Electron-transfer reactions of 17-electron and 19-electron organometallic radicals, tungsten complexes CpW(CO)3 and CpW(CO)3PPh3

Abstract

Laser flash photolysis of [CpW(CO)[sub 3]][sub 2] was carried out at 490 nm, where the primary photoprocess gives a 17-electron radical, CpW(CO)[sub 3]. The metal radical acts as both an electron donor and acceptor. The radical reduces ferrocenium ions (k = [1.89(4)] [times] 10[sup 7] L mol[sup [minus]1] s[sup [minus]1]) and benzoquinone (k = [2.7(1)] [times] 10[sup 7] L mol[sup [minus]1] s[sup [minus]1]) at 23[degrees]C in acetonitrile. Triphenylphosphine accelerates the reaction of the tungsten radical with ferrocenium ions, suggesting the formation of the more strongly reducing 19-electron radical CpW(CO)[sub 3]PPh[sub 3]. It reacts with ferrocenium ions; k = 3 [times] 10[sup 9] L mol[sup [minus]1] s[sup [minus]1]. The base-induced disproportionation of CpW(CO)[sub 3] was used to evaluate binding constants for CpW(CO)[sub 3] and PPh[sub 3] (K = 6 [+-] 1 L mol[sup [minus]1]) and pyridine (K = 0.16 [+-] 0.04 L mol[sup [minus]1]). Also, the CpW(CO)[sub 3] radical oxidizes decamethylferrocene (2.2 [times] 10[sup 8] L mol[sup [minus]1] s[sup [minus]1]) and N,N,N[prime],N[prime]-tetramethylphenylenediamine (2.6 [times] 10[sup 7] L mol[sup [minus]1] s[sup [minus]1]). In this system there is a secondary process responsible for the re-reduction of TMPD[sup [center dot]+]. We suggest the rapid formation of the radical anion [CpW(CO)[sub 3]][sub 2][sup [center dot][minus]],more » presumed to be a powerful electron donor, to account for back-electron transfer, but this has not been proved. 36 refs., 4 figs.« less