Effects of coordination sphere on unusually large zero field splitting and slow magnetic relaxation in trigonally...

Geometric control in mononuclear complexes has come to the forefront in the field of molecular magnets due to its profound effects on relaxation pathways and blocking temperature in single molecule magnets (SMMs). Herein we report the synthesis and magnetic characterization of six trigonally symmetric, divalent Fe, Co, and Ni molecules, with the rigid geometry enforced via the use of the tris-anionic, tetradentate ligand MST (N,N',N''-[2,2',2''-nitrilotris-(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamide)). A systematic study on the effect of converting between trigonal monopyramidal complexes, (Me₄N)[M(MST)], and trigonal bipyramidal complexes, (Me₄N)[M(MST)(OH₂)] was conducted experimentally and computationally. It was found that (Me₄N)[Ni(MST)] exhibits a very large, near record zero-field splitting parameter (D) value of -434 cm^-1, owing to an extremely low lying first excited state. The trigonal monopyramidal cobalt and iron complexes exhibit slow magnetic relaxation under applied fields, resulting in barriers of 45 K and 63.9 K respectively. Coordination of a single water molecule in the open axial site of the trigonal monopyramidal complexes exerts drastic dampening effects on the D value as well as slow relaxation. Computations reveal that coordination of water rotates the D axis away from the C ₃ axis of symmetry resulting in a smaller D value. The aquo species (Me₄N)[Co(MST)(OH₂)] also exhibits magnetic relaxation under an applied field, but the barrier is reduced to 9.9 K. Water coordination totally quenches the magnetic behavior in the iron complex, and reduces the D value for nickel to -185 cm^-1. These results showcase the drastic effect that a small change in the coordination environment can have on magnetic behavior, as well as that trigonal monopyramidal geometry can lead to near record D values.