Mononuclear, dinuclear, hexanuclear, and one-dimensional polymeric silver complexes having ligand-supported and unsupported argentophilic interactions stabilized by pincer-like 2,6-bis(5-pyrazolyl)pyridine ligands

Article abstract of DOI:10.1039/b710916d

The mononuclear complexes [Ag(H₂L¹)(Py) ₂](NO₃)·H₂O (1, H₂L ¹ = 2,6-bis(5-methyl-1H-pyrazol-3-yl)pyridine) and [Ag(NO ₃)(L²)] (2, L² = 2,6-bis(5-

Full text of DOI:10.1039/b710916d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Mononuclear, dinuclear, hexanuclear, and one-dimensional polymeric silver
complexes having ligand-supported and unsupported argentophilic interactions
stabilized by pincer-like 2,6-bis(5-pyrazolyl)pyridine ligands†‡
Yongbo Zhou,a Wanzhi Chen*a and Daqi Wangb
Received 27th July 2007, Accepted 13th December 2007
First published as an Advance Article on the web 23rd January 2008
DOI: 10.1039/b710916d
The mononuclear complexes [Ag(H2L1)(Py)2](NO3)·H2O (1, H2L1 = 2,6-bis(5-methyl-1H-pyrazol-
3-yl)pyridine) and [Ag(NO3)(L2)] (2, L2 = 2,6-bis(5-methyl-1-isopropyl-1H-pyrazol-3-yl)pyridine),
dinuclear complex [Ag2(H2L3)2(HL4)2] (3, H2L3 = 2,6-bis(5-phenyl-1H-pyrazol-3-yl)pyridine, HL4 =
6-(5-phenyl-1H-pyrazolyl-3-yl)picolinate), one-dimensional polymer {[Ag2(H2L1)2](NO3)2·H2O}n (4),
and hexanuclear clusters [Ag6(HL1)4](X)2 (X = NO3−, 5; BF4−, 6; ClO4−, 7) stabilized by pincer-like
bispyrazolyl ligands have been prepared and characterized using 1H NMR spectroscopy, elemental
analysis, IR spectroscopy, luminescence spectroscopy and X-ray diffraction. In complex 3, there is a
ligand unsupported Ag–Ag bond between the two silver atoms. Complex 4 displays a one-dimensional
polymer consisting of an infinite Ag–Ag chain and every two adjacent silver ions are bridged by an
H2L1 ligand. Complexes 5 and 7 have the same Ag6 cores in which six silver atoms are held together by
four HL1 and five Ag–Ag bonds, while complex 6 was held together by six Ag–Ag bonds. The
˚
silver–silver distances in these complexes are found in the range of 2.874(1)–3.333(2) A for ligand
˚
supported, and 3.040(1) A for ligand unsupported Ag–Ag bonds, respectively. Complexes 3–7 are
strongly luminescent due to either intraligand or metal–ligand charge transfer processes.
silver–silver interactions have been calculated to be relatively
Introduction
weaker.10 However, there are still many examples of argentophilic
interactions; most are ligand supported,5 while few are ligand-
unsupported.6
Direct metal–metal interactions are important because they are
often associated with many potentially useful chemical and
physical properties of materials such as catalytic behavior, or
magnetic, optical, or electronic properties.1 The attractive inter-
actions between closed-shell and pseudo closed-shell d10, s2, and
d8 metal centers have been becoming familiar and are termed
as metallophilicity.1e,2 The attraction between the closed-shell
metals promotes the aggregation of the metal centers having d10
d8, s2 configurations, which has been supported by spectroscopic
and structural evidence.2c,3 The weak interactions often lead to
materials with interesting properties and structural motifs.
Metallophilicity of an order-of-magnitude comparable to hy-
drogen bonds in the case of gold or weaker for other metals
has been used as an element for the construction of various
coordination polymers. Many metallic aggregates and coordina-
tion polymers containing Au–Au,4 Ag–Ag5,6, Ag–Au7 and Cu–
Cu8 bonds have recently been reported. The strengths of the
interactions have an order-of-magnitude comparable to those
of hydrogen bonds.2a,9 Compared with gold–gold interactions,
Pyrazolyl ligands are a kind of multifunctional organic lig-
and that often display an exo-bidentate coordination mode.
The structure and properties of closed-shell d10 coinage metal
pyrazolates are of significant interest.11–16 Numerous closed-
shell d10 coinage metal complexes with a variety of struc-
tures stabilized by monopyrazole ligands ranging from dimers,12
trimers,13 tetramers,13h,14 hexamers7b,c,15 to polymers13l,14d,16 have
been reported. When silver(I) pyrazolate is considered, sev-
eral trimers,6b,13k,n two tetramers,13h,14h and a one-dimensional
polymer13l have been structurally characterized using X-ray
diffraction. However, hexanuclear or higher nuclearity clusters are
not known so far.14h
Bispyrazolyl ligands, as a kind of potentially anionic multi-
dentate linker, would be expected to be suitable candidates to
connect transition metals into aggregates. Surprisingly, coinage
metal complexes stabilized by bispyrazolyl ligands have scarcely
been reported. To our knowledge, only a flexible porous silver(I)
coordination polymer17a and a copper(I) coordination polymer17b
supported by 3,3ꢀ,5,5ꢀ-tetramethyl-4,4ꢀ-bipyrazole have been re-
cently reported.
aDepartment of Chemistry, Zhejiang University, Xixi campus, Hangzhou
310028, China. E-mail: chenwzz@zju.edu.cn
bDepartment of Chemistry, Liaocheng University, Liaocheng 252059, China
We have been interested in the synthesis and reactivity of
multinuclear complexes and coordination polymers, and a number
† Structural parameters of the complexes 1–7. CCDC reference numbers
655894–655901. For crystallographic data in CIF or other electronic
format, see DOI: 10.1039/b710916d
‡ Electronic supplementary information (ESI) available: Molecular struc-
tures of H2L1, 2, 6 and 7, and emission spectral data for 3–7. See DOI:
10.1039/b710916d
of complexes containing Pt–Pt,18a Pt–Tl,18b,c Pt–Ag,18d Au–Au18e
,
and Ag–Ag18f interactions have been recently presented. As an
continuation of our studies on metal–metal interactions, in this
paper we report the synthesis and characterization of a family
1444 | Dalton Trans., 2008, 1444–1453
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of interesting silver complexes and a coordination polymer with
ligand-supported and ligand-unsupported silver–silver interac-
tions. These complexes are stabilized by the pincer-like bispyra-
zolyl ligands, 2,6-bis(5-methyl-1H-pyrazol-3-yl)pyridine (H2L1),
2,6-bis(5-methyl-1-isopropyl-1H-pyrazol-3-yl)pyridine (L2), 2,6-
bis(5-phenyl-1H-pyrazol-3-yl)pyridine (H2L3), and 6-(5-phenyl-
1H-pyrazolyl-3-yl)picolinate (HL4) generated in situ (see ESI‡).
