Ca[Ag₂(SCN)₄]·2H₂O

Calcium tetra­thio­cyanato­diargentate(I) dihydrate, Ca[Ag₂(SCN)₄]·2H₂O, contains eight-membered Ag₄S₄ rings bonded together through shared atoms to form layers parallel to (100). The thio­cyanate groups link the layers to Ca-O chains running parallel to the c axis. The Ca atom is located on a twofold rotation axis parallel to b and is surrounded by four water molecules of crystallization and four thio­cyanate N atoms in a distorted square antiprism.

S1. Comment

A large synthetic study concerning thiocyanates was published at the beginning of the last century (Wells, 1902), and the title compound, Ca[Ag₂(SCN)₄]·2H₂O, was then synthesized for the first time. Subsequently, the formation of the title compound has been studied concisely (McKerrow et al., 1946), but no other information about the title compound can be found in the literature. Both calcium and silver form simple thiocyanates. Ca(SCN)₂ crystallizes in space group C2/c (Wickerled & Larsen, 2002), and AgSCN crystallizes in two polymorphic forms in space groups Pmnn (Smith et al., 1982) and C2/c (Lindqvist, 1957; Zhu et al., 2003). Moreover, Ca(SCN)₂·2H₂O (Wickerled & Larsen, 2002) and Ca(SCN)₂·4H₂O (Held & Bohaty, 2001) are known and crystallize in space groups Pnma and P3₂21, respectively. We have synthesized some compounds similar to the title compound, such as potassium silver thiocyanates Ag K(SCN)₂ (Valkonen & Güneş, 2001; Güneş et al., 2002a) and Ag K₂(SCN)₃ (Güneş et al., 2002a), and triple thiocyanates Cs[AgZn(NCS)₄] (Güneş & Valkonen, 2002a), Cs₂[AgZn(SCN)₅] (Güneş & Valkonen, 2002b), Cs[Ag₄Zn₂(SCN)₉] (Güneş & Valkonen, 2002c) and CaCs₂[Ag₂(SCN)₆]·2H₂O (Güneş et al., 2002b), and have investigated their structural properties. Several compounds similar to the title compound are also known in the literature. The crystal structures of K[Ag(SCN)₂] (Krautscheid & Gerber, 2001; Valkonen & Güneş, 2001), K₃[Ag(SCN)₄] and K₄[Ag(SCN)₆] (Krautscheid & Gerber, 2001) have been published recently, whereas the structures of NH₄Ag(SCN)₂ (Lindqvist & Strandberg, 1957; Hall et al., 1983), Rb₂Ag(SCN)₃ and Cs₂Ag(SCN)₃ (Thiele & Kehr, 1984) have been known for decades.

Our interest in silver and other thiocyanates arises from the fact that some of these complexes, such as Cs₃Sr[Ag₂(SCN)₇] and Cs₃Ba[Ag₂(SCN)₇] (Bohaty & Fröhlich, 1992) along with other thiocyanates like ZnCd(SCN)₄, ZnHg(SCN)₄, CdHg(SCN)₄ and MnHg(SCN)₄ (Wang et al., 2001), have been found to have a non-centrosymmetric crystal structure, which can furthermore possess some very interesting optical, electro-optic and electrostrictive properties. These properties could be utilized, for example, in telecommunications, optical computing, optical information processing, optical disk data storage, laser remote sensing, laser-driven fusion, color displays, medical diagnostics and so on. The idea is based on the capability of these materials to convert IR-laser radiation efficiently to visible and UV wavelengths, and especially their highly efficient second-harmonic generation of blue–violet light (Wang et al., 2001). Another factor that makes thiocyanates such tempting materials for research is their chemical nature. Because thiocyanates are neither purely organic nor purely inorganic compounds, but a kind of semi-organic compound instead, they might possess advantages over both organic and inorganic materials. We hope to synthesize more of these compounds in the future, as they may also have the properties mentioned above.

In the present compound, the Ca atom is located on a twofold rotation axis at (1/2, 0.58417, 1/4), and is coordinated with four N atoms and four O atoms of the water molecules of crystallization. The coordination polyhedron of Ca is a distorted square antiprism. The Ag atom is surrounded by four S atoms and its coordination polyhedron is a distorted tetrahedron. There are two crystallographically independent thiocyanate groups. Each group is bonded to two Ag atoms through the S atom at one end and to one Ca atom through the N atom at the other end. The average S—C bond length of the thiocyanate group is 1.67 (4) Å and the average C—N bond length is 1.15 (5) Å. The angles of the thiocyanate groups at C are both close to 180 °.

