LnIII (Ln = La, Gd, and Dy) Benzimidazolium Tricarboxylate Coordination Polymers with Hydrogen Bonding Modulated Magnetic Relaxation

The novel LnIII {Ln = Dy (1 ‑Dy ), Gd (2 ‑Gd ), and La (3 ‑La )} benzimidazolium tricarboxylate coordination polymers have been synthesized, and their magnetic properties have been investigated. Single-crystal X-ray analysis revealed that 1 ‑Dy , 2 ‑Gd , and 3 ‑La are one-dimensional coordination polymers with general molecular formulas of {[Ln­(L)2(H2O)4]·(6Br)}∞ [L = 3,3′,3′′-((2,4,6-trimethylbenzene-1,3,5-triyl)­tris­(methylene))­tris­(1-(carboxymethyl)-benzimidazolium)], which crystallized in the monoclinic, P21 /c space group. The solid-state packing of these coordination polymers shows the spiral propagation in a one-dimensional direction. The direct current (dc) magnetic data (susceptibility and magnetization) were collected for 1 ‑Dy and 2 ‑Gd . The alternating current (ac) magnetic measurement for 1 ‑Dy at zero field shows the characteristic signature of single-molecule magnets (SMMs) at low temperatures but without clear maxima. Further, to rationalize the experimentally observed magnetic behavior and to understand the factors affecting the dynamic magnetic behavior of 1 ‑Dy , we performed detailed completed active space self-consistent field (CASSCF) based calculations on 1 ‑Dy . Our detailed theoretical analysis suggests that the hydrogen bonding interaction between the coordinated water molecule and the Br– counteranion increases the equatorial electron density, eradicating the slow relaxation in 1 ‑Dy .