Crystal structure of ammonium bis­(pyridine-2,6-di­carboxyl­ato-κ³O,N,O′)chromate(III) from synchrotron data

In the title complex, the Cr^III ion is coordinated by two nearly perpendicular pyridine-2,6-di­carboxyl­ate (pydc) dianions acting as tridentate ligands through the pyridine N atoms and two of the O atoms of each carboxyl­ate group, in a distorted octa­hedral geometry. The ammonium cation is linked to non-coordinating carbonyl O atoms from neighboring pydc groups through N—H...O hydrogen-bonding inter­actions.

Key indicators

• Single-crystal synchrotron study
• T = 301 K
• Mean [sigma](C-C) = 0.002 Å
• R factor = 0.028
• wR factor = 0.081
• Data-to-parameter ratio = 15.0

checkCIF/PLATON results

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Datablock: I


Alert level B
PLAT029_ALERT_3_B _diffrn_measured_fraction_theta_full Low .......      0.940 Note

Alert level C
PLAT911_ALERT_3_C Missing # FCF Refl Between THmin & STh/L=  0.600         35 Report
PLAT913_ALERT_3_C Missing # of Very Strong Reflections in FCF ....          6 Note

Alert level G
ABSMU01_ALERT_1_G  Calculation of _exptl_absorpt_correction_mu
                not performed for this radiation type.
PLAT002_ALERT_2_G Number of Distance or Angle Restraints on AtSite          2 Note
PLAT042_ALERT_1_G Calc. and Reported MoietyFormula Strings  Differ     Please Check
PLAT066_ALERT_1_G Predicted and Reported Tmin&Tmax Range Identical          ? Check
PLAT172_ALERT_4_G The CIF-Embedded .res File Contains DFIX Records          1 Report
PLAT860_ALERT_3_G Number of Least-Squares Restraints .............          1 Note
PLAT910_ALERT_3_G Missing # of FCF Reflection(s) Below Th(Min) ...          4 Report
PLAT912_ALERT_4_G Missing # of FCF Reflections Above STh/L=  0.600         12 Note
PLAT952_ALERT_5_G Calculated (ThMax) and CIF-Reported Lmax Differ           4 Units
PLAT958_ALERT_1_G Calculated (ThMax) and Actual (FCF) Lmax Differ           4 Units

   0 ALERT level A = Most likely a serious problem - resolve or explain
   1 ALERT level B = A potentially serious problem, consider carefully
   2 ALERT level C = Check. Ensure it is not caused by an omission or oversight
  10 ALERT level G = General information/check it is not something unexpected

   4 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   1 ALERT type 2 Indicator that the structure model may be wrong or deficient
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S1. Chemical context

Pyridine-2,6-di­carb­oxy­lic acid (also known as dipicolinic acid and abbreviated here as H₂pydc) can coordinate a metal center as a neutral molecule (H₂pydc), the univalent anion (Hpydc^-), or the divalent anion (pydc^2-). In particular, the pyridine-2,6-di­carboxyl­ate ligand frequently acts as a meridional tridentate ligand and sometimes also as a bidentate or bridging ligand (Park et al., 2007). The first [Cr(pydc)₂]^- complex was prepared as the Na⁺ salt according to the literature (Hoggard & Schmidtke, 1973) and its crystal structure determined using synchrotron data. Structural analysis showed the compound to be a dihydrate (Dai et al., 2006; González-Baró et al., 2008) rather than the 1.5 or 2.5 hydrates that had been suggested previously (Hoggard & Schmidtke, 1973; Fürst et al., 1979). The crystal structures of K[Cr(pydc)₂] (Hakimi et al., 2012) and Rb[Cr(pydc)₂] (Fürst et al., 1979) have also been reported previously but the structure of the ammonium salt is not known. Here we report the crystal structure of NH₄[Cr(pydc)₂] in order to clarify unambiguously the bonding mode of the two pyridine-2,6-di­carboxyl­ato ligands and the geometrical arrangement of this ammonium salt.

S2. Structural commentary

Counter-ionic species play a very important role in coordination chemistry. The structure reported here is another example of a [Cr(pydc)₂]^- salt but with a different cation. The structural analysis shows that the two tridentate pyridine-2,6-di­carboxyl­ate (pydc) dianions o­cta­hedrally coordinate to the Cr^III metal center through one N atom and two carboxyl­ate O atoms in a meridional arrangement. The Cr^III ion is located on a crystallographic fourfold rotoinversion axis (4). An ellipsoid plot of title complex together with the atomic numbering is illustrated in Fig. 1.

