Ring opening and displacement by chloride of the bidentate chelate ligand from dichloro[pyridine-2-(α-methoxymethanolato)]gold(III) - A kinetic and mechanistic study

Article abstract of DOI:10.1002/ejic.200500749

The kinetics of ring opening and displacement of the bidentate chelate ligand from dichloro[pyridine-2-(α-methoxymethanolato)]gold(III) [Au(N-O)Cl₂] (1) have been studied spectrophotometrically in methanol/water (95:5, v/v) at 25°C and constant ionic strength (I = 1 mol dm^-3, LiClO₄). In the presence of LiCl and perchloric acid the reaction consists of a pre-equilibrium protonation of the coordinated oxygen followed first by ring opening at oxygen accompanied by the entry of chloride or solvent and fast acetalisation of the hemiacetalic form of the ligand to give [AuCl₃(N-OMe)], and then by displacement of the N-bonded ligand to give [AuCl₄]^-. The ligand is not displaced in the absence of chloride and no reaction is observed in the presence of chloride alone. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200500749

FULL PAPER
DOI: 10.1002/ejic.200500749
Ring Opening and Displacement by Chloride of the Bidentate Chelate Ligand
from Dichloro[pyridine-2-(α-methoxymethanolato)]gold(III) – A Kinetic and
Mechanistic Study
[a]
[a]
[a]
[a]
Giampaolo Marangoni,* Bruno Pitteri, Giuliano Annibale, and Marco Bortoluzzi
Keywords: Chelate displacement / Gold / Kinetics / Reaction mechanism
The kinetics of ring opening and displacement of the biden-
tate chelate ligand from dichloro[pyridine-2-(α-methoxy- acetalic form of the ligand to give [AuCl
methanolato)]gold(III) [Au(N–O)Cl ] (1) have been studied by displacement of the N-bonded ligand to give [AuCl
spectrophotometrically in methanol/water (95:5, v/v) at 25 °C
entry of chloride or solvent and fast acetalisation of the hemi-
3
(N–OMe)], and then
–
2
4
] .
The ligand is not displaced in the absence of chloride and no
reaction is observed in the presence of chloride alone.
–3
4
and constant ionic strength (I = 1 moldm , LiClO ). In the
presence of LiCl and perchloric acid the reaction consists of
a pre-equilibrium protonation of the coordinated oxygen fol-
lowed first by ring opening at oxygen accompanied by the
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Introduction
In a recent study dealing with the interaction of pyridine-
–
2
-carbaldehyde (2CHO–py) with [AuCl ] in water we
4
The lability of oxygen-donor ligands in the substitution
[
6]
found that the initially formed N-coordinated [AuCl (2-
II
II
3
reactions at square planar tetra-coordinate Pt , Pd and
CHO–py)] complex adds a molecule of H O to the al-
III
2
Au complexes depends very greatly upon their nature,
dehydic group to form the corresponding diolic derivative
which, by elimination of HCl, gives rise to O-chelation at
the metal centre. Successive treatment with alcohols affords
the corresponding N–O-chelated hemiacetalic complexes
ranging from the highly labile coordinated water to the very
[
1]
inert hydroxide. Unfortunately, the lack of suitable sub-
III
strates of Au , because of the synthetic difficulties involved
in preparing appropriate complexes with monodentate oxy-
gen ligands, has hampered any extensive study in this field.
