Programmed multiple complexation for the creation of helical structures from acyclic phenol-bipyridine oligomer ligands

Article abstract of DOI:10.1039/c3dt51240a

Two new multidentate ligands, H₃L¹ and H ₄L², possessing bipyridine-phenol repetitive units were designed so that the multi-metal complexation could produce a single-helical structure in a pre-programmed fashion. The ligands were synthesized by successive palladium-catalyzed coupling reactions. The complexation of H ₃L¹ with zinc(ii) and nickel(ii) acetate afforded [L ¹Zn₂(OAc)] and [(L¹)₂Ni ₄](OAc)₂, respectively. Each of the ligand moieties in these complexes formed a one-turn single helix. The zinc(ii) complex [L ¹Zn₂(OAc)] underwent a helix compression-extension motion in solution. The complexation of the H₃L¹ ligand with iron(iii) chloride gave a dinuclear complex [(HL¹)₂Fe ₂Cl₂] with a non-helical dimeric structure. The longer ligand H₄L² afforded a trinuclear complex [L ²Zn₃(OAc)₂] with a 1.5-turn single-helical structure upon complexation with zinc(ii) acetate. The reaction of the H ₄L² ligand with cobalt(ii) acetate under aerobic conditions gave a mixed valence complex [L²Co₃(OAc) ₃(OMe)], which had two trivalent and one divalent cobalt ions. The structural features of the trinuclear complexes significantly depended on the metals; [L²Co₃(OAc)₃(OMe)] had a helical pitch of 7.6 A, which was almost twice that of [L²Zn ₃(OAc)₂] (4.0 A).

Full text of DOI:10.1039/c3dt51240a

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Programmed multiple complexation for the creation
of helical structures from acyclic phenol–bipyridine
oligomer ligands†
Cite this: DOI: 10.1039/c3dt51240a
Shigehisa Akine,*a,b Hiroki Nagumoa and Tatsuya Nabeshima*a,b
Two new multidentate ligands, H3L1 and H4L2, possessing bipyridine–phenol repetitive units were
designed so that the multi-metal complexation could produce a single-helical structure in a pre-
programmed fashion. The ligands were synthesized by successive palladium-catalyzed coupling reactions.
The complexation of H3L1 with zinc(II) and nickel(II) acetate afforded [L1Zn2(OAc)] and [(L1)2Ni4](OAc)2,
respectively. Each of the ligand moieties in these complexes formed a one-turn single helix. The zinc(II)
complex [L1Zn2(OAc)] underwent a helix compression–extension motion in solution. The complexation of
the H3L1 ligand with iron(III) chloride gave a dinuclear complex [(HL1)2Fe2Cl2] with a non-helical dimeric
structure. The longer ligand H4L2 afforded a trinuclear complex [L2Zn3(OAc)2] with a 1.5-turn single-
helical structure upon complexation with zinc(II) acetate. The reaction of the H4L2 ligand with cobalt(II)
acetate under aerobic conditions gave a mixed valence complex [L2Co3(OAc)3(OMe)], which had two
trivalent and one divalent cobalt ions. The structural features of the trinuclear complexes significantly
depended on the metals; [L2Co3(OAc)3(OMe)] had a helical pitch of 7.6 Å, which was almost twice that
of [L2Zn3(OAc)2] (4.0 Å).
Received 11th May 2013,
Accepted 21st June 2013
DOI: 10.1039/c3dt51240a
www.rsc.org/dalton
potential applications based on metal-derived functions such
as redox, magnetic, and photophysical properties or reacti-
Introduction
Metal coordination is a useful tool for making complex struc- vities such as catalysis.
tures from simple organic molecules. For example, the metal-
To obtain metal-assisted folded structures with a well-orga-
assisted assembly of organic molecules has now been one of nized conformation, the coordination of chelate ligands to
the most powerful methods for obtaining self-assembled struc- metal ions is particularly useful. When a chelate ligand forms
tures, such as double helicates,1 cage-like compounds2 etc. a metal complex in a multidentate fashion, two or more
Metal coordination can also be used to obtain a cyclic struc- coordination bonds are simultaneously formed. This complex
ture3 or a folded structure4,5 from a linear oligomeric molecule formation significantly reduces the flexibility of the chelate
without using the self-assembly of organic molecules. In this ligand. The conformational restriction caused by metal coordi-
case, metal coordination makes the flexible molecule adopt a nation is effective in allowing an organic molecule adopt a
well-defined folded conformation in a programmed fashion. It defined structure. If two or more chelate coordination sites are
is well known that the folding of polypeptides into a well- incorporated into an organic molecule in a pre-programmed
defined structure is crucial for the biofunctions of proteins, fashion, more complicated structures can be built simply by
and recently, various kinds of biomimetic and abiotic mole- complexation with multiple metal ions (Scheme 1). Such a
cules have been developed for spontaneous folding into a well-
defined structure.6 Among them, metal-containing abiotic fol-
damers have attracted much attention because of their
aFaculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai,
Tsukuba, Ibaraki 305-8571, Japan. E-mail: akine@chem.tsukuba.ac.jp,
nabesima@chem.tsukuba.ac.jp; Fax: +81 29-853-4507; Tel: +81 29-853-4507
bTsukuba Research Center for Interdisciplinary Materials Science, University of
Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan
†CCDC 938299–938303. For crystallographic data in CIF or other electronic
Scheme 1 Programmed formation of a helical structure from a chain-like
format see DOI: 10.1039/c3dt51240a
ligand with multiple chelate coordinating moieties.
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Scheme 2 Formation of helical complexes upon metal coordination of bipyri-
dine–phenol oligomer ligands, H3L1 and H4L2.
structural change is considered to be a pre-programmed trans-
formation, because we can recall the information recorded in
the organic molecule with the aid of metal addition. To date,
multi-metal complexation has been used for producing
various types of structures, such as a helix,7–9 and their func-
tion switching associated with the structural transformation
has been studied.
Scheme 3 Synthesis of H3L1. Reagents and conditions: (a) [Pd(PPh3)4], Na2CO3,
toluene–water–methanol, (b) (i) n-BuLi, THF, (ii) B(OMe)3, (iii) dil. HCl, (iv)
pinacol, toluene, (c) HBr, acetic acid.
In this study, we focused on a bipyridine–phenol repetitive
structure as a pre-programmed unit for forming a helical struc-
ture (Scheme 2).10,11 The constituent, 2,2′-(2,2′-bipyridine-6,6′-
diyl)diphenol,12,13 is known to be an N2O2-tetradentate ligand
that forms stable metal complexes with various kinds of metal
ions. The 2,2′-(2,2′-bipyridine-6,6′-diyl)diphenol units are con-
nected in such a way that the neighboring two units share one
phenol unit. This phenol unit could connect two metal ions
through μ-phenoxo bridging, as seen in most of the dinuclear
complexes of compartmental ligands.14 This coordinating
behavior could also contribute to the defined arrangement of
metal ions and the consequent programmed arrangement of
the organic molecules. In this paper, we describe the synthesis
and coordination behavior of bis- and tris(bipyridine) ligands,
H3L1 and H4L2 (Scheme 2). The multi-metal complexation of
these ligands with appropriate metal ions afforded a helical
structure in a pre-programmed fashion. The structural vari-
ation and dynamic behavior of the helical metal complexes
were studied.
