A two-dimensional cadmium(II) polymer based on nicotinic acid and thio­cyanate ligands

A cadmium–thio­cyanate complex, poly[[bis­(nicotinic acid-κN)di-μ-thio­cyanato-κ²N:S;κ²S:N-cadmium(II)] monohydrate], {[Cd(NCS)₂(C₆H₅NO₂)₂]·H₂O}_n, was synthesized by the reaction of nicotinic acid, cadmium nitrate tetra­hydrate and potassium thio­cyanide in aqueous solution. In the crystal structure, each Cd^II cation is in a distorted octa­hedral coordination environment, coordinated by the N and S atoms of nicotinic acid and thio­cyanate ligands. Neighbouring Cd^II cations are linked together by thio­cyanate bridges to form a two-dimensional network. Hydrogen-bond inter­actions between the uncoordinated solvent water mol­ecules and the organic ligands result in the formation of the three-dimensional supra­molecular network.

S1. Introduction

\ Recently, the design and synthesis of polymeric coordination networks have attracted intensive attention, not only owing to their intriguing architectures, but also due to their unique chemical and physical properties for potential applications (Moulton & Zaworotko, 2001; Gao et al., 2008). There are reports in which N- and S-donor bridging ligands have been used to form infinite polymeric frameworks (Kuniyasu et al., 1987; Liu et al., 2002; Zhang et al., 1999). The anionic thio­cyanate ligand (SCN^-) is a highly versatile ambidentate ligand with a polarizable π-system and it can coordinate to metal ions through either/both the N or/and the S atom (Eichele & Wasylishen, 1994). Different bridging modes of the thio­cyanate ligand and the Cd^II cation can generate various types of dimensional structures with particular properties, such as nonlinear optical behaviour (Liu et al., 2002). Furthermore, due to the general lability of Cd^II complexes, the formation of coordination bonds is reversible, which enables metal ions and ligands to rearrange during the process of polymerization to give highly ordered network structures (Shahverdizadeh & Morsali, 2011).

Most of the reported framework structures of polymeric cadmium complexes are linear polymeric chains having double thio­cyanate bridges, with two-dimensional networks being quite rare. Recently, however, a particularly inter­esting two-dimensional honeycomb-like {[Cd(NCS)]^-}_n anionic polymeric framework, in which the Cd—NCS—Cd units link to form eight-membered rings, was observed (Lai et al., 2007). Furthermore, the structure of a [Cd(SCN)₂(dmen)]_n (dmen is N,N-di­methyl­ethylenedi­amine) complex, composed of eight-membered (Cd—NCS—Cd links) and five-membered rings (C—N—Cd—N—C links) combining to form a novel two-dimensional polymeric sheet, was reported (Mondal et al., 2000). In the present paper, we report the use of nicotinic acid as a template for the formation of a new two-dimensional coordination polymer, which is assembled by [Cd(SCN)₂] and nitro­gen [nicotinic acid? nitro­gen-containing?] ligands, namely poly[[bis­(nicotinic acid-κN)di-µ-thio­cyanato-κ²N:S;κ²S:\ N-cadmium(II)] monohydrate], (I).

S2. Experimental

S2.1. Synthesis and crystallization

The title compound was obtained from an aqueous solution containing 1:1:3 molar equivalents of nicotinic acid, cadmium nitrate tetra­hydrate and potassium thio­cyanide. The resulting aqueous solution was kept at room temperature. Upon standing at room temperature for several days, suitable colourless single crystals of (I) were obtained by slow solvent evaporation.

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. C-bound H atoms were included in calculated positions and refined using a riding model, with C—H = 0.93 Å and U_iso (H) = 1.2U_eq(C). The water H atoms were located in a difference Fourier synthesis and refined isotropically, with O—H and H···H distance restraints of 0.82 (1) and 1.39 (1) Å, respectively.

S3. Results and discussion

\ Compound (I) crystallizes in the monoclinic space group C2/c. The independent unit is composed of one Cd^II cation, two nicotinic acid ligands, two anionic thio­cyanate (SCN^-) ligands and one free water molecule (Fig. 1).

