Kinetic studies on the reversible ring opening-closure reaction of the triazine ligand and structural properties of palladium(II) complexes

Article abstract of DOI:10.1016/j.ica.2007.03.024

A Pd(II) complex containing didentate triazine ligand L¹ (2-(2-methylphenyl)-3-(2-pyridyl)-2,3-dihydronaphtho[2,1-e][1,2,4]triazine) [PdCl₂(L¹)] (1) was prepared, and the structure was determined by X-ray crystallography. The absorption spectrum of complex 1 in dichloromethane changed gradually with isosbestic points when methanol was added to the solution, and [PdCl(L²)] (2) (L² = N-[methoxy(2-pyridyl)methyl]-1-(2-methylphenylazo)-2-naphthylamide) was obtained from the resulting solution after the reaction was completed. Addition of hydrogen chloride to the solution of complex 2 led to the recovery of complex 1. Thus, a reversible ring opening and closure reaction of the triazine ligand was observed. The progress of the ring opening reaction was monitored by observing the absorbance changes at several wavelengths of the visible spectra as a function of the concentration of methanol. The absorbance plots were fitted successfully with a mechanism that includes the consumption of two methanol molecules and the release of HCl, whose concentration is equivalent to that of the produced Pd complex ([PdCl₂ (L¹)] + 2 CH₃ OH ? [PdCl (L²)] + CH₃ OH₂⁺ · Cl^-). In dichloromethane with a low dielectric constant, the polar HCl molecule will be stabilized by forming an adduct with methanol. The equilibrium constant was determined as K = [PdCl (L²)] [CH₃ OH₂⁺ · Cl^-] [PdCl₂ (L¹)]^{- 1} [CH₃ OH]^{- 2} = (2.2 ± 0.4) × 10^{- 4} M^{- 1}at T = 25.0 °C. The kinetics of the reaction of [PdCl₂(L¹)] with methanol was investigated by monitoring the absorbance change of the reacting solution with time. We obtained rate constant values of k₁ = (2.40 ± 0.07) × 10^-3 s^-1 and k₂ = (5.8 ± 0.1) × 10^-3 M^-1 s^-1 at T = 25.0 °C on the observed pseudo-first-order rate constant of the forward reaction, k_f = k₁ + k₂ [CH₃OH].

Full text of DOI:10.1016/j.ica.2007.03.024

Inorganica Chimica Acta 360 (2007) 3040–3046
www.elsevier.com/locate/ica
Kinetic studies on the reversible ring opening–closure reaction of
the triazine ligand and structural properties of palladium(II) complexes
a
c
b
c,
*
Meishan Zhang , Masahiko Inamo , Masaaki Kojima , Kiyohiko Nakajima
a
Key Laboratory of Organism Functional Factors of the Changbai Mountain, Yanbian University, Ministry of Education, Yanji 133000, China
b
Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama, 700-8530, Japan
c
Department of Chemistry, Aichi University of Education, Kariya, Aichi 448-8542, Japan
Received 15 November 2006; received in revised form 22 February 2007; accepted 3 March 2007
Available online 23 March 2007
Abstract
A Pd(II) complex containing didentate triazine ligand L1 (2-(2-methylphenyl)-3-(2-pyridyl)-2,3-dihydronaphtho[2,1-e][1,2,4]triazine)
[PdCl2(L1)] (1) was prepared, and the structure was determined by X-ray crystallography. The absorption spectrum of complex 1 in
dichloromethane changed gradually with isosbestic points when methanol was added to the solution, and [PdCl(L2)] (2) (L2 = N-[meth-
oxy(2-pyridyl)methyl]-1-(2-methylphenylazo)-2-naphthylamide) was obtained from the resulting solution after the reaction was com-
pleted. Addition of hydrogen chloride to the solution of complex 2 led to the recovery of complex 1. Thus, a reversible ring opening
and closure reaction of the triazine ligand was observed. The progress of the ring opening reaction was monitored by observing the
absorbance changes at several wavelengths of the visible spectra as a function of the concentration of methanol. The absorbance plots
were fitted successfully with a mechanism that includes the consumption of two methanol molecules and the releaseꢂof HCl, whose con-
centration is equivalent to that of the produced Pd complex ð½PdCl2ðL1Þꢀ þ 2CH3OH ꢀ ½PdClðL2Þꢀ þ CH3OH2þ ꢁ Cl Þ. In dichlorometh-
ane with a low dielectric constant, the polar HCl molecule will be stabiliꢂz1ed by forming an adduct with methanol. The equilibrium
constant was determined as K ¼ ½PdClðL2Þꢀ½CH3OH þ ꢁ Clꢂꢀ½PdCl2ðL1Þꢀ ½CH3OHꢀꢂ2 ¼ ð2:2 ꢃ 0:4Þ ꢄ 10ꢂ4
M
ꢂ1at T = 25.0 ꢀC. The
2
kinetics of the reaction of [PdCl2(L1)] with methanol was investigated by monitoring the absorbance change of the reacting solution with
time. We obtained rate constant values of k1 = (2.40 0.07) · 10ꢂ3 sꢂ1 and k2 = (5.8 0.1) · 10ꢂ3 Mꢂ1 sꢂ1 at T = 25.0 ꢀC on the
observed pseudo-first-order rate constant of the forward reaction, kf = k1 + k2 [CH3OH].
