catena-Poly­[[(pyridin-4-ol-κN)silver(I)]-μ-pyridin-4-olato-κ²N:O]

The title complex, [Ag(C₅H₄NO)(C₅H₅NO)]_n, consists of a polymeric neutral chain involving both a neutral pyridin-4-ol ligand and a deprotonated pyridin-4-olate monoanion. The Ag^I atom shows a T-shaped coordination geometry, defined by one N atom of the pyridin-4-ol and one O and one N atom of two independent pyridin-4-olate bridges; the N—Ag—N moiety is approximately linear. The polymeric chains are connected via strong O—H...O hydrogen bonds and offset π–π interactions into a three-dimensional network.

S1. Comment

Hydroxypyridines (PyOH), including 2-, 3- and 4-PyOH, are widely used in pharmaceutical synthesis. Furthermore, they have attracted growing attention in the field of crystal engineering as good candidates for the construction of supramolecular systems. They are bifunctional ligands that are not only capable of binding to metal centres, but can also form classical hydrogen bonds, both as donors and acceptors (Breeze & Wang, 1993; Kawata et al., 1997).

Another important feature of 2- or 4-PyOH is tautomerization to their pyridone isomers, viz. 2- or 4-pyridone. In the solid state, the pyridone is the observed tautomeric form for 2- or 4-PyOH (Yang & Craven, 1998; Wheeler & Ammon, 1974; Trikoupis et al., 2002; Jones, 2001), whereas the 3-isomer is a true hydroxypyridine (Flakus et al., 2003). For 4-PyOH, the equilibrium between the two tautomers is dominated by the presence of the keto form in polar solvents (Johnson, 1984; Gilchrist, 1985). Because the N atom is protonated, 4-PyOH would be expected to coordinate, if at all, through the O atom, which has only weak donor properties (Masse & Le Fur, 1998). It is therefore predictably difficult to obtain complexes with the 4-PyOH tautomer. As a contribution to this field, we have recently reported the structures of two mononuclear Co complexes and one dimeric Cu complex coordinated by the O atoms of 4-pyridone ligands, namely [CoCl₂(4-pyridone)₂], [Co(NO₃)(4-pyridone)₂(H₂O)₂](NO₃) and [Cu₂(acetate)₄(4-pyridone)₂], in which the Co atoms exhibit tetrahedral and octahedral geometries, while the Cu atom displays a square-pyramidal coordination environment (Gao et al., 2004; Lu Gao Huo Zhang et al., 2004; Lu Gao Huo Zhao & Zhao, 2004).

In order to gain further insight into the metal-binding modes of the 4-PyOH ligand, we have now introduced the Ag^I ion into the coordination system of the 4-PyOH ligand. Our most unexpected discovery, reported here, is that the Ag^I atom favours coordination via the N atom and effectively reverses the usual tautomerization to the 4-pyridone tautomer, producing the title novel coordination polymer, [Ag(µ-pyridine-4-olate-N,O)(4-PyOH)]_n, (I), based on both neutral and deprotonated forms of the 4-PyOH tautomer. To the best of our knowledge, (I) is the first example of a metal complex containing the 4-PyOH tautomer in either form. \sch

The crystal structure of complex (I) consists of polymeric neutral [Ag(µ-pyridine-4-olate-N,O)(4-pyOH)]_n chains, in which the Ag^I atoms are bridged sequentially by N,O-bidentate pyridine-4-olate ligands (Figs. 1 and 2).

All the 4-PyOH ligands in (I) are coordinated to the Ag^I atoms in the 4-PyOH tautomer form, or its deprotonated counterpart, as evidenced by the C—O [1.300 (3) and 1.318 (3) Å] and C—N [1.340 (3)–1.349 (3) Å] bond lengths. A similar bond-length pattern is observed in free 3-PyOH [two independent molecules; C—O 1.3462 and 1.3466 (12) Å, and C—N 1.3361 (14)–1.3403 (13) Å; Flakus et al., 2003]. In the structure of free 4-pyridone (Jones, 2001), the C—O bond lengths [1.258 (2)–1.278 (2) Å] are significantly shorter than in (I), while the C—N bond distances [1.341 (3)–1.355 (3) Å] are slightly longer than in (I).

