Irreversibility of single electron transfer occurring from trivalent phosphorus compounds to Iron(III) complexes in the presence of ethanol

Various types of trivalent phosphorus compounds (1; Ph(3.n)P(OR)(n)) underwent single electron transfer (SET) to unsubstituted (2H) and 5-chloro-substituted tris(1, 10-phenanthroline)iron(III) complexes (20) in the presence of ethanol in acetonitrile, resulting in the reduction of 2 to the corresponding iron(II) complexes. The rate constants (k(p)) for the overall SET process were determined spectrophotometrically to show that within each series of 1 with an identical alkoxy group OR, log k(p) correlates linearly with the difference in the half-wave potentials (DeltaE(1/2)) between 1 and 2. The slope of each correlation line gave an a-value for each series of 1. The alpha-values were significantly smaller than unity, indicating that the SET step is irreversible, even though this step is endothermic. The trivalent phosphorus radical cation 1(.+) generated in the SET step undergoes a rapid ionic reaction with ethanol, which is certainly the origin of the irreversibility. Upon examining the alpha-values more closely, it was found that the transition state of the SET step becomes earlier with increasing bulkiness of the substituent OR. It is concluded that 1 and 2 form a tight encounter complex to undergo SET from the former to the latter.