Comment

Copper(I) cyanide frameworks, like those of other transition-metal cyanides, can be viewed as constructed from M(CN)_x structural building blocks. For copper(I), a range of potential building blocks are known, including simple species, such as linear [Cu(CN)₂]^-, trigonal [Cu(CN)₃]^2- and tetrahedral [Cu(CN)₄]^3- units, and larger fragments, such as -(CuCN)- chains. These units have well defined geometries and can be assembled to form new solids by combining with themselves, in association with charge-balancing species where necessary, or with other complex metal ions or organic species, e.g. Lewis bases such as amines, to generate one-, two- and three-dimensional frameworks.

The present work is a continuation of our investigations of copper(I) cyanide materials prepared in the presence of alkali-metal cations (Chippindale et al., 2004; Pohl et al., 2006). Cs₂Cu₃(CN)₅ reported here has the same layer structure as K₂Cu₃(CN)₅, prepared previously in acetonitrile under solvothermal conditions (Pohl et al., 2006).

The layer structure of Cs₂Cu₃(CN)₅ can be described in terms of -(Cu2CN)- chains running along the b axis and linked through bridging [Cu1(CN)₃]^2- units to generate a network of (CuCN)₈ rings within the layers. The layers stack in an ABAB fashion along the c axis (Fig. 1). Cs⁺ cations lie between the layers bonded to 12 cyanide groups, with Cs-C/N distances in the range 3.11 (2)-3.58 (3) Å.

There are two crystallographically distinct Cu atoms, both of which have approximately trigonal-planar coordination (Fig. 2). Atom Cu1, on a special position of site symmetry 2, is bonded to two equivalent bridging cyanide groups, Z3Z4, through the Z4 ends of the groups. The Z3Z4 unit shows partial `head-to-tail' disorder, as determined by refinement, with Z3 having occupancy 0.78 (4) for C3 and 0.22 (4) for N3 and Z4 having occupancy 0.22 (4) for C4 and 0.78 (4) for N4. The coordination around Cu1 is completed by a third cyanide group, C1N1, bonded as a terminal group to Cu1 through C1. Atom Cu2, sited on a general position, bonds directly to C2, N2 and Z3 and is also approximately trigonal planar, although the geometry around Cu2 is less regular than that found for Cu1. The refinement of site occupancies for the cyanide group C2N2 indicates that the C and N atoms are fully ordered. The greater deviation from linearity of the Cu2-N2C2 angle compared with the Cu2-C2N2 angle in Cu2-C2N2-Cu2ⁱⁱⁱ (symmetry code as in Table 1) confirms this assignment: strong - interactions between a metal and the C end of a cyanide usually result in a smaller deviation from linearity of the M-C-N angle than the M'-N-C angle (Vahrenkamp et al., 1997).

Figure 1 A projection of the crystal structure along the c axis, showing layers stacked as ABAB with Cs+ cations between the layers. Key: Cu atoms are black, Cs orange, C green, N blue and Z (C or N of a disordered cyanide group) cyan.

Figure 2 A (Cu(CN)8 ring from the [Cu3(CN)5]2- layer, showing the approximately trigonal-planar coordination of atoms Cu1 and Cu2. The terminal cyanide group C1N1 points into the centre of the ring. Key as given for Fig. 1. Displacement ellipsoids are drawn at the 50% probability level.

Experimental

Crystals of Cs₂Cu₃(CN)₅ were prepared at 293 K. KCN (1.30 g, 20.0 mmol), CuCN (0.46 g, 5.1 mmol) and CsNO₃ (1.94 g, 10.0 mmol) were dissolved in deionized water (15 ml) to form a colourless solution. On addition of 1 M H₂SO₄ (7.4 ml), a white precipitate formed immediately. This was subsequently identified as Cs₂Cu₃(CN)₅ using powder X-ray diffraction. The precipitate was allowed to stand in the solution at room temperature, and after three weeks colourless rectangular blocks of Cs₂Cu₃(CN)₅ had grown. The crystals were filtered off, washed with water and allowed to dry in the air. A powder X-ray diffraction pattern of the ground crystals confirmed that the product was monophasic. IR data (Nujol mull): (CN) 2140 (m), 2104 (s), 2098 (s) cm^-1.

Crystal data

Cs2Cu3(CN)5 Mr = 586.54 Monoclinic, C 2/c a = 17.8156 (9) Å b = 8.0962 (15) Å c = 8.3890 (8) Å = 91.771 (8)° V = 1209.4 (3) Å3 Z = 4 Dx = 3.221 Mg m-3 Mo K radiation = 11.13 mm-1 T = 150 K Block, colourless 0.24 × 0.12 × 0.08 mm

Data collection

Oxford Gemini S Ultra diffractometer /2 scans Absorption correction: multi-scan (ABSPACK; Oxford Diffraction, 2006) Tmin = 0.21, Tmax = 0.41 9301 measured reflections 1343 independent reflections 1123 reflections with I > 3(I) Rint = 0.021 max = 28.5°

Refinement

Refinement on F R[F2 > 2(F2)] = 0.017 wR(F2) = 0.020 S = 1.08 1123 reflections 71 parameters Modified Chebychev polynomial (Watkin, 1994; Prince, 1982) with coefficients: 16.0, -11.9, 11.5, 1.63 (/)max = 0.003 max = 0.88 e Å-3 min = -0.70 e Å-3

Table 1 Selected geometric parameters (Å, °) Z denotes a disordered cyanide group. Cu1-C1 1.915 (4) Cu1-Z4 1.951 (3) Cu2-C2 1.912 (3) Cu2-N2i 2.016 (3) Cu2-Z3 1.916 (3) N1-C1 1.151 (6) N2-C2 1.145 (4) Z3-Z4 1.156 (4) Z4ii-Cu1-Z4 114.60 (16) Z4-Cu1-C1 122.70 (8) N2i-Cu2-Z3 111.60 (11) N2i-Cu2-C2 110.50 (12) C2-Cu2-Z3 137.26 (12) Cu2iii-N2-C2 160.7 (3) Cu2-Z3-Z4 173.8 (3) Cu1-Z4-Z3 168.7 (3) N1-C1-Cu1 180 N2-C2-Cu2 176.7 (3) Symmetry codes: (i) ; (ii) ; (iii) .

The orientations of the three distinct CN groups were investigated as follows. Each CN was modelled as ZxZy with starting values for the occupancies of both Zx and Zy set to (0.5 C + 0.5 N). The site occupancies were then refined subject to the constraints that the total occupancy for each site was 1.00 and the displacement parameters of C and N on the same site were equal. Cyanide groups C1N1 and C2N2 were found to be fully ordered and the occupancies of these groups were fixed in subsequent refinements. The occupancies in the remaining bridging Z3Z4 group have refined values for Z3 of 0.78 (4) for C3 and 0.22 (4) for N3, and for Z4 of 0.22 (4) for C4 and 0.78 (4) for N4.

Data collection: CrysAlisPro, (Oxford Diffraction, 2006); cell refinement: CrysAlisPro; data reduction: CrysAlisPro; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: CRYSTALS.