A three-dimensional zinc-based coordination polymer built from 5-carboxyl­ato-1-carboxyl­atomethyl-2-oxidopyridinium with a 4-connected sra topology

A novel three-dimensional Zn^II complex, poly[aqua­(μ₄-5-car­boxyl­ato-1-carboxyl­ato­methyl-2-oxido­pyridinium)zinc(II)], [Zn(C₈H₅NO₄)(H₂O)]_n, has been prepared by hydro­thermal assembly of Zn(CH₃COO)₂·2H₂O and 5-carb­oxy-1-(carb­oxy­methyl)pyridin-1-ium-2-olate (H₂ccop). The ccop²⁻ anions bridge the Zn^II cations in a head-to-tail fashion via monodentate aromatic carboxylate and phenolate O atoms to form an extended zigzag chain which runs parallel to the [011] direction. One O atom of the aliphatic carboxylate group of the ccop²⁻ ligand coordinates to the Zn^II atom of a neighbouring chain thereby producing undulating layers which lie parallel to the (01) plane. A similar parallel undulating planar structure can be obtained if a path involving the other O atom of the aliphatic carboxylate group is considered. Thus, the aliphatic carboxylate group acts in a bridging bidentate mode to give extended –Zn–O–C–O–Zn– sequences running parallel to [001] which link the layers into an overall three-dimensional framework. The three-dimensional framework can be simplified as a 4-connected sra topology with a Schläfli symbol of 4².6³.8 if all the Zn^II centres and ccop²⁻ anions are regarded as tetra­hedral 4-connected nodes. The three-dimensional luminescence spectrum was measured at room temperature with excitation and emission wavelengths of 344–354 and 360–630 nm, respectively, at inter­vals of 0.15 and 2 nm, respectively.

S1. Introduction

Coordination polymers (CPs) based on multi­carboxyl­ate ligands and metal centres have received much attention in coordination chemistry due to their various topologies and potential applications, such as guest exchange, gas storage and separation, catalysis, drug delivery, luminescence etc. (He et al., 2014; Gandara et al., 2014; Furukawa et al., 2013; Li, Li, O'Keeffe & Yaghi, 2014; Cui et al., 2012; Betard & Fischer, 2012). Of these multi­carboxyl­ate based CPs, compounds having ligands with carboxyl­ate groups attached to aromatic rings are found to be widely utilized and dominate the literature (Furukawa et al., 2013; Cook et al., 2013; Li, Li, Zhou et al., 2014). These ligands tend to be quite rigid, as the carboxyl­ate groups are usually coplanar with the aromatic rings. In contrast, work on the construction of CPs based on flexible multi­carboxyl­ate ligands, where the carboxyl­ate is attached to an aliphatic C atom, is still scarce, especially when the multi­carboxyl­ate ligand involves a mixture of rigid (aromatic) and flexible (aliphatic) carboxyl­ate groups. The rational design of organic building blocks plays a key role in adjusting the coordination framework, and the nature of the coordinating donors may lead to the formation of unique networks with desired properties and functions (Li et al., 2010). 5-Carb­oxy-1-(carb­oxy­methyl)­pyridin-1-ium-2-olate (H₂ccop) possesses one rigid carboxyl­ate group, one flexible carboxyl­ate group and one phenolate O atom, so it can act as an excellent multidentate ligand with a range of versatile binding and coordination modes. Though it is a good multidentate ligand, H₂ccop has not been well exploited in building CPs and only a few metal complexes of H₂ccop have been reported, most of them being zero-, one- or two-dimensional structures (Zhan et al., 2010; Yuan et al., 2011; Jiang et al., 2009; Lin & He, 2007). On the basis of these considerations, we chose to react Zn(CH₃COO)₂·2H₂O with H₂ccop and produced the title compound, [Zn(ccop)(H₂O)]_n, (I).

