An unprecedented three-dimensional silver(I) cluster built up from pyrazine-2,3-dicarboxyl­ate ligands

The title complex, poly[(μ-3-carboxy­pyrazine-2-carboxyl­ato)(μ-pyrazine-2,3-dicarboxyl­ato)tris­ilver(I)], [Ag₃(C₆H₂N₂O₄)(C₆H₃N₂O₄)]_n or [Ag₃(pzdca)(Hpzdca)]_n (H₂pzdca is pyrazine-2,3-dicarboxylic acid), has a three-dimensional structure. The carboxyl­ate groups of the pzdca²⁻ and Hpzdca⁻ ligands adopt both bridging and chelating coordination modes. Although each Ag^I ion displays a tetra­hedral coordination, the coordination environment of each Ag atom is very different, viz. AgN₃O, AgNO₃ and AgO₄.

S1. Comment

The design and synthesis of inorganic–organic composite coordination polymers exhibiting novel structures and properties have provided exciting new prospects (Cingolani et al., 2005; Dikarev et al., 2005). To date, a number of monometallic extended inorganic–organic composite materials have been synthesized by the combination of organic spacers and inorganic metal salts (Fujita et al., 1995). At this stage, confidence in accomplishing this goal is based upon the sophisticated selection and utilization of suitable metal ions and multifunctional organic ligands with certain features, such as being a multiple donor and having versatile bonding modes or the ability to take part in hydrogen bonding (Davis, 2002; Hagrman et al., 1999).

Pyrazine-2,3-dicarboxylic acid (H₂pzdca) and its anions have been of interest in coordination and supramolecular chemistry and they generally tend to react with metal salts to yield insoluble polynuclear materials. The deprotonated pzdca anions (Hpzdca and pzdca) behave as polyfunctional ligands and they are well known to act as bridging ligands in metal complexes (Konar et al., 2004; Beobide et al., 2006; Maji et al., 2004). Various possible coordination modes for the pzdca ligand, such as bidentate, bis(bidentate) bridge, tridentate bridge and [Text missing?] have been reported (Wenkin et al., 1997; Kondo et al., 1999; Liu et al., 2007). Silver(I) is used as a favourable building block for many coordination architectures, on account of its metallophilic interaction, short M—M bond and rich variety of coordination geometry, such as linear, bent, trigonal planar, T-shaped, tetrahedral and trigonal pyramidal. Smith and Jaber and their co-workers have reported two-dimensional silver complexes containing H₂pzdca in which the Ag^I ions exhibit trigonal–planar geometries (Smith et al., 1995; Jaber et al., 1994). To date, as far as we are aware, there is no three-dimensional Ag^I example containing pyrazine-2,3-dicarboxylate. Here, we report the structure of the title compound, (I).

Compound (I) involves one pzdca dianion, one Hpzdca monoanion and three Ag^I atoms and has a three-dimensional structure. Although each Ag^I ion adopts a tetrahedral coordination, their coordination environments are very different (Fig. 1). Atom Ag1 displays an AgN₃O framework, with the pzdca dianion acting as a chelating ligand through atoms N4 and O5, the Hpzdca monoanion acting as a monodentate ligand through atom N1 and a symmetry-related pzdca dianion acting as a monodentate ligand through atom N3. Atom Ag2 exhibits an AgO₃N coordination environment, which consists of the N,O-chelating group from the Hpzdca monoanion and two O atoms from different symmetry-related pzdca dianions. Atom Ag3 adopts an AgO₄ coordination environment, which consists of two chelating O atoms, O5 and O6, from the same carboxylate group and two monodentate atoms, O2 and O9, from symmetry-related pzdca and Hpzdca ligands. The carboxylic Ag—O and Ag—N bond lengths are in good agreement with reported values (Dong et al., 2004).

The previously known structures based on the pzdca ligand, {[silver(I)(pyrazine-2,3-dicarboxylate)]ammonium} and [silver(I)(pyrazine-2,3-dicarboxylate)ammine] (Smith et al., 1995; Jaber et al., 1994), display two-dimensional polymeric sheets. In the two structures, the Ag^I centres exhibit trigonal–planar geometries very different from the coordination reported here for (I). Furthermore, the presence of an O-donor group of a smaller ammine molecule in the second structure may affect the final packing. In the present work, both N atoms bond to Ag^I ions and the carboxylates adopt diverse coordination modes, both chelating and bridging, which are responsible for the building of the three-dimensional structure.

