Synthesis, crystal structure and properties of the cyano-bridged heteropolynuclear [Cu(meso)]3[Co(CN)6]2•9.5H2O compound

Abstracts

The novel compound [Cu(meso)]3[Co(CN)6]2•9.5H2O was prepared and characterized structurally, magnetically and spectroscopically (UV-vis, FTIR and EPR). The crystal structure is result of the self-assembly of three [Cu(meso)]2+ and two [Co(CN)6]3- units leading to a polymeric zigzag chain. Isolated [Cu(meso)(H2O)2]2+ units counterbalance the charge of the whole structure. The crystal packing shows a large hollow space in the structure and a significant number of water molecules are found in these cavities. Magnetic properties exhibited a very weak antiferromagnetic exchange interaction between the copper ions, with J = -0.3 cm-1. FTIR spectrum showed typical frequencies of n(O-H) at 3604 cm-1 and bridging and non-bridging n(CN) at 2184 and 2128 cm-1, respectively. The electronic spectrum of the title complex presented a broad and symmetric band at 547 nm, assigned to the overlaid z² → x²-y², xy → x²-y² and (xz, yz) → x²-y² transitions. EPR data at 77 K presented a typical axial spectrum with g// = 2.33 and g⊥ = 2.01, in accordance with the tetragonal distortion of the copper(II) ion as supported by the single-crystal X ray diffraction data.

polynuclear; cyano-bridge; single crystal X-ray diffractometry; antiferromagnetism

O composto [Cu(meso)]3[Co(CN)6]2•9.5H2O foi preparado e caracterizado por difratometria de raios X de monocristal, e por suas propriedades espectrais (UV-vis, FTIR, EPR) e magnéticas. A estrutura cristalina mostrou a formação de um polímero com conformação zigzag produzido pela combinação espontânea das unidades [Cu(meso)]2+ e [Co(CN)6]3-, na proporção estequiométrica de 3:2. Unidades isoladas de [Cu(meso)(H2O)2]2+ completam o balanço de carga do polímero. O empacotamento cristalino exibe cavidades contendo um grande número de moléculas de água. As propriedades magnéticas do polímero mostraram um acoplamento antiferromagnético fraco entre os íons de cobre, com constante de acoplamento J = -0,3 cm-1. Espectros de absorção na região do infravermelho apresentaram bandas típicas dos modos vibracionais n(O-H) em 3604 cm-1 e n(CN) em 2184 e 2128 cm-1 referentes aos ligantes cianeto em ponte e equatoriais, respectivamente. O espectro eletrônico do composto exibiu uma banda larga e simétrica em 547 nm, atribuída à sobreposição das seguintes transições z² → x²-y², xy → x²-y² and (xz, yz) → x²-y². O espectro de EPR a 77 K apresentou um padrão típico de 4 linhas com valores de g// = 2,33 e g⊥ = 2,01 característicos de um sistema S = 1/2 com simetria axial, coerente com a distorção tetragonal observada para o íon cobre(II) pelas distâncias cristalográficas.

ARTICLE

Synthesis, crystal structure and properties of the cyano-bridged heteropolynuclear [Cu(meso)]₃[Co(CN)₆]₂•9.5H₂O compound

Fabiana H. O. Ishiruji^I; Nivaldo L. Speziali^II; Maria G. F. Vaz^III; Fábio S. Nunes^*,I

^IDepartamento de Química, Universidade Federal do Paraná, CP 19081, 81531-990 Curitiba-PR, Brazil

^IIDepartamento de Física, Universidade Federal de Minas Gerais-ICEx, CP 702, 30123-970 Belo Horizonte-MG, Brazil

^IIIInstituto de Química, Universidade Federal Fluminense, Outeiro de São João Batista s/n, Campus do Valonguinho, Centro, Niterói-RJ, Brazil

