Displacement of neutral nitrogen donors by chloride in AuCl3(am) (am = pyridines and amines): Kinetics and DFT calculations show the effects of basicity and π-acceptor ability

Article abstract of DOI:10.1002/ejic.200700457

The kinetics of the process AuCl₃(am) + Cl^- → AuCl₄^- + am (am = sp² N-donor isosteric pyridines with different π systems and sp³ amines; they cover a wide range of basicity) have been studied in methanol at 25°C. The reactions obey the usual two-term rate law observed in substitutions on square-planar complexes. The second-order rate constants, k₂, are very sensitive to the nature of the leaving group, and plots of log k₂ against the pK_a of the conjugate acids are linear, with the same slope, -0.68, for both normal pyridines and pyridines with a more extended π system, such as 4-cyanopyridine, isonicotinic acid, methyl isonicotinate and 4-acetylpyridine. The reactivity of the considered N-donors is different and follows the order sp³ N-donors > normal pyridines > π-extended pyridines. This result, with the support of ground-state DFT calculations on the AuCl₃(am) derivatives, is explained on the basis of an Au-N bond enforcement due to an increased π-back-donation contribution. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700457

FULL PAPER
DOI: 10.1002/ejic.200700457
Displacement of Neutral Nitrogen Donors by Chloride in AuCl3(am)
(am = Pyridines and Amines): Kinetics and DFT Calculations Show the
Effects of Basicity and π-Acceptor Ability
Bruno Pitteri*[a] and Marco Bortoluzzi[a]
Keywords: Gold / Kinetics / Nitrogen donors / Reaction mechanisms / DFT
–
The kinetics of the process AuCl3(am) + Cl– Ǟ AuCl4 + am
(am = sp2 N-donor isosteric pyridines with different π systems
and sp3 amines; they cover a wide range of basicity) have
been studied in methanol at 25 °C. The reactions obey the
usual two-term rate law observed in substitutions on square-
planar complexes. The second-order rate constants, k2, are
very sensitive to the nature of the leaving group, and plots
of logk2 against the pKa of the conjugate acids are linear,
with the same slope, –0.68, for both “normal” pyridines and
pyridines with a more extended π system, such as 4-cyano-
pyridine, isonicotinic acid, methyl isonicotinate and 4-acetyl-
pyridine. The reactivity of the considered N-donors is dif-
ferent and follows the order sp3 N-donors Ͼ “normal” pyr-
idines Ͼ “π-extended” pyridines. This result, with the sup-
port of ground-state DFT calculations on the AuCl3(am) de-
rivatives, is explained on the basis of an Au–N bond enforce-
ment due to an increased π-back-donation contribution.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Recently, we studied the kinetics of the reverse reaction
of process (1) using five-membered N-donor heterocycles
(thiazoles, oxazoles, imidazoles and their derivatives) as
leaving groups[5] and the results were compared with those
in a paper[3] where am = pyridines. It was observed that the
reactivity depends not only upon the ligand basicity but
also upon the nature of the ligand in the order: pyridines
Ͼ five-membered heterocycles and this behaviour was at-
tributed to different π interactions in the ground state be-
tween the metal centre and the N-donor ligands.
