Impact of Counteranion on Reversible Spin-State Switching in a Series of Cobalt(II) Complexes Containing a Redox-Active Ethylenedioxythiophene-Based Terpyridine Ligand.

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 = BF ₄ ( 1 ), ClO ₄ ( 2 ), and BPh ₄ ( 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 CoN ₆ coordination environment offered by the two L ligands in a bis-meridional fashion and BF ₄ ^- , ClO ₄ ^- , and BPh ₄ ^- as a counteranion, respectively. Complex 2 with ClO ₄ ^- 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 (BF ₄ ^- ) and 3 (BPh ₄ ^- ) 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.