Synthesis, spectroscopic, and structural aspects of two trinuclear cyano-bridged heterometallic complexes

Struct Chem (2006) 17:139–147DOI 10.1007/s11224-006-9045-x

ORIGINAL PAPER

Synthesis, spectroscopic, and structural aspects of two trinuclearcyano-bridged heterometallic complexesBrajagopal Samanta · Joy Chakraborty ·Nirmal Kumar Karan · R. K. Bhubon Singh ·Glean P. A. Yap · Christoph Marschner ·Judith Baumgartner · Samiran Mitra

Received: 28 November 2005 / Accepted: 3 March 2006 / Published online: 13 May 2006C© Springer Science+Business Media, Inc. 2006

Abstract Two new cyano-bridged trinuclear heterometal-lic complexes [Sr2(Phen)4(CF3CO2)(H2O)3Fe(CN)6]·2H2O(1) [Ca2(Phen)4(CF3CO2)(H2O)Co(CN)6]·2H2O (2) (wherePhen = 1,10-phenanthroline) have been synthesized andtheir crystal structures have been determined. The structureof complex (1) features a central [Fe(CN)6]3− unit that linksa monocation, [Sr(Phen)2(OH2)(OOCCF3)]+ and a dication,[Sr(Phen)2(OH2)2]2+ via two trans cyanide bridges. Thecomplex (2) features a central [Co(CN)6]3− unit that linkstwo monocations of [Ca(Phen)2(OH2)(OOCCF3)]+ (the po-sitions of the trifluoro acetate and water molecules are disor-dered over two positions) via two trans cyanide bridges. Eachmetal atom is seven coordinated and achieves pentagonalbipyramidal geometry. Two cocrystallized water moleculesare present in both the complexes. The presence of an ex-tensive network of hydrogen bonding imparts the overallstability to both the systems.

B. Samanta · J. Chakraborty · N. K. Karan · S. Mitra (�)Department of Chemistry, Jadavpur University,Kolkata 700032, Indiae-mail: smitra [email protected]

R. K. B. SinghDepartment of Chemistry, Nagaland University,Mokokchung 798601, Nagaland, India

G. P. A. YapDepartment of Chemistry and Biochemistry, University ofDelaware,Newark, DE 19716, USA

C. Marschner · J. BaumgartnerInstitut fur Anorganische Chemie, Technische Universitat Graz,Stremayrgasse 16,A-8010 Graz, Austria

Keywords Cyano bridge . Strontium–iron complex .

Calcium–cobalt complex

Introduction

The coordination chemistry of alkaline earth metals re-mained underdeveloped till date. Recently, there have been afew studies on these metals in both aqueous and nonaqueousmedia [1–5]. The alkoxides [6–8], β-diketonates [9–12],hydroxyapatites [13] and cucurbit[n]uril (the trivial name oforganic macrocyclic compounds obtained by condensationof formaldehyde with glycoluril) compounds [14] of groupII metals are of current interest because of their promisingapplications as Metal Organic Chemical Vapor Deposition(MOCVD) precursors for metal oxide superconductorsand functional ceramics such as biomaterials, fertilizers,conductors, sensors, etc. and supramolecular catalysts. Asalkaline earth metals, i.e. calcium, magnesium, strontium,etc. are essential biological elements because of theirinvolvement in DNA, protein synthesis [15–17], parathyroidhormone secretion activity and protector of life-energyproduction within the cell. They also play an important rolein the activation of enzyme by complexing with nucleic acidsinside the cell necessary for nerve impulse transmission,muscle contraction, and metabolism of carbohydrates [18,19]. Strontium specially helps to harden the calcium–magnesium–phosphorus structures of the body and alsocontrols the intake or structural use of calcium. Although thecarboxylato bridged bimetallic complex has been the subjectof extensive studies in mimetic bioinorganic chemistrybut carboxylato and cyano bridged complexes of barium,calcium, and strontium are of very few. Nowadays, thecyano bridged mixed valence complexes are being studiedbecause of their remarkable diversity of structural types. In

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most of these complexes, the bridging agent is [M(CN)6]n−

(n = 2, 3, 4 and M = Mn, Fe, Cr, etc). Recently, Wester-hausen et al. [20] have extended these studies in case ofalkaline-earth metals producing (THF)4Ca[M(CO)5)(CN)]2