Introduction
The reactivity of gold(III) derivatives with nitrogen-do-
nor bases has been widely studied, mainly involving pyr-
idine and substituted pyridines (am) as entering nucleo-
philes or leaving groups.[1–3] Substitution reactions of the
type (1) take place with an associative mechanism, whose
rate constants, in the forward as well as in the reverse reac-
tion, obey the general relationship kobs = k1 + k2[nucleo-
phile], which is usual for nucleophilic substitutions at
planar four-coordinate d8 metal complexes.[4]
In this work we tried to improve the knowledge about
the role of π interactions between N-donors and AuIII in
the kinetics of displacement by chloride of these ligands
from AuCl3(am). As for PtII substrates,[6] on considering
the second-order rate constants k2, the displaced nitrogen
donors can be divided into three groups: (i) “normal” pyr-
idines, (ii) pyridines having a more extended π system and
(iii) sp3 nitrogen-donor bases; and the reactivity towards
ligand substitution is in the order: sp3 nitrogen-donor bases
Ͼ “normal” pyridines Ͼ pyridines with a more extended π
system. These differences, which were mainly attributed to
a ground-state stabilization due to the π-back-donation
from the filled orbitals of the metal to the antibonding or-
bitals of the pyridine ligands, have been correlated in this
paper with the calculated Au–N bond lengths of the Au-
Cl3(am) complexes, obtained by performing ground-state
DFT B3PW91 calculations. By plotting the rAu–N values of
the AuCl3(am) complexes versus the corresponding pKa val-
ues, linear relationships comparable to those derived from
the kinetic measurements were obtained. Furthermore, a
–
AuCl4 + am i AuCl3(am) + Cl–
(1)
The first-order rate constant, k1, and the second-order rate
constant, k2, refer to the nucleophilic attack on the sub-
strate by, respectively, the solvent and the nucleophile. The
second-order rate constants, k2, are markedly influenced by
the nature of am, indicating that the discriminating ability
of gold(III) complexes is good. A linear free-energy rela-
tionship, logk2 = αpKa + constant, occurs between the rate
constant and the basicity of am, measured as the pKa of
its conjugated acid Ham+. Moreover, steric retardation is
observed for pyridines containing one or two ortho groups.
[a] Dipartimento di Chimica, Università “Cà Foscari” di Venezia,
Calle Larga S. Marta 2137, 30123 Venezia, Italy
E-mail: pitteri@unive.it
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Eur. J. Inorg. Chem. 2007, 4456–4461

Displacement of Neutral N Donors in Planar d8 Metal Complexes
systematic shortening of the Au–N bond length was ob- the rate of reaction is independent of the concentration of
served for pyridines having an extended π system.
acid over the range 0.02–0.2 moldm–3. The acid simply
serves to protonate the released N-donor ligand, thus pre-
venting the reverse reaction and forcing the reaction to
completion. Moreover, the presence of acid helps to avoid
Result and Discussion
When a methanolic solution of the neutral AuCl3(am) a slow reduction of the gold(III) derivatives of the most
substrate is allowed to react with an appropriate excess of basic amines. The rate constants have been determined in
chloride, repetitive scanning of the spectrum in the region the usual way from the change in absorbance as a function
240–360 nm shows that the reaction evolves in a first-order of time at convenient wavelength. All the reactions have
fashion leading to the substitution of the neutral base with been studied in the presence of a sufficient excess of chlo-
a chloride ion. The presence of well-maintained isosbestic ride over the substrate to provide pseudo-first-order condi-
points, whose positions depend only upon the spectrum of tions and the observed rate constants, kobs, collected in
the starting compound, and the comparison of the final Table 1, obey the general relationship kobs = k1 + k2[Cl–],
spectrum with that of original samples of the substituted which is usual for nucleophilic substitutions on gold(III)
products at the same concentration, clearly indicate a single complexes.
reaction stage. The addition of LiClO4 indicates that pri-
The k1 terms, referring to the pathway in which the rate-
mary salt effects are absent and secondary effects are negli- determining step is the nucleophilic attack of the solvent,
gible, thus no further attempt has been made to maintain followed by the rapid entry of chloride into the solvento
constant ionic strength. The reactions have been studied in complex,[7] are quite small and thus this reaction pathway
the presence of 0.1 moldm–3 CH3SO3H. Preliminary experi- makes a small contribution to the whole process. The k2
ments have shown that, at constant chloride concentration, terms are the second-order rate constants for the direct at-
Table 1. First-order rate constant, kobs, for the reaction AuCl3(am) + Cl– Ǟ AuCl4– + am in methanol at 25 °C (0.1 moldm–3 CH3SO3H).
