Decarboxylation and simultaneous reduction of silver(I) β-ketocarboxylates with three types of coordinations

Article abstract of DOI:10.1039/c5dt00773a

A series of silver(I) β-ketocarboxylates were prepared by reaction of β-ketocarboxylic acids with silver nitrate in the presence of diethanolamine. The silver(I) β-ketocarboxylates decomposed over a narrow temperature range to form metallic silver, CO₂, and the corresponding ketones. In addition, products derived from radical intermediates were detected by mass spectroscopic analysis for some silver(I) β-ketocarboxylates. Infrared and solid state ¹³C-NMR spectra of silver(I) β-ketocarboxylates suggested the presence of two types of structures involving a carbonyl group in addition to the dimeric eight-membered ring structure as in the structure of silver(I) stearate. The silver(I) β-ketocarboxylate model compound used was HCOCH₂COOAg and its structures were determined using density functional theory (DFT) and atoms-in-molecules (AIM) methods. Three types of coordinations around the Ag ion differing significantly in Ag-O bond strengths were found. Based on the calculated structures and experimental results, the relationships between the structures and decomposition temperatures are discussed in terms of the thermal decomposition process.

Full text of DOI:10.1039/c5dt00773a

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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C5DT00773A
Cite this: DOI: 10.1039/x0xx00000x
Decarboxylation and simultaneous reduction of
silver(I) β-ketocarboxylates with three types of
coordination
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
a
a
b
b
www.rsc.org/
Mariko Hatamura* , Shunro Yamaguchi* , Shinꢀya Takane , Yu Chen , and Katuaki
Suganuma
a
A series of silver(I) βꢀketocarboxylates were prepared by reaction of βꢀketocarboxylic acids with silver
nitrate in the presence of diethanolamine. The silver(I) βꢀketocarboxylates decomposed over a narrow
temperature range to form metallic silver, CO2, and the corresponding ketones. In addition, products
derived from radical intermediates were detected by mass spectroscopic analysis for some silver(I) βꢀ
ketocarboxylates. Infrared and solid state 13CꢀNMR spectra of silver(I) βꢀketocarboxylates suggested the
presence of two types of structures involving a carbonyl group in addition to the dimeric eightꢀmembered
ring structure as in the structure of silver(I) stearate. The silver(I) βꢀketocarboxylate model compound
used was HCOCH2COOAg and its structures were determined using density functional theory (DFT) and
atomsꢀinꢀmolecules (AIM) methods. Three types of coordination around the Ag ion differing
significantly in AgꢀO bond strengths were found. Based on the calculated structures and experimental
results, the relationship between the structures and decomposition temperatures are discussed in terms of
the thermal decomposition process.
products. The temperature and mechanism of thermal reductive
decomposition depends on the structure of the carboxylate moiety.
Introduction
Much attention has been paid to silver(I) carboxylates as
photographic and printed electronic materials because of their
unique structures and their reduction to metallic silver by thermal
decomposition.1ꢀ5 However, these silver(I) carboxylates need
temperature more than 200 °C for the thermal reduction. Silverꢀ
containing materials forming metallic silver at lower temperature
have been demanded in the industrial fields.
For example, silver(I) benzoate decomposed at 276 °C via a
complicated radical pathway to yield metallic silver and products
such as polyphenyls derived from the radical intermediate.14
Similarly, vacuum thermal decomposition of a series of silver
dicarboxylates, AgO2C(CH2)2nCO2Ag, n = 0–3, provided metallic
silver, CO2, and polymeric compounds derived from diradicals.15 In
contrast, silver(I) acetate thermally decomposed to give metallic
silver and CH3COOH via formation of silver oxide and anhydride
from the corresponding carboxylic acids.2,16,17 Silver
perfluorocarboxylate, CF3(CF2)nCOOAg, thermally decomposes in
a similar manner.2 Under decomposition conditions at temperatures
greater than 200 °C, even with Ag2O generation, Ag2O could be
thermally reduced to metallic silver.18 Therefore, uncertainty
remains about whether silver ion is reduced by electron transfer
during decarboxylation, or by thermal reduction of the Ag2O
formed during decomposition at temperatures above 200 °C.
In 1949, Vand reported a preliminary structure of silver salts of
fatty acids.6 The complete structure of silver perfluorobutyrate,
composed of an eightꢀmembered central ring containing two silver
atoms and two carboxyl groups, had been elucidated.7 Due to the
difficulty of crystallizing these salts and their light sensitivity, only
a few studies describing the structure of silver carboxylates have
been reported. Acland found that the silver salt of glycylglycine
adopted an eightꢀmembered ring structure about the silver
carboxylate moiety.8 A series of crystal structures of silver
carboxylate complexes containing an eightꢀmembered ring were
reported by Mak.9 Tolochko examined the structure of silver
stearate by extended Xꢀray absorption fine structure (EXAFS) and
reported that the AgꢀAg distance in the eightꢀmembered ring was
2.90 Å, similar to the bond distance in metallic silver.10 Other
structures of silver carboxylates that adopt an eightꢀmembered ring
have been reported,3,4,11ꢀ13 leading to the wide acceptance that silver
carboxylates tend to form a dimeric (RCO2)2Ag2 eightꢀmembered
ring structure.
The structural basis for the electron transfer process in silver
acetate was studied by Olson and colleagues.19 They concluded that
the Ag(I)ꢀAg(I) bond in silver acetate may be an important
precursor to metallic silver formation, as singleꢀelectron reduction
of the Ag2(carboxylate)2 species could result in the formation of
(Ag2)+• known from the chemistry of photographic processing to be
a precursor of metallic silver clusters.20
This body of fundamental research has promoted the
development of advanced silver nanoparticles. Silver carboxylates
have been used to generate silver nanoparticles, which require an
organic dispersant, such as nonꢀvolatile long alkyl fatty acids.
Silver(I) carboxylates tend to decarboxylate and thermally
decompose at ~300 °C to give metallic silver, CO2, and organic
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However, the nonꢀvolatile dispersant can contaminate the metallic
silver nanoparticles produced, which is undesirable.
