The synthesis, chemical and physical properties of silver(I) carboxylates and their use for joining of copper

Article abstract of DOI:10.1016/j.ica.2018.04.022

The synthesis of [AgO₂CCR₂OC(O)OMe] (6, R = H; 7, R = Me) by treatment of HO₂CCR₂OC(O)OMe (4, R = H; 5, R = Me) with [AgNO₃] in presence of NEt₃ is reported. The reaction of 6 and 7 with 2 equiv of PPh₃ gave [AgO₂CCR₂OC(O)OMe(PPh₃)₂] (8, R = H; 9, R = Me). The thermal behavior of 6 and 7 was studied by TG, DSC and TG-MS of which the latter evinced decarboxylation as initial decomposition step followed by Ag–C, C–C and C–O bond cleavages giving silver. For copper-to-copper joining, 6 and 7 were processed to spreadable pastes by mixing them with ethylene glycol (EG) or diethylene glycol (DEG). On contact of 7-EG with Cu sheets [Cu₂(O₂CCMe₂OC(O)OMe)₄·2H₂O] (12) was formed. Therefore, only 6-EG and 6-DEG were used for pressureless joining of Cu at 250 °C. Thru addition of Ag μm-particles a tensile shear strength of 19 MPa was achieved, showing a significant higher value as received with commercially Ag nanopastes. The solid state structure of 5, 8, 9 and 12 was determined by single crystal X-ray structure analysis, showing [3+1] (P₂O + O) (8), distorted tetrahedral (9) or octahedral (12) coordination spheres at the metal ion.

Full text of DOI:10.1016/j.ica.2018.04.022

Accepted Manuscript
Research paper
The Synthesis, Chemical and Physical Properties of Silver(I) Carboxylates and
their Use in Copper-to-Copper Joining
Peter Frenzel, Susann Hausner, Marcus Korb, Guntram Wagner, Heinrich Lang
PII:
S0020-1693(18)30261-5
https://doi.org/10.1016/j.ica.2018.04.022
ICA 18213
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
15 February 2018
12 April 2018
13 April 2018
Please cite this article as: P. Frenzel, S. Hausner, M. Korb, G. Wagner, H. Lang, The Synthesis, Chemical and
Physical Properties of Silver(I) Carboxylates and their Use in Copper-to-Copper Joining, Inorganica Chimica
Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.04.022
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The Synthesis, Chemical and Physical Properties of
Silver(I) Carboxylates and their Use in Copper-to-Copper
Joining
1
2
1
2
1 )
Peter Frenzel , Susann Hausner , Marcus Korb , Guntram Wagner , Heinrich Lang *
1
Chemnitz University of Technology, Faculty of Natural Sciences, Institute of Chemistry,
Inorganic Chemistry, 09107 Chemnitz, Germany
2
Chemnitz University of Technology, Faculty of Mechanical Engineering, Institute of Materials
Science and Engineering, Composites and Material Compounds, 09107 Chemnitz, Germany
*
)
Email: heinrich.lang@chemie.tu–chemnitz.de; Phone: +49(0)371–531–21210;
Fax.: +49(0)371–531–21219.
Received, …
Abstract
The synthesis of [AgO CCR OC(O)OMe] (6, R = H; 7, R = Me) by treatment of
2
2
HO CCR OC(O)OMe (4, R = H; 5, R = Me) with [AgNO ] in presence of NEt is
2
2
3
3
reported. The reaction of 6 and 7 with 2 equiv of PPh gave [AgO CCR OC-
3
2
2
1

(
O)OMe(PPh ) ] (8, R = H; 9, R = Me). The thermal behavior of 6 and 7 was studied
3 2
by TG, DSC and TG-MS of which the latter evinced decarboxylation as initial
decomposition step followed by Ag–C, C–C and C–O bond cleavages giving silver.
For copper-to-copper joining, 6 and 7 were processed to spreadable pastes by
mixing them with ethylene glycol (EG) or diethylene glycol (DEG). On contact of 7-EG
with Cu sheets [Cu (O CCMe OC(O)OMe) ∙2H O] (12) was formed. Hereafter, 6-EG
2
2
2
4
2
and 6-DEG were solely applied in pressureless Cu joining at 250 °C. Thru addition of
Ag µm-particles a tensile shear strength of 19 MPa was achieved, showing a
significant higher value as received with commercially Ag nanopastes. The solid state
structure of 5, 8, 9 and 12 was determined by single crystal X-ray structure analysis,
showing [3+1] (P O + O) (8), distorted tetrahedral (9) or octahedral (12) coordination
2
spheres at the metal ion.
_
__________________________________________________________________
KEYWORDS:
Silver(I) carboxylate, TG–(MS)/DSC, Solid state structure, Silver
sintering, Copper-to-copper joining.
1. Introduction
The continuous miniaturization process of electronic assemblies results in higher thermal
stresses of electronic modules, due to an increase of power densities [1]. As consequence
thereof, higher temperatures generated may result in the remelting of the joints and hence
sufficient reliability cannot be assured [1,2]. This is a major challenge for conventional soldering
materials. Likewise, a high content of lead in Sn–Pb solders restricts their use in electronic and
automobile industries [2,3] and hence the development of lead–free joining alternatives
for low temperature joining processes is a challenging task [4].
2

One approach for low temperature joining is given by using nanoparticles (NPs)
(high surface–to–volume ratio) [5–8], enabling joining processes at temperatures far
below the melting point of the corresponding bulk material, due to a reduced sintering
and melting temperature of the NPs with decreased particle size [9–13]. Beneficial of
NPs in the sintering process is the generation of high–strength and temperature
resistant joints at low temperature [10–16].
In this regard, different joining procedures of bulk copper were recently reported
[
10–14,16,17–27]. One methodology is based on the use of commercial available
silver NPs pastes [11–13,17]. However, these pastes show some limitations, e.g.
they are cost-expensive and reveal higher decomposition temperatures due to aging
processes during prolonged storage [11].
Silver NP-containing powders and pastes are accessible by the reduction of
AgNO by sodium citrate [18–20] or AgNO by ethylene glycol in presence of
3
2
polyvinylpyrrolidone as stabilizing component [19,21], and, for example, by admixing
AgNO with primary or secondary amines RR`NH containing different carbon chain
3
lengths R and R’, respectively, in the presence of ascorbic acid as reducing agent
[
28, 29]. A further methodology to produce silver NPs is given by the thermal
decomposition of amino silver(I) complexes of type [Ag(C H NH ) ]NO [22], or
12
25
2 2
3
applying silver(I) carboxylates such as [Ag(O CC H2n+1)] (n = 7, 13, 17) in a mixture
2
n
of RNH or NR’ (R = C H
(n = 8, 12, 18); R’ = C H
(n = 2, 8)) [23]. However,
2
3
n
2n+1
n
2n+1
the organic particle periphery can negatively impact the joining process, in particular
for large joining areas, since a high amount of organic materials can cause voids in
the joining seam. Out of this reason, either a thinning of the protecting layer
[
21,24,25], or the use of a mixture of nano- and micron-sized particles [26] is
beneficial.
3