(THF = Tetrahydrofuran, M = Cr, Mo, W). Some cyano-bridged alkaline earth metal complexes, i.e. ([Sr3(Phen)6

(H2O)6{Fe(CN)6}2]·Phen·6.5H2O)n [21], [Sr2(Phen)4

(H2O)3(NO3)Fe(CN)6]·H2O [22], [Ca2(Phen)4(H2O)3

Fe(CN)6(OH2)NO3]·H2O [23], [{Ba2(Phen)4(H2O)6Fe(CN)6·Cl2(Phen)·3H2O]n [24], [Ca2(Phen)4(H2O)6Fe(CN)6][{Ca(Phen)2(H2O)2Fe(CN)6}2]·3(Phen)·(PhenH)·14H2O·CH3OH [25] have already been reported. In con-tinuation of these studies, two cyano-bridged trimericseven-cordinated complexes of Sr(II) and Ca(II) with1,10-phenanthroline, trifluoroacetate, and [M(CN)6]3−

ligand are being reported in the present work. Synthesis,spectral, thermal, and structural characterization of both thecomplexes, [Sr2(Phen)4(CF3COO)(H2O)3Fe(CN)6]·2H2O(1) and [Ca2(Phen)4(CF3COO)(H2O)Co(CN)6]·2H2O (2)have been carried out.

Materials

All the chemicals and solvents used for the synthesiswere of reagent grade. Sr(CF3COO)2 and Ca(CF3COO)2

were prepared according to the ascribed procedure [26].1,10-Phenanthroline, K3[Fe(CN)6] (Loba Chemie, India),K3[Co(CN)6] (Fluka) were obtained commercially and usedas received without purification. All the solvents were driedusing standard methods before use.

Physical measurements

Elemental analyses were carried out using Perkin-Elmer2400 II elemental analyzer. The IR spectra were recorded ona Perkin-Elmer RXI-FT-IR spectrophotometer in the range4000–200 cm−1 as KBr pellets. Thermal investigation wascarried out on a Shimadzu TGA-50 thermal analyzer undera dynamic nitrogen environment.

Synthesis of the complexes

An aqueous solution (15 mL) of K3Fe(CN)6 (0.33 g, 1 mmol)was added to a stirred mixture of 10 mL of an aqueoussolution of Sr(CF3COO)2 (0.399 g, 2 mmol) and 10 mLof a methanolic solution of 1,10-phenanthroline monohy-drate (0.79 g, 4 mmol). On slow evaporation of the sol-vent at room temperature, the yellow crystals appearedafter 4–5 days. The crystals were washed with water,dried in air, and used for X-ray analysis. Anal. Calcd. forC56H42F3FeN14O7Sr2: C, 51.29; H, 3.22; N, 14.95, found:C, 51.41; H, 3.12; N, 14.99%. The complex 2 was prepared inthe same way as complex 1 by using K3[Co(CN)6] (330 mg,

1 mmol) K3[Fe(CN)6], Ca(CF3COO)2 (304 mg, 2 mmol),and 1,10-phenanthroline (790 mg, 4 mmol). Anal. Calc. forC56H38Ca2CoF3N14O5: C, 56.85; H, 3.23; N, 16.57, found:C, 57.12; H, 3.15; N, 17.1%.

X-ray data collection and structure refinement

A good-quality air-stable orange block type singlecrystal of 1 (size 0.22 mm × 0.18 mm × 0.15 mm)and colorless plate type single crystal of 2 (size0.48 mm × 0.25 mm × 0.10 mm) were selectedand mounted on APEX Bruker CCD area detector andBruker SMART APEX diffractometer for complexes 1and 2, respectively. Graphite-monochromatized Mo Kα

radiation (λ = 0.71073 Å) from a fine-focus sealed tube andthe ω scan technique was used to collect the data sets. Atotal of 28645 reflections (11346 independent reflections,Rint = 0.0273) for 1 and 9015 reflections (4390 independentreflections, Rint = 0.025) for 2 were collected in the range of2.41◦<θ<27.70◦ for 1 and 2.45◦<θ<24.71◦ for 2, applyingthe boundary condition I>2σ (I). The lattice componentswere determined by least-squares refinements of the angularsetting of 25 reflections near a θ value of 10◦. The stability ofthe crystals was checked by measuring standard reflectionsat fixed intervals during the data collection. However, nosignificant loss of intensity was noted. The data were alwayscorrected for Lorentz and polarization effects, crystal decayand absorption effects by the SADABS program [27a].Both the structures were solved by direct methods usingSHELXS-97 program [27b] and refined by full-matrix,least-squares methods using the SHELXL-97 program[27c]. The functions minimized were

