The title complex, [Ag(C
5H
4NO)(C
5H
5NO)]
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.
Supporting information
CCDC reference: 248998
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%.
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 Uiso(H) = 1.5Ueq(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 Uiso(H) = 1.2Ueq(C).
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.
catena-Poly[[(pyridin-4-ol-
κN)silver(I)]-µ-pyridin-4-olato-
κ2N:
O]
top
Crystal data top
[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) Å3 | 0.37 × 0.24 × 0.15 mm |
Z = 4 | |
Data collection top
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 top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.025 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.056 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.06 | w = 1/[σ2(Fo2) + (0.0316P)2 + 0.2855P] where P = (Fo2 + 2Fc2)/3 |
2234 reflections | (Δ/σ)max = 0.001 |
139 parameters | Δρmax = 0.59 e Å−3 |
1 restraint | Δρmin = −0.28 e Å−3 |
Crystal data top
[Ag(C5H4NO)(C5H5NO)] | V = 976.1 (4) Å3 |
Mr = 297.06 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 9.1673 (18) Å | µ = 2.04 mm−1 |
b = 10.401 (2) Å | T = 293 K |
c = 11.026 (2) Å | 0.37 × 0.24 × 0.15 mm |
β = 111.81 (3)° | |
Data collection top
Rigaku R-AXIS RAPID area-detector diffractometer | 2234 independent reflections |
Absorption correction: multi-scan (ABSCOR; Higashi, 1995) | 1929 reflections with I > 2σ(I) |
Tmin = 0.519, Tmax = 0.749 | Rint = 0.024 |
9387 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.025 | 1 restraint |
wR(F2) = 0.056 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.06 | Δρmax = 0.59 e Å−3 |
2234 reflections | Δρmin = −0.28 e Å−3 |
139 parameters | |
Special details top
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 (Å2) top | 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 (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
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) top
Ag1—N2 | 2.129 (2) | N1—C5 | 1.341 (3) |
Ag1—N1 | 2.131 (2) | N1—C1 | 1.349 (3) |
Ag1—O2i | 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—O2i | 91.5 (2) | C3—C2—H2 | 120.2 |
N2—Ag1—O2i | 92.9 (2) | C3—C4—H4 | 120.2 |
N2—Ag1—N1 | 174.12 (8) | C4—C3—C2 | 116.8 (2) |
Ag1i—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) top
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H11···O2ii | 0.84 (3) | 1.64 (3) | 2.476 (3) | 171 (4) |
Symmetry code: (ii) x+1, y−1, z. |
Experimental details
Crystal data |
Chemical formula | [Ag(C5H4NO)(C5H5NO)] |
Mr | 297.06 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 293 |
a, b, c (Å) | 9.1673 (18), 10.401 (2), 11.026 (2) |
β (°) | 111.81 (3) |
V (Å3) | 976.1 (4) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 2.04 |
Crystal size (mm) | 0.37 × 0.24 × 0.15 |
|
Data collection |
Diffractometer | Rigaku R-AXIS RAPID area-detector diffractometer |
Absorption correction | Multi-scan (ABSCOR; Higashi, 1995) |
Tmin, Tmax | 0.519, 0.749 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 9387, 2234, 1929 |
Rint | 0.024 |
(sin θ/λ)max (Å−1) | 0.649 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.025, 0.056, 1.06 |
No. of reflections | 2234 |
No. of parameters | 139 |
No. of restraints | 1 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.59, −0.28 |
Selected geometric parameters (Å, º) topAg1—N2 | 2.129 (2) | C6—C7 | 1.368 (4) |
Ag1—N1 | 2.131 (2) | C7—C8 | 1.406 (3) |
Ag1—O2i | 2.743 (2) | C8—C9 | 1.405 (3) |
O1—C3 | 1.318 (3) | C9—C10 | 1.362 (3) |
O2—C8 | 1.300 (3) | N1—C5 | 1.341 (3) |
C1—C2 | 1.375 (3) | N1—C1 | 1.349 (3) |
C2—C3 | 1.396 (3) | N2—C10 | 1.340 (3) |
C3—C4 | 1.393 (3) | N2—C6 | 1.349 (3) |
C4—C5 | 1.366 (4) | | |
| | | |
N1—Ag1—O2i | 91.5 (2) | N2—Ag1—N1 | 174.12 (8) |
N2—Ag1—O2i | 92.9 (2) | | |
Symmetry code: (i) −x+1, y−1/2, −z+3/2. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H11···O2ii | 0.84 (3) | 1.64 (3) | 2.476 (3) | 171 (4) |
Symmetry code: (ii) x+1, y−1, z. |
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 AgI ion into the coordination system of the 4-PyOH ligand. Our most unexpected discovery, reported here, is that the AgI 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 AgI 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 AgI 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 AgI centre of (I). The main interatomic bond distances and angles are listed in Table 1. Each AgI 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 AgI 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 AgI 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 AgI 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···Ohydroxyl interactions between adjacent chains [3.265 (3) Å]. These weak Ag···Ohydroxyl 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.