ꢁ 2007 Elsevier B.V. All rights reserved.
Keywords: Ring opening–closure reaction; Triazine; Palladium(II) complex; Crystal structure; Kinetics; Equilibrium
1. Introduction
reactions of the Ru(II) complexes with Schiff base
ligands containing phosphorus or sulfur donor atoms
[2]. Recently, we prepared new triazine and imidazole
ligands derived from 1-phenylazo-2-naphthylamine
and pyridine-2-aldehyde, and reported the structure and
reactivity of their Pd(II) complexes [PdCl2(L3)] and
[PdCl2(L4)] (Fig. 1) [3]. The absorption spectrum of the
triazine ligand L3, 2-phenyl-3-(2-pyridyl)-2,3-dihydro-
naphtho[2,1-e][1,2,4]triazine, remained unchanged in
dichloromethane or methanol. On the other hand, the
absorption spectrum of [PdCl2(L3)] in dichloromethane
changed rapidly on addition of methanol to the solution.
The complex [PdCl(L4)] was obtained from the resulting
Recently, compounds that show photochromism, ther-
mochromism, electrochromism, solvatochromism, and
vapochromism have attracted increasing attention in con-
nection with their application to switching devices,
molecular memory storage, etc. [1]. Our current research
interest is the structures and properties of the transition
metal complexes that show such chromotropism, and
we have investigated the photo and thermal substitution
*
Corresponding author. Tel.: +81 566 26 2355; fax: +81 566 26 2310.
E-mail address: knakajim@auecc.aichi-edu.ac.jp (K. Nakajima).
0020-1693/$ - see front matter ꢁ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.03.024

M. Zhang et al. / Inorganica Chimica Acta 360 (2007) 3040–3046
3041
Cl
Pd
7.50–7.60 (4H, m), 7.82 (1H, t), 8.03 (1H, m), 8.25 (1H,
d), 9.32 (1H, d).
Cl
Pd
N
N
N
_
Cl
R
N
CH3O
N
2.2.2. [PdCl2(L2)] (2)
N
N
_
N
Cl
CH3O
The complex was prepared according to a similar proce-
dure that was described for complex 1, however, the reac-
tion of the ligand and [PdCl2(PhCN)2] was run in
methanol. Pyridine-2-aldehyde (0.017 g, 1.6 · 10ꢂ4 mol)
was added to a methanol solution (20 cm3) of 1-toluene-
azo-2-naphthylamine (0.042 g, 1.6 · 10ꢂ4 mol). After the
solution was refluxed for 2 h, [PdCl2(PhCN)2] (0.058 g,
1.5 · 10ꢂ4 mol) was added, and the reaction mixture was
stirred for 30 min at room temperature. The solution was
concentrated to a small amount to yield red crystals, which
were filtered and air-dried. Yield 0.067 g. Anal. Calc. for
[PdCl2(L2)] (2) = C24H21Cl2N4OPd: C, 55.08; H, 4.04; N,
10.71. Found: C, 55.01; H, 3.55; N, 10.68%. UV–Vis
{dichloromethane, r/103 cmꢂ1 (loge/Mꢂ1 cmꢂ1)} 18.8
(3.88), 20.0 (3.70 sh), 26.5 (4.01), 27.5 (3.99 sh), 30.2
R
[PdCl2(L1)] (R=CH3) (1)
[PdCl2(L3)] (R=H)
[PdCl(L2)] (R=CH3) (2)
[PdCl(L4)] (R=H)
Fig. 1. Schematic representation of the triazine ring opening reactions of
the Pd(II) complexes.
solution. When an excess amount of (nBu)4NCl was
added to the solution of [PdCl(L4)] in dichloromethane,
the absorption spectrum reverted to that of [PdCl2(L3)].
