Acid-base behavior of substituted hydrazone complexes controlled by the coordination geometry

Article abstract of DOI:10.1246/bcsj.20100065

A new series of substituted hydrazone complexes, [Cu(Hpbph)I] (1), [Cu(Hpbph)PPh₃]PF₆ (2-PF₆), [NiCl- (Hpbph)]Cl (3-Cl), [PtCl(Hpbph)]ClO₄ (4-ClO₄), and [PtCl(pbph)] (4b) (Hpbph = 2-(diphenylphosphino)benzaldehyde 2-pyridylhydrazone) have been synthesized and characterized. X-ray crystallography revealed that the copper(I) complexes adopt pseudo-tetrahedral geometry, while the nickel(II) and platinum(II) complexes provide square-planar forms. All the complexes exhibit distinct color changes on the basis of the deprotonation/protonation on the ligand although their acid/base behaviors are largely different. The acidity constants (pK_a) in methanol were determined to be 11.4 (1), 12.5 (2), 7.7 (3), and 6.7 (4). The results indicate that the dissociation of the proton on the ligand strongly depends on the ligand deformation controlled by the coordination geometry of the complexes and ancillary ligands also somewhat affect the acidity.

Full text of DOI:10.1246/bcsj.20100065

© 2010 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 83, No. 8, 905–910 (2010)
905
Acid-Base Behavior of Substituted Hydrazone Complexes
Controlled by the Coordination Geometry
Mee Chang,1 Hiroyuki Horiki,1 Kiyohiko Nakajima,2 Atsushi Kobayashi,1
Ho-Chol Chang,1 and Masako Kato*1
1Division of Chemistry, Faculty and Graduate School of Science, Hokkaido University, Sapporo 060-0810
2Department of Chemistry, Aichi University of Education, Kariya 448-8542
Received March 8, 2010; E-mail: mkato@sci.hokudai.ac.jp
A new series of substituted hydrazone complexes, [Cu(Hpbph)I] (1), [Cu(Hpbph)PPh3]PF6 (2-PF6), [NiCl-
(Hpbph)]Cl (3-Cl), [PtCl(Hpbph)]ClO4 (4-ClO4), and [PtCl(pbph)] (4b) (Hpbph = 2-(diphenylphosphino)benzaldehyde
2-pyridylhydrazone) have been synthesized and characterized. X-ray crystallography revealed that the copper(I)
complexes adopt pseudo-tetrahedral geometry, while the nickel(II) and platinum(II) complexes provide square-planar
forms. All the complexes exhibit distinct color changes on the basis of the deprotonation/protonation on the ligand
although their acid/base behaviors are largely different. The acidity constants (pKa) in methanol were determined to be
11.4 (1), 12.5 (2), 7.7 (3), and 6.7 (4). The results indicate that the dissociation of the proton on the ligand strongly
depends on the ligand deformation controlled by the coordination geometry of the complexes and ancillary ligands also
somewhat affect the acidity.
Metal complexes with substituted hydrazones, R1HC=
NNHR2 (R1, R2 = alkyl, aryl, acyl, etc.) have attracted much
attention for their chromic behavior,1,2 catalytic properties,3-5
and biological applications.6-8 Such characteristics of hydra-
zone complexes are often related to the deprotonation/
protonation behavior of the hydrazone ligands, and thus it is
important to control the behavior from the viewpoints of the
construction of new materials having proton conductivity and
sensing and catalytic abilities. In order to control the deproto-
nation/protonation behavior, the geometry and ligand field
strength of hydrazone complexes should be important because
the deprotonation of uncoordinated hydrazones occurs only in
highly basic conditions. For example, the acidity constant (pKa)
of pyridine-2-aldehyde-2-pyridylhydrazone (i.e., R1, R2 =
pyridine) was estimated to be ca. 14.5 in water, while the metal
complexes were reported to provide colored deprotonated forms
as well as a colorless protonated form.1,9 In the case of a
palladium complex with 2-(diphenylphosphino)benzaldehyde
2-pyridylhydrazone (Hpbph), it was found that both protonated
and deprotonated forms coexist in the crystal as the composition
of [PdCl(Hpbph)][PdCl(pbph)]Cl¢2H2O.10 Thus various types
of hydrazone complexes have been prepared until now.
