Poly[diaqua­(μ-4,4′-bipyridine-κ²N:N′)bis­(μ-cyanido-κ²C:N)bis­(cyanido-κC)nickel(II)copper(II)]: a metal–organic cyanide-bridged framework

The structure of the title compound, [NiCu(CN)₄(C₁₀H₈N₂)(H₂O)₂]_n or [{Cu(H₂O)₂}(μ-C₁₀H₈N₂)(μ-CN)₂{Ni(CN)₂}]_n, was shown to be a metal–organic cyanide-bridged framework, composed essentially of –Cu–4,4′-bpy–Cu–4,4′-bpy–Cu– chains (4,4′-bpy is 4,4′-bipyridine) linked by [Ni(CN)₄]²⁻ anions. Both metal atoms sit on special positions; the Cu^II atom occupies an inversion center, while the Ni^II atom of the cyano­metallate sits on a twofold axis. The 4,4′-bpy ligand is also situated about a center of symmetry, located at the center of the bridging C—C bond. The scientific impact of this structure lies in the unique manner in which the framework is built up. The arrangement of the –Cu–4,4′-bpy–Cu–4,4′-bpy–Cu– chains, which are mutually perpendicular and non-inter­secting, creates large channels running parallel to the c axis. Within these channels, the [Ni(CN)₄]²⁻ anions coordinate to successive Cu^II atoms, forming zigzag –Cu—N[triple bond]C—Ni—C[triple bond]N—Cu– chains. In this manner, a three-dimensional framework structure is constructed. To the authors' knowledge, this arrangement has not been observed in any of the many copper(II)–4,4′-bipyridine framework complexes synthesized to date. The coordination environment of the Cu^II atom is completed by two water mol­ecules. The framework is further strengthened by O—H...N hydrogen bonds involving the water mol­ecules and the symmetry-equivalent nonbridging cyanide N atoms.

S1. Comment

The synthesis and characterization of multidimensional coordination networks has been an area of rapid growth in recent years. The aim of this intense activity is the deliberate design of materials with specific properties, for example electronic, magnetic, optical, catalytic, ion exchange and absorption (Chae et al., 2004; Janiak, 2003; Fujita et al., 1994; Noro et al., 2000; Tabares et al., 2001; Coronado et al., 2000). Among these materials, multidimensional cyano-bridged complexes, prepared by the self-assembly of specifically designed precursors (typically a cyanometallate complex that acts as a ligand and a transition metal complex with available coordination sites), are playing an important role in areas such as molecule-based magnets, magneto-optic materials, ion exchange, materials for storing gases and host–guest chemistry (Dunbar & Heintz, 1997; Ferlay et al., 1995; Cernák et al., 2002). Most approaches to the design of nanoporous coordination polymers have involved the employment of rigid bidentate heteroaromatic N-donor ligands, such as pyrazine (pyz) or 4,4'-bipyridine (4,4'-bpy), to connect metal ions, so giving cationic networks (Hagrman et al., 1999). Surprisingly, the ligand 4,4'-bpy has not been used extensively as a bridging ligand with metallocyanides to form multidimensional complexes. A search of the Cambridge Structural Database (Version 5.18, November 2007 update; Allen, 2002) revealed only 13 crystal structures of metallocyanide complexes involving 4,4'-bpy, and only five of these concerned first-row transition metals. A few three-dimensional coordination polymers based on 4,4'-bpy and cyanide compounds have been reported (Soma et al., 1994; Teichert & Sheldrick, 2000), but none of them involved tetracyanonickelate (II). Here we describe the synthesis and structure of a new metal–organic cyano-bridged framework, (I), formed from 4,4'-bipyridine, copper sulfate and tetracyanonickelate.

