Reversible multicomponent self-assembly mediated by bismuth ions

Article abstract of DOI:10.1039/c3dt50578b

Bi(iii) ions are capable of reversible, multicomponent self assembly with suitable tris-coordinate ligands. The nature of the self-assembled structures observed are dependent on the ligand coordination geometry, ligand protonation state and Bi concentration. These assemblies can exploit the maximum number of coordination sites at the Bi vertices (nine), and the self-assembly process has been studied by 1D NMR, Diffusion NMR, ESI-MS and X-ray crystallographic analysis. V-shaped coordinating ligands reversibly form discrete M ₂L₄, M₂L₃, and M₂L ₂ complexes dependent on ligand/bismuth concentration, whereas a linear coordinating ligand forms a single discrete M₃L₃ assembly.

Full text of DOI:10.1039/c3dt50578b

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Reversible multicomponent self-assembly mediated by
bismuth ions†
Cite this: Dalton Trans., 2013, 42, 8394
Amber M. Johnson, Michael C. Young and Richard J. Hooley*
Bi(III) ions are capable of reversible, multicomponent self assembly with suitable tris-coordinate ligands.
The nature of the self-assembled structures observed are dependent on the ligand coordination geome-
try, ligand protonation state and Bi concentration. These assemblies can exploit the maximum number of
coordination sites at the Bi vertices (nine), and the self-assembly process has been studied by 1D NMR,
Diffusion NMR, ESI-MS and X-ray crystallographic analysis. V-shaped coordinating ligands reversibly form
discrete M2L4, M2L3, and M2L2 complexes dependent on ligand/bismuth concentration, whereas a linear
coordinating ligand forms a single discrete M3L3 assembly.
Received 1st March 2013,
Accepted 14th April 2013
DOI: 10.1039/c3dt50578b
www.rsc.org/dalton
water stable diamagnetic assemblies with a single consistent
oxidation state.
Introduction
The self-assembly of organic ligands with metals is a powerful
Examples of self-assembled structures controlled by main
tool for the synthesis of complex three-dimensional struc- group metals are quite uncommon, however. Group 13 metals
tures.1 These assemblies offer advantages over covalent macro- such as aluminum and gallium have been shown to make self-
molecules in that they are held together by weaker bonds, assembled cages analogous to those of transition metals, but
allowing mismatches to break apart and reassemble into the are limited by their high charge : size ratio to hard, oxygen-
most thermodynamically stable structures.2 Typically tran- containing ligands.8 Larger main group metals can display a
sition metals, with their well-defined coordination spheres large range of coordination geometries and often lack the pre-
and numerous characterized Werner complexes, are utilized dictable angles associated with transition metals that allow for
for this purpose. By matching ligands and metals with comple- predictable multicomponent self-assembly. A limited number
mentary geometric constraints, many of these self-assembled of self-assembled structures of other main-group metals are
polygons3 and polyhedra4 have been synthesized. Polyhedral known,9 including those of arsenic,9a–d germanium,9e tin,9f,g
cages targeted in this method have shown uses ranging from lead,9h,i and antimony9j,k. A metal that has received relatively
selective guest binding5 to novel stereo and regioselectivity6 little attention in this field is the largest stable metal, bismuth.
not commonly observed with traditional catalysts. Transition Recent interest in the coordination chemistry of bismuth has
metal complexes can be challenging to study, however: tran- primarily focused on its potential medical applications.10
sition metals often display paramagnetism and can easily
The coordination sphere of bismuth, however, offers far
access varying oxidation states, leading to air and water sensi- more variety than that of smaller main-group metals.11 With
tivity of solution-phase cage complexes and complex (or in- multiple possibilities in coordination number, the use of
accessible) NMR spectra. As the size and complexity of the bismuth as a structural component in supramolecular self-
assemblies increases, the formation of a single discrete assembly could allow for controlled switchable self-assemblies
product can become challenging or even impossible.7 The use using a single metal type, something that can be difficult for
of main group metals as vertices for metal-mediated self- transition metals with more rigidly defined coordination geo-
assembly would solve these challenges by providing air and metry. The larger coordination sphere of bismuth(III) at first
seems reminiscent of the lanthanides, but nine-coordinate
Bi(III) species are rare, and have never been observed to
mediate the self-assembly of metal–organic cages.12 This is in
contrast to lanthanides, which in their +3 state can easily form
9-coordinate complexes.13
Bismuth ions have, to date, only been exploited for supra-
molecular assembly in their tri-coordinate state, as an analog
of As and Sb-controlled assembly involving bis-thiolate ligands
as single-point coordinators for two bismuth(III) ions.14
Department of Chemistry, University of California – Riverside, 501 Big Springs Rd,
Riverside, CA 92521, USA. E-mail: richard.hooley@ucr.edu; Fax: +1 951-827-4713;
Tel: +1 951-827-4924
†Electronic supplementary information (ESI) available: Synthetic procedures
and selected NMR, mass spectral, and X-ray diffraction data. CCDC 892045.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt50578b
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To access larger structures with predictable self-assembly prop-
erties, the full coordination sphere of Bi(III) must be exploited.