However, the acidity constants of hydrazone complexes have
not been determined exactly and the protonation/deprotonation
properties and their control have not been studied clearly.
In this study, we focus on the coordination geometry of
hydrazone complexes. Monovalent copper ion with d10
electronic configuration preferentially adopts tetrahedral coor-
dination geometry, while the divalent nickel and platinum
ions with d8 configuration take square-planar geometry.
We have prepared a series of new complexes, [Cu(Hpbph)I]
(1), [Cu(Hpbph)PPh3]PF6 (2-PF6), [NiCl(Hpbph)]Cl (3-Cl),
[PtCl(Hpbph)](ClO4) (4-ClO4), and the deprotonated form,
[PtCl(pbph)] (4b) (Scheme 1) and investigated their acid-base
properties as well as their structures.
Experimental
Materials. The ligand Hpbph was prepared by a slightly
modified method reported in the literature.10 A suspension of
2-(diphenylphosphino)benzaldehyde (199 mg, 0.68 mmol) and
2-hydrazinopyridine (79 mg, 0.72 mmol) in methanol (10 mL)
was stirred for 2 h at room temperature. After 2 h, white-yellow
precipitate was filtered, washed with methanol and diethyl
1
ether, and dried in vacuum. Yield 60%, H NMR (dmso-d6): ¤
10.98 (s, 1H, -NH), 8.58 (d, J = 4.3 Hz, 1H), 8.06 (d, J = 4.0
Hz, 1H), 7.95 (dd, J = 6.8, 4.3 Hz, 1H), 7.53 (t, J = 8.6 Hz,
1H), 7.45-7.35 (m, 7H), 7.30-7.10 (m, 1H), 7.03 (d, J = 8.6
Hz, 1H), 6.85-6.65 (m, 2H). Other chemicals were used as
those commercially available.
Ph Ph
n+
X
P
M
N
N
N
H
M = Cu+, X = I−, n = 0 (1)
M = Cu+, X = PPh3, n = 1 (2)
M = Ni2+, X = Cl−, n = 1 (3)
M = Pt2+, X = Cl−, n = 1 (4)
Scheme 1.
Published on the web July 15, 2010; doi:10.1246/bcsj.20100065

906
Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
Acid-Base Behavior of Hydrazone Complexes
Table 1. Crystallographic Data
1¢CH2Cl2
2-PF6
3-Cl
4b
Formula
Formula weight
Crystal system
Space group
a/¡
b/¡
c/¡
¡/°
¢/°
C25H22N3PCl2ICu
656.80
C42H35N3P3F6Cu
852.18
C24H20N3PCl2Ni
511.01
C24H19N3PClPt
610.95
triclinic
triclinic
monoclinic
P21/n (No. 14)
11.563(4)
12.435(4)
15.291(6)
90.00
90.687(2)
90.00
2198(1)
4
triclinic
ꢀ
ꢀ
ꢀ
P1 (No. 2)
P1 (No. 2)
P1 (No. 2)
8.800(6)
10.703(8)
14.23(1)
89.68(3)
74.92(2)
82.30(2)
1282(2)
2
11.689(1)
12.577(4)
13.952(4)
109.302(4)
93.545(4)
93.649(4)
1924(1)
2
7.431(2)
10.031(3)
14.797(5)
101.098(4)
95.702(4)
98.763(4)
1060.2(6)
2
£/°
V/¡3
Z
T/K
150
1.059
0.087
0.240
150
1.053
0.050
0.143
150
1.040
0.036
0.098
150
1.006
0.058
0.150
GOF on F2
R1 [I > 2·(I)]a)
b)
wR2
2
2 2
2 2 1/2
a) R1 = -««Fo« ¹ «Fc««/-«Fo«. b) wR2 = [-(w(Fo ¹ Fc ) )/-w(Fo ) ]
.
