Formation and transmetalation mechanisms of homo- and heterometallic (Fe/Zn) trinuclear triple-stranded side-by-side helicates

Article abstract of DOI:10.1021/ic502855g

A novel linear hybrid tris-bidentate neutral ligand having 2,2′-bipyridine and two terminal triazolylpyridine coordination sites (L) was efficiently synthesized and explored in the synthesis of trinuclear triple-stranded homometallic side-by-side helicates L₃Fe₃(OTf)₆ (1) and L₃Zn₃(OTf)₆ (2), in which the three metal centers display alternating λ and Δ configurations. Selective formation of the analogous heterometallic side-by-side helicate L₃Fe₂Zn(OTf)₆ (3) was achieved from a mixture of L, Fe(CH₃CN)₂(OTf)₂, and Zn(OTf)₂ (1:1:1) in acetonitrile at room temperature. Various analytical techniques, i.e., single-crystal X-ray diffraction and NMR and UV/vis spectroscopy, were used to elucidate the sequence of the metal atoms within the heterometallic helicate, with the Zn²⁺ at the central position. The formation of 3 was also achieved starting from either L₃Zn₃(OTf)₆ or L₃Fe₃(OTf)₆ by adding Fe(CH₃CN)₂(OTf)₂ or Zn(OTf)₂, respectively. ESI-MS and ¹H NMR studies elucidated different transmetalation mechanisms for the two cases: While a Zn²⁺-to-Fe²⁺ transmetalation occurs by the stepwise exchange of single ions on the helicate L₃Zn₃(OTf)₆ at room temperature, this mechanism is almost inoperative for the Fe²⁺-to-Zn²⁺ transmetalation in L₃Fe₃(OTf)₆, which is kinetically trapped at room temperature. In contrast, dissociation of L₃Fe₃(OTf)₆ at higher temperature is required, followed by reassembly to give L₃Fe₂Zn(OTf)₆. The reassembly follows an interesting mechanistic pathway when an excess of Zn(OTf)₂ is present in solution: First, L₃Zn₃(OTf)₆ forms as the high-temperature thermodynamic product, which is then slowly converted into the thermodynamic heterometallic L₃Fe₂Zn(OTf)₆ product at room temperature. The temperature-dependent equilibrium shift is traced back to significant entropy differences resulting from an enhancement of the thermal motion of the ligands at high temperature, which destabilize the octahedral iron terminal complex and select zinc in a more stable tetrahedral geometry.

Full text of DOI:10.1021/ic502855g

Article
pubs.acs.org/IC
Formation and Transmetalation Mechanisms of Homo- and
Heterometallic (Fe/Zn) Trinuclear Triple-Stranded Side-by-Side
Helicates
Bidyut Akhuli,† Luca Cera,‡ Barun Jana,† Subrata Saha,† Christoph A. Schalley,*,‡ and Pradyut Ghosh*,†
†Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Kolkata
700 032, India
‡Institut fur Chemie und Biochemie der Freien Universitat Berlin, Takustr. 3, 14195 Berlin, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A novel linear hybrid tris-bidentate neutral
ligand having 2,2′-bipyridine and two terminal triazolylpyr-
idine coordination sites (L) was efficiently synthesized and
explored in the synthesis of trinuclear triple-stranded
homometallic side-by-side helicates L3Fe3(OTf)6 (1) and
L3Zn3(OTf)6 (2), in which the three metal centers display
alternating Λ and Δ configurations. Selective formation of the
analogous heterometallic side-by-side helicate L3Fe2Zn(OTf)6
(3) was achieved from a mixture of L, Fe(CH3CN)2(OTf)2,
and Zn(OTf)2 (1:1:1) in acetonitrile at room temperature.
Various analytical techniques, i.e., single-crystal X-ray
diffraction and NMR and UV/vis spectroscopy, were used to
elucidate the sequence of the metal atoms within the
heterometallic helicate, with the Zn2+ at the central position. The formation of 3 was also achieved starting from either
1
L3Zn3(OTf)6 or L3Fe3(OTf)6 by adding Fe(CH3CN)2(OTf)2 or Zn(OTf)2, respectively. ESI-MS and H NMR studies
elucidated different transmetalation mechanisms for the two cases: While a Zn2+-to-Fe2+ transmetalation occurs by the stepwise
exchange of single ions on the helicate L3Zn3(OTf)6 at room temperature, this mechanism is almost inoperative for the Fe2+-to-
Zn2+ transmetalation in L3Fe3(OTf)6, which is kinetically trapped at room temperature. In contrast, dissociation of L3Fe3(OTf)6
at higher temperature is required, followed by reassembly to give L3Fe2Zn(OTf)6. The reassembly follows an interesting
mechanistic pathway when an excess of Zn(OTf)2 is present in solution: First, L3Zn3(OTf)6 forms as the high-temperature
thermodynamic product, which is then slowly converted into the thermodynamic heterometallic L3Fe2Zn(OTf)6 product at
room temperature. The temperature-dependent equilibrium shift is traced back to significant entropy differences resulting from
an enhancement of the thermal motion of the ligands at high temperature, which destabilize the octahedral iron terminal complex
and select zinc in a more stable tetrahedral geometry.
Besides homochiral helicates that have the same chirality at
all of the metal centers, there are heterochiral mesocates
comprising an even number of metal centers with a mirror
plane at the center of the helicate,52−55 which makes them
achiral meso forms. If one extends the mesocates into helicates
with an odd number of metal centers, another type of helicate is
formed in which the metal centers possess alternating Δ and Λ
chirality. The odd number of metal centers makes the helicate
chiral overall, and the ligands do not wind around the helix but
rather bind in a parallel zigzag fashion. These complexes are
called side-by-side helicates.10 In order to synthesize side-by-
side helicates rather than regular helicates, rigid chelating
ligands or ligands with spacers having an odd number of atoms
between the chelating units are used.56 Until now, only two
INTRODUCTION
■
The development of new supramolecular synthetic materials
through metal-ion-mediated self-assembly of polydentate
ligands is attracting significant interest and has shown great
potential for diverse applications in chemistry and materials
science.1−7 Self-assembled helicates are important8−12 because
of their fundamental properties in guest molecule recogni-
tion,13−16 their chirality,17−22 their magnetic properties,23−27
and their applications as sensors, DNA binders,17,28 and
anticancer agents.29 Since the discovery of the first homo-
metallic helicate by Lehn et al. in 1987,30 different bi- and
trinuclear homo- or heterochiral homometallic helicates bearing
different transition-metal ions31−40 as well as f-block
elements13,41−45 have been reported. However, selectively
formed heterometallic helicates are still rare in the
literature.35,46−51
Received: December 2, 2014
Published: April 16, 2015
© 2015 American Chemical Society
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examples of trinuclear triple-stranded Mn2+/Co2+ helicates with
alternating Δ and Λ chirality using a rigid ligand have been
reported.24,26 In contrast, the development of trinuclear triple-
stranded side-by-side homo- and heterometallic helicates with
flexible spacers has not yet been explored.
