Synthesis of metal-hydrazone complexes and vapochromic behavior of their hydrogen-bonded proton-transfer assemblies

Article abstract of DOI:10.1021/ja1063444

We synthesized and investigated a new series of metal-hydrazone complexes, including deprotonated [MX(mtbhp)] and protonated forms [MX(Hmtbhp)](ClO ₄) (M = Pd²⁺, Pt²⁺; X = Cl^-, Br ^-; Hmtbhp = 2-(2-(2-(methylthio)benzylidene)hydrazinyl)pyridine) and hydrogen-bonded proton-transfer (HBPT) assemblies containing [PdBr(mtbhp)] and bromanilic acid (H₂BA). The mtbhp hydrazone ligand acts as a tridentate SNN ligand and provides a high proton affinity. UV-vis spectroscopy revealed that these metal-hydrazone complexes follow a reversible protonation-deprotonation reaction ([MX(mtbhp)] + H⁺ [MX(Hmtbhp)]⁺), resulting in a remarkable color change from red to yellow. Reactions between proton acceptor [PdBr(mtbhp)] (A) and proton donor H₂BA (D) afforded four types of HBPT assemblies with different D/A ratios: for D/A = 1:1, {[PdBr(Hmtbhp)](HBA)?Acetone} and {[PdBr(Hmtbhp)](HBA)?2(1,4-dioxane)}; for D/A = 1:2, [PdBr(Hmtbhp)] ₂(BA); and for D/A = 3:2, {[PdBr(Hmtbhp)]₂(HBA) ₂(H₂BA)?2Acetonitrile}. The proton donor gave at least one proton to the acceptor to form the hydrogen bonded A...D pair of [PdBr(Hmtbhp)]⁺...HBA^-. The strength of the hydrogen bond in the pair depends on the kind of molecule bound to the free monoanionic bromanilate OH group. Low-temperature IR spectra (T < 150 K) showed that the hydrogen bond distance between [PdBr(Hmtbhp)]⁺ and bromanilate was short enough (ca. 2.58 A) to induce proton migration in the [PdBr(Hmtbhp)]₂(BA) assembly in the solid state. The hydrogen bonds formed not only between [PdBr(Hmtbhp)]⁺ and HBA^- but also between HBA^- and neutral H₂BA molecules in the {[PdBr(Hmtbhp)]₂(HBA)₂(H₂BA) ?2Acetonitrile} assembly. The H₂BA-based flexible hydrogen bond network and strong acidic host structure result in an interesting vapor adsorption ability and vapochromic behavior in this assembly because the vapor-induced rearrangement of the hydrogen bond network, accompanied by changes in π-π stacking interactions, provides a recognition ability of proton donating and accepting properties of the vapor molecule.

Full text of DOI:10.1021/ja1063444

Published on Web 10/12/2010
Synthesis of Metal-Hydrazone Complexes and Vapochromic
Behavior of Their Hydrogen-Bonded Proton-Transfer
Assemblies
Atsushi Kobayashi,*,† Masa-aki Dosen,† Mee Chang,† Kiyohiko Nakajima,‡
Shin-ichiro Noro,§ and Masako Kato*,†
Department of Chemistry, Faculty of Science, Hokkaido UniVersity, North-10 West-8, Kita-ku,
Sapporo 060-0810, Japan, Department of Chemistry, Aichi UniVersity of Education, Igaya,
Kariya, Aichi 448-8542, Japan, and Research Institute for Electronic Science, Hokkaido
UniVersity, North-20 West-10, Kita-ku, Sapporo 001-0020, Japan
Received July 17, 2010; E-mail: akoba@sci.hokudai.ac.jp; mkato@sci.hokudai.ac.jp
Abstract: We synthesized and investigated a new series of metal-hydrazone complexes, including
deprotonated [MX(mtbhp)] and protonated forms [MX(Hmtbhp)](ClO4) (M ) Pd2+, Pt2+; X ) Cl-, Br-; Hmtbhp
) 2-(2-(2-(methylthio)benzylidene)hydrazinyl)pyridine) and hydrogen-bonded proton-transfer (HBPT) as-
semblies containing [PdBr(mtbhp)] and bromanilic acid (H2BA). The mtbhp hydrazone ligand acts as a
tridentate SNN ligand and provides a high proton affinity. UV-vis spectroscopy revealed that these
metal-hydrazone complexes follow a reversible protonation-deprotonation reaction ([MX(mtbhp)] + H+
H [MX(Hmtbhp)]+), resulting in a remarkable color change from red to yellow. Reactions between proton
acceptor [PdBr(mtbhp)] (A) and proton donor H2BA (D) afforded four types of HBPT assemblies with different
D/A ratios: for D/A ) 1:1, {[PdBr(Hmtbhp)](HBA) ·Acetone} and {[PdBr(Hmtbhp)](HBA) ·2(1,4-dioxane)};
for D/A ) 1:2, [PdBr(Hmtbhp)]2(BA); and for D/A ) 3:2, {[PdBr(Hmtbhp)]2(HBA)2(H2BA)·2Acetonitrile}. The
proton donor gave at least one proton to the acceptor to form the hydrogen bonded A · · ·D pair of
[PdBr(Hmtbhp)]+ · · ·HBA-. The strength of the hydrogen bond in the pair depends on the kind of molecule
bound to the free monoanionic bromanilate OH group. Low-temperature IR spectra (T < 150 K) showed
that the hydrogen bond distance between [PdBr(Hmtbhp)]+ and bromanilate was short enough (ca. 2.58
Å) to induce proton migration in the [PdBr(Hmtbhp)]2(BA) assembly in the solid state. The hydrogen bonds
formed not only between [PdBr(Hmtbhp)]+ and HBA- but also between HBA- and neutral H2BA molecules
in the {[PdBr(Hmtbhp)]2(HBA)2(H2BA)·2Acetonitrile} assembly. The H2BA-based flexible hydrogen bond
network and strong acidic host structure result in an interesting vapor adsorption ability and vapochromic
behavior in this assembly because the vapor-induced rearrangement of the hydrogen bond network,
accompanied by changes in π-π stacking interactions, provides a recognition ability of proton donating
and accepting properties of the vapor molecule.
for their potential chemical sensing applications.2-12 Mann and
co-workers investigated the [Pt(aryl-isonitrile)4][Pd(CN)4] com-
plex and suggested that the intermolecular metallophilic interac-
tion between d8 metal ions plays a critical role in its vapochromic
behavior.2 Taking advantage of this intermolecular metallophilic
interaction, many researchers have focused on developing new
vapochromic materials applicable to chemical sensors2-7 and
found that the vapochromic behavior of these materials usually
originates from the structural change induced by the adsorption
of vapor molecules.3 The adsorption of vapor molecules changes
intermolecular metallophilic and/or π-π interactions, resulting
in the perturbation of the transition energy in the visible region.
Similarly, vapochromic materials using metallophilic interactions
1. Introduction
The design and synthesis of stable and reversible chemical
sensors have received considerable attention in the past decade.1
In particular, vapochromic materials showing pronounced and
reversible changes in color and/or emission in the presence of
volatile organic compounds have become increasingly attractive
† Department of Chemistry, Hokkaido University.
‡ Aichi University of Education.
§ Research Institute for Electronic Science, Hokkaido University.
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Vapochromic Hydrogen-Bonded Proton-Transfer Assemblies
A R T I C L E S
with Au(I) ions have been also reported.4 In addition to
vapochromism, some of the Au(I) complexes also showed
mechanochromism, the reversible color change induced by
applying mechanical pressure. Despite extensive studies on
vapochromic materials, it is still difficult to achieve vapor
selectivity and recognition capability in vapochromic materials
based on metallophilic interactions. This may be due to the fact
that most vapochromic complexes are molecule-based assembled
materials in which systematic control of the crystal structure is
difficult because of nondirectional intermolecular interactions,
such as van der Waals and Coulomb interactions. Much effort
has been devoted to achieving vapor selectivity and recognition
capability. Castellano and co-workers reported a Pt(II)-diimine
based sensor microarray in which the vapor recognition capabil-
ity was achieved by combining eighteen different vapochromic
Pt(II)-terpyridyl chloride complexes.7 Another approach is to
incorporate well-known photofunctional molecules into solid
state materials.8 For example, fluorescent metal-organic frame-
works built from photofunctional ligands and various metal ions
are promising because their rigidity and permanent porosity
enable shape and size selective sensing.8 In fact, this type of
materials shows guest-dependent luminescent properties.