Synthesis of Complexes. [Cu(Hpbph)I]¢CH2Cl2 (1¢CH2-
Cl2): To Hpbph (65 mg, 0.17 mmol) in dichloromethane
(115 mg, 0.3 mmol) in acetonitrile (30 mL) with a drop of
triethylamine was refluxed for 3 h. By concentration of the
solution, red crystals of [PtCl(pbph)] (4b) were deposited. The
crystals were collected by filtration, washed with diethyl ether,
and dried in vacuum. Next, 4b (61 mg, 0.1 mmol) was dissolved
in acetonitrile and after adding a drop of perchloric acid (60%),
yellow needle crystals of 4-ClO4 were deposited on standing
for several days. For 4-ClO4, Yield: 37 mg (52%). Anal. Calcd
for C24H20N3O4PCl2Pt¢1.3H2O: C, 39.23; H, 3.10; N, 5.72;
Cl, 9.65%. Found: C, 38.98; H, 2.81; N, 5.69; Cl, 9.63%.
1H NMR (CDCl3): ¤ 12.16 (t, J = 21 Hz, 1H, -NH), 9.04 (t,
J = 48 Hz, 1H), 8.75 (m, 1H), 7.96 (m, 1H), 7.89 (d, J = 7.4
Hz, 1H), 7.75 (t, J = 7.6 Hz, 1H), 7.70-7.35 (m, 13H), 7.09
(t, J = 6.7 Hz, 1H). IR (KBr): 3059 ¯(C-H) (aromatic), 1121
¯(ClO4) cm¹1. MS (ESI+): m/z = 612 [M]+. For 4b, Yield:
147 mg (80%). Anal. Calcd for C24H19N3ClPPt: C, 47.18; H,
3.13; N, 6.88; Cl, 5.80%. Found: C, 47.33; H, 3.32; N, 7.11; Cl,
(10 mL) was added solid CuI (32 mg, 0.17 mmol). The mixture
was stirred for an hour and orange precipitate was filtered. The
product was washed with diethyl ether and dried in vacuum.
Yield: 105 mg (94%). Anal. Calcd for C25H22N3PCl2ICu: C,
45.72; H, 3.38; N, 6.40%. Found: C, 45.35; H, 3.38; N, 6.47%.
IR (KBr): 3192 ¯(N-H), 3052 ¯(C-H, aromatic) cm¹1. 1H NMR
(dmso-d6): ¤ 11.68 (s, 1H, -NH), 8.34 (d, J = 5.9 Hz, 1H), 7.89
(s, 1H), 7.73 (t, J = 7.0 Hz, 2H), 7.60-7.30 (m, 12H), 6.97 (t,
J = 6.4 Hz, 1H), 6.80 (m, 4H).
[Cu(Hpbph)PPh3]PF6 (2-PF6):
A mixture of AgPF6
(6.2 mg, 0.25 mmol) and 1¢CH2Cl2 (14 mg, 0.25 mmol) in
dichloromethane (30 mL) was stirred for 30 min at room
temperature. The precipitate of AgI was removed by filtration.
After adding PPh3 (6.4 mg, 0.25 mmol), the mixture was stirred
for an hour and the solution was kept at room temperature.
Yellow crystals were formed after a few days. The crystals were
washed with diethyl ether and dried in vacuum. Yield: 202 mg
(95%). Anal. Calcd for C42H35N3P3F6Cu: C, 59.19; H, 4.14; N,
4.93%. Found: C, 59.41; H, 4.18; N, 4.78%. IR (KBr): 3322
1
6.12%. H NMR (CDCl3): ¤ 8.36 (m, 1H), 8.27 (t, J = 48 Hz,
1H), 7.85-7.25 (m, 15H), 6.82 (d, J = 8.1 Hz, 1H), 6.35 (t, J =
6.7 Hz, 1H). IR (KBr): 3051 ¯(C-H) (aromatic) cm¹1. MS
(ESI+): m/z = 612 [M + H]+.
1
¯(N-H), 3059 ¯(C-H, aromatic) cm¹1. H NMR (dmso-d6): ¤
X-ray Crystallography. All of the X-ray diffraction data
were collected on a Rigaku AFC-8/Mercury CCD system. All
structures were solved by direct methods and expanded using
Fourier technique. The non-hydrogen atoms were refined
anisotropically and hydrogen atoms were included and refined
using a riding model. The crystallographic data and final R
indices are summarized in Table 1. Calculations were carried
out using the program packages, Crystal Structure and
SHELXL-97.12 Crystallographic data have been deposited with
Cambridge Crystallographic Data Center: Deposition numbers
CCDC-766424-766427 for complexes, 2-PF6, 1¢CH2Cl2, 3-Cl,
and 4b in the order.