For compound 5a, the precipitate was soluble in water and thus was
carefully washed with the minimal amount of chilled water. The
silver βꢀketocarboxylates obtained were stored in brown bottles at ꢀ
The decarboxylation of carboxylic acids modified with certain
20 °C. The compounds obtained were characterized by elemental
analysis (C, H, N) (JM10; JꢀScience), Fourier transform infrared
spectroscopy (FTꢀIR) (FT/IR4100; Jasco), and solid state 13CꢀNMR
(Avance III 600WB; Bruker). Thermal analyses were conducted
using differential scanning calorimatry (DSC) (DSCꢀ50; Shimadzu)
and thermogravimetry (TG) (DTGꢀ50H; Shimadzu). Direct
analysis in real timeꢀmass spectrometry (DARTꢀMS) was
conducted using an AccuTOF DART (Jeol).
functional groups at the α or
formation of carbanions or radical intermediates.
acids, in particular, readily decarboxylate under ambient conditions
via a sixꢀmembered transition state.
β
position smoothly proceeds via the
β
ꢀKetocarboxylic
O
H
+
CO2
O
2) Thermolysis
O
O
Each silver(I) βꢀketocarboxylate compound (50 mg) was placed
Scheme 1 Mechanism of decarboxylation of βꢀketocarboxylic
in a 30ꢀmL round bottom flask. For compounds 1aꢀ3a, the flask
was heated in an oil bath at 150 °C; the silver salt foamed and
started to decompose within 10 seconds. Immediately after foaming
stopped, the products were extracted with acetone. For compound
4a, the flask was heated at 180 °C, and for compounds 5aꢀ8a, the
flask was heated at 130 °C. The other procedures were the same for
all products. Acetone extracts were analyzed by gas
acids.
The goal of this study was to produce metallic silver from silver
carboxylates at temperatures below 200 °C without the production
of undesirable organic compounds. Despite the promise of silver(I)
ꢀketocarboxylates for this purpose, their preparation has not been
reported, since ꢀketocarboxylic acids prepared from the
corresponding ester are unstable under hydrolysis conditions.21
β
β
chromatographyꢀmass spectrometry (GCꢀMS) (GCMSꢀQP2010;
Shimadzu) and the inorganic residues 1bꢀ8b were examined by Xꢀ
ray diffraction (XRD) (RINTꢀRAPID IIꢀS; Rigaku).
Preliminary results for the preparation of several silver(I)
βꢀ
ketocarboxylates have been published previously in a patent.22 The
present report describes the detailed preparation of these silver(I)
3) Computational Methods
β
ꢀketocarboxylates, decarboxylation in the solid state, and their
The DFT calculations were conducted at the B3LYP23 level of
theory. A 6ꢀ31+G(d) basis set was used for C, H, and O, and an
LANL2DZ24 effective core potential (ECP) was used for Ag. The
optimized geometries were verified by vibrational normal mode
analysis. All calculations were performed using the Gaussian 03
and Gaussian 09 software packages. 25 The AIM calculations were
performed using the AIMAll program26 to evaluate the nature of
the bonds in optimized structures.
structures based on infrared spectroscopy (IR) and 13C solid state
nuclear magnetic resonance (13CꢀNMR) spectra. The calculated
structures of a simple model compound and the relationship
between the calculated structures and thermal decomposition also
are discussed.
Experimental Section
1) Preparation
Results and Discussion
1) Preparation
All commercially available reagents and solvents were used
without further purification. Starting esters were purchased from
the following chemical companies: methylꢀpropionylacetate (1) and
ethylꢀ2ꢀbenzylacetoacetate (8) from Aldrich, ethylꢀbutyrylacetate
(2) and ethylꢀ2ꢀnꢀbutylacetoacetate (7) from Alfa Aesar, methylꢀ
isoꢀbutyrylacetate (3) and ethylꢀ2ꢀethylacetoacetate (6) from Wako,
methylꢀpivaloylacetate (4) from Nippon Fine Chemical, and ethylꢀ
2ꢀmethylacetoacetate (5) from Tokyo Kasei.
Silver(I)
2. Their preparation by treating sodium
silver nitrate in a oneꢀpot reaction resulted in the formation of
β
ꢀketocarboxylates were prepared as shown in Scheme
β
ꢀketocarboxylates with
inseparable byꢀproducts such as silver oxide.
βꢀKetocarboxylic
acids are unstable under hydrolysis conditions21 and thus were
carefully prepared according to a previously described procedure
on hydrolysis of acetoacetic ester.27 The free acid products were
immediately treated with silver nitrate in the presence of
General Reaction Procedure for Silver(I)
(1a-8a) Sodium hydroxide (0.12 mol) was added to water (30 mL).
Ethyl or methyl ꢀketocarboxylate (0.1 mol) was added dropꢀwise
β-Ketocarboxylates
β
diethanolamine to give the corresponding silver(I)
βꢀ
to the resulting solution at approximately 60 °C. The reaction
mixture was stirred for 1–4 hours at room temperature, followed by
TLC (silica, nꢀhexane:ethyl acetate = 3:1). When the starting ester
was detected, the reaction mixture was washed with ether to
remove it. After evaporation to remove the alcohol produced by
hydrolysis, the solution was cooled in an ice bath, followed by the
addition of ether (50 mL) and then sulfuric acid (0.06 mol) in water
(10 mL). After separation of the organic layer and extraction with
ether, the combined organic layers were cooled in an ice bath. A
solution of diethanolamine (0.105 mol) in water (10 mL) was
added, the ether layer was removed by decantation, then an
aqueous solution (10 mL) of silver(I) nitrate (0.095 mol) was added
dropꢀwise. The resulting white precipitate was filtered, washed
with chilled water and then with 2ꢀpropanol, and dried in vacuo.
ketocarboxylate. When R1 and R2 were alkyl groups (R3 = H),
hydrolysis of the esters proceeded smoothly to provide highꢀpurity
silver
βꢀketocarboxylates, listed in Table 1. The crystal shapes,
yields, and results of CHN analyses are described in the notes.
Although the preparation of silver(I) acetoacetates also proceeded
smoothly to give colorless precipitates, the resulting precipitates
appeared to be unstable and turned grey. When R1 and R2 were
electronꢀwithdrawing groups [(R1, R2) = (C6H5, H), (ClCH2, H),
(CH3, C6H5), and (CH3, F)] and R3 was H, silver(I)
ketocarboxylates were formed but not isolated due to byꢀproduct
formation resulting from nucleophilic attack at the ꢀcarbonyl
group. When (R1, R2, R3) = (CF3, H, H) and (CH3, C2H5, C2H5),
βꢀ
β
silver(I)
β
ꢀketocarboxylates were not formed. The prepared
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organic solvents or water; only compound 5a showed solubility in
silver(I)
from their IR and NMR spectra,notes which are listed in Table 1.