The Cr—N and Cr—O bond lengths to the pydc ligands are 1.9727 (15) and 1.9889 (9) Å, respectively, and these lengths agree well with the values observed in the literature for complexes with the same [Cr(pydc)₂]^- anion (Fürst et al., 1979; Dai et al., 2006; González-Baró et al., 2008; Zhou et al., 2009; Hakimi et al., 2012). The coordinating pyridine N atoms are in a mutually trans arrangement. Both tridendate pydc^2- ligands are nearly planar and are oriented perpendicular to one another. Bond angles about the central chromium atom are 79.10 (3) for N1—Cr1—O1, 100.90 (3) for N1—Cr1—O1ⁱ and 158.20 (5)° for O1^i-–Cr1—O1ⁱⁱ, indicating a distorted o­cta­hedral coordination environment [symmetry codes: (i) -y+5/4, x-3/4, -z+5/4; (ii) y+3/4, -x+5/4, -z+5/4]. The C1—O1 and C1—O2 bond lengths within the carboxyl­ate group of the pydc^2- ligand are 1.2941 (15) and 1.2223 (14) Å, respectively, and can be compared with values of 1.298 (5) and 1.224 (5) Å for Rb[Cr(pydc)₂] (Fürst et al., 1979). The ammonium cation, also lying on a crystallographic fourfold rotoinversion axis (4), shows a distorted tetra­hedral geometry of the hydrogen atoms around the central nitro­gen atom with N—H distances of 0.846 (9) Å and the H—N—H angles ranging from 105.36 (9) to 118.06 (9)°.

S3. Supra­molecular features

The pattern of hydrogen bonding around the cation is very similar to the coordination environment in the related potassium salt (Hakimi et al., 2012). The non-coordinating carbonyl O atom forms weak C—H···O hydrogen bonds that contribute to the crystal packing. The ammonium cation is also linked to the carbonyl O atoms from four neighboring pydc^2- ligands through classical N—H···O hydrogen bonds (Table 1). An extensive array of these contacts generate a three-dimensional network of molecules stacked along the a-axis direction (Fig. 2). π–π inter­actions involving adjacent pyridine rings further link the components of the structure into a three-dimensional network. The centroid–centroid distances between the π–π stacked rings (N1/C2–C4/C3^iv/C2^iv)···(N1^v/C2^v–C4^v/C3^vi/C2^vi) is 3.572 (2) Å [symmetry codes: (iv) 2 - x, 1/2 - y, z; (v) 1/2 + x, y, 3/2 - z; (vi) 5/2 -x, 1/2 - y, 3/2 - z].

S4. Database survey

A search of the Cambridge Structural Database (Version 5.35, May 2014 with one update; Groom & Allen, 2014) indicates a total of 16 hits for Cr^III complexes with a complex anion [Cr(pydc)₂]^- unit. Many crystal structures of [Cr(pydc)₂]^- with inorganic, organic or complex counter-cations such as K⁺ (Hakimi et al., 2012), Na⁺ (Dai et al., 2006; González-Baró et al., 2008; Zhou et al., 2009), Rb⁺ (Fürst et al., 1979), creatH⁺ (creat = creatinine; Aghabozorg et al., 2008), 4,4'-bpyH⁺ (bpy = bi­pyridine; Soleimannejad et al., 2008), dmpH⁺ (dmp = 2,9-di­methyl-1,10-phenanthrone; Aghajani et al., 2009), 2-apymH⁺ (2-apym = 2-amino­pyrimidine; Eshtiagh-Hosseini et al., 2010), [Cr(tpy)(pydc)]⁺ [tpy = 2,6-bis­(2-pyridyl)­pyridine; Casellato et al., 1991] and [Ag(atr)₂]⁺ (atr = 3-amino-1H-1,2,4-triazole; Tabatabaee et al., 2011) have been determined.

Alternative coordination behaviors of the pydc ligands are found in [Cu(Hpydc)₂]·3H₂O, which has one neutral H₂pydc and one divalent pydc^2- ligand, while [Ni(Hpydc)₂]·3H₂O (Nathan & Mai, 2000) has two meridional univalent Hpydc^- ligands. The ligands in [Ni(cyclam)(Hpydc)₂]·2H₂O (Park et al., 2007) and Na₂[Pt(Hpydc)₂]·6H₂O are monodentate and bidentate, respectively, while Hpydc^- is tridentate in the complexes [Cu(Hpydc)₂]·3H₂O and [Ni(Hpydc)₂]·3H₂O (Nathan & Mai, 2000).

S5. Synthesis and crystallization

All chemicals were reagent-grade materials and were used without further purification. The starting material, Na[Cr(pydc)₂]·2H₂O was prepared as described previously (Hoggard & Schmidtke, 1973). The sodium salt (0.20 g) was dissolved in 15 ml of water at 323 K and added to 3 ml of water containing 0.5 g of NH₄Cl. The resulting solution was filtered and allowed to stand at room temperature for several days to give brown block-like crystals of the ammonium salt NH₄[Cr(pydc)₂] suitable for X-ray structural analysis.