The simplest strategy is to apply tridentate chelator ligands,
three of which can assemble around each Bi ion. Recent work
with lanthanide-mediated self-assembly has shown that hydra-
zones derived from 2-pyridine carboxaldehyde and salicylalde-
hyde can be effective chelators for these non-transition metals
with larger coordination spheres.15 Here we exploit the variable
coordination sphere of bismuth to access reversible multi-
component self-assemblies upon addition of suitable triden-
tate chelating ligands.
Results and discussion
Two ligands (1 and 2) were used to study the self-assembly
properties of Bi(III) ions, providing different coordination
angles for self-assembly. V-shaped ligand 1 was synthesized in
three steps as shown in Fig. 1. Double Suzuki coupling
between 1,3-dibromobenzene and 4-ethoxycarbonylphenyl-
boronic acid gave the corresponding terphenyl diester in 64%
yield, which was converted to the corresponding hydrazide 3
in 86% yield by refluxing in anhydrous hydrazine. The desired
metal-coordinating hydrazone species 1 was formed by the
combination of 3 and commercially available 2-formylpyridine
under acidic conditions. The linear equivalent 2 was accessed
from commercially available biphenyl-4,4′-dicarboxylic acid. In
this case, formation of hydrazide 416 was achieved directly
from the diacid through HCTU coupling with anhydrous
hydrazine in 76% yield. The bis-hydrazide was then treated
with 2-formylpyridine as before to give bis-hydrazone 2.
Fig. 2 1H NMR titration of Bi(OTf )3 into ligand 1 (400 MHz, 298 K, CD3CN): (a)
0.5 eq.; (b) 0.67 eq.; (c) 1 eq.; (d) 2 eq.; (e) 3 eq.
in the downfield region of the spectrum (Fig. 2, for full
spectra, see ESI†). Upon further addition of Bi(OTf)3, peaks for
a second species grow in with concomitant decrease in inten-
sity for the original complex (Fig. 2b). After addition of 2 mol
eq. Bi(OTf)3, a third species is observed (Fig. 2d), and this
species becomes dominant after addition of 3 mol eq.
Bi(OTf)3. This shows the versatility of bismuth compared to tran-
sition metals, which do not show this variable coordination.17
The NMR titration data suggested that the three observed
complexes displayed M2L2, M2L3 and M2L4 stoichiometries,
shown in cartoon form in Fig. 2. All NH protons are retained
in the NMR spectra of the assemblies, suggesting that the
complex formation occurs with neutral ligand, leading to posi-
tively charged assemblies. The mixtures were also subjected to
ESI-MS analysis to corroborate this assignment. A sample of
M2L2 complex was prepared by combining ligand and Bi(OTf)3
in acetonitrile. After confirmation of the presence of a single
M2L2 complex in solution by 1H NMR analysis, the solution
was ionized under mild electrospray conditions. Upon ioniza-
tion, all three components were observed in the mass spec-
trum, as shown in Fig. 3. Each complex (12·Bi2, 13·Bi2, and
Initial self-assembly tests were analyzed by 1H NMR
spectroscopy. Hydrazone ligand 1 is only sparingly soluble in
acetonitrile, but is rapidly solubilized by addition of substoi-
chiometric amounts of bismuth(III) triflate. Upon titration of
Bi(OTf)3 into a CD3CN solution/suspension of 1, downfield
1
shifts were observed in the H NMR spectrum (see Fig. 2 and
ESI†), consistent with metal–ligand coordination. After
addition of 0.50 mol eq. Bi(OTf)3, only one species was
observed in the 1H NMR spectrum. The simplest analysis of
the coordination can be performed by studying ortho-pyridyl
proton H1 and imine proton H2 upon complexation, observed
Fig. 1 Synthesis of ligands 1 and 2. (a) 5% PdCl2(PPh3)2, Cs2CO3, DMF, 100 °C, 64%; (b) N2H4, 85 °C, 86%; (c) 2-formylpyridine, 5% AcOH, EtOH, reflux; (d) HCTU,
Et3N, N2H4, 76%.
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Fig. 4 (a) X-Ray diffraction structure of 12·Bi2·(OTf )6 in the solid state; (b)
expansion of the structure, indicating the coordination at Bi; (c) minimized struc-
ture of 13·Bi2 (SPARTAN, AM1 force field); (d) minimized structure of
14·Bi2·(OTf)2 (SPARTAN, AM1 force field).
Fig. 3 ESI-MS spectrum of 1 + Bi(OTf)3, indicating formation of the 12·Bi2,
13·Bi2 and 14·Bi2 complexes.
