Shape-persistent, ruthenium(ii)- and iron(ii)-bisterpyridine metallodendrimers: Synthesis, traveling-wave ion-mobility mass spectrometry, and photophysical properties

Article abstract of DOI:10.1039/c2nj20799k

A class of shape-persistent metallodendrimer has been developed through a self-assembly strategy, in which 〈tpy-Ru^II-tpy〉 or 〈tpy-Fe^II-tpy〉 connectivity is utilized, as branching moieties or nodes. These metallomacromolecules were fully characterized by ¹H and ¹³C NMR, traveling wave ion mobility mass spectrometry (TWIM MS), single crystal X-ray, UV-vis absorption, photoluminescence, and cyclic voltammetry. Significant increase in the drift times of the same charge states was observed with increasing generation of these complexes, which is in line with the change of molecular size. Moreover, the photophysical properties (molar extinction coefficients) and electrochemical stability of the complexes were noticeably different depending on size and metal ion center.

Full text of DOI:10.1039/c2nj20799k

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New Journal of Chemistry
A journal for new directions in chemistry
www.rsc.org/njc
Volume 36 | Number 2 | February 2012 | Pages 181–512
Themed issue: Dendrimers II
ISSN 1144-0546
COVER ARTICLE
Wesdemiotis and Newkome et al.
Shape-persistent ruthenium(II)-
and iron(II)-bisterpyridine
metallodendrimers
1144-0546(2012)36:2;1-I

Dynamic ArticVleiewLiAnrktiscle Online
NJC
Cite this: New J. Chem., 2012, 36, 484–491
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PAPER
Shape-persistent, ruthenium(II)- and iron(II)-bisterpyridine
metallodendrimers: synthesis, traveling-wave ion-mobility mass
spectrometry, and photophysical propertiesw
Jin-Liang Wang,a Xiaopeng Li,ab Carol D. Shreiner,a Xiaocun Lu,a
Charles N. Moorefield,a Sreedhar R. Tummalapalli,b Douglas A. Medvetz,b
Matthew J. Panzner,b Frank R. Fronczek,c Chrys Wesdemiotis*ab and
George R. Newkome*ab
Received (in Montpellier, France) 15th September 2011, Accepted 15th December 2011
DOI: 10.1039/c2nj20799k
A class of shape-persistent metallodendrimer has been developed through a self-assembly strategy,
in which htpy-RuII-tpyi or htpy-FeII-tpyi connectivity is utilized, as branching moieties or nodes.
1
These metallomacromolecules were fully characterized by H and 13C NMR, traveling wave ion
mobility mass spectrometry (TWIM MS), single crystal X-ray, UV-vis absorption,
photoluminescence, and cyclic voltammetry. Significant increase in the drift times of the same
charge states was observed with increasing generation of these complexes, which is in line with the
change of molecular size. Moreover, the photophysical properties (molar extinction coefficients)
and electrochemical stability of the complexes were noticeably different depending on size and
metal ion center.
configuration d4–d7 as well as dye-sensitized solar cells and other
Introduction
light-harvesting devices.7 Moreover, 2,20 :60,20 0-terpyridine (tpy)
ligands have gained considerable utility in metal–ligand coordi-
nation chemistry because of the structural diversity available by
using -htpy-MII-tpyi- connectivity; their relative ease to func-
tionalize is also critical to tailoring multinuclear assemblies.8
However, very few examples concerning their integration into
different, easily accessible -htpy-MII-tpyi- spacers in shape-
persistent metallodendrimers via self-assembly have been
reported; due, in part, to the difficulties associated with the
step-wise synthesis and separation of the desired heterometallic
product(s) from similar by-products.9 The characterization of
such complicated metallodendrimers is also a challenge because
it is not easy to obtain single crystals of such complex macro-
structures, especially at higher generations. A combination
of NMR spectroscopy and electrospray ionization (ESI) or
matrix-assisted laser desorption ionization (MALDI) mass
spectrometry (MS)10 is typically utilized; however, construction
of multinuclear assemblies always leads to some labile species
(several counterions and reversible transition-metal complexes),
which can easily generate numerous differently charged
fragments and their isotope peaks tend to overlap in ESI MS.
These problems obviously increase the difficulty in their precise
structural identification.11
Construction of highly ordered, 2D and 3D supramolecular
architectures employing self-assembly strategies based on
versatile building blocks has generated considerable attention
over the past few decades.1 One notable strategy is through
ligand–metal–ligand coordination to construct interesting
predesigned topologic architectures, including a variety of macro-
cycles,2 cages or spheres,3 prisms,4 and dendrimers.5 Among
various types of self-assembled topologies, metallo-dendrimers
are quite attractive due to their applications in catalysis, sensing,
and light-harvesting.6 Combination of various dyes or chromo-
phores into nanosized, shape-persistent metallo-dendrimers and
metallomacrocycles, formed by self-assembly methods, provides a
platform to investigate energy or charge transfer processes, which
are important requirements for prescreening their applications in
spin crossover based on light-induced excited-state spin trapping7
prevalent in transition metal complexes with electronic
a Department of Polymer Science, The University of Akron,
302 Buchtel Common, Akron, OH 44325, USA.
E-mail: newkome@uakron.edu; Fax: +1 330-972-2368
b Department of Chemistry, The University of Akron,
302 Buchtel Common, Akron, OH 44325, USA.
E-mail: wesdemiotis@uakron.edu
c Department of Chemistry, Louisiana State University,
Baton Rouge, LA 70803, USA
Recently, ion mobility spectroscopy-mass spectrometry
(IMS MS) and traveling wave ion mobility mass spectrometry
(TWIM MS) have been employed to detect and characterize
complicated supramolecular species.12 These techniques afford
w Electronic supplementary information (ESI) available. CCDC
reference number 855619. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2nj20799k
c
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Scheme 1 Synthetic route to ligands T3 and MT3 along with the
self-assembled metallomacromolecules.
