Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S010827010703185X/sf3044sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S010827010703185X/sf3044Isup2.hkl |
CCDC reference: 659116
For related literature, see: Ayyappan et al. (2001); Bernhard et al. (2003); Chen et al. (2001); Emseis et al. (2004); Harrowfield et al. (1983); Huang et al. (2001); Jia et al. (2004); Lin et al. (1998); Lu & Babb (2001); Lu et al. (2003); Majumder et al. (2002); Shi et al. (2005); Sugimori et al. (1997); Sun et al. (2005); Weng et al. (2002); Wu et al. (2007); Yamauchi et al. (1989); Yu et al. (2003); Zhu et al. (2005).
Single crystals of the title complex suitable for X-ray crystallographic analysis were obtained by solvothermal treatment of Zn(NO3)2·6H2O (0.2 mmol), H2-tyr (0.1 mmol), CH3OH (5 ml) and NH3 (0.2 ml). The reagents were placed in a thick Pyrex tube (ca 20 cm long). The tube was cooled with liquid N2 and the air evacuated. The sealed tube was heated at 413 k for 10 d to yield yellow block crystals in about 25% yield.
H atoms on C and N atoms were positioned geometrically and were allowed to ride on their parent atoms, with C—H = 0.93 Å and N—H = 0.90 Å, and Uiso(H) = 1.2Ueq(C) or 1.2Ueq(N). H atoms on O atoms were located in a difference map and refined. High displacement parameters suggest some disorder in the C8 atom of the tyr2- ligand, but this could not be resolved.
Data collection: CrystalClear (Molecular Structure Corporation & Rigaku, 2001); cell refinement: CrystalClear; data reduction: CrystalStructure (Rigaku/MSC, 2004); 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.
[Zn(C9H7NO3)(H2O)] | V = 484.81 (17) Å3 |
Mr = 260.54 | Z = 2 |
Triclinic, P1 | F(000) = 264 |
Hall symbol: -P 1 | Dx = 1.785 Mg m−3 |
a = 5.8633 (12) Å | Mo Kα radiation, λ = 0.71073 Å |
b = 9.681 (2) Å | θ = 2.3–26.0° |
c = 9.772 (2) Å | µ = 2.52 mm−1 |
α = 65.336 (3)° | T = 273 K |
β = 75.334 (3)° | Block, yellow |
γ = 78.440 (3)° | 0.14 × 0.10 × 0.06 mm |
Rigaku Mercury diffractometer | 1841 independent reflections |
Radiation source: fine-focus sealed tube | 1645 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.020 |
Detector resolution: 7.31 pixels mm-1 | θmax = 26.0°, θmin = 2.3° |
ω scans | h = −7→7 |
Absorption correction: multi-scan (Jacobson, 1998) | k = −11→11 |
Tmin = 0.719, Tmax = 0.863 | l = −12→12 |
3474 measured reflections |
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)] = 0.044 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.109 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.06 | w = 1/[σ2(Fo2) + (0.0592P)2 + 0.676P] where P = (Fo2 + 2Fc2)/3 |
