Dinuclear Complexes of Linear, Two-Coordinate Iron(II) and Cobalt(II): Geometric Control over Magnetic Exchange between Anisotropic Metal Centers.

Magnetic exchange coupling between high-anisotropy spin centers is a key requirement for single-molecule magnets with high operating temperatures. Here, we report the synthesis and characterization of the dinuclear iron(II) and cobalt(II) complexes [(iPr3Si)(Dipp)NFe]2(m-(NSiMe3)2C6Me3H) (m-Fe2), [(iPr3Si)(Dipp)NFe]2(p-(NSiMe3)2C6Me4) (p-Fe2), and [(iPr3Si)(Dipp)NCo]2(p-(NSiMe3)2C6Me4) (p-Co2), with each metal center residing in a linear, two-coordinate geometry. In these complexes, the two high-magnetic-anisotropy metal centers are linked through a meta- or para-bis(amido)arene, which mediates ferro- or antiferromagnetic superexchange, respectively. Variable-temperature magnetic susceptibility data reveal antiferromagnetic coupling in the para-substituted complexes, giving S = 0 ground states with exchange constants of -3.56(5) cm-1 for p-Fe2 and -9.82(15) cm-1 for p-Co2. In stark contrast, the meta-linker in m-Fe2 mediates weak ferromagnetic exchange (+0.44(1) cm-1), leading to an S = 4 ground state. Here, the coexistence of ferromagnetic coupling and strong single-ion anisotropy gives rise to single-molecule-magnet behavior with a relaxation barrier of 139 cm-1, the highest yet reported for a multinuclear transition metal complex, and the coupling markedly suppresses the fast through-barrier quantum relaxation that plagues related mononuclear species. To gain a microscopic understanding of the magnetic exchange in this family of complexes, we employed a combination of far-infrared magnetospectroscopy and computation. While a spin-only model reproduces the spectroscopic energy levels well, the substantial unquenched orbital angular momentum at the FeII and CoIIcenters inspired a fully ab initio treatment of the electronic structure, crystal-field splitting, and exchange constants. These calculations indicate that both the sign and the magnitude of the exchange constant depend strongly on the linker geometry.