The two dicarboxylates of the pzdca ligands of (I) exhibit different bridging modes in the packing. One carboxylate group (C7) of the pzdca dianion adopts bidentate-chelating and bridging modes, and the second (C10) shows a tridentate-bridging mode. The carboxylate group (C1) of the Hpzdca monoanion shows a tridentate-bridging mode. The carboxylic acid group (C4) of the Hpzdca monoanion is not involved in the coordination. Based on these observations, if we only consider atom Ag1, a one-dimensional V-shaped chain is built up from Ag^I ions and both pzdca ligands along the b axis, as shown in Fig. 2. The Ag1···Ag1 separation across the pzdca ligand bridge is 7.55 (8) Å. Including atom Ag2 in the V-shaped one-dimensional network leads to a two-dimensional network parallel to the (101) plane, as shown in Fig. 3. Including atom Ag3 links these two-dimensional sheets to build a three-dimensional network. To the best of our knowledge, this is the first example of an Ag–pzdca framework constructed from such a unique structural motif.

It may be noted that there is a hydrogen bond between the protonated carboxylate group (O4) and the carboxylate O atom (O9), further stabilizing the Ag–organic framework of (I) (Table 1).

In conclusion, this complicated three-dimensional Ag structure can be described as a new type of Ag coordination polymer, with the pzdca ligand acting in monodentate, bidentate and bridging modes.

S2. Experimental

A solution of Na₂(pzdca) (22.5 mg, 0.12 mmol) in methanol (15 ml) was layered onto an aqueous solution of silver acetate (10 mg, 0.025 mmol). The resultant system was kept at room temperature for three weeks and yielded block crystals of (I).

S3. Refinement

All H atoms attached to C and O atoms were fixed geometrically and treated as riding, with C—H = 0.93 Å (pyridine ring) and O—H = 0.82 Å, with U_iso(H) = 1.2U_eq(C) or 1.5U_eq(O).

Atoms Ag2 and Ag3 display rather large ellipsoids but attempts to model a disordered distribution of these Ag atoms failed. The large ellipsoids might be related to the fact that these two Ag atoms are not tightly bonded to O atoms and have sufficient room to move.

Computing details

Data collection: APEX2 (Bruker, 2004); cell refinement: APEX2 (Bruker, 2004); data reduction: APEX2 (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEPIII (Burnett & Johnson, 1996) and ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. A view of compound (I), with the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms have been omitted for clarity. [Symmetry codes: (i) 3/2 - x, y - 1/2, 3/2 - z; (ii) 1 + x, y - 1, z; (iii) 2 - x, 1 - y, 1 - z; (iv) 3/2 - x, 1/2 + y, 1/2 - z; (v) 1 - x, 1 - y, 1 - z.]
	Fig. 2. The one-dimensional V-shaped Ag1 chain of (I).
	Fig. 3. A partial packing view of (I), showing the formation of the two-dimensional sheet built up from Ag1 and Ag2 atoms and pzdca dianions.

poly[(µ-pyrazine-2,3-dicarboxylato)(µ-3-carboxypyrazine-2- carboxylato)trisilver(I)]

Crystal data

[Ag3(C6H2N2O4)(C6H3N2O4)]	F(000) = 1240
Mr = 656.81	Dx = 2.991 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 2583 reflections
a = 11.312 (3) Å	θ = 2.1–25.1°
b = 10.158 (3) Å	µ = 4.05 mm^−1
c = 13.348 (4) Å	T = 298 K
β = 108.004 (5)°	Block, colourless
V = 1458.7 (7) Å^3	0.26 × 0.19 × 0.11 mm
Z = 4

Data collection

Bruker APEXII area-detector diffractometer	2583 independent reflections
Radiation source: fine-focus sealed tube	1978 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.031
ϕ and ω scans	θmax = 25.1°, θmin = 2.1°
Absorption correction: multi-scan (SADABS; Bruker, 2004)	h = −13→6
Tmin = 0.419, Tmax = 0.664	k = −12→12
7029 measured reflections	l = −15→15

