Synthesis and characterization of heterobimetallic complexes of the type [Cu(pn)₂][MCl₄] where M = Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II)

2.1 Preparation of [Cu(pn)₂]Cl₂

[Cu(pn)₂]Cl₂ was synthesized by literature method with a slight modification (Siddiqi et al., 1993a–d). To a hot solution of CuCl₂·2H₂O (1.66 g, 0.01 mol) in MeOH (25 cm³) was added dropwise 1,3-diaminopropane (1.671 ml, 0.02 mol) dissolved in the same solvent. The dark blue precipitate formed instantaneously was refluxed for 5–6 h (Scheme 1) and left for nearly ten days which yielded dark blue flakes. It was washed with MeOH followed by Et₂O and dried in vacuum for seven days. M.P. ∼190–200 °C.

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Scheme 1

Synthesis of [Cu(pn)₂]Cl₂.

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2.2 Preparation of [Cu(pn)₂][MCl₄]

To a methanolic solution (15 cm³) of [Cu(pn)₂]Cl₂ (0.01 mol) was added divalent metal chloride dissolved in the same solvent (0.01 mol). On stirring the mixture for ten min dark colored precipitates (Table 1) were obtained (Scheme 2). It was filtered and washed with MeOH followed by Et₂O and dried in vacuum over anhydrous CaCl₂ for 7 days. Any attempt to prepare such complexes with Cr(II), Mn(II) and Fe(III) was unsuccessful. The quantity used and per cent yield of all the complexes are given below.

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Scheme 2

Synthesis of [Cu(pn)₂] [MCl₄].

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2.3 No. of moles, weight (g) of the reactants and per cent yield of the products

• $\underset{0.01\text{mol}(1.66\text{g})}{\text{Cu}{(\text{pn})}_2{\text{Cl}}_2}+\underset{0.01\text{mol}(2.91\text{g})}{{\text{CoCl}}_2·6{\text{H}}_2\text{O}}\to \underset{\text{yield}=32\%}{[\text{Cu}{(\text{pn})}_2][{\text{CoCl}}_4]}$

• $\underset{0.01\text{mol}(1.66\text{g})}{\text{Cu}{(\text{pn})}_2{\text{Cl}}_2}+\underset{0.01\text{mol}(2.71\text{g})}{{\text{NiCl}}_2·6{\text{H}}_2\text{O}}\to \underset{\text{yield}=41\%}{[\text{Cu}{(\text{pn})}_2][{\text{NiCl}}_4]}$

• $\underset{0.01\text{mol}(1.66\text{g})}{\text{Cu}{(\text{pn})}_2{\text{Cl}}_2}+\underset{0.01\text{mol}(1.70\text{g})}{{\text{CuCl}}_2·2{\text{H}}_2\text{O}}\to \underset{\text{yield}=45\%}{[\text{Cu}{(\text{pn})}_2][{\text{CuCl}}_4]}$

• $\underset{0.01\text{mol}(1.66\text{g})}{\text{Cu}{(\text{pn})}_2{\text{Cl}}_2}+\underset{0.01\text{mol}(1.66\text{g})}{\text{Zn}{\text{Cl}}_2}\to \underset{\text{yield}=45\%}{[\text{Cu}{(\text{pn})}_2][{\text{ZnCl}}_4]}$

• $\underset{0.01\text{mol}(1.66\text{g})}{\text{Cu}{(\text{pn})}_2{\text{Cl}}_2}+\underset{0.01\text{mol}(1.82\text{g})}{{\text{CdCl}}_2}\to \underset{\text{yield}=57\%}{[\text{Cu}{(\text{pn})}_2][{\text{CdCl}}_4]}$

• $\underset{0.01\text{mol}(1.66\text{g})}{\text{Cu}{(\text{pn})}_2{\text{Cl}}_2}+\underset{0.01\text{mol}(2.71\text{g})}{{\text{HgCl}}_2}\to \underset{\text{yield}=54\%}{[\text{Cu}{(\text{pn})}_2][{\text{HgCl}}_4]}$

2.4 Physical measurements

Elemental analyses were carried out with Carlo-Erba 1106. Molar conductance was measured with ELICO Conductivity Bridge CM 183. The IR spectra (600–4000 cm⁻¹) were recorded with Interspec 2020 FTIR SPECTROLAB, UK as KBr pellet. Far IR spectra (200–500 cm⁻¹) were recorded using CsBr. Electronic spectra were recorded with UV–vis spectrophotometer UV min-1240. Chlorine was analyzed as AgCl. The TGA was done with Mettler Star^e SW 8.10 under nitrogen atmosphere using Al₂O₃ as reference.