The hydrogen bond is one of the most effective interactions
for controlling crystal structure. It has been extensively utilized
in various fields such as supramolecular chemistry, molecular
recognition, and sensing.9 Eisenberg and co-workers have
reported the vapochromism of hydrogen bonded Pt(II)-diimine
complex [Pt(Nttpy)Cl](PF6)2 (Nttpy ) 4′-(p-nicotinamide-N-
methylphenyl)-2,2′:6′,2′′-terpyridine).10 Naota and co-workers
recently studied vapochromic organic S-shaped supramolecules
in which the hydrogen bond plays an important role in both
their recognition and vapor adsorption abilities.11 We have also
designed a hydrogen bonded vapochromic Pt(II) complex,
[Pt(CN)2(H2dcbpy)]·2H2O (H2dcbpy ) 4,4′-dicarboxy-2,2′-
bipyridine), and found that its vapochromism derives from the
formation/deformation and transformation of the hydrogen bond
network accompanied by significant changes in metallophilic
interactions.12 These studies suggest that hydrogen bonding
interactions are also useful for regulating the crystal structures
of vapochromic materials and may be promising for achieving
vapor selectivity and recognition capability.
affect both proton donating and accepting molecules. The proton
is known to move in the hydrogen bond when the donor-acceptor
distance is sufficiently short.13 In some cases, this proton
movement can provide interesting properties such as molecular
ferroelectricity.14 In this work, aiming at constructing a new
vapochromic system utilizing flexible hydrogen bonds and
proton transfer, we have designed a series of hydrogen-bonded
proton-transfer(HBPT)assembliesthatconsistofametal-hydrazone
unit as a proton acceptor and bromanilic acid as a proton donor.
In this paper, we report the structures and acid-base behaviors
of newly synthesized metal-hydrazone complexes [MX(mtbhp)]
(M ) Pd2+, X ) Cl- (1); M ) Pd2+, X ) Br- (2); M ) Pt2+
,
X ) Cl- (3); Hmtbhp )2-(2-(2-(methylthio)benzylidene)hy-
drazinyl)pyridine) and the physical properties of their HBPT
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Kobayashi et al.
Figure 1. Structural representation of metal-hydrazone complexes 1-3 and 1H and their proton transfer assemblies 4-8.
Hmtbhp. 2-Hydrazinopyridine (333.0 mg, 3.05 mmol) was added
to a 2-methylthio-benzaldehyde (462.8 mg, 3.04 mmol) solution
in methanol (10 mL) and refluxed for 1 h. The reaction solution
was cooled to room temperature, allowing a pale-yellow precipitate
to emerge gradually. The precipitate was filtered, washed using
diethylether, and vacuum-dried for 1 h to afford Hmtbhp as a pale
yellow crystalline powder. Yield: 558.4 mg, 75.2% (based on
2-methylthio-benzaldehyde). Elemental analysis calcd for C13-
H13N3S1: C, 64.17; H, 5.38; N, 17.27. Found: C, 64.05; H, 5.37;
N, 17.21.
assemblies {[PdBr(Hmtbhp)](HBA) ·Acetone} (4), {[PdBr(Hmt-
bhp)](HBA) ·2(1,4-dioxane)} (5), [PdBr(Hmtbhp)]2(BA) (6), and
{[PdBr(Hmtbhp)]2(HBA)2(H2BA)·2CH3CN} (7) (H2BA ) bro-
manilic acid; HBA ) monoanionic bromanilate) (Figure 1). We
also demonstrate that HBPT assembly 7 is a new vapochromic
material that can recognize the proton donating/accepting ability
of the vapor. To the best of our knowledge, this is the first proton
donor-acceptor based vapochromic material that does not
involve metallophilic interactions.
[PdCl(mtbhp)] (1). A solution of Hmtbhp ligand (31.7 mg, 0.13
mmol) in acetonitrile (10 mL) was added to a solution of
PdCl2(PhCN)2 (50 mg, 0.13 mmol) in acetonitrile (10 mL) in a
light resistant flask. Triethylamine (13.4 mg, 0.13 mmol) was added
slowly to the reaction mixture, which was then allowed to stand
for 1 d in the dark until reddish-purple platelets emerged. The
crystals were isolated by filtration, washed using a small amount
of acetonitrile, and then dried under vacuum. Yield: 31.3 mg, 63.5%
(based on PdCl2(PhCN)2). Elemental analysis calcd for C13H12-
ClN3SPd: C, 40.64; H, 3.15; N, 10.94. Found: C, 40.45; H, 3.11;
N, 10.98.
[PdBr(mtbhp)] (2). Bromide complex 2 was obtained through
a similar synthetic method as 1 but using PdBr2(PhCN)2 instead of
PdCl2(PhCN)2. Yield: 43.4 mg, 77.9% (based on PdBr2(PhCN)2).
Elemental analysis calcd for C13H12BrN3SPd: C, 36.43; H, 2.82;
N, 9.80. Found: C, 36.21; H, 2.74; N, 9.73.
2. Experimental Section
General Procedures. CAUTION! Although we experienced no
difficulties, perchlorate salts are potentially explosiVe and should
only be used in small quantities and handled with care.
All commercially available starting materials were used as
received, and solvents were used without any purification. Unless
otherwise stated, all manipulations were performed in air. 2-Me-
thylthio-benzaldehyde, 2-hydrazinopyridine, PdCl2, PdBr2, K2PtCl4,
and bromanilic acid (H2BA) were purchased from Wako Pure
Chemical Industries, Ltd. Japan. PdCl2(PhCN)2, PdBr2(PhCN)2, and
PtCl2(PhCN)2 (PhCN ) benzonitrile) were prepared according to
published methods.15
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[PtCl(mtbhp)] (3). A solution of Hmtbhp ligand (36.7 mg, 0.15
mmol) in acetonitrile (15 mL) was added to a solution of
PtCl2(PhCN)2 (71.1 mg, 0.15 mmol) in acetonitrile (15 mL) in a
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light resistant flask. Triethylamine (15.5 mg, 0.15 mmol) was added
slowly to the reaction mixture, which was refluxed for 3 h in the
dark and then allowed to cool to room temperature. The solvent
was evaporated to about 5 mL to give a red crystalline precipitate.
The precipitate was isolated by filtration, washed using a small
amount of acetonitrile, and then dried under vacuum. Yield: 49.3
mg, 51.5% (based on PtCl2(PhCN)2). Elemental analysis calcd for
C13H12ClN3SPt: C, 33.02; H, 2.56; N, 8.89. Found: C, 33.07; H,
2.84; N, 8.87.
[PdCl(Hmtbhp)](ClO4) (1H). Perchloric acid (10 equiv) was
added to a solution of 1 (3.0 mg, 7.83 µmol) in acetonitrile. The
solution color immediately changed from dark red to yellow. Diethyl
ether vapors were allowed to diffuse into the solution for 3 d to
afford [PdCl(Hmtbhp)](ClO4) as pale yellow crystals. The crystals
were filtered, washed using diethyl ether, and dried under vacuum
for 1 h. Yield: 1.6 mg, 42% (based on 1). Elemental analysis calcd
for C13H13Cl2N3O4SPd: C, 32.22; H, 2.70; N, 8.67. Found: C, 31.86;
H, 2.94; N, 8.85.
{[PdBr(Hmtbhp)](HBA) ·(CH3)2CO} (4). A solution of H2BA
(23.3 mg, 78.2 mmol) in acetone (10 mL) was added to a solution
of 2 (3.0 mg, 7.8 mmol), in acetone (10 mL) in a light resistance
flask. Reddish-purple needle crystals of 4 emerged by natural
evaporation for several days. The crystals were isolated by filtration,
washed with a small amount of acetonitrile, and air-dried. Yield:
3.2 mg, 53% (based on 2). Elemental analysis calcd for
C22H20Br3N3O5SPd: C, 33.68; H, 2.57; N, 5.36. Found: C, 33.41;
H, 2.40; N, 5.09.
{[PdBr(Hmtbhp)](HBA) ·2(1,4-dioxane)} (5). A solution of
H2BA (230 mg, 772.3 mmol) in 1,4-dioxane (30 mL) was added
to a solution of 2 (33.0 mg, 78.0 mmol), in 1,4-dioxane (70 mL) in
a light resistance flask. Dark-red platelet crystals of 5 emerged by
natural evaporation for several weeks. The crystals were isolated
by filtration, washed with a small amount of 1,4-dioxane, and air-
dried. Yield: 13.5 mg, 19.3% (based on 2). Elemental analysis calcd
for C21H18Br3N3O5SPd: C, 32.73; H, 2.35; N, 5.45. Found: C, 32.69;
H, 2.59; N, 5.26.
[PdBr(Hmtbhp)]2(BA) (6). Solutions of H2BA (2.3 mg, 7.8
mmol) and 10 equiv of pyrazine (3.1 mg, 39 mmol) in acetonitrile
(10 mL) were added to a solution of 2 (3.0 mg, 7.8 mmol), in
acetonitrile (30 mL) in a light resistance flask. Red platelets of 6
emerged by natural evaporation for several days. The crystals were
isolated by filtration, washed using a small amount of acetonitrile,
and then air-dried. Yield: 2.5 mg, 28% (based on 2). Elemental
analysis calcd for C32H26Br4N6O4Pd2S2: C, 33.27; H, 2.27; N, 7.28.