While crystallizations were attempted with a range of bismuth
14·Bi2) was observed as its +2 cation, with no triflate counter- concentrations, no other species were obtained. In the solid
ions observed. The charge state can be rationalized by loss of state, the 12·Bi2·(OTf)6 assembly forms as a quasi-coordination
multiple TfOH molecules from the self assembly upon ioniza- polymer, with two triflate counterions providing weak bridging
tion, with the additional proton provided by the NH in the interactions between each discrete M2L2 unit (Fig. 4a and 4b).
hydrazone group. This loss of the labile NH proton upon ioniza- This polymeric structure is more stable than the M2L3 or M2L4
tion is precedented for hydrazone-based self-assemblies.18
structures, which display fewer Bi–O contacts. Each Bi ion pro-
The most prevalent complex in the mass spectrum was the vides nine coordination sites, three for each ligand and one
12·Bi2 assembly, and the monocationic form was also for each of the three triflate counterions. Two triflates are
observed, with retention of one TfO− ion. Samples with bridged between each M2L2 complex and a third coordinates
varying concentrations of Bi were analyzed by ESI-MS, and in individually. There is also an additional triflate uncoordinated
all cases, the three complexes were observed. Mass spectra in the crystal structure, balancing the charge on the Bi(III)
were obtained from samples containing 0.5, 1, and 2 mol eq. ions. The presence of three coordinating (but labile) triflate
Bi(OTf)3 in an attempt to favor formation of the 13·Bi2 and ions indicates the probability of coordinating three equivalents
14·Bi2 complexes. Loss of ligand via fragmentation processes of ligand 1 around each Bi ion, forming the M2L3 system
2+
occurred in each, favoring the 12·Bi2 ion, although an observed in both the 1H NMR and ESI-MS spectra. The
increase in the amount of 14·Bi2 was observed in the sample expected geometry of the complexes is shown by molecular
with small amounts (0.5 eq.) of Bi(OTf)3.
minimization in Fig. 4c and 4d.
The combination of MS and NMR data indicates that the
The structure of the M2L4 species is less obvious, however.
binding energies of complexation are relatively weak: excess Bi Molecular minimizations suggest that self-assembly involving
was required to favor formation of the M2L2 complex. The four ligands is possible if the coordination occurs via only two
equilibrium process was analyzed to determine the relative atoms, the imine nitrogen and the hydrazide oxygen, with the
favorabilities. While the two equilibrium constants K1 (for the pyridine species uncoordinated as shown in Fig. 4d. Inte-
1
M2L4–M2L3 conversion) and K2 (for the M2L3–M2L2 conversion) gration of the H NMR spectrum shows that only one proton
are linked, the values can be calculated by analysis of the NMR (imine H2) is shifted downfield in the M2L4 assembly (as
spectra where only two species are observed (i.e. Fig. 2b and opposed to 12·Bi2 or 13·Bi2, where both H1 and H2 are shifted
2d), assuming (within error) that the concentration of any downfield more than 1 ppm). This coordination mode is not
other species is zero. The initial equilibrium constant K1 is the most favored, as might be expected, but is suitably stable
162 M−1 at ambient temperature, showing that the 14·Bi2 for observation at low concentrations of Bi(OTf)3. Bidentate
complex is indeed disfavored. The second equilibrium con- coordination via the two nitrogens of the pyridyl and imine
stant K2 is 8 M−1, indicating the similar stability of the 13·Bi2 groups is conceivable, but minimization of this possible struc-
and 12·Bi2 complexes. Due to the insolubility of 1 and its sub- ture was far less favorable due to geometrical constraints. A
sequent dissolution as Bi(OTf)3 is added, an equilibrium con- search of the Cambridge Crystallographic Database showed
stant for the initial formation of 14·Bi2 from 1 + Bi(OTf)3 could that four N–O bidentate ligands can form a BiL4 coordination
not reasonably be established by NMR.
complex, providing precedent for the proposed structure.19
X-Ray quality crystals of this complex were obtained from
The assembly process is reversible: once the 12·Bi2·(OTf)6
diffusion of chloroform into a solution of preformed complex complex is formed, addition of extra ligand 1 to the system
in acetonitrile. As the assembly process forms an equilibrium drives the equilibrium backwards (Fig. 5). Upon addition of
mixture of three products, solid state analysis was limited to small amounts of ligand 1, peaks for 13·Bi2 grew in to the spec-
that of complex 12·Bi2·(OTf)6, the most favorable assembly. trum, and a mixture of the 12·Bi2 and 13·Bi2 complexes was
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those of other metal–organic self-assemblies in CD3CN,
suggesting they exist as discrete complexes rather than
extended coordination polymers.20
The assemblies were not soluble in CDCl3 or D2O, and com-
plexation of Bi by the neutral ligand was not observed in com-
petitive solvents such as DMSO-d6. The deprotonated analog of
ligand 1 (i.e. 1·Na2) is a stronger coordinator, however, and was
accessible in quantitative yield upon treatment of 1 with base.
