The Mixed‐Valence, Mixed‐Ligand Complex Co₃(thd)₃(EtO)₄(tert‐BuCOO)

Abstract

The choice of solvent (alcohol or hydrocarbon), oxidising agent (air, pure oxygen or hydrogen peroxide), and temperature (ambient to boiling solution) is significant in the oxidation of Co(thd)2 [(thd)– = anion of H(thd) = C11H20O2 = 2,2,6,6‐tetramethylheptane‐3,5‐dione]. With EtOH as solvent, refluxing conditions, and oxygen atmosphere the solid reaction product was established as a 2:1 stoichiometric mixture of Co(thd)3 and Co3(thd)3(EtO)4(tert‐BuCOO). The relative amount of Co(thd)3 increases somewhat with decreasing reaction temperature. One third of the cobalt atoms in Co3(thd)3(EtO)4(tert‐BuCOO) retain their original CoII state (the rest being oxidised to CoIII). With MeOH as solvent negligible amounts of cobalt are available for oxidation since the CoII state instead becomes tied up in Co4(thd)4(MeO)4(MeOH)4 which is virtually insoluble in MeOH. Complete oxidation of CoII to CoIII occurs in 1‐ and 2‐propanol, but apart from Co(thd)3 the composition of the poorly crystalline products could not be identified. The crystal and molecular structures of Co3(thd)3(EtO)4(tert‐BuCOO) have been determined by single‐crystal X‐ray diffraction [a = 9.811(4), b = 22.528(8), c = 50.516(5) Å, β = 99.178(7)° at 150 K; space group P21/c]. The molecular structure comprises a triangular Co3 central cluster linked together by Co–O–Co bridges. The arrangement in the central cluster can be viewed as a cuboid‐resembling Co3O4 configuration (viz. a Co4O4 cuboid with one cobalt vertex vacant). The coordination of the CoIII atoms is approximately octahedral whereas that of the penta‐coordinated CoII atoms is distorted square pyramidal.