Self-assembled coordination structures are one of the most attractive areas of materials research due to their intriguing structural topologies and functional properties such as molecular adsorption, magnetism and luminescence (Biradha
et al., 2006
![[triangle]](/corehtml/pmc/pmcents/rtrif.gif)
; Wu
et al., 2009
; Blake, Brooks
et al., 1999
; Blake, Champness
et al., 1999
; Evans & Lin, 2002
; Kitagawa
et al., 2004
; Yaghi
et al., 2003
), and much attention has focused on their design and construction. However, the factors that govern the formation of such complexes are complicated and include not only the inherent properties of metal ions and ligand structure, but also anion-directed interactions and reaction conditions (Bu
et al., 2003
; Kong
et al., 2008
, 2009
). In addition to covalent bonds, noncovalent interactions, such as Ag
Ag, π–π, hydrogen-bond and cation
π interactions, also play important roles in controlling molecular packing (Pedireddi
et al., 1996
; Kolotuchin
et al., 1995
; Li
et al., 2006
; Sun
et al., 2003
; Lough
et al., 2000
; Massoud & Langer, 2009
; Goodgame
et al., 2002
). Because the central C—C bond of bipyridine (bipy) can rotate freely, bipy cannot just be regarded as a chelating ligand (Kaes
et al., 2000
; Marchetti
et al., 2007
), but also as a potential spacer between metal centers by acting as a bridging ligand with an
anti configuration (Yu
et al., 2007
; Forniés
et al., 1993
). Therefore, bipy and its derivatives are widely used in the construction of novel Ag
I-containing complexes incorporating diverse supramolecular interactions (Hung-Low
et al., 2009
; Ye
et al., 2005
). Recently, we have pursued systematic investigations into the assembly of Ag
I cations with different angular and linear bipodal
N-donor ligands, such as aminopyrimidine and aminopyrazine (
e.g. Luo, Huang, Chen
et al., 2008
; Luo, Huang, Zhang
et al., 2008
; Luo
et al., 2009
; Sun, Luo, Huang
et al., 2009
; Sun, Luo, Xu
et al., 2009
; Sun, Luo, Zhang
et al., 2009
), with the principal aim of obtaining supramolecular complexes or multifunctional coordination polymers. In an attempt to exploit the influence of synthesis conditions on the structures of the AgNO
3–bipy system, we successfully obtained the title compound, (I)
, and the known coordination polymer, (II) {
catena-poly[[(2,2′-bipyridine)silver(I)]-μ
2-nitrato]; Bowmaker
et al., 2005
}, in the same solvent system.
The asymmetric unit of (I)
contains one Ag
I cation, one bipy ligand, one coordinated ammonia molecule and one nitrate anion. As shown in Fig. 1, the Ag
I cation is coordinated in a trigonal–planar fashion by three N atoms from one bipy ligand and one ammonia molecule, with bond angles ranging from 72.36 (12) to 145.25 (14)°. The Ag—N
bipy bond lengths are identical within experimental error (Table 1) and comparable to reported values (Oxtoby
et al., 2002
; Fan
et al., 2007
; Nicola
et al., 2007
). The pyridyl rings of bipy are nearly coplanar with a twist angle of 4.8 (5)°. With an Ag—N bond length of 2.135 (4) Å, the coordinated ammonia molecule plays a role as terminator, obstructing aggregation of (I)
. Because of the labile nature of this Ag—N bond, the presence of ammonia in the coordination sphere of the metal center offers a potential coordination site in the molecule. Two symmetry-related Ag
I–bipy monomers aggregate to a dimer with a head-to-head arrangement through intra-dimer π–π stacking and a ligand-unsupported Ag
Ag interaction (Tong
et al., 1999
), where the Ag1
Ag1
i interaction [symmetry code: (i) −
x + 1,
y, −
z + ½] has a distance of 3.0456 (16) Å. This is significantly shorter than twice the van der Waals radius of Ag
I (3.44 Å; Bondi, 1964
) and the completed coordination sphere of the Ag centers can thus be described as capped trigonal planar. The other cases where such short contacts exist between nonbridged Ag
I cations are found in Ag(imidazole)
2ClO
4 (Eastland
et al., 1980
) and [Cu(ethylenediamine)
3][Ag
2(CN)
4] (Kappenstein
et al., 1988
). A crystallographic
C2 axis passes through the mid-point of and is perpendicular to this Ag1
Ag1
i axis. This weak bonding interaction between two
d
10 cations is possible
via the participation of 5
s and 5
p orbitals which are close in energy to the 4
d orbitals. Intra-dimer π–π stacking also contributes to the reinforcement of this Ag
Ag interaction (Venkatalakshmi
et al., 1992
). Moreover, the π–π stacking [
Cg1
Cg2
i = 3.628 (3) Å (intra-dimer) and
Cg1
Cg2
vi = 3.711 (3) Å (inter-dimer);
Cg1 and
Cg2 are the centroids of the N1/C1–C5 and N2/C6–C10 rings, respectively; symmetry codes: (i) −
x + 1,
y,
; (vi) −
x + 1, −
y + 1, −
z; Fig. 2] and weak Ag
C interactions [Ag1
C6
vi = 3.386 (4) Å and Ag1
C7
vi = 3.393 (5) Å] act as a ‘glue’ to reinforce the dimers, forming a column along the
c axis, in which the dimers are arranged in a head-to-tail orientation. In addition, the nitrate anion acts as an acceptor and is hydrogen bonded to three different symmetry equivalents of the ammonia molecules (Table 2). Nonclassical C
pyridyl—H
O hydrogen bonds [average C
O distance = 3.280 (5) Å; Table 2] and classical N—H
O hydrogen bonds link adjacent columns to form the resulting three-dimensional supramolecular framework (Fig. 3).
| Table 1Selected geometric parameters (Å, °) |
| Table 2Hydrogen-bond geometry (Å, °) |
The effects of the synthesis conditions on the structure of the AgNO
3–bipy system were investigated in the ultrasonic and stirred methods with the same solvent system (methanol–water; 15 ml, 1:2
v/
v). Under stirring, we could only obtain coordination polymer (II), first reported by Bowmaker
et al. (2005
). In the structure of (II), the nitrate anion not only acts as a ligand but also as a bridging anion to link Ag
I–bipy cationic units into one-dimensional zigzag chains. The difference between the structures of (I)
and (II) originates mainly from the different mechanical treatments which cause the ammonia molecule and nitrate anion to play different roles in the construction of (I)
and (II).
It is known that the free bipy molecule displays a weak luminescence at
circa 530 nm in the solid state at room temperature. As shown in Fig. 4, compound (I)
exhibits an intense emission maximum at 469 nm upon excitation at 345 nm, which may be attributed to the intraligand emission from bipy (Wang
et al., 2004
). Compared with that of the free bipy molecule, the blue shift and the luminescent enhancement of the emission at 469 nm may be due to the chelation of the bipy ligand to the Ag
I cation, which effectively increases the rigidity and coplanarity of the ligand and reduces the loss of energy by nonradiative decay of the intraligand emission excited state (Zhang
et al., 2003
; Qian & Wang, 2002
).
) axis. In addition, each AgI cation is coordinated by one chelating bipy ligand and one ammine ligand, giving a trigonal coordination environment capped by the symmetry-equivalent Ag atom. Molecules are assembled by Ag