One way to overcome the problem is to incorporate the oxy-
gen donor in a bi- or multidentate ligand so providing the
required ease of preparation and stability. Very recently,
[
(I), Scheme 1]. On the other hand, when the reaction be-
gold() complexes of the type [Au(N–O)Cl ] (N–O = pyri-
2
[2]
dine-2-methanolato,
substituted pyridine-2-carboxyl-
[3]
ato ) have been reported to be excellent catalysts in organic
synthesis. Some years ago we reported the preparation of
dichloro(pyridine-2-carboxylato)gold() and dichloro(pyri-
dine-2-methanolato)gold() neutral complexes and we
studied the replacement of N–O bidentate ligands by chlo-
ride in acidic solutions.[
4,5]
In the first case it was shown
that the chelate ring opened at nitrogen and the carboxylate
group was displaced in a slower subsequent step. In the sec-
ond case, because the change in the 2-substituent from
–
–
CO2 to CH O increases the basicity of the nitrogen by
2
more than three orders of magnitude and hence decreases
the lability of the Au–N bond by about two orders of mag-
nitude, and the bond to the alkoxide oxygen is considerably
more labile to direct chloride substitution than the bond to
carboxylate, the reaction sequence is reversed, ring opening
at oxygen occurring in the first stage of the reaction.
[
a] Department of Chemistry, Cà Foscari University of Venice,
Calle Larga S. Marta 2137, 30123 Venice, Italy
Scheme 1.
Eur. J. Inorg. Chem. 2006, 765–771
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
765

G. Marangoni, B. Pitteri, G. Annibale, M. Bortoluzzi
FULL PAPER
–
tween pyridine-2-carbaldehyde and [AuCl ] is carried out ther addition of excess LiCl results in a slower reaction,
4
in alcohols at 60 °C, the N-coordinated acetal complexes which leads to the displacement of the 2-substituted pyri-
–
trichloro(pyridine-2-carbaldehyde dialkyl acetal)gold() dine ligand and formation of [AuCl ] with a well-main-
4
are directly formed [(II), Scheme 1]. Some of the complexes tained isosbestic point at 296 nm until the end of the reac-
of both the series have been characterised by X-ray dif- tion (curves d, e). When an excess of both LiCl and perchlo-
[
6]
fractometry.
ric acid is added to a solution of 1, the change of ab-
As an extension of that work, we have now prepared the sorbance with time at 320 nm indicates a two-stage reac-
analogous neutral dichloro[pyridine-2-(α-methoxymeth- tion, the second stage being identical to that described
anolato)]gold() (1), [Au(N–O)Cl ], and have studied the above.
2
displacement of the new type of N–O chelate ligand by
chloride in acidic methanol containing 5% water.
Table 1. Observed rate constants: for the first stage (kobsf), for the
s
[a]
second stage (kobs ) of the reaction, and in the presence of per-
f
chloric acid alone (kЈobs ).
+
3
–
2
f
4
s
3
f
[
[
H ]
10 [Cl ]
10 kobs
10 kobs
10 kЈobs
Results
–
3
–3
–1
–1
–1
moldm ] [moldm ]
[s ]
[s ]
[s ]
The electronic spectrum in the wavelength range 250–
0
.15
3.69
4.47
5.64
6.40
7.60
9.33
70 nm of a 1×10–4 moldm solution of [Au(N–O)Cl ] (1) 0.2
–3
3
2
0
0
0
0
.25
in methanol/water (95:5 v/v) in the presence of a large ex-
cess of LiCl to suppress any solvolysis is shown in Figure 1
curve a). This spectrum does not change over long periods
.3
.4
.5
(
of time.
0.05
1.0
2.0
4.0
6.0
8.0
10
0.29
0.72
1.22
1.82
2.40
3.0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.05
.05
.05
.05
.05
.05
.1
.1
.1
.1
.1
12
3.70
0.96
1.67
2.85
4.05
5.11
6.40
7.58
1.56
2.30
3.20
4.05
4.61
5.30
6.14
6.89
7.60
8.40
2.50
3.51
4.82
5.82
6.61
7.82
8.90
9.85
11.4
12.55
3.22
4.84
6.0
1.0
2.0
4.0
6.0
8.0
10
.1
.1
12
.15
.15
.15
.15
.15
.15
.15
.15
.15
.15
.2
.2
.2
.2
.2
.2
.2
.2
.2
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10
1.0
2.0
3.0
4.0
5.0
1.0
2.0
3.0
4.0
5.0
0.99
1.85
2.56
2.91
3.90
5.40
6.20
7.93
Figure 1. Electronic spectra of 1.0×10–4 moldm–3 solutions of 1 in
methanol/water (95:5 v/v) at 25 °C under the different experimental
conditions reported in the text.