S6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C—H distances of 0.95 (ring H atoms) and with U_iso(H) = 1.2 U_eq(parent atom). The H atoms of the ammonium cation were located from difference Fourier maps and refined with restraints and a fixed N—H distance of 0.87 Å, with U_iso(H) = 1.2U_eq(N). One reflection with F_o<<<F_c was omitted from the final refinement cycles. The slightly low fraction of measured reflections results from the geometry of the 2D-SMC beamline goniostat.

Related literature

For related literature, see: Aghabozorg et al. (2008); Aghajani et al. (2009); Casellato et al. (1991); Dai et al. (2006); Eshtiagh-Hosseini, Yousefi, Mirzaei, Chen, Beyramabadi, Shokrollahi & Aghaei (2010); Fürst et al. (1979); González-Baró, Pis-Diez, Pis-Diez & Parajón-Costa (2008); Groom & Allen (2014); Hakimi et al. (2012); Hoggard & Schmidtke (1973); Nathan & Mai (2000); Park et al. (2007); Soleimannejad et al. (2008); Tabatabaee et al. (2011); Zhou et al. (2009).

Computing details

Data collection: PAL ADSC Quantum-210 ADX (Arvai & Nielsen, 1983); cell refinement: HKL3000sm (Otwinowski & Minor, 1997); data reduction: HKL3000sm (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXT2014/5 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2008, 2015b); molecular graphics: DIAMOND (Brandenburg, 2007); software used to prepare material for publication: publCIF (Westrip 2010).

Figures

	Fig. 1. The molecular structure of NH4[Cr(pydc)2], showing the atom-numbering scheme. Non-H atoms are shown as displacement ellipsoids drawn at the 50% probability level.
	Fig. 2. Crystal packing of NH4[Cr(pydc)2], viewed perpendicular to the bc plane. Dashed lines represent C—H···O (purple) and N—H···O (blue) hydrogen-bonding interactions.

Bis(pyridine-2,6-dicarboxylato-κ³O,N,O')chromate(III)

Crystal data

(NH4)[Cr(C7H3NO4)2]	Dx = 1.855 Mg m^−3
Mr = 400.25	Synchrotron radiation, λ = 0.62998 Å
Tetragonal, I41/a	Cell parameters from 20017 reflections
a = 7.0305 (10) Å	θ = 0.4–33.6°
c = 28.995 (6) Å	µ = 0.62 mm^−1
V = 1433.2 (5) Å^3	T = 301 K
Z = 4	Block, brown
F(000) = 812	0.15 × 0.10 × 0.10 mm

Data collection

ADSC Q210 CCD area detector diffractometer	903 reflections with I > 2σ(I)
Radiation source: PLSII 2D bending magnet	Rint = 0.052
ω scan	θmax = 26.0°, θmin = 4.0°
Absorption correction: empirical (using intensity measurements) (HKL3000sm SCALEPACK; Otwinowski & Minor, 1997)	h = −9→9
Tmin = 0.925, Tmax = 0.940	k = −9→9
6841 measured reflections	l = −36→36
943 independent reflections

Refinement

Refinement on F^2	1 restraint
Least-squares matrix: full	Hydrogen site location: mixed
R[F^2 > 2σ(F^2)] = 0.028	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.081	w = 1/[σ^2(Fo^2) + (0.0461P)^2 + 0.7347P] where P = (Fo^2 + 2Fc^2)/3
S = 1.21	(Δ/σ)max = 0.001
943 reflections	Δρmax = 0.31 e Å^−3
63 parameters	Δρmin = −0.86 e Å^−3

