Evidence for ionic samarium(II) species in THF/HMPA solution and investigation of their electron-donating properties

The fundamental nature of samarium(II) complexes in THF/HMPA (HMPA = hexame thylphosphoramide) solutions containing SmI2 has been clarified by means of cyclic voltammetry, conductivity measurements, UV spectroscopy, and kineti c measurements. The principal species is not CSmI2(hmpa)(4)] as previously suggested, but either the ionic cluster [Sm(hmpa)(4)(thf)(2)](2+)2I(-) if f our equivalents of HMPA is present in the THF solution or [Sm(hmpa)(6)](2+) 2I(-) in the presence of at least 10 equivalents of HMPA. The formal potent ial of the [Sm(hmpa)(4)(thf)(2)](3+)2I(-)/[Sm(hmpa)(4)(thf)(2)](2+)2I(-) re dox couple determined by cyclic voltammetry was - 1.79 +/- 0.08 V versus SC E, The order of reactivity of the samarium(II) complexes was found to be [S m(hmpa)(6)](2+) 2I(-) > [Sm(hmpa)(4)(thf)(2)](2+)2I(-) > SmI2 in their resp ective reactions with 1-iodobutane and with benzyl chloride. Very high rate enhancements, of the order of 1000 - 15000-fold, were observed upon additi on of HMPA to the THF solution containing SmI2. Comparison of these rate co nstants with the corresponding rate constants for electron transfer (ET) re actions involving aromatic radical anions revealed that none of the reactio ns studied can be classified as outer-sphere ET processes and that the inne r-sphere electron-donating abilities of the [Sm(hmpa)(4)(thf)(2)](2+)2I(-) and SmI2 complexes are comparable, The inner-sphere ET character of the tra nsition state increases on going from 1-iodobutane and benzyl bromide to be nzyl chloride and acetophenone.