Ancillary Ligand Coordination Directed Modes of Aggregation in Mixed-Valence Tetranuclear Cobalt Complexes: Synthesis, Structure, Field-Induced SIM Behavior, and Theoretical Insights

Abstract

The contrasting metal–ion coordination reactivity behavior for the multisite ligand H3L ([2,6-bis-{(2-hydroxymethyl-phenylimino)methyl}-4-methylphenol]) with Co(ClO4)2·6H2O in the presence of ancillary ligands NaPiv (Piv = (CH3)3CCO2–) and NH4SCN afforded two different types of mixed-valence tetranuclear cobalt complexes, [CoIII3CoII(HL)(L)(μ4-O)(μ-OH)((CH3)3CCOO)2]ClO4·H2O (1) and [CoIII2CoII2(L)2(NCS)4(CH3CN)2]·CH3CN (2). In the first case, the presence of bridging pivalate anions triggered the oxido-hydroxido-bridged dual control for aggregation, whereas in the second case, terminal thiocyanate coordination forced the dicubane type of aggregation. The connectivity for different numbers of CoIII and CoII ions are achieved through contrasting bridges by ancillary water-molecule-derived HO– plus O2– linkers and ligand-anion-based double phenolato bridges, where terminal monodentate coordinations finally fulfill the octahedral geometries around the CoII ions. Alternating current/direct current (AC/DC) magnetic studies revealed field-induced slow magnetic relaxation for complex 1 that arises from the single octahedral CoII ion connected to three other diamagnetic CoIII ions. Whereas in complex 2, having a tetranuclear CoIII2CoII2 structure, the two CoII centers showed an antiferromagnetic interaction (J = −0.39 cm–1). The structural distortions in these two types of aggregates lead to different magnitudes of easy-axis magnetic anisotropy (D = −51.31 cm–1 for 1 and −31.9 cm–1 for 2) and a small but non-negligible transverse component (E/D = 0.263 for 1 and 0.255 for 2). The static and dynamic magnetic data were analyzed using DFT and CASSCF-based calculations.