Unlike ligand 1, ligand 1·Na2 was able to self-assemble with Bi-
(OTf)3 in DMSO-d6 as well as CD3CN, but no appreciable
change in composition of the self-assembled products was
observed: all three distinct species were observed in the 1H
NMR spectrum upon titration with Bi(OTf)3, and the M2L2 was
favored at higher Bi concentrations, as with neutral 1 (see
Fig. 5 1H NMR titration of ligand 1 into the preformed 12·Bi2 complex: (a)
12·Bi2; (b) 1.0 eq.; (c) 1.8 eq.; (d) 2.9 eq.; (400 MHz, CD3CN).
observed. A similar pattern to the forward titration emerged,
and with increasing ligand concentration (decreased bismuth ESI†).
concentration), the 13·Bi2 complex was solely observed. The
The presence of triflate ions in the crystal structure leads to
the question of whether the self-assembly is mediated by the
pure M2L4 complex, but peaks were observed after addition of concentration of Bi(III) cations in the system alone, or whether
insolubility of the ligand prevented formation of a completely
an excess of ligand (for full spectra, see ESI†).
the triflate counterions themselves are competitive ligands for
the Bi ions. To study the assembly process in the absence of
The reversibility of the assembly in solution (and the pres-
ence of all three species in the ESI-MS) is somewhat at odds oxygen-containing counterions, Bi(BF4)3 was synthesized and
with the observation of a quasi-coordination polymer for applied to the ligand in CD3CN. In this case, only one complex
was observed, with a 1H NMR spectrum almost identical to
12·Bi2·(OTf)6 in the solid state. In solution, it is most likely
that 12·Bi2·(OTf)6 exists as a discrete species whereby no brid- that of the M2L4 species observed in Fig. 2a and b (see ESI†).
ging interactions occur between triflate ions. DOSY NMR This M2L4 aggregate was disrupted by addition of excess
NaOTf to the system, indicating that OTf− ions are indeed
(stimulated echo experiment, stebpgp1s pulse sequence,
Fig. 6) was performed to determine the relative size and mobi- good coordinators for Bi(III) and interrupt the formation of
lity of each complex. DOSY analysis of a mixture of the 13·Bi2 assemblies with greater ligand coordination. More strongly
coordinating counterions were less effective. When BiBr3 was
and 14·Bi2 complexes, gave observable, but small differences in
diffusion coefficient. Complex 14·Bi2 diffuses through CD3CN added to the neutral ligand 1 in CD3CN, no complexation
slightly faster than 13·Bi2 (D = 6.32 × 10−10 m2 s−1 for 14·Bi2, occurred. The bromide ions are far more strongly coordinating
D = 4.34 × 10−10 m2 s−1 for 13·Bi2). As expected, M2L2 complex
than triflate ions, and cannot be displaced by the neutral
12·Bi2 displayed a similar diffusion coefficient to the other ligand. Titration of BiBr3 into a DMSO-d6 solution of the more
species in CD3CN (D = 5.45 × 10−10 m2 s−1 for 12·Bi2). These strongly coordinating anionic ligand 1·Na2 (to more strongly
favor coordination) showed complex coordination at low con-
similar, albeit distinguishable, diffusion coefficients indicate
that the three assemblies are of similar size and charge, and centrations of Bi, but converged rapidly after addition of 1 mol
that the 12·Bi2·(OTf)6 is not a coordination polymer in solu- eq. BiBr3 to display a 1H NMR spectrum almost identical to
that of 12·Bi2·(OTf)6 (see ESI†). This system was not amenable
to ESI-MS analysis, but evidently the Br− ions are retained
upon complexation with 1·Na2 and cannot be displaced,
leading to formation of M2L2 complexes only.
tion. The values of the diffusion coefficients are similar to
The V-shaped ligand 1 is restricted by geometry to for-
mation of the M2L2–4 coordination described above. To access
larger polygonal assemblies, linear ligand 2 was used. Chan-
ging the ligand coordination angle removes the possibility of
forming M2Lx complexes of 2 for simple geometrical reasons:
while linear ligand 2 contains the requisite coordination motif
for Bi-mediated self-assembly, it is unable to provide suitable
coordination angles to form the observed M2Lx structures of 1.
Ligands with geometries such as this are precedented to form
either MnLn polygons,3 or larger self-assembled polyhedra.7
Ligand 2, similarly to ligand 1, displayed insolubility in
CD3CN until exposed to added bismuth triflate, which caused
dissolution. Ligand 2 was a much less effective coordinator for
Bi(OTf)3 than 1, presumably due to the inability to form
the favorable helix-type M2L3 assemblies above. Titration of
Fig. 6 (a) DOSY spectrum of 1 + 0.67 eq. Bi(OTf)3 (CD3CN, 600 MHz, 298 K,
Δ = 17.0 ms, δ = 7000 μs, Diffusion coefficient = 6.32 × 10−10 m2 s−1 for M2L4,
4.34 × 10−10 m2 s−1 for M2L3, and 3.06 × 10−9 m2 s−1 for CD3CN); (b) DOSY spec-
trum of 1 + 3 eq. Bi(OTf)3 (CD3CN, 600 MHz, 298 K, Δ = 17.0 ms, δ = 7000 μs,
diffusion coefficient = 5.45 × 10−10 m2 s−1 and 3.80 × 10−9 m2 s−1 for CD3CN).