∑w[|Fo|2 − [|Fc|2]2,

where w = [σ 2(I) + (0.0849P)2 + 0.3606P]−1 for 1and w = [σ 2(I) + (0.1188P)2 + 4.7636P]−1 for 2 withP = (|Fo|2 + 2|Fc|2)/3. All hydrogen atoms were computedand refined, using a riding model, with isotropic temperaturefactors equal to 1.2 times the equivalent temperature factorfor all atoms to which they are linked. The final R indiceswere 0.0462 for 1 and 0.0684 for 2 respectively for allthe observed reflections. A summary of the conditions forthe intensity data collection, structure solution, and somefeatures of the refinement parameters are given in Table 1.

Results and discussion

Infrared spectra

The IR spectra of the complexes 1 and 2 are consistent withthe structural data presented in this paper. The broad ab-sorption band for the ν(O–H) appeared in the region 3220–3420 cm−1 for both the complexes, indicating the pres-ence of water molecules in crystals. The sharp bands at

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Table 1 Crystallographic data and structure refinement details for complex 1 and 2

Complex 1 Complex 2

Empirical formula C56H42F3FeN14O7Sr2 C56H38Ca2CoF3N14O5

Formula weight 1311.13 1183.10Crystal system/space group Triclinic/P 1 Triclinic/P 1Unit cell dimensions

a (Å) 11.122(2) 9.804(2)b (Å) 12.504(3) 11.073(2)c (Å) 19.873(4) 12.390(3)α (◦) 77.852(3) 84.00(3)β (◦) 84.246(3) 78.43(3)γ (◦) 84.126(3) 84.21(3)

V (Å3)/Z 2679.0(9)/2 1306.1(5)/1Calculated density (mg/m3) 1.625 1.502Absorption coefficient (mm−1) 2.330 0.601F(0 0 0) 1322 604Crystal size (mm × mm × mm) 0.22 × 0.18 × 0.15 0.48 × 0.25 × 0.10Diffractometer used APEX Bruker CCD Bruker SMART APEXTemp. (K)/wavelength (Å) 120(2)/0.71073 100(2)/0.71073θ range for data collection (◦) 2.41–27.70 2.45–24.71Limiting indices − 13 ≤ h ≤ 14, − 16 ≤ k ≤ 16, − 24 ≤ l ≤ 25 − 11 ≤ h ≤ 11, − 13 ≤ k ≤ 13, − 14 ≤ l ≤ 14Total/uniq. data/R(int) 28645/11346/0.0273 9015/4390/0.025Observed data [I>2σ (I)] 8865 3963Absorption correction Multi-scan SADABSData/restraints/parameters 11346/33/778 4390/2/405Weighting scheme, w [σ 2(F2

o ) + (0.0849P)2 + 2.3606P]−1 [σ 2(F2o ) + (0.1188P)2 + 4.7636P]−1

Final R indices [I>2σ (I)] R1 = 0.0462, wR2 = 0.1301 R1 = 0.0684, wR2 = 0.1808R indices (all data) R1 = 0.0621, wR2 = 0.1401 R1 = 0.0728, wR2 = 0.1853Max. diff. peak and hole (e Å−3) 1.077 and − 1.046 2.015 and − 0.787

1680 and 1640 cm−1 for complexes 1 and 2 respectivelyindicate the δ(H–OH) mode. Again a sharp band at 399and 418 cm−1 for complexes 1 and 2 respectively may beassigned to ρ

w(H2O) and the bands at 843, 861 cm−1 and801, 848 cm−1 for complexes 1 and 2 respectively may beassigned to ρ

