OF THE CYANIDE-BRIDGED HETERONUCLEAR POLYMERIC COMPLEXES WITH 2-ETHYLIMIDAZOLE

Three new cyano-bridged heteronuclear polymeric complexes, [Cu(etim)3Ni(CN)4]n, {[Zn(etim)3Ni(CN)4]∙H2O}n and [Cd(etim)2Ni(CN)4]n (etim = 2-ethylimidazole, hereafter abbreviated as Cu–Ni– etim, Zn–Ni–etim and Cd–Ni–etim) have been prepared in powder form and characterized by FT-IR and Raman spectroscopies, thermal (TG, DTG and DTA) and elemental analyses. The spectral features of the complexes suggest that the Ni(II) ion is four coordinate with four cyanide-carbon atoms in a square planar geometry, whereas the Cu(II) and the Zn(II) ions of the Cu–Ni–etim and the Zn–Ni–etim complexes are completed by nitrogen atoms of two cyano groups of [Ni(CN)4] 2coordinated to the adjacent M(II) ions and three nitrogen atoms of the etim ligands. The Cd(II) ion of the Cd–Ni–etim complex is six-coordinate, completed with the two nitrogen atoms of the etim ligands and the four nitrogen atoms from bridging cyano groups. Polymeric structures of the Cu–Ni–etim and the Zn–Ni–etim complexes are 1D coordination polymer, while complex the Cd–Ni–etim presents a 2D network. The thermal decompositions in the temperature range 30–700 C of the complexes were investigated in the static air atmosphere.


INTRODUCTION
Coordination polymers include metal ions linked by coordinated ligands into an endless array.This endless net must be defined by coordination bonds.Coordination polymers can form surprising supramolecular contractions which indicate regulation one, two, three dimensional due to coordination bonds.Cyanometallate complexes in coordination polymers have an important place and they are known to be among the first coordination compounds that has been prepared, studied, described and used up to now.Therefore, they have still continued actuality in the coordination chemistry [1][2][3].Additionally, cyanometallate complexes exhibiting polymeric structures have found many application areas such as electrical conductivity [4], molecular sieves and ion exchange materials [5], molecular magnetic nanowires [6] and host-guest systems [7].

Measurements
Elemental analyses were performed using a CHNS-932 (LECO) analyzer for C, H and N at the Middle East Technical University Central Laboratory in Ankara, Turkey.The FT-IR spectra of the etim and the complexes were recorded as KBr pellets in the range of 4000-400 cm -1 (2 cm -1 resolution) on a Perkin Elmer 100 FT-IR spectrometer which was calibrated using polystyrene and CO 2 bands.The Raman spectra of the complexes were recorded in the range of 4000-250 cm -1 on a Bruker Senterra Dispersive Raman instrument using laser excitation of 785 nm.A Perkin Elmer Diamond TG/DTA thermal analyzer was used to record simultaneous TG, DTG and DTA curves of the complexes in a static air atmosphere at a heating rate of 10 K min -1 in the temperature range 30-700 ºC using platinum crucibles.