RuIII mono-capping adduct 4 was then obtained by refluxing a
mixture of 40-(p-methoxyphenyl)-2,20 :60,20 0-terpyridine (3) with
RuCl3ꢀ3H2O, according to the literature.16 The trinuclear complex
G0 was obtained (52%) by reaction of MT3 with 3.6 equiv. of
adduct 4 in a mixture of MeOH and CHCl3 in the presence of
N-ethylmorpholine for 12 h under reflux conditions and the
resultant crude product was column chromatographed (SiO2)
eluting with a solvent mixture of H2O/MeCN/sat.KNO3 (aq.)
(1/12/1 (v/v/v)) to give the desired product G0, as a dark red solid.
The 1H NMR spectrum of MT3 revealed a diagnostic
singlet at 1.96 ppm corresponding to a single type of methyl
proton on the central mesitylene moiety, which was shifted
downfield to 2.57 ppm in complex G0 due to the lower electron
density resulting from the coordination effect. Meanwhile, as
shown in Fig. 2, 1H NMR spectrum of G0 revealed two sets of
terpyridine resonances, consistent with the formation of the
symmetric trinuclear complex. For example, two singlets for
30,50-tpyH were shifted downfield to 9.21 and 9.14 ppm in
comparison with the peaks at 8.73 and 8.43 ppm, respectively.
As expected, a dramatic upfield shift for the 6,60 0 proton signal
Fig. 1 Structures of the RuII core (G0) and RuII/FeII metallodendrimers.
possible avenues to separate related supramolecular species
from fragments with the same mass-to-charge ratio (m/z) as
well as isomeric architectures with similar charge.13 The time
needed for an ion to travel through the ion mobility region is
defined as its drift time, which is affected by the mass, charge,
and shape of the ions. This affords information about the
conformation and geometry of the drifting ions.
Herein, we present the synthesis, via a self-assembly strategy,
isolation, and structural characterization of a family of small shape-
persistent metallodendrimers (Fig. 1), based on htpy-RuII-tpyi or
htpy-FeII-tpyi connectivity, in the form of either the branch (G0) or
core unit (G1Ru5 and G1Ru4Fe1). These metallomacromolecules
were identified by NMR, X-ray single crystal, TWIM MS, UV-vis
absorption, photoluminescence, and cyclic voltammetry providing
information about their purity, molecular size and shape, as well as
their photophysical and electrochemical properties, which were
notably modulated by dendron generation and metal ion center.
Results and discussion
Scheme 1 illustrates the synthetic approach to the complexes G0,
in which the key core ligands, tris-terpyridine T3 and MT3 with
three methyl groups, were prepared by condensing 2-acetyl-
pyridine with either 1,3,5-triformylbenzene to generate T3 or
1,3,5-triformyl-2,4,6-trimethylbenzene14 for MT3 under solvent-
free basic conditions.15 Each intermediate hexaketone was then
treated with ammonium hydroxide to produce the desired tris-
terpyridines in 31% and 25%, respectively. The paramagnetic,
Fig. 2 Aromatic regions of the 500 MHz 1H NMR spectra for
ligands MT3 and 3 (in CDCl3) and complexes G0 with NO3ꢁ, as the
counterion (in CD3OD/CDCl3, 3/1).
c
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occurred upon complexation due to the shielding effect of the
perpendicular terpyridine counterpart with metal center.
Moreover, two sharp peaks in the 13C NMR spectrum are
located at 56.1 (OCH3) and 20.9 ppm (CH3).
the shielding effect of metal centers relative to the uncomplexed
parts. The singlets at 8.42 ppm (in the spectrum of T3) for protons
of the central phenyl group showed successive downfield shifts and
split into two types due to their environment; this is most noticeable
for Ph-H, which shifts to 9.42 ppm in T1Ru2, and even higher than
the one for the coordinated 30,50-tpyH.
The key precursor for the next generation metallodendrimer
is the dinuclear ruthenium complex T1Ru2 possessing a free
terpyridine segment, which failed to form when a simple 1 : 1
ratio of the core T3 and the capping adduct 4 were utilized
under typical reaction conditions;9a however, using a 1 : 2 ratio
of T3 with 4 led to the desired product. With this method, the
dinuclear complexes T1Ru2 were isolated (48%) after chromato-
graphy [SiO2; H2O/MeCN/sat.KNO3(aq.), 1/15/1 (v/v/v)], while
the mononuclear complex T2Ru1 with two free terpyridine
segments was obtained in 18% isolated yield using [H2O/MeCN/
sat.KNO3(aq), 1/20/1 (v/v/v)], as eluent. These complexes are
readily soluble in common organic solvents, such as CHCl3,
MeOH, and DMSO (with Clꢁ or NO3ꢁ, as counterion) or MeCN
and acetone (with PF6ꢁ, as counterion). The structure and purity
of these dendrons were ascertained by 1H and 13C NMR, COSY,
as well as MALDI-ToF/ESI MS. As shown in Fig. 3, in
comparison with the two ligands, both T1Ru2 and T2Ru1 exhibit
more complex NMR patterns with the expected downfield shifts
due to the lower electron density resulting from the metal incor-
poration. Moreover, both complexes showed three diagnostic
peaks for 30,50-tpyH (singlet) and 3,300-tpyHs (doublet) in a
1 : 1 : 2 or 2 : 2 : 1 ratio, respectively. Although the two dendrons
exhibit a similar peak pattern for terpyridine, there are slight
variations, especially for the b pyridine ring protons, which have
a dramatic downfield shift (+0.14 ppm for bPy-H30,50) due to
coordination, in which the 30,50-tpyH protons are particularly
sensitive to their environment. Hence, it is a key discernable feature
for structural identification in such similar structures. Meanwhile,
the peaks for 6,60 0-tpyHs exhibited a dramatic upfield shift due to
The metallodendrimer G1Ru5 (Scheme 1) was generated by
refluxing a solution of T1Ru2 and 0.50 equivalents of
RuCl2(DMSO)417 in MeOH for 10 h followed by flash column
chromatography (SiO2) eluting with a H2O/MeCN/sat.KNO3(aq.)