Results and discussion
Ligand synthesis
The ligand framework of H3L1 and H4L2 was constructed by
successive palladium-catalyzed coupling reactions from a
dibromobipyridine derivative 2. The synthetic scheme for H3L1
is shown in Scheme 3. As the terminal unit of H3L1, the
anisole-linked bipyridine derivative 3 was synthesized by the
reaction of boronic acid 115 with dibromobipyridine 216 in
73% yield. The bis(bipyridine) derivative 6 was obtained in
67% yield by the Suzuki coupling reaction of 3 with the
Scheme 4 Synthesis of H4L2. Reagents and conditions: (a) [Pd(PPh3)4], Na2CO3,
toluene–water, (b) (i) n-BuLi, THF, (ii) n-Bu3SnCl, (c) Br2, dichloromethane,
(d) [Pd(PPh3)4], toluene, (e) HBr, acetic acid.
diboronate ester 5, which was prepared from the dibromide
4.17 The demethylation of 6 with hydrogen bromide in acetic
acid afforded the H3L1 ligand in 83% yield.
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For the synthesis of the H4L2 ligand (Scheme 4), the build- complex, having a 1 : 1 ligand–metal stoichiometry, was iso-
ing blocks 810a and 9 were prepared by Suzuki coupling of lated as a dinuclear complex [(HL1)2Fe2Cl2].
dibromobipyridine 2 and boronic acid 7.18 The mono-coupled
product 8 was converted into the stannane 10 by lithiation
followed by reaction with tributylchlorostannane. The bis-
coupled product 9 was converted into the dibromide 11 in
76% yield. The tris(bipyridine) derivative 12 was obtained by
Stille coupling reaction of the dibromide 11 with 2 equiv. of
the stannane 10 in 30% yield. The demethylation of 12 with
hydrogen bromide in acetic acid afforded the H4L2 ligand in
73% yield.
Crystal structure of metal complexes
The structure of the zinc(II) complex [L1Zn2(OAc)] was deter-
mined using X-ray crystallography (Fig. 1). As we expected, two
zinc(II) ions (Zn1, Zn2) sit in the 2,2′-(2,2′-bipyridine-6,6′-diyl)-
diphenol N2O2 coordination sites (N1–N2–O1–O2 and N3–N4–
O2–O3, respectively) and the central phenoxo group (O2)
bridged the two zinc(II) ions. One of the two terminal phenoxo
groups (O3) also bridged the two zinc(II) ions, forming a four-
membered ring (Zn1–O2–Zn2–O3). In addition, a monodentate
acetato ligand (O4) coordinated to one of the zinc(II) ions
(Zn2). As a result, the two zinc(II) ions had a pentacoordinate
environment, which was well described as an intermediate
geometry between a trigonal bipyramidal and a square pyrami-
dal geometry (Zn1, τ = 0.460; Zn2, τ = 0.218).19 The organic
ligand moiety (L1)3− formed a single-helical structure in which
the two terminal phenol rings stack on top of each other. The
pitch of the helix, which is defined as the centroid–centroid
distance between the two terminal phenol units, was around
3.7 Å (Table 1), indicating that there is no space to incorporate
a molecule or an ion in the helix groove. The helix winding
angle was 388°, which corresponds to a one-turn helix.
Complexation study of the bis(bipyridine) ligand, H3L1
Since the H3L1 ligand has two N2O2 binding pockets, H3L1 is
expected to accommodate 2 equiv. of metal ions. If each of the
N2O2 site coordinates to the metal ion in a tetradentate
fashion, a helical structure can be formed due to the newly
formed coordination bonds.
The formation of the metal complexes was clearly evidenced
by mass spectrometry. The mass spectrum of H3L1 in the pres-
ence of 2 equiv. of zinc(II) acetate showed a peak at m/z = 757.1
for [L1Zn2]+, indicative of the formation of a dinuclear species.
Similarly, the formation of a dinuclear nickel(II) species
was confirmed by the mass peak at m/z = 741.1 ([L1Ni2]+). In
contrast, a mononuclear species (m/z = 682.2 for [(HL1)Fe]+)
was formed upon reaction of H3L1 with 2 equiv. of iron(III)
chloride.
The zinc(II) and nickel(II) complexes, having a 1 : 2 ligand–
metal stoichiometry, were isolated from the reaction mixture
of H3L1 with 2 equiv. of M(OAc)2 (M = Zn, Ni) in 92 and 87%
yields, respectively (Scheme 5). These complexes had the for-
mulas of [L1Zn2(OAc)] and [(L1)2Ni4](OAc)2, which were deter-
mined using X-ray crystallography (see below). The iron(III)
Fig. 1 Crystal structure of [L1Zn2(OAc)] with thermal ellipsoids drawn at the
50% probability level. Hydrogen atoms are omitted for clarity.
Table 1 Structural parameters of helical metal complexes
Complex
Pitcha (Å)
Winding angleb,c (°)
[L1Zn2(OAc)]
[(L1)2Ni4](OAc)2
[L2Zn3(OAc)2]
3.678
3.700, 3.755
4.037
388
360, 359
579
d
[L2Co3(OAc)3(OMe)]
7.582
628
a The helical pitches were defined as the average of centroid–centroid
distances between ith and (i + 6)th benzene (or pyridine) rings. b The
helix winding angles of [L1M2] complexes were defined as the sum of
the six An–Mmid–An+1 angles, where points An (n = 1, 2, …, 7) are the
centroids of the aromatic rings (phenol and pyridine) of ligand (L1)3−
and point Mmid is the midpoint of the two metal ions, M1 and M2.
c The helix winding angles of [L2M3] complexes were defined as the
torsion angle A1–M1–M3–A10, where points A1 and A10 are the centroids
of the terminal benzene rings and points M1 and M3 are the terminal
metal ions. d Values for two helical units are shown.
Scheme 5 Synthesis of metal complexes of H3L1.
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Fig. 2 Crystal structure of [(L1)2Ni4)]2+ with thermal ellipsoids drawn at the
20% probability level. Hydrogen atoms are omitted for clarity.
The X-ray crystallographic analysis revealed that the nickel(II)
complex had
a
dimeric tetranuclear structure [(L1)2Ni4]-
Fig. 3 Crystal structure of [(HL1)2Fe2Cl2] with thermal ellipsoids drawn at the
(OAc)2 (Fig. 2). The terminal nickel(II) ions (Ni1, Ni4) adopted a
square planar geometry, while the other two (Ni2, Ni3)
adopted an octahedral geometry. In each monomer unit
[L1Ni2]+, the two phenoxo groups bridged the two nickel(II)
ions (O1 and O2 for Ni1–Ni2; O5 and O6 for Ni3–Ni4), forming
a Ni–O–Ni–O four-membered ring. The phenoxo group on the
other side (O3, O4) further coordinates to the counterpart
[L1Ni2]+ unit, resulting in a dimer with a doubly phenoxo
bridged structure (Ni2–O3–Ni3–O4). In each [L1Ni2]+ unit, the
organic ligand moiety adopted a single-helical structure as
seen in the zinc(II) complex, but the [L1Ni2]+ had a more
loosely wound structure (a helix winding angle of about 360°,
Table 1) than [L1Zn2(OAc)] (388°). The tetranuclear complex
was a meso-type dimer complex, in which the helical handed-
ness of the two helical units [L1Ni2]+ was opposite to each
other.
30% probability level. Hydrogen atoms are omitted for clarity.