Mechanistic investigations of the formation and trans-
metalation pathways of side-by-side helicates have not been
performed to date. A detailed understanding of these
mechanisms, in particular of the reversible interconversion
between homo- and heterometallic helicates, will certainly help
designing improved strategies for higher-order helicates with
desired properties.
Here we report a hybrid tris-bidentate ligand having one
central bipyridyl and two terminal triazolylpyridine chelating
units separated by methylene spacers. This ligand forms
trinuclear triple-stranded homometallic (Fe2+ or Zn2+) and
heterometallic (Fe2+ and Zn2+) side-by-side helicates. More-
over, detailed mechanistic insight into the transmetalation from
the homo- to the heterometallic species is presented.
bipyridine with N-bromosuccinimide (NBS) in CCl4 in the
presence of azobis(isobutyronitrile) (AIBN) gave 5,5′-dibro-
momethyl-2,2′-bipyridine (I). 5,5′-Diazidomethyl-2,2′-bipyri-
dine (II) was obtained by treating I with sodium azide in
N,N-dimethylformamide (DMF) at 80 °C. Finally, the click
reaction between 2-ethynylpyridine and II in tetrahydrofuran
(THF)/H2O at room temperature afforded the desired hybrid
ligand L. Overall, a yield of 77% was accomplished.
Trinuclear Triple-Stranded Homometallic Helicates.
To synthesize trinuclear triple-stranded homometallic helicates,
solutions of L in dry methanol were treated with 1 equiv of
either Fe(CH3CN)2(OTf)2 or Zn(OTf)2. With Fe2+, a deep-red
color was observed, consistent with the formation of low-spin
Fe2+ complexes that are stable under aerobic conditions. As
expected, a colorless complex was formed in case of Zn2+. The
recrystallized products were characterized by elemental
analysis; electrospray ionization mass spectrometry (ESI-MS);
1H, correlation spectroscopy (COSY), nuclear Overhauser
effect spectroscopy (NOESY), and diffusion-ordered spectros-
copy (DOSY) NMR studies; and single-crystal X-ray
diffraction.
RESULTS AND DISCUSSION
■
Figure 1 shows the electrospray ionization Fourier transform
ion cyclotron resonance (ESI-FTICR) mass spectra of 1 and 2.
Design Aspects and Ligand Synthesis. A careful choice
of the polytopic ligand and the metal ions is crucial for a
spontaneous formation of helicates. Following the “odd−even
rule”, one methylene group between two chelating units is
expected to favor the formation of a side-by-side helical
structure.56 Furthermore, 2,2′-bipyridine and triazolylpyridines
are suitable bidentate chelating ligands for complexation of
transition-metal ions. In order to generate heterometallic side-
by-side helicates, hybrid ligands combining both binding motifs
would be interesting in particular, as one might expect that the
preference of each of them for a certain metal ion would lead to
a subtle balance that may result in equilibria that can be shifted
from homo- to heterometallic helicates. Therefore, the tris-
bidentate ligand L under study here (Scheme 1) bears two
triazolylpyridine chelating units connected through methylene
bridges to a central 5,5′-disubstituted 2,2′-bipyridine.
Figure 1. FTICR-MS spectra of 20 μM methanol solutions of (top) 1
and (bottom) 2. Insets: calculated and experimental isotope patterns.
Another advantage of this design is the facile synthetic access
to triazoles by a Huisgen−Sharpless−Meldal click reaction. The
tris-bidentate ligand L was synthesized in three steps as
depicted in Scheme 1. The reaction of 5,5′-dimethyl-2,2′-
Characteristic peaks for three different charge states,
[L3M3(OTf)4]2+, [L3M3(OTf)3]3+, and [L3M3(OTf)2]4+, are
observed in both cases. The experimental isotope patterns
match well with those calculated on the basis of natural
abundances.
Scheme 1. Synthesis of Ligand L and the Trinuclear Triple-
Stranded Helicates 1 and 2
1H NMR spectra confirmed the purities of both complexes 1
and 2 (Figure 2). For 1, the 1H NMR signals of protons a, d, i,
and f are shifted upfield and the other signals downfield with
respect to those of free ligand L. Similar signal shifts are found
for 2, with the notable exception of the signal for proton i.
Clearly in both cases 1 and 2, the −CH2− protons are
diastereotopic and the corresponding resonance f (a singlet at
5.82 ppm for L) splits into an AB pattern (two doublets
centered at 5.58/5.48 ppm and 5.67/5.60 ppm, respectively). In
the DOSY NMR spectra of 1 and 2, all of the signals appear at
the same diffusion coefficient (3.88 × 10−10 and 4.15 × 10−10
m2/s, respectively) and thus belong to a single supramolecular
aggregate.
Furthermore, the NOESY spectra of 1 and 2 show two cross-
peaks that correspond to i−f and f−g interactions (Figure 3),
which are not present in the spectrum of the free ligand and
indicate the zigzag arrangement of the ligand along the three
metal centers. Each of the two diastereotopic protons f shows a
cross-peak to only one of the two protons g and i, in good
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monoclinic C2/c space group (Table S1 in the Supporting
Information). The internuclear Fe1−Fe2 and Fe2−Fe2
distances are found to be 7.67 and 15.33 Å, respectively. The
average metal−ligand bond distances for the terminal Fe2
atoms in 1 are Fe2−Ntriazole = 1.92(4) Å and Fe2−Npyridine
=
1.99(4) Å, while one obtains Fe1−N5 = 1.97(4) Å, Fe1−N10 =
1.96(4) Å, and Fe1−N11 = 1.98(4) Å for the central Fe1
(Table S2 in the Supporting Information).
Single-crystal X-ray diffraction analysis of L3Zn3(OTf)6
revealed the formation of a very similar helicate structure
(Figure 4, IV). Helicate 2, however, crystallizes in the triclinic
P1 space group with intramolecular Zn1−Zn2, Zn1−Zn3, and
̅
Zn2−Zn3 distances of 7.77, 7.63, and 15.39 Å, respectively.
The average Zn−N bond distances for the terminal Zn2 and
Zn3 atoms and the central Zn1 atom in 2 are 2.16 Å (Table S3
in the Supporting Information).
Photophysical Studies. The photophysical properties of
the two helicates were studied in acetonitrile. The UV/vis
spectrum of 1 shows strong bands at λmax = 422 nm (ε = 2.36 ×
103 M−1 cm−1) and 530 nm (ε = 8.13 × 102 M−1 cm−1) (Figure
5, I). The absorption band at 530 nm corresponds to the Fe−
bipyridine metal-to-ligand charge transfer (MLCT) transition.57
Correspondingly, the band at 422 nm is that of the Fe−
triazolylpyridine MLCT transition. As expected, the Fe2+
complexes do not show any emission when excited at 422
and 530 nm. Instead, complex 2 shows a strong emission at 455
nm upon excitation at 300 nm (Figure 5, II). The lifetime of the
excited state is about 10 ns. Furthermore, the decay pattern of
this complex indicates that only one excited-state species is
present (Figure 5, III).