ABSTRACT

The novel compound [Cu(meso)]₃[Co(CN)₆]₂•9.5H₂O was prepared and characterized structurally, magnetically and spectroscopically (UV-vis, FTIR and EPR). The crystal structure is result of the self-assembly of three [Cu(meso)]²⁺ and two [Co(CN)₆]^3- units leading to a polymeric zigzag chain. Isolated [Cu(meso)(H₂O)₂]²⁺ units counterbalance the charge of the whole structure. The crystal packing shows a large hollow space in the structure and a significant number of water molecules are found in these cavities. Magnetic properties exhibited a very weak antiferromagnetic exchange interaction between the copper ions, with J = -0.3 cm^-1. FTIR spectrum showed typical frequencies of n(O-H) at 3604 cm^-1 and bridging and non-bridging n(CN) at 2184 and 2128 cm^-1, respectively. The electronic spectrum of the title complex presented a broad and symmetric band at 547 nm, assigned to the overlaid z²→ x²-y², xy → x²-y² and (xz, yz) → x²-y² transitions. EPR data at 77 K presented a typical axial spectrum with g_// = 2.33 and g_⊥ = 2.01, in accordance with the tetragonal distortion of the copper(II) ion as supported by the single-crystal X ray diffraction data.

Keywords: polynuclear, cyano-bridge, single crystal X-ray diffractometry, antiferromagnetism

RESUMO

O composto [Cu(meso)]₃[Co(CN)₆]₂•9.5H₂O foi preparado e caracterizado por difratometria de raios X de monocristal, e por suas propriedades espectrais (UV-vis, FTIR, EPR) e magnéticas. A estrutura cristalina mostrou a formação de um polímero com conformação zigzag produzido pela combinação espontânea das unidades [Cu(meso)]²⁺ e [Co(CN)₆]^3-, na proporção estequiométrica de 3:2. Unidades isoladas de [Cu(meso)(H₂O)₂]²⁺ completam o balanço de carga do polímero. O empacotamento cristalino exibe cavidades contendo um grande número de moléculas de água. As propriedades magnéticas do polímero mostraram um acoplamento antiferromagnético fraco entre os íons de cobre, com constante de acoplamento J = -0,3 cm^-1. Espectros de absorção na região do infravermelho apresentaram bandas típicas dos modos vibracionais n(O-H) em 3604 cm^-1 e n(CN) em 2184 e 2128 cm^-1 referentes aos ligantes cianeto em ponte e equatoriais, respectivamente. O espectro eletrônico do composto exibiu uma banda larga e simétrica em 547 nm, atribuída à sobreposição das seguintes transições z²→ x²-y², xy → x²-y² and (xz, yz) → x²-y². O espectro de EPR a 77 K apresentou um padrão típico de 4 linhas com valores de g_// = 2,33 e g_⊥ = 2,01 característicos de um sistema S = 1/2 com simetria axial, coerente com a distorção tetragonal observada para o íon cobre(II) pelas distâncias cristalográficas.

Introduction

There has been a growing progress in the design of supramolecular architectures based on cyanometallate complexes for their relationship to molecular magnetism and electronic properties.^1-15 These building blocks, combined with coordinatively unsaturared systems such as cationic Schiff-bases, produce 1D, 2D or 3D network structures. These assemblies, also called Prussian blue analogues, gained importance due to the innumerous possibilities to prepare novel bimetallic extended lattices.

Most of these structures contain two different transition metal ions. Cyanide bridges are linear and suitable to communicate spin density as some compounds behave as high-temperature molecular magnets.⁵ The study of electronic and magnetic properties is necessary to a better understanding of potential metal-metal exchange interactions. These properties are intimately related to the structure and are a result of the nature of the building blocks, their relative orientation in the crystal lattice and interactions such as H-bonding and van der Waals. Therefore, controlling preparative methods is a key step in obtaining complexes with the desired structural and physical properties.^16,17

Our continuing interest in the tetradentate macrocyclic ligand meso = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (Scheme 1) encouraged us to investigate the reaction of [Co(CN)₆]^3- with [Cu(meso)]²⁺.^2,18,19 Herein we report the synthesis and characterization of the new trans-cyano-bridged [Cu(meso)]₃[Co(CN)₆]₂•9.5H₂O coordination polymer.

