Tuning Self-Aggregation and Nuclearity in Multinuclear Co^II Complexes: Synthesis, Structural Diversity, and Changes in Magnetic Anisotropy and Relaxation Behavior

The reaction of HL (2,6-bis-[{(methylthio)phenylimino}methyl]-4-methylphenol) with [Co2(μ-OH2)(O2CCMe3)4(HO2CCMe3)4] (Co2–Piv) in MeCN provided a trinuclear triangular-type complex [Co3L(μ3-OH)(μ1,3-O2CCMe3)3(O2CCMe3)]·MeCN (1), whereas in another reaction, using Co(ClO4)2·6H2O as the source of CoII ions with the additional employment of NaOAc in MeOH resulted in a tetranuclear cubane-type complex [Co4L2(μ3-OH)(μ3-OMe)(μ1,3-CH3CO2)2](ClO4)2·2.5H2O (2). The varying coordination behavior of the ligand was facilitated using two different CoII salts and the availability of different bridging anions, and the solvent medium used prompted two types of aggregations. Complex 1 has two CoII ions bound in the two adjacent pockets of L– in nearly octahedral geometry and one pivalate anion entangled tetrahedral CoII center. Complex 2 has four CoII centers, all having nearly octahedral geometry. From the alternating current (AC) susceptibility measurements, slow relaxation of magnetization for 1 becomes apparent on application of a DC field, but 2 does not exhibit slow magnetic relaxation, regardless of the applied DC field strength. The CASSCF/NEVPT2 calculations disclosed that the octahedral CoII sites in both 1 and 2 have easy-plane magnetic anisotropy of D in the +56.46–63.8 cm–1 range and the transverse component |E| in the 4.96–12.95 cm–1 range. The unique tetrahedral CoII center in 1 holds D = −6.46 cm–1 and |E| = 0.027 cm–1 with a negligible predisposition for quantum tunneling of magnetization, reinforcing the occurrence of field-induced slow magnetic relaxation of 1.