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)CH 3}] complexes incorporating anionic form of tridentate bis(8-quinolinyl)amine [N 1C 9H 6–N 2–C 9H 6N 3, L −] ligand

Abstract

The reaction of bis(8-quinolinyl)amine [N 1C 9H 6–N 2H–C 9H 6N 3, LH] with Co II(ClO 4) 2 · 6H 2O in methanol under aerobic conditions results in a new class of [Co IIIN 6] + ( 1 + ) chromophore incorporating an sp 2-amido nitrogen center (N 2) 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)CH 3}] ( 2). The same copper complex 2 is also obtained from Cu(ClO 4) · 6H 2O in presence of CH 3COONa as base. On the other hand, the reaction of Zn(ClO 4) · 6H 2O 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 p K 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 E 0, −1.0 V versus SCE.