A mixed-valence copper coordination polymer generated by a hydro­thermal metal/ligand redox reaction process

The title coordination polymer, poly[(μ₄-2-oxidoisophthalato-κ⁶O¹,O²:O²,O³:O^3′:O^3′)(μ₂-quinoxaline-κ²N:N′)copper(I)copper(II)], [Cu₂(C₈H₃O₅)(C₈H₆N₂)]_n, contains two crystallographically distinct Cu ions, one quinoxaline (QA) unit and one 2-oxidoisophthalate trianion (L) derived from 2-hydroxy­isophthalic acid (H₃L). The Cu^II ion is strongly coordinated by four O atoms in a distorted square geometry, of which two belong to two phenoxide groups and the other two to carboxyl­ate groups of two L ligands. In addition, the Cu^II cation inter­acts weakly with a symmetry-related carboxylate O atom which belongs to the L ligand in an adjacent layer, giving a square-pyramidal coordination geometry. The Cu^I ion is trigonally coordinated by two N atoms from two QA mol­ecules and one O atom from an L carboxyl­ate group. The Cu^I centres are bridged by QA ligands to give a chain along the c axis. Two Cu^II ions and two L ligands form a [Cu₂L₂]²⁻ `metallo-ligand', which coordinates two Cu^I ions. Thus, the chains of Cu^I and QA are linked by the [Cu₂L₂]²⁻ metallo-ligand to yield a two-dimensional (6,3) sheet. These sheets are further linked by symmetry-related carboxylate O atoms of neighbouring layers into a three-dimensional framework. The in situ reaction from benzene-1,2,3-tricarboxylic acid (H₃L1) to L in the present system has rarely been observed before, although a few novel in situ reactions, such as ligand oxidative coupling, hydrolysis and substitution, have been observed during the hydro­thermal process.

S1. Comment

Extended frameworks of coordination polymers, based on complexes of transition metals and multifunctional bridging ligands, are of great research interest (Eddaoudi et al., 2001; Hagrman et al., 1999; Noveron et al., 2002; Batten & Robson, 1998). The hydro(solvo)thermal method is a useful technique for the construction of highly stable robust metal–organic frameworks (Chen & Liu, 2002; Yang et al., 2007; Tong et al., 2000). It has been found that in situ reactions, such as ligand oxidative coupling, hydrolysis, substitution and the redox process of copper, can occur under hydro(solvo)thermal conditions (Maji et al., 2004). These reactions represent promising new routes for constructing novel coordination polymers.

The chemistry of mixed-valence Cu^I,II complexes is of great importance. We are interested in developing mixed-valence Cu^I,II complexes under hydrothermal conditions because of their superior electronic, optical and magnetic properties (Maji et al., 2004). In consideration of the fact that several Cu^I,II complexes contain N-heterocycle and carboxylate ligands, we postu that benzene-1,2,3-tricarboxylic acid (H₃L1) and quinoxaline (QA) are possibly good ligands for the construction of mixed-valence Cu^I,II complexes (Zhang & Fang, 2005). In this work, the hydrothermal reaction of H₃L1, QA and divalent copper(II) salts resulted in a new mixed-valence Cu^I,II coordination polymer, the title compound, [Cu^ICu^II(L)(QA)]_n, (I), where L1 was transformed into 2-hydroxyisophthalate (L).

Compound (I) was obtained under hydrothermal conditions at 453 K. Once formed, the compound is insoluble in most solvents, including water. As shown in Fig. 1, the asymmetric unit of (I) contains two crystallographically unique Cu atoms, one unique QA and one unique L. Atom Cu1 is primarily coordinated to four O atoms [O2, O3, O3ⁱ and O5ⁱ; symmetry code: (i) 2 - x, -y, 2 - z] in a distorted square geometry, of which two belong to two phenoxo groups and the other two to carboxylate groups of two L ligands. In addition, atom Cu1 interacts weakly with atom O1ⁱⁱⁱ [2.6852 (16) Å; symmetry code: (iii) 1 + x, y, z], which belongs to an L ligand in an adjacent layer. Therefore, Cu1 has square-pyramidal coordination geometry. Atom Cu2 is trigonally coordinated by two N atoms [N1 and N2ⁱⁱ; symmetry code: (ii) -1/2 + x, 1/2 - y, -1/2 + z] from two QA molecules and one O atom (O1) from an L carboxylate group.

