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The crystal structure of the title bimetallic cyanide-bridged complex, {K[HoRu(CN)6(H2O)2]·2H2O}n, was determined by means of single-crystal X-ray diffraction techniques. The coordination about the central holmium(III) ion is eightfold in a square-antiprismatic arrangement, while the ruthenium(II) ion is octahedrally coordinated. Channels permeating the crystal lattice contain the potassium cations and two zeolitic water mol­ecules. The HoIII and K atoms lie at sites with mm symmetry and the Ru atom is at a site with 2/m symmetry.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270103013350/ta1406sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270103013350/ta1406Isup2.hkl
Contains datablock I

Comment top

For over a century, hexacyanometalate ions have been extensively used as building blocks for bridged bimetallic assemblies. The rich chemistry of these prusside-type compounds has led to continued interest in cyanide-bridged lanthanide–transition metal complexes due to their varied uses in an array of practical applications. Mullica et al. (1996) demonstrated the structural properties of several early lanthanide double-salt hexacyanoruthenate(II) compounds having the general formula LnK[Ru(CN)6]·4H2O (Ln = La, Ce, Pr and Nd). These isomorphic complexes crystallize in the hexagonal space group P63/m with nine-coordinate lanthanide metal centers in a tricapped trigonal- prismatic geometry. Structural data for the title compound, (I), illustrate an eight-coordinate holmium complex that crystallizes in the orthorhombic space group Cmcm. These results are presented in order to provide a basis for future solid-state chemical studies dealing with the integration and association of diverse cations to applications of semipermeable membranes and the controlled hydration of zeolitic-type complexes.

Fig. 1 shows a representation of the polymeric structure of (I). The eight-coordinate holmium(III) ion is located in Wyckoff position 4c (0, y, 1/4) of the centrosymmetric space group Cmcm (No. 63), with its ligands arranged in a square-antiprismatic geometry (D4d). The holmium(III) metal center is linked to six [Ru(CN)6]4- fragments via the nitrogen ends of the cyano ligands in such a way as to form an infinite three-dimensional polymeric array. The two remaining coordination sites around the Ho atom are occupied by water molecules located trans to one another on one of the square faces.

The six-coordinate divalent Ru ions lie in the 4 b (0, 1/2, 1/2) positional site and are covalently bound by the cyano ligands in an octahedral fashion. The two crystallographically unique cyano groups are located in the special site 8f (C1, N1; axial) and the general site 16 h (C2, N2; equatorial).

Germane geometric parameters for (I) are included in Table 1, and the values of these parameters are reasonable when considering similar internuclear interactions in other eight-coordinate Ho species (Aslanov et al., 1971; Templeton et al., 1985; Du et al., 2000). The longer of the two Ho—N bonds corresponds to the isocyanide Ho—N1—C1 linkage, bridging to the ruthenium atom in an axial position, and most likely reflects a steric effect experienced by the bridging unit. The geometric parameters and coordination environment of the Ru atom are consistent with the expected directional (covalent) bonding nature of the CN- ligands to the Ru metal center. There is little directional influence in the bonding of the [Ru(CN)6]4- fragment to the Ho metal center, however, as indicated by the Ho—N—C bond angles of 167.9 (3) and 157.2 (4)°.

The potassium counter-ions and two uncoordinated zeolitic water molecules are contained in well defined channels that permeate the crystal lattice. These channels have a pore size of 5.518 (1) × 5.576 (1) Å, as estimated by calculating the Ru—CN—Ho distances that form the most restrictive channel parameters. The K+ ions are positioned in the channels so that the centroids of the six closest CN groups along with two O atoms (from the coordinated O1 atom) form a square-antiprismatic arrangement around the K atom. The two unique K···CN centroid separations are 2.99 and 2.92 Å for the C1—N1 and C2—N2 groups, respectively. The K—O1 distance is 2.936 (4) Å.