The C6—C7 and C9—C10 bond lengths [1.368 (4) and 1.362 (3) Å] are similar to the values observed in the terminal 4-PyOH ligand [1.375 (3) and 1.366 (4) Å]. The O2—C8 bond length [1.300 (3) Å] is shorter than the O1—C3 bond length [1.318 (3) Å], again indicating that the deprotonated 4-PyOH ligand assumes the pyridine-4-olate form. However, there is still a slight `pyridone-like' pattern to the C—C bond lengths, albeit much less marked than in 4-pyridone, with averages of 1.362 and 1.426 Å.

Fig. 1 shows the coordination environment of the Ag^I centre of (I). The main interatomic bond distances and angles are listed in Table 1. Each Ag^I atom shows a T-shaped coordination geometry, involving two N atoms, one each from a neutral terminal 4-PyOH ligand and a deprotonated pyridine-4-olate bridging ligand [N2—Ag1—N1 174.12 (8)°], and one deprotonated hydroxyl O atom of a pyridine-4-olate ligand in a neighbouring asymmetric unit. The N1—Ag1—O2 and N2—Ag1—O2 bond angles are 91.5 (2) and 92.9 (2)°, respectively. The Ag—N bond length is as expected [mean 2.130 (2) Å; Cai et al., 2001], while the Ag—O bond [2.743 (2) Å] is longer than common Ag—O distances reported for Ag^I complexes with a T-shaped geometry, which range from 2.235 (4) Å in [Ag₂(PPh₃)₄(SO₄)]·2H₂O (Bowmaker et al., 2001) to 2.613 (4) Å in [Ag(ODOQ)]_n(ClO₄)_n [ODOQ = O,O'-bis(8-quinolyl)-1,8-dioxaoctane] (Cai et al., 2001), but shorter than the value observed in the complex Ag₂CA (CA = cyanuric acid; 2.76 Å; Rao et al., 2000).

As a consequence of the pyridine-4-olate bridges, the adjacent Ag^I atoms are linked into a one-dimensional zigzag chain structure, which has terminal branches (the neutral hydroxypyridines). The intrachain Ag···Ag separation across the pyridine-4-olate ligand is 7.672 (3) Å. The polymeric chains are parallel to the b axis of the unit cell and show a corrugated arrangement, with an Ag···Ag···Ag angle of 85.40 (4)° between three successive Ag^I ions along the chain (Fig. 2).

The aromatic rings of the terminal and bridging ligands (those involving atoms N1 and O2, respectively) are almost coplanar, with an interplanar angle of 1.26 (3)°, while the mean planes of adjacent pyridine-4-olate bridges are almost perpendicular to each other [84.95 (6)°]. There are offset face-to-face interactions [(b) in the Scheme], with a `head-to-tail' arrangement between the terminal 4-PyOH ligands, with a centre-to-centre separation of 3.474 (3) Å (symmetry code: 2 − x, 1 − y, 1 − z). Such strong facial interactions may be considered as a cause of the weak Ag···O_hydroxyl interactions between adjacent chains [3.265 (3) Å]. These weak Ag···O_hydroxyl and the strong π–π interactions constitute small rectangular grids [Ag···Ag = 7.407 (3) Å] between adjacent chains and form a layer structure parallel to the bc plane (Fig. 2). Each layer includes the one-dimensional hydrogen-bonded chains via strong ionic O—H···O⁻ interactions between the O atoms of the terminal 4-PyOH ligand and the anionic pyridine-4-olate bridge [O···O 2.476 (3) Å]. The shortest Ag···Ag distance is 3.835 (3) Å, between adjacent hydrogen-bonded chains.