Accordingly, the reversible ring opening and closure
reaction of the triazine ligand was understood in terms
of competition between the CH3Oꢂ and Clꢂ ions.
However, the kinetics of the ring opening and closure
reaction of the triazine ligand were not elucidated. As
an extension of our study, we report here the prepara-
tion, structure, and properties of [PdCl2(L1)] (1) and
[PdCl(L2)] (2) complexes. The equilibrium and kinetics
of the triazine ring opening reaction are interpreted.
1
(4.09 sh), 31.6 (4.15), 38.0 (4.64 sh), 40.2 (4.71). H NMR
(chloroform-d) d = 2.46 (3H, s), 3.15 (3H, s), 6.71 (1H,
s), 7.24–7.33 (5H, m), 7.43 (2H, q), 7.62 (1H, d), 7.69
(2H, t), 7.81 (1H, d), 7.96 (1H, t), 8.38 (1H, d), 9.33 (1H,
d). Complex 2 was also prepared from complex 1 as a
starting material in methanol. The methanol solution
(20 cm3) of complex 1 (0.082 g, 1.3 · 10ꢂ4 mol) was stirred
for 30 min at room temperature and concentrated to a
small amount to yield red crystals of complex 2. Yield
0.061 g.
2. Experimental
2.1. General
[PdCl2(PhCN)2] was prepared according to the literature
procedure [4]. 1H NMR spectra were measured at
400 MHz on a JEOL JNM LA-400 spectrometer with
TMS as an internal reference. UV–Vis absorption spectra
were obtained on a Shimadzu UV-2500PC spectro-
photometer.
2.3. Collection of X-ray data and structure determination
A summary of the crystallographic data and structure
solution details for complexes 1 and 2 is listed in Table 1.
Single crystals were glued onto glass fibers. The crystal of
complex 1 was coated with epoxy resin to avoid intensity
changes during the data collection. On removal from the
mother liquor, the crystal of complex 1 deteriorated grad-
ually owing to loss of the solvent of crystallization. Inten-
sity data were collected with graphite-monochromated
2.2. Preparation of compounds
2.2.1. [PdCl2(L1)] (1)
Pyridine-2-aldehyde (0.017 g, 1.6 · 10ꢂ4 mol) was added
to a methanol solution (20 cm3) of 1-tolueneazo-2-naph-
thylamine (0.042 g, 1.6 · 10ꢂ4 mol). The solution was
refluxed for 2 h, and evaporated to dryness. The residue
was dissolved in dichloromethane (20 cm3) and [PdCl2-
(PhCN)2] (0.058 g, 1.5 · 10ꢂ4 mol) was added. The reaction
mixture was refluxed for 2 h to give an orange solution.
n-Hexane (10 cm3) was added with stirring, and the reac-
tion mixture was concentrated to a small volume to yield
red fine crystals, which were filtered and washed with
diethyl ether (10 cm3) and dried. Yield 0.072 g. Anal. Calc.
for [PdCl2(L1)] Æ CH2Cl2 (1) = C24H20Cl4N4Pd: C, 47.05;
H, 3.29; N, 9.14. Found: C, 46.98; H, 2.80; N, 9.07%.
UV–Vis {dichloromethane, r/103 cmꢂ1 (loge/Mꢂ1 cmꢂ1)}
20.9 (3.64 sh), 24.2 (4.04), 35.2 (4.39), 42.6 (4.71). 1H
NMR (chloroform-d) d = 2.68 (3H, s), 7.07 (1H, s), 7.09
(1H, t), 7.21 (1H, t), 7.28 (1H, d), 7.37–7.45 (3H, m),
MoKa radiation (k = 0.71073 A) on a RAXIS-RAPID dif-
˚
fractometer. A numerical absorption correction using the
program NUMABS [5] was applied to the intensity data.
