Synthesis and molecular structures of a novel tetranuclear silver(I) cluster [Ag2(Himdc)(PPh3)2]2 (H 3imdc = imidazole-4,5-dicarboxylic acid) and a mononuclear silver(I) complex [Ag(H2imdc)(PPh3)2]

Article abstract of DOI:10.1016/j.inoche.2005.09.012

A novel tetranuclear silver(I) cluster, [Ag₂(Himdc)(PPh ₃)₂]₂ 1 (H₃imdc = imidazole-4,5-dicarboxylic acid; O^N) was obtained by a reaction of the Ag-O bonding precursor [Ag(R-Hpyrrld)]₂ (H₂pyrrld = 2-pyrrolidone-5-carboxylic acid), PPh₃ and H₃imdc. The mononuclear complex, [Ag(H₂imdc)(PPh₃)₂] 2, was also formed by the reactions using increased amounts of PPh₃. In the solid state, complex 1 was a bivalve -like Ag₄ cluster, while complex 2 showed a supramolecular arrangement of the 4-coordinate AgP ₂(N^O) unit via the intermolecular hydrogen bonding interaction.

Full text of DOI:10.1016/j.inoche.2005.09.012

Inorganic Chemistry Communications 9 (2006) 107–110
www.elsevier.com/locate/inoche
Synthesis and molecular structures of a novel tetranuclear silver(I)
cluster [Ag2(Himdc)(PPh3)2]2 (H3imdc=imidazole-4,5-dicarboxylic acid)
and a mononuclear silver(I) complex [Ag(H2imdc)(PPh3)2]
*
Ryusuke Noguchi, Akiyoshi Sugie, Akihiro Hara, Kenji Nomiya
Department of Materials Science, Faculty of Science, Kanagawa University, Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan
Received 30 July 2005; accepted 7 September 2005
Available online 22 November 2005
Abstract
A novel tetranuclear silver(I) cluster, [Ag2(Himdc)(PPh3)2]2 1 (H3imdc = imidazole-4,5-dicarboxylic acid; OꢀN) was obtained by a
reaction of the Ag–O bonding precursor [Ag(R-Hpyrrld)]2 (H2pyrrld = 2-pyrrolidone-5-carboxylic acid), PPh3 and H3imdc. The mono-
nuclear complex, [Ag(H2imdc)(PPh3)2] 2, was also formed by the reactions using increased amounts of PPh3. In the solid state, complex 1
was a ‘‘bivalve’’-like Ag4 cluster, while complex 2 showed a supramolecular arrangement of the 4-coordinate AgP2(NꢀO) unit via the
intermolecular hydrogen bonding interaction.
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: Tetranuclear silver(I) cluster; Imidazole-4,5-dicarboxylic acid; Triphenylphosphine; 2-Pyrrolidone-5-carboxylic acid
There is currently considerable interest in the coordina-
tion chemistry of coinage metals such as silver(I) and
gold(I) with biological and/or medicinal activities [1–3].
In the structural viewpoint, silver(I) complexes with thiol
ligands have shown a tendency to form cluster and polymer
structures [1,4–10], whereas the corresponding gold(I) com-
plexes have shown supramolecular arrangements of 2-coor-
dinate linear units [1,11–14].
Recently, we have found that the chiral and achiral sil-
ver(I) complexes, [Ag(R or S-Hpyrrld)]2 and [Ag2(R-Hpy-
rrld) (S-Hpyrrld)] (H2pyrrld = 2-pyrrolidone-5-carboxylic
acid) [8,9] are light-stable and water-soluble Ag–O bonding
complexes, which can be used as useful precursors, in the
place of Ag2O, AgNO3, AgCH3CO2 and so on, for forma-
tion of novel coinage(I) metal complexes and silver(I) clus-
ters [10,14,15].