As shown in Fig. 2, the central Cd^II cation is six-coordinated by two N atoms from two nicotinic acid ligands, and by two N atoms and two S atoms from four different SCN^- ligands, adopting a distorted CdN₄S₂ o­cta­hedral coordinated geometry. Each of the central Cd^II cations is joined to the neighbouring Cd^II cations through a single thio­cyanate bridge. The Cd—N [2.321 (3)–2.352 (2) Å] and Cd—S [2.7550 (12) Å] distances are typical for Cd—N and Cd—S bonds (Gao et al., 2008; Wei et al., 2007). There are two sorts of angles around each Cd^II centre, namely orthogonal cis angles [88.49 (7)–91.51 (7)°] and linear trans angles (180°). In general, all the bond lengths and angles are within normal ranges, similar to those in other cadmium–thio­cyanate compounds. The Cd—S—C and Cd—N—C bond angles are consistent with those of other reported cadmium–thio­cyanate compounds (Wang et al., 2004; Liu et al., 2002).

Compound (I) is a new coordination polymer in which the six-coordinated Cd^II centre adopts a distorted o­cta­hedral coordination geometry. It displays a different coordination architecture compared with the similar compound, {[Cd₃(NCS)₂(3-pyc)₄(H₂O)₄].2H₂O}_n (Shahverdizadeh & Morsali, 2011; 3-Hpyc is pyridine-3-carb­oxy­lic acid), in which the central Cd2 cation is coordinated by two O atoms from two water molecules, two N atoms from nicotinic acid ligands and two N atoms from thio­cyanate ligands, rather than two N atoms of nicotinic acid ligands, plus the two N atoms and two S atoms of four different thio­cyanate ligands.

The two-dimensional network of (I) is a new coordination architecture compared with the related structure [Cd(SCN)₂(pyCN)₂] (pyCN is isonicotino­nitrile; Chen et al., 2002), where there is a one-dimensional infinite linear chain structure consisting of eight-membered rings of two Cd^II cations and two bridging SCN^- ligands. As with (I), the Cd^II cations are linked by the N atoms of nicotinic acid, two N:S-bridging and two S:N-bridging thio­cyanate ligands, forming an infinite two-dimensional coordination polymer, which we shall refer to as a two-dimensional puckered re­cta­ngular network (Fig. 2). This is different from what we found in our previous report on catena-poly[1-carb­oxy­methyl-4-(di­methyl­amino)­pyridinium [cadmium(II)-tri-µ-thio­cyanato-κ⁴N:S;κ²S:N] [[[4-(di­methyl­amino)­pyridinium-1-acetate-κ²O,O']cadmium(II)]\ di-µ-thio­cyanato-κ²N:S;κ²S:N]] (Wang & Zhou, 2015), where the Cd1 atoms are chelated by a carboxyl­ate group of a 4-(di­methyl­amino)­pyridinium-1-acetate ligand and linked by two N:S-bridging and two S:N-bridging thio­cyanate ligands, forming an infinite one-dimensional puckered coordination polymer. The Cd^II cations are alternately connected in one direction by M(NCS)M bridges, and in the other direction also by a single thio­cyanate bridge to form a two-dimensional network, with a Cd···Cd distance of 6.2247 (9) Å in the bc plane. In (I), the corrugated two-dimensional net structure has a puckered re­cta­ngular net structure where each loop within the grid is composed of a Cd₄(NCS)₄ ring, with four coordinated ligands in total per grid in the bc plane (Fig. 2). This also differs from the two-dimensional polymeric sheet of [Cd(SCN)₂(dmen)]_n (Mondal et al., 2000; dmen is N,N-di­methyl­ethylenedi­amine), which is formed through the bidentate dmen ligand and single SCN^- bridges. It should be pointed out that the bidentate dmen molecule coordinates to a Cd^II cation in a cis position, which is different from (I), where two monodentate nicotinic acid molecules coordinate in trans positions.

