Poly[[tetra­chlorido{μ₄-tetra­kis[(pyridin-4-yl)oxymeth­yl]methane-κ⁴N:N′:N′′:N′′′}dizinc(II)] di­methyl­formamide tetrasolvate]

A novel metal–organic framework, {[Zn₂Cl₄(C₂₅H₂₄N₄O₄)]·4C₃H₇NO}_n, has been synthesized solvothermally by assembling the semi-rigid tetra­hedral ligand tetra­kis­[(pyridin-4-yl)oxymeth­yl]methane (tpom) and zinc nitrate in di­methyl­formamide (DMF). The crystal structure is noncentrosymmetric (P2₁c). Each Zn^II cation has a tetra­hedral co­ordination environment (C₂ symmetry), which is formed by two chloride ligands and two pyridine N atoms from two tpom ligands. The tetra­hedral tetra­dentate tpom linker has a quaternary C atom located on the crystallographic axis. This linker utilizes all the peripheral pyridine N atoms to connect four Zn^II cations, thereby forming a wave-like two-dimensional sheet along the c axis. The two-dimensional layer can be topologically simplified as a typical uninodal 4-connected sql/Shubnikov net, which is represented by the Schläfli symbol {4⁴,6²}. Adjacent layers are further packed into a three-dimensional structure by C—H...Cl hydrogen bonds.

S1. Comment

The design and synthesis of coordination polymers containing transition metal cations and organic ligands in a metal–organic framework (MOF) have rapidly developed because of their various unique architectures and potential applications as absorbent, luminescent or nonlinear optical materials (Allendorf et al., 2009; Kuppler et al., 2009; Farrusseng et al., 2009; Maspoch et al., 2007). The creation of designed MOFs using various functional multidentate organic ligands with different lengths, geometries and degrees of rigidity provides a new route to producing novel inorganic–organic hybrid materials (Zhang et al., 2010; Long & Yaghi, 2009). N- and O-donor bridging linkers, such as carboxylic acid and pyridine derivatives, have been widely reported to funtion generally as rigid bridging coordination ligands (Janiak, 2003; Sun & Wang, 2009; Zhang et al., 2009). Among the known N-containing compounds, tetrakis[(pyridin-4-yl)oxymethyl]methane (tpom) is an important flexible tetradentate ligand used to produce diversely structured MOFs with various metal ions (Guo et al., 2011; Kitigawa & Uemura, 2005; Moulton & Zaworotko, 2001; Janiak, 2003; Hu et al., 2012). Tpom-containing crystal structures were not found in the Cambridge Structural Database (CSD, Version 5.32; Allen, 2002) until 2008, when Zhang et al. (2008) reported two metal clusters (MOF-9 and MOF-10) coordinated with tpom ligands (CSD refcode VOMCOQ [One refcode for two structures?]; Zhang et al., 2008). In later studies, tpom was used to produce various coordination assemblies with one-, two- or three-dimensional structures [References?]. Closed-shell d¹⁰ metal cations, such as Zn^II, Cd^II and Hg^II, have attracted much attention because of their coordination adaptability and significant luminescent properties (Zeng et al., 2010; Wang et al., 2009; Wei et al., 2006). Several structures with different three-dimensional frameworks containing tpom and Zn^II cations have been reported previously (Zeng et al., 2010; Zhang et al., 2010; Ren et al., 2010). However, to the best of our knowledge, no other studies have reported two-dimensional structures with Zn^II and tpom.

We previously synthesized a series of tpom-containing compounds, including porous coordination polymers built from tpom and transition metal sulfides with the three-dimensional PtS topology (Zhang et al., 2010), as well as a photoluminescent triply-interpenetrating diamondoid MOF based on Cu₄I₄ clusters and tpom (Ren et al., 2009). In this study, the synthesis and crystal structure of a new 4-connected two-dimensional wave-like sheet complex {[Zn₂Cl₄(tpom)].2DMF}_n (DMF is dimethylformamide), (I), is introduced.