The layers are further linked into a three-dimensional supramolecular framework (Fig. 3) via another offset π–π interaction [(c) in the Scheme] in a `head-to-head' fashion between the terminal 4-PyOH ligand and the monoanionic pyridine-4-olate bridge, with a centre-to-centre separation of 3.678 (3) Å (symmetry code: 1 − x, 1 − y, 1 − z), which is significantly longer than the value observed in the head-to-tail arrangement. This is explained by the fact that the head-to-tail arrangement may be energetically more favourable than the head-to-head orientation, due to the more effective charge separation, thus reducing the Coulombic repulsion between cationic centres. The facial interactions between the aromatic groups of the 4-PyOH ligands seem to play a critical role in the self-assembly process.

S2. Experimental

The title complex, (I), was synthesized by the addition of AgNO₃ (2 mmol) to an ethanol solution of 4-hydroxypyridine (6 mmol). The mixed solution was protected from light and allowed to evaporate slowly at room temperature, and colourless prismatic crystals of (I) were isolated after about 15 d. Analysis calculated for C₁₀H₉N₂O₂Ag: C 40.43, H 3.05, N 9.43%; found: C 40.20, H 3.11, N 9.47%.

S3. Refinement

The H atoms of the hydroxyl groups were located in a difference map and refined with O—H distance restraints of 0.85 (1) Å, and with U_iso(H) = 1.5U_eq(O). H atoms bound to C atoms were placed in calculated positions, with C—H = 0.93 Å, and refined using a riding model approximation, with U_iso(H) = 1.2U_eq(C).

Computing details

Data collection: RAPID-AUTO (Rigaku Corporation, 1998); cell refinement: RAPID-AUTO; data reduction: CrystalStructure (Rigaku/MSC, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The coordination environment of the Ag^I atom in (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. A perspective view of the chain and layer structure of (I). The weak Ag···O [3.265 (3) Å] contacts are denoted by dashed lines and the H atoms of the aromatic rings have been omitted.
	Fig. 3. A diagram of the three-dimensional supramolecular framework of (I), showing the two types of π–π pairs and hydrogen-bonding interactions. Hydrogen bonds and the weak Ag···O [3.265 (3) Å] contacts are denoted by dashed lines.

catena-Poly[[(pyridin-4-ol-κN)silver(I)]-µ-pyridin-4-olato-κ²N:O]

Crystal data

[Ag(C5H4NO)(C5H5NO)]	F(000) = 584
Mr = 297.06	Dx = 2.022 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 8987 reflections
a = 9.1673 (18) Å	θ = 3.1–27.5°
b = 10.401 (2) Å	µ = 2.04 mm^−1
c = 11.026 (2) Å	T = 293 K
β = 111.81 (3)°	Prism, colourless
V = 976.1 (4) Å^3	0.37 × 0.24 × 0.15 mm
Z = 4

Data collection

Rigaku R-AXIS RAPID area-detector diffractometer	2234 independent reflections
Radiation source: fine-focus sealed tube	1929 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.024
Detector resolution: 10 pixels mm^-1	θmax = 27.5°, θmin = 3.1°
ω scans	h = −11→11
Absorption correction: multi-scan (ABSCOR; Higashi, 1995)	k = −11→13
Tmin = 0.519, Tmax = 0.749	l = −14→14
9387 measured reflections

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)] = 0.025	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.056	H atoms treated by a mixture of independent and constrained refinement
S = 1.06	w = 1/[σ^2(Fo^2) + (0.0316P)^2 + 0.2855P] where P = (Fo^2 + 2Fc^2)/3
2234 reflections	(Δ/σ)max = 0.001
139 parameters	Δρmax = 0.59 e Å^−3
1 restraint	Δρmin = −0.28 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.