The iron(III) complex, having a 1 : 1 stoichiometry, turned
out to be a dimeric complex [(HL1)2Fe2Cl2] (Fig. 3) in which
two mononuclear [(HL1)FeCl] units were connected to each
other by a double phenoxo bridging (Fe1–O1–Fe1*–O1*). The
complex had a crystallographically imposed inversion center at
the midpoint of Fe1 and Fe1*. In each monomer unit, an iron(III)
ion (Fe1) occupied one of the two N2O2 sites (N1, N2, O1,
O2) while the other was vacant. A chloride ion completed the
octahedral geometry of the iron(III) ion (Fe1). Thus, the ligand
H3L1 did not form a helical structure upon complexation with
iron(III) chloride.
Scheme 6 Complexation of H3L1 with two equiv. of metal ions.
such an influence is significant, the first metalation at one site
could promote or suppress the metalation at the second site
during the complexation process. If the second metalation
becomes more favorable, the complexation can be described
as (positively) cooperative (Scheme 6).3,20
The cooperativity of the two-step complexation can be easily
discussed by analyzing the mole fractions of the mononuclear
and the dinuclear species upon complexation with 1 equiv. of
Complexation study using NMR
As described above, the crystal structural analysis revealed that metal ions. If the complexation process is highly cooperative,
the central phenol group of H3L1, shared by the two N2O2 0.5 mole of the dinuclear species and the uncomplexed ligand
coordination sites, acted as a bridging ligand in the zinc(II) will be observed as the dominant species, while the formation
and nickel(II) complexes. Due to the phenoxo bridging, the of the mononuclear species will be suppressed.
1
organic framework (L1)3− adopted a helical structure. At the
Fig. 4 shows the H NMR spectrum of H3L1 in the presence
same time, the two metal ions are fixed in close proximity, of 1 equiv. of zinc(II) ions. Although the spectrum showed
which would make the two metal ions affect each other. If broad peaks at room temperature, the spectrum became well-
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can be assigned to the dinuclear complex [L1Ni2]+ having para-
magnetic nickel(II) ions. When 1 equiv. of nickel(II) ions was
present in the solution, the spectrum showed another set of
small sharp signals, which can be assigned to the diamagnetic
mononuclear complex [(HL1)Ni]. The mole fraction of
[(HL1)Ni] was only 8% even in the presence of 1 equiv. of
nickel(II) acetate; that is, most of the added nickel(II) ions was
consumed to make the dinuclear species (Fig. 5). Conse-
quently, the complexation with nickel(II) ions proceeded in a
highly cooperative fashion.
The observed positive cooperative effects can be explained
by the following two factors. When the 2,2′-(2,2′-bipyridine-
6,6′-diyl)diphenol type N2O2 coordination site coordinates to a
metal ion in a tetradentate fashion, the two phenol groups
are converted into the phenoxo groups having a negative
charge.8,9,21 This could significantly promote the second com-
plexation with a positively charged metal ion. In addition, the
two phenoxo groups at the first site are fixed in such a way
that chelate coordination to another metal ion is favorable.
Therefore, this structural change by the first metalation
should also promote the second metalation. This cooperative
feature contributed to the easy and programmed formation of
the helical structures simply by mixing H3L1 with the appropri-
ate metal sources.
Fig. 4 Part of the 1H NMR spectrum of H3L1 (600 MHz, CDCl3–CD3OD, 4 : 1,
213 K) in the presence of 1 equiv. of zinc(II) acetate. The assignment of the
methyl and aromatic protons of the terminal phenol moieties is shown.
resolved at low temperatures. The spectrum showed a new set
of signals assignable to [L1Zn2(OAc)], in addition to the
uncomplexed H3L1. The ratio of the two sets of signals was
approximately 1 : 1. Noteworthy is that the signals assignable
to the mononuclear species were negligible. This indicates
that the complexation of H3L1 with zinc(II) was highly coopera-
tive; that is, the second metalation took place more easily than
the first metalation.
Similarly, the cooperativity during the complexation with
nickel(II) was studied by a 1H NMR titration experiment at
room temperature (Fig. 5). Upon the addition of 2 equiv. of
nickel(II) ions, significantly broad signals appeared and the
initial signals of H3L1 completely disappeared. Since the new
signals showed significant paramagnetic shifts, the signals
It is noteworthy that the zinc(II) complex showed signifi-
1
cantly broad signals in the H NMR spectra at room tempera-
ture (Fig. 6), which suggests a structural interconversion in
solution. The spectrum at 333 K showed relatively well-resolved
signals. This spectrum well accounts for the structure of
[L1Zn2(OAc)], in which relocation of the acetato ligands from
one zinc(II) ion to the other occurs fast on the NMR timescale.
Among the ten aromatic signals, three signals of the terminal
phenol rings were observed at high field (5.69, 6.42, and
6.86 ppm). In addition, the aromatic methyl group of the
Fig. 5 (a) 1H NMR spectral changes of H3L1 (400 MHz, CDCl3–CD3OD, 4 : 1)
upon the addition of nickel(II) acetate. (b) Mole fraction of H3L1, [(HL1)Ni], and
Fig. 6 1H NMR spectra of [L1Zn2(OAc)] at varying temperatures (600 MHz,
CDCl3–CD3OD, 1 : 1, 2 mM).
[L1Ni2]+ plotted versus equivalents of Ni2+
.
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Scheme 7 Proposed equilibrium between the low- and high-temperature
forms of [L1Zn2]+.
terminal phenol rings also appeared at
a higher field
(2.07 ppm) than that of the central one (2.45 ppm). These
observations suggest that the [L1Zn2(OAc)] adopts a structure
similar to that in the crystalline state in which the two termi-
nal phenol rings are closely stacked on top of each other.
Initially, we expected that the broadening of the 1H NMR
signals was attributed to the acetato ligand relocation and that
an unsymmetrical spectral pattern would be observed at low
temperatures. However, this was negated by the spectra at low
Scheme 8 Synthesis of trinuclear complexes from the tris(N2O2) ligand H4L2.
temperatures (253–233 K), again showing
a symmetrical
3 equiv. of metal ions is expected to give a more wound helical
structure having an approximately 1.5 turn. The zinc(II) trinuc-
lear complex [L2Zn3(OAc)2] was obtained by the reaction of the
H4L2 ligand with zinc(II) acetate in 90% yield (Scheme 8). The
formation of the trinuclear species was clearly demonstrated
by the mass peaks at m/z = 624.2 (for [L2Zn3]2+) and 1307.5 (for
[L2Zn3(OAc)]+). Similarly, the reaction of H4L2 with cobalt(II)
acetate in methanol–chloroform under aerobic conditions gave
a trinuclear complex (Scheme 8). The mass spectrum of this
complex showed peaks at m/z = 645.8 and 1322.5 which can be
assigned to [L2Co3(OMe)2]2+ and [L2Co3(OMe)3]+, respectively.
The charge balance indicated that this complex was a mixed-
valence complex containing two trivalent and one divalent
cobalt ions. The cobalt(II) ions were oxidized in part during
the preparation and isolation procedures. The X-ray crystallo-
graphy (see below) revealed that the complex had a compo-
sition of [L2Co3(OAc)3(OMe)], which further confirmed the
CoIII2CoII oxidation states.
pattern similar to those at high temperatures. It should also be
noted that the chemical shift of the acetato ligand (1.98 ppm)
exactly coincided with that of n-Bu4N(OAc), strongly indicating
that the acetato ligand did not coordinate to the metal centers
any more at low temperatures. Based on these observations, we
can suggest that [L1Zn2(OAc)] dissociates to give [L1Zn2Xn]+
and AcO− at low temperatures where X denotes solvent mole-
cules such as water (Scheme 7). This was also supported by the
fact that the mole fraction of the high-temperature form
[L1Zn2(OAc)] increased upon increasing the zinc(II) complex
concentration or upon the addition of the acetate ion.