Intermolecular hydrogen bonding is an important aspect of the molecular structure of (I). Details of the hydrogen-bonding interactions are tabulated in Table 2. An IR study clearly supports two distinct types of water-molecule environment, viz. sharp spikes at 3601 and 3527 cm-1 indicate free O—H stretching, while broad bands over the 3425–3243 cm-1 region suggest the occurrence of O—H···O hydrogen bonds. The presence of hydrogen bonding is generally accepted if H···O contact distances are appreciably shorter than the van der Waals contact distance between the H and O atoms, and if the appropriate angles are significantly greater than 90°. Based on the maximum of the broad ν(OH) band at 3349 cm-1, the empirical rules of Nakamoto et al. (1955) predict a corresponding O—H···O distance of 2.82 Å, which correlates with the O1—H1···O2 distance obtained experimentally. Although the internuclear separations involved in the O2—H2A···N1 interaction appear suitable for a weak hydrogen bond, such an interaction is doubtful, especially after careful consideration of geometric constraints. Clearly, the location of atom H2A in the difference Fourier maps indicates that the H-atom orbitals are not in a position that would accommodate an effective overlap with the prospective cyano ligands. In fact, both of the H atoms on atom O2 can be considered free (i.e. non-H bound), conceivably giving rise to the sharp spikes in the ν(OH) region of the IR spectrum.

Experimental top

Single crystals of (I) were prepared by allowing dilute aqueous solutions of K4[Ru(CN)6] and Ho(NO3)3 polyhydrate to diffuse into one another via a glass salt bridge. After six months, pale-purple crystals large enough for single-crystal X-ray analysis were obtained. IR (KBr disc, Mattson-Cygnus 100 F T–IR): ν(OH)/free water 3601 (s), 3527 (w); ν(OH)/hydrogen bonding 3349 (s, br) with shoulders at 3425 (m) and 3243 (m); ν(CN) 2080 (versus), 2047 (sh); δ(HOH) 1641 (m), 1612 (m), 1384 (w); δ(RuCN) 567 (m), 549 (m); ν(RuC) 481 (m), 470 (s), 462 (s), 434 (m) cm-1.

Refinement top

H atoms were placed near appropriate peaks in difference Fourier maps and then restrained to reasonable geometric constraints; isotropic displacement parameters were fixed [Uiso(H) = 1.2Uiso(O)].

Structure description top

For over a century, hexacyanometalate ions have been extensively used as building blocks for bridged bimetallic assemblies. The rich chemistry of these prusside-type compounds has led to continued interest in cyanide-bridged lanthanide–transition metal complexes due to their varied uses in an array of practical applications. Mullica et al. (1996) demonstrated the structural properties of several early lanthanide double-salt hexacyanoruthenate(II) compounds having the general formula LnK[Ru(CN)6]·4H2O (Ln = La, Ce, Pr and Nd). These isomorphic complexes crystallize in the hexagonal space group P63/m with nine-coordinate lanthanide metal centers in a tricapped trigonal- prismatic geometry. Structural data for the title compound, (I), illustrate an eight-coordinate holmium complex that crystallizes in the orthorhombic space group Cmcm. These results are presented in order to provide a basis for future solid-state chemical studies dealing with the integration and association of diverse cations to applications of semipermeable membranes and the controlled hydration of zeolitic-type complexes.

Fig. 1 shows a representation of the polymeric structure of (I). The eight-coordinate holmium(III) ion is located in Wyckoff position 4c (0, y, 1/4) of the centrosymmetric space group Cmcm (No. 63), with its ligands arranged in a square-antiprismatic geometry (D4d). The holmium(III) metal center is linked to six [Ru(CN)6]4- fragments via the nitrogen ends of the cyano ligands in such a way as to form an infinite three-dimensional polymeric array. The two remaining coordination sites around the Ho atom are occupied by water molecules located trans to one another on one of the square faces.

The six-coordinate divalent Ru ions lie in the 4 b (0, 1/2, 1/2) positional site and are covalently bound by the cyano ligands in an octahedral fashion. The two crystallographically unique cyano groups are located in the special site 8f (C1, N1; axial) and the general site 16 h (C2, N2; equatorial).

Germane geometric parameters for (I) are included in Table 1, and the values of these parameters are reasonable when considering similar internuclear interactions in other eight-coordinate Ho species (Aslanov et al., 1971; Templeton et al., 1985; Du et al., 2000). The longer of the two Ho—N bonds corresponds to the isocyanide Ho—N1—C1 linkage, bridging to the ruthenium atom in an axial position, and most likely reflects a steric effect experienced by the bridging unit. The geometric parameters and coordination environment of the Ru atom are consistent with the expected directional (covalent) bonding nature of the CN- ligands to the Ru metal center. There is little directional influence in the bonding of the [Ru(CN)6]4- fragment to the Ho metal center, however, as indicated by the Ho—N—C bond angles of 167.9 (3) and 157.2 (4)°.

