Rich redox-activity and solvatochromism in a family of heteroleptic cobalt complexes

Abstract

Solvatochromic models of tunable charge transfer bands illuminate environmental interactions that are key to potential sensing or switching applications for a family of cobalt complexes.

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

“…In MeCN and CH 2 Cl 2 , the visible region of the spectra of 1 2+ (PF 6 ) 2 and 3 2+ (PF 6 ) 2 is dominated by absorptions at approximately 25000 cm −1 arising from intraligand processes associated with the L 0 form of the Ar-BIAN ligand (Figures 5, S23, and S24). 48,83 The lack of features associated with Ar-BIAN •− confirms a HS-Co II -(L 0 ) 3 state in solution at room temperature for 1 2+ (PF 6 ) 2 and 3 2+ (PF 6 ) 2 , as observed in the solid state. Compound 1 2+ (PF 6 ) 2 also features a very weak broad band centered at 10160 cm −1 in the near infrared (NIR) region, assigned as a 4 T 1g → 4 T 2g d−d transition; the analogous transition was difficult to discern from the baseline in the case of 3 2+ (PF 6 ) 2 .…”

“…33,300 cm −1 ) and corresponds to the redshift of the high-energy π* orbitals of the ligands, presumably through charge effects. 48 The lower-energy absorbances are all very similar between the Co II species and the oxidized Co III form, but with increased intensities for the Co III containing species.…”

“…All compounds share vibrational patterns associated with the neutral L 0 ligand in the range 1670−1580 cm −1 , with the presence of ligands in different redox states in the neutral species accounting for the more complex spectra than their dicationic counterparts. 48,73 These observations are consistent with structural analysis that 1 2+ (PF 6 ) 2 •0.5DCE•0.5iPr 2 O and 3 2+ (PF 6 ) 2 •DCE exist as HS-Co II -(L 0 ) 3 at 100 K with no L •− present, and structural analysis of 1•0.8EtOH, 2, and 3•EtOH at 100 or 120 K that indicated a dominant HS-Co II -(L •− ) 2 (L 0 ) state in the solid state, which would give rise to both Ar-BIAN 0 and Ar-BIAN •− . For the neutral compounds, the ν(C�N) bands shift toward higher energies in the order 3 < 1 < 2 following the inductive effects of the ligand substituent.…”

“…The dominant high-energy features arise from a combination of Ar-BIAN 0 intraligand transitionsand Ar-BIAN •− intraligand transitions (Figures 5 and S25−S27). 48,83 The Ar-BIAN •− intraligand transitions decrease in energy from 3 (23,810 cm −1 ) to 1 (19,880 cm −1 ) and 2 (19,493 cm −1 ), corresponding to the shift from electron-donating to electron-withdrawing groups on the Ar-BIAN ligand. At lower energies (13,350− 14,250 cm −1 ), ligand-to-metal charge transfer (LMCT) features are also observed for all three neutral compounds (Figure 5).…”

“…47 The synthetic challenges associated with preparing the complexes used in that computational study prompted attention here to be focused on the preparation of a series of homoleptic [Co(Ar-BIAN) 3 ] n+ complexes to gain a deeper understanding of the redox chemistry of these species, and how charge distribution within the molecular framework may be influenced by the the formal redox state of the metal and ligand(s) and the electronic nature of the supporting aryl groups. 48 Pursuing a strategy to achieve VT or SCO behavior in homoleptic Ar-BIAN Co complexes, we adopted a dual approach examining: (1) the impact of the Ar-BIAN oxidation state on the electronic structure of [Co(Ar-BIAN) 3 ] n+ (n = 0− 2) and ( 2) the influence of substituents on the Ar-BIAN aryl (Ar) group. First, we focused on the case of complexes derived from Ph-BIAN with the redox ladder [Co(Ph-BIAN ).…”