Another approach for low temperature copper-to-copper joining processes is
based on the usage of single-source complexes such as silver(I) carboxylates, which
in situ generate silver NPs, when heated to temperatures around 250 °C without any
addition of reducing reagents and/or organic stabilizing components [27]. A further
advantage of this approach is the possibility to influence the decomposition behavior,
the metal content and the solubility of the silver(I) carboxylate complexes by their
modification. Hence, the in situ formation of NPs from silver(I) carboxylates during the
joining process facilitates a simplified handling of the material. Moreover, silver(I)
carboxylates are cost–effective and straightforward to prepare and remain stable for
a long time in solution and in the solid state. As recently shown, silver(I) carboxylates
are favored over silver(I)-n-alkylamines [30]. Thus, to complete the sinter process
prolonged holding times and/or higher temperatures would be required for the
respective silver(I)-n-alkylamines [30].
In this study, the synthesis, chemical and physical properties of silver(I)
carboxylates [AgO CCR OC(O)OMe] (R = H, Me) are described. They can serve as
2
2
filler material in pressureless joining processes of copper sheets at 250 °C, since the
O–carbomethoxy unit induces a low decomposition temperature and good solubility
of the respective complexes.
2. Experimental
2
.1. General
All synthetic methodologies were carried out under an atmosphere of argon with
the solvents degassed prior to use. Chloroform was dried by heating it with P O at
2
5
reflux and storing it afterwards over a 4 Å molecular sieve. Ethanol was distilled from
4

sodium/diethyl phthalate and acetonitrile was purified by distillation from calcium
hydride. Dichlormethane and diethyl ether were dried with a solvent purification
system (MB SPS–800, MBraun). All reagents were obtained from commercial
suppliers and were used without further purification.
1
13
31
NMR spectra (500.3 MHz for H, 125.8 MHz for C and 202.5 MHz for P) were
recorded with a Bruker Avance III 500 NMR spectrometer at ambient temperature.
Chemical shifts are reported in δ (ppm) downfield from tetramethylsilane with the
1
solvent as reference signal ( H NMR, CDCl δ = 7.26, DMSO–d δ = 2.50, D O δ =
3
6
2
13
1
4
.79, CD Cl δ = 5.32; C{ H} NMR, CDCl δ = 77.16 ppm, DMSO–d δ = 39.52,
2 2 3 6
31
1
CD Cl δ = 53.84) or by external standards ( P{ H} NMR relative to 85 % H PO at
2
2
3
4
0
.0 ppm and P(OMe) at 139.0 ppm).
3
The FTIR spectra were recorded with a Thermo Nicolet IR 200 instrument.
Elemental analyses were performed with a Thermo FLASHEA 1112 Series
instrument. High–resolution mass spectra were recorded with a Bruker Daltonik
micrOTOF–QII mass spectrometer performing in the ESI mode. The melting points
were determined by using a Gallenkamp MFB 595 010 M melting point apparatus.
The TG, DSC and TG–MS experiments were performed with a Mettler Toledo
TGA/DSC1 1600 system with a MX1 balance coupled with Pfeifer Vacuum MS
Thermostar GSD 301 T2 mass spectrometer.
2
.2. Single–Crystal X–ray Diffraction Analysis
Data of 5, 8, 9 and 12 were collected with an Oxford Gemini S diffractometer at
1
15 K (5), 110 K (8), 120 K (9) and 111 K (12) using Mo K (λ = 0.71073 Å) radiation
α
(for more details see Table S1, Supplementary Data). The molecular structures were
solved by direct methods using SHELXS–13 [31] and refined by full–matrix least
2
squares procedures on F with SHELXL–13 [32,33]. All non–hydrogen atoms were
5

refined anisotropically and a riding model was employed in the treatment of the
hydrogen atom positions. Graphics for the molecular structures have been created by
using ORTEP [34].
The carboxylic hydrogen atom in 5 has been refined as an idealized OH group
with the torsion angle derived from electron density (AIFX 147). The water–
hydrogens in 12 were derived from residual electron density and fixed by using DFIX
and DANG instructions. Crystallographic data for 5, 8, 9 and 12 have been deposited
with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road,
Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on
quoting the depository numbers CCDC 1815468 (5), 1815469 (8), 1815470
(
9∙CH Cl ) and 1815471 (12); (http://www.ccdc.cam.ac.uk).
2
2
2
.3. Synthesis of carboxylic acids 4 and 5
The preparation of the title complexes was carried out on the basis of the
procedure by Fischer et al. [35]. Since for the characterization of 4 only the elemental
analysis data were reported, high–resolution ESI–TOF mass spectrometric as well as
1
13
1
IR and NMR ( H, C{ H}) spectroscopic data of 4 are added here for completeness.
For the synthesis of 5, 2–hydroxy–2–methylpropanoic acid was used as reactant.
After appropriate work–up, 5 was obtained as analytically pure compound.
2
–((Methoxycarbonyl)oxy)acetic acid (4). 2–Hydroxyacetic acid (1) (7.00 g,
2.04 mmol), N,N–dimethylaniline (34.8 mL, 276 mmol) and methyl chloroformate
21.4 mL, 276 mmol) were reacted as described in reference [35]. Distillation (100
9
(
-2
°
C, 3 x 10 mbar) afforded the title compound as a colorless oil. Yield: 8.00 g (59.7
mmol, 65 % based on 1).
6