11.95 (s, 1H, -NH), 8.24 (d, J = 5.2 Hz, 1H), 8.02 (s, 1H),
7.77-7.62 (m, 3H), 7.45 (t, J = 7.3 Hz, 7H), 7.29 (t, J = 7.8 Hz,
9H), 7.07 (t, J = 9.0 Hz, 7H), 6.79 (m, 6H).
[NiCl(Hpbph)]Cl (3-Cl): To Hpbph (24 mg, 0.06 mmol) in
methanol (10 mL) was added solid NiCl2¢6H2O (15 mg,
0.06 mmol). The mixture was stirred for an hour and orange
precipitate was filtered. The product was washed with methanol
and dried in vacuum. Yield: 28 mg (90%). Anal. Calcd for
C24H20N3PCl2Ni: C, 56.41; H, 3.95; N, 8.22%. Found: C,
1
56.42; H, 3.93; N, 8.27%. H NMR (CDCl3): ¤ 14.96 (s, 1H,
-NH), 9.14 (s, 1H), 8.19 (d, J = 5.7 Hz, 1H), 7.78 (m, 10H),
7.54 (m, 4H), 7.32 (m, 2H), 6.86 (t, J = 6.8 Hz, 1H). IR (KBr):
Determination of Acidity Constants. Acid-base behavior
of the complexes was investigated in methanol solution
(2.3 © 10¹5 M) by the titration of 2.3 mM methanolic potas-
sium hydroxide or 2.3 mM perchloric acid. The absorption
spectral changes were measured monitoring the pH values with
a EUTECH pHScanWP2 pH meter. The pH values were
¹1
3182 ¯(N-H), 3049 ¯(C-H, aromatic) cm
.
[PtCl(Hpbph)]ClO4 (4-ClO4) and [PtCl(pbph)] (4b): The
starting material, [PtCl2(PhCN)2] (PhCN = benzonitrile) was
prepared by refluxing a mixture of PtCl2 and PhCN.11
A
mixture of [PtCl2(PhCN)2] (142 mg, 0.3 mmol) and Hpbph

M. Chang et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
907
(a)
(c)
(b)
(d)
Figure 1. Molecular structures of (a) [Cu(Hpbph)I] (1), (b) [Cu(Hpbph)PPh3]+ (2), (c) [NiCl(Hpbph)]+ (3), and (d) [PtCl(pbph)]
(4b). The thermal ellipsoids are drawn as the 50% probability level for non-hydrogen atoms.
corrected by using the eq 1 to the values in methanol, where
pHcorr and pHobs stand for the corrected and observed values,
respectively. Corrected coefficient value ¤ = ¹2.24 was used
for methanol solution.13,14
mononuclear complexes proving the flexibility of the ligand.
Only protonated forms of Hpbph were isolated for copper(I)
complexes (1¢CH2Cl2 and 2-PF6). It is remarkable that both
copper(I) complexes are very stable in air. On the other hand,
the nickel(II) and platinum(II) complexes were obtained as
protonated forms in organic solvents in the presence of a base
such as triethylamine, while the protonated forms were able to
be isolated from the acidic solution. Though the synthesis of
the deprotonated form of the nickel(II) complex, [NiCl(pbph)]
has been reported previously,10 the protonated form 3 and both
forms of the platinum(II) complexes 4 and 4b were obtained
and characterized in this work.
Structural Descriptions. Figure 1 shows ORTEP draw-
ings of molecular structures of complexes 1-3 and 4b.16
Relevant geometric parameters of the complexes are listed in
Table 2.
As shown in Figures 1a and 1b, the geometry around
copper(I) ion is a pseudo-tetrahedron for both 1 and 2. It is
interesting to note that the P1, N1, and I1 atoms for the
structure of 1 are located almost in a plane including Cu1.