1841 reflections | (Δ/σ)max = 0.070 |
144 parameters | Δρmax = 0.61 e Å−3 |
2 restraints | Δρmin = −0.51 e Å−3 |
[Zn(C9H7NO3)(H2O)] | γ = 78.440 (3)° |
Mr = 260.54 | V = 484.81 (17) Å3 |
Triclinic, P1 | Z = 2 |
a = 5.8633 (12) Å | Mo Kα radiation |
b = 9.681 (2) Å | µ = 2.52 mm−1 |
c = 9.772 (2) Å | T = 273 K |
α = 65.336 (3)° | 0.14 × 0.10 × 0.06 mm |
β = 75.334 (3)° |
Rigaku Mercury diffractometer | 1841 independent reflections |
Absorption correction: multi-scan (Jacobson, 1998) | 1645 reflections with I > 2σ(I) |
Tmin = 0.719, Tmax = 0.863 | Rint = 0.020 |
3474 measured reflections |
R[F2 > 2σ(F2)] = 0.044 | 2 restraints |
wR(F2) = 0.109 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.06 | Δρmax = 0.61 e Å−3 |
1841 reflections | Δρmin = −0.51 e Å−3 |
144 parameters |
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. |
x | y | z | Uiso*/Ueq | ||
N1 | 0.7359 (7) | 0.2206 (5) | 0.2087 (5) | 0.0400 (9) | |
H1B | 0.6048 | 0.2862 | 0.1883 | 0.048* | |
H1A | 0.6922 | 0.1362 | 0.2908 | 0.048* | |
Zn1 | 0.10104 (8) | −0.16052 (5) | 0.98560 (5) | 0.02613 (18) | |
O1 | 0.1409 (5) | 0.0629 (3) | 0.8754 (3) | 0.0317 (7) | |
O1W | 0.3249 (7) | −0.3060 (5) | 1.1269 (5) | 0.0491 (9) | |
H1W2 | 0.469 (6) | −0.292 (10) | 1.088 (9) | 0.12 (3)* | |
H1W1 | 0.296 (13) | −0.399 (3) | 1.158 (8) | 0.09 (2)* | |
O2 | 1.1516 (5) | 0.2756 (4) | 0.0164 (4) | 0.0344 (7) | |
O3 | 1.2600 (6) | 0.3971 (4) | 0.1327 (4) | 0.0464 (9) | |
C1 | 0.3115 (7) | 0.1270 (5) | 0.7566 (5) | 0.0274 (9) | |
C2 | 0.2607 (8) | 0.2616 (5) | 0.6356 (5) | 0.0383 (10) | |
H2 | 0.1059 | 0.3087 | 0.6380 | 0.046* | |
C3 | 0.4338 (10) | 0.3268 (6) | 0.5120 (5) | 0.0457 (12) | |
H3 | 0.3926 | 0.4169 | 0.4328 | 0.055* | |
C4 | 0.6654 (9) | 0.2635 (6) | 0.5017 (5) | 0.0436 (12) | |
C5 | 0.7176 (8) | 0.1297 (6) | 0.6229 (6) | 0.0447 (12) | |
H5 | 0.8733 | 0.0840 | 0.6201 | 0.054* | |
C6 | 0.5454 (8) | 0.0622 (5) | 0.7478 (6) | 0.0374 (10) | |
H6 | 0.5870 | −0.0279 | 0.8269 | 0.045* | |
C7 | 0.8563 (12) | 0.3332 (9) | 0.3650 (7) | 0.075 (2) | |
H7 | 0.9452 | 0.4037 | 0.3631 | 0.091* | |
C8 | 0.8947 (11) | 0.2886 (10) | 0.2421 (8) | 0.089 (3) | |
C9 | 1.1178 (7) | 0.3254 (5) | 0.1217 (5) | 0.0310 (9) |
U11 | U22 | U33 | U12 | U13 | U23 | |
N1 | 0.030 (2) | 0.056 (3) | 0.043 (2) | −0.0176 (18) | 0.0080 (17) | −0.030 (2) |
Zn1 | 0.0231 (3) | 0.0286 (3) | 0.0269 (3) | −0.00491 (18) | 0.00178 (19) | −0.0137 (2) |
O1 | 0.0320 (16) | 0.0268 (15) | 0.0299 (16) | −0.0078 (12) | 0.0120 (13) | −0.0130 (13) |
O1W | 0.038 (2) | 0.050 (2) | 0.055 (2) | −0.0049 (17) | −0.0040 (18) | −0.0190 (19) |
