Tetra­aqua­bis(D-camphor-10-sulfon­ato)calcium(II)

The structure of the title compound, [Ca(C₁₀H₁₅O₄S)₂(H₂O)₄], is the first example in which two D-camphor-10-sulfonate anions are coordinated to a metal ion, in this case with direct Ca-O bonding. The mol­ecule has crystallographically imposed twofold symmetry with the Ca atom on the twofold axis. Hydrogen bonds are formed between the coordinated water mol­ecules and the O atoms of the SO₃^- groups of adjacent mol­ecules, leading to the formation of a two-dimensional layered network. The compound displays sharp wavelength-selective transparency in the UV-visible spectrum, offering the potential for application as an optical filter.

S1. Comment

As part of our research on new optical materials, we recently prepared and characterized the magnesium salt of D-camphor-10-sulfonate (Jeremić et al., 2008). It was found to be similar to other divalent metal (Zn^II, Cu^II, Cd^II, Ni^II) salts of this anion that have been reported (Couldwell et al., 1978; Henderson & Nicholson, 1995; Schepke et al., 2007; Zhou et al., 2003). In none of these does the camphorsulfonate anion bond directly to the metal cation. Other examples in which the camphorsulfonate acts as a non-coordinating counterion include the salts of bis[(diphenylphosphino)ethane]rhodium(I) (Dorta et al., 2004), bis(imidazolidine-2-thione)gold(I) (Friedrichs & Jones, 2004) and tris(biguanide)chromium(III) (Brubaker & Webb, 1969). Only a few metal complexes with camphorsulfonate coordinated directly to the metal atom have been described to date (Xiao & Loh, 2007; Failer et al., 1995). These organometallic structures contain only one M—O_sulfonate bond (M = Mo and In). We now report the structure of the title compound, tetraaquabis(D-camphor-10-sulfonato)calcium(II), (I), the first example of a non-organometallic [In what sense is it not organometallic? Please clarify] complex with direct coordination between a metal ion and D-camphor-10-sulfonate. With the considerably low toxicity of camphorsulfonate (Baldacci, 1938; Sinha, 1940), this type of compound may be useful in medicine as a calcium source (Fantoni, 1940).

Compound (I) exists as discrete complexes (Fig. 1) containing one Ca^II cation coordinated in a fairly regular octahedral arrangement by four water molecules in equatorial positions and two sulfonate anions in axial positions. The cation rests on a twofold rotation axis, so the asymmetric unit consists of one half-complex. This is the first example of coordination by D-camphor-10-sulfonate to a divalent metal cation. In the related Mg^II salt (Jeremić et al., 2008) and other transition metal examples, the metal ion is hexacoordinated by water and the metal complexes interact with the sulfonate by hydrogen bonding only. The Ca—O_sulfonate bond lengths (Table 1) are a bit shorter than those of Ca—O_water, indicating the strong covalent interaction between Ca and the O atoms of the sulfonate ligands, contrasting with the ionic structure observed in the [M(H₂O)₆](C₁₀H₁₅O₄S)₂ analogues (M is Zn^II, Cu^II, Cd^II, Ni^II and Mg^II; Couldwell et al., 1978; Henderson & Nicholson, 1995; Schepke et al., 2007; Zhou et al., 2003; Jeremić et al. 2008). There are several known Ca^II complexes with Ca—O_sulfonate bonding, most of which exhibit catena structures (Denis et al., 2001; Francis et al., 2003; Kennedy et al., 2004), while a few are monomeric complexes with coordination number 6 or 7 (Shubnell et al., 1994; Fewings et al., 2001; Kennedy et al., 2001; Barboiu & van der Lee, 2003; Kennedy et al., 2004).

Solid state UV–Vis spectroscopy was carried out on crystals of (I) (Fig. 2). The transparency range is well defined for specific wavelengths and sharply separated from the non-transparent ranges. Considering these spectroscopic characteristsics, it may be possible for this compound to be used as an optical filter material, since the crystals are obtainable in dimensions up to 1 cm (Fig. 3).

