Synthesis and spectroscopic characterization of cobalt(III)-alkyl amine complexes showing surface affinity: Single crystal X-ray structure determinations

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Abstract

Structurally integrated cobalt(III) complexes showing interesting surface affinity in the interfacial electron transfer reactions were synthesized by incorporating alkyl amines into the coordination sphere of cis-[CoIII(en)2(RNH2)Cl]Cl2; (where RNH2 = MeNH2 (1), EtNH2 (2), PrnNH2 (3), BunNH2 (4), BuiNH2 (5), PennNH2 (6), HexnNH2 (7) and BzNH2 (8)) through a modified synthetic route. Such complexes are playing important role as electron acceptors in the interfacial electron transfer reactions taking place between metal complex and nanosized semiconductor particles in energy conversion schemes. The complexes were characterized by spectral, 1H NMR and 13C NMR techniques, which indicate the 1,2-diamino ethane site angles are closely similar forming five membered gauche configuration. Single crystal X-ray refinements were made to explore the structures of five complexes (2)–(5) and (7). The complexes under study crystallize either in monoclinic or orthorhombic structure and the space consists; (2) P21/n, (3) P212121, (4) Pbca, (5) P21 and (7) P21/n. The Co(III) ion does not have an electronic preference, however, the structures reflect the conformational preference of RNH2 ligand.

Highlights

► Synthesis of cobalt(III)-alkyl amine complexes showing surface affinity. ► Single crystal X-ray structure refinement of cobalt(III) complexes. ► Electron acceptor complexes for interfacial electron transfer reactions. ► Structural features of complexes can imply their surface affinity.

Introduction

Appropriate tailoring of the coordination geometry and specific incorporation of ligands around the transition metal ions are of key importances [1], [2], [3] in the modification of properties such as; spectroscopic, redox activity in interfacial electron transfer reactions, catalytic and photocatalytic properties of metal complexes, in addition, as biological model compounds. It is also well documented that a number of cobalt(III)-chelate complexes that are structurally different can function as efficient electron-transfer mediators [4] in solar energy conversion schemes. The donating ability of N atoms of amino alkane ligands (RNH2; n = 2–6 carbon atom –CH2/–CH units) are expected to be accountable for their contribution from an electronic point of view, as well as, dictate impacts such as steric hindrance and hydrophobic behavior. Such complex can become surface-active species do function as interfacial electron transfer mediator with enormous promises [5] in light harvesting, charge separation, conversion of electron transfer process into electrochemical potential energy, redox catalytic reaction and in the construction of organized system. Mixed ligand cobalt(III) complexes find potential applications in the fields of antitumor [6], antibacterial [7] antimicrobial [8], [9] and in cytotoxicity studies [10], they are probed to understand the functioning of micro organisms, and the mechanistic aspects of respiration processes in biological systems [11], [12], [13], [14]. Derivatization of RNH2 framework of cobalt(III) complexes containing 1,2-diamino ethane (en) and chloride ion result in substantial weakened metal–ligand interaction, which causes bond length and bond angle distortions. Recently, synthesis and structure refinement by X-ray crystallographic analysis of cis-[CoIII(en)2(RNH2)Cl]Cl2 complexes, in which RNH2 = methyl amine [15] by Miyoshi et al., n-pentyl amine [16], benzyl amine [17] and 4-methyl aniline [18] by Anbalagan et al. and aniline [19], by Deyrup et al. were reported. In continuation of our studies, we probed the variation in geometry of a series of cis-[CoIII(en)2(RNH2)Cl]Cl2 complexes, when the amino alkane ligand such as; RNH2 = EtNH2, PrnNH2, BunNH2, BuiNH2 and HexnNH2 is appended as an ancillary unit in the coordination sphere. Such mixed ligand cobalt(III)-chelates are interesting due to their role to moderate interfacial properties such as wettability, adsorption on surface, to create an appropriate potential gradient for vectorial electron transfer and as electrochemical response unit in heterogeneous photocatalysis.

Section snippets

Materials and instrumental methods

Reagent grade cobaltous chloride, 1,2-diamino ethane, methyl amine (MeNH2), ethyl amine (EtNH2), n-propyl amine (PrnNH2), n-butyl amine (BunNH2), isobutyl amine (BuiNH2), n-pentyl amine (PentnNH2), n-hexyl amine (HexnNH2) and benzyl amine (BzNH2) were obtained from Sigma–Aldrich and used as received. Water was triply distilled over alkaline potassium permanganate [20] in an all glass apparatus. Elemental analyses were performed on Elementar Vario EL III-Germany instrument. The FT-IR spectra of

Synthesis and characterization

Assembling of cobalt(III) complexes with 1,2-diamino ethane and alkyl amine was achieved in a modified synthetic method to obtain cis-[CoIII(en)2(RNH2)Cl]Cl2. The complexes (1)–(8) were synthesized by reacting 1:1 M ratio of trans-[CoIII(en)2Cl2]Cl with alkyl amine, RNH2 (where RNH2 = MeNH2, EtNH2, PrnNH2, BunNH2, BuiNH2, PennNH2, HexnNH2 and BzNH2). FTIR, UV–Vis, 1H NMR and 13C NMR were used to identify the bonding and geometry in the complexes. These complexes are anticipated to show varying

Conclusion

Modified methods have been developed allowing synthesis of a series of complexes possessing varying surface affinity such as: cis-[CoIII(en)2(RNH2)Cl]Cl2 in moderately higher yields. Structurally-controlled cobalt(III) complexes are anticipated to show varying redox activity in interfacial electron transfer reactions, which plays vital role in energy conversion schemes. The research article also explains the crystallographic refinement of five of them, in which, the ancillary ligand RNH2 was

Acknowledgements

KA expresses heartfelt thanks to the Department of Science and Technology (SR/S1/IC-13/2006/6.3.2007) and the Council of Scientific & Industrial Research (01(2140)/07/EMR-II/30.3.2007), New Delhi, for financial assistance. The authors thank DST-FIST, Department of Chemistry, and CIF, Pondicherry University. Authors are grateful to Dr. Babu Varghese and Mr. R. Jagan, SAIF, IIT Madras for their help in single crystal refinement.

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