.2
.3
.3
.3
In the absence of lithium chloride, addition of perchloric 0.3
7.48
9.01
3.57
5.30
7.60
9.10
10.38
0
0
0
0
0
.3
.4
.4
.4
.4
acid to a solution of 1 of the same concentration causes a
relatively fast change of the spectrum with time to finally
give the spectrum represented by curve b. Successive ad-
dition of 1:1 LiCl gives rise to a rapid change of the spec-
trum b to that of curve c, which is identical to that of the 0.4
trichloro(pyridine-2-carbaldehyde dimethyl acetal)gold()
2) measured under the same experimental conditions. Fur- MeOH/H
–3
, [substrate] = 1×10–4 moldm , 25 °C,
–3
[a] I = 1 moldm , LiClO
4
(
2
O (95:5 v/v); rate constants were accurate to within 5%.
766
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Eur. J. Inorg. Chem. 2006, 765–771

Ligand Displacement in a Gold Complex
FULL PAPER
Kinetics
The kinetics were evaluated from the change in ab-
sorbance at a fixed wavelength (320 nm) where both stages
lead to an increase of absorbance and as the two rate con-
stants differed by at least ten, their separate evaluation pre-
sented no problem. Values of the rate constants for the first,
kobsf, and second, kobss, stages of the reaction in the pres-
ence of both LiCl and perchloric acid under pseudo-first-
order conditions, as well as in the presence of perchloric
acid alone, are collected in Table 1. Rate constants were ac-
curate to within 5%.
+
f
–
At constant [H ], k
varies with [Cl ] such that a plot
obs
of kobsf versus [Cl ] is linear with a finite intercept (Fig-
–
ure 2), according to Equation (1).
Figure 3. Linear dependence of s–1 on [H ] . Data from Table 2.
+ –1
(
2)
Owing to the large uncertainty in the values of i, its de-
+
pendence on [H ] was better determined on studying the
reaction in the absence of added LiCl. The values of the
f
relative rate constants (kЈ
ϵ i) are collected in Table 1.
obs
kЈobsf increases as [H ] increases but appears to reach a lim-
iting value at the higher [H ] concentrations. A plot of
against [H ] is linear in the range of examined
H ] (Figure 4) with intercept e = (40± 6) s and gradient f
+
+
f –1
+ –1
(
[
kЈ
)
obs
+
–
3 –1
=
(35± 1) moldm s .
These results are in agreement with Equation (3).
f
–
Figure 2. Plots of kobs vs [Cl ] for the reaction between [Au(N–O)
+
(3)
Cl ] (1) and chloride at [H ] = 0.05 (᭺), 0.1 (ᮀ), 0.15 (◊), 0.2 (᭝),
2
–3
0
.3 (+), 0.4 (᭞) moldm .
On substituting Equations (3) and (2) into Equation (1),
the experimental rate law for the first step of the reaction
f
obs = i + s[Cl–]
k
(1)
+
Both i and s depend upon [H ] as shown in Figure 2 and
Table 2.
Table 2. Values of parameters i and s at constant added acid.