2-Ethylimidazole vibrations
The etim molecule containing two nitrogen atoms can donate electrons.The nitrogen atom can coordinate with a transition-metal ion and form complexes with different properties.The molecular structure of the etim is shown in Scheme 1.The FT-IR and Raman spectra of all of the complexes are given in Figures 1 and 2, respectively.The tentative assignments of the modes of vibration of the free etim and that of the etim molecules in the complexes are listed in Table 1.The FT-IR spectrum of the Zn-Ni-etim complex contains absorption bands originating from  (OH) and δ (OH) vibrations.In the Zn-Ni-etim complex, the absorption bands observed from 3700 to 3328 cm -1 may be attributed to the  (OH) stretching mode of the water molecule.The δ (OH) bending mode is also observed at about 1600 cm -1 in the Zn-Ni-etim complex.In all the heterocyclic compounds, the  (NH) stretching vibrations are typically located in the region 3500-3300 cm -1 [20].However, we have observed the  (NH) stretching vibrations of the free etim at ca. 3153 cm -1 in FT-IR spectrum.The  (NH) stretching vibrations of the etim ligand in the FT-IR and Raman spectra of the complexes were found to be between 3414 cm -1 and 3162 cm -1 and shifted to a higher frequency than the corresponding value of the etim molecule.This shift shows that the etim acts as monodentate ligand through the nitrogen atom of the imidazole ring.The in-plane (1375 cm -1 ) and out-ofplane (753 cm -1 ) bending vibrations of etim ligand are also supported to these shifts.The  (CH) stretching vibrations arising from imidazole ring of the free etim are assigned at about 3136 cm -1 in the FT-IR spectrum and these vibrations are observed in the frequency range of 3171-3134 cm -1 in the vibration spectra of the complexes.As seen in Table 1, the etim ligand shows infrared bands in the 3110-2894 cm -1 region which are assigned to the  (CH3) and  (CH2) asymmetric and symmetric stretching frequencies, respectively.The absorption bands of the  (CH3) and  (CH2) groups in the complexes are observed in the region 3141-2932 cm -1 , and significantly shifted to lower or higher frequencies compared to that of the free ligand.The ρ (CH2) in-plane rocking vibration frequency is identified at 1105 cm -1 in the FT-IR spectrum of the etim and shifted to higher frequency compared to the free ligand in the complexes.The  (CH3) in-plane and  (CH3) out of plane bending modes are found at 1165 and 1123 cm -1 in the FT-IR spectrum, respectively.The bands obtained at 1009 and 957 cm -1 in FT-IR spectrum of the etim are assigned to CH 3 in-plane and out-of plane rocking modes, respectively.The  (C=N) stretching vibration in the imidazole is assigned to the wavenumber 1675 cm -1 in FT-IR spectrum.The observed bands at 1576, 1483 and 1453 cm -1 in FT-IR spectrum of the etim ligand are assigned to  (C-N) stretching vibration.The  (CC) stretching vibrations are observed at 1431 and 1395 cm -1 in FT-IR spectrum and these vibrations in the complexes shifted to higher frequency region around 1-11 cm -1 at 1431 cm -1 and lower frequency region around 1-12 cm -1 at 1395 cm -1 , compared with the free etim molecule.These shifts can be explained as the coupling of the internal modes of the etim molecule with M-N(etim) vibrations.Similar shifts on coordination have been observed in the vibration spectra of the other metal-coordinated imidazole complexes [15][16][17][18][19][20][21].

[Ni(CN) 4 ] 2-group vibrations
The modes of vibration of the [Ni(CN) 4 ] 2-group of the complexes are given in Table 2.In order to assign the bands attributable to the [Ni(CN) 4 ] 2-ion in the spectra, we refer to the work of McCullough et al. who presented vibrational data for the salt Na 2 Ni(CN) 4 in the solid state [22].Symmetrical vibration band [ν(CN), E u ] of complex K 2 [Ni(CN) 4 ]H 2 O is observed at 2120 cm -1 , A 1g and B 1g symmetry stretching vibrations arising from Raman active ν (C≡N) vibration are observed at 2160 and 2137 cm -1 .In FT-IR and Raman spectra of the complexes the dominant feature is represented by strong and sharp well identifiable absorption bands due to the ν (C≡N) stretching vibrations [1,23].The cyanide groups exhibit strong and sharp absorption bands in vibrational spectra between 2200 and 2100 cm -1 , due to the stretching vibrations of the cyanide group.The ν (C≡N) stretching vibrations are an important tool to distinguish between terminal and bridging character in the cyanide complexes.The bridging cyanide frequencies in cyano-bridged complexes are usually observed at higher wavenumbers than those of the terminal cyanide.Shifting to higher wavenumbers of the cyanide stretching frequency upon the formation of cyanide bridges occur due to both the kinematic coupling and increasing the CN force constant when a second metal center is attached to the CN unit.In the FT-IR spectra of the complexes, the ν (C≡N) absorption bands are observed at 2173, 2160, 2135 and 2128 cm -1 (for the Cu-Nietim), 2173 and 2143 cm -1 (for the Zn-Ni-etim) and 2142 cm -1 (for the Cd-Ni-etim).Thus, in the Cu-Ni-etim complex, the two high wavenumbers, 2173 cm -1 and 2160 cm -1 , correspond to the bridging cyanide and the two low wavenumbers, 2135 cm -1 and 2128 cm -1 , correspond to the terminal cyanide.Additionally, in the Zn-Ni-etim complex, the two  (CN) bands, which appeared at 2173 and 2143 cm -1 , can be assigned to the bridging and terminal cyanide groups, respectively.However, the Cd-Ni-etim complex contains only one ν (C≡N) absorption band at 2142 cm -1 belonging to the bridging cyano groups.From the FT-IR spectra of the complexes, it is determined that the  (C≡N) stretching vibrational mode of the cyanide has shifted to approximately 22-53 cm -1 higher frequency (see Table 2).The cyanide stretching bands in the Raman spectra of the complexes were observed at 2166 and 2131 cm -1 in the Cu-Ni-etim, at 2182 and 2142 cm -1 in the Zn-Ni-etim, at 2156 cm -1 in the Cd-Ni-etim, respectively.On the other hand, the in-plane bending vibrational mode, δ (NiCN) was observed at 414 cm -1 in the FT-IR spectrum of the K 2 [Ni(CN) 4 ]H 2 O which was shifted to the higher frequency range around 1-12 cm -1 for the complexes.This result supports the shift of ν (C≡N) stretching vibrational band.Such frequency shifts have been observed for the other cyanide complexes [15,16,19,24].The representative illustrations of the complexes are shown in Scheme 2.
Table 1.The modes of vibration of the etim in the complexes (cm -1 ).
The symbols ν, δ and  refer to valence, in-plane and out-of-plane vibrations, respectively.s strong, m medium, w weak, sh shoulder, v very.The bands observed in the Raman spectra are given in parentheses.

