Monoclinic Cu₃(SeO₃)₂Cl₂: an oxo­halide with an unusual CuO₄Cl trigonal-bipyramidal coordination

In single crystals of a new monoclinic (C2/m) form of tricopper(II) diselenium(IV) dichloride hexa­oxide, Cu₃(SeO₃)₂Cl₂, the Se atom is in the 4i position, while the two Cu atoms are in 2a and 4i positions. The structure is based on layers of CuO₄Cl trigonal bipyramids, CuO₄ square planes and SeO₃E tetra­hedra. The Cu polyhedra are connected by edge- and corner-sharing to form [010] chains and these chains are bridged by the Se atoms to form (001) layers. The compound is isostructural with Cu₃(TeO₃)₂Br₂.

S1. Comment

The use of stereochemically active lone-pairs as spacers to open up structures is a concept which has proved very successful for synthesizing novel low-dimensional compounds (Johnsson et al., 2000, 2003; Takagi et al., 2006; Becker et al. 2006). Cations such as Te^IV or Se^IV receive an asymmetric coordination due to their stereochemically active lone pair and may then function as a chemical scissor.

This work describes a new layered monoclinic C2/m modification of Cu₃(SeO₃)₂Cl₂, (I), which was previously known in a triclinic form (Millet et al., 2000). The structure of triclinic Cu₃(SeO₃)₂Cl₂ is three-dimensional with three crystallographically distinct Cu atoms in square-planar, octahedral and square-pyramidal coordinations. The structure is made up of layers of Cu polyhedra sandwiched by Se atoms, and the layers are connected via CuO₄ square planes. By contrast, monoclinic Cu₃(SeO₃)₂Cl₂ is isostructural with Cu₃(TeO₃)₂Br₂ (Becker et al., 2005).

The Se atom in (I) shows a regular one-sided threefold coordination to one O2 and two O1 atoms. Its stereochemically active 4s² lone pair (E) completes the tetrahedral SeO₃E coordination. The two crystallographically distinct Cu atoms have different coordinations (Fig. 1). Atom Cu1 has a square-planar coordination involving four O1 atoms, and two further Cl atoms complete a distorted octahedral coordination. Atom Cu2 has a highly unusual distorted trigonal–bipyramidal CuO₄Cl coordination involving two apical O1 atoms and one Cl and two O2 equatorial atoms. To the best of our knowledge, (I) and Cu₃(TeO₃)₂Br₂ are the first compounds to show a trigonal–bipyramidal CuO₄X (X = Cl or Br) coordination polyhedron for Cu^II. Bond-valence sum calculations (Brese & O'Keeffe, 1991) confirm the coordination and oxidation states of all ions. A low value of 0.6 v.u. is calculated for Cl if only the primary Cu—Cl bond is included (0.8 v.u. if longer interactions are included). This suggests that Cl⁻ acts more as a counter-ion than as part of the ionic/covalent bond network within the structure.

The structure of (I) is made up of pairs of Cu2O₄Cl trigonal bipyramids sharing an O2···O2 equatorial edge. These pairs are connected by sharing O1 atoms with two Cu1O₄ square planes and form chains along the b axis (Fig. 1). The SeO₃E tetrahedra share an edge with the Cu2O₄ square plane and a corner with the Cu2O₄Cl bipyramid, so that they bridge the chains into layers (Fig. 2). The layers are parallel to the ab plane and are separated from each other by the Cl atoms and the lone pairs of the Se atoms (Fig. 3). The absence of significant contacts between the layers implies that only van der Waals interactions connect the layers to each other in the structure. As the layers have no net charge, they can be considered as infinite two-dimensional molecules.

The structural difference between (I) and Cu₃(TeO₃)₂Br₂ is most obvious when looking at the Cu···Cu distances. Within a chain, the shortest distance (Cu2···Cu2) is actually shorter in Cu₃(TeO₃)₂Br₂ than in (I) [3.145(s.u.?) versus 3.363(s.u.?) Å], and this is also reflected in the smaller Cu2—O2—Cu2 angle [96.09(s.u.?) versus 101.72 (10)°]. These effects are probably due to the larger Br atom pushing the Cu2 atoms closer together. The shortest inter-chain Cu···Cu distance (Cu1···Cu2) is shorter in (I) [3.607(s.u.?) versus 3.709(s.u.?) Å] due to the smaller size of Se^IV compared with Te^IV, thus allowing the chains to come closer. The shortest inter-layer Cu···Cu distance is substantially shorter in (I) [4.579 (s.u.?) versus 5.620(s.u.?) Å] which again can be explained by the relatively smaller sizes of the Se and Cl atoms. The denser chain and layer packing in (I) can also be seen in the length of the a and c axes. The a axis, or the chain stacking direction, is reduced from 9.319 (2) in Cu₃(TeO₃)₂Br₂ ? to 8.933 (1) Å in (I), and the c axis, or the layer stacking direction, is reduced from 8.200 (2) Å in Cu₃(TeO₃)₂Br₂ ? to 7.582 (1) Å in (I).