+
–3
–1
s [mol–1 dm s ]
3 –1
[
H ] [moldm ] i [s ]
0
0
0
0
0
0
.4
.3
.2
.15
.1
(1.9± 0.3)×10–2
17.4± 0.1
14.2± 0.4
10.9± 0.2
7.52± 0.08
5.95± 0.06
3.01± 0.05
(1.8± 0.1)×10–
2
–
2
(1.3± 0.1)×10
(0.87± 0.05)×10–
(0.43± 0.04)×10–
2
2
.05
(0.03± 0.04)×10–2
+
The dependence of s upon [H ] shows a tendency to slow
+
negative deviations as [H ] progressively increases, but a
–
1
+ –1
plot of s against [H ] is linear (Figure 3) with intercept
–
2 –1
c
=
(1.8± 0.5)×10
s
and
slope
d
=
–
2
–3 –1
(
1.6± 0.5)×10 moldm s , from which the empirical
f –1
+ –1
Figure 4. Plot of (kЈobs
)
against [H ] in the absence of added
Equation (2) can be derived.
chloride.
Eur. J. Inorg. Chem. 2006, 765–771
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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767

G. Marangoni, B. Pitteri, G. Annibale, M. Bortoluzzi
FULL PAPER
takes the form of Equation (4) which can be rearranged as
The reaction observed in the presence of HClO alone is
4
–
1
3
Equation (5) with e/f (1.14± 0.05 mol dm )
=
c/d described as a fast protonation pre-equilibrium AhB fol-
–
1
3
(1.1± 0.4 mol dm ) within the limit of the experimental er- lowed by a slow reversible reaction to give the solvato spe-
ror and an average value of 1.12± 0.03.
cies C (spectrum b in Figure 1). C is formulated as the ace-
talic species, assuming that acetylation of the hemiacetalic
group is a fast process in acidic media (see below). Addition
of an equimolar amount of LiCl rapidly affords the anation
product PЈ (spectrum c in Figure 1). The same species can
–
(
4) also be formed directly from B by reaction with Cl . In the
–
presence of higher [Cl ], such as those used under pseudo-
first-order conditions, displacement of the N-coordinated
ligand through two parallel reactions, solvolysis and direct
chloride substitution, occurs to give the final product PЈЈ.
This latter process corresponds to the second stage of the
reaction observed in the kinetic runs, better followed in the
presence of chloride ions in acidic solution. The derived
(
5)
s
s
s
–
rate law, kobs = k1 + k [Cl ] typical for the displacement
2
[
7]
of N-bonded pyridines is obeyed, with a solvolytic rate
constant k1 = (2± 1)×10
constant k2 = (7.6± 0.2)×10 mol dm s nearly equal,
within the limit of experimental error, to that relative to the
displacement of the N-coordinated dimethyl acetal ligand
from the trichloro(pyridine-2-carbaldehyde dimethyl acetal)
The rate constants for the displacement of the N-coordi-
(Table 1), are independent from
H ] and obey the usual relationship Equation (6) with g =
s
–4 –1
s
and the second-order rate
s
nated pyridine ligand, k
s
–2
–1
3 –1
obs
+
[
(
–
4
–1
–2
–1
3 –1
2± 1)×10
s
and h = (7.6± 0.2)×10 mol dm s (Fig-
ure 5).
gold()
complex
(2),
[k2
=
(7.875± 0.003)×
s
obs = g + h[Cl–]
–2
–1
3 –1
k
(6) 10 mol dm s ], kinetically measured under the same ex-
perimental conditions, as well as to those found for the dis-
placement of the strictly related pyridine-2-methanol (k =
2
–2
–1
3 –1
7
=
.58×10 mol dm s ) and 2-methoxymethylpyridine (k
2
7.93×10–2 mol dm s ). Values of kobs for the dis-
–1
3 –1 [5]
placement of the ligand from the trichloro(pyridine-2-car-
baldehyde dimethyl acetal)gold() complex (2) and the de-
rived rate constants are reported in Table 3.