-1
Anal. Calcd. for C H O (134.09 g mol ): C, 35.83; H, 4.51. Found: C, 35.41;
4
6
5
-2
-1
H, 4.74. Bp.: 100 °C at 3·10 mbar. IR (CHCl , NaCl; cm ): ꢀ = 2963 (br), 1764 (s,
3
C=O,MeOC(O)O), 1743 (s, C=O), 1446 (m), 1419 (w), 1293 (m), 1238 (m), 1009 (m),
1
9
13 (w), 783 (vs), 757 (vs). H NMR (CDCl , δ): 10.90 (s, br, 1H, CO H), 4.67 (s, 2H,
3
2
13
1
CH ), 3.82 (s, 3H, CH ). C{ H} NMR (CDCl , δ): 173.33 (C –CO H), 155.44 (C –
2
3
3
q
2
q
H COCO ), 62.97 (CH ), 55.60 (CH ). HRMS (ESI–TOF, m/z): calcd. for C H O Na
3
2
2
3
4
6
5
+
[
M + Na] : 157.0107, found 157.0079.
2
–((Methoxycarbonyl)oxy)–2–methylpropanoic acid (5). 2–Hydroxy–2–
methylpropanoic acid (2) (9.50 g, 91.3 mmol), N,N–dimethylaniline (34.6 mL, 274
mmol) and methyl chloroformate (21.2 mL, 274 mmol) were reacted on the basis of
the procedure reported by Fischer et al. [35]. The title compound was obtained as a
colorless solid. Yield: 12.0 g (74.0 mmol, 81 % based on 2).
–1
Anal. Calcd. for C H O (162.14 g mol ): C, 44.45; H, 6.22. Found: C, 44.52;
6
10
5
–1
H, 6.30. Mp.: 96 °C. IR (KBr; cm ):ꢁꢀ = 3019 (br), 2965 (m), 2907 (m), 2689 (w),
2
1
581 (w), 1746 (vs, C=O,MeOC(O)O), 1717 (vs, C=O), 1474 (m), 1456 (m), 1441 (s),
376 (m), 1297 (vs), 1246 (s), 1201 (s), 1152 (vs), 1095 (m), 964 (m), 928 (m), 890
1
(
m), 793 (m), 626 (m). H NMR (CDCl , δ): 11.63 (s, br, 1H, CO H), 3.77 (s, 3H,
3
2
13
1
OCH ), 1.62 (s, 6H, CH ). C{ H} NMR (CDCl , δ): 178.59 (C –CO H), 154.35 (C –
3
3
3
q
2
q
H COCO ), 79.92 (C –C(CH ) ), 54.96 (OCH ), 24.48 (CH ). HRMS (ESI–TOF, m/z):
3
2
q
3 2
3
3
+
calcd. for C H O Na [M + Na] : 185.0420, found 185.0409.
6
10
5
2
.4. Synthesis of silver(I) carboxylates 6 and 7
General procedure. Silver(I) nitrate was dissolved in 100 mL of ethanol and
acetonitrile (2.7 mL for the synthesis of 6, 3 mL for the synthesis of 7) at ambient
7

temperature. To this solution the carboxylic acid 4 or 5 and NEt in 10 mL of ethanol
3
were added at 0 °C with a syringe within 10 min. Immediately, a colorless solid began
to precipitate. Stirring of the reaction mixture was continued for 1 h at ambient
temperature. Subsequently, the colorless suspension was filtered off and washed
twice with cold ethanol (each 30 mL, 0 °C) and then twice with diethyl ether (each 10
mL,
2
0 °C). Removal of all volatiles under reduced pressure gave 6 or 7.
2
–((Methoxycarbonyl)oxy)–ethane–1–carboxylatosilver(I) (6). Compound 4
(
4.50 g, 33.6 mmol) and NEt (5.10 mL, 36.8 mmol) were reacted with silver(I) nitrate
3
(5.71 g, 33.6 mmol) as described above. After appropriate work–up, the title complex
was obtained as a colorless solid. Yield: 7.69 g (31.9 mmol, 95 % based on 4).
–1
Anal. Calcd. for C H AgO (240.95 g mol ): C, 19.94; H, 2.09. Found: C, 19.99;
4
5
5
–1
H, 1.98. Mp.: 215 °C (decomposition). IR (KBr; cm ): ꢀ = 2955 (w), 1753 (s,
C=O,MeOC(O)O), 1614 (vs, CO2,asym), 1448 (m, CO2,sym), 1422 (s), 1342 (m), 1282 (vs),
1
9
87 (m), 914 (m), 792 (m), 664 (m). H NMR (DMSO–d , δ): 4.39 (s, 2H, CH ), 3.68
6
2
13
1
(
s, 3H, OCH3). C{ H} NMR (DMSO–d , δ): 170.55 (C –CO Ag), 155.31 (C –
6 q 2 q
H COCO ), 65.97 (CH ), 54.43 (OCH ). HRMS (ESI–TOF, m/z): calcd. for
3
2
2
3
+
C H AgO Na [M + Na] : 262.9080, found 262.9099.
4
5
5
2
–((Methoxycarbonyl)oxy)–2–methylpropane–1–carboxylatosilver(I) (7). Car-
boxylic acid 5 (6.00 g, 37.0 mmol) and NEt (5.60 mL, 40.4 mmol) were reacted with
3
silver(I) nitrate (6.29 g, 37.0 mmol) as described above. After appropriate work–up,
the title complex was obtained as a colorless solid. Yield: 9.15 g (34.0 mmol, 92 %
based on 5).
8

–1
Anal. Calcd. for C H AgO (269.00 g mol ): C, 26.79; H, 3.37. Found: C, 26.88;
6
9
5
–1
H, 3.29. Mp.: 205 °C. IR (KBr; cm ): ꢀ= 2991 (w), 1740 (vs, C=O,MeOC(O)O), 1596 (vs,
CO2,asym), 1448 (m), 1407 (s, CO2,sym), 1367 (m), 1299 (vs), 1214 (m), 1144 (s), 1102
1
(
m), 961 (m), 820 (m), 794 (m). H NMR (DMSO–d , δ): 3.60 (s, 3H, OCH ), 1.48 (s,
6
3
1
3
1
6
8
H, CH ). C{ H} NMR (DMSO–d , δ): 175.11 (C –CO Ag), 153.75 (C –H COCO ),
3 6 q 2 q 3 2
2.35 (C –C(CH ) ), 53.76 (OCH ), 25.69 (CH ). HRMS (ESI–TOF, m/z): calcd. for
q
3 2
3
3
+
C H AgO Na [M + Na] : 290.9393, found 290.9407.
6
9
5
2
.5. Synthesis of silver(I) complexes 8 and 9
General procedure. The silver carboxylates 6 and 7, respectively, were dissolved
in 15 mL of dichloromethane and PPh was added in a single portion at 0 °C. The
3
respective reaction solution was stirred for 2 h at ambient temperature. Afterwards,
the reaction solution was filtered through a pad of Celite and all volatiles were
removed from the filtrate in vacuum, whereby a colorless solid of 8 or 9 remained.
2
–((Methoxycarbonyl)oxy)–ethane–1–carboxylato(bistriphenylphosphane)–
silver(I) (8). Complex 6 (200 mg, 0.83 mmol) and PPh (435 mg, 1.66 mmol) were
3
reacted as described above. The title complex was obtained as a colorless solid.
Yield: 574 mg (0.75 mmol, 90 % based on 6).
–1
Anal. Calcd. for C H AgO P (765.49 g mol ): C, 62.76; H, 4.61. Found: C,
40
35
5 2
–1
6
2.94; H, 4.58. Mp.: 171 °C. IR (KBr; cm ): ꢀ = 3051 (w), 3004 (w), 2955 (w), 1753
(vs,
C=O,MeOC(O)O), 1601 (vs, CO2,asym), 1479 (s, CO2,sym), 1435 (vs), 1408 (s,)
1
340 (m), 1254 (vs), 1183 (w), 1096 (s), 1070 (w), 1027 (m), 990 (m), 910 (m), 792
9

