catena-Poly[[disilver(I)(Ag—Ag)-bis[μ-1,1′-(butane-1,4-di­yl)diimidazole-κ²N³:N^3′]] bis­(perchlorate)]: a double helix stabilized by ligand-unsupported argentophilic inter­actions

The title compound, {[Ag₂(C₁₀H₁₄N₄)₂](ClO₄)₂}_n, is a one-dimensional coordination polymer formed by Ag^I atoms linearly bridged by 1,1′-(butane-1,4-di­yl)diimidazole mol­ecules. The chains have a helical arrangement and pairs of chains are held together by the rarely reported ligand-unsupported Ag—Ag inter­action [2.966 (1) Å], which results in a double-helix structure. The double helix contains twisted 24-membered metallomacrocycles, which are composed of four Ag atoms and two ligands. The Ag atoms lie on twofold axes.

S1. Comment

It is well known that weak interatomic interactions, such as hydrogen bonding and π–π stacking, have presented a synthetic paradigm for the rational design and synthesis of functional materials in supramolecular chemistry. The metallophilic attractions between the closed-shell d¹⁰ coinage metals also promote aggregation, as supported by spectroscopic and structural evidence (Codina et al., 2002). Such compounds are often associated with many potentially useful chemical and physical properties of materials, such as optical or electronic properties or catalytic behaviour, so the synthesis and properties of these metallophilic aggregates have attracted an ever-increasing level of attention. Compared with Au—Au interactions, Ag—Ag interactions have been reported as argentophilicity and are calculated to be relatively weak. They can also be used to control the conformation and topology of metallic aggregates. There are many examples of argentophilic interactions; most are ligand supported (Wang & Mak, 2001; Huang et al., 2006), while a few are reported to be ligand-unsupported (Liu et al., 2006; Singh et al., 1997). We are interested in the synthesis of coordination polymers organized by means of N-donor ligands (Lou et al., 2006; Li & Lou, 2007). In this paper, we report the synthesis and characterization of the title compound, (I), an interesting silver complex of double-helix chains stablized by ligand-unsupported argentophilic interactions based on 1,1'-(butane-1,4-diyl)diimidazole (hereinafter abbreviated to bbim).

Compound (I) is a one-dimensional coordination polymer. As shown in Fig. 1, the asymmetric unit consists of two symmetry-independent Ag^I ions, which lie on twofold axes, one bbim ligand and one perchlorate anion. As shown in Fig. 2, the two crystallographically unique Ag^I ions are bridged by the bbim ligands to form a one-dimensional infinite polymeric chain structure. Within a single strand of the chain, the coordination around each Ag^I ion is made up of two symmetry-related N atoms from different bbim ligands in a linear geometry, and the Ag—N distances are consistent with other reported Ag^I complexes of N-donor ligands (Singh et al., 1997; Carlucci et al., 1998). Interestingly, pairs of chains are held together by ligand-unsupported argentophilic interactions between symmetry-independent Ag atoms on the same twofold axis. The individual strands of the paired chains intertwine in a helical fashion to give a double-helix motif (Fig. 2). The Ag—Ag distance is 2.966 (1) Å, which is slightly longer than the Ag—Ag separation in metallic silver (2.889 Å; Reference?), but much shorter than the sum of the van der Waals radii (3.44 Å; Bondi, 1964). As a result of the Ag···Ag interactions, the total coordination geometry at each Ag^I atom is T-shaped.

Although Ag—Ag interactions are widely reported in silver coordination compounds, only a few examples of ligand-unsupported Ag^I aggregates are known and ligand-unsupported Ag^I coordination polymers are rare (Ming & Mak, 1991; Omary et al., 1998; Liu et al., 2006, 2005). The structural characterization 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 being stable in the absence of stabilizing ligands. The Ag—Ag contact in compound (I) is consistent with those of the known silver complexes having ligand-unsupported Ag—Ag bonds.

