Modulation of Magnetic Anisotropy and Exchange Interaction in Phenoxide-Bridged Dinuclear Co(II) Complexes

We report a new class of four dimeric Co­(II) complexes [Co₂(bbpen)­(X)₂] (H₂bbpen = N,N′-bis­(2-hydroxy­benzyl)-N,N′-bis­(2-methylpyridyl)­ethylene­diamine) [X^– = SCN (1), Cl (2), Br (3), and I (4)] with different coordination geometry of two Co­(II) centers (trigonal-prismatic and pseudo-tetrahedral) and their magnetic study. Interestingly, the two Co­(II) centers show two different types of magnetic anisotropy. State of the art ab initio CASSCF analysis reveals that the six-coordinate or the trigonal-prismatic Co­(II) center possesses a consistently large negative axial zero-field splitting (negative D) parameter (∼−60 cm^–1), while the four-coordinate or the pseudo-tetrahedral Co­(II) center exhibits a range of D values from +13 to −23 cm^–1. Ab initio calculations employing the lines model were used to estimate the magnetic exchange as both the Co­(II) centers possess significant magnetic anisotropy. All the complexes display rare ferromagnetic interaction, and the strength of this interaction decreases as the ligand field on the pseudo-tetrahedral Co­(II) center decreases from SCN^– > Cl^– > Br^– > I^–.