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solution. This indicates that the charge and size properties of
23·Bi3·(OTf)9 are similar to those of 12·Bi2·(OTf)6. Experiments
to favor increased coordination at the metal and formation of
larger constructs (e.g. use of Bi(BF4)3) were unsuccessful: the
linear coordination geometry is less favorable for Bi complexa-
tion than 1, and only the M3L3 triangle is observed.
Conclusion
In conclusion, we have shown that Bi(III) ions can be used for
mild, reversible self-assembly of suitable coordinating ligands,
while exploiting the maximum number of binding sites at the
metal. By varying the coordination angle of the ligands,
different self-assembled polygons can be accessed: V-shaped
ligands lead to either M2L2, M2L3 or M2L4 complexes, with the
proportions controlled by bismuth concentration. The com-
plexes are air and water stable, and soluble in common
organic solvents.
The formation of these complexes is rapid and reversible,
with addition of either ligand or Bi salts forcing the equili-
brium to reactants or products, respectively. In contrast, linear
coordinating ligands show formation of a mixture of unidenti-
fied oligomeric aggregates until, upon treatment with excess
Bi(III), formation of a single M3L3 triangle is favored. Further
study of reversible Bi-mediated supramolecular assembly and
its application in the formation of larger polyhedral structures
and switchable self-assemblies is currently underway in our
laboratory.
Fig. 7 Formation of an M3L3 triangle. (a) 1H NMR spectrum (CD3CN, 400 MHz,
298 K) of the complex 23·Bi3·(OTf)9; (b) predicted isotope pattern of
[23·Bi3·(OTf)]2+; (c) partial ESI-MS spectrum of M3L3 complex 23·Bi3·(OTf)9; (d)
minimized structure of 23·Bi3·(OTf)9 (SPARTAN).
Bi(OTf)3 into a CD3CN suspension of 2 caused ligand dissol-
ution, but no discrete self-assembled species were observed at
low concentrations of Bi, rather a series of unidentified poly-
meric aggregates. A single species was formed only after
addition of 2 equivalents of Bi(OTf)3, and only one discrete
species was formed in this case, rather than the reversible
coordination motif displayed by 1 (see Fig. 7a; for full spectra,
see ESI†). The complexation of Bi caused the expected down-
1
field shifts in ligand peaks in the H NMR spectrum, with the
Experimental
greatest shift observed in the imine peak H2 and the ortho-
pyridyl proton H1 with peaks in the complex at δ 9.80 and
9.35 ppm respectively. ESI-MS analysis (see ESI†) of the 2x·Bix
complex was less conclusive than for ligand 1, however. Many
species were observed in the ESI spectrum, and were assigned as
multiple fragments and MeCN adducts. The prevalent M+
species was a 23·Bi3 assembly, with peaks observed for a +3
species observed at m/z = 655 (corresponding to deprotonation of
ligand and loss of TfOH), and a +2 cation (with one TfO− anion)
of the same species present at m/z = 1057. Both peaks showed
isotope patterns consistent with the presence of three Bi atoms.
Fig. 7b and 7c show the predicted and observed isotope
pattern for 23·Bi3. While we were unsuccessful in obtaining a
single crystal of suitable quality for X-ray diffraction analysis,
molecular modeling suggests a self-assembled M3L3 triangle
with similar coordination geometry at Bi as for the crystal
structure obtained for 12·Bi2·(OTf)6, with three triflate counter-
ions occupying sites at the metal (Fig. 7d). While a quasi-poly-
meric structure might be expected in the solid state for
23·Bi3·(OTf)9, such a structure would contain large holes, poss-
ibly disfavoring crystallization. DOSY NMR analysis of the
23·Bi3·(OTf)9 species showed that the measured diffusion
coefficient (D = 4.88 × 10−10 m2 s−1) was similar to that
General information
1H and 13C spectra were recorded on a Varian Inova 400 or
Varian Inova 500 spectrometer. Proton (1H) chemical shifts are
reported in parts per million (δ) with respect to tetramethyl-
silane (TMS, δ = 0), and referenced internally with respect to the
protio solvent impurity. Deuterated NMR solvents were obtained
from Cambridge Isotope Laboratories, Inc., Andover, MA, and
used without further purification. Mass spectra were recorded
on an Agilent 6210 LC TOF mass spectrometer using electro-
spray ionization and processed with an Agilent MassHunter
Operating System. X-ray intensity data were collected at 100(2) K
on a Bruker APEX2 platform-CCD X-ray diffractometer system.
