Calcium dicaesium silver thio­cyanate dihydrate

The title compound, CaCs₂[Ag₂(SCN)₆]·2H₂O, forms a continuous structure where the Ag atoms form chains with S atoms in the c-axis direction. The chains are bonded together through Cs and Ca atoms. The crystal water of the structure is bonded to the Ca atoms, which lie on centers of symmetry.

S1. Comment

The title compound has been known since the beginning of the last century (Wells, 1902; Wells, 1922). At that time, most studies were synthetic and analytical. Since those days, many crystal structures of thiocyanates have been solved. The crystal structure of the title compound has not been reported before. Calcium, caesium and silver all form simple thiocyanates. CsSCN crystallizes in space group Pnma and AgSCN in two polymorphic forms in space groups Pmnn and C2/c. The crystal structure of Ca(SCN)₂·2H₂O is not available in the literature because while crystallizing it forms powder instead of large single crystals, and until recently it has not usually been possible to determine the crystal structure of a powder sample. However, a quite recently published crystal structure determination of Ca(SCN)₂·4H₂O (Held & Bohaty, 2001) is now available in the literature.

The main reason why we are so interested in thiocyanates like the title compound is that some of the thiocyanates of silver have very interesting optical, electro-optic and electrostrictive properties (Bohaty & Fröhlich, 1992), which are interrelated with non-centrosymmetric crystal structure. Earlier, we have reported the crystal structures of KAg(SCN)₂ (Valkonen & Güneş, 2001) and Cs₂[AgZn(SCN)₅] (Güneş & Valkonen, 2001) of which the Cs₂[AgZn(SCN)₅] has a non-centrosymmetric crystal structure. We presume that an odd number of thiocyanate groups could lead to a noncentrosymmetric crystal structure as there are also other thiocyanate complexes of silver such as Cs₃Sr[Ag₂(SCN)₇] and Cs₃Ba[Ag₂(SCN)₇] (Bohaty & Fröhlich, 1992), which have been found to have the same property. We look forward to synthesizing more of these compounds in the future as it might turn out that they will indeed have non-centrosymmetric crystal structures and as a consequence some chemically useful optical, electro-optic and electrostrictive properties.

In the present compound caesium is nine coordinated, with four S, four N and one O atom around it. Ag is tetrahedrally four coordinated being surrounded by four S atoms. The tetrahedron around Ag is slightly distorted (see the tables). Ca is octahedrally six coordinated with two O and four N atoms around it. The octahedron around Ca is nearly regular (see the tables). The average S—C distance of the thiocyanate group is 1.65 Å and the average C—N distance about 1.15 Å. The angles of the thiocyanate groups (at C) are all close to 180°.

The title compound consists of a continuous structure where the Ag atoms form chains (Fig. 1) in the direction of the c axis, bonded together through the S2 atoms of the bridging thiocyanate groups which bond through N to calcium atoms thereby connecting the Ag—S—Ag chains in the direction of the b axis. There are also two terminal thiocyanate groups attached through S to every silver of the Ag—S—Ag chains. One of the terminal thiocyanate groups of every other Ag of the chain is bonded through N to Ca atoms thereby also connecting the Ag—S—Ag chains. The bonding of the Ag—S—Ag chains is presented in Fig 1. The Cs atoms connect the Ag—S—Ag chains both in the direction of b axis and in the direction of the a axis. The crystal water of the structure is bonded to the Ca atoms. The water molecules are located in the capping positions of the coordination octahedron around calcium.

S2. Experimental

Silver thiocyanate was delivered by the Aldrich Chemical Company Inc. Calcium thiocyanate dihydrate and caesium thiocyanate were synthesized in the research laboratory.

CaCs₂[Ag₂(SCN)₆]·2H₂O was synthesized at room temperature by dissolving 1.45 g Ca(SCN)₂·2H₂O into 1.00 ml deionized water and 2.63 g CsSCN into 2.00 ml deionized water. The Ca(SCN)₂·2H₂O solution was added to CsSCN solution and 2.28 g AgSCN was dissolved into that solution. The dissolution of AgSCN was accelerated by heating the solution on a water bath. The final solution was filtered with a dense sinter glass (No: 4) while still hot. The filtered solution was allowed to cool and evaporate at room temperature. Within two days colorless crystals of CaCs₂[Ag₂(SCN)₆]·2H₂O were formed.