O2 | 0.0234 (14) | 0.0477 (18) | 0.0370 (17) | −0.0099 (13) | 0.0062 (13) | −0.0253 (15) |
O3 | 0.0335 (18) | 0.063 (2) | 0.056 (2) | −0.0206 (16) | 0.0110 (16) | −0.0400 (19) |
C1 | 0.026 (2) | 0.030 (2) | 0.029 (2) | −0.0093 (17) | 0.0047 (17) | −0.0162 (18) |
C2 | 0.035 (2) | 0.041 (3) | 0.034 (2) | −0.006 (2) | −0.002 (2) | −0.011 (2) |
C3 | 0.054 (3) | 0.051 (3) | 0.025 (2) | −0.023 (2) | −0.001 (2) | −0.003 (2) |
C4 | 0.047 (3) | 0.066 (3) | 0.028 (2) | −0.032 (3) | 0.010 (2) | −0.026 (2) |
C5 | 0.025 (2) | 0.063 (3) | 0.054 (3) | −0.012 (2) | 0.009 (2) | −0.036 (3) |
C6 | 0.034 (2) | 0.035 (2) | 0.037 (3) | −0.0042 (19) | 0.000 (2) | −0.013 (2) |
C7 | 0.083 (4) | 0.123 (6) | 0.047 (3) | −0.080 (4) | 0.042 (3) | −0.058 (4) |
C8 | 0.059 (4) | 0.185 (8) | 0.072 (4) | −0.087 (5) | 0.047 (3) | −0.100 (5) |
C9 | 0.025 (2) | 0.036 (2) | 0.034 (2) | −0.0082 (18) | 0.0033 (18) | −0.018 (2) |
N1—C8 | 1.409 (6) | O3—C9 | 1.236 (5) |
N1—Zn1i | 2.154 (4) | C1—C2 | 1.386 (6) |
N1—H1B | 0.9000 | C1—C6 | 1.387 (6) |
N1—H1A | 0.9000 | C2—C3 | 1.374 (7) |
Zn1—O1 | 2.006 (3) | C2—H2 | 0.9300 |
Zn1—O2i | 2.032 (3) | C3—C4 | 1.370 (8) |
Zn1—O1W | 2.041 (4) | C3—H3 | 0.9300 |
Zn1—O1ii | 2.075 (3) | C4—C5 | 1.385 (8) |
Zn1—N1i | 2.154 (4) | C4—C7 | 1.512 (7) |
O1—C1 | 1.339 (5) | C5—C6 | 1.383 (7) |
O1—Zn1ii | 2.075 (3) | C5—H5 | 0.9300 |
O1W—H1W2 | 0.85 (5) | C6—H6 | 0.9300 |
O1W—H1W1 | 0.86 (5) | C7—C8 | 1.392 (7) |
O2—C9 | 1.267 (5) | C7—H7 | 0.9300 |
O2—Zn1i | 2.032 (3) | C8—C9 | 1.510 (6) |
C8—N1—Zn1i | 110.5 (3) | C2—C1—C6 | 116.9 (4) |
C8—N1—H1B | 109.5 | C3—C2—C1 | 121.5 (5) |
Zn1i—N1—H1B | 109.5 | C3—C2—H2 | 119.2 |
C8—N1—H1A | 109.5 | C1—C2—H2 | 119.2 |
Zn1i—N1—H1A | 109.5 | C4—C3—C2 | 122.1 (5) |
H1B—N1—H1A | 108.1 | C4—C3—H3 | 119.0 |
O1—Zn1—O2i | 128.61 (13) | C2—C3—H3 | 119.0 |
O1—Zn1—O1W | 121.18 (15) | C3—C4—C5 | 116.7 (4) |
O2i—Zn1—O1W | 110.19 (15) | C3—C4—C7 | 122.4 (6) |
O1—Zn1—O1ii | 76.84 (12) | C5—C4—C7 | 120.9 (5) |
O2i—Zn1—O1ii | 91.12 (12) | C6—C5—C4 | 122.0 (5) |
O1W—Zn1—O1ii | 103.19 (14) | C6—C5—H5 | 119.0 |
O1—Zn1—N1i | 95.71 (14) | C4—C5—H5 | 119.0 |
O2i—Zn1—N1i | 79.16 (13) | C5—C6—C1 | 120.8 (5) |
O1W—Zn1—N1i | 96.35 (16) | C5—C6—H6 | 119.6 |
O1ii—Zn1—N1i | 160.22 (15) | C1—C6—H6 | 119.6 |
C1—O1—Zn1 | 127.1 (2) | C8—C7—C4 | 117.9 (5) |
C1—O1—Zn1ii | 128.8 (2) | C8—C7—H7 | 121.1 |
Zn1—O1—Zn1ii | 103.16 (12) | C4—C7—H7 | 121.1 |
Zn1—O1W—H1W2 | 113 (6) | C7—C8—N1 | 126.7 (5) |
Zn1—O1W—H1W1 | 111 (5) | C7—C8—C9 | 119.2 (4) |
H1W2—O1W—H1W1 | 114 (7) | N1—C8—C9 | 114.0 (4) |
C9—O2—Zn1i | 118.6 (3) | O3—C9—O2 | 123.6 (4) |