The diffraction data were processed by the PROCESS-AUTO
program package. The structures were solved by heavy-
atom Patterson methods (PATTY [6]) for complex 1, and
direct methods (SIR92 [7]) for complex 2, and expanded
using Fourier techniques (DIRDIF94 [6]). The structures were
refined by full-matrix least-squares based on F with aniso-
tropic displacement parameters for all non-hydrogen
atoms. Hydrogen atoms were placed at idealized positions
and included in the refinement with fixed isotropic thermal
parameters. All calculations were performed using the teX-
san crystallographic software package [8] of Molecular
Structure Corporation. The high GOF value of complex
2 is due to the bad crystallinity and the disorder of the
methyl group.

3042
M. Zhang et al. / Inorganica Chimica Acta 360 (2007) 3040–3046
Table 1
Crystallographic data and experimental details
Compound
Formula
M
1 Æ CH2Cl2
C24H20Cl4N4Pd
612.66
2
C24H21N4ClN4OPd
523.31
Crystal system
Space group
a (A)
triclinic
monoclinic
P21/c
9.0921(5)
26.362(2)
9.5214(5)
ꢀ
P1
˚
11.6997(4)
11.9763(5)
11.2163(2)
112.059(3)
114.076(2)
60.855(2)
1222.00(9)
2
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
101.071(2)
3
˚
V (A )
2239.7(2)
4
9.72
Fig. 2. ORTEP drawings of [PdCl2(L1)] (1) and [PdCl2(L3)].
Z
l(MoKa) (cmꢂ1
)
12.17
Transmission factor
Crystal color
0.7771–0.8368
orange
0.8061–0.9294
red
Crystal habit
prism
0.20 · 0.30 · 0.30
1.665
55.0
0.031
prism
0.08 · 0.10 · 0.40
1.552
55.0
0.081
Table 2
Selected bond distances (A) and angles (ꢀ) for complex 1
˚
Crystal size (mm)
Dcalc (g cmꢂ3
)
Pd(1)–Cl(1)
Pd(1)–Cl(2)
Pd(1)–N(1)
Pd(1)–N(2)
N(1)–C(5)
N(2)–C(6)
2.2928(7)
2.2836(7)
2.043(2)
2.047(2)
1.332(4)
1.453(3)
N(2)–C(7)
N(3)–N(4)
N(3)–C(16)
N(4)–C(6)
C(5)–C(6)
C(7)–C(16)
1.327(3)
1.328(3)
1.322(3)
1.461(3)
1.496(3)
1.452(3)
2hmax (ꢀ)
R1
a
b
Rw
S, Goodness-of-fitc
0.052
1.75
0.101
2.47
P
P
a
R1 = iFoj ꢂ jFci/ jFoj for I > 2r(I) data.
P
P
b
c
2 1/2
Rw = [ w(jFoj ꢂ jFcj)2/ wjFoj ]
.
Cl(1)–Pd(1)–Cl(2)
Cl(1)–Pd(1)–Cl(2)
Cl(1)–Pd(1)–N(1)
Cl(2)–Pd(1)–N(2)
N(1)–Pd(1)–N(2)
Pd(1)–N(1)–C(5)
Pd(1)–N(2)–C(6)
Pd(1)–N(2)–C(7)
C(6)–N(2)–C(7)
N(4)–N(3)–C(16)
N(3)–N(4)–C(6)
N(3)–N(4)–C(17)
89.55(4)
86.15(3)
93.46(7)
99.67(6)
80.91(8)
116.1(2)
113.8(1)
136.6(2)
109.6(2)
115.8(2)
115.6(2)
116.3(2)
Cl(2)–Pd(1)–N(2)
C(6)–N(4)–C(17)
N(1)–C(5)–C(6)
N(2)–C(6)–N(4)
N(2)–C(6)–C(5)
N(4)–C(6)–C(5)
N(2)–C(7)–C(8)
N(2)–C(7)–C(16)
C(8)–C(7)–C(16)
N(3)–C(16)–C(7)
N(3)–C(16)–C(15)
C(7)–C(16)–C(15)
96.2(1)
127.6(2)
116.9(2)
107.8(2)
112.2(2)
112.4(2)
123.6(2)
118.9(2)
117.3(2)
119.1(2)
116.8(2)
121.0(2)
P
S = [ w(jFoj ꢂ jFcj)2/(m ꢂ n)]1/2 (m is the number of used reflections,
n is the number of refined parameters).