In the 1H NMR spectrum of the low-temperature form
[L1Zn2Xn](OAc), the aromatic and methyl protons of the termi-
nal phenol rings appeared at lower field, indicating decreased
shielding effects. This strongly suggests that the two terminal
phenol rings became separated from each other at low temp-
eratures. At present, we assume that the low-temperature form
has two water molecules coordinating to zinc(II) ions at the
positions between the two terminal phenol rings. This means
that the [L1Zn2(OAc)] behaves as a molecular spring,11a,22
which compresses at high temperatures and extends at low
temperatures.
Crystal structure of the trinuclear complexes
The structure of the trinuclear zinc(II) complex [L2Zn3(OAc)2]
was determined using X-ray crystallography (Fig. 7). As we
expected, three zinc(II) ions (Zn1, Zn2, Zn3) are located in the
2,2′-(2,2′-bipyridine-6,6′-diyl)diphenol N2O2 coordination sites
(N1–N2–O1–O2, N3–N4–O2–O3, and N5–N6–O3–O4, respecti-
vely). The inner phenoxo groups (O2, O3) bridged the three
zinc ions forming a phenoxo-bridged trinuclear core. One of
the two terminal phenoxo group (O4) also bridged the two zinc
ions (Zn2–Zn3), resulting in a four-membered ring (Zn2–O3–
Zn3–O4). This bridging coordination by O4 completed the penta-
coordinate environment of the central zinc(II) ion (Zn2, τ =
The thermodynamic parameters for the conversion from
the low-temperature form to the high-temperature form were
determined to be ΔH = 54 kJ mol−1 and ΔS = 2.6 × 102 J K−1
mol−1 by using the van’t Hoff plots. The significantly large ΔS
can be well explained by the release of water molecules at high
temperatures, which also confirms the equilibrium shown in
Scheme 7.
Complexation of tris(bipyridine) ligands
The H4L2 ligand has three N2O2 binding sites that can simi- 0.516), which can be well described as an intermediate geome-
larly incorporate metal ions. The complexation of H4L2 with try between a square pyramidal and a trigonal bipyramidal
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geometry. In contrast, the two terminal zinc ions (Zn1 and respectively) and were bridged by the inner phenoxo groups
Zn3) adopted an almost ideal square pyramidal geometry (Zn1, (O2, O3). However, the terminal phenoxo group (O1, O4) no
τ = 0.080; Zn3, τ = 0.083), and the apical positions of Zn1 and longer participated in the bridging coordination. Instead, an
Zn3 were occupied by a monodentate acetato ligand (O5, O7). acetato ligand (O7–O8) and a methoxo ligand (O11) bridged
The helix winding angle was 579° (Table 1), which was about Co1–Co2 and Co2–Co3, respectively. The terminal Co1 and
1.5 times greater than that of [L1Zn2(OAc)]. In this helical Co3 had an additional monodentate acetato ligand. As a
structure, the four terminal aromatic rings (phenol–bipyri- result, all the three cobalt atoms had a hexacoordinate coordi-
dine–phenol) were stacked on top of those of the other side. nation environment. The Co2 was significantly distorted from
The helical pitch is around 4.0 Å (Table 1), which is not very the ideal octahedral geometry, while the other two (Co1 and
different from the value of the shorter analog [L1Zn2(OAc)].
Co3) adopted an almost ideal octahedral geometry. To assign
Single crystals of the cobalt mixed-valence trinuclear the oxidation state of the cobalt centers, bond valence sum
complex [L2CoIII2CoII(OAc)3(OMe)] suitable for X-ray crystallo- (BVS) analysis23 was carried out. The BVS value of 2.08 for the
graphy (Fig. 8) were obtained by vapor-phase diffusion of central Co2 clearly indicated that Co2 was divalent. The other
diethyl ether into a methanol solution of the complex. As two (Co1 and Co3), having much higher values (3.79 for Co1,
observed in the corresponding zinc(II) complex, three cobalt 3.94 for Co3),24 can be assigned to a trivalent form.
ions (Co1, Co2, Co3) occupied the three N2O2 coordination
As seen in the zinc(II) trinuclear complex, the (L2)4− ligand
sites (N1–N2–O1–O2, N3–N4–O2–O3, and N5–N6–O3–O4, in the [L2Co3(OAc)3(OMe)] formed a single helical structure.
The helix winding angle is 628° and the helical pitch is about
7.6 Å (Table 1). This significantly long helical pitch can be
explained by the fact that the bridging acetato and methoxo
ligands coordinated to the metal centers in an intercalating
fashion in the groove of the helical structure. Thus, the struc-
tural features of the [L2M3] complexes significantly depended
on the metal ions. The complexation with cobalt ions gave an
extended helical structure, while the complexation with zinc(II)
ions gave a compressed one.
Conclusions
The multidentate ligands, H3L1 and H4L2, having bipyridine–
phenol repetitive units were designed so that the multi-metal
Fig. 7 Crystal structure of [L2Zn3(OAc)2] with thermal ellipsoids drawn at the
complexation could produce a single-helical structure in a pre-
programmed fashion. The bis(N2O2) ligand, H3L1, was effective
20% probability level. Hydrogen atoms and less occupied disordered atoms are
omitted for clarity.
for obtaining a dinuclear complex having a one-turn single
helical structure; the helical zinc(II) and nickel(II) complexes
were isolated as [L1Zn2(OAc)] and [(L1)2Ni4](OAc)2, respectively.
Similarly, from the tris(N2O2) ligand H4L2, trinuclear com-
plexes [L2Zn3(OAc)2] and [L2Co3(OAc)3(OMe)] having a 1.5-turn
single-helical structure were obtained. These helical structures
were simply formed upon mixing the ligands with metal ions,
as if the ligand recalled the structural information pre-
installed in the ligand molecule. A spectroscopic investigation
of the synthesized complexes indicated that [L1Zn2(OAc)]
underwent a compression–extension motion in solution. Such
helical structures that change their structure upon stimulation
are now attracting much attention because of their potential
use as switching functional molecules in which the catalytic
activity, photochemical properties etc. change upon structural
conversion.
Experimental
General
Fig. 8 Crystal structure of [L2Co3(OAc)3(OMe)] with thermal ellipsoids drawn at
All experiments were carried out under an argon atmosphere
unless otherwise noted. For the organic syntheses, dehydrated
the 30% probability level. Hydrogen atoms are omitted for clarity.
