Di-μ-chloro-1:2κ²Cl;3:4κ²Cl-hexachloro-1κ³Cl,4κ³Cl-tetra-μ-dimethyl­glycine-2:3κ⁸O:O′-tetra­copper(II)

The title compound, [Cu₄Cl₈(C₄H₉NO₂)₄], crystallizes in the centrosymmetric space group P2₁/c with a unit cell containing two tetra­nuclear copper(II) complexes sited on crystallographic inversion centres. The coordination geometry around the central Cu atoms is square pyramidal, with four O atoms in the basal plane and a Cl atom in the apical position. The lateral CuCl₄ groups are flattened tetra­hedral. The bridging dimethyl­glycine mol­ecules are present in the dipolar zwitterionic form. The tetra­nuclear copper complexes exist as isolated entities since only intra­molecular hydrogen bonds are found.

S1. Comment

The title compound, (I), was synthesized within a project focused on the study of the magnetic properties of Cu^II compounds, in particular dimers and polymeric chains where the transition metal ions are bridged through organic diamagnetic ligands. Polynuclear Cu^II complexes are one of the most prominent families of compounds in modern chemistry, since a correlation between the magnetic properties and the molecular structure has been found in some cases. The most extensively studied have been hydroxo-bridged dinuclear compounds, with Hodson & Hatfield (Crawford et al., 1976) establishing a correlation between the exchange coupling constant and the Cu—O—Cu angle. The situation for compounds with higher nuclearity is not so clear, but is increasingly more interesting since their intermediate position between the simplest dinuclear species and bulk materials may generate an unexpected magnetic behaviour. Systems with intermediate nuclearity are also important in biology, where trinuclear and tetranuclear arrays of Cu atoms have been found in the active sites of enzymes (Haltia et al., 2003; Chen et al., 2004).

Compound (I) crystallizes in the centrosymmetric space group P2₁/c with a unit cell containing two tetranuclear Cu^II complexes. The molecular structure consists of a crystallographic centrosymmetric binuclear copper complex bridged by four dimethylglycine molecules and strongly coordinated to two [CuCl₄]²⁻ ions (Fig. 1). The latter orient themselves so as to have the four Cu and four Cl atoms (Cu1, Cu2, Cl1, Cl3 and their centrosymmetric counterparts) almost coplanar [mean deviation 0.0467 (4) Å].

The bridged Cu^II ions are a short distance [2.7303 (8) Å] from each other. The Cl1—Cu1—Cu1ⁱ angle is 164.13 (2)°, Cl1—Cu2—Cl3 is 142.88 (3)° and Cu1—Cl1—Cu2 is 93.45 (3)° [symmetry code: (i) 1 − x, −y, −z]. According to the procedure described by Addison et al. (1984), the shape of the polyhedron around Cu1 can be considered square-based pyramidal, since the τ value [τ= (θ1-θ2)/60°, where θ1 and θ2 are the largest angle values in the coordination sphere] is 0.04. The deviation of Cu1 from the least-squares plane of the base of the pyramid is 0.250 (1) Å towards the apical Cl atom. The angle between the plane that contains the Cu^II ions and the least-squares plane of the base of the pyramid is 85.75 (4)°.

The distances between Cu1 and the basal O atoms range from 1.9608 (18) to 1.9830 (17) Å, the apical Cl atom being 2.4625 (9) Å from Cu1; Cu2 is coordinated by four Cl atoms in a flattened tetrahedral geometry. The trans Cl—Cu2—Cl angles are opened to 136.30 (4) and 142.88 (3)°, which favours the yellow–green colour according to Willett & Zanchini (1990), while the other four range between 96.12 (3) and 97.48 (4)°. The Cu2—Cl distances vary in the range 2.2416 (9)–2.2925 (7) Å. This distortion has been observed in many [CuCl₄]²⁻ salts, and in some of them thermochromic properties are observed and explained by the deformation of [CuCl₄]²⁻ as a function of temperature. The neutral dimethylglycine molecules are present in the dipolar zwitterionic form. Inspection of the C—O distances in the carboxylate groups [between 1.246 (3) and 1.253 (3) Å] shows that these are deprotonated.

The carboxylate groups in (I) are twisted by 13.4 (3) and 16.4 (4)° around the C1—C2 and C5—C6 bonds, respectively. The main skeletons C1—C2—N1—C3 and C5—C6—N2—C8 deviate from planarity, as shown by the corresponding torsion angles of −160.2 (3) and 173.1 (2)°, respectively. The difference in the former angles might be explained by the closer proximity of Cu2 and Cl3 to N1 than to N2.