mixture was stirred until all sodium was consumed. Ethanol was
eliminated in vacuo. To the freshly prepared sodium ethoxide,
acetophenone (4.5 mL) in dry toluene (30 mL) was added,
followed quickly by dimethyl pyridine-2,6-dicarboxylate (2.50 g,
12.8 mmol). The mixture was stirred for 8 h and a yellowish sodium
salt precipitated out of the solution. After stirring at 60 ◦C for
4 h, the mixture was filtered and washed with diethyl ether before
drying. The dry solid was slowly added to a vigorously stirred
solution of acetic acid (15 mL), water (25 mL), and ice (50 g), and
the resulting solid was collected by filtration. The yellow solid was
dissolved in CH2Cl2 (150 mL) and filtered. The resulting solution
was washed with aqueous sodium bicarbonate solution (6%, 2 ×
30 mL) and distilled water (30 mL), then dried over anhydrous
magnesium sulfate. A yellow solid was obtained after removal
of CH2Cl2. Yield: 3.3 g, 69.9%. Anal. calcd for C23H17NO4: C,
74.38; H, 4.61; N, 3.77. Found: C, 74.42; H, 4.68; N, 3.75%.
1H NMR (DMSO-d6): d 16.54 (br), 16.50 (br), 8.29–8.21 (m),
8.10–8.07 (m), 7.72 (s), 7.69 (t), 7.61–7.53 (m), 7.19 (s), 4.97 (s),
4.50 (s). 13C NMR (100 MHz, DMSO-d6): d 196.4, 195.8, 185.5,
183.0, 182.9, 151.5, 151.4, 139.8, 136.2, 134.4, 133.9, 133.4, 129.1,
129.0, 128.4, 127.5, 127.0, 125.6, 124.9, 124.4, 93.9, 92.9, 49.4,
41.2. MS (m/z): 371 (M+), 266, 224, 105 (100%), 77. IR data
(KBr pellet, cm−1): 3743 w, 3085 w, 1610 s, 1565 s, 1550 s, 1485 s,
1304 m, 1262 m, 1230 s, 1070 s, 837 w, 805 m, 774 s, 685 m,
618 m.
Experimental
All chemicals were of reagent grade quality obtained from
commercial sources and used as received, unless stated otherwise.
1,1ꢀ-(2,6-Pyridyl)bis-1,3-butanedione19a and 2,6-bis(5-methyl-1H-
pyrazol-3-yl)pyridine19b,c (H2L1) were prepared according to the
reported procedures. 1H NMR (400 MHz) and 13C NMR
(100 MHz) spectra were recorded on a Bruker Avance (400 MHz)
spectrometer. Elemental analyses were determined with a Perkin-
Elmer 2400C instrument. IR spectra were measured as KBr pellets
using a Nicolet 5DX FX-IR spectrophotometer. Mass spectra
(EI, 70 eV) were recorded on a HP5989B mass spectrometer. The
photoluminescence study was carried out on powdered samples in
the solid-state at room temperature using a SHIMADZU RF-540
spectrometer.
Preparations
2,6-Bis(5-phenyl-1H-pyrazol-3-yl)pyridine (H2L3). A mixture
of 2,6-bis(1ꢀ-phenyl-1ꢀ,3ꢀ-dioxopropyl)pyridine (3.71 g, 10 mmol),
hydrazine hydrate (85%, 10 mL), and a few drops of concentrated
HCl in methanol (200 mL) was refluxed for 36 h. The resulting
solution was concentrated to ca. 20 mL. Addition of H2O (50 mL)
to the above solution resulted in a pale yellow precipitate. The
product was purified by recrystallization from CH3CH2OH and
CH2Cl2 (1 : 1). Yield: 2.1 g, 57.2%. Anal. calcd for C23H17N5: C,
76.01; H, 4.71; N, 19.27. Found: C, 75.87; H, 4.82; N, 19.32%. 1H
NMR (400 MHz, DMSO-d6): d 13.61 (br, 2H), 8.04 (t, J = 7.6,
1H), 7.92 (d, J = 8.0, 4H), 7.82 (d, J = 7.6, 2H), 7.52 (s, 2H),
7.46 (t, J = 7.6, 4H), 7.34 (t, J = 7.6, 2H). 13C NMR (100 MHz,
DMSO-d6): d 152.1, 147.5, 143.7, 143.1, 139.4, 133.9, 129.5, 129.2,
128.7, 128.1, 125.6, 118.9, 101.2. MS (m/z): 363 (M+, 100%), 334,
304, 239, 190, 129, 97, 71. IR data (KBr pellet, cm−1): 3200 s, 3030
m, 1602 m, 1566 s, 1453 s, 1313 m, 1219 m, 1186 m, 1156 m, 1074
m, 810 m, 765 s, 691 s.
2,6-Bis(5-methyl-1H-pyrazol-3-yl)pyridine hemihydrate (H2L1·
0.5H2O). 2,6-Bis(5-methyl-1H-pyrazol-3-yl)pyridine was pre-
pared from 2,6-bis(1,3-dioxobutyl)pyridine and hydrazine hydrate.
