Dinuclear Co(iii)/Co(iii) and Co(ii)/Co(iii) mixed-valent complexes: synthetic control of the cobalt oxidation level

Abstract

The reactivity of cobalt(II) salts towards H(3)L (2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine) was studied in different reaction conditions. Accordingly, the interaction of cobalt(II) acetate with H(3)L in methanol gives rise to the discrete complex [Co(III)(2)L(OAc)(2)(OMe)]*1.5H(2)O.MeOH, 1. Reaction of cobalt(II) acetylacetonate with H(3)L in the presence of dicarboxylic acids was also investigated. Thus, when cobalt(II) acetylacetonate and H(3)L are mixed with terephthalic or malonic acid in 4 : 2 : 1 molar ratios, the mixed valent [Co(II/III)(2)L(acac)(p-O(2)CC(6)H(4)CO(2)H)][Co(II/III)(2)L(acac)(OH)]*2H(2)O*2MeOH, 2 and [Co(II/III)(2)L(acac)(O(2)CCH(2)CO(2)H)][Co(II/III)(2)L(acac)(OH)]*7H(2)O, complexes are isolated. Decreasing the pH of the medium, by addition of a second mol of dicarboxylic acid, leads to [Co(II/III)(2)L(O(2)CCH(2)CO(2))(MeOH)]*2MeOH, 4, while the reaction with terephthalic acid does not proceed. 1, 2 and 4 were crystallographically characterised and all the complexes are dinuclear, with hydrogen bonds that expand the initial nodes. The magnetic characterisation, as well as the NMR spectroscopy, indicates a diamagnetic nature for 1, in agreement with the presence of Co(III), showing the aerial oxidation suffered by the cobalt(II) ions. Nevertheless, are paramagnetic. Temperature variable magnetic measurements were recorded for the crystallographically characterised complexes 2 and 4 and these studies confirm the mixed valence Co(II)/Co(III) nature of the compounds. The best fits of the magnetic data give an axial distortion parameter Delta = 628.7 cm(-1) for 2 and 698.8 cm(-1) for 4, and spin-orbit coupling constant lambda = -117.8 cm(-1) for 2 and -107.0 cm(-1) for 4. Therefore, this study shows that the oxidation degree of the initial cobalt(ii) salt by atmospheric oxygen can be controlled according to the pH of the medium.

Double‐CO32− Centered [CoII5] Wheel and Modeling of Its Magnetic Properties

“…Nevertheless, it can be expanded to include more metal ions and the coupling between them. [42,43] Complex 1 exhibits five nonequivalent Co II ions, which could, for simplicity, be treated using average values of the spin-orbit coupling parameters. However, the system contains metals with coordination numbers five and six.…”

Synthesis and Electrochemistry of Schiff Base Cobalt(III) Complexes and Their Catalytic Activity for Copolymerization of Epoxide and Carbon Dioxide

“…The chemical structures of the complexes as catalysts are related to the overall catalytic copolymerization performance of CO 2 with an epoxide. [8] Transition metal Schiffbase complexes with N,N,O,O-tetradentate ligands have been extensively studied, [9][10][11][12][13][14][15][16] because of their various applications [17][18][19] and importance in the area of coordination chemistry. [20] Cobalt(III) Schiff-base complexes are characterized by the asymmetric structure containing axial ligands, which are frequently labile, and exhibit high activities and yet a significant stability.…”

Binuclear cobalt(II/II) and cobalt(II/III) chelates with O2N2 ligands: synthesis, structure and magnetic studies

“…Weak antiferromagnetic interaction is taking place through phenoxo bridges in the present case. Co1-O-Co1# angle is 100.01°and the weak antiferromagnetism is associated with the structural parameters [2,4,5,[20][21][22][23].…”

“…Besides the isovalent dimeric cobalt complexes, the study of sparsely known high-spin, lowspin Co(II)/Co(III) mixed valence system [5][6][7][8][9] allows one to examine the physicochemical properties and the behavior of the unpaired electrons. Herein, we describe the synthesis and characterization of isovalent Co(II)/Co(II) and heterovalent Co(II)/Co(III) complexes using N 2 O 2 coordinating tetradentate ligands.…”