A few compounds where dimethylglycine coordinates to or bridges metal ions can be found in the Cambridge Structural Database (Version?; Allen, 2002). Structures have been described in which this substituted amino acid coordinates to the metal ion through the N and one of the O atoms or just by one of the carboxylate O atoms. Platinum (Gravenhorst et al., 1999), copper (Cameron et al., 1973), chromium (Darensbourg et al., 1997), tungsten (Darensbourg et al., 1994) and cobalt (Kojima et al., 1994) complexes are examples of the former, while the latter is found in tin (Khoo et al., 1994) and iron (Ramos Silva et al., 2003) coordination compounds. The bridging properties of dimethylglycine have been observed in an oxo-centred trinuclear iron compound (Ramos Silva et al., 2003) and again in the title compound. When in bridging mode, the carboxylate group is less rotated around C5—C6 than when in the (N,O)-coordination conformation, as perceived by the reported average torsion angles for N1—C2—C1—O1 and N1—C2—C1—O2, of 11 (5) and 171 (5)°, respectively, for the bridging conformation, and 27 (4) and 157 (2)°, respectively, for the (N,O)-coordination conformation. The simple O-coordination conformation presents the smallest degree of rotation, with average torsion angles of 5(3) and 174 (5)°, respectively.

The tetranuclear copper complexes in (I) exist almost as isolated dimeric entities, further stabilized by intramolecular N—H···Cl bonds between N2 and Cl4 [3.194 (3) Å and 162 (3)°] and N2 and Cl2 [3.188 (3) Å and 153 (3)°]. Among other interactions, there are weak C—H···Cl and C—H···O intermolecular interactions supporting the cohesion of the structure: one of the symmetry-independent organic ligands shares three H atoms with two neighbouring complexes, while the other ligand donates only one H atom to a neighbouring CuCl₄ Cl atom.

S2. Experimental

Dimethylglycine (5 mmol) was added to hydrated copper chloride (2.5 mmol) in an aqueous solution (Volume?). After a few months at room temperature, small single crystals of the title compound had grown, which were used for structure analysis. Surprisingly, after a few weeks, all these crystals had spontaneously dissolved in the solution and during the following few weeks new crystals formed, but this time of a quite different phase, which is currently under study.

S3. Refinement

H atoms were placed in calculated positions. Those not involved in hydrogen bonding were allowed to ride on their parent atoms, with C—H distances in the range 0.96–0.97 Å and with U_iso(H) = 1.5U_eq(C) [Please check added text], while atoms H1 and H2 were refined with isotropic displacement factors. Examination of the crystal structure with PLATON (Spek, 2003) showed that there are no solvent-accessible voids in the crystal lattice.

Computing details

Data collection: CAD-4 Software (Enraf–Nonius, 1989); cell refinement: CAD-4 Software; data reduction: PLATON (Spek, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97.

Figures

Fig. 1. A view of the title compound, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. Intramolecular hydrogen bonds are shown as dashed lines. [Symmetry code: (i) 1 − x, −y, −z.]

Octachloro-1κ⁴Cl,4κ⁴Cl-tetra-µ-dimethylglycine-2:3κ⁸O:O'- tetracopper(II)]

Crystal data

[Cu4Cl8(C4H9NO2)4]	F(000) = 952
Mr = 950.25	Dx = 1.812 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 9.759 (1) Å	Cell parameters from 25 reflections
b = 12.861 (1) Å	θ = 9.9–15.9°
c = 14.060 (1) Å	µ = 3.06 mm^−1
β = 99.20 (3)°	T = 293 K
V = 1741.8 (3) Å^3	Block, green
Z = 2	0.49 × 0.46 × 0.39 mm

Data collection

Enraf–Nonius CAD-4 diffractometer	2625 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.030
Graphite monochromator	θmax = 25.0°, θmin = 3.2°
profile data from ω/2θ scans	h = 0→11
Absorption correction: ψ scan (North et al., 1968)	k = 0→15
Tmin = 0.202, Tmax = 0.303	l = −16→16
3266 measured reflections	3 standard reflections every 180 min
3075 independent reflections	intensity decay: 4.1%