Found: C, 33.23; H, 2.15; N, 7.12.
{[PdBr(Hmtbhp)]2(HBA)2(H2BA)·2CH3CN} (7). A solution of
H2BA (23.0 mg, 77.2 mmol) in acetonitrile (10 mL) was added to
a solution of 2 (3.3 mg, 7.7 mmol) in acetonitrile (10 mL) in a
light resistance flask. Reddish-purple needle crystals of 7 emerged
by natural evaporation for several days. The crystals were isolated
by filtration, washed with a small amount of acetonitrile, and then
air-dried. Yield: 3.8 mg, 54% (based on 2). Elemental analysis calcd
for C24H18Br4N4O6SPd: C, 31.45; H, 1.98; N, 6.11. Found: C, 31.17;
H, 2.03; N, 5.93.
Single Crystal X-ray Diffraction Measurements. All single
crystal X-ray diffraction measurements were performed using a
Rigaku Mercury CCD diffractometer with graphite monochromated
Mo KR radiation (λ ) 0.710 69 Å) and a rotating anode generator.
Each single crystal was mounted on a glass fiber with epoxy resin.
The crystal temperature was cooled down using a N2-flow type
temperature controller. Diffraction data were collected and pro-
cessed using the CrystalClear software.16 Structures were solved
by the direct method using SIR-2004 for 1, 2, 5, 6, and 7; SIR-92
for 4; and SHELXS-97 for 1H.17-19 Structural refinements were
performed by full-matrix least-squares using SHELXL-97. Non-
(16) CrystalClear: Molecular Structure Corporation, Orem, UT, 2001.
(17) SIR2004: Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.;
Cascarano, G. L.; De Caro, L.; Giacovazzo, C.; Polidori, G.; Spagana,
R. J. Appl. Crystallogr. 2005, 38, 381–388.
9
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A R T I C L E S
Kobayashi et al.
Table 2. Selected Bond Lengths and Angles of the Hydrazone Ligand for Metal-Hydrazone Complexes 1, 2, and 1H and HBPT Assemblies
4, 5, 6, and 7
6 at 110 K
(R)a
6 at 150 K
(R)a
6 at 250 K
(R)a
1
2
1H
4
5
(S)
(S)
(S)
7
C6-C7
1.450(4) 1.448(3) 1.450(4) 1.450(5) 1.459(8) 1.464(5) 1.464(5) 1.460(5) 1.463(5) 1.443(5) 1.454(6) 1.463(7)
1.301(4) 1.304(3) 1.273(4) 1.291(4) 1.261(8) 1.283(6) 1.285(7) 1.293(6) 1.288(6) 1.310(7) 1.289(7) 1.279(6)
1.382(3) 1.375(2) 1.374(4) 1.378(4) 1.377(6) 1.378(4) 1.387(4) 1.380(4) 1.378(4) 1.366(4) 1.374(4) 1.394(6)
1.337(4) 1.342(3) 1.369(4) 1.360(4) 1.376(9) 1.357(7) 1.350(7) 1.352(6) 1.361(6) 1.370(7) 1.368(7) 1.351(6)
1.369(4) 1.372(2) 1.343(4) 1.346(2) 1.330(7) 1.344(6) 1.351(6) 1.348(6) 1.347(6) 1.341(7) 1.341(6) 1.355(7)
1.420(4) 1.423(3) 1.394(5) 1.396(4) 1.389(8) 1.404(5) 1.402(5) 1.402(5) 1.410(5) 1.392(6) 1.412(6) 1.387(8)
C7-N1
N1-N2
N2-C8
C8-N3
C8-C9
N2-H13
N2b-H13b
-
-
-
0.82(4)
0.82(2)
0.85(6)
1.34(7)
-
1.61(8)
1.22(7)
-
1.24(8)
1.11(7)
-
1.11(6)
0.88(4)
-
-
-
-
-
-
-
-
N1-N2-C8 111.8(2) 112.3(2) 119.2(3) 118.3(2) 119.6(5) 117.4(4) 117.7(3) 118.2(3) 117.4(3) 117.4(4) 117.8(4) 119.0(4)
a (R)-[PdBr(Hmtbhp)]+ unit in 6 was labeled with an additional character “b”.
Table 3. Selected Bond Lengths of the Bromanilate Anion in
HBPT Assemblies 4, 5, 6, and 7
6
4
5
110 K
150 K
250 K
7
C15-O1 1.327(2) 1.343(7) 1.275(7) 1.272(6) 1.276(7) 1.321(6)
C16-O2 1.239(3) 1.229(6) 1.224(6) 1.229(6) 1.237(7) 1.246(6)
C18-O3 1.256(2) 1.239(9) 1.274(7) 1.280(6) 1.281(7) 1.227(6)
C19-O4 1.215(3) 1.212(7) 1.227(7) 1.225(6) 1.228(7) 1.210(6)
Table 4. Hydrogen Bond (NH · · ·O) Distances between
[PdBr(Hmtbhp)]+ and HBA- in HBPT Assemblies 4, 5, 6, and 7
6
4
5
110 K
2.653(2) 2.597(8) 2.585(6) 2.584(5) 2.577(6) 2.679(5)
2.580(5) 2.588(5) 2.594(6)
150 K
250 K
7
N2-H13 · · · ·O3
N2b-H13b · · · ·O1
-
-
-
hydrogen atoms were refined anisotropically, hydrogen atoms bound
to nitrogen or oxygen atoms were refined isotropically, and other
hydrogen atoms were refined using the riding model. All calcula-
tions were performed using the Crystal Structure crystallographic
software package.20 Crystallographic data obtained for each
complex are summarized in Table 1. Selected bond lengths and
angles are shown in Tables 2 and 3 for metal-hydrazone and
bromanilate units, respectively. Hydrogen bond distances are
reported in Table 4.
Powder X-ray Diffraction. Powder X-ray diffraction measure-
ment was performed using a Rigaku SPD diffractometer at beamline
BL-8B, Photon Factory, KEK, Japan. The wavelength of the
synchrotron X-ray was 1.200(1) Å. Every sample was placed in a
glass capillary of 0.5 mmφ diameter.
UV-vis Spectroscopy. The UV-vis adsorption spectrum of
each complex was recorded on a Shimadzu UV-2400PC spectro-
photometer. The diffuse reflectance spectrum of each complex was
recorded on the same spectrophotometer equipped with an integrat-
ing sphere apparatus. Obtained reflectance spectra were converted
to absorption spectra using the Kubelka-Munk function F(R∞).
IR Spectroscopy. For IR absorption measurements, samples
were prepared by grinding single crystals with KBr and processing
the mixtures into pellets using a press (∼6 kbar) under vacuum.
Temperature dependent IR spectra were recorded on a Thermo-
Nicolet 6700 FT-IR spectrometer. The sample temperature was
controlled using an Oxford MicrostatHe instrument.
Figure 2. (a) Molecular structure of 1, (b) alternate π-π stacking of R-
and S-isomers along the a axis, and (c) packing diagram of 1 in the bc
plane with thermal vibrational ellipsoids at the 50% probability level.
Hydrogen atoms are omitted for clarity.
Thermogravimetric Analysis. Thermogravimetry and differ-
ential thermal analysis were performed using a Rigaku ThermoEvo
TG8120 analyzer.
Adsorption Isotherms. Adsorption isotherms of organic solvent
vapors were measured using an automatic volumetric adsorption
apparatus (BELSORP-MAX and BELSORP-mini; BEL Japan, Inc.).
Before each measurement, the sample was dried at 373 K under
vacuum.
3. Results and Discussion
3.1. Crystal Structures of Metal-Hydrazone Complexes.
First, we studied the effect of protonation on molecular and
crystal structures. Metal-hydrazone complexes 1-3 showed a
reversible protonation-deprotonation reaction in solution (see
below), and we succeeded in isolating both deprotonated
[PdCl(mtbhp)] (1) and protonated [PdCl(Hmtbhp)](ClO4) (1H)
forms as single crystals.
3.1.1. Deprotonated Form [PdX(mtbhp)] (X ) Cl, Br). Figure
2a shows the molecular structure of 1. The deprotonated form
1 crystallizes in the monoclinic P21/c space group. The tridentate
(18) SIR92: Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,
435–436.
(19) SHELX97: Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112–
122.
(20) CrystalStructure 3.7.0: Crystal Structure Analysis Package, Rigaku
and Rigaku/MSC (2000-2005).