Experimental

Physical measurements and instrumentation

X-ray diffraction data collection was performed on an Oxford-Diffraction GEMINI-Ultra diffractometer (LabCri) using graphite-Enhance Source MoK_a radiation (λ = 0.71069 Å) at T = 120(2) K. Data integration and scaling of the reflections were performed with the Crysalis suite.²⁰ Final unit cell parameters were based on the fitting of all reflection positions. Empirical multi-scan absorption corrections using equivalent reflections were performed with the program SCALE3 ABSPACK.²¹

The structure was solved by direct methods using the SHELXS program.²² The positions of all atoms could be unambiguously assigned on consecutive difference Fourier maps. Refinements were performed using SHELXL based on F² through full-matrix least square routine.²³ Most of the hydrogen atoms, including those bound to nitrogen atoms and those in water molecules, could be identified in the final Fourier difference maps. In the final model refinement those bound to carbon were geometrically positioned and refined using a riding model;²⁴ for the others, distance restraints were imposed.

The magnetization measurement was performed on a Cryogenic SX-600 SQUID magnetometer in the temperature range 2-298 K with an applied field of 1000 Oe. Diamagnetic correction was evaluated from Pascal's constants.

UV-Vis spectra in the range 190-820 nm were obtained on a SHIMADZU UVPC 2401 spectrophotometer in dmf solutions.

X-band electron paramagnetic resonance (EPR) spectra were recorded on a Bruker ESP300-E instrument from dmf frozen solutions at 77 K.

Infrared spectra were obtained with a FTS3500GX Bio-Rad Excalibur series spectrophotometer in the region 4000-400 cm^-1 in KBr pellets.

Materials

Reagent grade chemicals were used in this work. The macrocyclic meso was prepared as published elsewhere.¹⁹

Preparation of [Cu(meso)]₃[Co(CN)₆]₂•9.5H₂O

[Cu(meso)](ClO₄)₂²⁵ (1.23 g; 2.25 mmol) was dissolved under reflux in 50 mL of water. After, 0.50 g (1.52 mmol) of K₃[Co(CN)₆] dissolved in 10 mL of water was added dropwise. Upon standing overnight at room temperature, purple crystals were colleted by filtration, washed with cold water and dried under vacuum. Yield was 0.65 g (42%). IR absorptions ν_max/cm^-1: 3604m ν(OH), 3224s ν(NH), 2976m ν(CH₂CH₂), 2876m ν(CH), 2128vs ν(CN)-bridge, 2184vw ν(CN)-terminal, 1462w δ(NH), 1371w δ(CNC), 1289vw δ(CNC), 1189m γ(HCH), 824vw γ(HCC), 622m, ν(CoC), 484w ν(CuN) and 1090-1100vs ν(ClO). Caution! Perchlorate salts are potentially explosive in the presence of organic substances: any sample heating must be avoided.

Results and Discussion

Crystallographic structure

The crystal structure of [Cu(meso)]₃[Co(CN)₆]₂•9.5H₂O is the result of the self-assembly of three [Cu(meso)]²⁺ and two [Co(CN)₆]^3- units. Figure 1 shows the molecular assembly, where each [Co(CN)₆]^3– linked up to the two nearest [Cu(meso)]²⁺via cyano bridges with the following atomic bond distances: Co-C2 1.891(4) Å, C2-N2 1.155(5) Å, N2-Cu2 2.612(3) Å, Co-C5 1.887(4) Å, C5-N5 1.158(8) Å, N5-Cu3 2.737(3) Å. The average non-bridging CN distance is 1.152(3) Å. The Co^III ion lies in a fairly octahedral environment. Mean values for bond lengths of Co-C_axial and Co-C_equatorial are 1.886 and 1.903 Å, respectively, while C2-Co-C5, N5-C5-Co, C5-Co-C1 angles are 178.20(14)º, 176.6(3) and 87.23(14)º in this order. The Cu^II ion suffers a strong tetragonal distortion as is evidenced by the much longer Cu-N_axial bond distances (Cu2-N2 2.612(3) Å and Cu3-N5 2.737(3) Å) when compared to the Cu-N_equatoral values, in average 2.040(3) Å. The Cu2 and Cu3 ions lie very far apart, with a Cu...Cu distance of 9.609 Å, whereas for adjacent Co...Cu distance is 5.245(5) Å. The asymmetric unit for each string is given by [(meso)Cu-NC-Co(CN)₅]^- and the charge between two of them is balanced by one isolated [Cu(meso)(H₂O)₂]²⁺ unit. Bond distances and angles in this work are close to the values reported by Mondal and co-workers for [Cu(dmpn)₂]₃[Co(CN)₆]₂•12H₂O, which also shows a zigzag conformation of alternate [Co(CN)]₆]^3- and [Cu(dmpn)₂]²⁺ ions (dmpn = 1-dimethylamino-2-propylamine).¹⁴