2.4. Kinetic studies
Kinetics of the reaction was studied by monitoring the
UV–Vis absorption spectra as a function of time. The tem-
perature of the reaction solutions was controlled to within
0.1 ꢀC using a circulating thermostatted water bath. The
concentrations of the entering methanol ligand was kept in
large excess over that of the Pd(II) complex. The total con-
centration of the complex was less than 10ꢂ4 M. The
observed rate constant was evaluated by fitting the absor-
bance–time traces with a non-linear least-squares program.
The reproducibility of the kobs value was better than 5%.
small torsion angle of N(1)–C(5)–C(6)–N(2) (4.5(3)ꢀ) sug-
gests that the five-membered chelate ring is planar in com-
plex 1, while the chelate ring of [PdCl2(L3)] has a gauche
conformation, and makes a torsion angle of N(1)–C(5)–
C(6)–N(2) (34.5(3)ꢀ). In complex 1, the triazine ring is
folded, and the least-squares plane (C(6), N(2), C(7),
C(16)) makes a dihedral angle of 136.9(2)ꢀ with the mean
plane defined by C(6), N(4), N(3), and C(16). In
[PdCl2(L3)], on the other hand, the C(6) atom is displaced
3. Results and discussion
3.1. Preparation and structural characterization of
complexes 1 and 2
˚
by 0.337(4) A from the least-squares plane (N(2), C(7),
The triazine ligand (L1) was prepared in situ and used
for complex preparation without isolation. The reaction
of [PdCl2(PhCN)2] and L1 in dichloromethane yielded
complex 1; complex 2 was obtained by a similar reaction
procedure in methanol. Fig. 2 shows ORTEP drawings of
complex 1 (a) and [PdCl2(L3)] (L3 = 2-phenyl-3-(2-pyri-
dyl)-2,3-dihydronaphtho[2,1-e][1,2, 4]triazine) (b) for com-
parison of their structures, and selected bond distances and
angles of complex 1 are listed in Table 2. The coordination
geometry around the Pd atom is square planar for both
structures. The difference in Pd–Cl and Pd–N bond lengths
was hardly discernible between the two structures. The
C(16), N(3), N(4)). Accordingly, the conformation of the
triazine ring is different in each complex. The dihedral
angles of the phenyl ring (C(17)–C(22)) and pyridine ring
(N(1), C(1)–C(5)) (123.9(1)ꢀ) or naphthyl ring (C(7)–
C(16)) (127.7(1)ꢀ) are large in complex 1, but the phenyl
ring (C(17)–C(22)) and naphthyl ring (C(7)–C(16)) of
[PdCl2(L3)] make a small dihedral angle of 24.7(1)ꢀ. It is
also notable that the dihedral angle of the pyridine ring
(N(1), C(1)–C(5)) and naphthyl ring (C(7)–C(16)) is small
(22.6(1)ꢀ) in complex 1, although the corresponding angle
in [PdCl2(L3)] is large (108.8(1)ꢀ). Thus, the attachment
of a methyl group to a phenyl moiety caused a drastic

M. Zhang et al. / Inorganica Chimica Acta 360 (2007) 3040–3046
3043
structural difference between complex 1 and [PdCl2(L3)],
which caused a remarkable difference in their absorption
spectra, as described later.
N(3)–C(16) (1.33(1) A) are shorter than the common N–
C single bond. Accordingly, the negative charge of the
ligand is spread over the conjugated six-membered chelate
ring. A similar trend was also found in [PdCl(L4)], as
reported previously.