Anal. calcd for C13H14N5O0.5: C, 62.89; H, 5.68; N, 28.21. Found:
C, 63.07; H, 5.81; N, 28.37%. 1H NMR (CDCl3): d 12.57 (br, 2H),
7.54 (t, J = 7.6, 1H), 7.25 (d, J = 7.6, 2H), 6.27 (s, 2H), 2.18 (s,
6H). 13C NMR (100 MHz, DMSO-d6): d 149.5, 149.0, 141.9, 139.7,
121.0, 103.5, 10.8. MS (m/z): 239 (M+, 100%), 210, 167, 154, 141,
129, 77. IR data (KBr pellet, cm−1): 3185 s, 3127 s, 2976 s, 2927 s,
1574 s, 1475 s, 1302 s, 1160 s, 1007 s, 804 s, 691 w.
2,6-Bis(5-methyl-1-isopropyl-1H-pyrazol-3-yl)pyridine (L2).
A
mixture of 2,6-bis(5-methyl-1H-pyrazol-3-yl)pyridine (0.48 g,
2 mmol) and 60% NaH (0.32 g, 8 mmol) in dry DMF (15 mL) was
stirred for 2 h at room temperature. To the solution was added
2-bromopropane (0.98 g, 8 mmol). After stirring at 60 ◦C for
two days, the resulting solution was concentrated to 3 mL. Addi-
tion of H2O (15 mL) precipitated a pale yellow powder. Column
chromatography involved elution with ethyl acetate–petroleum
ether (1 : 5) which separated the compound as a white powder
(0.40 g). Yield: 61.0%. Anal. calcd for C19H25N5: C, 70.56; H, 7.79;
N, 21.65. Found: C, 70.52; H, 7.82; N, 21.57%. 1H NMR (CDCl3):
d 7.82 (d, J = 7.6, 2H), 7.68 (t, J = 7.6, 1H), 6.72 (s, 2H), 4.45 (m,
2H), 2.33 (s, 6H), 1.52 (d, J = 6.8, 12H). 13C NMR (100 MHz,
CDCl3): d 152.2, 150.4, 138.1, 136.7, 118.0, 104.0, 49.9, 22.5, 11.1.
MS (m/z): 323 (M+, 100%), 294, 279, 239, 210, 129, 77. IR data
(KBr pellet, cm−1): 2977 s, 2931 s, 1595 s, 1572 s, 1548 m, 1505
m, 1432 s, 1403 m, 1345 m, 1256 s, 990 m, 825 w, 792 s, 742 w,
694 w.
[Ag(H2L1)(Py)2](NO3)·H2O (1).
A solution of 2,6-bis(5-
methyl-1H-pyrazol-3-yl)pyridine (H2L1) (48 mg, 0.2 mmol) and
AgNO3 (34 mg, 0.2 mmol) in pyridine (5 mL) was stirred in the
dark at room temperature for 24 h. Then the mixture was filtered
through a short plug of Celite. Slow evaporation of diethyl ether
into the filtrate afforded colorless crystals suitable for X-ray single
diffraction. Yield: 87.1%. Anal. calcd for C23H25AgN8O4: C, 47.19;
H, 4.30; N, 19.14. Found: C, 47.02; H, 4.42; N, 19.26%. 1H NMR
(400 MHz, DMSO-d6): d 13.21 (s, 2H), 8.59 (d, J = 7.8, 4H), 8.05
(t, J = 8.0, 1H), 7.88 (d, J = 7.2, 2H), 7.84 (t, J = 7.8, 2H), 7.43
(t, J = 7.8, 4H), 6.86 (s, 2H), 2.30 (s, 6H). 13C NMR (100 MHz,
DMSO-d6): d 150.8, 149.8, 149.3, 142.0, 139.9, 137.9, 125.0, 120.9,
103.6, 11.0. IR data (KBr pellet, cm−1): 3446 s, 3200 s, 3138 m,
2985 w, 2929 w, 1578 s, 1501 m, 1382 s, 1323 m, 1227 w, 1160 m,
1019 m, 790 s, 735 w.
2,6-Bis(1ꢀ-phenyl-1ꢀ,3ꢀ-dioxopropyl)pyridine. Under a dinitro-
gen atmosphere, to a 100 cm3 Schlenk flask charged with sodium
(0.77 g, 33.4 mmol), dry ethanol (15 mL) was added and the
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[Ag(L2)(NO3)] (2). A solution of 2,6-bis(5-methyl-1-isopropyl-
1H-pyrazol-3-yl)pyridine (L2) (65 mg, 0.2 mmol) and AgNO3
(34 mg, 0.2 mmol) in THF (2 mL) was stirred, in darkness, for
ca. two days. The resulted precipitate was filtered and washed
with diethyl ether. Slow evaporation of diethyl ether into its clear
DMF solution yielded colorless crystals suitable for X-ray single
diffraction. Yield: 64.8%. Anal. calcd for C19H25AgN6O3: C, 46.26;
H, 5.11; N, 17.04. Found: C, 46.39; H, 5.20; N, 17.25%. 1H NMR
(400 MHz, DMSO-d6): d 8.08 (t, J = 7.6, 1H), 7.89 (d, J = 8.0,
2H), 6.82 (s, 2H), 4.34 (br, 2H), 2.24 (s, 6H), 0.99 (s, 12H). 13C
NMR (100 MHz, CDCl3): d 152.8, 150.7, 138.4, 136.9, 117.9,
103.9, 49.8, 22.4, 11.0. IR data (KBr pellet, cm−1): 2974 s, 2930
m, 1600 m, 1575 s, 1495 m, 1410 s, 1384 s, 1293 s, 1260 m, 803 m,
731 w.
calcd for C52H48Ag6B2F8N20: C, 35.21; H, 2.73; N, 15.79. Found:
C, 35.46; H, 2.82; N, 15.92%. IR data (KBr pellet, cm−1): 3354 s,
3144 m, 2928 w, 1577 s, 1477 m, 1438 m, 1384 m, 1293 w, 1082 s,
792 s.
[Ag6(HL1)4](ClO4)2·Et2O (7). Complex 7 was prepared by an
analogous method to that used for 6 from 2,6-bis(5-methyl-1H-
pyrazol-3-yl)pyridine (H2L1) (48 mg, 0.2 mmol) and AgClO4
(83 mg, 0.4 mmol) in CH3CN (8 mL). Yield: 42.2%. Anal. calcd for
C56H58Ag6Cl2N20O9: C, 35.90; H, 3.12; N, 14.95. Found: C, 35.73;
H, 3.21; N, 15.12%. IR data (KBr pellet, cm−1): 3287 s, 3145 m,
2924 w, 1575 s, 1481 m, 1434 m, 1383 m, 1115 s, 791 s.