As well as the coordination polymer, (I) also contains uncoordinated solvent water molecules. Each water molecule and carb­oxy­lic acid group are connected to two adjacent coordination polymers by means of O1W—H1W···O2 and O1—H1A···O2 hydrogen bonds (Fig. 3 and Table 2). Thus, each carb­oxy­lic acid group takes part in two hydrogen bonds, with the hydrogen bond linking the water to the nicotinic acid. These electrostatic and hydrogen-bonding inter­actions link the polymeric chains together, where the gaps between the chains are filled by the water molecules (Fig. 4). The O—H···O hydrogen-bonding inter­actions serve again as important driving forces to crosslink the puckered two-dimensional networks into a three-dimensional architecture in the ac plane (Fig. 4). Inter­molecular hydrogen-bonding inter­actions between the nicotinic acid ligands and the uncoordinated water molecules further stabilize the corrugated three-dimensional network.

In summary, a new cadmium–thio­cyanate coordination polymer with an inter­esting structural architecture has been prepared. The Cd^II cations are linked by N atoms from the nicotinic acid ligands and by bridging thio­cyanate ligands to form a two-dimensional coordination polymer which lies parallel to the bc plane. Hydrogen-bond inter­actions between the uncoordinated solvent water molecules and the organic ligands are involved in the formation of the three-dimensional architecture.

Computing details

Data collection: CrystalClear (Rigaku, 2005); cell refinement: CrystalClear (Rigaku, 2005); data reduction: CrystalClear (Rigaku, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg & Putz, 2005); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The coordination environment of the Cd^II cation in (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level and H atoms have been omitted for clarity. [Symmetry codes: (i) -x + 1/2, -y + 1/2, -z + 2; (ii) -x + 1/2, y - 1/2, -z + 3/2; (iii) x, -y + 1, z - 1/2; (iv) -x + 1/2, y + 1/2, -z + 3/2.]
	Fig. 2. The puckered rectangular net structure of complex (I) formed by the Cd^II centres, viewed in the bc plane.
	Fig. 3. The crystal packing of (I), showing the hydrogen-bonding interactions (dashed lines) among the nicotinic acid ligands and the water molecules.
	Fig. 4. An illustration of the three-dimensional supramolecular network of (I) in the ac plane, formed by the water molecules via hydrogen-bonding interactions (dashed lines).

Poly[[bis(nicotinic acid-κN)di-µ-thiocyanato-κ²N:S;κ²S:N-cadmium(II)] monohydrate]

Crystal data

[Cd(NCS)2(C6H5NO2)2]·H2O	F(000) = 976
Mr = 492.80	Dx = 1.807 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 2070 reflections
a = 24.174 (5) Å	θ = 3.3–27.5°
b = 9.850 (2) Å	µ = 1.47 mm^−1
c = 7.6135 (15) Å	T = 293 K
β = 92.44 (3)°	Block, colourless
V = 1811.2 (6) Å^3	0.28 × 0.25 × 0.20 mm
Z = 4

Data collection

Rigaku SCXmini diffractometer	1794 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.025
Graphite monochromator	θmax = 27.5°, θmin = 3.3°
ω scans	h = −31→31
Absorption correction: multi-scan ?	k = −12→11
Tmin = 0.684, Tmax = 0.758	l = −9→7
6122 measured reflections	3 standard reflections every 180 reflections
2070 independent reflections	intensity decay: none

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)] = 0.030	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.085	w = 1/[σ^2(Fo^2) + (0.050P)^2 + 3.P] where P = (Fo^2 + 2Fc^2)/3
S = 1.01	(Δ/σ)max < 0.001
2070 reflections	Δρmax = 0.44 e Å^−3
126 parameters	Δρmin = −0.47 e Å^−3
3 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008)
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0819 (12)