3.1 Electronic spectra

Two types of absorption bands were observed for all the complexes in their UV–visible spectra (Figs. 1a, 1b). The intense band ranging from 250 to 300 nm is unambiguously attributed to the L → M charge transfer (Bernhardt et al., 2001). All compounds of Cu(II) with diamines exhibit a band around 250 nm which is assigned to ligand to metal charge transfer. Zelenak and co-workers have recently reported from the electronic spectral study and crystal structure that the UV band at 250 nm in [Cu(pn)₂Cl₂] is attributed to ligand to metal charge transfer (Zelenak et al., 2006). Intraligand charge transfer does not occur in such complexes. Secondly, the Lapporte-forbidden d–d transition (400–800 nm) is due to d⁹ of Cu(II) ion (Nami et al., 2010). The precursor complex [Cu(pn)₂]Cl₂ shows a broad d–d band at 630 nm which indicates an octahedral geometry. However in bimetallic complex the same band appears at ca. 680 nm, indicating the presence of square-planar geometry (Gaura et al., 1982). The spectra slightly differ from each other with regard to the position of the d–d band, while the absorption band in the UV region remains nearly unaltered (Table 2). The peak in the visible region varies according to the nature of the metal. The λ_max in the visible region slightly increases with decreasing ionic radii of the metal ions.

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Figure 1a

UV spectra of complexes.

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Figure 1b

Visible spectra of complexes.

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Talbert et al. (1970), have suggested that [Cu(pn)₂][CuCl₄] is a complex salt with tetrahedrally coordinated chloro anion. We have observed a broad band at 279 nm in its electronic spectrum which is probably due to charge transfer from ligand to metal. It is also supported by additional (Cu–Cl) bands at 284 cm⁻¹ and 234 cm⁻¹ in its IR spectrum which is in agreement with values previously reported for [CuCl₄]²⁻ ion (Figgis, 1976). The geometry of [CuCl₄]²⁻ is, therefore, suggested to be tetrahedral stabilized by [Cu(pn)₂]²⁺ ion (Parent et al., 2007) .

3.2 IR spectra

As a consequence of the reaction of CuCl₂ with 1,3-diaminopropane a square-planar Cu chelate complex is formed bearing two chloride ions out of the coordination sphere. The IR spectrum of the 1,3-diaminopropane (Table 3) shows two strong (N-H) absorption bands in 3450–3250 cm⁻¹ region which undergoes a negative shift after chelation with copper(II) forming a six membered ring (Feerrari et al., 1991; Silverstien et al., 1981; Nakamoto, 1986). The in-plane vibration of the six membered rings so formed may be expected to be coupled mechanically to some extent with metal nitrogen vibrational modes.

On the basis of normal coordinate analysis of the [M(en)₂]²⁺ ion with C_2h symmetry Omura et al. (1971) have assigned the ν(Cu–N) band in 472–412 cm⁻¹ region. We have noted strong bands in 400–420 cm⁻¹ region for all the complexes (Malik et al., 1983). The M–Cl stretching frequency (Kabanos et al., 1992) in the 250–380 cm⁻¹ region in our case indicates the presence of tetrahedral tetrachlorometallate anion (Exarchos et al., 2001). The bands in the 600–400 cm⁻¹ are assigned to in-plane ring deformation and NH₂ rocking modes (Nami and Siddiqi, 2004). All the ν(Cu–N) and ν(M–Cl) were placed on a firm basis by the initial studies on stretching vibrations of the ${\text{MX}}_4^{2-}$ . However, it is clear from the previous work on ν(Cd–Cl) that these are invariably below 200 cm⁻¹ in octahedral complexes and only slightly above 200 cm⁻¹ in tetrahedral complexes. An additional IR band is observed in the region 230–350 cm⁻¹ which is assigned to M-Cl stretching frequency of [MCl₄]²⁻ moiety (Clark, 2010).

3.3 Thermogravimetry

The TGA profile of all the complexes are essentially similar and consists of four well defined stages implying their similar structural features. The weight loss data corresponding to various steps in thermogram were compared with those calculated. The thermal analysis of [Cu(pn)₂][CuCl₄] was studied (Fig. 2) in the temperature range 50–700 °C at a heating rate of 10 °C min⁻¹. During first break in the temperature range 150–250 °C, the weight loss of ∼6.15% (cal. 6.01%) could be accounted for the loss of 3/2 mol of H₂O from the complex which is believed to be absorbed during storage. In the second step (250–340 °C) the weight loss of ∼32.65% (cal. 33.33%) corresponds to the elimination of two moles of 1,3-diaminopropane moiety. In the third and fourth step two chloride ions are lost one by one at each step (340–430 °C) a weight loss of ∼14.81% (Cal. 15.98%) and (430–700 °C) a weight loss of 13.95% (Cal. 15.98%). In the end the residue 28.63% corresponds to Cu bimetals (Materazzi et al., 2002).

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Figure 2

Thermogram of [Cu(pn)₂][CuCl₄].

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