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)] = 0.032	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.082	H-atom parameters constrained
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0443P)^2] where P = (Fo^2 + 2Fc^2)/3
2583 reflections	(Δ/σ)max = 0.001
245 parameters	Δρmax = 0.84 e Å^−3
0 restraints	Δρmin = −1.12 e Å^−3

Special details

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

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

	x	y	z	Uiso*/Ueq
Ag1	0.91986 (4)	0.46303 (4)	0.63617 (3)	0.03579 (15)
Ag2	1.32928 (4)	0.00428 (5)	0.47964 (4)	0.05419 (18)
Ag3	0.58048 (7)	0.50046 (5)	0.33507 (4)	0.0730 (2)
N1	1.0494 (4)	0.3249 (4)	0.5918 (3)	0.0305 (10)
N2	1.2054 (4)	0.1422 (4)	0.5440 (3)	0.0321 (10)
N3	0.6917 (4)	0.8021 (4)	0.7866 (3)	0.0286 (9)
N4	0.8309 (3)	0.6320 (4)	0.7051 (3)	0.0266 (9)
O1	1.1342 (4)	0.0266 (4)	0.3475 (3)	0.0470 (10)
O2	0.9427 (3)	0.1015 (4)	0.3198 (3)	0.0492 (10)
O3	0.8651 (3)	0.3734 (4)	0.3674 (3)	0.0411 (9)
O4	0.8004 (3)	0.2162 (4)	0.4565 (3)	0.0438 (9)
H4	0.7309	0.2376	0.4191	0.066*
O5	0.7276 (3)	0.5285 (3)	0.5083 (3)	0.0383 (9)
O6	0.5609 (4)	0.6504 (4)	0.4844 (3)	0.0525 (11)
O8	0.5045 (3)	0.9023 (3)	0.5829 (3)	0.0412 (9)
O9	0.4278 (3)	0.7301 (4)	0.6421 (3)	0.0406 (9)
C1	1.0542 (5)	0.0888 (5)	0.3730 (4)	0.0320 (12)
C2	1.0914 (4)	0.1636 (4)	0.4776 (4)	0.0262 (11)
C3	1.0133 (4)	0.2536 (5)	0.5019 (3)	0.0254 (11)
C4	0.8838 (4)	0.2863 (5)	0.4316 (4)	0.0294 (12)
C5	1.1630 (5)	0.3016 (5)	0.6557 (4)	0.0364 (13)
H5	1.1913	0.3488	0.7183	0.044*
C6	1.2403 (5)	0.2105 (5)	0.6329 (4)	0.0365 (13)
H6	1.3187	0.1964	0.6808	0.044*
C7	0.6639 (4)	0.6092 (5)	0.5382 (4)	0.0284 (11)
C8	0.7140 (4)	0.6657 (4)	0.6482 (3)	0.0242 (11)
C9	0.6459 (4)	0.7504 (4)	0.6894 (4)	0.0252 (11)
C10	0.5153 (4)	0.7973 (5)	0.6310 (4)	0.0269 (11)
C11	0.8067 (4)	0.7677 (5)	0.8411 (4)	0.0294 (12)
H11	0.8416	0.8018	0.9085	0.035*
C12	0.8762 (4)	0.6835 (5)	0.8012 (4)	0.0285 (11)