am
[Cl–] /moldm–3
103kobs/s–1
am
[Cl–] /moldm–3
103kobs/s–1
4-Methoxypyridine
0.01
0.02
0.03
0.04
0.69Ϯ0.01
1.27Ϯ0.01
1.78Ϯ0.01
2.40Ϯ0.02
2.95Ϯ0.01
9.6Ϯ0.1
12.2Ϯ0.1
14.3Ϯ0.1
18.0Ϯ0.2
20.2Ϯ0.1
4.0Ϯ0.2
pyridine
0.01
0.02
0.03
0.04
4.17Ϯ0.02
7.54Ϯ0.01
10.7Ϯ0.04
13.9Ϯ0.03
18.1Ϯ0.02
48.0Ϯ0.1
62.7Ϯ0.7
71.0Ϯ0.1
83.6Ϯ0.3
93.0Ϯ0.2
0.76Ϯ0.01
1.58Ϯ0.02
2.21Ϯ0.03
3.11Ϯ0.07
3.84Ϯ0.03
0.15Ϯ0.01
0.29Ϯ0.01
0.41Ϯ0.02
0.53Ϯ0.01
0.64Ϯ0.01
2.51Ϯ0.06
5.01Ϯ0.06
7.71Ϯ0.07
9.93Ϯ0.06
12.61Ϯ0.03
5.29Ϯ0.04
6.44Ϯ0.03
7.78Ϯ0.04
8.98Ϯ0.02
10.02Ϯ0.03
1.2Ϯ0.1
0.05
0.05
Methyl isonicotinate
4-Acetylpyridine
4-Chloropyridine
4-Methylpyridine
4-Aminopyridine
0.001
0.002
0.003
0.004
0.005
0.001
0.002
0.003
0.004
0.005
0.001
0.002
0.003
0.004
0.005
0.01
0.02
0.03
0.04
0.05
0.02
0.04
0.07
0.10
4-cyanopyridine
0.0005
0.0010
0.0015
0.0020
0.0025
3,5-dimethylpyridine 0.01
5.6Ϯ0.1
7.1Ϯ0.1
9.0Ϯ0.2
0.02
0.03
0.04
0.05
0.1
0.2
0.3
0.4
0.5
0.001
0.002
0.003
0.004
0.005
0.0005
0.0010
0.0015
0.0020
0.0025
0.1
10.3Ϯ0.1
7.58Ϯ0.01
10.16Ϯ0.02
12.63Ϯ0.01
15.80Ϯ0.02
18.38Ϯ0.01
1.38Ϯ0.01
2.48Ϯ0.02
3.44Ϯ0.03
4.48Ϯ0.03
5.51Ϯ0.01
0.016Ϯ0.001
0.03Ϯ0.02
0.05Ϯ0.01
0.07Ϯ0.01
0.10Ϯ0.01
0.05Ϯ0.01
0.10Ϯ0.02
0.16Ϯ0.01
0.21Ϯ0.01
0.26Ϯ0.02
0.04Ϯ0.01
0.14Ϯ0.03
0.28Ϯ0.01
0.38Ϯ0.06
0.51Ϯ0.09
piperidine
isonicotinic acid
4-bromopyridine
cyclohexylamine
0.15
4-(Dimethylamino)pyridine 0.1
0.2
0.3
0.4
0.5
0.2
0.3
0.4
0.5
1.9Ϯ0.2
3.0Ϯ0.2
4.0Ϯ0.3
4.9Ϯ0.1
Morpholine
0.002
0.01
0.02
0.03
0.04
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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B. Pitteri, M. Bortoluzzi
FULL PAPER
tack of Cl– at the substrate and are summarized in Table 2.