Calcium thiocyanate dihydrate was synthesized at room temperature by suspending 60.00 g Ca(OH)₂ into 150 ml deionized water and 102.53 g NH₄SCN into 65.0 ml deionized water. The solutions were mixed and the mixture was heated under magnetic stirring until the smell of ammonia was not sensed anymore (about 2 h). The solution was filtered while still hot. The filtered solution was evaporated to dryness in 350–360 K using a vacuum pump on a water bath. Ca(SCN)₂·2H₂O was stored in a desiccator due to its hygroscopicity.

Caesium thiocyanate was synthesized by dissolving 6.98 g NH₄SCN into 20.0 ml deionized water and 15.0 g C s₂CO₃ into 55.0 ml deionized water. The solutions were mixed at room temperature and the mixture was heated under magnetic stirring until the smell of ammonia was not sensed anymore (about 2.5 h). The residue was evaporated close to dryness at room temperature. CsSCN was dried using a vacuum pump and was stored in an desiccator due to its hygroscopicity.

Calcium hydroxide was delivered by the Merck KGaA and ammonium thiocyanate and caesium carbonate by the Aldrich Chemical Company Inc.

S3. Refinement

The s.u.'s of the cell constants indicate the internal consistency of the measurements themselves i.e. the precision of the measurement, not their accuracy. During the crystal structure determination absorption correction for the measured intensities was calculated 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 SCALEPACK (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 2.1d (Brandenburg, 2000); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

Fig. 1. The Ag—S—Ag chains connected together through Ca atoms. The Cs atom is omitted from the figure for clarity. The displacement ellipsoids are represented in 50% probability.

dicaesium calcium disilver thiocyanate dihydrate

Crystal data

CaCs2[Ag2(SCN)6].2(H2O)	F(000) = 836
Mr = 906.15	Dx = 2.592 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 7.8696 (1) Å	Cell parameters from 5696 reflections
b = 19.0910 (2) Å	θ = 1.0–28.3°
c = 7.7411 (1) Å	µ = 5.55 mm^−1
β = 93.338 (1)°	T = 293 K
V = 1161.04 (2) Å^3	Prism, colorless
Z = 2	0.1 × 0.1 × 0.1 mm

Data collection

Nonius KappaCCD diffractometer	2588 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.024
Horizonally mounted graphite crystal monochromator	θmax = 28.3°, θmin = 2.1°
Detector resolution: 9 pixels mm^-1	h = −10→10
CCD scans	k = −25→25
11177 measured reflections	l = −10→10
2881 independent reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.033	All H-atom parameters refined
wR(F^2) = 0.059	w = 1/[σ^2(Fo^2) + 3.4217P] where P = (Fo^2 + 2Fc^2)/3
S = 1.12	(Δ/σ)max = 0.001
2881 reflections	Δρmax = 1.86 e Å^−3
124 parameters	Δρmin = −1.41 e Å^−3
0 restraints	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00268 (16)