(1 : 8 : 1 v/v/v) mixture in 70% isolated yield. Meanwhile, the
reaction of T1Ru2 (Scheme 1) with 0.50 equivalents of
FeCl2ꢀ4H2O in MeOH for 10 h gave (84%) the heteronuclear
dendrimer G1Ru4Fe1 after column chromatography (SiO2) eluting
with a solvent mixture of H2O/MeCN/sat. KNO3(aq.) (1:7:1,
1
v/v/v). The H NMR spectra of two dendrimers were assigned
(Fig. 3) with the aid of 2D COSY spectra (see ESIw) revealing three
sets of diagnostic absorptions for coordinated 30,50-tpyHs (singlet),
3,30 0-tpyHs (doublet), and 6,60 0-tpyHs (doublet) in a 2 : 2 : 1 ratio,
respectively. In comparison with the free terpyridine protons in
T1Ru2, the protons in G1Ru5 have an obvious downfield shift that
is even lower than the other two coordinated terpyridines. More-
over, when the central metal ion was changed from Ru(II) to Fe(II),
the 30,50-tpyH of G1Ru4Fe1 showed distinct downfield shifts
relative to the same protons in the G1Ru5, which resulted from
enhanced electron deficiency by htpy-FeII-tpyi connectivity.
Notably, the protons belonging to the central positions in
G1Ru4Fe1 showed an upfield shift compared with those
in G1Ru5. The 13C NMR spectra of these dendrimers were also
in full agreement with their symmetry resulting in the observation
of three peaks for 30,50-tpyCs (see ESIw).
In addition to NMR, MALDI-ToF and ESI MS coupled
with traveling wave ion mobility (TWIM) mass spectrometry
were applied to further verify the structures and purity of the
desired complexes.13 Both MALDIꢁ-ToF MS and ESI MS of
these metallodendrimers with NO3 counterions generated a
significant number of fragments (spectra not shown). In order
to obtain more informative mass spectra for idenꢁtification of
the structure and purity of the dendroꢁns, the NO3 salts were
converted to the corresponding PF6 salts, which produce
sharp molecular ion patterns with few fragments (Fig. 4,
Fig. S1 and S2, ESIw). For example, the structure and purity
of G0 are further corroborated by the two intense MALDI-ToF
MS peaks (Fig. S4, ESIw) at m/z 2860.8 (M ꢁ PF6)+ (calcd m/z =
2861.3) and 1358.4 (M ꢁ 2PF6)2+ (calcd m/z = 1358.2); similarly,
the MALDI-ToF mass spectrum of G1Ru5 displays a sharp
molecular ion peak at 4712.7 (M ꢁ PF6)+ (calcd m/z =
4713.3) and a +2 charge state signal at 2283.8 (M ꢁ 2PF6)2+
(calcd m/z = 2284.1). In contrast, MALDI peaks for intact
G1Ru4Fe1 were not detected due to the lower inherent stability
of the htpy-FeII-tpyi connectivity.13d These assembled complexes
were, however, confirmed by ESI MS, which is a milder analytical
MS procedure. As mentioned above, coordinatively bound self-
assemblies carrying neutral ligands are ionized to intact supra-
molecular ions by removal of one or more of their counterions. ESI
MSꢁof these complexes led to supramolecular ions missing 1–8
PF6 counterions and, hence, having 1–8 positive charges, which
showed the theoretically expected isotope patterns. For G1Ru4Fe1,
six intense ESI MS peaks were observed (Fig. S2a, ESIw) at
Fig. 3 Aromatic regions of the 500 MHz 1H NMR spectra for
ligands T3 and 3 (in CDCl3) and complexes T2Ru1, T1Ru2, G1Ru5,
and G1Ru4Fe1 with NO3ꢁ, as the counterion (in 3 : 1 CD3OD/CDCl3).
c
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G1Ru5, which is consistent with the concomitant increase in the
corresponding dendrimer sizes. Moreover, G1Ru5 and
G1Ru4Fe1 have similar drift times in charge states +3 to +7
due to their identical backbones, in spite of the different metal
ion cores. Such results indicate that drift time data can provide
valuable information about the differences or similarities
between unique supramolecular architectures.
To examine the relative stabilities of the metallodendrimers
with different metal ion cores, gradient tandem MS (gMS2)
experiments were performed on charge state +4 from both
G1Ru4Fe1 (m/z = 1058) as well as G1Ru5 (m/z = 1069),
which were subjected to collision-activated dissociation
(CAD) with Ar before ion mobility separation at collision
energies ranging from 6 to 65 eV (Fig. S2 and S3, ESIw). After
initially losing neutral PF5 units, these complexes dissociate
further, disappearing completely when the trap voltage reaches
45 V for G1Ru4Fe1 and 65 V for G1Ru5. From these results,
the center-of-mass collision energy (Ecm) of disappearance can
be calculated as 1.69 eV for G1Ru4Fe1 and 2.41 eV for G1Ru5,
which gives direct evidence for the lower stability of the
htpy-FeII-tpyi connecter under ESI MS2 conditions.
As mentioned in the Introduction, single crystals afford direct
evidence for structural identification. Single crystals of both the
initial ligand MT3 and complex G0 were realized. The single
crystal of ligand MT3 was grown from a saturated CHCl3
solution (Fig. 5a). The structure revealed distortions among the
pyridine rings of the mesitylene–terpyridines, which result from
the free rotation around the bonds; however, the steric inter-
actions between the 30,50 protons on the terpyridine moieties
and the protons on the methyl groups restrict rotation about the
terpyridine–mesitylene carbon–carbon connective bond.18
A single crystal of the complex G0 was obtained from a
MeOH solution. Each of the central pyridine rings for the
mesitylene terpyridines was found to be nearly perpendicular
(82.71) to the focal mesitylene, while the terminal terpyridine
Fig. 4 (a) ESI MS of G1Ru5. (b) Two-dimensional ESI-TWIM MS
plot (m/z vs. drift time) of G1Ru5, acquired using a traveling wave
velocity of 350 m sꢁ1 and a traveling wave height of 8.5 V.
m/z 1459.1 [M ꢁ 3PF6]3+ (calcd m/z = 1459.1), 1057.8 [M ꢁ
4PF6]4+ (calcd m/z = 1058.1), 817.4 [M ꢁ 5PF6]5+ (calcd m/z =
817.5), 657.0 [M ꢁ 6PF6]6+ (calcd m/z = 657.1), 542.5
[M ꢁ 7PF6]7+ (calcd m/z = 542.5), and 456.5 [M ꢁ
8PF6ꢁ]
8+ (calcd m/z = 456.6) with the correct isotope patterns.