X-Ray crystallography
[Ag2(H2L3)2(HL4)2] (3). A mixture of 2,6-bis(5-phenyl-1H-
pyrazol-3-yl)pyridine (H2L3) (36 mg, 0.1 mmol) and AgOAc
(35 mg, 0.1 mmol) in 20 mL of water in a sealed stainless vial
was heated at 170 ◦C for 48 h. Cooling the vial slowly afforded
colorless crystals. Yield: 32%. Anal. calcd for C76H54Ag2N16O4: C,
62.05; H, 3.70; N, 15.23. Found: C, 62.17; H, 3.83; N, 15.31%.
IR data (KBr pellet, cm−1): 3246 s, 3062 s, 1656 s, 1596 s,
1569 s, 1452 s, 1217 m, 1117 m, 1076 m, 809 m, 757 s, 693 m,
652 w.
Single-crystal X-ray diffraction data for the complexes were
collected at 298(2) K on a Siemens Smart/CCD area-detector
˚
diffractometer with MoKa radiation (k = 0.71073 A) using an
x-2h scan mode. Unit-cell dimensions were obtained with least-
squares refinement. Data collection and reduction were performed
using the SMART and SAINT software.20 All structures were
solved by direct methods and refined against F2 by the full-
matrix least squares techniques.21a All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were introduced in their
calculated positions. Disordered solvent molecules for 6 and
7 could not be modeled successfully and were removed from
[Ag2(H2L1)2](NO3)2·H2O (4). A solution of 2,6-bis(5-methyl-
1H-pyrazol-3-yl)pyridine (H2L1) (48 mg, 0.2 mmol) and AgNO3
(34 mg, 0.2 mmol) in DMF (5 mL) was stirred at room temperature
in the dark for ca. two days, then it was filtered through a short
plug of Celite. To the resulting clear solution, diethyl ether was
added, and the resulting precipitate was filtered and washed with
acetone. Complex 4 was obtained as a white powder. Yield: 78.6%.
Crystals suitable for X-ray single diffraction were obtained from
slow evaporation of diethyl ether into its DMF solution. Anal.
calcd for C26H28Ag2N12O7: C, 37.34; H, 3.37; N, 20.10. Found: C,
their reflection data with SQUEEZE21b (total potential solvent
3
˚
accessible void volume 270.0 A for 6 and solvent accessible
3
1
˚
void volume 118.00 A for 7). The structure of H2L ·0.5H2O was
checked for missing symmetry using the ADDSYM function of
PLATON,21b and this treatment resulted in a new space group
Pbna replacing the original space group Pnna. Its water hydrogen
cannot be located in a sensible position and has been left out
of the refinement model. Details of the X-ray experiments and
crystal data are summarized in Table 1. Selected bond distances
and angles are given in Tables 2–4.
1
37.39; H, 3.44; N, 19.94%. H NMR (400 MHz, DMSO-d6): d
13.18 (s, 2H), 8.07 (t, J = 8.0, 1H), 7.88 (d, J = 8.0, 2H), 6.86 (s,
2H), 2.30 (s, 6H). 13C NMR (100 MHz, DMSO-d6): d 149.8, 149.4,
142.2, 140.0, 121.2, 103.7, 11.0. IR data (KBr pellet, cm−1): 3525 s,
3189 s, 3139 s, 2981 s, 1578 s, 1502 m, 1437 m, 1370 s, 1321 s, 1282
m, 1021 m, 787 s.
Results and discussion
Synthesis
[Ag6(HL1)4](NO3)2·(CH3)2CO (5). A solution of 4 (84 mg,
0.1 mmol) and AgNO3 (34 mg, 0.2 mmol) in DMF (10 mL) and
acetone (0.3 mL) was stirred at room temperature in the dark
for ca. two days, then it was filtered off through a short plug of
Celite. Diethyl ether was slowly evaporated into the resulting clear
solution, and colorless crystals suitable for X-ray single diffraction
were obtained after several days. Yield: 68.2%. Anal. calcd for
C55H54Ag6N22O7: C, 37.06; H, 3.05; N, 17.29. Found: C, 36.89; H,
3.06; N, 17.39%. IR data (KBr pellet, cm−1): 3198 s, 3136 s, 2922 s,
1672 s, 1573 s, 1483 m, 1384 s, 1372 s, 1275 m, 1150 m, 1097 m,
1018 m, 811 m, 791 s.
1,1ꢀ-(2,6-Pyridyl)bis-1,3-butanedione and ligand 2,6-bis(5-methyl-
1H-pyrazol-3-yl)pyridine (H2L1) were synthesized according to
the known procedures. As shown in Scheme 1, 2,6-bis(1ꢀ-phenyl-
1ꢀ,3ꢀ-dioxopropyl)pyridine was prepared from dimethyl pyridine-
2,6-dicarboxylate, acetophenone, and freshly prepared EtONa.
2,6-Bis(5-phenyl-1H-pyrazol-3-yl)pyridine (H2L3) was prepared
from the condensation reaction of the 2,6-bis(1ꢀ-phenyl-1ꢀ,3ꢀ-
dioxopropyl)pyridine with hydrazine hydrate. The pincer lig-
and 2,6-bis(5-methyl-1-isopropyl-1H-pyrazol-3-yl)pyridine (L2)
was prepared by using 2,6-bis(5-methyl-1H-pyrazol-3-yl)pyridine,
NaH, and 2-bromopropane. All the compounds were character-
ized by elemental analyses, IR, MS, 1H NMR and 13C NMR
spectroscopy. 1H NMR spectra of 2,6-bis(diketonyl)pyridine com-
pounds are complicated since several tautomers exist in their
solutions including bis(keto-enol), enol-enol, bis(keto), and keto-
(keto-enol). The broad signals at ca. 16 ppm are assignable to the
enol OH protons. The resonance signals of acidic NH protons
appear at 12.6 and 13.6 ppm for H2L1 and H2L3 respectively.