The potassium counter-ions and two uncoordinated zeolitic water molecules are contained in well defined channels that permeate the crystal lattice. These channels have a pore size of 5.518 (1) × 5.576 (1) Å, as estimated by calculating the Ru—CN—Ho distances that form the most restrictive channel parameters. The K+ ions are positioned in the channels so that the centroids of the six closest CN groups along with two O atoms (from the coordinated O1 atom) form a square-antiprismatic arrangement around the K atom. The two unique K···CN centroid separations are 2.99 and 2.92 Å for the C1—N1 and C2—N2 groups, respectively. The K—O1 distance is 2.936 (4) Å.

Intermolecular hydrogen bonding is an important aspect of the molecular structure of (I). Details of the hydrogen-bonding interactions are tabulated in Table 2. An IR study clearly supports two distinct types of water-molecule environment, viz. sharp spikes at 3601 and 3527 cm-1 indicate free O—H stretching, while broad bands over the 3425–3243 cm-1 region suggest the occurrence of O—H···O hydrogen bonds. The presence of hydrogen bonding is generally accepted if H···O contact distances are appreciably shorter than the van der Waals contact distance between the H and O atoms, and if the appropriate angles are significantly greater than 90°. Based on the maximum of the broad ν(OH) band at 3349 cm-1, the empirical rules of Nakamoto et al. (1955) predict a corresponding O—H···O distance of 2.82 Å, which correlates with the O1—H1···O2 distance obtained experimentally. Although the internuclear separations involved in the O2—H2A···N1 interaction appear suitable for a weak hydrogen bond, such an interaction is doubtful, especially after careful consideration of geometric constraints. Clearly, the location of atom H2A in the difference Fourier maps indicates that the H-atom orbitals are not in a position that would accommodate an effective overlap with the prospective cyano ligands. In fact, both of the H atoms on atom O2 can be considered free (i.e. non-H bound), conceivably giving rise to the sharp spikes in the ν(OH) region of the IR spectrum.

Computing details top

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: XCAD4 (Harms, 1993); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Figures top
[Figure 1] Fig. 1. A view of the title compound showing the atom-numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 40% probability level. The potassium cation has been omitted for clarity. [Symmetry labels: (a) x, y, 1/2 - z; (b) 1/2 - x, 1/2 - y, 1 - z); (c) 1/2 - x, 1/2 - y, -1/2 + z); (d) -1/2 + x, 1/2 - y, 1 - z); (e) -1/2 + x, 1/2 - y, -1/2 + z; (f) -x, y, 1/2 - z); (g) x, 1 - y, 1 - z; (h) -x, y, z; (i) -x, 1 - y, 1 - z.]
[Figure 2] Fig. 2. A view of the crystal packing along the a-direction indicating the location of both the potassium cations and the uncoordinated water molecules in channels which permeate the lattice.
(I) top
Crystal data top
K[HoRu(CN)6]·2H2OF(000) = 992
Mr = 533.28Dx = 2.631 Mg m3
Orthorhombic, CmcmMo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2c 2Cell parameters from 25 reflections
a = 7.4019 (11) Åθ = 10–15°
b = 12.762 (3) ŵ = 7.28 mm1
c = 14.2535 (11) ÅT = 293 K
V = 1346.4 (4) Å3Parallelepiped, pale purple
Z = 40.15 × 0.11 × 0.07 mm
Data collection top
Enraf–Nonius CAD-4
diffractometer
982 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.021
Graphite monochromatorθmax = 29.9°, θmin = 2.9°
ω–2θ scansh = 210
Absorption correction: numerical
(SHELXTL/PC; Sheldrick, 1995)
k = 217
Tmin = 0.397, Tmax = 0.608l = 219
1795 measured reflections3 standard reflections every 120 min
1080 independent reflections intensity decay: 3.1%
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.018H-atom parameters constrained
wR(F2) = 0.041 w = 1/[σ2(Fo2) + (0.0119P)2 + 7.6136P]
where P = (Fo2 + 2Fc2)/3
S = 1.11(Δ/σ)max < 0.001
1080 reflectionsΔρmax = 0.73 e Å3
64 parametersΔρmin = 1.03 e Å3
5 restraintsExtinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: structure-invariant direct methodsExtinction coefficient: 0.00270 (12)
Crystal data top
K[HoRu(CN)6]·2H2OV = 1346.4 (4) Å3
Mr = 533.28Z = 4
Orthorhombic, CmcmMo Kα radiation
a = 7.4019 (11) ŵ = 7.28 mm1
b = 12.762 (3) ÅT = 293 K
c = 14.2535 (11) Å0.15 × 0.11 × 0.07 mm
Data collection top
Enraf–Nonius CAD-4
diffractometer
982 reflections with I > 2σ(I)
Absorption correction: numerical
(SHELXTL/PC; Sheldrick, 1995)
Rint = 0.021
Tmin = 0.397, Tmax = 0.6083 standard reflections every 120 min
1795 measured reflections intensity decay: 3.1%
1080 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0185 restraints
wR(F2) = 0.041H-atom parameters constrained
S = 1.11Δρmax = 0.73 e Å3
1080 reflectionsΔρmin = 1.03 e Å3
64 parameters
Special details top