Meanwhile, ESI MS of G0 and G1Ru5 generated a continuous
series of peaks with +2 to +6 charges and +3 to +8 charges,
respectively, which showed isotope distributions that are in
accord with the corresponding calculated distributions (Fig. 4,
Fig. S1, and S2, ESIw). In order to resolve any superimposed
fragments and determine whether overlapping isomers of the
complexes exist, ESI-TWIM MS was used as a second level of
MS analysis, for separation based on differences in ion mobi-
lities. In mobility experiments, ions are dispersed not only on
the basis of their m/z ratios but also according to their
molecular sizes and shapes. In all cases (Fig. 4, Fig. S1, and
S2, ESIw), the desired compounds are the predominant species;
a few minor fragments are observed, but the presence of isomers
or byproducts is excluded. The drift times of the charge states
formed by the Ru(II)/Fe(II) dendrimers can be obtained from
the two dimensional ESI-TWIM MS plots. Compact and
smaller ions drift faster, while more extended and larger ions
drift more slowly.12,13 For example, the ESI-TWIM MS plot in
Fig. 4 reveals that the [M ꢁ 3PF6]3+, [M ꢁ 4PF6]4+, [M ꢁ
5PF6]5+, [M ꢁ 6PF6]6+, and [M ꢁ 7PF6]7+ ions from G1Ru5
have drift times of 9.21, 5.14, 3.16, 1.99, and 1.35 ms, respec-
tively; whereas the +2 to +5 charge states of G0 have drift
times at 9.21, 3.70, 1.99, and 1.08 ms (see ESIw). The drift times
of the same charge states dramatically increased from G0 to
Fig. 5 (a) Single crystal X-ray structure of ligand MT3; (b) X-ray
single crystal of the network building block G0 viewed from the c-axis;
(c) honey-comb-like packing pattern of the crystalline network of
complex G0 viewed from the c-axis (see ESIw part 5 for pertinent X-ray
data for G0 and MT3).
c
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moieties were observed to be almost parallel to the mesitylene
moiety (Fig. 5b). This is most likely a result of a slight
distortion from the idealized octahedral geometry between
the binding terpyridines and Ru(II) center (the angle of the
hN–Ru–Ni band is about 176.51).
Table 1 Photophysical properties of the complexes and ligand TM3
in dilute MeOH (2.5 ꢂ 10ꢁ6 M) at 25 1C
lmax emis./
nm
Compounds lmax abs./nm (e ꢂ 10ꢁ5
)
TM3
280 (1.05)
340
The outer distances between the methyl groups on the capping
terpyridines were observed to be 31.8 A, forming an equilateral
triangle around the perimeter of the molecule. The Ru–Ru
distance for each of the three units was 12.99 A. The resultant
molecular packing revealed a unique honeycomb-like pattern
(Fig. 5c), which arises as a result of the stacking of the central
mesitylene rings, as well as a direct overlap between the 4,6 and
40 0,600 carbons in the terpyridine moiety. The interplanar distance
between the stacked mesitylene rings was calculated to be 10.8 A.
The substituents on the central ring are arranged in a gauche-like
conformation with an angle of 29.11 between the methyl groups
and 34.01 between the terpyridine rings (when viewed along the
c-axis). Notably, the internal methyl groups also restrict rotation,
as in MT3 and the molecule lies on a 3-fold axis. A Flack value of
0.32(6) indicates an inversion twin.
T2Ru1
T1Ru2
G0
282 (1.32), 492 (0.38)
308 (1.36), 497 (0.73)
284 (1.80), 308 (2.14), 490 (0.96)
G1Ru5
G1Ru4Fe1
293 (2.75), 307 (2.76), 497 (1.44)
286 (2.40), 324 (2.48), 495 (1.12), 568 (0.2)
transfer (MLCT) transition of htpy-RuII-tpyi chromophores.
Moreover, a slight blue shift is observed for G0 due to the steric
hindrance of the methyl group. Heterocomplexes G1Ru4Fe1 give
rise to a similar band at 495 nm plus the MLCT band of the
htpy-FeII-tpyi chromophore at 568 nm. Molar extinction coeffi-
cients progressively increase from ligand MT3 to metallo-
dendrimer G0 to G1Ru5, as expected from the increasing number
of ligand chromophores and metal ion centers. The broad and
strong absorption of three metallodendrimers is beneficial to the
light-harvesting process. Only the ligand is fluorescent in dilute
solution and the emission maximum in CHCl3 peaks at 340 nm.
Meanwhile, all Ru(II) or Ru/Fe(II)-complexes are non-emissive, as
a result of the fast relaxation of the Ru/Fe(II)-terpyridine complex
to the ground state.19
The absorption and emission spectra of the metallodendrimers
in dilute MeOH and the corresponding tris-ligand MT3 in dilute
CHCl3 are shown in Fig. 6 and summarized in Table 1. Ligand
MT3 exhibits a ligand-centered, p - p* transition (LC) at ca.
280 nm. Compared with the ligand, the Ru(II) or Ru/Fe(II)
complexes exhibited two distinct absorption bands. The one
possessing a shorter wavelength belongs to the absorption of
the tris-terpyridine ligands (T3 or MT3) at about 282–293 nm and
mono-capping ligand 3 at ca. 307 nm; the latter peak became
more distinct upon increasing the relative content of ligand 3 in
the complexes compared with tris-ligands. The other absorptions
for the Ru(II) complexes at longer wavelengths were centered at
490–497 nm, which is attributed to the metal-to-ligand charge
The electrochemical properties of these complexes and ligand
MT3 were investigated by cyclic voltammetry in DMF and
Ag/AgNO3, as reference electrode. Relevant electrochemical data
are listed in Table 2 and Fig. 7. The ligand MT3 showed only one
reversible reduction process that peaked at ca. ꢁ2.35 V; whereas,
the corresponding complexes showed two quasi-reversible one-
electron reduction processes at about ꢁ1.7 V and ꢁ1.9 V vs.
