Introduction

Now a days, it is known that various enzymatic reactions proceed through the formation of coordinated organic phenolate –radical that arises by one–electron oxidation of tyrosine residue of amino acid to the metal ion in metalloproteins.¹ Hence, the synthesis of radical–containing complexes and the study of transition metal–radical interactions² have drawn great attention to chemists. Fe–containing metalloenzymes such as Escherichia coli ribonucleotide reductase, which contains two subunits; protein B1 and protein B2.³ The active site of protein B2 contains a non–heme iron complex coordinated to a tyrosine radical, which is regenerated during the reduction process of ribose to deoxy–ribose sugar, and found for the enzyme activity.^3,4 The electronic interactions between the organic π–radical and iron center widely depend on structural distortion. Hence, it is interesting to synthesize and structurally characterize radical–coordinated Fe complexes as structural as well as functional models of metalloenzymes. Although, a large number of non–heme iron complexes where the central iron atom directly coordinated with –radicals are reported,⁵ but the non–heme iron complex with a tetradentate mixed ligand to provide monoradical–coordinated Fe complexes are rare.^5f

Tetradentate ligands coordinated with a Fe(III) ion are known to acquire a distorted square pyramidal geometry having an axially bound halogen atom. Structural distortion as well as the reactivity of the Fe(III) complexes can be tuned by introducing different substituent at the ligand backbone. As for an example, a Fe–porphyrin complex where hemoglobin acts as an oxygen carrier metalloenzyme in biology, however, it may act as an oxygen activation catalyst just by protein fine–tuning of the structure around the heme.⁶ Generally, ligand exists at the equatorial position of square pyramidal complexes form a strong metal–ligand bond, while, the axial ligand forms a weak coordinate bond with metal center. However, it has been well established using various model systems and hemoproteins that the electronic structure and function of the Fe(III)–heme complexes strongly depend on the ligands coordinated to the axial sites of the metal ion.⁷ A distorted square pyramidal Fe(III) complex, where the central Fe(III) can stabilize as S = ⁵/2 (HS); or, S = ½ (LS) ground state. Depending upon the structural distortion and displacement of metal center from the basal plane Fe(III) can show another interesting unknown ground state; S = ³/₂ (intermediate spin state).^5a,8

TH-1360_11612213

Page 132

Polynuclear metal complexes having different oxidation states of the metal ion are typically known as mixed valence compounds. Mixed valence compounds have useful importance to study of electron transfer process which is most fundamental reaction in physics, chemistry, and biology.⁹ A mononuclear transition metal complex having redox active metal center and is connected with two redox active ligands generates a mixed valence complex, L–M–L^+, upon one–electron oxidation at ligand center. In this system, electron transfer takes place from electron rich species L to the electron deficient species L^+ through redox active transition metal.¹⁰ Thus, the mixed valence assemblies containing multiple–

redox centers has significant implication in understanding the long range electron transfer process in biology and building molecular conducting devices.

In some instances mixed valence complexes are effective catalysts and reactive towards aerial oxygen to form metal–superoxo / peroxo intermediate during oxidation of various species by activating molecular oxygen.¹¹ Metal–oxo units have been proposed as active intermediates for several enzymatic and biomimetic C–H bond activation reactions.¹² Since then, transition metal mediated dioxygen activation as well as characterization of metal–oxo intermediates have drawn considerable attention of both chemists and biologists.

So, it’s important to investigate the interaction and activation of molecular oxygen by transition metal complexes. Cu(II) and Ni(II) peptide complexes can activate molecular oxygen where Cu(III)–superoxo and Ni(III)–superoxo were consider as an intermediate peptide complex.¹³

Both the steric effect and electronic effect play the crucial role to activate the molecular oxygen by the metal complexes. Some cases, the presence of the donor substituent at the ligand backbone increase the electron density at the metal center and consequently, the electron rich metal center activates the molecular oxygen to form a reactive intermediate metal–superoxo / or metal–oxo species.^11,12

To understand the effect of substituent at the ligand backbone, the steric effect as well as the electronic effect towards metal complexes, two new asymmetric tetradentate mixed ligands (with and without substituent as tert–butyl in the salicylidene unit) and their metal complexes have been synthesized. Tetradentate ligand will be presented here as mixed ligand, since it is composed of both redox active aminophenol (change in its oxidation state is favored in the presence of metal ion and molecular oxygen) and redox inactive salicylidene (change in its oxidation state is not favored in the presence of metal ion and oxygen)

TH-1360_11612213

Page 133

compartments connected via benzyl linker. Both the ligands differ from each other by the presence of tert–butyl group at 3,5–position of the salicylidene unit.

This chapter describes the synthesis and characterization of Fe(III), Ni(II), and Cu(II) metal complexes formed with these mixed ligands. Furthermore, substituent dependent reactivity towards aerial oxygen through metal–superoxo, and metal–peroxo intermediate formation is discussed.

 (a) [Fe(L^Mixed(H))Cl], (13) (b) [Fe(L^Mixed(tBu))Cl], (14)

 (a) [NiL^Mixed(H)], (15) (b) [NiL^Mixed(tBu)], (16) (c) [NiLMixed(tBu), oxidized

], (16a)

 (a) [CuL^Mixed(H)], (17) (b) [CuL^Mixed(tBu)], (18) (c) [CuLMixed(tBu), oxidized

], (18a)

TH-1360_11612213

Page 134