The resulting silver(I) ꢀketocarboxylates were not soluble in
βꢀketocarboxylates adopted the keto form as determined
1ꢀmethylꢀ2ꢀpyrrolidone
(NMP)
and
in
water.
β
Scheme 2 Preparation and thermolysis of silver(I)
βꢀketocarboxylates.
Table 1 Decomposition temperatures and FTꢀIR and solid state 13CꢀNMR data for silver(I)
β
ꢀketocarboxylates.
Compound
R1
R2
D.T.( °C)
FTꢀIRc) (cmꢀ1)
Solid state13C NMR (ppm)
C=O
COOꢀ
174.7
C=O
COOꢀ
1505
1505
1a
2a
C2H5ꢀ
Hꢀ
Hꢀ
130 ꢀ 138 a)
1715
1712
203.8
203.7
139 ꢀ 148 a
）
CH3(CH2)2ꢀ
175.1
3a
4a
5a
6a
7a
(CH3)2CHꢀ
(CH3)3Cꢀ
CH3ꢀ
Hꢀ
Hꢀ
139 ꢀ 150 a)
167 ꢀ 176 a)
101 ꢀ 109 a)
102 ꢀ 121 a)
110 ꢀ 119 a)
1712
1711
1715
1716
1716
1507
1532
1526
1540
1542
207.2
209.1
203.2
200.9
201.3
175.2
175.2
177.4
175.7
176.0
1703
1697
1698
1699
210.6
210.7
210.2
209.6
174.1
176
CH3ꢀ
CH3ꢀ
C2H5ꢀ
172.8
173.6
CH3ꢀ
CH3(CH2)3ꢀ
8a
CH3ꢀ
C6H5CH2ꢀ
109 ꢀ 114 a)
1694
1555
210.6
171.6
Silver acetate
210 ꢀ 330 b)
170 ꢀ 290 b)
95 ꢀ 240 b)
1509
1518
1512
Silver lauryrate
Siver neodecanoate
a) measured by DSC (rate of temperature increase 10°C/min in air)
b) measured by TG (rate of temperature increase 10°C/min in air)
c) attenuated total reflection (ATR)
decomposed over 100–250 °C. Therefore, results indicate that the
decomposition of silver(I) neodecanoate is a complicated and
gradual process that occurs over a wide temperature range.
2) Thermolysis
2-1) DSC, TG
The thermolysis of representative silver(I)
βꢀketocarboxylates
and silver(I) neodecanoate (used as a reference) was examined
using DSC and TG; results are shown in Figure 1 and the
decompsition temperatures determined by DSC are listed in Table
1. Silver(I) neodecanoate melted at approximately 100 °C and
gradually decomposed to give silver(0); the weight decrease
occured over a wide temperature range. Thermal analyses of
In contrast, all silver(I) βꢀketocarboxylates (1aꢀ8a) decomposed
without melting over a narrow temperature range with a rapid
decrease in weight. Following decomposition (with the exception
of compound 4a), TG showed a gradual decrease until 280 °C. The
gradual decrease in weight observed appeared to correspond to the
ketone with a high boiling point or the dimeric ketone which is
discussed in Mass spectroscopic analysis section. Exothermic peaks
silver(I)
longꢀchainꢀfatty
acylates
reported
previously12
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following decomposition. The observed weight loss at 97–104 °C
corresponded to the weight ratio of CO2 in compound 8a: 14.7%,
indicating that metallic silver and the corresponding ketone 8c (b.p.
235 °C) or the dimeric product remained at the decomposition
temperature.
were evident near 300 °C in silver(I) βꢀketocarboxylates (1aꢀ7a).
Because generated ketones and plausible dimeric products are
volatile and do not show exothermic peaks, these peaks could be
attributed to aggregated Ag particles, as suggested by
Logvinenko.28
The TG pattern of compound 4a showed 43.2% of the weight
ratio at 176 °C remained (exothermic peak in DSC Figure 1-i, -ii,
and notes), which corresponded to the theoretical weight ratio of
Ag(0): 43.0%. No gradual decrease in weight was observed
following decomposition. Little residue remained following
decomposition, likely because ketone 4c that formed had a low
boiling point (106 °C) and volatilized upon Ag(0) formation during
In compound 5a, 42.8% of the weight was lost at 109 °C (the
decomposition temperature), corresponding to 83.0% of the weight
of the organic moiety. Subsequently, the weight gradually
decreased to 50.6% of the original mass of compound 5a (98.1% of
the weight of the organic moiety) at 350 °C. As the theoretical
weight ratio of Ag(0) is 48.4%, the plausible dimeric or trimeric
ketone must have remained.
decomposition.
Silver(I)
β
ꢀketocarboxylates
4a
clearly
TG analysis of compounds 1aꢀ3a, 6a, and 7a showed patterns
similar to that of compound 5a. The theoretical weight ratio of Ag,
and the residual weight ratios at 280 °C, 350 °C, and just after
decomposition are described in the notes.
demonstrated simultaneous decomposition and reduction of Ag+.
The TG of compound 8a differed from the other silver(I)
ketocarboxylates: only a 14.6% weight loss was measured during
βꢀ
decomposition at 97–104 °C, and weight loss continued gradually
i)
40
35
30
25
20
15
10
35
30
25
20
50
40
30
20
10
0
20
15
10
5
15
10
5
0
0
50
100
150
200
250
300
350
5
0
ꢀ5
0
50
100
150
200
250
300
350
0
0
50
100
150
200
250
300
350
0
50
100
100
150
200
200
250
300
300
350
200
300
100
100
200
300
100
200
300
ꢀ5
ꢀ5
ꢀ10
ꢀ10
ii)
6
5
4
3
2
1
0
9
8
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
0
50
100
100
150
200
250
300
350
0
50
100
150
200
250
300
350
0
50
100
150
200
250
300
350
0
50
100
150
200
250
300
350
0
200
300
0
100
200
300
0
100
200
300
100
0
200
300
Silver neodecanate
8a
4a
5a
Figure 1 i) DSC of silver(I)
and silver(I) neodecanoate. Rate of temperature increase: 10 °C/min in air
β
ꢀketocarboxylates (4a, 5a, 8a) and silver(I) neodecanoate. ii) TG of silver(I)
β
ꢀketocarboxylates (4a, 5a, 8a)
150 °C, and a weak peak corresponding to the dimer (MW 226).
