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

A series of rhenium(0) radicals were spin trapped by 9,10-phenanthroquinone (C14H8O2) and its derivatives, [Re(˙O2C14H8 –nXn)(CO)4], [Re(˙O2C14H8 –nXn)(CO)3(PPh3)], and [Re(˙O2C14H8 –nXn)(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, ARe, 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 σp. Values of ARe for [Re(˙O2C14H8 –nXn)(CO)3(PPh3)] and AP for [Re(˙O2C14H8 –nXn)(CO)3(PPh3)] and [Re(˙O2C14H8 –nXn)(CO)2(PPh3)2] also correlate with the ARe values for [Re(˙O2C14H8 –nXn)(CO)4]. However, in the correlation of values of ARe 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(˙O2C14H8 –nXn)(CO)3(PPh3)2] induced by the electron-withdrawing group.