Halogen-Substituent Tuning of Magnetic Anisotropy and Exchange Coupling in Thiocyanate-Bridged Binuclear Cobalt(II) Complexes

We report the synthesis, structures, and magnetic properties of a new family of thiocyanate-bridged binuclear cobalt(II) complexes, [Co 2 (Xphtpy) 2 (μ-SCN) 2 (SCN) 2 ] (Xphtpy = 4′-(4Xphenyl)-2,2′:6′,2″-terpyridine, X = Cl, Br, I), with varying terminal halides in the halogen-functionalized terpyridine ligands. Single-crystal X-ray crystallography reveals that these complexes crystallize in the same monoclinic system P 1 space group with nearly identical structural features. Additionally, the structural distortion around the Co 2+ centers is highly similar, making the halogen-substituent effect play a critical role in tuning the magnetic anisotropy and couplings. Interestingly, the two Co(II) centers are bridged by two SCN – atoms in a reverse parallel mode. Magnetic studies reveal ferromagnetic interaction transferred by thiocyanates between the Co 2+ ions and the strength of this interaction decreases as the ligand field on the pseudo-octahedral Co(II) center decreases from Cl – > Br – > I – . Both density functional theory and ab initio calculations show that individual Co(II) centers show giant positive D values, and simulation of magnetic susceptibility predicts a dominant ferromagnetic interaction between these Co(II) centers in all complexes. Ab initio calculations predict that both the magnitude of the D value for individual Co(II) centers and the strength of this magnetic exchange interaction between the Co(II) centers increase as we move toward heavier halides.