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
Cs	0.24537 (4)	0.382640 (15)	0.47667 (4)	0.05609 (11)
Ag	0.72580 (5)	0.283818 (17)	0.51759 (5)	0.05416 (12)
Ca	0.0000	0.5000	0.0000	0.0419 (3)
S1	0.69567 (15)	0.41671 (5)	0.53277 (14)	0.0459 (3)
S2	0.94462 (14)	0.25482 (5)	0.29024 (13)	0.0431 (2)
S3	0.46489 (15)	0.21714 (6)	0.59864 (15)	0.0491 (3)
C1	0.8123 (5)	0.44028 (19)	0.3743 (5)	0.0391 (8)
C2	0.9913 (5)	0.1751 (2)	0.3620 (5)	0.0405 (9)
C3	0.4706 (5)	0.1525 (2)	0.4570 (5)	0.0417 (9)
N1	0.8973 (6)	0.4591 (2)	0.2690 (6)	0.0672 (12)
N2	1.0237 (5)	0.1192 (2)	0.4068 (6)	0.0620 (11)
N3	0.4720 (6)	0.1056 (2)	0.3633 (6)	0.0623 (11)
O	0.2849 (5)	0.4839 (2)	0.1070 (5)	0.0572 (9)
H1	0.336 (9)	0.461 (4)	0.044 (9)	0.09 (2)*
H2	0.335 (8)	0.515 (3)	0.122 (8)	0.07 (2)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs	0.05030 (17)	0.04272 (16)	0.0739 (2)	0.00388 (12)	−0.00835 (13)	−0.00223 (13)
Ag	0.0530 (2)	0.03568 (17)	0.0745 (3)	−0.00441 (14)	0.01010 (17)	−0.00121 (16)
Ca	0.0456 (6)	0.0324 (5)	0.0492 (7)	−0.0003 (5)	0.0156 (5)	0.0114 (5)
S1	0.0604 (6)	0.0301 (5)	0.0496 (6)	−0.0031 (4)	0.0237 (5)	−0.0016 (4)
S2	0.0601 (6)	0.0352 (5)	0.0337 (5)	−0.0037 (4)	0.0016 (4)	0.0008 (4)
S3	0.0552 (6)	0.0408 (5)	0.0526 (6)	−0.0079 (5)	0.0142 (5)	−0.0019 (5)
C1	0.043 (2)	0.0300 (18)	0.045 (2)	0.0024 (16)	0.0068 (17)	0.0067 (16)
C2	0.040 (2)	0.041 (2)	0.040 (2)	−0.0005 (17)	0.0014 (16)	−0.0068 (17)
C3	0.040 (2)	0.039 (2)	0.047 (2)	−0.0067 (17)	0.0124 (17)	0.0068 (18)
N1	0.072 (3)	0.064 (3)	0.068 (3)	−0.001 (2)	0.029 (2)	0.023 (2)
N2	0.064 (3)	0.039 (2)	0.082 (3)	0.0062 (18)	−0.002 (2)	0.003 (2)
N3	0.069 (3)	0.052 (2)	0.068 (3)	−0.009 (2)	0.025 (2)	−0.011 (2)
O	0.052 (2)	0.050 (2)	0.070 (2)	−0.0027 (17)	0.0101 (17)	−0.0203 (18)

Geometric parameters (Å, º)