r(H2O). These results show the presence ofboth coordinated and lattice water molecules [26]. For theν(C≡N) mode of [M(CN)6]3−, two main band systems ofcomparable intensities are observed. The low-intensity bandat 2112 and 2160 cm−1 for complexes 1 and 2 respectivelyare assigned to the inter-metallic C≡N stretching and high-intensity band around 2049 and 2128 cm−1 for complexes 1and 2 respectively are assigned to the terminal C≡N stretch-ing mode. So our results suggest that the contribution ofthe terminal cyanide group is high. The characteristic ab-sorption bands at 1588, 1502, 1420, 1136, 1091, 853, 738,623 cm−1 may be attributed to 1,10-phenanthroline. The IRspectrum contains νasym(COO−) and νsym(COO−) bands for1 appear at 1625 and 1343 cm−1 and for 2 appear at 1640and 1346 cm−1 respectively, giving a frequency difference(�ν) of 282 and 294 cm−1 respectively which reflects theunidentate nature of the carboxylate group [28]. The M–N

stretching frequency for 1 and 2 appears at 259, 241 cm−1

and 242, 225 cm−1 respectively [29].

Thermal analysis

The TGA trace of the complex 1 is stable upto 60◦C afterwhich it undergoes dehydration in two steps in betweenthe temperature range 61–115 and 116–221◦C. The firststep corresponds to the loss of two molecules of latticewater and next step corresponds to the loss of threemolecules of coordinated water. Above 260◦C it undergoesdecomposition in two steps. The first step correspondsto the loss of one trifluoro acetate ligand in between260–380◦C and the next step corresponds to the loss offour 1,10-phenanthroline molecules in a single step in thetemperature range 380–431◦C. The complex 2 is stable upto56◦C and beyond this temperature most of the mass lossproceeds in two temperature ranges 57–140 and 141–212◦C.The first process corresponds to the loss of one moleculeof lattice water and next step is consistent with the loss oftwo molecules of coordinated water. Above 260◦C it losesone trifluoro acetate ligand and four 1,10-phenanthroline

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Fig. 1 Perspective view of 1with atom numbering scheme.Thermal ellipsoids are drawn atthe 30% probability level

molecules in the temperature range 250–301◦C and302–340◦C respectively without any intermediate.

Description of crystal structures

The crystal structure with atom numbering scheme and im-portant bond lengths, bond angles of 1 are presented in Fig. 1and Table 2 respectively and that of 2 are presented in Fig. 2and Table 3 respectively.

[Sr2(Phen)4(CF3COO)(H2O)3Fe(CN)6]·2H2O (1)

The asymmetric unit of 1 consists of one[Sr(Phen)2(CF3COO)(H2O)]+ monocation, [Fe(CN)6]3−

anion, [Sr(Phen)2(H2O)2]2+ dication, and two watermolecules as lattice water. The central [Fe(CN)6]3− unitlinks the rest two cation units via two trans cyano bridges.The asymmetric unit is completed by two solvent watermolecules of crystallization and there is no molecularsymmetry owing to the presence of different strontiumco-ordination environment and thereby obtaining differentgeometries. Within the complex the [Fe(CN)6]3− ion hasthe usual expected 6-coordinated octahedral arrangementwith slight distortions as indicated by the variation ofthe cis angles [85.10(13)–94.12(13)◦] and trans angles[178.23(13)–179.22(13)◦] from the ideal value of 90 and180◦ respectively. The Fe–C and C≡N distances are groupedtogether in the narrow ranges [1.934(3)–1.961(4) Å] and[1.142(5)–1.155(4) Å] respectively. The Fe–C–N bondangles are almost linear, as indicated by the angles

[175.4(3)–179.6(3)◦], whereas the Sr–N–C bond angles arelargely deviated from linearity [142.6(2)–143.0(3)◦].