The first group includes the complexes with pyridine: 4-
For a better comparison of the different reactivity of the bromopyridine, 4-chloropyridine, 4-methylpyridine, 4-meth-
groups of amines, we have again examined some of the reac- oxypyridine, 4-aminopyridine, 4-(dimethylamino)pyridine
tions in ref.[3], in the same experimental conditions, increas- and 3,5-dimethylpyridine, while the second group includes
ing both the number of pyridines and the pKa range.
the derivatives of 4-cyanopyridine: isonicotinic acid, methyl
isonicotinate and 4-acetylpyridine. The lability of pyridines
Table 2. Second-order rate constants, k2,[a] and pKa of Ham+ for is considerably lower than that of the third group of deriva-
the reaction AuCl3(am) + Cl– Ǟ AuCl4– + am in methanol at 25 °C
tives, that is, the complexes of the sp3-ligands piperidine,
cyclohexylamine and morpholine. Unfortunately, the dif-
ferent steric hindrance around the N atom of these sp3 N-
donors avoids a direct relationship between logk2 and the
corresponding pKa values.
(0.1 moldm–3, CH3SO3H).
am
k2/dm3 mols–1
pKa of
Ham+[b]
4-Methoxypyridine
Methyl isonicotinate
4-Acetylpyridine
4-Chloropyridine
4-Methylpyridine
4-Aminopyridine
4-Bromopyridine
Pyridine
4-Cyanopyridine
3,5-Dimethylpyridine
Piperidine
Isonicotinic acid
4-(Dimethylamino)pyridine
Morpholine
0.0565Ϯ0.0008
2.7Ϯ0.1
1.60Ϯ0.04
6.47
3.26
3.52
3.84
6.02
9.12
3.84
5.25
1.90
6.34
11.1
3.26
Both “normal” (᭿ in Figure 1) and “π extended” (᭺ in
Figure 1) pyridines obey the linear relationship logk2
=
2.73Ϯ0.06
αpKa + constant, with α = –0.68. For the entry of pyridines
0.103Ϯ0.001
0.00065Ϯ0.00001
2.40Ϯ0.06
0.34Ϯ0.01
22Ϯ1
0.078Ϯ0.002
0.00122Ϯ0.00003
2.51Ϯ0.04
–
on AuCl4 in the same experimental conditions, a similar
free-energy relationship was found,[1] with α = 0.16. The
parameter α is a measure of the ability of a substrate to
discriminate among isosteric nitrogen donors and can be
assumed to be a relative measure of its electrophilicity. The
asynchronous substitution, characteristic of planar-tetraco-
ordinate gold(III) complexes, offers the opportunity to de-
scribe separately the energetic contributions of bond forma-
tion and bond rupture to the free energy of activation.[7]
The relatively low discrimination among nitrogen donors in
the entry reaction (α = 0.16), together with the relatively
high dependence of reactivity upon the basicity of the leav-
ing nitrogen bases in the reverse processes (α = –0.68, this
work), strongly suggests a transition state for all these sub-
stitutions in which the Au–N bond is only partially formed
in the rate-determining transition state, whereas the Au–Cl
bond is practically formed to the same extent in the ground
and transition states.
0.000537Ϯ0.000007 9.37
0.0122Ϯ0.0003
0.0095Ϯ0.0003
8.33
10.66
Cyclohexylamine
[a] Determined by linear regression of kobs values vs. chloride con-
centration. [b] pKa values were taken from ref.[8]
The reacting species can be divided into three groups, as
depicted in the plot of logk2 versus pKa of Ham+ (Fig-
ure 1):
i: “normal” pyridines;
ii: pyridines with a more extended π system;
iii: sp3 nitrogen-donor bases.
The lower lability of pyridine ligands, as compared with
the sp3 N-donors, allows the assumption that the Au–N
bond in the case of pyridine derivatives may be reinforced
by π bonding, that is, by the π-back-donation from the
filled orbitals of the metal to the antibonding orbitals of
the ligand, leading to a relative stabilization of the corre-
sponding ground state. Moreover, π interactions of this type
can also be invoked to explain the small but significant dif-
ference between the reactivity of “normal” and “π-ex-
tended” pyridines, which is ∆logk2 ≈ 0.4 (∆ in Figure 1),
corresponding to about 2.3 kJmol–1. To support this hy-
pothesis, ground-state DFT calculations on the AuCl3(am)
(am = pyridines) have been performed and the rAu–N bond
lengths, collected in Table 3, have been measured.
