Spectroscopy and Electrochemistry of Cobalt(III) Schiff Base Complexes

The structural, spectroscopic, and electrochemical properties of cobalt(III) derivatives of acacen (H₂acacen = bis(acetylacetone) ethylenediimine) and related ligands have been investigated. Electronic structure calculations indicate that the absorption between 340 and 378 nm in Co^III(acacen) spectra is attributable to the lowest π-π^* intraligand charge-transfer transition. Equatorial ligand substitutions affect reduction potentials less than axial ligand changes, consistent with an electronic structural model in which d_Z² is populated in forming cobalt(II). The crystal structure of [Co(3-Cl-acacen)(NH₃)₂]BPh₄ has been determined: The compound crystallizes in the monoclinic space group P2₁/m (No. 11) with a = 9.720(2) Å, b = 18.142(4) Å, c = 10.046(2) Å, β= 100.11(3)°, D_c = 1.339 g cm^-3, and Z = 2; the complex cation, [Co(3-Cl-acacen)(NH₃)₂]⁺, exhibits a slightly distorted octahedral coordination geometry. The distances between the cobalt atom and the two axial nitrogen donor atoms differ only slightly (1.960(6) and 1.951 (6) Å) and are similar to Co-N distances found in cobalt-arnmine complexes as well as the axial Co-N distances in [Co(acacen)(4-MeIm)₂]Br·1.5H₂O; the latter compound crystallizes in the triclinic space group P1̄ (No. 2) with a = 18.466(9) Å, b= 14.936(7) Å, c = 10.111(5)Å, α = 96.27(5)°, β = 94.12(5)°, γ = 112.78(5)°, D_c = 1.447 g cm^-3, and Z = 4.