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Vapochromic Hydrogen-Bonded Proton-Transfer Assemblies
A R T I C L E S
mtbhp ligand is coordinated to Pd2+ ions through two pyridyl
and hydrazone N atoms and the methyl-thio S atom. All atoms
are located on the same plane except for the methyl group.
Owing to the chirality of the mtbhp ligand S atom, the
[PdCl(mtbhp)] molecule adopts a spiral configuration to produce
two (R)-[PdCl(mtbhp)] and (S)-[PdCl(mtbhp)] chiral molecules.
As shown in Figure 2b, these two enantiomers are alternately
stacked and form one-dimensional (1-D) columns along the a
axis, resulting in the formation of the racemic crystal (R,S)-
[PdCl(mtbhp)]. Within the 1-D columns, the long distance
between Pd ions (Pd · · ·Pd > 4.9 Å) indicates no metallophilic
interactions. Because of the steric effect of the methyl group,
the [PdCl(mtbhp)] units are weakly dimerized through relatively
strong π-π interactions, consistent with the 3.32 Å distance
between two [PdCl(mtbhp)] molecular planes. In dimers formed
by pairs of (R)- and (S)-isomers, two [PdCl(mtbhp)] molecular
planes overlap and the methyl groups of the mtbhp ligands are
directed to the outside of the dimer. As a result, the π-π
interactions are stronger in the dimer than between dimers (3.47
Å). The bromide complex [PdBr(mtbhp)] (2) obtained by ligand
substitution has an isomorphic structure of 1, as shown by its
same space group of P21/c. The bond distances in the mtbhp
ligand of 1 and 2 are almost the same (Table 2), suggesting
that substitution of the chloride ion by a bromide ion hardly
affects the molecular structure of the metal-hydrazone complex.
However, the distances between [PdBr(mtbhp)] molecular planes
were slightly shorter within the 1-D column of 2 (3.31 and 3.45
Å) than in that of 1 (3.32 and 3.47 Å). This difference may be
due to the steric effect of the larger bromide ions, which causes
each molecule to slightly slip toward opposite directions relative
to each other in the bc plane to reduce the steric effect. As a
result, the absorption band observed around 600 nm in the
diffuse reflectance spectrum of 2 slightly shifted by about 20
nm to longer wavelengths compared to that of 1, consistent with
the shorter π-π stacking distances (see Figure S1 in the
Supporting Information).
Figure 3. (a) Stacking structure and (b) packing diagram of 1H along the
b axis with thermal vibrational ellipsoids at the 50% probability level. Dotted
lines show hydrogen bonds. Carbon-bound hydrogen atoms are omitted for
clarity.
Figure 4. Changes in the absorption of 1 in acetonitrile upon addition of
70% aq. HClO4. Spectra were acquired for 0, 0.25, 0.5, 0.60, 0.70, 0.80,
1.00, and 1.20 equiv additions of acid.
3.1.2. Protonated Form [PdCl(mtbhp)](ClO4) (1H). The mo-
lecular framework and the chirality of 1H are almost the same
as those for 1. In particular, the bond lengths and angles of the
methyl-thio-benzene ring are almost the same as those in 1H
and 1. However, protonation of the hydrazone N2 atom
significantly affected the bond lengths around hydrazone and
pyridine groups. As shown in Table 2, the N2-C8 bond
lengthened by about 0.03 Å, while C7-N1, C8-N3, and
C8-C9 bonds shortened by about 0.03 Å upon protonation.
The N1-N2-C8 bond angle (119.2(3)°) also increased by about
7° to almost reach 120°, which is an ideal bond angle of sp2
hybridized atoms. Bond lengths around the pyridine ring of 1H
are therefore very close to typical values for pyridine and related
substances.21 These differences suggest that protonation of the
hydrazone N2 atom affects the degree of the π electron
delocalization in the mtbhp ligand. In other words, the pyridine
π electrons are delocalized not only in the ring but also in the
hydrazone of 1, whereas the delocalization is limited to the
pyridine ring upon protonation of the hydrazone moiety. The
planes (3.40 Å) increased by about 0.09 Å in 1H, suggesting
that π-π interactions are weaker in 1H than in 1. This may be
due to either Coulomb repulsion between the [PdCl(Hmtbhp)]+
cations or steric hindrance of the perchlorate anion.
3.2. Protonation and Deprotonation Reactions of Metal-
Hydrazone Complexes. Strong bases are known to remove a
proton that binds to the hydrazone group, indicating that the
acid dissociation constant (pKa) of hydrazones is not so low.22
In fact, the proton bound to the Hmtbhp ligand N2 atom could
not be removed until the pH reached 10.4 (see Figure S2),
suggesting that the pKa value of the Hmtbhp ligand is higher
than 10.4. The proton affinity of the metal-hydrazone complex
was examined through acidic and basic titrations in solution.
The reactions were monitored by UV-vis absorption spectros-
copy. Figure 4 shows the UV-vis absorption spectral changes
of 1 upon addition of perchloric acid in acetonitrile. The
absorption maximum (λmax), molar absorption coefficient (ε),
and acid-dissociation constant (pKa) are given in Table 5 for 1,
[PdBr(mtbhp)] (2), and [PtCl(mtbhp)] (3). Before the addition
of HClO4, an absorption band attributable to the intraligand
charge transfer (ILCT) transition in the hydrazone ligand was
observed at 474 nm.22 The ILCT band decreased and a new
[PdCl(Hmtbhp)]+ cations are hydrogen-bonded to the ClO4
-
counteranions (2.838(4) Å). Similar to the case for 1, both (R)-
and (S)-[PdCl(Hmtbhp)]+ units are stacked alternately along the
b axis in 1H (Figure 3). Compared with deprotonated form 1,
the distance between two adjacent [PdCl(Hmtbhp)]+ molecular
(22) (a) Kohata, K.; Kawamonzen, Y.; Odashima, T.; Ishii, H. Bull. Chem.
Soc. Jpn. 1990, 63, 3398–3404. (b) Lub, Y.-H.; Lua, Y.-W.; Wu, C.-
L.; Shao, Q.; Chen, X.-L.; Bimbong, R. N. B. Spectrochim. Acta A
2006, 65, 695–701.
(21) (a) Siedle, A. R.; Pignolet, L. H. Inorg. Chem. 1982, 21, 135–141.
(b) Tebbe, K.-F.; Grafe-Kavoosian, A.; Freckmann, B. Z. Naturforsch.,
B: Chem. Sci. 1996, 51, 999–1007.
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A R T I C L E S
Kobayashi et al.
Table 5. Electronic Transition Data and Acid Dissociation
Constants of [MX(mthbhp)] Complexes
Complex
λ
max./nm
ε/M-1cm-1
pKaa
1
2
3
474
477
490
1.78 × 104
2.45 × 104
2.50 × 104
3.6
3.6
3.4
a Measured in 1:1 CH3CN/H2O.
absorption band appeared at 365 nm upon acid addition, while
the isosbectic points were maintained at 327 and 411 nm.
Conversely, the original spectrum of 1 was recovered upon
addition of a base like triethylamine. These changes clearly show
that the protonation of 1 and deprotonation of 1H occur without
any decomposition. The pKa value of 1 was determined to be
3.6 in a 1:1 acetonitrile/water mixture using the pH-dependent
absorbance at 477 nm (see Figure S3). [PdBr(mtbhp)] (2) and
[PtCl(mtbhp)] complexes (3) also exhibited reversible proton-
ation/deprotonation reaction upon acid or base addition (see
Figure S3). The ILCT band wavelengths of 2 and 3 were found
to be 477 and 490 nm, respectively. The pKa value of 2 was
the same as that of 1, while that of 3 was found to be slightly
smaller. The differences between the three [MX(mtbhp)]
complexes indicate that, unlike the halide ligand substitution,
the substitution of the metal ions affects the electronic property
of these complexes. This result may originate from the difference
in electronegativity between Pt and Pd atoms (2.28 and 2.20,
respectively). Compared to other metal-hydrazone complexes,
such as [CuI(Hpbph)] or [PtCl(pbph)] (Hpbph ) 2-(diphe-
nylphosphino)benzaldehyde-2-pyridylhydrazone),23 these com-
plexes have relatively small pKa values. The reasons for these
strong acid properties may be the high planarity of the hydrazone
ligand and/or electron donating ability of the methyl-thio group.
We have previously reported that the high planarity contributes
to the stabilization of the deprotonated form in which π-electrons
are delocalized in the entire ligand.23
Figure 5. (a) A-D-S type hydrogen bond network, (b) stacking structure,
and (c) packing diagram of 4 along the b axis with thermal vibrational
ellipsoids at the 50% probability level. Acetone molecules are shown as
space filling models. Dotted lines represent hydrogen bonds. Carbon-bound
hydrogen atoms are omitted for clarity.