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toluene, diethyl ether and tetrahydrofuran (THF) were combined organic layer was washed with brine, dried over
purchased and used without further purification. Dichloro- anhydrous magnesium sulfate, filtered, and concentrated to
methane and N,N,N’,N’-tetramethylethylenediamine were dis- dryness. The residue was subjected to column chromatography
tilled under an argon atmosphere from calcium hydride prior (SiO2, chloroform–hexane, 2 : 1) to afford
3 (234 mg,
1
to use. Saturated aqueous solution of sodium carbonate was 0.658 mmol, 73%) as colorless crystals, mp 83–84 °C; H NMR
degassed prior to use. Commercial methanol and chloroform (400 MHz, CDCl3) δ 2.38 (s, 3H), 3.82 (s, 3H), 6.90 (d, J =
were used without purification for the synthesis of metal com- 8.3 Hz, 1H), 7.17 (dd, J = 8.3, 2.3 Hz, 1H), 7.44 (d, J = 7.7 Hz,
plexes. All chemicals were of reagent grade and used as 1H), 7.62 (t, J = 7.7 Hz, 1H), 7.74 (d, J = 2.3 Hz, 1H), 7.78 (t, J =
received. Column chromatography was performed with Kanto 7.8 Hz, 1H), 7.89 (d, J = 7.8 Hz, 1H), 8.31 (d, J = 7.8 Hz, 1H),
Chemical silica gel 60 N (spherical, neutral). Preparative re- 8.52 (d, J = 7.7 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 20.58
cycling gel permeation chromatography (GPC) was performed (CH3), 55.76 (CH3), 111.62 (CH), 119.24 (CH), 119.88 (CH),
with a JAI LC-9201 equipped with JAIGEL 1H + 2H columns 125.78 (CH), 127.70 (CH), 128.51 (C), 130.17 (C), 130.47 (CH),
using chloroform as an eluent. Melting points were deter- 131.73 (CH), 136.53 (CH), 139.09 (CH), 141.40 (C), 153.86 (C),
mined on a Yanaco melting point apparatus MP-J13 and 155.18 (C), 155.31 (C), 157.74 (C); Anal. Calcd for
not corrected. NMR spectra were recorded on a Bruker AC C18H15BrN2O: C, 60.86; H, 4.26; N, 7.89. Found: C, 61.10; H,
300 (1H, 300 MHz), an Avance400 (1H, 400 MHz; 13C, 100 MHz) 4.32; N, 7.85.
or
spectrometer. Mass spectra were recorded on an AB Sciex (4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (5). Under an argon
QStar Pulsar i (ESI-TOF; positive mode) spectrometer or an atmosphere, n-butyllithium (2.64 in hexane, 8.4 mL,
a
Bruker Avance600 (1H, 600 MHz; 13C, 150 MHz)
Synthesis of 2,2′-(2-methoxy-5-methyl-1,3-phenylene)bis-
M
AB Sciex TOF/TOF 5800 (MALDI-TOF; positive mode) spectro- 22 mmol) was added to a solution of dibromide 4 (2.79 g,
meter. Elemental analyses were performed on a Yanaco 9.97 mmol) in THF (75 mL) at −78 °C. After stirring the solu-
CHN-corder MT-6.
tion for 10 min, trimethyl borate (3.4 mL, 31 mmol) was added
and the mixture was stirred for 30 min at −78 °C and then
allowed to warm to 0 °C over a period of 1 h. The mixture was
Materials
6,6′-Dibromo-2,2′-bipyridine (2),16 1,3-dibromo-2-methoxy-5- acidified with hydrochloric acid (2 M, 40 mL) and stirred for
methylbenzene (4),17 and 5-tert-butyl-2-methoxyphenylboronic 1 h at rt. After the addition of diethyl ether, the organic layer
acid (7)18 were prepared according to the literature.
was separated and the aqueous layer was extracted with diethyl
Synthesis of 2-methoxy-5-methylphenylboronic acid (1). ether. The combined organic layer was concentrated to
Under an argon atmosphere, n-butyllithium (2.64 M in hexane, dryness to yield crude diboronic acid. This was mixed with
32 mL, 84 mmol) and N,N,N’,N’-tetramethylethylenediamine pinacol (2.37 g, 20.1 mmol) and dissolved in toluene (100 mL).
(14 mL, 93 mmol) were added to a solution of 1-methoxy-4- The solution was heated under reflux for 3 h using a Dean-
methylbenzene (8.57 g, 70.2 mmol) in diethyl ether (150 mL) Stark trap to remove water. The solvent was evaporated under
and the resulting solution was stirred overnight at room temp- reduced pressure and the residue was subjected to column
erature. Trimethyl borate (7.8 mL, 70 mmol) was added to this chromatography (SiO2, ethyl acetate–hexane, 2 : 5) to yield
mixture at 0 °C, and the mixture was stirred for 3 h at room diboronate ester 5 (1.37 g, 3.67 mmol, 37%) as colorless crys-
temperature. After acidification (pH = 3) with hydrochloric tals, mp 132–136 °C; 1H NMR (400 MHz, CDCl3) δ 1.34 (s,
acid (4 M), the organic layer was separated and the aqueous 24H), 2.29 (s, 3H), 3.80 (s, 3H), 7.58 (s, 2H); 13C NMR
layer was extracted with diethyl ether. The combined organic (100 MHz, CDCl3) δ 20.35 (CH3), 24.75 (CH3), 63.70 (CH3),
layer was washed with brine, dried over anhydrous magnesium 83.32 (C), 122 (br, C), 132.00 (C), 139.87 (CH), 169.04 (C); Anal.
sulfate, filtered, and concentrated to dryness. The crude Calcd for C20H32B2O5: C, 64.21; H, 8.62. Found: C, 64.06; H,
product was recrystallized from hexane to afford boronic acid 1 8.56.
(5.50 g, 33.2 mmol, 47%) as pale brown crystals, mp 90–93 °C
Synthesis of 6′,6′′-(2-methoxy-5-methyl-1,3-phenylene)bis-
(lit15 90–91 °C); H NMR (600 MHz, CDCl3) δ 2.30 (s, 3H), 3.87 (6-(2-methoxy-5-methylphenyl)-2,2′-bipyridine) (6). Under an
(s, 3H), 6.71 (br s, 2H), 6.80 (d, J = 8.6 Hz, 1H), 7.23 (dd, J = argon atmosphere, bromide 3 (2.41 g, 6.78 mmol), diboronate
8.6, 2.2 Hz, 1H), 7.67 (d, J = 2.2 Hz, 1H); 13C NMR (150 MHz, ester 5 (1.28 g, 3.42 mmol), and tetrakis(triphenylphosphine)-
CDCl3) δ 20.29 (CH3), 55.51 (CH3), 109.86 (CH), 118 (br, C), palladium(0) (194 mg, 0.168 mmol) were dissolved in a
1
130.25 (C), 133.14 (CH), 137.23 (CH), 162.54 (C).
mixture of toluene (40 mL), methanol (5 mL), and saturated
Synthesis of 6-bromo-6′-(2-methoxy-5-methylphenyl)-2,2′- aqueous solution of sodium carbonate (10 mL). This mixture
bipyridine (3). Under an argon atmosphere, a mixture of was stirred at 80 °C for 24 h. After the reaction completed, the
boronic acid 1 (150 mg, 0.906 mmol), dibromide 2 (316 mg, organic layer was separated and the aqueous layer was
1.01 mmol), and tetrakis(triphenylphosphine)palladium(0) extracted with chloroform. The combined organic layer was
(59.2 mg, 0.0512 mmol) was dissolved in a mixture of toluene dried over anhydrous magnesium sulfate, filtered, and concen-
(10 mL), methanol (1 mL), and saturated aqueous solution of trated to dryness. The residue was subjected to column chrom-
sodium carbonate (2 mL). The mixture was stirred at 80 °C for atography (SiO2, chloroform–ethanol, 99 : 1) to afford 6 (1.52 g,
48 h. After the reaction completed, the organic layer was separ- 2.27 mmol, 67%) as colorless crystals, mp 194–196 °C;
ated and the aqueous layer was extracted with chloroform. The 1H NMR (400 MHz, CDCl3) δ 2.40 (s, 6H), 2.52 (s, 3H), 3.43
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(s, 3H), 3.83 (s, 6H), 6.91 (d, J = 8.4 Hz, 2H), 7.18 (dd, J = 8.4,
Synthesis of [(HL1)2Fe2Cl2]. A solution of H3L1 (8.1 mg,
2.3 Hz, 2H), 7.81 (s, 2H), 7.83 (t, J = 7.7 Hz, 2H), 7.83 (d, J = 12.5 μmol) in chloroform was mixed with a solution of iron(III)
2.3 Hz, 2H), 7.87 (d, J = 7.7 Hz, 2H), 7.91 (dd, J = 7.7, 1.0 Hz, chloride (4.1 mg, 25.4 μmol) in methanol. Vapor-phase
2H), 7.95 (dd, J = 7.7, 1.0 Hz, 2H), 8.51 (dd, J = 7.7, 1.0 Hz, 2H), diffusion of diethyl ether into this solution afforded
8.56 (dd, J = 7.7, 1.0 Hz, 2H); 13C NMR (100 MHz, CDCl3) [(HL1)2Fe2Cl2] (9.7 mg, 4.95 μmol, 79%) as black crystals;
δ 20.62 (CH3), 21.08 (CH3), 55.82 (CH3), 61.77 (CH3), 111.67 ESI-MS: m/z 682.2 ([(HL1)Fe]+); Anal. Calcd for C41H30-ClFeN4O3·
(CH), 119.13 (CH), 119.43 (CH), 124.56 (CH), 125.14 (CH), 2.2CHCl3 (= HL1FeCl·2.2CHCl3): C, 52.91; H, 3.31; N, 5.71.