S2. Experimental

S2.1. Synthesis and crystallization

A mixture of zinc acetate dihydrate (0.148 g, 0.5 mmol), H₂ccop (0.10 g, 0.5 mmol), NaOH (0.08 g, 0.2 mmol) and H₂O (12 ml) was placed in a 23 ml Teflon reactor, which was heated to 403 K for 3 d and then cooled to room temperature at a rate of 10 K h^-1. The colourless crystals of (I) obtained were washed with methanol and dried in air (yield 0.12 g, 89%). IR (KBr, ν, cm^-1): 3460 (s), 1662 (s), 1438 (m), 1396 (s), 1326 (w), 1267 (w), 1211 (w), 1141 (m), 997 (w), 937 (w), 858 (w), 792 (w), 663 (w), 607 (w), 522 (w).

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. C-bound H atoms were placed at calculated positions and treated as riding, with C—H = 0.93 and 0.97 Å, and with U_iso(H) = 1.2U_eq(C). Water H atoms were tentatively located in difference Fourier maps and were refined with distance restraints of O—H = 0.82 Å and H···H = 1.39 Å, and with U_iso(H) = 1.5U_eq(O). The hightest residual electron-density peak is located 0.92 Å from atom O6 and the deepest hole is located 0.75 Å from atom Zn1.

S3. Results and discussion

The asymmetric unit of (I) contains one unique Zn^II cation, one coordinated water molecule and one ccop^2- ligand (Fig. 1). The Zn^II centre is five-coordinated by four carboxyl­ate O atoms from four different ccop^2- ligands and one water molecule, adopting a distorted trigonal–biyramidal geometry. The Zn—O bond lengths and O—Zn—O bond angles are in the ranges 1.964 (3)–2.097 (3) Å, and 86.48 (11)–176.09 (13) and 111.27 (12)–125.25 (12)° [Why two angle ranges?], respectively (Table 2), which are within the ranges of observed values for other five-coordinated Zn^II complexes with oxygen-donating ligands (Ma et al., 2013; Kumagai et al., 2003; Dong et al., 2012). In the polymeric structure of (I), the ccop^2- ligands adopt the same µ₄-bridging mode to link four Zn^II cations. The ccop^2- ligand bridges these metal centres in a head-to-tail fashion via the monodentate aromatic carboxyl­ate O1 amd phenolate O5 atoms, to form a one-dimensional zigzag chain along the bc plane (Fig. 2a). The aliphatic carboxyl­ate group of the ccop^2- ligand adopts a bidentate-bridging mode, linking chains to form a two-dimensional layer network parallel to the c axis (Fig. 2b), which is further extended to give rise to a three-dimensional framework via additional Zn—O—Zn linkages (Fig. 2c). The coordinated water molecules provide additional hydrogen bonds to stabilize the crystal structure (Table 3).

Better insight into this framework can be achieved by topology analysis. In this structure, all the Zn^II centres and the ccop^2- ligands can be regarded as tetra­hedral 4-connected nodes. The three-dimensional framework of (I) can then be simplified as a 4-connected sra topology with a Schläfli symbol of 4².6³.8 (Blatov 2012; Alexandrov et al., 2011; Rosi et al., 2005) (Fig. 3).

Besides (I), there is only one three-dimensional CP based on ccop and a transition metal (Zhan et al., 2010), viz. [Cd(ccop)(H₂)]_n, (II). In (II), the aromatic and aliphatic carboxyl­ate groups within the ccop^2- ligand adopt bidentate–chelate and chelate–bridging modes, respectively, differing from those described in (I). Similar to (II), (I) can also be regarded as a further product for the phenolate O atom [This phrase is not at all clear - please revise as necessary], leading to the final three-dimensional architecture. Moreover, the coordination modes of the ccop^2- ligands in (I) and (II) are unique, differing from those in other known ccop^2--based complexes (Lin & He, 2007; Jiang et al., 2009; Yuan et al., 2011; Zhan et al., 2010). In contrast with (I), the three-dimensional framework structure of (II) includes left- and right-handed helical chains.