[Ag6(HL1)4](BF4)2 (6). A solution of 2,6-bis(5-methyl-1H-
pyrazol-3-yl)pyridine (H2L1) (48 mg, 0.2 mmol) and AgBF4
(78 mg, 0.4 mmol) in DMF (8 mL) was stirred at room temperature
in the dark for ca. two days, then it was filtered through a short plug
of Celite. Diethyl ether was slowly evaporated into the resulting
clear solution, and colorless crystals suitable for X-ray single
diffraction were obtained after several days. Yield: 34.3%. Anal.
1446 | Dalton Trans., 2008, 1444–1453
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As shown in Schemes 2 and 3, the reactions of H2L1 in
pyridine and L2 with silver nitrate in THF in 1 : 1 metal-to-
ligand ratios yielded mononuclear silver pincer complexes with
the formulae of [Ag(H2L1)(Py)2](NO3)·H2O (1) and [Ag(L2)(NO3)]
(2), respectively, whereas the reaction of H2L1 and silver nitrate
in DMF afforded {[Ag2(H2L1)2](NO3)2·H2O}n (4). Variation of
metal salt and ligand ratios from 1 : 3 to 1 : 1 did not change
the composition of the products. The resonance signals in the 1H
NMR spectra of 1, 2 and 4 are similar to their corresponding
ligands. Reaction of 4 with silver nitrate at a ratio of 1 : 2 afforded
[Ag6(HL1)4](NO3)2 (5). [Ag6(HL1)4]X2 (X = BF4−, 6, or ClO4
,
−
7) were obtained similarly from the reactions of AgX and H2L1.
−
−
Variation of the ratio of the two reactants (X = BF4 and ClO4
)
from 1 : 1 to 3 : 1 did not change the products, and the reactions
are also anion i◦ndependent. Hydrothermal reaction of AgOAc
and H2L3 at 170 C yielded complex 3 as a neutral dinuclear silver
complex. Complexes 3, 5, 6, and 7 are not soluble in common
organic solvents such as CHCl3, CH3COCH3, CH3CN, or DMSO,
and their 1H NMR spectra were not obtained.
Structures
The structure of H2L1·0.5H2O was further characterized from X-
ray single crystal diffraction (see ESI‡). The results show that the
two pyrazole rings and the pyridine ring are nearly coplanar.
Single-crystal X-ray analysis of 1 reveals that the asymmet-
ric unit consists of one Ag center, one H2L1, one uncoordinated
nitrate, one water molecule and two pyridines (Fig. 1 and Table 2).
H2L1 acts as a tridentate pincer ligand. The silver centre is
pentacoordinated in a distorted bipyramidal geometry to three
pyridyl nitrogen atoms and two pyrazolyl nitrogen atoms. The
three equatorial Ag–Npyridyl bond distances are in the range of
˚
2.292(4)–2.382(4) A, which are much shorter than those of the
˚
two axial Ag–Npyrazolyl bonds which are 2.457(5) and 2.595(5) A.
Compound 2 is also a discrete mononuclear complex (see
ESI‡). The asymmetric unit of 2 is composed of two independent
molecules of the formula [Ag(L2)(NO3)] with essentially the same
structure as 1. As expected, L2 also acts as a tridentate pincer
ligand. The silver center also displays a distorted bipyramidal
geometry and is surrounded by a tridentate L2 and a bidentate
nitrate anion. The two axial Ag–Npyrazolyl bonds (2.515(8) and
˚
2.592(7) A) are also much longer than the equatorial Ag–Npyridyl
˚
bond (2.299(7) A).
The asymmetric unit of complex 3 (Fig. 2) consists of one metal
ion, one H2L3, and one 6-(5-phenyl-1H-pyrazolyl-3-yl)picolinate
(HL4) anion. Obviously HL4 resulted from the in situ oxidation
of H2L3. The IR spectrum shows a strong stretching band at
1656 cm−1 assignable to the carboxylate group. Although H2L3
is potentially pentadentate, it acts as a monodentate ligand by
using one of the pyrazolyl nitrogen atoms. HL4 acts as a chelating
ligand and forms a 5-membered ring with the central silver atom.
Thus the silver atom exhibits Y-shaped geometry. The Ag(1)–
˚
N(1) and Ag(1)–N(2) bond distances are 2.362(3) and 2.311(3) A,
respectively, which are much longer than that of Ag(1)–N(6)
˚
which is 2.196(3) A. The silver ion interacts very weakly with
˚
the carboxylate group with a Ag(1)–O(1) distance of 2.864(2) A,
which is not considered to be bonding.
It is interestingly observed that there is a ligand unsupported
Ag–Ag bond in the dimeric complex. The Ag–Ag distance is
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Scheme 1 Synthesis of the pincer-like bispyrazolyl ligands and the pincer ligand L2.
Scheme 2 Synthesis of 1, 4, 5, 6 and 7.
˚
The structure of 4 crystallizes in the centrosymmetric space
group P21/n and features a 1-Dinfinite chain structure constructed
through ligand supported Ag–Ag interactions. A section of the
chain is shown in Fig. 3a. The asymmetric unit contains two crys-
tallographically independent Ag atoms, two 2,6-bis(5-methyl-1H-
pyrazol-3-yl)pyridine (H2L1) ligands, two uncoordinated nitrate
anions and one molecule of water. Both Ag(1) and Ag(2) atoms
are coordinated by two pyrazolyl rings from two different H2L1
ligands forming a [Ag2(H2L1)2] unit. The Ag–N bond distances
3.040(1) A, which is slightly longer than the Ag–Ag separation
˚
in metallic silver (2.889 A), but shorter than the sum of the van
der Waals radii (3.44 A). Although silver–silver interactions are
˚
a common feature in silver coordination compounds,5,6 only a
few examples of ligand unsupported Ag(I) aggregates have been
reported, such as dimeric complexes,6b,22 trimeric complexes,6b and
polymeric chains.6a,c,23 The preparation and structural character-
ization of silver complexes having ligand unsupported Ag–Ag
interactions is of importance, because this has been a matter of
some debate due to the scarcity of unambiguous experimental
evidence of argentophilicity that is stable in the absence of
stabilizing ligands. The Ag–Ag contact in this compound is
comparable to those of known silver complexes having ligand
unsupported Ag–Ag bonds.6
˚
fall into a narrow range of 2.118(3)–2.139(3) A. The pyrazole
rings are slightly twisted from the pyridine rings which is reflected
by the dihedral angles ranging from 8.8 to 21.7◦. Along the chain
every two adjacent Ag atoms are singly bridged by one H2L1,
thus the compound displays a supramolecular motif consisting
1448 | Dalton Trans., 2008, 1444–1453
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Scheme 3 Synthesis of 2 and 3.