128.89 (C), 130.21 (C), 130.31 (CH), 131.84 (CH), 132.51 (CH), Found: C, 52.69; H, 3.39; N, 5.74.
133.87 (C), 134.22 (C), 136.41 (CH), 136.83 (CH), 154.25 (C),
Synthesis of 8 and 9. 6,6′-Dibromo-2,2′-bipyridine (2)
155.10 (C), 155.23 (C), 155.51 (C), 155.88 (C), 156.19 (C); Anal. (517 mg, 1.65 mmol), 5-tert-butyl-2-methoxyphenylboronic
Calcd for C44H38N4O3·H2O: C, 76.72; H, 5.85; N, 8.13. Found: acid (7) (286 mg, 1.37 mmol), and tetrakis(triphenyl-
C, 77.03; H, 5.66; N, 8.03.
phosphine)palladium(0) (89.2 mg, 0.0772 mmol) were placed
Synthesis of 2,2′-(6′,6′′-(2-hydroxy-5-methyl-1,3-phenylene)bis- in a 50 mL two-necked flask. Toluene (15 mL) and saturated
(2,2′-bipyridine-6′,6-diyl))bis(4-methylphenol)(H3L1). Amixture aqueous solution of sodium carbonate (3 mL) were added to
of the precursor 6 (102 mg, 0.153 mmol), acetic acid (25 mL), the mixture in one portion. The mixture was stirred at 75 °C
and hydrobromic acid (48%, 1 mL) was heated under reflux for 60 h. After cooling, water and chloroform were added and
for 24 h. After cooling to rt, the solution was neutralized with the organic layer was separated. The aqueous layer was
saturated aqueous solution of sodium hydrogencarbonate extracted with chloroform. The combined organic layer was
and extracted with chloroform. The organic layer was dried dried over anhydrous magnesium sulfate and concentrated to
over anhydrous magnesium sulfate, filtered, and concentrated dryness. The crude product was purified by column chromato-
to dryness. The residue was recrystallized from chloroform– graphy on silica gel using chloroform–hexane (2/1, v/v) as an
methanol to yield H3L1 (81.7 mg, 0.126 mmol, 83%) as yellow eluent to give mono-coupled product 8 (276 mg, 0.695 mmol,
1
crystals, mp 293–295 °C; H NMR (600 MHz, CDCl3) δ 2.38 (s, 51%) and di-coupled product 9 (102 mg, 0.213 mmol, 15%). 8:
1
6H), 2.50 (s, 3H), 6.99 (d, J = 8.3 Hz, 2H), 7.17 (dd, J = 8.3, colorless crystals, mp 112–113 °C; H NMR (400 MHz, CDCl3)
1.6 Hz, 2H), 7.67 (d, J = 1.6 Hz, 2H), 7.87 (s, 2H), 7.97–8.01 (m, δ 1.38 (s, 9H), 3.86 (s, 3H), 6.97 (d, J = 8.6 Hz, 1H), 7.42 (dd, J =
4H), 8.02 (t, J = 7.8 Hz, 2H), 8.15 (dd, J = 7.8, 0.8 Hz, 2H), 8.18 8.6, 2.6 Hz, 1H), 7.47 (d, J = 7.6 Hz, 1H), 7.66 (t, J = 7.6 Hz, 1H),
(dd, J = 7.8, 0.8 Hz, 2H), 8.32 (dd, J = 7.8, 0.8 Hz, 2H), 14.28 (s, 7.82 (t, J = 7.9 Hz, 1H), 7.91 (dd, J = 7.9, 0.9 Hz, 1H), 7.98 (d, J =
2H), 14.88 (s, 1H); 13C NMR (150 MHz, CDCl3) δ 20.79 (CH3), 2.6 Hz, 1H), 8.34 (dd, J = 7.6, 0.9 Hz, 1H), 8.52 (d, J = 7.9 Hz,
20.94 (CH3), 118.26 (CH), 118.53 (C), 118.93 (CH), 119.34 (CH), 1H); 13C NMR (100 MHz, CDCl3) δ 31.56 (CH3), 34.23 (C), 55.73
119.43 (CH), 123.06 (CH), 123.85 (C), 126.72 (CH), 128.00 (C), (CH3), 111.22 (CH), 119.19 (CH), 119.74 (CH), 125.84 (CH),
128.03 (C), 131.17 (CH), 132.55 (CH), 138.14 (CH), 138.91 (CH), 126.90 (CH), 127.71 (CH), 128.09 (C), 128.36 (CH), 136.57 (CH),
152.83 (C), 152.99 (C), 155.86 (C), 156.84 (C), 157.53 (C), 157.60 139.13 (CH), 141.41 (C), 143.54 (C), 153.82 (C), 155.07 (C),
(C); ESI-MS: m/z 629.3 for [H3L1 + H]+; Anal. Calcd for 155.60 (C), 157.80 (C); Anal. Calcd for C21H21BrN2O: C, 63.48;
C41H32N4O3·H2O: C, 76.14; H, 5.30; N, 8.66. Found: C, 76.40; H, 5.33; N, 7.05. Found: C, 63.31; H, 5.49; N, 6.87. 9: colorless
H, 5.27; N, 8.69.