H12	0.9566	0.6621	0.8422	0.034*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ag1	0.0279 (2)	0.0412 (2)	0.0373 (2)	0.00940 (18)	0.00870 (18)	−0.00252 (18)
Ag2	0.0313 (3)	0.0572 (3)	0.0711 (4)	0.0182 (2)	0.0115 (2)	0.0085 (2)
Ag3	0.1189 (5)	0.0528 (3)	0.0294 (3)	−0.0139 (3)	−0.0033 (3)	0.0032 (2)
N1	0.022 (2)	0.040 (2)	0.028 (2)	0.0010 (18)	0.0045 (18)	−0.0030 (18)
N2	0.020 (2)	0.045 (2)	0.029 (2)	0.0051 (19)	0.0041 (18)	0.0045 (19)
N3	0.022 (2)	0.031 (2)	0.029 (2)	−0.0002 (18)	0.0028 (18)	−0.0029 (17)
N4	0.018 (2)	0.028 (2)	0.030 (2)	0.0029 (17)	0.0022 (17)	−0.0010 (17)
O1	0.037 (2)	0.055 (2)	0.045 (2)	0.0152 (19)	0.0070 (19)	−0.0141 (18)
O2	0.031 (2)	0.071 (3)	0.036 (2)	0.010 (2)	−0.0036 (17)	−0.023 (2)
O3	0.036 (2)	0.045 (2)	0.035 (2)	0.0027 (18)	0.0007 (16)	0.0060 (18)
O4	0.023 (2)	0.060 (2)	0.043 (2)	0.0010 (19)	0.0026 (18)	0.0101 (19)
O5	0.036 (2)	0.048 (2)	0.028 (2)	0.0138 (18)	0.0046 (17)	−0.0019 (16)
O6	0.034 (2)	0.076 (3)	0.034 (2)	0.024 (2)	−0.0086 (18)	−0.0136 (19)
O8	0.024 (2)	0.039 (2)	0.052 (2)	0.0027 (16)	−0.0012 (17)	0.0149 (18)
O9	0.020 (2)	0.052 (2)	0.047 (2)	0.0018 (17)	0.0051 (17)	0.0078 (18)
C1	0.032 (3)	0.032 (3)	0.031 (3)	0.004 (2)	0.008 (2)	0.002 (2)
C2	0.018 (3)	0.033 (3)	0.025 (3)	−0.001 (2)	0.003 (2)	0.004 (2)
C3	0.017 (3)	0.028 (2)	0.030 (3)	0.000 (2)	0.006 (2)	0.003 (2)
C4	0.026 (3)	0.036 (3)	0.024 (3)	−0.001 (2)	0.005 (2)	−0.007 (2)
C5	0.027 (3)	0.051 (3)	0.025 (3)	−0.004 (2)	−0.001 (2)	−0.006 (2)
C6	0.023 (3)	0.054 (3)	0.029 (3)	0.005 (2)	0.003 (2)	0.001 (2)
C7	0.026 (3)	0.030 (3)	0.026 (3)	−0.002 (2)	0.003 (2)	0.005 (2)
C8	0.016 (3)	0.027 (2)	0.025 (3)	0.0020 (19)	−0.002 (2)	0.0045 (19)
C9	0.019 (3)	0.023 (2)	0.029 (3)	−0.0013 (19)	0.001 (2)	0.002 (2)
C10	0.023 (3)	0.031 (3)	0.024 (3)	0.001 (2)	0.004 (2)	−0.002 (2)
C11	0.019 (3)	0.034 (3)	0.031 (3)	−0.003 (2)	0.002 (2)	−0.005 (2)
C12	0.017 (3)	0.034 (3)	0.027 (3)	0.000 (2)	−0.004 (2)	0.002 (2)