Though the geometry around iron is found regular butgeometries around strontium are irregular. Four nitrogendonor atoms from two phen units, one nitrogen fromCN of [Fe(CN)6]3− and one oxygen atom from each ofthe two water molecules form N5O2 donor environmentaround Sr(2) atom in a pentagonal bipyramidal geome-try in which five nitrogen atoms form a distorted pen-tagonal plane with two oxygen atoms from water occupythe trans axial positions [O(4)–Sr(2)–O(5) = 160.61(8)◦].Seven coordination is rather uncommon [22] for stron-tium complexes, whereas eight-coordination is very com-mon [29–31]. Sr(2)–O(H2O) and Sr(2)–N(phen) distanceslie in the range of 2.0496(2)–2.498(3) average 2.497 Åand 2.619(3)–2.692(3) Å, average 2.649 Å respectively.So, phen ligands are relatively weakly bonded to Sr(2).This Sr(2)–N(phen) and Sr(2)–O bond lengths are lowerthan that of the other reported Sr compounds [22, 31, 32].This difference in behavior is most probably due to theless steric crowding around seven-coordinated Sr atom. TheSr(1) atom is chelated by two phen ligands and directlybonded to one water molecule and one bridging cyano groupof [Fe(CN)6]3− anion and linked to one trifluoro acetateanion (CF3COO−]. The distances Sr(1)–N(1), Sr(1)–N(2),Sr(1)–N(3), Sr(1)–N(4) involving the phen ligand 2.665(3),2.681(3), 2.708(3), 2.661(3) Å, respectively, are quite dif-ferent from the other complex [31] with Sr–N (phen) dis-tances in the range 2.776(1) − 2.809(9) Å. The averageSr(1)–O(H2O) chelating distance [2.41(2) Å] is slightlyless than that of Sr(2)–O(H2O), average 2.497 Å. Whereas

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Table 2 Bond lengths (Å) andbond angles (◦) for complex 1

Bond lengths (Å)Sr(1)–O(1) 2.419(2) Sr(1)–O(3) 2.491(2) Sr(1)–N(9) 2.635(3)Sr(1)–N(4) 2.661(3) Sr(1)–N(1) 2.665(3) Sr(1)–N(2) 2.681(3)Sr(1)–N(3) 2.708(3) Sr(2)–O(4) 2.496(2) Sr(2)–O(5) 2.498(3)Sr(2)–N(10) 2.616(3) Sr(2)–N(5) 2.619(3) Sr(2)–N(8) 2.633(3)Sr(2)–N(6) 2.653(3) Sr(2)–N(7) 2.692(3) Fe–C(56) 1.934(3)Fe–C(54) 1.934(3) Fe–C(51) 1.941(3) Fe–C(52) 1.941(3)Fe–C(55) 1.950(3) Fe–C(53) 1.961(4) N(9)–C(51) 1.154(4)N(10)–C(52) 1.155(4) N(11)–C(53) 1.153(4) N(12)–C(54) 1.149(4)N(13)–C(55) 1.142(5) N(14)–C(56) 1.152(4)

Bond angles (◦)O(1)–Sr(1)–O(3) 159.55(9) O(1)–Sr(1)–N(9) 82.85(9)O(3)–Sr(1)–N(9) 83.15(9) O(1)–Sr(1)–N(4) 78.74(9)O(3)–Sr(1)–N(4) 114.48(9) N(9)–Sr(1)–N(4) 84.06(10)O(1)–Sr(1)–N(1) 83.77(9) O(3)–Sr(1)–N(1) 80.42(9)N(9)–Sr(1)–N(1) 85.90(9) N(4)–Sr(1)–N(1) 160.77(10)O(1)–Sr(1)–N(2) 81.79(9) O(3)–Sr(1)–N(2) 101.79(8)N(9)–Sr(1)–N(2) 145.40(9) N(4)–Sr(1)–N(2) 122.64(9)N(1)–Sr(1)–N(2) 61.76(9) O(1)–Sr(1)–N(3) 121.25(9)O(3)–Sr(1)–N(3) 79.20(8) N(9)–Sr(1)–N(3) 128.80(9)N(4)–Sr(1)–N(3) 61.35(10) N(1)–Sr(1)–N(3) 136.46(9)N(2)–Sr(1)–N(3) 85.49(9) O(4)–Sr(2)–O(5) 160.61(8)O(4)–Sr(2)–N(10) 82.24(8) O(5)–Sr(2)–N(10) 96.07(9)O(4)–Sr(2)–N(5) 78.21(8) O(5)–Sr(2)–N(5) 82.42(9)N(10)–Sr(2)–N(5) 87.73(9) O(4)–Sr(2)–N(8) 121.70(8)O(5)–Sr(2)–N(8) 76.83(8) N(10)–Sr(2)–N(8) 81.72(9)N(5)–Sr(2)–N(8) 155.51(9) O(4)–Sr(2)–N(6) 98.04(8)O(5)–Sr(2)–N(6) 73.75(9) N(10)–Sr(2)–N(6) 149.78(9)N(5)–Sr(2)–N(6) 63.05(9) N(8)–Sr(2)–N(6) 121.68(9)O(4)–Sr(2)–N(7) 82.94(8) O(5)–Sr(2)–N(7) 113.22(9)N(10)–Sr(2)–N(7) 124.26(9) N(5)–Sr(2)–N(7) 140.07(9)N(8)–Sr(2)–N(7) 62.08(9) N(6)–Sr(2)–N(7) 85.49(9)C(56)–Fe–C(54) 179.22(13) C(56)–Fe–C(51) 94.12(13)C(54)–Fe–C(51) 85.10(13) C(56)–Fe–C(52) 86.28(13)C(54)–Fe–C(52) 94.50(13) C(51)–Fe–C(52) 178.23(13)C(56)–Fe–C(55) 89.82(14) C(54)–Fe–C(55) 90.14(13)C(51)–Fe–C(55) 89.44(14) C(52)–Fe–C(55) 88.84(13)C(56)–Fe–C(53) 89.53(13) C(54)–Fe–C(53) 90.52(13)C(51)–Fe–C(53) 91.37(14) C(52)–Fe–C(53) 90.36(13)C(55)–Fe–C(53) 179.00(13)