For compounds 1aꢀ6a, the corresponding ketones 1cꢀ6c were not
detected as their boiling point is lower than the temperature used for
thermolysis.
2-2) Mass spectroscopic analysis of products formed during
thermolysis
The products formed during thermolysis were analyzed by mass
spectroscopy; detailed results are presented in the notes.
Powdery silver(I)
βꢀketocarboxylates were analyzed by DARTꢀ
The products formed by thermolysis from compounds 1aꢀ8a at
their decomposition temperatures (see Experimental Section) were
extracted with acetone and examined by GCꢀMS. Thermolysis of
compound 8a at 130 °C provided a peak corresponding to 8c: 4ꢀ
phenylꢀ2ꢀbutanone with a boiling point of 235 °C. The gradual
decrease between 200 and 250 °C in the TG curve of compound 8a
in Figure 1-ii was attributed to thermal behavior of 8c. Similarly,
MS using a 300 °C ion source. Compounds 1aꢀ8a gave peaks
corresponding to the molecular weights (M1c-8c + 1) of their
corresponding ketones, 1c-8c. In addition, intense peaks with a
molecular weight of (2M1c-8c ꢀ 1), due to dimerized radical
intermediates, and peaks corresponding to the trimer (3M1cꢀ8c ꢀ 3)
were observed. Although GCꢀMS data of peaks corresponding to
dimers do not prove their formation during thermolysis, the results
suggest the formation of ketoꢀradicals during thermolysis.
thermolysis of compound 7a at 130 °C provided
a peak
corresponding to 7c, 2ꢀheptanone (MW 114), with a boiling point of
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In contrast, the IR spectra of compounds 4aꢀ7a contained two
bands corresponding to β
ꢀcarbonyl groups, at 1711–1716 cmꢀ¹ and at
1697–1703 cmꢀ¹, and bands for their carboxylate groups at 1526–
1542 cmꢀ¹. The solid state 13CꢀNMR spectra of compounds 4aꢀ7a
showed two signal groups for the
βꢀcarbonyl and carboxylate groups.
The IR spectrum of compound 8a showed a band corresponding to
the β
ꢀcarbonyl group at 1694 cmꢀ¹ and of the carboxylate group at
1555 cmꢀ¹. In contrast with the solid state 13CꢀNMR spectra of
compounds 1aꢀ3a, there is a signal at lower magnetic field (210.6
ppm) for the βꢀcarbonyl group and a signal for the carboxylate group
at higher magnetic field (171.6 ppm). This signal pattern was simpler
than those of compounds 5aꢀ7a. The signals near 210.6 ppm seen in
Figures 2-8a were attributed to side bands.
Scheme
ketocarboxylates.
3 Mechanism of thermolysis of silver(I) βꢀ
Absorption bands and 13CꢀNMR signals arising from the carbonyl
group at the βꢀposition and from the carboxylate group suggested the
Formation of the ketone and dimer may explain the gradual
decrease in weight following decomposition and the exothermic
peaks around 300 °C in the TG data. Volatilization of the resulting
ketones and dimer provided endothermic peaks. Thus, in each case,
the gradual decrease in weight following decarboxylation
corresponded to volatilization of the dimer and resulting ketone with
boiling points at temperatures greater than 200 °C, whereas the
exothermic peak near 300 °C can be attributed to aggregation of the
Ag particles.
presence of a number of coordination modes involving the carbonyl
group.
3-2) Proposed Structures
The carbonyl group in silver(I)
βꢀketocarboxylates may
coordinate the silver ion. This suggests structures other than the
eightꢀmembered ring coordination of silver salts of fatty acids, as
shown in Figure 3. Type I is the same type as silver salts of fatty
acids. In Type II, one of the two carbonyl groups is involved in the
coordination. Type III coordinates the silver ion between two COOꢀ
groups and two C=O groups. Spectroscopic features of these
different types of coordination can be predicted empirically as
2-3) Chemical composition analysis of the inorganic residue
Silver(I)
βꢀketocarboxylates 1aꢀ8a thermally decomposed at
temperatures below 190 °C. At these temperatures, if the mechanism
of decomposition occurred via acid anhydride and Ag2O, similar to
those in silver carboxylates,2,16,17 Ag2O should be formed. The Ag2O
was not thermally reduced to afford metallic silver at temperatures
below 190 °C.18 The inorganic residues were examined using XRD
after conducting thermolysis at the decomposition temperatures. No
silver oxides or silver salt (Ag2O, AgO, or AgCO3) were observed
after thermolysis: only Ag(0) was found.
follows. For the Type III structure, the IR absorption bands of the βꢀ
carbonyl group would shift to lower wavenumbers and those of the
carboxylate groups would shift to higher wavenumbers compared
with the corresponding absorption bands of Type I compounds. In
Type II coordination, two IR absorption bands are seen. Solid state
13CꢀNMR of Type III compounds show the
βꢀcarbonyl group signal
shifted to lower magnetic field and the carboxylate group signal
shifted to higher magnetic field compared with the corresponding
signals in Type I compounds. In Type II compounds, two signals
for each C=O group and each COOꢀ group can be seen.
Decomposition by decarboxylation was complete at temperatures
below 190 °C. The resulting products included metallic silver and
organic compounds derived from the corresponding ketoradical.
These results clearly suggest that silver oxide was not involved in
these reactions and that metallic silver, ketoradicals, and CO2 were
formed.
3-3) Classification of silver(I) β-ketocarboxylates
Compounds 1aꢀ8a were classified into Types IꢀIII based on their
spectra. Compounds 1a, 2a, and 3a showed spectra similar to
silver(I) aliphatic carboxylates and were therefore classified into the
Type I structure. The spectroscopic patterns obtained for compound
8a corresponded with the expected pattern for Type III compounds:
3) Structure
3-1) FT-IR and solid state 13C-NMR data for silver(I)
ketocarboxylates
β-
the absorption bands of the
βꢀcarbonyl group shifted to lower
Characteristic spectroscopic data and decomposition temperatures
wavenumbers and those of the carboxylate groups shifted to higher
wavenumbers than the corresponding absorption bands of
of the isolated silver(I) βꢀketocarboxylates 1aꢀ8a are listed in Table
1. Representative solid state 13CꢀNMR spectra are shown in Figure 2.
The IR spectra of compounds 1a, 2a, and 3a showed a band
compounds 1a, 2a, and 3a. The solid state 13CꢀNMR signal of the
βꢀ
corresponding to β
ꢀcarbonyl groups at 1712–1715 cmꢀ¹ and strong
carbonyl group shifted to lower magnetic field and the signal from
the carboxylate group shifted to higher magnetic field compared
with Type I compounds. When compound 8a adopted a Type III
structure with the twelveꢀmembered ring, steric repulsion by the
benzyl groups may decrease compared to a Type I structure.
bands corresponding to carboxylate groups at 1505–1507 cmꢀ¹. Solid
state 13CꢀNMR spectra of these compounds showed single signals
for the βꢀcarbonyl and carboxylate groups.
