Experimental and theoretical insights into Co-Ln magnetic exchange and the rare slow-magnetic relaxation behavior of [Co^II<inf>2</inf>Pr]²⁺ in a series of linear [Co^II<inf>2</inf>Ln]²⁺ complexes

We herein report a series of near-linear trinuclear complexes [Co2Ln(HL)4(NO3)](NO3)2 (where HL = (2-methoxy-6-[(E)-2′-hydroxymethyl-phenyliminomethyl]-phenolate) with Ln(iii) = La (1), Ce (2), Pr (3)). For the comparative study, we have also included the recently reported analogous complexes of Gd(iii), Tb(iii), and Dy(iii) (complexes 4-6) with the same H2L ligand. The experimental nature of the dc magnetic susceptibilities profile and an empirical approach revealed that the magnetic exchange interaction between Co(ii) and Ln(iii) having <4f7 (complexes 2 and 3) is antiferromagnetic while the dominant interaction between Co(ii) and Ln(iii) having ≥4f7 (complexes 4-6) is ferromagnetic. Dynamic magnetic relaxation studies on complexes 1-3 revealed the field induced single-molecule magnetic (SMM) behavior of 1 and 3 with effective energy barriers of 10.65 K and 15.03 K respectively, for magnetic relaxation. To the best of our knowledge, 3d-Pr(iii) based zero or field induced SMMs have not been reported to date. CASSCF/SO-RASSI/SINGLE_ANISO based ab initio calculations on the X-ray structures of complexes 1-6, followed by POLY_ANISO simulations, estimated the magnetic exchange coupling constants JCo-Ln and JCo-Co and also rationalized our experimental findings for the dynamic magnetic properties.