Sr(1)–O(carboxylate) monodentate distance [2.419(2) Å] isslightly shorter than that of the reported complexes [33–35].The Sr(1) atom also exists in a seven-coordinated geom-etry defined by a N5O2 donor environment similar to Sr(2)center. But there is significant distortion away from a pentag-onal bipyramidal geometry as indicated by the O–Sr(1)–Oaxial angle 159.55(9)◦. The Sr–N–C angle is143.0(3)◦ forSr(1) and 142.6(2)◦ for Sr(2). As can be seen from Fig. 3,the structure is stabilized by a complicated network of hydro-gen bond interaction. The key hydrogen bonding contacts for1 are listed in Table 4. The O(3) water molecule forms twodonor interactions and bridges two cyanide nitrogen atoms,one intramolecularly to N(12) and another intermolecularlyto N(12) that results in the formation of a centrosymmet-

ric eight-membered (H–O–H···N···)2 ring. The O(4) watermolecule behaves in the same way, forming a similar eight-membered ring but involving two N14 atoms. The coordi-nated water molecule, O(5), forms an interaction with nonco-ordinating oxygen atoms of the trifluoroacetate coordinatingin the monodentate mode and forms a donor interaction tothe lattice water molecule of solvation, i.e. O(6). These O(6)and O(7) lattice water molecules are intermolecularly hy-drogen bonded to each other. Again the O(7) water moleculeforms one intermolecular donor interaction and bridges oneoxygen atoms of water, O(1). The H–acceptor and donor–acceptor distances are in the range of 2.724(4)–3.220(5) Åand 1.838(15)–2.38(4) Å respectively which fall within thegenerally accepted limits of H bonds. The donor–H–acceptor

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Fig. 2 ORTEP view of thecomplex 2 indicating the atomnumbering scheme. Thermalellipsoids are drawn at the 50%probability level

