Di­aqua­bis(4-nitro­benzoato)­zinc(II): a redetermination

The title compound, [Zn(O₂CC₆H₄NO₂)₂(H₂O)₂], has zinc in a coordination geometry intermediate between tetrahedral and octahedral, with each carboxyl­ate ligand forming a primary and a secondary Zn-O bond; the lengths of these differ by more than 0.5 Å. The mol­ecule has crystallographic twofold rotation symmetry. Intermolecular hydrogen bonds are formed between the aqua ligands as donors and carboxyl­ate O atoms as acceptors, linking the mol­ecules together into sheets, the closest contacts between which involve the nitro groups.

Key indicators

• Single-crystal X-ray study
• T = 160 K
• Mean [sigma](C-C) = 0.004 Å
• R factor = 0.028
• wR factor = 0.074
• Data-to-parameter ratio = 9.8

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level C:



REFLT_03
         From the CIF: _diffrn_reflns_theta_max           25.35
         From the CIF: _reflns_number_total               1278
         TEST2: Reflns within _diffrn_reflns_theta_max
         Count of symmetry unique reflns         1414
         Completeness (_total/calc)             90.38%
          Alert C: < 95% complete

General Notes



ABSTM_02  When printed, the submitted absorption T values will be replaced
          by the scaled T values. Since the ratio of scaled T's is
          identical to the ratio of reported T values, the scaling does
          not imply a change to the absorption corrections used in the
          study.
          Ratio of Tmax expected/reported      1.077
          Tmax scaled      0.672 Tmin scaled      0.579


 0 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 1 Alert Level C = Please check

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S1. Comment

As part of a study of zinc complexes of carboxylic acids, we have attempted to prepare anhydrous zinc 4-nitrobenzoate and have obtained the title compound, (I), which incorporates two molecules of water as ligands to the Zn atom in addition to the two 4-nitrobenzoate groups.

The crystal structure of this complex has been reported previously by Guseinov et al. (1983), from data collected at room temperature. Although the space group was given as P1, with a complete molecule as the asymmetric unit, use of the ADSYM procedure of PLATON (Spek, 2000) shows that this should be C2/c, with molecules lying on twofold rotation axes and an asymmetric unit containing only one ligand of each kind as well as the Zn atom. The transformed structure is essentially the same as that reported here, which has been determined from low-temperature data and is of higher precision.

The monomeric molecule (Fig. 1) has a highly distorted coordination geometry for zinc, intermediate between tetrahedral and octahedral. Each of the two symmetry-equivalent 4-nitrobenzoate ligands coordinates to Zn through a primary Zn—O bond and a considerably weaker secondary Zn—O bond, the difference in the bond lengths being >0.5 Å. The asymmetry of bonding by the two O atoms of the carboxylate function is also reflected in the C—O bond lengths, which differ by 0.04 Å, the shorter C—O bond being the one more weakly coordinated to zinc and having greater double-bond character.

The coordinated water molecules are involved in hydrogen bonding, acting as donors but not acceptors. One H atom of each aqua ligand hydrogen bonds to a carboxylate O atom in the next molecule along the short b axis, generating chains of molecules in this direction (Fig. 2). The other forms a hydrogen bond to a carboxylate O atom in an adjacent molecule, such that the chains of molecules are further crosslinked into sheets (Fig. 3). These sheets are stacked with shortest contacts involving their nitro groups.

Diaqua complexes of bis(carboxylato)zinc(II) are known for many different carboyxlates, and there are about 20 examples in the Cambridge Structural Database (Allen & Kennard, 1993). The structure of the title compound is typical, but the second Zn—O bond for each carboxlato ligand varies considerably in length, from a situation in which the two Zn—O bonds are equivalent in a symmetrically chelating carboxylate to over 2.8 Å for ligands which are essentially monodentate. The degree of asymmetry in the title compound lies towards the higher end of this observed range.

S2. Experimental

The title compound was prepared from the reaction of ZnSO₄·7H₂O (0.01 mol) and sodium 4-nitrobenzoate (0.02 mol) solutions; sodium 4-nitrobenzoate was first obtained by adding 0.02 mol of 4-nitrobenzoic acid to an aqueous solution of 0.02 mol NaHCO₃. The mixture was set aside to crystallize at ambient temperature for several days, giving suitable colourless single crystals.

S3. Refinement

H atoms were placed geometrically and refined with a riding model (including free rotation about C—C bonds), and with U_iso constrained to be 1.2U_eq of the carrier atom, in the organic ligands. For the coordinated water molecules, H atoms were refined with the restraint of similar O—H distances, and with U_iso = 1.5U_eq(O).

Computing details

Data collection: SMART (Siemens, 1995); cell refinement: local programs; data reduction: SAINT (Siemens, 1995); program(s) used to solve structure: SHELXTL (Sheldrick, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and local programs.

Figures

	Fig. 1. The molecular structure of (I) with atom labels and 50% probability ellipsoids for non-H atoms. The view is down the b axis and the secondary Zn—O bonds are shown broken.
	Fig. 2. Chains of molecules linked along the b axis by O—H···O hydrogen bonds (shown dashed).
	Fig. 3. The crosslinking of chains of molecules by further O—H···O hydrogen bonds. The view direction is as for Fig. 1.