By plotting the rAu–N values of the AuCl3(am) complexes
versus the corresponding pKa values, linear relationships
comparable to those derived from the kinetic measurements
have been obtained, as depicted in Figure 2. “Normal” pyr-
idines (᭿ in Figure 2) respond to the equation rAu–N
(–0.0027Ϯ0.0001)pKa + (2.1119Ϯ0.0005), while the equa-
tion for “π-extended” pyridines (᭺ in Figure 2) is rAu–N
(–0.0027Ϯ0.0001)pKa + (2.1077Ϯ0.0004). The increase of
electron density on the N atom, quite well represented by
the pKa values of the free ligands, appears to lead to an
enforcement of the Au–N σ bond. The effect of growing
=
Figure 1. Plots of logk2 for the reaction AuCl3(am) + Cl– Ǟ AuCl4
–
=
+ am against pKa of Ham+: (᭿) pyridine, 4-chloropyridine, 4-meth-
oxypyridine, 4-methylpyridine, 4-bromopyridine, 3,5-dimethyl-
pyridine, 4-aminopyridine, 4-(dimethylamino)pyridine; (᭺) 4-cyano-
pyridine, 4-acetylpyridine, methyl isonicotinate, isonicotinic acid;
(᭹) morpholine, cyclohexylamine, piperidine.
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Eur. J. Inorg. Chem. 2007, 4456–4461

Displacement of Neutral N Donors in Planar d8 Metal Complexes
Table 3. Calculated rAu–N bond lengths of AuCl3(am) derivatives
(am = pyridines) and pKa of Ham+.[a]
Conclusion
In summary, the results of kinetic measurements on the
am
rAu–N /Å
pKa of Ham+
–
process AuCl3(am) + Cl– Ǟ AuCl4 + am (am = sp2 N-
donor isosteric pyridines with different π systems and sp3
amines) reported in this paper highlight the role of π-back-
donation from the gold centre to the N-donor ligands in
the stabilization of the AuCl3(am) complexes and in the re-
activity of these species towards substitution reactions. Be-
sides the great difference between sp2 and sp3 N-donors, the
small but significant difference observed between “normal”
and “π-extended” pyridines, which has found a nice corre-
lation with the calculated rAu–N bond lengths, should be
noted.
4-Methoxypyridine
Methyl isonicotinate
4-Acetylpyridine
4-Chloropyridine
4-Methylpyridine
4-Aminopyridine
4-Bromopyridine
Pyridine
4-Cyanopyridine
3,5-Dimethylpyridine
Isonicotinic acid
4-(Dimethylamino)pyridine
2.09397
2.09899
2.09800
2.10118
2.09490
2.08734
2.10175
2.09816
2.10250
2.09429
2.09871
2.08601
6.47
3.26
3.52
3.84
6.02
9.12
3.84
5.25
1.90
6.34
3.26
9.37
[a] More details on the geometries and energies of the optimized
systems are available in the electronic supporting information.
Experimental Section
Materials: The compound KAuCl4·2H2O was prepared from pure
gold foil (99.99%). Pure reagent-grade LiCl, LiClO4 and CH3SO3H
(Aldrich) were used without further purification. Pyridines and
amines were recrystallized or distilled before use when necessary.
Anhydrous MeOH was obtained by distillation over Mg wires, but
traces of water did not appear to have any appreciable effect upon
the reactions. Acetone and dimethylformamide were pure reagent-
grade products (Aldrich and BDH).
pKa on the metal–pyridine bond shortening was, however,
already partly observed in a paper regarding PtII-pyridines
derivatives, where it was confirmed on the basis of NMR
measurements.[10]
Instruments: Electronic spectra and kinetic measurements were ob-
tained on a Perkin–Elmer Lambda 15 spectrophotometer. 1H
NMR spectra were taken on Bruker Avance 300 and/or Bruker AC
200 spectrometers at 298 K and are referred to internal tetramethyl-
silane. The conductivity of 1ϫ10–3 moldm–3 solutions of the com-
plexes in dimethylformamide at 25 °C was measured with a Radi-
ometer CDM 83 instrument. Elemental analyses were performed
by the Microanalytical Laboratory of the Faculty of Pharmaceuti-
cal Sciences of the University of Padua.