Cu^II ions with d⁹ configurations tend to have a square-pyramidal or axially elongated octahedral coordination geometry, while Cu^I ions with d¹⁰ configurations often adopt a trigonal or tetrahedral coordination geometry (Zhang & Fang, 2005). The coordination geometry of the copper centres, in combination with the charge balance, indicates that in compound (I) atom Cu1 is di-positive and atom Cu2 is uni-positive. The Cu^I ions are bridged by QA ligands to give a chain along the c axis. Two Cu^II ions and two L ligands form a [Cu₂(L)₂]^2- `metallo-ligand', which coordinates two Cu^I ions. Thus, the chains of Cu^I and QA units are linked by the [Cu₂(L)₂]^2- metallo-ligand to yield a (6,3) sheet in the (101) plane (Fig. 2). These sheets are further linked by the Cu1—O1 interaction into a three-dimensional framework.

It is worth noting that a new in situ reaction occurs in the CuCl₂.2H₂O/H₃L1/QA system under hydrothermal conditions. The ligand L1 was transformed into L via decarboxylation and hydroxylation steps. The in situ reaction from L1 to L in the present system has rarely been observed before, although a similar reaction process whereby mixed-valence Cu^I,II species and in situ synthesis of L are simultaneously generated under the hydrothermal reaction of isophthalate and 4,4'-bipyridine with Cu(NO₃).3H₂O has been reported (Tao et al., 2002). So far, a few novel in situ reactions such as ligand oxidative coupling, hydrolysis and substitution have been observed during the hydrothermal process (Tao et al., 2002), in which many factors, including the nature of the metal ion and the temperature, pressure and pH, have been found to influence the reaction outcome significantly. As far as the present system is concerned, the pH value may play a key role in the transformation of L1.

Related literature

For related literature, see: Batten & Robson (1998); Chen & Liu (2002); Eddaoudi et al. (2001); Hagrman et al. (1999); Maji et al. (2004); Noveron et al. (2002); Tao et al. (2002); Tong et al. (2000); Yang et al. (2007); Zhang & Fang (2005).

S2. Experimental

A mixture of CuCl₂.2H₂O (0.085 g, 0.5 mmol), H₃L1 (0.070 g, 0.33 mmol) and QA (0.065 g, 0.5 mmol) was dissolved in 10 ml distilled water; this was followed by the addition of triethylamine until the pH was in the range 5.5–6.3. The resulting solution was stirred for about 1 h at room temperature, sealed in a 23 ml Teflon-lined stainless steel autoclave and heated at 453 K for 5 d under autogenous pressure. Afterwards, the reaction system was slowly cooled to room temperature. Dark-red block crystals of (I) suitable for single-crystal X-ray diffraction analysis were collected from the final reaction system by filtration, washed several times with distilled water and dried in air at ambient temperature (yield: 29% based on Cu).

S3. Refinement

The carbon-bound H atoms were generated geometrically (C—H = 0.93 Å) and refined as riding, with U_iso(H) = 1.2U_eq(C). No H atoms could be found in the vicinity of the carboxyl or hydroxyl O atoms, consistent with the relevant C—O distances. The small voids within the structure were found to contain no significant electron density.

Computing details

Data collection: SMART (Bruker, 1997); cell refinement: SAINT (Bruker, 1999); data reduction: SAINT (Bruker, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL-Plus (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

Fig. 1. A view of the local coordination of the Cu^I,II atoms in (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radii. [Symmetry codes: (i) 2 - x, -y, 2 - z; (ii) -1/2 + x, 1/2 - y, -1/2 + z; (iii) 1 + x, y, z.]

Fig. 2. A view of the layer structure of (I) in the (101) plane. For the sake of clarity, all H atoms have been omitted. [Symmetry codes: (i) 2 - x, -y, 2 - z; (ii) 5/2 - x, y - 1/2, 5/2 - z; (iii) 3 - x, -y, 3 - z; (iv) 1 + x, y, 1 + z; (v) 1/2 + x, 1/2 - y, 1/2 + z; (vi) 5/2 - x, 1/2 + y, 5/2 - z; (vii) 2 - x, 1 - y, 2 - z; (viii) 3/2 - x, 1/2 + y, 3/2 - z; (ix) 1 - x, 1 - y, 1 - z; (x) x - 1, 1 + y, z - 1; (xi) x - 1/2, 3/2 - y, z - 1/2; (xii) x, 1 + y, z; (xiii) x - 1/2, 1/2 - y, z - 1/2.]