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)] = 0.024	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.064	H atoms treated by a mixture of independent and constrained refinement
S = 1.09	w = 1/[σ^2(Fo^2) + (0.0325P)^2 + 1.4695P] where P = (Fo^2 + 2Fc^2)/3
3075 reflections	(Δ/σ)max < 0.001
191 parameters	Δρmax = 0.48 e Å^−3
0 restraints	Δρmin = −0.43 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
Cu1	0.59767 (3)	−0.05778 (2)	0.05727 (2)	0.02322 (9)
Cu2	0.80484 (3)	0.03701 (2)	0.26202 (2)	0.03025 (10)
Cl1	0.76619 (7)	−0.12398 (5)	0.19292 (5)	0.03222 (15)
Cl2	0.72292 (8)	0.00060 (6)	0.39923 (5)	0.04465 (19)
Cl3	0.73336 (9)	0.20340 (6)	0.24919 (5)	0.0481 (2)
Cl4	1.01405 (8)	0.06124 (7)	0.21906 (7)	0.0547 (2)
O1	0.71701 (18)	0.06279 (13)	0.03822 (14)	0.0305 (4)
O2	0.55297 (18)	0.16049 (14)	−0.04852 (14)	0.0351 (4)
O3	0.50695 (19)	0.02369 (15)	0.14862 (13)	0.0340 (4)
O4	0.34662 (19)	0.12127 (14)	0.05746 (13)	0.0340 (4)
N1	0.9150 (2)	0.2101 (2)	0.03939 (18)	0.0368 (6)
H1	0.922 (3)	0.167 (3)	0.083 (3)	0.044*
N2	0.4146 (2)	0.07026 (17)	0.31278 (16)	0.0302 (5)
H2	0.509 (3)	0.065 (2)	0.320 (2)	0.036*
C1	0.6723 (3)	0.14454 (19)	−0.00322 (18)	0.0256 (5)
C2	0.7680 (3)	0.2372 (2)	0.0039 (2)	0.0311 (6)
H2A	0.7616	0.2692	−0.0591	0.037*
H2B	0.7375	0.2880	0.0471	0.037*
C3	0.9954 (4)	0.3040 (3)	0.0776 (3)	0.0716 (12)
H3A	1.0905	0.2850	0.0991	0.107*
H3B	0.9565	0.3322	0.1306	0.107*
H3C	0.9911	0.3553	0.0276	0.107*
C4	0.9797 (4)	0.1568 (4)	−0.0356 (3)	0.0670 (11)
H4A	1.0726	0.1363	−0.0090	0.101*
H4B	0.9823	0.2032	−0.0888	0.101*
H4C	0.9263	0.0963	−0.0576	0.101*
C5	0.4099 (3)	0.0885 (2)	0.13612 (19)	0.0272 (5)
C6	0.3643 (3)	0.1331 (2)	0.22518 (19)	0.0293 (6)
H6A	0.3994	0.2035	0.2349	0.035*
H6B	0.2638	0.1362	0.2157	0.035*
C7	0.3818 (4)	0.1243 (3)	0.4001 (2)	0.0500 (8)
H7A	0.4158	0.0838	0.4563	0.075*
H7B	0.2831	0.1324	0.3948	0.075*
H7C	0.4253	0.1915	0.4054	0.075*
C8	0.3573 (4)	−0.0379 (2)	0.3081 (3)	0.0484 (8)
H8A	0.3968	−0.0754	0.3650	0.073*
H8B	0.3801	−0.0725	0.2522	0.073*
H8C	0.2583	−0.0351	0.3041	0.073*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu1	0.02030 (16)	0.02385 (16)	0.02461 (17)	0.00118 (11)	0.00088 (12)	0.00023 (12)
Cu2	0.02925 (18)	0.03154 (18)	0.03020 (18)	−0.00199 (13)	0.00552 (13)	0.00063 (13)
Cl1	0.0326 (3)	0.0281 (3)	0.0330 (3)	0.0040 (3)	−0.0039 (3)	0.0034 (3)
Cl2	0.0481 (4)	0.0552 (5)	0.0329 (4)	0.0049 (4)	0.0134 (3)	0.0090 (3)
Cl3	0.0725 (5)	0.0322 (4)	0.0379 (4)	0.0059 (4)	0.0033 (4)	−0.0035 (3)
Cl4	0.0254 (4)	0.0719 (6)	0.0660 (5)	−0.0070 (4)	0.0053 (3)	0.0184 (4)
O1	0.0260 (9)	0.0270 (9)	0.0374 (10)	−0.0041 (7)	0.0018 (8)	0.0028 (8)
O2	0.0272 (10)	0.0310 (10)	0.0443 (11)	−0.0048 (8)	−0.0027 (8)	0.0072 (8)
O3	0.0294 (10)	0.0404 (10)	0.0329 (10)	0.0098 (8)	0.0068 (8)	−0.0049 (8)
O4	0.0351 (10)	0.0366 (10)	0.0301 (10)	0.0071 (8)	0.0043 (8)	−0.0036 (8)
N1	0.0297 (12)	0.0405 (14)	0.0382 (14)	−0.0100 (10)	−0.0009 (10)	0.0089 (11)
N2	0.0279 (12)	0.0323 (12)	0.0310 (12)	−0.0017 (10)	0.0066 (9)	−0.0003 (9)
C1	0.0263 (13)	0.0268 (13)	0.0241 (13)	−0.0024 (10)	0.0056 (10)	−0.0016 (10)
C2	0.0289 (14)	0.0293 (14)	0.0344 (15)	−0.0049 (11)	0.0029 (11)	0.0017 (11)
C3	0.057 (2)	0.060 (2)	0.087 (3)	−0.0336 (19)	−0.019 (2)	0.005 (2)
C4	0.044 (2)	0.094 (3)	0.069 (3)	0.012 (2)	0.0251 (18)	0.005 (2)
C5	0.0238 (13)	0.0269 (12)	0.0315 (14)	−0.0032 (11)	0.0061 (11)	−0.0037 (11)
C6	0.0290 (13)	0.0280 (13)	0.0324 (14)	0.0030 (11)	0.0094 (11)	0.0010 (11)
C7	0.069 (2)	0.0505 (19)	0.0328 (17)	0.0014 (16)	0.0150 (15)	−0.0033 (14)
C8	0.054 (2)	0.0321 (16)	0.060 (2)	−0.0071 (14)	0.0105 (16)	0.0066 (14)