The title compound contains Ag₄S₄ eight-membered rings in chair conformations. These rings are bonded together through two shared Ag atoms and one shared S atom to form layers parallel to (100) (Fig. 1). On the other hand, two Ca atoms and two O atoms form a Ca₂O₂ rhombus. The rhombuses are further linked together at the shared Ca atoms to form chains parallel to the c axis (Fig. 2). The chains are combined with the Ag₄S₄ layer via the thiocyanate groups.

S2. Experimental

Silver thiocyanate was obtained from the Aldrich Chemical Company Inc. Calcium thiocyanate dihydrate was synthesized as described by Güneş et al., 2002b. All of the chemicals used were of analytical reagent grade. Ca[Ag₂(SCN)₄]·2H₂O was synthesized by dissolving Ca(SCN)₂·2H₂O (4.35 g) in deionized water (3 ml) and dissolving AgSCN (1.30 g) into this solution. The solution was filtered through a dense sinter glasss (No. 4). Within 2 d, colorless blocks of Ca[Ag₂(SCN)₄]·2H₂O crystallized out of the filtrate.

S3. Refinement

Multi-scan absorption correction (Blessing, 1995) was performed but not applied. The absorption correction was found to have no significant effect on the refinement results.

Computing details

Data collection: Collect (Nonius, 1997–2000); cell refinement: HKL DENZO (Otwinowski & Minor 1997); data reduction: HKL DENZO and SCALEPACK (Otwinowski & Minor 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 2000); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. A view of the layer formed by Ag/S eight-membered rings, sharing two common Ag atoms and one common S atom with every adjacent Ag/S ring. Displacement ellipsoids are shown at the 50% probability level.
	Fig. 2. A view of the Ca/O chain, with 50% probability displacement ellipsoids. [Symmetry codes: (i) 1/2 − x,1/2 + y,1/2 − z; (ii) x,1 + y,z]

calcium tetrathiocyanato diargentate dihydrate

Crystal data

CaAg2(SCN)4·2H20	F(000) = 1000
Mr = 524.17	Dx = 2.575 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 5864 reflections
a = 23.0272 (5) Å	θ = 1.0–30.0°
b = 7.5684 (2) Å	µ = 3.89 mm^−1
c = 7.8595 (2) Å	T = 293 K
β = 99.266 (2)°	Block, colourless
V = 1351.87 (6) Å^3	0.15 × 0.1 × 0.1 mm
Z = 4

Data collection

Nonius KappaCCD diffractometer	Rint = 0.054
Detector resolution: 9 pixels mm^-1	θmax = 30.0°, θmin = 1.8°
CCD rotation images, thick slices scans	h = −32→29
6337 measured reflections	k = −9→10
1958 independent reflections	l = −11→9
1552 reflections with I > 2σ(I)

Refinement

Refinement on F^2	All H-atom parameters refined
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0591P)^2] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.035	(Δ/σ)max < 0.001
wR(F^2) = 0.112	Δρmax = 1.39 e Å^−3
S = 1.17	Δρmin = −0.96 e Å^−3
1958 reflections	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
87 parameters	Extinction coefficient: 0.0168 (8)
0 restraints

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ag	0.286151 (13)	0.08328 (5)	0.27223 (4)	0.04434 (19)
Ca	0.5000	0.58417 (12)	0.2500	0.0206 (2)
S1	0.20652 (4)	−0.10335 (12)	0.10494 (11)	0.0290 (2)
S2	0.35314 (4)	0.10044 (11)	0.01757 (11)	0.0274 (2)
C1	0.14304 (14)	−0.0523 (4)	0.1697 (4)	0.0239 (7)
C2	0.40177 (15)	−0.0561 (5)	0.0875 (4)	0.0272 (7)
N1	0.09838 (13)	−0.0207 (5)	0.2093 (4)	0.0339 (7)
N2	0.43670 (14)	−0.1614 (5)	0.1360 (4)	0.0395 (7)
O	0.47943 (13)	0.3247 (4)	0.0484 (3)	0.0275 (6)
H1	0.495 (2)	0.248 (5)	0.059 (5)	0.024 (12)*
H2	0.448 (2)	0.291 (7)	0.036 (5)	0.048 (15)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ag	0.0320 (2)	0.0485 (3)	0.0514 (3)	−0.00843 (13)	0.00335 (15)	−0.00640 (13)
Ca	0.0162 (4)	0.0247 (5)	0.0220 (4)	0.000	0.0061 (3)	0.000
S1	0.0200 (4)	0.0330 (5)	0.0362 (5)	−0.0019 (3)	0.0108 (3)	−0.0044 (3)
S2	0.0250 (4)	0.0255 (5)	0.0319 (4)	0.0021 (3)	0.0055 (3)	0.0008 (3)
C1	0.0197 (15)	0.0241 (18)	0.0282 (15)	0.0001 (13)	0.0046 (12)	0.0023 (12)
C2	0.0198 (16)	0.032 (2)	0.0307 (16)	−0.0025 (13)	0.0060 (12)	−0.0014 (13)
N1	0.0218 (14)	0.0378 (19)	0.0431 (16)	0.0048 (13)	0.0083 (12)	0.0045 (14)
N2	0.0306 (17)	0.039 (2)	0.0487 (18)	0.0095 (15)	0.0059 (13)	0.0023 (15)
O	0.0222 (13)	0.0305 (16)	0.0306 (12)	0.0004 (12)	0.0062 (9)	−0.0006 (11)

