A copper–polyol complex: [Na₂(C₂H₆O₂)₆][Cu(C₂H₄O₂)₂]

The ionic title complex, bis(μ-ethyl­ene glycol)-κ³O,O′:O′;κ³O:O,O′-bis[(ethyl­ene glycol-κ²O,O′)(ethyl­ene glycol-κO)sodium] bis(ethyl­ene glycolato-κ²O,O′)copper(II), [Na₂(C₂H₆O₂)₆][Cu(C₂H₄O₂)₂], was obtained from a basic solution of CuCl₂ in ethyl­ene glycol and consists of discrete ions inter­connected by O—H...O hydrogen bonds. This is the first example of a disodium–ethyl­ene glycol complex cation cluster. The cation lies about an inversion center and the Cu^II atom of the anion lies on another independent inversion center.

S1. Comment

There has been considerable interest in the coordination chemistry of metal ions in polyol media as this has relevance to our understanding of the reaction kinetics and solvent and ion exchange during reduction reactions. Several papers have investigated the mechanism of the polyol process by the study of kinetic data obtained by UV–visible and IR spectroscopy (Bonet et al., 2000; Fievet et al., 1988; Pasquarello et al., 2001; Salmon et al., 1992). From these data, metal glycolates have been postulated as possible intermediates. However, these studies do not provide definitive structural details for these species. We report here the structure of a Cu^II glycolate, [Na₂(C₂H₆O₂)₆][Cu(C₂H₄O₂)₂], (I), which was obtained from a basic solution of CuCl₂ in ethylene glycol. A typical poylol reduction protocol was employed in which a basic ethylene glycol solution is used to reduce a metal such as Cu^II or Ni^II to the elemental form. The structure of the resulting complex consists of a di-chelate bis-ethylene glycolate dianion of Cu^II stabilized by ethylene glycol solvated Na ions. A survey of the literature reveals that only four examples of complexes containing the bis-ethylene glycolate Cu^II2- dianion have been crystallographically characterized (Habermann et al., 1992; Love et al., 1992; Pico et al., 1997). These complexes contain Li⁺, Ba²⁺ or Sr²⁺ as counterions. To the best of our knowledge, this is the first report of a crystal structure of an ethylene glycolate copper(II) dianion stabilized by ethylene glycol coordinated sodium counterions.

The molecular structure of complex (I) is illustrated in Fig. 1. Selected bond lengths are listed in Table 1. The anionic moiety consists of a bis-ethylene glycolate Cu^II2- dianion with the Cu atom on an inversion center, while the cation comprises two inversion-related Na ions complexed by chelating, bridging and monodentate ethylene glycol units.

The anion has square-planar coordination with fully deprotonated ethylene glycolate units which chelate the Cu²⁺ ion. The Cu1—O1 [1.9317 (13) Å] and Cu1—O2 [1.9252 (13) Å] distances fall between the previously reported Cu—O distances for Cu[C₂H₄O₂]^2- (Habermann et al., 1992; Love et al., 1992; Pico et al., 1997). A comparison of the Cu—O distances and angles from the previously reported structures of both ethylene glycolate and ethylene glycol copper complexes is given in Table 3. The Cu—O distances are consistently longer in the ethylene glycol complexes versus the glycolate complexes. More significantly, the bite angle of the glycolate ligand is much larger than the protonated form by an average of 8°.

In the [Na(C₂H₆O₂)₃]₂²⁺ cation the unique Na ion is bound by two chelating ethylene glycols, one bridging oxygen from an ethylene glycol that is chelating to the other inversion-related Na atom, and one monodentate ethylene glycol. The unique Na ion has a distorted octahedral coordination environment with Na—O distances in the range 2.3816 (16)–2.4545 (15) Å with the shortest distances coming from the non-bridging chelating O atoms Na1—O6, and the longest distances arising from the bridging interaction of a chelating ethylene glycol Na1—O3 (-x,-y,-z). A search of the Cambridge Structural Database (CSD, Version 5.30 plus three updates; Allen, 2002) reveals only one previously reported structure with ethylene glycol bound to Na ions (CSD ref. code: OBOBAI) (Schubert et al., 2000). In this structure the Na ions are directly attached to the anionic portion of the structure through bridging ethylene glycol units and are therefore not part of an independent sodium–ethylene glycol cationic cluster as reported here.

