Substituent effects on rhenium and phosphorus hyperfine coupling constants of organometallic rhenium(0) radicals spin trapped by 9,10-phenanthroquinone and its derivatives

Abstract

A series of rhenium(0) radicals were spin trapped by 9,10-phenanthroquinone (C₁₄H₈O₂) and its derivatives, [Re(˙O₂C₁₄H_{8 –n}X_n)(CO)₄], [Re(˙O₂C₁₄H_{8 –n}X_n)(CO)₃(PPh₃)], and [Re(˙O₂C₁₄H_{8 –n}X_n)(CO)₂(PPh₃)₂](n= 1, X = H, OMe-3, Cl-3, Br-3, Prⁱ-3, CN-3, NO₂-2, or NO₂-4; n= 4, X₄= Me₄-1,3,6,8 or -2,4,5,7), and were studied by e.s.r. spectroscopy. The rhenium hyperfine coupling constants, A_Re, for [Re(˙O₂C₁₄H₇X)(CO)₄](X = H, OMe-3, Cl-3, Br-3, Prⁱ-3, CN-3, NO₂-2, or NO₂-4) correlate very well with the Hammett parameter σ_p. Values of A_Re for [Re(˙O₂C₁₄H_{8 –n}X_n)(CO)₃(PPh₃)] and A_P for [Re(˙O₂C₁₄H_{8 –n}X_n)(CO)₃(PPh₃)] and [Re(˙O₂C₁₄H_{8 –n}X_n)(CO)₂(PPh₃)₂] also correlate with the A_Re values for [Re(˙O₂C₁₄H_{8 –n}X_n)(CO)₄]. However, in the correlation of values of A_Re for bis(phosphine) and tetracarbonyl radicals, the data for radicals with a strong electron-withdrawing group attached to 9,10-phenanthroquinone are irregular. It is proposed that the irregularity is due to the stereochemical change of [Re(˙O₂C₁₄H_{8 –n}X_n)(CO)₃(PPh₃)₂] induced by the electron-withdrawing group.