1:2:1 molar-ratio reaction in organic media of [Ag(R-
Hpyrrld)]2, PPh3 and the water-insoluble and organic sol-
vent-insoluble nitrogen-containing heterocycle ligand,
H3imdc [16]. By reactions using increased amounts of
PPh3, the mononuclear complex, [Ag(H2imdc)(PPh3)2] 2,
was also obtained [17]. Herein, we report the synthesis of
1 and 2 and their unequivocal characterization with ele-
mental analysis, TG/DTA, FTIR, solution (1H, 13C and
31P) NMR and solid state 31P CPMAS NMR, and X-ray
crystallography.
Compound 1 was formed in the 1:2:1 molar-ratio reac-
tion of [Ag(R-Hpyrrld)]2, PPh3 and H3imdc in a 1:4 mixed
CHCl3/EtOH solvent, and colorless plate crystals of 1 were
obtained in 47.3% (0.50 g scale) yield. Upon complexation
with the silver(I) source, the water-insoluble and organic
solvent-insoluble H3imdc ligand dissolved in the mixed or-
ganic media. The synthetic reaction of 1 is shown in Eq. (1).
In this work, the novel tetranuclear silver(I) cluster [Ag2-
(Himdc)(PPh3)2]2 1 (H3imdc = imidazole-4,5-dicarboxylic
acid) with a ‘‘bivalve’’-like skeleton was obtained in the
2½AgðR-HpyrrldÞꢀ2 þ 4PPh3 þ 2H3imdc
! ½Ag2ðHimdcÞðPPh3Þ2ꢀ2 þ 4R-H2pyrrld
ð1Þ
*
In Eq. (1), by changing the amounts of PPh3 and H3
imdc, the products were changed: the 1:3:2 molar-ratio
Corresponding author.
E-mail address: nomiya@chem.kanagawa-u.ac.jp (K. Nomiya).
1387-7003/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2005.09.012

108
R. Noguchi et al. / Inorganic Chemistry Communications 9 (2006) 107–110
reaction gave 2 as well as 1 (0.35 g scale combined yield of
1 + 2), the 1:4:2 molar-ratio and 1:6:2 molar-ratio reac-
tions gave only 2 in 59.0% (0.93 g scale) yield and in
54.8% (0.25 g scale) yield, respectively. Complex 1 was also
obtained by a reaction using the racemic precursor [Ag2 (R-
Hpyrrld)(S-Hpyrrld)] in 26.0% (0.12 g scale) yield, showing
that chirality of the silver(I) precursor does not influence
the product. Complex 1 was also obtained in 66.9%
(0.30 g scale) yield from the 1:1 molar-ratio reaction of
the separately prepared intermediate [Ag2(R-Hpyrrld)2
(H2O)(PPh3)2] [15] with H3 imdc. As a matter of fact,
the Ag–O bonding complex [Ag(Hpyrrld)]2 can be con-
verted to several triphenylphosphinesilver(I) complexes,
i.e., [Ag2(R-or S-Hpyrrld)2(H2O)(PPh3)2]H2O, [Ag(R or
S-Hpyrrld)(PPh3)2]2, [Ag(R, S-Hpyrrld)(PPh3)]2 and [Ag-
(R, S-Hpyrrld)(PPh3)2], which comprises different Ag–O
bonding modes, depending on the number of PPh3 ligands
and the chirality of the Hpyrrldꢁ ligand [15]. On the other
hand, complex 2 was also obtained in 40.9% (0.32 g scale)
yield from the 1:2 molar-ratio reaction of another PPh3-
containing intermediate [Ag(R-Hpyrrld)(PPh3)2]2 [15] with
H3 imdc. Complex 2 was not obtained from 1:4 molar-ratio
reaction of 1 with PPh3. Thus, the formation of 1 and 2
proceeds via the different PPh3-coordinated intermediates
[15], and the Hpyrrldꢁ ligand would provide only an
appropriate reaction field for the silver(I) complex forma-
tion, since this ligand eventually disappeared from the final
products [16,17]. Complex 2 was also given by a reaction of
the separately prepared, polymeric species [Ag(H2imdc)]n 3
[18] with PPh3, but complex 1 was not formed by such a
reaction. Complexes 1 and 2 were not derived directly from
silver(I) salt, H3imdc and PPh3, i.e., from the system with-
out Hpyrrldꢁ ligand. Thus, the use of the precursor [Ag(H-
pyrrld)]2 is crucial in the formation of 1. The composition
and molecular formulas of 1 and 2 were consistent with
elemental analysis, TG/DTA, FTIR, solution (1H, 13C
and 31P) NMR, solid-state 31P CPMAS NMR and X-ray
crystallorgraphy.
