Resonance Raman and Infrared Spectra of Molecular Oxygen Adducts of N,N'-Ethylenebis(2,2-diacetylethylideneaminato)cobalt(II)

Article abstract of DOI:10.1021/ja00376a024

The infrared spectrum of base-free Co(J-en)O2 (H2(J-en) = N,N'-ethylenebis(2,2-diacetylideneamine)) was measured by using matrix cocondensation techniques.Its O2 stretching frequency, ν(O2), at 1260 cm^-1 is much higher than that of Co(acacen)O2 (H2(acacen) N,N'-ethylenebis(acetylacetone imine)) at 1146 cm^-1.This high-frequency shift is attributed to the electron-withdrawing effect of the acetyl groups in Co(J-en) which results in a much less negatively charged O2 relative to that of Co(acacen)O2.Resonance Raman spectra of the O2 adducts of Co(J-en) in solution were obtained and studied as a function of the nature of the axial ligand, solvent polarity, temperature, and O2 pressure.The ν(O2) of the 1:1 and 1:2 (O2/Co) adducts were located in the regions 1150-1130 and 800-820 cm^-1, respectively, by ¹⁶O2-¹⁸O2 substitution.The following conclusions have been obtained by using these ν(O2) as the marker bands: (1) Formation of the 1:1 adduct is favored with a stronger base (axial ligand), in a more polar solvent, at lower temperature, and under higher O2 pressure. (2) The ν(O2) of the 1:2 adduct decreases linearly as the pK_a of the axial ligand increases.Different linear relationships are obtained, howerver, depending upon whether the ligand is a pure ? donor, ? as well as ? donor, or ? acceptor.The 1:2 adduct, 2O2, and its 18O2 analogue have been isolated as crystals and their ν(O2) and ν_s(CoO) (symmetric stretch) assigned at 834 and 562 cm^-1, respectively.The Co-O2 CT bands of the 1:1 and 1:2 adducts in solution equilibria (n-butylamine as the axial ligand) were located at ca. 550 and 610 nm, respectively, via excitation profile studies of their ν(O2) vibrations.Possible origins of these CT bands are discussed in terms of rudimentary MO schemes.