Figure 2. (I, II, IV) Partial 1H NMR spectra of (I) L, (II) 1, and (IV)
2 in MeOH-d4 at 298 K, showing typical complexation-induced signal
shifts. (III, V) DOSY spectra of (III) 1 and (V) 2 with respective
diffusion coefficient (D) values, indicating the formation of a single
supramolecular aggregate in MeOH-d4 for each helicate.
agreement with the solid-state structure discussed below, in
which the Ntriazole−CH2−C−CHi dihedral angle is 70−95°.
Single-Crystal X-ray Diffraction. Single crystals of 1 and 2
suitable for X-ray diffraction studies were obtained when a
methanolic solution of the complex was layered over benzene at
room temperature. Single-crystal X-ray diffraction analysis of 1
revealed a trinuclear triple-stranded helicate with three Fe2+
centers arranged in a linear fashion (Figure 4, I). The structure,
which is D3-symmetric in solution, is slightly distorted in the
solid state by packing effects. The side-by-side binding mode of
the three nonplanar C2-symmetric tris-bidentate ligands around
the three octahedral Fe2+ centers leads to a racemic mixture of
heterochiral triple helicates with alternating (Λ, Δ, Λ) and (Δ,
Λ, Δ) chirality (Figure 4, II). Helicate 1 crystallizes in the
Furthermore, UV/vis titration experiments with Fe2+ and
Zn2+ (∼5 × 10−4 M solution in MeOH) and the tris-bidentate
chelating ligand L (∼5 × 10−5 M solution in 1:1 CHCl3/
MeOH) were carried out at room temperature to evaluate the
formation constants for the trinuclear triple-stranded homo-
helicates in solution. The two titration experiments showed a
similar spectral feature with a red-shifted absorption band of L
in the UV region upon titration with the respective metal ions
(Figures S29 and S30 in the Supporting Information). New
peaks at 305 and 315 nm appeared during the titrations with
Fe2+ and Zn2+, respectively. An isosbestic point at ∼295 nm in
1
Figure 3. 2D NOESY spectra of (I) 1 and (II) 2, in which two through-space H−1H couplings are clearly seen.
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Figure 4. Single-crystal X-ray structures of trinuclear triple-stranded helicates 1 (I, front view; II, side view; III, space-filling model) and 2 (IV, front
view) showing each ligand strand and the metal coordination octahedra with Λ and Δ chirality. Hydrogen atoms, counterions, and solvent molecules
are not shown for clarity.
Figure 5. (I) UV/vis spectra of complexes 1 (5 × 10−4 M solution) and 2 (5 × 10−5 M solution). (II) Emission spectrum of complex 2 (10−6
M
solution). (III) Lifetime measurement for complex 2 (10−6 M solution) with respect to a sodium dodecyl sulfate (SDS) prompt in acetonitrile at
room temperature.
both cases indicated a single equilibrium that supports the
formation of one type of complex in the solution, which could
be the formation of the M3L3 helicate. The saturation point of
the absorbance was achieved upon consumption of ∼1.0 equiv
of the metal ion. Addition of more than 1 equiv of metal ion did
not change the absorbance, and thus, no further complexation
takes place beyond 1.0 equiv of metal ion. Further, the
equivalence plots (plots of absorbance vs equivalents of metal
ion; Figures S29 and S30) clearly indicate the formation of 1:1
(i.e., M2+:L = 3:3) complexes in solution, which is consistent
with the solid-state single-crystal X-ray structures of the
trinuclear triple-stranded helicates of Fe2+ and Zn2+, in which
M2+:L = 3:3. The UV/vis titration data of L with Fe2+ and Zn2+
ions were fitted using the nonlinear regression analysis program
SPECFIT,58 which gave good fits for 3:3 stoichiometry
(ligand:metal) in both the cases. The overall formation
constants (K) for M3L3 complexes of Fe2+ and Zn2+ were
calculated from the above analyses and were found to be
Synthesis and Characterization of the Trinuclear
Triple-Stranded Heterometallic Helicate. In this section,
our choice of metal ions is Fe2+ and Zn2+ for the formation of
heterometallic helicates. This choice was made because Zn2+
and low-spin Fe2+ complexes and the heterometallic complex
formed with these ions can easily be monitored by NMR
spectroscopy.
The reaction of L with Fe(CH3CN)2(OTf)2 and Zn(OTf)2
in a 1:1:1 ratio in methanol at room temperature yielded a
yellowish-red solution, which has a different color compared to
the one of L3Fe3(OTf)6 (dark red) and L3Zn3(OTf)6
(colorless) (Figure S31 in the Supporting Information).
FTICR-MS results show characteristic peaks at m/z 1095.1,
680.4, and 473.1, corresponding to [L3Fe2Zn(OTf)4]2+,
[L3Fe2Zn(OTf)3]3+, and [L3Fe2Zn(OTf)2]4+, respectively
(Figure 6). The experimental and calculated isotope patterns
match well and confirm that complex 3 contains two Fe2+ ions
and one Zn2+ ion. Furthermore, energy-dispersive X-ray (EDX)
analysis of 3 also shows the presence of both Fe and Zn within
the complex (Figure S32 in the Supporting Information).
However, two isomers of [L3Fe2Zn(OTf)6] are possible with
respect to the sequence of metal ions: the two Fe2+ ions at the
0.2
0.6
1026.5
M−5 and 1025.6
M−5, respectively. Thus, the Fe3L3
helicate shows a slightly higher overall formation constant value
than the Zn3L3 helicate.
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(CH3CN)2(OTf)2, and Zn(OTf)2 in acetonitrile-d3 was
1
monitored over time by H NMR spectroscopy (Figure 9). A
1
comparison of the time-dependent H NMR spectra recorded
during the reaction with those of independently prepared
samples of helicates 2 and 3 clearly shows the exclusive
formation of 3 over the period of a day. Nevertheless, the
assembly of other complexes certainly occurs, and over time
these complexes converge to 3 and thus must be assembly
intermediates. Because of superimposed signals, however, the
time-dependent NMR experiments are difficult to interpret in
detail. To get more insight into the mechanistic aspects of the
formation of the heterometallic helicate, ESI-FTICR mass
spectrometry was used to monitor the changes in the solution
composition.
First, a 1:2:2 mixture of L, Zn(OTf)2, and Fe-
(CH3CN)2(OTf)2 in acetonitrile was monitored over time.