Compound (I) crystallizes in the noncentrosymmetric space group P42₁c and is constructed using two-dimensional polymeric layers, with the four axes directed along the tetragonal c axis. Each layer is a regular tpom array located on the 4 axes, which are bridged by ZnCl₂ units on the twofold axes. The asymmetric unit contains half a Zn^II cation, one Cl^- anion, half a tpom ligand and one solvent DMF molecule. Each Zn^II cation (Fig. 1) is tetracoordinated by two terminal Cl atoms (Cl1 and Cl1^iv) and two N atoms (N1 and N1^iv) of two tpom ligands [symmetry code: (iv) x, y - 1, z]. The coordination environment of Zn^II (Fig. 1 and Table 1) is a distorted tetrahedron, with bond angles in the range 101.15 (2)–122.41 (1)°. The Zn—N bond length is similar to those found in related structures (Zeng et al., 2010; Hu et al., 2010). Each tetradentate tpom ligand adopts a distorted tetrahedral geometry and coordinates four ZnCl₂ units via its N-donor atoms, thereby forming a two-dimensional framework. The central C atoms of adjacent ligands in this framework are linked to a common ZnCl₂ unit. These atoms are separated by a C_core···C_core distance of 12.290 (5) Å, whereas the Zn···Zn distances spanned by the tpom ligand are 12.290 (5) and 14.423 (7) Å (Fig. 2). The extended layer in the ab plane is constructed by connecting the quaternary bridges and the ZnCl₂ units.

As mentioned above, tpom is a flexible ligand and its various coordination configurations cause the observed structural differences. A previous study reported that the tpom conformation can be assessed from the distortion of the tetrahedral orientation by comparing the N—C_core—N angles and the C_core—CH₂—O—C_py torsion angles (py is pyridine; Ryan et al., 2009). Guo et al. (2011) concluded that tpom ligands can bend and rotate freely when coordinated to metal atoms because of the flexible nature of the spacers between the four pyridine rings (Guo et al., 2011). Li & Yu (2012) discussed the effect of the additional rotation of the pyridine rings on the variable conformations (Li & Yu, 2012). The N—C_core—N angles of the tpom ligand in (I) range from 91.68 (5) to 119.03 (2)° (Fig. 2), whereas the C_core—CH₂—O—C_py torsion angles are 179.16 (2)°. Therefore, the ligand deviates slightly from tetrahedral geometry and adopts a fully extended configuration. A similar distortion was observed in the three-dimensional tpom complex [Fe(SCN)₂(tpom)].2[HN(CH₃)₂].4H₂O (Li & Yu, 2012).

A better understanding of the framework of (I) can be achieved through topological analysis. The tetradentate tpom ligands are considered as four connecting `X' nodes, and the neutral ZnCl₂ units are linear linkers that are connected to two tpom ligands. Therefore, (I) can be reduced to the topology of a uninodal 4-connected two-dimensional sql/Shubnikov tetragonal plane net with the Schläfli symbol {4⁴,6²}, as calculated by the TOPAS software (Blatov, 2006) (Fig. 3). The two-dimensional interleaved sheets stack on top of each other along the c axis, with a repeat distance of 8.066 (5) Å, such that they block the solvent molecules. The DMF solvent molecules fill the space between two layers of the stable framework structure via weak C—H···O2 hydrogen bonds (Table 2). PLATON (Spek, 2009) calculates that 1.6% of the unit cell is occupied by the solvent. The three-dimensional packing of the structure is governed by classical C—H···Cl1 hydrogen bonds between pyridine groups and terminal chloride ligands (Fig. 4).

In summary, we have synthesized the unprecedented 4-connected two-dimensional metal–organic framework {[Zn₂Cl₄(tpom)].2DMF}_n from the assembly of a semi-rigid tetrahedral tpom ligand, in which hydrogen bonds interweave two-dimensional wave-like sheets into a three-dimensional architecture. Based on both our previous work and the present study, a new route to the diverse assembly of MOF structures is via adjustment of the configurations of flexible ligands. The flexible nature of the tpom ligand allows for changes in its coordination modes. Further studies will involve the synthesis of a new series using this ligand with other functional groups and coordination complexes.

Related literature

For related literature, see: Allen (2002); Allendorf et al. (2009); Blatov (2006); Farrusseng et al. (2009); Guo et al. (2011); Hu et al. (2010, 2012); Janiak (2003); Kitigawa & Uemura (2005); Kuppler et al. (2009); Li & Yu (2012); Long & Yaghi (2009); Maspoch et al. (2007); Moulton & Zaworotko (2001); Ren et al. (2009, 2010); Ryan et al. (2009); Spek (2009); Sun & Wang (2009); Wang et al. (2009); Wei et al. (2006); Zeng et al. (2010); Zhang et al. (2009, 2010).

S2. Experimental

The tetrahedral tetradentate linker tpom ligand was synthesized by reacting pentaerythrityl tetrabromide with pyridin-4-ol in N,N'-dimethylformamide (DMF) containing K₂CO₃ and KI under reflux.

