Synthesis of Ag+-selective dipalladium(II) metallohost based on O-alkyloxime bis(N2SO) ligands

Article abstract of DOI:10.1080/10426501003772110

Novel bis(N₂SO) ligands 2a (= H₄L^2a) and 2b (= H₄L^2b) comprising O-alkyloxime, thiophenol, and phenol moieties were synthesized. The complexation of H₄L ^2a and H₄L^2b with palladium(II) acetate afforded the dinuclear metallohosts [L^2aPd₂] and [L ^2bPd₂], which have a recognition site consisting of two sulfur and four oxygen donor atoms. The metallohost [L^2bPd ₂] strongly binds Ag⁺ to give [(L^2bPd ₂)₂Ag]⁺, whereas no detectable interaction between [L^2bPd₂] and hard metal cations (Na⁺, Ca²⁺, Y³⁺, La³⁺, and Lu³⁺) was observed. Copyright Taylor & Francis Group.

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Synthesis of Ag+-Selective Dipalladium(II)
Metallohost Based on O-Alkyloxime
Bis(N2SO) Ligands
Shigehisa Akine a , Ayako Akimoto a & Tatsuya Nabeshima a
a Graduate School of Pure and Applied Sciences, University of
Tsukuba, Tsukuba, Ibaraki, Japan
Published online: 27 May 2010.
To cite this article: Shigehisa Akine , Ayako Akimoto & Tatsuya Nabeshima (2010): Synthesis of Ag+-
Selective Dipalladium(II) Metallohost Based on O-Alkyloxime Bis(N2SO) Ligands, Phosphorus, Sulfur,
and Silicon and the Related Elements, 185:5-6, 1000-1007
To link to this article: http://dx.doi.org/10.1080/10426501003772110
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Phosphorus, Sulfur, and Silicon, 185:1000–1007, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426501003772110
SYNTHESIS OF Ag+-SELECTIVE DIPALLADIUM(II)
METALLOHOST BASED ON O-ALKYLOXIME BIS(N2SO)
LIGANDS
Shigehisa Akine, Ayako Akimoto, and Tatsuya Nabeshima
Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba,
Ibaraki, Japan
Novel bis(N2SO) ligands 2a (= H4L2a) and 2b (= H4L2b) comprising O-alkyloxime, thio-
phenol, and phenol moieties were synthesized. The complexation of H4L2a and H4L2b with
palladium(II) acetate afforded the dinuclear metallohosts [L2aPd2] and [L2bPd2], which
have a recognition site consisting of two sulfur and four oxygen donor atoms. The metallo-
host [L2bPd2] strongly binds Ag+ to give [(L2bPd2)2Ag]+, whereas no detectable interaction
between [L2bPd2] and hard metal cations (Na+, Ca2+, Y3+, La3+, and Lu3+) was observed.
Keywords Metallohost; oxime; palladium; silver; thiol
INTRODUCTION
Metallohosts, which have a metal complex moiety in their framework, have recently
attracted much interest. The metal atom is useful to form a host structure suitable for
guest recognition1 or to output signals by changing the properties of the metal upon the
guest recognition.2,3 We have investigated the design, synthesis, and complexation be-
havior of a series of bis(N2O2) type ligands (1 and its analogs, Scheme 1), which are
expected to afford dinuclear metallohosts.4,5 We have demonstrated that H2salamo (= 1,2-
bis(salicylideneaminooxy)ethane)6,7 based on O-alkyloxime functionality is useful as the
N2O2 coordination site to synthesize these types of bis(N2O2) ligands because of the high
stability toward C N bond recombination. Upon metalation, the bis(N2O2) ligand affords
a trinuclear complex in a cooperative fashion, in which the third metal ion sits in the central
O6 site in addition to the two metal ions in the N2O2 sites. The trinuclear complex acts
as a metallohost based on novel transmetalation strategy, in which guest recognition takes
place with concomitant release of one of the metal ions. The guest ion nicely fits into the
metallohost cavity comprising six oxygen donor atoms of four phenoxo and two methoxy
groups.4
Received 25 November 2008; accepted 27 December 2008.
Dedicated to Professor Naomichi Furukawa on the occasion of his 70th birthday.
This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
Address correspondence to Tatsuya Nabeshima, Graduate School of Pure and Applied Sciences, University
of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan. E-mail: nabesima@chem.tsukuba.ac.jp
1000