After just 15 s of reaction time, the main product is
homohelicate 2 (m/z 1104.2, 687.1, and 477.6 for the +2,
+3, and +4 charge states). At this stage, only traces of the
heterohelicate L3FeZn2(OTf)6 at m/z 683.8 and an intense
signal for the 1:1 LFe(OTf)2 complex at m/z 677.8 are
observed (Figure 10, I), while L3Fe2Zn(OTf)6 and
L3Fe3(OTf)6 are virtually absent. The initially formed
homohelicate 2 is then gradually converted into
L3FeZn2(OTf)6 (m/z 683.8) (Figure 10, II). After 2 days, the
mixture is almost completely converged into the heterohelicate
L3Fe2Zn(OTf)6 (m/z 680.4). Not even a trace of the
homohelicate L3Fe3(OTf)6 is detected at m/z 677.4 (Figure
10, III). In marked contrast, the iron homohelicate
L3Fe3(OTf)6 forms much more slowly when a solution of
Fe(CH3CN)2(OTf)2 and L is monitored over time in absence
of Zn(OTf)2 in the system. Interestingly, the reaction proceeds
through an L3Fe2(OTf)4 intermediate, which forms within less
than 1 h and converges into the final L3Fe3(OTf)6 significantly
more slowly (Figure S33 in the Supporting Information). From
these results, two conclusions can be drawn: (i) L3Fe2Zn-
(OTf)6 is the thermodynamic product, and (ii) it is generated
through a very quickly formed L3Zn3(OTf)6 intermediate that
is afterward converted into the final heterometallic helicate
L3Fe2Zn(OTf)6 by two consecutive error-correcting trans-
metalation reactions.
Figure 6. FTICR-MS spectrum of a 20 μM methanol solution of 3 and
(inset) the corresponding isotope pattern of the quadruply charged
ion.
terminal triazolylpyridine sites and the Zn2+ ion at the central
bipyridine position (Figure 7, I) or the two Fe2+ ions placed
next to each other and the Zn2+ ion in one of the terminal
1
triazolylpyridine sites (Figure 7, III). A comparison of the H
1
NMR spectrum of heterometallic helicate 3 with the H NMR
spectra of homometallic helicates 1 and 2 (Figure 7, II) clearly
supports the symmetric distribution of the metal ions with Zn2+
1
at the central position. A close match between the H NMR
spectra of 3 and 1 is found for the set of proton signals a, b, c, d,
and e belonging to the pyridine−triazole aromatic moiety, while
1
very similar chemical shifts in the H NMR spectra of 3 and 2
are observed for protons f and g corresponding to the
bipyridine protons.
Furthermore, the solution UV/vis spectrum of L3Fe2Zn-
(OTf)6 in acetonitrile reveals an absorption band at λmax = 422
nm (ε = 1.75 × 104 M−1 cm−1) (Figure 8, I). The absence of
the absorption at 530 nm, which is characteristic of the
bipyridine−Fe MLCT transition, demonstrates the presence of
Zn2+ at the bipyridine center of the complex. Finally, single-
crystal X-ray diffraction of heterometallic helicate 3 confirms
the formation of an isostructural trinuclear triple-stranded
helicate with two Fe2+ ions located in the two terminal
pyridine−triazole centers and Zn2+ at the central bipyridine
position (Figure 8, II). The Fe1−Fe2, Fe1−Zn1, and Fe2−Zn1
internuclear distances are 15.46, 7.68, and 7.78 Å, respectively.
The average Zn−N bond distance is 2.14 Å, which nicely
matches with the corresponding average bond distance of the
central Zn2+ ion of the homometallic L3Zn3(OTf)6 helicate
(Table S4 in the Supporting Information). The average Fe−
Ntriazole and Fe−Npyridine bond distances are 1.93 and 2.00 Å,
respectively, in line with the corresponding distances in
L3Fe3(OTf)6 (Table S4).
If this holds true, the formation of the heterometallic helicate
L3Fe2Zn(OTf)6 would also be expected from a solution of
L3Zn3(OTf)6 and Fe(CH3CN)2(OTf)2 at room temperature.
Mechanistic Studies. Formation Pathway of Helicate 3
at Room Temperature. An equimolar solution of L, Fe-
Figure 7. (I, III) Graphical representations of the two possible metal ion sequences in L3Fe2Zn(OTf)6. (II) 1H NMR spectra of the three complexes
in MeOH-d4 (4.0 mM) at room temperature. (IV) Partial chemical structure of L showing the proton assignments.
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Figure 8. (I) UV/vis spectra of the three helicates in acetonitrile at room temperature. The λmax values are shown. (II) Single-crystal X-ray structure
of L3Fe2Zn(OTf)6.
Figure 9. Time-dependent 1H NMR (4.0 mM) spectra recorded for a mixture of L with Zn(OTf)2 and Fe(CH3CN)2(OTf)2 (1:1:1) in acetonitrile-
d3 at room temperature and a comparison with the spectra of L3Zn3(OTf)6 and L3Fe2Zn(OTf)6 (4.0 mM).
1
Indeed, time-dependent H NMR studies carried out on a 1:3
mixture of L3Zn3(OTf)6 and Fe(CH3CN)2(OTf)2 revealed
complete conversion of 2 into 3 after 24 h. Figure 11 shows the
1
time-dependent decrease in the H NMR signal intensities for
protons a, b, c, d, e, and i belonging to L3Zn3(OTf)6 and the
complementary increase in the intensities of a new set of signals
closely matching those of the corresponding protons of the
heterometallic L3Fe2Zn(OTf)6 helicate. The peak positions of
the bipyridine protons g and h do not change during the course
of the experiment, confirming that the central Zn2+ ion does
not transmetalate at all during the reaction.
Solution-state UV/vis studies were also carried out to follow
the formation of the heterometallic helicate L3Fe2Zn(OTf)6
(Figure 12). Upon addition of Fe(CH3CN)2(OTf)2 to the
solution of L3Zn3(OTf)6 in acetonitrile, time-dependent UV/
vis analysis clearly showed the increase in the absorption band
at 422 nm (λmax) characteristic of the Fe2+−triazolylpyridine
MLCT transition. Hardly any absorption band around 530 nm
characteristic of the Fe2+−bipyridine MLCT transition was
detected, again ruling out the formation of any L3Fe3(OTf)6
helicate in this conversion.
Figure 10. (I) ESI-FTICR mass spectrum of a mixture of L,
Zn(OTf)2, and Fe(CH3CN)2(OTf)2 (1:2:2) in acetonitrile solution
(20 μM in L) after 15 s. (II) Gradual changes in the composition as
monitored for the triply charged helicate ions. (III) Final mass
spectrum recorded after 48 h.
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Figure 11. Time-dependent 1H NMR spectra for the reaction of L3Zn3(OTf)6 with Fe(CH3CN)2(OTf)2 (1:3) in acetonitrile-d3 at room
temperature and a comparison with the spectra of the helicates L3Zn3(OTf)6 and L3Fe2Zn(OTf)6 (4.0 mM).
