Introduction

4. The ligand provided square pyramidal diradical–containing Co(III) complex and square planar Ni(II) and Cu(II) complexes. In all cases a strong antiferromagnetic coupling

3.1: Introduction

Organic moieties containing at least one –radical and coordinated to a metal ion, especially transition metal ion, have achieved great importance to the synthesis of catalysts for metal complex–catalyzed organic small molecules oxidation reactions, for example, oxidation of primary alcohols to aldehydes, catechol to quinone, primary amines to aldehydes, etc.¹ The advantage of having –radical in a molecule is the easy acceptance of electron from substrates. In this note, the syntheses of redox–active organic compounds, known as non–innocent ligands, and their corresponding transition metal complexes as catalysts have drawn considerable attention of chemists.^1,2 Enzymatic reactions are specific and selective³ and therefore, the design of catalysts, i.e., radicalcontaining transition metal complexes, is often influenced by the structure of either active site or radical–containing intermediate of metalloeznyzes.^1,2 Furthermore, aerial oxidation could also be performed using those catalysts.

Copper–containing Amine Oxidases (AOs)⁴ catalyzes oxidation of primary amines to their corresponding aldehydes with concomitant production of one equivalent of ammonia and hydrogen peroxide. The active site of the primary amine oxidase contains a Cu(II) ion which is surrounded by three histidine imidazole units and a closely located topa quinone organic cofactor.⁴ The topa quinone part accepts two electrons from an amine substrate and oxidizes amine to aldehyde in the presence of water. The reduced topa quinone, which undergoes to aminoquinol form, is reoxidized to its initial quinone form by molecular oxygen (air).^4d Therefore, for the design of catalysts for the oxidation of primary amines to the corresponding aldehydes, Cu(II)–quinone complexes would be ideal. However, the synthesis of Cu(II) complex coordinated to a quinone moiety is a highly challenging task as Cu(II)–

quinone complex will not be very stable due to weak coordination property of quinone moiety and will succumb to easy decomposition. Furthermore, reoxidation of catecholate^2–

form, coordinated to a Cu(II) ion, to its two–electron oxidized o–quinone⁰ form by aerial oxygen would not be convenient. Therefore, Cu(II)–quinone complexes can rarely be used for the amine oxidation process. However, radical–containing Mn(IV) complex,⁵ or in situ generated radical–Cu(II) complexes,^4c,6 as proposed, have previously used as catalysts for the oxidation of primary amines. In addition, Fe(III), Cu(I), and Cu(II) salts with additive organic radical, TEMPO, have used as catalysts for the aerial oxidation of primary, secondary amine and anilines.⁷ It is to note that well defined –radical–coordinated Cu(II) complex as functional model for primary amine oxidases is unknown. Hence, we have initially attempted TH-1360_11612213

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to understand the oxidation–mechanism of primary amines by employing a –radical–

containing Cu(II) complex for the betterment in designing of amine–oxidation catalysts.

2–Anilino–4,6–di–tert–butylphenol is a non–innocent ligand and provides a radical–

containing Cu(II), Ni(II), Co(III) complexes. Incorporation of a –CH₂NH₂ group at the – ortho position to the aniline moiety of the ligand would provide a new non–innocent ligand H₄L^CH2NH

2 (Scheme 3.1). This ligand would be a combination of benzylamine, the substrate part, and 4,6–di–tert–butyl–2–aminophenol, the radical generating part (Scheme 3.1). The ligand H₄L^CH2NH

2 provided diradical–coordinated octahedral Co(III) complex; 9, and a ligand backbone modified distorted square planar Ni(II); 10, and Cu(II); 11, complexes. Synthesis of Ni(II) complex under aerobic condition, the ligand should provide a diradical–coordinated square planar Ni(II) complex, that further underwent to aryl migration to generate the Ni(II) complex. Synthesis of Cu(II) complex under air employing the ligand should provide a –

radical–coordinated Cu(II) complex with aldehyde as the –ortho substituent due to the oxidation of –CH2NH2 group. Thereafter, the mechanistic study of the Cu(II) complex formation would get insight the primary amine oxidation process and consequently, would facilitate the design of –radical–coordinated Cu(II) complexes as functional models for primary amine oxidation.

Furthermore, the formation of complexes 10, and 11 where a complete modification of the new ligand propagated via a rearrangement in the position of 2,4–di–tert–butylphenol unit from the aniline part of the ligand to the benzylamine part. Therefore, to understand the modification as well as the rearrangement paths ligand H₃L1 and H₄L2 have also been synthesized (Scheme 3.1). The ligand H₄L2 is the geometrical isomer of H₄L^CH2NH

2, while, H₃L1 is the two–electron oxidized (amine to imine) form of H₄L2. Interesting, both ligands H3L1, and H4L2 reacted with CuCl22H2O in the presence of Et3N under air and provided complex 11 (Scheme 3.1). Similar type result was also obtained in case of Ni(II) complex.

In this chapter, Co, Ni, and Cu complexes using of H4L^CH2NH

2, H3L1 and H4L2 ligands will be described. Furthermore, it has been noticed that ligand H3L1 reacted with salicylaldehyde only in the presence of Lewis acidic Zn(II) ion and provided fluorescent dinuclear complex. The synthesis and characterization of the complex is also described in this chapter.

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Scheme 3.1: Different tridentate ligands and the metal complexes formed by using the ligands.

 (a){[C42H56CoN4O2]Cl}CH3CN●H2O; (9CH3CNH2O)

 (b) [C28H31N3NiO]; (10)

 (c) [C28H31CuN3O]; (11)

 (d) [C56H60N4O4Zn2]; (12)

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