The IR spectrum exhibits the following significant bands: weak–medium (CH–, CH₂–, CH₃– aliphatic) 2965 cm^-1; very strong (coordinated water) 3401 cm^-1; medium (C═O) 1735 cm^-1; strong, two bands (R—SO₃¯) 1174 and 1052 cm^-1. The strong band belonging to the coordinated water molecules does not exist in the IR spectrum of the pure acid. The presence of the SO₃^- group (two bands in the IR spectrum) may be confirmed by the fact that the –SO₃H group gives only one band. Band positions for the aliphatic alkyl groups (CH–, CH₂– and CH₃–) and for C═O are not significantly changed compared with the free acid.

The crystal structures of the related complexes [M(H₂O)₆](C₁₀H₁₅O₄S)₂ (M = Zn^II, Cu^II, Cd^II, Ni^II and Mg^II; Couldwell et al., 1978; Henderson & Nicholson, 1995; Schepke et al., 2007; Zhou et al., 2003; Jeremić et al. 2008) consist of [M(H₂O)₆]²⁺ cations and two crystallogaphically independent D-camphor-10-sulfonate anions arranged in alternating layers held together by hydrogen bonds. All the complexes crystallize in the chiral monoclinic space group P2₁.

In contrast, the title Ca^II complex, [Ca(C₁₀H₁₅O₄S)₂(H₂O)₄], (I), crystallizes in the orthorhombic chiral space group C222₁ with four molecules in the elemental cell. A set of nearly linear robust hydrogen bonds (H···O ca 2.0 Å) between coordinated water molecules and the sulfonate O atoms of neighbouring complexes stabilizes the structure (Table 2). Each water molecule coordinated to a given Ca^II ion interacts via hydrogen bonds to two neighbouring D-camphor-10-sulfonate anions (Fig. 4), generating a layered packing of molecules in the unit cell. The complexes are positioned such that the layers, which are parallel to the ab plane, are effectively sulfonate–Ca(H₂O)₄–sulfonate sandwiches that then stack along the c direction. A few weak C—H···O interactions may play a role in holding these layers together to form a kind of three-dimensional network (Table 2; Fig. 4).

S2. Experimental

Elemental analyses were carried out with an Elemental Vario EL III microanalyser. IR spectra were recorded on a Perkin–Elmer FTIR 31725X spectrometer (4000–400 cm^-1). The UV–Vis spectrum was recorded using the double-beam tehnique in the 200–800 nm range on a Specord M40 instrument (Carl Zeiss, Jena). Crystals of appropriate size, form and shape (typically 5 × 5 × 3 mm ) were used for the UV–Vis experiment.

D-Camphorsulfonic acid monohydrate (25.00 g) was dissolved in deionized water (80 ml). Calcium chips were added and the suspension was left at room temperature until vigorous reaction was finished. The solution was filtered, and a further quantity of the acid (1 g) was added to lower the pH to about 2. A titanium wire (0.5 mm diameter, 80 mm long) was added to give a controlled cooling zone as well as to provide nucleation centres. The solution was left at room temperature for two weeks. By this method it was possible to produce crystals with large dimensions (Fig. 3) that are transparent to visible light and suitable for X-ray analysis. Analysis, calculated for C₂₀H₃₈O₁₂S₂Ca: C 41.80, H 6.66, S 11.16%; found: C 41.24, H 6.41, S 10.84%.

S3. Refinement

The water H atoms were refined isotropically and yielded reasonable bond lengths and angles [O—H = 0.76 (3)–0.83 (3) Å]. All other H atoms were positioned geometrically and treated as riding, with C—H = 0.98–1.00 Å. U_iso(H) = 1.2U_eq(C) or 1.5U_eq(O). [Please check added text]

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction Ltd, 2008); cell refinement: CrysAlis RED (Oxford Diffraction Ltd, 2008); data reduction: CrysAlis RED (Oxford Diffraction Ltd, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997) and PLATON (Spek, 2009); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The molecular structure of compound (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. [Symmetry code: (i) Please complete.]
	Fig. 2. Solid-state UV–VIS spectrum of (I). The thickness of the crystal used was 3.853 mm.
	Fig. 3. Crystals of (I).
	Fig. 4. A packing diagram for (I), with hydrogen-bonding interactions shown as dashed lines. H atoms not involved in hydrogen bonding have been omitted for clarity.