Similar tendency can be seen also for 2. The deviations of the
four atoms (P1, N1, I1, and Cu1 for 1, and P1, N1, P2, and Cu1
for 2) from the least-square plane are within 0.17 ¡ for 1 and
0.16 ¡ for 2. The sum of the three angles formed by these
atoms around Cu1 is calculated to be 359.6(2)° for 1 and
pHcorr ¼ pHobs ꢀ ¤
ð1Þ
On the basis of the pH dependence of the absorbance data, the
limiting absorbance method was employed to determine the
acidity constants (pKa) of the complexes.15 The pKa values
were determined from the intercepts of the least-square lines of
the plots of log{(A ¹ Amin)/(Amax ¹ A)} versus pH, where the
slopes were adjusted to be 1.00 (Figure S1). These pKa values
are consistent with those estimated by another method, the half
height method.15
Other Measurements. 1H NMR spectra were measured on
a JEOL JNM-EX270 spectrophotometer. IR spectra were
measured as KBr pellets with a JASCO FT/IR-4100 spectro-
photometer. UV-vis absorption and emission spectra were
recorded on Shimadzu UV-2500 and JASCO F6000 spectro-
photometers, respectively.
Results and Discussion
Synthesis of the Complexes.
The ligand Hpbph was
coordinated to metal ions such as Cu+, Ni2+, and Pt2+ with
different geometries as a tridentate ligand to form a series of

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Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
Acid-Base Behavior of Hydrazone Complexes
Table 2. Selected Bond Lengths/¡, Bond Angles/°, and
Dihedral Angles/°a)
1¢CH2Cl2
2-PF6
3-Cl
4b
M-N1
M-N3
M-P1
M-X
2.045(8) 2.065(2) 1.946(2) 2.051(7)
2.273(8) 2.127(2) 1.886(2) 1.965(9)
2.193(3) 2.237(1) 2.162(7) 2.219(2)
2.555(1) 2.230(1) 2.167(8) 2.297(3)
N1-M-N3
N1-M-P1
N1-M-X
N3-M-P1
N3-M-X
P1-M-X
75.7(3) 78.06(9) 83.81(7)
125.1(2) 121.55(7) 162.97(6) 173.0(2)
102.2(2) 110.98(7) 94.36(5)
80.6(2) 84.37(6) 94.54(5)
120.6(2) 120.33(7) 178.11(5) 173.4(2)
132.31(8) 125.82(3) 87.07(3) 90.80(9)
79.3(3)
94.1(2)
95.8(2)
Dihedral angleb)
21.9(4)
19.1(1) 12.12(7)
13.2(4)
Figure 2. Dimeric structure of 3-Cl: Ni£Cl2 = 2.9115(6),
N2£Cl2 = 3.030(2), N2-H2£Cl2 = 2.18 ¡.
a) M = Cu1 for 1 and 2, Ni1 for 3, Pt1 for 4b; X = I1 for 1, P2
for 2, Cl1 for 3 and 4b. b) The dihedral angles denote those
between least-squares planes for phenylene and pyridine rings
center in the crystal (Ni£Cl2 = 2.9115(6) ¡). For the corre-
sponding palladium(II) complex, which was found to be a co-
crystal of the protonated and deprotonated forms, such dimeric
structure was not formed though there was the hydrogen bond
¹
of the ligands Hpbph or pbph .
358.4(1)° for 2, respectively. Therefore, the coordination
geometry of these hydrazone copper(I) complexes can be said
to be an inclined trigonal pyramid rather than a distorted
tetrahedron. It would be due to the structural restraint of this
PNN tridentate ligand, Hpbph, which prefers planar coordina-
tion for its conjugated structure (phenyl-HC=NNH-pyridyl)
even for the protonated form. In fact, the Cu1-N3 bond
distance is elongated largely for 1 and 2 (2.273(8) and
2.127(2) ¡, respectively) compared with those for the com-
plexes having planar geometry (Table 2). The dihedral angle
¹
between the amine proton and the Cl ion in the crystal.10
Absorption Spectra and Acid-Base Behavior.
The
electronic absorption spectra at various pH values in methanol
are shown in Figure 3 and the data are summarized in Table 3.
All the complexes exhibit characteristic absorption bands
arising from the intraligand transition of the pbph ligand in the
visible region.2,18 The copper(I) complexes 1 and 2 in methanol
exhibited similar spectra with the maximum at 289 nm and a
shoulder around 350 nm. By the titration of a methanolic KOH
solution, the decrease in the bands and the appearance of new
bands occurred at 420 and 453 nm for 1 and 2, respectively,
showing the color change from yellow to orange. On the other
hand, the nickel(II) and platinum(II) complexes 3 and 4 exhibit
red color in methanol and spectral changes occurred by adding
perchloric acid showing the color change from red to yellow.