Fig. 1. Molecular structure of 1 with a ‘‘bivalve’’-like skeleton (symmetry
operation i = ꢁx+2, ꢁy, ꢁz+2), in which hydrogen atoms were omitted
and the PPh3 ligands were simplified for clarity. Selected interatomic
i
˚
distances (A) and angles (ꢂ): Ag1–N1 2.183(2), Ag1–O1 2.535(2), Ag1-O3
2.5639(19), Ag1–P1 2.3259(11), Ag2–N3 2.171(2), Ag2–O3 2.577(2), Ag2–
i
˚
P2 2.3467(11) A; N1–Ag1–O1 71.75(6), N1–Ag1–O3 94.75(7), N1–Ag1–
P1 141.32(5), O1–Ag1–O3i 89.72(6), O3i–Ag1–P1 117.36(5), O1–Ag1–P1
125.53(5), N3–Ag2–O3 71.09(6), O3–Ag2–P2 135.03(4), N3–Ag2–P2
153.30(6)ꢂ.
Fig. 2. Crystal and molecular structures of 2. Selected interatomic
˚
distances (A) and angles (ꢂ): Ag1–N3 2.337(3), Ag1–O1 2.497(3), Ag1–
P1 2.4420(16), Ag1–P2 2.4065(15), O1. . .N1i 2.707 A; N3–Ag1–P1
˚
106.60(8), N3–Ag1–P2 11508(8), N3–Ag1–O1 69.37(10), P1–Ag1–P2
133.90(5), O1–Ag1–P1 98.79(6), O1–Ag1–P2 113.28(7)ꢂ.
complex 2 also showed the intermolecular hydrogen-bond-
ing interaction between the carboxyl oxygen atom and the
imidazole nitrogen atom of the neighboring complex
i
˚
(O1 ꢂ ꢂ ꢂ N1 2.707 A).
The solid-state 31P CPMAS NMR spectra showed phos-
phorus resonances of the PPh3 ligand coordinating to the
silver(I) atom as two doublet peaks (four lines) for 1 and
as two double doublet peaks (six lines) for 2. These splitting
peaks are due to 1J(Ag–P) and 2J(P–P) couplings, and these
results are consistent with the solid-state structures re-
vealed by X-ray crystallography.
X-ray structure analysis revealed the tetranuclear sil-
ver(I) cluster 1 (Fig. 1) and mononuclear silver(I) complex
2 (Fig. 2) [19]. The molecular structure of 1 was an Ag4
cluster with S2 symmetry, which was constructed with
two 4-coordinated AgO(OꢀN)P units (Ag1, Ag1i) and
two 3-coordinated Ag(OꢀN)P units (Ag2, Ag2i). The dian-
ionic ligand, Himdc2ꢁ, coordinated to the silver(I) center
via chelation of the carboxyl oxygen and aromatic nitrogen
atoms. In 1, there is no stacking interaction between two
inverted imidazole units. Two unequivalent silver(I) atoms
present in 1 are based on their different coordination num-
bers arisen from bridging by one of the two carboxyl
groups in the [Ag2(Himdc)(PPh3)2] unit. On the other
hand, the distorted 4-coordinate complex 2, formed with
chelation of the monoanionic ligand H2 imdcꢁ (N3–Ag1–
O1 69.37(1), P1–Ag1–P2 133.90(5)ꢂ) and monodentate
coordination of two PPh3 ligands, contained the intramo-
lecular hydrogen bonding interaction within the two car-
Solution 31P NMR in CD2Cl2 of 1 showed two 31P res-
onances at 15.9 and 19.9 ppm on the basis of coordination
to the two unequivalent silver(I) atoms, suggesting that the
solid-state Ag4 cluster will be maintained in solution. On
the other hand, solution 31P NMR in CD2Cl2 of 2 showed
only one 31P resonance at 10.3 ppm, the chemical shift of
which was consistent with those of the complexes contain-
ing two PPh3 ligands per silver(I) atom [15].