The molecular structure of the asymmetric unit of (I) is shown in Fig. 1, and selected geometrical parameters are given in Table 1. The crystal structure analysis of (I) revealed that it has a neutral three-dimensional framework, built up of [Cu(4,4'-bpy)₂(H₂O)₂]²⁺ cations and [Ni(CN)₄]^2- anions. The copper(II) atom is located on an inversion center, while the nickel(II) atom sits on a twofold rotation axis. The 4,4'-bpy ligand is also situated about a center of symmetry located at the center of the bridging C—C bond. The Cu atom has a distorted octahedral geometry, being coordinated to four N atoms in the equatorial plane, two from cyanide ligands and two from the 4,4'-bpy ligands. The axial positons are occupied by two water molecules. The coordination polyhedron of the Cu atoms can be described as Cu(N)₂(H₂O)₂(NC)₂ or CuN₄O₂. Two of the four C≡N groups of the [Ni(CN)₄]^2- anion are nonbridging, while the other two bond to the Cu1 atoms, giving rise to Ni—C≡N—Cu bridges and forming zigzag –Cu—N≡C—Ni—C≡N—Cu– chains extending in the c direction (Fig. 2a). The Ni atom has a square-planar arrangement, and the mean Ni—C and C—N bond lengths are similar to the values reported for other tetracyanonickelate salts (Miyoshi et al., 1973; Cernák & Lipkowski,1999; Akitsu & Einaga, 2006; Broring et al., 2007). The Ni—C≡N bond angles are almost linear and there is no difference between those involving the bridging N atom (N8) and the nonbridging N atom (N10). The Cu1—N≡C bond angles are slightly bent.

The centrosymmetric 4,4'-bpy ligands bonded to the Cu atoms in trans positions give rise to the formation of –Cu–4,4'-bpy–Cu–4,4'-bpy– chains, which run at right-angles to one another (Fig. 2b). They are separated by a distance of ca 3.27 Å. These chains are connected to one another via two of the four C≡N groups of the [Ni(CN)₄]^2- anions, so giving rise to the three-dimensional nature of the compound (Fig. 2c). The shortest Ni···Niⁱ distance is 3.7514 (6) Å. The structure is further stabilized by O—H···N hydrogen bonds involving the coordinated water molecules and the N atoms of the nonbridging cyano groups (Table 2). There are also two C—H···O interactions involving the water molecules and two H atoms of symmetry-related 4,4'-bpy ligands.

Related literature

For related literature, see: Akitsu & Einaga (2006); Allen (2002); Broring et al. (2007); Cernák & Lipkowski (1999); Cernák et al. (2002); Chae et al. (2004); Coronado et al. (2000); Dunbar & Heintz (1997); Fujita et al. (1994); Hagrman et al. (1999); Janiak (2003); Miyoshi et al. (1973); Noro et al. (2000); Soma et al. (1994); Tabares et al. (2001); Teichert & Sheldrick (2000).

S2. Experimental

To an aqueous solution of CuSO₄^.5H₂O (2 mmol, 0.499 g, 20 ml) was added with stirring K₂[Ni(CN)₄] (2 mmol, 0.489 g) in 20 ml of water. A blue precipitate formed at once and was dissolved by adding appropriate amounts of citric acid (1.8 g) and 2-aminoethanol (1.6 ml) to adjust the pH to 9, giving a final volume of ca 100 ml. A portion of this solution (30 ml) was very carefully layered onto an ethylene glycol solution (20 ml) of 4,4'-bpy (1 mmol, 0.156 g). Green crystals of (I) appeared at the interface of the two solutions after several weeks. Analysis calculated for C₁₄H₁₂CuN₆NiO₂: C 40.18, H 2.89, N 20.08%; found: C 40.64, H 3.09, N 20.72%. IR (KBr disk): ν(O—H) 3513 (s), 3456 (s), 3379 (sh); ν(ArC—H) 3144 (w), 3109 (w), 3088 (w); ν(C—H) 3061 (s), 3045 (s); ν(C≡N) 2142 (s), 2121 (s); ν(ArC—C) 1641 (vs), 1615 (vs), 1537 (m), 1493 (m), 1435 (s); ν(O—H, C—C, ArC—H in-plane) 1388 (s), 1369 (s), 1238 (m), 1212 (m), 1153 (w), 1099 (m), 1012 (w); ν(ArC—H out-of-plane) 866 (w), 728 (m), 688 (m), 660 (sh); ν(Ni—C, Cu—N) 553 (w), 483 (m), 454 (m).