The development of the double-helical strands containing the Ag···Ag interactions results in the formation of 24-membered metallacycles containing four Ag atoms and two bbim ligands (Fig. 1). Each of these metallacycles occupies one half of a turn of the helix. Helicity is a fundamental structural feature of great current interest in inorganic and coordination chemistry (Shang et al., 2007; Berti et al., 2005; Erxleben, 2001; Carlucci et al., 1998). It has a fundamental role in biology, and potential applications in the fields of supramolecular chemistry, asymmetric catalysis and nonlinear optical materials. Ag^I complexes containing infinite double helices are interesting but have been investigated only rarely (Lee et al., 2005; Erxleben, 2001). The twisted 24-membered metallomacrocyclic ring and the T-shaped Ag coordination environment in (I) lead to chains with an almost rectangular cross section. Coordination compounds having specially shaped metallomacrocycles show promise in various applications, such as selective sensors, gas storage, sorters, catalysis and luminescent materials.

S2. Experimental

All reagents were of analytical grade and used without further purification. Bbim was prepared by the general procedure of Ma et al. (2003). A solution of pymim (0.4 mmol, 76 mg) in MeOH (4 ml) was added dropwise to a stirred solution of AgClO₄ (0.8 mmol, 167 mg) in MeOH (8 ml). The white precipitate that formed immediately was collected, washed with MeOH and dried. Colourless single crystals of (I) were grown by vapour diffusion of diethyl ether into a dimethylformamide solution containing the silver complex [yield 38%; m.p. 540–543 K (decomposition)]. Analysis, found: C 30.11, H 3.69, N 14.23%; calculated for Ag(C₁₀H₁₄N₄)(ClO₄): C 30.21, H 3.55, N 14.09%.

S3. Refinement

The Cl—O bond lengths and O···O separations in the perchlorate anion were restrained to 1.44 (1) and 2.35 (2) Å, respectively. The U^ij components for these atoms were also restrained using similarity and approximate isotropic restraints. The H atoms were positioned geometrically and treated as riding, with C—H bond lengths constrained to 0.93 (aromatic CH) or 0.97 Å (methylene CH₂), and with U_iso(H) = 1.2U_eq(C).

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXTL (Sheldrick, 2001); program(s) used to refine structure: SHELXTL (Sheldrick, 2001); molecular graphics: SHELXTL (Sheldrick, 2001); software used to prepare material for publication: SHELXTL (Sheldrick, 2001) and publCIF (Westrip, 2007).

Figures

	Fig. 1. One metallomacrocycle in the double-helix chain of (I), showing the atom-numbering scheme of the asymmetric unit. Displacement ellipsoids are drawn at the 30% probability level. [Symmetry codes: (i) x − 1, y, z; (ii) x + 1, y, z.]
	Fig. 2. The double-helix structure in (I). H atoms and perchlorate anions have been omitted. [Symmetry codes:(i) x − 1, y, z; (ii) x + 1, y, z; (iii) −x, y, −z + 3/2 (iv) −x + 2, y, −z + 3/2.]

catena-Poly[[disilver(I)(Ag—Ag)]-bis[µ-1,1'-(butane-1,4-diyl)diimidazole- κ²N:N'] bis(perchlorate)]

Crystal data

[Ag2(C10H14N4)2](ClO4)2	F(000) = 1584
Mr = 397.57	Dx = 1.887 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 2054 reflections
a = 11.090 (2) Å	θ = 2.4–21.7°
b = 13.293 (2) Å	µ = 1.65 mm^−1
c = 18.989 (3) Å	T = 298 K
β = 90.488 (3)°	Prism, colourless
V = 2799.3 (8) Å^3	0.40 × 0.22 × 0.11 mm
Z = 8

Data collection

Bruker SMART 1K CCD area-detector diffractometer	2520 independent reflections
Radiation source: fine-focus sealed tube	1733 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.037
ϕ and ω scans	θmax = 25.3°, θmin = 2.2°
Absorption correction: multi-scan (SADABS; Sheldrick, 2002)	h = −13→13
Tmin = 0.558, Tmax = 0.839	k = −15→15
7211 measured reflections	l = −15→22

