Tuning of Dimensionality and Nuclearity as a Function of Ligand Field Modulation Resulting in Field-Induced Cobalt(II) Single-Ion Magnet

Abstract

The present study reveals that predetermined diverse topology and desired properties can be achieved by strategically designing the ligand and it confirms that the structural and magnetic features of cobalt(II) complexes have a significant impact of the ligand modulation and proper utilization of a coligand. In this work, tactically, two new Pyclen-based macrocyclic N4 ligands have been designed to demonstrate the effect of the ligand field on structure and magnetic properties. Using these macrocyclic N4 ligands, a series of cobalt complexes, [Co(L1–N4)(μ-NCS)]2·[Co(NCS)4] (1), [Co4(L1–N4)2{μ-N(CN)2}8]n·2CH2Cl2 (2), and [Co(L2–N4)(NCS)2] (3), have been synthesized and characterized by different physicochemical techniques, including single-crystal X-ray diffraction, magnetic, and various spectroscopic methods. The dinuclear plus mononuclear (2 + 1) cobalt complex (1) and 2D cobalt coordination polymer (2) are synthesized using the same macrocyclic N4 ligand: 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane (L1–N4) with a variation of coligand from thiocyanato to dicyananamido. A penta-coordinated mononuclear cobalt complex (3) is synthesized by substituting one methyl group of L1–N4 with a carboxaldehyde group, which leads to a [3,9-dimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane-6-carbaldehyde (L2–N4)] ligand. Single-crystal X-ray diffraction studies reveal the impact of strategic modification of ligands and variation of coligand on the coordination environment, dimensionality, and topology of Co(II) complexes. Whereas, detailed magnetic studies disclose the occurrence of Co(II) ions in a high-spin electronic state for all three complexes, yet, only complex 3 displays exciting slow relaxation of magnetization using an external dc magnetic field and acts as a single-ion magnet, while complexes 1 and 2 display no such property. Detailed theoretical studies have been carried out to better understand and justify the experimentally obtained result.