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
Cd1	0.2500	0.2500	1.0000	0.02973 (12)
S1	0.31494 (4)	0.70616 (9)	0.80081 (12)	0.0435 (2)
O1	0.43099 (13)	0.4946 (4)	0.6234 (5)	0.0754 (9)
H1A	0.4511 (18)	0.553 (4)	0.578 (6)	0.090*
O2	0.48551 (11)	0.3293 (3)	0.5279 (4)	0.0601 (7)
N1	0.33088 (10)	0.1962 (3)	0.8535 (3)	0.0319 (5)
N2	0.26096 (12)	0.4774 (3)	0.9296 (4)	0.0409 (6)
C1	0.36200 (13)	0.2910 (4)	0.7766 (4)	0.0352 (6)
H1B	0.3506	0.3811	0.7806	0.042*
C2	0.40952 (13)	0.2612 (3)	0.6928 (4)	0.0349 (7)
C3	0.42573 (14)	0.1251 (4)	0.6869 (5)	0.0434 (8)
H3	0.4576	0.1004	0.6308	0.052*
C4	0.39424 (15)	0.0288 (4)	0.7642 (5)	0.0469 (8)
H4	0.4045	−0.0621	0.7617	0.056*
C5	0.34731 (13)	0.0679 (3)	0.8459 (4)	0.0386 (7)
H5	0.3261	0.0015	0.8980	0.046*
C6	0.44482 (13)	0.3656 (4)	0.6082 (5)	0.0428 (8)
C7	0.28255 (12)	0.5729 (3)	0.8770 (4)	0.0302 (6)
O1W	0.5000	0.1570 (6)	0.2500	0.127 (2)
H1W	0.486 (4)	0.205 (2)	0.328 (8)	0.191*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.03461 (18)	0.02221 (17)	0.03330 (19)	−0.00492 (11)	0.01249 (12)	0.00013 (11)
S1	0.0516 (5)	0.0345 (4)	0.0436 (5)	−0.0168 (4)	−0.0062 (4)	0.0131 (4)
O1	0.0660 (19)	0.069 (2)	0.093 (3)	−0.0151 (16)	0.0246 (17)	0.0188 (18)
O2	0.0486 (15)	0.0675 (19)	0.0669 (18)	−0.0031 (13)	0.0311 (13)	0.0123 (14)
N1	0.0329 (13)	0.0321 (13)	0.0314 (13)	−0.0069 (10)	0.0097 (10)	−0.0007 (11)
N2	0.0494 (15)	0.0290 (14)	0.0449 (16)	−0.0045 (12)	0.0111 (12)	0.0073 (12)
C1	0.0388 (16)	0.0304 (14)	0.0369 (17)	−0.0061 (13)	0.0085 (13)	0.0009 (13)
C2	0.0302 (14)	0.0432 (19)	0.0315 (16)	−0.0064 (12)	0.0039 (12)	0.0025 (12)
C3	0.0340 (16)	0.052 (2)	0.045 (2)	0.0018 (14)	0.0119 (14)	−0.0029 (16)
C4	0.0459 (18)	0.0365 (18)	0.059 (2)	0.0013 (15)	0.0122 (16)	−0.0032 (16)
C5	0.0396 (16)	0.0328 (16)	0.0441 (18)	−0.0085 (13)	0.0103 (13)	0.0015 (14)
C6	0.0328 (15)	0.054 (2)	0.0425 (19)	−0.0094 (14)	0.0082 (13)	0.0100 (15)
C7	0.0365 (15)	0.0269 (14)	0.0271 (14)	0.0004 (11)	0.0016 (11)	0.0027 (11)
O1W	0.149 (6)	0.081 (4)	0.157 (7)	0.000	0.067 (5)	0.000

Geometric parameters (Å, º)

