Synthesis, electrochemistry, and magnetic properties of binuclear cobalt complexes containing the Co2(µ-X)(µ-carboxylato)2n+ core (X = OH, Cl, or Br; n= 1–3). The crystal structures of [Co2II(µ-ClH2CCO2)2(µ-Cl)L2]PF6 and [CoIICoIII(µ-MeCO2)2(µ-OH)L2][ClO4]2·0.5H2O (L =N,N′,N″-trimethyl-1,4,7-triazacyclononane)

Abstract

A series of novel binuclear cobalt(II) complexes containing the µ-hydroxo-bis(µ-carboxylato)-dicobalt(II) core have been prepared where each cobalt(II) centre is capped by the tridentate macrocycle N,N′,N″-trimethyl-1,4,7-triazacyclononane (L, C₉H₂₁N₃): [Co^II₂(µ-RCO₂)₂L₂(µ-OH)]PF₆[R = Me (1); ClH₂C (2), or Cl₂HC (3)]. The hydroxo bridge may be replaced by a chloro or bromo bridge: [Co^II₂(µ-X)(µ-RCO₂)₂L₂]PF₆[X = Cl, R = Me (4) or ClH₂C (5); X = Br, R = Me (6)]. Air oxidation of (1) and (2) affords the mixed-valence complexes [Co^IICo^III(µ-RCO₂)₂(µ-OH)L₂][PF₆]₂[R = Me (7) or ClH₂C (8)]. Finally, the dicobalt(III) complex [Co^III₂(µ-MeCO₂)₂(µ-OH)L₂][ClO₄]₃(9) has been prepared. Cyclic voltammograms of complexes (1)–(6) in acetonitrile display a reversible and a quasi-reversible one-electron transfer process (Co^II₂⇌ Co^IICo^III+ e^–⇌ Co^III₂+ e^–). The crystal structures of (5) and (7) have been determined by X-ray analyses. Crystals of (5) are orthorhombic, space group Pnam, with a= 13.043(2), b= 14.824(2), c= 17.827(3)Å, and Z= 4. Crystals of (7) are triclinic, space group P, with a= 12.479(8), b= 16.27(1), c= 19.11(2)Å, α= 84.40(6), β= 74.13(6), γ= 68.60(5)°, and Z= 4. In (7) the cobalt-(II) and -(III) centres are clearly identified. From the electronic spectra and structural results it is concluded that in the mixed-valence Co^IICo^III complexes the valences are localized. In the dicobalt(II) complexes weak intramolecular antiferromagnetic coupling of the two cobalt centres (d⁷, high spin) has been detected from temperature-dependent susceptibility measurements in the range 98–298 K.