All other materials were obtained from Aldrich Chemical
Company, St. Louis, MO or Combi-Blocks, San Diego, CA and
were used as received. Solvents were dried through a commer-
cial solvent purification system (Pure Process Technologies,
Inc.). Molecular modeling (semi-empirical calculations) was per-
formed using the AM1 force field using SPARTAN.21
Experimental procedures
Synthesis of compounds
1,1′:3′,1′′-Terphenyl-4,4′′-dicarbohydrazide (3). To
a round
observed for the previously analyzed
12–4·Bi2 species in
bottom flask equipped with stir bar and reflux condenser was
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added 1,3-dibromobenzene (600 mg, 2.54 mmol), 4-ethoxycarb- which was then rinsed with deionized water (200 mL), tritu-
onylphenylboronic acid (1.23 g, 6.35 mmol), bis(triphenylpho- rated in dichloromethane (50 mL), and refiltered to give 4 as
1
sphine)palladium(II) chloride (90 mg, 0.13 mmol), and cesium an off-white solid (848 mg, 76%). H NMR (400 MHz, DMSO-
carbonate (2.49 g, 7.65 mmol). The mixture was placed under d6) δ 9.91 (br, 2H); 7.94 (d, J = 8.4 Hz, 4H); 7.84 (d, J = 8.4 Hz,
nitrogen and anhydrous N,N-dimethylformamide (10 mL) was 4H); 4.83 (br, 4H); 13C NMR (100 MHz, DMSO-d6) δ 165.4;
added. The reaction was heated under nitrogen at 100 °C for 141.6; 132.5; 127.7; 126.7; HRMS (ESI) m/z calcd for
16 h. Methanol (20 mL) was added and the reaction was fil- C14H14N4O2 (M + H)+ 271.1189; found 271.1204.
tered. The filtrate was allowed to cool in an ice bath, and the
resultant precipitate was filtered. The crude product was dis-
solved in methylene chloride (5 mL) and filtered through a
celite plug topped with a thin layer of silica gel. The filtrate
was evaporated under reduced pressure to yield diethyl
1,1′:3′,1′′-terphenyl-4,4′′-dicarboxylate as a white solid (610 mg,
64%). 1H NMR (400 MHz, CDCl3) δ 8.14 (d, J = 8.4 Hz, 4H);
7.85 (t, J = 2.0 Hz, 1H); 7.71 (d, J = 8.4 Hz, 4H); 7.65 (dd, J = 1.6,
6.8 Hz, 2H); 7.57 (dd, J = 6.8, 8.4 Hz, 1H); 4.42 (q, J = 7.1 Hz,
4H); 1.43 (t, J = 7.1 Hz, 6H); 13C NMR (100 MHz, CDCl3)
δ 166.6; 145.3; 141.0; 130.3; 129.70; 129.65; 127.3; 127.2; 126.4;
61.2; 14.5; HRMS (ESI) m/z calcd for C24H23O4 (M + H)+
375.1591; found 375.1601.
This ester (254 mg, 0.68 mmol) and hydrazine (3 mL) were
then added to a round bottom flask equipped with stir bar and
reflux condenser. The reaction was stirred at 85 °C for 18 h
before water (5 mL) was added and the precipitate filtered to
yield 3 as a white solid (203 mg, 86%). 1H NMR (400 MHz,
DMSO-d6) δ 9.86 (s, 2H); 8.02 (s, 1H); 7.76 (d, J = 8.2 Hz, 4H);
7.88 (d, J = 8.3 Hz, 4H); 7.75 (d, J = 7.7 Hz, 2H); 7.60 (t, J =
7.8 Hz, 1H); 4.54 (br s, 4H); 13C NMR (100 MHz, DMSO-d6)
δ 165.6; 142.5; 140.0; 132.3; 129.8; 127.6; 126.9; 126.6; 125.5;
HRMS (ESI) m/z calcd for C20H19N4O2 (M + H)+ 347.1503;
found 347.1536.
1,1′:3′,1′′-Terphenyl-4,4′′-dicarboxylic acid, bis-(2-pyridinyl-
methylene)-hydrazide (1). To a round bottom flask equipped
with stir bar and reflux condenser was added hydrazide
3 (176 mg, 0.51 mmol), 2-pyridinecarboxaldehyde (112 μL,
1.27 mmol), ethanol (5 mL), and acetic acid (2 drops). The
reaction was refluxed for 16 h, cooled in an ice bath, and
washed with cold ethanol to yield 1 as a white solid (208 mg,
78%). 1H NMR (400 MHz, DMSO-d6) δ 12.17 (s, 2H); 8.62 (d, J =
4.8 Hz, 2H); 8.52 (s, 2H); 8.08 (m, 5H); 8.03 (d, J = 8.0 Hz, 2H);
7.98 (d, J = 7.7 Hz, 4H); 7.90 (t, J = 7.6 Hz, 2H); 7.81 (d, J =
7.6 Hz, 2H); 7.64 (t, J = 7.8 Hz, 1H); 7.43 (t, J = 6.3 Hz, 2H);
13C NMR (100 MHz, DMSO-d6) δ 163.3; 153.4; 149.7; 148.2;
143.5; 140.1; 137.1; 132.2; 130.0; 128.6; 127.2; 127.0; 125.7;
124.6; 120.2; HRMS (ESI) m/z calcd for C32H25N6O2 (M + H)+
525.2034; found 525.1983.
