Electron spin resonance studies of organometallic manganese(0) radicals spin trapped by 9,10-phenanthroquinone and its derivatives

Abstract

A series of manganese(0) radicals spin trapped by 9,10-phenanthroquinone (C₁₄H₈O₂) and its derivatives, [Mn(˙O₂C₁₄H₇X)(CO)₄], [Mn(˙O₂C₁₄H₇X)(CO)₃(PPh₃)], and [Mn(˙O₂C₁₄H₇X)(CO)₂(PPh₃)₂](X = H, 3-CN, 2-NO₂, 4-NO₂, 3-Cl, 3-Br, 3-OMe, or 3-Prⁱ), have been studied by e.s.r. spectroscopy. The hyperfine coupling constants A_Mn and A_P correlate well with the Hammett parameter σ_p. The larger the σ_p, the larger the coupling constant A_Mn or A_P. The proton hyperfine coupling constants, A_H for protons at positions 1, 3, 6, and 8 also correlate with σ_p, however, the trend is opposite to that observed for A_Mn or A_P. These correlations can be explained qualitatively by considering the mixing between the O₂C₁₄H₇X π* orbital which is occupied by an unpaired electron and the dπ orbital of the Mn(CO)_4–n(PPh₃)_n fragment, the influence of substituent X on the energy of the π* orbital of 9,10-phenanthroquinone, and the influence of PPh₃ on the energy of the dπ orbital. It was estimated that ca. 30% spin density is delocalized from ˙O₂C₁₄H₇X onto Mn(CO)_4–n(PPh₃)_n, depending on X and n.