As part of a continuing programme dedicated to luminescent d¹⁰ systems, the three-dimensional fluorescence of (I) was investigated at room temperature with excitation and emission wavelengths of 344–354 and 360–630 nm, respectively, with inter­vals of 0.15 and 2 nm, respectively. Compound (I) exhibits blue photoluminescence with an emission maximum at 450 nm upon excitation in the range 344–354 nm. Compared with the emission peak of free H₂ccop at 632 nm (Jiang et al., 2009), the emission maximum of (I) is clearly blue-shifted, due to the n–π* electronic transition of the ligand (Yang et al., 2008; Roy et al., 2009). The emission of (I) probably originates from a ligand-to-metal charge transfer (LMCT) (Qiu et al., 2010) excited state, because the ccop^2- ligand has a π-conjugated system in the pyridine ring and uses oxygen donors to coordinate to the Zn^II cations, which enhances the `rigidity' of the ligand and thus reduces the loss of energy through a radiationless pathway (Fu et al., 2002).

In summary, we have successfully prepared a new three-dimensional zinc(II)-based coordination polymer with sra topology, which is constructed from the ccop^2- ligand under hydro­thermal conditions.

Related literature

For related literature, see: Alexandrov et al. (2011); Betard & Fischer (2012); Blatov (2012); Cook et al. (2013); Cui et al. (2012); Dong et al. (2012); Fu et al. (2002); Furukawa et al. (2013); Gandara et al. (2014); He et al. (2014); Jiang et al. (2009); Kumagai et al. (2003); Li et al. (2010); Li, Li, O'Keeffe & Yaghi (2014); Li, Li, Zhou, Li & O'Keeffe (2014); Lin & He (2007); Ma et al. (2013); Qiu et al. (2010); Rosi et al. (2005); Roy et al. (2009); Yang et al. (2008); Yuan et al. (2011); Zhan et al. (2010).

Computing details

Data collection: CrysAlis PRO (Agilent, 2011); cell refinement: CrysAlis PRO (Agilent, 2011); data reduction: CrysAlis PRO (Agilent, 2011); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. A view of the asymmetric unit of (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms have been omitted for clarity. [Symmetry codes: (i) x - 1/2, -y + 3/2, z + 1; (ii) x - 1/2, -y + 3/2, z; (iii) -x + 3/2, y + 1/2, z + 1/2.]
	Fig. 2. (a) The infinite zigzag-shaped chain of (I) along the bc plane, (b) a view of the two-dimensional layer structure of (I) parallel to the c axis and (c) a view of the three-dimensional framework of (I) parallel to the c axis.
	Fig. 3. View of the sra-type 4-connected architecture of (I). Colour key: Zn^2+ cations are shown as turquoise balls and ccop^2- ligands as blue balls.

Poly[aqua(µ₄-5-carboxylato-1-carboxylatomethyl-2-oxidopyridinium)zinc(II)]

Crystal data

[Zn(C8H5NO4)(H2O)]	F(000) = 560
Mr = 278.52	Dx = 2.020 Mg m^−3
Orthorhombic, Pna21	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2c -2n	Cell parameters from 5300 reflections
a = 16.9663 (13) Å	θ = 1.3–28.0°
b = 10.8788 (8) Å	µ = 2.70 mm^−1
c = 4.9606 (4) Å	T = 293 K
V = 915.59 (12) Å^3	Block, colourless
Z = 4	0.31 × 0.26 × 0.19 mm

Data collection

Agilent Xcalibur diffractometer with Eos Gemini detector	1569 independent reflections
Radiation source: fine-focus sealed tube	1444 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.041
ω scans	θmax = 25.2°, θmin = 3.1°
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2011)	h = −20→20
Tmin = 0.451, Tmax = 0.613	k = −13→13
5989 measured reflections	l = −5→5

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.029	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.064	w = 1/[σ^2(Fo^2) + (0.030P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.00	(Δ/σ)max = 0.001
1569 reflections	Δρmax = 0.35 e Å^−3
151 parameters	Δρmin = −0.29 e Å^−3
4 restraints	Absolute structure: Flack (1983), with 647 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.015 (19)