S3. Refinement

Water H atoms were located in difference Fourier maps and were refined with O—H distant restraints of 0.88 (2) Å. The remainder of the H atoms were included in calculated positions and treated as riding atoms [C—H = 0.95 Å, with U_iso(H) = 1.2U_eq(C)].

Computing details

Data collection: EXPOSE in IPDS Software (Stoe & Cie, 2000); cell refinement: CELL in IPDS Software (Stoe & Cie, 2000); data reduction: INTEGRATE in IPDS Software (Stoe & Cie, 2000); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

Fig. 1. A view of the asymmetric unit of (I), showing the atom-numbering scheme and displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) -x + 1, y, -z + 1/2; (ii) -x + 1, -y, -z + 1; (iii) x - 1/2, y + 1/2, z - 1; (iv) -x + 1/2, -y + 1/2, -z.]

Fig. 2. (a) A view down the a axis of the zigzag –Cu—N≡ C–Ni—C≡N—Cu– chains, extending in the c direction. (b) A view down the c axis of the mutually perpendicular arrangement of the –Cu–4,4'-bpy–Cu-4,4'–bpy– chains (the 4,4'-bpy H atoms have been omitted for clarity). (c) The crystal packing of (I), viewed along the c axis. The O—H···N hydrogen bonds are shown as dashed lines; see Table 2 for details (the 4,4'-bpy H atoms have been omitted for clarity).

Poly[diaqua(µ-4,4'-bipyridine-κ²N:N')bis(µ₂-cyanido-κ²C:N)bis(cyanido-κC)nickel(II)copper(II)]

Crystal data

[NiCu(CN)4(C10H8N2)(H2O)2]	F(000) = 844
Mr = 418.55	Dx = 1.791 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 1389 reflections
a = 15.158 (2) Å	θ = 2.0–25.6°
b = 14.8108 (15) Å	µ = 2.60 mm^−1
c = 7.4512 (11) Å	T = 173 K
β = 111.892 (11)°	Block, green
V = 1552.2 (4) Å^3	0.50 × 0.45 × 0.40 mm
Z = 4

Data collection

Stoe IPDS diffractometer	1389 independent reflections
Radiation source: fine-focus sealed tube	1248 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.037
Detector resolution: 0.81Å pixels mm^-1	θmax = 25.2°, θmin = 2.0°
φ oscillation scans	h = −18→18
Absorption correction: multi-scan (MULscanABS; Spek, 2003)	k = −17→16
Tmin = 0.246, Tmax = 0.352	l = −8→8
8011 measured reflections

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)] = 0.021	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.054	w = 1/[σ^2(Fo^2) + (0.0333P)^2 + 0.7853P] where P = (Fo^2 + 2Fc^2)/3
S = 1.04	(Δ/σ)max < 0.001
1389 reflections	Δρmax = 0.22 e Å^−3
120 parameters	Δρmin = −0.26 e Å^−3
2 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0011 (3)