The molecular structure of 1 is quite different from those
of the previously reported Ag4 clusters formed with the S-
donor atom and PPh3 ligand such as [Ag(2-Hmba)(PPh3)]4
[10], [Ag(SPh)(PPh3)]4 (SPhꢁ = benzenethiolate) [20] and
[Ag(l-SC{O}R)(PPh3)]4 (R = Me, Ph) [21]. Unique Ag4
˚
boxyl groups (O4 ꢂ ꢂ ꢂ O2 2.477 A). In the solid state,

R. Noguchi et al. / Inorganic Chemistry Communications 9 (2006) 107–110
109
in water, light petroleum, acetone, MeOH, EtOAc, acetonitrile, Et2O.
This complex is thermally- and light-stable both in solution and in the
solid-state. Anal. Found: C, 54.80; H, 3.40; N, 3.18. Calcd. for
cluster 1 can be also compared with other Ag4 clusters
formed with various donor atoms [22–29].
Finally, the present Ag–O bonding precursor would be
useful for formation of novel silver(I) clusters, in the place
of the traditional silver(I) sources such as AgNO3, Ag2O,
AgCH3CO2 and so on. In fact, we have very recently
obtained a novel tetranuclear silver(I) cluster [Ag(2-Hmba)
(PPh3)]4 with a three-leaves propeller (C3 symmetry) in the
1:2:2 molar-ratio reaction of [Ag2(R-Hpyrrld)(S-Hpyrrld)],
PPh3 and 2-H2mba in a 1:4 mixed CHCl3/EtOH solvent
[10]. By the AgCl elimination reaction in CHCl3 between
[AuCl(PPh3)] and [Ag(Hpyrrld)]2, the novel, light-stable
gold(I) complex consisting of the hard O donor and soft
P donor atoms, [Au(Hpyrrld)(PPh3)]CHCl3 with high pur-
ity has been successfully obtained in good yield [14].
C
82H64N4O8P4Ag4 or [Ag2(Himdc)(PPh3)2]2: C, 55.06; H, 3.61; N,
3.13%. TG/DTA data: no weight loss was observed before decom-
position. Decomposition gradually began around 217 ꢂC with an
endothermic peak at 255 ꢂC. Prominent IR bands at 1800–400 cmꢁ1
region (KBr disk): 1585m, 1559s, 1519s, 1480vs (PPh3), 1435vs
(PPh3), 1407m, 1246m, 1097s (PPh3), 997w, 754m (PPh3), 745m
(PPh3), 692s (PPh3), 522m (PPh3), 505s (PPh3), 491m cmꢁ1. 1H NMR
(CD2Cl2, 23.6 ꢂC): d 7.47–7.61 (62 H, m, Aryl and H2). 13C NMR
(CD2Cl2, 24.0 ꢂC): d 129.5 (JCP 10.8 Hz, Ph), 130.7 (JCP 39.8 Hz, Ph),
131.4 (Ph), 134.2 (JCP 15.8 Hz, Ph), 135.7 (C4, C5), 144.2 (C2), 168.2
(CO2). 31P NMR (CD2Cl2, 23.4 ꢂC): d 15.9, 19.9. 31P NMR (CD2Cl2,
ꢁ80.3 ꢂC): d 16.4 (JAg–P 689.3 Hz), 16.7 (JAg–P 688.4 Hz). Solid-state
31P CPMAS NMR (substitution method with (NH4)2HPO4, 25.0 ꢂC):
d 15.3 (JAg–P 669.3 Hz), 18.7 (JAg–P 699.7 Hz).