1
(
m), 746 (vs), 695 (vs), 516 (vs). H NMR (CDCl , δ): 7.40–7.28 (m, 30H, C H ), 4.56
3 6 5
1
3
1
(
s, 2H, CH ), 3.78 (s, 3H, OCH ). C{ H} NMR (CDCl , δ): 172.89 (C –CO Ag),
2 3 3 q 2
1
56.12 (C –H COCO ), 134.05 (d, J = 16.54 Hz, C H ) 132.21 (d, J = 28.15 Hz,
q 3 2 PC 6 5 PC
C –C H ), 130.32 (s, C H ), 128.99 (d, J = 9.76 Hz, C H ), 66.54 (CH ), 54.65
q
6
5
6
5
PC
6
5
2
3
1
1
(
OCH ). P{ H} NMR (CDCl , δ): 9.40 (s).
3 3
2
–((Methoxycarbonyl)oxy)–2–methylpropane–1–carboxylato(bistriphenyl–
phosphane)silver(I) (9). Complex 7 (300 mg, 1.12 mmol) and PPh (588 mg, 2.24
3
mmol) were reacted as described above. The title complex was obtained in
analytically pure form as a colorless solid. Yield: 841 mg (1.06 mmol, 95 % based on
7
).
–1
Anal. Calcd. for C H AgO P (793.57 g mol ): C, 63.57; H, 4.95. Found: C,
42
39
5 2
–
1
6
2
1
4.04; H, 4.97. Mp.: 160 °C (decomposition). IR (KBr; cm ): ꢀ = 3054 (m), 3000 (w),
972 (w), 1751 (vs, C=O,MeOC(O)O), 1592 (vs, CO2,asym), 1479 (s, CO2,sym), 1434 (vs),
401 (s,) 1362 (m), 1272 (vs), 1210 (m), 1181 (w), 1147 (s), 1097 (s), 1027 (m), 997
1
(
w), 960 (m), 932 (w), 817 (m), 784 (m), 746 (vs), 695 (vs), 516 (s). H NMR (CD Cl ,
2
2
13
1
δ): 7.47–7.33 (m, 30H, C H ), 3.66 (s, 3H, OCH ) 1.62 (s, 6H, CH ). C{ H} NMR
6
5
3
3
(
CD Cl , δ): 177.64 (C – CO Ag), 154.79 (C –H COCO ), 134.32 (d, J = 16.71 Hz,
2 2 q 2 q 3 2 PC
C H ) 132.73 (d, J = 27.41 Hz, C –C H ), 130.61 (C H ), 129.23 (d, J = 9.72 Hz,
6
5
PC
q
6
5
6
5
PC
31
1
C H ), 83.49 (C –C(CH ) ), 54.08 (OCH ), 26.14 (CH ). P{ H} NMR (CD Cl , δ):
6
5
q
3 2
3
3
2
2
8
.92 (s).
2
.6. Synthesis of sodium carboxylates 10 and 11 (Reference substances for IR
studies)
1
0

General procedure. Sodium hydroxide was dissolved in 12 mL of methanol at
ambient temperature. To this solution carboxylic acid 4 or 5 in 5 mL of methanol was
added in a single portion at ambient temperature. Stirring of the reaction mixture was
continued for 10 h at ambient temperature. Subsequently, all volatiles were removed
under reduced pressure to give 10 or 11.
Sodium 2–((methoxycarbonyl)oxy)–ethane–1–carboxylate (10). Sodium
hydroxide (86.0 mg, 2.15 mmol) and carboxylic acid 4 (288 mg, 2.15) were reacted
as described above. The title complex was obtained as a colorless solid. Yield: 335
mg (2.15 mmol, 100 % based on 4).
–1
Anal. Calcd. for C H NaO (156.07 g mol ): C, 30.78; H, 3.23. Found: C, 30.77;
4
5
5
–1
H, 3.16. Mp.: 250 °C decomposition. IR (KBr; cm ): ꢀ = 2982 (w), 1751 (vs,
C=O,MeOC(O)O), 1607 (vs, CO2,asym), 1457 (s), 1439 (s, CO2,sym), 1352 (m), 1291 (vs),
1
1
253 (s), 993 (s), 952 (m), 923 (m), 786 (s), 675 (m). H NMR (D O, δ): 4.44 (s, 2H,
2
13
1
CH ), 3.78 (s, 3H, OCH ). C{ H} NMR (D O, δ): 175.03 (C –CO Na), 156.38 (C –
2
3
2
q
2
q
H COCO ), 65.64 (CH ), 55.24 (OCH ). HRMS (ESI–TOF, m/z): calcd. for
3
2
2
3
+
C H NaO H [M + H] : 157.0107, found 157.0103.
4
5
5
Sodium 2–((methoxycarbonyl)oxy)–2–methylpropane–1–carboxylate (11).
Sodium hydroxide (95.0 mg, 2.38 mmol) and carboxylic acid 5 (386 mg, 2.38) were
reacted as described above. The title complex was obtained as a colorless solid.
Yield: 438 mg (2.38 mmol, 100 % based on 5).
1
1

–1
Anal. Calcd. for C H NaO (184.12 g mol ): C, 39.14; H, 4.93. Found: C, 39.13;
6
9
5
–1
H, 4.94. Mp.: 138 °C. IR (KBr; cm ): ꢀ = 2992 (w), 1738 (vs, C=O,MeOC(O)O), 1601 (vs,
CO2,asym), 1447 (s), 1409 (s, CO2,sym), 1367 (m), 1302 (vs), 1214 (m), 1145 (s), 1103
1
(
m), 962 (m), 935 (w), 794 (m). H NMR (D O, δ): 3.71 (s, 3H, OCH ), 1.48 (s, 6H,
2
3
13
1
CH ). C{ H} NMR (D O, δ): 179.71 (C –CO Ag), 155.02 (C –H COCO ), 83.38 (C –
3
2
q
2
q
3
2
q
C(CH ) ), 54.69 (OCH ), 24.29 (CH ). HRMS (ESI–TOF, m/z): calcd. for C H NaO H
3
2
3
3
6
9
5
+
[
M + H] : 185.0420, found 185.0420.
2
.7. Tetra–µ–(2–((methoxycarbonyl)oxy)–2–methylpropane–1–carboxylato)–
bis(aqua)–dicopper(II) (12). A paste made from 7 and ethylene glycol was applied
on a copper sheet. After an application time of 1 h the paste underwent a color
change from off–white to turquoise. The formed turquoise colored solid was
suspended in water, filtered off and the obtained aqueous solution was subsequently
evaporated at ambient temperature. The title complex was obtained as turquoise
crystals.
–1
Anal. Calcd. for C H Cu O (807.64 g mol ): C, 35.69; H, 4.99. Found: C,
24
40
2
22
–1
3
5.77; H, 4.82. Mp.: 121 °C. IR (KBr; cm ): ꢀ = 3495 (br), 2995 (w), 1758 (vs,
C=O,MeOC(O)O), 1627 (vs, CO2,asym), 1564 (m), 1478 (s), 1462 (s), 1423 (s, CO2,sym),
1
368 (m), 1308 (vs), 1215 (s), 1163 (vs), 1102 (m), 966 (m), 935 (m), 829 (m), 795
+
(
m), 783 (vs), 652 (m). HRMS (ESI–TOF, m/z): calcd. for C H Cu O [2 M* + Cu] :
24
36
3
20
+
8
34.9668, found 834.9676; C H Cu O [3 M + Cu] : 1219.9866, found 1219.9851.
36 54 4 30
M* = [Cu(O CCMe OC(O)OMe) ].
2
2
2
1
2