Figure 12. Time-dependent UV/vis studies (15 min intervals) of the
reaction of L3Zn3(OTf)6 with Fe(CH3CN)2(OTf)2 (1:3) in
acetonitrile at room temperature.
Figure 13. (I) ESI-FTICR mass spectrum of a mixture of
L3Zn3(OTf)6 and Fe(CH3CN)2(OTf)2 (1:3) in acetonitrile solution
(20 μM in L3Zn3(OTf)6) after 1 min. (II) Gradual changes in the
composition as monitored for the doubly charged helicate ions. (III)
Final mass spectrum recorded after 60 h.
The same reaction was also monitored by ESI-FTICR mass
spectrometry. As expected, L3Zn3(OTf)6 (m/z 1104.1) was
completely consumed over a period of 3 days, leading to the
generation of L3Fe2Zn(OTf)6 (m/z 1095.2) via the L3Zn2Fe-
(OTf)6 intermediate (Figure 13).
In conclusion, L3Fe2Zn(OTf)6 is formed as the thermody-
namic product at room temperature, starting from either an
equimolar solution of Zn(OTf)2, Fe(CH3CN)2(OTf)2, and L
or a solution of L3Zn3(OTf)6 and Fe(CH3CN)2(OTf)2. One
would therefore also expect the formation of L3Fe2Zn(OTf)6
from a mixture of L3Fe3(OTf)6 and Zn(OTf)2. However, when
L3Fe3(OTf)6 and Zn(OTf)2 were mixed in acetonitrile, hardly
any transmetalation was observed (Figure 14). Even after a few
days at room temperature, L3Fe3(OTf)6 was still by far the
dominant component. A small percentage of the helicates
underwent transmetalation to give L3Fe2Zn(OTf)6. Even more
surprising was the formation of some L3Zn3(OTf)6, while the
second intermediate L3FeZn2(OTf)6 was not observed at all.
While the significantly slower Fe2+-to-Zn2+ transmetalation
can easily be understood by invoking a higher kinetic stability of
the Fe2+ complex over the Zn2+ complexes, the surprising
distribution of the transmetalation products is more difficult to
interpret. Therefore, high-temperature experiments were
Figure 14. ESI-FTICR mass spectra of a mixture of L3Fe3(OTf)6 and
Zn(OTf)2 (1:3) in acetonitrile solution (20 μM in L3Fe3(OTf)6) at
room temperature after (I) 10 h and (II) 96 h.
performed to increase the transmetalation rates and study
this reaction in more detail.
Formation Pathway of Helicate 3 at High Temperature.
For the high-temperature studies, different metal ion
stoichiometries were investigated in order to arrive at a more
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complete picture of the helicate formation and transmetalation
pathways.
When a 1:18 mixture of L3Fe3(OTf)6 and Zn(OTf)2 in
acetonitrile was heated to 80 °C for 1 day and monitored after
different time intervals by ESI-FTICR mass spectrometry, the
complete transmetalation of all of the Fe2+ ions to Zn2+ was
observed. After 3 h, L3Fe3(OTf)6 was mainly converted into
L3Fe2Zn(OTf)6 and L3FeZn2(OTf)6, while only a small
amount of L3Zn3(OTf)6 was detected. After 6 h,
L3FeZn2(OTf)6 accumulated and subsequently converted into
L3Zn3(OTf)6 (Figure 15).
Figure 16. ESI-FTICR mass spectra of a mixture of L3Fe3(OTf)6 and
Zn(OTf)2 (1:3) in acetonitrile solution (20 μM in L3Fe3(OTf)6) at 80
°C after (I) 1 h, (II) 8 h, and (III) 36 h.
concentration. It is unreasonable to assume that this third Fe2+-
to-Zn2+ exchange becomes faster at lower Zn2+ concentration.
Therefore, the L3Zn3(OTf)6 homohelicate observed after 8 h
(Figure 16, II) must be formed through a pathway different
from a stepwise triple Fe2+-to-Zn2+ transmetalation.
Thus, the comparison of the two mixtures monitored at high
temperature leads to the conclusion that there are two different
pathways for transmetalation (Figure 17). One is certainly a
Figure 15. (I) ESI-FTICR mass spectrum of a mixture of L3Fe3(OTf)6
and Zn(OTf)2 (1:18) in acetonitrile solution (20 μM in L3Fe3(OTf)6)
at 80 °C after 3 h. (II) Gradual changes in the composition as
monitored for the doubly charged helicate ions. (III) Final mass
spectrum recorded after 24 h.
Considering the structure of the helicate and the room-
temperature results for the Zn2+-free solution discussed above,
it is reasonable to assume that the first two exchange steps
involve predominantly, if not exclusively, the Fe−triazolylpyr-
idine sites and are followed by a much slower exchange of the
central Fe2+ with Zn2+. The same experiment was also
Figure 17. (left) Comparison of the ESI-FTICR mass spectra of 1:3
(Fe2+:Zn2+ = 1:1) and 1:18 (Fe2+:Zn2+ = 1:6) mixtures of L3Fe3(OTf)6
and Zn(OTf)2 at 80 °C acquired after 6 and 8 h, respectively. (right)
Representation of the two competing pathways for the formation of
L3Zn3(OTf)6.
1
monitored by H NMR spectroscopy, and while the formation
of all of the intermediates was hard to follow in detail, the
overall replacement of the central Fe2+ was clearly indicated by
the disappearance of the signals of protons c and d (Figure S34
in the Supporting Information).
To further investigate the high-temperature transmetalation,
L3Fe3(OTf)6 was subjected to the same heating experiment in
the presence of a stoichiometric amount of Zn2+. When a 1:3
mixture of L3Fe3(OTf)6 and Zn(OTf)2 in acetonitrile was
heated to 80 °C and monitored by ESI-FTICR mass
spectrometry, the Fe2+-to-Zn2+ transmetalation was again
observed, this time finally leading to an equilibrated mixture
of all of the possible homo- and heterohelicates within 36 h
(Figure 16).
Interestingly, L3Fe3(OTf)6, L3Fe2Zn(OTf)6, and
L3Zn3(OTf)6 were again the only species detected after an
intermediate reaction time of 8 h, while L3FeZn2(OTf)6 was
absent as an intermediate on the way to L3Zn3(OTf)6. This
behavior again is surprising in view of the experiment
performed with the 1:18 mixture of L3Fe3(OTf)6 and
Zn(OTf)2, in which the L3FeZn2(OTf)6 intermediate slowly
converted into the L3Zn3(OTf)6 homohelicate at high Zn2+
stepwise metal ion exchange in which initially two terminal Fe2+
ions are exchanged with Zn2+ ions, followed by the much
slower exchange of the central Fe2+ ion with Zn2+. The second
pathway must circumvent this stepwise reaction sequence, and
we propose a disassembly−reassembly mechanism. Partial
dissociation of the helicates leads to the presence of free
ligands. As discussed above, the formation of L3Zn3(OTf)6 is
much faster than the formation of any helicates containing Fe2+
ions. Thus, the formation of free ligands is quickly followed by
the formation of the Zn2+ homohelicate, which over longer
reaction times also back-exchanges to give the L3FeZn2(OTf)6
helicate that appears in the final mixture (Figure 16, III).