crystals, mp 212–215 °C; 1H NMR (400 MHz, CDCl3) δ 1.40
Synthesis of [L1Zn2(OAc)]. A solution of H3L1 (8.4 mg, (s, 18H), 3.88 (s, 6H), 6.98 (d, J = 8.6 Hz, 2H), 7.42 (dd, J = 8.6,
13.0 μmol) in chloroform was mixed with a solution of zinc(II) 2.6 Hz, 2H), 7.83 (t, J = 7.7 Hz, 2H), 7.90 (dd, J = 7.1, 1.1 Hz,
acetate dihydrate (5.6 mg, 25.4 μmol) in methanol. Vapor- 2H), 8.05 (d, J = 2.6 Hz, 2H), 8.50 (dd, J = 7.7, 1.1 Hz, 2H); 13C
phase diffusion of diethyl ether into this solution afforded NMR (100 MHz, CDCl3) δ 31.56 (CH3), 34.23 (C), 55.78 (CH3),
[L1Zn2(OAc)] (10.1 mg, 11.9 μmol, 92%) as yellow crystals; 111.29 (CH), 118.85 (CH), 124.99 (CH), 126.63 (CH), 128.53
1H NMR (600 MHz, CDCl3–CD3OD, 1 : 1, 223 K) δ 1.98 (s, 3H), (CH and C), 136.34 (CH), 143.54 (C), 155.14 (C), 155.22 (C),
2.26 (s, 6H), 2.41 (s, 3H), 6.60 (d, J = 8.5 Hz, 2H), 7.00 (d, J = 155.95 (C); Anal. Calcd for C32H36N2O2: C, 79.96; H, 7.55; N,
8.5 Hz, 2H), 7.42 (s, 2H), 7.47 (s, 2H), 7.99 (d, J = 7.9 Hz, 2H), 5.83. Found: C, 79.85; H, 7.73; N, 5.56.
8.01 (d, J = 7.6 Hz, 2H), 8.24 (t, J = 7.9 Hz, 2H), 8.35 (t, J =
Synthesis of 6-(5-tert-butyl-2-methoxyphenyl)-6′-(tributyl-
7.6 Hz, 2H), 8.37 (t, J = 7.9 Hz, 2H), 8.46 (d, J = 7.6 Hz, 2H); stannyl)-2,2′-bipyridine (10). Under an argon atmosphere,
ESI-MS: m/z 757.1 ([L1Zn2]+); Anal. Calcd for C43H32N4O5Zn2· n-butyllithium (1.67 M in hexane, 2.15 mL, 3.6 mmol) was
MeOH (= L1Zn2(OAc)·MeOH): C, 62.35; H, 4.28; N, 6.61. Found: added to a solution of monosubstituted compound 8 (1.19 g,
C, 62.19; H, 4.14; N, 6.78.
3.00 mmol) in THF (70 mL) at −78 °C and the solution was
Synthesis of [(L1)2Ni4](OAc)2. A solution of H3L1 (8.0 mg, stirred for 1 h. Tributylchlorostannane (0.90 mL, 3.3 mmol)
12.4 μmol) in chloroform was mixed with a solution of was then added to the solution and the mixture was further
nickel(II) acetate tetrahydrate (6.3 mg, 25.4 μmol) in methanol. stirred for 1 h at −78 °C and allowed to warm to rt. After
Vapor-phase diffusion of diethyl ether into this solution hexane (30 mL) and water (30 mL) were added, the mixture
afforded [(L1)2Ni4](OAc)2 (10.04 mg, 5.39 μmol, 87%) as red was separated and the aqueous layer was extracted with
crystals; ESI-MS: m/z 741.1 ([L1Ni2]+); Anal. Calcd for hexane. The combined organic layer was dried over anhydrous
C43H32N4O5Ni2·7H2O (= L1Ni2(OAc)·7H2O): C, 55.64; H, 5.00; N, magnesium sulfate, filtered, and concentrated to dryness.
6.04. Found: C, 55.49; H, 4.81; N, 5.91.
Column chromatography (SiO2, diethyl ether–hexane, 2 : 1) of
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the residue afforded compound 10 (1.82 g), which was used for (CH), 118.95 (CH), 119.25 (CH), 119.30 (CH), 124.59 (CH),
the next step without further purification although the 124.75 (CH), 125.16 (CH), 126.72 (CH), 128.52 (CH and C),
product still contained a small amount of impurity. 10: 1H 129.15 (CH), 129.23 (CH), 133.81 (C), 133.88 (C), 136.46 (CH),
NMR (300 MHz, CDCl3) δ 0.90 (t, 9H), 1.13–1.19 (m, 6H), 136.88 (CH), 136.95 (CH), 143.59 (C), 146.99 (C), 154.18 (C),
1.30–1.44 (m, 6H), 1.38 (s, 9H), 1.58–1.68 (m, 6H), 3.86 (s, 3H), 155.17 (C), 155.39 (C), 155.81 (C), 155.85 (C), 155.98 (C), 156.03
6.97 (d, J = 8.7 Hz, 1H), 7.39 (dd, J = 7.6, 1.3 Hz, 1H), 7.41 (dd, (C), 156.19 (C); Anal. Calcd for C74H76N6O4·H2O: C, 78.55; H,
J = 8.7, 2.6 Hz, 1H), 7.63 (t, J = 7.6 Hz, 1H), 7.81 (t, J = 7.6 Hz, 6.95; N, 7.43. Found: C, 78.18; H, 6.84; N, 7.32.
1H), 7.87 (dd, J = 7.6, 1.3 Hz, 1H), 8.01 (d, J = 2.6 Hz, 1H), 8.39
(dd, J = 7.6, 1.3 Hz, 1H), 8.45 (dd, J = 7.6, 1.3 Hz, 1H).
Synthesis of 6,6′-(2,2′-bipyridine-6,6′-diyl)bis(4-tert-butyl-2-
(6′-(5-tert-butyl-2-hydroxyphenyl)-2,2′-bipyridine-6-yl)phenol)
Synthesis of 6,6′-bis(3-bromo-5-tert-butyl-2-methoxyphenyl)- (H4L2). A mixture of the precursor 12 (145 mg, 0.128 mmol),
2,2′-bipyridine (11). Under an argon atmosphere, a solution of acetic acid (5 mL), and hydrobromic acid (48%, 1 mL) was
bromine (0.25 mL, 4.9 mmol) in dichloromethane (10 mL) was heated under reflux for 10 h. After cooling to rt, the solution
slowly added to a solution of disubstituted compound 9 was treated with dichloromethane and neutralized with satu-
(361 mg, 0.750 mmol) in dichloromethane (20 mL). The solu- rated aqueous solution of sodium hydrogencarbonate. The
tion was stirred at 40 °C for 48 h. After the addition of organic layer was separated and the aqueous layer was
aqueous solution of sodium thiosulfate (10 mL) and then satu- extracted with dichloromethane. The combined organic layer
rated aqueous solution of sodium hydrogencarbonate (10 mL), was dried over anhydrous magnesium sulfate, filtered, and
the solution was stirred for 1 h at rt. The organic layer was concentrated to dryness. The residue was treated with metha-
separated and the aqueous layer was extracted with dichloro- nol and the insoluble materials were collected by filtration to
methane. The combined organic layer was dried over an- yield H4L2 (101 mg, 0.0933 mmol, 73%) as yellow crystals, mp
hydrous magnesium sulfate, filtered, and concentrated to 215–219 °C; 1H NMR (400 MHz, CDCl3) δ 1.39 (s, 18H), 1.49 (s,
dryness. The recrystallization of the residue from chloroform– 18H), 7.03 (d, J = 8.6 Hz, 2H), 7.42 (dd, J = 8.6, 2.3 Hz, 2H),
methanol afforded 11 (362.7 mg, 0.568 mmol, 76%) as color- 7.87 (d, J = 2.3 Hz, 2H), 7.98–8.02 (m, 6H), 8.05 (dd, J = 7.8,
less crystals, mp 225–228 °C; 1H NMR (400 MHz, CDCl3) δ 1.39 1.6 Hz, 2H), 8.08 (d, J = 2.4 Hz, 2H), 8.12 (d, J = 2.4 Hz, 2H),
(s, 18H), 3.62 (s, 6H), 7.64 (d, J = 2.4 Hz, 2H), 7.90 (t, J = 7.6 Hz, 8.17 (d × 2, J = 7.8 Hz, 4H), 8.22 (d, J = 7.8 Hz, 2H), 8.34 (d, J =
2H), 7.90 (d, J = 2.4 Hz, 2H), 7.93 (dd, J = 7.6, 1.5 Hz, 2H), 8.52 7.8 Hz, 2H), 8.36 (dd, J = 7.8, 1.6 Hz, 2H), 14.34 (s, 2H), 14.97
(dd, J = 7.6, 1.5 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 31.38 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 31.54 (CH3), 31.63 (CH3),
(CH3), 34.70 (C), 61.19 (CH3), 117.75 (C), 119.65 (CH), 124.65 34.15 (C), 34.37 (C), 117.97 (CH), 118.02 (C), 118.61 (CH),
(CH), 127.87 (CH), 131.00 (CH), 134.51 (C), 137.27 (CH), 148.74 118.99 (CH), 119.06 (CH), 119.18 (CH), 122.13 (CH), 122.44 (C),
(C), 152.74 (C), 155.00 (C), 155.89 (C); Anal. Calcd for 122.76 (CH), 123.60 (CH), 124.32 (C), 126.90 (CH), 128.21 (CH),
C32H34Br2N2O2: C, 60.20; H, 5.37; N, 4.39. Found: C, 59.99; H, 128.98 (CH), 137.78 (CH), 138.12 (CH), 138.75 (CH), 141.15 (C),
5.35; N, 4.25.