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.049	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.141	H-atom parameters constrained
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0748P)^2 + 3.7187P] where P = (Fo^2 + 2Fc^2)/3
2520 reflections	(Δ/σ)max < 0.001
183 parameters	Δρmax = 0.78 e Å^−3
100 restraints	Δρmin = −0.42 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. Highest peak 0.78 at 0.1600 0.5210 0.0793 [1.22 A from O4] Deepest hole −0.42 at 0.2276 0.4327 0.0552 [0.54 A from O4]

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ag1	0.0000	0.10567 (6)	0.7500	0.0662 (3)
Ag2	1.0000	0.32882 (6)	0.7500	0.0688 (3)
N1	0.2791 (4)	0.1012 (4)	0.5998 (3)	0.0460 (12)
N2	0.1159 (5)	0.1067 (4)	0.6625 (3)	0.0536 (14)
N3	0.6758 (5)	0.3379 (4)	0.6314 (3)	0.0569 (14)
N4	0.8580 (5)	0.3250 (4)	0.6750 (3)	0.0572 (14)
C1	0.4077 (6)	0.0902 (5)	0.5819 (4)	0.0586 (17)
H1A	0.4140	0.0522	0.5385	0.070*
H1B	0.4477	0.0520	0.6188	0.070*
C2	0.4727 (6)	0.1905 (5)	0.5731 (4)	0.0550 (17)
H2A	0.5553	0.1775	0.5593	0.066*
H2B	0.4341	0.2278	0.5352	0.066*
C3	0.4738 (7)	0.2538 (6)	0.6376 (5)	0.068 (2)
H3A	0.5079	0.2150	0.6762	0.081*
H3B	0.3913	0.2702	0.6498	0.081*
C4	0.5448 (6)	0.3510 (6)	0.6304 (5)	0.084 (3)
H4A	0.5216	0.3833	0.5865	0.100*
H4B	0.5228	0.3958	0.6685	0.100*
C5	0.2334 (6)	0.0965 (5)	0.6634 (3)	0.0512 (16)
H5	0.2791	0.0869	0.7041	0.061*
C6	0.0866 (6)	0.1214 (6)	0.5939 (4)	0.0621 (19)
H6	0.0090	0.1326	0.5766	0.074*
C7	0.1874 (6)	0.1174 (6)	0.5543 (4)	0.0602 (18)
H7	0.1923	0.1243	0.5056	0.072*
C8	0.7446 (7)	0.3283 (6)	0.6901 (4)	0.0654 (19)
H8	0.7143	0.3245	0.7356	0.079*
C9	0.8645 (6)	0.3323 (5)	0.6038 (4)	0.0583 (17)
H9	0.9353	0.3308	0.5779	0.070*