Cs—N3^i	3.401 (5)	Ca—H2	2.76 (6)
Cs—N1^ii	3.421 (5)	S1—C1	1.637 (4)
Cs—O	3.483 (4)	S1—Cs^iii	3.8599 (10)
Cs—S1	3.6042 (12)	S2—C2	1.655 (4)
Cs—C1^iii	3.609 (4)	S2—Ag^ix	2.7461 (11)
Cs—C1^ii	3.625 (4)	S2—Cs^x	3.6389 (11)
Cs—S2^ii	3.6389 (11)	S3—C3	1.653 (4)
Cs—S3	3.6970 (12)	S3—Cs^i	3.9706 (11)
Cs—N1^iii	3.811 (5)	C1—N1	1.142 (5)
Cs—C2^iv	3.848 (4)	C1—Cs^iii	3.609 (4)
Cs—N2^iv	3.848 (5)	C1—Cs^x	3.625 (4)
Cs—S1^iii	3.8599 (10)	C2—N2	1.146 (5)
Ag—S3	2.5259 (11)	C2—Cs^xi	3.848 (4)
Ag—S1	2.5513 (10)	C3—N3	1.153 (5)
Ag—S2	2.5927 (11)	N1—Ca^x	2.407 (4)
Ag—S2^i	2.7461 (11)	N1—Cs^x	3.421 (5)
Ca—O^v	2.365 (4)	N1—Cs^iii	3.811 (5)
Ca—O	2.365 (4)	N2—Ca^xii	2.397 (4)
Ca—N2^vi	2.397 (4)	N2—Cs^xi	3.848 (5)
Ca—N2^vii	2.397 (4)	N3—Cs^ix	3.401 (5)
Ca—N1^viii	2.407 (4)	O—H1	0.78 (7)
Ca—N1^ii	2.407 (4)	O—H2	0.71 (6)
Ca—H1	2.75 (7)
N3^i—Cs—N1^ii	138.57 (10)	N2^vi—Ca—N1^viii	89.35 (15)
N3^i—Cs—O	128.26 (10)	N2^vii—Ca—N1^viii	90.65 (15)
N1^ii—Cs—S1	137.24 (7)	O^v—Ca—N1^ii	89.19 (15)
O—Cs—S1	82.21 (6)	O—Ca—N1^ii	90.81 (15)
N3^i—Cs—S2^ii	133.54 (7)	N2^vi—Ca—N1^ii	90.65 (15)
N1^ii—Cs—S2^ii	67.38 (8)	N2^vii—Ca—N1^ii	89.35 (15)
O—Cs—S2^ii	97.67 (7)	N1^viii—Ca—N1^ii	180
S1—Cs—S2^ii	141.35 (2)	C1—S1—Ag	100.44 (14)
N3^i—Cs—S3	67.56 (7)	C1—S1—Cs	123.32 (15)
N1^ii—Cs—S3	146.52 (8)	Ag—S1—Cs	84.74 (3)
O—Cs—S3	128.43 (7)	C1—S1—Cs^iii	68.88 (13)
S1—Cs—S3	71.45 (2)	Ag—S1—Cs^iii	167.00 (4)
S2^ii—Cs—S3	79.14 (2)	Cs—S1—Cs^iii	107.17 (3)
N3^i—Cs—N1^iii	69.41 (10)	C2—S2—Ag	96.42 (15)
N1^ii—Cs—N1^iii	69.66 (13)	C2—S2—Ag^ix	97.35 (14)
O—Cs—N1^iii	91.58 (9)	Ag—S2—Ag^ix	99.64 (4)
S1—Cs—N1^iii	96.43 (7)	C2—S2—Cs^x	110.94 (14)
S2^ii—Cs—N1^iii	122.15 (7)	Ag—S2—Cs^x	91.72 (3)
S3—Cs—N1^iii	133.86 (7)	Ag^ix—S2—Cs^x	148.13 (4)
N1^ii—Cs—N2^iv	91.12 (9)	C3—S3—Ag	99.06 (14)
O—Cs—N2^iv	141.52 (9)	C3—S3—Cs	119.92 (15)
S1—Cs—N2^iv	112.92 (7)	Ag—S3—Cs	83.13 (3)
S2^ii—Cs—N2^iv	91.15 (6)	C3—S3—Cs^i	99.24 (13)
S3—Cs—N2^iv	90.00 (6)	Ag—S3—Cs^i	145.06 (4)
N3^i—Cs—S1^iii	83.90 (7)	Cs—S3—Cs^i	112.42 (3)
N1^ii—Cs—S1^iii	70.21 (8)	N1—C1—S1	176.6 (4)
S1—Cs—S1^iii	72.83 (3)	N2—C2—S2	178.0 (4)
S2^ii—Cs—S1^iii	137.23 (3)	N3—C3—S3	177.2 (4)
S3—Cs—S1^iii	142.95 (3)	C1—N1—Ca^x	163.6 (4)
N2^iv—Cs—S1^iii	94.95 (6)	C1—N1—Cs^x	91.0 (3)
S3—Ag—S1	114.24 (4)	Ca^x—N1—Cs^x	104.19 (14)
S3—Ag—S2	129.99 (4)	C1—N1—Cs^iii	71.2 (3)
S1—Ag—S2	108.10 (3)	Ca^x—N1—Cs^iii	108.29 (13)
S3—Ag—S2^i	98.98 (4)	Cs^x—N1—Cs^iii	110.34 (13)
S1—Ag—S2^i	106.71 (4)	C2—N2—Ca^xii	162.7 (4)
S2—Ag—S2^i	93.03 (3)	C2—N2—Cs^xi	81.4 (3)
O^v—Ca—O	180	Ca^xii—N2—Cs^xi	107.41 (13)
O^v—Ca—N2^vi	83.87 (14)	C3—N3—Cs^ix	118.6 (4)
O—Ca—N2^vi	96.13 (14)	Ca—O—Cs	103.39 (12)
O^v—Ca—N2^vii	96.13 (14)	Ca—O—H1	111 (5)
O—Ca—N2^vii	83.87 (14)	Cs—O—H1	106 (5)
N2^vi—Ca—N2^vii	180	Ca—O—H2	117 (5)
O^v—Ca—N1^viii	90.81 (15)	Cs—O—H2	113 (5)
O—Ca—N1^viii	89.19 (15)	H1—O—H2	106 (7)

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