Table 3 Bond lengths (Å) and bond angles (◦) for complex 2

Bond lengthsCa(1)–O(2) 2.341(4) Ca(1)–O(3) 2.393(3) Ca(1)–N(6) 2.506(4)Ca(1)–N(3) 2.522(4) Ca(1)–N(2) 2.533(3) Ca(1)–N(4) 2.558(4)Ca(1)–N(1) 2.594(4) Co(1)–C(25) 1.900(4) Co(1)–C(25)a 1.900(4)Co(1)–C(26)a 1.907(4) Co(1)–C(26) 1.907(4) Co(1)–C(27)a 1.913(5)Co(1)–C(27) 1.913(5) N(3)–C(24) 1.366(6) N(4)–C(22) 1.330(8)N(4)–C(23) 1.354(8) N(5)–C(25) 1.152(5) N(6)–C(26) 1.150(5)N(7)–C(27) 1.152(6) C(1)–C(2) 1.396(7)Bond anglesO(2)–Ca(1)–O(3) 164.10(13) O(2)–Ca(1)–N(6) 92.46(15)O(3)–Ca(1)–N(6) 81.10(11) O(2)–Ca(1)–N(3) 84.44(13)O(3)–Ca(1)–N(3) 80.62(11) N(6)–Ca(1)–N(3) 85.77(13)O(2)–Ca(1)–N(2) 77.34(13) O(3)–Ca(1)–N(2) 115.78(11)N(6)–Ca(1)–N(2) 82.25(11) N(3)–Ca(1)–N(2) 157.66(13)O(2)–Ca(1)–N(4) 77.68(13) O(3)–Ca(1)–N(4) 100.72(11)N(6)–Ca(1)–N(4) 150.04(15) N(3)–Ca(1)–N(4) 65.33(15)N(2)–Ca(1)–N(4) 121.88(14) O(2)–Ca(1)–N(1) 116.56(15)O(3)–Ca(1)–N(1) 78.50(11) N(6)–Ca(1)–N(1) 127.05(12)N(3)–Ca(1)–N(1) 136.83(12) N(2)–Ca(1)–N(1) 64.40(12)N(4)–Ca(1)–N(1) 82.05(14) C(25)–Co(1)–C(25)a 180.0C(25)–Co(1)–C(26)b 86.50(16) C(25)a–Co(1)–C(26)a 93.50(16)C(25)–Co(1)–C(26) 93.50(16) C(25)a–Co(1)–C(26) 86.50(16)C(26)a–Co(1)–C(26) 179.999(1) C(25)–Co(1)–C(27)a 89.41(17)C(25)a–Co(1)–C(27)a 90.59(17) C(26)a–Co(1)–C(27)a 89.61(18)C(26)–Co(1)–C(27)a 90.39(18) C(25)–Co(1)–C(27) 90.59(17)C(25)a–Co(1)–C(27) 89.41(17) C(26)a–Co(1)–C(27) 90.39(18)C(26)–Co(1)–C(27) 89.61(18) C(27)a–Co(1)–C(27) 180.0C(26)–Co(1)–C(27) 89.61(18) C(27)a–Co(1)–C(27) 180.0

aSymmetry transformations used to generate equivalent atoms: − x, − y + 2, − zbSymmetry transformations used to generate equivalent atoms: − x, − y + 1, − z + 1

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Table 4 Key hydrogenbonding contacts (Å,◦) forcomplex 1

D–H···A d(D–H) d(H···A) d(D···A) ∠D − H···AO(3)–H(1)···N(12) 0.949(15) 2.022(16) 2.958(4) 169(3)O(3)–H(2)···N(12)a 0.958(15) 1.855(15) 2.804(4) 170(3)O(4)–H(3)···N(14) 0.955(15) 2.091(18) 3.002(4) 159(3)O(4)–H(4)···N(14)b 0.961(15) 1.838(15) 2.793(4) 172(3)O(5)–H(5)···O(2)c 0.911(15) 1.847(18) 2.724(4) 161(3)O(5)–H(6)···O(6) 0.998(16) 1.841(16) 2.810(4) 163(3)O(6)–H(7)···O(7) 0.931(19) 2.38(4) 3.029(7) 126(4)O(7)–H(9)···O(6) 1.09(2) 2.28(5) 3.029(7) 124(4)O(7)–H(10)···O(2)c 1.05(2) 2.17(5) 2.788(6) 116(4)O(7)–H(10)···O(1)c 1.05(2) 2.25(4) 3.220(5) 53(5)

aSymmetry transformationsused to generate equivalentatoms: − x + 2, − y, − zbSymmetry transformationsused to generate equivalentatoms: − x + 2, − y, − z + 1cSymmetry transformationsused to generate equivalentatoms: x − 1, y + 1, z

Fig. 3 Packing diagram of 1viewed down crystallographic aaxis

angle is in the range of 116(4)-172(3)◦ which indicates all un-symmetrical hydrogen bonds with donor H–acceptor angle indifferent ranges. Thus, this extensive network of H-bondingimparts overall stability to the crystal.