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

	x	y	z	Uiso*/Ueq
Ag1	0.66638 (2)	0.620516 (17)	0.580211 (19)	0.03813 (8)
O1	1.0789 (3)	0.19027 (19)	0.47214 (19)	0.0497 (5)
O2	0.2413 (2)	1.05781 (16)	0.66040 (17)	0.0376 (4)
N1	0.7977 (2)	0.46500 (18)	0.5463 (2)	0.0333 (4)
N2	0.5168 (2)	0.77112 (18)	0.5953 (2)	0.0350 (4)
C1	0.9023 (3)	0.3973 (2)	0.6448 (2)	0.0341 (5)
C2	0.9982 (3)	0.3038 (2)	0.6265 (2)	0.0332 (5)
C3	0.9902 (3)	0.2759 (2)	0.5003 (2)	0.0327 (5)
C4	0.8820 (3)	0.3456 (2)	0.3986 (2)	0.0361 (5)
C5	0.7912 (3)	0.4368 (2)	0.4257 (2)	0.0349 (5)
C6	0.4125 (3)	0.8382 (2)	0.4961 (2)	0.0383 (5)
C7	0.3180 (3)	0.9336 (2)	0.5114 (2)	0.0368 (5)
C8	0.3251 (3)	0.9673 (2)	0.6370 (2)	0.0289 (5)
C9	0.4313 (3)	0.8944 (2)	0.7391 (2)	0.0350 (5)
C10	0.5215 (3)	0.8013 (2)	0.7149 (2)	0.0360 (5)
H1	0.9099	0.4148	0.7296	0.041*
H2	1.0679	0.2595	0.6977	0.040*
H4	0.8716	0.3301	0.3127	0.043*
H5	0.7202	0.4822	0.3560	0.042*
H6	0.4043	0.8183	0.4116	0.046*
H7	0.2488	0.9762	0.4386	0.044*
H9	0.4402	0.9100	0.8246	0.042*
H10	0.5905	0.7560	0.7856	0.043*
H11	1.140 (3)	0.152 (3)	0.539 (2)	0.075*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ag1	0.03988 (12)	0.03334 (11)	0.04500 (12)	0.00966 (8)	0.02019 (9)	−0.00097 (8)
O1	0.0598 (12)	0.0504 (11)	0.0414 (11)	0.0288 (10)	0.0218 (9)	0.0062 (9)
O2	0.0411 (10)	0.0332 (9)	0.0393 (9)	0.0105 (7)	0.0157 (8)	−0.0006 (7)
N1	0.0333 (10)	0.0299 (9)	0.0400 (11)	0.0049 (8)	0.0176 (9)	0.0026 (8)
N2	0.0370 (11)	0.0301 (10)	0.0396 (11)	0.0046 (8)	0.0162 (9)	−0.0023 (9)
C1	0.0361 (12)	0.0337 (13)	0.0360 (12)	0.0008 (9)	0.0174 (10)	−0.0001 (9)
C2	0.0337 (12)	0.0328 (12)	0.0332 (12)	0.0033 (9)	0.0126 (10)	0.0055 (9)
C3	0.0334 (12)	0.0294 (11)	0.0382 (12)	0.0047 (9)	0.0165 (10)	0.0015 (9)
C4	0.0402 (13)	0.0391 (13)	0.0303 (12)	0.0068 (10)	0.0147 (10)	0.0007 (10)
C5	0.0350 (12)	0.0343 (13)	0.0342 (13)	0.0073 (10)	0.0114 (10)	0.0058 (10)
C6	0.0475 (14)	0.0374 (13)	0.0326 (12)	0.0067 (11)	0.0179 (11)	−0.0014 (10)
C7	0.0401 (13)	0.0346 (13)	0.0350 (13)	0.0073 (10)	0.0131 (10)	0.0030 (10)
C8	0.0277 (11)	0.0241 (10)	0.0354 (12)	−0.0017 (8)	0.0123 (9)	−0.0021 (9)
C9	0.0374 (12)	0.0349 (13)	0.0306 (11)	0.0039 (10)	0.0102 (10)	−0.0044 (9)
C10	0.0348 (13)	0.0343 (12)	0.0359 (13)	0.0064 (10)	0.0095 (10)	0.0005 (10)