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) of 1 and 2
1
2
Ag(1)–N(6)
Ag(1)–N(7)
Ag(1)–N(1)
Ag(1)–N(2)
Ag(1)–N(4)
2.292(4)
2.330(5)
2.382(4)
2.457(5)
2.595(5)
Ag(1)–N(1)
Ag(1)–N(2)
Ag(1)–N(4)
Ag(1)–O(1)
Ag(1)–O(2)
2.299(7)
2.592(7)
2.515(8)
2.333(8)
2.678(11)
N(6)–Ag(1)–N(7)
N(6)–Ag(1)–N(1)
N(7)–Ag(1)–N(1)
N(6)–Ag(1)–N(2)
N(7)–Ag(1)–N(2)
N(1)–Ag(1)–N(2)
N(6)–Ag(1)–N(4)
N(7)–Ag(1)–N(4)
N(1)–Ag(1)–N(4)
N(2)–Ag(1)–N(4)
108.76(15)
130.21(14)
120.34(13)
110.77(16)
101.28(17)
68.81(15)
98.12(17)
99.95(17)
67.07(15)
135.88(14)
N(1)–Ag(1)–O(1)
N(1)–Ag(1)–N(4)
O(1)–Ag(1)–N(4)
N(1)–Ag(1)–N(2)
O(1)–Ag(1)–N(2)
N(4)–Ag(1)–N(2)
N(1)–Ag(1)–O(2)
O(1)–Ag(1)–O(2)
N(4)–Ag(1)–O(2)
N(2)–Ag(1)–O(2)
146.4(3)
70.0(2)
122.7(3)
68.7(2)
99.7(3)
136.4(2)
164.6(3)
48.5(3)
106.0(3)
110.1(3)
Fig. 1 Molecular structure of [Ag(H2L1)(Py)2](NO3)·H2O (1) with H
atoms and the water molecule omitted.
of infinite zigzag Ag chains. The neighboring silver ions interact
˚
via relatively short Ag(I)–Ag(I) bonds [2.889(1) and 2.893(1) A],
which are much shorter than the unsupported one in complex 3.
The silver separations exhibited by silver complexes containing
silver–silver bonds which are stabilized by bridging or capping
5,6
˚
ligands are normally in a wide range of 2.8–3.4 A. The two
independent Ag(1)#2–Ag(2)–Ag(1) and Ag(2)#1–Ag(1)–Ag(2)
angles are 136.41(2) and 158.83(2)◦, respectively, and the bent
metallic axes lead to the extended zigzag chain.
Numerous Ag(I) coordination polymers with bridging ligands24
have been reported; however, silver coordination polymers with
infinite metallic backbones are very rare. One family of these
Fig. 2 Molecular structure of [Ag2(H2L3)2(HL4)2] (3) with H atoms
omitted for clarity.
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Fig. 3 (a) ORTEP drawing of a segment of the infinite Ag chain of {[Ag2(H2L1)2](NO3)2·H2O}n (4). (b) Arrangement of the infinite chains along the
crystallographic c axis.
compounds are the silver complexes stabilized by 4ꢀ-thiomethyl-
◦
˚
2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine with short silver–silver contacts (3.03–
Table 3 Selected bond lengths (A) and angles ( ) of 3 and 4
25
13l
˚
3.15 A), and another one is [Ag(l-pz)]n with the Ag–Ag
3a
4b
˚
separation of 3.40 A. Such compounds are potentially useful as
nanowire materials.
Ag(1)–Ag(1)#1
Ag(1)–N(6)
Ag(1)–N(2)
Ag(1)–N(1)
Ag(1)–O(1)
3.040(1)
2.196(3)
2.311(3)
2.362(3)
2.864(2)
Ag(1)–Ag(2)#1
Ag(1)–Ag(2)
Ag(1)–N(9)#1
Ag(1)–N(4)
Ag(2)–N(7)
Ag(2)–N(2)
2.889(1)
2.893(1)
2.118(3)
2.125(3)
2.125(3)
2.139(3)
The adjacent chains are connected by weak hydrogen bonding
involving nitrate anions, water molecules, and the NH groups of
the pyrazolyl rings. Two nitrate anions and two water molecules
form four-membered rings via weak hydrogen bonds. These
connections generate a 2-D network structure (Fig. 3b).
N(6)–Ag(1)–N(2)
N(6)–Ag(1)–N(1)
N(2)–Ag(1)–N(1)
N(6)–Ag(1)–Ag(1)#1 116.93(7) N(9)#1-Ag(1)–Ag(2)
N(2)–Ag(1)–Ag(1)#1 63.61(7) N(4)–Ag(1)–Ag(2)
N(1)–Ag(1)–Ag(1)#1 99.62(6) N(7)–Ag(2)–N(2)
N(7)–Ag(2)–Ag(1)#2
152.10(10) N(9)#1-Ag(1)–N(4)
131.99(10) N(9)#1-Ag(1)–Ag(2)#1 101.02(9)
71.26(9) N(4)–Ag(1)–Ag(2)#1
169.97(13)
Although hexanuclear silver clusters 5, 6, and 7 have the similar
formula of [Ag6(HL1)4]X2 (X = NO3−, 5; BF4−, 6; and ClO4
,
−
82.16(10)
89.66(9)
90.47(9)
144.87(12)
97.45(9)
98.78(9)
100.73(9)
88.48(9)
7), the three complexes crystallize in different space groups.
Complexes 5 and 7 crystallize in an orthorhombic space group
Pbcn, while complex 6 crystallizes in a monoclinic space group
C2/c. The molecular structure of the cationic complex of 5 is
depicted in Fig. 4a, and for those of 6 and 7, see the ESI‡. Some
important bond distances and angles of these are summarized in
Table 4.