Ag/AgNO3 that can be assigned to a sequential one-electron
reduction of the two terpyridine ligands surrounding the
complexed Ru/Fe(II) metal ions (tpy/tpyꢁ and tpyꢁ/tpy2ꢁ).
Inspection of the cathodic region revealed that a third irreversible
reduction peak followed the two-wave reduction process that is
related to a further reduction at the coordinated terpyridine units.
Interestingly for T1Ru2 and T2Ru1, there is a fourth cathodic
peak whose intensity decreased from T2Ru1 to T1Ru2. These
results suggested that the electron was from the uncoordinated
terminal terpyridine groups, which is consistent with ligand MT3.
All complexes exhibit one quasi-reversible oxidation wave due to
the RuIII/RuII or FeIII/FeII couples. From the measured Eox(onset)
values, the energies of the corresponding highest occupied
molecular orbital (HOMO) can be calculated according to eqn (1).
Fig. 6 Absorption and emission spectra of ligand MT3 and the complexes,
acquired in dilute MeOH (2.5 ꢂ 10ꢁ6 M) at 25 1C. The excitation
wavelength in the emission experiments of ligand MT3 was 280 nm.
EHOMO = ꢁe(Eox(onset) + 4.71) (eV)
(1)
Table 2 Electrochemical peaks, onset potentials and energy levels of metallodendrimers and liganda
Compound
Eox(peak)/V
Eox(onset)/V
Ered(peak)/V
Ered(onset)/V
EHOMO/eV
ELUMO/eV
Eg(cv)/eV
Eg(opt)/eV
MT3
—
—
ꢁ2.42/ꢁ2.60
ꢁ2.35
ꢁ1.55
ꢁ1.55
ꢁ1.55
ꢁ1.52
ꢁ1.46
ꢁ6.11
ꢁ5.39
ꢁ5.42
ꢁ5.40
ꢁ5.43
ꢁ5.39
ꢁ2.36
ꢁ3.16
ꢁ3.16
ꢁ3.16
ꢁ3.19
ꢁ3.25
—
3.75
2.17
2.18
2.19
2.17
2.08
T2Ru1
T1Ru2
G0
G1Ru5
G1Ru4Fe1
0.82
0.83
0.85
0.85
0.84
0.68
0.71
0.69
0.72
0.68
ꢁ1.67, ꢁ1.92, ꢁ2.32, ꢁ2.44
ꢁ1.65, ꢁ1.92, ꢁ2.37, ꢁ2.52
ꢁ1.67, ꢁ1.93, ꢁ2.42
ꢁ1.64, ꢁ1.92, ꢁ2.38
ꢁ1.68, ꢁ1.93, ꢁ2.37
2.23
2.26
2.24
2.24
2.14
a
Scan rate: 100 mV sꢁ1; working electrode: Pt disc; auxiliary electrode: Pt wire; reference electrode: Ag/AgNO3; supporting electrolyte: Bu4NPF6
(0.1 M, DMF).
c
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Fig. 7 Cyclic voltammogram of metallodendrimers and ligand in a 0.1 M solution of Bu4NPF6 in DMF, acquired at a scan rate of 100 mV sꢁ1. For
both scans shown, the bottom traces (reduction) were scanned from right to left and the upper traces (oxidation) were scanned from left to right.
Eox(onset) is the onset of oxidation potential vs. Ag/AgNO3.20
Similarly, the energy of the lowest unoccupied molecular orbital
can be estimated from the value of Ered(onset) via eqn (2).
were conducted on a Perkin Elmer LS55 luminescence spectro-
meter. Cyclic voltammetry was performed using a BASi EC
epsilon workstation; scan rate: 100 mV sꢁ1; working electrode:
Pt disc; auxiliary electrode: Pt wire; reference electrode:
Ag/AgNO3 (Ag/AgNO3 chosen for comparison,11 notably,
decamethylferrocene may prove more useful); supporting
electrolyte: Bu4NPF6 (0.1 M, DMF). ESI mass spectra were
obtained on a Bruker Esquire quadrupole ion trap mass
ELUMO = ꢁe(Ered(onset) + 4.71) (eV)
(2)
The electrochemical band gaps Eg(cv), calculated by subtracting
the HOMO from the LUMO energy, albeit slightly larger, are
spectrometer (ESI MS) or
a Waters Synapt HDMS
consistent with the corresponding optical band gaps, Eg(opt)
,
which corresponds well with the work of Baurele et al.21 Notably,
quadrupole/time-of-flight (Q/ToF) tandem mass spectrometer.
MALDI-ToF MS measurements were performed with a Bruker
UltraFlex III ToF/ToF mass spectrometer, equipped with a
Nd:YAG laser emitting at a wavelength of 355 nm. The TWIM
MS experiments were performed on the Synapt mass spectro-
meter using the following parameters: ESI capillary voltage:
1.0 kV; sample cone voltage: 7 V; extraction cone voltage: 3.2 V;
desolvation gas flow: 800 L hꢁ1 (N2); trap collision energy (CE):
1 eV; transfer CE: 1 eV; trap gas flow: 1.5 mL minꢁ1 (Ar); ion
mobility cell gas flow: 22.7 mL minꢁ1 (N2); sample flow rate:
5 mL minꢁ1; source temperature: 30 1C; desolvation temperature:
40 1C; IM travelling wave height: 8.5 V; and IM travelling wave
velocity: 350 m sꢁ1. The sprayed solution was prepared
by dissolving B0.3 mg of sample in 1 mL of a MeCN/MeOH
(1 : 1, v/v) mixture. Data analyses were conducted using the
MassLynx 4.1 and DriftScope 2.1 programs provided by Waters.
¨
electrostatic interactions in solution that result from a wide range
of oxidation state variations are under consideration.
