Comment

In recent years, studies of the synthesis and properties of semiconductor materials such as CdS and CdSe have become an area of interest owing to the great scope for fundamental understanding of materials as well as potential technological applications (Zhang et al., 1999), such as light-emitting devices, non-linear optical devices, solar cells and biological labels. As a result, the search for new precursors, such as salts containing [Cd(SCN)₃]^-, is receiving much attention. As the d¹⁰ configuration and softness of Cd^II permit a wide variety of geometries and coordination numbers, especially with the ambidentate ligand thiocyanate (SCN^-), various structural types have been observed. Which structural type occurs depends on the size, shape and symmetry of the counter-cations and also on the ratio of Cd²⁺ to SCN^- ions. Thus, the structures of a number of one-dimensional single chains (Zhang et al., 2001), two-dimensional networks (Zhang et al., 1997) and three-dimensional structures (Thiele & Messer, 1980) have been reported and reviewed (Sun et al., 2001). Of special interest are the low-dimensional structural motifs, since these relate to highly anisotropic physical properties. In continuation of our interest in the supramolecular chemistry of salts of simple metal complexes (Sharma et al., 2005, 2006), the synthesis and characterization of the title compound, (I), was undertaken.

For (I), structure determination revealed the presence of four crystallographically independent components in the solid state: two [(C₆H₅CH₂)N(CH₃)₃]⁺ cations and two [Cd(SCN)₃]^- anions (Fig. 1). Each Cd^II ion is 3N,3S-hexacoordinated, and adopts a slightly deformed fac octahedral geometry. Thus, each S atom is trans to an N atom. One of the thiocyanate ions (S6/C6/N6) appears to be rotationally disordered about its central C atom, which modifies the Cd^II coordination geometry at 9% of the Cd2 metal sites. Both the Cd-S and Cd-N bond lengths show considerable variation (Table 1). Similar distances (Cd-S = 2.688-2.743 Å and Cd-N = 2.279-2.379 Å) are observed in [(CH₃)₄N][Cd(SCN)₃], which is also 3N,3S-coordinated (Kuniyasu et al., 1987). The average Cd-N-C and Cd-S-C angles in (I) (142.11 and 98.93°, respectively) are also comparable with those in [(CH₃)₄N][Cd(SCN)₃]. The {[Cd(SCN)₃]^-}_n chains (Fig. 2) propagate along the b-axis direction, with [Cd(SCN)₆] octahedra linked in a face-sharing manner via the shared SCN^- ligands. The [(C₆H₅CH₂)N(CH₃)₃]⁺ cations occupy positions between the chains. It is generally believed that the relative arrangement of the anionic {[Cd(SCN)₃]^-}_n chains is strongly influenced by the size and shape of the cation. With larger cations, parallel alignment of the {[Cd(SCN)₃]^-}_n chains is expected; this is observed in (I).

Figure 1 The contents of the asymmetric unit of (I), with displacement ellipsoids drawn at the 50% probability level for non-H atoms. The minor disorder component is indicated by dashed bonds.

Figure 2 The polymeric {[Cd(SCN)3]-}n chains extending along the b-axis direction.

Experimental

Analytical grade reagents were used without any further purification. Benzyltrimethylammonium chloride (1.0 g, 0.005 mol) was dissolved in 10 ml water, while CdCl₂ (0.98 g, 0.004 mol) and ammonium thiocyanate (1.22 g, 0.016 mol) were dissolved in 20 ml water by mechanical stirring. The solutions were mixed and a curd-like white solid precipitated immediately. This was filtered off and dried in air. Crystals of (I) were obtained after redissolving the white solid in an acetone-water mixture (1:1) at room temperature. The salt decomposes at 393 K and is insoluble in organic solvents (C₂H₅OH, CCl₄ and CH₃Cl), but soluble in DMSO and hot water. IR (KBr, , cm^-1): 2116 (s), 2087 (s, SCN), 1660 (m), 1553 (m), 1081 (s), 1028 (s), 1002 (s). ¹H NMR (d₆-DMSO, 298 K): 7.2 (s, 5H, HAr), 4.2 (s, 2H, ArCH₂), 2.6 (s, 9H, CH₃). ¹³C NMR (d₆-DMSO, 298 K): 128-133 (Ar), 126 (SCN), 68 (ArC), 25 (CH₃).

Crystal data

(C10H16N)[Cd(SCN)3] Mr = 436.88 Monoclinic, P 21 a = 9.9668 (3) Å b = 10.8210 (3) Å c = 16.5299 (5) Å = 102.351 (2)° V = 1741.50 (9) Å3 Z = 4 Dx = 1.666 Mg m-3 Mo K radiation = 1.61 mm-1 T = 123 (2) K Needle, colourless 0.35 × 0.08 × 0.06 mm

Data collection

Nonius KappaCCD diffractometer and scans Absorption correction: multi-scan (SORTAV; Blessing, 1997) Tmin = 0.880, Tmax = 0.908 31141 measured reflections 7120 independent reflections 5777 reflections with I > 2(I) Rint = 0.060 max = 27.1°

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.041 wR(F2) = 0.067 S = 1.06 7120 reflections 394 parameters H-atom parameters constrained w = 1/[2(Fo2) + (0.0138P)2 + 2.3495P] where P = (Fo2 + 2Fc2)/3 (/)max = 0.001 max = 0.92 e Å-3 min = -0.57 e Å-3 Absolute structure: Flack (1983), 3074 Friedel pairs Flack parameter: -0.04 (2)

Table 1 Selected geometric parameters (Å, °) Cd1-N1i 2.293 (5) Cd1-N3ii 2.320 (5) Cd1-N2ii 2.369 (5) Cd1-S3 2.6749 (15) Cd1-S1 2.7231 (15) Cd1-S2 2.7350 (15) Cd2-N4iii 2.294 (5) Cd2-N5iv 2.341 (5) Cd2-N6iv 2.361 (6) Cd2-S5 2.6925 (15) Cd2-S4 2.7097 (15) Cd2-S6 2.762 (2) C1-S1-Cd1 94.81 (18) C2-S2-Cd1 99.67 (17) C3-S3-Cd1 99.07 (18) C4-S4-Cd2 95.24 (18) C5-S5-Cd2 98.12 (18) C1-N1-Cd1ii 155.1 (4) C2-N2-Cd1i 144.8 (4) C3-N3-Cd1i 146.7 (4) C4-N4-Cd2iv 155.3 (4) C5-N5-Cd2iii 149.3 (4) C6-S6-Cd2 102.2 (2) C6-N6-Cd2iii 140.2 (6) Symmetry codes: (i) ; (ii) ; (iii) ; (iv) .

One SCN^- ligand (S6/C6/N6) was modelled as disordered by a rotation about the C atom, giving two S and two N sites. The site occupancies of the two components were refined to 0.911 (7):0.089 (7). All H atoms were placed in geometrically idealized positions and refined using a riding model: C-H = 0.95 Å for CH, 0.99 Å for CH₂ and 0.98 Å for CH₃; U_iso(H) = 1.2U_eq(C) for CH and CH₂, and U_iso(H) = 1.5U_eq(C) for CH₃. We have noted that many crystals from the sample were twinned so that they appeared C-centred monoclinic.

Data collection: COLLECT (Hooft, 1988) and DENZO (Otwinowski & Minor, 1997); cell refinement: DENZO and COLLECT; data reduction: DENZO; 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.