Cd1—N2^i	2.321 (3)	N1—C1	1.349 (4)
Cd1—N2	2.321 (3)	N2—C7	1.155 (4)
Cd1—N1^i	2.352 (2)	C1—C2	1.369 (4)
Cd1—N1	2.352 (2)	C1—H1B	0.9300
Cd1—S1^ii	2.7550 (12)	C2—C3	1.398 (5)
Cd1—S1^iii	2.7550 (12)	C2—C6	1.499 (4)
S1—C7	1.646 (3)	C3—C4	1.365 (5)
S1—Cd1^iv	2.7550 (12)	C3—H3	0.9299
O1—C6	1.321 (5)	C4—C5	1.372 (5)
O1—H1A	0.835 (10)	C4—H4	0.9301
O2—C6	1.233 (4)	C5—H5	0.9300
N1—C5	1.327 (4)	O1W—H1W	0.840 (10)
N2^i—Cd1—N2	180.0	C7—N2—Cd1	157.4 (3)
N2^i—Cd1—N1^i	90.16 (10)	N1—C1—C2	123.2 (3)
N2—Cd1—N1^i	89.84 (10)	N1—C1—H1B	118.4
N2^i—Cd1—N1	89.84 (10)	C2—C1—H1B	118.4
N2—Cd1—N1	90.16 (10)	C1—C2—C3	117.5 (3)
N1^i—Cd1—N1	180.0	C1—C2—C6	123.9 (3)
N2^i—Cd1—S1^ii	88.52 (8)	C3—C2—C6	118.6 (3)
N2—Cd1—S1^ii	91.48 (7)	C4—C3—C2	119.3 (3)
N1^i—Cd1—S1^ii	88.49 (7)	C4—C3—H3	120.3
N1—Cd1—S1^ii	91.51 (7)	C2—C3—H3	120.3
N2^i—Cd1—S1^iii	91.48 (8)	C3—C4—C5	119.2 (3)
N2—Cd1—S1^iii	88.52 (7)	C3—C4—H4	120.4
N1^i—Cd1—S1^iii	91.51 (7)	C5—C4—H4	120.4
N1—Cd1—S1^iii	88.49 (7)	N1—C5—C4	122.8 (3)
S1^ii—Cd1—S1^iii	180.0	N1—C5—H5	118.6
C7—S1—Cd1^iv	98.93 (10)	C4—C5—H5	118.6
C6—O1—H1A	118 (4)	O2—C6—O1	122.2 (3)
C5—N1—C1	117.9 (3)	O2—C6—C2	119.7 (3)
C5—N1—Cd1	119.5 (2)	O1—C6—C2	118.1 (3)
C1—N1—Cd1	122.7 (2)	N2—C7—S1	178.3 (3)
N2^i—Cd1—N1—C5	1.0 (2)	Cd1—N1—C1—C2	179.2 (2)
N2—Cd1—N1—C5	−179.0 (2)	N1—C1—C2—C3	0.5 (5)
N1^i—Cd1—N1—C5	145 (100)	N1—C1—C2—C6	−179.4 (3)
S1^ii—Cd1—N1—C5	−87.5 (2)	C1—C2—C3—C4	−0.3 (5)
S1^iii—Cd1—N1—C5	92.5 (2)	C6—C2—C3—C4	179.6 (3)
N2^i—Cd1—N1—C1	−178.6 (2)	C2—C3—C4—C5	0.1 (6)
N2—Cd1—N1—C1	1.4 (2)	C1—N1—C5—C4	0.2 (5)
N1^i—Cd1—N1—C1	−34 (100)	Cd1—N1—C5—C4	−179.4 (3)
S1^ii—Cd1—N1—C1	92.8 (2)	C3—C4—C5—N1	0.0 (6)
S1^iii—Cd1—N1—C1	−87.2 (2)	C1—C2—C6—O2	−176.4 (3)
N2^i—Cd1—N2—C7	−178 (100)	C3—C2—C6—O2	3.7 (5)
N1^i—Cd1—N2—C7	171.7 (7)	C1—C2—C6—O1	3.8 (5)
N1—Cd1—N2—C7	−8.3 (7)	C3—C2—C6—O1	−176.1 (3)
S1^ii—Cd1—N2—C7	−99.8 (7)	Cd1—N2—C7—S1	−1 (11)
S1^iii—Cd1—N2—C7	80.2 (7)	Cd1^iv—S1—C7—N2	111 (10)
C5—N1—C1—C2	−0.5 (5)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···O2^v	0.84 (1)	2.11 (2)	2.934 (4)	169 (5)
O1W—H1W···O2	0.84 (1)	1.96 (5)	2.746 (5)	156 (11)

Symmetry code: (v) −x+1, −y+1, −z+1.