O1—C1—C2 | 121.1 (4) | O3—C9—C8 | 119.4 (4) |
O1—C1—C6 | 122.0 (4) | O2—C9—C8 | 117.0 (4) |
O2i—Zn1—O1—C1 | −109.6 (3) | C3—C4—C5—C6 | 0.5 (7) |
O1W—Zn1—O1—C1 | 72.1 (4) | C7—C4—C5—C6 | −178.8 (4) |
O1ii—Zn1—O1—C1 | 169.7 (4) | C4—C5—C6—C1 | −0.2 (7) |
N1i—Zn1—O1—C1 | −28.9 (3) | O1—C1—C6—C5 | 179.2 (4) |
O2i—Zn1—O1—Zn1ii | 80.73 (17) | C2—C1—C6—C5 | −0.3 (7) |
O1W—Zn1—O1—Zn1ii | −97.61 (17) | C3—C4—C7—C8 | −89.0 (9) |
O1ii—Zn1—O1—Zn1ii | 0.0 | C5—C4—C7—C8 | 90.3 (8) |
N1i—Zn1—O1—Zn1ii | 161.41 (15) | C4—C7—C8—N1 | 19.8 (14) |
Zn1—O1—C1—C2 | 139.5 (4) | C4—C7—C8—C9 | −165.6 (7) |
Zn1ii—O1—C1—C2 | −53.4 (5) | Zn1i—N1—C8—C7 | −175.7 (8) |
Zn1—O1—C1—C6 | −39.9 (5) | Zn1i—N1—C8—C9 | 9.4 (8) |
Zn1ii—O1—C1—C6 | 127.2 (4) | Zn1i—O2—C9—O3 | −179.8 (4) |
O1—C1—C2—C3 | −178.9 (4) | Zn1i—O2—C9—C8 | 2.6 (6) |
C6—C1—C2—C3 | 0.5 (7) | C7—C8—C9—O3 | −1.4 (11) |
C1—C2—C3—C4 | −0.3 (8) | N1—C8—C9—O3 | 173.8 (6) |
C2—C3—C4—C5 | −0.2 (7) | C7—C8—C9—O2 | 176.3 (7) |
C2—C3—C4—C7 | 179.0 (5) | N1—C8—C9—O2 | −8.4 (9) |
Symmetry codes: (i) −x+1, −y, −z+1; (ii) −x, −y, −z+2. |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1B···O3iii | 0.90 | 2.18 | 3.056 (6) | 165 |
O1W—H1W1···O3iv | 0.86 (5) | 2.15 (5) | 2.950 (6) | 156 (6) |
O1W—H1W2···O2v | 0.85 (5) | 2.21 (5) | 3.051 (6) | 175 (9) |
Symmetry codes: (iii) x−1, y, z; (iv) x−1, y−1, z+1; (v) −x+2, −y, −z+1. |
Experimental details
Crystal data | |
Chemical formula | [Zn(C9H7NO3)(H2O)] |
Mr | 260.54 |
Crystal system, space group | Triclinic, P1 |
Temperature (K) | 273 |
a, b, c (Å) | 5.8633 (12), 9.681 (2), 9.772 (2) |
α, β, γ (°) | 65.336 (3), 75.334 (3), 78.440 (3) |
V (Å3) | 484.81 (17) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 2.52 |
Crystal size (mm) | 0.14 × 0.10 × 0.06 |
Data collection | |
Diffractometer | Rigaku Mercury diffractometer |
Absorption correction | Multi-scan (Jacobson, 1998) |
Tmin, Tmax | 0.719, 0.863 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 3474, 1841, 1645 |
Rint | 0.020 |
(sin θ/λ)max (Å−1) | 0.617 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.044, 0.109, 1.06 |
No. of reflections | 1841 |
No. of parameters | 144 |
No. of restraints | 2 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.61, −0.51 |
Computer programs: CrystalClear (Molecular Structure Corporation & Rigaku, 2001), CrystalClear, CrystalStructure (Rigaku/MSC, 2004), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), ORTEPII (Johnson, 1976), SHELXL97.
Zn1—O1 | 2.006 (3) | Zn1—O1ii | 2.075 (3) |
Zn1—O2i | 2.032 (3) | Zn1—N1i | 2.154 (4) |
Zn1—O1W | 2.041 (4) | ||
O1—Zn1—O2i | 128.61 (13) | O1W—Zn1—O1ii | 103.19 (14) |