The cell unit reveals (Figure 2) the formation of polymeric zigzag chains with a very bent C5-N5-Cu3 angle of 132.08(3)º. In the [Cu(meso)]²⁺ units, each Cu atom is in an inversion center in the P-1 space group and, consequently, only half of the N and C atoms are non equivalent; the Cu and the four N atoms are on perfect planes. The crystal packing also shows a large hollow space in the structure, which accommodates a significant number of water molecules. There is an extended H-bonding network between the hydrogen and nitrogen atoms of terminal cyanide ligands. Crystallographic parameters and details of data collection and refinement are given in Table 1 and selected bonds lengths and angles are given in Table 2.

Other linear chain arrangements have been previously described in the literature. For example, Triki and coworkers²⁶ reported on the [{Cu(tnH)₂(H₂O)₂}{Fe(CN)₆}]•2H₂O (tn = 1,2-diaminopropane). Two copper(II)-N₄O₂ units are connected via hexacianoferrate(II) bridges, where cyanides are in a trans configuration and the iron ions show an almost perfect octahedral coordination. An interesting case is seen for [{(323)₂Cu₂(Cl)Co(CN)₆}_n]•2nH₂O (323 = N,N'-bis(3-aminopropyl)ethylenediamine), which forms 1-D zigzag chains along the crystallographic axes a and b. The propagation is completed by hydrogen bonding interactions between adjacent strings of atoms to form a highly symmetric 3-D network.²⁷

Magnetic properties

Magnetic measurements were carried out in the temperature range 2-298 K. The plot of χ_M as a function of temperature is displayed in Figure 3. The effective magnetic moment, µ_eff, at room temperature was determined as being 2.45 µ_B, close to the expected spin-only value (2.83 µ_B) for two non-interacting copper(II) (S=1) ions and the diamagnetism of [Co(CN)₆]^3– (low spin cobalt(III) ion, S = 0). The trinuclear [Cu(dmpn)₂]₂[Co(CN)₆]ClO₄•3H₂O complex reported by Mondal and co-workers exhibited a µ_eff of 2.50 µ_B.¹⁴ From room temperature down to 3.0 K, µ_eff remains practically constant, corresponding to two non-interacting Cu^II ions. Below this point, µ_eff drops abruptly and reaches 2.20 µ_B at 1.5 K and under an applied field of 1000 Oe. Analysis of the molar susceptibility was done using the isotropic Heisenberg-Dirac-van Vleck model with the Hamiltonian H = -2JS₁•S₂ and the theoretical expression for χ is given elsewhere.²⁸

χ_M = [C(1-p)(2e^2x)/1+3e^2x] + ¼Cp + T.I.P.

with x = |J|/kT.

Temperature-independent paramagnetism (TIP) was considered to be 1.00 × 10^-6 mL mol^-1. The expression of χ_M was corrected to include the fraction p of any paramagnetic impurity present and the Weiss term θ to account for any intermolecular interactions. Non-linear fittings carried out by varying J, g, p and θ led to estimated values for parameters of the theoretical expression to the experimental data.

Best fitting lines shown in Figure 3 yielded J = -0.30 cm^-1 and g = 2.13 in accordance with a very weak antiferromagnetic interaction between the copper ions. The magnetic interactions are practically insignificant, mainly due to the large value of the Cu...Cu separation (9.609 Å). Also, the magnetic orbital of the copper(II) ion is of the type d_x²-y², with x and y axes defined by the short Cu-N bonds, and the axial positions, which are occupied by cyanide ions, have a low-spin density. Although no further experimental support is available, intermolecular interactions between copper(II) in the chain and the isolated [Cu(meso)]²⁺ can not be ruled out as the Cu...Cu separation in this case is slightly lower, 8.467 Å.