˚
Fig. 3 displays the perspective views of complex 2 and
[PdCl(L4)] (L4 = N-[methoxy(2-pyridyl)methyl]-1-(phenyl-
azo)-2-naphthylamide). The selected bond distances and
angles of complex 2 are listed in Table 3. The coordination
geometry around the Pd atom is also square planar for
both structures. The methyl group attached to a phenyl
moiety is disordered to two different positions (C(23) and
C(24)). A methoxy group is attached to the sp3-C(6) atom,
and the N(4) atom of the azo moiety is coordinated to the
Pd atom. The overall structure of complex 2 is similar to
that of [PdCl(L4)]; however, the dihedral angle of the phe-
nyl ring (C(17)–C(22)) and the coordination plane (Pd(1),
Cl(1), N(1), N(2), and N(4),) (98.4(3)ꢀ) in complex 2 is lar-
ger than that of [PdCl(L4)] (54.2(1)ꢀ) in order to avoid a
steric hindrance between the methyl group and the coordi-
nated Cl(1) atom. The bond length of N(2)–C(6)
3.2. Equilibria of the ring opening reaction of the triazine
ligand
Fig. 4 shows the absorption spectra of complexes 1 and
2 in dichloromethane. We reported previously that solu-
tions of [PdCl2(L3)] and [PdCl(L4)] in dichloromethane
exhibit absorption bands (MLCT) at 570 and 540 nm,
respectively. It is notable that the absorption maximum
of complex 1 is shifted to a shorter wavelength relative to
[PdCl2(L3)] and observed at 413 nm. It seems that complex
1 has a strong d–p* interaction. On the other hand, com-
plex 2 exhibits an absorption band at 531 nm, and the spec-
tral pattern is similar to that of [PdCl(L4)].
On addition of methanol to the dichloromethane solu-
tion of complex 1, the absorption spectrum changed grad-
ually with isosbestic points at 489, 387, 309, and 278 nm.
The absorption spectra observed after the completion of
the reaction are shown in Fig. 5. These findings can be
interpreted by the mechanism that the reaction of complex
1 with methanol proceeds via a unimolecular process with
respect to the Pd complex. During the reaction of complex
1 with methanol, one of two chloride ions binding to the Pd
atom should be displaced with the N(4) atom of ligand L2
that was derived from the nucleophilic attack of the meth-
anol oxygen atom on the C(6) atom of ligand L1. After the
nucleophilic substitution reaction, the ligand has a struc-
ture that includes the CH3O-moiety at the C(6) atom,
and the consecutive release of hydrogen ions from the
attacking methanol molecule can be trapped by chloride
ions that are released from the complex. The finding that
the addition of hydrogen chloride to the solution of com-
plex 2 leads to the recovery of complex 1 is in accord with
this mechanism. The progress of the reaction was moni-
˚
(1.46(1) A) indicates an N–C single bond, and that of
˚
N(3)–N(4) (1.29(1) A) indicates an N=N double bond.
˚
However, the bond lengths of N(2)–C(7) (1.31(1) A) and
Fig. 3. ORTEP drawings of [PdCl2(L2)] (2) and [PdCl2(L4)].
Table 3
˚
Selected bond distances (A) and angles (ꢀ) for complex 2
Pd(1)–Cl(1)
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–N(4)
O(1)–C(6)
O(1)–C(25)
N(1)–C(5)
2.319(2)
2.056(9)
1.990(7)
1.946(9)
1.42(1)
1.44(2)
1.36(1)
N(2)–C(6)
N(2)–C(7)
N(3)–N(4)
N(3)–C(16)
C(5)–C(6)
C(7)–C(16)
1.46(1)
1.31(1)
1.29(1)
1.33(1)
1.54(2)
1.44(2)
60000
50000
40000
30000
20000
Cl(1)–Pd(1)–N(1)
Cl(1)–Pd(1)–N(4)
N(1)–Pd(1)–N(2)
N(2)–Pd(1)–N(4)
Pd(1)–N(1)–C(5)
Pd(1)–N(2)–C(6)
Pd(1)–N(2)–C(7)
C(6)–N(2)–C(7)
N(4)–N(3)–C(16)
Pd(1)–N(4)–N(3)
Pd(1)–N(4)–C(17)
93.3(2)
93.1(2)
83.1(3)
N(3)–N(4)–C(17)
N(1)–C(5)–C(6)
O(1)–C(6)–N(2)
O(1)–C(6)–C(5)
N(2)–C(6)–C(5)
N(2)–C(7)–C(8)
N(2)–C(7)–C(16)
C(8)–C(7)–C(16)
N(3)–C(16)–C(7)
N(3)–C(16)–C(15)
C(7)–C(16)–C(15)
111.0(8)
119(1)
112.5(9)
109(1)
109.4(8)
121(1)
122.8(8)
116.4(9)
125.4(9)
113.2(9)
121.4(8)
90.7(3)
112.3(7)
115.5(7)
126.0(7)
118.5(8)
126.1(9)
127.4(6)
121.6(6)
10000
0
300
400
500
600
700
Wavelength / nm
Fig. 4. Absorption spectra of complexes [PdCl2(L1)] (1) (—) and
[PdCl2(L2)] (2) (- - -) in dichloromethane.