The close resemblance of the electronic spectra of the
intermediate formed at the end of the first stage of the reac-
tion and that of complex 2, as well as the values of their
second-order rate constants quoted above for the final dis-
placement of the ligand, strongly support the conclusion
that the hemiacetalic form of the chelated ligand, once pro-
tonated and then O-opened, undergoes a rapid process of
acetalisation in the presence of the solvent MeOH under
acidic conditions to give the derived N-coordinated di-
methyl acetal as the true leaving 2-substituted pyridine in
the second stage of the reaction. This behaviour is in agree-
ment with the instability of the hemiacetals of the free or-
[
8]
ganic carbaldehydes that, under comparable experimental
conditions, react with alcohols to rapidly give the corre-
sponding acetals. This conclusion has been fully confirmed
1
by the series of H NMR experiments reported in Figure 6.
s
–
Figure 5. Plot of kobs against [Cl ]. Data from Table 1.
When dichloro[pyridine-2-(α-methoxymethanolato)]gol-
d() (1) in CD NO [integrated proton ratio C–H (δ =
3
2
5
.45 ppm)/O–CH (δ = 3.56 ppm) of 1:3] (spectrum A) is
3
added to methanol and DCl [molar ratio 5:1 and 1:1 with
respect to the concentration of complex
2.56×10–2 moldm )] its spectrum rapidly changes (spec-
trum B) to that of an authentic sample of the trichloro(pyri-
1
–3
(
Discussion
The spectroscopic evidence together with the kinetic re- dine-2-carbaldehyde dimethyl acetal)gold() (2) in
–2
–3
sults are fully consistent with the reaction depicted in CD NO at a similar concentration (2.34×10 moldm
3
2
Scheme 2.
(spectrum C), both of them showing an integrated proton
768
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Ligand Displacement in a Gold Complex
FULL PAPER
3
–1
f
3
–1 –1
f
0.025± 0.001 s–1,
s
(2± 1)×10–4 s–1,
s
Scheme 2.
K
=
1.12± 0.03 dm mol ,
k
3
=
56± 17 dm mol s ,
k
1
=
f
k
1
=
k
2
=
f
–2
3
–1 –1
(7.6± 0.2)×10 dm mol s .
Table 3. Observed rate constants (kobs) and derived rate constants (k–1f + k [Cl ]) ϾϾ k }, the following dependence of k
–
f
2
1
obs
–
–
(
k
1
and k
2
) for the reaction [Au(N–OMe)Cl
3
] + Cl Ǟ [AuCl
4
] +
+
–
on [H ] and [Cl ] is derived [Equation (7)].
+
HN–OMe at 25 °C in MeOH/H
2
O (95:5 v/v).
Because under our experimental conditions in the pres-
Cl–]
moldm ]
10 kobs [s ]
3
–1
k
[s ]
–1
k
[mol–1 dm– s ]
3
–1
+
–3
[
[
1
2
ence of [H ] Ն 0.05 moldm alone, the reaction is com-
–
3
pletely shifted towards the solvato species C, the condition
k–1f ϽϽ k must hold and hence, in the presence of Cl , the
f
–
0
0
0
0
0
0
.01
.02
.03
.04
.08
.1
0.84
1.68
2.44
3.13
6.34
7.96
1
f
–
f
condition k [Cl ] ϾϾ k must be satisfied. Under these
conditions Equation (7) reduces to Equation (8).
2
–1
(5± 3)×10–5
(7.875±0.003)×10–2
Equation (8) is identical to the empirical Equation (5) if
f
f
k K = 1/f, K = e/f, k K = 1/d, from which the values of
1
3
3
–1
the equilibrium constant (K = 1.12± 0.03 dm mol ) and of
the rate constants (k = 56± 17 dm mol s , k
f
3
–1 –1
f
=
3
1
–1
0.025± 0.001 s ) can be calculated. These values together
ratio C–H (δ = 6.15 ppm)/O–CH (δ = 3.66 ppm) of 1:6. with the rate constants for the second stage of the reaction
3
s
s
Further addition of DCl [molar ratio 10:1 with respect to (k
the concentration of complex 1] allows the reaction to go
and k
) are also reported in Scheme 2.