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ARTICLE
DOI: 10.1039/C5DT00773A
3a
4a
5a
Figure 2 Typical solid state 13CꢀNMR spectra.
8a
Figure 3 Plausible structures of silver(I) βꢀketocarboxylates.
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results are in good agreement with the experimental and
Steric repulsion by the substituents at the α positions of
compounds 5a, 6a, and 7a should be less than that in compound
8a. The spectra suggest that compounds 5a, 6a, and 7a may adopt
a Type II or mixture of Type I and III structures. A sharp
exothermic peak was present at low temperatures. If these
compounds were a mixture of Type I and III structures, the
exothermic peak would be broad. But from this point, they are
more likely to adopt Type II.
theoretical results of related complexes reported previously.19
Type 2 structures have one side that forms the same fiveꢀ
membered ring as in Type 1 compounds, whereas the other side
forms threeꢀoxygen coordination. Carboxylate oxygen O(4)
coordinated to Ag(1), and carboxylate oxygen O(5) and carbonyl
oxygen O(6) coordinated to Ag(2), as shown in Figure 4(c). Two
rotational conformers existed around the αꢀcarbon and
carboxylate carbon bond that differed only slightly in energy (<
0.1 kcal/mol). The AgꢀAg bond length was calculated to be 2.988
Å, indicating metallophilic interaction29.
The spectrum of compound 4a showed patterns similar to those
of compounds 5aꢀ7a, with some differences. In the solid state 13Cꢀ
NMR of compounds 5aꢀ7a, two broad signals for C=O and COOꢀ
groups were present. In the IR spectra of compounds 5aꢀ7a, the
differences between the wavenumbers of the two absorption bands
for carbonyl groups C=O were in the range of 17–18 cmꢀ1. In
contrast, in the solid state 13CꢀNMR of compound 4a, two sharp
signals and a small chemical shift difference between the carbonyl
signals were observed. In the IR spectrum of compound 4a, two
absorption bands for a carbonyl group C=O were observed at
wavenumbers above 1700 cmꢀ1, with a difference between the
wavenumbers of 8 cmꢀ1, which differed from the shifts: 17–18 cmꢀ
In Type 3, both carboxylates coordinate with two silver atoms
through threeꢀoxygen coordination. Two distinct structures were
evident with significantly different Ag(1)ꢀAg(2) distances (3.297
Å and 3.669 Å). Type 3a has C2 symmetry, whereas Type 3b has
Ci symmetry. AIM results indicated that Type 3a had a weak BCP,
whereas Type 3b had no BCP between Ag(1) and Ag(2).
Of the types of structures obtained, Type 1 is the most stable,
followed by structure Type 2, whose calculated energy is about 5
kcal/mol greater than that of Type 1a. The calculated energy of
Type 3b is about 9 kcal/mol greater than that of Type 1a; that of
Type 3a, the least stable structure is about 10 kcal/mol greater
than that of Type 1a. These relative energies indicate that all four
structure types can exist stably or isomerize easily due to the small
energy differences between them.
1
as seen in Type II: compounds 5aꢀ7a. These spectroscopic
differences might be attributed to stereoisomers of compound 4a
with a Type I rather than Type II structure in the crystals.
Based on the spectroscopic data, compounds 1aꢀ4a, compounds
5aꢀ7a, and compound 8a were categorized as Type I, Type II,
and Type III, respectively. Definitive assignment required
determining the structures of the proposed types by Xꢀray analysis
and EXAFS, but this was not possible due to practical difficulties.
The calculated vibrational frequencies of the COOꢀ and C=O
groups are listed in Table 2. Type 2 compounds showed two
peaks arising from C=O stretching of the
contrast, for Type 3, the peak of the ꢀcarbonyl group shifted to
lower wavenumber compared to the corresponding ꢀcarbonyl
βꢀcarbonyl groups. In
β
β
3-4) DFT calculations and AIM analysis of silver(I)
ketocarboxylates
β-
group of Type 1. From these results, it is possible that the Type 1
structure calculated here corresponds to Type I of the plausible
structure shown in Figure 3, Type 2 corresponds to Type II, and
Type 3 corresponds to Type III.
Possible structures of silver(I)
βꢀketocarboxylates were
investigated using structural calculations of a model compound
lacking substituents. The compound, HCOCH2COOAg [R1 = R2 =
R3 = H], was chosen to investigate a wide range of skeletal
structures due to flexibility (no rotational hindrance caused by
substituents).
These computational results allowed the reꢀexamination of the
structures of the compounds synthesized in this study. The above
discussion regarding two isomers, such as Type 1a and Type 1b,
in the eightꢀmembered ring structure is pertinent to compound 4a.
DFT calculations clearly showed the presence of isomers with
very similar energy levels; furthermore, the difference observed
between the wavenumbers of the C=O groups, 8 cmꢀ1, was
consistent with the calculated difference. Thus, the observed
spectroscopic differences can be attributed to stereoisomers of
compound 4a with a Type I structure. As predicated by the
calculated isomers, each carbon showed two signals, as seen in
Figures 2-4a.
The structures optimized for symmetry and potential energy and
the results of AIM calculations are illustrated in Figure 4. The
bond critical points (BCP) are shown as green dots and the ring
critical points (RCP) are shown as red dots. The bond paths with
an electron density value (ρ) at a BCP below 0.025 a.u. are drawn
as dashed lines instead of solid lines. Optimized energies of the
five resulting categories of structures, their relative energies, and
the calculated IR vibrational frequencies of the intense COOꢀ and
C=O stretching modes are listed in Table 2. Selected bond lengths
and AIM properties at each BCP are listed in Table 3 (supporting
information).