141.37 (C), 152.83 (C), 152.90 (C), 153.11 (C), 155.84 (C), 156.77
Synthesis of 6,6′-bis(5-tert-butyl-3-(6′-(5-tert-butyl-2-methoxy- (C), 156.93 (C), 157.43 (C), 157.65 (C); MALDI-MS: m/z 1057.3
phenyl)-2,2′-bipyridine-6-yl)-2-methoxyphenyl)-2,2′-bipyridine for [H4L2 + H]+; Anal. Calcd for C70H68N6O4·1.5H2O: C, 77.53;
(12). Under an argon atmosphere, stannane 10 (1.55 g, H, 6.60; N, 7.75. Found: C, 77.66; H, 6.44; N, 7.71.
2.55 mmol), dibromide 11 (766 mg, 1.20 mmol), and tetrakis
Synthesis of [L2Zn3(OAc)2]. A solution of H4L2 (6.84 mg,
(triphenylphosphine)palladium(0) (139 mg, 0.120 mmol) were 6.31 μmol) in chloroform was mixed with a solution of zinc(II)
dissolved in toluene (12 mL). This mixture was stirred at acetate dihydrate (4.99 mg, 22.7 μmol) in methanol. Vapor-
100 °C for 24 h. An aqueous solution of cesium fluoride phase diffusion of diethyl ether into this solution afforded
(20 mL) was added and then the mixture was stirred for 1 h at [L2Zn3(OAc)2] (8.7 mg, 5.65 μmol, 90%) as yellow crystals,
rt. The mixture was filtered through a Celite pad and the 1H NMR (600 MHz, DMSO-d6) δ 1.18 (s, 36H), 2.0 (br, 6H), 5.39
organic layer was separated. The aqueous layer was extracted (br, 2H), 6.76 (br, 2H), 7.21 (br, 2H), 7.34 (br, 4H), 7.56 (s, 2H),
with chloroform and the combined organic layer was dried 7.65 (br, 2H), 7.87 (br, 2H), 8.01 (br, 4H), 8.14 (br, 2H), 8.24
over anhydrous magnesium sulfate, filtered, and concentrated (br, 2H), 8.48 (t, J = 7.8 Hz, 2H), 8.78 (d, J = 7.8 Hz, 2H);
to dryness. The crude product was purified by column ESI-MS: m/z 624.2 ([L2Zn3]2+), 1307.5 ([L2Zn3(OAc)]+); Anal.
chromatography (SiO2, chloroform) and then GPC to afford 12 Calcd for C74H70N6O8Zn3·CHCl3·3H2O (= L2Zn3(OAc)2·CHCl3·
(401.2 mg, 0.355 mmol, 30%) as colorless crystals, mp 3H2O): C, 58.45; H, 5.04; N, 5.45. Found: C, 58.31; H, 4.96; N,
306–310 °C; 1H NMR (400 MHz, CDCl3) δ 1.41 (s, 18H), 1.50 (s, 5.26.
18H), 3.45 (s, 6H), 3.88 (s, 6H), 6.98 (d, J = 8.6 Hz, 2H), 7.43
Synthesis of [L2Co3(OAc)3(OMe)]. A solution of H4L2
(dd, J = 8.6, 2.7 Hz, 2H), 7.86 (t, J = 7.7 Hz, 2H), 7.90 (t, J = (7.8 mg, 7.19 μmol) in chloroform was mixed with a solution
7.7 Hz, 2H), 7.92 (t, J = 7.7 Hz, 2H), 7.92 (dd, J = 7.7, 1.1 Hz, of cobalt(II) acetate tetrahydrate (5.7 mg, 22.9 μmol) in metha-
2H), 7.96 (dd, J = 7.7, 1.1 Hz, 2H), 7.98 (dd, J = 7.7, 1.1 Hz, 2H), nol. The mixture was stirred for 5 min and then the solvent
8.03 (ABq, 4H), 8.07 (d, J = 2.7 Hz, 2H), 8.52 (dd, J = 7.7, was evaporated over a period of 2 d under an aerobic atmos-
1.1 Hz, 2H), 8.57 (dd, J = 7.7, 1.1 Hz, 2H), 8.59 (dd, J = 7.7, phere. The residue was dissolved in methanol and vapor
1.1 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 31.58 (CH3), 31.59 phase diffusion of diethyl ether into this solution afforded
(CH3), 34.26 (C), 34.73 (C), 55.82 (CH3), 61.70 (CH3), 111.35 [L2Co3(OAc)3(OMe)] (8.1 mg, 5.18 μmol, 72%) as reddish brown
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crystals, ESI-MS: m/z 645.8 ([L2Co3(OMe)2]2+), 1322.5
([L2Co3(OMe)3]+); Anal. Calcd for C77H76Co3N6O11·7H2O
(=L2Co3(OAc)3(OMe)·7H2O): C, 59.12; H, 5.80; N, 5.37. Found:
C, 58.86; H, 5.53; N, 5.13.
X-ray crystallography
Intensity data were collected at 120 K on a Rigaku R-AXIS
Rapid diffractometer with Mo Kα radiation (λ = 0.71069 Å). The
data were corrected for Lorentz and polarization factors, and
for absorption by semiempirical methods based on symmetry-
equivalent and repeated reflections. The structures were solved
by Patterson methods (DIRDIF 99)25 or direct methods
(SHELXS 97)26 and refined by full-matrix least-squares on F 2
using SHELXL 97.26 The crystallographic data are summarized
in Table 2. CCDC-938299 ([L1Zn2(OAc)]·2CHCl3·1.5MeOH·
0.5H2O), CCDC-938300 ([(L1)2Ni4](OAc)2·6.5MeOH·4H2O), CCDC-
938301 ([(HL1)2Fe2Cl2]·2MeCN), CCDC-938302 ([L2Zn3(OAc)2]·
CHCl3·3.5MeOH·H2O), and CCDC-938303 ([L2Co3(OAc)3(OMe)]·
1.25MeOH·0.5Et2O·H2O) contain the supplementary crystallo-
graphic data for this paper.
Acknowledgements
This work was supported by Grants-in-Aid for Scientific
Research on Innovative Areas (Coordination Programming
Area 2107, no. 22108505 and 24108707 to S.A.) from the
Ministry of Education, Culture, Sports, Sciences and Tech-
nology, Japan.
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