2
.8. Joining process
For the joining experiments, copper sheets (ISO: Cu–ETP, EN: CW004A, DIN
material number 2.0060) with a purity of ≥ 99.9 % were used. Due to the most
suitable thermal behavior (see Section Results and Discussion), complex 6 was
selected as filler material for the joining experiments. To ensure an efficient and
hence straightforward application, 6 (initial state: solid) was mixed with two different
solvents (ethylene glycol (EG) and diethylene glycol (DEG)) to prepare the
corresponding joining pastes 6–EG and 6–DEG, respectively. With these spreadable
pastes, an application similar to conventional soldering or brazing pastes is possible.
Furthermore, the addition of Ag particles (0.7 – 1.2 μm, 99.9 %, Chempur) to 6 in
different mass ratios was examined to analyze the influence on the achievable
strengths and the resulting microstructure of the joints.
All joining experiments were carried out pressureless with a joining temperature of
2
50 °C and a holding time of 10 min. The heating of the samples was carried out in
air by means of induction. The determination of the tensile shear strength as well as
the microstructure investigations were performed using the sample geometry shown
in Figure 1. The joining area was 9.0 mm x 7.5 mm. The examination of all tensile
specimens – at least five individual samples per variation – was performed with a
universal tensile testing machine Zwick/Roell Z020 at ambient temperature and at a
-3
-1
strain rate of 10 s . The microstructure of the joints was analyzed with a scanning
electron microscope (LEO 1455VP) to correlate the resulting microstructure with the
achievable strengths.
1
3

Figure 1. Sample geometry for joining experiments.
3. Results and Discussion
3
.1. Synthesis and characterization
The synthesis of carboxylic acids 4 and 5 was carried out as shown in Scheme 1
according to reference [35]. Subsequent treatment of 4 and 5 with equimolar
amounts of [AgNO ] in presence of NEt in ethanol afforded silver(I) carboxylates 6
3
3
and 7 in excellent yields as colorless solids (Scheme1). Complexes 6 and 7 are
soluble in polar organic solvents such as acetonitrile and dimethyl sulfoxide.
Scheme 1. Synthesis of 4 – 7.
Since, from 6 and 7 no single crystals suitable for X–ray diffraction analysis could
be obtained, they were reacted with 2 equiv of PPh in dichloromethane at ambient
3
temperature to give [AgO CCR OC(O)OMe(PPh ) ] (8, R = H; 9, R = Me) (Equation
2
2
3 2
(1)) in a yield of 90 % (8) or 95 % (9).
1
4

1
13
1
The identity of 4–9 was confirmed by elemental analysis, IR, H, C{ H} and
31
1
P{ H} NMR spectroscopy and high–resolution ESI–TOF mass spectrometry (4–7)
(Experimental). The molecular structure of 5, 8 and 9 in the solid state was
determined by single crystal X–ray structure analysis.
The IR spectra of 6–9 show the symmetric and asymmetric stretching vibrations
-1
(
ꢀCO2,sym; ꢀCO2,asym) of the carboxylato groups at about 1407 and 1614 cm (Table 1).
-1
-
The obtained separations ∆ꢀCO2 = ꢀCO2,asym – ꢀCO2,sym are 166 cm for 6 and 189 cm
1
for 7. These values are close to the differences of the corresponding sodium
carboxylates 10 and 11 (reference substances, Table 1 and Experimental), implying
a bidentate bridging coordination mode [36–38]. For 8 and 9 differences of ∆ꢀCO2
=
-1
1
22 and 113 cm were found (Table 1), indicating a chelating binding mode [36–38],
which was also verified by single crystal X–ray structure analysis (vide infra).
Table 1. ꢀCO2 Absorptions and ∆ꢀCO2 values for 6–11.
-1
-1
a)
-1
Compd. ꢀCO2,asym [cm ]
ꢀCO2,sym [cm ]
∆ꢀCO2 [cm ]
6
7
8
9
1614
1596
1601
1592
1607
1601
1448
1407
1479
1479
1439
1409
166
189
122
113
168
192
1
0
1
1
1
5

a) ∆ꢀCO2 = ꢀCO2,asym – ꢀCO2,sym
1
H NMR spectroscopy allows to trace the reaction of 4/5 to 6/7 since upon
treatment of 4/5 with [AgNO ] and NEt the carboxylic proton is removed. The
3
3
introduction of PPh in 6 and 7 to produce 8 and 9 is accompanied by a typical
3
31
downfield shift of the P NMR resonance signal from -5.47 ppm (free PPh [39]) to
3
9
.40 ppm for 8, and 8.92 ppm for 9 [40, 41] (Experimental).
3
.2. Solid state structures
Single crystals of 5 (Figure 2), 8 and 9 (Figure 3) suitable for X–ray diffraction
studies were obtained by layering a dichloromethane solution containing the
respective compounds with hexane at ambient temperature. Selected bond distances
(Å) and bond angles (°) are summarized in the captions of Figures 2 and 3.
Compounds 5 and 9 crystallize in the monoclinic space group P2 /c and 8 in triclinic
1
P–1, each with one molecule in the asymmetric unit. The carboxylic functionality in 5
forms hydrogen bridge bonds to an adjacent molecule, resulting in the formation of a
dimer (Figure 2). The C–O single bond (C6–O4) of 1.3053(15) Å can clearly be
distinguished from the C=O double bond (C6–O5) with 1.2190(16) Å.
Figure 2. ORTEP (50 % probability level) of the molecular structure of 5 involved in
intermolecular hydrogen bridge–bond formation (pale blue). C–bonded hydrogen atoms have
been omitted for clarity. Selected bond distances (Å) and angles (°): C6–O4 1.3053(15), C6–
1
6