N(2)–Ag(2)–Ag(1)#2
N(7)–Ag(2)–Ag(1)
N(2)–Ag(2)–Ag(1)
Ag(2)#1-Ag(1)–Ag(2) 158.83(2)
Ag(1)#2-Ag(2)–Ag(1) 136.41(2)
The asymmetric unit of 5 consists of half of the molecule
with three independent Ag(I) centers, one nitrate anion, and one
acetone molecule. There are two mono-deprotonated HL1 ligands
a Symmetry code: #1 −x + 1, −y, −z + 2. b Symmetry codes: #1 −x + 3/2,
y − 1/2, −z + 3/2; #2 −x + 3/2, y + 1/2, −z + 3/2.
1450 | Dalton Trans., 2008, 1444–1453
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In hexanuclear silver complex 6, six Ag ions are held together
with six Ag–Ag bonds. Two trimetallic units in complex 6 are
connected by two Ag–Ag bonds. The distances of the three pairs
of symmetrically related Ag ions (Ag1–Ag1A, Ag2–Ag2A, and
˚
Ag3–Ag3A) are 2.893(1), 3.411(2), and 3.333(2) A, respectively.
The interaction of the two central silver ions (Ag1 and Ag1A) is
much stronger than those of others,
The structure of 7 consists of essentially the same Ag6 core
as 5. The separation of the two symmetrically related central ions
˚
(Ag1–Ag1A) is of 2.897(1) A, whereas the Ag–Ag distances within
˚
the linear trimetallic chain are 2.916(1) and 2.938(1) A, which are
slightly longer than those of 5.
So far several hexanuclear silver clusters are known, displaying
either a “cyclohexane-chair” conformation26a or an octahedron
arrangement.26b–e The two linear trimetallic chains of 5–7 are
arranged in a crossed fashion. This conformation for Ag6 clusters
is novel and not well modelled. These complexes also represent
the first hexanuclear clusters supported by pyrazolate ligands.
The silver–silver distances in the known complexes cover a wide
range depending on whether the Ag–Ag bonds are ligand bridged
or ligand unsupported, the donating atoms, and the coordination
˚
geometry. For instance, Ag–Ag contacts of 2.75 A in a trimetallic
Ag(I) complex stabilized by N-heterocyclic carbenes27a and 3.31
˚
and 3.02 A in a 3-cyanopyridine supported tetranuclear complex
with linearly arranged array27b were observed. The Ag–Ag contacts
in these complexes are also consistent with those of our recently
reported silver clusters containing multidentate N-heterocyclic
carbene ligands.18e,f
Emission properties
The solid-state photoluminescent spectra of complexes 3–7 and
their corresponding ligands at room temperature have been
measured, respectively. All the silver complexes and the ligands
are intensely emissive. The solid-state emission spectra of H2L1
and H2L3 show similar single sharp bands at ∼366 nm upon
excitation at 338 nm. Complexes 3–7 have similar solid-state
emission spectra (for emission data, see ESI‡); the solid-state
excitation and emission spectra of 3, 4, and 5 are shown in Fig. 5.
The unusual multiple emission bands at ∼406, ∼440, ∼453, ∼470,
and ∼493 nm were found upon excitation at 220 nm. The low-
energy bands at around 520 nm for 3–5 were also observed as
shoulder peaks in their emission spectra.
Fig. 4 (a) Molecular structure of the cation of [Ag6(HL1)4](NO3)2 (5)
with H atoms omitted for clarity. (b) A simplified diagram showing the
coordination geometry of the cation of [Ag6(HL1)4](NO3)2 (5).
in the asymmetric unit, and both act as anionic tridentate bridging
ligands with the nitrogen atoms of the pyridyl rings uncoordinated.
The coordination around each Ag ion is made up of two pyrazolyl
nitrogen atoms in nearly linear geometry with an average N–Ag–N
angle of 170◦. The Ag–N bond distances ranging from 2.081(9)
This multiple-band emission could suggest the presence of
different weakly coupled relaxation channels in these complexes,
as found in other compounds containing closed shell d10 metal
centres.12,26b,28a Vibronically structured bands with a spacing be-
˚
to 2.126(9) A are observed for the three independent silver ions.
Because of deprotonation and complexation, the ligands become
twisted and the dihedral angles between pyrazole and pyridine
rings are 10.9–21.5◦.
The silver cations in the hexanuclear core are held together
with five Ag–Ag bonds through four mono-deprotonated HL1
ligands (Fig. 4b). Three independent silver atoms are bridged by
two HL1 ligands, forming a nearly linear trimetallic chain with
tween the local maxima of the emission bands of about 1450 cm−1,
14f
=
=
characteristic of the m(C N) or m(N N) stretch, were also
observed. The appearance of vibrational progressions is suggestive
of involvement of the pyrazolate ligands in the emission process.
The fact that the ligands H2L1 and H2L3 are luminescent and
all the silver complexes exhibit high energy emission bands at
almost the same positions suggests that these emissions probably
originate from the same electronic states assignable to intraligand
(IL) transition and MLCT (d–p) transition.
˚
Ag–Ag separations of 2.905(1) and 2.908(1) A, and an Ag–Ag–
Ag angle of 167.01(4)◦. Such a connection mode is quite similar
to the trimetallic segment in the infinite chain complex 4. The
central silver ions of the two symmetrically related trimetallic units
interact each other via an argentophilic attraction with a relatively
It has been reported that for [Ag2(L2)(ClO4)2] (L = 4,5-
diazospirobifluorene) the origin of the fluorescence at ca. 510 nm
can be attributed to the coupling of a MLCT transition and a
˚
short Ag(1)–Ag(1A) bond at 2.874(1) A.