The absorption bands for the deprotonated form of 3 and 4
appear at much longer wavelength (500 nm) compared with
those of copper(I) complexes. This would be related to the
planarity of the Hpbph ligand in the complex (vide infra). The
intraligand transitions must be lowered in the square-planar
complexes because of the extension of ³-conjugation. All the
spectral changes gave the isosbestic points indicating the
equilibrium between two components. The initial spectrum
recovered completely by returning the pH values to the original
points (Figure 3). These results indicate that the deprotonation/
protonation of the complexes occurs reversibly in the different
pH regions (Scheme 2). For the platinum complex 4, lumines-
cence spectra with maxima at 602 and 465 nm were also
observed for both deprotonated and protonated forms, respec-
tively (Figure S2), though other complexes exhibit no lumi-
nescence at room temperature.19
¹
between pyridine and phenylene rings in Hpbph (or pbph )
also indicates the deformation of the ligand due to the
coordination to the monovalent copper ion (Table 2). Never-
theless, it is noteworthy that these copper(I) complexes are
stable in air. In the crystal structure of 1, the intermolecular
hydrogen bond between the secondary amine (N2-H) and the
¹
I
ligand in the adjacent complex was found (N2-H2£I1 =
2.85, N2£I1 = 3.723(8) ¡).17 In the case of 2, the amine
¹
proton interacts with one of the fluorine atoms of PF6
(N2-H2£F1 = 2.09, N2£F1 = 2.940(9) ¡).
In contrast to the copper(I) complexes, the nickel(II) and
platinum(II) complexes, 3 and 4b, take square-planar structures
as is usual for d8 metal complexes (Figures 1c and 1d). As
shown in Table 2, it is clear that the deformations of the
hydrazone ligand in 3 and 4b are much less than those for the
copper(I) complexes on the basis of the M-N bond lengths and
the dihedral angles between pyridine and phenylene rings in the
ligand. Comparing 3 with 4b, the planarity of the coordination
plane is better for the neutral platinum(II) complex 4b. The
maximum deviation from the least-square plane made by
five atoms (Cl1, P1, N1, N3, and Metal ion (Ni1 or Pt1) is
0.162(9) ¡ for 4b while 0.488(2) ¡ for 3. The larger deviation
for the nickel(II) complex would be due to the formation of a
dimeric structure linked by the chloride ions as shown in
Figure 2. In the crystal structure of the nickel(II) complex
3-Cl, the amine proton makes a strong hydrogen bond
Figure 4 shows the plots of absorbance changes against pH
at 409, 460, 500, and 505 nm for 1, 2, 3, and 4, respectively.
The acidity constants (pKa) of the complexes determined by the
limiting absorbance method are listed in Table 3. Considering
that the free Hpbph ligand never dissociates its proton even
under very highly basic conditions in methanol (Figure S3),
¹
with the counter anion, Cl (N2-H2£Cl2 = 2.18, N2£Cl2 =
3.030(2) ¡),17 which is also located just on the axial position of
the Ni atom of the adjacent complex related by the inversion

M. Chang et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
909
3
2
1
0
pH
9.6
10.6
pH
9.4
1
2
10.1
10.5
10.7
11.1
11.2
11.4
11.5
11.7
12.0
12.1
2
11.1
11.7
12.1
12.3
12.5
12.7
12.8
12.9
13.1
13.2
13.3
13.4
13.5
13.6
13.8
1
ε
ε
0
300
400
500
600
700
300
400
500
600
700
Wavelength / nm
Wavelength / nm
4
3
2
1
0
4
3
2
pH
pH
5.4
6.2
7.1
7.5
7.8
8.3
8.7
9.3
9.8
10.2
5.4
6.0
6.3
6.6
6.8
7.1
8.2
9.2
4
3
1
ε
ε
0
300
400
500
600
700
300
400
500
600
700
Wavelength / nm
Wavelength / nm
Figure 3. Absorption spectral changes of 1-4 at various pH values in methanol at room temperature.