An atom should be positioned no further away than to contribute at least 4% of the bond valence to be regarded as bonded (Brown, 2002). For Se^IV and Te^IV, this means a longest primary bonding distance to O of 2.5–2.7 Å. The possibility of forming isostructural Se and Te analogues is largely dependent upon the coordination of these atoms. Se^IV has only been observed in SeO₃ coordination with bond distances of around 1.7 Å, while Te^IV may be coordinated by three, four, or even five ligands, with the nearest three ligands at approximately 1.9 Å and additional ligands at more than 2 Å (Zemann, 1971). This means that a TeO₃ coordination environment is required for the formation of isostructural Te and Se analogues. This is the case in Cu₃Bi(TeO₃)₂O₂Cl (Becker & Johnsson, 2005) and Cu₃Bi(SeO₃)₂O₂Cl (Pring et al., 1990), where the fourth Te—O and Se—O distances are both around 3 Å and thus clearly outside the primary coordination sphere. A different case is observed for Ni₅(TeO₃)₄Cl₂ (Johnsson et al., 2003) and Ni₅(SeO₃)₄Cl₂ (Shen et al., 2005), for which the fourth Te—O distance (2.65 Å) can be regarded as a bond, whereas the fourth Se—O distance (2.94 Å) clearly lies outside the primary coordination sphere.

S2. Experimental

Single crystals of Cu₃(SeO₃)₂Cl₂ were synthesized via a chemical vapour transport reaction in a sealed evacuated silica tube. CuO (Alfa Aesar, 99.7%), SeO₂ (Alfa Aesar, 99.4%) and CuCl₂ (Alfa Aesar, 99%) were used as starting materials. The crystals were grown from the off-stoichiometric molar ratio CuO:SeO₂:CuCl₂ = 1:1:1. The powders were placed at one end of a silica tube which was then evacuated to 10⁻⁵ Torr (1 Torr = 133.322 Pa). Electronic grade HCl gas was introduced into the tube before sealing it off. The ampoule was then heated in a two-zone furnace with charge and growth zone temperatures of 733 and 623 K, respectively. After six weeks, two different kinds of single crystals were observed in the ampoule, namely yellow-green Cu₃(SeO₃)₂Cl₂ platelets with a maximum size of 10 × 6 × 0.2 mm⁻³ in the centre of the ampoule, and thin brown air-sensitive CuCl₂ needles with a maximum size of 20 × 1 × 0.1 mm⁻³ in the charge zone. The synthesis products were characterized in a scanning electron microscope (SEM, Jeol 820) with an energy-dispersive spectrometer (EDS, LINK AN10000), confirming the presence and stoichiometry of Cu, Se and Cl.

S3. Refinement

The value for the maximum residual electron density is 1.34 e Å⁻³, located 0.58 Å away from Cu2 at the fractional coordinates (0.3610 0.0000 0.1582), and the value of the minimum is −0.73 e Å⁻³, located 0.72 Å away from Cu2 at (0.3864 0.0000 0.0808).

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2006); cell refinement: CrysAlis RED (Oxford Diffraction, 2006); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The asymmetric unit of Cu3(SeO3)2Cl2, with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) x, −1y, z [Please check]; (ii) 1/2 − x, 1/2 + y, 2 − z; (iii) 1 − x, 1 − y, 2 − z; (iv) 1 − z, y, 2 − z; (v) 1 − x, −y, 2 − z; (vi) x, −1 + y, z; (vii) 1 − x, −1 + y, 2 − z; (viii) x, −y, z.]
	Fig. 2. Chains of Cu-centred polyhedra and SeO3E tetrahedra. O atoms, Cl atoms and Se lone pairs (E) are represented by open, crossed and solid circles, respectively.
	Fig. 3. The (001) layers in Cu3(SeO3)2Cl2, viewed along the b axis. Atoms and polyhedra are drawn as in Fig. 2.

tricopper(II) diselenium(IV) dichloride hexaoxide

Crystal data

Cu3(SeO3)2Cl2	F(000) = 474
Mr = 515.44	Dx = 4.333 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C2y	Cell parameters from 2959 reflections
a = 8.9333 (12) Å	θ = 4.1–30.0°
b = 6.2164 (7) Å	µ = 17.88 mm^−1
c = 7.5815 (12) Å	T = 292 K
β = 110.238 (13)°	Thin plate, green
V = 395.03 (10) Å^3	0.30 × 0.12 × 0.06 mm
Z = 2

