Diaqua­bis(quinoxaline-2-carboxyl­ato-[kappa]²N¹,O)copper(II)

In the title compound, [Cu(C₉H₅N₂O₂)₂(H₂O)₂], the Cu^II ion lies on an inversion centre and has an elongated centrosymmetric octa­hedral environment, equatorially trans-coordinated by two N,O-bidentate quinoxaline-2-carboxyl­ate ligands and axially coordinated by two water O atoms. Symmetry-related mol­ecules are linked by strong O-H...O hydrogen bonds, involving the uncoordinated carboxyl O atom of the carboxyl­ate group and the coordinated water mol­ecules, to form a two-dimensional network. Weak inter­molecular C-H...N inter­actions also stabilize the crystal structure.

S1. Comment

Copper complexes play an important role in catalysing enzymatic activity, and much interest has been shown in copper complexes of organic acids because of their special biocatalytic functions (Şengül & Büyükgüngör, 2005). It is well known that the incorporation of carboxylic acid groups into coordination compounds gives interesting supramolecular architectures (Puddephatt et al., 2002). Quinoxaline-2-carboxylic acid (Hqlc) is a potential chelate with interesting possibilities, exhibiting N,O-chelation (through the quinoxaline N atom and the carboxylate group, forming a five-membered chelate ring). Chen & Tong (2007) reported several important solvothermal (including hydrothermal) in situ metal/ligand reactions and their mechanisms, including oxidation of aldehydes into the corresponding carboxylates in the presence of Cu^II (Evans & Lin, 2002), and suggested that a new bridge had been created between coordination chemistry and synthetic organic chemistry. We found a similar phenomenon under mild conditions by conventional solution methods. In the reaction, Cu^II ions may act as an oxidative agent to oxidize quinoxaline-2-carbaldehyde into Hqlc in alkaline medium. To the best of our knowledge, no crystal structures of metal complexes with the Hqlc ligand have been reported to date, although there are several crystal structures of complexes with analogous ligands (Okabe et al., 2004; Stachová et al., 2006). As an extension of our works (Xie et al., 2007), we report here our efforts to establish the preferred coordination mode of Hqlc to copper(II) in the structure of the title compound, (I).

The X-ray analysis reveals that atom Cu1 (located on an inversion center) has a Jahn–Teller-elongated octahedral coordination geometry, completed by three pairs of symmetry-related atoms, equatorially by two carboxylate O atoms [Cu1—O2 = 1.9228 (12) Å] and two quinoxaline N atoms [Cu1—N2 = 2.1249 (11) Å], and axially by two symmetry-equivalent water O atoms [Cu1—O1 = 2.3408 (11) Å]. As shown in Fig. 1, the Cu1—O2 distance is slightly shorter than the Cu1—N2 distance. This effect is usually observed for the metal complexes of analogous compounds, such as (5-n-butylpyridine-2-carboxylato)copper(II) [Cu—O_eq = 1.952 (2) Å and Cu—N = 1.977 (2) Å; Okabe et al., 2002] and trans-bis(isoquinoline-3-carboxylato-κ²N,O)bis(methanol-κO)-copper(II) [Cu—O_eq = 1.963 (2) Å and Cu—N = 1.979 (2) Å; Okabe et al., 2004]. The coordination bond length in the axial direction is longer than the bond lengths in the equatorial plane. A similar behaviour is found for (5-n-butylpyridine-2-carboxylato)copper(II) [2.596 (3) and 1.952 (2) Å respectively; Okabe et al., 2002]. The coordination polyhedron displays a tetragonality factor T = 0.86, which indicates an additional elongation of the octahedron along the O1—Cu1—O1ⁱ axis. This distortion must be attributed to the Jahn–Teller effect and is consistent with a d_x²_-y² ground state of the Cu^II ion.

In the title compound, the carboxylic acid groups are deprotonated, which means that the ligand also serves as a counter-ion and the overall complex ion is neutral. In the molecule of (I), the angles around the Cu atom are slightly distorted from an ideal octahedral configuration [O2—Cu1—N2 = 81.54 (4)°]. Similar bond angles are observed in some related metal complexes, such as trans-bis-(isoquinoline-3-carboxylato-κ²N,O)bis(methanol-κO)copper(II) [O—Cu—N = 83.77 (7)°; Okabe et al., 2004] and trans-di-aquabis(3-hydroxypicolinato)zinc(II) [O—Zn—N = 79.3 (1)°; Di Marco et al., 2004]. The Cu^II ion chelates the Hqlc ligand via atoms N2 and O2 to form a five-membered ring. The N2/O2/Cu1/N2ⁱⁱⁱ/O2ⁱⁱⁱ unit [symmetry code: (iii) -x + 1, -y + 1, -z + 1] is strictly planar. Both Hqlc ligands are practically planar, the greatest deviations from the mean planes being 0.0678 (s.u.?) Å (for atom O2) for the exocyclic atoms and 0.1445 (s.u.?) Å (O3) for the endocyclic ones. The ligand plane forms an angle of 5.5° with the five-membered chelate ring plane, which results in a copper out-of-plane distance from the Hqlc ligand of 0.2087 Å.

