A stair-shaped molecular silver(0) chain

Article abstract of DOI:10.1002/anie.200803465

Bound in chains: The molecular Ag⁰ chain [Ag₄py ₂]_n (py=C₅H₅N) is formed from two covalently linked infinite zigzag chains of Ag, in which Ag-Ag bonds alternate between long and short (see picture; blue Ag, gray C, orange N). Theoretical calculations indicate that partially positively (+0.35) and negatively (-0.30) charged Ag atoms are alternately located along the chain, which has a bandgap of 4.1 eV. (Figure Presented)

Full text of DOI:10.1002/anie.200803465

Communications
DOI: 10.1002/anie.200803465
Metal–Metal Interactions
A Stair-Shaped Molecular Silver(0) Chain**
Hoi Ri Moon, Cheol Ho Choi, and Myunghyun Paik Suh*
Compounds formed by metal–metal interactions, such as
obtain crystals. Therefore, we have modified the synthetic
procedure to obtain the crystals in a shorter period of time
with higher yields.
[
1,2]
[3,4]
metal clusters
and extended metal–atom chains,
are
important for the development of new electrical and elec-
tronic devices. In particular, silver-containing compounds are
interesting because silver has one of the highest electric
conductivities of all metals. However, no extended chains of
Colorless flat parallelepiped crystals of [Ag py ] (1) were
4
2 n
prepared by slow diffusion of NaOH solution into the water/
pyridine solution of AgNO in a refrigerator for three weeks.
3
0
I
I
0
Ag or Ag centers formed purely by Ag–Ag interactions
without any bridging ligands have been reported to date.
In this context, NaOH appears to reduce Ag to Ag . NaOH is
known to reduce, or promote reduction of, Ag, Pd, and Pt ions
to result in metallic clusters; stabilizers or alcoholic solvents
should be additionally used to obtain monodispersed nano-
0
Silver clusters of several (2–23) Ag atoms have been
reported, but their structures were derived only from
[
5,6]
[11]
theoretical calculations.
Surprisingly, there has been no
particles. To obtain crystals of 1, a small reaction scale, the
0
report of the isolation of an Ag coordination compound,
correct amount of NaOH, and a slow diffusion rate are very
important, otherwise significant amounts of a brown precip-
itate of Ag nanoparticles form, as confirmed by high-
resolution transmission electron microscopy (HRTEM).
The crystals of 1 are stable only in their mother liquor, and
they instantly lose pyridine molecules upon exposure to air,
which results in the formation of yellowish [Ag ] , as shown
0
although
the
Ag
species
[Ag (m-(Bzim)Ph P) ]
2 2 3
(
(Bzim)Ph P = 1-benzyl-2-imidazolyldiphenylphosphine)
2
was observed as a transient species during the electrochemical
I
[7]
0
reduction of Ag compounds, and some Ag complexes of p-
donor ligands were trapped in various matrices and charac-
[8]
terized by EPR spectroscopy.
4
n
Herein we report the X-ray single crystal structure of a
by elemental analysis data. Therefore, except the X-ray
structure, no characterization data was obtained for 1. The X-
ray structure of [Ag ] could not be determined because it
stair-shaped infinite silver atom chain, [Ag py ] (1; py =
4
2 n
0
C H N), as the first Ag coordination compound without
5
5
4 n
bridging ligands. It is formed from two covalently linked 1D
does not form single crystals, even though the crystal
morphology was retained. The EPR spectrum of 1, which is
likely to be that of [Ag ] because 1 loses pyridine molecules
zigzag Ag chains, with alternating long and short AgÀAg
bonds. Theoretical calculations indicate that partially pos-
itively (+ 0.35) and negatively (À0.30) charged Ag atoms are
also alternately located and that 1 has a HOMO–LUMO gap
of 4.1 eV (HOMO = highest occupied molecular orbital,
LUMO = lowest unoccupied molecular orbital).
4
n
as soon as it is removed from the mother liquor, was silent
(see the Supporting Information). This indicates that an
0
unpaired electron of each Ag atom is coupled with an
0
electron from an adjacent Ag atom.
We have been interested in coordination polymers and
The X-ray single crystal structure of 1 is shown in Figure 1.
The diffraction data were collected in the presence of the
mother liquor in a glass capillary. The asymmetric unit of 1
consists of four crystallographically independent silver atoms
(Ag1–Ag4). These are linked by covalent bonds to form a
[9,10]
their inclusion of metal nanoparticles.