Tetraaquabis(D-camphor-10-sulfonato)calcium(II)

Crystal data

[Ca(C10H15O4S2(H2O)4]	F(000) = 1224
Mr = 574.7	Dx = 1.451 Mg m^−3
Orthorhombic, C2221	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: C 2c 2	Cell parameters from 13622 reflections
a = 7.5020 (2) Å	θ = 3.1–32.2°
b = 10.8274 (3) Å	µ = 0.46 mm^−1
c = 32.3927 (9) Å	T = 130 K
V = 2631.17 (12) Å^3	Plate, colourless
Z = 4	0.4 × 0.3 × 0.1 mm

Data collection

Oxford Xcalibur S CCD diffractometer	3263 independent reflections
Graphite monochromator	3008 reflections with I > 2σ(I)
Detector resolution: 16.356 pixels mm^-1	Rint = 0.048
ω and ϕ scans	θmax = 28.3°, θmin = 3.3°
Absorption correction: multi-scan [CrysAlis RED (Oxford Diffraction Ltd, 2008); empirical (using intensity measurements) absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm]	h = −10→10
Tmin = 0.964, Tmax = 1	k = −14→14
30549 measured reflections	l = −43→43

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.032	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.065	w = 1/[σ^2(Fo^2) + (0.0295P)^2 + 1.4883P] where P = (Fo^2 + 2Fc^2)/3
S = 1.05	(Δ/σ)max < 0.001
3263 reflections	Δρmax = 0.35 e Å^−3
177 parameters	Δρmin = −0.38 e Å^−3
0 restraints	Absolute structure: Flack (1983), with how many Friedel pairs?
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.00 (4)

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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
Ca1	0.5	0.73448 (4)	0.25	0.01223 (11)
S1	0.89629 (6)	0.73117 (4)	0.315586 (12)	0.01384 (9)
O1	0.71388 (16)	0.74021 (14)	0.30086 (4)	0.0222 (3)
O2	0.9902 (2)	0.62686 (12)	0.29748 (4)	0.0215 (3)
O3	0.9921 (2)	0.84680 (11)	0.31151 (4)	0.0222 (3)
O4	1.04866 (19)	0.77761 (16)	0.44314 (4)	0.0295 (3)
O5	0.6544 (2)	0.88730 (14)	0.21355 (5)	0.0242 (4)
O6	0.3454 (2)	0.57848 (14)	0.28396 (5)	0.0238 (4)
C1	0.6059 (3)	0.72417 (18)	0.41675 (5)	0.0188 (4)
C2	0.7847 (2)	0.77265 (17)	0.39874 (5)	0.0141 (3)
C3	0.8907 (3)	0.79110 (16)	0.43885 (5)	0.0197 (4)
C4	0.7580 (3)	0.8298 (2)	0.47151 (6)	0.0281 (5)
H4A	0.7877	0.912	0.483	0.034*
H4B	0.7525	0.7689	0.4943	0.034*
C5	0.5843 (3)	0.83333 (18)	0.44733 (6)	0.0239 (4)
H5	0.4743	0.8297	0.4647	0.029*