1,1′-Biphenyl-4,4′-dicarboxylic acid, bis-(2-pyridinylmethylene)-
hydrazide (2). To a round bottom flask equipped with stir bar
and reflux condenser was added hydrazide 4 (500 mg,
5.27 mmol), 2-pyridinecarboxaldehyde (500 μL, 5.27 mmol),
absolute ethanol (15 mL), and acetic acid (2 drops). The reac-
tion was refluxed 24 h under N2. After cooling to room temp-
erature, a white precipitate was filtered and washed with 95%
ethanol (100 mL) to give product 2 as a white solid (857 mg,
91%). 1H NMR (400 MHz, DMSO-d6) δ 12.15 (s, 2H); 8.62 (d, J =
4.3 Hz, 2H); 8.54 (s, 2H); 8.08 (d, J = 8.1 Hz, 4H); 8.00 (d, J = 7.6
Hz, 2H); 7.94 (d, J = 8.1 Hz, 4H); 7.88 (t, J = 7.4 Hz, 2H); 7.42 (t,
J = 5.4 Hz, 2H); 13C NMR (100 MHz, DMSO-d6) δ 162.9; 153.3;
149.5; 148.2; 142.3; 136.9; 132.6; 128.5; 127.0; 124.4; 119.9;
HRMS (ESI) m/z calcd for C26H20N6O2 (M + H)+ 449.1721;
found 449.1708.
Synthesis and characterization of self-assembled complexes
12Bi2·(OTf)6 complex. Ligand 1 (5.0 mg, 0.01 mmol) and Bi-
(OTf)3 (18.8 mg, 0.03 mmol) were combined in an NMR tube
with 0.5 mL CD3CN. The mixture was shaken for 10 s for quan-
1
titative formation of 12Bi2·(OTf)6. H NMR (400 MHz, CD3CN)
δ 9.77 (s, 2H); 9.34 (s, 2H); 8.49 (t, J = 7.8 Hz, 2H); 8.35 (d, J =
7.6 Hz, 2H); 8.31 (d, J = 8.0 Hz, 4H); 8.14 (s, 1H); 8.08 (m, 6H);
7.87 (d, J = 8.0 Hz, 2H); 7.70 (t, J = 7.8 Hz, 1H); 13C NMR
(100 MHz, CD3CN) δ 169.0; 154.1; 153.0; 148.4; 144.1; 140.7;
131.4; 131.1; 131.0; 129.2; 128.9; 127.5; 126.7; 122.8; 119.6.
HRMS (ESI) m/z calcd for C78H54Bi3N18O6 (M-4(TfOH)-2-
(OTf))2+ 731.1602; found 655.1290.
23Bi3·(OTf)9 complex. Ligand 2 (10.0 mg, 0.02 mmol) and Bi-
(OTf)3 (29.3 mg, 0.04 mmol) were combined in an NMR tube
with 0.5 mL CD3CN. The mixture was shaken for 10 s for quanti-
1
tative formation of 23Bi3·(OTf)9. H NMR (400 MHz, CD3CN) δ
9.79 (s, 2H), 9.37 (d, J = 5.1 Hz, 2H); 8.51 (t, J = 7.7 Hz, 2H);
8.38 (d, J = 7.6 Hz, 2H); 8.31 (d, J = 8.4 Hz, 4H); 8.11 (partially
overlapped t, J = 6.6 Hz, 2H); 8.08 (d, J = 8.4 Hz, 4H); 5.58 (br,
2H); (100 MHz, CD3CN) δ 168.9; 154.4; 153.0; 148.4; 146.7;
144.2; 131.1; 129.6; 127.9; 125.9; 121.1 (q, J = 318.6 Hz); HRMS
(ESI) m/z calcd for C78H54Bi3N18O6 (M-6(TfOH)-3(OTf))3+
655.1289; found 655.1290.
NMR titration procedure. Ligand 1 (4.4 mg, 0.008 mmol)
and 0.5 mL CD3CN were combined in an NMR tube, and a
solution of Bi(OTf)3 was added at 0.25, 0.33, 0.50, 0.67, 0.75 1,
1.5, and 2 eq. with shaking to mix at each addition.