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 (Å2) top
xyzUiso*/Ueq
Ho0.00000.16990 (2)0.25000.01019 (9)
N10.00000.2812 (3)0.3921 (3)0.0223 (8)
O10.2531 (5)0.2885 (3)0.25000.0296 (7)
H10.32120.28800.29620.060 (16)*
K0.00000.46556 (12)0.25000.0244 (3)
C10.00000.3580 (3)0.4353 (3)0.0164 (8)
Ru0.00000.50000.50000.01067 (10)
C20.1949 (4)0.4489 (2)0.58943 (19)0.0156 (5)
O20.50000.3321 (4)0.3957 (3)0.0438 (10)
H2A0.50000.29740.44420.053*
H2B0.50000.39310.41470.053*
N20.3000 (4)0.4221 (2)0.64496 (19)0.0229 (5)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Ho0.00991 (12)0.01051 (12)0.01014 (12)0.0000.0000.000
N10.027 (2)0.0167 (16)0.0235 (18)0.0000.0000.0066 (16)
O10.0219 (15)0.0409 (19)0.0259 (16)0.0146 (16)0.0000.000
K0.0287 (7)0.0260 (7)0.0186 (6)0.0000.0000.000
C10.0170 (18)0.0178 (18)0.0143 (17)0.0000.0000.0016 (15)
Ru0.01170 (19)0.01066 (18)0.00964 (19)0.0000.0000.00031 (15)
C20.0177 (12)0.0162 (12)0.0128 (11)0.0017 (11)0.0011 (11)0.0002 (10)
O20.047 (2)0.050 (2)0.034 (2)0.0000.0000.000 (2)
N20.0205 (12)0.0293 (13)0.0189 (11)0.0087 (12)0.0018 (10)0.0016 (11)
Geometric parameters (Å, º) top
Ho—O12.409 (3)Ru—C2viii2.032 (3)
Ho—O1i2.409 (3)Ru—C2ix2.032 (3)
Ho—N12.474 (4)Ru—C1vii2.034 (4)
Ho—N1ii2.474 (4)C2—N21.161 (4)
Ho—N2iii2.411 (3)O2—H2A0.82
Ho—N2iv2.411 (3)O2—H2B0.82
Ho—N2v2.411 (3)N2—Hoiv2.411 (3)
Ho—N2vi2.411 (3)K—O12.936 (4)
N1—C11.157 (5)K—C12.977 (4)
O1—H10.83K—N13.105 (4)
C1—Ru2.034 (4)K—C2vii2.917 (3)
Ru—C2vii2.032 (3)K—N2vii3.038 (3)
Ru—C22.032 (3)
O1—Ho—O1i102.15 (19)N2v—Ho—N1ii142.01 (7)
O1—Ho—N2iii141.61 (6)N2vi—Ho—N1ii76.77 (10)
O1i—Ho—N2iii80.11 (10)N1—Ho—N1ii109.93 (19)
O1—Ho—N2iv80.11 (10)C1—N1—Ho157.2 (4)
O1i—Ho—N2iv141.61 (6)H1—O1—Ho117.9
N2iii—Ho—N2iv121.70 (13)N1—C1—Ru174.8 (4)
O1—Ho—N2v141.61 (6)C2vii—Ru—C2180.0
O1i—Ho—N2v80.11 (10)C2vii—Ru—C2viii90.41 (16)
N2iii—Ho—N2v76.78 (13)C2—Ru—C2viii89.59 (16)
N2iv—Ho—N2v75.77 (13)C2vii—Ru—C2ix89.59 (16)
O1—Ho—N2vi80.12 (10)C2—Ru—C2ix90.41 (16)
O1i—Ho—N2vi141.61 (6)C2viii—Ru—C2ix180.0
N2iii—Ho—N2vi75.77 (13)C2vii—Ru—C1vii89.90 (11)
N2iv—Ho—N2vi76.78 (13)C2—Ru—C1vii90.10 (11)
N2v—Ho—N2vi121.70 (13)C2viii—Ru—C1vii89.90 (11)
O1—Ho—N168.86 (7)C2ix—Ru—C1vii90.10 (11)
O1i—Ho—N168.86 (7)C2vii—Ru—C190.10 (11)
N2iii—Ho—N1142.01 (7)C2—Ru—C189.90 (11)
N2iv—Ho—N176.77 (10)C2viii—Ru—C190.10 (11)
N2v—Ho—N176.77 (10)C2ix—Ru—C189.90 (11)
N2vi—Ho—N1142.01 (7)C1vii—Ru—C1180.0
O1—Ho—N1ii68.86 (7)N2—C2—Ru175.8 (3)
O1i—Ho—N1ii68.86 (7)H2A—O2—H2B103
N2iii—Ho—N1ii76.77 (10)C2—N2—Hoiv167.9 (3)
N2iv—Ho—N1ii142.01 (7)H1—O1—H1ii105
Symmetry codes: (i) x, y, z+1/2; (ii) x, y, z+1/2; (iii) x1/2, y+1/2, z1/2; (iv) x+1/2, y+1/2, z+1; (v) x1/2, y+1/2, z+1; (vi) x+1/2, y+1/2, z1/2; (vii) x, y+1, z+1; (viii) x, y+1, z+1; (ix) x, y, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O1—H1···O20.832.022.821 (4)162
O2—H2A···N1iv0.822.543.353 (6)171
Symmetry code: (iv) x+1/2, y+1/2, z+1.