[Ca2(Phen)4(CF3COO)(H2O)Co(CN)6]·2H2O (2)

X-ray crystallographic study shows that the complex 2(Fig. 2) consists of a central [Co(CN)6]3− unit which linkstwo cations, [Ca(Phen)2(CF3CO2)(H2O)]+ via two transcyanide bridges. The Co(1) atom has an almost regular oc-tahedral geometry. The Co–C and C–N (cyano) distanceslie in the narrow ranges 1.900(4)–1.913(5) Å and 1.149(6)–1.151(7) Å respectively. The C–Co–C angle involving thetrans CN groups is perfectly 180◦ and those involving thecis groups vary from 86.52(19) to 93.48(19)◦ from the idealvalue of 90◦. These values are in well agreement with thoseobtained in other complexes [36–40]. There is only one tri-fluoro acetate and three water molecules in the asymmet-ric unit. Each calcium(II) ion is chelated to two bidentate

phen ligands, one partially occupied trifluoroacetate, onewater molecule and one trans bridging CN group of the[Co(CN)6]3− anion. The position of the trifluoro acetate isdisordered over two positions. So there are actually one waterand one trifluoroacetate molecule between the two calciumatoms. The positions of all these atoms except the coordinat-ing oxygen atoms therefore are only half occupied.

So, the Ca(II) center is seven-coordinated in a distortedpentagonal bipyramidal geometry. This type of geometryis very common for cyano-bridged calcium complexes [23,24]. The Ca(2)–N distances involving the two chelatedphen ligands [2.526(4)–2.592(4) Å] are much longer thanthat involving the bridging cyano group [2.506(4) Å]. TheCa–O(H2O) and Ca–(trifluoroacetate) distances also showsome variations [2.393(3)–2.341(4)Å]. The fact that the av-erage Ca(2)–N(phen) bond [2.553 Å] is much longer thanthe average Ca(2)–O bond [2.367 Å] and suggests that Caatoms are relatively weakly bonded to the phen ligands thanthe others. These Ca(2)–N(phen) and Ca(2)–O(H2O) dis-tances are well matched with the other similar complexes,

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Fig. 4 Packing diagram of 2

Table 5 Key hydrogen bonding contacts (Å,◦) for complex2

D–H···A d(D–H) d(H···A) d(D···A) ∠D–H···AO3–H1···N5 0.9800 2.0800 3.031(4) 166.00O3–H4···N5 1.0700 1.8100 2.828(4) 157.00C2–H8···N7 0.9500 2.4500 3.249(8) 142.00C21–H21···F3 0.9500 2.2200 2.789(14) 118.00C22–H22···F3 0.9500 2.2400 2.809(11) 118.00

i.e. (2.567(3)–2.617(3) Å [23], 2.512(2)–2.586(2) Å [24])and (2.435(2)–2.558(3) Å [23], 2.379(2)–2.382(2) Å [24])respectively. This variations and asymmetry in Ca–O(H2O)and Ca–N(phen) chelate bonds are most probably due tothe steric crowding around the Ca atoms. Each metal atomis seven coordinated and achieves pentagonal bipyramidalgeometry.

As can be seen from Fig. 4, the structure is stabilized by acomplicated network of H-bonding interaction. Table 5 sum-marizes key hydrogen bonding contacts for 2. The coordi-nated water molecule, O(3), forms two donor interactions andbridges two noncoordinated cyanide nitrogen atoms, one in-tramolecularly to N(5) and another intermolecularly to N(5)that in the formation of a centrosymmetric eight-membered(H–O–H···N···)2 ring.

Conclusion

In this paper, we presented the structures of twonovel cyanide-bridged heterometallic assemblies basedon hexacyanometalate as the building block, [Sr2(Phen)4

(CF3CO2)(H2O)3Fe(CN)6]·2H2O (1) and [Ca2(Phen)4(CF3

CO2)(H2O)Co(CN)6]·2H2O (2). The structural analysesshow that both the complexes consist of a trinuclear unit,M(Phen)2–NC–M′(CN)4–CN–M(Phen)2 [M = Sr or Ca andM′ = Fe or Co respectively]. The results of this studyof compounds 1 and 2 provide the evidence that hex-acyanometalate and bidentate phen ligands can be usedfor the extraction of strontium and calcium from aqueoussolutions.

Supplementary material

Crystallographic data has been deposited at theCambridge Crystallographic Data Centre with depositionNumber 289453 and 289454. Copies of the informationmay be obtained free of charge from The Director, CCDC,12 Union Road, Cambridge, CB2 IEZ, UK (fax: + 44-1223-336033; e-mail: [email protected] or www:http://www.ccdc.cam.ac.uk).

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Acknowledgement Financial assistance from DRDO and UGC, NewDelhi, India, is gratefully acknowledged.

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