O1—Zn1—O1W | 121.18 (15) | O1—Zn1—N1i | 95.71 (14) |
O2i—Zn1—O1W | 110.19 (15) | O2i—Zn1—N1i | 79.16 (13) |
O1—Zn1—O1ii | 76.84 (12) | O1W—Zn1—N1i | 96.35 (16) |
O2i—Zn1—O1ii | 91.12 (12) | O1ii—Zn1—N1i | 160.22 (15) |
Symmetry codes: (i) −x+1, −y, −z+1; (ii) −x, −y, −z+2. |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1B···O3iii | 0.90 | 2.18 | 3.056 (6) | 165 |
O1W—H1W1···O3iv | 0.86 (5) | 2.15 (5) | 2.950 (6) | 156 (6) |
O1W—H1W2···O2v | 0.85 (5) | 2.21 (5) | 3.051 (6) | 175 (9) |
Symmetry codes: (iii) x−1, y, z; (iv) x−1, y−1, z+1; (v) −x+2, −y, −z+1. |
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The rational design and assembly of metal–organic coordination frameworks have received much attention in recent years, owing to their intriguing structural topologies and potential or practical applications in the areas of catalysis, magnetism, gas storage, nonlinear optics, electronics and others (Bernhard et al., 2003; Lin et al., 1998; Sun et al., 2005; Zhu et al., 2005; Wu et al., 2007). A number of fascinating metal–organic coordination polymers are known to be constructed by the combination of symmetrical or asymmetrical bridging ligands as the 'spacer', and metal ions or a metal cluster as the 'node'. Carboxylate-containing ligands acting as the 'spacer' have attracted much attention because of the diversity of the binding modes of the carboxylate group (Zhu et al., 2005; Shi et al., 2005). The tyr ligand, with a carboxylate group, and its derivatives are good spacers because they often behave similarly to isonicotinic acid, acting as a chelating/bridging ligand via the N and O atoms. Diverse topologies can be achieved with the tyr ligand (Ayyappan et al., 2001; Lu & Babb, 2001; Lu et al., 2003). However, the reported complexes of the tyr ligand are generally mononuclear (Emseis et al., 2004; Harrowfield et al., 1983; Majumder et al., 2002), with only two coordination polymers being described in the literature, namely [Cu(H-tyr)2]n and {[Cu2(H-tyr)2(4,4-bipy). 2H2O].2 ClO4}n (Weng et al., 2002). Interestingly, the phenolate O atom in these two complexes is uncoordinated. In order to learn more about the coordination mode of the tyrosinato ligand, we have chosen the zinc(II) salts as the node. The zinc coordination polymers exhibit rich structural diversity because of the variable coordination behaviours of the d10 metal ion ZnII. We have successfully obtained the 1-D zinc(II) polymer [Zn(C9H7NO3)(H2O)]n, (I), containing the unusual coordinating phenolate O atom, via synthesis under solvothermal conditions.