[17] 2: It was prepared by a 1:4:2 molar-ratio reaction of [Ag(R-Hpyrrld)]2
(0.472 g, 1.00 mmol), PPh3 (1.05 g, 4.00 mmol) and H3imdc (0.312 g,
2.00 mmol) with the similar work-up described above. Colorless plate
crystals of 2 obtained in 59.0% (0.93 g scale) yield were soluble in
CHCl3 and CH2Cl2, and sparingly soluble in EtOH, and insoluble in
water and Et2O. Anal. Found: C, 62.27; H, 4.01; N, 3.61. Calc. for
C41H33N2P2O4Ag or [Ag(H2imdc)(PPh3)2]: C, 62.53; H, 4.22; N,
3.56%. TG/DTA data: no weight loss was observed before decom-
position. Decomposition gradually began around 219 ꢂC with endo-
thermic peaks at 251, 257 and 267 ꢂC. Prominent IR bands at 1800–
400 cmꢁ1 region (KBr disk): 1703w, 1560s, 1509s, 1496s, 1480s,
1435vs (PPh3), 1373s, 1311w, 1239m, 1095s (PPh3), 1026w, 998w,
836w, 753m (PPh3), 742s (PPh3), 693vs (PPh3), 657w, 514s (PPh3),
Acknowledgments
This work was supported by a Grant-in-Aid for Scien-
tific Research and also by a High-tech Research Center
Project, both from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
References
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H2O was added 0.156 g (1.00 mmol) of H3imdc, followed by stirring for
2 h. A white powder was formed, which was collected on a membrane
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C5H3.5N2O4.25Ag or [Ag(H2imdc)]0.25H2O: C, 22.45; H, 1.32; N,
10.47%. TG/DTA data: decomposition gradually began around 220 ꢂC
with an endothermic peak at 294 ꢂC and an exothermic peak at 365 ꢂC.
Prominent IR bands at 1700–400 cmꢁ1 region (KBr disk): 1602vs,
1559vs, 1519vs, 1469s, 1427s, 1388vs, 1317w, 1243m, 1166m, 1084m,
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clinic, space group P2(1)/n, a = 13.475(8), b = 12.395(7), c =
[16] 1: To a suspension of 0.472 g (1.00 mmol) of [Ag(R-Hpyrrld)]2
dispersed in 20 ml of 1:4 (v/v) solvent mixture of CH2Cl2 and EtOH
was added 0.524 g (2.00 mmol) of PPh3. After stirring the turbid
solution for 30 min, 0.156 g (1.00 mmol) of H3imdc was added. After
stirring for 1 h, the solution was filtered through a folded filter paper
(Whatman #5). Along an interior wall of the beaker containing the
clear colorless filtrate was slowly spread 85 ml of light petroleum. The
beaker containing the two phases consisting of the filtrate in the lower
layer and the light petroleum in the upper layer was sealed and stood
in the dark. After one day, clear colorless plate crystals deposited,
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vacuo for 2 h. Colorless plate crystals, obtained in 47.3% (0.50 g scale)
yield, were soluble in EtOH, DMSO, CH2Cl2, CHCl3, and insoluble
3
˚
˚
22.511(13) A, b = 105.424(7)ꢂ, V = 3624(4) A , Z = 2, Dcalcd.
=
1.639 g cmꢁ3
, , 38492 reflections collected, 8689
l = 1.214 mmꢁ1
independent (Rint = 0.0497), R1 = 0.0319, wR2 = 0.0792 for I > 2r
(I), R1 = 0.0397, wR2 = 0.0820, GOF = 1.023 for all data. Crystal
data for 2:C41H33N2P2O4Ag; M = 787.50, monoclinic, space group
˚
C2/c, a = 19.509(13), b = 13.630(9), c = 27.695(18) A, b = 92.544(9)ꢂ,
3
V = 7357(8) A , Z = 8, Dc = 1.422 g cmꢁ3, l = 0.679 mmꢁ1, 38574
˚
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wR2 = 0.1313 for I > 2r (I), R1 = 0.0667, wR2 = 0.1413, GOF =
1.009 for all data. The details of the crystal data have been deposited
with Cambridge crystallographic data centre as supplementary
publication no. CCDC 270002 for 1 and CCDC 270003 for 2.

110
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