(I)

Crystal data

[Cu2(C8H3O5)(C8H6N2)]	F(000) = 868
Mr = 436.33	Dx = 1.974 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 3402 reflections
a = 5.2011 (2) Å	θ = 1.1–28.3°
b = 23.5553 (11) Å	µ = 2.93 mm^−1
c = 12.2402 (6) Å	T = 293 K
β = 101.719 (1)°	Block, dark red
V = 1468.33 (11) Å^3	0.31 × 0.27 × 0.19 mm
Z = 4

Data collection

Bruker APEX CCD area-detector diffractometer	3402 independent reflections
Radiation source: fine-focus sealed tube	2852 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.029
ω scans	θmax = 28.3°, θmin = 1.7°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −6→6
Tmin = 0.391, Tmax = 0.575	k = −31→30
8982 measured reflections	l = −16→13

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.028	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.071	H-atom parameters constrained
S = 0.96	w = 1/[σ^2(Fo^2) + (0.0427P)^2] where P = (Fo^2 + 2Fc^2)/3
3402 reflections	(Δ/σ)max = 0.001
226 parameters	Δρmax = 0.48 e Å^−3
0 restraints	Δρmin = −0.29 e Å^−3

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
C1	0.3875 (4)	0.07382 (8)	0.87479 (16)	0.0236 (4)
C2	0.3891 (4)	0.01664 (8)	0.81996 (17)	0.0246 (4)
C3	0.5985 (4)	−0.02297 (8)	0.84865 (16)	0.0239 (4)
C4	0.5854 (4)	−0.07520 (8)	0.78954 (17)	0.0258 (4)
C5	0.3694 (4)	−0.08508 (9)	0.70422 (19)	0.0340 (5)
H5	0.3614	−0.1189	0.6645	0.041*
C7	0.1646 (5)	−0.04639 (9)	0.6757 (2)	0.0372 (5)
H7	0.0230	−0.0542	0.6179	0.045*
C8	0.1755 (4)	0.00368 (9)	0.73481 (19)	0.0306 (5)
H8	0.0374	0.0294	0.7175	0.037*
C9	0.5675 (5)	0.20908 (9)	1.08446 (18)	0.0328 (5)
H9	0.6585	0.1786	1.0625	0.039*
C10	0.6686 (4)	0.23533 (9)	1.18769 (18)	0.0320 (5)
H10	0.8240	0.2216	1.2309	0.038*
C11	0.3229 (4)	0.29778 (9)	1.15748 (17)	0.0275 (4)
C12	0.1873 (5)	0.34460 (9)	1.19042 (19)	0.0358 (5)
H12	0.2490	0.3619	1.2590	0.043*
C13	−0.0349 (5)	0.36457 (10)	1.1214 (2)	0.0412 (6)
H13	−0.1223	0.3957	1.1433	0.049*
C14	−0.1323 (5)	0.33878 (10)	1.01838 (19)	0.0383 (5)
H14	−0.2829	0.3530	0.9723	0.046*
C15	−0.0086 (5)	0.29320 (9)	0.98517 (18)	0.0329 (5)
H15	−0.0764	0.2760	0.9170	0.039*
C16	0.2213 (4)	0.27178 (8)	1.05309 (17)	0.0271 (4)
C6	0.7921 (4)	−0.12175 (8)	0.81325 (18)	0.0283 (4)
N1	0.3490 (4)	0.22601 (7)	1.01771 (14)	0.0287 (4)
N2	0.5513 (4)	0.27873 (7)	1.22558 (14)	0.0292 (4)