Ag+-SELECTIVE DIPALLADIUM(II) METALLOHOST
1001
If sulfur donor atoms are introduced into a host molecule instead of oxygen, the
binding selectivity is expected to change. Indeed, thiacrown ethers prefer softer metal ions
such as Ag+, whereas the oxygen analogs bind harder cations.8 Thus, we introduce sulfur
donor atoms into the bis(N2O2) framework of 1 in order to change the binding selectivity
of our novel ion recognition system. We have already reported the synthesis, reactivity,
and complexation ability of sulfur analogs of the salamo ligand, which have one or two
thiophenol moieties instead of the phenol groups.9 In this article we report the synthesis of
new bis(N2SO) ligands 2a and 2b, which can be converted into the dinuclear metallohost
(Scheme 1). The complexation ability of the metallohost is also investigated.
Scheme 1 Design of bis(N2SO) ligands and the metallohost for guest recognition.
RESULTS AND DISCUSSION
Synthesis of Bis(N2SO) Ligand
Synthesis of the ligands 2a and 2b was first investigated according to route A starting
from the thiosalicylaldehydes 3 (Scheme 2). This route is analogous to the synthesis of
1 previously reported in the literature.4 2-Mercaptobenzaldehyde (3a) was synthesized
by the reaction of thiophenol with n-butyllithium followed by N,N-dimethylformamide,
according to the literature.10 The methoxy derivative, 2-mercapto-3-methoxybenzaldehyde
(3b),11 was also prepared in a similar manner. The reaction of thiosalicylaldehydes 3a and
3b with 1,2-bis(aminooxy)ethane12 gave a mixture containing compound 5. However, the
product was found to be difficult to purify. Hence, we instead prepared the central unit 6
by the reaction of dialdehyde 44d,13 and 5 equiv of 1,2-bis(aminooxy)ethane in 72% yield
(route B). The bis(N2SO) ligands 2a and 2b were successfully synthesized in 48% and
75% yield, respectively, by the condensation of the diamine 6 with the terminal unit 3.
These compounds were obtained as stable colorless crystals, which were characterized
by 1H 13C NMR and elemental analysis. We have previously reported that unsymmetrical
N2SO ligands based on two C N nitrogen, thiophenol, and phenol groups are difficult to
Scheme 2 Synthetic routes to bis(N2SO) ligands 2a and 2b.

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S. AKINE ET AL.
synthesize unless an O-alkyloxime group is used instead of an imine group.9 Thiol groups
may react with imine groups but not with O-alkyloxime groups. The low reactivity of the
oxime groups also retards the C N bond recombination, which facilitates the synthesis of
the unsymmetrical salen-type ligands.
The bis(N2SO) ligands 2a and 2b were stable in solution. These compounds remained
1
unchanged in the solution at room temperature after 1 month. In the H NMR spectrum
of 2a, the signals for the thiol and phenol protons were observed at 4.55 and 9.71 ppm,
respectively, in CDCl3. The thiol protons of the methoxy analog 2b appeared at lower field
(4.91 ppm), as observed for the parent N2SO ligands.9
Synthesis of Bis(N2SO)-Pd2 Metallohost
We first investigated the metalation of the bis(N2SO) ligands 2a (= H4L2a) and 2b
(= H4L2b) to obtain the dinuclear metallohosts. We have previously reported that the parent
mono(N2SO) ligand gave a diamagnetic mononuclear complex upon complexation with
palladium(II), while the N–O bond cleavage took place when the ligand was allowed to
react with nickel(II) or copper(II).9 Consequently, palladium(II) is suitable for the metal to
be incorporated into the metallohost.
When the N2SO sites of ligands H4L2a and H4L2b are metalated with palladium(II),
dinuclear complexes [L2aPd2] and [L2bPd2] are formed, respectively. In this process, four
new coordination bonds were formed in each of the N2SO moieties. This makes the
metallohost molecule less flexible and fixes the conformation in a C-shaped fashion. Thus,
the metallohosts were expected to have a cavity consisting of the S2O2 or S2O4 donor sets.
The palladium(II) complexes [L2aPd2] and [L2bPd2] were prepared in 74% and 55%
yields by the reaction of palladium(II) acetate with the corresponding free ligands H4L2a and
H4L2b, respectively. The methoxy derivative [L2bPd2] is soluble in DMSO, while [L2aPd2]
without methoxy groups has low solubility. The 1H NMR spectra in DMSO-d6 showed two
singlets for the oxime protons at 8.28 and 8.76 ppm for [L2aPd2] and 8.26 and 8.80 ppm for
[L2bPd2], indicating a symmetrical structure. The formation of dinuclear complex [L2bPd2]
was confirmed by the mass spectrum (m/z 847.1 for [(L2bPd2)Na]+).
We have reported that it is difficult to obtain a dinuclear metallohost of the oxygen
analog 1, because the formation process of the trinuclear complexes of 1 with zinc(II),
cobalt(II), manganese(II), and nickel(II) was highly cooperative. Even if the dinuclear
metallohosts are formed, they should readily bind to an additional metal ion in the central
cavity to afford the trinuclear complexes. In the case of the sulfur analogs 2 in this study, a
dinuclear metallohost having a vacant cavity was obtained by using the combination of the
N2SO coordination sites and palladium(II).
Complexation Study of Dinuclear Metallohost
We investigated the complexation ability of the dinuclear metallohost [L2bPd2], which
1
has better solubility in DMSO. The H NMR spectra of [L2bPd2] showed no change in
chemical shifts upon the addition of 1 equiv of La(OTf)3, Lu(OTf)3, Y(OTf)3, Ca(ClO4)2,
and NaClO4. This result indicates that the metallohost [L2bPd2] has low affinity for these
hard cations. When Pb2+ was added to the solution, small chemical shift changes were
observed.

