Synthesis, structure, spectral and electron-transfer properties of octahedral-[Co^III(L)₂]⁺/[Zn^II(L)₂] and square planar-[Cu^II(L){OC(=O)CH₃}] complexes incorporating anionic form of tridentate bis(8-quinolinyl)amine [N¹C₉H₆-N²-C₉H₆N³, L^-] ligand

Abstract

The reaction of bis(8-quinolinyl)amine [N¹C₉H₆-N²H-C₉H₆N³, LH] with Co^II(ClO₄)₂ . 6H₂O in methanol under aerobic conditions results in a new class of [Co^IIIN₆]⁺ (1⁺) chromophore incorporating an sp²-amido nitrogen center (N²) in the ligand frame. During the course of the reaction, the cobalt ion has been oxidized from its starting +2 oxidation state to +3 state in 1. The reaction of LH with the Cu-acetate yields monomeric square planar complex, [Cu^II(L){OC(=O)CH₃}] (2). The same copper complex 2 is also obtained from Cu(ClO₄) . 6H₂O in presence of CH₃COONa as base. On the other hand, the reaction of Zn(ClO₄) . 6H₂O with LH results in octahedral complex Zn^II(L)₂ (3). The Cu(II) complex 2 displays a four-line EPR spectrum at room temperature. Crystal structure of the free ligand (LH) shows that the amine proton [N(2)H] is hydrogen-bonded with the terminal quinoline nitrogen centers [N(1) and N(3)]. The crystal structure of 1 confirms the meridional geometry of the complex cation. The square planar geometry of copper complex 2 is confirmed by its crystal structure where the acetate function behaves as a monodentate ligand. The free ligand, LH, is found to be highly acidic in acetonitrile-water (1:1) medium and correspondingly the amine proton (NH) readily dissociates leading to its L^- form even in absence of any external base. The pK_b value of L^- is determined to be 2.6. Both cobalt and copper complexes do not show any expected spin-allowed d-d transitions, possibly have masked by the intense charge-transfer transitions. However, in case of cobalt complex 1, one very weak unusual spin-forbidden ¹A_1g → ³T_1g transition has been observed at 935 nm. The quasi-reversible cobalt (III)↔ cobalt(II) reduction of 1 is observed at E⁰, -1.0 V versus SCE. The reactions of bis(8-quinolinyl)amine [N¹C₉H₆-N²H-C₉H₆N³, LH] with Co^II(ClO₄)₂ . 6H₂O, Zn^II(ClO₄)₂ . 6H₂O and Cu^II-acetate result in octahedral-[Co^III(L^-)₂]⁺ and [Zn^II(L^-)₂] and square planar-[Cu^II(L^-){^-OC(=O)CH₃}] complexes, respectively, incorporating an sp²-amido nitrogen center (N²) in the coordinated ligand frame of L. The structural, spectral and electrochemical aspects of the complexes have been described.