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

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

	x	y	z	Uiso*/Ueq
Cr1	1.0000	0.2500	0.6250	0.01282 (14)
O1	0.88412 (13)	0.50247 (12)	0.63797 (3)	0.0205 (2)
O2	0.78279 (14)	0.69538 (13)	0.69398 (4)	0.0261 (2)
N1	1.0000	0.2500	0.69303 (5)	0.0133 (3)
C1	0.86229 (15)	0.55077 (15)	0.68071 (4)	0.0163 (2)
C2	0.93527 (14)	0.40403 (15)	0.71495 (4)	0.0142 (2)
C3	0.93425 (15)	0.41005 (17)	0.76260 (4)	0.0189 (2)
H3	0.8912	0.5171	0.7783	0.023*
C4	1.0000	0.2500	0.78645 (6)	0.0201 (3)
H4	1.0000	0.2500	0.8185	0.024*
N1S	0.5000	0.7500	0.6250	0.0223 (4)
H1S	0.502 (3)	0.8529 (19)	0.6099 (7)	0.027*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cr1	0.01591 (16)	0.01591 (16)	0.0066 (2)	0.000	0.000	0.000
O1	0.0286 (4)	0.0193 (4)	0.0134 (5)	0.0055 (3)	−0.0013 (3)	0.0013 (3)
O2	0.0304 (5)	0.0222 (4)	0.0258 (5)	0.0098 (3)	−0.0009 (3)	−0.0037 (3)
N1	0.0146 (5)	0.0171 (6)	0.0082 (7)	0.0008 (4)	0.000	0.000
C1	0.0159 (4)	0.0170 (5)	0.0160 (6)	0.0004 (3)	−0.0013 (4)	−0.0005 (4)
C2	0.0133 (4)	0.0175 (4)	0.0117 (5)	0.0001 (3)	−0.0005 (3)	−0.0016 (3)
C3	0.0171 (5)	0.0258 (5)	0.0137 (6)	−0.0006 (4)	0.0005 (3)	−0.0059 (4)
C4	0.0199 (7)	0.0324 (8)	0.0078 (8)	−0.0021 (5)	0.000	0.000
N1S	0.0220 (6)	0.0220 (6)	0.0229 (12)	0.000	0.000	0.000

Geometric parameters (Å, º)

Cr1—N1	1.9727 (15)	N1—C2	1.3355 (12)
Cr1—N1^i	1.9727 (15)	C1—C2	1.5208 (15)
Cr1—O1	1.9889 (9)	C2—C3	1.3824 (16)
Cr1—O1^ii	1.9889 (9)	C3—C4	1.3993 (15)
Cr1—O1^i	1.9889 (9)	C3—H3	0.9300
Cr1—O1^iii	1.9889 (9)	C4—C3^iii	1.3992 (15)
O1—C1	1.2941 (15)	C4—H4	0.9300
O2—C1	1.2223 (14)	N1S—H1S	0.846 (9)
N1—C2^iii	1.3354 (12)
N1—Cr1—N1^i	180.0	C2^iii—N1—C2	123.19 (15)
N1—Cr1—O1	79.10 (3)	C2^iii—N1—Cr1	118.40 (7)
N1^i—Cr1—O1	100.90 (3)	C2—N1—Cr1	118.41 (7)
N1—Cr1—O1^ii	100.90 (3)	O2—C1—O1	125.02 (11)
N1^i—Cr1—O1^ii	79.10 (3)	O2—C1—C2	120.87 (11)
O1—Cr1—O1^ii	92.049 (10)	O1—C1—C2	114.02 (9)
N1—Cr1—O1^i	100.90 (3)	N1—C2—C3	120.14 (11)
N1^i—Cr1—O1^i	79.10 (3)	N1—C2—C1	110.77 (10)
O1—Cr1—O1^i	92.049 (10)	C3—C2—C1	129.04 (10)
O1^ii—Cr1—O1^i	158.20 (5)	C2—C3—C4	117.88 (11)
N1—Cr1—O1^iii	79.10 (3)	C2—C3—H3	121.1
N1^i—Cr1—O1^iii	100.90 (3)	C4—C3—H3	121.1
O1—Cr1—O1^iii	158.20 (5)	C3^iii—C4—C3	120.77 (16)
O1^ii—Cr1—O1^iii	92.049 (10)	C3^iii—C4—H4	119.6
O1^i—Cr1—O1^iii	92.049 (10)	C3—C4—H4	119.6
C1—O1—Cr1	117.64 (7)
Cr1—O1—C1—O2	−175.18 (9)	O1—C1—C2—N1	−2.76 (12)
Cr1—O1—C1—C2	1.39 (12)	O2—C1—C2—C3	−3.27 (17)
C2^iii—N1—C2—C3	0.47 (7)	O1—C1—C2—C3	−179.99 (10)
Cr1—N1—C2—C3	−179.53 (7)	N1—C2—C3—C4	−0.90 (14)
C2^iii—N1—C2—C1	−177.04 (9)	C1—C2—C3—C4	176.10 (9)
Cr1—N1—C2—C1	2.96 (9)	C2—C3—C4—C3^iii	0.44 (7)
O2—C1—C2—N1	173.96 (9)

Symmetry codes: (i) y+3/4, −x+5/4, −z+5/4; (ii) −y+5/4, x−3/4, −z+5/4; (iii) −x+2, −y+1/2, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C3—H3···O2^iv	0.93	2.50	3.4071 (15)	167
N1S—H1S···O2^v	0.85 (1)	2.04 (1)	2.8462 (11)	158 (2)

Symmetry codes: (iv) −x+3/2, −y+3/2, −z+3/2; (v) −y+5/4, x+1/4, −z+5/4.