Ag+-SELECTIVE DIPALLADIUM(II) METALLOHOST
1003
Figure 1 1H NMR spectra changes of [L2bPd2] by the addition of AgNO3 in DMSO-d6, [L2bPd2] = 1.0 mM,
400 MHz. (a) 0 equiv; (b) 0.25 equiv; (c) 0.5 equiv; (d) 1.0 equiv; (e) 3.0 equiv.
In contrast, the addition of Ag+ to the solution of [L2bPd2] caused significant changes
1
in the H NMR spectra (Figure 1). When 0.25 equiv of Ag+ is present, the spectrum
of [L2bPd2] was considerably broadened, probably because the process of complexa-
1
tion/decomplexation with Ag+ is slow on the H NMR timescale. However, 0.5 equiv
of Ag+ caused the appearance of sharp signals, which were observed at different chemical
shifts from those of the free ligand H4L2b. For example, two signals of the OCH2CH2O
moiety at 4.48 and 4.79 ppm shifted to 5.15 and 4.41 ppm, respectively, and the oxime
protons at 8.26 and 8.80 shifted to 8.66 and 8.75 ppm, respectively. Further addition of
Ag+ caused a slight change in the chemical shifts of the oxime protons. These results
clearly indicate a 2:1 stoichiometry ([L2bPd2]:Ag+), which corresponds to the 2:1 adduct
[(L2bPd2)2Ag]+ (Scheme 3).
In the ESI-MS of [L2bPd2] in the presence of 1 equiv of Ag+, a strong peak was
observed at m/z 930.8, whose isotope pattern is consistent with [(L2bPd2)Ag]+ (Figure 2).
This result indicates that the 1:1 adduct [(L2bPd2)Ag]+ is present under the highly diluted
conditions for the ESI-MS measurement.
Consequently, dinuclear complex [L2bPd2] acts as a metallohost that efficiently binds
to Ag+ in the S2O4 cavity. The thiolato groups in the N2SO–Pd complex moieties contribute

1004
S. AKINE ET AL.
Scheme 3 Plausible structure of the metallohost-guest complexes between [L2bPd2] and Ag+.
Figure 2 ESI mass spectrum of [L2bPd2] in the presence of Ag+. The inset shows the observed and calculated
isotope patterns of [(L2bPd2)Ag]+.
to the Ag+-binding, probably because distance between the two thiolato groups is suitable
for binding to Ag+.
CONCLUSION
Bis(N2SO) ligands 2a and 2b based on the oxime chelate coordination moiety were
synthesized. The complexation of the 2b (= H4L2b) with palladium(II) acetate afforded the
dinuclear metallohost [L2bPd2], which has a recognition site consisting of the two sulfur
and four oxygen donor atoms. The metallohost [L2bPd2] binds strongly to Ag+ to give
[(L2bPd2)2Ag]+, whereas interactions between [L2bPd2] and hard metal cations were not
detected.
EXPERIMENTAL
All experiments were carried out under aerobic conditions unless otherwise noted.
Hexane was distilled from calcium hydride prior to use. Commercial methanol, ethanol,