O5 1.2190(16), O2–C2–O1 126.71(13), O2–C2–O3 125.72(13), O1–C2–O3 107.57(12), O5–
C6–O4 124.35(13). Symmetry operation for generating equivalent atoms (‘): 1–x, 2–y, –z.
Hydrogen bridge–bond properties: O4∙∙∙O5’ 2.6155(13) Å, O1–H1∙∙∙O2 178 °.
Silver complexes 8 and 9 consist of the bidentate bonded carboxylato and two
datively–bonded PPh groups. In 8, the atoms P1, P2 and O1 form a 3–fold
3
coordinated planar geometry at Ag1, whereby the P1–Ag1–P2 angle of 128.70(2) ° is
the largest one observed in 8. This angle and P–Ag1–O1 (122.35(5), 107.72(5) °) are
in the range to recently published phosphane coordinated silver(I) carboxylates
[
40,42]. The O2 atom stabilizes the complex as a weakly coordinating ligand with
.955(2) Å above the P O1 plane (Figure 3). This causes a slight shift of Ag1 out of
1
2
this P O1 plane towards O2 by 0.1552(7) Å resulting in a [3+1] coordination
2
geometry. This is also supported by different Ag–O bond distances, which are
decreased to 2.3238(18) Å for O1, compared to more weakly bonded O2 (2.6194(16)
Å). This is due to the presence of a C–O single bond between C1 and O1 (1.255(3)
Å) and a carbonyl moiety for the C1–O2 bond (1.225(3) Å).
Figure 3. ORTEPs (probability level: 50 %, 8 and 9) of the molecular structures of 8 (left) and
9
(right) with the atom numbering schemes. All hydrogen atoms and one disordered
molecule of dichloromethane (9) have been omitted for clarity. Selected bond distances (Å)
and angles (°) for 8: C1–O1 1.255(3), C1–O2 1.225(3), Ag1–O1 2.3238(18), Ag1–O2
2.6194(16), Ag1–P1 2.4270(6), Ag1–P2 2.4181(6), O1–Ag1–O2 52.59(6), P1–Ag1–P2
128.70(2), P2–Ag1–O1 122.35(5), P1–Ag1–O1 107.72(5) and 9: C1–O1 1.226(9), C1–O2
1.250(9), Ag1–O1 2.465(5), Ag1–O2 2.444(5), Ag1–P1 2.431(3), P1–Ag1–P2 126.59(6), P1–
1
7

Ag1–O2 119.81(15), P2–Ag1–O2 109.34(15), P1–Ag1–O1 110.80(15), P2–Ag1–O1
114.64(16), O1–Ag1–O2 53.44(15).
In contrast, complex 9 exhibits equal Ag1–O distances of 2.465(5) (O1) and
2
.444(5) Å (O2) and thus, a symmetrical complexation of the Ag ion by the
carboxylato group in a distorted tetrahedral environment is present (Figure 3). This is
supported by shifts of O1 and O2 out of the AgP plane by 1.259(7) (O1) and
2
0
.922(7) (O2) Å, which are in close range compared to 8, where O1 is solely shifted
by 0.485(2) Å and O2 by 1.5213(19) Å (Figure 4). The plane intersection of the
AgCO unit towards the AgP fragment in 9 is 82.01(19) °, further evidencing a
2
2
tetrahedral coordination sphere at Ag1. In 8, the AgCO plane is rotated by 66.18(5) °
2
towards the AgP unit, due to the weakly bonded O2. However, the carboxylato
2
group restricts the O1–Ag1–O2 angle to 53.44(15) ° in 9, causing a large distortion
from the ideal value of 109.5 °.
1
8

Figure 4. ORTEP (50 % probability level) of the molecular structures of 8 (top) and 9
(
(
bottom) showing the distances of the carboxylato oxygen atoms towards the AgP plane
grey). All hydrogen atoms and the phenyl rings have been omitted for clarity.
2
3
.3. Thermal properties
The thermal behavior of 6 and 7 in the solid state and as pastes (processing 6
and 7 into pastes using ethylene glycol (EG) or diethylene glycol (DEG)) as the
solvents was studied by thermogravimetry (= TG/DTG) and differential scanning
calorimetry (= DSC). In addition, TG–coupled mass-spectrometric (= TG–MS)
experiments were carried out, to study the detailed decomposition behavior of 6 and
7
.
The respective TG/DTG and DSC studies of 6 and 7 were performed in a stream
-1
-1
of O and N (gas flow 20 mL min ) with a heating rate of 10 °C min in the
2
2
temperature range of 100 – 500 °C. For all measurements, a continuous nitrogen
-1
carrier gas flow of 40 mL min was used. The corresponding TG/DTG and DSC
traces of solid 6 are depicted in Figure 5.

300
[°C]
 [°C]
(
a)
(b)
1
00
200
400
500
100
100
200
300
400
500
1
00
DTG
TG
DTG
8
6
4
0
80
60
40
0
0
TG
4
0
3
2
1
-2
-4
1
00
200
300
[°C]
400
500
100
200
300
400
500


[°C]
Figure 5. TG/DTG (top) and DSC (bottom) traces for the thermal behavior of 6 in O (a)
2
-1
-1
and in N (b) (gas flow 20 mL min , N carrier gas flow 40 mL min , heating rate 10 °C
min ).
2
2
-1
1
9

The decomposition of 6 in oxygen occurs in consecutive steps starting at 215 °C
(onset temperature), also detectable by DSC as an endothermic process and
following by further exothermic events at 236 °C and 247 °C, respectively (Figure
a). The decomposition of 6 is completed at 260 °C (according to DSC) which was
quantified with a residue of 44.9 %, matching the formation of elemental silver
5
(theoretical silver content of 6, 44.8 %) as it was confirmed by PXRD measurements
(Figure S1, ESI).
A similar thermal behavior of 6 was observed in an atmosphere of nitrogen,
except that the decomposition of 6 was completed at a slightly higher temperature
280 °C, according to DSC) (Figure 5b). The as–obtained residue was 44.6 %, which
(
agrees with the theoretical silver content of 44.8 %.
The appropriate TG/DTG and DSC traces of solid 7 (in O as in N ) are depicted
2
2
in Figure 6.

[°C]

300
[°C]
(
a)
(b)
1
00
200
400
500
100
100
200
300
400
500
1
00
DTG
TG
DTG
8
6
4
0
80
6
0
0
0
0
TG
4
2
-
0,5
1,0
1,5
2,0
1
0
-
-
-
-1
-
2
1
00
200
300
[°C]
400
500
100
200
300
[°C]
400
500