Conclusions
A family of well-defined shape-persistent and small, rigid
metallodendrimers G0, G1Ru5, and G1Ru4Fe1, based on
htpy-RuII-tpyi or htpy-FeII-tpyi connectivity as either the
branch or core center has been prepared from two simple
ligands via a self-assembly approach. The resulting products
were purified by column chromatography and their molecular
structures as well as sizes were elucidated by NMR and
ESI-TWIM MS, and single crystal X-ray data. Introduction
of higher generation dendritic structures and multiple metal ion
centers results in higher molar extinction coefficients in a broad
absorption region. The photo-physical and electrochemical
properties of these complexes exhibited noticeable changes that
are quantitatively expected based on the number of electro-
active moieties within dendron generation. These materials
provide interesting platforms for fine-tuning spin crossover
applications as well as for sensitizer components of solar cells.
Syntheses
Tris-terpyridines—T3 and MT3. To 1,3,5-triformylbenzene
(600 mg, 3.7 mmol), 2-acetylpyridine (3.02 g, 24.4 mmol) and
NaOH (1.21 g, 29.6 mmol) were added and ground using a
mortar and pestle; the yellow medium was aggregated until a
yellow solid resulted. The solid was transferred to a 250 mL
flask and EtOH (80 mL) was added. After stirring at 25 1C for
2 h, aqueous NH4OH (80 mL, 28–30%) was added. The
mixture was refluxed for 20 h, then the precipitate was filtered
and the solid was purified by column chromatography (Al2O3)
eluting with CHCl3 to give T3, as a white solid: 880 mg (31%);
mp 316–318 1C; 1H NMR (500 MHz): d 8.89 (s, 6H, Py-H30,50),
8.73–8.74 (d, J = 4.5 Hz, 6H, Py-H6,600), 8.70–8.72 (d, J =
8.0 Hz, 6H, Py-H3,300), 8.42 (s, 3H, Ph-H), 7.89–7.92 (dt, 6H, J =
8.0, 1.5 Hz, Py-H4,400), 7.36–7.38 (m, 6H, Py-H5,500); 13C NMR
(125 MHz): d 156.32, 156.29, 150.1, 149.4, 140.8, 137.1, 121.3,
124.1, 121.6, 119.7; MALDI-ToF MS (m/z): calcd for C51H33N9:
771.3. Found: 772.3 ([M + H]+).
Experimental section
General procedures
Chemicals were commercially purchased and used without
further purification. Thin layer chromatography (TLC) was
conducted on flexible sheets (Baker-flex) precoated with Al2O3
(IB-F) or SiO2 (IB2-F) and the separated products were visua-
lized by UV light. Column chromatography was conducted
using basic Al2O3, Brockman Activity I (60–325 mesh) or SiO2
(60–200 mesh) from Fisher Scientific. 1H and 13C NMR spectra
were recorded on a Varian NMRS 500 spectrometer using
CDCl3, except where noted. UV-vis spectra were recorded on
a Perkin Elmer Lambda 35 UV-vis spectrometer. PL spectra
To 1,3,5-triformyl-2,4,6-trimethylbenzene (1.50 g, 7.35 mmol),
2-acetylpyridine (5.87 g, 48.4 mmol) and NaOH (2.35 g, 58.8 mmol)
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 484–491 489

View Article Online
were added and then ground using a mortar and pestle. The yellow
medium was aggregated until a yellow solid was formed. The solid
was transferred to a 500 mL flask and was dissolved in EtOH
(150 mL). After stirring at 25 1C for 2 h, aqueous NH4OH
(160 mL, 28–30%) was added into the mixture. After refluxing
for 20 h, the precipitate was filtered, purified by column chromato-
graphy (Al2O3) using CHCl3, as eluent, to give MT3, as a white
Tris-metallated tris-terpyridine—G0. Tris-terpyridine ligand
MT3 (40.6 mg, 50 mmol) was added to a suspension of
mono(RuIII) adduct 4 (97.0 mg, 180 mmol) in MeOH and
CHCl3 (45 mL, 2 : 1), then N-ethylmorpholine (4 drops) was
added. The mixture was refluxed for additional 15 h. The red
solution was filtered through Celite and concentrated in vacuo
to give a residue, which was flash column chromatographed
(silica gel) eluting with (H2O/MeCN/sat. KNO3(aq), 1/12/1) to
afford the product with salt. After washing with water, the red
solid was dissolved in MeOH, and was concentrated as a red
solid: 65 mg (52%); 1H NMR (CD3OD/CDCl3, 3/1,
1
solid: 1.50 g (25%); mp 378–380 1C; H NMR (500 MHz): d
8.75–8.76 (d, J = 4.0 Hz, 6H, Py-H6,600), 8.71–8.72 (d, J = 8.0 Hz,
6H, Py-H3,300), 8.43 (s, 6H, Py-H30,50), 7.89–7.92 (m, 6H, Py-H4,400),
7.35–7.37 (m, 6H, Py-H5,500), 1.96 (s, 9H, CH3); 13C NMR (125
MHz): d 156.3,156.0, 152.3, 149.4, 138.4, 136.9, 132.4, 123.9, 122.3,
121.3, 19.8; MALDI-ToF MS (m/z): calcd for C54H39N9: 813.3.
Found: 814.4 ([M + H]+).
a
b
500 MHz): d 9.21 (s, 6H, Py-H30,50), 9.14 (s, 6H, Py-H30,50),
a
8.89–8.91 (d, 6H, J = 8.0 Hz, Py-H3,300), 8.82–8.84 (d, 6H,
J = 8.0 Hz, bPy-H3,300), 8.20–8.22 (d, J = 8.5 Hz, 6H, Ph-H),
7.98–8.03 (m, 12H, a,bPy-H4,400), 7.50–7.53 (m, 12H,
a
a,bPy-H6,600), 7.33–7.36 (m, 6H, Py-H5,500), 7.29–7.31 (m, 6H,
Mono- and bis-metallated tris-terpyridines—T2Ru1 and
T1Ru2. Tris-ligand T3 (154.4 mg, 200 mmol) was added to a
suspension of mono(RuIII) adduct 4 (218.4 mg, 400 mmol) in
MeOH and CHCl3 (75 mL, 2 : 1), then N-ethylmorpholine
(4 drops) was added. The mixture was refluxed for additional
15 h. The red solution was filtered through Celite, then the
filtrate was concentrated in vacuo to give a residue, which was
flash column chromatographed (Al2O3) using H2O/MeCN/
sat.KNO3(aq.), as the mobile phase to successively elute
T2Ru1 (solvent composition, 1/20/1, v/v/v), and T1Ru2 (solvent
composition, 1/15/1, v/v/v), which were isolated as red solids.