C6	0.5947 (3)	0.94547 (19)	0.41859 (6)	0.0274 (5)
H6C	0.6328	1.0202	0.4338	0.033*
H6D	0.4781	0.9617	0.4053	0.033*
C7	0.7357 (3)	0.90719 (17)	0.38631 (6)	0.0201 (4)
H7A	0.8413	0.9618	0.3877	0.024*
H7B	0.6857	0.9101	0.358	0.024*
C8	0.4570 (3)	0.7147 (2)	0.38492 (7)	0.0276 (5)
H8A	0.4837	0.6477	0.3655	0.041*
H8B	0.4475	0.7928	0.3698	0.041*
H8C	0.344	0.6974	0.399	0.041*
C9	0.6239 (3)	0.59867 (18)	0.43798 (6)	0.0265 (5)
H9A	0.5185	0.5833	0.4551	0.04*
H9B	0.7307	0.5985	0.4554	0.04*
H9C	0.6342	0.5337	0.417	0.04*
C10	0.8863 (3)	0.69230 (16)	0.36858 (5)	0.0166 (4)
H10A	0.8354	0.6081	0.3705	0.02*
H10B	1.0107	0.6869	0.3787	0.02*
H5A	0.613 (4)	0.957 (3)	0.2118 (9)	0.039 (8)*
H5B	0.760 (5)	0.877 (3)	0.2063 (9)	0.057 (10)*
H6A	0.368 (4)	0.513 (3)	0.2901 (9)	0.038 (8)*
H6B	0.242 (4)	0.585 (2)	0.2880 (8)	0.033 (8)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca1	0.0105 (2)	0.0116 (2)	0.0146 (2)	0	−0.00157 (18)	0
S1	0.0111 (2)	0.01715 (19)	0.01331 (17)	−0.00032 (18)	−0.00066 (16)	0.00037 (16)
O1	0.0132 (6)	0.0354 (8)	0.0182 (6)	0.0008 (6)	−0.0034 (5)	−0.0016 (6)
O2	0.0189 (8)	0.0223 (6)	0.0234 (6)	0.0000 (6)	0.0029 (6)	−0.0042 (5)
O3	0.0195 (7)	0.0192 (6)	0.0279 (7)	−0.0015 (6)	0.0035 (7)	0.0045 (5)
O4	0.0249 (8)	0.0384 (8)	0.0251 (7)	−0.0015 (7)	−0.0088 (6)	−0.0010 (7)
O5	0.0196 (10)	0.0159 (8)	0.0372 (9)	0.0000 (6)	0.0062 (7)	0.0051 (6)
O6	0.0152 (10)	0.0187 (8)	0.0375 (9)	0.0001 (6)	0.0042 (7)	0.0074 (6)
C1	0.0177 (9)	0.0165 (8)	0.0222 (8)	0.0011 (9)	0.0025 (8)	0.0031 (7)
C2	0.0147 (9)	0.0135 (7)	0.0142 (8)	−0.0006 (8)	−0.0001 (6)	0.0009 (7)
C3	0.0265 (11)	0.0151 (9)	0.0174 (8)	−0.0020 (8)	−0.0006 (8)	0.0004 (6)
C4	0.0399 (14)	0.0260 (11)	0.0183 (10)	0.0012 (10)	0.0007 (9)	−0.0027 (8)
C5	0.0305 (13)	0.0205 (9)	0.0206 (9)	0.0049 (9)	0.0076 (9)	0.0030 (7)
C6	0.0374 (14)	0.0177 (9)	0.0270 (10)	0.0105 (9)	0.0099 (10)	0.0045 (8)
C7	0.0252 (11)	0.0143 (9)	0.0209 (9)	0.0025 (8)	0.0027 (8)	0.0027 (7)
C8	0.0153 (10)	0.0336 (12)	0.0338 (11)	−0.0025 (9)	0.0004 (8)	0.0027 (9)
C9	0.0299 (13)	0.0207 (10)	0.0288 (10)	−0.0025 (9)	0.0083 (10)	0.0068 (8)
C10	0.0193 (10)	0.0180 (8)	0.0124 (7)	0.0033 (8)	−0.0014 (8)	0.0028 (6)