Data collection

Oxford Xcalibur3 diffractometer	628 independent reflections
Radiation source: fine-focus sealed tube	610 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.051
ω scan at different ϕ	θmax = 30.0°, θmin = 4.1°
Absorption correction: gaussian CrysAlis CCD (Oxford Diffraction, 2006) and CrysAlis RED (Oxford Diffraction, 2006)	h = −12→12
Tmin = 0.113, Tmax = 0.554	k = −8→8
2959 measured reflections	l = −10→10

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)] = 0.026	w = 1/[σ^2(Fo^2) + (0.0353P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.076	(Δ/σ)max = 0.001
S = 1.65	Δρmax = 1.34 e Å^−3
628 reflections	Δρmin = −0.73 e Å^−3
39 parameters	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0254 (18)

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
Se	0.34244 (4)	0.5000	1.26285 (5)	0.00972 (18)
Cu1	0.5000	0.5000	1.0000	0.0165 (2)
Cu2	0.43034 (6)	0.0000	1.17961 (10)	0.0213 (2)
Cl	0.28908 (14)	0.0000	1.37017 (19)	0.0257 (3)
O1	0.41995 (18)	0.3090 (3)	1.1506 (3)	0.0146 (4)
O2	0.3508 (3)	0.0000	0.8657 (4)	0.0131 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Se	0.0098 (2)	0.0092 (2)	0.0115 (2)	0.000	0.00539 (15)	0.000
Cu1	0.0247 (4)	0.0076 (3)	0.0268 (4)	0.000	0.0211 (3)	0.000
Cu2	0.0347 (3)	0.0068 (3)	0.0356 (4)	0.000	0.0288 (3)	0.000
Cl	0.0313 (5)	0.0245 (5)	0.0321 (6)	0.000	0.0247 (5)	0.000
O1	0.0187 (9)	0.0085 (8)	0.0223 (8)	0.0006 (5)	0.0144 (7)	−0.0003 (7)
O2	0.0086 (9)	0.0148 (10)	0.0174 (11)	0.000	0.0063 (8)	0.000

Geometric parameters (Å, º)

Se—O2^i	1.663 (2)	Cu1—Se^iv	2.8095 (5)
Se—O1	1.7379 (17)	Cu2—O1^v	1.9321 (18)
Se—O1^ii	1.7379 (17)	Cu2—O1	1.9321 (18)
Se—Cu1	2.8095 (5)	Cu2—O2^vi	2.099 (2)
Cu1—O1^ii	1.9468 (17)	Cu2—Cl	2.2229 (15)
Cu1—O1^iii	1.9468 (17)	Cu2—O2	2.236 (3)
Cu1—O1	1.9468 (17)	O2—Se^i	1.663 (2)
Cu1—O1^iv	1.9468 (17)	O2—Cu2^vi	2.099 (2)
O2^i—Se—O1	102.81 (9)	Se^iv—Cu1—Se	180.0
O2^i—Se—O1^ii	102.81 (9)	O1^v—Cu2—O1	167.71 (13)
O1—Se—O1^ii	86.18 (11)	O1^v—Cu2—O2^vi	89.56 (5)
O2^i—Se—Cu1	104.93 (9)	O1—Cu2—O2^vi	89.56 (5)
O1^ii—Cu1—O1^iii	180.000 (1)	O1^v—Cu2—Cl	93.33 (5)
O1^ii—Cu1—O1	75.15 (10)	O1—Cu2—Cl	93.33 (5)
O1^iii—Cu1—O1	104.85 (10)	O2^vi—Cu2—Cl	151.25 (9)
O1^ii—Cu1—O1^iv	104.85 (10)	O1^v—Cu2—O2	83.91 (6)
O1^iii—Cu1—O1^iv	75.15 (10)	O1—Cu2—O2	83.91 (6)
O1—Cu1—O1^iv	180.0	O2^vi—Cu2—O2	78.28 (10)
O1^ii—Cu1—Se^iv	142.37 (5)	Cl—Cu2—O2	130.47 (7)
O1^iii—Cu1—Se^iv	37.63 (5)	Se—O1—Cu2	129.43 (10)
O1—Cu1—Se^iv	142.37 (5)	Se—O1—Cu1	99.21 (8)
O1^iv—Cu1—Se^iv	37.63 (5)	Cu2—O1—Cu1	131.30 (9)
O1^ii—Cu1—Se	37.63 (5)	Se^i—O2—Cu2^vi	137.82 (15)
O1^iii—Cu1—Se	142.37 (5)	Se^i—O2—Cu2	120.45 (12)
O1—Cu1—Se	37.63 (5)	Cu2^vi—O2—Cu2	101.72 (10)
O1^iv—Cu1—Se	142.37 (5)

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