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 = BF4 (1), ClO4 (2), and BPh4 (3)) (L = 4 '-(3,4-ethylenedioxythiophene)-2,2 ':6 ',2 ''-terpyridine). The impact of various counteranions on stabiliza-tion 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.

Automated summary

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. The impact of various counteranions on stabilization and spin-state switching of the cobalt(II) center was explored. The results revealed that the non-covalent cation-anion interactions played a significant role in stabilizing the spin-state and the complexes exhibited reversible spin-state switching behavior.