It is worth note few examples of similar weak magnetic behavior that have been recently published. For example, the one-dimensional copper(II) coordination polymer linked by nitroprusside bridges, [Cu(L₂)][Fe(CN)₅(NO)]•3H₂O (L = tetraazamacrocycle), with g = 2.12 and J = -0.12 cm^-1;²⁹ the heteropolynuclear complex [{(Cu(dien))₂Co(CN)₆}_n] [Cu(dien)(H₂O)Co(CN)₆]•5nH₂O, where g = 2.14 and J = –1.02 cm^-1;¹⁵ [{Cu(tren)NC}₂Co(tren)](ClO₄)₅•2H₂O (tren = tris(2-aminoethyl)amine), with g = 2.06 and J = –0.094 cm^-1;³⁰ and [{Cu(tnH)₂(H₂O)₂}{Fe(CN)₆}]•2H₂O, which exhibits a Curie-Weiss constant of –0.98 K.²⁶

Infrared, electronic and EPR spectra

The infrared spectrum in KBr is consistent with the structural data presented and has molecular features of the macrocyclic ligand and the cyanometalate core. The occurrence of lattice water in the crystal structure is consistent with the band at 3604 cm^-1. Frequencies of the CN stretching mode provide information about the ligand-bridging mode. The high intensity and lowest-frequency ν(CN) band at 2128 cm^-1 was assigned to the equatorial terminal cyano ligands and the low intensity high-frequency vibration at 2184 cm^-1 to the intermetallic bridging cyanides. For comparison see the polynuclear complex [Cu(dmpn)₂]₃[Co(CN)₆]₂•12H₂O, that shows very similar values with bands at 2130 and 2180 cm^-1.¹⁴ Further, [Fe(tmphen)₂]₃[Co(CN)₆]₂ (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) exhibits bands at 2127 and 2167 cm^-1, respectively, while the free [Co(CN)₆]^3-ν(CN) band appears at 2126 cm^-1.⁵

The electronic spectra of [Cu(meso)]²⁺ in Figure 4A shows a low intensity structureless broad band centered at 587 nm, typical of copper(II), and has contributions from three ligand-field transitions: z²→ x²-y², xy → x²-y² and (xz, yz) → x²-y².³¹ The [Cu(meso)]₃[Co(CN)₆]₂•9.5H₂O compound exhibited a more symmetric band at 547 nm with a higher intensity, consistent with a significant change in the environment around the copper(II) ion after reacting with [Co(CN)₆]^3-. Complex [Cu(dmpn)₂]₃[Co(CN)₆]₂•12H₂O shows a band with a maximum at 590 nm with the same assignment.¹⁴

EPR spectrum in dmf at 77 K (Figure 4B) exhibits a four line EPR spectrum for Cu²⁺, I = 3/2, with parameters g_// = 2.33 and g_⊥ = 2.01. Since g_// > g_⊥, the unpaired electron resides in the x²-y² orbital. These results are consistently interpreted as being due to a tetragonal distortion around the copper(II) ion, which was also evidenced in the crystal structure by the longer Cu-N_axial bond lengths, as discussed above. The hexacyanocobaltate ion is diamagnetic and EPR silent.

Conclusions

In conclusion, hexacyanocobaltate(III) reacts with [Cu^II(meso)](ClO₄)₂ in water producing a 1D-polymeric aggregate formed by two units of [Cu^II(meso)]²⁺ bridged by one (µ-CN)Co^III(CN)₅^3– anion. Single crystal X-ray diffraction analysis revealed a zigzag arrangement of Co-CN-Cu atoms and a packing space in the crystallographic framework filled with water molecules. The copper(II) centers (S=1/2) are too far apart and the solid features a small antiferromagnetism. EPR, UV-vis data and bond distances are well in accordance with a significant tetragonal distortion at the copper center.

Supplementary Information

CCDC-706872 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK; fax: [phone]; email: [e-mail]).

Acknowledgments

Brazilian Research Councils CNPq, FAPEMIG, FINEP and FAPERJ (Brazil) supported this work. F. H. O. I., F. S. N., M. G. F. V. and N. S. thank CNPq for research fellowships. We also thank LDRX-UFF and Dr. Miguel A. Novak (UFRJ) for magnetic measurements.

Received: September 23, 2009

Web Release Date: March 11, 2010

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Publication Dates

• Publication in this collection
  05 Aug 2010
• Date of issue
  2010

History

• Received
  23 Sept 2009
• Accepted
  11 Mar 2010