Geometric parameters (Å, º)

Ca1—O1^i	2.3005 (12)	C2—C7	1.555 (2)
Ca1—O1	2.3005 (12)	C3—C4	1.512 (3)
Ca1—O6	2.3256 (15)	C4—C5	1.521 (3)
Ca1—O6^i	2.3256 (15)	C4—H4A	0.99
Ca1—O5^i	2.3394 (15)	C4—H4B	0.99
Ca1—O5	2.3394 (15)	C5—C6	1.532 (3)
S1—O3	1.4495 (14)	C5—H5	1
S1—O1	1.4525 (13)	C6—C7	1.544 (3)
S1—O2	1.4546 (14)	C6—H6C	0.99
S1—C10	1.7690 (17)	C6—H6D	0.99
O4—C3	1.202 (2)	C7—H7A	0.99
O5—H5A	0.82 (3)	C7—H7B	0.99
O5—H5B	0.83 (3)	C8—H8A	0.98
O6—H6A	0.76 (3)	C8—H8B	0.98
O6—H6B	0.79 (3)	C8—H8C	0.98
C1—C8	1.524 (3)	C9—H9A	0.98
C1—C9	1.529 (3)	C9—H9B	0.98
C1—C5	1.551 (3)	C9—H9C	0.98
C1—C2	1.554 (3)	C10—H10A	0.99
C2—C10	1.514 (2)	C10—H10B	0.99
C2—C3	1.536 (2)
O1^i—Ca1—O1	176.91 (8)	C3—C4—C5	102.16 (16)
O1^i—Ca1—O6	90.60 (6)	C3—C4—H4A	111.3
O1—Ca1—O6	91.64 (6)	C5—C4—H4A	111.3
O1^i—Ca1—O6^i	91.64 (6)	C3—C4—H4B	111.3
O1—Ca1—O6^i	90.60 (6)	C5—C4—H4B	111.3
O6—Ca1—O6^i	86.85 (9)	H4A—C4—H4B	109.2
O1^i—Ca1—O5^i	89.83 (5)	C4—C5—C6	106.82 (18)
O1—Ca1—O5^i	87.98 (5)	C4—C5—C1	102.74 (17)
O6—Ca1—O5^i	91.61 (5)	C6—C5—C1	102.16 (15)
O6^i—Ca1—O5^i	177.88 (6)	C4—C5—H5	114.6
O1^i—Ca1—O5	87.98 (5)	C6—C5—H5	114.6
O1—Ca1—O5	89.83 (5)	C1—C5—H5	114.6
O6—Ca1—O5	177.88 (6)	C5—C6—C7	103.50 (16)
O6^i—Ca1—O5	91.61 (5)	C5—C6—H6C	111.1
O5^i—Ca1—O5	89.97 (9)	C7—C6—H6C	111.1
O3—S1—O1	112.27 (9)	C5—C6—H6D	111.1
O3—S1—O2	113.19 (8)	C7—C6—H6D	111.1
O1—S1—O2	112.10 (8)	H6C—C6—H6D	109
O3—S1—C10	108.35 (9)	C6—C7—C2	103.79 (15)
O1—S1—C10	107.16 (9)	C6—C7—H7A	111
O2—S1—C10	103.11 (8)	C2—C7—H7A	111
S1—O1—Ca1	152.94 (8)	C6—C7—H7B	111
Ca1—O5—H5A	120.2 (19)	C2—C7—H7B	111
Ca1—O5—H5B	121 (2)	H7A—C7—H7B	109
H5A—O5—H5B	117 (3)	C1—C8—H8A	109.5
Ca1—O6—H6A	134 (2)	C1—C8—H8B	109.5
Ca1—O6—H6B	120.4 (19)	H8A—C8—H8B	109.5
H6A—O6—H6B	105 (3)	C1—C8—H8C	109.5
C8—C1—C9	108.03 (17)	H8A—C8—H8C	109.5
C8—C1—C5	114.02 (17)	H8B—C8—H8C	109.5
C9—C1—C5	113.53 (15)	C1—C9—H9A	109.5
C8—C1—C2	113.68 (14)	C1—C9—H9B	109.5
C9—C1—C2	113.12 (17)	H9A—C9—H9B	109.5
C5—C1—C2	94.16 (15)	C1—C9—H9C	109.5
C10—C2—C3	111.08 (15)	H9A—C9—H9C	109.5
C10—C2—C1	118.87 (16)	H9B—C9—H9C	109.5
C3—C2—C1	99.98 (13)	C2—C10—S1	120.71 (12)
C10—C2—C7	119.36 (14)	C2—C10—H10A	107.1
C3—C2—C7	102.68 (14)	S1—C10—H10A	107.1
C1—C2—C7	102.08 (14)	C2—C10—H10B	107.1
O4—C3—C4	127.00 (18)	S1—C10—H10B	107.1
O4—C3—C2	126.32 (17)	H10A—C10—H10B	106.8
C4—C3—C2	106.68 (17)

Symmetry code: (i) −x+1, y, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O5—H5A···O2^ii	0.82 (3)	2.02 (3)	2.834 (2)	175 (3)
O5—H5B···O3^iii	0.83 (4)	1.97 (4)	2.808 (2)	178 (4)
O6—H6A···O3^iv	0.76 (3)	2.14 (3)	2.881 (2)	166 (3)
O6—H6B···O2^v	0.79 (3)	1.97 (3)	2.751 (2)	172 (2)
C4—H4B···O4^vi	0.99	2.59	3.386 (2)	138
C10—H10B···O4	0.99	2.32	2.858 (2)	113

Symmetry codes: (ii) −x+3/2, y+1/2, −z+1/2; (iii) −x+2, y, −z+1/2; (iv) x−1/2, y−1/2, z; (v) x−1, y, z; (vi) x−1/2, −y+3/2, −z+1.