Self-assembly, structures, magnetic properties and solution behaviors of six mixed-valence cobalt clusters

Abstract

The introduction of multidentate chelating ligands and secondary small inorganic anions into a variable-valence cobalt synthesis system led to the formation of six novel mixed-valence cobalt clusters, namely, [Co^II₂Co^III₂(mdea)₂(OH)₂(2,2′-bipy)₂(CH₃CN)₄]·4ClO₄·2H₂O (SD/Co1), [Co^II₂Co^III₂(dmhmp)₆(CH₃CN)₄]·4ClO₄·4CH₃CN (SD/Co2), [Co^IICo^III₄(mdea)₄(N₃)₄(piv)₂] (SD/Co3), [Co^II₂Co^III₂Na(mdea)₂(N₃)(piv)₈] (SD/Co4), [Co^II₄Co^III₂(dea)₂(Hdea)₄(piv)₄]·2piv (SD/Co5), and [Co^II₂Co^III₂(mdea)₂(dmpea)₂(OH)₂(CH₃CN)₂(H₂O)₂]·4ClO₄·2H₂O (SD/Co6) (H₂dea = diethanolamine, H₂mdea = N-methyldiethanolamine, 2,2′-bipy = 2,2′-bipyridine, Hdmhmp = 3,5-dimethyl-1-(hydroxymethyl)-pyrazole, Hpiv = pivalic acid and dmpea = 1-(3,5-dimethyl-1H-pyrazol-1-yl)-ethanimine). Among them, compounds SD/Co1, SD/Co2 and SD/Co6 have a similar cationic tetranuclear core motif {Co₄O₆} consisting of two Co^III and two Co^II mixed-valence ions. Compound SD/Co3 has two similar neutral pentanuclear {Co₅O₈} clusters co-crystallized in one asymmetric unit that each contains four Co^III ions and one Co^II ion. Compound SD/Co4 consists of a bimetallic pentanuclear calyx-like {Co₄Na} skeleton including two Co^III and two Co^II ions, while compound SD/Co5 has a cationic hexanuclear rod-like cluster {Co₆O₈} consisting of two Co^III and four Co^II ions. In addition, the high-resolution electrospray ionization mass spectrometry (HRESI-MS) technique was introduced to detect their solution behaviors and reveal their evolutional processes. Magnetic measurements indicated that these cobalt clusters show distinct magnetic behaviors at low temperature.