To a solution of tpom (0.022 g, 0.045 mmol) and Zn(NO₃)₂.6H₂O (0.030 g, 0.10 mmol) in DMF (10 ml) were added two drops of aqueous HCl (1:1 v/v) (pH ~5). The mixture was transferred into a vial, which was sealed and heated at 373 K for 3 d. The vial was allowed to cool to room temperature. Colourless octahedral crystals of (I) (yield 0.028 g) were obtained by filtration, and washed with DMF and acetic ether three times each, respectively (final yield 63.6%, based on tpom). Spectroscopic analysis: FT–IR (KBr, cm^-1): 3132 (w), 3104 (w), 3052 (w), 1672 (s), 1611 (s), 1567 (s), 1512 (s), 1470 (s), 1439 (s), 1409 (m), 1386 (s), 1334 (w), 1298 (s), 1256 (m), 1209 (s), 1168 (w), 1095 (s), 1060 (s), 1031 (s), 949 (w), 875 (s), 847 (s), 829 (s), 669 (m), 537 (m).

S3. Refinement

All H atoms were placed in geometrically idealized position and treated as riding, with C—H = 0.96 (methyl), 0.97 (methylene) or 0.93 Å (pyridine), and with U_iso(H) = 1.2U_eq(C) for pyridine and methylene H atoms or 1.5U_eq(C) for methyl H atoms.

Computing details

Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and DIAMOND (Brandenburg & Putz, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008), PLATON (Spek, 2009) and publCIF (Westrip, 2010).

Figures

	Fig. 1. The coordination environment of the Zn^II cation of (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. The DMF solvent molecules and H atoms have been omitted for clarity. [Symmetry codes: (i) -x + 1, -y + 1, z; (ii) y, -x + 1, -z + 1; (iii) -y + 1, x, -z + 1; (iv) x, y - 1, z; (v) -x + 1, -y + 2, z.]
	Fig. 2. A view of the adjacent tpom units in (I), showing the bridging mode and the coordination geometry. Selected distances are Zn1···Zn1^vi = 14.423 (7) Å and Zn1···Zn1^vii = C7···C7^vii = 12.290 (5) Å, and angles are N1—C7—N1^i = 91.685° and N1—C7—N1^viii = 119.03 (2)°. [Symmetry codes: (i) -x + 1, -y + 1, z; (vi) -x + 2, y - 1, -z + 1; (vii) x, y + 1, z; (viii) x, -y + 1, -z + 1.]
	Fig. 3. A perspective view of the two-dimensional sheet of (I), extending in the ab plane. The balls and thick lines represent the topological net.
	Fig. 4. A perspective view of the three-dimensional supramolecular network in (I), formed via C—H···Cl hydrogen bonds (dashed lines).

Poly[[tetrachlorido{µ₄-tetrakis[(pyridin-4-yl)oxymethyl]methane-κ⁴N:N':N'':N'''}dizinc(II)] dimethylformamide tetrasolvate]

Crystal data

[Zn2Cl4(C25H24N4O4)]·4C6H14NO2	F(000) = 1044
Mr = 1009.41	Dx = 1.376 Mg m^−3
Tetragonal, P421c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P -4 2 n	θ = 2.1–28.2°
a = 12.2905 (14) Å	µ = 1.26 mm^−1
c = 16.133 (4) Å	T = 291 K
V = 2437.0 (7) Å^3	Block, colourless
Z = 2	0.20 × 0.19 × 0.18 mm

Data collection

Bruker APEXII CCD area-detector diffractometer	2980 independent reflections
Radiation source: fine-focus sealed tube	2149 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.033
ϕ and ω scans	θmax = 28.2°, θmin = 2.1°
Absorption correction: multi-scan (SADABS; Sheldrick, 2008)	h = −16→14
Tmin = 0.787, Tmax = 0.806	k = −16→16
14763 measured reflections	l = −21→18

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)] = 0.031	H-atom parameters constrained
wR(F^2) = 0.086	w = 1/[σ^2(Fo^2) + (0.0446P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.07	(Δ/σ)max = 0.001
2980 reflections	Δρmax = 0.27 e Å^−3
136 parameters	Δρmin = −0.21 e Å^−3
0 restraints	Absolute structure: Flack (1983), with 1291 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.006 (18)