Geometric parameters (Å, º)

Ag—S1	2.5119 (9)	S1—C1	1.669 (3)
Ag—S1^i	2.5560 (10)	S1—Ag^ix	2.5560 (10)
Ag—S2^ii	2.6595 (9)	S2—C2	1.661 (4)
Ag—S2	2.7197 (9)	S2—Ag^x	2.6595 (9)
Ca—N1^iii	2.469 (3)	C1—N1	1.147 (4)
Ca—N1^i	2.469 (3)	C2—N2	1.153 (5)
Ca—N2^iv	2.493 (3)	N1—Ca^xi	2.469 (3)
Ca—N2^v	2.493 (3)	N2—Ca^xii	2.493 (3)
Ca—O	2.520 (3)	O—Ca^vii	2.561 (2)
Ca—O^vi	2.520 (3)	O—H1	0.68 (4)
Ca—O^vii	2.561 (2)	O—H2	0.75 (5)
Ca—O^viii	2.561 (2)
S1—Ag—S1^i	135.87 (2)	O—Ca—O^vii	71.21 (10)
S1—Ag—S2^ii	112.01 (3)	O^vi—Ca—O^vii	137.90 (9)
S1^i—Ag—S2^ii	102.47 (3)	N1^iii—Ca—O^viii	120.94 (9)
S1—Ag—S2	95.42 (3)	N1^i—Ca—O^viii	70.06 (10)
S1^i—Ag—S2	102.88 (3)	N2^iv—Ca—O^viii	69.30 (10)
S2^ii—Ag—S2	103.12 (3)	N2^v—Ca—O^viii	86.41 (10)
N1^iii—Ca—N1^i	142.52 (17)	O—Ca—O^viii	137.90 (9)
N1^iii—Ca—N2^iv	78.42 (12)	O^vi—Ca—O^viii	71.21 (10)
N1^i—Ca—N2^iv	134.20 (11)	O^vii—Ca—O^viii	148.75 (14)
N1^iii—Ca—N2^v	134.20 (11)	C1—S1—Ag	108.17 (12)
N1^i—Ca—N2^v	78.42 (12)	C1—S1—Ag^ix	96.17 (12)
N2^iv—Ca—N2^v	78.85 (17)	Ag—S1—Ag^ix	109.02 (3)
N1^iii—Ca—O	75.61 (11)	C2—S2—Ag^x	98.53 (12)
N1^i—Ca—O	75.38 (10)	C2—S2—Ag	98.89 (12)
N2^iv—Ca—O	150.20 (10)	Ag^x—S2—Ag	100.20 (3)
N2^v—Ca—O	109.65 (10)	N1—C1—S1	177.6 (3)
N1^iii—Ca—O^vi	75.38 (10)	N2—C2—S2	178.2 (3)
N1^i—Ca—O^vi	75.61 (11)	C1—N1—Ca^xi	169.7 (3)
N2^iv—Ca—O^vi	109.65 (10)	C2—N2—Ca^xii	171.7 (3)
N2^v—Ca—O^vi	150.20 (10)	Ca—O—Ca^vii	108.79 (10)
O—Ca—O^vi	77.59 (13)	Ca—O—H1	123 (3)
N1^iii—Ca—O^vii	70.06 (10)	Ca^vii—O—H1	99 (3)
N1^i—Ca—O^vii	120.94 (9)	Ca—O—H2	115 (4)
N2^iv—Ca—O^vii	86.41 (10)	Ca^vii—O—H2	107 (3)
N2^v—Ca—O^vii	69.30 (10)	H1—O—H2	102 (5)

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