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M. Zhang et al. / Inorganica Chimica Acta 360 (2007) 3040–3046
shown in Fig. 6 were analyzed using a non-linear least-
squares fitting program based on Eq. (2), but the least-
squares calculation did not converge to give appropriate
parameters. Instead, a successful fit was obtained only
when the mechanism included the consumption of two
methanol molecules and the release of HCl whose concen-
tration is equivalent to that of the produced Pd complex as
shown in Eq. (3)
½PdCl2ðL1Þꢀ þ 2CH3OH ꢀ ½PdClðL2Þꢀ þ CH3OH2þ ꢁ Clꢂ
ð3Þ
In dichloromethane with a low dielectric constant, the
polar HCl molecule will be stabilized by forming an adduct
with methanol, probably giving CH3OH2þ ꢁ Clꢂ. The cal-
culated curves based on Eq. (3) are given as solid lines in
Fig. 6 and satisfactorily explain the experimental results.
The equilibrium constant of reaction (3) was determined as
Fig. 5. UV–Vis absorption spectra of the dichloromethane solution of
[PdCl2(L1)] (1) in the presence of methanol. CPd = 4.66 · 10ꢂ5 M.
Cmethanol/M = 0 (a), 0.101 (b), 0.203 (c), 0.397 (d), 0.701 (e), 0.997 (f).
T = 25.0 ꢀC.
K ¼ ½PdClðL2Þꢀ½CH3OH þ ꢁ Clꢂꢀ½PdCl2ðL1Þꢀꢂ1½CH3OHꢀꢂ2
¼
2
ð2:2 ꢃ 0:4Þ ꢄ 10ꢂ4 Mꢂ1 at T = 25.0 ꢀC. The fact that two
equivalent methanol molecules participate in the reaction
can be explained by these mechanisms.
3.3. Kinetics of the reaction
The kinetics of the reaction of complex 1 with methanol
were investigated by monitoring the absorbance change of
the reacting solution with time using a UV–Vis spectropho-
tometer. The reaction rate was measured under conditions,
where the entering methanol molecules were present in
large excess over the Pd complex and under the conditions
of a reversible reaction where the kinetics of the reverse
reaction contribute to the overall rate law of the reaction.
The forward reaction can be considered as a pseudo-first-
order reaction with respect to complex 1 and the reverse
one as a second-orꢂder reaction with respect to complex 2
and CH3OH2þ ꢁ Cl . Thus, the rate law can be expressed
by Eq. (4) [9]
Fig. 6. Dependence of the absorbance of the dichloromethane solution of
[PdCl2(L1)] (1) on the methanol concentration. CPd = 4.66 · 10ꢂ5 M.
T = 25.0 ꢀC.