If this complicated yet intriguing mechanistic scenario holds
true, the stoichiometry dependence certainly needs an
explanation. If one assumes that the triflate counterions
compete with the coordinating ligands to some extent, a
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higher concentration of Zn(OTf)2 causes stronger competition,
in line with the observation that the two terminal Fe2+ ions are
exchanged more quickly in the 1:18 mixture than in the 1:3
mixture. We further propose that the disassembly−reassembly
pathway is less strongly affected by the triflate concentration.
Therefore, the relative fractions of the helicates that trans-
metalate through each of the pathways change with the
Zn(OTf)2 concentration.
Finally, additional evidence for a strong temperature-
dependent equilibrium shift comes from an experiment in
which the 1:3 mixture of L3Fe3(OTf)6 and Zn(OTf)2 was first
equilibrated at high temperature for 89 h and then stirred at
room temperature for additional 40 h. The time-dependent
FTICR-MS measurements indeed showed the gradual con-
version of L3Zn3(OTf)6 and L3FeZn2(OTf)6 into L3Fe2Zn-
(OTf)6 and no further conversion into L3Fe3(OTf)6 (Figure 18,
II).
Figure 19. UV/vis spectra of (I) L3Fe3(OTf)6 in acetonitrile and (II) a
mixture of L3Fe3(OTf)6 and Zn(OTf)2 (1:3) equilibrated at high
temperature in acetonitrile after stirring for 40 h at room temperature.
is accessible, which is accomplished by a more stable Fe2+
complex in an octahedral geometry.
CONCLUSIONS
■
We have demonstrated the synthesis of two rare examples of
linear trinuclear triple-stranded side-by-side helicates via Fe2+/
Zn2+-assisted self-assembly of a newly synthesized linear tris-
bidentate hybrid ligand having a central 2,2′-bypyridine unit
and two terminal triazolylpyridine chelating units. Furthermore,
we have reported the first example of heterometallic trinuclear
triple-stranded side-by-side helicates. The self-sorting behavior
of the hybrid ligand toward the two different metal ions selects
the specific formation of L3Fe2Zn(OTf)6, which can be
achieved either at room temperature by mixing of the free
components or by conversion of the L3Fe3(OTf)6 helicate in
the presence of Zn(OTf)2 at higher temperature. Indeed, the
fast formation of L3Zn3(OTf)6 is followed by a slower Zn-to-Fe
back-exchange at the terminal positions that leads to the
formation of L3Fe2Zn(OTf)6 and no further conversion of the
central metal ion. Alternatively, L3Fe2Zn(OTf)6 can be
achieved from a solution of L3Fe3(OTf)6 by providing the
thermal energy necessary to dissemble the helicate scaffold and
allowing the replacement of the central Fe2+ with Zn2+.
Moreover we have found an unexpectedly pronounced
temperature-dependent equilibrium shift between the Zn2+
and Fe2+ at the triazolylpyridine metal complex that allows
the controlled and reversible formation of L3Zn3(OTf)6 at high
temperature.
Figure 18. (I) ESI-FTICR mass spectrum of a mixture of L3Fe3(OTf)6
and Zn(OTf)2 (1:3) in acetonitrile solution (20 μM in L3Fe3(OTf)6)
equilibrated at high temperature. (II) Gradual changes in the
composition as monitored for the doubly charged helicate during
the Zn2+-to-Fe2+ transmetalation at room temperature. (III) Final mass
spectrum recorded after 40 h at room temperature.
As expected, the terminal Zn−triazolylpyridine complexes
undergo a slow Zn2+-to-Fe2+ back-transmetalation, while no
further replacement of the central Zn2+ ion is detected. The
presence of a Zn2+ ion at the central position is further
indicated by a dramatic decrease in the absorption band at 530
nm corresponding to the Fe−bipyridine MLCT transition
(Figure 19).
The back-shift of the equilibrium was also observed in the
case of a solution of L3Fe3(OTf)6 containing excess of
Zn(OTf)2 (Figure S35 in the Supporting Information). As
expected, the L3Zn3(OTf)6 homohelicate forms predominantly
at high temperature and undergoes back-exchange of the two
terminal Zn2+ at room temperature to become L3Fe2Zn(OTf)6
after 60 h.
The temperature-dependent equilibrium shift can be
explained in terms of the different abilities of Fe2+ and Zn2+
to form complexes with different coordination numbers. At
higher temperature, the increased thermal motion of the ligand
preferentially selects the complex with a lower coordination
number, which can only be provided by a more stable
tetrahedral triazolylpyridine−Zn2+ complex. At room temper-
ature, however, the complex with a higher coordination number
EXPERIMENTAL SECTION
■
Materials and Methods. ESI-FTICR-MS experiments were done
on a Varian/IonSpec QFT-7 instrument. ESI-MS experiments were
carried out on a Waters QToF model YA 263 spectrometer in positive-
ion ESI mode. 1D and 2D NMR spectra were recorded on 300/400/
500 MHz Bruker DPX NMR spectrometers. All of the reactions were
carried out in a dry nitrogen gas atmosphere. The following workup
procedures were carried out under ambient conditions. Acetonitrile
and methylene chloride were refluxed over CaH2 and collected prior to
use. DMF was refluxed over CaH2, vacuum-distilled, and stored over
molecular sieves. Zn(OTf)2 was purchased from Aldrich and used as
received. Fe(CH3CN)2(OTf)2 was synthesized according to a
literature procedure.59 NBS and AIBN were recrystallized from
water and methanol, respectively, prior to use. Deuterated solvents and
all other chemicals were used as received from the supplier without
further purification.