Preparation of the Complexes: Trichloro(pyridine)gold(III) was first
characterized by Renz,[9] trichloro(3,5-dimethylpyridine)gold(III)
and trichloro(4-methylpyridine)gold(III) by Cattalini and Tobe[3]
and trichloro(4-cyanopyridine)gold(III) by Cattalini et al.,[1] but
the other derivatives of the type AuCl3(am) are described here for
the first time. All the complexes were prepared by the following
method: KAuCl4·2H2O (0.415 g, 1.0 mmol) was dissolved in water
(20 mL) and a stoichiometric amount of the nitrogen-donor base,
dissolved in a small volume of water (2–3 mL), was added dropwise
whilst stirring. The yellow precipitate, formed almost immediately,
was filtered off, washed three times with water (10 mL) and dried
in vacuo. For the most basic ligands, piperidine, cyclohexylamine,
4-aminopyridine, 4-(dimethylamino)pyridine and morpholine, it
was necessary to keep them at 0 °C and in the dark to prevent a
possible reduction of gold(III). All the complexes were nonconduc-
tive in organic solvents. Yields were in any case nearly quantitative
(Ͼ95%). Analytical and 1H NMR spectra are collected in Tables 4
and 5, respectively.
Kinetics: The reactions were initiated by adding a 0.015 moldm–3
acetone solution (10–20 µL) of the substrate complex, AuCl3(am),
to a methanolic solution of chloride ion, previously brought to the
reaction temperature (25 °C) in a thermostatted cell in the spectro-
photometer. The concentration of the entering group was always
large enough to provide pseudo-first-order conditions. After pre-
liminary repetitive scan experiments in the range 240–360 nm to
search for isosbestic points and spectral changes, the kinetics were
studied by measuring the changing absorbance at suitable wave-
Figure 2. Plots of rAu–N bond lengths against pKa of Ham+: (᭿)
pyridine, 4-chloropyridine, 4-methoxypyridine, 4-methylpyridine, 4-
bromopyridine, 3,5-dimethylpyridine, 4-aminopyridine, 4-(dimeth-
ylamino)pyridine; (᭺) 4-cyanopyridine, 4-acetylpyridine, methyl
isonicotinate, isonicotinic acid.
The small difference between the reactivity of the “nor-
mal” and “p-extended” groups of pyridines appears to be
correlated to a small difference between the corresponding
ground-state rAu–N bond lengths, which is about 0.004 Å.
As all the considered pyridines are isosteric, it seems rea-
sonable to conclude that this difference could be ascribed
to an enforcement of the Au–N bond because of a greater
π-back-donation contribution for the second group of li-
gands, as supposed from the results of the kinetic measure-
ments.
Eur. J. Inorg. Chem. 2007, 4456–4461
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4459

B. Pitteri, M. Bortoluzzi
FULL PAPER
Table 4. Analytical data for the complexes (calculated values in parentheses).