The structure of compound 8a was assigned to Type III based
on the spectroscopic patterns. DFT calculations showed two
possible coordination types: Types 3a and 3b, depicted in Figure
4. The AgꢀO=C bond length in Types 2 and 3a was 2.603 Å and
2.641 Å, respectively; however, a shorter bond length of 2.410 Å
was calculated for Type 3b. Thus, the bond strength of AgꢀO=C
in Type 3b is stronger than in Types 2 and 3a. In accordance with
the strong AgꢀO=C bond strength, the FTꢀIR spectrum of
compound 8a shows C=O shifted to lower wavenumber. In
addition, the observed COO absorption bands are similar to the
calculated bands. Therefore, the structure of compound 8a is more
likely Type 3b rather than Type 3a.
The resulting structures were divided into three main categories.
Type 1 is an eightꢀmembered ring structure, which is the same
structure as the wellꢀknown silver carboxylate dimers.8 Several
conformers of this type differing in rotational bond angle between
the αꢀcarbon and carboxylate carbon were found in this study.
Figures 4(a) and (b) show examples of Ci symmetry and C2h
symmetry structures, respectively. Only small differences in the
energy and ring structural parameters exist in Type I conformers;
in addition, the Ag(1)ꢀAg(2) distances were all approximately
2.83 Å, and BCPs were found between Ag(1) and Ag(2). Two
RCPs were found at each AgOCOAg fiveꢀmembered ring. These
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O(6)
O(6)
Ag(1)
O(4)
O(4)
Ag(1)
C(4)
C(4)
O(1)
O(1)
C(3)
O(5)
C(3)
O(5)
C(2)
C(1)
C(2)
C(1)
Ag(2)
Ag(2)
O(2)
O(2)
O(3)
O(3)
(a) Type 1a (Ci symmetry)
(b) Type 1b (C2h)
Ag(1)
O(4)
O(5)
C(3)
O(6)
O(1)
C(1)
C(2)
C(4)
Ag(2)
O(2)
O(3)
(c) Type 2
O(6)
Ag(1)
Ag(1)
O(1)
O(1)
O(6)
C(3)
C(1)
C(1)
O(2)
C(2)
O(4)
O(2)
C(2)
C(4)
O(4)
C(4)
C(3)
O(5)
Ag(2)
Ag(2)
O(5)
O(3)
O(3)
(d) Type 3a (C2)
(e) Type 3b (Ci)
Figure 4 Optimized structures of silver βꢀketocarboxylates. The green dots show BCPs and small red dots show RCPs. The electron
density threshold value for defining “weak” BCPs is 0.025 a.u.
Table 2 Calculated energies and vibrational frequencies of COOꢀ and CO stretching modes with strong IR intensities.
ꢁE a)
(kcal/mol)
ꢁEZPE
ν
(COOꢀ)
(cmꢀ1)
ν
(CO)
a,b)
E (hartree)
ꢀ975.256926
ꢀ975.254536
ꢀ975.248007
ꢀ975.239913
ꢀ975.242649
(kcal/mol)
(cmꢀ1)
1810
1819
Type 1a
Type 1b
Type 2
(0)
1.5
(0)
1.5
1621
1631
5.6
5.4
1628 1778, 1808
Type 3a
Type 3b
10.7
9.0
10.3
8.6
1625
1645
1787
1778
a) Relative energies to Type 1a.
b) Including vibrational zero point energy correction.
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electron transfer, successive bondꢀcleavages yield products
derived from the keto radical, CO2, and metallic silver.
4) Relationship between thermal decomposition and
The short AgꢀAg bond suggested by Tolochko10 is likely to be
advantageous for the formation of (Ag2)+• by electron transfer.
The AgꢀAg distance in the dimeric structure in Types 1a and 1b,
Type 2, and Type 3a obtained by AIM calculations were 2.831–
3.29 Å, similar to that of simple silver aliphatic carboxylates.
However, Type 3b compounds showed a nonꢀbonding AgꢀAg
bond distance (3.669 Å). Compound 8a belongs to Type 3b and
yet decomposed abruptly at 109–114 °C. Consequently, the effect
of the bond distance between AgꢀAg needs to be reconciled with
the calculations for structural changes of compound 8a and/or the
model compound with the observed formation of silver radical
cation.
structure
The factors affecting the formation of metallic silver upon
thermal decomposition of silver(I) carboxylates have been
discussed in terms of bonding strength to the Ag+, the structure of
the intermediate,19 and the length of the AgꢀAg bond.10 Similarly,
the thermal decomposition of silver(I)
βꢀketocarboxylates 1aꢀ8a
were examined. As discussed above, the thermal decomposition
temperature of compounds 1aꢀ4a (130–176 °C), compounds 5aꢀ7a
(101–121 °C), and compound 8a (109–114 °C), coupled with
spectroscopic data and the results of AIM calculations, suggest
these compounds be categorized as Type 1a or 1b, Type 2, and
Type 3a or Type 3b, respectively.
Initially, silver(I)
βꢀketocarboxylates were expected to
Burleva examined the relationship between decomposition
decompose inherently via the wellꢀknown sixꢀmembered ring
transition state, similar to that shown in Scheme 1, or silver(I)
carboxylates were expected to decompose via Olson`s
intermediate. The calculated structures suggested that the
decomposition of dimeric Type 2 and Type 3a, b compounds may
proceed via a deformed sixꢀmembered ring transition state
involving the C=O group, and that in Type 1a and b compounds,
the structures may be deformed to a Type 2 or Type 3ꢀlike
structure. Thus, the substituent of R2 decides the Types of the
structures and in Type 1: compound 1a-4a, increasing the
bulkiness of R1, transformation to the expected intermediate: Type
2ꢀ or Type 3aꢀlike intermediate requires higher temperature. The
temperature
and
structure
of
silver
dicarboxylates
AgO2C(CH2)nCO2Ag and determined that decomposition
temperature depended on the strength of the AgꢀO bond.15 The
bond strength between Ag and the oxygen atom of the carboxylate
group in Type 3a [ρ value: 0.0477 for Ag(1)ꢀO(4)], Type 3b [ρ
value: 0.0217 for Ag(1)ꢀO(2)], and Type 2 [ρ value: 0.0467 for
Ag(2)ꢀO(5)] are weaker than those of Types 1a and 1b (ρ value:
ca. 0.068). The decrease in ρ value of Type 3a, Type 3b, and
Type 2 compounds was attributed to the coordination of Ag+ by
the C=O group. According to the relationship suggested by
Burleva, a decrease in the bond strength between the Ag ions and
carboxylates decreases the decomposition temperature.
