Synthesis and characterization of ternary complexes of chromium(III) with l-histidine and various diols

1 Introduction

Ternary transition metal ion complexes of amino acids are of great importance from the biological point of view (Abdel-Mawgoud and Abdel-Hamid, 1987; Seng et al., 2012; Faheim et al., 2013). A thorough literature survey has revealed that little attention has been paid to ternary metal complexes containing amino acids. Equilibrium studies on the Schiff base complex systems of Co, Ni, Cu and Zn(II)-vanillin(van)(A)-l-valine(val)/l-glutamine(gln)/l-glutamic acid(glu)/l-histidine (his)(B) have been studied (Nair et al., 2007). Ternary complexes of Co(II), Ni(II), Zn(II), Cd(II), Mg(II) and Ca(II) with adenosine-5′-triphosphate (ATP) as the primary ligand and glycine, alanine, valine, norvaline, leucine, serine, methionine, threonine, aspartic acid, uracil and thymine as secondary ligands have also been studied by other workers (Sastry and Gupta, 1998). Copper(II) complexes of amino acids and peptides with the chelating bis(imidazolyl) residues have been reviewed by other researchers (Sovago et al., 2003). Ahmed et al. (1998) studied the mixed ligand complex of Cu, Ni and Co using dicarboxylic amino acids as primary ligands and 8-hydroxyquinoline as secondary ligand (Ahmed et al., 1998). The interaction of some metal ions with aspartic and glutamic acid has also been investigated using infrared photodissociation spectroscopy (O-Brien et al., 2008). Siu et al. (2008) studied the dissociations of two types of copper(II)-containing complexes of tryptophan, tyrosine and phenylalanine (Siu et al., 2008). Mixed metal Zn(II)-molybdenum(IV) peroxo complexes containing glycylglycine, glycine and acetic acid have also been studied (Sastry and Gupta, 1997). A new series ternary complexes of Cu(II), Ni(II), Co(II) and Zn(II) have also been designed and synthesized using a Schiff base derived from 4-aminoantipyrine and o-phenylenediamine (Raman et al., 2008). Konig has also analyzed and reported the spectral data on the complexes of transition metal ions (Konig, 1971). Transition metal ion complexes of several compounds have also been investigated for various biological activities (Josyphus and Nair, 2010; Mahmud et al., 2010; Gwaram et al., 2016; Zhou et al., 2013). The stability constants of mixed ligand ternary complexes of Ni(II) with histidine and various diols were also determined in the previous literature (Gesawat et al., 2010). Most of these studies have been performed with divalent ions, paying little attention on transition metal ion complexes with trivalent ions like chromium (Mandlik and Aswar, 2003; Maples et al., 2009). In the present research work, we performed the synthesis, evaluated the results of analytical, spectral and magnetic studies and characterization of ternary complexes of Cr(III) (M) with l-histidine (A) and various diols (B) of the type MAB, MAB₂ and MA₂B. The three spin allowed and one spin forbidden transitions were experimentally observed in these complexes. In this treatment, we have tested and applied the methods which may be used to obtain a numerical fit to the relevant experimental data. An interesting check on the accuracy of the method has been provided by calculating the extra band energy.

2 Experimental

3 Results and discussion

The d³ configuration of Cr(III) gave two quartet state and five doublet state ⁴F, ⁴P, ²P, ²G, ²D, ²H and ²F. In an octahedral field, ⁴F was split up into ⁴A₂, ⁴T₁ and ⁴T₂ states (Fowles et al., 1967; Henning et al., 1967). The only other ligand field states that have been observed in Cr(III) spectra are ⁴T₁(P), ²E₁, ²T₁ and ²T₂ derived from ²G state. Experimentally, we have observed three spin allowed transition ⁴A₂ → ⁴T₂ at around 18,000 cm⁻¹, ⁴A₂ → ⁴T₁(F) at 34,000 cm⁻¹, ⁴A₂ → ⁴T₁(P) at around 39,000 cm⁻¹ and a spin forbidden due to transition ⁴A₂ → ²E at around 12,800 cm⁻¹. These transitions have also been calculated by using numerical fitting method by employing following equations (Figgis and Lewis, 1959; Fowles et al., 1967; Henning et al., 1967; Konig, 1971; Vogel, 1971; Raman et al., 2008):

The nephelauxetic ratio β₃₅ was computed by using the relationship:

The value of β for complexes was less than that of free ion clearly indicating that as a result of complexation, the metal orbitals expand and this cause the nephelauxetic effect. The magnitude of nephelauxetic effect (1 − β) showed the trend in the incidence of covalent character in the metal–ligand bond.

Cr(III) ion (d³ ion) is a hard metal-ion and thus naturally prefers hard ligand centers for coordination. l-histidine has additional coordination sites in addition to normal amino nitrogen and carboxylate oxygen informed secondary and tertiary nitrogen of imidazole moiety. The pH titration studies have been carried out in detail and their results point out that the secondary nitrogen of imidazole moiety liberates proton even at pH lower than 7. Thus the complexes which have been isolated in the pH range 6.5–7.5 have unidentate carboxylate coordinate and coordination from secondary nitrogen from imidazole moiety. The structure of histidine is such that all the three nitrogen atoms present in the molecule cannot interact simultaneously as these do not lie in a plane. The presence of diols moiety with oxygen donor atoms restricts the further coordination of nitrogen atom, as the coordination affinity of oxygen for Cr(III) is much more than that of nitrogen. The pseudo-aromaticity of imidazole contributes partially for increased covalency of histidine chelates as compared to similar chelates with alanine. The possible structural formulae of these complexes are given in Fig. 1.

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Figure 1 Possible structural formulae of ternary complexes of Cr (III) with l-histidine and various diols at different ratios A (1:1:1), B (1:2:1) and C (1:1:2).

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4 Conclusions

Ternary complexes of Cr(III) with l-histidine monohydrochloride and various diols were successfully synthesized and characterized. The values of Dq/B were of greater magnitude for MA₂B type species as compared to MAB and MAB₂ species. Values of nephelauxetic ratio β₅₅ (for spin forbidden band) were found more than β₃₅ (for spin allowed transition).