Complex
Formula
C
H
N
Cl
(pyridine)AuCl3
C5H5AuCl3N
C6H7AuCl3N
C7H9AuCl3N
C5H4AuCl4N
C5H4AuBrCl3N
C6H7AuCl3NO
C5H6AuCl3N2
C7H10AuCl3N2
C6H4AuCl3N2
C7H7AuCl3NO
C7H7AuCl3NO2
C6H5AuCl3NO2
C5H11AuCl3N
C4H9AuCl3NO
C6H13AuCl3N
15.9 (15.7)
18.3 (18.2)
20.4 (20.5)
14.2 (14.4)
12.9 (13.0)
17.5 (17.5)
15.1 (15.1)
19.6 (19.8)
17.6 (17.7)
19.7 (19.8)
19.3 (19.1)
16.7 (16.9)
15.3 (15.5)
12.3 (12.3)
18.0 (17.9)
1.67 (1.32)
1.65 (1.78)
2.18 (2.21)
1.27 (0.97)
1.04 (0.87)
1.62 (1.71)
1.39 (1.52)
2.21 (2.37)
1.23 (0.99)
1.49 (1.66)
1.61 (1.60)
0.99 (1.18)
2.76 (2.85)
2.54 (2.32)
3.27 (3.26)
3.71 (3.66)
3.49 (3.53)
3.33 (3.41)
3.32 (3.36)
3.12 (3.04)
3.31 (3.40)
7.13 (7.05)
6.47 (6.58)
6.72 (6.88)
3.34 (3.30)
3.24 (3.18)
3.37 (3.28)
3.71 (3.61)
3.32 (3.59)
3.46 (3.48)
27.5 (27.8)
26.7 (26.8)
25.7 (25.9)
34.1 (34.0)
23.0 (23.1)
25.6 (25.8)
26.9 (26.8)
24.8 (25.0)
17.6 (17.7)
25.0 (25.1)
24.3 (24.1)
25.0 (24.9)
27.2 (27.4)
27.1 (27.2)
26.3 (26.4)
(4-methylpyridine)AuCl3
(3,5-dimethylpyridine)AuCl3
(4-chloropyridine)AuCl3
(4-bromopyridine)AuCl3
(4-methoxypyridine)AuCl3
(4-aminopyridine)AuCl3
[4-(dimethylamino)pyridine]AuCl3
(4-cyanopyridine)AuCl3
(4-acetylpyridine)AuCl3
(methyl isonicotinate)AuCl3
(isonicotinic acid)AuCl3
(piperidine)AuCl3
(morpholine)AuCl3
(cyclohexylamine)AuCl3
1
Table 5. H NMR spectroscopic data for the complexes.
Complex
1H NMR, (CD3)2CO, 298 K
[a]
3
4
3
4
(pyridine)AuCl3
8.89 (dd, 2 H, JHH = 6.5, JHH = 1.5 Hz, Hα); 8.33 (tt, 1 H, JHH = 7.8, JHH = 1.5 Hz, Hγ);
3
3
7.89 (dd, 2 H, JHH = 6.5, J = 7.8 Hz, Hβ)
3
3
(4-methylpyridine)AuCl3
(3,5-dimethylpyridine)AuCl3
(4-chloropyridine)AuCl3
(4-bromopyridine)AuCl3
(4-methoxypyridine)AuCl3
(4-aminopyridine)AuCl3
8.90 (d, 2 H, JHH = 6.2 Hz, Hα); 7.80 (d, 2 H, JHH = 6.2 Hz, Hβ); 2.62 (s, 3 H, CH3)
8.74 (s, 2 H, Hα); 8.05 (s, 1 H, Hγ); 2.52 (s, 6 H, CH3)
9.14 (d, 2 H, JHH = 7.0 Hz, Hα); 8.13 (d, 2 H, JHH = 7.0 Hz, Hβ)
9.04 (d, 2 H, JHH = 6.8 Hz, Ha); 8.27 (d, 2 H, JHH = 6.8 Hz, Hβ)
8.85 (d, 2 H, JHH = 7.3 Hz, Hα); 7.46 (d, 2 H, JHH = 7.3 Hz, Hβ); 4.16 (s, 3 H, CH3)
3
3
3
3
3
3
3
3
8.25 (d, 2 H, JHH = 7.1 Hz, Hα); 6.91 (d, 2 H, JHH = 7.1 Hz, Hb); 7.06 (br. s, 2 H, NH2)
3
3
[4-(dimethylamino)pyridine]AuCl3 8.28 (d, 2 H, JHH = 7.8 Hz, Hα); 6.91 (d, 2 H, JHH = 7.8 Hz, Hβ); 3.26 (s, 6 H, NMe2)
(4-cyanopyridine)AuCl3
(4-acetylpyridine)AuCl3
3
3
4
4
9.47, 8.46 (AAЈBBЈ spin system, 4 H, JAB = JAЈBЈ = 6.5, JAAЈ = JBBЈ = 2.0 Hz, aromatic protons)
9.33, 8.37 (AAЈBBЈ spin system, 4 H, JAB = JAЈBЈ = 7.0, JAAЈ = JBBЈ = 2.0 Hz, aromatic protons);
2.80 (s, 3 H, CH3)
9.36, 8.43 (AAЈBBЈ spin system, 4 H, JAB = JAЈBЈ = 6.9, JAAЈ = JBBЈ = 1.5 Hz, aromatic protons)
9.36, 8.42 (AAЈBBЈ spin system, 4 H, JAB = JAЈBЈ = 7.3, JAAЈ = JBBЈ = 1.0 Hz, aromatic protons);