The cations are linked by O—H···O hydrogen bonds through interactions involving O5—H5···O4^iv and O8—H8···O7^v (see Table 3) to form sheets parallel to the (001) plane. The deprotonated O atoms, O1 and O2, of the anion are each hydrogen bonded to two different ethylene glycol O atoms of the cation as shown in Fig. 1 with details in Table 2. The association of the two cations and anions via classical O—H···O hydrogen bonding (Table 2) results in a three-dimensional network with alternating layers of cations and anions down the c axis (Fig. 2).

The IR spectrum of (I) revealed weak C—O stretches at 1245 and 1218 cm^-1 and weak M—O stretches at 541 and 606 cm^-1. These values are consistent with data obtained for the Cu—Sr structure, [Sr(C₂H₆O₂)₅][Cu(C₂H₄O₂)₂] (Pico et al., 1997).

The synthesis of this complex provides insight into complexes formed during the polyol process before metal reduction takes place. This study provides structural evidence of previously proposed metal glycolate formation during the polyol process.

Related literature

For related literature, see: Allen (2002); Bonet et al. (2000); Fievet et al. (1988); Habermann et al. (1992); Love et al. (1992); Pasquarello et al. (2001); Pico et al. (1997); Salmon & Lond (1992); Schubert et al. (2000).

S2. Experimental

All reagents were used as received without further purification. For the preparation of the title compound, (I), CuCl₂(H₂O)₂ (1 g, 5.87 mmol) and 4 equivalents of powdered NaOH (0.94 g, 23.46 mmol) were dissolved in ethylene glycol (25 ml). The solution was heated under reflux under dry dinitrogen and stirred magnetically (40 min); during this time the solution turned dark blue. The solution was then allowed to cool to room temperature. Crystals suitable for X-ray analysis were obtained from the solution by cooling to 248 K for 3 d in a sealed flask under nitrogen. The ethylene glycol was decanted off and the crystals were washed with diethyl ether (2 x 50 ml) and hexane (2 x 50 ml). Unoptimized yield: 2.4 g, 68% (based on Cu). M.p. 113–115 (dec.) FT–IR (cm^-1, ATR) v(CO) 1245 (w), 1218 (w); v(MO) 541 (w), 529 (w).

The crystals are deliquescent and readily form droplets within an hour of sitting in the atmosphere. The crystals are insoluble in common organic solvents, but dissolve readily with decomposition in alcohols and water. Attempts to isolate a nickel intermediate from an analogous reaction using NiCl₂(H₂O)₆ were unsuccessful.

S3. Refinement

The pendant oxygen from the non-chelating ethylene glycol bound to sodium was disordered over two positions, O8 and O8B, whose occupancy factors were refined and then fixed at 0.911 and 0.089, respectively. The C8—O8 and C8—O8B were restrained to be equivalent. All methylene H-atom positions were calculated using the appropriate riding model, with U_iso(H) = 1.2U_eq (parent atom) and with C—H distances of 0.99 Å. The hydroxy H atoms were located in a difference Fourier map and refined freely.

Computing details

Data collection: CrystalClear (Rigaku, 2008); cell refinement: CrystalClear (Rigaku, 2008); data reduction: CrystalClear (Rigaku, 2008); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: CIFTAB (Sheldrick, 2008).

Figures

	Fig. 1. The molecular structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 40% probability level. The disordered atom O8B and methylene H atoms have been omitted for clarity. [Symmetry codes: (i) -x,-y,-z; (ii) -x,-y, 1 - z.]
	Fig. 2. A packing diagram of (I) viewed approximately down the a axis showing the hydrogen-bonding interactions between the anionic and cationic units. H atoms not involved in hydrogen bonding and the disordered atom O8B have been omitted for clarity. [Symmetry codes: (ii) -x,-y,-z; (iii) -x,-y, 1 - z; (v) 1 - x, 1 - y,-z; (vi) 1-x, 1-y, 1-z; (vii) x, y, 1+z.]

bis(µ-ethylene glycol)-κ³O,O':O';κ³O:O,O'- bis[(ethylene glycol-κ²O,O')(ethylene glycol-κO)sodium] bis(ethylene glycolato-κO,O')copper(II)

Crystal data

[Na2(C2H6O2)6][Cu(C2H4O2)2]	F(000) = 319
Mr = 602.03	Dx = 1.425 Mg m^−3
Triclinic, P1	Melting point: 386 K
a = 8.7666 (12) Å	Mo Kα radiation, λ = 0.71069 Å
b = 9.5089 (13) Å	Cell parameters from 5829 reflections
c = 9.7804 (13) Å	θ = 6.8–54.8°
α = 99.152 (4)°	µ = 0.88 mm^−1
β = 101.983 (4)°	T = 100 K
γ = 113.910 (4)°	Prism, blue
V = 701.61 (16) Å^3	0.09 × 0.09 × 0.08 mm
Z = 1