In the crystal structure, there are three intermolecular hydrogen bonds, and no evidence was observed for the existence of intermolecular aromatic π–π interaction in the complex. The molecular units are linked by weak intermolecular C—H···N interactions (C3—H3A···N1) to form infinite zigzig chains running along the b axis. Adjacent chains are further connected into a two-dimensional network by strong intermolecular hydrogen bonds between the coordinated water molecule and the uncoordinated O atom (O3) of the carboxylate group (Table 2), as shown in Fig. 2.

Related literature

For related literature, see: Chen & Tong (2007); Di Marco, Tapparo, Dolmella & Bombi (2004); Evans & Lin (2002); Okabe et al. (2002, 2004); Puddephatt et al. (2002); Stachová et al. (2006); Xie et al. (2007); Şengül & Büyükgüngör (2005).

S2. Experimental

To a 15 ml me thanol solution of Cu(ClO₄)₂·6H₂O (1 mmol, 371 mg) and quinoxaline-2-carbaldehyde (2 mmol, 316 mg) was added a 5 ml me thanol solution of triethylenetetramine (1 mmol, 146.2 mg). The mixture was stirred for three hours with refluxing at 333 K. The precipitate was then filtered off, washed with methanol and dried in air. The deep-green filtrate was allowed to evaporate slowly, affording X-ray quality crystals of the title complex, (I). M.p. >573 K (decomposition). Analysis calculated for C₁₈H₁₄CuN₄O₆: C 48.49, H 3.16, N 12.57%; found: C 48.40, H 3.22, N 12.64%. IR (KBr pellet, cm^-1): 3590 (b), 3063 (s), 2942 (s), 2849 (s), 2010 (m), 1839 (b),1707(sh), 1611 (s, b), 1572 (s), 1530 (s), 1494 (s), 1465 (s), 1420 (s), 1368 (s), 1339 (s), 1214 (s), 1134 (b, s), 935 (s), 769 (s), 627 (s), 554 (m), 431 (m).

S3. Refinement

H atoms on O atoms were located in a difference Fourier map and refined freely; the other H atoms were positioned geometrically and refined as riding on their attached C atoms, with C—H bond distances of 0.95 Å and with U_iso(H) value s of 1.2U_eq(C).

Computing details

Data collection: RAPID-AUTO (Rigaku Corporation, 2004); cell refinement: RAPID-AUTO (Rigaku Corporation, 2004); 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: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. A view of the molecular structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level.
	Fig. 2. The packing of the title compound, viewed down the a axis. Hydrogen bonds are shown as dashed lines.

Diaquabis(quinoxaline-2-carboxylato-κ²N¹,O)copper(II)

Crystal data

[Cu(C9H5N2O2)2(H2O)2]	F(000) = 454
Mr = 445.87	Dx = 1.688 Mg m^−3
Monoclinic, P21/c	Melting point > 573 K
Hall symbol: -P 2ybc	Mo Kα radiation, λ = 0.71073 Å
a = 5.9278 (5) Å	Cell parameters from 7421 reflections
b = 22.256 (2) Å	θ = 3.1–27.5°
c = 7.2456 (6) Å	µ = 1.29 mm^−1
β = 113.393 (2)°	T = 153 K
V = 877.34 (12) Å^3	Platelet, green
Z = 2	0.40 × 0.38 × 0.35 mm

Data collection

Rigaku R-AXIS SPIDER diffractometer	2019 independent reflections
Radiation source: Rotating Anode	1889 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.047
ω scans	θmax = 27.5°, θmin = 3.2°
Absorption correction: empirical (using intensity measurements) (ABSCOR; Higashi, 1995)	h = −7→7
Tmin = 0.594, Tmax = 0.627	k = −28→28
8445 measured reflections	l = −9→9

Refinement

Refinement on F^2	H atoms treated by a mixture of independent and constrained refinement
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0543P)^2 + 0.1652P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.029	(Δ/σ)max < 0.001
wR(F^2) = 0.086	Δρmax = 0.46 e Å^−3
S = 1.08	Δρmin = −0.70 e Å^−3
2019 reflections	Extinction correction: SHELXL97
142 parameters	Extinction coefficient: 0.018 (3)
0 restraints

