Pentagonal-bipyramidal 4d and 5d complexes with unquenched orbital angular momentum as a unique platform for advanced single-molecule magnets: current state and perspectives.

This article overviews the current state and prospects of the concept of advanced single-molecule magnets (SMMs) based on low-spin ( = 1/2) pentagonal-bipyramidal (PBP) 4d and 5d complexes with unquenched orbital angular momentum. This approach is based on the unique property of PBP 4d and 5d complexes to cause highly anisotropic spin coupling of perfect uniaxial symmetry, -zizj - (xixj + yiyj), regardless of the local geometric symmetry. The M(4d/5d)-M(3d) exchange-coupled pairs in the apical positions of the PBP complexes produce Ising-type exchange interactions (|| > ||), which serve as a powerful source of uniaxial magnetic anisotropy of a SMM cluster. In polynuclear heterometallic 4d/5d-3d complexes embodying PBP 4d/5d units and high-spin 3d ions, anisotropic Ising-type exchange interactions produce a double-well potential with high energy barriers , which is controlled by the anisotropic exchange parameters , . Theoretical analysis shows that the barrier is proportional to the difference | - | and to the number of the apical 4d/5d-3d pairs in a SMM cluster, ∝ | - |, which provides an opportunity to scale up the barrier and blocking temperature up to the record values. A novel family of 4d/5d complexes with forced PBP coordination provided by structurally rigid planar pentadentate Schiff-base ligands in the equatorial plane is discussed as a better alternative to the cyanometallates. The possibility of a significant increase in the anisotropic exchange parameters , in PBP complexes with monoatomic apical μ-bridging ligands is examined. The basic principles of molecular engineering the highest barrier through anisotropic exchange interactions of PBP 4d/5d complexes are formulated. The theoretical and experimental results taken together indicate that the concept of high-performance SMMs based on 4d/5d PBP complexes with unquenched orbital angular momentum is an attractive alternative to the currently dominant lanthanide-based SMM strategy.