(HBA-).24 The remaining monoanionic bromanilate OH group
is hydrogen-bonded to the acetone solvent molecule (O-H· · ·O
) 2.796 Å), leading to an A · · ·D· · ·S (S: solvent) type
hydrogen-bonded network. The NH · · ·O hydrogen bond distance
between [PdBr(Hmtbhp)]+ and HBA- is 2.653 Å, which is
shorter than the typical value (ca. 2.89 Å).25 The three com-
ponents are stacked into three separate columns along the b axis
as shown in Figure 5b and c. The (R)- and (S)-[PdBr(Hmtbhp)]+
ions are stacked alternately as in 1 and 1H. Because of the steric
hindrance of the [PdBr(Hmtbhp)]+ methyl group, there are no
π-π stacking interactions within the HBA- columns. The
distance between methyl-thio-benzene and pyridine rings within
the [PdBr(Hmtbhp)]+ columns (ca. 3.37 Å) suggests moderate
π-π interactions. As shown in Figure 5b, each enantiomer is
stacked alternately along the column, similarly to 1, 2, and 1H.
It is noted that the molecular plane of [PdBr(Hmtbhp)]+ is not
parallel to the plane of HBA- in 4, showing that the electronic
contribution to π-π stacking is not effective between these
units.
3.3. Crystal Structures of Hydrogen-Bonded Proton-
Transfer Assemblies. Because of its ability to accept one electron
and donate two protons, H2BA has been widely used to construct
hydrogen bonded supramolecular architectures and molecular
ferroelectric materials.13,14 To investigate the hydrogen bonding
and proton-transfer property of the [MX(mtbhp)] complexes,
we synthesized new HBPT assemblies using the metal-hydrazone
complex and H2BA. In this section, we discuss the molecular
and crystal structures of these assemblies from the view points
of proton transfer and hydrogen bonding.
On the other hand, the reaction between 2 and H2BA in 1,4-
dioxane afforded the different 1:1 adduct {[PdBr(Hmtbhp)]-
(HBA)·2(1,4-dioxane)} (5). Figure 6a shows the hydrogen bond
j
network at 250 K. Complex 5 crystallized in the triclinic P1
j
space group. Since the inversion center of the P1 space group
is located on the midpoint between two bromanilic acid
molecules, only one crystallographically independent Pd com-
plex and a bromanilic acid molecule are present in the unit cell.
Judging from the bond distances around the hydrazone ligand
and bromanilic acid shown in Tables 3 and 4, the proton donor
H2BA gave one proton to the Pd complex to form a D· · ·A
pair as well as the HBPT assembly 4. Unlike in the case of 4,
the two D · · ·A pairs are hydrogen-bonded to each other,
resulting in the formation of the A · · ·D· · ·D· · ·A type hydrogen
bond network. The NH · · ·O hydrogen bond distance between
[PdBr(Hmtbhp)]+ and HBA- is 2.597 Å which is shorter than
those of 4 and 7 and slightly longer than that of 6. Interestingly,
the [PdBr(Hmtbhp)]+ and HBA- units are stacked alternately
3.3.1. 1:1 Assembly between [PdBr(mtbhp)] and H2BA. The
reaction between deprotonated form 2 and H2BA in acetone
afforded the 1:1 adduct {[PdBr(Hmtbhp)](HBA) ·Acetone} (4)
as a crystalline material. Figure 5a shows the three-component
hydrogen bond network between the Pd complex, bromanilic
acid, and acetone for 4. The HBPT assembly (4) crystallizes in
the monoclinic C2/c space group. The bond lengths and angles
of the hydrazone ligand are very close to those of protonated
complex 1H (Table 2), indicating that the [PdBr(mtbhp)]
molecule accepts one proton from the bromanilic acid to form
a proton donor-acceptor (D · · ·A) pair. Table 3 also shows three
shorter CdO and one longer CsO bond lengths for bromanilic
acid, consistent with typical values for monoanionic bromanilate
(24) (a) Andersen, E. K. Acta Crystallogr. 1967, 22, 188–191. (b) Andersen,
E. K. Acta Crystallogr. 1967, 22, 196–201. (c) Ishida, H.; Kashino,
S. Acta Crystallogr., Sect. C 1999, 55, 1149–1152.
(25) Kuleshova, L. N.; Zorkii, P. M. Acta. Crystallogr., Sect. B 1981, 37,
1363–1366.
(23) Chang, M.; Horiki, H.; Nakajima, K.; Kobayashi, A.; Chang, H.-C.;
Kato, M. Bull. Chem. Soc. Jpn. 2010, in press. DOI: 10.1246/
bcsj.20100065.
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Vapochromic Hydrogen-Bonded Proton-Transfer Assemblies
A R T I C L E S
Figure 6. (a) A-D-D-A type hydrogen bond network, (b) stacking structure,
and (c) packing diagram of 5 along the a axis with thermal vibrational
ellipsoids at the 50% probability level. 1,4-Dioxane molecules are shown
as space filling models. Dotted lines represent hydrogen bonds. Carbon-
bound hydrogen atoms are omitted for clarity.
Figure 7. (a) A-D-A type hydrogen bond network, (b) stacking structure,
and (c) packing diagram of 6 at 250 K along the b axis with thermal
vibrational ellipsoids at the 50% probability level. Dotted lines represent
hydrogen bonds. Carbon-bound hydrogen atoms are omitted for clarity.
along the a axis with the ca. 3.4 Å intermolecular distances
between the methyl-thio-benzene or pyrizine rings of [Pd-
Br(Hmtbhp)]+ and HBA- as shown in Figure 6b. This suggests
the intermolecular π-π stacking interaction between these
proton donor and acceptor. The electronic absorption band
appeared at longer wavelengths than those of 4 and 6 but shorter
wavelengths than that of 7, probably due to the alternately
stacking manner and π-π stacking interaction between the
proton donor and acceptor. It should be noted that the solvated
1,4-dioxane molecules were not hydrogen-bonded to any other
molecules and formed the layers on the ac plane as shown in
Figure 6c.
3.3.2. 2:1 Assembly of [PdBr(mtbhp)] and H2BA. The reac-
tion between deprotonated form 2 and H2BA in a 1:1 molar
ratio in acetonitrile in the presence of 10 equiv of pyrazine
afforded crystals of [PdBr(Hmtbhp)]2(BA) (6). Figure 7a shows
the hydrogen bond network of 6 at 250 K. Complex 6
crystallizes in the monoclinic C2/c space group. There are no
acetonitrile or pyrazine molecules in the crystal. It is noteworthy
that two crystallographically independent [PdBr(mtbhp)] units
are observed, meaning that the structural features of these two
molecules are different. As judged from the bond distances and
angles of the hydrazone ligand shown in Table 2, each
[PdBr(mtbhp)] unit can accept one proton from H2BA to form
an A· · ·D· · ·A structure. Each structural component is stacked
along the b axis in the same manner as that in HBPT assembly
4 and protonated complex 1H. Within the [PdBr(Hmtbhp)]+
columns, the (R)- and (S)-enantiomers are stacked alternately
along the column (Figure 7b). Similar to 4, the steric hindrance
of the methyl group prevents the bromanilates from forming
π-π stacking interactions within bromanilate columns. Since
the torsion angles between [PdBr(Hmtbhp)] and bromanilate
molecular planes are about 72°, the electronic interaction
between these units is thought to be very weak. Distances
between methyl-thio-benzene and pyridine rings within [Pd-
Br(Hmtbhp)]+ columns (ca. 3.36 Å) in 6 are slightly shorter
than those in 4, suggesting relatively stronger π-π interactions.
This difference was confirmed in the diffuse reflectance spectra
of 4 and 6, which show that the absorption band around 600
nm of 6 shifted to longer wavelengths by about 26 nm compared
to that of 4 (see Figure S4). It should be noted that the observed
bromanilate CdO bond lengths (ca. 1.27 Å) do not match those
of HBA- or those of BA2- (ca. 1.243 Å) (Table 3), suggesting
that the two protons, H13 and H13b, may be disordered between
the hydrazone N-site and the bromanilate O-site. As shown in
Table 4, the NH · · ·O hydrogen bond distances between bro-
manilate and [PdBr(Hmtbhp)]+ units are shorter than those in
the (Chloranilic acid)(1,2-Diazine)2 proton-transfer complex
(2.615 Å) in which hydrogen bond protons can migrate in the
solid state.24b In fact, the hydrazone N2-H bond distances in
two [PdBr(Hmtbhp)]+ units whose hydrogen atoms have been
refined isotropically are much longer (1.11(6) and 1.18(7) Å)
than the normal value of the N-H bond (ca. 0.85 Å). Thus,
hydrogen bond protons are expected to migrate between
bromanilate and hydrazone units.