dx=dt ¼ kf ða ꢂ xÞ ꢂ krx2
ð4Þ
tored by observing the absorbance changes at several wave-
lengths of the visible spectra as a function of the concentra-
tion of methanol. The absorbance of the solution is plotted
as a function of the concentration of methanol that exists
in large excess over the Pd complex in Fig. 6. First, we ana-
lyzed the equilibrium of the reaction using the following
reaction scheme:
where the initial concentration of complex 1 is represented
by a and the extent of reaction in terms of concentration is
given by x. kf and kr are the pseudo-first-order rate constant
of the forward reaction and the second-order rate constant
of the reverse reaction, respectively. At equilibrium, dx/dt
is equal to 0, and the kr term can be given by Eq. (5) using
xe as an equilibrium value of x:
[PdCl2(L1)] + CH3OH ꢀ [PdCl(L2)] + HCl
ð1Þ
kr ¼ kf ða ꢂ xeÞ=x2e
ð5Þ
In this case, the absorbance, A, of the solution can be ex-
pressed by Eq. (2)
These equations can be integrated to give Eq. (6)
A¼ðe1 þe2K½CH3OHꢀ½HClꢀꢂ1Þð1þK½CH3OHꢀ½HClꢀꢂ1Þꢂ1CPd
ð2Þ
where e1 and e2 represent the molar extinction coefficient of
[PdCl2(L1)] (1) and [PdCl(L2)] (2), respectively, and K and
CPd are equilibrium constants of reaction (1) and the total
concentration of the Pd complex, respectively. The results
lnfaxe þ xða ꢂ xeÞgfaðxe ꢂ xÞgꢂ1 ¼ kf ð2a ꢂ xeÞxꢂ1
t
ð6Þ
e
Using Eq. (6), the absorbance of the reaction solution at
time t, At, can be given by Eq. (7)
0
0
At ¼ A0 þ ðeB ꢂ eAÞaxeðek t ꢂ 1Þða ꢂ xe þ aek tÞꢂ1
ð7Þ

M. Zhang et al. / Inorganica Chimica Acta 360 (2007) 3040–3046
3045
In this equation k0 represents kf ð2a ꢂ xeÞxꢂe 1, and the molar
absorption coefficients of complexes 1 and 2 are given by eA
and eB, respectively. Reaction curves obtained are analyzed
by a non-linear least-squares fitting program using Eq. (7)
to determine the rate constants. Fig. 7 shows examples of
the absorbance–time traces, and the observed absorbance
changes are well explained by the calculated curves.
The pseudo-first-order rate constant of the forward
reaction, kf, was evaluated using the k0 value obtained.
The methanol concentration dependence of kf is shown in
Fig. 8. The reaction rate increases with the increase in
methanol concentration, and kf can be expressed by Eq. (8)
includes methanol participation during the activation pro-
cess, is in accord with the mechanism of the nucleophilic
attack of methanol oxygen atoms on the ligand L1. Mean-
while, the former reaction path is first-order with respect to
the Pd complex and does not depend on the concentration
of attacking methanol molecules. The methanol molecule
itself participates in the overall substitution reaction, and
therefore it is involved in the reaction that occurs after
the rate-determining step of the corresponding reaction
path. It has been demonstrated that a water molecule can
act as a nucleophile just like methanol for this type of reac-
tion and that the ring opening reaction of the triazine li-
gand occurs to give the product including an OHꢂ
moiety at the C(6) atom instead of CH3Oꢂ moiety [3b].
Since the water concentration is quite low (less than
1 mM), and once produced, a complex having an OHꢂ
moiety may easily be transferred to the final product of
complex 2 via substitution of an OH group by methanol
under the present experimental conditions in which metha-
nol exists in large excess over the Pd complex. This type of
water-assisted mechanism path could be operative for the
k1 path of the forward reaction.
kf ¼ k1 þ k2½CH3OHꢀ
ð8Þ
k1 and k2 represent the first- and second-order rate
constants of the reaction paths, respectively. By applying
the least-squares fitting calculation to the results shown
in Fig. 8, we obtained values of each rate constant of
k1 = (2.40 0.07) · 10ꢂ3 sꢂ1 and k2 = (5.8 0.1) · 10ꢂ3
M
ꢂ1 sꢂ1 at T = 25.0 ꢀC. The latter reaction path, which
The rate constant of the reverse reaction can be expressed
as kr = (k1 + k2[CH3OH])Kꢂ1[CH3OH]ꢂ2. Although kr
shows a complicated methanol concentration dependence
due to the parallel reaction paths, the estimated kr value
using Eq. (5) was successfully explained by the above equa-
tion with obtained values of K, k1, and k2.
Acknowledgements
The present work was supported by a Grant-in-Aid for
Scientific Research Nos. 17350028, 17550056, and
18550052 from the Ministry of Education, Science, Sports,
and Culture of Japan.
Fig. 7. The absorbance–time traces of the reaction of [PdCl2(L1)] (1) with
methanol. k = 530 nm. CPd = 5.06 · 10ꢂ5 M. Cmethanol/M = 0.370 (a),
0.641 (b). T = 25.0 ꢀC.
Appendix A. Supplementary material
CCDC 624843 and 624844 contain the supplementary
crystallographic data for 1 and 2. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at
doi:10.1016/j.ica.2007.03.024.
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