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◦
˚
Table 4 Selected bond lengths (A) and angles ( ) of 5, 6, and 7
5a
6b
7c
Ag(1)–Ag(1)#1
Ag(1)–Ag(2)
Ag(1)–Ag(3)
Ag(1)–N(4)
Ag(1)–N(7)#1
Ag(2)–N(5)#1
Ag(2)–N(2)
2.874(1)
2.908(1)
2.905(1)
2.105(8)
2.126(9)
2.081(9)
2.114(9)
2.092(9)
2.110(11)
Ag(1)–Ag(1)#1
Ag(1)–Ag(2)
Ag(1)–Ag(3)
Ag(3)–Ag(3)#1
Ag(1)–N(2)
Ag(1)–N(7)
Ag(2)–N(8)#1
Ag(2)–N(4)
2.893(1)
2.891(1)
2.918(1)
3.333(2)
2.120(6)
2.133(6)
2.082(7)
2.110(7)
2.084(6)
2.119(7)
Ag(1)–Ag(1)#1
Ag(1)–Ag(2)
Ag(1)–Ag(3)
Ag(1)–N(2)
Ag(1)–N(7)
Ag(2)–N(3)#1
Ag(2)–N(4)
2.897(1)
2.938(1)
2.916(1)
2.098(6)
2.119(7)
2.081(7)
2.124(7)
2.087(7)
2.134(7)
Ag(3)–N(8)
Ag(3)–N(9)#1
Ag(3)–N(8)#1
Ag(3)–N(9)
Ag(3)–N(3)#1
Ag(3)–N(9)
Ag(3)–Ag(1)–Ag(2)
Ag(1)#1–Ag(1)–Ag(3)
Ag(1)#1–Ag(1)–Ag(2)
N(4)–Ag(1)–N(7)#1
N(5)#1–Ag(2)–N(2)
N(8)–Ag(3)–N(9)#1
N(4)–Ag(1)–Ag(1)#1
N(7)–Ag(1)–Ag(1)#1
N(4)–Ag(1)–Ag(3)
N(7)–Ag(1)–Ag(3)
N(4)–Ag(1)–Ag(2)
N(7)–Ag(1)–Ag(2)
N(5)#1–Ag(2)–Ag(1)
N(2)–Ag(2)–Ag(1)
N(8)#1–Ag(3)–Ag(1)
N(9)–Ag(3)–Ag(1)
167.01(4)
83.81(4)
83.49(4)
172.7(3)
169.6(3)
167.8(4)
93.1(2)
94.0(3)
83.7(2)
95.8(3
99.6(2)
82.4(2)
96.7(2)
93.5(2)
97.4(2)
93.2(3)
Ag(2)–Ag(1)–Ag(3)
Ag(2)–Ag(1)–Ag(1)#1
Ag(1)#1–Ag(1)–Ag(3)
Ag(1)–Ag(3)–Ag(3)#1
N(2)–Ag(1)–N(7)
167.34(3)
84.296(19)
83.04(2)
75.75(2)
176.4(3)
Ag(3)–Ag(1)–Ag(2)
Ag(1)#1–Ag(1)–Ag(3)
Ag(1)#1–Ag(1)–Ag(2)
N(2)–Ag(1)–N(7)
166.07(3)
84.26(3)
82.60(3)
171.6(2)
94.63(18)
93.51(18)
84.64(18)
94.24(18)
100.98(18)
82.00(18)
171.1(3)
96.75(19)
92.1(2)
N(2)–Ag(1)–Ag(1)#1
N(7)–Ag(1)–Ag(1)#1
N(2)–Ag(1)–Ag(3)
N(7)–Ag(1)–Ag(3)
N(2)–Ag(1)–Ag(2)
N(7)–Ag(1)–Ag(2)
N(3)#1–Ag(2)–N(4)
N(3)#1–Ag(2)–Ag(1)
N(4)–Ag(2)–Ag(1)
N(8)#1–Ag(3)–N(9)
N(8)#1–Ag(3)–Ag(1)
N(9)–Ag(3)–Ag(1)
N(2)–Ag(1)–Ag(2)
N(7)–Ag(1)–Ag(2)
N(2)–Ag(1)–Ag(1)#1
N(7)–Ag(1)–Ag(1)#1
N(2)–Ag(1)–Ag(3)
N(7)–Ag(1)–Ag(3)
N(8)#1–Ag(2)–N(4)
N(8)#1–Ag(2)–Ag(1)
N(4)–Ag(2)–Ag(1)
N(3)#1–Ag(3)–N(9)
N(3)#1–Ag(3)–Ag(1)
N(9)–Ag(3)–Ag(1)
N(3)#1–Ag(3)–Ag(3)#1
N(9)–Ag(3)–Ag(3)#1
98.44(16)
82.91(17)
92.48(17)
90.96(18)
82.10(16)
97.31(17)
173.6(3)
95.96(18)
89.94(18)
171.9(3)
96.66(18)
91.13(18)
75.45(18)
104.6(2)
162.7(3)
97.45(19)
98.2(2)
a Symmetry code: #1 −x + 1, y, −z + 1/2. b Symmetry code: #1 −x + 2, y, −z + 1/2. c Symmetry code: #1 −x + 1, y, −z + 1/2.
pincer complexes could be prepared from both pincer-like ligands
H2L1 and L2. Under different reaction conditions, three hexamer
clusters and a 1-D infinite Ag chain compound are also obtained
with the pincer-like ligand H2L1. The hexamer clusters are the
first examples supported by pyrazolate ligands. The infinite Ag
chain compound featuring Ag(I)–Ag(I) interactions have Ag–Ag
distances equal to the Ag–Ag separation in metallic silver. Under
hydrothermal conditions, reaction of AgOAc with H2L3 yielded
a dinuclear neutral complex having a ligand unsupported argen-
tophilic interaction, which is relatively rare. In these complexes, the
pyridine linked bis(pyrazolyl) ligands are bonded in monodentate,
bidentate, or tridentate anionic fashions, respectively. In addition,
these oligomeric and polymeric complexes are intensely emissive,
which are potentially luminescent materials. Further studies to
clarify the nature of the emissions are required.
Acknowledgements
Fig. 5 Solid-state emission (right) and excitation (left) spectra of 3, 4 and
5.
We gratefully acknowledge the financial support of the NSF of
China (Grant 20572096) and the Zhejiang Provincial Natural
Science Foundation (R405066).
metal-centered transition (AgI, ds/dp) disturbed by argentophilic
interactions.28b The nature of the low-energy emission bands at
around 520 nm of 3, 4, and 5 may also be attributed to metal-
centered processes.
Notes and references
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Conclusions
Through ligand functionalities and the argentophilic attraction
some new silver architectures have been made. Mononuclear
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