2
1
A point that deserves to be underlined is the different
to completion with release of the ligand in the form of the reactivity of complex 1 with respect to that of the previously
[
5]
N-protonated pyridine-2-carbaldehyde dimethyl acetal [in- studied dichloro(pyridine-2-methanolato)gold() as re-
–
tegrated proton ratio C–H (δ = 6.02 ppm)/O–CH3 (δ = gards the reaction with Cl in the absence of acid. In the
3.64 ppm) of 1:6], as comes from a comparison of its spec- first case no reaction takes place, whereas in the second case
trum (spectrum D) with the spectrum of the released ligand the reaction goes to completion with ring opening at oxygen
from 2 under the same experimental conditions (spectrum followed by substitution of the pyridine ligand. This dif-
E). Spectrum B shows also the presence of some free N- ferent behaviour, which implies a stronger Au–O bond in
protonated pyridine-2-carbaldehyde dimethyl acetal as a re- complex 1, would suggest a remarkable effect on the reac-
sult of the incoming development of the second stage of the tivity as a result of the replacement of a hydrogen atom
reaction.
with a methoxy group, possibly related to their different
From the reaction in Scheme 2, on assuming species C inductive effects. Accordingly, both the values of the pro-
–1
3
as a steady-state intermediate {dC/dt = 0 which implies that tonation constant (K = 1.12 mol dm ) and the rate con-
Eur. J. Inorg. Chem. 2006, 765–771
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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769

G. Marangoni, B. Pitteri, G. Annibale, M. Bortoluzzi
FULL PAPER
1
Figure 6. H NMR spectrum of 1 in CD
3
NO
2
(spectrum A); spectrum of 1 in methanol and DCl [molar ratio 5:1 and 1:1 with respect
3 2
to the concentration of the complex] (spectrum B); spectrum of an authentic sample of 2 in CD NO (spectrum C); spectrum of the
released ligand at the end of the reaction in the presence of methanol and an excess of DCl [molar ratio 5:1 and 10:1 with respect to the
concentration of complex (1)] (spectrum D); spectrum of the released ligand from 2 under the same experimental conditions (spectrum
E).
Concluding Remarks
The kinetics of ring opening and displacement of the bi-
dentate chelate ligand from dichloro[pyridine-2-(α-meth-
(
(
7)
8)
oxymethanolato)]gold(), [Au(N–O)Cl ] (1) have been
2
studied spectrophotometrically in methanol/water (95:5 v/
v). Because of the relative inertness of the Au–alkoxide oxy-
gen bond, complex 1 does not react with chloride alone. In
the presence of LiCl and perchloric acid the reaction pro-
ceeds through a pre-equilibrium protonation of the coordi-
nated oxygen followed first by ring opening at oxygen ac-
stant relative to the ring opening of the protonated species
f
–1
3 –1
(
k3 = 56 mol dm s ) for the hemiacetalic complex (1) are
companied by the entry of chloride or solvent and then by
lower than the corresponding constants for the complex
with pyridine-2-methanol (K
26 mol dm s ).
The second stage of the reaction, that is the displacement
of the 2-substituted pyridine by Cl from the species PЈ, can
be compared with other reactions of this sort as it is known
that the second-order rate constant for the reaction with
chloride depends upon the basicity of the leaving pyridine,
–
displacement of the N-bonded ligand to give [AuCl ] .