The crystal structure of (I) consists of neutral [Zn(C9H7NO3) (H2O)] 1-D chains (see Fig. 1). Two Zn2+ ions are linked by two O atoms of the phenolate groups to give rise to a dimeric unit, which displays an inversion centre sited in the middle of the Zn2O2 cores. The Zn1—Zn1ii [symmetry code: (ii) -x, -y, -z + 2] intramolecular separation is 3.1981 (9) Å. Each zinc(II) centre in the dimeric unit is coordinated to three O atoms and one N atom from three tyr ligands, and one aqua ligand in a distorted square-pyramidal coordination geometry. These dimeric units are connected by tyr ligands to form a 1-D chain coordination polymer propagating along the crystallographic c axis. The Zn—O bonds, varying from 2.006 (3) to 2.075 (3) Å, are in good agreement with the corresponding bond lengths in [Zn2(Rsala)2 (H2O)2]·2H2O (Vittal & Yang, 2002), [Zn(BTZ)2]2 (Yu et al., 2003), Zn2(H2SB)2·3H2O·Me2CO (Matalobos et al., 2004) and [Zn2C22H20N4O2(H2O)2] (ClO4)2 (Huang et al., 2001). The Zn1—N1i [symmetry code: (i) -x + 1, -y, -z + 1] bond length is 2.154 (4) Å, comparable with those reported for [M(en)3]2Sn2S6 [2.14 (3)–2.23 (1) Å; Jia et al., 2004] and [Zn(en)3]4In16(Te2)4(Te3)Te22 [2.12 (3)–2.32 (3) Å; Chen et al., 2001]. The hydrogen bonds between the O atoms of the carboxylate groups and the H atoms of the water molecules [O1W—H···O3iv, symmetry code: (iv) x - 1, y - 1, z + 1] link the adjacent chains to form a 2-D sheet within the (100) plane (Fig. 2); the chains further interact via the formation of N1—H···O3iii and O1W—H···O2v [symmetry code: (iii) x - 1, y, z; (v) -x + 2, -y, -z + 1] hydrogen bonds, resulting in a three-dimensional H-bonding network structure.
The possible coordination modes of tyr2- and H-tyr- are shown in the scheme below. The usual chelating mode is (b), as exemplified by [Co(en)(2-N-eth-en) (H-tyr)]·2ClO4·2H2O (Harrowfield et al., 1983), [Cu(hista)(H-tyr) (ClO4)] (Yamauchi et al., 1989), [Ru(bpy)2 (H-tyr)]ClO4 (Majumder et al., 2002), [Cu(H-tyr)(phen)ClO4]·2.5H2O (Sugimori et al., 1997) and [Co(picchxn)(H-tyr)] Br2·3.5H2O (Emseis et al., 2004). The bridging mode (c) is less common, with only two examples found, [Cu(H-tyr)2]n and {[Cu2(H-tyr)2(4,4-bipy). 2H2O].2 ClO4}n (Weng et al., 2002). The most remarkable feature of (I) is the unusual coordination mode (a) of the tyr2- ligand. The tyr ligand adopts a chelating/bridging coordination mode, in which its amino and carboxylate bind a Zn2+ ion to form a chelating five-membered ring. In addition, the phenolate –OH bridges another two Zn2+ ions, resulting in the formation of a planar Zn2O2 four-membered ring which contributes to the distortion of the square-pyramidal geometry around the metal ion.