Data collection

Rigaku AFC-12 with Saturn 724+CCD diffractometer	3177 independent reflections
Radiation source: fine-focus sealed tube	2863 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.029
Detector resolution: 28.5714 pixels mm^-1	θmax = 27.4°, θmin = 3.4°
ω–scan	h = −11→10
Absorption correction: multi-scan (ABSCOR; Higashi, 1995)	k = −12→12
Tmin = 0.573, Tmax = 0.9	l = −12→12
7103 measured reflections

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)] = 0.034	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.080	H atoms treated by a mixture of independent and constrained refinement
S = 1.07	w = 1/[σ^2(Fo^2) + (0.023P)^2 + 0.6667P] where P = (Fo^2 + 2Fc^2)/3
3177 reflections	(Δ/σ)max < 0.001
190 parameters	Δρmax = 0.36 e Å^−3
1 restraint	Δρmin = −0.40 e Å^−3

Special details

Experimental. Instruments: Melting points were determined on an Electrothermal Melting Point apparatus in sealed capillaries under nitrogen (1 atm.) and are uncorrected. The IR data was collected on a Nicolet Avater 330 F T—IR with Smart Performer ATR attachment.

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.

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	Occ. (<1)
Cu1	0.0000	0.0000	0.5000	0.01124 (10)
Na1	0.23160 (9)	0.10932 (9)	0.03314 (8)	0.01381 (16)
O1	−0.02964 (17)	−0.16279 (15)	0.60199 (14)	0.0148 (3)
O2	−0.02654 (18)	−0.15463 (16)	0.33218 (14)	0.0156 (3)
O3	0.03541 (17)	−0.12615 (16)	0.08085 (15)	0.0143 (3)
H3	0.027 (4)	−0.127 (3)	0.153 (3)	0.038 (9)*
O4	0.18312 (18)	−0.12527 (16)	−0.14889 (15)	0.0141 (3)
H4	0.135 (4)	−0.130 (3)	−0.223 (3)	0.029 (7)*
O5	0.50462 (19)	0.12573 (18)	0.16616 (16)	0.0198 (3)
H5	0.579 (4)	0.115 (3)	0.146 (3)	0.033 (8)*
O6	0.29490 (19)	0.23625 (18)	0.28251 (15)	0.0186 (3)
H6	0.219 (4)	0.210 (3)	0.313 (3)	0.031 (8)*
O7	0.32505 (19)	0.26319 (17)	−0.13052 (16)	0.0184 (3)
H7	0.239 (4)	0.233 (3)	−0.199 (3)	0.039 (8)*
O8	0.6992 (3)	0.4700 (2)	−0.0151 (3)	0.0431 (6)	0.911 (4)
H8	0.678 (4)	0.526 (4)	0.019 (4)	0.040 (10)*	0.911 (4)
O8B	0.531 (3)	0.450 (3)	−0.296 (2)	0.044 (5)*	0.089 (4)
C1	−0.0800 (3)	−0.3121 (2)	0.5009 (2)	0.0179 (4)
H1A	−0.2091	−0.3711	0.4615	0.021*
H1B	−0.0381	−0.3784	0.5507	0.021*
C2	−0.0010 (3)	−0.2803 (2)	0.3782 (2)	0.0179 (4)
