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

Abstract

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.

Incorporation of Switchable Inorganic Building Blocks into Heterometallic Coordination Polymers

“…Co II in a bis-terpyridine environment is well-known to give spin crossover, although it is often, but not always, ,− a gradual transition. ,, Changes in counterion are well-known to impact SCO transitions due to the change in packing interactions in the crystalline lattice that this causes, ,− and this is also well-known in Co II terpy complexes. ,, …”

“…2 Analogous iron(II) and ruthenium-(II) complexes of terpyridine ligands often show reversible redox behavior, and as such they too have potential as molecular switches. 7 Furthermore, ditopic terpyridine-based ligands have found use in higher order supramolecular assemblies such as in grids, 8 interlocked molecules, 9 dendrimers, 10 coordination polymers, 11 metal organic frameworks, and large self-assembled architectures. 12,13 Synthesis of 4-substituted terpyridines via the Kroḧnke synthesis 14,15 allows for the incorporation of a secondary binding site "out the back" of the terpyridine unit.…”

“…Complexes of terpyridine and 4-substituted terpyridines have been used for their interesting magnetic, electrochemical, photophysical, and catalytic properties. − Relevant herein, cobalt­(II) complexes of terpyridine, of the type [Co­(tpy) 2 ] 2+ , are well-known to undergo a spin crossover from the low spin ( S = 1/2) to high spin ( S = 3/2) state on application of a thermal stimulus, opening them up for use as molecular switches . Analogous iron­(II) and ruthenium­(II) complexes of terpyridine ligands often show reversible redox behavior, and as such they too have potential as molecular switches …”

“…37,48,49 Changes in counterion are well-known to impact SCO transitions due to the change in packing interactions in the crystalline lattice that this causes, 7,49−51 and this is also well-known in Co II terpy complexes. 7,49,52 Solid-state magnetic measurements were carried out, between 50 and 400 K, on solid samples of CoBB-BF 4 •2H 2 O and CoBB-PF 6 (Figure 5). For CoBB-BF 4 •2H 2 O, on the first cooling cycle a gradual decrease in χ m T from 1.14 cm 3 K mol −1 (300 K) to 0.47 cm 3 K mol −1 (50 K, fully low spin) is seen (Figure S43).…”

Spin-state switching: chemical modulation and the impact of intermolecular interactions in manganese(iii) complexes

“…Spin-state switching in molecular iron(II and III) SCO compounds has been well explored with respect to the less common manganese(III) and cobalt(II) systems in an octahedral environment, where in the case of manganese(III) spin conversion happens between the LS (t 4 2g e 0 g , S = 1, 3 T 1 ) and HS (t 3 2g e 1 g , S = 2, 5 E) states. [39][40][41][42][43][44][45][46][47][48][49][50][51] In addition, the alternation of structural features due to SCO in the manganese(III) center is significantly different from that of other SCO metal systems; the HS state contains singly occupied e g orbitals and undergoes more pronounced Jahn-Teller distortion. Further, the small energy difference between the potential surfaces of HS and LS states describes the fast and more sensitive SCO dynamics for manganese(III).…”