Ag+-SELECTIVE DIPALLADIUM(II) METALLOHOST
1005
dichloromethane, and chloroform were used without purification. Silica gel chromatogra-
phy was performed by Kanto Silica Gel 60N (spherical, neutral). Gel permeation chro-
matography (GPC) was performed on an LC908 (Japan Analytical Industry Co., Ltd.)
equipped with JAIGEL 1H and 2H columns using chloroform as eluent. Melting points
1
were determined on a Yanaco melting point apparatus and are not corrected. H and 13C
NMR spectra were recorded on a Bruker ARX 400 or an Avance 400 spectrometer (400
and 100 MHz, respectively), using tetramethylsilane as an internal standard. Mass spec-
tra (ESI-TOF, positive mode) were recorded on an Applied Biosystems QStar/Pulsar i
spectrometer.
Materials
1,2-Bis(aminooxy)ethane12 and 2,3-dihydroxybenzene-1,4-dicarbaldehyde (4)4d,13
were prepared according to the reported procedures. 2-Mercaptobenzaldehyde (3a) was pre-
pared10 and stored as a dichloromethane solution under nitrogen. 2-Methoxybenzenethiol
was purchased from Wako Pure Chemical Industries Ltd.
Synthesis of 2-Mercapto-3-methoxybenzaldehyde (3b)
n-Butyllithium (2.6 M, in hexane, 13.4 mL, 35 mmol) was added slowly over a
period of 30 min to a stirred solution of 2-methoxybnzenethiol (2.0 mL, 16 mmol) and
N,N,Nꢁ,Nꢁ-tetramethylethylenediamine (5.6 mL, 37 mmol) in hexane (50 mL) at 0◦C under
nitrogen atmosphere. After the reaction mixture was stirred overnight at room temperature,
N,N-dimethylformamide (3.4 mL, 44 mmol) was added, and the mixture was stirred for
further 6 h. After acidification with 1 M hydrochloric acid, the mixture was extracted with
ether, dried over anhydrous magnesium sulfate, filtered, and concentrated to dryness. The
residue was subjected to chromatography on silica gel (eluent, dichloromethane/hexane,
1:1) to afford 3b (1.00 g, 6.0 mmol, 37%) as yellow crystals, 39–41◦C,11 1H NMR (400
MHz, CDCl3) δ 3.96 (s, 3H), 5.71 (s, 1H), 7.07 (dd, J = 7.9, 1.2 Hz, 1H), 7.26 (t, J = 7.9
Hz, 1H), 7.40 (dd, J = 7.9, 1.2 Hz, 1H), 10.11 (s, 1H).
Synthesis of Dioxime 6
A solution of dialdehyde 4 (140 mg, 0.84 mmol) in ethanol (30 mL) was added
dropwise to a solution of 1,2-bis(aminooxy)ethane (465 mg, 5.1 mmol) in ethanol (70 mL)
with stirring at 50–55◦C. The solution was stirred at the temperature for 30 min and then
concentrated to dryness. The crude product was purified by GPC to afford dioxime 6 (190
mg, 0.60 mmol, 72%) as colorless crystals, mp 102–106◦C, 1H NMR (400 MHz, CDCl3)
δ 3.97–3.99 (m, 4H), 4.39–4.41 (m, 4H), 5.52 (brs, 4H), 6.76 (s, 2H), 8.21 (s, 2H), 9.77
(s, 2H); 13C NMR (100 MHz, CDCl3) δ 72.8 (CH2), 73.6 (CH2), 117.5 (C), 120.8 (CH),
145.7 (C), 151.1 (CH). Anal. Calcd for C12H18N4O6: C, 45.86; H, 5.77; N, 17.83. Found:
C, 46.05; H, 5.64; N, 17.53.
Synthesis of Bis(N2SO) Ligand 2a (= H4L2a)
A solution of dioxime 6 (65.6 mg, 0.21 mmol) in dichloromethane (2 mL) was
added to a solution of 2-mercaptobenzaldehyde (3a, 0.42 mmol) in dichloromethane, and
the resulting mixture was stirred for 3 h at room temperature. After the solution was
concentrated, the residual crude product was purified by GPC to afford 2a (55.2 mg, 0.099
mmol, 48%) as colorless crystals, mp 109–110◦C, 1H NMR (400 MHz, CDCl3) δ 4.49–4.54

