The self-assembly of a redox-active ethylenedioxythiophene (EDOT)-terpyridine-based tridentate ligand and cobalt(II) unit with different counteranions has led to a series of new cobalt(II) complexes [Co(L)2](X)2 (X = BF4 (1), ClO4 (2), and BPh4 (3)) (L = 4′-(3,4-ethylenedioxythiophene)-2,2′:6′,2″-terpyridine). The impact of various counteranions on stabilization and spin-state switching of the cobalt(II) center was explored through detailed magneto-structural investigation using variable temperature single-crystal X-ray diffraction, magnetic, spectroscopic, electrochemical, and spectroelectrochemical studies. All three complexes 1–3 consisted of an isostructural dicationic distorted octahedral CoN6 coordination environment offered by the two L ligands in a bis-meridional fashion and BF4 –, ClO4 –, and BPh4 – as a counteranion, respectively. Complex 2 with ClO4 – counteranion showed a reversible, gradual, and nearly complete spin-state switching between low-spin (LS) (S = 1/2) and high-spin (HS) (S = 3/2) states, while an incomplete spin-state switching behavior was observed for complexes 1 (BF4 –) and 3 (BPh4 –) in the measured temperature range of 350–2 K. The non-covalent cation–anion interactions played a significant role in stabilizing the spin-state in 1–3. Additionally, complexes 1–3 also exhibited interesting redox-stimuli-based reversible paramagnetic HS cobalt(II) (S = 3/2) to diamagnetic LS cobalt(III) (S = 0) conversion, offering an alternate way to switch the magnetic properties.
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, 2), and [Co(L2−N4)(NCS) 2 ] (3), have been synthesized and characterized by different physicochemical techniques, including singlecrystal 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.
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