C10	0.7533 (6)	0.3418 (5)	0.5766 (4)	0.0586 (18)
H10	0.7331	0.3496	0.5292	0.070*
Cl1	0.28074 (16)	0.99123 (15)	0.85980 (10)	0.0655 (5)
O1	0.3670 (5)	1.0303 (6)	0.8122 (3)	0.110 (2)
O2	0.3173 (8)	0.8917 (6)	0.8799 (5)	0.150 (3)
O3	0.1705 (5)	0.9784 (6)	0.8220 (4)	0.120 (2)
O4	0.2697 (9)	1.0500 (7)	0.9192 (4)	0.166 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ag1	0.0752 (6)	0.0541 (5)	0.0699 (5)	0.000	0.0309 (4)	0.000
Ag2	0.0691 (6)	0.0607 (5)	0.0762 (6)	0.000	−0.0221 (4)	0.000
N1	0.043 (3)	0.046 (3)	0.050 (3)	0.003 (2)	0.004 (2)	−0.001 (2)
N2	0.053 (4)	0.054 (3)	0.054 (3)	0.003 (3)	0.016 (3)	0.003 (3)
N3	0.043 (3)	0.055 (3)	0.073 (4)	−0.001 (3)	0.000 (3)	−0.008 (3)
N4	0.052 (4)	0.065 (4)	0.054 (3)	0.000 (3)	−0.001 (3)	0.000 (3)
C1	0.049 (4)	0.058 (4)	0.069 (4)	0.002 (3)	0.007 (3)	−0.008 (4)
C2	0.045 (4)	0.059 (4)	0.062 (4)	0.000 (3)	0.007 (3)	0.009 (3)
C3	0.048 (4)	0.060 (4)	0.095 (6)	−0.002 (3)	0.016 (4)	−0.006 (4)
C4	0.051 (5)	0.055 (5)	0.145 (8)	0.006 (4)	−0.005 (5)	−0.009 (5)
C5	0.061 (4)	0.050 (4)	0.042 (4)	−0.007 (3)	−0.001 (3)	−0.001 (3)
C6	0.052 (4)	0.077 (5)	0.057 (4)	0.014 (4)	0.000 (3)	0.006 (4)
C7	0.058 (4)	0.075 (5)	0.047 (4)	0.009 (4)	−0.002 (3)	0.003 (3)
C8	0.064 (5)	0.078 (5)	0.054 (4)	−0.008 (4)	0.012 (4)	−0.005 (4)
C9	0.053 (4)	0.060 (4)	0.062 (4)	−0.004 (3)	0.013 (3)	−0.008 (4)
C10	0.062 (5)	0.062 (5)	0.052 (4)	−0.001 (4)	−0.007 (3)	0.005 (3)
Cl1	0.0554 (11)	0.0831 (13)	0.0579 (10)	−0.0164 (9)	0.0009 (8)	0.0018 (9)
O1	0.085 (4)	0.167 (6)	0.079 (4)	−0.048 (4)	−0.006 (3)	0.024 (4)
O2	0.134 (6)	0.156 (6)	0.159 (6)	0.031 (5)	0.008 (5)	0.044 (5)
O3	0.067 (4)	0.147 (5)	0.145 (5)	−0.017 (4)	−0.004 (4)	0.011 (4)
O4	0.207 (7)	0.170 (7)	0.122 (5)	−0.044 (6)	0.038 (5)	−0.044 (5)