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S. AKINE ET AL.
(m, 8H), 4.55 (s, 2H), 6.76 (s, 2H), 7.14–7.22 (m, 4H), 7.32 (dd, J = 7.4, 1.6 Hz, 2H), 7.53
(dd, J = 7.4, 1.6 Hz, 2H), 8.23 (s, 2H), 8.44 (s, 2H), 9.71 (s, 2H); 13C NMR (100 MHz,
CDCl3) δ 72.7 (CH2), 73.3 (CH2), 117.6 (C), 120.7 (CH), 125.7 (CH), 129.5 (C), 129.6
(CH), 129.8 (CH), 131.5 (CH), 132.1 (C), 145.8 (C), 149.4 (CH), 151.2 (CH). Anal Calcd
for C26H26N4O6S2·H2O: C, 54.53; H, 4.93; N, 9.78. Found: C, 54.66; H, 5.02; N, 9.56.
Synthesis of Bis(N2SO) Ligand 2b (= H4L2b)
A solution of dioxime 6 (15.4 mg, 0.049 mmol) in dichloromethane (2 mL) was
added to a solution of 2-mercapto-3-methoxybenzaldehyde (3b, 17.8 mg, 0.11 mmol) in
dichloromethane (2 mL), and the resulting mixture was stirred for 4.5 h at room temperature.
After the solution was concentrated, the residue was subjected to column chromatography
on silica gel (eluent, dichloromethane:hexane, 1:1 then 1:0) to afford 2b as colorless crystals
(22.5 mg, 0.037 mmol, 75%), mp 128–130◦C, 1H NMR (400 MHz, CDCl3) δ 3.91 (s, 6H),
4.49–4.54 (m, 8H), 4.91 (s, 2H), 6.76 (s, 2H), 6.85 (dd, J = 8.0, 0.9 Hz, 2H), 7.10 (t, J =
8.0 Hz, 2H), 7.22 (dd, J = 8.0, 0.9 Hz, 2H), 8.23 (s, 2H), 8.43 (s, 2H), 9.68 (s, 2H); 13C
NMR (100 MHz, CDCl3) δ 56.2 (CH3), 72.5 (CH2), 73.3 (CH2), 111.1 (CH), 117.6 (C),
120.7 (CH), 121.4 (CH), 122.3 (C), 125.1 (CH), 129.8 (C), 145.8 (C), 148.8 (CH), 151.2
(CH), 154.7 (C). Anal Calcd for C28H30N4O8S2: C, 54.71; H, 4.92; N, 9.11. Found: C,
54.40; H, 4.92; N, 8.64.
Synthesis of Metallohost [L2aPd2]
A solution of ligand H4L2a (30.9 mg, 0.056 mmol) in dichloromethane (25 mL)
was added to a solution of palladium(II) acetate (23.8 mg, 0.11 mmol) in methanol (25
mL). After the solution was allowed to stand at room temperature, brown precipitates were
1
collected to give [L2aPd2] (30.1 mg, 0.039 mmol, 74%) as brown powder, H NMR (400
MHz, DMSO-d6) δ 4.48 (brt, 4H), 4.74 (brt, 4H), 6.50 (s, 2H), 7.07 (t, J = 7.4 Hz, 2H),
7.35 (t, J = 7.4 Hz, 2H), 7.57 (d, J = 7.4 Hz, 2H), 7.64 (d, J = 7.4 Hz, 2H), 8.28 (s, 2H),
8.76 (s, 2H). Anal. Calcd for C26H22N4O6Pd2S2·1.5CH2Cl2: C, 37.08; H, 2.83; N, 6.29.
Found: C, 36.71; H, 3.16; N, 6.47.
Synthesis of Metallohost [L2bPd2]
A solution of ligand H4L2b (36.4 mg, 0.059 mmol) in chloroform (100 mL) was added
to a solution of palladium(II) acetate (30.9 mg, 0.14 mmol) in chloroform (100 mL). After
the solution was allowed to stand at room temperature, brown precipitates were collected
1
to give [L2bPd2] (27.0 mg, 0.033 mmol, 55%) as brown powder, H NMR (400 MHz,
DMSO-d6) δ 3.89 (s, 6H), 4.48 (brt, 4H), 4.79 (brt, 4H), 6.49 (s, 2H), 7.05–7.07 (m, 4H),
7.23–7.26 (m, 2H), 8.26 (s, 2H), 8.80 (s, 2H). Anal. Calcd for C28H26N4O8Pd2S2·2CH2Cl2:
C, 36.27; H, 3.04; N, 5.64. Found: C, 35.95; H, 3.06; N, 5.62.
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