O1	0.1729 (3)	0.10073 (6)	0.85540 (13)	0.0331 (4)
O2	0.5921 (3)	0.09585 (6)	0.93331 (13)	0.0307 (3)
O3	0.8022 (3)	−0.01133 (6)	0.93043 (12)	0.0282 (3)
O4	0.7721 (3)	−0.16162 (6)	0.74691 (13)	0.0392 (4)
O5	0.9752 (3)	−0.12024 (6)	0.90078 (13)	0.0344 (4)
Cu1	0.90826 (5)	0.059211 (10)	1.00466 (2)	0.02654 (9)
Cu2	0.20383 (5)	0.187416 (11)	0.87622 (2)	0.03198 (9)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0264 (10)	0.0225 (9)	0.0219 (10)	0.0013 (8)	0.0046 (8)	0.0030 (8)
C2	0.0271 (10)	0.0211 (9)	0.0256 (10)	0.0007 (8)	0.0053 (8)	0.0007 (8)
C3	0.0256 (10)	0.0233 (10)	0.0224 (10)	−0.0019 (8)	0.0037 (8)	0.0011 (8)
C4	0.0273 (11)	0.0240 (10)	0.0260 (10)	−0.0008 (8)	0.0050 (8)	−0.0025 (8)
C5	0.0360 (13)	0.0284 (11)	0.0345 (12)	−0.0011 (9)	−0.0005 (10)	−0.0084 (9)
C7	0.0319 (12)	0.0364 (12)	0.0376 (13)	0.0014 (10)	−0.0064 (10)	−0.0088 (10)
C8	0.0281 (11)	0.0286 (11)	0.0329 (12)	0.0023 (9)	0.0011 (9)	−0.0003 (9)
C9	0.0385 (13)	0.0255 (11)	0.0322 (12)	0.0050 (9)	0.0020 (10)	−0.0030 (9)
C10	0.0331 (12)	0.0280 (11)	0.0302 (11)	0.0030 (9)	−0.0051 (9)	0.0016 (8)
C11	0.0327 (12)	0.0252 (10)	0.0237 (10)	−0.0006 (8)	0.0031 (9)	0.0000 (8)
C12	0.0406 (13)	0.0343 (12)	0.0304 (12)	0.0039 (10)	0.0021 (10)	−0.0094 (9)
C13	0.0449 (15)	0.0358 (13)	0.0419 (14)	0.0120 (11)	0.0061 (11)	−0.0070 (10)
C14	0.0365 (13)	0.0411 (13)	0.0340 (13)	0.0119 (10)	−0.0009 (10)	0.0032 (10)
C15	0.0377 (12)	0.0319 (12)	0.0258 (11)	0.0024 (10)	−0.0010 (9)	−0.0005 (9)
C16	0.0331 (12)	0.0234 (10)	0.0241 (10)	0.0010 (8)	0.0038 (9)	0.0007 (8)
C6	0.0302 (11)	0.0228 (10)	0.0311 (11)	−0.0009 (8)	0.0044 (9)	−0.0015 (8)
N1	0.0362 (10)	0.0230 (9)	0.0243 (9)	0.0014 (7)	0.0001 (8)	−0.0020 (7)
N2	0.0352 (10)	0.0249 (9)	0.0247 (9)	−0.0026 (8)	−0.0008 (8)	−0.0010 (7)
O1	0.0305 (8)	0.0248 (8)	0.0404 (9)	0.0066 (6)	−0.0015 (7)	−0.0036 (6)
O2	0.0285 (8)	0.0225 (7)	0.0373 (9)	0.0036 (6)	−0.0027 (7)	−0.0049 (6)
O3	0.0301 (8)	0.0211 (7)	0.0292 (8)	0.0041 (6)	−0.0040 (6)	−0.0055 (6)
O4	0.0450 (10)	0.0301 (8)	0.0377 (9)	0.0054 (7)	−0.0028 (7)	−0.0141 (7)
O5	0.0363 (9)	0.0240 (7)	0.0375 (9)	0.0049 (6)	−0.0054 (7)	−0.0086 (6)
Cu1	0.02696 (15)	0.02070 (14)	0.02882 (15)	0.00291 (10)	−0.00173 (11)	−0.00478 (10)
Cu2	0.03764 (17)	0.02884 (16)	0.02625 (16)	0.00393 (11)	−0.00110 (12)	−0.00446 (10)

Geometric parameters (Å, º)