During the syn-
thesis of silver carboxylate frameworks, we unexpectedly
obtained crystals that were found to be 1. This compound was
originally prepared by the addition of an aqueous solution of
Na C DC (C DC = 1,6-hexanedicarboxylate) or Na BPDC
rhombic tetranuclear Ag cluster unit, as well as two pyridine
2
6
6
2
4
(
BPDC = 4,4’- biphenyldicarboxylate) to AgX (X = NO or
molecules coordinating to the Ag3 and Ag4 atoms (see the
Supporting Information). The observed structure of the
3
CF SO ) in a water/pyridine solution. However, this method
3
3
gave an extremely poor yield and it took several months to
tetranuclear Ag cluster unit is similar to that derived from
theoretical calculations, although the bond distances and
4
[
6]
angles of the observed cluster unit are much larger. The Ag4
[
*] H. R. Moon, Prof. M. P. Suh
Department of Chemistry, Seoul National University
Seoul 151-747 (Korea)
Fax: (+82)2-886-8516
E-mail: mpsuh@snu.ac.kr
units are further linked with those of the adjacent Ag cluster
4
units by AgÀAg bonds, with the inversion center in the middle
of two units. This gives rise to a stair-shaped molecular silver
chain formed from two infinite 1D zigzag Ag chains. The
Prof. C. H. Choi
Department of Chemistry, Kyungpook National University
Taegu 702-701 (Korea)
angles between the adjacent Ag planes are in the range of
4
1
26–1278. The AgÀAg bond lengths along the infinite 1D
chain alternate between long (2.876–2.897 ꢀ) and short
2.827–2.830 ꢀ), with an average Ag–Ag distance of
.858(1) ꢀ. The average Ag–Ag separation between the two
[
**] This work was supported by a Korea Research Foundation Grant
funded by the Korean Government (MOEHRD Basic Research
Promotion Fund, KRF-2005-084-C00020), and by a Korea Science
and Engineering Foundation (KOSEF) grant funded by the Korea
government (MEST, No. R11-2005-008-00000-0).
(
2
Ag chains is 2.890(1) ꢀ. These Ag–Ag distances are well
below the sum of the van der Waals radii of silver atoms
(
3.44 ꢀ), and are close to the Ag–Ag separation in bulk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803465.
metallic silver (2.89 ꢀ). AgÀAg bonds are also formed
8390
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Angewandte
Chemie
(Figure 2) indicate that 1, after the release of pyridine
molecules in a vacuum chamber, contains only silver; in
contrast, [Ag I py ] on release of pyridine molecules contains
2
2
2 n
a 1:1 ratio of silver and iodine atoms. The Ag L and Ag Lb
a
Figure 2. Energy dispersive X-ray spectroscopy (EDS) data for a) [Ag4]n
(1) showing that silver is the only chemical element present and
b) [Ag I ] showing that silver and iodine elements coexist. During the
2
2 n
measurement in the vacuum chamber, pyridine molecules are released
from 1 and [Ag py
Figure 1. The X-ray structure of [Ag py ] (1). a) ORTEP drawing of 1
2
I
2
] .
2 n
4
2 n
with atomic numbering scheme (thermal ellipsoids at 30% probabil-
ity). Symmetry operations: #1, Àx+2, Ày, Àz+1; #2, Àx+1, Ày,
Àz+1. b) View seen along the bc plane. c) View seen along the ab
plane, showing the stair-shaped molecular silver chain running along
the a direction. Blue Ag, gray C, orange N.
peaks for 1 are found at 2.98 keV and 3.15 keV, respectively,
but those for [Ag I py ] appear at 3.00 keV and 3.16 keV,
2
2
2 n
respectively. The EDS data for [Ag I py ] show additional L
2
2
2
n
a
and L peaks for iodine at 3.94 keVand 4.24 keV, respectively.
b
On heating at 5008C, 1 has a metallic silver luster while
between the pyridine-coordinated Ag atoms (Ag3 and Ag4),
with an average bond length of 3.135(1) ꢀ. Therefore, Ag1
and Ag2 are bound to three Ag atoms, and Ag3 and Ag4 are
bound to five Ag atoms and a pyridine molecule. The average
Ag–N(py) distances are 2.349(6) ꢀ and the dihedral angle
between the two pyridine rings coordinated to Ag3 and Ag4 is
[Ag I py ] becomes a yellowish powder (see the Supporting
2
2
2 n
Information). Although mass spectrometry would show any
differences between the two compounds, mass spectra could
not be recorded because of the nonvolatile nature of the
samples. It seems that 1 and [Ag I py ] have very similar
2
2
2 n
structures because silver and iodine atoms have similar
covalent radii, 1.34 ꢀ and 1.33 ꢀ, respectively.