1,1′-Biphenyl-4,4′-dicarbohydrazide (4). To a round bottom
flask equipped with stir bar was added 4,4′-biphenyl di-
carboxylic acid (1.00 g, 4.13 mmol), 2-(6-chloro-1H-benzotri-
azole-1-yl)-1,1,3,3-tetramethylaminium hexafluoro-phosphate
[HCTU] (3.44 g, 8.32 mmol), acetonitrile (40 mL), and triethyl-
amine (2.5 mL, 17.8 mmol). The reaction was stirred at room
X-ray structure determination
Crystal data. Crystals of 12·Bi2·(OTf)6 suitable for X-ray diffr-
temperature for 5 h, followed by addition of triethylamine action analysis were obtained by slow diffusion of chloroform
(2.5 mL, 17.8 mmol) and anhydrous hydrazine (300 μL, into a solution of complex in acetonitrile. A colorless fragment
9.60 mmol). The reaction was stirred at room temperature for of a prism (0.35 × 0.17 × 0.07 mm3) was used for the single
an additional 16 h, followed by filtering a white precipitate, crystal X-ray diffraction study of [C32H24N6O2]2[Bi]2[SO3CF3]6
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Dalton Trans., 2013, 42, 8394–8401 | 8399

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Dalton Transactions
(CCDC submission #892045). The crystal was coated with para- complex from a mixture of 9.91 × 10−3 mmol 1, 1.98 × 10−2
tone oil and mounted on to a cryo-loop glass fiber. X-ray inten- mmol Bi(OTf)3, and 0.620 mL DMSO were determined by inte-
sity data were collected at 100(2) K on a Bruker APEX2 grating the 1H NMR from 9.78–9.65 ppm and 9.65–9.51 ppm.
platform-CCD X-ray diffractometer system (fine focus Mo-radi-
½13 ꢀ Bi2ꢁ ¼ 3:27 ꢂ 10ꢃ3 M; ½12 ꢀ Bi2ꢁ ¼ 3:1 ꢂ 10ꢃ3 M;
ation, λ = 0.71073 Å, 50 kV/35 mA power). The CCD detector
was placed at a distance of 5.0000 cm from the crystal.
½Biꢁ ¼ 1:92 ꢂ 10ꢃ2
M
[C32H24N6O2]2[Bi]2[SO3CF3]6[partial solvents],
M
=
1391.94
2 M2L3 þ 2 M ! 3 M2L2
ˉ
[including partially occupied solvents], triclinic space group P1
(no. 2), a = 10.5697(3) Å, b = 13.7043(4) Å, c = 20.0249(6) Å,
α = 98.057(1), β = 92.112(1)°, γ = 96.452(1), V = 2849.71(14) Å3,
Z = 2, calculated density Dc = 1.622 g cm−3, V = 2849.71(14) Å3,
Z = 2, calculated density Dc = 1.622 g cm−3, colorless prism
fragment (0.35 × 0.17 × 0.07 mm3) coated with paratone oil,
T = 100(2) K, 68 323 reflections measured (0.77 Å resolution),
13 040 unique (Rint = 0.0256, completeness = 99.8%), final R1 =
3
½M2L2ꢁ
K2
¼
¼ 8 + 1 Mꢃ1
2
2
½M2L3ꢁ ½Mꢁ
Acknowledgements
0.0293, wR2 = 0.0774 with intensity I > 2σ (I) (see ESI† for full The authors would like to thank Dr Fook Tham for X-ray
experimental).
crystallographic analysis, Ron New for mass spectral analysis,
Dr Dan Borchardt for assistance with DOSY-NMR, and the
National Science Foundation (CHE-1151773) for support of
this research.
Procedure for diffusion experiments
Diffusion experiments were performed using a Bruker Avance
600 MHz NMR spectrometer equipped with a broadband
inverse probe with x-, y-, and z-gradients. Chemical shifts were
referenced to the acetonitrile-d(2) resonance (1.94) ppm.
Diffusion datasets were acquired using the stimulated echo
experiment with bipolar gradients (stebpgp1s) included with
the Topspin release version 1.3. Gradient amplitudes were
incremented as a square dependence from 5% to 95% into 64
or 96 gradient increments. A spoil gradient pulse of length
1 ms and amplitude of −17.13% was used to effectively remove
transverse magnetization following the encode period of the
pulse program. Spectra were acquired with 128 transients
coadded and 28 672 data points per transient for each of the
64 or 96 increments. Diffusion (Δ) and gradient pulse times (δ)
were optimized using a one-dimensional version of the stimu-
lated echo pulse sequence, stebpgp1s1d to give values of 17
and 7 ms, respectively. Diffusion coefficients were calculated
using the processing features in Topspin by Bruker.
Notes and references
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observed at equilibrium so contributions from 1 and 12·Bi2
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½Biꢁ ¼ 3:30 ꢂ 10ꢃ3
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8400 | Dalton Trans., 2013, 42, 8394–8401
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This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 8394–8401 | 8401