Diaquabis(4-nitrobenzoato)zinc(II)

Crystal data

[Zn(C7H4NO4)2(H2O)2]	F(000) = 880
Mr = 433.63	Dx = 1.866 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 26.519 (4) Å	Cell parameters from 3034 reflections
b = 5.1011 (9) Å	θ = 2.2–25.3°
c = 12.143 (2) Å	µ = 1.66 mm^−1
β = 110.019 (4)°	T = 160 K
V = 1543.4 (5) Å^3	Block, colourless
Z = 4	0.52 × 0.32 × 0.24 mm

Data collection

Siemens SMART 1K CCD diffractometer	1278 independent reflections
Radiation source: sealed tube	1262 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.037
Detector resolution: 8.192 pixels mm^-1	θmax = 25.4°, θmin = 1.6°
ω rotation with narrow frames scans	h = −30→25
Absorption correction: multi-scan (XPREP; Sheldrick, 1997)	k = −5→4
Tmin = 0.538, Tmax = 0.624	l = −13→14
3124 measured reflections

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)] = 0.028	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.074	w = 1/[σ^2(Fo^2) + (0.0351P)^2 + 3.132P] where P = (Fo^2 + 2Fc^2)/3
S = 1.09	(Δ/σ)max < 0.001
1278 reflections	Δρmax = 0.26 e Å^−3
130 parameters	Δρmin = −0.43 e Å^−3
1 restraint	Extinction correction: SHELXTL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: heavy-atom method	Extinction coefficient: 0.0018 (4)

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

	x	y	z	Uiso*/Ueq
Zn	0.5000	0.56751 (7)	0.2500	0.02271 (17)
O1	0.56045 (6)	0.8270 (3)	0.30048 (13)	0.0235 (4)
O2	0.53729 (7)	0.7466 (3)	0.45494 (14)	0.0264 (4)
C1	0.56563 (9)	0.8670 (5)	0.40835 (19)	0.0207 (5)
C2	0.60557 (9)	1.0695 (4)	0.4735 (2)	0.0194 (5)
C3	0.60243 (10)	1.1777 (5)	0.5757 (2)	0.0241 (5)
H3	0.5752	1.1201	0.6046	0.029*
C4	0.63833 (9)	1.3682 (5)	0.6361 (2)	0.0238 (5)
H4	0.6366	1.4406	0.7068	0.029*
C5	0.67691 (9)	1.4501 (4)	0.5903 (2)	0.0215 (5)
C6	0.68118 (9)	1.3479 (5)	0.4885 (2)	0.0254 (5)
H6	0.7081	1.4085	0.4593	0.030*
C7	0.64518 (9)	1.1542 (5)	0.4299 (2)	0.0246 (5)
H7	0.6475	1.0793	0.3602	0.030*
N	0.71401 (8)	1.6588 (4)	0.65244 (17)	0.0249 (5)
O3	0.75082 (7)	1.7158 (4)	0.61735 (15)	0.0332 (4)
O4	0.70634 (7)	1.7640 (4)	0.73614 (16)	0.0352 (5)
O5	0.52937 (8)	0.2914 (3)	0.17327 (15)	0.0286 (4)
H5A	0.5345 (13)	0.287 (7)	0.112 (2)	0.043*
H5B	0.5414 (13)	0.159 (6)	0.206 (3)	0.043*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn	0.0221 (2)	0.0131 (2)	0.0325 (3)	0.000	0.00889 (17)	0.000
O1	0.0272 (8)	0.0197 (9)	0.0209 (8)	−0.0006 (7)	0.0048 (7)	−0.0042 (7)
O2	0.0287 (9)	0.0209 (9)	0.0271 (9)	−0.0063 (7)	0.0061 (7)	0.0009 (7)
C1	0.0221 (11)	0.0132 (11)	0.0225 (11)	0.0033 (10)	0.0021 (9)	0.0014 (9)
C2	0.0203 (11)	0.0165 (12)	0.0195 (11)	0.0015 (9)	0.0042 (9)	0.0015 (8)
C3	0.0264 (12)	0.0223 (13)	0.0257 (12)	−0.0027 (10)	0.0115 (10)	−0.0010 (10)
C4	0.0266 (12)	0.0215 (12)	0.0234 (12)	−0.0023 (10)	0.0087 (10)	−0.0029 (10)
C5	0.0218 (12)	0.0174 (12)	0.0213 (11)	−0.0008 (9)	0.0022 (9)	0.0016 (9)
C6	0.0234 (12)	0.0267 (13)	0.0263 (12)	−0.0040 (10)	0.0090 (10)	0.0008 (10)
C7	0.0254 (12)	0.0265 (13)	0.0228 (12)	0.0000 (10)	0.0093 (10)	−0.0022 (10)
N	0.0247 (10)	0.0194 (11)	0.0252 (10)	−0.0022 (9)	0.0016 (8)	0.0042 (9)
O3	0.0325 (10)	0.0347 (11)	0.0303 (9)	−0.0149 (8)	0.0079 (8)	0.0027 (8)
O4	0.0358 (10)	0.0306 (10)	0.0374 (11)	−0.0069 (8)	0.0102 (8)	−0.0143 (8)
O5	0.0413 (10)	0.0193 (9)	0.0261 (9)	0.0076 (8)	0.0128 (8)	0.0029 (7)