X-ray Crystallographic Refinement Details. Crystals suitable
for single-crystal X-ray diffraction studies were selected from the
mother liquor, immersed in Paratone oil, mounted on the tip of a glass
fiber, and cemented using epoxy resin. Intensity data for the crystals of
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1, 2, and 3 were collected using Mo Kα radiation (λ = 0.7107 Å) on a
Bruker SMART APEX II diffractometer equipped with a CCD area
detector at 150 K. The data integration and reduction were processed
with SAINT software60 provided with the SMART APEX II software
package. An empirical absorption correction was applied to the
collected reflections using SADABS.61 The structures were solved by
direct methods using SHELXL62 and were refined on F2 by the full-
matrix least-squares technique using the SHELXL-9763 program
package. Graphics were generated using PLATON64 and MERCURY
2.3.65 Non-hydrogen atoms were refined anisotropically until
convergence was reached. In the cases of complexes 1, 2, and 3, the
hydrogen atoms were geometrically fixed at idealized positions. For
complexes 2 and 3, we were unable to assign electron density for some
solvent molecules in the unit cell, even though the data were collected
at 150 K several times. The routine SQUEEZE66 was applied to the
intensity data for both complexes 2 and 3 to take into account the
disordered solvent molecules. Again, for complex 2, one triflate
300 MHz): δ (ppm) 5.81 (s, 4H, -CHf2), 7.32−7.36 (m, 2H, Ar−
CHc), 7.86−7.94 (m, 4H, Ar−CHb,g), 8.01−8.04 (m, 2H, Ar−CHi),
8.39 (d, 2H, J = 8.1 Hz, Ar−CHh), 8.58−8.60 (m, 2H, Ar−CHd),
1
8.76−8.78 (m, 4H, Ar−CHa,e). H NMR (MeOH-d4, 300 MHz): δ
(ppm) 5.82 (s, 4H, CHf 2), 7.33−7.38 (m, 2H, Ar−CHc), 7.88−7.97
(m, 4H, Ar−CHb,g), 8.06−8.09 (m, 2H, Ar−CHi), 8.39 (d, 2H, J = 8.1
Hz, Ar−CHh), 8.52−8.56 (m, 4H, Ar−CHe,d), 8.74 (s, 2H, Ar−CHa).
13C NMR (DMSO-d6,75 MHz): δ (ppm) 50.93, 120.03, 121.19,
123.65, 124.17, 137.72, 137.80, 138.02, 148.21, 149.63, 149.77, 150.19,
150.36.
Synthesis of Helicate 1. A dry methanol solution of Fe-
(CH3CN)2(OTf)2 (39 mg, 0.089 mmol) was added to a dry methanol
(5 mL) suspension of L (42 mg, 0.089 mmol) in a round-bottom flask
at room temperature. The reaction mixture was allowed to stir in an
inert gas atmosphere, and a clear, dark-red solution was obtained after
4 h. The reaction mixture was stirred for an additional 24 h and then
filtered through a filter paper. The filtrate was dried, and the red solid
was dissolved using 5 mL of methanol. The filtrate was concentrated
to 2 mL and layered over benzene (2 mL) in a test tube. Dark-red
needle-shaped crystals suitable for single-crystal X-ray measurements
were obtained at the interlayer between the two solvents in 12 h. The
solvents were removed using a pipet, and the residue was dried under
vacuum to get 1 in analytically pure form. Yield: 48 mg (65%). ESI-
FTICR-MS: m/z calcd for [Fe3L3·4OTf]2+ 1090.5882, found
1090.5895. Elemental analysis for C84H60F18N30O18S6Fe3: Found: C,
40.74; H, 2.53; N, 17.03. Calcd: C, 40.69; H, 2.44; N, 16.95. 1H NMR
(MeOH-d4, 300 MHz): δ (ppm) 5.48 (d, 2H, J = 14.5 Hz, CHf2), 5.58
(d, 2H, J = 14.0 Hz, CHf 2), 7.45 (t, 2H, J = 6.5 Hz, Ar−CHc), 7.51 (s,
2H, Ar−CHi), 7.79 (d, 2H, J = 5.5 Hz, Ar−CHd), 8.12 (t, 2H, J = 7.5
Hz, Ar−CHb), 8.22 (d, 2H, J = 8.0 Hz, Ar−CHa), 8.49 (d, 2H, J = 7.0
Hz, Ar−CHg), 8.82 (d, 2H, J = 8.5 Hz, Ar−CHh), 9.19 (s, 2H, Ar−
CHe). 13C NMR (MeOH-d4, 75 MHz): δ (ppm) 52.95, 121.16,
123.51, 125.91, 127.02, 127.25, 136.58, 140.51, 141.47, 151.45, 154.27,
155.86, 160.72.
−
(CF3SO3 ) counteranion (consisting of S8, O13, O14, O15, C84, F10,
F11, and F19) was disordered at two sites, and the occupancy factors
were refined using the FVAR command of the SHELXTL program
and isotropically refined.
Synthesis of 5,5′-Bis(bromomethyl)-2,2′-bipyridine (I). This
compound was prepared by a modified version of the procedure
developed by Tian et al.67 5,5′-Dimethyl 2,2′-bipyridine (1 g, 5.43
mmol), NBS (1.932 g, 10.86 mmol), AIBN (445.7 mg, 2.71 mmol),
and 200 mL of dry CCl4 were mixed in a 250 mL round-bottom flask.
The reaction mixture was put on a photoreactor (200 W light bulb) for
1.5 h and then heated to 45−50 °C for 24 h. Subsequently, the
reaction mixture was filtered, and the filtrate was dried in vacuo. The
crude reaction mixture was separated using silica gel column
chromatography in a 25:1 CH2Cl2/MeOH mixture. Yield: 1.6 g
(86%). ESI-MS(+): m/z calcd for C12H11Br2N2 [M + H]+ 343.04,
1
found 343.08. H NMR (300 MHz, CDCl3): δ (ppm) 4.55 (s, 4H,
CH2), 8.02 (dd, 2H, J = 8.1 and 2.1 Hz, Ar−CH), 8.63 (d, 2H, J = 8.1
Hz, Ar−CH), 8.78 (d, 2H, J = 2.1 Hz, Ar−CH). 13C NMR (75 MHz,
CDCl3): δ (ppm) 29.56, 121.41, 134.13, 137.89, 149.35, 155.18.
Synthesis of 5,5′-Bis(azidomethyl)-2,2′-bipyridine (II). This
compound was prepared by a modified version of the procedure
developed by Sauvage and co-workers.68 Dibromide I (220 mg, 0.64
mmol) and NaN3 (125 mg, 1.93 mmol) were dissolved in 4 mL of dry
DMF. The reaction mixture was heated to 80−90 °C for about 2 days
and then poured into 150 mL of water. The water phase was extracted
with chloroform (3 × 10 mL). The combined organic phases were
dried over anhydrous Na2SO4, and the solvent was removed in vacuo
to get an off-white solid that was used in the next step without further
purification. Yield: 160 mg (94%). ESI-MS(+): m/z calcd for
C12H10N8Na [M + Na]+ 289.25, found 289.16. 1H NMR (500
MHz, CDCl3): δ (ppm) 4.44 (s, 4H, CH2), 7.79 (dd, 2H, J = 10 and 3
Hz, Ar−CH), 8.44 (d, 2H, J = 10 Hz, Ar−CH), 8.62 (s, 2H, J = 2.1
Hz, Ar−CH). 13C NMR (125 MHz, CDCl3): δ (ppm) 52.09, 121.31,
131.50, 136.93, 148.84, 155.66.