To gain more insight about the proton migration, single-
crystal X-ray diffraction measurements at lower temperature
were conducted for HBPT assembly 6. Crystallographic pa-
rameters obtained at 110 and 150 K are shown in Table 1. At
these temperatures, the space groups were similar to that
9
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A R T I C L E S
Kobayashi et al.
shorter CdO bonds and one longer CsO bond, in close
agreement with typical values for HBA- (Table 3). Interestingly,
these two D · · ·A pairs are bridged by two hydrogen bonds
between one H2BA molecule and two HBA- units to form an
A· · ·D· · ·D· · ·D· · ·A type hydrogen bond network (Figure 8a).
The bridging H2BA molecule is located at the inversion center
and has two shorter CdO (1.229 Å) and two longer CsO bonds
(1.319 Å), consistent with its neutrality. As shown in Figure
8b, structural components [PdBr(Hmtbhp)]+, HBA-, and H2BA
are stacked separately. Interestingly, the (R)- and (S)-[Pd-
Br(Hmtbhp)]+ enantiomers are also stacked separately unlike
in HBPT assemblies 4 and 6. The NH · · ·O hydrogen bond
distance between [PdBr(Hmtbhp)]+ and HBA- is 2.673(5) Å,
in close agreement with that in the 55DMBP-H2CA complex
(2.683 Å),14c which exhibits a proton-transfer-driven dielectric
phase transition in the hydrogen bond. This result implies that
the proton between [PdBr(Hmhtbp)]+ and HBA- may move in
the hydrogen bond as a result of some external stimuli such as
light, electric field, and temperature. In contrast, the hydrogen
bond distance between HBA- and H2BA (2.827(5) Å) is
significantly longer than the other hydrogen bond distances in
this HBPT assembly. The planar [PdBr(Hmtbhp)]+ and HBA-
units are in the same plane whereas the torsion angle between
HBA- and H2BA molecular planes is very close to 90°,
suggesting that the π-π stacking-induced electronic interactions
are effective between [PdBr(Hmtbhp)]+ and HBA- units but
ineffective between HBA- and H2BA units. Intermolecular
distances within columns show moderate π-π interactions
within HBA- columns (ca. 3.38 Å) but relatively weak π-π
interactions in [PdBr(Hmhtbp)]+ and H2BA columns (over 3.45
Å). Interestingly, Figure 8b displays significantly short π-π
interactions (ca. 3.0 Å) between [PdBr(Hmhtbp)]+ and HBA-
units located above the lower [PdBr(Hmhtbp)]+ molecular plane.
The absorption band observed for 7 in the solid state diffuse
reflectance spectrum appeared at a longer wavelength compared
to 4, 5, and 6 (see Figure S4). Considering that π-π interactions
between adjacent HBA- units are only effective in 7, the
absorption band observed at longer wavelengths (around 650
nm) is attributable to the intermolecular charge transfer transition
from HBA- to [PdBr(Hmhtbp)]+ or the intramolecular π-π*
transition in HBA-, which is stabilized by the intermolecular
π-π interactions. Acetonitrile molecules also form a 1-D
channel along the b axis but do not contribute to the hydrogen
bond network of this HBPT assembly.
Figure 8. (a) A-D-D-D-A type hydrogen bond network, (b) stacking
structure, and (c) packing diagram of 7 at 150 K along the b axis with
thermal vibrational ellipsoids at the 50% probability level. Acetonitrile
molecules are shown as space filling models. Dotted lines represent hydrogen
bonds. Carbon-bound hydrogen atoms are omitted for clarity.
obtained at 250 K and the molecular structures were almost
the same. However, it may be noteworthy that the N2-C8 bond
in the (R)-enantiomer shortened slightly with decreasing tem-
perature (see Table 2) and was closer to that of deprotonated
form 1 than that of protonated form 1H at 110 K. The hydrogen
bond distance between the bromanilate and (S)-[PdBr(Hmt-
bhp)]+ unit was about 0.017 Å shorter than that between the
bromanilate and (R)-[PdBr(Hmtbhp)]+ unit at 250 K. At 110
K, these two hydrogen bond distances were the same within
experimental error, implying that the electrostatic potential is
affected by temperature for the proton. In fact, the N2b-H13b
bond was found to lengthen by about 0.5 Å with decreasing
temperature, allowing the H13b proton to attach to the broma-
nilate at 110 K. The N2-H13 bond length also lengthened with
decreasing temperature, and the H13 proton was located at the
midpoint of the N2 · · ·O3 hydrogen bond at 110 K. In addition,
the IR spectrum of 6 displayed an absorption band corresponding
to the ν(O-H) mode at 3200 cm-1 below 150 K (see below).
These results suggest that at least one proton can move in the
hydrogen bond depending on temperature.
3.3.3. 2:3 Assembly of [PdBr(mtbhp)] and H2BA. The reac-
tion between the deprotonated form 2 and H2BA in acetonitrile
solution afforded the 3:2 adduct {[PdBr(Hmtbhp)]2(HBA)2-
(H2BA)·2CH3CN} (7) as crystals. It is noteworthy that the
reaction conditions of this HBPT assembly were almost the same
as those for 4 except for the solvent. Figure 8a shows the
hydrogen bond network of 7, which consists of two [PdBr(Hmt-
bhp)]+ ions, two HBA- ions, and one neutral H2BA molecule.
HBPT assembly 7 crystallizes in the monoclinic P21/n space
group. The bond lengths and angles of the hydrazone and
pyridine parts are very close to those of protonated form 1H,
indicating that [PdBr(mtbhp)]+ accepts one proton from the
bromanilic acid to form a D · · ·A pair. In addition, the bromanilic
acid that is hydrogen-bonded to [PdBr(Hmhtbp)]+ exhibits three
3.4. Proton Migration in the Solid State. The crystal struc-
tures of HBPT assemblies 4, 5, 6, and 7 revealed that hydrogen
bonds between [PdBr(Hmtbhp)]+ and HBA- depend on what
kind of molecule is hydrogen-bonded to the remaining broma-
nilate OH group in these assemblies (Table 4). As discussed
above, the NH · · ·O hydrogen bond distances (2.577-2.653 Å)
suggest that the hydrogen-bonded proton can move toward the
O-site of the bromanilate in 6 to form an O-H· · ·N hydrogen
bond at low temperature. Proton migration was therefore
examined in 6 using IR spectroscopy, one of the most powerful
techniques for investigating this phenomenon in hydrogen
bonds.26 Figure 9 shows IR spectra of 6 at various temperatures.
Unlike assemblies 4, 5, and 7 (see Figure S5), assembly 6
exhibited a very broad band around 2400 cm-1 at 300 K. The
energies of the stretching modes of N-H and O-H bonds are
(26) (a) Somorjai, R. L.; Hornig, D. F. J. Chem. Phys. 1962, 36, 1980–
1987. (b) Hadzi, D.; Bratos, S. In The Hydrogen Bond; Schuster, P.;
Zundel, G.; Sandorfy, C., Eds.; North-Holland Publishing Co.:
Amsterdam, 1976; Vol. 2, p 565.
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Vapochromic Hydrogen-Bonded Proton-Transfer Assemblies
A R T I C L E S
Scheme 1. Schematic Representation of Proton Transfer in HBPT
Assembly 6a
Figure 9. Temperature dependent IR spectra of assembly 6 in the
4000-1700 cm-1 region. Red, green, blue, and black lines represent spectra
at 300, 150, 135, and 14 K, respectively. Inset: temperature dependent
absorbance of the ν(O-H) mode (3218 cm-1) (black square) and ν(N-H)
mode (2282 cm-1) (red circle). The marked sharp peak at 2360 cm-1 is
attributed to the vibration of CO2 in the air.
a (a) Two-proton-transfer state above 150 K. (b) One-proton-transfer state
below 150 K.
organic solvent vapors and that this behavior is deeply related
to the hydrogen bonding ability of vapor molecules. We
therefore examined this behavior in detail.
known to strongly depend on the hydrogen bond distance.