Geometric parameters (Å, º)

Ag1—N1	2.237 (4)	O2—C1	1.248 (5)
Ag1—N4	2.319 (4)	O3—C4	1.204 (6)
Ag1—O5	2.406 (3)	O4—C4	1.304 (6)
Ag1—N3^i	2.476 (4)	O4—H4	0.8200
Ag2—O8^ii	2.283 (3)	O5—C7	1.237 (6)
Ag2—N2	2.326 (4)	O6—C7	1.237 (5)
Ag2—O1	2.372 (4)	O8—C10	1.232 (6)
Ag2—O8^iii	2.470 (4)	O9—C10	1.248 (6)
Ag3—O2^iv	2.249 (3)	C1—C2	1.530 (7)
Ag3—O9^v	2.368 (4)	C2—C3	1.377 (6)
Ag3—O5	2.412 (3)	C3—C4	1.512 (6)
Ag3—O6	2.572 (4)	C5—C6	1.370 (7)
N1—C5	1.326 (6)	C5—H5	0.9300
N1—C3	1.352 (5)	C6—H6	0.9300
N2—C6	1.326 (6)	C7—C8	1.513 (6)
N2—C2	1.337 (6)	C8—C9	1.378 (7)
N3—C11	1.326 (6)	C9—C10	1.518 (6)
N3—C9	1.346 (6)	C11—C12	1.375 (6)
N4—C12	1.333 (5)	C11—H11	0.9300
N4—C8	1.350 (5)	C12—H12	0.9300
O1—C1	1.234 (6)
N1—Ag1—N4	165.82 (14)	Ag2^viii—O8—Ag2^iii	103.37 (13)
N1—Ag1—O5	120.52 (13)	C10—O9—Ag3^v	128.3 (3)
N4—Ag1—O5	70.68 (12)	O1—C1—O2	126.4 (5)
N1—Ag1—N3^i	98.96 (14)	O1—C1—C2	119.2 (4)
N4—Ag1—N3^i	89.07 (14)	O2—C1—C2	114.4 (4)
O5—Ag1—N3^i	90.73 (13)	N2—C2—C3	120.6 (4)
O8^ii—Ag2—N2	124.09 (14)	N2—C2—C1	117.4 (4)
O8^ii—Ag2—O1	157.94 (13)	C3—C2—C1	121.9 (4)
N2—Ag2—O1	71.85 (13)	N1—C3—C2	121.6 (4)
O8^ii—Ag2—O8^iii	76.63 (13)	N1—C3—C4	113.7 (4)
N2—Ag2—O8^iii	120.12 (13)	C2—C3—C4	124.7 (4)
O1—Ag2—O8^iii	110.02 (13)	O3—C4—O4	126.9 (5)
O2^iv—Ag3—O9^v	124.88 (14)	O3—C4—C3	121.7 (4)
O2^iv—Ag3—O5	132.61 (13)	O4—C4—C3	111.1 (4)
O9^v—Ag3—O5	92.07 (12)	N1—C5—C6	122.4 (5)
O2^iv—Ag3—O6	115.36 (15)	N1—C5—H5	118.8
O9^v—Ag3—O6	118.02 (13)	C6—C5—H5	118.8
O5—Ag3—O6	52.16 (12)	N2—C6—C5	121.2 (4)
C5—N1—C3	116.2 (4)	N2—C6—H6	119.4
C5—N1—Ag1	121.7 (3)	C5—C6—H6	119.4
C3—N1—Ag1	121.9 (3)	O6—C7—O5	125.1 (5)
C6—N2—C2	117.9 (4)	O6—C7—C8	116.0 (4)
C6—N2—Ag2	126.8 (3)	O5—C7—C8	118.9 (4)
C2—N2—Ag2	114.9 (3)	N4—C8—C9	120.3 (4)
C11—N3—C9	116.5 (4)	N4—C8—C7	117.3 (4)
C11—N3—Ag1^vi	119.0 (3)	C9—C8—C7	122.4 (4)
C9—N3—Ag1^vi	124.4 (3)	N3—C9—C8	122.2 (4)
C12—N4—C8	117.5 (4)	N3—C9—C10	113.4 (4)
C12—N4—Ag1	126.0 (3)	C8—C9—C10	124.4 (4)
C8—N4—Ag1	115.7 (3)	O8—C10—O9	125.5 (4)
C1—O1—Ag2	115.9 (3)	O8—C10—C9	117.6 (4)
C1—O2—Ag3^vii	106.6 (3)	O9—C10—C9	116.7 (4)
C4—O4—H4	109.5	N3—C11—C12	122.2 (4)
C7—O5—Ag1	116.3 (3)	N3—C11—H11	118.9
C7—O5—Ag3	95.1 (3)	C12—C11—H11	118.9
Ag1—O5—Ag3	148.42 (16)	N4—C12—C11	121.4 (4)
C7—O6—Ag3	87.6 (3)	N4—C12—H12	119.3
C10—O8—Ag2^viii	129.4 (3)	C11—C12—H12	119.3
C10—O8—Ag2^iii	123.1 (3)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O4—H4···O9^v	0.82	1.76	2.568 (5)	169

Symmetry code: (v) −x+1, −y+1, −z+1.