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	0.5000	1.0000	0.85678 (2)	0.05180 (12)
Cl1	0.34442 (8)	0.97835 (7)	0.92316 (6)	0.1075 (3)
O1	0.54728 (13)	0.62201 (13)	0.60884 (9)	0.0569 (4)
O2	0.7769 (3)	1.0492 (4)	0.6489 (3)	0.198 (2)
N1	0.51928 (16)	0.87304 (14)	0.77596 (11)	0.0528 (5)
N2	0.7964 (2)	0.8929 (2)	0.5793 (2)	0.0926 (8)
C1	0.6057 (2)	0.8061 (2)	0.77321 (17)	0.0626 (6)
H1	0.6623	0.8177	0.8105	0.075*
C2	0.61440 (19)	0.7234 (2)	0.71932 (17)	0.0618 (7)
H2	0.6750	0.6782	0.7205	0.074*
C3	0.53285 (18)	0.70591 (18)	0.66208 (14)	0.0487 (5)
C4	0.44309 (19)	0.7715 (2)	0.66406 (16)	0.0569 (6)
H4	0.3857	0.7603	0.6274	0.068*
C5	0.43936 (18)	0.8540 (2)	0.72117 (17)	0.0606 (7)
H5	0.3785	0.8989	0.7220	0.073*
C6	0.46106 (17)	0.59462 (17)	0.55325 (13)	0.0469 (5)
H6A	0.4431	0.6566	0.5186	0.056*
H6B	0.3966	0.5739	0.5841	0.056*
C7	0.5000	0.5000	0.5000	0.0429 (8)
C8	0.8988 (5)	0.8688 (6)	0.6135 (3)	0.199 (3)
H8A	0.9539	0.8772	0.5718	0.298*
H8B	0.8991	0.7952	0.6334	0.298*
H8C	0.9134	0.9175	0.6587	0.298*
C9	0.7535 (4)	0.8170 (4)	0.5198 (4)	0.1611 (19)
H9A	0.7640	0.7441	0.5397	0.242*
H9B	0.7907	0.8259	0.4679	0.242*
H9C	0.6772	0.8303	0.5121	0.242*
C10	0.7416 (4)	0.9791 (6)	0.5987 (3)	0.1448 (19)
H10	0.6737	0.9897	0.5746	0.174*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.0626 (2)	0.0476 (2)	0.0452 (2)	−0.0026 (2)	0.000	0.000
Cl1	0.1149 (7)	0.0881 (6)	0.1196 (6)	−0.0043 (5)	0.0668 (6)	0.0056 (5)
O1	0.0504 (8)	0.0615 (10)	0.0589 (10)	0.0118 (8)	−0.0075 (8)	−0.0171 (8)
O2	0.146 (3)	0.217 (5)	0.231 (4)	−0.008 (3)	0.030 (3)	−0.142 (4)
N1	0.0574 (12)	0.0519 (10)	0.0491 (10)	0.0001 (9)	−0.0049 (9)	−0.0035 (9)
N2	0.0869 (19)	0.0880 (18)	0.103 (2)	0.0136 (16)	−0.0005 (18)	0.0050 (18)
C1	0.0522 (13)	0.0759 (17)	0.0596 (16)	0.0093 (13)	−0.0146 (12)	−0.0102 (14)
C2	0.0460 (13)	0.0692 (16)	0.0703 (17)	0.0154 (11)	−0.0118 (12)	−0.0165 (14)
C3	0.0504 (12)	0.0487 (12)	0.0470 (13)	0.0037 (10)	−0.0014 (10)	−0.0029 (10)
C4	0.0562 (14)	0.0581 (14)	0.0564 (16)	0.0098 (12)	−0.0157 (12)	−0.0109 (12)
C5	0.0559 (14)	0.0600 (14)	0.0660 (16)	0.0113 (12)	−0.0111 (13)	−0.0116 (13)
C6	0.0461 (11)	0.0470 (12)	0.0477 (13)	0.0028 (10)	−0.0038 (10)	−0.0028 (10)
C7	0.0419 (12)	0.0419 (12)	0.045 (2)	0.000	0.000	0.000
C8	0.171 (5)	0.194 (6)	0.231 (6)	0.035 (5)	−0.089 (5)	−0.011 (5)
C9	0.188 (5)	0.147 (4)	0.148 (5)	−0.030 (4)	−0.005 (4)	−0.018 (4)
C10	0.089 (3)	0.194 (5)	0.151 (5)	0.026 (4)	0.020 (3)	0.005 (4)

Geometric parameters (Å, º)

