Exchange Couplings and Magneto-Structural Correlations of Trinuclear MII-UIV-MII (MII=Co, Ni, Cu) Complexes

The magnetic properties of the trinuclear Schiff base complexes M₂UL⁷ (M^II=Co, Ni, Cu; L⁷=N,N’-bis(3-hydroxysalicylidene)-2,2-dimethyl-1,3-propanediamine), exhibiting the [M(μ-O)₂]₂U core structure (3d-5 f-3d subsystem), have been investigated theoretically using scalar relativistic ZORA/DFT computations combined with the broken symmetry (BS) approach. The calculated coupling constants J_MU between the adjacent M1−U and M2−U agree with the observed ferromagnetic (Ferro) character observed in the case of the Cu₂UL⁷ complex, the antiferromagnetic (AF) character of the Ni₂UL⁷ one is consistent with the experimentally observed AF behaviour for Co₂UL⁷. The structural parameters, in particular the M−U distances and the M−O_b−U angles, as well as the electronic factors driving the superexchange couplings are discussed. The bond orders and the magnetic molecular orbital analyses reveal that the U(5 f) covalent contribution to the bonding within the M−O−U coordination is more important in the Co₂UL⁷ and Ni₂UL⁷ complexes than in the Cu₂UL⁷ congener, thus favouring AF coupling between the transition metal and the uranium magnetic centers, in the first complexes. The analyses are supported by the study of the mixed ZnMUL⁷ and M₂ThL⁷ systems, where the Co^II (3d⁷) and Ni^II (3d⁸) paramagnetic ions are replaced by the diamagnetic Zn^II(3d¹⁰) one, whereas in the second complex, the U^IV(5f²) paramagnetic center is replaced by the diamagnetic Th^IV(5f⁰) one. The Natural Populations Analyses confirm the crucial role of spin delocalization that is at work in favour of the AF vs. Ferro magnetic character of the M−U−M (M=Ni, Co) and Cu−U−Cu coordination, respectively.