Geometric parameters (Å, º)

Cu1—O4^i	1.9608 (19)	N2—C6	1.489 (3)
Cu1—O2^i	1.9656 (18)	N2—C8	1.496 (4)
Cu1—O3	1.9735 (18)	N2—H2	0.91 (3)
Cu1—O1	1.9830 (17)	C1—C2	1.508 (4)
Cu1—Cl1	2.4625 (9)	C2—H2A	0.9700
Cu1—Cu1^i	2.7303 (8)	C2—H2B	0.9700
Cu2—Cl4	2.2416 (9)	C3—H3A	0.9600
Cu2—Cl3	2.2490 (8)	C3—H3B	0.9600
Cu2—Cl2	2.2523 (9)	C3—H3C	0.9600
Cu2—Cl1	2.2925 (7)	C4—H4A	0.9600
O1—C1	1.246 (3)	C4—H4B	0.9600
O2—C1	1.252 (3)	C4—H4C	0.9600
O2—Cu1^i	1.9656 (18)	C5—C6	1.507 (4)
O3—C5	1.253 (3)	C6—H6A	0.9700
O4—C5	1.250 (3)	C6—H6B	0.9700
O4—Cu1^i	1.9608 (19)	C7—H7A	0.9600
N1—C4	1.482 (5)	C7—H7B	0.9600
N1—C2	1.482 (4)	C7—H7C	0.9600
N1—C3	1.493 (4)	C8—H8A	0.9600
N1—H1	0.82 (3)	C8—H8B	0.9600
N2—C7	1.489 (4)	C8—H8C	0.9600
O4^i—Cu1—O2^i	88.37 (9)	O2—C1—C2	114.9 (2)
O4^i—Cu1—O3	165.63 (8)	N1—C2—C1	113.1 (2)
O2^i—Cu1—O3	89.35 (8)	N1—C2—H2A	109.0
O4^i—Cu1—O1	88.19 (8)	C1—C2—H2A	109.0
O2^i—Cu1—O1	165.15 (8)	N1—C2—H2B	109.0
O3—Cu1—O1	90.40 (8)	C1—C2—H2B	109.0
O4^i—Cu1—Cl1	104.74 (6)	H2A—C2—H2B	107.8
O2^i—Cu1—Cl1	102.49 (6)	N1—C3—H3A	109.5
O3—Cu1—Cl1	89.61 (6)	N1—C3—H3B	109.5
O1—Cu1—Cl1	92.35 (6)	H3A—C3—H3B	109.5
O4^i—Cu1—Cu1^i	90.04 (6)	N1—C3—H3C	109.5
O2^i—Cu1—Cu1^i	83.35 (6)	H3A—C3—H3C	109.5
O3—Cu1—Cu1^i	75.60 (6)	H3B—C3—H3C	109.5
O1—Cu1—Cu1^i	82.21 (5)	N1—C4—H4A	109.5
Cl1—Cu1—Cu1^i	164.13 (2)	N1—C4—H4B	109.5
Cl4—Cu2—Cl3	97.48 (4)	H4A—C4—H4B	109.5
Cl4—Cu2—Cl2	136.30 (4)	N1—C4—H4C	109.5
Cl3—Cu2—Cl2	96.81 (3)	H4A—C4—H4C	109.5
Cl4—Cu2—Cl1	96.12 (3)	H4B—C4—H4C	109.5
Cl3—Cu2—Cl1	142.88 (3)	O4—C5—O3	127.1 (2)
Cl2—Cu2—Cl1	96.80 (3)	O4—C5—C6	115.9 (2)
Cu2—Cl1—Cu1	93.45 (3)	O3—C5—C6	117.0 (2)
C1—O1—Cu1	123.61 (16)	N2—C6—C5	112.2 (2)
C1—O2—Cu1^i	123.19 (17)	N2—C6—H6A	109.2
C5—O3—Cu1	132.04 (18)	C5—C6—H6A	109.2
C5—O4—Cu1^i	115.13 (16)	N2—C6—H6B	109.2