bPy-H5,500), 7.22–7.24 (d, J = 8.5 Hz, 6H, Ph-H), 3.96 (s, 9H,
OCH3). 2.57 (s, 9H, CH3); 13C NMR (CD3CN, 125 MHz): d
162.9, 159.2, 156.8, 156.2, 152.9, 152.8, 150.9, 149.9, 139.3,
138.2, 135.5, 130.0, 129.5, 129.0, 128.9, 126.5, 126.3, 125.8,
121.7, 115.9, 56.1, 20.9; MALDI-ToF MS (m/z): calcd for
C
120H90F36N18O3P3Ru3: 3006.2. Found: 2860.8 (M
ꢁ
PF6ꢁ)+, 1358.4 (M ꢁ 2PF6
)
ꢁ 2+; ESI MS (m/z): 1358.3
ꢁ 2+
ꢁ 3+
[M ꢁ 2PF6
]
(calcd m/z = 1358.2), 857.2 [M ꢁ 3PF6
]
ꢁ 4+
(calcd m/z =
ꢁ 5+ (calcd m/z = 456.3), 355.9 [M ꢁ
(calcd m/z = 857.1), 606.6 [M ꢁ 4PF6
]
606.6), 456.1 [M ꢁ 5PF6
]
ꢁ 6+
(calcd m/z = 356.1).
Data for T2Ru1. 54 mg (18%); 1H NMR (CD3OD/CDCl3, 3/
1, 500 MHz): d 9.52 (s, 2H, bPy-H30,50), 9.17 (s, 6H, aPy-H30,50),
6PF6
]
b
c
9.03–9.06 (m, 6H, Py-H3,300, Py-H30,50), 8.98 (d, J = 1.0 Hz,
2H, Ph-Hx,y), 8.83–8.84 (d, 2H, J = 8.0 Hz, aPy-H3,300),
Homo-
and
heterogenous
multinuclear
dendritic
8.73–8.75 (m, 9H, cPy-H3,300 cPy-H6,600, Ph-Hz), 8.20–8.22
,
materials—G1Ru5. Solid RuCl2(DMSO)4 (12.2 mg, 25.2 mmol)
was added to a stirred solution of T1Ru2 (95.1 mg, 50 mmol) in
MeOH (100 mL). After the mixture refluxing for 10 h, the red
solution was concentrated in vacuo to give a residue, which was
purified by flash column chromatography (SiO2) using a
H2O/MeCN/sat. KNO3(aq.) (1/8/1 v/v/v) mixture, as eluent,
to afford the red solid: 71 mg (70%); 1H NMR (CD3OD/
CDCl3, 3/1, 500 MHz): d 9.80 (s, 4H, cPy-H30,50), d 9.77 (s, 8H,
bPy-H30,50), 9.45–9.46 (m, 6H, Ph-Hx,y,z), 9.18 (s, 8H, aPy-H30,50),
9.15–9.16 (d, 4H, J = 8.0 Hz, cPy-H3,300), 9.11–9.13 (d, 8H, J =
(d, J = 9.0 Hz, 2H, Ph-H), 8.01–8.06 (m, 8H, a,b,cPy-H4,400),
7.50–7.54 (m, 8H, a,bPy-H6,600 cPy-H5,500), 7.30–7.32 (m, 4H,
,
a,bPy-H5,500), 7.25–7.27 (d, J = 9.0 Hz, 2H, Ph-H), 3.98 (s, 3H,
OCH3); 13C NMR (CD3OD/CDCl3, 3/1, 125 MHz): d 163.2,
159.4, 159.2, 157.4, 156.9, 156.8, 156.2, 153.0, 152.9, 150.2,
150.05, 150.00, 148.5, 141.7, 139.6, 139.5, 139.3, 138.6, 130.1,
129.6, 128.9, 128.8, 128.4, 128.3, 126.1, 125.9, 125.4, 123.0,
122.5, 121.8, 120.2, 116.1, 56.2; MALDI-ToF MS (m/z): calcd
for C73H50F12N12OP2Ru: 1502.3. Found: 1357.5 (M ꢁ PF6ꢁ)+,
1212.5 (M + H+ ꢁ 2PF6ꢁ)+.