Geometric parameters (Å, º)

Ag1—N2	2.110 (5)	C2—H2B	0.9700
Ag1—Ag2^i	2.966 (1)	C3—C4	1.520 (11)
Ag2—N4	2.115 (5)	C3—H3A	0.9700
N1—C5	1.315 (8)	C3—H3B	0.9700
N1—C7	1.346 (8)	C4—H4A	0.9700
N1—C1	1.476 (8)	C4—H4B	0.9700
N2—C5	1.310 (8)	C5—H5	0.9300
N2—C6	1.353 (9)	C6—C7	1.354 (10)
N3—C8	1.352 (9)	C6—H6	0.9300
N3—C10	1.356 (9)	C7—H7	0.9300
N3—C4	1.463 (9)	C8—H8	0.9300
N4—C8	1.293 (9)	C9—C10	1.339 (10)
N4—C9	1.358 (8)	C9—H9	0.9300
C1—C2	1.525 (9)	C10—H10	0.9300
C1—H1A	0.9700	Cl1—O4	1.379 (6)
C1—H1B	0.9700	Cl1—O1	1.420 (5)
C2—C3	1.488 (11)	Cl1—O3	1.422 (5)
C2—H2A	0.9700	Cl1—O2	1.434 (6)
N2^ii—Ag1—N2	179.2 (3)	C4—C3—H3B	108.7
N2—Ag1—Ag2^i	89.62 (15)	H3A—C3—H3B	107.6
N4^iii—Ag2—N4	177.2 (3)	N3—C4—C3	114.4 (6)
N4—Ag2—Ag1^iv	88.62 (16)	N3—C4—H4A	108.7
C5—N1—C7	107.6 (5)	C3—C4—H4A	108.7
C5—N1—C1	125.9 (5)	N3—C4—H4B	108.7
C7—N1—C1	126.5 (6)	C3—C4—H4B	108.7
C5—N2—C6	105.0 (5)	H4A—C4—H4B	107.6
C5—N2—Ag1	127.0 (4)	N2—C5—N1	112.0 (6)
C6—N2—Ag1	128.0 (5)	N2—C5—H5	124.0
C8—N3—C10	106.3 (6)	N1—C5—H5	124.0
C8—N3—C4	125.1 (7)	N2—C6—C7	109.6 (6)
C10—N3—C4	128.4 (7)	N2—C6—H6	125.2
C8—N4—C9	106.2 (6)	C7—C6—H6	125.2
C8—N4—Ag2	124.8 (5)	N1—C7—C6	105.8 (6)
C9—N4—Ag2	128.5 (5)	N1—C7—H7	127.1
N1—C1—C2	113.4 (5)	C6—C7—H7	127.1
N1—C1—H1A	108.9	N4—C8—N3	111.3 (6)
C2—C1—H1A	108.9	N4—C8—H8	124.3
N1—C1—H1B	108.9	N3—C8—H8	124.3
C2—C1—H1B	108.9	C10—C9—N4	109.6 (6)
H1A—C1—H1B	107.7	C10—C9—H9	125.2
C3—C2—C1	113.8 (6)	N4—C9—H9	125.2
C3—C2—H2A	108.8	C9—C10—N3	106.7 (6)
C1—C2—H2A	108.8	C9—C10—H10	126.7
C3—C2—H2B	108.8	N3—C10—H10	126.7
C1—C2—H2B	108.8	O4—Cl1—O1	112.3 (5)
H2A—C2—H2B	107.7	O4—Cl1—O3	113.5 (5)
C2—C3—C4	114.0 (7)	O1—Cl1—O3	107.6 (4)
C2—C3—H3A	108.7	O4—Cl1—O2	109.3 (6)
C4—C3—H3A	108.7	O1—Cl1—O2	108.4 (5)
C2—C3—H3B	108.7	O3—Cl1—O2	105.3 (5)
Ag2^i—Ag1—N2—C6	−79.7 (6)	C5—N2—C6—C7	1.4 (8)
Ag1^iv—Ag2—N4—C8	93.3 (6)	Ag1—N2—C6—C7	179.4 (5)
Ag1^iv—Ag2—N4—C9	−96.3 (6)	C5—N1—C7—C6	−0.2 (8)
C5—N1—C1—C2	−97.5 (7)	C1—N1—C7—C6	179.4 (6)
C7—N1—C1—C2	82.9 (8)	N2—C6—C7—N1	−0.8 (8)
N1—C1—C2—C3	61.2 (8)	C9—N4—C8—N3	−0.2 (8)
C1—C2—C3—C4	176.6 (6)	Ag2—N4—C8—N3	171.9 (5)
C8—N3—C4—C3	−82.0 (10)	C10—N3—C8—N4	−0.7 (8)
C10—N3—C4—C3	104.4 (9)	C4—N3—C8—N4	−175.4 (6)
C2—C3—C4—N3	−74.4 (9)	C8—N4—C9—C10	1.1 (8)
C6—N2—C5—N1	−1.6 (7)	Ag2—N4—C9—C10	−170.6 (5)
Ag1—N2—C5—N1	−179.6 (4)	N4—C9—C10—N3	−1.5 (8)
C7—N1—C5—N2	1.2 (8)	C8—N3—C10—C9	1.3 (8)
C1—N1—C5—N2	−178.5 (6)	C4—N3—C10—C9	175.8 (7)

Symmetry codes: (i) x−1, y, z; (ii) −x, y, −z+3/2; (iii) −x+2, y, −z+3/2; (iv) x+1, y, z.