Figure 6. TG/DTG (top) and DSC (bottom) traces for the thermal behavior of 7 in O (a)
2
-1
-1
and in N (b) (gas flow 20 mL min , N carrier gas flow 40 mL min , heating rate 10 °C
min ).
2
2
-1
The onset temperature of 7 in O is 206 °C, decomposition is completed at 400
2
°
C. The appropriate DSC trace shows that decomposition of 7 occurs in consecutive
steps (Figure 6a). Under N , the decomposition starts at 196 °C, which is about 10 °C
2
2
0

lower as compared to its decomposition in O and is completed at 310 °C (Figure 6b).
2
st
According to the DTG and DSC traces, 7 decomposes in three steps in which the 1
rd
one at 206 °C and the 3 one at 260 °C constitute to endothermic events, whereas
nd
the 2 process at 210 °C is exothermic. The obtained residues at 500 °C correspond
to 40.6 %, which points to the formation of elemental silver (40.1 % theoretical).
The presence of the α–positioned Me groups in 7 leads to slightly lower onset
temperatures, when compared to 6. This can be explained with a higher
thermodynamic stability of the decarboxylation products (organic radicals) of 7, as
recently evidenced for [Cu(O CCMe (C H O) OMe) ] [43,44]. The decomposition of 7
2
2
2
4
2
2
takes place within a broader temperature range in O and N .
2
2
To obtain a deeper insight into the thermal decomposition behavior of 6 and 7,
TG–coupled mass spectrometric experiments were carried out (Figure 7).
T [°C]
300
T [°C]
300
(
a)
(b)
1
00
200
400
500
100
100
200
400
500
1
00
8
6
4
0
8
6
4
0
0
0
0
0
15
28
31
44
15
31
44
58
5
9
100
200
300
400
500
100
200
300
400
500
T [°C]
T [°C]
Figure 7. TG trace (top) and selected mass spectrometric scans (bottom) of 6 (a): m/z =
+
+
+
+
+
1
5 (CH ), m/z = 28 (CO ), m/z = 31 (CH O ), m/z = 44 (CO ) and 7 (b): m/z = 15 (CH ),
3 3 2 3
+
+
+
+
m/z = 31 (CH O ), m/z = 44 (CO ), m/z = 58 (C H O ), m/z = 59 (C H O ) (nitrogen gas
flow 20 mL min , heating rate 5 °C min ).
3
2
3
6
2
3
2
-1
-1
st
+
2
+
In the 1 decomposition step of 6 and 7 CO (m/z = 44) and CO (m/z = 28) as
typical fragments could be detected, indicating that initially most probably the
cleavage of the Ag–O bond takes place, forming [AgCR OC(O)OMe] (R = H, Me).
2
This assumption is in good agreement with previous investigations on, for example,
2
1

n
[
AgO C(CH OCH ) H] and [AuO CCH (OCH CH ) OMe( Bu P)] [45–47]. Subse-
2 2 2 3 2 2 2 2 2 3
quently, [AgCR OC(O)OMe] decomposes via Ag–C, C–C and C–O homolytical bond
2
cleavages, which was verified by the detection of characteristic fragments (6: m/z =
+
+
+
3
+
2
+
2
8 (CO ), 31 (CH O ), 15 (CH ); 7: m/z = 59 (C H O = CH CO ), 58 (C H O ), 31
3
2
3
3
2
3
6
+
+
3
(
CH O ), 15 (CH )) (Figure 7).
3
3
.4. Copper-to-copper joining
To ensure a simplified application for copper-to-copper joining, the formation of
spreadable pastes of 6 and 7 based on ethylene glycol (EG) and diethylene glycol
DEG) were applied. For the pastes with 7 a slow color change from off–white to
(
turquoise was observed, when it was applicated to the Cu sheets (Figure S2, ESI).
Obviously a redox reaction took place between 7 and the copper surface according
to Equation 2.
A possible explanation for this behavior is the improved solubility of 7 in the
pastes, due to the presence of the α–methyl groups, which increase an interaction of
7
with the Cu surface.
To investigate more closely this behavior, the formed turquoise colored solid was
suspended in water, filtered off and the obtained aqueous solution was subsequently
evaporated to give suitable single crystals for X–ray diffraction study. For further
characterization (IR, ESI-TOF, XRD) of 12 see the ESI.
2
2

Based on the results obtained from the thermal behavior, complex 6 represents
the most eligible precursor for the low temperature joining of copper sheets, due to a
narrow decomposition temperature range and a higher metal content in comparison
to 7.
To select the suitable solvent (EG or DEG) for paste formation, the appropriate
pastes were investigated by TG/DTG and DSC analysis (Figure 8).
T [°C]
200
T [°C]
200
(a)
(b)
100
300
400
100
300
400
1
00
100
DTG
TG
DTG
TG
8
6
4
0
0
80
60
40
0
1
1
0
0
0
-
2
-
1
-1
-2
400
-2
-4
100
200
T [°C]
300
100
200
T [°C]
300
400
Figure 8. TG/DTG (top) and DSC (bottom) traces for the thermal behavior of 6–EG (a) and
-
1
-1
-1
6
–DEG (b) (O flow 20 mL min , N carrier gas flow 40 mL min , heating rate 10 °C min ).
2 2
As it can be seen from Figure 8, the solvents shift the onset temperature to lower
values for 6–EG (100 °C) and 6–DEG (45 °C) as compared to solid 6. This is related
to the reducing properties of the respective solvents, which are responsible for
Ag(I)/Ag(0) reduction forming, e.g. acetaldehyde on the example of EG [50]. In case
of 6–EG, the decomposition starts at 116 °C and is completed at 260 °C (Figure 8a).
According to the DTG and DSC traces, the decomposition involves three events in
st
which the 1 process at 135 °C is endothermic, whereas the following two processes
at 186 and 230 °C are exothermic.
The decomposition process of 6–DEG starts at 170 °C and is completed at 310 °C
(Figure 8b). The DSC trace comprises several endothermic and exothermic peaks.
The obtained residue after the thermal heating of the paste with EG is 33.2 % and
with DEG 30.6 %, respectively, corresponding to the formation of elemental silver,
2
3

which was proven by PXRD measurements.
Copper-to-copper joining experiments with pastes 6–EG and 6–DEG were carried
out. The joining process was performed at 250 °C for 10 min without applying a
pressure. The strength tests (Figure 9) show that with 6–DEG higher joint strengths
(8 MPa) could be achieved in comparison to 6–EG (5 MPa). This is most likely a
result of the much denser microstructure of the joining seam received with 6–DEG
than with 6–EG (Figure 10).
The reason for this different behavior may be the difference between the joining
temperature and decomposition temperature of the two pastes. Presumably, for the
complete decomposition of 6–EG the last two exothermic processes (186, 230 °C)
st
are crucial, which indicates a higher mass loss compared to the 1 process at 135
°
C. In contrast, with 6–DEG the thermal processes, which are relevant for the
complete decomposition, occur at a lower temperature thus correlating with the onset
temperature of 170 °C.
The seams of the as–joint Cu sheets produced with paste 6–DEG exhibit in
comparison with 6–EG higher tensile shear strengths and a denser microstructure at
lower temperature. Therefore, only DEG as solvent was used for further joining
experiments.
2
4

Figure 9. Tensile shear strengths of joints Figure 10. SEM images of joints
obtained with 6–EG and 6–DEG (joining obtained with 6–EG (a) and 6–DEG (b)
temperature 250 °C, holding time 10 min, (joining temperature 250 °C, holding time
joining pressure 0 MPa).
10 min, joining pressure 0 MPa).
Furthermore, the addition of Ag particles of size 0.7 – 1.2 µm to 6 was examined.
Therefore, the influence of different mass ratios (Table in Figure 11) on the strength
behavior and the resulting microstructure of the joints was investigated. The addition
of µm–sized Ag particles to increase the metal content in conventional nanoparticle–
containing pastes is known [2,11,19,51] and commercialized [11,52].
Figure 11 shows that the amount of the added particles has an essential
influence on the strength behavior. The highest strengths of about 19 MPa could be
achieved with 6–DEG:Ag particles in the mass ratio of 1:1 (paste 2). The strengths
are more than twice as high when compared to the pure precursor paste (joining
paste 1) with a strength of 8 MPa.
Moreover, with joining paste 2 at 250 °C higher strength values (19 MPa) could be
achieved as compared to a commercial
Ag nanopaste (10 MPa) (300 °C)
(same sample geometry, Figure 11)
[
11,13].
2
5