H2A	0.1257	−0.2494	0.4120	0.022*
H2B	−0.0578	−0.3782	0.2961	0.022*
C3	0.1027 (2)	−0.2374 (2)	0.0444 (2)	0.0142 (4)
H3A	0.0375	−0.3378	0.0686	0.017*
H3B	0.2277	−0.1916	0.1007	0.017*
C4	0.0827 (2)	−0.2712 (2)	−0.1154 (2)	0.0155 (4)
H4A	0.1241	−0.3509	−0.1432	0.019*
H4B	−0.0423	−0.3158	−0.1714	0.019*
C5	0.5744 (3)	0.2480 (3)	0.2984 (2)	0.0337 (6)
H5A	0.6151	0.3539	0.2784	0.040*
H5B	0.6756	0.2445	0.3623	0.040*
C6	0.4348 (3)	0.2236 (4)	0.3726 (2)	0.0367 (6)
H6A	0.3922	0.1168	0.3908	0.044*
H6B	0.4836	0.3054	0.4672	0.044*
C7	0.4533 (3)	0.2471 (3)	−0.1933 (2)	0.0228 (4)
H7A	0.3994	0.1976	−0.2993	0.027*
H7B	0.4938	0.1758	−0.1510	0.027*
C8	0.6074 (3)	0.4068 (3)	−0.1660 (3)	0.0315 (5)
H8A	0.6854	0.3940	−0.2217	0.038*	0.911 (4)
H8B	0.5654	0.4807	−0.2011	0.038*	0.911 (4)
H8C	0.6293	0.4825	−0.0736	0.038*	0.089 (4)
H8D	0.7153	0.3976	−0.1688	0.038*	0.089 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu1	0.01155 (16)	0.01460 (17)	0.00850 (16)	0.00675 (13)	0.00358 (12)	0.00261 (12)
Na1	0.0126 (4)	0.0164 (4)	0.0126 (4)	0.0067 (3)	0.0038 (3)	0.0039 (3)
O1	0.0192 (7)	0.0140 (7)	0.0112 (6)	0.0077 (5)	0.0057 (5)	0.0018 (5)
O2	0.0214 (7)	0.0182 (7)	0.0105 (6)	0.0121 (6)	0.0051 (5)	0.0036 (5)
O3	0.0170 (7)	0.0194 (7)	0.0132 (7)	0.0119 (6)	0.0083 (6)	0.0067 (5)
O4	0.0137 (7)	0.0165 (7)	0.0121 (7)	0.0065 (5)	0.0044 (5)	0.0047 (5)
O5	0.0137 (7)	0.0251 (8)	0.0200 (7)	0.0104 (6)	0.0048 (6)	0.0010 (6)
O6	0.0138 (7)	0.0277 (8)	0.0169 (7)	0.0105 (6)	0.0077 (6)	0.0060 (6)
O7	0.0134 (7)	0.0262 (8)	0.0178 (7)	0.0095 (6)	0.0066 (6)	0.0080 (6)
O8	0.0207 (9)	0.0245 (11)	0.0667 (15)	0.0121 (8)	−0.0102 (9)	−0.0075 (10)
C1	0.0239 (10)	0.0149 (9)	0.0145 (9)	0.0088 (8)	0.0067 (8)	0.0020 (7)
C2	0.0248 (10)	0.0208 (10)	0.0147 (9)	0.0157 (8)	0.0076 (8)	0.0046 (8)
C3	0.0156 (9)	0.0154 (9)	0.0169 (9)	0.0105 (7)	0.0066 (7)	0.0066 (7)
C4	0.0154 (9)	0.0136 (9)	0.0175 (9)	0.0065 (7)	0.0061 (7)	0.0031 (7)
C5	0.0162 (10)	0.0536 (16)	0.0211 (11)	0.0161 (11)	−0.0011 (9)	−0.0082 (10)
C6	0.0293 (12)	0.076 (2)	0.0140 (10)	0.0352 (13)	0.0051 (9)	0.0074 (11)
C7	0.0178 (10)	0.0266 (11)	0.0283 (11)	0.0117 (9)	0.0111 (9)	0.0091 (9)
C8	0.0196 (11)	0.0294 (13)	0.0537 (16)	0.0122 (9)	0.0166 (11)	0.0219 (11)