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
Cu1	0.5000	0.5000	0.5000	0.01337 (13)
C1	0.5008 (3)	0.33153 (6)	0.1582 (2)	0.0186 (3)
O1	0.4182 (2)	0.44592 (5)	0.74364 (16)	0.0210 (2)
N1	0.2657 (2)	0.33312 (5)	0.01881 (19)	0.0222 (3)
C2	0.6584 (3)	0.28434 (6)	0.1498 (2)	0.0245 (3)
H2	0.5990	0.2549	0.0467	0.029*
O2	0.1530 (2)	0.50851 (5)	0.34433 (18)	0.0171 (2)
N2	0.4439 (2)	0.42096 (5)	0.32275 (16)	0.0139 (2)
C3	0.8939 (3)	0.28119 (6)	0.2889 (2)	0.0253 (3)
H3	0.9977	0.2494	0.2826	0.030*
O3	−0.17128 (17)	0.46885 (5)	0.10101 (15)	0.0196 (2)
C4	0.9850 (3)	0.32475 (7)	0.4425 (3)	0.0221 (3)
H4	1.1498	0.3219	0.5386	0.027*
C5	0.8400 (3)	0.37113 (6)	0.4553 (2)	0.0188 (3)
H5	0.9040	0.4003	0.5590	0.023*
C6	0.5947 (3)	0.37531 (6)	0.3134 (2)	0.0154 (3)
C7	0.1260 (3)	0.37658 (6)	0.0368 (2)	0.0195 (3)
H7	−0.0406	0.3785	−0.0561	0.023*
C8	0.2150 (2)	0.42052 (6)	0.1896 (2)	0.0143 (3)
C9	0.0497 (2)	0.46961 (6)	0.2104 (2)	0.0147 (3)
H1A	0.555 (4)	0.4461 (9)	0.845 (3)	0.030 (5)*
H1B	0.329 (4)	0.4679 (10)	0.771 (3)	0.034 (6)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu1	0.01036 (18)	0.01502 (18)	0.01273 (18)	0.00147 (7)	0.00246 (12)	−0.00315 (7)
C1	0.0251 (7)	0.0147 (6)	0.0190 (7)	−0.0013 (5)	0.0119 (6)	−0.0005 (5)
O1	0.0170 (5)	0.0267 (6)	0.0180 (5)	0.0006 (4)	0.0054 (4)	−0.0011 (4)
N1	0.0263 (6)	0.0189 (6)	0.0218 (6)	−0.0043 (5)	0.0098 (5)	−0.0062 (5)
C2	0.0359 (8)	0.0160 (6)	0.0261 (8)	0.0016 (6)	0.0169 (7)	−0.0034 (5)
O2	0.0125 (5)	0.0202 (5)	0.0165 (6)	0.0023 (4)	0.0034 (5)	−0.0036 (4)
N2	0.0161 (5)	0.0136 (5)	0.0127 (5)	−0.0005 (4)	0.0065 (4)	−0.0004 (4)
C3	0.0349 (8)	0.0170 (7)	0.0294 (8)	0.0081 (6)	0.0187 (7)	0.0036 (6)
O3	0.0120 (5)	0.0281 (5)	0.0163 (5)	−0.0009 (4)	0.0030 (4)	0.0001 (4)
C4	0.0232 (7)	0.0211 (7)	0.0231 (7)	0.0065 (6)	0.0103 (5)	0.0062 (6)
C5	0.0209 (7)	0.0182 (6)	0.0178 (6)	0.0021 (5)	0.0083 (6)	0.0012 (5)
C6	0.0197 (7)	0.0137 (6)	0.0157 (6)	0.0006 (5)	0.0101 (6)	0.0014 (5)
C7	0.0188 (7)	0.0213 (7)	0.0170 (7)	−0.0041 (5)	0.0055 (6)	−0.0033 (5)
C8	0.0151 (6)	0.0147 (6)	0.0140 (6)	−0.0018 (5)	0.0066 (5)	0.0007 (5)
C9	0.0140 (6)	0.0187 (6)	0.0129 (6)	−0.0009 (5)	0.0070 (5)	0.0021 (5)

Geometric parameters (Å, º)

