Rational Design of a Confacial Pentaoctahedron Anisotropic Exchange in a Linear ZnIIFeIIIFeIIIFeIIIZnII Complex

The first confacial pentaoctahedron comprised of transition metal ions namely ZnII FeIIIA FeIIIB FeIIIA ZnII has been synthesized by using a dinucleating nonadentate ligand. The face-sharing bridging mode enforces short ZnII ⋅⋅⋅FeIIIA and FeIIIA ⋅⋅⋅FeIIIB distances of 2.83 and 2.72 A, respectively. Ab-initio CASSCF/NEVPT2 calculations provide significant negative zero-field splittings for FeIIIA and FeIIIB with |DA |>|DB | with the main component along the C3 axis. Hence, a spin-Hamiltonian comprised of anisotropic exchange, zero-field, and Zeeman term was employed. This allowed by following the boundary conditions from the theoretical results the simulation in a theory-guided parameter determination with Jxy =+0.37, Jz =-0.32, DA =-1.21, EA =-0.24, DB =-0.35, and EB =-0.01 cm-1 supported by simulations of high-field magnetic Mossbauer spectra recorded at 2 K. The weak but ferromagnetic FeIIIA FeIIIB interaction arises from the small bridging angle of 84.8° being at the switch from anti- to ferromagnetic for the face-sharing bridging mode.