Spin Interactions and Magnetic Anisotropy in a Triangular Nickel(II) Complex with Triaminoguanidine Ligand Framework

The trinuclear nickel(II) complex Ni₃(saltag^tBu)(bpy)₃(H₂O)₃]Cl (H₅saltag^tBu = 1,2,3-tris(5-tert-butylsalicylidene)amino]guanidine) was synthesized and characterized by experimental as well as theoretical methods. The complex salt crystallizes with three molecules of dimethylformamide (dmf) and water as Ni₃(saltag^tBu)(bpy)₃(H₂O)₃]Cl · 3dmf · 3H₂O ( 1 ) in the trigonal space group P3 , with the complex located on a threefold rotation axis, which is consistent with the molecular C₃ symmetry of the complex cation. Magnetic measurements reveal an antiferromagnetic coupling (J = –35.9 cm^–1) between the nickel(II) ions leading to a diamagnetic ground state for the trinuclear complex cation. Theoretical investigations based on broken-symmetry DFT confirm the antiferromagnetic exchange within the complex cation of 1 . Additional single-ion CASSCF ab initio studies reveal that magnetic anisotropy is present in the system. The experimental and theoretical results for 1 are compared with those of a structurally similar nickel(II) complex that is based on the bromo-substituted derivative of the triaminoguanidine ligand. The differences in their magnetic properties can be attributed to the stronger elongation of the pseudo-octahedral coordination sphere at the nickel(II) centers in case of 1 . The analysis of the magnetic properties of 1 clearly shows that for such exchange coupled systems reliable parameters for the magnetic anisotropy cannot be extracted from experimental data alone.