Cu1—O2^i	1.9228 (12)	O2—C9	1.2628 (17)
Cu1—O2	1.9228 (12)	N2—C8	1.3163 (18)
Cu1—N2^i	2.1249 (11)	N2—C6	1.3725 (17)
Cu1—N2	2.1249 (11)	C3—C4	1.411 (2)
Cu1—O1	2.3408 (11)	C3—H3	0.9500
Cu1—O1^i	2.3408 (11)	O3—C9	1.2328 (17)
C1—N1	1.359 (2)	C4—C5	1.370 (2)
C1—C2	1.423 (2)	C4—H4	0.9500
C1—C6	1.4235 (19)	C5—C6	1.412 (2)
O1—H1A	0.85 (2)	C5—H5	0.9500
O1—H1B	0.80 (2)	C7—C8	1.4132 (19)
N1—C7	1.3128 (19)	C7—H7	0.9500
C2—C3	1.361 (2)	C8—C9	1.5144 (18)
C2—H2	0.9500
O2^i—Cu1—O2	180.0	C9—O2—Cu1	117.56 (9)
O2^i—Cu1—N2^i	81.54 (4)	C8—N2—C6	117.41 (11)
O2—Cu1—N2^i	98.46 (4)	C8—N2—Cu1	108.50 (8)
O2^i—Cu1—N2	98.46 (4)	C6—N2—Cu1	133.95 (9)
O2—Cu1—N2	81.54 (4)	C2—C3—C4	120.50 (13)
N2^i—Cu1—N2	180.0	C2—C3—H3	119.8
O2^i—Cu1—O1	90.03 (5)	C4—C3—H3	119.8
O2—Cu1—O1	89.97 (5)	C5—C4—C3	121.13 (14)
N2^i—Cu1—O1	89.96 (4)	C5—C4—H4	119.4
N2—Cu1—O1	90.04 (4)	C3—C4—H4	119.4
O2^i—Cu1—O1^i	89.97 (5)	C4—C5—C6	119.52 (14)
O2—Cu1—O1^i	90.03 (5)	C4—C5—H5	120.2
N2^i—Cu1—O1^i	90.04 (4)	C6—C5—H5	120.2
N2—Cu1—O1^i	89.96 (4)	N2—C6—C5	121.02 (12)
O1—Cu1—O1^i	180.0	N2—C6—C1	119.18 (13)
N1—C1—C2	118.98 (13)	C5—C6—C1	119.80 (12)
N1—C1—C6	122.24 (12)	N1—C7—C8	122.27 (13)
C2—C1—C6	118.78 (14)	N1—C7—H7	118.9
Cu1—O1—H1A	103.8 (13)	C8—C7—H7	118.9
Cu1—O1—H1B	103.2 (15)	N2—C8—C7	122.33 (12)
H1A—O1—H1B	106 (2)	N2—C8—C9	116.19 (11)
C7—N1—C1	116.52 (12)	C7—C8—C9	121.48 (12)
C3—C2—C1	120.27 (13)	O3—C9—O2	124.79 (13)
C3—C2—H2	119.9	O3—C9—C8	119.38 (12)
C1—C2—H2	119.9	O2—C9—C8	115.82 (12)
C2—C1—N1—C7	178.10 (13)	C8—N2—C6—C1	2.27 (17)
C6—C1—N1—C7	−1.5 (2)	Cu1—N2—C6—C1	−172.99 (9)
N1—C1—C2—C3	−179.21 (13)	C4—C5—C6—N2	179.57 (12)
C6—C1—C2—C3	0.4 (2)	C4—C5—C6—C1	−0.2 (2)
N2^i—Cu1—O2—C9	177.12 (10)	N1—C1—C6—N2	−0.34 (19)
N2—Cu1—O2—C9	−2.88 (10)	C2—C1—C6—N2	−179.97 (12)
O1—Cu1—O2—C9	−92.92 (11)	N1—C1—C6—C5	179.45 (13)
O1^i—Cu1—O2—C9	87.08 (11)	C2—C1—C6—C5	−0.18 (19)
O2^i—Cu1—N2—C8	−174.72 (9)	C1—N1—C7—C8	1.5 (2)
O2—Cu1—N2—C8	5.28 (9)	C6—N2—C8—C7	−2.40 (18)
O1—Cu1—N2—C8	95.24 (9)	Cu1—N2—C8—C7	174.00 (10)
O1^i—Cu1—N2—C8	−84.76 (9)	C6—N2—C8—C9	176.96 (10)
O2^i—Cu1—N2—C6	0.84 (12)	Cu1—N2—C8—C9	−6.64 (13)
O2—Cu1—N2—C6	−179.16 (12)	N1—C7—C8—N2	0.5 (2)
O1—Cu1—N2—C6	−89.19 (12)	N1—C7—C8—C9	−178.81 (13)
O1^i—Cu1—N2—C6	90.81 (12)	Cu1—O2—C9—O3	179.05 (10)
C1—C2—C3—C4	−0.3 (2)	Cu1—O2—C9—C8	0.10 (15)
C2—C3—C4—C5	−0.1 (2)	N2—C8—C9—O3	−174.09 (11)
C3—C4—C5—C6	0.4 (2)	C7—C8—C9—O3	5.28 (19)
C8—N2—C6—C5	−177.51 (12)	N2—C8—C9—O2	4.92 (17)
Cu1—N2—C6—C5	7.23 (19)	C7—C8—C9—O2	−175.71 (13)

Symmetry code: (i) −x+1, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···O3^ii	0.85 (2)	1.98 (2)	2.8076 (14)	162.5 (19)
O1—H1B···O3^iii	0.80 (2)	2.10 (2)	2.8839 (15)	166 (2)

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