Modulation of Charge Distribution in Cobalt-α-Diimine Complexes toward Valence Tautomerism

Valence tautomerism (VT) and spin crossover (SCO) are
promising
avenues for developing a range of molecular materials for sensing,
memory, and optoelectronic applications. However, these phenomena
arise only when specific metal–ligand combinations are employed.
The underexplored combination of cobalt(II/III) paired with bis((aryl)imino)acenapthene
(Ar-BIAN) ligands, which can exist as neutral Ar-BIAN0 (L0), monoanionic radical Ar-BIAN•– (L•–), and dianionic Ar-BIAN2– (L2–) forms, has potential to afford both VT and
SCO. Aiming to develop a new family of switchable molecules, we systematically
explored a dual-tuning approach by varying the redox state and aryl
substituents in a series of homoleptic [Co(Ar-BIAN)3]n+ complexes (Ar = Ph, n =
2 (12+), 1 (1+), 0 (1); Ar = 3,5-CF3-Ph, n = 0 (2); Ar = 4-MeO-Ph, n = 2 (32+), 0 (3)). As a prelude to synthetic and experimental studies, density
functional theory (DFT) calculations were used to explore the structure
and relative energies of the different electronic forms of each complex,
comprising different cobalt oxidation and spin states and different
ligand oxidation states. Except for compound 3, DFT identified
a HS-CoII-L0 containing ground state for all
complexes, precluding thermally induced SCO or VT. For 3, calculations suggested a possible thermally accessible LS-CoIII-(L•–)3 ⇌ HS-CoII-(L•–)2(L0) VT interconversion. Experimentally, structural and magnetic data
reveal a HS-CoII-L0 containing ground state
for all six compounds in the solid state, including 3, discounting thermally induced VT or SCO. In solution, electrochemical
and spectroscopic analysis also indicate that all compounds exist
as the HS-CoII-L0-containing electromer at 298
K. Intervalence charge transfer (IVCT) bands observed for neutral 1, 2, and 3 at room temperature
suggest the mixed-valence HS-CoII-(L•–)2(L0) charge distribution. However, cooling 3 to 243 K in acetonitrile uniquely affords a substantial
reduction in the intensity of this IVCT band, consistent with thermally
induced VT interconversion to the LS-CoIII-(L•–)3 ground state as predicted by DFT calculations. This
study emphasizes the utility of computationally guided molecular design
for complicated systems with redox activity at the metal and multiple
ligands, thus opening new avenues for tuning electronic structure
and developing new families of switchable molecules.