Experimental details

Crystal data
Chemical formulaK[HoRu(CN)6]·2H2O
Mr533.28
Crystal system, space groupOrthorhombic, Cmcm
Temperature (K)293
a, b, c (Å)7.4019 (11), 12.762 (3), 14.2535 (11)
V3)1346.4 (4)
Z4
Radiation typeMo Kα
µ (mm1)7.28
Crystal size (mm)0.15 × 0.11 × 0.07
Data collection
DiffractometerEnraf–Nonius CAD-4
Absorption correctionNumerical
(SHELXTL/PC; Sheldrick, 1995)
Tmin, Tmax0.397, 0.608
No. of measured, independent and
observed [I > 2σ(I)] reflections
1795, 1080, 982
Rint0.021
(sin θ/λ)max1)0.701
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.018, 0.041, 1.11
No. of reflections1080
No. of parameters64
No. of restraints5
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.73, 1.03

Computer programs: CAD-4 Software (Enraf-Nonius, 1989), CAD-4 Software, XCAD4 (Harms, 1993), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), ORTEP-3 for Windows (Farrugia, 1997), SHELXL97.

Selected geometric parameters (Å, º) top
Ho—O12.409 (3)C2—N21.161 (4)
Ho—N12.474 (4)K—O12.936 (4)
Ho—N2i2.411 (3)K—C12.977 (4)
N1—C11.157 (5)K—N13.105 (4)
C1—Ru2.034 (4)K—C2ii2.917 (3)
Ru—C22.032 (3)K—N2ii3.038 (3)
O1—Ho—O1iii102.15 (19)C1—N1—Ho157.2 (4)
O1—Ho—N2iv141.61 (6)N1—C1—Ru174.8 (4)
O1—Ho—N2v80.11 (10)C2ii—Ru—C2180.0
N2iv—Ho—N2v121.70 (13)C2ii—Ru—C2vii90.41 (16)
N2iv—Ho—N2vi76.78 (13)C2—Ru—C2vii89.59 (16)
N2v—Ho—N2vi75.77 (13)C2ii—Ru—C1ii89.90 (11)
O1—Ho—N2i80.12 (10)C2—Ru—C1ii90.10 (11)
O1—Ho—N168.86 (7)C2—Ru—C189.90 (11)
N2iv—Ho—N1142.01 (7)N2—C2—Ru175.8 (3)
N2v—Ho—N176.77 (10)C2—N2—Hov167.9 (3)
Symmetry codes: (i) x+1/2, y+1/2, z1/2; (ii) x, y+1, z+1; (iii) x, y, z+1/2; (iv) x1/2, y+1/2, z1/2; (v) x+1/2, y+1/2, z+1; (vi) x1/2, y+1/2, z+1; (vii) x, y+1, z+1.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O1—H1···O20.832.022.821 (4)162
O2—H2A···N1v0.822.543.353 (6)171
Symmetry code: (v) x+1/2, y+1/2, z+1.
 

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