C4—N1—C2	111.7 (2)	C5—C6—H6B	109.2
C4—N1—C3	111.8 (3)	H6A—C6—H6B	107.9
C2—N1—C3	110.8 (3)	N2—C7—H7A	109.5
C4—N1—H1	103 (2)	N2—C7—H7B	109.5
C2—N1—H1	112 (2)	H7A—C7—H7B	109.5
C3—N1—H1	108 (2)	N2—C7—H7C	109.5
C7—N2—C6	110.0 (2)	H7A—C7—H7C	109.5
C7—N2—C8	110.0 (2)	H7B—C7—H7C	109.5
C6—N2—C8	113.4 (2)	N2—C8—H8A	109.5
C7—N2—H2	107 (2)	N2—C8—H8B	109.5
C6—N2—H2	109.5 (19)	H8A—C8—H8B	109.5
C8—N2—H2	107.0 (19)	N2—C8—H8C	109.5
O1—C1—O2	127.1 (2)	H8A—C8—H8C	109.5
O1—C1—C2	118.0 (2)	H8B—C8—H8C	109.5
Cl4—Cu2—Cl1—Cu1	−105.25 (3)	Cu1^i—Cu1—O3—C5	0.7 (2)
Cl3—Cu2—Cl1—Cu1	5.77 (6)	Cu1—O1—C1—O2	−8.9 (4)
Cl2—Cu2—Cl1—Cu1	116.58 (3)	Cu1—O1—C1—C2	168.39 (18)
O4^i—Cu1—Cl1—Cu2	133.21 (6)	Cu1^i—O2—C1—O1	3.9 (4)
O2^i—Cu1—Cl1—Cu2	−135.17 (7)	Cu1^i—O2—C1—C2	−173.54 (17)
O3—Cu1—Cl1—Cu2	−45.92 (6)	C4—N1—C2—C1	74.5 (3)
O1—Cu1—Cl1—Cu2	44.46 (6)	C3—N1—C2—C1	−160.2 (3)
Cu1^i—Cu1—Cl1—Cu2	−24.95 (10)	O1—C1—C2—N1	14.5 (3)
O4^i—Cu1—O1—C1	97.4 (2)	O2—C1—C2—N1	−167.9 (2)
O2^i—Cu1—O1—C1	20.7 (4)	Cu1^i—O4—C5—O3	3.1 (4)
O3—Cu1—O1—C1	−68.3 (2)	Cu1^i—O4—C5—C6	−177.53 (17)
Cl1—Cu1—O1—C1	−157.96 (19)	Cu1—O3—C5—O4	−2.6 (4)
Cu1^i—Cu1—O1—C1	7.06 (19)	Cu1—O3—C5—C6	177.95 (17)
O4^i—Cu1—O3—C5	−1.7 (5)	C7—N2—C6—C5	173.1 (2)
O2^i—Cu1—O3—C5	−82.6 (2)	C8—N2—C6—C5	−63.2 (3)
O1—Cu1—O3—C5	82.5 (2)	O4—C5—C6—N2	163.9 (2)
Cl1—Cu1—O3—C5	174.9 (2)	O3—C5—C6—N2	−16.6 (3)

Symmetry code: (i) −x+1, −y, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C6—H6B···Cl4^ii	0.97	2.63	3.529 (3)	155
C7—H7C···O2^iii	0.96	2.31	3.254 (4)	167
C2—H2A···Cl3^iv	0.97	2.69	3.625 (3)	162
C8—H8C···Cl4^ii	0.96	2.78	3.616 (4)	146
N1—H1···Cl4	0.82 (3)	2.40 (4)	3.194 (3)	162 (3)
N2—H2···Cl2	0.91 (3)	2.35 (3)	3.188 (3)	153 (3)

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