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()](X) (X = BF (), ClO (), and BPh ()) ( = 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 - consisted of an isostructural dicationic distorted octahedral CoN coordination environment offered by the two ligands in a bis-meridional fashion and BF, ClO, and BPh as a counteranion, respectively. Complex with ClO counteranion showed a reversible, gradual, and nearly complete spin-state switching between low-spin (LS) ( = 1/2) and high-spin (HS) ( = 3/2) states, while an incomplete spin-state switching behavior was observed for complexes (BF) and (BPh) 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 -. Additionally, complexes - also exhibited interesting redox-stimuli-based reversible paramagnetic HS cobalt(II) ( = 3/2) to diamagnetic LS cobalt(III) ( = 0) conversion, offering an alternate way to switch the magnetic properties.