C1—O1	1.264 (2)	C11—C16	1.419 (3)
C1—O2	1.267 (2)	C12—C13	1.369 (3)
C1—C2	1.506 (3)	C12—H12	0.9300
C2—C8	1.394 (3)	C13—C14	1.399 (3)
C2—C3	1.423 (3)	C13—H13	0.9300
C3—O3	1.330 (2)	C14—C15	1.356 (3)
C3—C4	1.422 (3)	C14—H14	0.9300
C4—C5	1.389 (3)	C15—C16	1.404 (3)
C4—C6	1.522 (3)	C15—H15	0.9300
C5—C7	1.392 (3)	C16—N1	1.381 (3)
C5—H5	0.9300	C6—O4	1.232 (2)
C7—C8	1.379 (3)	C6—O5	1.281 (3)
C7—H7	0.9300	Cu1—O1^i	2.6852 (16)
C8—H8	0.9300	Cu1—O2	1.9047 (14)
C9—N1	1.320 (3)	Cu1—O3^ii	1.9198 (14)
C9—C10	1.409 (3)	Cu1—O3	1.9201 (13)
C9—H9	0.9300	Cu1—O5^ii	1.8678 (14)
C10—N2	1.322 (3)	Cu2—N2^iii	2.0176 (17)
C10—H10	0.9300	Cu2—N1	1.9659 (17)
C11—N2	1.380 (3)	Cu2—O1	2.0599 (14)
C11—C12	1.411 (3)
O1—C1—O2	120.62 (18)	C14—C13—H13	119.6
O1—C1—C2	116.71 (18)	C15—C14—C13	120.5 (2)
O2—C1—C2	122.59 (18)	C15—C14—H14	119.7
C8—C2—C3	119.64 (18)	C13—C14—H14	119.7
C8—C2—C1	116.99 (18)	C14—C15—C16	120.4 (2)
C3—C2—C1	123.34 (18)	C14—C15—H15	119.8
O3—C3—C4	120.66 (18)	C16—C15—H15	119.8
O3—C3—C2	120.22 (17)	N1—C16—C15	120.04 (19)
C4—C3—C2	119.12 (18)	N1—C16—C11	120.29 (19)
C5—C4—C3	118.38 (19)	C15—C16—C11	119.66 (19)
C5—C4—C6	117.06 (18)	O4—C6—O5	121.41 (19)
C3—C4—C6	124.56 (19)	O4—C6—C4	117.5 (2)
C4—C5—C7	122.7 (2)	O5—C6—C4	121.07 (18)
C4—C5—H5	118.7	C9—N1—C16	116.76 (18)
C7—C5—H5	118.7	C9—N1—Cu2	122.28 (14)
C8—C7—C5	118.7 (2)	C16—N1—Cu2	120.94 (15)
C8—C7—H7	120.6	C10—N2—C11	116.20 (18)
C5—C7—H7	120.6	C10—N2—Cu2^iv	120.17 (15)
C7—C8—C2	121.4 (2)	C11—N2—Cu2^iv	123.63 (14)
C7—C8—H8	119.3	C1—O1—Cu2	115.47 (13)
C2—C8—H8	119.3	C1—O2—Cu1	128.42 (13)
N1—C9—C10	122.7 (2)	C3—O3—Cu1^ii	128.93 (12)
N1—C9—H9	118.6	C3—O3—Cu1	130.02 (12)
C10—C9—H9	118.6	Cu1^ii—O3—Cu1	100.76 (6)
N2—C10—C9	122.6 (2)	C6—O5—Cu1^ii	129.19 (13)
N2—C10—H10	118.7	O5^ii—Cu1—O2	94.39 (6)
C9—C10—H10	118.7	O5^ii—Cu1—O3^ii	93.86 (6)
N2—C11—C12	120.01 (19)	O2—Cu1—O3^ii	170.96 (6)
N2—C11—C16	121.40 (19)	O5^ii—Cu1—O3	170.10 (7)
C12—C11—C16	118.6 (2)	O2—Cu1—O3	92.11 (6)
C13—C12—C11	120.0 (2)	O3^ii—Cu1—O3	79.24 (6)
C13—C12—H12	120.0	N1—Cu2—N2^iii	129.17 (7)
C11—C12—H12	120.0	N1—Cu2—O1	125.01 (7)
C12—C13—C14	120.8 (2)	N2^iii—Cu2—O1	105.81 (7)
C12—C13—H13	119.6

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