b
a
8.0 Hz, Py-H3,300), 8.83–8.85 (d, 8H, J = 8.0 Hz, Py-H3,300),
8.22–8.24 (d, J = 8.5 Hz, 8H, Ph-H), 8.00–8.08 (m, 20H, a,b,cPy-
H4,400), 7.74–7.75 (d, 4H, J = 5.5 Hz, Py-H6,600), 7.69–7.70 (d,
Data for T1Ru2. 210 mg (48%); 1H NMR (CD3OD/CDCl3,
c
b
3/1, 500 MHz): d 9.66 (s, 4H, Py-H30,50), 9.42 (s, 1H, Ph-Hy),
8H, J = 5.5 Hz, bPy-H6,600), 7.53–7.54 (d, 8H, J = 5.5 Hz,
aPy-H6,600), 7.38–7.41 (m, 4H, cPy-H5,500), 7.34–7.37 (m, 8H,
bPy-H5,500), 7.27–7.30 (m, 8H, aPy-H5,500), 7.26–7.27 (d, J =
8.5 Hz, 8H, Ph-H), 3.99 (s, 12H, OCH3); 13C NMR (CD3OD/
CDCl3, 3/1, 125 MHz): d 163.1, 159.6, 159.5, 159.3, 156.92,
156.90, 156.4, 153.5, 153.2, 152.7, 150.0, 148.9, 148.8, 140.9,
140.8, 139.5, 139.3, 130.1, 129.6, 128.9, 128.8, 126.5, 125.7,
123.7, 123.6, 121.7, 116.0, 56.1; MALDI-ToF MS (m/z): calcd
9.20 (m, 6H, aPy-H30,50, Ph-Hx,z), 9.11–9.13 (d, 4H, J = 8.0 Hz,
bPy-H3,300), 9.10 (s, 2H, cPy-H30,50), 8.85–8.87 (d, 4H, J =
8.0 Hz, aPy-H3,300), 8.81–8.85 (m, 4H, cPy-H3,300 cPy-H6,600),
,
8.24–8.26 (d, J = 8.5 Hz, 2H, Ph-H), 8.09–8.11 (m, 2H,
cPy-H4,400), 8.01–8.08 (m, 10H, a,bPy-H4,40), 7.64–7.65 (d, 4H, J =
,
5.5 Hz, bPy-H6,600), 7.55–7.59 (m, 6H, aPy-H6,600 cPy-H5,500),
7.31–7.35 (m, 8H, a,bPy-H5,500), 7.27–7.29 (d, J = 8.5 Hz, 4H,
Ph-H), 3.99 (s, 6H, OCH3); 13C NMR (CD3OD/CDCl3, 3/1,
125 MHz): d 163.3, 159.7, 159.6, 157.6, 157.2, 157.1, 156.5,
153.4, 153.1, 150.5, 150.2, 150.1, 148.9, 142.2, 140.8, 139.5,
139.4, 139.2, 130.3, 129.9, 129.7, 129.1, 129.02, 129.00, 126.5,
125.9, 125.8, 123.41, 123.38, 121.93, 120.3, 116.2, 56.2;
MALDI-ToF MS (m/z): calcd for C95H67F24N15O2P2Ru2:
for
C190H134F60N30O4P10Ru5: 4858.3. Found: 4712.7
(M ꢁ PF6ꢁ)+, 2283.8 (M ꢁ 2PF6
)
ꢁ 2+; ESI MS (m/z): 1474.1
ꢁ 3+
ꢁ 4+
[M ꢁ 3PF6
]
(calcd m/z = 1474.1), 1069.3 [M ꢁ 4PF6
]
ꢁ 5+
(calcd m/z =
(calcd m/z = 1069.4), 826.5 [M ꢁ 5PF6
]
(calcd m/z = 664.6), 548.9
ꢁ 6+
826.5), 664.5 [M ꢁ 6PF6
]
ꢁ 7+
ꢁ 8+
[M ꢁ 7PF6
]
(calcd m/z = 548.9), 462.2 [M ꢁ 8PF6
]
2233.2. Found: 2087.4 (M ꢁ PF6ꢁ)+, 1940.4 (M + H+
ꢁ
(calcd m/z = 462.2), 394.7 [M ꢁ 9PF6
]
ꢁ 9+ (calcd m/z = 394.7),
2PF6ꢁ)+.
and 340.8 [M ꢁ 10PF6
]
ꢁ 10+
(calcd m/z = 340.8).
c
490 New J. Chem., 2012, 36, 484–491
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

View Article Online
G1Ru4Fe1. A solution of FeCl2ꢀ4H2O (50.0 mg, 25.1 mmol)
in MeOH (100 mL) was added to a stirred solution of T1Ru2
(95.1 mg, 50 mmol) in MeOH (100 mL). After stirring the
mixture at 25 1C for 10 h, the purple solution was concentrated
in vacuo to give a residue, which was purified by flash column
chromatography (SiO2) using a H2O/MeCN/sat. KNO3(aq.)
(1/7/1 v/v/v) mixture as eluent to afford the dark solid: 84 mg
(84%); 1H NMR (CD3OD/CDCl3, 3/1, 500 MHz): d 9.97
(s, 4H, cPy-H30,50), d 9.79 (s, 8H, bPy-H30,50), 9.57 (s, 4H,
Ph-Hx,z), 9.50 (s, 2H, Ph-Hy), 9.15 (s, 8H, aPy-H30,50),
9.12–9.14 (d, 8H, J = 8.0 Hz, bPy-H3,300), 9.09–9.11 (d, 4H,
J = 8.0 Hz, cPy-H3,300), 8.81–8.83 (d, 8H, J = 8.0 Hz, aPy-H3,300),
8.19–8.21 (d, J = 8.5 Hz, 8H, Ph-H), 8.00–8.06 (m, 20H, a,b,cPy-
H4,400), 7.68–7.70 (d, 4H, J = 5.5 Hz, bPy-H6,600), 7.48–7.50
(m, 12H, a,cPy-H6,600), 7.35–7.37 (m, 8H, bPy-H5,500), 7.29–7.31
(m, 12H, a,cPy-H5,500), 7.25–7.26 (d, J=8.5 Hz, 8H, Ph-H), 3.98
(s, 12H, OCH3); 13C NMR (CD3OD/CDCl3, 3/1, 125 MHz): d
163.1, 161.9, 159.7, 159.41, 159.38, 157.0, 156.4, 153.9, 153.5,
152.8, 151.0, 150.0, 148.8, 141.0, 140.8, 140.1, 139.5, 139.3, 130.4,
130.2, 130.1, 129.7, 129.0, 128.8, 126.5, 125.8, 123.7, 123.5, 121.8,
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]
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(calcd m/z = 1058.1), 817.4
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ꢁ 4+
1459.1), 1058.1 [M ꢁ 4PF6
]
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ꢁ 5+
ꢁ 6+
[M ꢁ 5PF6
]
(calcd m/z = 817.5), 657.0 [M ꢁ 6PF6
]
(calcd m/z = 657.1), 542.5 [M ꢁ 7PF6
]
ꢁ 7+ (calcd m/z = 542.5),
456.5 [M ꢁ 8PF6-]8+ (calcd m/z = 456.6).
Acknowledgements
The authors gratefully thank the National Science Foundation
(DMR-0812337 and DMR-0705015 to GRN; CHE-1012636
and DMR-0821313 to CW) and the Ohio Board of Regents
for financial support.
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c
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