5
7.4(3)8, whilst the dihedral angles between the Ag plane and
4
the pyridine molecules coordinated to Ag3 and Ag4 are
6.2(2)8 and 66.7(3)8, respectively. Each pyridine molecule
We have performed ab initio energy calculations on
[14]
6
[Ag py ] (1) (n = 2–5) by the HF/SBKJC(d) method,
4 2 n
interacts with six adjacent pyridine molecules, two from its
own chain and four from two neighboring chains, by edge-to-
face p–p interactions (see the Supporting Information). The
thickness of the stair-shaped silver chain is 0.46 nm ꢁ 0.58 nm
and each chain extends along the [100] direction. The axis-to-
axis distances between the neighboring molecular chains are
employing the general atomic and molecular electronic
[15]
structure system (GAMESS). The geometries of 1 were
taken from the X-ray crystal data. Mulliken population
analysis of 1 (Figure 3a) clearly indicates that the pyridine-
coordinated Ag atoms (Ag3 and Ag4) have partial positive
charges and the uncoordinated Ag atoms (Ag1 and Ag2) have
partial negative charges. The calculated atomic charges for
the Ag atoms are a function of model cluster size (n = 2–5),
and the limiting values are obtained by extrapolating to a
system where n = infinity (1/n = 0), which gives charges of
+ 0.35 and À0.30. Such alternating charge pattern, together
with the alternating long and short Ag–Ag distances, can be
attributed to structural dimerizations along the chain, which
could arise from Peierls instability of one-dimensional
1
7.0 ꢀ along the [010] direction, and 9.50 ꢀ along the [001]
direction (Figure 1b and c). The shortest molecular chain-to-
chain distance is 9.74 ꢀ along the [011] direction.
The X-ray structure of [Ag I py ] , which was obtained
2
2
2 n
from recrystallization of AgI in pyridine, has been reported
previously. Surprisingly, the structure of [Ag I py ] was very
2
2
2 n
[
12]
similar to that of 1. To check if [Ag I py ] was the same as
2
2
2 n
1, we independently prepared colorless rectangular prismatic
[16]
crystals of [Ag I py ] and determined its X-ray crystal
chains. The limiting value of the HOMO–LUMO gap in
the [Ag py ] model cluster is 4.1 eV (Figure 3b). It is noted,
2
2
2 n
[
13]
structure (see the Supporting Information). Although the
4
2 n
X-ray crystal structure is identical to that of 1, [Ag I py ] is
different from 1 in terms of energy dispersive X-ray spec-
troscopy (EDS) data and crystal properties. The EDS data
however, that the estimated gap value should be considered
qualitatively because the theoretical methods adopted here
tend to overestimate the HOMO–LUMO gap. In any case, it
2
2
2 n
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Communications
well as the synthesis of molecular chains formed from other
metallic elements.
Experimental Section
1
: An aqueous solution (3 mL) of NaOH (0.004 g, 0.057 mmol) was
added dropwise to a solution of AgNO (0.021 g, 0.059 mmol) in H O/
3
2
pyridine (1:3 v/v, 4 mL). The solution was allowed to stand in a
refrigerator for approximately three weeks until colorless flat
parallelepiped crystals formed. The crystals were filtered and briefly
washed with the mother liquor. On exposure to air the crystal lost
single crystallinity and became a yellowish solid. FTIR (KBr pellet):
À1
n˜ = 1591(s), 1482(s), 1441(s), 1213(s) cm . Yield: 1.98 mg (27%).
Elemental anal. calcd for Ag C H N : C, 20.37; H, 1.71; N, 4.75.
4
10 10
2
Found: C, 0.1159; H, 0.0; N, 0.0. The elemental analysis data indicates
that 1 lost pyridine molecules as soon as it was exposed to air.
Crystal data for 1: Ag C H N , M = 589.50, crystal size 0.3 ꢁ
4
10 10
2
r
3
0
1
.1 ꢁ 0.1 mm , monoclinic, space group P2 /n, a = 9.2810(3), b =
7.0351(10), c = 9.5013(5) ꢀ, b = 106.346(3)8, V= 1441.46(12) ꢀ ,
1
3
À3
Z = 4, 1calcd = 2.717 gcm
, l = 0.71073 ꢀ, T= 293(2) K, 2qmax =
2
7.468, R = 0.0597 (I > 2s(I), 3271 reflections), wR = 0.2396 [all
1
2
data, 5655 reflections], GOF = 1.083. CCDC 633448 (1) contains the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif
Received: July 17, 2008
Revised: August 23, 2008
Published online: October 1, 2008
Keywords: ab initio calculations · Ag–Ag bonds ·
.
cluster compounds · metal-atom chains · silver
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coordination compound. The chain is formed from rhombic
[
[
tetranuclear Ag cluster units that are linked linearly by AgÀ
4
Ag bonds to give a stair-shaped infinite chain of Ag atoms
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8393