Compounds categorized as Types 2, 3a, and 3b, which have lower
ρ values, are expected to decompose at lower temperatures than
Type 1a and 1b compounds. In fact, compounds 1aꢀ4a,
categorized as Type 1a or 1b, decomposed at 130–176 °C, and
compounds 5aꢀ8a, categorized as Types 2, 3a, and 3b,
decomposed at 101–121 °C. Thus, the thermal decomposition
characteristic sharp thermal decompositions of silver(I)
βꢀ
ketocarboxylate at lower temperatures can be attributed to the
coordination of the C=O group to the Ag+ ion. Viewed from
another perspective, the decomposition temperature can be
controlled by intramolecular or intermolecular coordination to the
Ag+ ion.30 To understand the decomposition processes in detail,
the structures of silver(I)
substituents are currently being determined.
β
ꢀketocarboxylates with R1 and R2
temperature of silver(I)
βꢀketocarboxylates depended on the
strength of the bond between Ag and the oxygen atom of the
carboxylate group; a decrease in strength due to C=O group
participation caused a decrease in the decomposition temperature.
Compounds 1aꢀ4a were categorized as Types 1a and 1b; no
coordination occurred between the Ag+ and the C=O groups, but
C=O group participation might occur during the thermal
decomposition. As shown in Table 1, the decomposition
temperature increased with the bulkiness of R1. The small
differences in potential energy (< 10 kcal/mol) indicate that each
type could be converted to another type. Thus, the calculations
conducted on the model compound suggested that the C=O group
may participate in coordination to Ag+ during thermal
decomposition. It is likely that increased bulkiness may disrupt
coordination by the C=O group. In the Type I compounds 1aꢀ4a,
an increase in the bulkiness of R1 increased the decomposition
temperature. Furthermore, simple silver aliphatic carboxylates
lacking a C=O group decomposed at temperatures above 200 °C,
indicating that the coordination of C=O to Ag ion lowers the
decomposition temperature.
Conclusions
A series of silver(I)
reaction of ꢀketocarboxylic acids from the corresponding esters
with silver nitrate in the presence of diethanolamine. The obtained
silver(I) ꢀketocarboxylates thermally decomposed at lower
βꢀketocarboxylates were prepared by
β
β
temperature than silver(I) aliphatic carboxylates over a narrow
temperature range and afforded metallic silver, volatile ketones,
and CO2. Product analysis by DARTꢀMS and GCꢀMS indicated
the involvement of radical intermediates during decomposition.
Differences in the decomposition temperatures of silver(I)
βꢀ
ketocarboxylates, together with IR and solid 13CꢀNMR spectral
data, allowed their classification into three structural categories:
Type I, as well as silver(I) stearate with an eightꢀmembered ring
structure, Type II, and Type III. The
βꢀcarbonyl group is
proposed to be involved in coordinating Ag+ in Type II and Type
III compounds. The proposed structures were verified by
Silver(I) βꢀketocarboxylates 1aꢀ8a were characterized by both
their lower decomposition temperature and abrupt decomposition
observed by thermal analysis. The abrupt decomposition indicates
the decomposition mechanism. Products derived from ketoꢀ
radicals were observed by mass analysis. Observation of ketoꢀ
radicalꢀderived products coupled with the results of DFT and AIM
calculating the structures of
a simple model compound,
HCOCH2COOAg, using DFT and AIM methods. Three types of
calculated structures, Types 1a-b, Type 2, and Type 3b,
corresponded to the proposed structures Type I, Type II, and
Type III, respectively. The three categories differed significantly
in AgꢀO bond strength. The decreased bond strength of the AgꢀO
bond caused by C=O group participation lowered the
decomposition temperature. Coordination of C=O to Ag ion
resulted in smooth electron transfer during decomposition to yield
(Ag2)+•. The apparent similarity of the Type 2 and Type 3a
structures to the intermediate made compounds 5aꢀ8a favorable
J. Name., 2012, 00, 1-3 | 9
calculations allowed the proposal of
a
mechanism of
decomposition. The coordination of C=O to Ag ion may lead to
formation of (Ag2) +• by electron transfer, as well as by the single
electronꢀreduction proposed by Olson.19 In particular, the structure
of Type 2 and Type 3a compounds are similar to that of the
intermediate, making them conducive to electron transfer. After
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Journal Name
for electron transfer. After electron transfer, successive bondꢀ
cleavages yielded products derived from the keto radical, as well
as CO2 and metallic silver.
(600MHz, Solid); δ 14.6(CH3CH), 28.7, 29.0(CH3CO), 56.4,
57.7(COCHCO2), 176.0, 177.4(CO2ꢀ), 202.9, 203.2, 210.7,
211.1(CO); Elemental analysis (measured values; C 26.64, H 2.88,
calculated values; C 26.93, H 3.16); TG(109°C 57.2%, 280°C
51.1%, 350°C 50.6% theoretical silver content 48.4%); DARTꢀ
MS (ESI+, 12V) m/z 73 (M’ + 1)61, 143 (2M’ ꢀ 1)100, 195(2M +
51)42, (M’ = M ꢀ (CO2Ag) + 1: CH3COC2H5);
Silver(I) βꢀketocarboxylates thermally decompose into volatile
ketones and metallic silver without the need for a chemical
reducing reagent. Consequently, silver(I) βꢀketocarboxylates are
potent compounds for the formation of metallic silver by thermal
decomposition and are a useful source of metallic silver for
industrial materials.
Silver(I) β-ethylacetoacetate (Silver(I) 2-ethyl 3-oxobutyrate)
(6a): colorless microcrystals; 71% yield; 13C NMR (600MHz,
Acknowledgements
Solid);
30.9(CH3CO), 60.6, 63.0, 63.9, 68.3(COCHCO2), 172.8, 173.5,
175.7(CO2ꢀ), 200.9, 204.2, 210.2(CO); Elemental
δ
13.0(CH3CH2), 22.2, 25.0(CH3CH2CH), 29.0,
We wish to express our gratitude to Ms. Asano, Dr. Shu, Mr.