Special details

Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles

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
Cu1	0.25000	0.25000	0.00000	0.0192 (1)
Ni1	0.50000	0.48517 (2)	0.25000	0.0212 (1)
O1W	0.19674 (10)	0.29463 (10)	0.2554 (2)	0.0281 (5)
N1	0.33744 (11)	0.15805 (11)	0.1921 (2)	0.0206 (5)
N8	0.35317 (11)	0.34047 (11)	0.0831 (2)	0.0239 (5)
N10	0.34939 (15)	0.62688 (13)	0.0778 (3)	0.0377 (6)
C2	0.32545 (14)	0.06920 (13)	0.1632 (3)	0.0240 (6)
C3	0.38581 (14)	0.00581 (13)	0.2811 (3)	0.0237 (6)
C4	0.46528 (13)	0.03304 (13)	0.4376 (3)	0.0208 (5)
C5	0.47620 (14)	0.12552 (13)	0.4718 (3)	0.0240 (6)
C6	0.41181 (14)	0.18454 (13)	0.3492 (3)	0.0244 (6)
C7	0.40931 (14)	0.39594 (13)	0.1439 (3)	0.0232 (6)
C9	0.40768 (15)	0.57362 (14)	0.1433 (3)	0.0284 (6)
H1W	0.2405 (17)	0.3173 (17)	0.353 (3)	0.047 (8)*
H2W	0.178 (2)	0.2458 (14)	0.287 (4)	0.048 (8)*
H2	0.27240	0.04870	0.05550	0.0290*
H3	0.37310	−0.05670	0.25530	0.0280*
H5	0.52810	0.14780	0.57980	0.0290*
H6	0.42020	0.24730	0.37670	0.0290*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu1	0.0142 (2)	0.0125 (2)	0.0231 (2)	−0.0007 (1)	−0.0021 (1)	0.0034 (1)
Ni1	0.0155 (2)	0.0128 (2)	0.0283 (2)	0.0000	0.0000 (2)	0.0000
O1W	0.0270 (8)	0.0209 (8)	0.0287 (8)	−0.0014 (6)	0.0016 (7)	0.0002 (6)
N1	0.0171 (8)	0.0176 (8)	0.0226 (8)	0.0003 (6)	0.0023 (7)	0.0028 (6)
N8	0.0182 (8)	0.0184 (8)	0.0267 (9)	0.0003 (7)	−0.0013 (7)	0.0049 (7)
N10	0.0368 (11)	0.0283 (10)	0.0420 (11)	0.0139 (9)	0.0077 (9)	0.0064 (9)
C2	0.0182 (9)	0.0197 (10)	0.0278 (10)	0.0000 (7)	0.0014 (8)	0.0008 (8)
C3	0.0209 (10)	0.0165 (9)	0.0284 (10)	0.0001 (7)	0.0031 (8)	0.0021 (8)
C4	0.0189 (9)	0.0191 (9)	0.0227 (9)	0.0024 (8)	0.0057 (8)	0.0037 (8)
C5	0.0218 (10)	0.0218 (10)	0.0212 (9)	0.0011 (8)	−0.0004 (8)	0.0013 (8)
C6	0.0249 (10)	0.0171 (10)	0.0253 (10)	0.0007 (8)	0.0025 (8)	−0.0002 (8)
C7	0.0188 (10)	0.0182 (10)	0.0268 (10)	0.0056 (8)	0.0017 (8)	0.0055 (8)
C9	0.0282 (11)	0.0215 (10)	0.0307 (11)	−0.0024 (9)	0.0055 (9)	−0.0014 (9)

Geometric parameters (Å, º)