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
Zn1	0.52286 (2)	0.73045 (3)	0.98693 (15)	0.01778 (13)
O1	0.62537 (15)	0.6711 (3)	0.8570 (6)	0.0314 (7)
O2	0.63979 (18)	0.5394 (3)	1.1931 (7)	0.0496 (10)
O3	0.99355 (19)	0.7618 (3)	0.3783 (7)	0.0281 (7)
O4	0.93744 (16)	0.7035 (2)	0.7561 (5)	0.0212 (6)
O5	0.93402 (13)	0.4107 (2)	0.5035 (8)	0.0275 (6)
O6	0.47204 (18)	0.5548 (3)	0.9763 (11)	0.0466 (8)
H6A	0.4321 (17)	0.549 (5)	0.883 (8)	0.070*
H6B	0.471 (3)	0.514 (5)	1.117 (7)	0.070*
N1	0.83676 (16)	0.5516 (2)	0.5328 (7)	0.0164 (8)
C1	0.66003 (19)	0.5847 (3)	0.9750 (12)	0.0246 (8)
C2	0.7332 (2)	0.5349 (3)	0.8482 (9)	0.0214 (9)
C3	0.7683 (2)	0.4267 (3)	0.9438 (9)	0.0218 (10)
H3A	0.7455	0.3860	1.0891	0.026*
C4	0.8339 (2)	0.3803 (3)	0.8311 (9)	0.0256 (10)
H4A	0.8547	0.3066	0.8945	0.031*
C5	0.8718 (2)	0.4430 (3)	0.6158 (8)	0.0220 (9)
C6	0.7692 (2)	0.5944 (3)	0.6444 (8)	0.0184 (9)
H7A	0.7472	0.6667	0.5786	0.022*
C7	0.8822 (2)	0.6292 (3)	0.3497 (8)	0.0193 (8)
H8A	0.8469	0.6842	0.2541	0.023*
H8B	0.9089	0.5783	0.2178	0.023*
C8	0.94277 (19)	0.7043 (3)	0.5069 (12)	0.0156 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.0154 (2)	0.0195 (2)	0.0184 (2)	0.00249 (15)	0.0006 (3)	−0.0017 (4)
O1	0.0222 (15)	0.0302 (17)	0.0417 (18)	0.0104 (13)	0.0062 (15)	0.0057 (14)
O2	0.037 (2)	0.073 (3)	0.039 (2)	0.0101 (18)	0.0175 (17)	0.017 (2)
O3	0.0305 (17)	0.0330 (17)	0.0208 (16)	−0.0115 (13)	0.0055 (13)	−0.0014 (14)
O4	0.0235 (15)	0.0242 (15)	0.0160 (16)	−0.0046 (12)	−0.0020 (12)	−0.0015 (12)
O5	0.0202 (13)	0.0206 (12)	0.0418 (17)	0.0057 (10)	0.0101 (19)	0.001 (2)
O6	0.052 (2)	0.0319 (16)	0.056 (2)	−0.0157 (14)	−0.030 (2)	0.012 (3)
N1	0.0171 (15)	0.0145 (14)	0.018 (2)	0.0003 (11)	0.0031 (16)	−0.0009 (14)
C1	0.0177 (19)	0.0268 (19)	0.029 (2)	−0.0001 (14)	0.005 (3)	−0.003 (3)
C2	0.0153 (19)	0.023 (2)	0.026 (2)	0.0011 (16)	0.0048 (17)	−0.0050 (19)
C3	0.023 (2)	0.0185 (18)	0.024 (3)	−0.0036 (15)	0.0050 (19)	0.0066 (17)
C4	0.024 (2)	0.016 (2)	0.036 (2)	0.0006 (17)	0.010 (2)	0.0059 (19)
C5	0.025 (2)	0.0136 (18)	0.027 (2)	−0.0031 (17)	−0.0031 (18)	−0.0025 (17)
C6	0.016 (2)	0.0166 (18)	0.022 (2)	−0.0009 (16)	−0.0022 (18)	−0.0010 (17)
C7	0.020 (2)	0.018 (2)	0.020 (2)	−0.0009 (16)	0.0028 (18)	0.0044 (17)
C8	0.0154 (16)	0.0155 (16)	0.016 (2)	0.0032 (13)	0.000 (3)	0.001 (3)