Haneoka, and Mr. Matsuzaki of the Comprehensive Analysis
Center at ISIR, Osaka University. This work was partially
supported by NEDO JAPERA project in 2014.
analysis(measured values; C 30.11, H 3.68, calculated values; C
30.40, H 3.84); TG(121°C 53.2%, 280°C 47.3%, 350°C 46.4%
theoretical silver content 45.6%) DARTꢀMS (ESI+, 12V) m/z 87
(M’ + 1)38, 171 (2M’ ꢀ 1)100, 255 (3M’ ꢀ 3)7, (M’ = M ꢀ
(CO2Ag) + 1: CH3CO(CH2)2CH3);
Notes
Silver(I) n-propionylacetate (Silver(I) 3-oxopentanoate) (1a):
colorless microcrystals; 80% yield; 13C NMR (600MHz, Solid); δ
6.8(CH3), 34.9(CH3CH2CO), 51.2(COCH2CO2), 174.7(CO2ꢀ),
203.8(CO); Elemental analysis (measured values; C 26.87, H 2.95,
Silver(I)
β-n-butylacetoacetate
(Silver(I)
2-n-butyl-3-
oxobutyrate) (7a): colorless microcrystals; 84% yield; 13C NMR
(600MHz, Solid); δ 13.8(CH3CH2), 22.2, 23.5, 25.5, 26.5(CH2),
29.9, 31.5(CH3CO), 60.4, 62.2, 62.5, 65.0(COCHCO2), 172.5,
173.6, 176.0, 177.6(CO2ꢀ), 201.3, 202.9, 209.6, 21.06, 211.0(CO);
Elemental analysis (measured values; C 36.14, H 4.86, calculated
values; C 36.25, H 4.94); TG(119°C 53.2%, 280°C 41.7%, 350°C
40.0% theoretical silver content 40.8%)DARTꢀMS (ESI+, 12V)
m/z 115 (M’ + 1)59, 227 (2M’ ꢀ 1)100, 339 (3M’ ꢀ 3)9, (M’ = M ꢀ
(CO2Ag) + 1: CH3CO(CH2)4CH3);
calculated values; C 26.93, H 3.16); TG(138°C 55.7%, 280°C
51.5%, 350°C 50.5%, theoretical silver content 48.4%); DARTꢀ
MS (ESI+, 12V) m/z 73 (M’ + 1) 9, 143 (2M’ ꢀ 1)100, 213 (3M’ ꢀ
3)44, (M’ = M ꢀ (CO2Ag) + 1: C2H5COCH3);
Silver(I) n-butyrylacetate (Silver(I) 3-oxohexanoate) (2a):
colorless microcrystals; 76% yield; 13C NMR (600MHz, Solid); δ
12.9(CH3), 16.5(CH3CH2), 44.2(CH2CH2CO), 52.2(COCH2CO2),
175.1(CO2ꢀ), 203.7(CO); Elemental analysis (measured values; C
30.39, H 3.69, calculated values; C 30.40, H 3.84); TG(148°C
55.7%, 280°C 48.9%, 350°C 47.8%, theoretical silver content
45.6%); DARTꢀMS (ESI+, 12V) m/z 87 (M ’+ 1) 6, 171 (2M’ ꢀ
7c: GCꢀMS M’ ꢀ 71(43) = 100, M’ ꢀ 56(58) = 58.14, M’ ꢀ 43(71)
= 15.12, M’ ꢀ 29(85) = 2.32, M’ ꢀ 15(99) = 1.74, M’(114) = 2.91
1)100, 255 (3M’
CH3(CH2)2COCH3);
ꢀ
3)31, (M’
=
M
ꢀ
(CO2Ag)
+
1:
Silver(I)
oxobutyrate) (8a): colorless microcrystals; 83% yield; 13C NMR
(600MHz, Solid); 33.8(CH3), 35.3, 36.6(CH2),
64.3(COCHCO2), 126.0, 129.0, 139.9, 141.3(C6H5), 171.6(CO2ꢀ),
210.6(CO); Elemental analysis (measured values; C 44.02, H 3.52,
calculated values; C 44.18, H 3.71); TG(97°C 99.4%, 104°C
84.8% 114°C 81.0% 280°C 46.5%, 350°C 43.6% theoretical silver
content 36.1%); DARTꢀMS (ESI+, 12V) m/z 149 (M’ + 1)36, 295
(2M’ꢀ 1)100, 441 (3M’ꢀ 3)12, (M’ = M ꢀ (CO2Ag) + 1:
CH3CO(CH2)2C6H5);
β-benzylacetoacetate
(Silver(I)
2-n-benzyl-3-
δ
Silver(I)
iso-butyrylacetate
(Silver(I)
4-methyl
3-
oxopentanoate) (3a): colorless microcrystals; 83% yield; 13C
NMR (600MHz, Solid);
δ
17.5(CH3CH), 18.7(CH3CH),
40.2(CHCO), 50.4(COCH2CO2), 175.2(CO2ꢀ), 207.2(CO);
Elemental analysis: (measured values C 30.33 H 3.65 calculated
values C 30.41 H 3.84); TG(150°C 51.9%, 280°C 46.3%, 350°C
45.5% theoretical silver content 45.6%); DARTꢀMS (ESI+, 12V)
m/z 87 (M’ + 1)15, 171 (2M’ ꢀ 1)100, 255 (3M’ ꢀ 3)51, (M’ = M ꢀ
(CO2Ag) + 1: (CH3)2CH2COCH3);
8c: GCꢀMS M’ ꢀ 105(43) = 100, M’ ꢀ 97(51) = 15.85, M’ ꢀ 83(65)
= 12.20, M’ ꢀ 71(77) = 21.95, M’ ꢀ 57(91) = 63.41, M’ ꢀ 43(105) =
92.68, M’ ꢀ 33(115) = 4.88, M’ ꢀ 15(133) = 14.63, M’(148) =
52.44
Silver(I)
t-pivaloylacetate
(Silver(I)
4,4-dimethyl
3-
oxopentanoate) (4a): colorless microcrystals; 84% yield; 13C
NMR (600MHz, Solid); δ 27.0, 27.2(CH3), 44.0, 44.3(CCO), 48.0,
49.4(COCH2CO2), 174.1, 175.2(CO2ꢀ), 209.1, 210.6(CO);
Elemental analysis: (measured values C 33.42 H 4.47 calculated
values C 33.49 H4.42); TG(176°C 43.2%, 280°C 42.3%, 350°C
41.8% theoretical silver content 43.0%); DARTꢀMS (ESI+, 12V)
m/z 101 (M’ + 1)31, 199 (2M’ ꢀ 1)67, 279 (2M’ + 79, 3M’ – 3 –
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