Crystal structure of ammonium bis-(pyridine-2,6-di-carboxyl-ato-κ(3) O,N,O')chromate(III) from synchrotron data.

The structure of the title compound, (NH4)[Cr(pydc)2] (pydc is pyridine-2,6-di-carboxyl-ate, C7H3NO4), has been determined from synchrotron data. The Cr(III) ion and the N atom of the ammonium cation are located on a crystallographic fourfold rotoinversion axis (-4). The Cr(III) cation is coordinated by four O atoms and the two N atoms of two meridional pydc ligands, displaying a distorted octa-hedral geometry. The Cr-N and Cr-O bond lengths are 1.9727 (15) and 1.9889 (9) Å, respectively. The crystal structure is stabilized by inter-molecular hydrogen bonds involving the N-H groups of the ammonium cation and pyridine C-H groups as donors and the non-coordinating carbonyl O atoms as acceptors.

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 (pydc2−). 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 structural arrangement of this ammonium salt.

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 CrIII metal center through one N atom and two carboxyl­ate O atoms in a meridional arrangement. The CrIII ion is located on a crystallographic fourfold rotoinversion axis (). An ellipsoid plot of title complex together with the atomic numbering is illustrated in Fig. 1 ▸.

Figure 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.

The Cr—N and Cr—O bond lengths to the pydc2− 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 pydc2− 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 O1i—Cr1—O1ii, indicating a distorted octa­hedral coordination environment [symmetry codes: (i) −y + , x − , −z + ; (ii) y + , −x + , −z + ]. The C1—O1 and C1—O2 bond lengths within the carboxyl­ate group of the pydc2− 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 (), 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)°.

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 pydc2− 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 distance between the π–π stacked rings (N1/C2–C4/C3iv/C2iv)⋯(N1v/C2v–C4v/C3vi/C2vi) is 3.572 (2) Å [symmetry codes: (iv) 2 − x,  − y, z; (v)  + x, y,  − z; (vi)  − x,  − y,  − z].

Table 1

Hydrogen-bond geometry (, )

DHA	DH	HA	D A	DHA
C3H3O2i	0.93	2.50	3.4071(15)	167
N1SH1SO2ii	0.85(1)	2.04(1)	2.8462(11)	158(2)

Symmetry codes: (i) ; (ii) .

Figure 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.

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 CrIII 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 pydc2− 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 ▸).

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.

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<<

Table 2

Experimental details

Crystal data
Chemical formula	(NH4)[Cr(C7H3NO4)2]
M r	400.25
Crystal system, space group	Tetragonal, I41/a
Temperature (K)	301
a, c ()	7.0305(10), 28.995(6)
V (3)	1433.2(5)
Z	4
Radiation type	Synchrotron, = 0.62998
(mm1)	0.62
Crystal size (mm)	0.15 0.10 0.10
Data collection
Diffractometer	ADSC Q210 CCD area detector
Absorption correction	Empirical (using intensity measurements) (HKL3000sm SCALEPACK; Otwinowski Minor, 1997 ▸)
T min, T max	0.925, 0.940
No. of measured, independent and observed [I > 2(I)] reflections	6841, 943, 903
R int	0.052
(sin /)max (1)	0.695
Refinement
R[F 2 > 2(F 2)], wR(F 2), S	0.028, 0.081, 1.21
No. of reflections	943
No. of parameters	63
No. of restraints	1
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
max, min (e 3)	0.31, 0.86

Computer programs: PAL ADSC Quantum-210 ADX (Arvai Nielsen, 1983 ▸), HKL3000sm (Otwinowski Minor, 1997 ▸), SHELXT2014/5 (Sheldrick, 2015a ▸), SHELXL2014/7 (Sheldrick, 2008 ▸, 2015b ▸), DIAMOND (Brandenburg, 2007 ▸) and publCIF (Westrip 2010 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989015001152/sj5438sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989015001152/sj5438Isup2.hkl

CCDC reference: 1044324

Additional supporting information: crystallographic information; 3D view; checkCIF report

(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) Å3	T = 301 K
Z = 4	Block, brown
F(000) = 812	0.15 × 0.10 × 0.10 mm

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 on F2	1 restraint
Least-squares matrix: full	Hydrogen site location: mixed
R[F2 > 2σ(F2)] = 0.028	H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.081	w = 1/[σ2(Fo2) + (0.0461P)2 + 0.7347P] where P = (Fo2 + 2Fc2)/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

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.

	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*

	U11	U22	U33	U12	U13	U23
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

Cr1—N1	1.9727 (15)	N1—C2	1.3355 (12)
Cr1—N1i	1.9727 (15)	C1—C2	1.5208 (15)
Cr1—O1	1.9889 (9)	C2—C3	1.3824 (16)
Cr1—O1ii	1.9889 (9)	C3—C4	1.3993 (15)
Cr1—O1i	1.9889 (9)	C3—H3	0.9300
Cr1—O1iii	1.9889 (9)	C4—C3iii	1.3992 (15)
O1—C1	1.2941 (15)	C4—H4	0.9300
O2—C1	1.2223 (14)	N1S—H1S	0.846 (9)
N1—C2iii	1.3354 (12)
N1—Cr1—N1i	180.0	C2iii—N1—C2	123.19 (15)
N1—Cr1—O1	79.10 (3)	C2iii—N1—Cr1	118.40 (7)
N1i—Cr1—O1	100.90 (3)	C2—N1—Cr1	118.41 (7)
N1—Cr1—O1ii	100.90 (3)	O2—C1—O1	125.02 (11)
N1i—Cr1—O1ii	79.10 (3)	O2—C1—C2	120.87 (11)
O1—Cr1—O1ii	92.049 (10)	O1—C1—C2	114.02 (9)
N1—Cr1—O1i	100.90 (3)	N1—C2—C3	120.14 (11)
N1i—Cr1—O1i	79.10 (3)	N1—C2—C1	110.77 (10)
O1—Cr1—O1i	92.049 (10)	C3—C2—C1	129.04 (10)
O1ii—Cr1—O1i	158.20 (5)	C2—C3—C4	117.88 (11)
N1—Cr1—O1iii	79.10 (3)	C2—C3—H3	121.1
N1i—Cr1—O1iii	100.90 (3)	C4—C3—H3	121.1
O1—Cr1—O1iii	158.20 (5)	C3iii—C4—C3	120.77 (16)
O1ii—Cr1—O1iii	92.049 (10)	C3iii—C4—H4	119.6
O1i—Cr1—O1iii	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)
C2iii—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)
C2iii—N1—C2—C1	−177.04 (9)	C1—C2—C3—C4	176.10 (9)
Cr1—N1—C2—C1	2.96 (9)	C2—C3—C4—C3iii	0.44 (7)
O2—C1—C2—N1	173.96 (9)

D—H···A	D—H	H···A	D···A	D—H···A
C3—H3···O2iv	0.93	2.50	3.4071 (15)	167
N1S—H1S···O2v	0.85 (1)	2.04 (1)	2.8462 (11)	158 (2)