Cu1—O1W	2.4199 (15)	N1—C2	1.335 (3)
Cu1—N1	2.0605 (16)	N8—C7	1.148 (3)
Cu1—N8	1.9754 (17)	N10—C9	1.149 (3)
Cu1—O1W^i	2.4199 (15)	C2—C3	1.375 (3)
Cu1—N1^i	2.0605 (16)	C3—C4	1.388 (3)
Cu1—N8^i	1.9754 (17)	C4—C5	1.392 (3)
Ni1—C7	1.858 (2)	C4—C4^iii	1.483 (3)
Ni1—C9	1.865 (2)	C5—C6	1.373 (3)
Ni1—C7^ii	1.858 (2)	C2—H2	0.9500
Ni1—C9^ii	1.865 (2)	C3—H3	0.9500
O1W—H2W	0.84 (2)	C5—H5	0.9500
O1W—H1W	0.85 (2)	C6—H6	0.9500
N1—C6	1.346 (3)
Ni1···Ni1^iv	3.7514 (6)
O1W—Cu1—N1	87.19 (6)	Cu1—O1W—H1W	113.5 (17)
O1W—Cu1—N8	91.46 (6)	C2—N1—C6	116.57 (17)
O1W—Cu1—O1W^i	180	Cu1—N1—C6	121.66 (13)
O1W—Cu1—N1^i	92.81 (6)	Cu1—N1—C2	121.73 (13)
O1W—Cu1—N8^i	88.54 (6)	Cu1—N8—C7	173.32 (17)
N1—Cu1—N8	90.09 (7)	N1—C2—C3	123.50 (19)
O1W^i—Cu1—N1	92.81 (6)	C2—C3—C4	120.05 (18)
N1—Cu1—N1^i	180	C4^iii—C4—C5	121.76 (19)
N1—Cu1—N8^i	89.91 (7)	C3—C4—C5	116.52 (19)
O1W^i—Cu1—N8	88.54 (6)	C3—C4—C4^iii	121.72 (18)
N1^i—Cu1—N8	89.91 (7)	C4—C5—C6	119.87 (19)
N8—Cu1—N8^i	180	N1—C6—C5	123.37 (18)
O1W^i—Cu1—N1^i	87.19 (6)	Ni1—C7—N8	178.18 (19)
O1W^i—Cu1—N8^i	91.46 (6)	Ni1—C9—N10	178.6 (2)
N1^i—Cu1—N8^i	90.09 (7)	N1—C2—H2	118.00
C7—Ni1—C9	89.98 (9)	C3—C2—H2	118.00
C7—Ni1—C7^ii	89.30 (9)	C2—C3—H3	120.00
C7—Ni1—C9^ii	179.23 (10)	C4—C3—H3	120.00
C7^ii—Ni1—C9	179.23 (10)	C4—C5—H5	120.00
C9—Ni1—C9^ii	90.74 (10)	C6—C5—H5	120.00
C7^ii—Ni1—C9^ii	89.98 (9)	N1—C6—H6	118.00
H1W—O1W—H2W	110 (2)	C5—C6—H6	118.00
Cu1—O1W—H2W	103.4 (19)
O1W—Cu1—N1—C2	114.82 (16)	C2—N1—C6—C5	3.0 (3)
O1W—Cu1—N1—C6	−67.81 (16)	N1—C2—C3—C4	−1.4 (3)
N8—Cu1—N1—C2	−153.72 (17)	C2—C3—C4—C5	3.4 (3)
N8—Cu1—N1—C6	23.65 (16)	C2—C3—C4—C4^iii	−176.9 (2)
O1W^i—Cu1—N1—C2	−65.18 (16)	C3—C4—C5—C6	−2.3 (3)
O1W^i—Cu1—N1—C6	112.19 (16)	C4^iii—C4—C5—C6	178.0 (2)
N8^i—Cu1—N1—C2	26.28 (17)	C3—C4—C4^iii—C3^iii	−180.0 (2)
N8^i—Cu1—N1—C6	−156.35 (16)	C3—C4—C4^iii—C5^iii	0.3 (3)
Cu1—N1—C2—C3	175.67 (17)	C5—C4—C4^iii—C3^iii	−0.3 (3)
C6—N1—C2—C3	−1.8 (3)	C5—C4—C4^iii—C5^iii	−180.0 (2)
Cu1—N1—C6—C5	−174.48 (17)	C4—C5—C6—N1	−1.0 (3)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1W—H1W···N10^v	0.85 (2)	2.04 (2)	2.886 (3)	177 (2)
O1W—H2W···N10^vi	0.84 (2)	2.15 (2)	2.976 (3)	169 (3)
C2—H2···N8^i	0.95	2.43	2.964 (3)	115
C3—H3···O1W^vi	0.95	2.43	3.342 (3)	160
C5—H5···O1W^vii	0.95	2.55	3.431 (3)	155
C6—H6···N8	0.95	2.46	2.957 (3)	112

Symmetry codes: (i) −x+1/2, −y+1/2, −z; (v) x, −y+1, z+1/2; (vi) −x+1/2, y−1/2, −z+1/2; (vii) x+1/2, −y+1/2, z+1/2.