Thermal analysis
Thermal decomposition behaviors of the complexes were performed in temperature range 30-700 C in static air atmosphere.Thermoanalytical data and thermal decomposition curves of the complexes are given in Table 3 and  and Zn(II) ions are five coordinate with the three etim ligands and the two bridging cyanide groups, whereas the Cd(II) ion is six coordinate with the two etim ligands and the four bridging cyanide groups.We also showed that thermal analyses of the complexes support the spectroscopic results.

Scheme 1 .
Scheme 1.The molecular structure of the etim.

Figure 1 .
Figure 1.The FT-IR spectra of the etim and the complexes.

Figure 2 .
Figure 2. The Raman spectra of the complexes.

Scheme 2 .
Scheme 2. The representative illustration of the Cu-Ni-etim and the Zn-Ni-etim (a) and the Cd-Ni-etim (b).The water molecule is not shown in the Zn-Ni-etim.
Figures 3-5, respectively.Thermal decomposition of the complexes proceeds in the three stages.In the first stage, the Cu-Ni-etim and the Cd-Ni-etim complexes lose the one etim ligand in a single step in the temperature range of 161-192 C for the Cu-Ni-etim and 98-291 C for the Cd-Ni-etim, but the Zn-Ni-etim complex loses one water molecule between 30 and 160 C.In the second stage, the endothermic peaks in the temperature range 192-407 ºC for the Cu-Ni-etim and 160-463 ºC for the Zn-Ni-etim correspond to the loss of the two etim ligands but in the temperature range 291-407 ºC for the Cd-Ni-etim are released the one etim ligand.These values were found as following; the two etim ligands for the Cu-Ni-etim and the Zn-Ni-etim, the one etim ligand for the Cd-Ni-etim [found (calcd.)(%) = 35.17(37.35) for the Cu-Ni-etim, 33.05 (35.96) for the Zn-Ni-etim and 19.64 (20.56) for the Cd-Ni-etim].In the following stages of the Cu-Ni-etim and the Cd-Nietim complexes, the remaining four cyanide groups decompose as exothermic in the 407-476 ºC temperature range for the Cu-Ni-etim and in the 407-435 ºC temperature range for the Cd-Nietim.In the next step in the Zn-Ni-etim complex, the remaining four cyanide groups and the one etim ligand decompose between 463 and 504 C.The final decomposition products were identified as CuO and NiO for the Cu-Ni-etim, ZnO and NiO for the Zn-Ni-etim and CdO and NiO for the Cd-Ni-etim [found (calcd.)(%) = 28.78(29.96) for the Cu-Ni-etim, 28.42 (29.19) for the Zn-Ni-etim and 42.08 (43.45) for the Cd-Ni-etim].The thermal decomposition products were identified by FT-IR spectroscopy.

Figure 5 .
Figure 5.The TG, DTG and DTA curves of the Cd-Ni-etim.