Geometric parameters (Å, º)

Cu1—O2	1.9252 (13)	O8—H8	0.69 (3)
Cu1—O2^i	1.9252 (13)	O8B—C8	1.507 (15)
Cu1—O1	1.9317 (13)	C1—C2	1.518 (3)
Cu1—O1^i	1.9317 (13)	C1—H1A	0.9900
Na1—O6	2.3816 (16)	C1—H1B	0.9900
Na1—O7	2.3858 (16)	C2—H2A	0.9900
Na1—O3	2.3945 (16)	C2—H2B	0.9900
Na1—O5	2.4052 (16)	C3—C4	1.503 (3)
Na1—O4	2.4453 (15)	C3—H3A	0.9900
Na1—O3^ii	2.4545 (15)	C3—H3B	0.9900
O1—C1	1.433 (2)	C4—H4A	0.9900
O2—C2	1.424 (2)	C4—H4B	0.9900
O3—C3	1.438 (2)	C5—C6	1.507 (3)
O3—Na1^ii	2.4545 (15)	C5—H5A	0.9900
O3—H3	0.72 (3)	C5—H5B	0.9900
O4—C4	1.442 (2)	C6—H6A	0.9900
O4—H4	0.74 (3)	C6—H6B	0.9900
O5—C5	1.419 (3)	C7—C8	1.504 (3)
O5—H5	0.75 (3)	C7—H7A	0.9900
O6—C6	1.414 (3)	C7—H7B	0.9900
O6—H6	0.75 (3)	C8—H8A	0.9900
O7—C7	1.436 (2)	C8—H8B	0.9900
O7—H7	0.81 (3)	C8—H8C	0.9900
O8—C8	1.421 (3)	C8—H8D	0.9900
O2—Cu1—O2^i	180.0	O2—C2—H2A	109.9
O2—Cu1—O1	87.11 (5)	C1—C2—H2A	109.9
O2^i—Cu1—O1	92.89 (5)	O2—C2—H2B	109.9
O1—Cu1—O1^i	180.0	C1—C2—H2B	109.9
O6—Na1—O7	120.74 (6)	H2A—C2—H2B	108.3
O6—Na1—O3	85.75 (6)	O3—C3—C4	108.68 (15)
O7—Na1—O3	151.04 (6)	O3—C3—H3A	110.0
O6—Na1—O5	71.56 (5)	C4—C3—H3A	110.0
O7—Na1—O5	100.58 (6)	O3—C3—H3B	110.0
O3—Na1—O5	98.93 (6)	C4—C3—H3B	110.0
O6—Na1—O4	147.89 (6)	H3A—C3—H3B	108.3
O7—Na1—O4	87.66 (5)	O4—C4—C3	109.59 (15)
O3—Na1—O4	71.16 (5)	O4—C4—H4A	109.8
O5—Na1—O4	89.86 (5)	C3—C4—H4A	109.8
O6—Na1—O3^ii	102.35 (5)	O4—C4—H4B	109.8
O7—Na1—O3^ii	78.69 (5)	C3—C4—H4B	109.8
O3—Na1—O3^ii	84.59 (5)	H4A—C4—H4B	108.2
O5—Na1—O3^ii	172.59 (6)	O5—C5—C6	109.26 (19)
O4—Na1—O3^ii	97.47 (5)	O5—C5—H5A	109.8
C1—O1—Cu1	109.42 (11)	C6—C5—H5A	109.8
C2—O2—Cu1	108.70 (10)	O5—C5—H5B	109.8
C3—O3—Na1	102.62 (10)	C6—C5—H5B	109.8
C3—O3—Na1^ii	133.07 (11)	H5A—C5—H5B	108.3
Na1—O3—Na1^ii	95.41 (5)	O6—C6—C5	108.96 (19)
C3—O3—H3	106 (2)	O6—C6—H6A	109.9
Na1—O3—H3	122 (2)	C5—C6—H6A	109.9
Na1^ii—O3—H3	100 (2)	O6—C6—H6B	109.9
C4—O4—Na1	111.25 (10)	C5—C6—H6B	109.9
C4—O4—H4	107 (2)	H6A—C6—H6B	108.3
Na1—O4—H4	112 (2)	O7—C7—C8	111.16 (18)
C5—O5—Na1	108.46 (12)	O7—C7—H7A	109.4
C5—O5—H5	109 (2)	C8—C7—H7A	109.4
Na1—O5—H5	135 (2)	O7—C7—H7B	109.4
C6—O6—Na1	112.10 (12)	C8—C7—H7B	109.4
C6—O6—H6	109 (2)	H7A—C7—H7B	108.0
Na1—O6—H6	116 (2)	O8—C8—C7	109.9 (2)
C7—O7—Na1	118.73 (12)	C7—C8—O8B	96.4 (9)
C7—O7—H7	105 (2)	O8—C8—H8A	110.5
Na1—O7—H7	106 (2)	C7—C8—H8A	109.2
C8—O8—H8	113 (3)	O8—C8—H8B	110.0
O1—C1—C2	109.13 (16)	C7—C8—H8B	109.2
O1—C1—H1A	109.9	H8A—C8—H8B	107.9
C2—C1—H1A	109.9	C7—C8—H8C	112.5
O1—C1—H1B	109.9	O8B—C8—H8C	111.9
C2—C1—H1B	109.9	C7—C8—H8D	112.4
H1A—C1—H1B	108.3	O8B—C8—H8D	113.1
O2—C2—C1	108.82 (15)	H8C—C8—H8D	110.0

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O6—H6···O1^i	0.75 (3)	1.93 (3)	2.683 (2)	175 (3)
O4—H4···O1^iii	0.74 (3)	1.89 (3)	2.6153 (19)	168 (3)
O5—H5···O4^iv	0.75 (3)	2.05 (3)	2.779 (2)	164 (3)
O7—H7···O2^ii	0.81 (3)	1.81 (3)	2.612 (2)	171 (3)
O3—H3···O2	0.72 (3)	1.94 (3)	2.6566 (19)	172 (3)
O8—H8···O7^v	0.69 (3)	2.14 (3)	2.815 (3)	167 (4)

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