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 ( S = 1/2) pentagonal-bipyramidal (PBP) 4d 3 and 5d 3 complexes with unquenched orbital angular momentum. This approach is based on the unique property of PBP 4d 3 and 5d 3 complexes to cause highly anisotropic spin coupling of perfect uniaxial symmetry, - J z S zi S zj - J xy ( S xi S xj + S yi S yj), 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 (| J z | > | J xy |), 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 U eff , which is controlled by the anisotropic exchange parameters J z , J xy . Theoretical analysis shows that the barrier is proportional to the difference | J z - J xy | and to the number n of the apical 4d/5d-3d pairs in a SMM cluster, U eff ∝ | J z - J xy | n , which provides an opportunity to scale up the barrier U eff and blocking temperature T B 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 J z , J xy 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.