Geometric parameters (Å, º)

Zn1—O1	1.964 (3)	N1—C6	1.355 (5)
Zn1—O4^i	1.982 (3)	N1—C5	1.385 (4)
Zn1—O3^ii	2.006 (3)	N1—C7	1.460 (5)
Zn1—O5^iii	2.095 (2)	C1—C2	1.494 (5)
Zn1—O6	2.097 (3)	C2—C6	1.347 (5)
O1—C1	1.253 (5)	C2—C3	1.401 (5)
O2—C1	1.238 (6)	C3—C4	1.345 (5)
O3—C8	1.241 (5)	C3—H3A	0.9300
O3—Zn1^iv	2.006 (3)	C4—C5	1.421 (5)
O4—C8	1.239 (6)	C4—H4A	0.9300
O4—Zn1^v	1.982 (3)	C6—H7A	0.9300
O5—C5	1.244 (5)	C7—C8	1.527 (5)
O5—Zn1^vi	2.095 (2)	C7—H8A	0.9700
O6—H6A	0.823 (10)	C7—H8B	0.9700
O6—H6B	0.826 (10)
O1—Zn1—O4^i	125.25 (12)	C6—C2—C1	121.2 (4)
O1—Zn1—O3^ii	123.43 (13)	C3—C2—C1	121.0 (4)
O4^i—Zn1—O3^ii	111.27 (12)	C4—C3—C2	121.8 (4)
O1—Zn1—O5^iii	90.66 (11)	C4—C3—H3A	119.1
O4^i—Zn1—O5^iii	86.48 (11)	C2—C3—H3A	119.1
O3^ii—Zn1—O5^iii	90.52 (14)	C3—C4—C5	120.4 (4)
O1—Zn1—O6	93.22 (14)	C3—C4—H4A	119.8
O4^i—Zn1—O6	90.88 (13)	C5—C4—H4A	119.8
O3^ii—Zn1—O6	87.75 (17)	O5—C5—N1	118.1 (4)
O5^iii—Zn1—O6	176.09 (13)	O5—C5—C4	125.8 (4)
C1—O1—Zn1	120.6 (3)	N1—C5—C4	116.0 (3)
C8—O3—Zn1^iv	133.7 (3)	C2—C6—N1	121.6 (3)
C8—O4—Zn1^v	121.4 (2)	C2—C6—H7A	119.2
C5—O5—Zn1^vi	125.3 (2)	N1—C6—H7A	119.2
Zn1—O6—H6A	115 (4)	N1—C7—C8	110.3 (3)
Zn1—O6—H6B	119 (4)	N1—C7—H8A	109.6
H6A—O6—H6B	114.1 (19)	C8—C7—H8A	109.6
C6—N1—C5	122.3 (3)	N1—C7—H8B	109.6
C6—N1—C7	120.1 (3)	C8—C7—H8B	109.6
C5—N1—C7	116.9 (3)	H8A—C7—H8B	108.1
O2—C1—O1	125.2 (4)	O4—C8—O3	124.5 (4)
O2—C1—C2	117.0 (4)	O4—C8—C7	117.2 (3)
O1—C1—C2	117.7 (4)	O3—C8—C7	118.3 (5)
C6—C2—C3	117.7 (3)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O6—H6B···O6^vii	0.83 (1)	2.17 (2)	2.911 (4)	150 (4)
O6—H6A···O2^viii	0.82 (1)	1.82 (3)	2.574 (5)	152 (5)

Symmetry codes: (vii) −x+1, −y+1, z+1/2; (viii) −x+1, −y+1, z−1/2.