Specifically, the stronger the hydrogen bond, the lower the
energy of these bands. With the consideration that the hydrogen
bond distance between [PdBr(Hmtbhp)]+ and HBA- increases
according to the order 6 < 5 < 4 < 7 (see above), the broad
band around 2400 cm-1 observed for 6 is probably attributed
to the ν(N-H) mode, which would be remarkably weakened
upon hydrogen bond formation. In fact, the deuterated assembly
[PdBr(Dmtbhp)]2(BA) (6D) showed the broad absorption band
attributable to the ν(N-D) mode at around 1830 cm-1 (see
Figure S6). Although spectra measured above 150 K were
almost identical to the spectrum obtained at 300 K, a drastic
but reversible spectral change occurred below 150 K, resulting
in the appearance of a new absorption band assigned to the
ν(O-H) mode around 3200 cm-1. The appearance of this mode
As mentioned above, acetonitrile molecules form a 1-D
channel along the b axis in HBPT assembly 7. Thermogravi-
metric analysis revealed that these solvent molecules were easily
removed by heating at 373 K in an Ar atmosphere. This
acetonitrile removal resulted in a guest-free assembly (8) and
was accompanied by a significant color change from dark red
to reddish-purple (see Figure S7). Elemental analysis and an
IR spectrum of 8 also indicated that the acetonitrile was
completely removed (aee Figure S8). Interestingly, the original
color of 7 was recovered after exposing 8 to acetonitrile vapor
at room temperature, suggesting that HBPT assembly 8 is a
vapochromic material that can adsorb acetonitrile vapor revers-
ibly. In order to clarify this behavior in detail, UV-vis diffuse
reflectance spectra of assembly 8 exposed to several organic
vapors were measured. As shown in Figure 10, the acetonitrile-
bound assembly (7) exhibits a broad absorption band around
650 nm that shifts to shorter wavelengths by about 47 nm upon
acetonitrile removal. After exposing 8 to methanol, ethanol, and
1,4-dioxane (p-DO) vapors, the absorption band underwent a
red shift of about 26, 10, and 64 nm, respectively. On the other
hand, exposure to dimethylformamide (DMF), pyridine (Py),
dimethylacetoamide (DMA), and dimethylsulfoxide (DMSO)
vapors caused the absorption band of 8 to shift to shorter
wavelengths by about 16, 32, 45, and 45 nm, respectively.
To confirm whether this vapochromic behavior originates
from structural transformations induced by vapor adsorption or
not, powder X-ray diffraction (PXRD) patterns were measured.
Figure 11 shows the PXRD patterns of 8 exposed to several
organic vapors for 2 days. Acetonitrile removal significantly
changed the PXRD pattern of 7, suggesting that the crystal lattice
of 7 is not robust enough to retain its structure without the
acetonitrile molecules. The original pattern of 7 was recovered
after exposing 8 to acetonitrile vapor, showing that HBPT
assembly 8 is vapochromic. PXRD patterns obtained after
exposing 8 to other organic vapors differed from each other,
suggests that the proton moves from the [PdBr(Hmtbhp)]+
N
site to the bromanilate O-site. On the other hand, the ν(N-H)
mode around 2400 cm-1 was still observed below this temper-
ature (see inset, Figure 9). These results are consistent with
X-ray diffraction observations indicating that only one of the
two [PdBr(Hmtbhp)]+ units gives a proton to the bromanilate
at 110 K. In other words, one proton moves to the bromanilate
O-site to form [PdBr(mtbhp)] · · ·HBA- · · ·[PdBr(Hmtbhp)]+
below 150 K (Scheme 1). The pKa values of bromanilic acid
(pKa1 ) 0.80, pKa2 ) 3.10)27 indicate that the proton donating
ability of the monoanionic bromanilate is comparable to that
of protonated form 1H. The reason why the proton moved from
the hydrazone N-site to the bromanilate O-site with decreasing
temperature may be the shrinkage of the crystal accompanied
by a change in hydrogen bond distance.
3.5. Vapochromism Derived from the Rearrangement of
the Hydrogen Bond Network. We have found that HBPT
assembly 7 exhibits an interesting vapochromic behavior toward
(27) Wallenfels, K.; Friedrich, K. Chem. Ber. 1960, 93, 3070–3082.
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A R T I C L E S
Kobayashi et al.
Figure 10. UV-vis diffuse reflectance spectra of 8 after exposure to several
organic vapors for 2 days at room temperature. The black solid line shows
the spectrum of the guest-free assembly (8). Red, brown, green, and blue
solid lines represent the spectra of 8 after exposure to 1,4-dioxane (p-DO),
acetonitrile (AN), dimethylformamide (DMF), and dimethylsurfoxide
(DMSO), respectively.
Figure 12. IR spectra of 8 before and after exposure to several organic
vapors for 2 days at room temperature.
Table 6. Vapochromic Response of HBPT Assembly 8 and Donor
and Acceptor Numbers
Vapochromic
shift/nm
Vapor
DNa
ANa
1,4-dioxane (p-DO)
acetonitrile (AN)
methanol (MeOH)
ethanol (EtOH)
dimethylformamide (DMF)
pyridine (Py)
14.8
14.1
19.0
22.9
26.6
33.1
27.8
29.8
10.8
18.9
41.3
37.1
16.0
14.2
13.6
19.3
64
47
26
10
-12
-32
-45
-45
dimethylacetoamide (DMA)
dimethylsurfoxide (DMSO)
a See ref 28.
H2BA will adsorb and insert itself between HBA- and H2BA
to form a more stable A · · ·D· · ·S type hydrogen bond with the
HBA- OH group. In fact, the V(SdO) mode of adsorbed DMSO
was observed at 1008 cm-1 when assembly 8 was exposed to
DMSO vapor, which is a ca. 42 cm-1 shift to lower energy
compared to liquid DMSO. Likewise, the V(CdO) modes
observed for adsorbed DMF (1604 cm-1) and DMA molecules
(1600 cm-1) also shifted to lower energy by about 70 and 46
cm-1 compared to the liquids, respectively. In addition, TG
analysis for 8 after exposure to DMF vapor for 1 day revealed
that the assembly 8 can adsorb about 2 mol ·mol-1 DMF vapor
(see Figure S10). These results provide more evidence for the
contribution of the adsorbed vapor molecules to the hydrogen
bond network. Thus, the bands observed within 2800-2400
cm-1 after exposing 8 to DMSO, DMF, DMA, and Py vapors
correspond to the stretching mode of the HBA- O-H bond
whose strength changes upon vapor adsorption. In contrast,
vapor molecules like acetonitrile and p-DO that cannot form
such a strong hydrogen bond will adsorb without inserting itself
between HBA- and H2BA. Table 6 shows the donor and
acceptor numbers of solvent molecules and the observed
chromic shift induced by the vapor molecules. Vapochromic
shifts observed for 8 seem related to the donor and acceptor
numbers of the vapor molecules. Vapors that make the absorp-
tion band around 600 nm of 8 shift to longer wavelengths have
relatively small donor numbers and Vice Versa. Molecules with
a large donor number and a small acceptor number are known
to be good proton acceptors. Thus, this tendency suggests that
the hydrogen bonding ability of the vapor is deeply related to
the vapochromic behavior of HBPT assembly 8. In other words,
Figure 11. Powder X-ray diffraction patterns of 8 before and after exposure
to several organic vapors for 2 days at room temperature.
indicating that the adsorption of these vapors transformed the
structure of 8. It is noteworthy that the PXRD pattern of 8
changed in one step in the presence of acetonitrile vapor but in
two steps under exposure to p-DO vapor (see Figure S9).
To gain more information about the vapochromic behavior
of 8, IR spectral changes caused by the exposure of 8 to organic
vapors were measured. As shown in Figure 12, the vibration
modes of bound vapor molecules were observed for each vapor,
indicating that the vapochromic behavior of 8 originates from
structural transformation induced by vapor adsorption. The band
observed at 3178 cm-1 corresponding to the ν(O-H) mode of
H2BA was observed for both 8 and 7, suggesting that the
A· · ·D· · ·D· · ·D· · ·A type hydrogen bond network is retained
after acetonitrile removal. Assembly 8 showed a new broad
absorption band in the 2800-2400 cm-1 region when exposed
to DMF, DMSO, DMA, and Py vapors, but not to EtOH,
acetonitrile, and p-DO vapors. Additionally, the ν(O-H) mode
of H2BA observed at 3178 cm-1 for 8 clearly disappeared after
exposure to DMF and DMSO vapors. As discussed above, the
energies of N-H and O-H vibration modes are known to
strongly depend on hydrogen bond strength. Taking into account
the fact that the hydrogen bond formed between HBA- and
H2BA is relatively weaker than the other hydrogen bonds in 7,
a vapor molecule that can form a stronger hydrogen bond than
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Vapochromic Hydrogen-Bonded Proton-Transfer Assemblies
A R T I C L E S
above), half of the amount of acetonitrile used to reach saturation
may be enough to transform 8 into 7. The adsorption process
of p-DO vapor was more complicated than that for acetonitrile
vapor. After a small adsorption (ca. 0.67 mol mol-1) below P/P0
) 0.54, the isotherm showed a sudden increase in vapor uptake
at P/P0 ) 0.54 and reached a saturation point (2.7 mol mol-1
)
that was remarkably larger than that for acetonitrile adsorption.