Geometric parameters (Å, º)

Zn—O5	1.9888 (18)	C3—H3	0.9500
Zn—O5^i	1.9888 (18)	C4—C5	1.386 (3)
Zn—O1	2.0062 (16)	C4—H4	0.9500
Zn—O1^i	2.0062 (16)	C5—C6	1.381 (3)
Zn—O2	2.5141 (17)	C5—N	1.471 (3)
Zn—O2^i	2.5141 (17)	C6—C7	1.389 (4)
O1—C1	1.286 (3)	C6—H6	0.9500
O2—C1	1.246 (3)	C7—H7	0.9500
C1—C2	1.497 (3)	N—O3	1.226 (3)
C2—C3	1.386 (3)	N—O4	1.226 (3)
C2—C7	1.397 (3)	O5—H5A	0.80 (2)
C3—C4	1.382 (4)	O5—H5B	0.79 (2)
O5—Zn—O5^i	89.83 (11)	C7—C2—C1	120.1 (2)
O5—Zn—O1	102.13 (7)	C4—C3—C2	120.9 (2)
O5^i—Zn—O1	136.40 (7)	C4—C3—H3	119.6
O5—Zn—O1^i	136.40 (7)	C2—C3—H3	119.6
O5^i—Zn—O1^i	102.13 (7)	C3—C4—C5	117.9 (2)
O1—Zn—O1^i	97.44 (9)	C3—C4—H4	121.1
O5—Zn—O2	129.54 (6)	C5—C4—H4	121.1
O5^i—Zn—O2	82.99 (6)	C6—C5—C4	122.9 (2)
O1—Zn—O2	57.01 (6)	C6—C5—N	119.1 (2)
O1^i—Zn—O2	93.74 (6)	C4—C5—N	117.9 (2)
O5—Zn—O2^i	82.99 (6)	C5—C6—C7	118.3 (2)
O5^i—Zn—O2^i	129.54 (6)	C5—C6—H6	120.8
O1—Zn—O2^i	93.74 (6)	C7—C6—H6	120.8
O1^i—Zn—O2^i	57.01 (6)	C6—C7—C2	120.0 (2)
O2—Zn—O2^i	137.39 (8)	C6—C7—H7	120.0
C1—O1—Zn	102.22 (14)	C2—C7—H7	120.0
C1—O2—Zn	79.85 (13)	O3—N—O4	124.2 (2)
O2—C1—O1	120.9 (2)	O3—N—C5	117.8 (2)
O2—C1—C2	121.8 (2)	O4—N—C5	118.1 (2)
O1—C1—C2	117.2 (2)	Zn—O5—H5A	132 (2)
C3—C2—C7	120.0 (2)	Zn—O5—H5B	121 (2)
C3—C2—C1	119.9 (2)	H5A—O5—H5B	106 (3)
O5—Zn—O1—C1	−130.69 (14)	O2—C1—C2—C7	164.6 (2)
O5^i—Zn—O1—C1	−27.90 (19)	O1—C1—C2—C7	−17.4 (3)
O1^i—Zn—O1—C1	88.49 (14)	C7—C2—C3—C4	−0.4 (4)
O2—Zn—O1—C1	−1.14 (13)	C1—C2—C3—C4	−178.9 (2)
O2^i—Zn—O1—C1	145.69 (14)	C2—C3—C4—C5	1.0 (4)
O5—Zn—O2—C1	78.98 (15)	C3—C4—C5—C6	−0.7 (4)
O5^i—Zn—O2—C1	162.93 (14)	C3—C4—C5—N	178.1 (2)
O1—Zn—O2—C1	1.16 (13)	C4—C5—C6—C7	−0.1 (4)
O1^i—Zn—O2—C1	−95.29 (14)	N—C5—C6—C7	−178.9 (2)
O2^i—Zn—O2—C1	−52.60 (12)	C5—C6—C7—C2	0.7 (4)
Zn—O2—C1—O1	−1.8 (2)	C3—C2—C7—C6	−0.4 (4)
Zn—O2—C1—C2	176.1 (2)	C1—C2—C7—C6	178.0 (2)
Zn—O1—C1—O2	2.2 (2)	C6—C5—N—O3	−7.1 (3)
Zn—O1—C1—C2	−175.78 (16)	C4—C5—N—O3	174.1 (2)
O2—C1—C2—C3	−17.0 (3)	C6—C5—N—O4	173.2 (2)
O1—C1—C2—C3	161.0 (2)	C4—C5—N—O4	−5.6 (3)

Symmetry code: (i) −x+1, y, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O5—H5A···O2^ii	0.80 (2)	1.95 (2)	2.737 (2)	172 (3)
O5—H5B···O1^iii	0.79 (2)	2.01 (3)	2.794 (2)	169 (3)

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