4
–
1
3
= 9.7 mol dm , k =
However, once opened the hemiacetalic N-coordinated li-
gand is rapidly transformed in the corresponding 2-(pyri-
dinecarbaldehyde dimethyl acetal), which is the true leaving
ligand, to be displaced in the second stage of the reaction
–
1
3 –1 [5]
1
–
as a 2-substituted pyridine with a pK value of 5.86.
a
according to the linear relationship log k = a(pK ) + C Experimental Section
2
a
(
pK is that of the protonated base in water at 25 °C) and
a
Materials: Pyridine-2-carbaldehyde (Aldrich) was vacuum distilled
and stored at 4 °C. Potassium tetrachloroaurate() dihydrate, the
inorganic salts and the solvents were reagent grade products (Ald-
rich) used without further purification.
is independent from steric hindrance of the ring substitu-
ents. In the mixture methanol/water (95:5, v/v) this rela-
[
7]
tionship is also obeyed and the values of a and C, already
measured, are –0.67 and 2.81 respectively. A k2s value of
[9]
Instruments: UV/Vis spectra and kinetics were recorded with a Per-
kin–Elmer Lambda 5 spectrophotometer. IR spectra (4000–
–
2
–1
3 –1
7
.6×10 mol dm s leads therefore to a pK of 5.86 for
a
the leaving pyridine-2-carbaldehyde dimethyl acetal, which
closely compares to those of pyridine-2-methanol and 2-
–1
–1
5
00 cm , KBr disks; 600–50 cm , polyethylene pellets) were col-
1
lected with a Nicolet Magna FTIR 750 spectrometer. H NMR
(
5
methoxymethyl)pyridine, as they both have a pK value of spectra in high purity CD NO (Aldrich) were obtained with a
a
3 2
.6.[
5]
Bruker AC 200 spectrometer. Chemical shifts (ppm) were internally
770
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 765–771

Ligand Displacement in a Gold Complex
FULL PAPER
referenced to the residual undeuterated solvent resonances and re-
lated to tetramethylsilane (δ = 0 ppm). Conductivity measurements
were carried out in dmf with a CDM 83 Radiometer Copenhagen
conductivity meter and a CDC 334 immersion cell. Elemental
analyses were performed by the Microanalytical Laboratory of the
University of Padua.
function of time. They were initiated by adding 10–20 µL of a
–
3
0.015 moldm dimethylformamide (dmf) solution of 1 or 2 to a
3
methanol/water (95:5 v/v) solution (3 cm ) of the appropriate rea-
4 4
gent, either HClO or HClO , and LiCl, previously brought to the
reaction temperature (25 °C) in the thermostatted cell in the spec-
trophotometer. All reactions were carried out at constant ionic
–
3
strength (I = 1.0 moldm , LiClO
4
). Pseudo-first-order rate con-
stants (kobs/s ) were obtained either from the gradients of plots of
log(D – D ) versus time or from a nonlinear least-squares fit of
experimental data to the equation D = D + [D – D exp(–kobst)],
where D , D and kobs are the parameters to be optimised (D is
absorbance after mixing of reactants, D is absorbance at comple-
Synthesis of Complexes 1 and 2 and of the Ligand Released at the
End of the Process (3)
–
1
t
ϱ
Dichloro[pyridine-2-(α-methoxymethanolato)]gold(III
)
(1): [Spec-
t
ϱ
0
ϱ
trum A in Figure 6] This complex was prepared following the same
0
ϱ
0
procedure as for the ethoxy species previously reported[ using
4]
ϱ
methanol instead of ethanol as
AuCl NO (406.02): calcd. C 20.71, H 1.99, Cl 17.46, N 3.45;
found C 20.64, H 1.72, Cl 17.32, N 3.46. H NMR (200 MHz,
a solvent. Yield (80%).
tion of reaction). Rate constants were accurate within 5%.
C
7
H
8
2
2
1
3
Acknowledgments
CD
J
H
3
NO
HH = 7.6, JHH = 1.4 Hz, 1 H, py-H
3,5), 5.45 (s, 1 H, CH), 3.56 (s, 3 H, OCH
bands: 1609 (νC=N), 1079 (νO–CH3), 370, 349 (νAu–Cl) cm . Λ
2
, 25 °C): δ = 9.04 (d, JHH = 5.2 Hz, 1 H, py-H
6
), 8.37 (td,
), 7.92–7.77 (m, 2 H, py-
) ppm. Selected IR
3
4
4
We thank the Cà Foscari University of Venice for financial support
(Ateneo fund 2004).