This difference in saturation points between acetonitrile and
p-DO vapors may be related to the number of hydrogen bonding
sites in the vapor molecule. The adsorption profile for pyridine
vapor, which has a larger donor number than the other three
vapors, is similar to that for p-DO vapor. The pyridine vapor
was first adsorbed around P/P0 ≈ 0.1 (adsorbed amount ∼0.5
mol mol-1), and then the adsorbed amount was increased rapidly
above P/P0 ) 0.55. Below P/P0 ) 0.55, the adsorption process
was similar to that of acetonitrile vapor although pyridine
occupies a larger molecular volume than acetonitrile. The
saturation point for pyridine vapor was 4.91 mol mol-1, which
is significantly larger than that for other vapors. This larger
adsorption amount may originate from the high basicity of
pyridine. No steep decrease was observed during the desorption
process, even at vapor pressures as low as 0.14 P/P0, suggesting
that the pyridine vapor molecules are tightly bound to the crystal
lattice due to the potential formation of hydrogen bonds with
HBA- or H2BA. The adsorption profile obtained for MeOH
vapor, which has a larger acceptor number, was very different
from the other organic vapors. Adsorption of the MeOH vapor
was observed only at pressures exceeding P/P0 > 0.76, which
are remarkably higher than those of acetonitrile, p-DO, and
Figure 13. Adsorption isotherms obtained for acetonitrile (red square),
methanol (blue circle), 1,4-dioxane (green triangle), and pyridine (yellow
diamond) vapor in 8. Closed and open symbols represent the adsorption
and desorption processes, respectively.
the hydrogen bonding mode of 8 was thought to change from
the A· · ·D· · ·D· · ·D· · ·A type to the A· · ·D· · ·S type by
adsorbing a solvent vapor with a high donor number, but not a
vapor with a low donor number and/or a high acceptor number.
Actually, in the crystal structure of 4, the adsorbed acetone
molecule has moderate donor (17.0) and acceptor numbers
(12.5) and forms a hydrogen bond with the HBA- OH group.
In addition, the diffuse reflectance spectrum of 8 observed after
exposure to DMSO vapor was very similar to that of 4 (see
Figure S11) in which the π-π stacking interactions within
HBA- columns are not effective. These results suggest that the
vapochromism of HBPT assembly 8, induced by vapors with
high donor and low acceptor numbers, may originate from
structural transformations involving significant changes in the
hydrogen bond network and π-π stacking within each structural
component. Although there are many reports about vapo-
chromism, to the best of our knowledge, assembly 8 is the first
vapochromic proton-transfer complex comprised of proton donor
and acceptor molecules and its behavior reflects the hydrogen
bonding ability of the vapor.
3.6. Vapor Adsorption Property of the Vapochromic HBPT
Assembly 8. As discussed above, HBPT assembly 8 exhibits a
vapochromic behavior that strongly depends on the proton
accepting ability of the vapor. The vapor recognition ability of
8 was therefore analyzed from the viewpoint of the adsorption
property of the vapor. If HBPT assembly 8 can recognize the
hydrogen bonding capability of the vapor, the adsorption
property may reflect a difference in this capability.
Figure 13 shows the adsorption isotherms of the acetonitrile,
p-DO, MeOH, and pyridine vapor of 8. Before each measure-
ment, the sample was dried at 373 K under vacuum to remove
the adsorbed acetonitrile from 7. Although guest-free assembly
8 could adsorb these four vapors, the adsorption profiles for
each vapor were significantly different. The adsorption process
of acetonitrile vapor proceeded in two steps: after a first
adsorption at low pressure (below P/P0 ) 0.22, ca. 0.56
mol·mol-1), the isotherm gradually reached a saturation point
at 1.2 mol mol-1, which corresponds to the number of adsorbed
acetonitrile molecules in 7. Because the PXRD pattern of 8
changed in one step in the presence of acetonitrile vapor (see
pyridine vapors. The saturation amount (about 1.5 mol mol-1
)
was slightly larger than that for acetonitrile vapor, probably due
to the smaller volume of MeOH compared to acetonitrile. During
the desorption process, the adsorbed amount slightly decreased
to P/P0 ) 0.26 first and then in two steps via a narrow plateau
region to between 0.12 and 0.18. Considering that no steep
decrease in adsorption amount was observed for the other
organic vapors, the structure of the adsorbed MeOH was less
stable than that of other adsorbed vapor. The EtOH vapor
adsorption isotherm of 8 revealed the saturation amount (0.55
mol mol-1) was quite smaller by about one-third than that of
MeOH (see Figure S12). This result might be consistent with
the smaller vapochromic shift for EtOH vapor.
As discussed above, the adsorption behavior of 8 clearly
reflects the proton donating/accepting ability of the vapor.
Essentially, HBPT assembly 8 can adsorb proton accepting
vapors with a large donor number more easily than proton
donating vapors with a large acceptor number. This may be
due to the fact that assembly 8 consists of a neutral H2BA
molecule and a hydrogen bond network involving [PdBr(Hmt-
bhp)]+, HBA-, and H2BA. In other words, proton accepting
molecules can easily form strong hydrogen bonds with HBA-
or H2BA in HBPT assembly 8 to form stable vapor-bound
structures.
4. Conclusion
We have synthesized a new series of [MX(mtbhp)] metal-
hydrazone complexes, which exhibit a reversible protonation/
deprotonation reaction ([MX(mtbhp)] + H+ H [MX(Hmtbhp)]+)
in solution (Hmtbhp ) 2-(2-(2-(methylthio)benzylidene)hy-
drazinyl)pyridine; M ) Pd2+, X ) Cl- for 1; M ) Pd2+, X )
Br- for 2; M ) Pt2+, X ) Cl- for 3). Spectroscopic titration
measurements revealed that the acid dissociation constants of
Pd(II) and Pt(II) complexes are 3.6 and 3.3, respectively, almost
(28) Gutmann, V. The Donor-Acceptor Approach to Molecular Interaction;
Plenum Press: 1978.
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A R T I C L E S
Kobayashi et al.
independently of the halide bound to the metal ion. By reacting
[PdBr(mtbhp)] and bromanilic acid (H2BA), we have succeeded
in synthesizing hydrogen-bonded proton-transfer (HBPT) as-
semblies, {[PdBr(Hmtbhp)](HBA) ·Acetone} (4), {[PdBr(Hmt-
bhp)](HBA) ·2(1,4-dioxane)} (5), [PdBr(Hmtbhp)]2(BA) (6), and
{[PdBr(Hmtbhp)]2(HBA)2(H2BA)·2Acetonitrile} (7). In these
HBPT assemblies, each structural component was stacked
separately along the b axis, except for 5 in which the proton
donor and acceptor were stacked alternately along the a axis.
The proton donor H2BA gave at least one proton to the accep-
tor [PdBr(mtbhp)] to form the hydrogen-bonded [PdBr-
(Hmtbhp)]+ · · ·HBA- pair. The hydrogen bond distance between
the [PdBr(Hmtbhp)]+ N-site and the HBA- O-site strongly
depended on the molecule that was hydrogen-bonded to the other
bromanilate anion OH group. In assembly 6, the hydrogen bond
distance between [PdBr(Hmtbhp)]+ and bromanilate was short
enough to enable proton migration in the hydrogen bond,
resulting in the transformation of the two-proton-transfer state
[PdBr(Hmtbhp)]+ · · ·BA2- · · ·[PdBr(Hmtbhp)]+ into a one-pro-
ton-transfer state [PdBr(mtbhp)] · · ·HBA- · · ·[PdBr(Hmtbhp)]+-
at 150 K. HBPT assembly 7 could release the adsorbed
acetonitrile upon heating to 373 K to form guest-free assembly
8, which was found to be an interesting vapochromic material
that recognizes the proton donating/accepting ability of the vapor
molecule. Further development of these HBPT assemblies is
now in progress.
Acknowledgment. The authors thank Prof. H. Kitagawa (Kyoto
University) and Emeritus Prof. R. Ikeda (Univ. of Tsukuba) for
enlightening discussions and valuable support for low temperature
IR spectral measurements. This work is supported by a Grant-in-
Aid for Scientific Research (Photochromoism (No.471)), Young
Scientists (B) (19750050), and the Global COE Program (Project
No. B01: Catalysis as the Basis for Innovation in Materials Science)
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan.
Supporting Information Available: X-ray crystallographic
files in CIF format of 1, 2, 1H, 4, 5, 6, and 7; diffuse reflectance
spectra of 4, 5, 6, and 7; IR spectra of deuterated and non-
deuterated 6; thermogravimetric analyses of 7 and 8 exposed
to DMF vapor; exposure time dependence of PXRD pattern of
7; UV-vis spectra of 2 and 3 in solution; and EtOH vapor
isotherm of 8. This material is available free of charge via the
Internet at http://pubs.acs.org.
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15298 J. AM. CHEM. SOC. VOL. 132, NO. 43, 2010