Zn1—N1	2.0472 (18)	C4—C5	1.371 (4)
Zn1—N1^i	2.0472 (18)	C4—H4	0.9300
Zn1—Cl1^i	2.2077 (8)	C5—H5	0.9300
Zn1—Cl1	2.2077 (8)	C6—C7	1.523 (2)
O1—C3	1.354 (3)	C6—H6A	0.9700
O1—C6	1.428 (2)	C6—H6B	0.9700
O2—C10	1.260 (6)	C7—C6^ii	1.523 (2)
N1—C5	1.342 (3)	C7—C6^iii	1.523 (2)
N1—C1	1.344 (3)	C7—C6^iv	1.523 (2)
N2—C10	1.293 (6)	C8—H8A	0.9600
N2—C8	1.407 (6)	C8—H8B	0.9600
N2—C9	1.439 (5)	C8—H8C	0.9600
C1—C2	1.342 (3)	C9—H9A	0.9600
C1—H1	0.9300	C9—H9B	0.9600
C2—C3	1.380 (3)	C9—H9C	0.9600
C2—H2	0.9300	C10—H10	0.9300
C3—C4	1.366 (3)
N1—Zn1—N1^i	100.88 (10)	O1—C6—C7	107.52 (15)
N1—Zn1—Cl1^i	107.49 (6)	O1—C6—H6A	110.2
N1^i—Zn1—Cl1^i	108.50 (6)	C7—C6—H6A	110.2
N1—Zn1—Cl1	108.50 (6)	O1—C6—H6B	110.2
N1^i—Zn1—Cl1	107.49 (6)	C7—C6—H6B	110.2
Cl1^i—Zn1—Cl1	121.96 (6)	H6A—C6—H6B	108.5
C3—O1—C6	118.73 (17)	C6^ii—C7—C6	108.55 (8)
C5—N1—C1	116.74 (19)	C6^ii—C7—C6^iii	111.32 (16)
C5—N1—Zn1	117.89 (15)	C6—C7—C6^iii	108.55 (8)
C1—N1—Zn1	125.37 (16)	C6^ii—C7—C6^iv	108.55 (8)
C10—N2—C8	123.0 (5)	C6—C7—C6^iv	111.32 (16)
C10—N2—C9	120.1 (4)	C6^iii—C7—C6^iv	108.55 (8)
C8—N2—C9	116.9 (4)	N2—C8—H8A	109.5
C2—C1—N1	123.2 (2)	N2—C8—H8B	109.5
C2—C1—H1	118.4	H8A—C8—H8B	109.5
N1—C1—H1	118.4	N2—C8—H8C	109.5
C1—C2—C3	119.6 (2)	H8A—C8—H8C	109.5
C1—C2—H2	120.2	H8B—C8—H8C	109.5
C3—C2—H2	120.2	N2—C9—H9A	109.5
O1—C3—C4	124.7 (2)	N2—C9—H9B	109.5
O1—C3—C2	116.64 (19)	H9A—C9—H9B	109.5
C4—C3—C2	118.6 (2)	N2—C9—H9C	109.5
C3—C4—C5	118.6 (2)	H9A—C9—H9C	109.5
C3—C4—H4	120.7	H9B—C9—H9C	109.5
C5—C4—H4	120.7	O2—C10—N2	122.4 (5)
N1—C5—C4	123.2 (2)	O2—C10—H10	118.8
N1—C5—H5	118.4	N2—C10—H10	118.8
C4—C5—H5	118.4
N1^i—Zn1—N1—C5	−58.04 (17)	C1—C2—C3—C4	−2.4 (4)
Cl1^i—Zn1—N1—C5	−171.58 (17)	O1—C3—C4—C5	−179.7 (2)
Cl1—Zn1—N1—C5	54.73 (18)	C2—C3—C4—C5	2.0 (4)
N1^i—Zn1—N1—C1	122.3 (2)	C1—N1—C5—C4	−0.3 (4)
Cl1^i—Zn1—N1—C1	8.7 (2)	Zn1—N1—C5—C4	180.0 (2)
Cl1—Zn1—N1—C1	−125.0 (2)	C3—C4—C5—N1	−0.6 (4)
C5—N1—C1—C2	−0.1 (4)	C3—O1—C6—C7	178.82 (17)
Zn1—N1—C1—C2	179.6 (2)	O1—C6—C7—C6^ii	173.86 (15)
N1—C1—C2—C3	1.5 (4)	O1—C6—C7—C6^iii	−65.00 (8)
C6—O1—C3—C4	−3.7 (3)	O1—C6—C7—C6^iv	54.43 (11)
C6—O1—C3—C2	174.6 (2)	C8—N2—C10—O2	1.2 (8)
C1—C2—C3—O1	179.2 (2)	C9—N2—C10—O2	−178.3 (5)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C5—H5···O2^i	0.93	2.33	3.137 (4)	144
C6—H6A···Cl1^v	0.97	2.81	3.725 (2)	158

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