Synthesis of Ligand L. 200 mg (0.75 mmol) of II was dissolved in
a mixture of 6 mL of THF and 2 mL of H2O in a 25 mL round-bottom
flask. 2-Ethynylpyridine (160 μL, 1.58 mmol) was added to the
solution, followed by sodium ascorbate (310 mg, 1.56 mmol) in 1 mL
of H2O and CuSO4 (25 mg, 0.1 mmol) in another 1 mL of H2O. The
reaction mixture was allowed to stir for 12 h under a nitrogen
atmosphere. Then 10 mL of a saturated aqueous EDTA solution was
added, and the mixture was stirred for another 24 h. The water phase
was extracted with dichloromethane (3 × 50 mL). The combined
organic phases were washed two times with brine and dried over
Na2SO4. The solvent was removed in vacuo to obtain a light-yellow
solid that was purified by silica gel (60−120 mesh size) column
chromatography using 10% MeOH in CH2Cl2 with 100 μL of
triethylamine in each 100 mL of solvent system. The pure ligand is
poorly soluble in all common organic solvents such as dimethyl
sulfoxide (DMSO), DMF, MeOH, CHCl3, and CH3CN but shows
moderate solubility in a solvent mixture such as DMSO/CHCl3 or
MeOH/CHCl3. Yield: 340 mg (95%). ESI-MS(+): m/z calcd for
C26H20N10Na [M + Na]+ 495.18, found 495.18. 1H NMR (DMSO-d6,
Synthesis of Helicate 2. A methanol solution of Zn(OTf)2 (36.9
mg, 0.10 mmol) was added to a methanol (5 mL) suspension of L (48
mg, 0.10 mmol) in a round-bottom flask at room temperature. The
reaction mixture was allowed to stir, and a clear, colorless solution was
obtained after 30 min. The reaction mixture was stirred for additional
3 h and then filtered through a filter paper. The filtrate was
concentrated to 2 mL and layered over benzene (2 mL) in a test tube.
Colorless needle-shaped crystals suitable for single-crystal X-ray
measurements were obtained at the bottom of the test tube and at
the interlayer between the two solvents in 12 h. The solvents were
removed using a pipet, and the residue was dried under vacuum to get
2 in analytically pure form. Yield: 67 mg (80%). ESI-FTICR-MS: m/z
calcd for [Zn3L3·4OTf]2+ 1104.1393, found 1104.1412. Elemental
analysis for C84H60F18N30O18S6Zn3: Found: C, 40.31; H, 2.49; N,
1
16.71 Calcd: C, 40.22; H, 2.41; N, 16.75. H NMR (MeOH-d4, 500
MHz): δ (ppm) 5.59 (d, 2H, J = 14 Hz, CHf2), 5.66 (d, 2H, J = 14.5
Hz, CHf2), 7.64 (t, 2H, J = 6.5 Hz, Ar−CHc), 8.14 (d, 2H, J = 8.5 Hz,
Ar−CHa), 8.24−8.27 (m, 6H, Ar−CHb,d,i), 8.59 (d, 2H, J = 8.0 Hz,
Ar−CHg), 8.81 (d, 2H, J = 8.5 Hz, Ar−CHh), 9.10 (s, 2H, Ar−CHe).
13C NMR (MeOH-d4, 75 MHz,): δ (ppm) 52.39, 124.12, 125.84,
126.25, 127.68, 136.15, 143.20, 143.90, 144.36, 146.42, 149.50, 150.15,
150.75.
Synthesis of Helicate 3. A 5 mL methanol solution of Zn(OTf)2
(36.9 mg, 0.10 mmol) and Fe(CH3CN)2(OTf)2 (44 mg, 0.10 mmol)
was added to a methanol (5 mL) suspension of L (48 mg, 0.10 mmol)
in a 25 mL round-bottom flask in a dry glovebox at room temperature.
A clear straw-yellow solution was obtained after 30 min. The reaction
mixture was stirred for additional 24 h and then filtered through a filter
paper. The filtrate was dried, and the yellowish-red solid was extracted
using 5 mL of methanol. The filtrate was concentrated to 2 mL and
layered over benzene (2 mL) in a test tube. Yellowish-red needle-
shaped crystals suitable for single-crystal X-ray measurements were
obtained at the interlayer between the two solvents in 12 h. The
solvents were removed using a pipet, and the residue was dried under
vacuum to get 3 in analytically pure form. Yield: 51 mg (62%). ESI-
FTICR-MS: m/z calcd for [Fe2ZnL3·4OTf]2+ 1095.0842, found
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1095.0860. Elemental analysis for C84H60F18Fe2N30O18S6Zn: Found:
C, 40.71; H, 2.49; N, 17.01. Calcd: C, 40.53; H, 2.43; N, 16.88. H
(16) Custelcean, R.; Bonnesen, P. V.; Roach, B. D.; Duncan, N. C.
Chem. Commun. 2012, 48, 7438−7440.
1
NMR (MeOH-d4, 300 MHz): δ (ppm) 5.47 (s, 4H, CHf2), 7.38 (t,
2H, J = 6.0 Hz, Ar−CHc), 7.66 (d, 2H, J = 6.0 Hz, Ar−CHd), 7.90 (d,
2H, J = 3.0 Hz, Ar−CHi), 8.07 (t, 2H, J = 9.0 Hz, Ar−CHb), 8.15 (d,
2H, J = 6.0 Hz, Ar−CHa), 8.52 (dd, 2H, J = 9.0 and 3.0 Hz, Ar−CHg),
8.65 (d, 2H, J = 9.0 Hz, Ar−CHh), 8.97 (s, 2H, Ar−CHe). 13C NMR
(MeOH-d4, 75 MHz): δ (ppm) 52.57, 123.18, 125.37, 125.89, 126.88,
135.60, 140.15, 143.72, 149.61, 150.19, 150.86, 153.63, 155.77.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data (CIF), NMR data, mass data, UV/vis
titration experiments, and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
Engeser, M.; Lutzen, A. J. Am. Chem. Soc. 2014, 136, 11830−11838.
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Wernsdorfer, W.; Chibotaru, L. F.; Murugesu, M. Chem. Sci. 2012, 3,
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: c.schalley@fu-berlin.de.
*E-mail: icpg@iacs.res.in.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.G. gratefully acknowledges the Science and Engineering
Research Board (SERB), New Delhi (Project SR/S1/IC-39/
2012) for financial support. B.A. acknowledges CSIR for a SRF.
Single-crystal X-ray diffraction data were collected at the DBT-
funded CEIB Program (Project BT/01/CEIB/11/V/13)
awarded to the Department of Organic Chemistry, IACS,
Kolkata, India. C.A.S. thanks the Deutsche Forschungsgemein-
schaft and Freie Universitat Berlin for financial support. We are
̈
grateful to the Alexander von Humboldt Foundation for a
return fellowship to P.G. We also thank Shahi Imam Reja, Guru
Nanak Deb University, India, for his help with the formation
constant calculations.
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