Speciation and transformation of Co(II)/Ni(II)–citrate–imidazole ternary system – synthesis, spectroscopic and structural studies

Yuan-Fu Deng, Zhao-Hui Zhou*, Ze-Xing Cao and Khi-Rui Tsai

Department of Chemistry, State Key Laboratory for Physical Chemistry of Solid Surface, Xiamen University, Xiamen 361005, China

Received 2 February 2004;
Revised 19 March 2004;
accepted 23 March 2004.
Available online 22 April 2004.

Abstract

The cobalt and nickel imidazole citrate complexes [Co(Im)6][CoΔ(Im)3(Hcit)] [CoΛ(Im)3(Hcit)] · 4H2O (1) and [Ni(Im)6][NiΔ(Im)3(Hcit)] [NiΛ(Im)3(Hcit)] · 4H2O (2) (Im=imidazole, H4cit=citric acid) were synthesized in aqueous solutions, which were isolated in pure crystalline forms and characterized spectroscopically. The X-ray structural analyses revealed that both compounds are isomorphous and consist of two types of metal ions in different coordination modes. One is octahedrally bound by six imidazole ligands, and the other is quasi-octahedrally coordinated by three imidazole and one citrate ligands, in which the citrate ion coordinates to the metal ions tridentately through greek small letter alpha-hydroxyl, greek small letter alpha-carboxyl and one of the β-carboxyl groups, leaving the other deprotonated β-carboxyl group free, which form intramolecular strong hydrogen bond with greek small letter alpha-hydroxyl group. The co-existences of [M(Im)6]2+ and [M(Im)3(Hcit)]− in the double salts (1) and (2) (M=Co2+, Ni2+) show an equilibrium between imidazole and imidazole–citrate species, which is further supported by the conversions of imidazole or citrate complexes to (1) and (2). The formations of the mixed ligand complexes present a case that the Co2+ and Ni2+ might interact competitively with monodentate imidazole and tridentate citrate ligands.

Author Keywords: Cobalt; Nickel; Imidazole; Citrate
Article Outline

1. Introduction
2. Experimental
2.1. Preparation of [Co(Im)6][CoΔ(Im)3(Hcit)] [CoΛ(Im)3(Hcit)] · 4H2O (1)
2.2. Preparation of [Ni(Im)6][NiΔ(Im)3(Hcit)][NiΛ(Im)3(Hcit)] · 4H2O (2)
2.3. Transformation of [Co(Im)6](NO3)2 to (1)
2.4. Transformation of [Ni(Im)6](NO3)2 to (2)
2.5. Transformation of (NH4)4[Co(Hcit)2] to (1)
2.6. Transformation of (NH4)4[Ni(Hcit)2] · 2H2O to (2)
2.7. Transformation of (1) to (NH4)4[Co(Hcit)2]
2.8. Transformation of (2) to (NH4)4[Ni(Hcit)2]  · 2H2O
2.9. X-ray crystallographic study
3. Results and discussion
3.1. Synthesis
3.2. Crystal structures
3.3. Spectroscopic properties
3.4. Supporting information
Acknowledgements
References

这篇文章是我博士期间的额外收获，当初由于博士课题遇到一些麻烦，所以顺便找了一个简单三元体系来研究物种形成和分布。过渡金属、柠檬酸、咪唑这三种物质都存在与生物体内，所以选择这三元体系，在接近人体内的pH值范围内，从合成化学的角度出发，考察这三元体系在水容溶液中的物种分布。从合成的到物种分析得出：柠檬酸作为螯合配体，它的配位能力比咪唑强，当柠檬酸过量时，柠檬酸能够置换咪唑配体，形成金属柠檬酸络合物；