Table 3. Absorption Data and the pKa Values in Methanol
¹1
-
max/nm (¾ © 10¹4/M¹1 cm
)
a)
Compounds
pKa
Protonated form
Deprotonated form
Hpbph
337 (2.72)
®
1
2
3
4
289 (1.95), 350sh (1.11)
289 (2.25), 350sh (0.83)
252 (2.18), 291sh (1.63), 365 (1.06)
260 (2.21), 291sh (1.20), 396 (1.21)
338 (1.20), 420 (1.10), 460sh (0.60)
316 (1.60), 453 (0.83)
279 (2.61), 336 (1.33), 502 (1.14)
283 (1.75), 325 (1.16), 506 (1.69)
11.4
12.5
7.7
6.7
a) The estimated errors are «0.1.
Ph
Ph
Ph
Ph
X
X
P
N
P
N
- H+
H+
M
M
N
N
N
N
H
Scheme 2.
the effect of coordination is clear. The pKa values of the
compounds increase in the following order:
As a result, a distinct difference can be seen in the pKa values
between the complexes containing the much distorted ligand 1
and 2 and the complexes containing the planar geometry 3 and
4. The easiness of deprotonation would be closely related to the
electron density on the secondary amine of the ligand Hpbph.
PtII complex 4 < NiII complex 3
ꢁ CuI complexes 1 and 2 ꢁ free Hpbph
ð2Þ

910
Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
Acid-Base Behavior of Hydrazone Complexes
geometry of metal ions. The findings would be useful for
biological and catalytic application. In fact, we found that the
substituted hydrazone ligand dimerized forming a covalent
bond on copper(I) complexes by controlling the acidity. The
reactions are now under investigation.
4
3
1
2
This work was partially supported by Grants-in-Aid for
Scientific Research on priority area “New Frontiers in
Photochromism (No. 471)” and for Exploratory Research
(No. 20655010), and Elements Science and Technology Project
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan.
Supporting Information
5
6
7
8
9
10
11
12
13
14
The absorbance vs. pH curves with the plots for the limiting
absorbance method, luminescence spectra of 4, UV-vis spectra
of free Hpbph in basic methanol (Figures S1-S3). This
material is available free of charge on the Web at: http://
www.csj.jp/journals/bcsj/.
pH
Figure 4. Absorbance-pH curves for 1-4 monitored at 409,
460, 500, and 505 nm, respectively: 1 ( ), 2 ( ), 3 ( ), and
4 ( ).
As mentioned in the section of Structural Descriptions, the
deformed geometries of Hpbph for 1 and 2 result in weaker
coordination, where the effect of the donation from the ligand
to metal would be small compared with that for the complexes
containing Hpbph with planar geometry. In fact, the chemical
shifts of the 1H NMR for 1 and 2 (¤ 10.50 and 10.25 in CDCl3,
respectively) appear at higher magnetic field compared with
those for 3 and 4 (¤ 14.06 and 12.16 in CDCl3, respectively).
In comparison of two square-planar complexes, the pKa value
of the PtII complex 4 is slightly smaller than that of the NiII
complex 3. The higher acidity for the PtII complex would be
ascribed to the higher stability of the planar geometry due
to the stronger ligand field compared with the NiII complex.
Furthermore, for the CuI complexes, the pKa value of 2 is
somewhat larger than that of 1 (Table 3). This can be explained
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All of the complexes containing a substituted hydrazone
exhibit acid-base behavior followed by remarkable color
changes. The dissociation of proton in the hydrazone ligand
is induced by the coordination to metal ions. However, the
acidity strongly depends on the coordination geometry charac-
teristic of the metal ions. The copper(I) complexes in pseudo-
tetrahedral conformation stabilize the protonated forms com-
pared with the corresponding nickel(II) and platinum(II)
complexes with square-planar geometry. One of the main
reasons would be the distorted coordination structures by
the rigid PNN tridentate ligand where the coordination of N
atom in the middle of the ligand weakens. In addition, for
the deprotonated form, the planar geometry would be more
favorable to get an extended ³-conjugation. In conclusion, we
elucidated that the acidity of the substituted hydrazone ligand
can be controlled by using the characteristics of coordination
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to the visible light. The luminescence properties will be reported
elsewhere.