Theoretical studies on {3d-Gd} and {3d-Gd-3d} complexes: Effect of metal substitution on the effective exchange interaction

Series of DFT calculations on [L¹Ni(H₂O) ₂Gd(NO₃)₃] (1) and their transition metal substituted models {Cr^IIIGd^III}, {Fe^IIGd ^III}, {Mn^IIGd^III} and {Cu^IIGd ^III} have been performed to probe the electronic effects of the metal substitution on the sign and strength of magnetic coupling. Our results unfold an interesting observation that the strength of the exchange is related to the d-orbital occupancy with occupancy on the e_g orbitals resulting in stronger ferromagnetic exchange while progressively filling the t_2g orbitals leads to a reduction in the computed J values. Further, we extend our studies to the trinuclear {3d-Ln-3d} [L²M₂Ln][ClO ₄] (LH₃ = (S)P[N(Me)N = CH-C₆H ₃-2-OH-3-OMe]₃) (here M = Cu²⁺, Ni²⁺ and Mn²⁺ and Ln = Gd^III and La^III) complexes possessing phosphorus, salicylideneaminooxy and Schiff base supported ligands to corroborate our findings in dinuclear models. By computing the nearest neighbor and next-nearest neighbor interactions in all three complexes, our studies reveal that the observation of strong exchange with e_g occupancy is preserved also in the trinuclear complexes. This will have implications in the area of molecular coolants where Cu^II and Gd^III are the preferred choice as they inherit no/negligible anisotropy, but our finding suggests that Cu^II invariably also offers strong exchange interaction - a parameter unsuitable for ideal molecular coolants. Metals ions such as Mn^II and Cr^III are perhaps better choice for molecular refrigeration as they possess large spin values, negligible anisotropy and mediate relatively a weak magnetic exchange interaction.