Figure 11. Left: Tensile shear strengths of joints obtained using 6–DEG with addition of Ag
µm particles in different mass ratios (joining temperature 250 °C, holding time 10 min, joining
pressure 0 MPa) and a commercial Ag nanopaste (joining temperature 300 °C, holding time
10 min, joining pressure 0 MPa). Right: Mass ratios of the joining pastes.
A comparison of the microstructure of joints obtained with pastes 1 and 2 shows
that with paste 2 a lower and finer porosity in the joining seam could be achieved
Figure 12). This is most likely the result of the Ag particle addition, which results in a
(
higher metal content of paste 2 and therefore produces a denser microstructure and
hence higher strength values of the joints. In addition, due to the addition of Ag
particles, the joining seam with paste 2 is significantly thicker (≈16 µm) compared
with the one obtained by using paste 1 (4 – 5 µm, Figure 12).
Figure 12. Comparison of SEM images of joints obtained with joining pastes 1 (a) and 2 (b)
(
joining temperature 250 °C, holding time 10 min, joining pressure 0 MPa). (Composition of
pastes 1 and 2, see Figure 11).
2
6

It is interesting to note that the other mixture ratios (joining pastes 3 and 4) lead to
lower strengths (Figure 11). Paste 3 shows with 8 MPa the same strength as the
pure precursor paste (paste 1). This value is significantly lower than the strengths
with joining paste 2 (19 MPa). The image of the microstructure (Figure 13a) shows
that with paste 3 the joining seam exhibits a very porous structure, which is the
reason for the lower strength values compared to joining paste 2. The seam seems
like fluffed and is with approx. 80 µm comparatively thick. With paste 4, the strengths
of 11 MPa are only slightly higher compared to pastes 1 and 3, but decidedly lower
than paste 2. The microstructure of the seam with paste 4 (Figure 13b) exhibits a
denser microstructure when compared with the one of joining paste 3 (Figure 13a),
but possesses a higher porosity compared to the seam with paste 2 (Figure 12b).
Therefore, also the strength values with paste 4 are between the values of joining
pastes 2 and 3 (Figure 11).
Figure 13. Comparison of SEM images of joints obtained with joining pastes 3 (a) and 4 (b)
(
joining temperature 250 °C, holding time 10 min, joining pressure 0 MPa). (Composition of
pastes 3 and 4, see Figure 11).
4. Conclusion
2
7

The synthesis of silver(I) carboxylates [AgO CCR OC(O)OMe] (6, R = H; 7, R =
2
2
Me) is reported. For investigation of their molecular structure in the solid state by
single crystal X–ray diffraction analysis the respective PPh -coordinated complexes
3
[
AgO CCR OC(O)OMe(PPh ) ] (8, R = H; 9, R = Me) were prepared. In 8 a [3+1]
2 2 3 2
(
P O + O) coordination geometry is present, due to an unsymmetrical bonding of the
2
carboxylato moiety to the silver ion. Therefore, the oxygen atom, involved in a C–O
single bond acts as the strong and the carbonyl oxygen as a weaker bonded donor.
Complex 9 exhibits a distorted tetrahedral environment with a symmetrical
complexation of the carboxylato group towards the Ag ion and similar C–O distances
within the CO fragment. IR spectra of 8 and 9 confirm the chelate binding of the
2
carboxylato group. In contrast, 6 and 7 show a bidentate bridging coordination motif
as evidenced by IR spectroscopy. The investigation of the thermally induced
decomposition of 6 and 7 in the solid state by TG/DSC indicated lower onset
temperatures for 7 in comparison to 6. The decomposition of 7 is finished at higher
temperatures, with 400 (O ) and 310 °C (N ), when compared to 6 (260 (O ), 280 °C
2
2
2
(
N )). TG–MS experiments of 6 and 7 confirmed that initially decarboxylation occurs,
2
followed by Ag–C, C–C and C–O bond cleavages. The obtained residues were
characterized as silver by PXRD analysis.
To enable a simplified application for copper-to-copper sheet joining, 6 and 7 were
mixed with ethylene glycol (EG) or diethylene glycol (DEG) to spreadable pastes. By
application of 7–EG to Cu sheets a redox reaction was observed resulting in the
formation of [Cu (O CCMe OC(O)OMe) ∙2H O] (12). In comparison to 7, complex 6
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2
showed the most suitable thermal behavior for low temperature joining. Therefore,
the influence of the used solvents (EG and DEG) on the decomposition behavior of 6
was investigated by TG/DTG and DSC. Due to the reducing properties of the
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solvents EG and DEG towards the respective Ag(I) complexes, the onset
temperature of the pure precursor was shifted to significant lower values. In addition,
pressureless joining experiments (250 °C, 10 min) with 6–EG and 6–DEG were
carried out and the resulting joining seams were investigated regarding to the
strength behavior and the microstructure to select the convenient solvent. These
results indicated that 6–DEG positively impacts the joining results, enabling a denser
microstructure and higher strengths compared to 6–EG. Additionally, to increase the
metal content in the joining pastes, Ag µm–sized particles in different mass ratios
were added to 6–DEG (1:1, 1:2 and 2:1) and their influence was examined regarding
the achievable strengths and the resulting microstructure of the joints. The results
showed that the mass ratio 6–DEG:Ag µm particles has an essential influence on the
strength behavior. The highest strengths of about 19 MPa were achieved with
particles in the mass ratio of 1:1 (paste 2). The value of 19 MPa (250 °C, 10 min,
pressureless) is significantly higher compared to 10 MPa, which is characteristic for a
commercial Ag nanopaste (300 °C, 10 min, pressureless) [11,13]. In combination
with the described advantages of the presented silver(I) carboxylate precursors
(cost–effective and straightforward synthesis, long–term stability and considerably
simplified handling), they pose a promising alternative for conventional nanopastes.
Comparing the silver(I) carboxylates reported herein with recently reported silver(I)-n-
alkylamine complexes, discussed as potentially joining materials for copper-to-copper
silver joints [30], show better decomposition properties, a higher chemical stability
and, for example, an enhanced resistance against light. In addition, it was reported
that after sintering of silver(I)-n-alkylamines large amounts of tar-like organic residues
remained on the generated silver layers, which may result in the formation of less
strong adhesions between the silver-substrate interface [30].
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