3
–
1
M
(dmf)
2
–1
=
2.7 Ω–1 cm mol .
[
[
[
1] F. Basolo, R. G. Pearson, Mechanisms of Inorganic Reactions,
rd ed., Wiley, New York, 1967, p. 384.
2] A. S. K. Hashmi, M. Rudolph, J. P. Weyrauch, M. Wölfle, W.
Frey, J. W. Bats, Angew. Chem. Int. Ed. 2005, 44, 2798–2801.
3] A. S. K. Hashmi, J. P. Weyrauch, M. Rudolph, E. Kurpejovic,
Angew. Chem. Int. Ed. 2004, 43, 6545–6547.
Trichloro(pyridine-2-carbaldehyde dimethyl acetal)gold(III
)
(2):
3
[
Spectra B, C in Figure 6] The complex was prepared as previously
[
6]
8 3 2
reported. Yield (75%). C H11AuCl NO (456.51): calcd. C 21.05,
H 2.43, Cl 23.30, N 3.07; found C 20.94, H 2.39, Cl 23.61, N 3.10.
1
3
H NMR (200 MHz, CD
3
NO
2
, 25 °C): δ = 8.96 (d, JHH = 6.0 Hz,
3
4
1
H, py-H
6
), 8.36 (td, JHH = 7.9, JHH = 1.4 Hz, 1 H, py-H
4
), 8.22
[4] G. Annibale, L. Canovese, L. Cattalini, G. Marangoni, G.
Michelon, M. L. Tobe, J. Chem. Soc., Dalton Trans. 1984,
1641–1646.
3
4
3
(dd, JHH = 7.9, JHH = 1.8 Hz, 1 H, py-H
3
); 7.87 (ddd, JHH
), 6.15 (s, 1 H, CH), 3.66
2
) ppm. Selected IR bands: 1611 (νC=N), 1098
=
3
4
6
.0, JHH = 7.9, JHH = 1.8 Hz 1 H, py-H
5
[
5] L. Canovese, L. Cattalini, G. Marangoni, M. L. Tobe, J. Chem.
Soc., Dalton Trans. 1985, 731–735.
6] V. Bertolasi, G. Annibale, G. Marangoni, G. Paolucci, B. Pit-
teri, J. Coord. Chem. 2003, 397–406.
(
(
s, 6 H, OMe
νO–CH3), 366, 349 (νAu–Cl) cm . Λ
–
1
–1
2
–1
M
(dmf) = 2.3 Ω cm mol .
[
2
-CH(OMe)
2
py·HCl (3): [Spectra D, E in Figure 6] 1H NMR
3
(200 MHz, CD
3
3
NO
2
, 25 °C): δ = 8.81 (d, JHH = 6.0 Hz, 1 H, py-
[7] L. Cattalini, M. L. Tobe, Inorg. Chem. 1966, 5, 1145–1150.
), 8.30–8.08 [8] J. March, Advanced Organic Chemistry. Reactions, Mechanisms,
4
H
6
), 8.75 (td, JHH = 7.9, JHH = 1.6 Hz, 1 H, py-H
+H
) ppm.
4
and Structure, 3rd ed., Wiley, New York, 1985, p. 790.
9] L. Cattalini, G. Chessa, G. Marangoni, B. Pitteri, M. L. Tobe,
J. Chem. Soc., Dalton Trans. 1985, 2091–2094.
Received: August 25, 2005
(
(
m, 2 H, py-H
s, 6 H, OMe
3
5
), 6.91 (br, 1 H, NH), 6.02 (s, 1 H, CH), 3.64
[
2
Kinetics: The reactions were followed spectrophotometrically by
measuring the changing absorbance at a suitable wavelength as a
Published Online: December 28, 2005
Eur. J. Inorg. Chem. 2006, 765–771
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
771