the complete absence of
unique m/z signals for pentameric and heptameric species
provided additional support that the NMR-detected symmetric
structures were hexameric. In both mass spectra, highly abundant
signals can be observed at low m/z, due to possible overlap of the
Fig. 2 Two-dimensional ESI-TWIM-MS plot for m/z 1320 from 2. Ion
mobility separation gave signals at 4.50, 2.89, 1.99, and 1.71 ms,
corresponding to linear [2L+2Pd+3BF4]1+, linear [4L+4Pd+6BF4]2+
,
linear [6L+6Pd+9BF4]3+, and cyclic [6L+6Pd+9BF4]3+, respectively.
The pattern of BF3 losses observed from each species agrees well with the
assigned charge state.
c
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could be a cluster of low charge states, such as the combina-
tion of [3L+3Pd+5BF4]1+ and [6L+6Pd+10BF4]2+; similar
clusters were also observed in previous studies.12a,c
1834; (c) I. Krivokapic, M. Zerara, M. L. Daku, A. Vargas,
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The ion mobility separation of m/z 1915 from 3 gave rise to
three species, corresponding to [3L+3Cd+5NO3]1+
,
[6L+6Cd+10NO3]2+, and [9L+9Cd+15NO3]3+ appearing
at 8.66, 4.78, and 3.70 ms, respectively (Fig. S6w). The cyclic
[6L+6Cd+10NO3]2+ of 3 drifted at the same time (4.78 ms)
as the cyclic [6L+6Pd+10BF4]2+ (Fig. S5w). Notably from 3,
there is a complete absence of the linear [6L+6Cd+10NO3]2+
isomer.
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The DOSY NMR spectra of metallomacrocycles 2 and 3
clearly show the presence of only one species in solution for
each. The experimental hydrodynamic radius (rH), calculated
via the Stokes–Einstein equation for each complex, is in
excellent agreement with the mean radii obtained from the
respective calculated structures (Table S4w). The DOSY NMR
data unambiguously reveal that these self-assembled complexes
have uniform macrocyclic architectures. The UV-vis and photo-
luminescence data for the ligand and complexes are also shown
in the supporting information (Fig. S7 and Table S5w).
In conclusion, the self-assembled, hexameric, PdII- and
CdII-polypyridine macrocycles 2 and 3 have been generated
by taking advantage of the novel bis-functional ligand,
40-(3-pyridinyl)-4,40 0-di(tert-butyl)-2,20 : 60,20 0-terpyridine. These
hexameric PdII and CdII metallomacrocycles 2 and 3 were
characterized using 1H and 13C NMR, 2D DOSY NMR,
TWIM-MS, and molecular modeling. TWIM-MS completely
deconvolutes the isotope patterns of different charge states,
which avoids the isomer superposition prevalent in regular
ESI-MS and FTMS. Further, the solution-phase hydro-
dynamic radii obtained by DOSY NMR are in full accord
with the values predicted by molecular modeling.
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The authors gratefully acknowledge support from the National
Science Foundation (GRN: DMR-0812337, DMR-0705015,
CW: DMR-0821313, CHE-1012636).
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c
4660 Chem. Commun., 2011, 47, 4658–4660
This journal is The Royal Society of Chemistry 2011