4.05 (s, 3 H, CH3)
3
3
4
4
3
3
4
4
(isonicotinic acid)AuCl3
(methyl isonicotinate)AuCl3
3
3
4
4
(piperidine)AuCl3
(morpholine)AuCl3
(cyclohexylamine)AuCl3
3.63 (br. s, 1 H, NH); 3.56 (m, 4 H, CH2-N); 2.00–1.50 (m, 6 H, CH2)
4.03 (m, 4 H, CH2-O); 3.56 (m, 4 H, CH2-N); 3.80 (br. m, 1 H, NH)
3.92 (s, very br., 2 H, NH2); 2.48 (br. m, 1 H, CH-N); 1.62 (m, 4 H, CH2); 1.39 (m, 4 H, CH2);
1.19 (m, 2 H, CH2)
[a] CD3NO2, 298 K.
–
lengths as a function of time. Pseudo-first-order rate constants
[Cl]y– and [pyridine](y x)+, leading to uncorrected results; for
(kobs, s–1) were obtained either from the gradient of plots of example the apparent d10 configuration of the gold centres. For this
log(Dt – Dϱ) versus time or from a nonlinear least-squares fit of
experimental data to Dt = Dϱ + (D0 – Dϱ)exp(–kobst) with D0, Dϱ
and kobs as the parameters to be optimized (D0 is absorbance after
mixing the reactants, Dϱ is absorbance at completion of reaction).
reason, no discussion based on the NBO analysis was helpful for
this work.
Supporting Information (see also the footnote on the first page of
this article): Selected geometry data and energies for the prepared
gold(III) complexes.
Computational Details: The computational geometry optimizations
of the AuCl3(am) complexes were performed in vacuo using the
hybrid DFT B3PW91 method[11] without symmetry constraints, in
combination with the polarized, single-ζ core, triple-ζ valence 6-
311G** basis set on the elements of the first rows of the periodic
table and the ECP-based SDD basis set on Au.[12] The “restricted”
formalism was applied in all calculations, together with tight con-
vergence criteria. All the resultant stationary points were charac-
terized as true minima (i.e., no imaginary frequencies). All calcula-
tions were carried out at CINECA (Centro Italiano di Supercal-
colo, Bologna, Italy) using IBM p5-575 computers with 64-bit IBM
Power5 processors, which operated at 1.9 GHz frequency. The soft-
ware used was the Gaussian ‘03.[13] The Natural Bond Orbitals
analysis[14] was performed during our studies, using the NBO ver-
sion 3.1 implemented in Gaussian 03. Unfortunately, the software
Acknowledgments
We sincerely thank the Ca’ Foscari University of Venice for finan-
cial support (Ateneo fund 2005).
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Received: April 26, 2007
Published Online: July 31, 2007
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Please note: Minor changes have been made to this manuscript since its publi-
cation in the European Journal of Inorganic Chemistry Early View. The Editor.
Eur. J. Inorg. Chem. 2007, 4456–4461
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