Geometric parameters (Å, º)

Ag1—N2	2.129 (2)	N1—C5	1.341 (3)
Ag1—N1	2.131 (2)	N1—C1	1.349 (3)
Ag1—O2^i	2.743 (2)	N2—C10	1.340 (3)
O1—C3	1.318 (3)	N2—C6	1.349 (3)
O2—C8	1.300 (3)	O1—H11	0.84 (3)
C1—C2	1.375 (3)	C1—H1	0.9300
C2—C3	1.396 (3)	C2—H2	0.9300
C3—C4	1.393 (3)	C4—H4	0.9300
C4—C5	1.366 (4)	C5—H5	0.9300
C6—C7	1.368 (4)	C6—H6	0.9300
C7—C8	1.406 (3)	C7—H7	0.9300
C8—C9	1.405 (3)	C9—H9	0.9300
C9—C10	1.362 (3)	C10—H10	0.9300
N1—Ag1—O2^i	91.5 (2)	C3—C2—H2	120.2
N2—Ag1—O2^i	92.9 (2)	C3—C4—H4	120.2
N2—Ag1—N1	174.12 (8)	C4—C3—C2	116.8 (2)
Ag1^i—O2—C8	113.8 (2)	C4—C5—H5	117.9
O1—C3—C4	118.6 (2)	C5—C4—C3	119.7 (2)
O1—C3—C2	124.5 (2)	C5—C4—H4	120.2
O2—C8—C9	121.1 (2)	C5—N1—C1	116.1 (2)
O2—C8—C7	124.1 (2)	C5—N1—Ag1	121.33 (16)
N1—C1—C2	123.5 (2)	C6—N2—Ag1	126.88 (17)
N1—C1—H1	118.2	C6—C7—C8	120.1 (2)
N1—C5—C4	124.2 (2)	C6—C7—H7	119.9
N1—C5—H5	117.9	C8—C7—H7	119.9
N2—C6—C7	124.4 (2)	C8—C9—H9	119.4
N2—C6—H6	117.8	C7—C6—H6	117.8
N2—C10—C9	124.0 (2)	C9—C8—C7	114.8 (2)
N2—C10—H10	118.0	C9—C10—H10	118.0
C1—N1—Ag1	122.26 (16)	C10—C9—C8	121.1 (2)
C1—C2—C3	119.6 (2)	C10—C9—H9	119.4
C1—C2—H2	120.2	C10—N2—C6	115.5 (2)
C2—C1—H1	118.2	C10—N2—Ag1	117.59 (16)
C3—O1—H11	113 (3)
Ag1—N1—C5—C4	174.3 (2)	C1—N1—C5—C4	0.2 (4)
Ag1—N1—C1—C2	−174.3 (2)	C2—C3—C4—C5	0.5 (4)
Ag1—N2—C10—C9	−179.3 (2)	C3—C4—C5—N1	−0.3 (4)
Ag1—N2—C6—C7	179.4 (2)	C5—N1—C1—C2	−0.2 (3)
O1—C3—C4—C5	−178.6 (3)	C6—C7—C8—O2	178.9 (2)
O2—C8—C9—C10	−178.6 (2)	C6—C7—C8—C9	−1.3 (4)
N1—C1—C2—C3	0.4 (4)	C6—N2—C10—C9	−1.2 (4)
N2—C6—C7—C8	−0.2 (4)	C7—C8—C9—C10	1.6 (3)
C1—C2—C3—O1	178.5 (2)	C8—C9—C10—N2	−0.4 (4)
C1—C2—C3—C4	−0.5 (3)	C10—N2—C6—C7	1.5 (4)

Symmetry code: (i) −x+1, y−1/2, −z+3/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H11···O2^ii	0.84 (3)	1.64 (3)	2.476 (3)	171 (4)

Symmetry code: (ii) x+1, y−1, z.